Lenses of SPARC-null mice exhibit an abnormal cell surface-basement membrane interface

SPARC (secreted protein acidic and rich in cysteine) is a matricellular pro tein involved in cell-matrix interactions. We have shown previously that mi ce deficient in SPARC develop posterior cortical cataract early in life tha t progresses to a mature opacity and capsule rupture. To evaluate the prima ry effects of SPARC deficiency in the lens, we examined the lenses of SPARC -null and wild-type mice by electron microscopy and immunohistochemistry to investigate whether ultrastructural abnormalities occur at the basement me mbrane (capsule)-lens cell interface in SPARC-null mice. The most notable f eature in the lenses of SPARC-null mice, relative to wild-type animals, was the modification of the basal surface of the lens epithelial and fiber cel ls at the basement membrane (capsule) interface. Electron microscopy reveal ed numerous filopodial projections of the basal surface of the lens epithel ial and fiber cells into the extracellular matrix of the anterior, posterio r, and equatorial regions of the lens capsule. In 1 week old precataractous lenses, basal invasive filopodia projecting into the capsule were small an d infrequent. Both the size and frequency of these filopodia increased in p recataractous 3-4 week old lenses and were prominent in the cataractous 5-6 week old lenses, By rhodamine-phalloidin labeling, we confirmed the presen ce of basal invasive filopodia projecting into the lens capsule and demonst rated that the projections contained actin filaments. In contrast to the ob vious abnormal projections at the interface between the basal surface of th e lens epithelial and fiber cells and the lens capsule, the apical and late ral plasma membranes of lens epithelial cells and lens fibers in SPARC-null mice were as smooth as those of wild-type mice. We conclude that the absen ce of SPARC in the murine lens is associated with a filopodial protrusion o f the basal surface of the lens epithelium and differentiating fiber cells into the lens capsule. The altered structures appear prior to the opacifica tion of the lens in the SPARC-null model. These observations are consistent with one or more functions previously proposed for SPARC as a modulator of cell shape and cell-matrix interactions. (C) 2000 Academic Press.