Review Meprin α and meprin β: Procollagen proteinases in health and disease Johannes Prox a, Philipp Arnold b and Christoph Becker-Pauly a a - Unit for Degradomics of the Protease Web, Institute of Biochemistry, University of Kiel, Kiel, Germany b - Anatomical Institute, University of Kiel, Kiel, Germany Correspondence to Christoph Becker-Pauly: Unit for Degradomics of the Protease Web, Institute of Biochemistry, University of Kiel, 24229 Kiel, Germany. Tel.: +49 431 880 7118; fax: +49 431 880 5007. [email protected] kiel.de. http://dx.doi.org/10.1016/j.matbio.2015.01.010 Edited by W.C. Parks and S. Apte Abstract Metalloproteases meprin α and meprin β were recently discovered as procollagen proteinases, capable of cleaving off the globular C- and N-terminal prodomains of fibrillar collagen type I and type III. This proteolytic process is indeed sufficient to induce collagen fibril assembly as visualized by transmission electron microscopy. The biological relevance was demonstrated with the help of meprin α and meprin β knock-out mice, which exhibit decreased collagen deposition in skin resulting in impaired tensile strength. On the other hand, overexpression of meprin metalloproteases was found under fibrotic conditions in the skin (keloids) and the lung (pulmonary hypertension). Thus, regulation of meprin activity by specific inhibition to reduce collagen maturation might be a suitable approach for the treatment of certain pathological conditions. © 2015 Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons. org/licenses/by-nc-nd/4.0/). Introduction [12,13]. This cleavage event at the so-called β-site enables γ-secretase to further cleave the remaining Meprin α and meprin β are multidomain metallopro- C-terminal fragment of APP within the membrane, teases with distinct functions during various physiolog- thereby releasing Aβ-peptides, which are known to be ical and pathophysiological conditions. They are involved in the onset and progression of Alzheimer's involved in inflammation by the release and maturation disease. Meprin β itself can be shed from the cell of cytokines [1,2] and proteoglycans [3], they induce surface by a disintegrin and metalloprotease 10 and 17 extracellular matrix assembly and fibrosis [4,5], and (ADAM10/17) [14]. As a soluble protein meprin β is no enhance cancer progression through trans-activation longer capable of cleaving APP at the β-site [12],thus of EGF receptors [6]. Several studies indicated that indicating a complex regulation of proteolytic activity by meprin α and meprin β may be reminiscent of matrix ectodomain shedding. Meprin α instead is constitutively metalloproteases (MMPs) “simply” degrading extracel- shed by furin during the secretory pathway and lular matrix (ECM) components, thereby promoting cell secreted into extracellular space [15]. Interestingly, migration and metastasis [7,8]. But meprins are much we and others could show that meprin α tends to more discriminating between substrates than expect- oligomerize to huge complexes up to the mega Dalton ed, which is reflected by defined cleavage specificity range, which makes it the largest extracellular protease and structural features unique among all proteases [9– [16,17] (See Fig. 1). These fascinating ring and chain 11]. For instance, meprin β is expressed as a dimeric like structures can easily be visualized by transmission type 1 transmembrane protein and acts as a sheddase electron microscopy (TEM), but structure-function at the cell surface where it releases the entire relationships are still ambiguous. Besides the different ectodomain of amyloid precursor protein (APP) cellular localization, meprin α and meprin β were found 0022-2836/© 2015 Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons. org/licenses/by-nc-nd/4.0/). Matrix Biol. (2015) 44–46,7–13 8 Procollagen proteinases in health and disease Fig. 1. Activation, shedding and ECM substrates of meprin metalloproteases. Meprins are secreted as zymogens and are activated by trypsin-like serine proteases (e.g., human kallikrein related peptidases, KLK). Meprin β is intrinsically membrane bound and can be shed from the cell surface by ADAM10/17. Several components of the ECM have been described as meprin substrates. These include, for example, MMP1, which is inactivated by meprins, or nidogen and fibronectin, which were shown to be cleaved in vitro. For procollagen type 1 it was demonstrated that cleavage of the prodomain leads to spontaneous fibril formation. Meprin deficient mice show a reduced tensile strength pointing towards an important in vivo function in collagen deposition. Left box shows the domain structure of meprin α and meprin β. Both consist of a propeptide (PRO), a catalytic domain (CAT), a MAM (meprin A5 protein tyrosine phosphatase μ) domain, a TRAF (tumor-necrosis-factor-receptor-associated factor) domain, an EGF (epidermal growth factor) like domain, a transmem- brane region and a C-terminal part. Additionally, there is a so called inserted (I) domain found in meprin α between the TRAF and the EGF like domain. This inserted domain is cleaved by furin resulting in secretion into extracellular space. Meprin α forms large oligomers up to 6.4 MDa through a yet unknown oligomerisation site. This makes it the largest secreted protease known as depicted in the transmission electron microscopy image. to be differentially expressed in the small and large From proteomics to substrates intestine, leucocytes, and several tumors [18].In normal dermal skin, meprin α is constitutively higher In the last years several approaches were initiated expressed than meprin β, but both proteases are highly to characterize meprin metalloproteases in more up-regulated in keloid tissue [19]. detail in matters of substrate identification, specificity Recently, we demonstrated that meprin α and and structure [5,21]. Employing a mass spectro- meprin β are C- and N-procollagen proteinases metry-based proteomics technique called PICS thereby inducing collagen fibril assembly [4] (See (Proteomic Identification of Protease Cleavage Fig. 1). This biological activity is directly associated Sites) [22], we identified a unique cleavage site with increased expression of meprin metallopro- specificity for meprin α and meprin β with a pref- teases in fibrotic conditions [19,20]. These obser- erence for negatively charged amino acids (aspar- vations are summarized herein and provide tate and glutamate) particularly in the P1′ position evidence for important functions of meprin α and [10] (nomenclature by Schechter and Berger) [23]. meprin β in ECM homeostasis in health Terminal amine isotopic labeling of substrates and disease. (TAILS) [24], another proteomics approach, enabled Procollagen proteinases in health and disease 9 us to identify a multitude of specific meprin Thus compensatory mechanisms by other proteolytic substrates, among these proteases, cytokines, enzymes with regard to procollagen maturation, or extracellular matrix proteins, growth factors and other BMP-1-associated activities, such as myostatin adhesion molecules [14]. However, the biological activation, are needed in different organs. Metallo- roles of meprin α and meprin β with regard to their proteases ADAMTS-2, 3, and 14 were shown to substrates, inhibitors, or other regulatory proteins as release the N-propeptides of fibrillar collagens [35] part of the protease web have to be further (review by Bekhouche and Colige). However, as investigated. shown for BMP-1, ADAMTS-2, 3, and 14 deficient Meprin knockout mice as loss of function models cells exhibited remaining N-procollagen proteinase are viable, fertile and do not show a striking devel- activity, indicating the existence of other compensat- opmental phenotype despite the broad substrate ing proteolytic enzymes [36,37]. spectrum identified [25,26]. Initial studies of Employing the proteomics based approach TAILS, meprins in the intestine and kidney of mice, we identified human procollagen type I as a sub- where these proteases are found to be highly strate of meprin α and meprin β [14]. Incubation of expressed, revealed that meprin α and meprin β purified recombinant procollagen I with meprin α and are somehow involved in pro- and/or anti-inflam- meprin β and subsequent analysis by western matory processes (LPS-sepsis-, colitis-models) but blotting identified both meprins as C-and N-procolla- the detailed molecular pathways initiated or regu- gen proteinases [4] (See Fig. 1). Moreover it was lated by meprins remain elusive [2,27]. Except of shown that incubation of procollagen I with meprin α the few inflammatory models the roles of meprins or meprin β led to spontaneous fibril formation that during challenged or pathological conditions in can be observed using transmission electron mi- mice have not been investigated in detail yet. croscopy (TEM). Full maturation of procollagen I by However, under unchallenged conditions defects in single enzymes as observed with meprins is in collagen assembly in skin could be identified due to contrast to the previously described procolla- alterations in N- and C-propeptide processing of genases BMP-1, tolloid1/2, and ADAMTS-2, 3, and collagen I in the dermis of meprin α and meprin β 14 [31,35]. As BMP-1 cleaves off only the C-terminal deficient mice [4]. and ADAMTS proteases only the N-terminal propep- tides of procollagen I this is not sufficient for spontaneous fibril formation as demonstrated by Meprins in collagen maturation