The AMPA-type glutamate receptors are ligand-gated cation channels that mediate fast excitatory neurotransmission in the brain, and thus are critically involved in all aspects of brain function including cognition, movement, learning, and memory. The function and number of postsynaptic AMPA receptors are dynamically regulated to control the strength of synaptic connections, and this plasticity is a key feature of cellular models of learning and memory. Signals that trigger synaptic plasticity lead to phosphorylation of AMPA receptors by protein kinases, and this phosphorylation controls AMPA receptor trafficking and function. Phosphorylation by protein kinase C (PKC) or Ca2+/calmodulin dependent kinase II (CamKII) of an intracellular serine residue (Ser831) located on the GluR1 subunit enhances AMPA receptor function to increase synaptic strength during expression of long-term potentiation (LTP), one model of synaptic plasticity. Although previous studies observed that CamKII phosphorylation of GluR1 enhances the single channel conductance, no conceptual or structural mechanism has been described for this unique form of ion channel regulation. The goal of the experiments proposed here is to understand functionally, structurally, and conceptually how phosphorylation of GluR1 Ser831 potentiates AMPA receptor function. We will focus on Ser831 in GluR1 because of the unique mechanism of potentiation (increased unitary conductance), and will expand the study to evaluate for the first time how three nearby phosphorylation sites (Ser818, Thr840, Ser845) might functionally interact with phospho-Ser831. Furthermore, we will test whether the effects of phospho-Ser831 reflect either intra-protein interactions between the phospho-Ser831 and intracellular portions of the receptor, or inter-protein interactions between phospho-Ser831 and GluR1 binding partners. Completion of these studies will provide a comprehensive functional and structural understanding of an under-studied feature of synaptic plasticity-phosphorylation mediated changes in postsynaptic AMPA channel function. The proposed experiments address three questions: 1. What is the mechanism by which phosphorylation regulates AMPA receptor function? Single channel currents will be recorded to determine how phosphorylation of Ser831 controls GluR1 function. We will also evaluate the interactions of Ser831 with nearby phosphorylation sites, and validate our conclusions in neurons. 2. What is the structural basis for phospho-serine regulation of AMPA receptor function? We will identify intracellular GluR1 residues as phospho-Ser831 hydrogen bonding partners. We will additionally search for inter-protein interactions involving GluR1 that depend on the phosphorylation of Ser831. 3. Can models of independent subunit gating describe AMPA receptor regulation by phosphorylation? We will analyze the response of patches with one active GluR1 channel (plus stargazin) to the rapid application of a maximally effective concentration of glutamate. These data will be used to develop a novel model of subunit gating that can account for the potentiation of GluR1 channel function by phosphorylation of Ser831.
AMPA receptors mediate communication between neurons in the central nervous system, and thus play an important role in virtually all brain functions. The AMPA receptors are comprised of four different subunits (GluR1-4). Among these, the GluR1 subunit has been shown to play a unique role in activity-dependent synaptic plasticity. GluR1 is subject to C-terminal phosphorlyation by a variety of kinases, and this phosphorylation can influence trafficking to the membrane and AMPA receptor function. In this proposal we examine CamKII phosphorylation of GluR1-Ser831. CamKII activation has been shown to be a critical step in some forms of synaptic plasticity, presumably through phosphorylation of the GluR1 subunit. Phosphorylation of GluR1-Ser831 increases single channel conductance by an unknown mechanism. This proposal describes three series of experiments that will evaluate the underlying functional and structural mechanisms of the effects on receptor function following phosphorylation at GluR1-Ser831. Understanding how AMPA receptor function is sculpted by intracellular signaling pathways is an important step towards understanding the mechanisms of synaptic plasticity, which likely underlie higher order functions such as learning and memory.
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