Tyrosine kinases can affect growth, differentiation, and survival of neurons and glia. Furthermore, in animal models, they have been shown to affect learning and memory. Receptor tyrosine kinases transduce a variety of extracellular signals resulting in activation of their intracellular tyrosine kinase domain. A new member of this receptor family, Tyro-3, is of special interest given its localization, structure and potential activation by neural activity. Preliminary evidence presented here indicates that Tyro-3 is preferentially localized to dendrites of cortical and hippocampal neurons of area Ca1. Unique among receptor tyrosine kinases is the strong structural similarity between its extracellular domains and those of neural cell adhesion molecules suggesting a functional role in adhesive cell-cell interactions. These interactions are crucial for establishing correct functional neuroanatomy during development and in the adult animal. Tyro-3 can be phosphorylated by the binding of its ligand gas6 to the extracellular domain of this receptor. Gas6 (growth arrest specific gene-6) is a molecule shown to prevent apoptosis and to have mitogenic properties, functions that in the nervous system could be mediated by Tyro-3. As first shown in this proposal, Tyro-3 can also become activated by the excitatory neurotransmitter glutamate when added to hippocampal cultures. We propose: (a) To determine the developmental patterns of expression of Tyro-3 and its ligand gas6 in the nervous system, and the cellular and subcellular distribution of Tyro-3. This will provide important initial cues for elucidating cellular functions of the gas6/Tyro-3 ligand/receptor pair and clarify its involvement in synaptic or extrasynaptic processes. (b) To test the hypothesis that Tyro-3 can bind to itself or other molecules at the cell surface functioning as a cell adhesion molecule, and to examine how gas6 can modulate these interactions. We will also determine if adhesive events result in activation of the tyrosine kinase domain. (c) To identify the critical parameters that affect tyrosine phosphorylation of Tyro-3 upon stimulation of hippocampal neurons by the excitatory neurotransmitter glutamate, and to determine if Tyro-3 activation occurs in the absence of extracellular binding ligands. Through these efforts we hope to uncover the role of Tyro-3 in molecular mechanisms of cell adhesion, intracellular signaling and neuroplasticity in the normal and diseases nervous system.
