Fibrillin degradation by matrix metalloproteinases: implications for connective tissue remodelling

Fibrillin is the principal structural component of the 10-12 nm diameter el astic microfibrils of the extracellular matrix. We have previously shown th at both fibrillin molecules and assembled microfibrils are susceptible to d egradation by serine proteases. In this study, we have investigated the pot ential catabolic effects of six matrix metalloproteinases (MMP-2, MMP-3, MM P-9, MMP-12, MMP-13 and MMP-14) on fibrillin molecules and on intact fibril lin-rich microfibrils isolated from ciliary zonules, Using newly synthesize d recombinant fibrillin molecules, major cleavage sites within fibrillin-1 were identified. In particular, the six different MMPs generated a major de gradation product of similar to 45 kDa from the N-terminal region of the mo lecule, whereas treatment of truncated, unprocessed and furin-processed C-t ermini also generated large degradation products. Introduction of a single ectopia lentis-causing amino acid substitution (E2447K; one-letter symbols for amino acids) in a calcium- binding epidermal growth factor-like domain, predicted to disrupt calcium binding, markedly altered the pattern of C-te rminal fibrillin-1 degradation. However, the fragmentation pattern of a mut ant fibrillin-1 with a comparable E --> K substitution in an upstream calci um-binding epidermal growth factor-like domain was indistinguishable from w ild-type molecules. Ultrastructural examination highlighted that fibrillin- rich microfibrils isolated from ciliary zonules were grossly disrupted by M MPs. This is the first demonstration that fibrillin molecules and fibrillin -rich microfibrils are degraded by MMPs and that certain amino acid substit utions change the fragmentation patterns. These studies have important impl ications for physiological and pathological fibrillin catabolism and for lo ss of connective tissue elasticity in ageing and disease.