View metadata, citation and similar papers at core.ac.uk brought to you by CORE

provided by Elsevier - Publisher Connector

Biochimica et Biophysica Acta 1792 (2009) 903–914

Contents lists available at ScienceDirect

Biochimica et Biophysica Acta

journal homepage: www.elsevier.com/locate/bbadis

Review Vacuolar H+-ATPase meets glycosylation in patients with cutis laxa

Mailys Guillard a,1, Aikaterini Dimopoulou b,1, Björn Fischer b,1, Eva Morava c, Dirk J. Lefeber a, Uwe Kornak b,d,1,⁎, Ron A. Wevers a,1

a Laboratory of Pediatrics and Neurology, Radboud University Nijmegen Medical Centre, PO Box 9101, 6500 HB, Nijmegen, The Netherlands b Institute for Medical Genetics, Charité Universitaetsmedizin Berlin, Campus Virchow Augustenburger Platz 1, 13353 Berlin, Germany c Department of Pediatrics, Radboud University Nijmegen Medical Center, Nijmegen, The Netherlands d Max Planck Institute for Molecular Genetics, Berlin, Germany

article info abstract

Article history: Glycosylation of proteins is one of the most important post-translational modifications. Defects in the glycan Received 7 November 2008 biosynthesis result in congenital malformation syndromes, also known as congenital disorders of Received in revised form 22 December 2008 glycosylation (CDG). Based on the iso-electric focusing patterns of plasma transferrin and apolipoprotein Accepted 29 December 2008 C-III a combined defect in N- and O-glycosylation was identified in patients with autosomal recessive cutis Available online 8 January 2009 laxa type II (ARCL II). Disease-causing mutations were identified in the ATP6V0A2 gene, encoding the a2 + Keywords: subunit of the vacuolar H -ATPase (V-ATPase). The V-ATPases are multi-subunit, ATP-dependent proton Glycosylation pumps located in membranes of cells and organels. In this article, we describe the structure, function and Cutis laxa regulation of the V-ATPase and the phenotypes currently known to result from V-ATPase mutations. A V-ATPase clinical overview of cutis laxa syndromes is presented with a focus on ARCL II. Finally, the relationship Congenital disorders of glycosylation between ATP6V0A2 mutations, the glycosylation defect and the ARCLII phenotype is discussed. OMIM 219200 © 2009 Elsevier B.V. All rights reserved. Apolipoprotein C III

1. Introduction in the plasma or organel membrane, is responsible for transfer of protons across the membrane into vesicles or into the extracellular Detailed clinical and laboratory characterization of autosomal space. As shown in Fig. 1, the two domains are connected by a central recessive cutis laxa phenotypes has revealed a surprising connection and two peripheral stalks, which couple ATP hydrolysis to proton between glycosylation defects and cutis laxa. The discovery of translocation through a rotary mechanism similar to the mitochon- mutations in the ATP6V0A2 gene, encoding the a2 subunit of the drial F-ATPase complex [2,3]. The detailed mechanism through which + vacuolar H -ATPase (V-ATPase), in autosomal recessive cutis laxa these processes are coupled has already been described extensively type II (ARCL II) furthermore highlighted the crucial role of the V- and will therefore not be repeated here [4–6]. ATPase in glycosylation processes within the Golgi [1]. In this article, V-ATPases were originally described as intracellular membrane wewillreviewthecurrentknowledgeofV-ATPasecomplex proteins, present in endosomes, lysosomes, Golgi-derived vesicles, composition, its function and regulation. Known diseases associated clathrin-coated vesicles and secretory vesicles. However, they have with V-ATPase malfunction will be described with a special focus on also been identified at the surface of a variety of cell types such as ARCL II. Finally, we will discuss the possible links between a2 renal intercalated cells and osteoclasts, where they play an essential dysfunction, aberrant glycosylation and the disease pathophysiology. role in proton transport across the membrane [2]. In mammalian cells, several subunits of the V1 and V0 domains are expressed as multiple + 2. Vacuolar H -ATPases isoforms. Subunits B, C, E, H, d and e are expressed as two isoforms, subunit G as three isoforms and subunit a even exists in 4 different + Vacuolar H -ATPases are ATP-dependent proton pumps composed isoforms (Table 1) [7–11]. Expression analysis has revealed that many of two multi-subunit domains, V1 and V0, containing eight (A through isoforms are ubiquitously expressed while other isoforms are limited H, total of 640 kDa) and five different subunits (a, c, c″, d and e in to certain cell types [12]. This isoform complexity is thought to be mammals, total of 260 kDa), respectively. The structural organization necessary for tissue- and organel specific targeting of the V-ATPases of the V-ATPase complex is shown in Fig. 1. The cytoplasmic V1 and may also contribute to the control of their activity. domain carries out ATP hydrolysis whereas the V0 domain, embedded 2.1. Structure of the V1 domain and function of the subunits

⁎ Corresponding author. E-mail address: [email protected] (U. Kornak). The subunits of the V1 domain of the ATPase are organized in (i) a 1 These authors contributed equally to this work. ring shaped hexameric head consisting of three copies of A and B, (ii) a

0925-4439/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.bbadis.2008.12.009 904 M. Guillard et al. / Biochimica et Biophysica Acta 1792 (2009) 903–914

complexes could be bound to different sets of other stator subunits such as C and H, while Fethiere et al. provided evidence for a complex of E, G and C subunits. Thus, the first peripheral stalk contains an EGC complex and the second one an EGH complex, both interacting with the N-terminal domain of subunit a [22,23]. The E and G subunits are expressed as more than one isoform. G1 is ubiquitously expressed, while G2 is brain specific and G3 is detected exclusively in kidney [12,27]. Similarly, E2 is expressed in all tissues, as opposed to E1 which is a testis-specific isoform [28]. Subunit C, which is located near the interface of the V1 and V0 domains, is suggested to control the interaction between the two domains [29]. Whereas the C1 isoform of this subunit is expressed throughout several different tissues, the C2 isoform is specifically expressed in placenta and kidney [12]. Subunit H has also been shown to regulate V-ATPase activity, as it suppresses the ATP hydrolytic activity of the soluble V1 domain [30]. Based on cross-linking and electron microscopy studies, subunit H is suggested to be localised to the peripheral stator near the interface between V1 and V0 [31]. Mutation analysis has demonstrated that subunit H bridges the peripheral and central stalks and serves as a mechanical brake to prevent ATP-driven rotation [32]. All subunit isoforms are encoded by different genes, except the two known H isoforms, which seem to be splice variants of the same mRNA [33,34]. Fig. 1. Structure and function of the V-ATPase. ATP binds to and is hydrolysed by the A and B subunits of the hexameric head (light grey). This generates a rotational movement of the head and the central stalk or rotor, composed of subunits D, F and d 2.1.3. Central stalk or rotor: subunits d, D, and F (dark grey), which is transferred to the proteolipid ring (subunits c and c″, white). The main function of the central stalk, composed of D and F Subunit a (black) and the peripheral stalks (subunits C, E, G and H, medium grey) are subunits of V1 and the d subunit of V0, is coupling of the ATP- static. The conformational changes generated by the rotational movement stimulate the hydrolysis and proton transport. Cross-linking experiments have transfer of a proton along the a-subunit and the proteolipid ring. proven the strong interaction between these subunits in bovine V- ATPases [35,36]. In addition, cysteine-mediated cross-linking studies central stalk composed of subunits D and F of V1 and subunit d of V0 showed that D interacts with B subunits at sites predicted to be and (iii) peripheral stalks composed of subunits C, E, G and H of V1 oriented toward the centre of the A3B3 hexameric head [21,25]. and the N-terminal domain of the V0 subunit a. The two types of stalks connect the V1 domain to the V0 domain (Fig. 1 and Table 1) [13]. 2.2. Structure of the V0 domain and function of the subunits

2.1.1. Subunits A and B 2.2.1. The proteolipid ring: subunits c and c″ Three copies of A and three copies of B alternate in the ring-shaped The main component of the V0 domain is a proteolipid ring, head of V1. The six ATP binding sites are located at the interfaces formed by 4 or 5 copies of subunit c (17 kDa) and a single copy of between the A and B subunits, whereas the three catalytic sites are subunit c″ (23 kDa). These highly hydrophobic proteins contain a located solely on the A subunits. Some residues of the B subunit make number of transmembrane helices with buried glutamic acids that contact with these sites, but this is not an essential interaction for are important for proton translocation across the membrane catalysis. In the B subunit, several non-catalytic sites have been [13,37–40]. described that are hypothesized to play a role in the regulation of ATP hydrolysis [14–19]. A unique domain of the A subunit, called the 2.2.2. Subunits d and e “non-homologous region” plays an important role in the control of Subunit d interacts directly with the D and F subunits of the ATP-hydrolysis, associated with proton translocation, through the central stalk, and is also known to associate with the proteolipid ring reversible dissociation of the two V-ATPase domains [20]. This non- of the V0 domain [9,41]. Its specific shape provides the contact homologous region forms a separate domain on the outer surface of surface needed to couple the stalk to the proteolipid ring. Because of V1 and may contribute to peripheral connection between V1 and V0, its location, the d subunit is likely to play the connecting role in the controlling the tightness of coupling of the two domains. Two rotary mechanism of the ATPase [9]. In man, two isoforms of subunit isoforms of the B subunit are known in humans: the B1 isoform is d exist, a d1 isoform that is ubiquitously expressed and a d2 isoform present in kidney but has not been detected in pancreas, muscle, liver, essentially expressed in kidney and osteoclasts [12]. The membrane- lung, brain or heart, whereas the B2 isoform is expressed at a basal bound subunit e is known to be essential for V-ATPase function, yet level in all tissues analyzed, with a higher level in the human brain and its exact location and function within the V0 domain are still kidney [7]. unknown [42,43].

2.1.2. Peripheral stalk or stator: subunits a, C, E, G, and H 2.2.3. Subunit a The peripheral stalk, which spans nearly the entire length of the The V0 subunit a is a 100 kDa protein that presents a two-domain outer surface of V1, is composed of subunits C, E, G, H and the N- structure: an N-terminal, hydrophilic domain extending along V1, and terminal region of V0 subunit a [21]. Cross-linking and electron a hydrophobic C-terminal domain containing multiple transmem- microscopy studies have provided evidence that there are two brane helices [44]. The N-terminal domain is responsible for peripheral stalks in the V-ATPase [22–24]. The E and G subunits are intracellular targeting of the V-ATPase towards the appropriate essentially structural elements. Whereas E spans the entire length of organel membrane. In addition, experiments have shown that in the V1 outer surface, the G subunit is present near the top of the yeast, in vivo dissociation of V-ATPase in response to glucose complex [25,26]. Recent studies by Ohira et al. have shown that the deprivation is controlled by this N-terminal domain [45]. The last two peripheral stalks connecting the two V-ATPase domains are both two transmembrane helices of the C-terminal domain contain critical based on an EG dimer [24]. Venzke et al. suggested that the EG arginine residues that are required for proton transport. Through M. Guillard et al. / Biochimica et Biophysica Acta 1792 (2009) 903–914 905

Table 1 Tissue-specific localisation and function of human V-ATPase subunits.

Subunit Gene Chromosomal Swiss-Prot entry Swiss-Prot acc. Tissue Function References isoform name localisation namea numbera V1 domain A ATP6V1A1b 3q13.2–q13.31 VATA_HUMAN P38606 Ubiquitous ATP binding and hydrolysis [130] Control of ATP hydrolysis B1 ATP6V1B1 2p13.1 VATB1_HUMAN P153131 Kidney, epididymis ATP binding [131] B2 ATP6V1B2 8p22–p21 VATB2_HUMAN P2128 Ubiquitous Regulation of ATP hydrolysis C1 ATP6V1C1 8q22.3 VATC1_HUMAN P21283 Ubiquitous Regulation of V1–V0 interaction [132–135] C2 ATP6V1C2 Chr. 2 VATC2_HUMAN Q8NEY4 Placenta, lung, kidney Element of peripheral stalk/stator D ATP6V1D 14q23–q24.2 VATD_HUMAN Q9Y5K8 Ubiquitous Element of central stalk/rotor [136,137] E1 ATP6V1E1 22q11.1 VATE1_HUMAN P36543 Testis Structural element of peripheral stalk/ [28,133,138] E2 ATP6V1E2 2p21 VATE2_HUMAN Q96A05 Ubiquitous stator F ATP6V1F 7q32 VATF_HUMAN Q16864 Ubiquitous Element of central stalk/rotor [139,140] G1 ATP6V1G1 9q32 VATG1_HUMAN O75348 Ubiquitous Structural element of peripheral stalk/ [140] G2 ATP6V1G2 6p21.3 VATG2_HUMAN O95670 Brain, neuronal tissue stator G3 ATP6V1G3 1q31.3 VATG3_HUMAN Q96LB4 Kidney, epididymis H1 ATP6V1Hc 8q11.2 VATH_HUMAN Q9UI12 Unknown Regulation of ATP-hydrolysis [31,84] H2 Element of peripheral stalk/stator

V0 domain a1 ATP6V0A1 17q21 VPP1_HUMAN Q93050 Neuronal tissue Intracellular targeting of ATPase, [79,141– 146] a2 ATP6V0A2 12q24.31 VPP2_HUMAN Q9Y487 Ubiquitous Proton pumping across membranes a3 ATP6V0A3b 11q13.2 VPP3_HUMAN Q13488 Osteoclast Regulation of versicle formation, (TCIRG1) a4 ATP6V0A4 7q33–q34 VPP4_HUMAN Q9HBG4 Kidney, inner ear, pH sensor and regulator epididymis c ATP6V0C 16p13.3 VATL_HUMAN P27449 Unknown Proteolipid ring, proton transfer [147–149] Vesicle budding c″ ATP6V0B 1p32.3 VATO_HUMAN Q99437 Unknown Proteolipid ring, proton transfer [150,151] d1 ATP6V0D1 16q22 VA0D1_HUMAN P61421 Ubiquitous Connection of rotor with proteolipid [133,152– d2 ATP6V0D2 Chr. 8 VA0D2_HUMAN Q8N8Y2 Kidney, epididymis ring 154] e1 ATP6V0E1 5q35.2 VA0E1_HUMAN O15342 Unknown Unknown [43,135,155] e2 ATP6V0E2 7q36.1 VA0E2_HUMAN Q8NHE4

In order to reflect our interest in the structure of the V-ATPase, we have chosen to retain the ATP6V⁎ nomenclature. a Source: www.expasy.org/. b Smith A.N. et al. [136] defined different official names for these subunits. c Subunit H has two isoforms, generated by two different splice variants of the same gene. channels, the protons are allowed to access the buried glutamic acid to the kidney and the inner ear. An apical localisation of the a4 isoform residues of the proteolipid ring [44]. The models for the transmem- in α-intercalated cells is known [56,57]. Interestingly, the a4 and B1 brane topology of the a subunit either predict eight or nine distribution are highly similar [57]. transmembrane helices [46]. However, an uneven number of helices is suggested by the fact that residues at the C-terminus cannot be 3. Localisation, function and regulation of V-ATPases modified by membrane impermeable agents [47]. Leng et al. have shown in yeast that mutations of specific residues in this domain not 3.1. Cellular and subcellular localisation of V-ATPases only affect the assembly of the V-ATPase, but also result in mistargeting of the subunit [48]. V-ATPases are present in various subcellular compartments in all Four isoforms of subunit a are known, and their role in targeting mammalian cells. As described in the previous section, different theV-ATPasetospecific locations is supported by data from isoforms of numerous subunits are known. The composition of these mammalian cells. Mammalian isoforms share about 50% overall isoforms within the V-ATPase complex and the expression levels are identity, with higher conservation within the transmembrane regions varying in a cell type- and subcellular compartment-specific manner [49]. Broad and overlapping mRNA expression patterns were demon- [58,59]. Thus, there are different levels of regulation to ensure the strated for a1, a2 and a3, but each of the subunits is known to optimal fine tuning of the V-ATPase activity according to the needs of predominate in special compartments of some cell types [49–51].In each subcellular compartment. contrast, the protein distribution is less well characterized and data High V-ATPase expression levels are found in some specialized cell are often contradictory due to the different specificity and sensitivity types. The osteoclast is a prominent example, but also renal of the antibodies used. The a1 isoform has been described to be highly intercalated cells, macrophages, neutrophils and epididymal cells expressed in brain and to reside in synaptic vesicles, which was show strong V-ATPase expression. In osteoclasts, the vacuolar H+- corroborated by mass spectrometric analysis [52]. However, a1 is not ATPase is essential for bone resorption. The ATPase complex is specific for this compartment as indicated by the fact that it was recruited to the plasma membrane by fusion of late endosomes which detected in the Golgi proteome, together with a2 [53]. Fractions leads to the formation of the ruffled border membrane (Fig. 2) [54,60]. enriched for kidney brush border contain subunits a1, a2 and a4 [51]. Renal intercalated cells strongly express V-ATPase to control acid-base In kidney-derived MDCK cells, a2 does not reside in the Golgi, but balance by secretion of acid via the plasma membrane (Fig. 2) [59].In rather in early endosomes [50]. The a3 isoform is mostly expressed in rat epididymis and vas deference the V-ATPase complex is found on late endosomes and lysosomes and in the osteoclast ruffled the apical membrane of epididymal clear cells. In neutrophils V- membrane that is derived from these vesicles [54]. Other data suggest ATPases are localised in intracellular vesicles and at the plasma a localisation of a3 in secretory granules in pancreatic beta cells [55]. membrane. Further cell types that express V-ATPases on their plasma In contrast to the other a-subunits, expression of a4 is highly restricted membrane are the insect midgut cells and some invasive tumour cells 906 M. Guillard et al. / Biochimica et Biophysica Acta 1792 (2009) 903–914

Fig. 2. Multiple roles of V-ATPase in pH regulation and vesicle function. The main function of V-ATPases is the regulation of the luminal pH of different intracellular compartments, the extracellular space and the cytoplasm. (a) In osteoclasts, V-ATPase is recruited from late endosomes/lysosomes to the cell surface membrane where it acidifies the resorption lacuna, also called an “extracellular lysosome”. (b) V-ATPase controls the acid-base balance by pumping protons via the plasma membrane of alpha-intercalated cells into the renal tubule. (c) Receptor-mediated endocytosis: V-ATPase provides the acidic pH in endosomes necessary for the release of internalized ligands from their receptors and the recycling of the unoccupied receptors back to the cell surface. The sorting and trafficking of different kinds of endocytosed physiologically (e.g. growth factors) and pathologically (e.g. toxins) relevant cargo is also influenced by V-ATPases. (d) The V0 domain is suggested to play a role in different exocytic events: (d1) V-ATPase with a1-subunit has special importance for neurotransmitter release in neuronal cells. (d2) The a3 subunit has been shown to be involved in insulin secretion. (d3) Possible role of the a2 subunit for the secretion of extracellular matrix components as indicated by defective elastic fibres in patients with autosomal recessive cutis laxa type 2.

[3]. Hence, the cell types with the most prominent expression are in disruption of the microtubular network inhibits glucose-dependent most cases those that show a plasma membrane localisation of the V- dissociation of V-ATPase without affecting reassembly [65].In ATPase complex. contrast, reassembly requires a novel protein complex called RAVE This plasma membrane localisation, however, is a more recent (regulator of the ATPase of vacuolar and endosomal membranes) evolutionary invention since the vacuolar-type ATPases got their [66]. RAVE interacts with dissociated V1 sectors via the subunits name because they were identified in the yeast vacuole. In contrast, E and/or G [67]. the proton pumps localised at the yeast plasma membrane belong to Aldolase is also proposed to regulate the V1–V0 complex the P-ATPase family. In mammalian cells, the V-ATPase was found in assembly in a glucose-dependent fashion [68]. This function depends many intracellular compartments. Localisation in vesicles of the on the interaction of aldolase with the B subunit, but does not endocytic pathway like early, late and sorting endosomes as well as require enzymatic activity [69]. An interaction of the a-subunit of the lysosomes is long known [61]. Many groups have shown that the V- V0 domain (possibly the N-terminal cytoplasmic domain) and the A ATPase is also localised in secretory and synaptic vesicles and in the subunit of the V1 domain has also been implicated in glucose Golgi compartment [50,53,55,62,63]. In all these intracellular com- sensing [70]. partments, V-ATPase is involved in regulation of pH-homeostasis, The density of proton pumps at the surface of several cell types is membrane trafficking and vesicle fusion. controlled by reversible exo- and endocytosis. In epididymal clear cells, recycling of V-ATPases between intracellular compartments and 3.2. Regulation of V-ATPases the apical membrane is regulated by adenylyl cyclase (sAC) [71]. The presence of a sAC in numerous secretory epithelia, including renal Several mechanisms are described for V-ATPase regulation, intercalated cells, suggests that this might represent a common including V1/V0 domain dissociation, change of their cellular mechanism for V-ATPase regulation. The actin cytoskeleton also localisation and the efficiency of proton transport and ATP hydrolysis. appears to have an important role in the regulation of V-ATPase Reversible dissociation of the V1 and V0 domains takes place in localisation. Modulation of the actin cytoskeleton via gelsolin is response to different stimuli such as glucose depletion and proposed to regulate V-ATPase recycling [72]. Alterations of the activation of antigen processing [64]. It is suggested that dissocia- coupling efficiency between proton transport and ATP hydrolysis have tion and assembly are two independently controlled processes. A been proposed to modulate acidification [61]. Although there is M. Guillard et al. / Biochimica et Biophysica Acta 1792 (2009) 903–914 907 important evidence to support this hypothesis, the mechanisms 3.3.2. Functions of plasma membrane V-ATPases responsible in vivo remain to be further investigated. In a variety of cell types, particularly cells of epithelial origin, V- ATPases are present in the plasma membrane where their major 3.3. Physiological functions of V-ATPases function is acid secretion. In the kidney, this applies to proximal tubular cells, as well as both alpha- and beta-intercalated cells of the V-ATPases serve a number of functions in normal physiology distal tubule and the collecting duct [3]. Proximal tubule cells reabsorb mainly by regulating the pH of different intracellular compartments, bicarbonate, by first converting it into CO2 that can cross the apical the extracellular space and the cytoplasm. membrane. This conversion takes place when acid is secreted into the lumen by the proximal tubular cells. The secretion of acid is carried 3.3.1. Functions of intracellular V-ATPases out by plasma membrane V-ATPases and apical membrane Na+/H+ The acidic pH that V-ATPases produce in endosomes generated exchangers. In the distal tubule and the collecting duct, the V-ATPases by receptor-mediated endocytosis induces the release of inter- are involved in secretion of acid or base equivalents into the urine. nalized ligands from their receptors [61]. This facilitates the Alpha-intercalated cells contain V-ATPases at the apical membrane − − recycling of the unoccupied receptors back to the cell surface and and Cl /HCO3 exchangers at the basolateral membrane. V-ATPases the targeting of the released ligands to the lysosome for are responsible for proton secretion from the cytoplasm into the − − − degradation [3]. Typical examples for this type of transport are, lumen, whereas the Cl /HCO3 exchanger releases HCO3 across the among others, the cholesterol carrier low density lipoprotein (LDL), basolateral membrane and prevents alkalinisation of the cytoplasm asialoglycoproteins, peptide hormones (e.g. insulin) and growth [60]. To the contrary, intercalated cells which express the V-ATPase factors (e.g. EGF) (Fig. 2) [5]. The lysosomal enzymes synthesized basolaterally serve to reabsorb acid equivalents. in the Golgi compartment are transported to the lysosome partially In osteoclasts, V-ATPase complexes are found at the cell surface by a similar mechanism by binding to the mannose-6-phosphate and have a crucial role in bone resorption [60]. Osteoclasts attach to receptor [3,73]. the bone surface and seal off a portion of the extracellular space. Acid V-ATPases also appear to play a role in the formation of endosomal and digestive enzymes that dissolve the bone matrix are secreted into carrier vesicles from sorting endosomes that transport the uncoupled this space that thereby becomes a resorption lacuna. V-ATPases in this ligands to later endosomal compartments. Budding of these vesicles domain of the osteoclast cell surface, called the ruffled border, are requires the binding of beta-COP coat proteins to the early endosomal responsible for acid secretion [80]. The a3 subunit of the Vo domain is membrane, and is dependent upon the acidic lumenal pH generated prerequisite to direct the V-ATPase complexes to the ruffled border by the V-ATPase [3]. [54]. Furthermore, V-ATPases present at the apical plasma membrane Hurtado-Lorenzo et al. suggested that the V-ATPase also operates of clear cells in the vas deferens and epididymus play the main role in as a scaffold protein to recruit factors involved in trafficking [50]. maintenance of luminal pH at a relatively low value. This low pH is It was shown that ARNO (ARF nucleotide-binding site opener) important for sperm maturation [71]. In macrophages and neutro- binds to the N-terminus of subunit a2 of the V-ATPase, which phils, V-ATPases provide proton efflux from the cell. This is necessary localises to early endosomes and the Golgi compartment [53]. because these types of cells are often found in highly acidic ARNO is the guanine exchange factor (GEF) for Arf6 (ADP- environments such as infected regions or tumours. Thus, they demand ribosylation factor 6) which in the same study was shown to great acid secretion capacity in order to maintain their neutral pH [13]. bind to the subunit c of the V0 domain in a pH-dependent manner. V-ATPases also seem to participate in angiogenesis for which invasion This interaction between V-ATPase, Arf6 and ARNO implicates a and migration of microvascular endothelial cells is essential. In these direct, pH-dependent influence of the V-ATPase on membrane cells, plasma membrane V-ATPases have been identified at the leading budding for which Arf6-recruitment is a crucial step. After budding edge, where they appear to play a role in both processes [81]. and uncoating, the vesicles are tethered to the acceptor compart- ment via tethering factors and v- and t-SNARE proteins [74].After 3.3.3. Pathophysiological aspects the vesicle membrane has been brought in close contact with the Envelope viruses and bacterial toxins, such as influenza virus, target membrane both membranes must fuse to form a pore. The Semliki forest virus, vesicular stomatitus virus (VSV), diphtheria toxin V0 domain of V-ATPases has been implicated in this fusion process and anthrax toxin, enter cells via acidic endosomal compartments according to studies about fusion between vacuoles in yeast [75].It (Fig. 2). In these compartments low pH induces the formation of a is suggested that the V0 domains of opposing membranes form membrane pore through which the viral mRNA or cytotoxic portions pairs with each other, which leads to the assembly of fusion pores of the toxin molecules can be translocated into the cytoplasm [82].For [76]. Furthermore, V-ATPases have been shown to play a role in example, the binding of influenza virus to a cell surface receptor is exocytic processes like synaptic vesicle release in Drosophila [77], followed by internalization via clathrin-coated vesicles. Reaching early exosome release of Hedgehog-related proteins in Caenorhabditis endosomes, the acidic environment produced by the V-ATPase elegans [78] and secretion of insulin by pancreatic islet cells [55].In triggers the fusion of the viral and endosomal membranes, which some of these studies, it was demonstrated that the effect of the creates a fusion pore and results in release of the viral RNA into the loss of the a-subunit in the mutants was different from the effect of cytoplasm of the host cell [61]. In the case of toxins, entry occurs either the V-ATPase inhibitor bafilomycin. This indicates that the V0 directly from sorting endosomes, or first into the vesicles contained in domain might be able to directly mediate vesicle fusion by multivesicular bodies followed by release into the cytoplasm from late catalyzing the mixing of the two lipid bilayers due to its highly endosomes [83]. Furthermore, V-ATPases have been implicated in hydrophobic subunit composition. infection of cells by HIV. Subunit H of the V-ATPase appears to play a Lumenal pH regulation is also crucial for posttranslational role in the internalization of the HIV receptor CD4 [84]. In addition to modification in the Golgi compartment. While the pH of the cis this role in entry of viruses and toxins, it has been shown that the E5 cisterna is close to neutral, the trans cisterna has a pH around 6. Like oncoprotein of bovine papilloma virus causes an inhibition of all enzymes, glycosyltransferases have a pH optimum for function and endosomal and Golgi acidification through binding to the c subunit therefore disturbance of pH regulation in the Golgi could lead to of the V-ATPase [36]. Mutations in E5 that disrupt interaction with the glycosylation defects. The pH gradient formed by V-ATPases is also c subunit both prevent inhibition of acidification and block the ability used for different types of secondary active transport. Several of E5 to transform cells [85]. Martinez-Zaguilan et al. found that highly neurotransmitter-proton antiporters are essential for the transport invasive MB231 breast-tumour cells exhibited significant plasma of neurotransmitters into synaptic vesicles [79]. membrane V-ATPase activity in comparison to the poorly metastatic 908 M. Guillard et al. / Biochimica et Biophysica Acta 1792 (2009) 903–914

MCF7 cells [86]. It has been suggested that tumour-cell invasiveness marrow failure, nystagmus, hepatosplenomegaly, hypertelorism, could be enhanced by a low extracellular pH created by V-ATPases psychomotor retardation, and nasal congestion [92]. Seventy-five [61,87]. In these cells V-ATPases may play an additional role by percent of patients, when untreated, die by the age of 4 years as a improving drug resistance. It has been shown that drug-resistant consequence of recurrent infections such as osteomyelitis and tumour cell lines show increased expression of V-ATPase subunits pancytopenia [93]. The diagnosis can be made on the basis of [87]. Thus, V-ATPases could be a potential drug target to increase the characteristic radiologic changes [94] and should be confirmed by drug sensitivity of tumours. molecular genetic analysis. Diffuse sclerosis of the spine and long bones [95] and modelling defects at the metaphyseal ends of long 3.4. Phenotypes elicited by V-ATPase mutations not found in humans bones, resulting in typically widened and blunted diaphyses and metaphyses (‘Erlenmeyer flask’ deformity) are the most typical Two isoforms of the d subunit, d1 and d2, exist in human and mice radiographic features [94]. ‘Bone-in-bone’ appearance can be present which exhibit high similarity [10]. The d2 isoform is expressed mainly in the vertebrae, phalanges, long bones, and pelvic bones. Mutations in the kidney and at lower levels in heart, spleen, skeletal muscle, and in the a3 subunit (ATP6V0A3, TCIRG1, MIM: 604592) of the V-ATPase testis, whereas d1 was detected in various tissues [88]. have been found to be causative for ARO [54,91]. Most mutations It was shown that the inactivation of Atp6v0d2 in mice results in entail a loss of function of the encoded protein. Since early bone normal growth and development [89]. Despite the high expression of marrow transplantation provides the only curative treatment for d2 in the kidney urine pH was normal. However, Atp6v0d2−/− malignant forms of osteopetrosis an early clinical, radiologic and animals showed a strikingly increased bone mass reminiscent of genetic diagnosis is crucial for the prognosis [96]. osteopetrosis. Increased cartilage remnants and reduced serum type 1 collagen cross-linked C-terminal telopeptide indicated a decreased 4.2. Autosomal recessive renal tubular acidosis with and without bone resorption activity. Although d2 was not detectable in sensorineural deafness osteoblasts, osteoblast number and bone formation rate were significantly increased. In vitro investigations revealed that Distal renal tubular acidosis (dRTA) is a rare genetic disease in Atp6v0d2−/− osteoclasts were much smaller due to impaired which the intercalated cells in the collecting duct fail to secrete the progenitor cell fusion. When the in vitro resorption activity of mutant protons required for urinary acid excretion. The dRTA is classified into osteoclasts was normalized by the number of multinucleated four different types according to the mode of inheritance and the osteoclasts it was undistinguishable from wildtype. Therefore, the presence of hearing impairment. Different forms have been described: d2 subunit, in contrast to the a3 subunit (see above), does not play a a) a dominant type of dRTA, b) a recessive type without hearing loss, role in acid secretion via the ruffled border, but by a yet unknown c) a recessive type with early onset of sensorineural deafness (MIM: mechanism regulates fusion of osteoclast progenitors. 267300) and d) a recessive type with later onset of sensorineural Peri and Nuesslein-Volhard demonstrated that knock down of the deafness (MIM: 602722) [97]. The clinical severity is very variable, V-ATPase a1 subunit in zebra fish leads to defects in brain ranging from compensated mild acidosis, absence of symptoms, and development due to the reduced phagocytotic removal of apoptotic the incidental finding of kidney stones and/or renal tract calcification neurons by microglial cells [63]. Normally, the phagolysosomes to major effects in infancy with severe acidosis, impaired growth, and required for this process are formed by fusion of many acidic vesicles. early nephrocalcinosis causing eventual renal insufficiency in combi- The defective generation of phagolysosomes after knock down of the nation with progressive and irreversible bilateral sensorineural a1 subunit shows that the a1 subunit mediates fusion between hearing loss [98]. If untreated, this acidosis may result in dissolution phagosomes and lysosomes during microglial phagocytosis, a function of bone, leading to osteomalacia and rickets. In 1999, Karet et al. first that is independent of its proton pump activity. associated mutations in the AT6B1 gene to dRTA with sensorineural In Drosophila mutations in the vha-100 gene encoding the a1 deafness [99]. ATP6V1B1 encodes the V-ATPase B1 subunit. Northern subunit lead to impaired photoreceptor function due to a defect Blot analysis revealed high expression of ATP6V1B1 in human kidney in synaptic transmission [77]. Subsequent analysis demonstrated and cochlea. By immunohistochemistry the protein was detected in that lack of a1 unexpectedly did not impair loading of synaptic the apical membrane of the renal α-intercalated cells of the distal vesicles with neurotransmitters, but their fusion with the tubule [99]. Mutations introduce premature termination codons, presynaptic membrane. frameshift mutations, splice-site mutations and non-conservative missense substitutions. Subsequently, mutations in another V-ATPase 4. Human phenotypes arising from mutations in gene, ATP6V0A4, were identified in kindreds with recessive renal V-ATPase subunits tubular acidosis [57]. The gene encodes subunit a4 of the vacuolar H+- ATPase, which was found to be exclusively expressed in foetal and Genetic defects in V-ATPase genes have been associated with adult kidney. While ATP6V1B1 mutations are associated with early hereditary human diseases. Mutations in four known genes encoding sensorineural hearing loss (SNHL), ATP6V0A4 mutations are classically specific subunits of V-ATPases have been identified so far. associated with either late-onset SNHL or normal hearing. Vargas- Poussou R. et al. assessed the phenotype and genotype of 39 families 4.1. Infantile malignant autosomal recessive osteopetrosis (ARO) with recessive dRTA [100]. Unexpectedly, 7 out of the 21 probands with mutations in ATP6V0A4 developed severe hearing loss at young Infantile malignant osteopetrosis (MIM: 259700) is a rare auto- age. This study demonstrates that mutations in either of these genes somal recessive disease with an average incidence of 1:200,000 to may cause early deafness. 1:300,000 [90]. It is characterized by high bone density due to a failure of osteoclasts to resorb bone. The clinical features comprise abnormal 5. Cutis laxa bone remodelling, deficient haematopoiesis, and neurologic impair- ment owing to narrowing of the foramina [90]. Bony encroachment Recently, defects in the V-ATPase a2 subunit due to mutations in can cause facial nerve entrapment resulting in facial palsy, visual the ATP6V0A2 gene have been shown to cause an autosomal recessive impairment and optic atrophy, hearing loss, and difficulties with form of cutis laxa in combination with glycosylation defects [1,101]. swallowing and feeding [90,91]. The disease is most commonly Cutis laxa is a rare inherited disorder of the connective tissue diagnosed soon after birth or within the first years of life with characterized by excessive sagging of skin folds and decreased symptoms of variable severity such as anaemia caused by bone elasticity of the skin. Patients suffering from cutis laxa present with M. Guillard et al. / Biochimica et Biophysica Acta 1792 (2009) 903–914 909 an aged appearance. Most inherited types occur in syndromic forms in variable degree with or without mental retardation. Their perinatal associated with variable organ involvement and severity. One of the history is mostly normal, and generalized cutis laxa is present already major diagnostic criteria is the detection of decreased amounts of at birth. Skin biopsy in the most severe cases demonstrates an structurally abnormal elastin fibres in the skin biopsy, which can, abnormal, broken, shortened and fuzzy elastic fibre structure with a however be missing in some clinically similar, mostly autosomal significantly decreased amount of elastin. The skin anomalies became recessively inherited cases. In most patients diagnosed with cutis laxa less obvious, and sometimes disappear with age. Cardiac anomalies are syndrome the underlying genetic cause is not known. Due to overlap rare. Many of the patients have congenital hip dislocations and in the phenotypes and significant clinical variability the exact clinical increased joint laxity. The systemic involvement is very mild, including diagnosis remains challenging [102]. Dominant, recessive and x- elevated liver enzyme activities and slightly abnormal coagulation in a chromosomal modes of inheritance have been described. very few patients. Common ophthalmologic abnormalities are stra- The autosomal dominant form of cutis laxa syndrome (ADCL; MIM: bismus, myopia or amblyopia. Congenital or progressive microcephaly 123700) is relatively mild, compared to the recessive neonatal form is the most common associated feature in patients. Congenital brain [103]. Systemic involvement seldom occurs except for cardiac malformations are extremely rare. Although developmental delay can symptoms [104]. Some patients with autosomal dominant inheritance be observed in the majority of the children the motor development carry fibulin-5 (FBLN5) gene mutations, or deletions in the elastin gene improves in most cases with age. One might suspect that the motor (ELN) [104,105]. The X-linked variant (XLCL; MIM: 304150) is due to a delay was partially due to muscle hypotonia, and in some degree to the defect of the cellular copper transport and has a wide phenotypic hyperelastic joints. Most of the children demonstrate a normal mental spectrum, and a combination of skeletal and internal organ involve- development and only a minority shows intellectual disability [1,101]. ment [106]. This disorder is allelic to Menkes disease, and is caused by So far, all patients with ATP6V0A2 mutations show a combined defect mutations in the copper transporter gene ATP7A [107]. in the biosynthesis of N- and O-linked glycans. This can easily be picked up with the technique of plasma transferrin isofocusing (see below in 5.1. Autosomal recessive cutis laxa “Defects in the ATP6V0A2 gene lead to a combined defect in N- and O- glycosylation”). No specific clinical features have been found distin- The most severe forms of cutis laxa are inherited in an autosomal guishing the group of patients having the glycosylation abnormalities, recessive fashion and are frequently lethal in the neonatal period and remaining patients within the diagnostic group of ARCL II. (ARCL I; MIM: 219199). Besides cutis laxa, patients show arterial Apparently, the frequency of central nervous system malformations tortuosity, emphysema, diverticulae of urinary and gastrointestinal is somewhat higher in the group with CDG, but congenital brain tract. Mutations in the fibulin-4 (FBLN4, EFEMP2) gene [108] or the malformations have been described in the group without glycosylation fibulin-5 (FBLN5) gene [109,110] have been detected in children with defect as well [101]. ARCL I. Patients classified as autosomal recessive cutis laxa type II (ARCL II, 5.3. CNS abnormalities in ARCL II ARCL Debré type; MIM: 219200) [111,112] have a much better clinical outcome than those with a severe, multisystemic form (ARCL I; MIM: Congenital brain anomalies seldom occur in ARCL II. Motor 219199). In ARCL II children, cutis laxa is associated with prenatal and developmental delay is common but low normal in most cases. A postnatal growth retardation, delayed closure of large fontanels, unique finding is the presence of developmental brain abnormalities congenital hip dislocation, osteoporosis, dental caries and a char- such as migration defects (cobble-stone like brain dysgenesis) in a acteristic facial appearance (down-slanting palpebral fissures, broad couple of cases [1]. These are comparable to the brain anomalies found flat nasal bridge, anteverted nostrils, large ears). There is a develop- in congenital O-mannosylation defects (Walker–Warburg syndrome, mental delay in some of the cases. Osteoporosis and decreased bone Muscle–Eye–Brain disease) [116]. In general, CNS symptoms are less density have been reported in many patients. Within the same clinical frequent in ARCL II (including ATP6V0A2 defects) than in patients group there is biochemical heterogeneity: a congenital combined N- with O-mannosylation defects or in patients with classic disorders of and O-linked disorder of glycosylation can be detected in some of the N-glycosylation. In the latter patient group cerebellar hypoplasia, patients (see below) [1,101,112]. spasticity, epilepsy, hearing- and visual loss occur relatively frequent. Patients with De Barsy syndrome, included in ARCL III (MIM: Neuronal migration is a complex developmental process. Several 219150), show significant phenotype overlap with ARCL II, but no disorders of neuronal migration are known, including a group of patient has been reported so far with combined glycosylation disorders caused by defective O-mannosyl glycan synthesis. The most anomalies. The underlying genetic aetiology of this unique, progres- severe form, Walker–Warburg syndrome (WWS; MIM: 236670) is a sive disorder is still unknown [113]. One should mention two recessive disorder characterized by severe brain malformations, additional clinical syndromes in the clinical spectrum of ARCL muscular dystrophy, and congenital eye abnormalities. Another two syndromes. Gerodermia osteodysplastica (GO; MIM: 231070) [114], similar, but less fatal, diseases are the so called muscle eye brain presents with several features similar to ARCL II, including congenital disease, (MEB; MIM: 253280) and Fukuyama congenital muscular cutis laxa and skeletal findings, including generalized osteoporosis dystrophy (FCMD; MIM: 253800). and increased fracture rate. GO is caused by mutations in the gene In brain, several different proteins and glycosylated molecules have SCYL1BP1 encoding a novel Golgi protein [115]. Another similar entity been hypothesized to be responsible for the occurrence of develop- is wrinkly skin syndrome (WSS; MIM: 278250) [114], a mild variant of mental defects. Failed interactions with extracellular matrix compo- ARCL II, without significant histological abnormalities, and a nents may underlie the severe neuronal migration that causes the biochemical and genetic heterogeneity. congenital anomalies in the MEB disease spectrum, such as in the currently described novel glycosylation defect. 5.2. Clinical features in ARCL II 6. Glycosylation defects Recently, missense and nonsense mutations were discovered in the ATP6V0A2 gene in several families diagnosed with autosomal recessive Glycosylation is one of the most variable post-translational cutis laxa type II and wrinkly skin syndrome, suggesting that both are modifications of proteins. It plays an important role in cell migration, phenotypic variants of the same disorder [1,101,112]. ARCL II and WSS pathway finding and cellular connectivity, as well as localisation, patients with a ATP6V0A2 mutation have generalized skin abnormal- interaction and function of glycoproteins. Since 50% of the proteome ities, skeletal, neuromuscular and central nervous system involvement consists of glycoproteins that play many different roles in the 910 M. Guillard et al. / Biochimica et Biophysica Acta 1792 (2009) 903–914

Fig. 3. N-glycosylation of transferrin (Tf). Left: N-glycans present on tetrasialotransferrin (▪ = GlcNAc, = Man, ○ = Gal, = sialic acid); right: iso-electric focusing of human plasma transferrin. Lane 1: normal IEF profile, lane 2: type I IEF profile, characteristic for N-glycan biosynthesis defects localised in the cytoplasm or the ER (decreased tetrasialo-Tf, increased disialo- and monosialo-Tf), lane 3: asialo type II IEF profile, characteristic for Golgi based glycosylation defects (decreased tetrasialo-Tf, increase of all hypoglycosylated forms), lane 4: disialo type II IEF profile, characteristic for patients with cutis laxa due to ATP6V0A2 defect (decreased tetrasialo-Tf, increased trisialo- and disialo-Tf). The double banding pattern observed in the IEF profiles of lane 3 and 4 is due to a polymorphism of transferrin [adapted from 122]. organism, it is not surprising that inborn errors of glycosylation result glycan) are increased (Fig. 3). If the defect occurs after transfer of the in a wide variety of congenital malformation syndromes, collectively oligosaccharide to the protein, the result is a deficient processing of called congenital disorders of glycosylation (CDG) [102]. Two types of the glycan antennae and undersialylation thereof. The defect can glycosylation are known, based on the attachment site of the glycan influence biosynthesis and transport of nucleotide sugars or the on the protein: N- and O-glycosylation. activity and localisation of glycosidases and glycosyltransferases. In addition, proper transport of the glycoprotein from ER to cis-, median 6.1. N-glycosylation defects and trans-Golgi cisternae is important for the correct sequential removal of terminal glucoses and mannoses and the addition of new The biosynthesis of N-glycans is a multistep process taking place monosaccharides to the glycan. Ultimately, complex N-glycans, such in different cellular compartments. It starts in the cytosol, where as those present on human transferrin, may present truncated nucleotide sugars are synthesized from monosaccharides derived antennae, thus lacking one or two sialic acids. This leads to a typical from dietary sources and recycling processes. Next, in the endoplas- type II transferrin iso-electric focusing pattern, as seen in Fig. 3. matic reticulum, a lipid-linked oligosaccharide (LLO), consisting of Further biochemical analysis to find the exact diagnosis of the CDG dolichol-PP-GlcNAc3-Man9-Glc3, is constructed and transferred to an subtype include enzyme measurements, analysis of lipid linked asparagine of a nascent protein. Defects in the assembly and transfer oligosaccharides and mutation analysis [118]. of the LLO result in a congenital disorders of glycosylation type I (CDG-I) [117]. Finally, the glycoprotein leaves the ER and enters the 6.2. O-glycosylation defects Golgi, where N-glycans are processed through the sequential action of a number of specific glycosyltransferases and glycosidases in a highly The onset of O-glycosylation takes place in the ER or in the Golgi controlled way. Defects in the processing of N-glycans result in CDG apparatus, after folding and oligomerisation of proteins. O-glycans are type II (CDG-II). Fully processed N-glycans present two, three or four attached to the hydroxyl group of serine or threonine, and antennae terminated by a negatively charged sialic acid (Fig. 3) [117]. occasionally to hydroxylysine. In humans, mucin-type glycans are Iso-electric focusing (IEF) of plasma transferrin is an important the most common form. They are attached to the protein via a GalNAc tool in diagnosing patients with congenital disorders of glycosylation. and are usually limited in size (1 to 10 residues). Within the mucin- Human plasma transferrin contains two N-glycans, each with 2 or 3 type O-glycans, 8 core structures can be distinguished, depending on antennae, usually with a negatively charged terminal sialic acid the second sugar and its glycosidic linkage [119]. Like N-glycosylation, (Fig. 3). In human plasma of healthy individuals, the tetrasialotrans- O-glycosylation starts with the biosynthesis of activated nucleotide ferrin fraction is the most abundant form, containing two biantennary, sugars in the cytosol. In the ER and Golgi, O-glycans are assembled in fully sialylated N-glycans. When a genetic defect is present in the early a highly controlled way by the sequential action of specific, steps of N-glycan biosynthesis, in the cytoplasm or endoplasmic membrane-bound transferases, whose localisation is very important. reticulum, the flux through the biosynthetic pathway of the LLO will Complex processes are involved in vesicular transfer between be overall insufficient. The newly built glycoproteins therefore cannot different cisternae of the Golgi and thus the localisation of the be adequately N-glycosylated. The transferrin molecule, normally transferases. Defects in trafficking within the Golgi can affect acquiring two N-glycans, will then obtain a single or no N-glycan. structure and function of the whole apparatus, leading to impaired Thus, in patients with CDG-I, the percentages of asialotransferrin (no biosynthesis of glycoconjugates, intracellular protein sorting, and N-glycan) and disialotransferrin (only one fully built biantennary N- protein secretion [119]. M. Guillard et al. / Biochimica et Biophysica Acta 1792 (2009) 903–914 911

Wopereis et al. [120] developed a screening method to demon- Fig. 3, lane 4 [116]. Alongside the N-glycan biosynthesis defect, strate defects in the biosynthesis of core 1 mucin-type O-glycans, patients also have a core 1 mucin-type O-glycan biosynthesis defect. based on the iso-electric focusing of serum apolipoprotein C-III (apoC- This is shown by the typical apoC-III1 profile (Fig. 4, lane 3), III). ApoC-III carries one glycan: a core 1 mucin-type O-glycan. As characterized by decreased apoC-III2 and increased apoC-III1. It has shown in Fig. 4, this glycan presents with two sialic acids in its fully been observed that patients may have a normal transferrin isofocusing synthesized form. The corresponding apoC-III isoform is called apoC- profile in the first months of life, but develop the typical transferrin

III2. Two other isoforms exist, based on the number of sialic acids: abnormality later on. In these patients, the apoC-III isofocusing was apoC-III0 (nonsialylated form) and apoC-III1 (monosialylated form) already abnormal in the first months of life [124]. [120]. Compared to the isofocusing profile of apoC-III of healthy individuals, the apoC-III profile of patients with disturbed core 1 7. ATP6V0A2 mutations and their influence on glycosylation and mucin-type O-glycan biosynthesis shows increased amounts of apoC- elastic fibre formation

III0 and/or apoC-III1. While it might be that O-glycosylation defects are responsible for 6.3. Defects in the ATP6V0A2 gene lead to a combined defect of the brain abnormalities found in ARCL II in analogy to known N- and O-glycosylation disorders of O-glycosylation, the connection between loss of the a2 subunit, Golgi dysfunction and the elastic fibre defect remains elusive. As described above, processing of N-glycans and mucin O- Also, the exact mechanism by which mutations in the V-ATPase glycosylation both occur in the Golgi apparatus and defects in this a2 subunit affect glycosylation remains to be elucidated. Two major part of the biosynthetic route are characterized by a type II transferrin functions of the V-ATPase V0 domain are (i) maintenance of the pH isofocusing pattern. Some CDG-II patients also have an abnormal gradient along the secretory pathway by proton transport and (ii) the apoC-III isofocusing profile. A number of factors involved in glycan regulation of protein transport through the facilitation of vesicle biosynthesis are common to the N and O-glycosylation process, such fusion [75]. Both processes are closely related, as vesicle acidification as nucleotide sugars and their transporters. Also, Golgi trafficking and can regulate the recruitment of factors involved in vesicle budding structural integrity are crucial for both types of glycosylation. and fusion. Combined glycosylation defects have been observed in several Axelsson et al. [125] have proven that neutralization of the pH patients, including a group with mutations in one of the subunits of gradient of the secretory pathway results in the relocalisation of the Conserved Oligomeric Golgi (COG) complex. This octameric glycosyltransferases such as N-acetylgalactosaminyltransferase 2, β1,2 complex is essential for the structure of the Golgi complex and is N-acetylglucosaminyltransferase I and β1,4 galactosyltransferase 1. thought to have a role in trafficking between ER and Golgi and They are normally localised in the medial/trans-Golgi and the trans- amongst Golgi cisternae [121,122]. Based on transferrin iso-electric Golgi/TGN, respectively. Bafilomycin A1, a specific V-ATPase inhibitor, focusing patterns as well as apoC-III iso-electric focusing and SDS- causes a relocalisation of these enzymes to the cell surface, while the PAGE profiles of serum from CDG-II patients, Wopereis et al. [123] general morphology of the ER and Golgi apparatus is retained. In defined 6 biochemical subgroups among these patients. Surprisingly, colorectal carcinoma cells, in vitro neutralization of the pH of whole 3 out of 4 patients of the subgroup showing disialotransferrin IEF and cells with NH4Cl leads to a relocalisation of the enzymes to apoC-III1 profiles, displayed an ARCL II phenotype. These patients endosomes and an altered O-glycosylation of gel-forming mucins. turned out to have a defect in the a2 subunit of the V-ATPase [1]. Also, these cells show a structurally abnormal Golgi apparatus, Subsequently, all patients with cutis laxa due to ATP6V0A2 consisting of fragmented and separate elements scattered around mutations known to date have been shown to have an abnormal the nucleus. The same unusual structures were observed in acidifica- transferrin isofocusing profile [1,101,120]. The typical so-called disialo- tion-defective breast cancer cell lines. In these cells, both N and O type II transferrin isoform profile seen in these patients is shown in glycosylation were abnormal, as well as glycosylation of glycolipids [126]. Thus, alterations in the pH in the Golgi may cause changes in glycosylation in two ways: through the alteration of the localisation of glycosyltransferases and through changes in their enzymatic activity. Abnormal Golgi trafficking is often paralleled by alterations of the cisternal structures. No major alterations were observed in Golgi morphology in skin fibroblasts of patients with cutis laxa due to the ATP6V0A2 defect. This is in accordance with experiments where V- ATPase was inhibited specifically with bafilomycin A1 [125]. Delayed retrograde translocation of Golgi membranes to the ER was observed upon brefeldin A treatment, which indicates a disturbed Golgi-ER retrograde transport. This mechanism has also been described in patients with CDG due to a COG complex defect [127]. Therefore, defective retrograde transport seems to be a general mechanism that could result in abnormal N- and O-glycosylation, either directly or via alterations in pH of acidic organels. Disruption of the interaction between the a2-isoform and other proteins that regulate vesicular trafficking as described by Hurtado-Lorenzo et al. [50] may provide another pathophysiological basis for defects in vesicular transport. Functional defects of the elastic fibre system lead to wrinkled skin or cutis laxa. In the skin extracellular matrix (ECM), non-glycosylated Fig. 4. O-glycosylation of apolipoprotein C-III. Lane 1: iso-electric focusing pattern of a (e.g. elastin) and glycosylated proteins (e.g. the fibulins, fibronectin, fi control; lane 2: apoC-III0 pro le of CDG-II patient (decreased apoC-III2, increased apoC- and collagen) have important functions in establishing and maintain- fi III0 and apoC-III1); lane 3: apoC-III1 pro le characteristic for cutis laxa patients with an ing elastic fibres. The final structure and adequate function of collagen ATP6V0A2 defect (decreased apoC-III , increased apoC-III ). The picture also shows the 2 1 and elastin fibres is dependent on the function and anchoring of these probable O-glycan structure present on the apoC-III isoforms. The nomenclature of the two apoC-III IEF profiles is derived from Wopereis et al. [123].(□ = GalNAc, ○ = Gal, = essential matrix molecules. It can be speculated that either abnormal sialic acid) [adapted from 123]. glycosylation, other changes in post-translational processing or simply 912 M. Guillard et al. / Biochimica et Biophysica Acta 1792 (2009) 903–914 impaired secretion of these important ECM proteins as a result of [19] E. Vasilyeva, Q. Liu, K.J. MacLeod, J.D. Baleja, M. Forgac, Cysteine scanning mutagenesis of the noncatalytic nucleotide binding site of the yeast V-ATPase, J. ATP6V0A2 mutations, leads to the cutis laxa phenotype. Reduced Biol. Chem. 275 (2000) 255. glycosylation of fibulin-5 was shown to enhance binding of the protein [20] E. Shao, T. Nishi, S. Kawasaki-Nishi, M. Forgac, Mutational analysis of the non- to tropoelastin and elastic fibre formation in vitro [128]. Acute homologous region of subunit A of the yeast V-ATPase, J. Biol. Chem. 278 (2003) 12985. treatment of cultured cells with several agents that disturb the pH [21] Y. Arata, J.D. Baleja, M. Forgac, Cysteine-directed cross-linking to subunit B gradient in all cellular compartments was shown to severely reduce suggests that subunit E forms part of the peripheral stalk of the vacuolar H+- tropoelastin secretion [129]. Although direct evidence for a role of the ATPase, J. Biol. Chem. 277 (2002) 3357. a2 subunit in tropoelastin-containing secretory vesicles is missing, the [22] J. Fethiere, D. Venzke, D.R. Madden, B. Bottcher, Peripheral stator of the yeast V- ATPase: stoichiometry and specificity of interaction between the EG complex and localisation in the Golgi compartment makes it likely that a2 directly subunits C and H, Biochemistry 44 (2005) 15906. or indirectly influences the formation of Golgi-derived vesicles, either [23] D. Venzke, I. Domgall, T. Kocher, J. Fethiere, S. Fischer, B. Bottcher, Elucidation of for anterograde or retrograde transport. the stator organization in the V-ATPase of Neurospora crassa, J. Mol. Biol. 349 (2005) 659. In conclusion, patients presenting with an autosomal recessive [24] M. Ohira, A.M. Smardon, C.M. Charsky, J. Liu, M. Tarsio, P.M. Kane, The E and G cutis laxa type II phenotype with a combined defect in N- and O- subunits of the yeast V-ATPase interact tightly and are both present at more than glycosylation, have a mutation in the ATP6V0A2 gene. This gene one copy per V1 complex, J. Biol. Chem. 281 (2006) 22752. + [25] Y. Arata, J.D. Baleja, M. Forgac, Localization of subunits D, E, and G in the yeast V- encodes the a2 subunit isoform of the vacuolar H -ATPase. Defects in ATPase complex using cysteine-mediated cross-linking to subunit B, Biochemistry pH regulation of acidic organels or vesicular transport within the cell 41 (2002) 11301. are hypothesized to be involved in the underlying pathomechanism. [26] Y. Arata, T. Nishi, S. Kawasaki-Nishi, E. Shao, S. Wilkens, M. Forgac, Structure, subunit function and regulation of the coated vesicle and yeast vacuolar (H(+))- Further investigations are necessary to elucidate the relation ATPases, Biochim. Biophys. Acta 1555 (2002) 71. between V-ATPase dysfunction, glycosylation defects and the cutis [27] B.P. Crider, P. Andersen, A.E. White, Z. Zhou, X. Li, J.P. Mattsson, L. Lundberg, D.J. laxa phenotype. Keeling, X.S. Xie, D.K. Stone, S.B. Peng, Subunit G of the vacuolar proton pump. Molecular characterization and functional expression, J. Biol. Chem. 272 (1997) 10721. Acknowledgment [28] Y. Imai-Senga, G.H. Sun-Wada, Y. Wada, M. Futai, A human gene, ATP6E1, encoding a testis-specific isoform of H(+)-ATPase subunit E, Gene 289 (2002) 7. fi This work was supported by the European Commission, contract [29] P.M. Kane, The where, when, and how of organelle acidi cation by the yeast vacuolar H+-ATPase, Microbiol. Mol. Biol. Rev. 70 (2006) 177. number LSHM-CT2005-512131 (Euroglycanet). [30] K.J. Parra, P.M. Kane, Reversible association between the V1 and V0 domains of yeast vacuolar H+-ATPase is an unconventional glucose-induced effect, Mol. Cell. Biol. 18 (1998) 7064. References [31] S. Wilkens, T. Inoue, M. Forgac, Three-dimensional structure of the vacuolar ATPase. Localization of subunit H by difference imaging and chemical cross- [1] U. Kornak, E. Reynders, A. Dimopoulou, R.J. van, B. Fischer, A. Rajab, B. linking, J. Biol. Chem. 279 (2004) 41942. Budde,P.Nurnberg,F.Foulquier,D.Lefeber,Z.Urban,S.Gruenewald,W. [32] K.C. Jefferies, M. Forgac, Subunit H of the vacuolar (H+) ATPase inhibits ATP Annaert, H.G. Brunner, B.H. van, R. Wevers, E. Morava, G. Matthijs, M.L. hydrolysis by the free V1 domain by interaction with the rotary subunit F, J. Biol. Van, S. Mundlos, Impaired glycosylation and cutis laxa caused by Chem. 283 (2008) 4512. mutations in the vesicular H+-ATPase subunit ATP6V0A2, Nat. Genet. 40 [33] D. Hui, A. Deppe, G. Wen, T. Leeb, J. Masabanda, A. Robic, B.G. Baumgartner, R. (2008) 32. Fries, B. Brenig, Molecular cloning and chromosomal assignment of the porcine [2] T. Nishi, M. Forgac, The vacuolar (H+)-ATPases—nature's most versatile proton 54 and 56 kDa vacuolar H(+)-ATPase subunit gene (V-ATPase), Mamm. Genome pumps, Nat. Rev. Mol. Cell Biol. 3 (2002) 94. 10 (1999) 266. [3] K.C. Jefferies, D.J. Cipriano, M. Forgac, Function, structure and regulation of the [34] Z. Zhou, S.B. Peng, B.P. Crider, C. Slaughter, X.S. Xie, D.K. Stone, Molecular vacuolar (H(+))-ATPases, Arch. Biochem. Biophys. 476 (1) (2008) 33. characterization of the 50- and 57-kDa subunits of the bovine vacuolar proton [4] M. Grabe, H. Wang, G. Oster, The mechanochemistry of V-ATPase proton pumps, pump, J. Biol. Chem. 273 (1998) 5878. Biophys. J. 78 (2000) 2798. [35] T. Xu, M. Forgac, Subunit D (Vma8p) of the yeast vacuolar H+-ATPase plays a role [5] D.J. Cipriano, Y. Wang, S. Bond, A. Hinton, K.C. Jefferies, J. Qi, M. Forgac, Structure in coupling of proton transport and ATP hydrolysis, J. Biol. Chem. 275 (2000) and regulation of the vacuolar ATPases, Biochim. Biophys. Acta 1777 (2008) 599. 22075. [6] K.W. Beyenbach, H. Wieczorek, The V-type H+ ATPase: molecular structure and [36] T. Xu, E. Vasilyeva, M. Forgac, Subunit interactions in the clathrin-coated vesicle function, physiological roles and regulation, J. Exp. Biol. 209 (2006) 577. vacuolar (H(+))-ATPase complex, J. Biol. Chem. 274 (1999) 28909. [7] B. van Hille, H. Richener, P. Schmid, I. Puettner, J.R. Green, G. Bilbe, Heterogeneity [37] H. Arai, G. Terres, S. Pink, M. Forgac, Topography and subunit stoichiometry of the of vacuolar H(+)-ATPase: differential expression of two human subunit B coated vesicle proton pump, J. Biol. Chem. 263 (1988) 8796. isoforms, Biochem. J. 303 (1994) 191. [38] B. Powell, L.A. Graham, T.H. Stevens, Molecular characterization of the yeast [8] G.H. Sun-Wada, Y. Imai-Senga, A. Yamamoto, Y. Murata, T. Hirata, Y. Wada, M. vacuolar H+-ATPase proton pore, J. Biol. Chem. 275 (2000) 23654. Futai, A proton pump ATPase with testis-specific E1-subunit isoform required for [39] R. Hirata, L.A. Graham, A. Takatsuki, T.H. Stevens, Y. Anraku, VMA11 and VMA16 acrosome acidification, J. Biol. Chem. 277 (2002) 18098. encode second and third proteolipid subunits of the Saccharomyces cerevisiae [9] A.N. Smith, R.W. Francis, S.L. Sorrell, F.E. Karet, The d subunit plays a central role vacuolar membrane H+-ATPase, J. Biol. Chem. 272 (1997) 4795. in human vacuolar H(+)-ATPases, J. Am. Soc. Nephrol. 16 (2005) 1245. [40] A.R. Flannery, L.A. Graham, T.H. Stevens, Topological characterization of the c, c′, [10] G.H. Sun-Wada, T. Yoshimizu, Y. Imai-Senga, Y. Wada, M. Futai, Diversity of mouse and c″ subunits of the vacuolar ATPase from the yeast Saccharomyces cerevisiae,J. proton-translocating ATPase: presence of multiple isoforms of the C, d and G Biol. Chem. 279 (2004) 39856. subunits, Gene 302 (2003) 147. [41] C. Bauerle, M.N. Ho, M.A. Lindorfer, T.H. Stevens, The Saccharomyces cerevisiae [11] Y. Murata, G.H. Sun-Wada, T. Yoshimizu, A. Yamamoto, Y. Wada, M. Futai, VMA6 gene encodes the 36-kDa subunit of the vacuolar H(+)-ATPase Differential localization of the vacuolar H+ pump with G subunit isoforms (G1 membrane sector, J. Biol. Chem. 268 (1993) 12749. and G2) in mouse neurons, J. Biol. Chem. 277 (2002) 36296. [42] J. Ludwig, S. Kerscher, U. Brandt, K. Pfeiffer, F. Getlawi, D.K. Apps, H. Schagger, [12] A.N. Smith, K.J. Borthwick, F.E. Karet, Molecular cloning and characterization of Identification and characterization of a novel 9.2-kDa membrane sector- novel tissue-specific isoforms of the human vacuolar H(+)-ATPase C, G and d associated protein of vacuolar proton-ATPase from chromaffin granules, J. Biol. subunits, and their evaluation in autosomal recessive distal renal tubular Chem. 273 (1998) 10939. acidosis, Gene 297 (2002) 169. [43] M. Sambade, P.M. Kane, The yeast vacuolar proton-translocating ATPase contains [13] A. Hinton, S. Bond, M. Forgac, V-ATPase functions in normal and disease a subunit homologous to the Manduca sexta and bovine e subunits that is processes, Pflugers Arch. (2007). essential for function, J. Biol. Chem. 279 (2004) 17361. [14] E. Vasilyeva, M. Forgac, 3′-O-(4-Benzoyl)benzoyladenosine 5′-triphosphate [44] T. Nishi, M. Forgac, Molecular cloning and expression of three isoforms of the inhibits activity of the vacuolar (H+)-ATPase from bovine brain clathrin-coated 100-kDa a subunit of the mouse vacuolar proton-translocating ATPase, J. Biol. vesicles by modification of a rapidly exchangeable, noncatalytic nucleotide Chem. 275 (2000) 6824. binding site on the B subunit, J. Biol. Chem. 271 (1996) 12775. [45] S. Kawasaki-Nishi, K. Bowers, T. Nishi, M. Forgac, T.H. Stevens, The amino- [15] Y. Feng, M. Forgac, A novel mechanism for regulation of vacuolar acidification, J. terminal domain of the vacuolar proton-translocating ATPase a subunit controls Biol. Chem. 267 (1992) 19769. targeting and in vivo dissociation, and the carboxyl-terminal domain affects [16] Q. Liu, P.M. Kane, P.R. Newman, M. Forgac, Site-directed mutagenesis of the yeast coupling of proton transport and ATP hydrolysis, J. Biol. Chem. 276 (2001) V-ATPase B subunit (Vma2p), J. Biol. Chem. 271 (1996) 2018. 47411. [17] Q. Liu, X.H. Leng, P.R. Newman, E. Vasilyeva, P.M. Kane, M. Forgac, Site-directed [46] V. Marshansky, The V-ATPase a2-subunit as a putative endosomal pH-sensor, mutagenesis of the yeast V-ATPase A subunit, J. Biol. Chem. 272 (1997) 11750. Biochem. Soc. Trans. 35 (2007) 1092. [18] K.J. MacLeod, E. Vasilyeva, J.D. Baleja, M. Forgac, Mutational analysis of the [47] X.H. Leng, T. Nishi, M. Forgac, Transmembrane topography of the 100-kDa a nucleotide binding sites of the yeast vacuolar proton-translocating ATPase, J. Biol. subunit (Vph1p) of the yeast vacuolar proton-translocating ATPase, J. Biol. Chem. Chem. 273 (1998) 150. 274 (1999) 14655. M. Guillard et al. / Biochimica et Biophysica Acta 1792 (2009) 903–914 913

[48] X.H. Leng, M.F. Manolson, M. Forgac, Function of the COOH-terminal domain of [78] S. Liegeois, A. Benedetto, J.M. Garnier, Y. Schwab, M. Labouesse, The V0-ATPase Vph1p in activity and assembly of the yeast V-ATPase, J. Biol. Chem. 273 (1998) 6717. mediates apical secretion of exosomes containing Hedgehog-related proteins in [49] S.B. Peng, X. Li, B.P. Crider, Z. Zhou, P. Andersen, S.J. Tsai, X.S. Xie, D.K. Stone, Caenorhabditis elegans, J. Cell Biol. 173 (2006) 949. Identification and reconstitution of an isoform of the 116-kDa subunit of the [79] N. Morel, J.C. Dedieu, J.M. Philippe, Specific sorting of the a1 isoform of the vacuolar proton translocating ATPase, J. Biol. Chem. 274 (1999) 2549. V-H+ATPase a subunit to nerve terminals where it associates with both synaptic [50] A. Hurtado-Lorenzo, M. Skinner, A.J. El, M. Futai, G.H. Sun-Wada, S. Bourgoin, J. vesicles and the presynaptic plasma membrane, J. Cell Sci. 116 (2003) 4751. Casanova, A. Wildeman, S. Bechoua, D.A. Ausiello, D. Brown, V. Marshansky, V- [80] Y.P. Li, W. Chen, Y. Liang, E. Li, P. Stashenko, Atp6i-deficient mice exhibit severe ATPase interacts with ARNO and Arf6 in early endosomes and regulates the osteopetrosis due to loss of osteoclast-mediated extracellular acidification, Nat. protein degradative pathway, Nat. Cell Biol. 8 (2006) 124. Genet. 23 (1999) 447. [51] N. Schulz, M.H. Dave, P.A. Stehberger, T. Chau, C.A. Wagner, Differential [81] J.D. Rojas, S.R. Sennoune, D. Maiti, K. Bakunts, M. Reuveni, S.C. Sanka, G.M. localization of vacuolar H+-ATPases containing a1, a2, a3, or a4 (ATP6V0A1-4) Martinez, E.A. Seftor, C.J. Meininger, G. Wu, D.E. Wesson, M.J. Hendrix, R. subunit isoforms along the nephron, Cell Physiol. Biochem. 20 (2007) 109. Martinez-Zaguilan, Vacuolar-type H+-ATPases at the plasma membrane [52] S. Takamori, M. Holt, K. Stenius, E.A. Lemke, M. Gronborg, D. Riedel, H. Urlaub, S. regulate pH and cell migration in microvascular endothelial cells, Am. J. Physiol. Schenck, B. Brugger, P. Ringler, S.A. Muller, B. Rammner, F. Grater, J.S. Hub, B.L. De Heart Circ. Physiol. 291 (2006) H1147–H1157. Groot, G. Mieskes, Y. Moriyama, J. Klingauf, H. Grubmuller, J. Heuser, F. Wieland, [82] J. Gruenberg, F.G. van der Goot, Mechanisms of pathogen entry through the R. Jahn, Molecular anatomy of a trafficking organelle, Cell 127 (2006) 831. endosomal compartments, Nat. Rev., Mol. Cell Biol. 7 (2006) 495. [53] A. Gilchrist, C.E. Au, J. Hiding, A.W. Bell, J. Fernandez-Rodriguez, S. Lesimple, H. [83] L. Abrami, M. Lindsay, R.G. Parton, S.H. Leppla, F.G. van der Goot, Membrane Nagaya, L. Roy, S.J. Gosline, M. Hallett, J. Paiement, R.E. Kearney, T. Nilsson, J.J. insertion of anthrax protective antigen and cytoplasmic delivery of lethal Bergeron, Quantitative proteomics analysis of the secretory pathway, Cell 127 factor occur at different stages of the endocytic pathway, J. Cell Biol. 166 (2006) 1265. (2004) 645. [54] A. Frattini, P.J. Orchard, C. Sobacchi, S. Giliani, M. Abinun, J.P. Mattsson, D.J. [84] M. Geyer, O.T. Fackler, B.M. Peterlin, Subunit H of the V-ATPase involved in Keeling, A.K. Andersson, P. Wallbrandt, L. Zecca, L.D. Notarangelo, P. Vezzoni, A. endocytosis shows homology to beta-adaptins, Mol. Biol. Cell. 13 (2002) 2045. Villa, Defects in TCIRG1 subunit of the vacuolar proton pump are responsible for a [85] F. Schapiro, J. Sparkowski, A. Adduci, F. Suprynowicz, R. Schlegel, S. Grinstein, subset of human autosomal recessive osteopetrosis, Nat. Genet. 25 (2000) 343. Golgi alkalinization by the papillomavirus E5 oncoprotein, J. Cell Biol. 148 (2000) [55] G.H. Sun-Wada, T. Toyomura, Y. Murata, A. Yamamoto, M. Futai, Y. Wada, The a3 305. isoform of V-ATPase regulates insulin secretion from pancreatic beta-cells, J. Cell [86] S.R. Sennoune, K. Bakunts, G.M. Martinez, J.L. Chua-Tuan, Y. Kebir, M.N. Attaya, R. Sci. 119 (2006) 4531. Martinez-Zaguilan, Vacuolar H+-ATPase in human breast cancer cells with [56] T. Oka, Y. Murata, M. Namba, T. Yoshimizu, T. Toyomura, A. Yamamoto, G.H. Sun- distinct metastatic potential: distribution and functional activity, Am. J. Physiol. Wada, N. Hamasaki, Y. Wada, M. Futai, a4, a unique kidney-specific isoform of Cell Physiol. 286 (2004) C1443–C1452. mouse vacuolar H+-ATPase subunit a, J. Biol. Chem. 276 (2001) 40050. [87] T. Torigoe, H. Izumi, H. Ishiguchi, H. Uramoto, T. Murakami, T. Ise, Y. Yoshida, M. [57] A.N. Smith, J. Skaug, K.A. Choate, A. Nayir, A. Bakkaloglu, S. Ozen, S.A. Hulton, S.A. Tanabe, M. Nomoto, H. Itoh, K. Kohno, Enhanced expression of the human Sanjad, E.A. Al-Sabban, R.P. Lifton, S.W. Scherer, F.E. Karet, Mutations in ATP6N1B, vacuolar H+-ATPase c subunit gene (ATP6L) in response to anticancer agents, J. encoding a new kidney vacuolar proton pump 116-kD subunit, cause recessive Biol. Chem. 277 (2002) 36534. distal renal tubular acidosis with preserved hearing, Nat. Genet. 26 (2000) 71. [88] A.N. Smith, F. Jouret, S. Bord, K.J. Borthwick, R.S. Al-Lamki, C.A. Wagner, D.C. [58] G.H. Sun-Wada, Y. Wada, M. Futai, Diverse and essential roles of mammalian Ireland, V. Cormier-Daire, A. Frattini, A. Villa, U. Kornak, O. Devuyst, F.E. Karet, vacuolar-type proton pump ATPase: toward the physiological understanding of Vacuolar H+-ATPase d2 subunit: molecular characterization, developmental inside acidic compartments, Biochim. Biophys. Acta 1658 (2004) 106. regulation, and localization to specialized proton pumps in kidney and bone, J. [59] C.A. Wagner, K.E. Finberg, S. Breton, V. Marshansky, D. Brown, J.P. Geibel, Renal Am. Soc. Nephrol. 16 (2005) 1245. vacuolar H+-ATPase, Physiol. Rev. 84 (2004) 1263. [89] S.H. Lee, J. Rho, D. Jeong, J.Y. Sul, T. Kim, N. Kim, J.S. Kang, T. Miyamoto, T. Suda, S.K. [60] C. Supanchart, U. Kornak, Ion channels and transporters in osteoclasts, Arch. Lee, R.J. Pignolo, B. Koczon-Jaremko, J. Lorenzo, Y. Choi, v-ATPase V0 subunit d2- Biochem. Biophys. 473 (2008) 161. deficient mice exhibit impaired osteoclast fusion and increased bone formation, [61] M. Forgac, Vacuolar ATPases: rotary proton pumps in physiology and pathophy- Nat. Med. 12 (2006) 1403. siology, Nat. Rev. Mol. Cell Biol. 8 (2007) 917. [90] W. Balemans, W.L. Van, H.W. Van, A clinical and molecular overview of the [62] T. Toyomura, Y. Murata, A. Yamamoto, T. Oka, G.H. Sun-Wada, Y. Wada, M. Futai, human osteopetroses, Calcif. Tissue Int. 77 (2005) 263. From lysosomes to the plasma membrane: localization of vacuolar-type H+- [91] U. Kornak, A. Schulz, W. Friedrich, S. Uhlhaas, B. Kremens, T. Voit, C. Hasan, U. ATPase with the a3 isoform during osteoclast differentiation, J. Biol. Chem. 278 Bode, T.J. Jentsch, C. Kubisch, Mutations in the a3 subunit of the vacuolar H(+)- (2003) 22023. ATPase cause infantile malignant osteopetrosis, Hum. Mol. Genet. 9 (2000) 2059. [63] F. Peri, C. Nusslein-Volhard, Live imaging of neuronal degradation by microglia [92] S.R. Phadke, A. Gupta, J. Pahi, A. Pandey, P. Gautam, S.S. Agarwal, Malignant reveals a role for v0-ATPase a1 in phagosomal fusion in vivo, Cell 133 (2008) 916. recessive osteopetrosis, Indian Pediatr. 36 (1999) 69. [64] E.S. Trombetta, M. Ebersold, W. Garrett, M. Pypaert, I. Mellman, Activation of [93] J. Tolar, S.L. Teitelbaum, P.J. Orchard, Osteopetrosis, N. Engl. J. Med. 351 (2004) lysosomal function during dendritic cell maturation, Science 299 (2003) 1400. 2839. [65] T. Xu, M. Forgac, Microtubules are involved in glucose-dependent dissociation of [94] H.W. Van, F. Vanhoenacker, W. Balemans, K. Janssens, A.M. De Schepper, the yeast vacuolar [H+]-ATPase in vivo, J. Biol. Chem. 276 (2001) 24855. Molecular and radiological diagnosis of sclerosing bone dysplasias, Eur. J. Radiol. [66] J.H. Seol, A. Shevchenko, A. Shevchenko, R.J. Deshaies, Skp1 forms multiple 40 (2001) 198. protein complexes, including RAVE, a regulator of V-ATPase assembly, Nat. Cell [95] F.A. Del, A. Cappariello, A. Teti, Genetics, pathogenesis and complications of Biol. 3 (2001) 384. osteopetrosis, Bone 42 (2008) 19. [67] A.M. Smardon, M. Tarsio, P.M. Kane, The RAVE complex is essential for stable [96] C.G. Steward, I. Pellier, A. Mahajan, M.T. Ashworth, A.G. Stuart, A. Fasth, D. Lang, A. assembly of the yeast V-ATPase, J. Biol. Chem. 277 (2002) 13831. Fischer, W. Friedrich, A.S. Schulz, Severe pulmonary hypertension: a frequent [68] M. Lu, Y.Y. Sautin, L.S. Holliday, S.L. Gluck, The glycolytic enzyme aldolase complication of stem cell transplantation for malignant infantile osteopetrosis, mediates assembly, expression, and activity of vacuolar H+-ATPase, J. Biol. Br. J. Haematol. 124 (2004) 63. Chem. 279 (2004) 8732. [97] A.C. Fry, F.E. Karet, Inherited renal acidoses, Physiology (Bethesda.) 22 (2007) [69] M. Lu, D. Ammar, H. Ives, F. Albrecht, S.L. Gluck, Physical interaction between 202. aldolase and vacuolar H+-ATPase is essential for the assembly and activity of the [98] F.E. Karet, Inherited renal tubular acidosis, Adv. Nephrol. Necker Hosp. 30 (2000) proton pump, J. Biol. Chem. 282 (2007) 24495. 147. [70] E. Shao, M. Forgac, Involvement of the nonhomologous region of subunit A of the [99] F.E. Karet, K.E. Finberg, R.D. Nelson, A. Nayir, H. Mocan, S.A. Sanjad, J. Rodriguez- yeast V-ATPase in coupling and in vivo dissociation, J. Biol. Chem. 279 (2004) Soriano, F. Santos, C.W. Cremers, P.A. Di, B.I. Hoffbrand, J. Winiarski, A. 48663. Bakkaloglu, S. Ozen, R. Dusunsel, P. Goodyer, S.A. Hulton, D.K. Wu, A.B. Skvorak, [71] N. Pastor-Soler, V. Beaulieu, T.N. Litvin, S.N. Da, Y. Chen, D. Brown, J. Buck, L.R. C.C. Morton, M.J. Cunningham, V. Jha, R.P. Lifton, Mutations in the gene encoding Levin, S. Breton, Bicarbonate-regulated adenylyl cyclase (sAC) is a sensor that B1 subunit of H+-ATPase cause renal tubular acidosis with sensorineural regulates pH-dependent V-ATPase recycling, J. Biol. Chem. 278 (2003) 49523. deafness, Nat. Genet. 21 (1999) 84. [72] V. Beaulieu, S.N. Da, N. Pastor-Soler, C.R. Brown, P.J. Smith, D. Brown, S. Breton, [100] R. Vargas-Poussou, P. Houillier, P.N. Le, L. Strompf, C. Loirat, V. Baudouin, M.A. Modulation of the actin cytoskeleton via gelsolin regulates vacuolar H+-ATPase Macher, M. Dechaux, T. Ulinski, F. Nobili, P. Eckart, R. Novo, M. Cailliez, R. recycling, J. Biol. Chem. 280 (2005) 8452. Salomon, H. Nivet, P. Cochat, I. Tack, A. Fargeot, F. Bouissou, G.R. Kesler, S. Lorotte, [73] F.R. Maxfield, T.E. McGraw, Endocytic recycling, Nat. Rev. Mol. Cell Biol. 5 (2004) 121. N.Godefroid,V.Layet,G.Morin,X.Jeunemaitre,A.Blanchard,Genetic [74] J.S. Bonifacino, B.S. Glick, The mechanisms of vesicle budding and fusion, Cell 116 investigation of autosomal recessive distal renal tubular acidosis: evidence for (2004) 153. early sensorineural hearing loss associated with mutations in the ATP6V0A4 [75] C. Peters, M.J. Bayer, S. Buhler, J.S. Andersen, M. Mann, A. Mayer, Trans-complex gene, J. Am. Soc. Nephrol. 17 (2006) 1437. formation by proteolipid channels in the terminal phase of membrane fusion, [101] E. Morava, D.J. Lefeber, Z. Urban, M.L. de, P. Meinecke, K.G. Gillessen, J. Sykut- Nature 409 (2001) 581. Cegielska, M. Adamowicz, I. Salafsky, J. Ranells, E. Lemyre, R.J. van, H.G. Brunner, [76] D.K. Clare, E.V. Orlova, M.A. Finbow, M.A. Harrison, J.B. Findlay, H.R. Saibil, An R.A. Wevers, Defining the phenotype in an autosomal recessive cutis laxa expanded and flexible form of the vacuolar ATPase membrane sector, Structure syndrome with a combined congenital defect of glycosylation, Eur. J. Hum. Genet. 14 (2006) 1149. 16 (2008) 28. [77] P.R. Hiesinger, A. Fayyazuddin, S.Q. Mehta, T. Rosenmund, K.L. Schulze, R.G. Zhai, [102] J. Jaeken, G. Matthijs, Congenital disorders of glycosylation: a rapidly expanding P. Verstreken, Y. Cao, Y. Zhou, J. Kunz, H.J. Bellen, The v-ATPase V0 subunit a1 is disease family, Annu. Rev. Genomics Hum. Genet. 8 (2007) 261. required for a late step in synaptic vesicle exocytosis in Drosophila, Cell 121 [103] P. Beighton, The dominant and recessive forms of cutis laxa, J. Med. Genet. 9 (2005) 607. (1972) 216. 914 M. Guillard et al. / Biochimica et Biophysica Acta 1792 (2009) 903–914

[104] Z. Szabo, M.W. Crepeau, A.L. Mitchell, M.J. Stephan, R.A. Puntel, L.K. Yin, R.C. Kirk, [130] B. van Hille, H. Richener, D.B. Evans, J.R. Green, G. Bilbe, Identification of two Z. Urban, Aortic aneurysmal disease and cutis laxa caused by defects in the subunit A isoforms of the vacuolar H(+)-ATPase in human osteoclastoma, J. Biol. elastin gene, J. Med. Genet. 43 (2006) 255. Chem. 268 (1993) 7075. [105] D. Markova, Y. Zou, F. Ringpfeil, T. Sasaki, G. Kostka, R. Timpl, J. Uitto, M.L. Chu, [131] T.G. Paunescu, S.N. Da, V. Marshansky, M. McKee, S. Breton, D. Brown, Expression Genetic heterogeneity of cutis laxa: a heterozygous tandem duplication within of the 56-kDa B2 subunit isoform of the vacuolar H(+)-ATPase in proton- the fibulin-5 (FBLN5) gene, Am. J. Hum. Genet. 72 (2003) 998. secreting cells of the kidney and epididymis, Am. J. Physiol. Cell Physiol. 287 [106] S.G. Kaler, L.K. Gallo, V.K. Proud, A.K. Percy, Y. Mark, N.A. Segal, D.S. Goldstein, C.S. (2004) C149–C162. Holmes, W.A. Gahl, Occipital horn syndrome and a mild Menkes phenotype [132] H. Nelson, S. Mandiyan, T. Noumi, Y. Moriyama, M.C. Miedel, N. Nelson, Molecular associated with splice site mutations at the MNK locus, Nat. Genet. 8 (1994) 195. cloning of cDNA encoding the C subunit of H(+)-ATPase from bovine chromaffin [107] L.B. Moller, Z. Tumer, C. Lund, C. Petersen, T. Cole, R. Hanusch, J. Seidel, L.R. Jensen, granules, J. Biol. Chem. 265 (1990) 20390. N. Horn, Similar splice-site mutations of the ATP7A gene lead to different [133] B. van Hille, M. Vanek, H. Richener, J.R. Green, G. Bilbe, Cloning and tissue phenotypes: classical Menkes disease or occipital horn syndrome, Am. J. Hum. distribution of subunits C, D, and E of the human vacuolar H(+)-ATPase, Genet. 66 (2000) 1211. Biochem. Biophys. Res. Commun. 197 (1993) 15. [108] V. Hucthagowder, N. Sausgruber, K.H. Kim, B. Angle, L.Y. Marmorstein, Z. Urban, [134] O. Drory, F. Frolow, N. Nelson, Crystal structure of yeast V-ATPase subunit C Fibulin-4: a novel gene for an autosomal recessive cutis laxa syndrome, Am. J. reveals its stator function, EMBO Rep. 5 (2004) 1148. Hum. Genet. 78 (2006) 1075. [135]K.G.Blake-Palmer,Y.Su,A.N.Smith,F.E.Karet,Molecularcloningand [109] B. Loeys, M.L. Van, G. Mortier, P. Coucke, S. Gerniers, J.M. Naeyaert, P.A. De, characterization of a novel form of the human vacuolar H+-ATPase e-subunit: Homozygosity for a missense mutation in fibulin-5 (FBLN5) results in a severe an essential proton pump component, Gene 393 (2007) 94. form of cutis laxa, Hum. Mol. Genet. 11 (2002) 2113. [136]J.A.Kennell,N.W.Richards,P.E.Schaner,D.L.Gumucio,cDNAcloning, [110] E. Elahi, R. Kalhor, S.S. Banihosseini, N. Torabi, H. Pour-Jafari, M. Houshmand, S.S. chromosomal localization and evolutionary analysis of mouse vacuolar ATPase Amini, A. Ramezani, B. Loeys, Homozygous missense mutation in fibulin-5 in an subunit D, Atp6m, Cytogenet. Cell Genet. 92 (2001) 337. Iranian autosomal recessive cutis laxa pedigree and associated haplotype, J. [137] A.N. Smith, R.C. Lovering, M. Futai, J. Takeda, D. Brown, F.E. Karet, Revised Invest. Dermatol. 126 (2006) 1506. nomenclature for mammalian vacuolar-type H+-ATPase subunit genes, Mol. [111] A. Rajab, U. Kornak, B.S. Budde, K. Hoffmann, J. Jaeken, P. Nurnberg, S. Mundlos, Cell. 12 (2003) 801. Geroderma osteodysplasticum hereditaria and wrinkly skin syndrome in 22 [138] V. Baud, A.J. Mears, V. Lamour, C. Scamps, A.M. Duncan, H.E. McDermid, M. patients from Oman, Am. J. Med. Genet., A 146A (2008) 965. Lipinski, The E subunit of vacuolar H(+)-ATPase localizes close to the [112] M.A. Patton, J. Tolmie, P. Ruthnum, S. Bamforth, M. Baraitser, M. Pembrey, centromere on human chromosome 22, Hum. Mol. Genet. 3 (1994) 335. Congenital cutis laxa with retardation of growth and development, J. Med. Genet. [139] T. Fujiwara, A. Kawai, F. Shimizu, H. Hirano, S. Okuno, S. Takeda, K. Ozaki, Y. 24 (1987) 556. Shimada, M. Nagata, T. Watanabe, Cloning, sequencing and expression of a novel [113] A.M. de Barsy, E. Moens, L. Dierckx, Dwarfism, oligophrenia and degeneration of cDNA encoding human vacuolar ATPase (14-kDa subunit), DNA Res. 2 (1995) the elastic tissue in skin and cornea. A new syndrome? Helv. Paediatr. Acta 23 107. (1968) 305. [140] T.H. Stevens, M. Forgac, Structure, function and regulation of the vacuolar (H+)- [114] L.I. Al-Gazali, L. Sztriha, F. Skaff, D. Haas, Gerodermia osteodysplastica and ATPase, Annu. Rev. Cell Dev. Biol. 13 (1997) 779. wrinkly skin syndrome: are they the same? Am. J. Med. Genet. 101 (2001) 213. [141] E. Ntrivalas, R. Derks, A. Gilman-Sachs, J. Kwak-Kim, R. Levine, K. Beaman, Novel [115] H.C. Hennies, U. Kornak, H. Zhang, J. Egerer, X. Zhang, W. Seifert, J. Kuhnisch, B. role for the N-terminus domain of the a2 isoform of vacuolar ATPase in Budde, M. Natebus, F. Brancati, W.R. Wilcox, D. Muller, P.B. Kaplan, A. Rajab, G. interleukin-1beta production, Hum. Immunol. 68 (2007) 469. Zampino, V. Fodale, B. Dallapiccola, W. Newman, K. Metcalfe, J. Clayton-Smith, M. [142] S. Kawasaki-Nishi, T. Nishi, M. Forgac, Yeast V-ATPase complexes containing Tassabehji, B. Steinmann, F.A. Barr, P. Nurnberg, P. Wieacker, S. Mundlos, Nat. different isoforms of the 100-kDa a-subunit differ in coupling efficiency and in Genet. 40 (2008) 1410–1412. vivo dissociation, J. Biol. Chem. 276 (2001) 17941. [116] E. Morava, S. Wopereis, P. Coucke, G. Gillessen-Kaesbach, T. Voit, J. Smeitink, R. [143] Y.P. Li, W. Chen, P. Stashenko, Molecular cloning and characterization of a Wevers, S. Grunewald, Defective protein glycosylation in patients with cutis laxa putative novel human osteoclast-specific 116-kDa vacuolar proton pump syndrome, Eur. J. Hum. Genet. 13 (2005) 414. subunit, Biochem. Biophys. Res. Commun. 218 (1996) 813. [117] T. Marquardt, J. Denecke, Congenital disorders of glycosylation: review of their [144] T. Heinemann, G.C. Bulwin, J. Randall, B. Schnieders, K. Sandhoff, H.D. Volk, E. molecular bases, clinical presentations and specific therapies, Eur. J. Pediatr. 162 Milford, S.R. Gullans, N. Utku, Genomic organization of the gene coding for (2003) 359. TIRC7, a novel membrane protein essential for T cell activation, Genomics 57 [118] E. Morava, H. Wosik, J. Karteszi, M. Guillard, M. Adamowicz, J. Sykut-Cegielska, K. (1999) 398. Hadzsiev, R.A. Wevers, D.J. Lefeber, Congenital disorder of glycosylation type Ix: [145] F.E. Karet, K.E. Finberg, A. Nayir, A. Bakkaloglu, S. Ozen, S.A. Hulton, S.A. Sanjad, review of clinical spectrum and diagnostic steps, J. Inherit. Metab. Dis. 31 (2008) E.A. Al-Sabban, J.F. Medina, R.P. Lifton, Localization of a gene for autosomal 450. recessive distal renal tubular acidosis with normal hearing (rdRTA2) to 7q33–34, [119] S. Wopereis, D.J. Lefeber, E. Morava, R.A. Wevers, Mechanisms in protein O-glycan Am. J. Hum. Genet. 65 (1999) 1656. biosynthesis and clinical and molecular aspects of protein O-glycan biosynthesis [146] E.H. Stover, K.J. Borthwick, C. Bavalia, N. Eady, D.M. Fritz, N. Rungroj, A.B. Giersch, defects: a review, Clin. Chem. 52 (2006) 574. C.C. Morton, P.R. Axon, I. Akil, E.A. Al-Sabban, D.M. Baguley, S. Bianca, A. [120] S. Wopereis, S. Grunewald, E. Morava, J.M. Penzien, P. Briones, M.T. Garcia-Silva, Bakkaloglu, Z. Bircan, D. Chauveau, M.J. Clermont, A. Guala, S.A. Hulton, H. Kroes, P.N. Demacker, K.M. Huijben, R.A. Wevers, Apolipoprotein C-III isofocusing in the V.G. Li, S. Mir, H. Mocan, A. Nayir, S. Ozen, S.J. Rodriguez, S.A. Sanjad, V. Tasic, C.M. diagnosis of genetic defects in O-glycan biosynthesis, Clin. Chem. 49 (2003) Taylor, R. Topaloglu, A.N. Smith, F.E. Karet, Novel ATP6V1B1 and ATP6V0A4 1839. mutations in autosomal recessive distal renal tubular acidosis with new evidence [121] R. Zeevaert, F. Foulquier, J. Jaeken, G. Matthijs, Deficiencies in subunits of the for hearing loss, J. Med. Genet. 39 (2002) 796. Conserved Oligomeric Golgi (COG) complex define a novel group of Congenital [147] G.A. Gillespie, S. Somlo, G.G. Germino, D. Weinstat-Saslow, S.T. Reeders, CpG Disorders of Glycosylation, Mol. Genet. Metab 93 (2008) 15. island in the region of an autosomal dominant polycystic kidney disease locus [122] D.M. Kingsley, K.F. Kozarsky, M. Segal, M. Krieger, Three types of low density defines the 5′ end of a gene encoding a putative proton channel, Proc. Natl. Acad. lipoprotein receptor-deficient mutant have pleiotropic defects in the synthesis of Sci. U. S. A. 88 (1991) 4289. N-linked, O-linked, and lipid-linked carbohydrate chains, J. Cell Biol. 102 (1986) [148] A.M. Simckes, S.K. Swanson, R.A. White, Chromosomal localization of three 1576. vacuolar-H+-ATPase 16 kDa subunit (ATP6V0C) genes in the murine genome, [123] S. Wopereis, E. Morava, S. Grunewald, M. Adamowicz, K.M. Huijben, D.J. Lefeber, Cytogenet. Genome Res. 97 (2002) 111. R.A. Wevers, Patients with unsolved congenital disorders of glycosylation type II [149] P. Malkus, L.A. Graham, T.H. Stevens, R. Schekman, Role of Vma21p in assembly can be subdivided in six distinct biochemical groups, Glycobiology 15 (2005) and transport of the yeast vacuolar ATPase, Mol. Biol. Cell 15 (2004) 5075. 1312. [150] T. Oka, R. Yamamoto, M. Futai, Three vha genes encode proteolipids of [124] S. Wopereis, E. Morava, S. Grunewald, P.B. Mills, B.G. Winchester, P. Clayton, P. Caenorhabditis elegans vacuolar-type ATPase. Gene structures and preferential Coucke, K.M. Huijben, R.A. Wevers, A combined defect in the biosynthesis of N- expression in an H-shaped excretory cell and rectal cells, J. Biol. Chem. 272 and O-glycans in patients with cutis laxa and neurological involvement: the (1997) 24387. biochemical characteristics, Biochim. Biophys. Acta 1741 (2005) 156. [151] H. Nishigori, S. Yamada, H. Tomura, A.A. Fernald, M.M. Le Beau, T. Takeuchi, J. [125] M.A. Axelsson, N.G. Karlsson, D.M. Steel, J. Ouwendijk, T. Nilsson, G.C. Hansson, Takeda, Identification and characterization of the gene encoding a second Neutralization of pH in the Golgi apparatus causes redistribution of glycosyl- proteolipid subunit of human vacuolar H(+)-ATPase (ATP6F), Genomics 50 transferases and changes in the O-glycosylation of mucins, Glycobiology 11 (1998) 222. (2001) 633. [152] A.K. Agarwal, P.C. White, Structure of the VPATPD gene encoding subunit D of the [126] S. Kellokumpu, R. Sormunen, I. Kellokumpu, Abnormal glycosylation and altered human vacuolar proton ATPase, Biochem. Biophys. Res. Commun. 279 (2000) Golgi structure in colorectal cancer: dependence on intra-Golgi pH, FEBS Lett. 543 %20. 516 (2002) 217. [153] T. Nishi, S. Kawasaki-Nishi, M. Forgac, Expression and function of the mouse V- [127] R.D. Smith, V.V. Lupashin, Role of the conserved oligomeric Golgi (COG) complex ATPase d subunit isoforms, J. Biol. Chem. 278 (2003) 46396. in protein glycosylation, Carbohydr. Res. 343 (2008) 2024. [154] C. Pietrement, G.H. Sun-Wada, N.D. Silva, M. McKee, V. Marshansky, D. Brown, M. [128] Q. Hu, J.L. Reymond, N. Pinel, M.T. Zabot, Z. Urban, Inflammatory destruction of Futai, S. Breton, Distinct expression patterns of different subunit isoforms of the elastic fibers in acquired cutis laxa is associated with missense alleles in the V-ATPase in the rat epididymis, Biol. Reprod. 74 (2006) 185. elastin and fibulin-5 genes, J. Invest. Dermatol. 126 (2006) 283. [155] M. Nanji, V.A. Coronado, D.W. Cox, ATP6H, a subunit of vacuolar ATPase involved [129] E.C. Davis, R.P. Mecham, Intracellular trafficking of tropoelastin, Matrix Biol. 17 in metal transport: evaluation in canine copper toxicosis, Mamm. Genome 12 (1998) 245. (2001) 617.