Roles of GSK-3beta and PYK2 signaling pathways in synaptic plasticity

Abstract

Activity-dependent modification of synapses, as in long term potentiation (LTP) or long term depression (LTD), is widely believed to be a crucial mechanism for learning and memory. Molecular perturbations in these processes may underlie certain neuropsychiatric conditions. This thesis examines the role of two signaling pathways, glycogen synthase kinase 3 beta (GSK- 3beta) and proline-rich tyrosine kinase 2 (PYK2), in LTD at rat hippocampal synapses. GSK-3beta, a serine/threonine kinase implicated in the pathophysiology of schizophrenia, mood disorders, and Alzheimer's disease, is known to play a critical role in LTD. Here we report that GSK-3beta phosphorylates the postsynaptic scaffold protein PSD-95, a major determinant of synaptic strength, at the Thr- 19 residue. In hippocampal neurons, this promotes the activity-dependent dispersal of synaptic PSD-95 clusters. We found that overexpression of a phospho-null mutant (Ti 9A-PSD-95), but not a phospho-mimic mutant, blocks LTD without affecting basal synaptic function relative to wild type PSD-95 overexpression. Thus PSD-95 phosphorylation by GSK-3beta is a necessary step in LTD. [This project is a collaboration with Myung Jong Kim, and I am second author of the manuscript.] PYK2 is a calcium-dependent tyrosine kinase that is activated in cerebral ischemia and seizures. PYK2 is also known to bind PSD-95 at a region implicated in LTD signaling. Here we report a novel role for PYK2 in LTD. Chemical LTD treatment induces PYK2 phosphorylation at Tyr-402, and small hairpin RNA-mediated knockdown of PYK2 blocks LTD, but not LTP. We identify both enzymatic and non-enzymatic (scaffolding) roles for PYK2 in LTD, and find that PYK2 is required to suppress activity-dependent phosphorylation of the mitogen activated protein kinase ERK. ERK activity is believed to promote glutamate receptor insertion at synapses. Overexpression of WT-PYK2 further depresses activity-dependent ERK phosphorylation, and inhibits LTP, but not LTD. Our studies support a model whereby PYK2 antagonizes ERK signaling to promote LTD, at the expense of LTP, in hippocampal neurons. [This project is a collaboration with Myung Jong Kim and Chi-Fong Wang, and I am first author of the manuscript.]