SPARC and the Differentiation of Transparent Lens Fibers

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DESCRIPTION (provided by applicant): SPARC (secreted protein acidic and rich in cysteine) is a major component of remodeling tissues and, as such, features prominently in morphogenesis, development, injury, and repair. It belongs to the matricellular class of secreted glycoproteins that, although structurally dissimilar, regulate interactions between cells and extracellular matrix (ECM). SPARC has been shown specifically to a) inhibit the cell cycle, b) prevent or disrupt cell adhesion, c) inactivate cellular responses to certain growth factors including basic fibroblast growth factor (bFGF or FGF2), d) regulate ECM production, e) bind to specific collagens including those of the basement membrane, and f) promote a rounded cell shape and reorganization of the actin cytoskeleton. These properties provide a strong rationale for a targeted disruption of the SPARC gene. Although SPARC -/- mice were viable and fertile, there were phenotypic abnormalities associated with connective tissue. The major, dominant phenotype, however, was the appearance of lenticular opacity at 1-2 mo. after birth and progression to mature cataracts by 8 mo. The explanation for the effect of SPARC on lens transparency in SPARC-null mice is unknown and forms the basis for this proposal. We will test the hypothesis that SPARC regulates lens epithelial cell differentiation via 1) its modulation of signaling pathway(s) activated by bFGF, and 2) its effects on lens cell-cell and cell-ECM (lens capsule) interactions. Our hypothesis is based on the following premises: a) throughout lens fiber differentiation, the basal surface of proliferating, migrating, and elongating fiber is directly attached to the ECM of the capsule, which contains SPARC, b) development and fiber differentiation are known to be regulated by FGF2, and c) cell differentiation in the lens is rigorously controlled from early in fetal development throughout the life of the animal (even minor perturbances are likely to be "recorded" as errors in the development of the lens that might not be apparent until adulthood, e.g., cataracts). The experiments proposed thus address the role of dysregulation in cell epithelium-ECM-interaction in lens transparency and afford a novel model for perturbations in differentiation that could be manifested later in life.