Opticin (OPTC) is a a member of the small leucine-rich repeat proteoglycan family (Reardon et al. 2000). It is found in the vitreous cavity of the eye where it co-localizes with the fine network of collagen fibrils that maintains the gel state of the vitreous and the inner-limiting lamina, and in other tissues, including the brain, heart, and cartilage (Le Goff et al. 2012). It forms a homodimer in solution through its leucine-rich repeats (Le Goff et al. 2003). Opticin has anti-angiogenic activity which is mediated by binding to vitreous collagen fibrils, which are composed of collagens II, IX, and V/XI (Bishop 2000). This binding competitively inhibits endothelial cell interactions with collagen I via Alpha-1Beta-1and Alpha-2Beta-1 integrins, preventing proangiogenic signaling via these integrins (Le Goff et al. 2012). OPTC is expressed and translocated to the nucleus of chronic lymphocytic leukemia cells (Mikaelsson et al.2013).
OPTC can be degraded by Matrix metalloprotease (MMP) -1, -2, -3, -7, -8, -9, -13 and by ADAMTS-4 and ADAMTS-5, with MMP2 and MMP7 having highest activity towards the recombinant protein (Montfort et al. 2008, Ma et al. 2012, Tio et al. 2014). MMP13 cleaves recombinant bovine OPTC at G104/L105 (major product), and P109/A110 (minor product) (Montfort et al. 2008).The major cleavage site corresponds to G114/L115 in human opticin.