Cleavage site-directed antibodies reveal the prion protein in humans is shed by ADAM10 at Y226 and associates with misfolded protein deposits in neurodegenerative diseases.

Proteolytic cell surface release ('shedding') of the prion protein (PrP), a broadly expressed GPI-anchored glycoprotein, by the metalloprotease ADAM10 impacts on neurodegenerative and other diseases in animal and in vitro models. Recent studies employing the latter also suggest shed PrP (sPrP) to be a ligand in intercellular communication and critically involved in PrP-associated physiological tasks. Although expectedly an evolutionary conserved event, and while soluble forms of PrP are present in human tissues and body fluids, for the human body neither proteolytic PrP shedding and its cleavage site nor involvement of ADAM10 or the biological relevance of this process have been demonstrated thus far. In this study, cleavage site prediction and generation (plus detailed characterization) of sPrP-specific antibodies enabled us to identify PrP cleaved at tyrosin 226 as the physiological and apparently strictly ADAM10-dependent shed form in humans. Using cell lines, neural stem cells and brain organoids, we show that shedding of human PrP can be stimulated by PrP-binding ligands without targeting the protease, which may open novel therapeutic perspectives. Site-specific antibodies directed against human sPrP also detect the shed form in brains of cattle, sheep and deer, hence in all most relevant species naturally affected by fatal and transmissible prion diseases. In human and animal prion diseases, but also in patients with Alzheimer`s disease, sPrP relocalizes from a physiological diffuse tissue pattern to intimately associate with extracellular aggregated deposits of misfolded proteins characteristic for the respective pathological condition. Findings and research tools presented here will accelerate novel insight into the roles of PrP shedding (as a process) and sPrP (as a released factor) in neurodegeneration and beyond.

PCSK9 acts as a key regulator of Aß clearance across the blood-brain barrier.

Despite the neurodegenerative disorder Alzheimer's disease (AD) is the most common form of dementia in late adult life, there is currently no therapy available to prevent the onset or slow down the progression of AD. The progressive cognitive decline in AD correlates with a successive accumulation of cerebral amyloid-ß (Aß) due to impaired clearance mechanisms. A significant percentage is removed by low-density lipoprotein receptor-related protein 1 (LRP1)-mediated transport across the blood-brain barrier (BBB) into the periphery. Circulating proprotein convertase subtilisin/kexin type 9 (PCSK9) binds to members of the low-density lipoprotein receptor protein family at the cell surface and targets them for lysosomal degradation, which reduces the number of functional receptors. However, the adverse impact of PCSK9 on LRP1-mediated brain Aß clearance remains elusive. By using an established BBB model, we identified reduced LRP1-mediated brain-to-blood Aß clearance due to PCSK9 across different endothelial monolayer in vitro. Consequently, the repetitive application of FDA-approved monoclonal anti-PCSK9 antibodies into 5xFAD mice decreased the cerebral Aß burden across variants and aggregation state, which was not reproducible in brain endothelial-specific LRP1-/- 5xFAD mice. The peripheral PCSK9 inhibition reduced Aß pathology in prefrontal cortex and hippocampus-brain areas critically involved in memory processing-and prevented disease-related impairment in hippocampus-dependent memory formation. Our data suggest that peripheral inhibition of PCSK9 by already available therapeutic antibodies may be a novel and easily applicable potential AD treatment.

Meprin ß cleaves TREM2 and controls its phagocytic activity on macrophages.

The triggering receptor expressed on myeloid cells 2 (TREM2) is a multifunctional surface protein that affects survival, migration, and phagocytic capacity of myeloid cells. Soluble TREM2 levels were found to be increased in early stages of sporadic and familial Alzheimer's disease (AD) probably reflecting a defensive microglial response to some initial brain damage. The disintegrin and metalloproteases (ADAM) 10 and 17 were identified as TREM2 sheddases. We demonstrate that meprin ß is a direct TREM2 cleaving enzyme using ADAM10/17 deficient HEK293 cells. LC-MS/MS analysis of recombinant TREM2 incubated with meprin ß revealed predominant cleavage between Arg136 and Asp137, distant to the site identified for ADAM10/17. We further demonstrate that the metalloprotease meprin ß cleaves TREM2 on macrophages concomitant with decreased levels of soluble TREM2 in the serum of Mep1b-/- mice compared to WT controls. Isolated BMDMs from Mep1b-/- mice showed significantly increased full-length TREM2 levels and enhanced phagocytosis efficiency compared to WT cells. The diminished constitutive shedding of TREM2 on meprin ß deficient macrophages could be rescued by ADAM stimulation through LPS treatment. Our data provide evidence that meprin ß is a TREM2 sheddase on macrophages and suggest that multiple proteases may be involved in the generation of soluble TREM2.

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.

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 ß 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 ß. In this study, we identified human meprin α and meprin ß 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 ß 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 ß 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.

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

Meprin ß 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 ß, which influences its substrate repertoire and subsequent biologic functions. Proteomic analysis also revealed several ADAMs as putative meprin ß substrates. Here, we demonstrate specific N-terminal processing of ADAM9, 10, and 17 by meprin ß and identify cleavage sites within their prodomains. Because ADAM prodomains can act as specific inhibitors, we postulate a role for meprin ß in the regulation of ADAM activities. Indeed, prodomain cleavage by meprin ß caused increased ADAM protease activities, as observed by peptide-based cleavage assays and demonstrated by increased ectodomain shedding activity. Direct interaction of meprin ß and ADAM proteases could be shown by immunofluorescence microscopy and immunoprecipitation experiments. As demonstrated by a bacterial activator of meprin ß and additional measurement of TNF-α shedding on bone marrow-derived macrophages, meprin ß/ADAM protease interactions likely influence inflammatory conditions. Thus, we identified a novel proteolytic pathway of meprin ß 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 ß induces activities of A disintegrin and metalloproteinases 9, 10, and 17 by specific prodomain cleavage.

Regulation of the alternative ß-secretase meprin ß by ADAM-mediated shedding.

Alzheimer's Disease (AD) is the sixth-leading cause of death in industrialized countries. Neurotoxic amyloid-ß (Aß) plaques are one of the pathological hallmarks in AD patient brains. Aß accumulates in the brain upon sequential, proteolytic processing of the amyloid precursor protein (APP) by ß- and γ-secretases. However, so far disease-modifying drugs targeting ß- and γ-secretase pathways seeking a decrease in the production of toxic Aß peptides have failed in clinics. It has been demonstrated that the metalloproteinase meprin ß acts as an alternative ß-secretase, capable of generating truncated Aß2-x peptides that have been described to be increased in AD patients. This indicates an important ß-site cleaving enzyme 1 (BACE-1)-independent contribution of the metalloprotease meprin ß within the amyloidogenic pathway and may lead to novel drug targeting avenues. However, meprin ß 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 ß. Overall, we highlight the current cellular, molecular and structural understanding of meprin ß as alternative ß-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 ß 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 ß, 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.

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 ß as well as a disintegrin and metalloproteinase 10 (ADAM10) and ADAM17 are also associated with these diseases. Interestingly, meprin ß, 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 ß, to elucidate the potential mutual regulations of both enzymes. Herein, we show that besides ADAM10/17, MT1-MMP is also able to shed meprin ß from the plasma membrane, leading to the release of soluble meprin ß. Mass spectrometry-based cleavage site analysis revealed that the cleavage of meprin ß by all three proteases occurs between Pro602 and Ser603 , N-terminal of the EGF-like domain. Furthermore, only inactive human pro-meprin ß is shed by MT1-MMP, which is again in accordance with the shedding capability observed for ADAM10/17. Vice versa, meprin ß 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.

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

The metalloproteases meprin α and meprin ß 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 ß 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/ß heterodimers. Galectin-3-deficient and meprin α/ß double knockout mice show similar alterations in their microbiome in comparison to wild-type mice. We further demonstrate that meprin α/ß 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/ß heterodimers may play a key role in colon host-microbiome homeostasis.

Meprin ß: A novel regulator of blood-brain barrier integrity.

The metalloprotease meprin ß (Mep1b) is capable of cleaving cell-adhesion molecules in different tissues (e.g. skin, kidney and intestine) and is dysregulated in several diseases associated with barrier breakdown (Alzheimer´s disease, kidney disruption, inflammatory bowel disease). In this study, we demonstrate that Mep1b is a novel regulator of tight junction (TJ) composition and blood-brain barrier (BBB) integrity in brain endothelium. In Mep1b-transfected mouse brain endothelial cells (bEnd.3), we observed a reduction of the TJ protein claudin-5, decreased transendothelial electrical resistance (TEER) and an elevated permeability to paracellular diffusion marker [14C]-inulin. Analysis of global Mep1b knock-out (Mep1b-/-) mice showed increased TJ protein expression (claudin-5, occludin, ZO-1) in cerebral microvessels and increased TEER in cultivated primary mouse brain endothelial compared to wild-type (wt) mice. Furthermore, we investigated the IgG levels in cerebrospinal fluid (CSF) and the brain water content as additional permeability markers and detected lower IgG levels and reduced brain water content in Mep1b-/- mice compared to wt mice. Showing opposing features in overexpression and knock-out, we conclude that Mep1b plays a role in regulating brain endothelial TJ-proteins and therefore affecting BBB tightness in vitro and in vivo.

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

Meprin ß 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 ß, both exhibiting a single amino acid exchange, namely, meprin ß G45R and G89R. Based on the structural data of meprin ß 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 ß, 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 ß and its variants. In transfected HEK cells, the G89R variant was found to generate less soluble IL-6R, whereas the expression of meprin ß G45R resulted in increased shedding of the IL-6R compared to wild-type meprin ß and the G89R variant. A similar tendency of the induced shedding capacity of G45R was seen for the well-described meprin ß substrate CD99. Furthermore, employing an assay for cell migration in a collagen IV matrix, we observed that the transfection of wild-type meprin ß and the G45R variant resulted in increased migration of HeLa cells, while the G89R variant led to diminished mobility.

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

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

Loss of ADAMTS19 causes progressive non-syndromic heart valve disease.

Valvular heart disease is observed in approximately 2% of the general population1. Although the initial observation is often localized (for example, to the aortic or mitral valve), disease manifestations are regularly observed in the other valves and patients frequently require surgery. Despite the high frequency of heart valve disease, only a handful of genes have so far been identified as the monogenic causes of disease2-7. Here we identify two consanguineous families, each with two affected family members presenting with progressive heart valve disease early in life. Whole-exome sequencing revealed homozygous, truncating nonsense alleles in ADAMTS19 in all four affected individuals. Homozygous knockout mice for Adamts19 show aortic valve dysfunction, recapitulating aspects of the human phenotype. Expression analysis using a lacZ reporter and single-cell RNA sequencing highlight Adamts19 as a novel marker for valvular interstitial cells; inference of gene regulatory networks in valvular interstitial cells positions Adamts19 in a highly discriminatory network driven by the transcription factor lymphoid enhancer-binding factor 1 downstream of the Wnt signaling pathway. Upregulation of endocardial Krüppel-like factor 2 in Adamts19 knockout mice precedes hemodynamic perturbation, showing that a tight balance in the Wnt-Adamts19-Klf2 axis is required for proper valve maturation and maintenance.

Meprin ß and BMP-1 are differentially regulated by CaCl2.

The two metalloproteases meprin ß and bone morphogenetic protein 1 (BMP-1) are both members of the astacin protease family. They share specificity for negatively charged residues around the scissile bond and they are expressed in overlapping compartments of the human body. One important proteolytic substrate they share is pro-collagen I. Ablation of one of the two proteases however leads to different collagen I associated phenotypes in vivo. Over the last years calcium emerged as a regulator for the proteolytic activity of both enzymes. For meprin ß a reduction and for BMP-1 an increase in activity was reported under increasing calcium concentrations. Here we revisit different compartments that rely on pro-collagen I maturation and explore the crystal structure of both proteases to highlight possible calcium binding sites. With this we aim to emphasize a to date underestimated regulator that influences both proteases.

Structure and evolution of N-domains in AAA metalloproteases.

Metalloproteases of the AAA (ATPases associated with various cellular activities) family play a crucial role in protein quality control within the cytoplasmic membrane of bacteria and the inner membrane of eukaryotic organelles. These membrane-anchored hexameric enzymes are composed of an N-terminal domain with one or two transmembrane helices, a central AAA ATPase module, and a C-terminal Zn(2+)-dependent protease. While the latter two domains have been well studied, so far, little is known about the N-terminal regions. Here, in an extensive bioinformatic and structural analysis, we identified three major, non-homologous groups of N-domains in AAA metalloproteases. By far, the largest one is the FtsH-like group of bacteria and eukaryotic organelles. The other two groups are specific to Yme1: one found in plants, fungi, and basal metazoans and the other one found exclusively in animals. Using NMR and crystallography, we determined the subunit structure and hexameric assembly of Escherichia coli FtsH-N, exhibiting an unusual α+ß fold, and the conserved part of fungal Yme1-N from Saccharomyces cerevisiae, revealing a tetratricopeptide repeat fold. Our bioinformatic analysis showed that, uniquely among these proteins, the N-domain of Yme1 from the cnidarian Hydra vulgaris contains both the tetratricopeptide repeat region seen in basal metazoans and a region of homology to the N-domains of animals. Thus, it is a modern-day representative of an intermediate in the evolution of animal Yme1 from basal eukaryotic precursors.