Synthesis of a thrombospondin-like cytoadhesion molecule by cells of the trabecular meshwork

Thrombospondin is a large extracellular-matrix protein that is released by smooth muscle cells and platelets and is distributed widely in mammalian brain. By using western blotting with a monoclonal antibody against the calcium-binding domain of thrombospondin, the authors detected a 180-kD glycated polypeptide in the trabecular meshwork tissue of normal human and porcine eyes. The epitope-bearing polypeptide was soluble in sodium dodecylsulfate/urea (SDS/urea), and apparently it was stabilized in part by disulfide bonding to the Triton X-100 and SDS/urea-insoluble pellet of this tissue. Treatment of the insoluble matrix with beta-mercaptoethanol led to an enriched extraction of the approximately 160-kD form of thrombospondin. On immunohistochemical study, the thrombospondin antibody also reacted positively with the extracellular matrix and intracellular structures of trabecular cells in primary monolayer culture. This suggested that the presence of thrombospondin in the trabecular meshwork was probably due to local synthesis. These findings are relevant to the establishment of a model in vitro for assessment of adhesion of trabecular cells. Because thrombospondin is a cell-substrate adhesion molecule, its role in the loss of cellularity that occurs in the trabecular meshwork of the aging eye and in eyes with primary open-angle glaucoma is worthy of further investigation.