Localization and signaling of the receptor protein tyrosine kinase Tyro3 in cortical and hippocampal neurons.

Protein phosphorylation serves as a critical biochemical regulator of short-term and long-term synaptic plasticity. Receptor protein tyrosine kinases (RPTKs) including members of the trk, eph and erbB subfamilies have been shown to modulate signaling cascades that influence synaptic function in the central nervous system (CNS). Tyro3 is one of three RPTKs belonging to the "TAM" receptor family, which also includes Axl and Mer. Tyro3 is the most widely expressed of these receptors in the CNS. Despite recent advances suggesting roles for members of this receptor family in the reproductive and immune systems, their functions in the CNS remain largely unexplored. In an effort to elucidate the roles of Tyro3 and its ligand, the protein growth arrest-specific gene6 (Gas6) in the hippocampus and cortex, we performed a detailed study of the localization and signaling of Tyro3 polypeptides in rat hippocampal and cortical neurons. Tyro3 was readily detected in dendrites and in the soma where it was distributed in a punctate pattern. Tyro3 exhibited only a limited level of co-localization with postsynaptic density protein-95 (PSD-95), suggesting that while located within dendrites, it was not confined to the postsynaptic compartment. In addition, Tyro3 was also identified in the axons and growth cones of immature neurons. The prominent expression of Tyro3 in dendrites suggested that it may be capable of modulating signaling pathways triggered by synaptic transmission. We have provided evidence in support of this role by demonstrating that Gas6 induced the phosphorylation of Tyro3 in cortical neurons in vitro, resulting in the recruitment of the mitogen-activated protein kinase (MAPK) and the phosphoinositide-3 kinase (PI(3)K) signaling pathways. As these pathways play critical roles in the induction of hippocampal long-term potentiation (LTP), these findings suggest that Tyro3 signaling may influence synaptic plasticity in the dendritic compartment of hippocampal and cortical neurons.