The establishment of neural circuitry requires vast numbers of synapses to be generated during a specific window of brain development, but it is not known why the developing mammalian brain has a much greater capacity to generate new synapses than the adult brain. Here we report that immature but not mature astrocytes express thrombospondins (TSPs)-1 and -2 and that these TSPs promote CNS synaptogenesis in vitro and in vivo. TSPs induce ultrastructurally normal synapses that are presynaptically active but postsynaptically silent and work in concert with other, as yet unidentified, astrocyte-derived signals to produce functional synapses. These studies identify TSPs as CNS synaptogenic proteins, provide evidence that astrocytes are important contributors to synaptogenesis within the developing CNS, and suggest that TSP-1 and -2 act as a permissive switch that times CNS synaptogenesis by enabling neuronal molecules to assemble into synapses within a specific window of CNS development.

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R current appreciation of the involvement of hrombospondin 1 (TSP1) 1 in diverse biological processes (Fig. 1; 1, 3, 25) extends far beyond the role initially attributed to the protein in platelet aggregation and coagulation. This diversity of function has led to considerable confusion in the literature and skepticism among scientists that a single protein can subserve such protean, sometimes conflicting, functions. In this Commentary I will attempt to show that the ability of matricellular proteins, as defined below, to interact with a wide range of both matrix proteins and cell surface receptors can explain the complex biological functions of TSP1 and resolve many of the controversies regarding its mode of action. A conclusion of this analysis is that, even if allowances are made for some errors in experimentation and interpretation, the majority of the reported functions of TSP1, divergent as they appear to be, are likely to be correct.The term "matricellular" is used in this analysis to refer to a group of modular, extracellular proteins whose functions are achieved by binding to matrix proteins as well as to cell surface receptors, or to other molecules such as cytokines and proteases that interact, in turn, with the cell surface. In addition to TSP1, this group is likely to include members of the tenascin protein family, SPARC/osteonectin and its relatives, and osteopontin. Although matricellular proteins can be associated with structural elements such as collagen fibrils or basement membranes, it is presumed that they do not contribute to the structural integrity of these elements. An association could, nevertheless, serve to sequester matricellular proteins, and provide a source of the proteins for subsequent recruitment to the cell surface. It should be noted that the distinction between structural matrix and matricellular proteins is not complete, since proteins such as fibronectin and laminin, which do serve as integral components of structural elements, also have adhesive functions and play biological roles that partially overlap those of matricellular proteins. Furthermore, matricellular proteins may participate in the formation of structural complexes under some circum

Thrombospondin (TSP) 2, and its close relative TSP1, are extracellular proteins whose functions are complex, poorly understood, and controversial. In an attempt to determine the function of TSP2, we disrupted the Thbs2 gene by homologous recombination in embryonic stem cells, and generated TSP2-null mice by blastocyst injection and appropriate breeding of mutant animals. Thbs2−/− mice were produced with the expected Mendelian frequency, appeared overtly normal, and were fertile. However, on closer examination, these mice displayed a wide variety of abnormalities. Collagen fiber patterns in skin were disordered, and abnormally large fibrils with irregular contours were observed by electron microscopy in both skin and tendon. As a functional correlate of these findings, the skin was fragile and had reduced tensile strength, and the tail was unusually flexible. Mutant skin fibroblasts were defective in attachment to a substratum. An increase in total density and in cortical thickness of long bones was documented by histology and quantitative computer tomography. Mutant mice also manifested an abnormal bleeding time, and histologic surveys of mouse tissues, stained with an antibody to von Willebrand factor, showed a significant increase in blood vessels. The basis for the unusual phenotype of the TSP2-null mouse could derive from the structural role that TSP2 might play in collagen fibrillogenesis in skin and tendon. However, it seems likely that some of the diverse manifestations of this genetic disorder result from the ability of TSP2 to modulate the cell surface properties of mesenchymal cells, and thus, to affect cell functions such as adhesion and migration.