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

Increased expression of metalloprotease meprin beta 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 beta 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 beta and suggest transmembrane serine protease 6 [TMPRSS6 or matriptase-2 (MT2)] as a new potent activator, cleaving off the propeptide of meprin beta 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 beta at the cell surface, analysed by specific fluorogenic peptide cleavage assay, Western blotting and confocal laser scanning microscopy (CLSM). Maturation of full-length meprin beta 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).