Roles of Tyrosine Phosphorylation in The Development and Plasticity of The Central Nervous System

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We performed three types of studies that are (1) the search of the novel tyrosine kinases expressed in the central nervous system, (2) identification of proteins that are respectively involved in Lyn and ALK tyrosine kinases-mediated signaling pathways in the central nervous system, (3) characterization of a protein-tyrosine phosphatase PTPMEG that associates with ionotropic glutamate receptors. The followings are the summary of the study.(1) By performing differential screening of RT-PCR products and in silico screening, we identified novel kinases that included the one termed BREK (brain enriched kinase). We also identified a homologue of BREK termed BREK2. BREK and BREK2 are similar to AATYK that has been reported to be a tyrosine kinase involved in regulation of apoptotic death of neurons. Unlike AATYK, both BREK and BREK2 are dual specificity kinases showing strong serine/threonine and weak tyrosine kinase activities. Expression of BREK is high in the central nervous system, such as in cerebral cortex and olfactory bulb. BREK becomes phosphorylated upon NGF stimulation of cultured neuronal cells, suggesting that BREK is involved in neurite extension.(2) By employing filter kinase assay and yeast two-hybrid screening, we identified BANK and SNT2 as substrates of Lyn and ALK, respectively. BANK is an adaptor protein that links Lyn to IP_3 receptor (IP_3R). We showed that formation of Lyn/BANK/IP_3R resulted in tyrosine phosphorylation of IP_3R, which is known to upregulate the channel activity of IP_3R. Regarding SNT2, we showed that SNT2 was critically important for ALK-mediated signaling.(3) We also identified various signaling molecules that were associated with ionotropic glutamate receptors. One of them is PTPMEG that associates with GluRd2 and NMDA receptor subunits NR2A. PTPMEG is expressed high in the thalamus and affects the level of tyrosine phosphorylation of NR2A, suggesting that PTPMEG is involved in controlling synaptic plasticity.