Cell surface heparan sulfate proteoglycans (HSPGs) are abundant, ubiquitous molecules, that bind via heparan sulfate (HS) chains to growth factors, extracellular matrix proteins, and other important cellular effectors. The major cell surface HSPGs are products of two gene families: the syndecans, which are transmembrane proteins, and the glypicans, which are glycosylphosphatidylinositol-anchored. Four syndecans and at least five glypicans are expressed in mammals, including man. The biological functions of cell surface HSPGs are poorly understood, but at least one function involves regulation of the responses of cells to polypeptide growth factors. Recently, a human birth defects syndrome associated with dysregulated tissue growth was shown to be caused by null mutations in glypican-3. Genetic studies in the fruitfly, Drosophila, have also linked loss of glypican function to defects in tissue growth and patterning. One tissue in which glypicans are likely to play especially important development roles is the nervous system. Experiments with cultured neutral cells and lower organisms have long pointed to crucial roles for HSPGs in neutral patterning, axon guidance, and synapse formation. Previous work on this project has demonstrated that glypicans-1 and -2 are among the most abundant HSPGs in the mammalian brain, and are expressed in locations such as neutral precursor zones, growing axons, and synaptic terminal fields, that are consistent with playing such developmental roles. The goals for the next project period focus on uncovering the functions of glypicans-1 and -2 in the mammalian nervous system, and on relating glypican function to structural features of the glypicans, such as the presence of HS chains, the mode of membrane anchorage, and presence of a large N- terminal globular domain that does not carry HS, yet has been highly conserved throughout evolution. Functional experiments involve the analysis of mice engineered to lack glypican-1, glypican-2, or both. Glypican-2 null mice have already been made, and the generation of glypican-1 null mice is in progress. Analysis of structure-function relationships in glypicans will use the fruitfly as an experimental system in which genes encoding altered glypicans can be rapidly and quantitatively tested for function. Finally, the results of observing mutant phenotypes and making structure/function correlations will be used to direct a genetic and biochemical search for the ligands with which glypicans interact in mammalian nervous system development. These studies should provide insights into how an important and poorly understood class of molecules regulates basic developmental processes, particularly in the nervous system.
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