An extracellular matrix-associated glycoprotein expressed in a variety
of tissues during embryogenesis and repair, SPARC contains modular do
mains that can function independently to bind cells and matrix compone
nts. Because SPARC can selectively disrupt cellular contacts with matr
ix and thereby effect changes in cell shape, it has been referred to a
s an antiadhesin. Inhibition of the expression of SPARC altered axial
development in frogs, and deregulated expression in nematode worms res
ulted in a derangement of muscle attachment and embryonic lethality. S
PARC also inhibits cell cycle progression in vitro, in part through a
cationic, disulfide-bonded region that is homologous to a repeated dom
ain in the cytokine inhibitor, follistatin. Moreover, SPARC binds spec
ifically to the B chain of platelet-derived growth factor and alters t
he response of cells to several cytokines. Although information concer
ning the expression, biochemical properties, and cellular activities o
f SPARC has increased significantly over the last decade, the precise
function of the protein has not been resolved. Goals of future studies
include characterization of cell-surface receptors for SPARC and the
interactions with morphogens and growth factors that regulate specific
activities during animal development.