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P.N. Bishop, J.T. Gallagher, V.J. Hindson; Opticin Binds Heparan Sulfate and May Thus Provide a Molecular Basis for Vitreoretinal Adhesion . Invest. Ophthalmol. Vis. Sci. 2005;46(13):5153.
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Purpose: Opticin is a class III member of the small leucine–rich repeat proteoglycan/protein (SLRP) family. It is a glycoprotein that is highly expressed in the eye where it associates with vitreous collagen fibrils. Immunolocalisation experiments have demonstrated that opticin particularly concentrates at the vitreoretinal interface where it appeared to co–localise with type XVIII collagen. Type XVIII collagen is a heparan sulfate proteoglycan so we investigated whether opticin binds heparan sulfate. Methods: Recombinant human and bovine opticin were expressed in 293–EBNA cells and purified to apparent homogeneity. Solid phase assays and surface plasmon resonance (SPR) using a BIAcore 3000 were used to investigate the interaction between opticin and immobilised heparin/HS. Solution based competition assays were then used to test the specificity of the interactions. Results: Solid phase and SPR data revealed that opticin bind heparin and HS. A KD of ∼100 nM was obtained by analysis of SPR data for binding to heparin and porcine intestinal HS, but opticin bound less strongly to HS derived from bovine kidney. The inhibition studies showed that (a) a 6–mer heparin oligosaccharide had a lower affinity for opticin than an 8–mer or larger oligosaccharides (b) that removal of the sulfate group at ring position two of iduronate had little effect upon the affinity of opticin for heparin and (c) that heparin and PIM HS compete with immobilized heparin for binding to opticin very strongly, whereas bovine kidney HS, chondroitin sulphate (CS)–A and CS–B show moderate competition, and CS–C and hyaluronan show minimal competition. Conclusions: Opticin by binding both vitreous collagen fibrils and HS proteoglycans in the ILL (e.g. type XVIII collagen) may form the molecular "glue" that results in postbasal vitreoretinal adhesion. These interactions could provide a new therapeutic target for pharmacological vitreoretinal disinsertion.
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