SPARC, a matricellular protein that functions in cellular differentiation and tissue response to injury

Vertebrate SPARC binds to a number of different ECM components including thrombospondin 1, vitronectin, entactin/nidogen, fibrillar collagens (types I, II, III, and V), and collagen type IV, the prevalent collagen in basement membranes (1). Therefore, SPARC has the potential to contribute to the organization of matrix in connective tissue as well as basement membranes. Interestingly, SPARC is expressed abundantly in basement membranes and in capsules that surround a variety of organs and tissues. In this regard, SPARC-null mice display early cataractogenesis, a phenotype with 100% penetrance (2). Transmission electron microscopy of lens epithelial cells in SPARC-null mice shows an intrusion of cellular processes into the basement membrane of the lens capsule, whereas wild-type lens epithelial cells exhibit a precise border at the cell-matrix interface (3). We have proposed that this phenotype reflects aberrant cell behavior or differentiation resulting from altered composition or structure of the basement membrane formed in the absence of SPARC.