Proteolysis of fibrillin-2 microfibrils is essential for normal skeletal development
The embryonic extracellular matrix (ECM) undergoes transition to mature ECM as development progresses, yet specific transition mechanisms ensuring ECM proteostasis and their regulatory impact are poorly defined. Fibrillin microfibrils are macromolecular ECM complexes serving structural and regulatory roles. In mice, Fbn1 and Fbn2, encoding major microfibrillar components, are strongly expressed during embryogenesis, but fibrillin-1 is the major component observed in adult tissue microfibrils. Here, analysis of mouse Adamts6 and Adamts10 mutant embryos, lacking these homologous secreted metalloproteases individually and in combination, along with in vitro analysis of microfibrils, measurement of ADAMTS6-fibrillin affinities and N-terminomics identification of ADAMTS6-cleaved sites, demonstrates a transcriptionally adapted system for fibrillin-2 proteolysis that contributes to postnatal fibrillin-1 dominance. The lack of ADAMTS6, alone and in combination with ADAMTS10 led to excess fibrillin-2 in perichondrium, with impaired skeletal development resulting from a drastic reduction of aggrecan, cartilage link protein and impaired BMP, but not TGF{beta} signaling in cartilage. Although ADAMTS6 cleaves fibrillin-1 and fibrillin-2 as well as fibronectin, which provides the initial scaffold for microfibril assembly, primacy of the protease-substrate relationship between ADAMTS6 and fibrillin-2 was unequivocally established by reversal of these defects in Adamts6-/- embryos by genetic reduction of Fbn2, but not Fbn1.