Analyzing the protease web in skin: meprin metalloproteases are activated specifically by KLK4, 5 and 8 vice versa leading to processing of proKLK7 thereby triggering its activation

Comparative analyses of Netherton syndrome patients and Spink5 conditional knock-out mice uncover disease-relevant pathways

Netherton syndrome (NS) is a rare skin disease caused by loss-of-function mutations in the serine peptidase inhibitor Kazal type 5 (SPINK5) gene. Disease severity and the lack of efficacious treatments call for a better understanding of NS mechanisms. Here we describe a novel and viable, Spink5 conditional knock-out (cKO) mouse model, allowing to study NS progression. By combining transcriptomics and proteomics, we determine a disease molecular profile common to mouse models and NS patients. Spink5 cKO mice and NS patients share skin barrier and inflammation signatures defined by up-regulation and increased activity of proteases, IL-17, IL-36, and IL-20 family cytokine signaling. Systemic inflammation in Spink5 cKO mice correlates with disease severity and is associated with thymic atrophy and enlargement of lymph nodes and spleen. This systemic inflammation phenotype is marked by neutrophils and IL-17/IL-22 signaling, does not involve primary T cell immunodeficiency and is independent of bacterial infection. By comparing skin transcriptomes and proteomes, we uncover several putative substrates of tissue kallikrein-related proteases (KLKs), demonstrating that KLKs can proteolytically regulate IL-36 pro-inflammatory cytokines. Our study thus provides a conserved molecular framework for NS and reveals a KLK/IL-36 signaling axis, adding new insights into the disease mechanisms and therapeutic targets.

Variants in KLK11, affecting signal peptide cleavage of kallikrein-related peptidase 11, cause an autosomal-dominant cornification disorder

BACKGROUND

Mendelian disorders of cornification (MeDOC) are a group of heterogeneous genodermatoses with different genetic bases. The pathogenesis of a substantial group of MeDOC remains to be elucidated.

OBJECTIVES

To identify a new causative gene and the pathogenesis of a previously undescribed autosomal-dominant cornification disorder.

METHODS

Whole-exome sequencing was performed in three families with the novel cornification disorder to identify the disease-causing variants. As the variants were located around the signal peptide (SP) cleavage site of a kallikrein-related peptidase, SP cleavage, subcellular localization and extracellular secretion of the variants were evaluated in eukaryotic overexpression systems by Western blotting or immunocytochemistry. Then the trypsin-like and chymotrypsin-like proteolytic activity of the peptidase and degradation of its catalytic substrate were assayed using the patients' stratum corneum (SC) samples. The morphology of the lamellar bodies and corneodesmosomes (CDs) in the patients' SC was ultrastructurally examined. A mouse model harbouring the equivalent variant was constructed and evaluated histologically.

RESULTS

We identified two heterozygous variants affecting Gly50 in kallikrein-related peptidase (KLK)11 in a familial case and two sporadic cases with the new disorder, which is characterized by early-onset ichthyosiform erythroderma or erythrokeratoderma. KLK11 belongs to the family of kallikrein-related peptidases participating in skin desquamation by decomposing CDs, a process essential for shedding of the SC. In vitro experiments demonstrated that the variants perturbed the SP cleavage of KLK11, leading to subcellular mislocalization and impaired extracellular secretion of the KLK11 Gly50Glu variant. Both trypsin-like and chymotrypsin-like proteolytic activities were significantly decreased in the patients' SC samples. Reduced proteolysis of desmoglein 1 and delayed degeneration of CDs were detected in patients' SC, indicating delayed skin desquamation. Consistently, the patients showed a thickened, dense SC, indicating abnormal skin desquamation. Mice harbouring the homozygous c.131G>A (p.Gly44Glu) Klk11 variant, which is equivalent to KLK11 c.149G>A (p.Gly50Glu) in humans, exhibited hyperkeratosis and abnormal desquamation, partially recapitulating the phenotype.

CONCLUSIONS

We provide evidence that variants at Gly50 affecting the SP cleavage of KLK11 cause a new autosomal-dominant cornification disorder with abnormal desquamation. Our findings highlight the essential role of KLKs in maintaining homeostasis of skin keratinization and desquamation.

MT1-MMP and ADAM10/17 exhibit a remarkable overlap of shedding properties

Membrane-type-I matrix metalloproteinase (MT1-MMP) is one of six human membrane-bound MMPs and is responsible for extracellular matrix remodelling by degrading several substrates like fibrillar collagens, including types I-III, or fibronectin. Moreover, MT1-MMP was described as a key player in cancer progression and it is involved in various inflammatory processes, as well as in the pathogenesis of Alzheimer's disease (AD). The membrane-tethered metalloprotease meprin β as well as a disintegrin and metalloproteinase 10 (ADAM10) and ADAM17 are also associated with these diseases. Interestingly, meprin β, ADAM10/17 and MT1-MMP also have a shared substrate pool including the interleukin-6 receptor and the amyloid precursor protein. We investigated the interaction of these proteases, focusing on a possible connection between MT1-MMP and meprin β, to elucidate the potential mutual regulations of both enzymes. Herein, we show that besides ADAM10/17, MT1-MMP is also able to shed meprin β from the plasma membrane, leading to the release of soluble meprin β. Mass spectrometry-based cleavage site analysis revealed that the cleavage of meprin β by all three proteases occurs between Pro₆₀₂ and Ser₆₀₃ , N-terminal of the EGF-like domain. Furthermore, only inactive human pro-meprin β is shed by MT1-MMP, which is again in accordance with the shedding capability observed for ADAM10/17. Vice versa, meprin β also appears to shed MT1-MMP, indicating a complex regulatory network. Further studies will elucidate this well-orchestrated proteolytic web under distinct conditions in health and disease and will possibly show whether the loss of one of the above-mentioned sheddases can be compensated by the other enzymes.

Remodelling of the tumour microenvironment by the kallikrein-related peptidases

Kallikrein-related peptidases (KLKs) are critical regulators of the tumour microenvironment. KLKs are proteolytic enzymes regulating multiple functions of bioactive molecules including hormones and growth factors, membrane receptors and the extracellular matrix architecture involved in cancer progression and metastasis. Perturbations of the proteolytic cascade generated by these peptidases, and their downstream signalling actions, underlie tumour emergence or blockade of tumour growth. Recent studies have also revealed their role in tumour immune suppression and resistance to cancer therapy. Here, we present an overview of the complex biology of the KLK family and its context-dependent nature in cancer, and discuss the different therapeutic strategies available to potentially target these proteases.

Meprin and ADAM proteases as triggers of systemic inflammation in sepsis

Systemic inflammatory disorders (SIDs) comprise a broad range of diseases characterized by dysregulated excessive innate immune responses. Severe forms of SIDs can lead to organ failure and death, and their increasing incidence represents a major issue for the healthcare system. Protease-mediated ectodomain shedding of cytokines and their receptors represents a central mechanism in the regulation of inflammatory responses. The metalloprotease A disintegrin and metalloproteinase (ADAM) 17 is the best-characterized ectodomain sheddase capable of releasing TNF-α and soluble IL-6 receptor, which are decisive factors of systemic inflammation. Recently, meprin metalloproteases were also identified as IL-6 receptor sheddases and activators of the pro-inflammatory cytokines IL-1β and IL-18. In different mouse models of SID, particularly those mimicking a sepsis-like phenotype, ADAM17 and meprins have been found to promote disease progression. In this review, we summarize the role of ADAM10, ADAM17, and meprins in the onset and progression of sepsis and discuss their potential as therapeutic targets.

Reconstructing the epidermal proteolytic cascades in health and disease

The epidermis is the outer stratified epithelium of the skin, forming the physical barrier that is indispensable for homeostasis. Epidermal proteolysis, mainly but not exclusively executed by kallikrein-related peptidases (KLKs), is tightly regulated to ensure maintenance of physiological skin renewal and an intact skin barrier. Perturbation of epidermal proteolytic networks is implicated in a wide array of rare and common skin pathologies of diverse genetic backgrounds. Recent studies of monogenic human skin diseases and newly developed animal models have revealed new mechanisms of regulation of proteolytic pathways in epidermal physiology and in disease states. These new data have challenged some accepted views, for example the role of matriptase in epidermal desquamation, which turned out to be restricted to mouse skin. The significance of PAR2 signaling in skin inflammation should also be reconsidered in the face of recent findings. Cumulatively, recent studies necessitate a sophisticated redefinition of the proteolytic and signaling pathways that operate in human skin. We elaborate how epidermal proteolysis is finely regulated at multiple levels, and in a spatial manner that was not taken into consideration so far, in which specific proteases are confined to distinct epidermal sublayers. Of interest, transglutaminases have emerged as regulators of epidermal proteolysis and desquamation by spatially fixing endogenous protease inhibitors, constituting regulatory factors that were not recognized before. Furthermore, new evidence suggests a link between proteolysis and lipid metabolism. By synthesis of established notions and recent discoveries, we provide an up-to-date critical parathesis of current knowledge and the extended complexity of proteolysis regulation and signaling pathways in skin. This article is protected by copyright. All rights reserved.

Characterization of the Cancer-Associated Meprin Βeta Variants G45R and G89R

Meprin β is a metalloprotease associated with neurodegeneration, inflammation, extracellular matrix homeostasis, transendothelial cell migration, and cancer. In this study, we investigated two melanoma-associated variants of meprin β, both exhibiting a single amino acid exchange, namely, meprin β G45R and G89R. Based on the structural data of meprin β and with regard to the position of the amino acid exchanges, we hypothesized an increase in proteolytic activity in the case of the G45R variant due to the induction of a potential new activation site and a decrease in proteolytic activity from the G89R variant due to structural instability. Indeed, the G89R variant showed, overall, a reduced expression level compared to wild-type meprin β, accompanied by decreased activity and lower cell surface expression but strong accumulation in the endoplasmic reticulum. This was further supported by the analysis of the shedding of the interleukin-6 receptor (IL-6R) by meprin β and its variants. In transfected HEK cells, the G89R variant was found to generate less soluble IL-6R, whereas the expression of meprin β G45R resulted in increased shedding of the IL-6R compared to wild-type meprin β and the G89R variant. A similar tendency of the induced shedding capacity of G45R was seen for the well-described meprin β substrate CD99. Furthermore, employing an assay for cell migration in a collagen IV matrix, we observed that the transfection of wild-type meprin β and the G45R variant resulted in increased migration of HeLa cells, while the G89R variant led to diminished mobility.

The Swedish dilemma - the almost exclusive use of APPswe-based mouse models impedes adequate evaluation of alternative β-secretases

Alzheimer's disease (AD) is the most common form of dementia, however incurable so far. It is widely accepted that aggregated amyloid β (Aβ) peptides play a crucial role for the pathogenesis of AD, as they cause neurotoxicity and deposit as so-called Aβ plaques in AD patient brains. Aβ peptides derive from the amyloid precursor protein (APP) upon consecutive cleavage at the β- and γ-secretase site. Hence, mutations in the APP gene are often associated with autosomal dominant inherited AD. Almost thirty years ago, two mutations at the β-secretase site were observed in two Swedish families (termed Swedish APP (APPswe) mutations), which led to early-onset AD. Consequently, APPswe was established in almost every common AD mouse model, as it contributes to early Aβ plaque formation and cognitive impairments. Analyzing these APPswe-based mouse models, the aspartyl protease BACE1 has been evolving as the prominent β-secretase responsible for Aβ release in AD and as the most important therapeutic target for AD treatment. However, with respect to β-secretase processing, the very rare occurring APPswe variant substantially differs from wild-type APP. BACE1 dominates APPswe processing resulting in the release of Aβ1-x, whereas N-terminally truncated Aβ forms are scarcely generated. However, these N-terminally truncated Aβ species such as Aβ2-x, Aβ3-x and Aβ4-x are elevated in AD patient brains and exhibit an increased potential to aggregate compared to Aβ1-x peptides. Proteases such as meprin β, cathepsin B and ADAMTS4 were identified as alternative β-secretases being capable of generating these N-terminally truncated Aβ species from wild-type APP. However, neither meprin β nor cathepsin B are capable of generating N-terminally truncated Aβ peptides from APPswe. Hence, the role of BACE1 for the Aβ formation during AD might be overrepresented through the excessive use of APPswe mouse models. In this review we critically discuss the consideration of BACE1 as the most promising therapeutic target. Shifting the focus of AD research towards alternative β secretases might unveil promising alternatives to BACE1 inhibitors constantly failing in clinical trials due to ineffectiveness and harmful side effects.

Structural and functional properties of meprin β metalloproteinase with regard to cell signaling

The metalloproteinase meprin β plays an important role during collagen I deposition in the skin, mucus detachment in the small intestine and also regulates the abundance of different cell surface proteins such as the interleukin-6 receptor (IL-6R), the triggering receptor expressed on myeloid cells 2 (TREM2), the cluster of differentiation 99 (CD99), the amyloid precursor protein (APP) and the cluster of differentiation 109 (CD109). With that, regulatory mechanisms that control meprin β activity and regulate its release from the cell surface to enable access to distant substrates are increasingly important. Here, we will summarize factors that alternate meprin β activity and thereby regulate its proteolytic activity on the cell surface or in the supernatant. We will also discuss cleavage of the IL-6R and TREM2 on the cell surface and compare it to CD109. CD109, as a substrate of meprin β, is cleaved within the protein core, thereby releasing defined fragments from the cell surface. At last, we will also summarize the role of proteases in general and meprin β in particular in substrate release on extracellular vesicles.

Regulation of meprin metalloproteases in mucosal homeostasis

Mucus is covering the entire epithelium of the gastrointestinal tract (GIT), building the interface for the symbiosis between microorganisms and their host. Hence, a disrupted mucosal barrier or alterations of proper mucus composition, including the gut microbiota, can cause severe infection and inflammation. Meprin metalloproteases are well-known to cleave various pro-inflammatory molecules, contributing to the onset and progression of pathological conditions including sepsis, pulmonary hypertension or inflammatory bowel disease (IBD). Moreover, meprins have an impact on migration and infiltration of immune cells like monocytes or leukocytes during intestinal inflammation by cleaving tight junction proteins or cell adhesion molecules, thereby disrupting epithelial cell barrier and promoting transendothelial cell migration. Interestingly, both meprin α and meprin β are susceptibility genes for IBD. However, both genes are significantly downregulated in inflamed intestinal tissue in contrast to healthy donors. Therefore, a detailed understanding of the underlying molecular mechanisms is the basis for developing new and effective therapies against manifold pathologies like IBD. This review focuses on the regulation of meprin metalloproteases and its impact on physiological and pathological conditions related to mucosal homeostasis.

Syndecan-1 shedding by meprin β impairs keratinocyte adhesion and differentiation in hyperkeratosis

Dysregulation of proteolytic enzymes has huge impact on epidermal homeostasis, which can result in severe pathological conditions such as fibrosis or Netherton syndrome. The metalloprotease meprin β was found to be upregulated in hyperproliferative skin diseases. AP-1 transcription factor complex has been reported to induce Mep1b expression. Since AP-1 and its subunit fos-related antigen 2 (fra-2) are associated with the onset and progression of psoriasis, we wanted to investigate if this could partially be attributed to increased meprin β activity. Here, we demonstrate that fra-2 transgenic mice show increased meprin β expression and proteolytic activity in the epidermis. To avoid influence by other fra-2 regulated genes, we additionally generated a mouse model that enabled tamoxifen-inducible expression of meprin β under the Krt5-promotor to mimic the pathological condition. Interestingly, induced meprin β expression in the epidermis resulted in hyperkeratosis, hair loss and mottled pigmentation of the skin. Employing N-terminomics revealed syndecan-1 as a substrate of meprin β in skin. Shedding of syndecan-1 at the cell surface caused delayed calcium-induced differentiation and impaired adhesion of keratinocytes, which was blocked by the meprin β inhibitor fetuin-B.

Phosphorylation of meprin β controls its cell surface abundance and subsequently diminishes ectodomain shedding

Meprin β is a zinc-dependent metalloprotease exhibiting a unique cleavage specificity with strong preference for acidic amino acids at the cleavage site. Proteomic studies revealed a diverse substrate pool of meprin β including the interleukin-6 receptor (IL-6R) and the amyloid precursor protein (APP). Dysregulation of meprin β is often associated with pathological conditions such as chronic inflammation, fibrosis, or Alzheimer's disease (AD). The extracellular regulation of meprin β including interactors, sheddases, and activators has been intensively investigated while intracellular regulation has been barely addressed in the literature. This study aimed to analyze C-terminal phosphorylation of meprin β with regard to cell surface expression and proteolytic activity. By immunoprecipitation of endogenous meprin β from the colon cancer cell line Colo320 and subsequent LC-MS analysis, we identified several phosphorylation sites in its C-terminal region. Here, T694 in the C-terminus of meprin β was the most preferred residue after phorbol 12-myristate 13-acetate (PMA) stimulation. We further demonstrated the role of protein kinase C (PKC) isoforms for meprin β phosphorylation and identified the involvement of PKC-α and PKC-β. As a result of phosphorylation, the meprin β activity at the cell surface is reduced and, consequently, the extent of substrate cleavage is diminished. Our data indicate that this decrease of the surface activity is caused by the internalization and degradation of meprin β.

A Suite of Activity-Based Probes To Dissect the KLK Activome in Drug-Resistant Prostate Cancer

Kallikrein-related peptidases (KLKs) are a family of secreted serine proteases, which form a network (the KLK activome) with an important role in proteolysis and signaling. In prostate cancer (PCa), increased KLK activity promotes tumor growth and metastasis through multiple biochemical pathways, and specific quantification and tracking of changes in the KLK activome could contribute to validation of KLKs as potential drug targets. Herein we report a technology platform based on novel activity-based probes (ABPs) and inhibitors enabling simultaneous orthogonal analysis of KLK2, KLK3, and KLK14 activity in hormone-responsive PCa cell lines and tumor homogenates. Importantly, we identifed a significant decoupling of KLK activity and abundance and suggest that KLK proteolysis should be considered as an additional parameter, along with the PSA blood test, for accurate PCa diagnosis and monitoring. Using selective inhibitors and multiplexed fluorescent activity-based protein profiling (ABPP), we dissect the KLK activome in PCa cells and show that increased KLK14 activity leads to a migratory phenotype. Furthermore, using biotinylated ABPs, we show that active KLK molecules are secreted into the bone microenvironment by PCa cells following stimulation by osteoblasts suggesting KLK-mediated signaling mechanisms could contribute to PCa metastasis to bone. Together our findings show that ABPP is a powerful approach to dissect dysregulation of the KLK activome as a promising and previously underappreciated therapeutic target in advanced PCa.

Discovery and Optimization of Selective Inhibitors of Meprin α (Part I)

Meprin α and β are zinc-dependent proteinases implicated in multiple diseases including cancers, fibrosis, and Alzheimer’s. However, until recently, only a few inhibitors of either meprin were reported and no inhibitors are in preclinical development. Moreover, inhibitors of other metzincins developed in previous years are not effective in inhibiting meprins suggesting the need for de novo discovery effort. To address the paucity of tractable meprin inhibitors we developed ultrahigh-throughput assays and conducted parallel screening of >650,000 compounds against each meprin. As a result of this effort, we identified five selective meprin α hits belonging to three different chemotypes (triazole-hydroxyacetamides, sulfonamide-hydroxypropanamides, and phenoxy-hydroxyacetamides). These hits demonstrated a nanomolar to micromolar inhibitory activity against meprin α with low cytotoxicity and >30-fold selectivity against meprin β and other related metzincincs. These selective inhibitors of meprin α provide a good starting point for further optimization.

Protease propeptide structures, mechanisms of activation, and functions

Proteases are a diverse group of hydrolytic enzymes, ranging from single-domain catalytic molecules to sophisticated multi-functional macromolecules. Human proteases are divided into five mechanistic classes: aspartate, cysteine, metallo, serine and threonine proteases, based on the catalytic mechanism of hydrolysis. As a protective mechanism against uncontrolled proteolysis, proteases are often produced and secreted as inactive precursors, called zymogens, containing inhibitory N-terminal propeptides. Protease propeptide structures vary considerably in length, ranging from dipeptides and propeptides of about 10 amino acids to complex multifunctional prodomains with hundreds of residues. Interestingly, sequence analysis of the different protease domains has demonstrated that propeptide sequences present higher heterogeneity compared with their catalytic domains. Therefore, we suggest that protease inhibition targeting propeptides might be more specific and have less off-target effects than classical inhibitors. The roles of propeptides, besides keeping protease latency, include correct folding of proteases, compartmentalization, liganding, and functional modulation. Changes in the propeptide sequence, thus, have a tremendous impact on the cognate enzymes. Small modifications of the propeptide sequences modulate the activity of the enzymes, which may be useful as a therapeutic strategy. This review provides an overview of known human proteases, with a focus on the role of their propeptides. We review propeptide functions, activation mechanisms, and possible therapeutic applications.

Role of meprin metalloproteases in metastasis and tumor microenvironment

A crucial step for tumor cell extravasation and metastasis is the migration through the extracellular matrix, which requires proteolytic activity. Hence, proteases, particularly matrix metalloproteases (MMPs), have been discussed as therapeutic targets and their inhibition should diminish tumor growth and metastasis. The metalloproteases meprin α and meprin β are highly abundant on intestinal enterocytes and their expression was associated with different stages of colorectal cancer. Due to their ability to cleave extracellular matrix (ECM) components, they were suggested as pro-tumorigenic enzymes. Additionally, both meprins were shown to have pro-inflammatory activity by cleaving cytokines and their receptors, which correlates with chronic intestinal inflammation and associated conditions. On the other hand, meprin β was identified as an essential enzyme for the detachment and renewal of the intestinal mucus, important to prevent bacterial overgrowth and infection. Considering this, it is hard to estimate whether high activity of meprins is generally detrimental or if these enzymes have also protective functions in certain cancer types. For instance, for colorectal cancer, patients with high meprin β expression in tumor tissue exhibit a better survival prognosis, which is completely different to prostate cancer. This demonstrates that the very same enzyme may have contrary effects on tumor initiation and growth, depending on its tissue and subcellular localization. Hence, precise knowledge about proteolytic enzymes is required to design the most efficient therapeutic options for cancer treatment. In this review, we summarize the current findings on meprins' functions, expression, and cancer-associated variants with possible implications for tumor progression and metastasis.

Kallikreins: Essential epidermal messengers for regulation of the skin microenvironment during homeostasis, repair and disease

As the outermost layer of the skin, the epidermis is playing a major role in organism homeostasis providing the first barrier against external aggressions. Although considered as an extracellular matrix (ECM)-poor subtissue, the epidermal microenvironment is a key regulator of skin homeostasis and functionality. Among the proteins essential for upholding the epidermal microenvironment are the members of the kallikrein (KLK) family composed of 15 secreted serine proteases. Most of the members of these epithelial-specific proteins are present in skin and regulate skin desquamation and inflammation. However, although epidermal products, the consequences of KLK activities are not confined to the epidermis but widespread in the skin. In this review starting with the location and proteolytic activation cascade of KLKs, we present KLKs involvement in skin homeostasis, regeneration and pathology. KLKs have a large variety of substrates including ECM proteins, and evidence suggests that they are involved in the different steps of skin wound healing as discussed here. KLKs are also used as prognosis/diagnosis markers for many cancer types and we are focusing later on KLKs in cutaneous cancers, although their pathogenicity remains to be fully elucidated. Dysregulation of the KLK cascade is directly responsible for skin diseases with heavy inflammatory aspects, highlighting their involvement in skin immune homeostasis. Future studies will be needed to support the therapeutic potential of adjusting KLK activities for treatment of inflammatory skin diseases and wound healing pathologies.

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Regulation of the microenvironment even in an extracellular matrix-poor tissue can heavily impact organ function.

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Extracellular activities of kallikreins maintain skin homeostasis by regulating desquamation and inflammation.

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The activation of skin epidermal-specific kallikrein family of proteases is regulated by an intricate proteolytic cascade.

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Kallikreins are emerging as players during skin wound healing.

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Dysregulated kallikrein expression and activity occur in cancers and inflammatory skin diseases.

Regulation of the alternative β-secretase meprin β by ADAM-mediated shedding

Alzheimer's Disease (AD) is the sixth-leading cause of death in industrialized countries. Neurotoxic amyloid-β (Aβ) plaques are one of the pathological hallmarks in AD patient brains. Aβ accumulates in the brain upon sequential, proteolytic processing of the amyloid precursor protein (APP) by β- and γ-secretases. However, so far disease-modifying drugs targeting β- and γ-secretase pathways seeking a decrease in the production of toxic Aβ peptides have failed in clinics. It has been demonstrated that the metalloproteinase meprin β acts as an alternative β-secretase, capable of generating truncated Aβ2-x peptides that have been described to be increased in AD patients. This indicates an important β-site cleaving enzyme 1 (BACE-1)-independent contribution of the metalloprotease meprin β within the amyloidogenic pathway and may lead to novel drug targeting avenues. However, meprin β itself is embedded in a complex regulatory network. Remarkably, the anti-amyloidogenic α-secretase a disintegrin and metalloproteinase domain-containing protein 10 (ADAM10) is a direct competitor for APP at the cell surface, but also a sheddase of inactive pro-meprin β. Overall, we highlight the current cellular, molecular and structural understanding of meprin β as alternative β-secretase within the complex protease web, regulating APP processing in health and disease.

The cancer-associated meprin β variant G32R provides an additional activation site and promotes cancer cell invasion

The extracellular metalloprotease meprin β is expressed as a homodimer and is primarily membrane bound. Meprin β can be released from the cell surface by its known sheddases ADAM10 and ADAM17. Activation of pro-meprin β at the cell surface prevents its shedding, thereby stabilizing its proteolytic activity at the plasma membrane. We show that a single amino acid exchange variant (G32R) of meprin β, identified in endometrium cancer, is more active against a peptide substrate and the IL-6 receptor than wild-type meprin β. We demonstrate that the change to an arginine residue at position 32 represents an additional activation site used by furin-like proteases in the Golgi, which consequently leads to reduced shedding by ADAM17. We investigated this meprin β G32R variant to assess cell proliferation, invasion through a collagen IV matrix and outgrowth from tumor spheroids. We found that increased meprin β G32R activity at the cell surface reduces cell proliferation, but increases cell invasion.

Tethering soluble meprin α in an enzyme complex to the cell surface affects IBD-associated genes

Biologic activity of proteases is mainly characterized by the substrate specificity, tissue distribution, and cellular localization. The human metalloproteases meprin α and meprin β share 41% sequence identity and exhibit a similar cleavage specificity with a preference for negatively charged amino acids. However, shedding of meprin α by furin on the secretory pathway makes it a secreted enzyme in comparison with the membrane-bound meprin β. In this study, we identified human meprin α and meprin β as forming covalently linked membrane-tethered heterodimers in the early endoplasmic reticulum, thereby preventing furin-mediated secretion of meprin α. Within this newly formed enzyme complex, meprin α was able to be activated on the cell surface and detected by cleavage of a novel specific fluorogenic peptide substrate. However, the known meprin β substrates amyloid precursor protein and CD99 were not shed by membrane-tethered meprin α. On the other hand, being linked to meprin α, activation of or substrate cleavage by meprin β on the cell surface was not altered. Interestingly, proteolytic activity of both proteases was increased in the heteromeric complex, indicating an increased proteolytic potential at the plasma membrane. Because meprins are susceptibility genes for inflammatory bowel disease (IBD), and to investigate the physiologic impact of the enzyme complex, we performed transcriptome analyses of intestinal mucosa from meprin-knockout mice. Comparison of the transcriptional gene analysis data with gene analyses of IBD patients revealed that different gene subsets were dysregulated if meprin α was expressed alone or in the enzyme complex, demonstrating the physiologic and pathophysiological relevance of the meprin heterodimer formation.-Peters, F., Scharfenberg, F., Colmorgen, C., Armbrust, F., Wichert, R., Arnold, P., Potempa, B., Potempa, J., Pietrzik, C. U., Häsler, R., Rosenstiel, P., Becker-Pauly, C. Tethering soluble meprin α in an enzyme complex to the cell surface affects IBD-associated genes.

Regulation of kallikrein-related peptidases in the skin – from physiology to diseases to therapeutic options

Kallikrein-related peptidases (KLKs) constitute a family of 15 highly conserved serine proteases, which show a tissue-specific expression profile. This made them valuable tumour expression markers. It became evident that KLKs are involved in many physiological processes like semen liquefaction and skin desquamation. More recently, we have learnt that they are involved in many pathophysiological conditions and diseases making them promising target of therapeutic intervention. Therefore, regulation of KLKs raised the interest of numerous reports. Herein, we summarise the current knowledge on KLKs regulation with an emphasis on skin-relevant KLKs regulation processes. Regulation of KLKs takes place on the level of transcription, on protease activation and on protease inactivation. A variety of protease inhibitors has been described to interact with KLKs including the irreversible serine protease inhibitors (SERPINs) and the reversible serine protease inhibitors of Kazal-type (SPINKs). In an attempt to integrate current knowledge, we propose that KLK regulation has credentials as targets for therapeutic intervention.

Top-down proteomics for the analysis of proteolytic events - Methods, applications and perspectives

Mass spectrometry based proteomics is an indispensable tool for almost all research areas relevant for the understanding of proteolytic processing, ranging from the identification of substrates, products and cleavage sites up to the analysis of structural features influencing protease activity. The majority of methods for these studies are based on bottom-up proteomics performing analysis at peptide level. As this approach is characterized by a number of pitfalls, e.g. loss of molecular information, there is an ongoing effort to establish top-down proteomics, performing separation and MS analysis both at intact protein level. We briefly introduce major approaches of bottom-up proteomics used in the field of protease research and highlight the shortcomings of these methods. We then discuss the present state-of-the-art of top-down proteomics. Together with the discussion of known challenges we show the potential of this approach and present a number of successful applications of top-down proteomics in protease research. This article is part of a Special Issue entitled: Proteolysis as a Regulatory Event in Pathophysiology edited by Stefan Rose-John.

Meprin metalloproteases: Molecular regulation and function in inflammation and fibrosis

The zinc-endopeptidases meprin α and meprin β are extracellular proteases involved in connective tissue homeostasis, intestinal barrier function and immunological processes. Meprins are unique among other extracellular proteases with regard to cleavage specificity and structure. Meprin α and meprin β have a strong preference for negatively charged amino acids around the scissile bond, reflected by cleavage sites identified in procollagen I, the amyloid precursor protein (APP) and the interleukin-6 receptor (IL-6R). In this review we report on recent findings that summarize the complex molecular regulation of meprins, particular folding, activation and shedding. Dysregulation of meprin α and meprin β is often associated with pathological conditions such as neurodegeneration, inflammatory bowel disease and fibrosis. Based on mouse models and patient data we suggest meprins as possible key regulators in the onset and progression of fibrotic disorders, leading to severe diseases such as pulmonary hypertension. This article is part of a Special Issue entitled: Proteolysis as a Regulatory Event in Pathophysiology edited by Stefan Rose-John.

The Metalloprotease Meprin β Is an Alternative β-Secretase of APP

The membrane bound metalloprotease meprin β is important for collagen fibril assembly in connective tissue formation and for the detachment of the intestinal mucus layer for proper barrier function. Recent proteomic studies revealed dozens of putative new substrates of meprin β, including the amyloid precursor protein (APP). It was shown that APP is cleaved by meprin β in distinct ways, either at the β-secretase site resulting in increased levels of Aβ peptides, or at the N-terminus releasing 11 kDa, and 20 kDa peptide fragments. The latter event was discussed to be rather neuroprotective, whereas the ectodomain shedding of APP by meprin β reminiscent to BACE-1 is in line with the amyloid hypothesis of Alzheimer's disease, promoting neurodegeneration. The N-terminal 11 kDa and 20 kDa peptide fragments represent physiological cleavage products, since they are found in human brains under different diseased or non-diseased states, whereas these fragments are completely missing in brains of meprin β knock-out animals. Meprin β is not only a sheddase of adhesion molecules, such as APP, but was additionally demonstrated to cleave within the prodomain of ADAM10. Activated ADAM10, the α-secretase of APP, is then able to shed meprin β from the cell surface thereby abolishing the β-secretase activity. All together meprin β seems to be a novel player in APP processing events, even influencing other enzymes involved in APP cleavage.

Putative functions of tissue kallikrein-related peptidases in vaginal fluid

Cervical-vaginal fluid (CVF) is a complex biological fluid that hydrates the mucosa of the lower female reproductive system. In-depth proteomic and biochemical studies on CVF have revealed that it contains large amounts of endogenous proteases and protease inhibitors, including an abundance of several members of the tissue kallikrein-related peptidase (KLK) family. Despite their ubiquitous presence in human tissues and fluids, KLK expression levels vary considerably, with maximum expression observed in reproduction-related tissues and fluids. The roles of KLKs in the lower female reproductive system are not fully understood. The activation of KLKs in CVF is dependent on pH and various modes of KLK regulation in the vagina exist. KLKs have been postulated to have roles in physiological functions related to antimicrobial processes, vaginal and cervical epithelial desquamation, sperm transport, and the processing of fetal membranes as observed in preterm premature rupture of membranes. Increased understanding of the functional roles of KLKs in the lower female reproductive system could lead to new diagnostic and therapeutic modalities for conditions such as vaginal infections and vaginal atrophy.

Proteolytic degradation of neuropeptide Y (NPY) from head to toe: Identification of novel NPY-cleaving peptidases and potential drug interactions in CNS and Periphery

The bioactivity of neuropeptide Y (NPY) is either N-terminally modulated with respect to receptor selectivity by dipeptidyl peptidase 4 (DP4)-like enzymes or proteolytic degraded by neprilysin or meprins, thereby abrogating signal transduction. However, neither the subcellular nor the compartmental differentiation of these regulatory mechanisms is fully understood. Using mass spectrometry, selective inhibitors and histochemistry, studies across various cell types, body fluids, and tissues revealed that most frequently DP4-like enzymes, aminopeptidases P, secreted meprin-A (Mep-A), and cathepsin D (CTSD) rapidly hydrolyze NPY, depending on the cell type and tissue under study. Novel degradation of NPY by cathepsins B, D, L, G, S, and tissue kallikrein could also be identified. The expression of DP4, CTSD, and Mep-A at the median eminence indicates that the bioactivity of NPY is regulated by peptidases at the interphase between the periphery and the CNS. Detailed ex vivo studies on human sera and CSF samples recognized CTSD as the major NPY-cleaving enzyme in the CSF, whereas an additional C-terminal truncation by angiotensin-converting enzyme could be detected in serum. The latter finding hints to potential drug interaction between antidiabetic DP4 inhibitors and anti-hypertensive angiotensin-converting enzyme inhibitors, while it ablates suspected hypertensive side effects of only antidiabetic DP4-inhibitors application. The bioactivity of neuropeptide Y (NPY) is either N-terminally modulated with respect to receptor selectivity by dipeptidyl peptidase 4 (DP4)-like enzymes or proteolytic degraded by neprilysin or meprins, thereby abrogating signal transduction. However, neither the subcellular nor the compartmental differentiation of these regulatory mechanisms is fully understood. Using mass spectrometry, selective inhibitors and histochemistry, studies across various cell types, body fluids, and tissues revealed that most frequently DP4-like enzymes, aminopeptidases P, secreted meprin-A (Mep-A), and cathepsin D (CTSD) rapidly hydrolyze NPY, depending on the cell type and tissue under study. Novel degradation of NPY by cathepsins B, D, L, G, S, and tissue kallikrein could also be identified. The expression of DP4, CTSD, and Mep-A at the median eminence indicates that the bioactivity of NPY is regulated by peptidases at the interphase between the periphery and the CNS. Detailed ex vivo studies on human sera and CSF samples recognized CTSD as the major NPY-cleaving enzyme in the CSF, whereas an additional C-terminal truncation by angiotensin-converting enzyme could be detected in serum. The latter finding hints to potential drug interaction between antidiabetic DP4 inhibitors and anti-hypertensive angiotensin-converting enzyme inhibitors, while it ablates suspected hypertensive side effects of only antidiabetic DP4-inhibitors application.

Metalloprotease meprin β is activated by transmembrane serine protease matriptase-2 at the cell surface thereby enhancing APP shedding

Metalloprotease meprin β is a sheddase of transmembrane proteins. We identified serine protease matriptase-2 (MT2) as a specific activator of meprin β at the cell surface. This provides mechanistic insight for the regulation of meprin β activity and demonstrates clear differences in proenzyme activation.

Metastasis of ovarian cancer is mediated by kallikrein related peptidases

Ovarian cancer, in particular epithelial ovarian cancer (EOC), is commonly diagnosed when the tumor has metastasized into the abdominal cavity with an accumulation of ascites fluid. Combining histopathology and genetic variations, EOC can be sub-grouped into Type-I and Type-II tumors, of which the latter are more aggressive and metastatic. Metastasis and chemoresistance are the key events associated with the tumor microenvironment that lead to a poor patient outcome. Kallikrein-related peptidases (KLKs) are aberrantly expressed in EOC, in particular, in the more metastatic Type-II tumors. KLKs are a family of 15 serine proteases that are expressed in diverse human tissues and involved in various patho-physiological processes. As extracellular enzymes, KLKs function in the hydrolysis of growth factors, proteases, cell membrane bound receptors, adhesion proteins, and cytokines initiating intracellular signaling pathways and their downstream events. High KLK levels are differentially associated with the prognosis of ovarian cancer patients, suggesting that they not only have application as biomarkers but also function in disease progression, and therefore are potential therapeutic targets. Recent studies have demonstrated the function of these proteases in promoting and/or suppressing the invasive behavior of ovarian cancer cells in metastasis in vitro and in vivo. Both conventional cell culture methods and three-dimensional platforms have been applied to mimic the ovarian cancer microenvironment of patients, such as the solid stromal matrix and ascites fluid. Here we summarize published studies to provide an overview of our understanding of the role of KLKs in EOC, and to lay the foundation for future research directions.

The metalloproteases meprin α and meprin β: unique enzymes in inflammation, neurodegeneration, cancer and fibrosis

The metalloproteases meprin α and meprin β exhibit structural and functional features that are unique among all extracellular proteases. Although meprins were discovered more than 30 years ago, their precise substrates and physiological roles have been elusive. Both enzymes were originally found to be highly expressed in kidney and intestine, which focused research on these particular tissues and associated pathologies. Only recently it has become evident that meprins exhibit a much broader expression pattern, implicating functions in angiogenesis, cancer, inflammation, fibrosis and neurodegenerative diseases. Different animal models, as well as proteomics approaches for the identification of protease substrates, have helped to reveal more precise molecular signalling events mediated by meprin activity, such as activation and release of pro-inflammatory cytokines. APP (amyloid precursor protein) is cleaved by meprin β in vivo, reminiscent of the β-secretase BACE1 (β-site APP-cleaving enzyme 1). The subsequent release of Aβ (amyloid β) peptides is thought to be the major cause of the neurodegenerative Alzheimer's disease. On the other hand, ADAM10 (a disintegrin and metalloprotease domain 10), which is the constitutive α-secretase, was shown to be activated by meprin β, which is itself shed from the cell surface by ADAM10. In skin, both meprins are overexpressed in fibrotic tumours, characterized by massive accumulation of fibrillar collagens. Indeed, procollagen III is processed to its mature form by meprin α and meprin β, an essential step in collagen fibril assembly. The recently solved crystal structure of meprin β and the unique cleavage specificity of these proteases identified by proteomics will help to generate specific inhibitors that could be used as therapeutics to target meprins under certain pathological conditions.

Meprin A metalloproteinase and its role in acute kidney injury

Meprin A, composed of α- and β-subunits, is a membrane-associated neutral metalloendoprotease that belongs to the astacin family of zinc endopeptidases. It was first discovered as an azocasein and benzoyl-l-tyrosyl-p-aminobenzoic acid hydrolase in the brush-border membranes of proximal tubules and intestines. Meprin isoforms are now found to be widely distributed in various organs (kidney, intestines, leukocytes, skin, bladder, and a variety of cancer cells) and are capable of hydrolyzing and processing a large number of substrates, including extracellular matrix proteins, cytokines, adherens junction proteins, hormones, bioactive peptides, and cell surface proteins. The ability of meprin A to cleave various substrates sheds new light on the functional properties of this enzyme, including matrix remodeling, inflammation, and cell-cell and cell-matrix processes. Following ischemia-reperfusion (IR)- and cisplatin-induced acute kidney injury (AKI), meprin A is redistributed toward the basolateral plasma membrane, and the cleaved form of meprin A is excreted in the urine. These studies suggest that altered localization and shedding of meprin A in places other than the apical membranes may be deleterious in vivo in acute tubular injury. These studies also provide new insight into the importance of a sheddase involved in the release of membrane-associated meprin A under pathological conditions. Meprin A is injurious to the kidney during AKI, as meprin A-knockout mice and meprin inhibition provide protective roles and improve renal function. Meprin A, therefore, plays an important role in AKI and potentially is a unique target for therapeutic intervention during AKI.

Kallikrein-related peptidase 4 (KLK4) mRNA predicts short-term relapse in colorectal adenocarcinoma patients

The members of the kallikrein-related peptidase (KLK) family are aberrantly expressed in cancer, including colorectal adenocarcinoma. KLK4 is an endogenous activator of protease-activated receptor 1 (PAR1) in HT-29 colorectal adenocarcinoma cells, inducing PAR1 signaling and subsequent ERK1/2 activation. The aim of this study was to analyze KLK4 mRNA expression in colorectal adenocarcinoma and to examine its prognostic value as a novel molecular tissue biomarker in this malignancy. Therefore, total RNA was isolated from primary tumors of 81 colorectal adenocarcinoma patients, cDNA was prepared, and KLK4 mRNA expression analysis was performed using quantitative real-time PCR. KLK4 mRNA was significantly associated with the Dukes stage, tumor invasion, size, and histological grade. Survival analysis demonstrated that KLK4 mRNA expression constitutes an unfavorable prognostic biomarker in colorectal adenocarcinoma, predicting poor disease-free survival (DFS), independently of the nodal status and tumor size. Furthermore, KLK4 mRNA predicts short-term relapse of lymph node-negative patients or those with tumors of early Dukes stage. In conclusion, KLK4 mRNA expression can be regarded as a novel potential tissue biomarker in colorectal adenocarcinoma.

Meprins process matrix metalloproteinase-9 (MMP-9)/gelatinase B and enhance the activation kinetics by MMP-3

Meprin α and β, members of the astacin family of zinc metalloproteinases, are unique plasma membrane and secreted proteases known to cleave a wide range of biological substrates involved in inflammation, cancer and fibrosis. In this study, we identified proMMP-9 as a novel substrate and show that aminoterminal meprin-mediated clipping improves the activation kinetics of proMMP-9 by MMP-3, an efficient activator of proMMP-9. Interestingly, the NH(2)-terminus LVLFPGDL, generated by incubation with meprin α, is identical to the form produced in conditioned media from human neutrophils and monocytes. Hence, this meprin-mediated processing and enhancement of MMP-9 activation kinetics may have biological relevance in the context of in vivo inflammatory processes.

Structural basis for the sheddase function of human meprin β metalloproteinase at the plasma membrane

Ectodomain shedding at the cell surface is a major mechanism to regulate the extracellular and circulatory concentration or the activities of signaling proteins at the plasma membrane. Human meprin β is a 145-kDa disulfide-linked homodimeric multidomain type-I membrane metallopeptidase that sheds membrane-bound cytokines and growth factors, thereby contributing to inflammatory diseases, angiogenesis, and tumor progression. In addition, it cleaves amyloid precursor protein (APP) at the β-secretase site, giving rise to amyloidogenic peptides. We have solved the X-ray crystal structure of a major fragment of the meprin β ectoprotein, the first of a multidomain oligomeric transmembrane sheddase, and of its zymogen. The meprin β dimer displays a compact shape, whose catalytic domain undergoes major rearrangement upon activation, and reveals an exosite and a sugar-rich channel, both of which possibly engage in substrate binding. A plausible structure-derived working mechanism suggests that substrates such as APP are shed close to the plasma membrane surface following an "N-like" chain trace.

The substrate degradome of meprin metalloproteases reveals an unexpected proteolytic link between meprin β and ADAM10

The in vivo roles of meprin metalloproteases in pathophysiological conditions remain elusive. Substrates define protease roles. Therefore, to identify natural substrates for human meprin α and β we employed TAILS (terminal amine isotopic labeling of substrates), a proteomics approach that enriches for N-terminal peptides of proteins and cleavage fragments. Of the 151 new extracellular substrates we identified, it was notable that ADAM10 (a disintegrin and metalloprotease domain-containing protein 10)-the constitutive α-secretase-is activated by meprin β through cleavage of the propeptide. To validate this cleavage event, we expressed recombinant proADAM10 and after preincubation with meprin β, this resulted in significantly elevated ADAM10 activity. Cellular expression in murine primary fibroblasts confirmed activation. Other novel substrates including extracellular matrix proteins, growth factors and inhibitors were validated by western analyses and enzyme activity assays with Edman sequencing confirming the exact cleavage sites identified by TAILS. Cleavages in vivo were confirmed by comparing wild-type and meprin(-/-) mice. Our finding of cystatin C, elafin and fetuin-A as substrates and natural inhibitors for meprins reveal new mechanisms in the regulation of protease activity important for understanding pathophysiological processes.

Meprinα transactivates the epidermal growth factor receptor (EGFR) via ligand shedding, thereby enhancing colorectal cancer cell proliferation and migration

Meprinα, an astacin-type metalloprotease is overexpressed in colorectal cancer cells and is secreted in a non-polarized fashion, leading to the accumulation of meprinα in the tumor stroma. The transition from normal colonocytes to colorectal cancer correlates with increased meprinα activity at primary tumor sites. A role for meprinα in invasion and metastatic dissemination is supported by its pro-angiogenic and pro-migratory activity. In the present study, we provide evidence for a meprinα-mediated transactivation of the EGFR signaling pathway and suggest that this mechanism is involved in colorectal cancer progression. Using alkaline phosphatase-tagged EGFR ligands and an ELISA assay, we demonstrate that meprinα is capable of shedding epidermal growth factor (EGF) and transforming growth factor-α (TGFα) from the plasma membrane. Shedding was abrogated using actinonin, an inhibitor for meprinα. The physiological effects of meprinα-mediated shedding of EGF and TGFα were investigated with human colorectal adenocarcinoma cells (Caco-2). Proteolytically active meprinα leads to an increase in EGFR and ERK1/2 phosphorylation and subsequently enhances cell proliferation and migration. In conclusion, the implication of meprinα in the EGFR/MAPK signaling pathway indicates a role of meprinα in colorectal cancer progression.

Protease-armed bacteria in the skin

The skin constitutes a formidable barrier against commensal and pathogenic bacteria, which permanently and transiently colonise the skin, respectively. Commensal and pathogenic species inhabiting skin both express proteases. Whereas proteases secreted by commensals contribute to homeostatic bacterial coexistence on skin, proteases from pathogenic bacteria are used as virulence factors, helping them colonise skin with breached integrity of the epithelial layer. From these initial sites of colonisation, pathogens can disseminate into deeper layers of skin, possibly leading to the spread of infection. Secreted bacterial proteases probably play an important role in this process and in the deterrence of innate defence mechanisms. For example, Staphylococcus aureus proteases are essential for changing the bacterial phenotype from adhesive to invasive by degrading adhesins on the bacterial cell surface. Secreted staphylococcal proteases mediate pathogen penetration by degrading collagen and elastin, essential components of connective tissue in the dermis. The activation of the contact system and kinin generation by Streptococcus pyogenes and S. aureus proteases contributes to an inflammatory reaction manifested by oedema, redness and pain. Kinin-enhanced vascular leakage might help bacteria escape into the circulation thereby causing possible systemic dissemination of the infection. The inflammatory reaction can also be fueled by the activation of protease-activated receptors on keratinocytes. Concomitantly, bacterial proteases are involved in degrading antimicrobial peptides, disarming the complement system and neutrophils and preventing the infiltration of the infected sites with immune cells by inactivation of chemoattractants. Together, this provides protection for colonising and/or invading pathogens from attack by antibacterial forces of the skin.

Doxycycline indirectly inhibits proteolytic activation of tryptic kallikrein-related peptidases and activation of cathelicidin

The increased abundance and activity of cathelicidin and kallikrein 5 (KLK5), a predominant trypsin-like serine protease (TLSP) in the stratum corneum, have been implicated in the pathogenesis of rosacea, a disorder treated by the use of low-dose doxycycline. Here we hypothesized that doxycycline can inhibit activation of tryptic KLKs through an indirect mechanism by inhibition of matrix metalloproteinases (MMPs) in keratinocytes. The capacity of doxycycline to directly inhibit enzyme activity was measured in surface collections of human facial skin and extracts of cultured keratinocytes by fluorescence polarization assay against fluorogenic substrates specific for MMPs or TLSPs. Doxycycline did inhibit MMP activity but did not directly inhibit serine protease activity against a fluorogenic substrate specific for TLSPs. However, when doxycycline or other MMP inhibitors were added to live keratinocytes during the production of tryptic KLKs, this treatment indirectly resulted in decreased TLSP activity. Furthermore, doxycycline under these conditions inhibited the generation of the cathelicidin peptide LL-37 from its precursor protein hCAP18, a process dependent on KLK activity. These results demonstrate that doxycycline can prevent cathelicidin activation, and suggest a previously unknown mechanism of action for doxycycline through inhibiting generation of active cathelicidin peptides.

Toward an integrative analysis of the tumor microenvironment in ovarian epithelial carcinoma

Ovarian epithelial carcinomas are heterogeneous malignancies exhibiting great diversity in histological phenotypes as well as genetic and epigenetic aberrations. A general early event in tumorigenesis is regional dissemination into the peritoneal cavity. Initial spread to the peritoneum is made possible by cooperative signaling between a wide array of molecules constituting the tissue microenvironment in the coelomic epithelium. Changes in the activity of key microenvironmental components not constitutively expressed in normal tissue, including several disclosed adhesion molecules, growth factors, proteases, and G-protein coupled receptors (GPCRs), coordinate the transition. Remodeling of the extracellular matrix (ECM) and subsequent cell surface interactions enable transformation by promoting chromosomal instability (CIN) and stimulating several common signal transduction cascades to prepare the tissue for harboring and facilitating growth, angiogenesis and metastasis of the developing tumor.

Enhanced activity of meprin-α, a pro-migratory and pro-angiogenic protease, in colorectal cancer

Meprin-α is a metalloprotease overexpressed in cancer cells, leading to the accumulation of this protease in a subset of colorectal tumors. The impact of increased meprin-α levels on tumor progression is not known. We investigated the effect of this protease on cell migration and angiogenesis in vitro and studied the expression of meprin-α mRNA, protein and proteolytic activity in primary tumors at progressive stages and in liver metastases of patients with colorectal cancer, as well as inhibitory activity towards meprin-α in sera of cancer patient as compared to healthy controls. We found that the hepatocyte growth factor (HGF)- induced migratory response of meprin-transfected epithelial cells was increased compared to wild-type cells in the presence of plasminogen, and that the angiogenic response in organ-cultured rat aortic explants was enhanced in the presence of exogenous human meprin-α. In patients, meprin-α mRNA was expressed in colonic adenomas, primary tumors UICC (International Union Against Cancer) stage I, II, III and IV, as well as in liver metastases. In contrast, the corresponding protein accumulated only in primary tumors and liver metastases, but not in adenomas. However, liver metastases lacked meprin-α activity despite increased expression of the corresponding protein, which correlated with inefficient zymogen activation. Sera from cancer patients exhibited reduced meprin-α inhibition compared to healthy controls. In conclusion, meprin-α activity is regulated differently in primary tumors and metastases, leading to high proteolytic activity in primary tumors and low activity in liver metastases. By virtue of its pro-migratory and pro-angiogenic activity, meprin-α may promote tumor progression in colorectal cancer.

Morpholino knockdown of the ubiquitously expressed transmembrane serine protease TMPRSS4a in zebrafish embryos exhibits severe defects in organogenesis and cell adhesion

Abstract Over the past years the members of the type II transmembrane serine protease (TTSP) family have emerged as new players in mammalian biology. TMPRSS4 (transmembrane protease/serine) is overexpressed in several human cancer tissues, promoting invasion, migration, and metastasis. However, the physiological function has not yet been elucidated. Here, we present morpholino knockdown studies targeting TMPRSS4a, a homolog of human TMPRSS4 in zebrafish embryos. By RT-PCR, we could demonstrate an expression of this protease already 5 h post-fertilization, suggesting important functions in the early stages of embryonic development. Indeed, in vivo gene silencing caused severe defects in tissue development and cell differentiation including a disturbed skeletal muscle formation, a decelerated heartbeat, and a degenerated vascular system. Scanning electron microscopy revealed strong defects in epidermal skin organization, with clearly altered cell-cell contacts, resulting in the detachment of keratinocytes from the underneath tissue. The disturbed organogenesis in general is consistent with RT-PCR results which exhibited a ubiquitous expression of TMPRSS4a, predominantly in kidney, skin, heart, and gills. Our results demonstrate the importance of TMPRSS4a in tissue development and cell differentiation. Whether its proteolytic activity is directed towards adhesion molecules or leads to the activation of other proteases needs to be investigated further.

Metalloprotease meprin beta generates nontoxic N-terminal amyloid precursor protein fragments in vivo

Identification of physiologically relevant substrates is still the most challenging part in protease research for understanding the biological activity of these enzymes. The zinc-dependent metalloprotease meprin β is known to be expressed in many tissues with functions in health and disease. Here, we demonstrate unique interactions between meprin β and the amyloid precursor protein (APP). Although APP is intensively studied as a ubiquitously expressed cell surface protein, which is involved in Alzheimer disease, its precise physiological role and relevance remain elusive. Based on a novel proteomics technique termed terminal amine isotopic labeling of substrates (TAILS), APP was identified as a substrate for meprin β. Processing of APP by meprin β was subsequently validated using in vitro and in vivo approaches. N-terminal APP fragments of about 11 and 20 kDa were found in human and mouse brain lysates but not in meprin β(-/-) mouse brain lysates. Although these APP fragments were in the range of those responsible for caspase-induced neurodegeneration, we did not detect cytotoxicity to primary neurons treated by these fragments. Our data demonstrate that meprin β is a physiologically relevant enzyme in APP processing.

Proteomic analyses reveal an acidic prime side specificity for the astacin metalloprotease family reflected by physiological substrates

Astacins are secreted and membrane-bound metalloproteases with clear associations to many important pathological and physiological processes. Yet with only a few substrates described their biological roles are enigmatic. Moreover, the lack of knowledge of astacin cleavage site specificities hampers assay and drug development. Using PICS (proteomic identification of protease cleavage site specificity) and TAILS (terminal amine isotopic labeling of substrates) degradomics approaches >3000 cleavage sites were proteomically identified for five different astacins. Such broad coverage enables family-wide determination of specificities N- and C-terminal to the scissile peptide bond. Remarkably, meprin α, meprin β, and LAST_MAM proteases exhibit a strong preference for aspartate in the peptide (P)1′ position because of a conserved positively charged residue in the active cleft subsite (S)1′. This unparalleled specificity has not been found for other families of extracellular proteases. Interestingly, cleavage specificity is also strongly influenced by proline in P2′ or P3′ leading to a rare example of subsite cooperativity. This specificity characterizes the astacins as unique contributors to extracellular proteolysis that is corroborated by known cleavage sites in procollagen I+III, VEGF (vascular endothelial growth factor)-A, IL (interleukin)-1β, and pro-kallikrein 7. Indeed, cleavage sites in VEGF-A and pro-kallikrein 7 identified by terminal amine isotopic labeling of substrates matched those reported by Edman degradation. Moreover, the novel substrate FGF-19 was validated biochemically and shown to exhibit altered biological activity after meprin processing.