Regulation of AKAP79 Postsynaptic Targeting and Signaling Long-term potentiation (LTP) and depression (LTD) at hippocampal CA1 synapses are widely studied due to their involvement in spatial learning and memory processes that are altered in diseases including schizophrenia, Alzheimer's, and post-traumatic stress disorder. LTP and LTD are induced by postsynaptic NMDA glutamate receptor signaling and expressed by long-lasting increases or decreases, respectively, in AMPA glutamate receptor (AMPAR) function. AMPARs are tetrameric assemblies of GluA1-GluA4 subunits, and most synaptic AMPARs are composed of GluA1/2 or GluA2/3, with the GluA2 subunit preventing Ca2+ influx. However, a small number of Ca2+-permeable GluA1 homomers (CP-AMPARs) reside in extrasynaptic locations where they can be recruited to synapses during some forms of plasticity. One leading model proposes that phosphorylation of GluA1 S845 by the cAMP-dependent protein kinase (PKA) promotes GluA1 endosomal recycling leading to accumulation of CP-AMPARs in the extrasynaptic plasma membrane, where they are primed for synaptic insertion during LTP in response to additional signals from Ca2+-calmodulin- dependent protein kinase II (CaMKII). In contrast, during LTD, it is proposed that the Ca2+-activated protein phosphatase-2B/calcineurin (CaN) dephosphorylates extrasynaptic GluA1 and remove these CP-AMPARs via endocytosis. However, the importance of CP-AMPARs for plasticity at CA1 synapses is still controversial, and the mechanisms regulating AMPAR subunit composition are not well understood. This uncertainty has arisen in part because many studies only target AMPARs as the endpoint of plasticity regulation without elucidating how upstream signaling is controlled. Here we precisely target specific postsynaptic signaling pathways to overcome this limitation. PKA and CaN are both targeted to GluA1 through binding to the postsynaptic scaffold protein, A-kinase anchoring protein (AKAP) 79/150. During the last funding period we generated and characterized AKAP150 ?PIX and ?PKA mice knock-in mice that selectively disrupt CaN or PKA anchoring in vivo, respectively. Through characterization of these AKAP mutant mice we confirmed parts of this model by showing postsynaptic CaN anchoring is critical for LTD and constrains LTP, while PKA anchoring modulates both LTP and LTD. However, a number of our recent findings challenge current models by showing that CP- AMPARs may be transiently recruited to synapses and required for LTD, as in some forms of LTP. Also challenging conventional wisdom, we found that CaMKII activation is not only required for LTP but also for LTD induction. Thus, In Aim 1 we will test the novel hypothesis that LTD induction requires AKAP-PKA and CaMKII-dependent CP-AMPAR recruitment to synapses that is followed by AKAP-CaN mediated removal.
In Aim 2 we further explore whether AKAP79/150 palmitoylation by DHHC2, which targets it to recycling endosomes, is required to coordinate this PKA/CaN regulation of GluA1 CP-AMPAR trafficking during LTP/LTD. Finally, In Aim 3 we will test the hypothesis that alterations in CP-AMPAR regulation of hippocampal synaptic plasticity in AKAP150 mutant mice result in changes in contextual fear learning and memory. We will test these hypotheses using a combination of biochemical, fluorescence imaging, electrophysiological, and behavioral approaches.
The AKAP79/150-organized neuronal excitatory postsynaptic signaling processes we are studying that regulate AMPA-type glutamate receptor trafficking and subunit composition control Ca2+-permeable AMPA receptor activity are believed to be relevant for mechanisms of altered synaptic plasticity, impaired cognition, and neuronal cell loss in neurological disorders such as Alzheimer's, epilepsy, and in mental health disorders such as Down syndrome, schizophrenia, autism, post-traumatic stress disorder (PTSD), and drug addiction. These same pathways also have relevance for understanding how excessive glutamate receptor activation leads to excitotoxic neuronal death in traumatic brain injury and stroke. In particular, regulation of AMPA receptor subunit composition, to increase the synaptic activity of Ca2+-permeable AMPA receptors, has been implicated in changes in plasticity associated with drug addiction, stroke, epilepsy, and PTSD. Thus, understanding the role of AKAP79/150 in controlling Ca2+- permeable AMPA receptor activity and trafficking is important for understanding basic synaptic processes that underlie normal learning and memory and are altered in human disease.
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