N-benzoyl-L-tyrosyl-p-aminobenzoic acid hydrolase: a metalloendopeptidase of the human intestinal microvillus membrane which degrades biologically active peptides

Distinct neurofunctional alterations during motivational and hedonic processing of natural and monetary rewards in depression - a neuroimaging meta-analysis

Reward processing dysfunctions are considered a candidate mechanism underlying anhedonia and apathy in depression. Neuroimaging studies have documented that neurofunctional alterations in mesocorticolimbic circuits may neurally mediate these dysfunctions. However, common and distinct neurofunctional alterations during motivational and hedonic evaluation of monetary and natural rewards in depression have not been systematically examined. Here, we capitalized on pre-registered neuroimaging meta-analyses to (1) establish general reward-related neural alterations in depression, (2) determine common and distinct alterations during the receipt and anticipation of monetary v. natural rewards, and, (3) characterize the differences on the behavioral, network, and molecular level. The pre-registered meta-analysis (https://osf.io/ay3r9) included 633 depressed patients and 644 healthy controls and revealed generally decreased subgenual anterior cingulate cortex and striatal reactivity toward rewards in depression. Subsequent comparative analyses indicated that monetary rewards led to decreased hedonic reactivity in the right ventral caudate while natural rewards led to decreased reactivity in the bilateral putamen in depressed individuals. These regions exhibited distinguishable profiles on the behavioral, network, and molecular level. Further analyses demonstrated that the right thalamus and left putamen showed decreased activation during the anticipation of monetary reward. The present results indicate that distinguishable neurofunctional alterations may neurally mediate reward-processing alterations in depression, in particular, with respect to monetary and natural rewards. Given that natural rewards prevail in everyday life, our findings suggest that reward-type specific interventions are warranted and challenge the generalizability of experimental tasks employing monetary incentives to capture reward dysregulations in everyday life.

Predictive value of MEP1A in cancer prognosis: A protocol for systematic review and meta-analysis

BACKGROUND

Meprin is a member of the astaxanthin family; it performs many functions through a wide range of proteolytic enzyme activities during health and disease, including tumors and inflammatory conditions. The purpose of this systematic review was to evaluate the predictive value of MEP1A in tumor prognosis.

METHODS

A comprehensive search was conducted on PubMed, Cochrane library, and Web of Science Database using a developed search strategy. The Newcastle-Ottawa Scale (NOS) or the Cochrane Collaboration's tool for assessing risk of bias will be used to access the methodological quality of included studies, and GRADE will be applied to evaluate evidence quality of outcomes. All analyses were performed by Stata 15.0.

RESULTS

The results will systematically summarize and display the currently collected evidence on the predictive value of MEP1A in different tumor prognosis.

CONCLUSION

This study may play a certain role in predicting the prognosis of cancer patients in the future, and may prompt clinicians to make necessary treatment or prevention plans as soon as possible.

ETHICS AND COMMUNICATION

It is not necessary because the present systematic review is based on published studies.

INPLASY REGISTRATION NUMBER

INPLASY2020100005.

Characterisation of proguanylin expressing cells in the intestine - evidence for constitutive luminal secretion

Guanylin, a peptide implicated in regulation of intestinal fluid secretion, is expressed in the mucosa, but the exact cellular origin remains controversial. In a new transgenic mouse model fluorescent reporter protein expression driven by the proguanylin promoter was observed throughout the small intestine and colon in goblet and Paneth(-like) cells and, except in duodenum, in mature enterocytes. In Ussing chamber experiments employing both human and mouse intestinal tissue, proguanylin was released predominantly in the luminal direction. Measurements of proguanylin expression and secretion in cell lines and organoids indicated that secretion is largely constitutive and requires ER to Golgi transport but was not acutely regulated by salt or other stimuli. Using a newly-developed proguanylin assay, we found plasma levels to be raised in humans after total gastrectomy or intestinal transplantation, but largely unresponsive to nutrient ingestion. By LC-MS/MS we identified processed forms in tissue and luminal extracts, but in plasma we only detected full-length proguanylin. Our transgenic approach provides information about the cellular origins of proguanylin, complementing previous immunohistochemical and in-situ hybridisation results. The identification of processed forms of proguanylin in the intestinal lumen but not in plasma supports the notion that the primary site of action is the gut itself.

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.

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

The host metalloprotease meprin β is required for mucin 2 (MUC2) cleavage, which drives intestinal mucus detachment and prevents bacterial overgrowth. To gain access to the cleavage site in MUC2, meprin β must be proteolytically shed from epithelial cells. Hence, regulation of meprin β shedding and activation is important for physiological and pathophysiological conditions. Here, we demonstrate that meprin β activation and shedding are mutually exclusive events. Employing ex vivo small intestinal organoid and cell culture experiments, we found that ADAM-mediated shedding is restricted to the inactive pro-form of meprin β and is completely inhibited upon its conversion to the active form at the cell surface. This strict regulation of meprin β activity can be overridden by pathogens, as demonstrated for the bacterial protease Arg-gingipain (RgpB). This secreted cysteine protease potently converts membrane-bound meprin β into its active form, impairing meprin β shedding and its function as a mucus-detaching protease.

Metalloproteases meprin-ɑ (MEP1A) is a prognostic biomarker and promotes proliferation and invasion of colorectal cancer

BACKGROUND

Meprin displays multiple functions in both health and disease, due in part to its broad proteolytic activity. In this report, we explored the clinical significance and functional relevance of the expression of meprin-ɑ (MEP1A) in colorectal cancer (CRC).

METHODS

The mRNA and protein expression levels of MEP1A in tumor specimens obtained from CRC patients was determined by quantitative real-time PCR and Western blot assay and comparatively paired with adjacent mucosa that presented as normal tissue. ShRNA was used to knock-down MEP1A expression in CRC cell-lines and the effects of dampened expression of MEP1A on the proliferation and invasion were determined by colony formation assays, Cell Counting Kit-8 assays and matrigel invasion assays. Moreover, nude mouse xenograft models were designed to investigate the same effect in vivo. In order to determine whether MEP1A expression correlated with CRC clinicopathologic factors and survival, immunohistochemical staining of a tissue microarray containing 88 paired CRC specimens was performed.

RESULTS

In CRC, enhanced expression of MEP1A was seen. Additionally, both in vitro and in vivo, CRC cellular proliferation and invasiveness was inhibited by dampened MEP1A expression. Several parameters were associated with enhanced MEP1A expression including tumor size (P = 0.023), staging of CRC by the American Joint Committee on Cancer (AJCC) (P = 0.024), and T (P = 0.032) and N stages (P = 0.001). Moreover, the expression of MEP1A is an independent prognostic factor for overall survival in CRC (HR 3.643; 95 % CI 0.305-5.842; P = 0.007).

CONCLUSION

MEP1A was not only found to be functionally important, but it might also serve as an important and unique indicator of patient prognosis and therapeutic targeting in CRC.

MEP1A contributes to tumor progression and predicts poor clinical outcome in human hepatocellular carcinoma

Although many staging classifications have been proposed for hepatocellular carcinoma (HCC), determining a patient's prognosis in clinical practice is a challenge due to the molecular diversity of HCC. We investigated the relationship between MEP1A, a candidate oncogene, and clinical outcomes of HCC patients; furthermore, we explored the role of MEP1A in HCC. In this report, it was demonstrated by quantitative real-time polymerase chain reaction that MEP1A messenger RNA levels were significantly elevated in HCC tumor tissues compared with matched adjacent nonneoplastic tissues and nonmalignant liver disease tissues. Immunohistochemical analyses of tissue samples from two independent groups of 394 HCC patients showed that positive expression of MEP1A in tumor cells was an independent and significant risk factor affecting survival after curative resection in both cohort 1 (hazard ratio = 2.05, 95% confidence interval 1.427-2.946; P < 0.001) and cohort 2 (hazard ratio = 1.89, 95% confidence interval 1.260-2.833; P = 0.002). Analysis of Barcelona Clinic Liver Cancer stage 0-A subgroup further showed that patients with positive MEP1A expression in tumor cells had poorer surgical prognoses than those with negative MEP1A expression in tumor cells (cohort 1 P = 0.001, cohort 2 P < 0.001). Both in vitro and in vivo assays showed that MEP1A promoted HCC cell proliferation, migration, and invasion. Further analyses found that MEP1A played an important role in regulating cytoskeletal events and induced epithelial-mesenchymal transition in HCC cells.

CONCLUSION

MEP1A is a novel prognostic predictor in HCC and plays an important role in the development and progression of HCC.

The anti-inflammatory peptide Ac-SDKP is released from thymosin-β4 by renal meprin-α and prolyl oligopeptidase

N-acetyl-seryl-aspartyl-lysyl-proline (Ac-SDKP) is a natural tetrapeptide with anti-inflammatory and antifibrotic properties. Previously, we have shown that prolyl oligopeptidase (POP) is involved in the Ac-SDKP release from thymosin-β4 (Tβ4). However, POP can only hydrolyze peptides shorter than 30 amino acids, and Tβ4 is 43 amino acids long. This indicates that before POP hydrolysis takes place, Tβ4 is hydrolyzed by another peptidase that releases NH2-terminal intermediate peptide(s) with fewer than 30 amino acids. Our peptidase database search pointed out meprin-α metalloprotease as a potential candidate. Therefore, we hypothesized that, prior to POP hydrolysis, Tβ4 is hydrolyzed by meprin-α. In vitro, we found that the incubation of Tβ4 with both meprin-α and POP released Ac-SDKP, whereas no Ac-SDKP was released when Tβ4 was incubated with either meprin-α or POP alone. Incubation of Tβ4 with rat kidney homogenates significantly released Ac-SDKP, which was blocked by the meprin-α inhibitor actinonin. In addition, kidneys from meprin-α knockout (KO) mice showed significantly lower basal Ac-SDKP amount, compared with wild-type mice. Kidney homogenates from meprin-α KO mice failed to release Ac-SDKP from Tβ4. In vivo, we observed that rats treated with the ACE inhibitor captopril increased plasma concentrations of Ac-SDKP, which was inhibited by the coadministration of actinonin (vehicle, 3.1 ± 0.2 nmol/l; captopril, 15.1 ± 0.7 nmol/l; captopril + actinonin, 6.1 ± 0.3 nmol/l; P < 0.005). Similar results were obtained with urinary Ac-SDKP after actinonin treatment. We conclude that release of Ac-SDKP from Tβ4 is mediated by successive hydrolysis involving meprin-α and POP.

Isoform-specific interactions between meprin metalloproteases and the catalytic subunit of protein kinase A: significance in acute and chronic kidney injury

Meprin metalloproteases are abundantly expressed in the brush-border membranes of kidney proximal tubules. Meprins are implicated in ischemia-reperfusion (IR)-induced renal injury and diabetic nephropathy. The protein kinase A (PKA) signaling pathway modulates extracellular matrix metabolism in diabetic kidneys. The present study evaluated isoform-specific interactions between the catalytic subunit of PKA (PKA C) and meprins. To this end, cytosolic-enriched kidney proteins from meprin αβ double knockout mice, and purified forms of recombinant mouse PKA Cα, Cβ1, and Cβ2, were incubated with activated forms of either homomeric meprin A or meprin B. The cleaved protein products were subjected to SDS-PAGE and analyzed by Coomassie staining and Western blot analysis. While meprin A only cleaved PKA Cβ1, meprin B cleaved all three PKA C isoforms. Analysis of the proteolytic fragments by mass spectrometry revealed that meprin A and B cleave the PKA C isoforms at defined sites, resulting in unique cleavage products. Michaelis-Menten enzyme kinetics demonstrated that meprin B-mediated cleavage of PKA Cα occurs at a rate consistent with that of other physiologically relevant meprin substrates. Meprin cleavage decreased the kinase activity of PKA Cα, Cβ1, and Cβ2. PKA C levels were higher in diabetic kidneys, with evidence of in vivo fragmentation in wild-type diabetic kidneys. Confocal microscopy showed localization of meprin A in the glomeruli of diabetic kidneys. At 3 h post-IR, PKA C levels in proximal tubules decreased compared with distal tubules, which lack meprins. These data suggest that meprins may impact kidney injury, in part, via modulation of PKA signaling pathways.

ADAM10 is the major sheddase responsible for the release of membrane-associated meprin A

Meprin A, composed of α and β subunits, is a membrane-bound metalloproteinase in renal proximal tubules. Meprin A plays an important role in tubular epithelial cell injury during acute kidney injury (AKI). The present study demonstrated that during ischemia-reperfusion-induced AKI, meprin A was shed from proximal tubule membranes, as evident from its redistribution toward the basolateral side, proteolytic processing in the membranes, and excretion in the urine. To identify the proteolytic enzyme responsible for shedding of meprin A, we generated stable HEK cell lines expressing meprin β alone and both meprin α and meprin β for the expression of meprin A. Phorbol 12-myristate 13-acetate and ionomycin stimulated ectodomain shedding of meprin β and meprin A. Among the inhibitors of various proteases, the broad spectrum inhibitor of the ADAM family of proteases, tumor necrosis factor-α protease inhibitor (TAPI-1), was most effective in preventing constitutive, phorbol 12-myristate 13-acetate-, and ionomycin-stimulated shedding of meprin β and meprin A in the medium of both transfectants. The use of differential inhibitors for ADAM10 and ADAM17 indicated that ADAM10 inhibition is sufficient to block shedding. In agreement with these results, small interfering RNA to ADAM10 but not to ADAM9 or ADAM17 inhibited meprin β and meprin A shedding. Furthermore, overexpression of ADAM10 resulted in enhanced shedding of meprin β from both transfectants. Our studies demonstrate that ADAM10 is the major ADAM metalloproteinase responsible for the constitutive and stimulated shedding of meprin β and meprin A. These studies further suggest that inhibiting ADAM 10 activity could be of therapeutic benefit in AKI.

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.

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.

Villin and actin in the mouse kidney brush-border membrane bind to and are degraded by meprins, an interaction that contributes to injury in ischemia-reperfusion

Meprins, metalloproteinases abundantly expressed in the brush-border membranes (BBMs) of rodent proximal kidney tubules, have been implicated in the pathology of renal injury induced by ischemia-reperfusion (IR). Disruption of the meprin β gene and actinonin, a meprin inhibitor, both decrease kidney injury resulting from IR. To date, the in vivo kidney substrates for meprins are unknown. The studies herein implicate villin and actin as meprin substrates. Villin and actin bind to the cytoplasmic tail of meprin β, and both meprin A and B are capable of degrading villin and actin present in kidney proteins as well as purified recombinant forms of these proteins. The products resulting from degradation of villin and actin were unique to each meprin isoform. The meprin B cleavage site in villin was Glu(744)-Val(745). Recombinant forms of rat meprin B and homomeric mouse meprin A had K(m) values for villin and actin of ∼1 μM (0.6-1.2 μM). The k(cat) values varied substantially (0.6-128 s(-1)), resulting in different efficiencies for cleavage, with meprin B having the highest k(cat)/K(m) values (128 M(-1)·s(-1) × 10(6)). Following IR, meprins and villin redistributed from the BBM to the cytosol. A 37-kDa actin fragment was detected in protein fractions from wild-type, but not in comparable preparations from meprin knockout mice. The levels of the 37-kDa actin fragment were significantly higher in kidneys subjected to IR. The data establish that meprins interact with and cleave villin and actin, and these cytoskeletal proteins are substrates for meprins.

Specific processing of tenascin-C by the metalloprotease meprinbeta neutralizes its inhibition of cell spreading

The metalloprotease meprin has been implicated in tissue remodelling due to its capability to degrade extracellular matrix components. Here, we investigated the susceptibility of tenascin-C to cleavage by meprinbeta and the functional properties of its proteolytic fragments. A set of monoclonal antibodies against chicken and human tenascin-C allowed the mapping of proteolytic fragments generated by meprinbeta. In chicken tenascin-C, meprinbeta processed all three major splicing variants by removal of 10kDa N-terminal and 38kDa C-terminal peptides, leaving a large central part of subunits intact. A similar cleavage pattern was found for large human tenascin-C variant where two N-terminal peptides (10 or 15kDa) and two C-terminal fragments (40 and 55kDa) were removed from the intact subunit. N-terminal sequencing revealed the exact amino acid positions of cleavage sites. In both chicken and human tenascin-C N-terminal cleavages occurred just before and/or after the heptad repeats involved in subunit oligomerization. In the human protein, an additional cleavage site was identified in the alternative fibronectin type III repeat D. Whereas all these sites are known to be attacked by several other proteases, a unique cleavage by meprinbeta was located to the 7th constant fibronectin type III repeat in both chicken and human tenascin-C, thereby removing the C-terminal domain involved in its anti-adhesive activity. In cell adhesion assays meprinbeta-digested human tenascin-C was not able to interfere with fibronectin-mediated cell spreading, confirming cleavage in the anti-adhesive domain. Whereas the expression of meprinbeta and tenascin-C does not overlap in normal colon tissue, inflamed lesions of the mucosa from patients with Crohn's disease exhibited many meprinbeta-positive leukocytes in regions where tenascin-C was strongly induced. Our data indicate that, at least under pathological conditions, meprinbeta might attack specific functional sites in tenascin-C that are important for its oligomerization and anti-adhesive activity.

Disruption of the meprin alpha and beta genes in mice alters homeostasis of monocytes and natural killer cells

Meprin metalloproteases are implicated in inflammatory bowel disease, which involves dysfunction of immune cells. However, the roles of meprins in the immune and hematological system remain uncharacterized. In this report, we demonstrate that meprins were expressed in the hematological system, and meprin alpha/beta null (alpha(-/-)/beta(-/-)) mice had decreased prevalence of resident monocytes and natural killer (NK) cells in blood, with a concomitant accumulation of inflammatory monocytes and NK cells in bone marrow. In contrast, T and B lymphocytes were not affected by meprin deficiency. In response to acute inflammation induced by intraperitoneal injection of thioglycollate, meprin-deficient mice exhibited higher body temperature than wild-type mice, which was correlated with retention of inflammatory monocytes, but persistent low prevalence of NK cells in blood. These results indicate that meprin metalloproteases play important roles in the homeostasis of monocytes and NK cells, and possibly are involved in egress of these two type cells from bone marrow and homing to the periphery. Our findings are the first report to demonstrate that metalloproteases affect homeostasis of leukocytes, which have important implications for understanding physiology of and pathogenesis in the hematological system.

Meprins, membrane-bound and secreted astacin metalloproteinases

The astacins are a subfamily of the metzincin superfamily of metalloproteinases. The first to be characterized was the crayfish enzyme astacin. To date more than 200 members of this family have been identified in species ranging from bacteria to humans. Astacins are involved in developmental morphogenesis, matrix assembly, tissue differentiation and digestion. Family members include the procollagen C-proteinase (BMP1, bone morphogenetic protein 1), tolloid and mammalian tolloid-like, HMP (Hydra vulgaris metalloproteinase), sea urchin BP10 (blastula protein) and SPAN (Strongylocentrotus purpuratus astacin), the 'hatching' subfamily comprising alveolin, ovastacin, LCE, HCE ('low' and 'high' choriolytic enzymes), nephrosin (from carp head kidney), UVS.2 from frog, and the meprins. In the human and mouse genomes, there are six astacin family genes (two meprins, three BMP1/tolloid-like, one ovastacin), but in Caenorhabditis elegans there are 40. Meprins are the only astacin proteinases that function on the membrane and extracellularly by virtue of the fact that they can be membrane-bound or secreted. They are unique in their domain structure and covalent subunit dimerization, oligomerization propensities, and expression patterns. They are normally highly regulated at the transcriptional and post-translational levels, localize to specific membranes or extracellular spaces, and can hydrolyse biologically active peptides, cytokines, extracellular matrix (ECM) proteins and cell-surface proteins. The in vivo substrates of meprins are unknown, but the abundant expression of these proteinases in the epithelial cells of the intestine, kidney and skin provide clues to their functions.

Activation of the epidermal growth factor receptor (EGFR) by a novel metalloprotease pathway

Neutrophil Elastase (NE) is a pro-inflammatory protease present at higher than normal levels in the lung during inflammatory disease. NE regulates IL-8 production from airway epithelial cells and can activate both EGFR and TLR4. TACE/ADAM17 has been reported to trans-activate EGFR in response to NE. Here, using 16HBE14o-human bronchial epithelial cells we demonstrate a new mechanism by which NE regulates both of these events. A high molecular weight soluble metalloprotease activity detectable only in supernatants from NE-treated cells by gelatin and casein zymography was confirmed to be meprin alpha by Western immunoblotting. In vitro studies demonstrated the ability of NE to activate meprin alpha, which in turn could release soluble TGFalpha and induce IL-8 production from 16HBE14o- cells. These effects were abrogated by actinonin, a specific meprin inhibitor. NE-induced IL-8 expression was also inhibited by meprin alpha siRNA. Immunoprecipitation studies detected EGFR/TLR4 complexes in NE-stimulated cells overexpressing these receptors. Confocal studies confirmed colocalization of EGFR and TLR4 in 16HBE14o- cells stimulated with meprin alpha. NFkappaB was also activated via MyD88 in these cells by meprin alpha. In bronchoalveolar lavage fluid from NE knock-out mice infected intra-tracheally with Pseudomonas aeruginosa meprin alpha was significantly decreased compared with control mice, and was significantly increased and correlated with NE activity, in bronchoalveolar lavage fluid from individuals with cystic fibrosis but not healthy controls. The data describe a previously unidentified lung metalloprotease meprin alpha, and its role in NE-induced EGFR and TLR4 activation and IL-8 production.

A novel 2D-based approach to the discovery of candidate substrates for the metalloendopeptidase meprin

In the past, protease-substrate finding proved to be rather haphazard and was executed by in vitro cleavage assays using singly selected targets. In the present study, we report the first protease proteomic approach applied to meprin, an astacin-like metalloendopeptidase, to determine physiological substrates in a cell-based system of Madin-Darby canine kidney epithelial cells. A simple 2D IEF/SDS/PAGE-based image analysis procedure was designed to find candidate substrates in conditioned media of Madin-Darby canine kidney cells expressing meprin in zymogen or in active form. The method enabled the discovery of hitherto unknown meprin substrates with shortened (non-trypsin-generated) N- and C-terminally truncated cleavage products in peptide fragments upon LC-MS/MS analysis. Of 22 (17 nonredundant) candidate substrates identified, the proteolytic processing of vinculin, lysyl oxidase, collagen type V and annexin A1 was analysed by means of immunoblotting validation experiments. The classification of substrates into functional groups may propose new functions for meprins in the regulation of cell homeostasis and the extracellular environment, and in innate immunity, respectively.

Metalloprotease meprin beta in rat kidney: glomerular localization and differential expression in glomerulonephritis

Meprin (EC 3.4.24.18) is an oligomeric metalloendopeptidase found in microvillar membranes of kidney proximal tubular epithelial cells. Here, we present the first report on the expression of meprin beta in rat glomerular epithelial cells and suggest a potential involvement in experimental glomerular disease. We detected meprin beta in glomeruli of immunostained rat kidney sections on the protein level and by quantitative RT-PCR of laser-capture microdissected glomeruli on the mRNA level. Using immuno-gold staining we identified the membrane of podocyte foot processes as the main site of meprin beta expression. The glomerular meprin beta expression pattern was altered in anti-Thy 1.1 and passive Heymann nephritis (PHN). In addition, the meprin beta staining pattern in the latter was reminiscent of immunostaining with the sheep anti-Fx1A antiserum, commonly used in PHN induction. Using Western blot and immunoprecipitation assays we demonstrated that meprin beta is recognized by Fx1A antiserum and may therefore represent an auto-antigen in PHN. In anti-Thy 1.1 glomerulonephritis we observed a striking redistribution of meprin beta in tubular epithelial cells from the apical to the basolateral side and the cytosol. This might point to an involvement of meprin beta in this form of glomerulonephritis.

The metalloprotease meprinbeta processes E-cadherin and weakens intercellular adhesion

Background

Meprin (EC 3.4.24.18), an astacin-like metalloprotease, is expressed in the epithelium of the intestine and kidney tubules and has been related to cancer, but the mechanistic links are unknown.

Methodology/Principal Findings

We used MDCK and Caco-2 cells stably transfected with meprinα and or meprinβ to establish models of renal and intestinal epithelial cells expressing this protease at physiological levels. In both models E-cadherin was cleaved, producing a cell-associated 97-kDa E-cadherin fragment, which was enhanced upon activation of the meprin zymogen and reduced in the presence of a meprin inhibitor. The cleavage site was localized in the extracellular domain adjacent to the plasma membrane. In vitro assays with purified components showed that the 97-kDa fragment was specifically generated by meprinβ, but not by ADAM-10 or MMP-7. Concomitantly with E-cadherin cleavage and degradation of the E-cadherin cytoplasmic tail, the plaque proteins β-catenin and plakoglobin were processed by an intracellular protease, whereas α-catenin, which does not bind directly to E-cadherin, remained intact. Using confocal microscopy, we observed a partial colocalization of meprinβ and E-cadherin at lateral membranes of incompletely polarized cells at preconfluent or early confluent stages. Meprinβ-expressing cells displayed a reduced strength of cell-cell contacts and a significantly lower tendency to form multicellular aggregates.

Conclusions/Significance

By identifying E-cadherin as a substrate for meprinβ in a cellular context, this study reveals a novel biological role of this protease in epithelial cells. Our results suggest a crucial role for meprinβ in the control of adhesiveness via cleavage of E-cadherin with potential implications in a wide range of biological processes including epithelial barrier function and cancer progression.

Successive action of meprin A and neprilysin catabolizes B-type natriuretic peptide

Natriuretic peptides such as B-type natriuretic peptide (BNP) are important cardioprotective hormones with essential functions in sodium excretion, water balance and blood pressure regulation. Consequently, the catabolism of these peptides is in the focus of clinical research. In previous studies, we demonstrated that BNP, in contrast to the structurally related atrial and C-type natriuretic peptide, was not hydrolyzed by neprilysin (NEP). Because membrane preparations of several organs of NEP-knockout mice rapidly degrade BNP, the aim of this study was to identify BNP-catabolizing peptidases responsible for this fast clearance. Using kidney membranes of wild-type and NEP-knockout mice, as well as several peptidase inhibitors, we monitored the catabolism of BNP and analyzed its degradation products. We identified meprin A, a multimeric metalloprotease expressed in the brush borders of kidney proximal tubules, to initially truncate mouse BNP in the N terminus to mBNP7-32, a BNP metabolite with conserved biological activity. Consequently, in vivo experiments with the meprin inhibitor actinonin successfully elevated plasma BNP concentration in rats. We further demonstrated that the generation of mBNP7-32 is the prerequisite to catabolize BNP and identified NEP as the peptidase degrading the truncated BNP. Thus, the cooperative, successive action of the 2 transmembranal peptidases meprin A and NEP is crucial for rapid renal BNP inactivation. Therefore, the inhibition of meprin A could be a potent tool for increasing circulating BNP levels.

Two alpha subunits and one beta subunit of meprin zinc-endopeptidases are differentially expressed in the zebrafish Danio rerio

Meprins are members of the astacin family of metalloproteases expressed in epithelial tissues, intestinal leukocytes and certain cancer cells. In mammals, there are two homologous subunits, which form complex glycosylated disulfide-bonded homo- and heterooligomers. Both human meprin alpha and meprin beta cleave several basement membrane components, suggesting a role in epithelial differentiation and cell migration. There is also evidence that meprin beta is involved in immune defence owing to its capability of activating interleukin-1beta and the diminished mobility of intestinal leukocytes in meprin beta-knockout mice. Here we show for the first time by reverse transcription PCR, immunoblotting and immunofluorescence analyses that meprins are expressed not only in mammals, but also in the zebrafish Danio rerio. In contrast to the human, mouse and rat enzymes, zebrafish meprins are encoded by three genes, corresponding to two homologous alpha subunits and one beta subunit. Observations at both the mRNA and protein level indicate a broad distribution of meprins in zebrafish. However, there are strikingly different expression patterns of the three subunits, which is consistent with meprin expression in mammals. Hence, D. rerio appears to be a suitable model to gain insight into the basic physiological functions of meprin metalloproteases.

The alpha and beta subunits of the metalloprotease meprin are expressed in separate layers of human epidermis, revealing different functions in keratinocyte proliferation and differentiation

The zinc endopeptidase meprin (EC 3.4.24.18) is expressed in brush border membranes of intestine and kidney tubules, intestinal leukocytes, and certain cancer cells, suggesting a role in epithelial differentiation and cell migration. Here we show by RT-PCR and immunoblotting that meprin is also expressed in human skin. As visualized by immunohistochemistry, the two meprin subunits are localized in separate cell layers of the human epidermis. Meprin alpha is expressed in the stratum basale, whereas meprin beta is found in cells of the stratum granulosum just beneath the stratum corneum. In hyperproliferative epidermis such as in psoriasis vulgaris, meprin alpha showed a marked shift of expression from the basal to the uppermost layers of the epidermis. The expression patterns suggest distinct functions for the two subunits in skin. This assumption is supported by diverse effects of recombinant meprin alpha and beta on human adult low-calcium high-temperature keratinocytes. Here, beta induced a dramatic change in cell morphology and reduced the cell number, indicating a function in terminal differentiation, whereas meprin alpha did not affect cell viability, and may play a role in basal keratinocyte proliferation.

Compartmentalised expression of meprin in small intestinal mucosa: enhanced expression in lamina propria in coeliac disease

Epithelial cells in the human small intestine express meprin, an astacin-like metalloprotease, which accumulates normally at the brush border membrane and in the gut lumen. Therefore, meprin is targeted towards luminal components. In coeliac disease patients, peptides from ingested cereals trigger mucosal inflammation in the small intestine, disrupting epithelial cell differentiation and function. Using in situ hybridisation on duodenal tissue sections, we observed a marked shift of meprin mRNA expression from epithelial cells, the predominant expression site in normal mucosa, to lamina propria leukocytes in coeliac disease. Meprin thereby gains access to the substrate repertoire present beneath the epithelium.

Strategies to improve plasma half life time of peptide and protein drugs

Due to the obvious advantages of long-acting peptide and protein drugs, strategies to prolong plasma half life time of such compounds are highly on demand. Short plasma half life times are commonly due to fast renal clearance as well as to enzymatic degradation occurring during systemic circulation. Modifications of the peptide/protein can lead to prolonged plasma half life times. By shortening the overall amino acid amount of somatostatin and replacing L: -analogue amino acids with D: -amino acids, plasma half life time of the derivate octreotide was 1.5 hours in comparison to only few minutes of somatostatin. A PEG(2,40 K) conjugate of INF-alpha-2b exhibited a 330-fold prolonged plasma half life time compared to the native protein. It was the aim of this review to provide an overview of possible strategies to prolong plasma half life time such as modification of N- and C-terminus or PEGylation as well as methods to evaluate the effectiveness of drug modifications. Furthermore, fundamental data about most important proteolytic enzymes of human blood, liver and kidney as well as their cleavage specificity and inhibitors for them are provided in order to predict enzymatic cleavage of peptide and protein drugs during systemic circulation.

Human meprin alpha and beta homo-oligomers: cleavage of basement membrane proteins and sensitivity to metalloprotease inhibitors

Meprin is a zinc endopeptidase of the astacin family, which is expressed as a membrane-bound or secreted protein in mammalian epithelial cells, in intestinal leucocytes and in certain cancer cells. There are two types of meprin subunits, alpha and beta, which form disulphide-bonded homo- and hetero-oligomers. Here we report on the cleavage of matrix proteins by hmeprin (human meprin) alpha and beta homo-oligomers, and on the interactions of these enzymes with inhibitors. Despite their completely different cleavage specificities, both hmeprin alpha and beta are able to hydrolyse basement membrane components such as collagen IV, nidogen-1 and fibronectin. However, they are inactive against intact collagen I. Hence the matrix-cleaving activity of hmeprin resembles that of gelatinases rather than collagenases. Hmeprin is inhibited by hydroxamic acid derivatives such as batimastat, galardin and Pro-Leu-Gly-hydroxamate, by TAPI-0 (tumour necrosis factor alpha protease inhibitor-0) and TAPI-2, and by thiol-based compounds such as captopril. Therapeutic targets for these inhibitors are MMPs (matrix metalloproteases), TACE (tumour necrosis factor alpha-converting enzyme) and angiotensin-converting enzyme respectively. The most effective inhibitor of hmeprin alpha in the present study was the naturally occurring hydroxamate actinonin ( K(i)=20 nM). The marked variance in the cleavage specificities of hmeprin alpha and beta is reflected by their interaction with the TACE inhibitor Ro 32-7315, whose affinity for the beta subunit (IC50=1.6 mM) is weaker by three orders of magnitude than that for the alpha subunit ( K(i)=1.6 microM). MMP inhibitors such as the pyrimidine-2,4,6-trione derivative Ro 28-2653 that are more specific for gelatinases do not bind to hmeprin, presumably due to the subtle differences in the mode of zinc binding and active-site structure between the astacins and the MMPs.

Differences in the activation mechanism between the alpha and beta subunits of human meprin

Meprins are zinc-endopeptidases of the astacin family, which are expressed as membrane-bound or secreted forms in renal and intestinal brush-border membranes of mouse, rat and man. There are two types of meprin subunits, alpha and beta, which form disulfide-bonded homo- and heterodimers; further oligomerization is mediated by non-covalent interactions. Both subunits are translated as proenzymes that have to be activated by removal of an N-terminal propeptide. In the gut, the most probable activator is trypsin. In addition, plasmin has been shown to activate the human alpha subunit in colorectal cancer tissue. In the present study we have overexpressed the human meprin alpha subunit and a His-tagged soluble tail-switch-mutant of meprin beta in Baculovirus-infected insect cells. The recombinant homo-oligomeric proteins were purified by gel filtration and affinity chromatography with yields of up to 10 mg/l cell culture medium and analyzed with regard to their activation mechanism. While both alpha and beta homo-oligomers are activated by trypsin, only meprin alpha homo-oligomers are processed to their mature form by plasmin. These results indicate a different accessibility of the propeptide in meprin homo-oligomers and suggest an explanation for the appearance of meprin hetero-oligomers consisting of active alpha, but latent beta subunits.

Human meprin beta: O-linked glycans in the intervening region of the type I membrane protein protect the C-terminal region from proteolytic cleavage and diminish its secretion

Human meprin (hmeprin; N -benzoyl-L-tyrosyl-p-aminobenzoic acid hydrolase; EC 3.4.24.18) is a member of the astacin family of zinc metalloendopeptidases. The major site of expression is the brush border membrane of small intestinal and kidney epithelial cells. The enzyme is a type I integral membrane protein composed of two distinct subunits, alpha and beta, which are linked by disulphide bridges. The enzyme complex is attached to the plasma membrane only via the beta-subunit. The alpha-subunit is cleaved in the endoplasmic reticulum in a constitutive manner to remove the C-terminal membrane anchor which leads to secretion of the protein. While the beta-subunit of hmeprin remains largely attached to the brush-border membrane some proteolytic processing occurs intracellularly as well as at the cell surface and results in the release of this subunit from the cell. In the present paper, we report that the beta-subunit bears multiple O-linked sugar residues in the intervening domain. In contrast, the alpha-subunit does not contain O-linked oligosaccharides. Our results show that the O-linked carbohydrate side chains in hmeprinbeta are clustered around a 13 amino acid sequence that contains the main cleavage site for proteolytic processing of the subunit. Prevention of O-glycosylation by specific inhibitors leads to enhanced proteolytic processing and the consequence is an increased release of hmeprinbeta into the culture medium.

Activation mechanism of pro-astacin: role of the pro-peptide, tryptic and autoproteolytic cleavage and importance of precise amino-terminal processing

Astacin (EC 3.4.24.21) is a prototype for the astacin family and for the metzincin superfamily of zinc peptidases, which comprise membrane-bound and secreted enzymes involved in extracellular proteolysis during tissue development and remodelling. Generally, metzincins are translated as pro-enzymes (zymogens), which are activated by removal of an N-terminal pro-peptide. In astacin, however, the mode of zymogen activation has been obscured, since the pro-form does not accumulate in vivo. Here we report the detection of pro-astacin in midgut glands of brefeldin A-treated crayfish (Astacus astacus) by immunoprecipitation and mass spectrometry. We demonstrate that the pro-peptide is able to shield the active site of mature astacin as a transient inhibitor, which is degraded slowly. In vitro studies with recombinant pro-astacin in the absence of another protease reveal a potential of auto-proteolytic activation. The initial cleavage in this autoactivation appears to be an intramolecular event. This is supported by the fact that the mutant E93A-pro-astacin is incapable of autoactivation, and completely resistant to cleavage by mature astacin. However, this mutant is cleaved by Astacus trypsin within the pro-peptide. This probably reflects the in vivo situation, where Astacus trypsin and astacin work together during pro-astacin activation. In a first step, trypsin produces amino-terminally truncated pro-astacin derivatives. These are trimmed subsequently by each other and by astacin to yield the mature amino terminus, which forms a salt-bridge with Glu103 in the active site. The disruption of this salt-bridge in the mutants E103A and E103Q results in extremely heat labile proteins, whose catalytic activities are not altered drastically, however. This supports a concept according to which the linkage of Glu103 to the precisely trimmed amino terminus is a crucial structural prerequisite throughout the astacin family.

Activation of human meprin-alpha in a cell culture model of colorectal cancer is triggered by the plasminogen-activating system

The activation of latent proenzymes is an important mechanism for the regulation of localized proteolytic activity. Human meprin-alpha, an astacin-like zinc metalloprotease expressed in normal colon epithelial cells, is secreted as a zymogen into the intestinal lumen. Here, meprin is activated after propeptide cleavage by trypsin. In contrast, colorectal cancer cells secrete meprin-alpha in a non-polarized way, leading to accumulation and increased activity of meprin-alpha in the tumor stroma. We have analyzed the activation mechanism of promeprin-alpha in colorectal cancer using a co-culture model of the intestinal mucosa composed of colorectal adenocarcinoma cells (Caco-2) cultivated on filter supports and intestinal fibroblasts grown in the companion dish. We provide evidence that meprin-alpha is activated by plasmin and show that the presence of plasminogen in the basolateral compartment of the co-cultures is sufficient for promeprin-alpha activation. Analysis of the plasminogen-activating system in the co-cultures revealed that plasminogen activators produced and secreted by fibroblasts converted plasminogen to active plasmin, which in turn generated active meprin-alpha. This activation mechanism offers an explanation for the observed meprin-alpha activity in the tumor stroma, a prerequisite for a potential role of this protease in colorectal cancer.

Developmental expression of meprin metalloprotease subunits in ICR and C3H/He mouse kidney and intestine in the embryo, postnatally and after weaning

Meprins are secreted and membrane-bound metalloendopeptidases highly expressed in kidney and intestinal epithelial cells. They are oligomeric glycoproteins composed of evolutionarily related alpha and/or beta subunits. The present work revealed that the messages for both meprin subunits were expressed in intestine and kidney in ICR and C3H/He mouse embryos (as early as day 11), indicating developmental functions for both subunits. During the first 2 weeks after birth, the mRNA levels for both subunits increased in ICR mice, but between 10 days and 3 weeks (time of weaning) the alpha subunit level in the intestine fell markedly. In adult ICR mice, meprin beta mRNA was consistently expressed in both kidney and intestine, whereas meprin alpha mRNA was highly expressed in kidney but only present at low levels in intestine. In C3H/He mice, the pattern of meprin alpha and beta subunit mRNA expression was similar to that of ICR mice, except that meprin alpha was barely detectable in kidney after birth. The results of postnatal studies indicate that the meprin alpha subunit has a role in the intestine during suckling but is not essential after weaning, and that the beta homooligomer is the major meprin form after weaning in both kidney and intestine.

The roles of Glu93 and Tyr149 in astacin-like zinc peptidases

The catalytic zinc of astacin, a prototype of the astacin family and the metzincin superfamily of metalloproteinases is coordinated by three histidines, a glutamate bound water and a tyrosine. In order to assess the roles of active site key residues, two mutants, Glu93Ala-astacin and Tyr149Phe-astacin, were expressed in Escherichia coli, affinity-purified and renatured. While the Glu93Ala mutant was inactive, the Tyr149Phe mutant retained about 2. 5% residual activity toward Dns-Pro-Lys-Arg*Ala-Pro-Trp-Val, based on the k(cat)/K(m) value for recombinant wild-type astacin. These results support a model in which Glu93 is the general base in substrate hydrolysis, whereas Tyr149 contributes to transition state binding.

Structure and expression of the human MEP1A gene encoding the alpha subunit of metalloendopeptidase meprin A

The human genome contains several sequences that are similar to the MEP1A gene encoding the alpha subunit of metalloendopeptidase meprin A. We now report the first genomic structure for the human MEP1A gene that maps to chromosome 6p21. The gene spans approximately 45 kb and consists of 14 exons and 13 introns. Overall, about 6.7% (3 kb) of the MEP1A gene corresponds to the exon sequences. Tissue specificity of the MEP1A gene expression was examined by dot blot analysis of poly(A) RNA from 50 different human tissues. The MEP1A mRNA was detected for the first time in kidney and appendix in addition to colon and small intestine previously known to express the gene. The elucidated gene structure and tissue-specific expression of the MEP1A gene set the stage for investigating regulation and function of the gene and related sequences in the human genome.

Heterologously overexpressed, affinity-purified human meprin alpha is functionally active and cleaves components of the basement membrane in vitro

Meprins are astacin-like metalloproteases of renal and intestinal epithelia and embryonic neuroepithelial cells. The full length cDNA of the human meprin alpha subunit has been overexpressed in baculovirus-infected insect cells yielding the tetrameric proprotein which could be proteolytically activated and affinity-purified to homogeneity. Recombinant meprin alpha hydrolyzes the synthetic substrate N-benzoyl-tyrosyl-p-aminobenzoic acid (PABA-peptide) and cleaves by limited proteolysis the basement membrane constituents laminin 1 and laminin 5. This supports a concept that meprin alpha, when basolaterally secreted by human colon carcinoma epithelial cells, increases the proteolytic capacity for tumor progression in the stroma.

Meprin mRNA in rat intestine during normal and glucocorticoid-induced maturation: divergent patterns of expression of alpha and beta subunits

Meprin is a zinc-metalloendopeptidase expressed in intestinal epithelial cells. In rat jejunum collected from postnatal day 4 (P4) through P25 meprins alpha mRNA exhibited uniform levels for the first three postnatal weeks and then declined, whereas meprin beta mRNA showed a biphasic pattern with high levels in the first postnatal week followed by low levels from P7 through P19 and then a marked rise at P22 and P25. Dexamethasone treatment beginning at P10 had no significant effect on levels of meprins a mRNA, whereas this treatment caused a precocious increase in expression of meprin beta mRNA. These divergent patterns of expression of meprins alpha and beta mRNA suggest distinct roles for the two subunits during the suckling and weaning phases of rodent intestinal development.

N-Benzoyl-L-tyrosyl-p-aminobenzoic acid hydrolase beta (human meprinbeta). A 13-amino-acid sequence is required for proteolyticprocessing and subsequent secretion

N-Benzoyl-L-tyrosyl-p-aminobenzoic acid hydrolase or human meprin (PPH) is a brush-border membrane enzyme of small intestinal epithelial cells. It is a type I integral membrane protein composed of two disulphide-bridged subunits (alpha and beta). PPH and its homologous counterparts in rodents belong to the astacin family of zinc-metalloendopeptidases. Although the amino-acid sequence of the beta subunits is 80-90% identical in these three species, processing is different. Expression of PPHbeta in simian virus 40-transformed African green monkey kidney cells (COS-1) and Madin Darby canine kidney (MDCK) cells results in its cell surface localization and secretion, whereas mouse meprinbeta is only found at the plasma membrane. To investigate proteolytic processing of PPHbeta and to identify the cleavage site, different C-terminal domains of wild-type PPHbeta were exchanged with the homologous domains of mouse meprinbeta. We identified a 13-amino-acid sequence (QIQLTPAPSVQDL) necessary for cleavage and subsequent secretion of PPHbeta. Using brefeldin A, the site of processing was identified as being after passage through the Golgi compartment. Proteolytic processing of PPHbeta thus provides a means for secretion of alphabeta heterodimers.

Secretion of human meprin from intestinal epithelial cells depends on differential expression of the alpha and beta subunits

Human meprin (N-benzoyl-l-tyrosyl-p-aminobenzoic acid hydrolase, EC 3.4.24.18), an astacin-type metalloprotease, is expressed by intestinal epithelial cells as a dimeric protein complex of alpha and beta subunits. In transfected cells, intracellular proteolytic removal of the membrane anchor from the alpha subunit results in its secretion, while the beta subunit and alpha/beta heterodimers are retained at the cell membrane. We investigated the consequence of differential intracellular processing of alpha and beta subunits in the human small and large intestine using subunit-specific immunohistochemistry, in situ hybridization and biosynthetic studies in organ culture. In the ileum, both subunits localize to the brush-border membrane of villus enterocytes. In contrast, the beta subunit is not expressed in the colon, which leads to the secretion of the alpha subunit. We conclude that differential expression of meprin alpha and beta subunits is a unique means of targeting the proteolytic activity of the alpha subunit either to the brush-border membrane in the ileum or to the lumen in the colon, suggesting dual functions of cell-associated and luminal meprin. Meprin alpha and beta subunits are also coexpressed in distinct lamina propria leukocytes, suggesting an additional role for this protease in leukocyte function in the intestinal mucosa.

Activation mechanism of meprins, members of the astacin metalloendopeptidase family

Meprins are mammalian zinc metalloendopeptidases with protease domains structurally related to astacin, the prototype of the "astacin family" of metalloproteases. Mature, active astacins are produced by proteolytic removal of an activation peptide to generate a new NH2-terminal residue. Structural studies indicate that the NH2-terminal ammonium group inserts into a water-filled cavity adjacent to the active site to form a salt bridge with a Glu residue that is conserved in all astacins. A similar interaction is known to play a crucial role in the activation of trypsin, resulting in the hypothesis that this salt bridge is required for the activation of astacin-like proteases. In this study, we have used the mouse meprin alpha subunit as a model to test this hypothesis of zymogen activation of the astacins. Mutants were generated to vary the NH2-terminal residue of the mature meprin alpha subunit (Asn78) and its putative salt bridge partner (Glu178). In addition, mutants creating NH2-terminal extensions and truncations were expressed in human embryonic kidney 293 cells. The recombinant proteins were activated by limited protease digestion and assayed for enzymatic activity and thermal stability. Point mutations of Asn78 resulted in enzymes with activity comparable to the wild-type enzyme, indicating that the structure of this side chain is not essential for activity. NH2-terminal extension mutants of meprin alpha retained partial activity, with greater decreases against peptide relative to protein substrates. A mutant with a deletion of Asn78 to disrupt salt bridge formation with Glu178 had full activity, indicating that the putative salt bridge with Glu178 is not essential for enzyme activity. However, all changes in meprin alpha subunit NH2-terminal structure were found to decrease the thermal stability of the enzyme. These observations and additional data indicate that the zymogen activation mechanism of meprin and other astacins differs from that of the trypsin family of enzymes, and has some features in common with matrixins. It is proposed that prosequence removal of astacins allows the formation of hydrogen bonds involving the two NH2-terminal residues that are critical for enzyme structure.

Polarised expression of human intestinal N-benzoyl-L-tyrosyl-p-aminobenzoic acid hydrolase (human meprin) alpha and beta subunits in Madin-Darby canine kidney cells

N-Benzoyl-L-tyrosyl-p-aminobenzoic acid hydrolase (PPH, human meprin), is a peptidase found in the microvillus membrane of human small intestinal epithelial cells. PPH belongs to the astacin family of zinc-metalloendopeptidases and is a protein complex composed of two glycosylated subunits, alpha and beta. The present report describes the cloning of the complete beta subunit and the remaining N2-terminal end of the alpha subunit for analysis of their primary structures in addition to the examination of their biogenesis in transfected cell cultures. The complete open reading frame of the PPH beta cDNA translates into 700 amino acid residues compared with 746 residues for the PPH alpha cDNA. The primary structure of beta and alpha subunits are 44% identical and 61% similar. As predicted from their primary structure, the two subunits of PPH have identical modular structures; starting at the N2-terminus both contain a signal peptide, a propeptide, a protease domain containing the astacin signature, a meprin A5 protein tyrosine phospatase mu (MAM) and a meprin and TRAF homology domain (MATH) domain, an epidermal growth factor(EGF)-like domain, a putative transmembrane anchor domain and a short cytosolic tail. Pulse/chase labelling and immuno-Gold electronmicroscopy of recombinant PPH beta and alpha subunits expressed in transfected Madin-Darby canine kidney (MDCK) cells show that post-translational processing and transport of the two subunits are very different. When expressed alone, the beta subunit acquired complex glycan residues, readily formed homodimers and was transported to the plasma membrane. Small amounts of PPH beta were found in the culture medium. In contrast, the cell-bound alpha subunit, when expressed alone, remained primarily in the high-mannose form, was aggregated and not expressed at the cell surface. However, the bulk of mostly endo-beta-N-acetylglucosaminidase H-resistant alpha subunit was found in the filtered culture medium. The proteolytic event that leads to the formation of this soluble transport-competent form occurs in the endoplasmic reticulum (ER). Coexpression of the alpha subunit with the beta subunit allowed the localisation of the alpha subunit to the plasma membrane. These studies indicate that assembly of the two subunits of PPH is required for the localisation of the alpha subunit to the plasma membrane. In contrast to rodent meprin, both PPH subunits are apically secreted from MDCK cells.

C-cytosolic and transmembrane domains of the N-benzoyl-L-tyrosyl-p-aminobenzoic acid hydrolase alpha subunit (human meprin alpha) are essential for its retention in the endoplasmic reticulum and C-terminal processing

N-benzoyl-L-tyrosyl-p-aminobenzoic acid hydrolase (PPH, human meprin) is a member of the astacin family of Zn-metalloendopeptidases and is highly expressed in the microvillus membrane of human small intestinal epithelial cells. It is a type I transmembrane protein consisting of differentially processed glycosylated alpha and beta subunits. Biosynthesis experiments using transfected, metabolically labelled simian virus 40 (SV40) transformed african green monkey kidney cells (COS-1) and Madin Darby canine kidney (MDCK) cells, have previously shown that PPH alpha was retained in the endoplasmic reticulum (ER) and that for subsequent secretion removal of the alpha-tail was necessary [Grünberg, J., Dumermuth, E., Eldering, J. A. & Sterchi, E. E. (1993) FEBS Lett. 335, 376-379]. We proposed an involvement of the alpha-tail in ER retention. To investigate the possible role of the transmembrane and/or the C-terminal domain of the alpha-subunit, tailswitch mutants were constructed in which these domains were exchanged between the alpha and beta subunits. Biosynthesis and post-translational processing of these mutants were investigated in transiently transfected COS-1 cells. The beta/alpha tailswitch mutant, in which the transmembrane and C-cytosolic parts of PPH beta were substituted by the corresponding parts of the PPH alpha subunit, was transported much slower compared with the wild-type PPH beta subunit. In addition, fusion of the alpha-tail to a C-terminally truncated secretory form of PPH alpha leads to its retention in the ER. This mutant, but not the secretory form, coimmunoprecipitated with calnexin, indicating an involvement of this molecular chaperone in retaining PPH alpha in the ER. The alpha/beta tailswitch mutant, in which the transmembrane domain and the C-cytosolic part of PPH alpha were substituted by the corresponding parts of PPH beta, was processed less efficiently in comparison with PPH alpha, resulting in a lower secretion rate. Taken together these data suggest a role of the alpha-tail in mediating association with ER-resident machinery, facilitating C-terminal processing.

Unveiling the substrate specificity of meprin beta on the basis of the site in protein kinase A cleaved by the kinase splitting membranal proteinase

The kinase splitting membranal proteinase (KSMP) is a metalloendopeptidase that inactivates the catalytic (C) subunit of protein kinase A (PKA) by clipping off its carboxyl terminal tail. Here we show that this cleavage occurs at Glu332-Glu333, within the cluster of acidic amino acids (Asp328-Glu334) of the kinase. The Km values of KSMP and of meprin beta (which reproduces KSMP activity) for the C-subunit are below 1 microM. The Km for peptides containing a stretch of four Glu residues are in the micromolar range, illustrating the significant contribution of this cluster to the substrate recognition of meprin beta. This conclusion is supported by a systematic study using a series of the C-subunit mutants with deletions and mutations in the cluster of acidics. Hydrophobic amino acids vicinal to the cleavage site increase the Kcat of the proteinase. These studies unveil a new specificity for meprin beta, suggesting new substrates that are 1-2 orders of magnitude better in their Km and Kcat than those commonly used for meprin assay. A search for substrates having such a cluster of acidics and hydrophobics, which are accessible to meprin under physiological conditions, point at gastrin as a potential target. Indeed, meprin beta is shown to cleave gastrin at its cluster of five glutamic acid residues and also at the M-D bond within its WMDF-NH2 sequence, which is indispensable for all the known biological activities of gastrins. The latter meprin cleavage will lead to the inactivation of gastrin and thus to the control of its activity.

Tachykinins in the gut. Part I. Expression, release and motor function

The preprotachykinin-A gene-derived peptides substance P and neurokinin (NK) A are expressed in distinct neural pathways of the mammalian gut. When released from intrinsic enteric or extrinsic primary afferent neurons, tachykinins have the potential to influence both nerve and muscle by way of interaction with three different types of tachykinin receptor, termed NK1, NK2 and NK3 receptors. Most prominent among the effects of tachykinins is their excitatory action on gastrointestinal motor activity, which is seen in virtually all regions and layers of the mammalian gut. This action depends not only on a direct activation of the muscle through NK1 and/or NK2 receptors, but also on stimulation of excitatory enteric motor pathways through NK3 and/or NK1 receptors. In addition, tachykinins can inhibit motor activity by stimulating either inhibitory neuronal pathways or interrupting excitatory relays. A synopsis of the available data indicates that endogenous substance P and NKA interact with other enteric transmitters in the physiological control of gastrointestinal motor activity. Derangement of the regulatory roles of tachykinins may be a factor in the gastrointestinal dysmotility associated with infection, inflammation, stress and pain. In a therapeutic perspective, it would seem conceivable, therefore, that tachykinin agonists and antagonists are adjuncts to the treatment of motor disorders that involve pathological disturbances of the gastrointestinal tachykinin system.

Cysteine mutations in the MAM domain result in monomeric meprin and alter stability and activity of the proteinase

Meprins are oligomeric, glycosylated cell surface or secreted metalloendopeptidases that are composed of multidomain disulfide-linked subunits. To investigate whether subunit oligomerization is critical for intracellular transport or for the enzymatic and/or physical properties of the proteinase, specific cysteine residues were mutated, and the mutants were expressed in 293 cells. Mutation of mouse meprin alpha Cys-320 to Ala in the MAM domain (an extracellular domain found in meprin, A-5 protein, and receptor protein-tyrosine phosphatase mu) resulted in expression of a monomeric form of meprin, as determined by SDS-polyacrylamide gel electrophoresis and nondenaturing gel electrophoresis. The monomeric subunits were considerably more vulnerable to proteolytic degradation and heat inactivation in vitro compared with the oligomeric form of the enzyme. Proteolytic activity of the monomeric meprin using a bradykinin analog or aminobenzoyl-Ala-Ala-Phe-p-nitroanilide as substrate was similar to that of disulfide-linked oligomeric meprin; however, activity against azocasein was markedly decreased. Mutation of another cysteine residue in the MAM domain (C289A), predicted to be involved in intrasubunit disulfide bridging, resulted in disulfide-linked oligomers and monomers. These results indicated that this mutant was capable of forming intersubunit disulfide bonds but less efficiently than wild-type meprin subunits. Mutant C289A also retained activity toward peptides but not the protein substrate and was more vulnerable to proteolytic degradation and heat inactivation compared with the wild-type enzyme. Both Cys mutants were expressed and secreted into the medium at levels comparable with the wild type and had slightly altered glycosylation. This work indicates that 1) Cys-320 of mouse meprin alpha is most likely responsible for the covalent interactions of the subunits; 2) covalent dimerization of subunits is not essential for efficient biosynthesis, trafficking, or posttranslational processing of the secreted protease; and 3) mutations in the MAM domain affect noncovalent interactions of the subunits and the stability and activity of the protease domain, indicating that domain-domain interactions are critical for structure and function of the enzyme.

Identification of the cysteine residues implicated in the formation of alpha 2 and alpha/beta dimers of rat meprin

Meprin (endopeptidase-24.18; EC 3.4.24.18) is a multisubunit zinc-metallopeptidase found in the brush-border membranes of rodent kidney and human intestine. The alpha and beta subunits of meprin are disulphide-linked to form either soluble alpha 2 homodimers or membrane-associated alpha/beta heterodimers. The aim of the present study was to identify the cysteine residue(s) implicated in the formation of alpha 2 and alpha/beta dimers and to investigate the effects of dimerization on intracellular transport and processing of the alpha subunit. Three cysteine residue candidates for the formation of disulphide bonds in the alpha subunit were selected by hydrophobic cluster analysis. These residues, located at positions 309, 560 and 562, were mutated to serine residues. When the resulting alpha subunit mutants were expressed alone in COS-1 cells, the alpha C560S and alpha C562S mutants were found to be secreted as alpha 2 homodimers whereas the alpha C309S mutant was found as monomers in the culture medium. In double-transfection experiments with the wild-type beta subunit, the alpha C560S and alpha C562S mutants behaved exactly as the wild-type alpha subunit and formed membrane-bound alpha/beta heterodimers. In contrast, the alpha C309S mutant was not retained at the cell surface but rather secreted as monomers in the culture medium, as was found in the simple transfection experiment. These results show that, despite the normal expression level and folding of the protein in a transport-competent from, the alpha C309S mutant is unable to form alpha 2 homodimers or alpha/beta heterodimers. This suggests that Cys309 is the unique residue of the alpha subunit implicated in the alpha 2 and alpha/beta dimerizations. Hydrophobic cluster analysis of the alpha and beta subunit sequences predicts that Cys309 is similar to Cys306 of the beta subunit. We mutated the latter residue to a serine and expressed the beta C306S mutant and the wild-type alpha subunit in the same COS-1 cells. No beta 2 or alpha/beta dimers were observed on immunoblotting, showing that Cys306 of the beta subunit is required for the formation of intermolecular disulphide bonds both in beta 2 homodimers and in alpha/beta heterodimers. Taken together, these results suggest that the alpha/beta heterodimeric form of meprin is held together by a single disulphide bond linking Cys309 in the alpha subunit to Cys306 in the beta subunit.

Proteolytic processing of the alpha-subunit of rat endopeptidase-24.18 by furin

Endopeptidase-24.18 (EC 3.4.24.18; meprin) is a multisubunit metallopeptidase of the astacin family. It is found in brush-border membranes of rodent kidney and human intestine. The membrane-bound enzyme is composed of alpha/beta dimers. Molecular cloning has shown that both subunits have a similar structural domain organization. Soluble alpha 2 dimers have also been observed in vivo and in transfected cells. The structures of all known alpha-subunits contain, upstream from the transmembrane domain, the sequence RXKR, which corresponds to the RXK/RR consensus sequence for specific cleavage by furin. In order to investigate the involvement of this putative cleavage site in the secretion process of endopeptidase-24,.18 alpha-subunit, we expressed in COS-1 cells rat alpha-subunits in which residues R655 or S656 (within the sequence R652PKRS656) were mutated to valine or leucine respectively. In contrast to the wild-type protein, the alpha R655V and alphaS656L mutants were not secreted in the culture medium. Moreover, when cells expressing the alpha-subunit were infected with a furin-encoding vaccinia virus, immunoblotting showed a shift of the major cell-associated form of endopeptidase-24.18 alpha-subunit from 98 kDa to 85 kDa and an increase in the amounts of secreted alpha-subunit. This shift in molecular mass was not observed with the mutant alpha-subunits. As observed for the 98 kDa species, the 85 kDa cell-associated protein was sensitive to endoglycosidase H treatment, suggesting that the proteolytic cleavage occurred in the endoplasmic reticulum or in an early Golgi compartment. Similar experiments using PACE4 and PC5 instead of furin showed that these enzymes were not able to generate the 85 kDa species. We conclude that furin is most probably the cellular enzyme involved in the proteolysis resulting in secretion of rat endopeptidase-24.18 alpha-subunit.

The astacin family of metalloendopeptidases

The astacin family of metalloendopeptidases was recognized as a novel family of proteases in the 1990s. The crayfish enzyme astacin was the first characterized and is one of the smallest members of the family. More than 20 members of the family have now been identified. They have been detected in species ranging from hydra to humans, in mature and in developmental systems. Proposed functions of these proteases include activation of growth factors, degradation of polypeptides, and processing of extracellular proteins. Astacin family proteases are synthesized with NH2-terminal signal and proenzyme sequences, and many (such as meprins, BMP-1, tolloid) contain multiple domains COOH-terminal to the protease domain. They are either secreted from cells or are plasma membrane-associated enzymes. They have some distinguishing features in addition to the signature sequence in the protease domain: HEXXHXXGFXHEXXRXDR. They have a unique type of zinc binding, with pentacoordination, and a protease domain tertiary structure that contains common attributes with serralysins, matrix metalloendopeptidases, and snake venom proteases; they cleave peptide bonds in polypeptides such as insulin B chain and bradykinin and in proteins such as casein and gelatin; and they have arylamidase activity. Meprins are unique proteases in the astacin family, and indeed in the animal kingdom, in their oligomeric structure; they are dimers of disulfide-linked dimers and are highly glycosylated, type I integral membrane proteins that have many attributes of receptors or integrins with adhesion, epidermal growth factor-like, and transmembrane domains. The alpha and beta subunits are differentially expressed and processed to yield latent and active proteases as well as membrane-associated and secreted forms. Meprins represent excellent models of hetero- and homo-oligomeric enzymes that are regulated at the transcriptional and posttranslational levels.

COOH-terminal proteolytic processing of secreted and membrane forms of the alpha subunit of the metalloprotease meprin A. Requirement of the I domain for processing in the endoplasmic reticulum

Cell surface isoforms of meprin A (EC 3.4.24.18) from mice and rats contain beta subunits that are type I integral membrane proteins and alpha subunits that are disulfide-linked to or noncovalently associated with membrane-anchored meprin subunits. Both alpha and beta subunits are synthesized with COOH-terminal domains predicted to be cytoplasmic, transmembrane, and epidermal growth factor-like; these domains are retained in beta subunits but are removed from alpha during maturation. The present studies establish that an inserted 56-amino acid domain (the "I" domain), present in alpha but not in beta, is necessary and sufficient for COOH-terminal proteolytic processing of the alpha subunit. This was demonstrated by expression of mutant meprin subunits (deletion mutants, chimeric alpha beta subunits, and beta mutants containing the I domain) in COS-1 cells. Mutations of two common processing sites present in the I domain (a dibasic site and a furin site) did not prevent COOH-terminal proteolytic processing, indicating that the proteases responsible for cleavage are distinct from those having these specificities. Deletion of the I domain from the alpha subunit resulted in accumulation of unprocessed subunits in a preGolgi compartment. Furthermore, COOH-terminal proteolytic processing of wild-type alpha subunits occurred before acquisition of endoglycosidase H resistance. Pulse-chase experiments and expression of an alpha subunit transcript containing a c-myc epitope tag, confirmed that proteolytic processing at the COOH terminus occurs in the endoplasmic reticulum. This work identifies the region of the alpha subunit that is essential for COOH-terminal processing and demonstrates that the differential processing of the evolutionarily-related subunits of meprin A that results in a structurally unique tetrameric protease begins in the endoplasmic reticulum.