Long-term potentiation (LTP) and depression (LTD) at hippocampal CA1 synapses are widely studied due to their involvement in learning and memory processes that are altered in diseases including post- traumatic stress disorder, schizophrenia, Down syndrome, and Alzheimer's. LTP and LTD are induced by Ca2+ influx through NMDA glutamate receptors (NMDAR) and expressed by long-lasting increases or decreases, respectively, in AMPA glutamate receptor (AMPAR) function. AMPAR are tetrameric assemblies of GluA1- GluA4 subunits, and most synaptic AMPAR are composed of GluA1/2 or GluA2/3 with GluA2 preventing Ca2+ influx. However, a small number of Ca2+-permeable GluA1 homomers reside in extrasynaptic locations where they can be recruited to synapses during normal plasticity or under pathophysiological conditions. Phosphorylation of GluA1 S845 by the kinase PKA promotes GluA1 endosomal recycling exocytosis leading to accumulation of receptors in the extrasynaptic plasma membrane where they are primed for synaptic insertion during LTP. During LTD, the Ca2+-activated phosphatase calcineurin (CaN) dephosphorylates S845 and removes GluA1 via endocytosis. PKA and CaN are targeted to GluA1 through binding to A-kinase anchoring protein (AKAP) 79/150 (human79/rodent150). During the last funding period, we uncovered a novel role for palmitoylation of the AKAP N-terminal targeting domain in mediating its localization to postsynaptic lipid rafts and dendritic recycling endosomes. Importantly, we found that a palmitoylation deficient AKAP mutant altered regulation of spine enlargement, endosome recycling, and GluA1 trafficking associated with LTP. We also generated AKAP150?PIX knock-in mice that selectively disrupted CaN anchoring in vivo. We found that ?PIX mice lack LTD but express enhanced LTP at CA1 synapses. Basal GluA1 S845 phosphorylation was elevated and LTD-induced dephosphorylation and synaptic removal of GluA1 was impaired in ?PIX mice. Finally, basal synaptic activity of GluA2-lacking AMPAR was increased and pharmacologic antagonism of these receptors restored normal LTD and inhibited enhanced LTP in ?PIX mice. Based on these findings we propose a model where AKAP150-anchored CaN opposes PKA phosphorylation of GluA1 to limit trafficking and synaptic incorporation of Ca2+-permeable AMPAR. This model will be tested here using a variety of biochemical, fluorescence imaging, electrophysiological, and behavioral approaches in three specific aims.
Aim 1 will characterize regulation of AKAP79 and GluA1 endosomal trafficking and extrasynaptic vs. synaptic localization by palmitoyl-acyl transferases (PATs) focusing on DHHC2.
Aim 2 will use PKA-anchoring deficient AKAP150 ?PKA mice and CaN-anchoring deficient ?PIX mice to study the roles of anchored PKA and CaN in controlling accumulation of GluA2-lacking receptors in extrasynaptic locations from where they are recruited to synapses in LTP or removed in LTD.
Aim 3 will extend studies of AKAP150-PKA/CaN control of AMPAR phosphorylation and subunit composition to regulation of LTP by ?-adrenergic receptor activation of PKA and regulation of contextual fear memory extinction in vivo.
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