Soluble Interleukin-6 receptor (sIL-6R) mediated trans-signaling is an important pro-inflammatory stimulus associated with pathological conditions, such as arthritis, neurodegeneration and inflammatory bowel disease. The sIL-6R is generated proteolytically from its membrane bound form and A Disintegrin And Metalloprotease (ADAM) 10 and 17 were shown to perform ectodomain shedding of the receptor in vitro and in vivo. However, under certain conditions not all sIL-6R could be assigned to ADAM10/17 activity. Here, we demonstrate that the IL-6R is a shedding substrate of soluble meprin α and membrane bound meprin β, resulting in bioactive sIL-6R that is capable of inducing IL-6 trans-signaling. We determined cleavage within the N-terminal part of the IL-6R stalk region, distinct from the cleavage site reported for ADAM10/17. Interestingly, meprin β can be shed from the cell surface by ADAM10/17 and the observation that soluble meprin β is not capable of shedding the IL-6R suggests a regulatory mechanism towards trans-signaling. Additionally, we observed a significant negative correlation of meprin β expression and IL-6R levels on human granulocytes, providing evidence for in vivo function of this proteolytic interaction.

ABSTRACT:The adhesion molecule CD99 is essential for the transendothelial migration of leukocytes. In this study, we used biochemical and cellular assays to show that CD99 undergoes ectodomain shedding by the metalloprotease meprin b and subsequent intramembrane proteolysis by g-secretase. The cleavage site in CD99 was identified by mass spectrometry within an acidic region highly conserved through different vertebrate species. This finding fits perfectly to the unique cleavage specificity of meprin b with a strong preference for aspartate residues and suggests coevolution of protease and substrate. We hypothesized that limited CD99 cleavage by meprin b would alter cellular transendothelial migration (TEM) behavior in tissue remodeling processes, such as inflammation and cancer. Indeed, meprin b induced cell migration of Lewis lung carcinoma cells in an in vitro TEM assay. Accordingly, deficiency of meprin b in Mep1b 2/2 mice resulted in significantly increased CD99 protein levels in the lung. Therefore, meprin b could serve as a therapeutic target, given that in a proof-of-concept approach we showed accumulation of CD99 protein in lungs of meprin b inhibitortreated mice.-Bedau, T., Peters, F., Prox, J., Arnold, P., Schmidt, F., Finkernagel, M., Köllmann, S., Wichert, R., Otte, A., Ohler, A., Stirnberg, M., Lucius, R., Koudelka, T., Tholey, A., Biasin, V., Pietrzik, C. U., Kwapiszewska, G., Becker-Pauly, C. Ectodomain shedding of CD99 within highly conserved regions is mediated by the metalloprotease meprin b and promotes transendothelial cell migration. FASEB J. 31, 1226-1237 (2017). www.fasebj.orgRegulated intramembrane proteolysis (RIP) of cell adhesion molecules, such as junctional adhesion molecule (JAM)-A, intercellular adhesion molecule (ICAM)-1, and L-selectin, was shown to be essential for transendothelial migration (TEM) of inflammatory or cancer cells (1). Meprin b, a multidomain type I transmembrane metalloprotease, is an initiator of RIP, and structural studies revealed dimeric formation of the protease with the active site in proximity to the cell surface (2-4). In addition, meprin b can be shed from the cell surface by ADAM10/ 17, resulting in a soluble active protease, which for instance is important for mucus detachment in the small intestine (5). Meprin b is characterized by a unique cleavage specificity, with a preference for negatively charged amino acids (6). These structural features provide all requirements that meprin b must have to act as an ectodomain sheddase at the cell surface. Indeed, membrane-bound amyloid precursor protein (APP), for instance, is cleaved by meprin b, resulting in the release of sAPP-b fragments and neurotoxic Ab peptides (4,7,8). Many of the known substrates of meprin b have been identified by mass spectrometry (MS)-based proteomic approaches (9).

Increased expression of metalloprotease meprin β is associated with fibrotic syndromes and Alzheimer's disease (AD). Hence, regulation of meprin activity might be a suitable strategy for the treatment of these conditions. Meprin β is a type 1 transmembrane protein, but can be released from the cell surface by ectodomain shedding. The protease is expressed as an inactive zymogen and requires proteolytic maturation by tryptic serine proteases. In the present study, we demonstrate, for the first time, the differences in the activation of soluble and membrane bound meprin β and suggest transmembrane serine protease 6 [TMPRSS6 or matriptase-2 (MT2)] as a new potent activator, cleaving off the propeptide of meprin β between Arg(61) and Asn(62) as determined by MS. We show that MT2, but not TMPRSS4 or pancreatic trypsin, is capable of activating full-length meprin β at the cell surface, analysed by specific fluorogenic peptide cleavage assay, Western blotting and confocal laser scanning microscopy (CLSM). Maturation of full-length meprin β is required for its activity as a cell surface sheddase, releasing the ectodomains of transmembrane proteins, as previously shown for the amyloid precursor protein (APP).

The meprin b metalloproteinase is an important enzyme in extracellular matrix turnover, inflammation, and neurodegeneration in humans and mice. Previous studies showed a diminished cleavage of certain meprin b substrates in the presence of calcium, although the mechanism was not clear. With the help of a specific fluorogenic peptide assay and the human amyloid precursor protein as substrate, we demonstrated that the influence of calcium is most likely a direct effect on human meprin b itself. Analyzing the crystal structures of pro-and mature meprin b helped to identify a cluster of negatively charged amino acids forming a potential calcium binding site. Mutation of 2 of these residues (D204A and D245A) led to severe differences in proteolytic activity and cellular localization of meprin b. D245A was almost completely inactive and largely stored into intracellular vesicles, indicating severe misfolding of the protein. Astonishingly, D204A was not transported to the cell surface, but exhibited strong b-secretase activity, resulting in massive accumulation of Ab-peptides. This could be explained by constitutive maturation of this meprin b mutant already in the early secretory pathway. We hypothesize that lacking D204 abrogates the capability of binding calcium in the catalytic domain, an important step for proper folding of the propeptide and subsequent inhibition of the protease. This is supported by the inhibition constant of calcium for meprin b (inhibitory constant 50 = 11 mM), which resembles the physiologic concentrations found in the endoplasmic reticulum. For instance, it was shown for amyotrophic lateral sclerosis that a loss of calcium in the endoplasmic reticulum leads to the misfolding of calcium-dependent proteins, which might also be relevant for proper function of meprin b.-Arnold, P., Schmidt, F., Prox, J., Zunke, F., Pietrzik, C., Lucius, R., Becker-Pauly, C. Calcium negatively regulates meprin b activity and attenuates substrate cleavage. FASEB J. 29, 3549-3557 (2015). www.fasebj.org Key Words: protease • APP • amyloid b • protein folding MEPRIN b BELONGS TO THE zinc-endopeptidases of the metzincin superfamily. It has a defined cleavage specificity with a high preference for negatively charged amino acids around the scissile bond (1). Meprin b is expressed as a dimer and is primarily membrane bound. Nevertheless, it can be shed from the cell surface by A disintegrin and metalloproteinase proteases (2, 3). It is a multidomain enzyme starting with an N-terminal propeptide that shields the active site and needs to be removed for activation. The propeptide is followed by the catalytic domain containing the active site, a meprin A5 proteine tyrosine phosphatase m domain, a TNF receptor-associated factor domain, an epidermal growth factor-like domain, a transmembrane region, and a small cytosolic part. Different components of the extracellular matrix have been described to be cleaved by meprin b such as procollagen (4, 5), dentin sialophosphoprotein (DSPP) and its cleavage fragment dentin phosp...

Transendothelial cell migration (TEM) is crucial for inflammation and metastasis. The adhesion molecule CD99 was shown to be important for correct immune cell extravasation and is highly expressed on certain cancer cells. Recently, we demonstrated that ectodomain shedding of CD99 by the metalloprotease meprin β promotes TEM in vitro.In this study, we employed an acute inflammation model (air pouch/carrageenan) and found significantly less infiltrated cells in meprin β knock-out animals validating the previously observed pro-inflammatory activity. To further analyze the impact of meprin β on CD99 shedding with regard to cell adhesion and proliferation we characterized two lung cancer associated CD99 variants (D92H, D92Y), carrying point mutations at the main cleavage site. Interestingly, ectodomain shedding of these variants by meprin β was still detectable. However the cleavage site shifted to adjacent positions. Nevertheless, expression of CD99 variants D92H and D92Y revealed partial misfolding and proteasomal degradation. A previously observed influence of CD99 on Src activation and increased proliferation could not be confirmed in this study, independent of wild-type CD99 or the variants D92H and D92Y. However, we identified meprin β as a potent inducer of Src phosphorylation. Importantly, we found significantly increased cell migration when expressing the cancer-associated CD99 variant D92H compared to the wild-type protein.