Proteoglycans (PGs), proteins that bear distinctive polysaccharides known as glycosaminoglycans, are widely distributed on cell surfaces and in the extracellular matrix. Many studies suggest that cell-surface PGs play critical roles in development, and this is thought to reflect the specific interactions of PGs and glycosaminoglycans with growth factors and extracellular matrix proteins. In the nervous system in particular, evidence suggests that PGs are involved in a variety of developmental events. To understand the specific roles of PGs in nervous system development, the structures, locations and mechanisms of action of neural PGs must be defined. Toward this end, two of the major cell-surface PGs of the developing rat brain have been isolated and cDNA clones that encode their protein cores have been obtained. Both molecules are heparan sulfate PGs, both are expressed by neurons, and one is expressed exclusively in the nervous system and exclusively during development. Interestingly, the nervous system-specific PG can be isolated in a form that exhibits a high binding affinity for the extracellular matrix molecule fibronectin, suggesting that it might be a neuronal receptor for fibronectin. The temporal and spatial pattern of expression of mRNA for both PGs suggest that both are present in cells that interact with fibronectin in the developing brain. Experiments are proposed to address key questions about the biological properties of these two brain PGs and their roles in nervous system development. The studies will focus on the fetal rat forebrain, where these molecules are strongly expressed, and where fibronectin is suspected to play a role in guiding nerve fiber growth during development. The questions that will be addressed are: What forebrain cells express these PGs, and what cells make forms with high and low affinity for fibronectin? Monospecific antibodies will be used to identify and isolate PGs from cultured cells and measure PG- fibronectin affinities. What role do these PGs play in the interactions of forebrain neurons with fibronectin? Tissue culture assays will be used to pursue evidence that heparan sulfate PGs influence axon extension by fetal cerebral cortex neurons in response to fibronectin. What is the structural basis for PG forms that exhibit high affinity for fibronectin? Experiments will determine whether high affinity binding is mediated by glycosaminoglycan chains, core proteins, or both. The possibility that variations in glycosaminoglycan structure are important for high affinity binding will be studied, and the role of core proteins in specifying these variations will be tested. Such studies should provide insights into molecular mechanisms of mammalian brain development and will be relevant to the pathogenesis and treatment of developmental abnormalities.
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