Diverse distributions and functions of SPARC during Xenopus laevis early development.

摘要

SPARC is a highly conserved calcium-binding glycoprotein with dynamic transient patterns of expression during early Xenopus laevis embryonic development. Injection of mature SPARC into the blastocoel cavity of blastula embryos leads to a dose dependent reduction in anterior to posterior structures (ventralization/posteriorization). This phenotype is mimicked by a synthetic peptide (peptide 4.2) corresponding to the C-terminal EF-hand of SPARC. Molecular and morphological studies indicate that SPARC induces involuting chordomesodermal cells to round up, reducing their posterior to anterior migration. This activity on cell-matrix interaction provides the first in vivo evidence of a counter-adhesive activity for SPARC.; Although SPARC has been characterized as a secreted extracellular matrix (ECM) glycoprotein, whole mount immunocytochemical analysis indicated that SPARC is associated with cilia of the embryonic/larval ectoderm of Xenopus. SPARC is concentrated in the outer surface layer, particularly in cilia. Immunogold electron microscopy shows that SPARC is associated with the 9 + 2 microtubule arrays of the ciliary axonemes. Tubulin can be immunoprecipitated with antibodies that cross-react with Xenopus SPARC, indicating a direct or indirect interaction of SPARC and tubulin.; SPARC is also concentrated circumferentially at the apicolateral plasma membranes of surface ectodermal cells. Injection of anti-sense Xenopus SPARC morpholinos leads to the dissociation of surface ectoderm by mid-tailbud. A similar dissociation is observed with animal cap explants taken from morpholino injected embryos. These studies indicate that SPARC promotes epithelial cell-cell adhesion and is required for maintaining the integrity of the embryonic epidermis of Xenopus embryos. This represents the first evidence of an adhesive function for SPARC.; Collectively, my studies indicate that SPARC functions intracellularly as a calcium-dependent regulator of ciliary movement and that SPARC has adhesive activity in epithelial tissues, in contrast to an anti-adhesive activity in mesenchymal tissues. Since the associations of SPARC with cilia and circumferential enrichment are observed in amphibians and mammals, it is likely that these intracellular and extracellular functions of this glycoprotein are evolutionarily conserved.