Posttranslational regulation of AMPA receptor trafficking and function. Curr Opin Neurobiol

In the mammalian central nervous system, the majority of fast excitatory synaptic transmission is mediated by glutamate acting on AMPA-type ionotropic glutamate receptors. The abundance of AMPA receptors at the synapse can be modulated through receptor trafficking, which dynamically regulates many fundamental brain functions, including learning and memory. Reversible posttranslational modifications, including phosphorylation, palmitoylation and ubiquitination of AMPA receptor subunits are important regulatory mechanisms for controlling synaptic AMPA receptor expression and function. In this review, we highlight recent advances in the study of AMPA receptor posttranslational modifications and discuss how these modifications regulate AMPA receptor trafficking and function at synapses. Introduction The amino acid glutamate is the predominant excitatory neurotransmitter in the brain. After release from presynaptic nerve terminals, glutamate binds to postsynaptic ionotropic glutamate receptors to mediate excitatory synaptic transmission. These ionotropic glutamate receptors are tetrameric cation channels consisting of three distinct subtypes: AMPA, NMDA and kainate receptors [1]. Among them, AMPA receptors conduct the majority of fast, moment-to-moment synaptic transmission and are the primary driving force for postsynaptic depolarization. The abundance of AMPA receptors at synapses is a crucial factor in determining synaptic efficacy. Convincing evidence has demonstrated that AMPA receptors undergo activity-dependent recruitment to or removal from synapses during synaptic plasticity, including long-term potentiation (LTP), long-term depression (LTD) or homeostatic plasticity, to ensure proper synaptic communication [2][3] AMPA receptors are assembled from four subunits, GluA1-A4 (previously termed GluR1-4 or GluR-A to GluR-D) to form homomeric or heteromeric channels [1, Here we will review the latest developments of AMPA receptor phosphorylation, palmitoylation and ubiquitination, in which there has been significant progress over the past few years. We will discuss the modulation of channel biophysical properties by AMPA receptor phosphorylation and examine the role of phosphorylation of individual AMPA receptor subunits in receptor trafficking and synaptic plasticity. In addition, we will review recent studies on AMPA receptor palmitoylation and ubiquitination. Finally, we will highlight how different posttranslational modifications interact with each other to modulate AMPA receptors. Other AMPA receptor Phosphorylation Phosphorylation is a ubiquitous mechanism crucial for the rapid regulation of trafficking and function of ion channels. Indeed, AMPA receptor phosphorylation is an important regulatory mechanism that controls many aspects of receptor function GluA1 Previous studies have shown that phosphorylation of the GluA1 C-tail directly modulates the receptor's biophysical properties. For example, phosphorylation at S831, a PKC and CaMKII site S1 S1 Current Opinion in Neurobiology Loop2 Loop2 Topology and posttranslational modification sites of AMPA receptor subunits. Each AMPA receptor subunit is an integral membrane protein with an extracellular amino terminal domain (NTD), three transmembrane domains (M1, M3 and M4), a hydrophobic hairpin domain (M2), which underlies the formation of the channel pore, and three intracellular domains, Loop1, Loop2 and the carboxyl-tail (C-tail). The S1 domain in the N-terminus and the S2 domain that links M3 and M4 underlie the formation of glutamate binding domain. TARPs In addition to modulating AMPA receptor channel biophysics, phosphorylation of GluA1 functions in synaptic plasticity through the regulation of receptor trafficking. The role of S831 phosphorylation in AMPA receptor trafficking remains unclear. It has been shown that GluA1 lacking phosphorylation on S831 can be delivered to synapses in a CaMKII-dependent manner In contrast to LTP, LTD is crucially dependent on S845, but not on S831 phosphorylation, as knockin mice lacking phosphorylation at S845, but not at S831, have deficits in hippocampal LTD More recently, two additional phosphorylation sites in the GluA1 C-tail, S818 and T840, have been identified In addition to the multiple phosphorylation sites on the GluA1 C-tail, recently the GluA1 Loop1 domain was shown to be phosphorylated on S567. Biochemical studies demonstrated that CaMKII phosphorylates S567, although it is a relatively weak substrate compared to S831 in the C-tail It largely remains unclear how phosphorylation events on GluA1 engage AMPA receptor trafficking machinery to regulate receptor targeting and synaptic plasticity. One possibility is that phosphorylation may modulate GluA1-mediated protein-protein interactions that function in AMPA receptor trafficking. Indeed, one elegant study recently showed that PKC phosphorylation at GluA1 S818 enhances the GluA1 interaction with a neuronal specific actin-binding protein 4.1N, which leads to increased exocytosis of AMPA receptors at extrasynaptic membranes and underlies synaptic delivery of AMPA receptors during LTP induction The regulation of AMPA receptor trafficking and synaptic plasticity by GluA1 phosphorylation has important implications for animal cognition. For example, in double knockin mice lacking phosphorylation at the S831 and S845 sites, both the retention of newly acquired spatial memory and norepinephrine-mediated enhancement of contextual learning are impaired [34,35]. In addition, a recent study from Huganir's group uncovers an imperative role for GluA1 S845 phosphorylation in fear memory erasure GluA2 The vast majority of AMPA receptors in the forebrain contain GluA2 Phosphorylation at S880, mediated by PKC, differentially regulates the GluA2 interaction with GRIP/ABP and PICK1 In addition to S880, recent studies implicate a role for GluA2 tyrosine phosphorylation in AMPA receptor trafficking and plasticity. An early study demonstrated that GluA2 is tyrosine phosphorylated at tyrosine 876 (Y876) both in vitro and in vivo by the Src family tyrosine kinases Palmitoylation Protein S-palmitoylation is a posttranslational modification whereby palmitic acid is covalently attached to intracellular cysteine residues of the protein substrates by palmitoyl acyltransferase (PAT), primarily mediated by a large DHHC (Asp-His-His-Cys) protein family All four AMPA receptor subunits are palmitoylated at two conserved sites, one within the pore domain (C585 in GluA1), and the other one in the C-tail juxtamembrane region (C811 in GluA1) AMPA receptor palmitoylation is highly regulated by neuronal activity. Glutamate stimulation rapidly induces depalmitoylation of AMPA receptors without affecting total receptor levels in neuronal cultures Ubiquitination Ubiquitination is an evolutionally conserved posttranslational process that attaches a single ubiquitin or polymeric ubiquitin chains to lysine residues of a substrate protein (reviewed in Early studies have indicated a crucial role for ubiquitination-mediated signaling in regulating AMPA receptor trafficking and turnover Which E3 ligase(s) mediates AMPA receptor subunit ubiquitination? Recently, Nedd4 (neural precursor cell expressed, developmentally downregulated) has been identified as an E3 ligase that ubiquitinates GluA1 [76 ,78 ]. Nedd4 was originally isolated from a screen from embryonic mouse brain www.sciencedirect. Cross-talk among posttranslational modifications An emerging concept is that different posttranslational modifications functionally cross-talk with each other to provide fine regulation of trafficking and function of the target proteins. For example, it has been demonstrated that palmitoylation of the b2-adrenergic receptor, the kainate receptor GluK2 subunit or NMDA receptor GluN2 subunits modulates their phosphorylation levels, thus altering receptor trafficking and function Recent studies also demonstrate the significance of such cross-talk among posttranslational modifications in the regulation of AMPA receptor trafficking. One elegant example is the functional interaction between C811 palmitoylation and nearby S818 phosphorylation in the GluA1 C-tail in controlling AMPA receptor trafficking In addition to phosphorylation, palmitoylation, and ubiquitination, two other posttranslational modifications, sumoylation and nitrosylation, have recently been demonstrated to play key roles in modulating synaptic protein function Conclusions Reversible posttranslational modifications, including phosphorylation, palmitoylation and ubiquitination, have been shown to control various aspects of AMPA receptor trafficking and functional modulation. Recent advances highlight how posttranslational modifications regulate proteinprotein interactions that mediate receptor trafficking and how different posttranslational modifications interact with each other to precisely regulate AMPA receptor-mediated synaptic transmission. It will continue to be important to identify molecular and biochemical mechanisms underlying posttranslational modifications of AMPA receptor trafficking and function, which will provide key insights into the mechanisms for synaptic plasticity. In addition, it will be equally important to determine the local signal transduction pathways that give rise to posttranslational modifications of AMPA receptors in a highly spatially and temporally controlled manner