374 Review TRENDS in Cell Biology Vol.12 No.8 August 2002 Altered trafficking and epithelial cell polarity in disease Mary-Pat Stein, Angela Wandinger-Ness and Tamara Roitbak Establishment and maintenance of a polarized epithelium relies on the non-polarized expression of normally polarized integration of signaling cascades, acquisition of specialized trafficking circuits molecules. Thus, identifying the signals and and establishment of a unique cytoarchitecture. Defects in any of these molecular machinery required to appropriately processes can adversely affect cell polarity and cause defects in specific organs maintain the polarized expression of newly and systemic disease. Mutations that disrupt the proper transport of individual synthesized, endocytosed or transcytosed proteins, plasma membrane proteins, or inactivate components of the epithelial-specific is essential to expose the potential underlying trafficking machinery, have severe functional consequences. Links between causes of human diseases. renal diseases and defects in trafficking, differentiation or signaling, highlight Specific cytoplasmic signal sequences or the delicate balance between these parameters which, when altered, conformational determinants mediate the transport precipitates a loss of renal function. of newly synthesized molecules from the trans-Golgi to either the basolateral or apical membrane domains The barrier and transport functions provided by (Fig. 1). Basolateral delivery is specified by either epithelia depend on their highly polarized phenotype. tyrosine-based sorting signals (e.g. YXXφ, where φ Polarization of epithelial cells is a complex process denotes a bulky hydrophobic amino acid, or NPXY) directed by external cues such as cell–cell and or dileucine motifs [2]. These sorting signals cell–extracellular matrix (ECM) interactions [1]. promote clustering of the molecules and their These interactions initiate the formation of association with vesicle-associated adaptor proteins specialized junctions that demarcate apical and [3], leading to the inclusion of proteins exiting the basolateral plasma membrane domains. Once Golgi in discrete, basolaterally targeted vesicles. polarity is established, a diverse array of cellular Similarly, apical sorting and retention signals are machinery is used to ensure maintenance of epithelial encoded in protein transmembrane domains, polarity, with specialized intracellular trafficking attached lipid moieties, carbohydrate domains pathways directing the domain-specific targeting of or C-terminal PDZ domains [4,5]. Sorting of newly synthesized, endocytosed and transcytosed apically targeted proteins into detergent-insoluble molecules. The accurate delivery of molecules to their glycosylphosphatidylinositol–cholesterol (DIG) appropriate membrane domains depends on the membrane rafts, mediated either by interaction of differentiation state of the cell. Consequently, defects their glycosylphosphatidylinositol or glycan moieties in trafficking pathways that are used to maintain with DIG components, or by glycan interaction with epithelial polarity or alterations in epithelial putative carbohydrate-specific receptors in the differentiation can cause disease in organs in which trans-Golgi network (TGN) [6–11], facilitates apical epithelial cell polarity is crucial; for example, the targeting. In kidney and intestinal epithelia, both liver, kidney and intestines. DIG-dependent and DIG-independent routes of Here, we focus on diseases resulting from the transport have been documented [11]. Hepatocytes mistargeting of proteins or the loss of polarity, either can use a third route for apical transport, in which of which can lead to the inappropriate expression or many (but perhaps not all) apical plasma membrane loss of molecules from specific cell-surface domains proteins are first delivered to the basolateral with pathological consequences (Table 1). Although membrane and subsequently reach the apical many significant diseases result from point membrane by transcytosis [12,13]. mutations that lead to protein misfolding and Maintenance of epithelial cell polarity not only retention in the endoplasmic reticulum (ER), requires regulated insertion of membrane proteins discussion of diseases that affect de novo biosynthesis into the apical and basolateral domains, but also is beyond the scope of this review. tight control of endocytic recycling and transcytosis Mary-Pat Stein (Fig. 1). Similar to de novo basolateral sorting, Angela Wandinger-Ness* Molecular sorting endocytosis and transcytosis of cell-surface molecules Tamara Roitbak Molecular Trafficking Defects in the ability to sort or transport molecules to depends largely on cytoplasmic sorting cassettes. Laboratory, Dept of their appropriate cellular destinations can render Pathological conditions can arise as a result of the Pathology, University of epithelial cells non-functional with respect to barrier inappropriate removal or insertion of itinerant New Mexico School of and transport functions, thereby causing disease. proteins, stressing the importance of proper endocytic Medicine, Albuquerque, NM 87131, USA. Genetic mutations affecting either sorting signals or and transcytotic transport in the maintenance of *e-mail: [email protected] epithelial trafficking machinery can result in the polarized protein expression. http://tcb.trends.com 0962-8924/02/$ – see front matter © 2002 Elsevier Science Ltd. All rights reserved. PII: S0962-8924(02)02331-0 Review TRENDS in Cell Biology Vol.12 No.8 August 2002 375 Table 1. Diseases resulting from altered trafficking or cell polaritya Classification Disease Defect Refs Sorting signals Sucrase–isomaltase deficiency type IV N- and O-linked glycans [9,10,19–22] Cystic fibrosis CFTR PDZ-binding domains [23–25] Wilson disease (copper toxicity) ATP7B [26,27] Familial hypercholesterolemia LDLR [14–18] Liddle’s syndrome ENaC [30–32] Nephrogenic diabetes insipidus AQP2 [28,29] Trafficking machinery Lowe syndrome Phosphatidylinositol 5-phosphatase [38–41] Tuberous sclerosis Tuberin (Rab5 GAP) and hamartin [46–49] Differentiation ADPKD, ARPKD Dedifferentiation [73–77,84–87] aAbbreviations: ADPKD, autosomal dominant polycystic kidney disease; AQP2, aquaporin-2; ARPKD, autosomal recessive polycystic kidney disease; ATP7B, a P-type ATPase; CFTR, cystic fibrosis transmembrane conductance regulator; ENaC, amiloride-sensitive epithelial Na+ channel; GAP, GTPase-activating protein; LDLR, low-density lipoprotein receptor. Sorting signal defects IV results in mispolarization of SI from its normal Familial hypercholesterolemia (FH) is an autosomal intestinal brush-border membrane to the basolateral dominant disorder resulting from the inability of membrane in intestinal epithelial cells [19]. patients to remove low-density lipoprotein (LDL) Recognition of glycosylation and incorporation into from their plasma, leading to elevated levels of serum DIGs enables efficient apical transport of SI and cholesterol bound to LDL, premature atherosclerosis dipeptidyl peptidase IV (DPPIV), another apically and coronary heart disease. Defects in LDL uptake targeted intestinal enzyme. Sorting of SI is dependent are attributed to genetic variants of the LDL receptor on a heavily O-glycosylated stalk region of the protein (LDLR) that affect receptor internalization, and on the transmembrane domain [10]. Inhibition of LDL binding and appropriate targeting of LDLR in O- or N-linked glycosylation results in random hepatocytes [14–16]. The inappropriately targeted distribution of SI and DPPIV, respectively, on both mutant is of interest for this review as it results in the apical and basolateral membranes [9,10]. In addition, loss of LDLR from the basolateral (sinusoidal) surface a P-domain or trefoil motif [20] in SI interacts with and expression on the apical membrane. the O-glycosylated stalk region and provides a spatial LDLR is normally sorted and transported to the determinant for SI sorting and apical targeting [21]. basolateral surface of hepatocytes. Two tyrosine-based A point mutation in the P-domain is also linked to sorting motifs facilitate the basolateral-membrane non-polarized membrane targeting [22]. The targeting of LDLR: a membrane proximal motif importance of post-translational modifications and containing a single tyrosine residue that is also conformational determinants in the apical sorting of crucial for the endocytosis of LDLR; and a more distal SI is clearly apparent and illustrates how disease C-terminal motif containing a crucial G–Y amino acid might result from defects in protein modification. pair [16,17]. A naturally occurring point mutation Proper apical trafficking of the cystic fibrosis identified in the FH–Turku LDLR allele leads to transmembrane conductance regulator (CFTR), substitution of the glycine residue in this motif with a cAMP-activated chloride channel, is dependent on aspartic acid (i.e. D–Y) [18]. This mutation results in two C-terminal domains [5] and a C-terminal the predominant expression of LDLR on the apical PDZ-binding motif [23]. Interactions between the bile canalicular surface, with only low levels of LDLR PDZ domains of CFTR and the Golgi-associated being expressed on the basolateral membrane. The CAL protein (CFTR-associated ligand) promote Golgi importance of proper basolateral targeting of LDLR retention, whereas interactions between CFTR and in hepatic cells is demonstrated by the severe NHE-RF (Na+/H+ exchange regulatory factor) [24] consequences resulting from its apical expression in promote