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Astrocytes regulate synaptic connectivity in the CNS through secreted signals. Here we identified two astrocyte-secreted proteins, hevin and SPARC, as regulators of excitatory synaptogenesis in vitro and in vivo. Hevin induces the formation of synapses between cultured rat retinal ganglion cells. SPARC is not synaptogenic, but specifically antagonizes synaptogenic function of hevin. Hevin and SPARC are expressed by astrocytes in the superior colliculus, the synaptic target of retinal ganglion cells, concurrent with the excitatory synaptogenesis. Hevin-null mice had fewer excitatory synapses; conversely, SPARC-null mice had increased synaptic connections in the superior colliculus. Furthermore, we found that hevin is required for the structural maturation of the retinocollicular synapses. These results identify hevin as a positive and SPARC as a negative regulator of synapse formation and signify that, through regulation of relative levels of hevin and SPARC, astrocytes might control the formation, maturation, and plasticity of synapses in vivo.F ormation of the correct type and number of synaptic connections is crucial for the proper development and function of our nervous systems. In the past decade, astrocytes have emerged as important regulators of synaptic connectivity (1, 2).By using a purified retinal ganglion cell (RGC) culture system (3), we previously showed that astrocyte-secreted factors, including a family of ECM proteins, thrombospondins (TSPs), significantly increase the number of synapses formed between RGCs (4-6). These in vitro findings paved the way for recognition of astrocytes, and the TSPs they secrete, as important regulators of synapse formation and injury-mediated synaptic remodeling in vivo (4,5,7).TSPs belong to a subclass of secreted proteins called matricellular proteins. Matricellular proteins function by modulation of cell-cell and cell-matrix interactions, and thereby regulate the adhesion state of cells (8). Astrocytes express a number of matricellular proteins in addition to TSPs, and their expression is developmentally regulated and overlaps with early postnatal periods of synaptic development in the CNS (9, 10).In the present study we investigated whether other astrocytesecreted matricellular proteins could modulate synapse formation. Gene expression profiling of astrocytes suggested the matricellular proteins hevin [also known as secreted protein acidic and rich in cysteine (SPARC)-like 1] and SPARC as possible candidates. Hevin and SPARC are members of the SPARC family (11). Hevin was first identified as a synaptic glycoprotein and was initially termed synaptic cleft-1, or SC1 (12). It is localized to excitatory CNS synapses (13). Astrocytes in the developing brain express high levels of hevin and SPARC mRNA, with hevin mRNA being one of the highest-level mRNAs expressed by astrocytes (10). Unlike TSP1 and TSP2, the expression of which is decreased during maturation, hevin and SPARC mRNA levels remain high in the adult (9, 14-16).Here we investigated whether hevin and SPARC p...

An extracellular matrix-associated glycoprotein expressed in a variety of tissues during embryogenesis and repair, SPARC contains modular domains that can function independently to bind cells and matrix components. Because SPARC can selectively disrupt cellular contacts with matrix and thereby effect changes in cell shape, it has been referred to as an antiadhesin. Inhibition of the expression of SPARC altered axial development in frogs, and deregulated expression in nematode worms resulted in a derangement of muscle attachment and embryonic lethality. SPARC also inhibits cell cycle progression in vitro, in part through a cationic, disulfide-bonded region that is homologous to a repeated domain in the cytokine inhibitor, follistatin. Moreover, SPARC binds specifically to the B chain of platelet-derived growth factor and alters the response of cells to several cytokines. Although information concerning the expression, biochemical properties, and cellular activities of 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.

To delineate potential angiogenic roles of platelet-derived growth factor (PDGF), we have investigated PDGF and its receptors on bovine aortic endothelial cells that exhibit spontaneous angiogenesis in vitro (angiogenic endothelial cells). Initiation of cord/tube formation by angiogenic endothelial cells required bovine or human serum. Neutralization of PDGF-BB in human serum with a monoclonal anti-PDGF-BB antibody reduced cord/tube formation by 37 +/- 10%, whereas neutralizing anti-PDGF-AA and an IgG isotype-matched control antibody had no effect. DNA synthesis in response to PDGF-BB increased as the cords and tubes developed; furthermore, PDGF-BB induced the incorporation of BrdU in the nuclei of cells associated with these structures. PDGF beta-receptor (PDGF-beta) mRNA increased concomitantly with cord/tube formation, and PDGFR-beta were specifically localized by immunocytochemistry to developing and mature cords and tubes. However, PDGFR-beta transcripts and protein were undetectable in nonangiogenic endothelial cells, and PDGF alpha-receptor mRNA was not expressed in either endothelial cell strain. In contrast to nonangiogenic endothelial cells, angiogenic endothelial cells did not express the PDGF B-chain, the required ligand for the PDGFR-beta. We conclude that (a) PDGF-BB can contribute to angiogenesis in vitro, (b) PDGFR-beta are specific for cord/tube-forming endothelial cells and mediate endothelial proliferation and cord/tube formation, and (c) in angiogenic and nonangiogenic endothelial cells, the expression of PDGFR-beta and PDGF B-chain is inversely correlated. We therefore suggest that paracrine PDGF might amplify angiogenesis via direct action on endothelially expressed PDGFR-beta.

The concept of a matricellular protein was first proposed by Paul Bornstein in the mid-1990s to account for the non-lethal phenotypes of mice with inactivated genes encoding thrombospondin-1, tenascin-C, or SPARC. It was also recognized that these extracellular matrix proteins were primarily counter or de-adhesive. This review reappraises the matricellular concept after nearly two decades of continuous investigation. The expanded matricellular family as well as the diverse and often unexpected functions, cellular location, and interacting partners/receptors of matricellular proteins are considered. Development of therapeutic strategies that target matricellular proteins are discussed in the context of pathology and regenerative medicine.