The long-term goal of this research is to determine the role of tyrosine phosphatases and dual-specificity phosphatases in neurological disorders of the basal ganglia and related structures. Our recent findings indicate that a striatal enriched tyrosine phosphatase, STEP, specifically expressed in the neurons of the cortex, hippocampus and striatum may participate in cell survival following an excitotoxic insult. The activity of STEP is itself regulated by the neurotransmitters dopamine and glutamate through phosphorylation and dephosphorylartion of a critical serine residue within the kinase interacting motif or KIM domain. Glutamate/NMDA receptor mediated influx of Ca2+ activates STEP, whereas dopamine/D1 receptor mediated PKA activation leads to inactivation of STEP. The proposed study will now test whether STEP is activated through NR2B-NMDA receptors, a pool of NMDA receptor that is associated with glutamate excitotoxicity. It will also investigate whether active STEP in turn can down regulate or inhibit multiple interrelated pathways that are involved in glutamate/NMDA receptor-mediated cell death. We anticipate that active STEP, as part of a feed-back loop, can down regulate NR2B-NMDA receptor channel activity through tyrosine dephosphorylation of NR2B subunit, thereby inhibiting Ca2+ overload and subsequent cell damage. It can also inhibit the activation of the p38 MAP kinase signaling pathway that has been attributed to cell death in multiple neurodegenerative disorders. In this way STEP may promote cell survival following an initial insult. However depending upon the severity of the insult STEP may be proteolytically cleaved and eventually degraded, thereby facilitating activation of cell death pathways. The study will further investigate if a constitutively active form of STEP that cannot be proteolytically cleaved and can be delivered in vivo will be able to attenuate ischemic brain damage, where the involvement of glutamate excitotoxicity is well established. These studies will involve neuron culture experiments of glutamate toxicity and an animal model of ischemic stroke and will utilize biochemical, immunocytochemical and molecular biology techniques. The findings will help us to determine whether the tyrosine phosphatase STEP may be therapeutically beneficial and can be considered for the treatment of ischemic stroke and related neurological disorders. Project Narrative The goal of the proposed study is to understand the role of a brain-enriched and neuron-specific tyrosine phosphatase, STEP, in neuronal cell death following an excitotoxic insult. The findings will help us to determine whether STEP can attenuate excitotoxic neuronal cell death and can be considered as a potential therapeutic target for the treatment of ischemic stroke and related neurological disorders.
