BINDING AND FUNCTIONS OF RECEPTOR TYROSINE PHOSPHATASE B

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Cellular tyrosine phosphorylation plays a crucial role in the control of normal development and neoplasia. RPTP beta is a receptor protein tyrosine phosphatase that is expressed in glia in a pattern suggesting a role in morphogenesis and plasticity of the nervous system. Moreover, this protein binds to the extracellular matrix protein tenascin and to the neural cell adhesion molecules N-CAM and Ng. CAM/L1. The goal of this project is to analyze interactions of RPTP beta with various ligands and to study the consequences of these interactions on cell adhesion, and on transmembrane signalling that may modulate glial differentiation and interactions with neurons. The first specific aim is to characterize the expression patterns of RPTP beta in tissues and cells, and its interactions with different ligands during development. Then to analyze the binding properties of the different extracellular domains in RPTP beta cDNA constructs encoding different regions in RPTP beta will be used to express secreted and membrane-anchored forms of RPTP beta by transfection into mammalian cells. Molecular binding assays for secreted forms and cellular adhesion assays for forms expressed on the cell surface will be used to analyze which domains are important for binding of RPTP beta to different ligands such as tenascin and Ng-CAM/L1. To test potential functions of RPTP beta and its different domains in cells, molecular cloning techniques will be used to express or suppress expression of RFTP beta. In each case, effects of the treatment will be analyzed to detect changes in RPTP beta expression which will be correlated with alterations in cellular responses to ligands (i.e. tenascin and Ng-CAM) including cell adhesion, cell shape and cell division. RPTP beta is the first receptor/phosphatase with identified heterophilic ligands, and therefore it is important to investigate whether ligand binding to RPTP beta is involved in signal transduction by altering the specific activity of the phosphatase, by causing a redistribution of the phosphatase to alter its activity locally, or by changing the pattern of RPTP beta expression. This experimental approach will provide new information on the structure of binding regions of RPTP beta that may be involved in adhesion and growth regulation of normal and transformed astroglial cells. The results may also provide important clues for understanding development of radial glial cells and astrocytes, and their interactions with developing neurons, as well as potential roles of RPTP beta in growth of brain tumors.