Mucus Detachment by Host Metalloprotease Meprin β Requires Shedding of Its Inactive Pro-form, which Is Abrogated by the Pathogenic Protease RgpB

Neonatal hypoxia leads to impaired intestinal function and changes in the composition and metabolism of its microbiota

Neonatal hypoxia, a prevalent complication during the perinatal period, poses a serious threat to the health of newborns. The intestine, as one of the most metabolically active organs under stress conditions, is particularly vulnerable and susceptible to hypoxic injury. Using a neonatal hypoxic mouse model, we systematically investigated hypoxia-induced intestinal barrier damage and underlying mechanisms. Hypoxia caused significant structural abnormalities in the ileum and distal colon of neonatal mice, including increased numbers of F4/80+ cells (p = 0.0031), swollen mucus particles (p = 0.0119), and disrupted tight junction. At the genetic level, hypoxia caused dysregulation of the expression of genes involved in intestinal barrier function, including antimicrobial activity, immune response, intestinal motility, and nutrient absorption. Further 16 S rDNA sequencing revealed hypoxia-driven gut microbiota dysbiosis with general reduced microbial abundance and diversity (Chao1 = 0.1143, Shannon = 0.0571, and Simpson = 0.3429). Structural dysbiosis of the gut microbiota consequently perturbed metabolic homeostasis, especially enhancing the activity of glycolipid metabolism. Notably, results showed that hypoxia may interfere with neurotransmitter metabolism, thereby increasing the risk of neurological disorders.

Targeted approach to determine the impact of cancer-associated protease variants

Several steps of cancer progression, from tumor onset to metastasis, critically involve proteolytic activity. To elucidate the role of proteases in cancer, it is particularly important to consider single-nucleotide variants (SNVs) that affect the active site of proteases, thereby influencing cleavage specificity, substrate processing, and thus cancer cell behavior. To facilitate systematic studies, we here present a targeted approach to determine the impact of cancer-associated protease variants (TACAP). Starting with the semiautomated identification of potential specificity-modulating SNVs, our workflow comprises mass spectrometry-based cleavage specificity profiling and substrate identification, localization, and inhibitor studies, followed by functional analyses investigating cancer cell properties. To demonstrate the feasibility of TACAP, we analyzed the meprin β R238Q variant. This amino acid exchange R238Q leads to a loss of meprin β's characteristic cleavage preference for acidic amino acids at P1' position, accompanied with changes in substrate pool and inhibitor affinity compared to meprin β wild type.

Alzheimer's Disease and Porphyromonas gingivalis: Exploring the Links

Recent research highlights compelling links between oral health, particularly periodontitis, and systemic diseases, including Alzheimer's disease (AD). Although the biological mechanisms underlying these associations remain unclear, the role of periodontal pathogens, particularly Porphyromonas gingivalis, has garnered significant attention. P. gingivalis, a major driver of periodontitis, is recognized for its potential systemic effects and its putative role in AD pathogenesis. This review examines evidence connecting P. gingivalis to hallmark AD features, such as amyloid β accumulation, tau hyperphosphorylation, neuroinflammation, and other neuropathological features consistent with AD. Virulence factors, such as gingipains and lipopolysaccharides, were shown to be implicated in blood-brain barrier disruption, neuroinflammation, and neuronal damage. P. gingivalis-derived outer membrane vesicles may serve to disseminate virulence factors to brain tissues. Indirect mechanisms, including systemic inflammation triggered by chronic periodontal infections, are also supposed to exacerbate neurodegenerative processes. While the exact pathways remain uncertain, studies detecting P. gingivalis virulence factors and its other components in AD-affected brains support their possible role in disease pathogenesis. This review underscores the need for further investigation into P. gingivalis-mediated mechanisms and their interplay with host responses. Understanding these interactions could provide critical insights into novel strategies for reducing AD risk through periodontal disease management.

The MUC2 Gene Product: Polymerisation and Post-Secretory Organisation-Current Models

MUC2 mucin, the primary gel-forming component of intestinal mucus, is well researched and a model of polymerisation and post-secretory organisation has been published previously. Recently, several significant developments have been made which either introduce new ideas or challenge previous theories. New ideas include an overhaul of the MUC2 C-terminal globular structure which is proposed to harbour several previously unobserved domains, and include a site for an extra intermolecular disulphide bridge dimer between the cysteine 4379 of adjacent MUC2 C-termini. MUC2 polymers are also now thought to be secreted attached to the epithelial surface of goblet cells in the small intestine and removed following secretion via a metalloprotease meprin β-mediated cleavage of the von Willebrand D2 domain of the N-terminus. It remains unclear whether MUC2 forms intermolecular dimers, trimers, or both, at the N-termini during polymerisation, with several articles supporting either trimer or dimer formation. The presence of a firm inner mucus layer in the small intestine is similarly unclear. Considering this recent research, this review proposes an update to the previous model of MUC2 polymerisation and secretion, considers conflicting theories and data, and highlights the importance of this research to the understanding of MUC2 mucus layers in health and disease.

Modulation of engineered nanomaterial interactions with organ barriers for enhanced drug transport

The biomedical use of nanoparticles (NPs) has been the focus of intense research for over a decade. As most NPs are explored as carriers to alter the biodistribution, pharmacokinetics and bioavailability of associated drugs, the delivery of these NPs to the tissues of interest remains an important topic. To date, the majority of NP delivery studies have used tumor models as their tool of interest, and the limitations concerning tumor targeting of systemically administered NPs have been well studied. In recent years, the focus has also shifted to other organs, each presenting their own unique delivery challenges to overcome. In this review, we discuss the recent advances in leveraging NPs to overcome four major biological barriers including the lung mucus, the gastrointestinal mucus, the placental barrier, and the blood-brain barrier. We define the specific properties of these biological barriers, discuss the challenges related to NP transport across them, and provide an overview of recent advances in the field. We discuss the strengths and shortcomings of different strategies to facilitate NP transport across the barriers and highlight some key findings that can stimulate further advances in this field.

The putative pleiotropic functions of meprin β in gastric cancer

BACKGROUND

The gastric microbiome and inflammation play a key role in gastric cancer (GC) by regulating the immune response in a complex manner and by inflammatory events supporting carcinogenesis. Meprin β is a zinc endopeptidase and participates in tissue homeostasis, intestinal barrier function and immunological processes. It influences local inflammatory processes, dysbiosis and the microbiome. Here, we tested the hypothesis that meprin β is expressed in GC and of tumor biological significance.

PATIENTS AND METHODS

Four hundred forty whole mount tissue sections of patients with therapy-naive GC were stained with an anti-meprin β antibody. The histoscore and staining pattern were analyzed for each case. Following dichotomization at the median histoscore into a "low" and "high" group, the expression was correlated with numerous clinicopathological patient characteristics.

RESULTS

Meprin β was found intracellularly and at the cell membrane of GC. Cytoplasmic expression correlated with the phenotype according to Lauren, microsatellite instability and PD-L1 status. Membranous expression correlated with intestinal phenotype, mucin-1-, E-cadherin-, β-catenin status, mucin typus, microsatellite instability, KRAS mutation and PD-L1-positivity. Patients with cytoplasmic expression of meprin β showed a better overall and tumor-specific survival.

CONCLUSIONS

Meprin β is differentially expressed in GC and has potential tumor biological relevance. It might function as a tumor suppressor or promotor depending on histoanatomical site and context.

Proteolytic processing of galectin-3 by meprin metalloproteases is crucial for host-microbiome homeostasis

The metalloproteases meprin α and meprin β are highly expressed in the healthy gut but significantly decreased in inflammatory bowel disease, implicating a protective role in mucosal homeostasis. In the colon, meprin α and meprin β form covalently linked heterodimers tethering meprin α to the plasma membrane, therefore presenting dual proteolytic activity in a unique enzyme complex. To unravel its function, we applied N-terminomics and identified galectin-3 as the major intestinal substrate for meprin α/β heterodimers. Galectin-3-deficient and meprin α/β double knockout mice show similar alterations in their microbiome in comparison to wild-type mice. We further demonstrate that meprin α/β heterodimers differentially process galectin-3 upon bacterial infection, in germ-free, conventionally housed (specific pathogen-free), or wildling mice, which in turn regulates the bacterial agglutination properties of galectin-3. Thus, the constitutive cleavage of galectin-3 by meprin α/β heterodimers may play a key role in colon host-microbiome homeostasis.

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.

Helical ultrastructure of the metalloprotease meprin α in complex with a small molecule inhibitor

The zinc-dependent metalloprotease meprin α is predominantly expressed in the brush border membrane of proximal tubules in the kidney and enterocytes in the small intestine and colon. In normal tissue homeostasis meprin α performs key roles in inflammation, immunity, and extracellular matrix remodelling. Dysregulated meprin α is associated with acute kidney injury, sepsis, urinary tract infection, metastatic colorectal carcinoma, and inflammatory bowel disease. Accordingly, meprin α is the target of drug discovery programs. In contrast to meprin β, meprin α is secreted into the extracellular space, whereupon it oligomerises to form giant assemblies and is the largest extracellular protease identified to date (~6 MDa). Here, using cryo-electron microscopy, we determine the high-resolution structure of the zymogen and mature form of meprin α, as well as the structure of the active form in complex with a prototype small molecule inhibitor and human fetuin-B. Our data reveal that meprin α forms a giant, flexible, left-handed helical assembly of roughly 22 nm in diameter. We find that oligomerisation improves proteolytic and thermal stability but does not impact substrate specificity or enzymatic activity. Furthermore, structural comparison with meprin β reveal unique features of the active site of meprin α, and helical assembly more broadly.

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.

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.

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.

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.

The expression of P2Y6 receptor promotes the quality of mucus in colitic mice

In the intestine, mucins are expressed and secreted by goblet cells and enterocytes in a constitutive manner and in response to secretagogues to form a protective mucus layer. This protective barrier is often lost in inflammatory bowel disease (IBD). Interestingly, extracellular nucleotides, through P2Y receptors, were identified as mucin secretagogues in mucinous epithelia. These nucleotides are found in the intestine's extracellular milieu under basal conditions and in higher concentrations in pathologies such as IBD. It was observed that the mucus layer was affected in P2ry6 knockout mice suffering from dextran sodium sulfate (DSS)-induced colitis. P2ry6^-/- mice were more sensitive to DSS-induced colitis, resulting in larger ulcers and increased disease activity index. Interestingly, the absence of P2Y₆ receptor expression negatively affected the mucus quality, as shown by a reduction in sulfomucin staining and the absence of a dense internal fucosylated mucin layer in P2ry6^-/- mice. Hence, we cannot rule out that the absence of P2Y₆ receptors in knockout animals could negatively impact mucin secretion. However, we did not measure a reduction in the number of goblet cells, as previously reported. Instead, the results suggest that goblet cells rapidly discharged mucins to compensate for the mucus layer's increased lability, which resulted in empty goblet cells that are less visible to mucin staining. This study's results, along with previous reports, point toward a protective role for the P2Y₆ receptor in IBD.

Jejunum: The understudied meeting place of dietary lipids and the microbiota

Although the jejunum is the main intestinal compartment responsible for lipid digestion and absorption, most of the studies assessing the impact of dietary lipids on the intestinal microbiota have been performed in the ileum, colon and faeces. This lack of interest in the jejunum is due to the much lower number of microbes present in this intestinal region and to the difficulty in accessing its lumen, which requires invasive methods. Recently, several recent publications highlighted that the whole jejunal microbiota or specific bacterial members are able to modulate lipid absorption and metabolism in enterocytes. This information reveals new strategies in the development of bacterial- and metabolite-based therapeutic interventions or nutraceutical recommendations to treat or prevent metabolic-related disorders, including obesity, cardiovascular diseases and malnutrition. This review is strictly focused on the following triad: dietary lipids, the jejunal epithelium and the jejunal microbiota. First, we will describe each member of the triad: the structure and functions of the jejunum, the composition of the jejunal microbiota, and dietary lipid handling by enterocytes and by microorganisms. Then, we will present the mechanisms leading to lipid malabsorption in small intestinal bacterial overgrowth (SIBO), a disease in which the jejunal microbiota is altered and which highlights the strong interactions among this triad. We will finally review the recent literature about the interactions among members of the triad, which should encourage research teams to further explore the mechanisms by which specific microbial strains or metabolites, alone or in concert, can mediate, control or modulate lipid absorption in the jejunum.

The Role of the Gastrointestinal Mucus System in Intestinal Homeostasis: Implications for Neurological Disorders

Mucus is integral to gut health and its properties may be affected in neurological disease. Mucus comprises a hydrated network of polymers including glycosylated mucin proteins. We propose that factors that influence the nervous system may also affect the volume, viscosity, porosity of mucus composition and subsequently, gastrointestinal (GI) microbial populations. The gut has its own intrinsic neuronal network, the enteric nervous system, which extends the length of the GI tract and innervates the mucosal epithelium. The ENS regulates gut function including mucus secretion and renewal. Both dysbiosis and gut dysfunction are commonly reported in several neurological disorders such as Parkinson's and Alzheimer's disease as well in patients with neurodevelopmental disorders including autism. Since some microbes use mucus as a prominent energy source, changes in mucus properties could alter, and even exacerbate, dysbiosis-related gut symptoms in neurological disorders. This review summarizes existing knowledge of the structure and function of the mucus of the GI tract and highlights areas to be addressed in future research to better understand how intestinal homeostasis is impacted in neurological disorders.

Necroptosis, ADAM proteases and intestinal (dys)function

Recently, an unexpected connection between necroptosis and members of the a disintegrin and metalloproteinase (ADAM) protease family has been reported. Necroptosis represents an important cell death routine which helps to protect from viral, bacterial, fungal and parasitic infections, maintains adult T cell homeostasis and contributes to the elimination of potentially defective organisms before parturition. Equally important for organismal homeostasis, ADAM proteases control cellular processes such as development and differentiation, immune responses or tissue regeneration. Notably, necroptosis as well as ADAM proteases have been implicated in the control of inflammatory responses in the intestine. In this review, we therefore provide an overview of the physiology and pathophysiology of necroptosis, ADAM proteases and intestinal (dys)function, discuss the contribution of necroptosis and ADAMs to intestinal (dys)function, and review the current knowledge on the role of ADAMs in necroptotic signaling.

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.

Degradome of soluble ADAM10 and ADAM17 metalloproteases

Disintegrin and metalloproteinases (ADAMs) 10 and 17 can release the extracellular part of a variety of membrane-bound proteins via ectodomain shedding important for many biological functions. So far, substrate identification focused exclusively on membrane-anchored ADAM10 and ADAM17. However, besides known shedding of ADAM10, we identified ADAM8 as a protease capable of releasing the ADAM17 ectodomain. Therefore, we investigated whether the soluble ectodomains of ADAM10/17 (sADAM10/17) exhibit an altered substrate spectrum compared to their membrane-bound counterparts. A mass spectrometry-based N-terminomics approach identified 134 protein cleavage events in total and 45 common substrates for sADAM10/17 within the secretome of murine cardiomyocytes. Analysis of these cleavage sites confirmed previously identified amino acid preferences. Further in vitro studies verified fibronectin, cystatin C, sN-cadherin, PCPE-1 as well as sAPP as direct substrates of sADAM10 and/or sADAM17. Overall, we present the first degradome study for sADAM10/17, thereby introducing a new mode of proteolytic activity within the protease web.

Phosphorylation of the amyloid precursor protein (APP) at Ser-675 promotes APP processing involving meprin β

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by abnormal deposition of β-amyloid (Aβ) peptides. Aβ is a cleavage product of the amyloid precursor protein (APP), and aberrant posttranslational modifications of APP can alter APP processing and increase Aβ generation. In the AD brain, seven different residues, including Ser-675 (APP695 numbering) in the APP cytoplasmic domain has been found to be phosphorylated. Here, we show that expression of a phosphomimetic variant of Ser-675 in APP (APP-S675E), in human neuroblastoma SK-N-AS cells, reduces secretion of the soluble APP ectodomain (sAPPα), even though the total plasma membrane level of APP was unchanged compared with APP levels in cells expressing APPwt or APP-S675A. Moreover, the level of an alternative larger C-terminal fragment (CTF) increased in the APP-S675E cells, whereas the CTF form that was most abundant in cells expressing APPwt or APP-S675A decreased in the APP-S675E cells. Upon siRNA-mediated knockdown of the astacin metalloprotease meprin β, the levels of the alternative CTF decreased and the CTF ratio was restored back to APPwt levels. Our findings suggest that APP-Ser-675 phosphorylation alters the balance of APP processing, increasing meprin β-mediated and decreasing α-secretase-mediated processing of APP at the plasma membrane. As meprin β cleavage of APP has been shown to result in formation of highly aggregation-prone, truncated Aβ2-40/42 peptides, enhanced APP processing by this enzyme could contribute to AD pathology. We propose that it would be of interest to clarify in future studies how APP-Ser-675 phosphorylation promotes meprin β-mediated APP cleavage.

Meprin β induces activities of A disintegrin and metalloproteinases 9, 10, and 17 by specific prodomain cleavage

Meprin β is a membrane-bound metalloprotease involved in extracellular matrix assembly and inflammatory processes in health and disease. A disintegrin and metalloproteinase (ADAM)10 and ADAM17 are physiologic relevant sheddases of inactive promeprin β, which influences its substrate repertoire and subsequent biologic functions. Proteomic analysis also revealed several ADAMs as putative meprin β substrates. Here, we demonstrate specific N-terminal processing of ADAM9, 10, and 17 by meprin β and identify cleavage sites within their prodomains. Because ADAM prodomains can act as specific inhibitors, we postulate a role for meprin β in the regulation of ADAM activities. Indeed, prodomain cleavage by meprin β caused increased ADAM protease activities, as observed by peptide-based cleavage assays and demonstrated by increased ectodomain shedding activity. Direct interaction of meprin β and ADAM proteases could be shown by immunofluorescence microscopy and immunoprecipitation experiments. As demonstrated by a bacterial activator of meprin β and additional measurement of TNF-α shedding on bone marrow-derived macrophages, meprin β/ADAM protease interactions likely influence inflammatory conditions. Thus, we identified a novel proteolytic pathway of meprin β with ADAM proteases to control protease activities at the cell surface as part of the protease web.-Wichert, R., Scharfenberg, F., Colmorgen, C., Koudelka, T., Schwarz, J., Wetzel, S., Potempa, B., Potempa, J., Bartsch, J. W., Sagi, I., Tholey, A., Saftig, P., Rose-John, S., Becker-Pauly, C. Meprin β induces activities of A disintegrin and metalloproteinases 9, 10, and 17 by specific prodomain cleavage.

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.

Differences in Shedding of the Interleukin-11 Receptor by the Proteases ADAM9, ADAM10, ADAM17, Meprin α, Meprin β and MT1-MMP

Interleukin-11 (IL-11) has been associated with inflammatory conditions, bone homeostasis, hematopoiesis, and fertility. So far, these functions have been linked to classical IL-11 signaling via the membrane bound receptor (IL-11R). However, a signaling cascade via the soluble IL-11R (sIL-11R), generated by proteolytic cleavage, can also be induced. This process is called IL-11 trans-signaling. A disintegrin and metalloprotease 10 (ADAM10) and neutrophil elastase were described as ectodomain sheddases of the IL-11R, thereby inducing trans-signaling. Furthermore, previous studies employing approaches for the stimulation and inhibition of endogenous ADAM-proteases indicated that ADAM10, but not ADAM17, can cleave the IL-11R. Herein, we show that several metalloproteases, namely ADAM9, ADAM10, ADAM17, meprin β, and membrane-type 1 matrix metalloprotease/matrix metalloprotease-14 (MT1-MMP/MMP-14) when overexpressed are able to shed the IL-11R. All sIL-11R ectodomains were biologically active and capable of inducing signal transducer and activator of transcription 3 (STAT3) phosphorylation in target cells. The difference observed for ADAM10/17 specificity compared to previous studies can be explained by the different approaches used, such as stimulation of protease activity or making use of cells with genetically deleted enzymes.

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.

Mammalian plasma fetuin-B is a selective inhibitor of ovastacin and meprin metalloproteinases

Vertebrate fetuins are multi-domain plasma-proteins of the cystatin-superfamily. Human fetuin-A is also known as AHSG, α₂-Heremans-Schmid-glycoprotein. Gene-knockout in mice identified fetuin-A as essential for calcified-matrix-metabolism and bone-mineralization. Fetuin-B deficient mice, on the other hand, are female infertile due to zona pellucida ‘hardening’ caused by the metalloproteinase ovastacin in unfertilized oocytes. In wildtype mice fetuin-B inhibits the activity of ovastacin thus maintaining oocytes fertilizable. Here we asked, if fetuins affect further proteases as might be expected from their evolutionary relation to single-domain-cystatins, known as proteinase-inhibitors. We show that fetuin-A is not an inhibitor of any tested protease. In stark contrast, the closely related fetuin-B selectively inhibits astacin-metalloproteinases such as meprins and ovastacin, but not astacins of the tolloid-subfamily, nor any other proteinase. The analysis of fetuin-B expressed in various mammalian cell types, insect cells, and truncated fish-fetuin expressed in bacteria, showed that the cystatin-like domains alone are necessary and sufficient for inhibition. This report highlights fetuin-B as a specific antagonist of ovastacin and meprin-metalloproteinases. Control of ovastacin was shown to be indispensable for female fertility. Meprin inhibition, on the other hand, renders fetuin-B a potential key-player in proteolytic networks controlling angiogenesis, immune-defense, extracellular-matrix-assembly and general cell-signaling, with implications for inflammation, fibrosis, neurodegenerative disorders and cancer.

Role of meprin metalloproteinases in cytokine processing and inflammation

Meprin metalloendopeptidases, comprising α and β isoforms, are widely expressed in mammalian cells and organs including kidney, intestines, lungs, skin, and bladder, and in a variety of immune cells and cancer cells. Meprins proteolytically process many inflammatory mediators, including cytokines, chemokines, and other bioactive proteins and peptides that control the function of immune cells. The knowledge of meprin-mediated processing of inflammatory mediators and other target substrates provides a pathophysiologic link for the involvement of meprins in the pathogenesis of many inflammatory disorders. Meprins are now known to play important roles in inflammatory diseases including acute kidney injury, sepsis, urinary tract infections, bladder inflammation, and inflammatory bowel disease. The proteolysis of epithelial and endothelial barriers including cell junctional proteins by meprins promotes leukocyte influx into areas of tissue damage to result in inflammation. Meprins degrade extracellular matrix proteins; this ability of meprins is implicated in the cell migration of leukocytes and the invasion of tumor cells that express meprins. Proteolytic processing and maturation of procollagens provides evidence that meprins are involved in collagen maturation and deposition in the fibrotic processes involved in the formation of keloids and hypertrophic scars and lung fibrosis. This review highlights recent progress in understanding the role of meprins in inflammatory disorders in both human and mouse models.

Proteolytic ectodomain shedding of membrane proteins in mammals-hardware, concepts, and recent developments

Proteolytic removal of membrane protein ectodomains (ectodomain shedding) is a post-translational modification that controls levels and function of hundreds of membrane proteins. The contributing proteases, referred to as sheddases, act as important molecular switches in processes ranging from signaling to cell adhesion. When deregulated, ectodomain shedding is linked to pathologies such as inflammation and Alzheimer's disease. While proteases of the "a disintegrin and metalloprotease" (ADAM) and "beta-site APP cleaving enzyme" (BACE) families are widely considered as sheddases, in recent years a much broader range of proteases, including intramembrane and soluble proteases, were shown to catalyze similar cleavage reactions. This review demonstrates that shedding is a fundamental process in cell biology and discusses the current understanding of sheddases and their substrates, molecular mechanisms and cellular localizations, as well as physiological functions of protein ectodomain shedding. Moreover, we provide an operational definition of shedding and highlight recent conceptual advances in the field. While new developments in proteomics facilitate substrate discovery, we expect that shedding is not a rare exception, but rather the rule for many membrane proteins, and that many more interesting shedding functions await discovery.

Undiagnosed Kidney Injury in Uninsured and Underinsured Diabetic African American Men and Putative Role of Meprin Metalloproteases in Diabetic Nephropathy

Diabetes is the leading cause of chronic kidney disease. African Americans are disproportionately burdened by diabetic kidney disease (DKD) and end stage renal disease (ESRD). Disparities in DKD have genetic and socioeconomic components, yet its prevalence in African Americans is not adequately studied. The current study used multiple biomarkers of DKD to evaluate undiagnosed DKD in uninsured and underinsured African American men in Greensboro, North Carolina. Participants consisted of three groups: nondiabetic controls, diabetic patients without known kidney disease, and diabetic patients with diagnosed DKD. Our data reveal undiagnosed kidney injury in a significant proportion of the diabetic patients, based on levels of both plasma and urinary biomarkers of kidney injury, namely, urinary albumin to creatinine ratio, kidney injury molecule-1, cystatin C, and neutrophil gelatinase-associated lipocalin. We also found that the urinary levels of meprin A, meprin B, and two kidney meprin targets (nidogen-1 and monocytes chemoattractant protein-1) increased with severity of kidney injury, suggesting a potential role for meprin metalloproteases in the pathophysiology of DKD in this subpopulation. The study also demonstrates a need for more aggressive tests to assess kidney injury in uninsured diabetic patients to facilitate early diagnosis and targeted interventions that could slow progression to ESRD.