AMPA receptor

The α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor is an ionotropic glutamate receptor and predominantly sodium ion channel that mediates fast excitatory neurotransmission in the central nervous system. Its activation by the neurotransmitter glutamate facilitates rapid neuronal communication, essential for various brain functions, including learning and memory. Its name is derived from the ability to be activated by the artificial glutamate analog AMPA. The receptor was initially named the "quisqualate receptor" by Watkins and colleagues after the naturally occurring agonist quisqualate. Later, the receptor was designated as the "AMPA receptor" following the development of the selective agonist AMPA by Tage Honore and colleagues at the Royal Danish School of Pharmacy in Copenhagen. The GRIA2-encoded AMPA receptor ligand binding core was the first glutamate receptor ion channel domain to be crystallized.

Structure and function

Subunit composition

AMPARs are composed of four types of subunits encoded by different genes, designated as GRIA1, GRIA2, GRIA3, and GRIA4, which combine to form a tetrameric structure. Most AMPARs are heterotetrameric, consisting of symmetric 'dimer of dimers' of GluA2 and either GluA1, GluA3 or GluA4. Dimerization starts in the endoplasmic reticulum with the interaction of N-terminal LIVBP domains, then "zips up" through the ligand-binding domain into the transmembrane ion pore.
The conformation of the subunit protein in the plasma membrane caused controversy for some time. While the amino acid sequence of the subunit indicated that there seemed to be four transmembrane protein domains, proteins interacting with the subunit indicated that the N-terminus were extracellular, while the C-terminus were intracellular. However, if each of the four transmembrane domains went all the way through the plasma membrane, then the two termini would have to be on the same side of the membrane. It was eventually discovered that the second "transmembrane" domain does not fully traverse the membrane but instead forms a reentrant helix-loop, contributing to the ion-conducting pore of the receptor. The domain kinks back on itself within the membrane and returns to the intracellular side. When the four subunits of the tetramer come together, this second membranous domain forms the ion-permeable pore of the receptor. The M2 loop plays a crucial role in forming the ion channel's selectivity filter, with the helical portions of M2 contributing to hydrophobic interfaces between AMPAR subunits in the ion channel.
AMPAR subunits differ most in their C-terminal sequence, which determines their interactions with scaffolding proteins. All AMPARs contain PDZ-binding domains, but which PDZ domain they bind to differs. For example, GluA1 binds to SAP97 through SAP97's class I PDZ domain, while GluA2 binds to PICK1 and GRIP/ABP. Of note, AMPARs cannot directly bind to the common synaptic protein PSD-95 owing to incompatible PDZ domains, although they do interact with PSD-95 via stargazin.
Phosphorylation of AMPARs can regulate channel localization, conductance, and open probability. GluA1 has four known phosphorylation sites at serine 818, S831, threonine 840, and S845. S818 is phosphorylated by protein kinase C and is necessary for long-term potentiation. S831 is phosphorylated by CaMKII and PKC during LTP, which helps deliver GluA1-containing AMPAR to the synapse, and increases their single channel conductance. The T840 site was more recently discovered, and has been implicated in LTD. Finally, S845 is phosphorylated by protein kinase A which regulates its open probability.

Mechanism of Action

AMPA receptors are integral to fast excitatory neurotransmission in the CNS. Each receptor is a tetramer composed of four subunits, each providing a binding site for agonists like glutamate. The ligand-binding domain is formed by the N-terminal tail and the extracellular loop between transmembrane domains three and four. The subunit composition significantly influences the receptor's functional properties, including ion permeability and gating kinetics.

Agonist Binding and Channel Activation

Upon glutamate binding, these two loops move towards each other, leading to pore opening. The channel opens when two sites are occupied, and increases its current as more binding sites are occupied. This opening allows the influx of sodium and, depending on subunit composition, calcium ions into the postsynaptic neuron, leading to depolarization and the propagation of excitatory signals. Once open, the channel may undergo rapid desensitization, stopping the current.

Desensitization Mechanism

The mechanism of desensitization is due to a small change in angle of one of the parts of the binding site, closing the pore. AMPARs open and close quickly, and are thus responsible for most of the fast excitatory postsynaptic transmission in the central nervous system.

Subunit Composition and Ion Permeability

The AMPAR's permeability to calcium and other cations, such as sodium and potassium, is governed by the GluA2 subunit. If an AMPAR lacks a GluA2 subunit, then it will be permeable to sodium, potassium, and calcium. The presence of a GluA2 subunit will render the channel impermeable to calcium. This is determined by post-transcriptional modification — RNA editing — of the Q-to-R editing site of the GluA2 mRNA. Here, A→I editing alters the uncharged amino acid glutamine to the positively charged arginine in the receptor's ion channel. The positively charged amino acid at the critical point makes it energetically unfavorable for calcium to enter the cell through the pore. Almost all of the GluA2 subunits in CNS are edited to the GluA2 form. This means that the principal ions gated by AMPARs are sodium and potassium, distinguishing AMPARs from NMDA receptors, which also permit calcium influx. Both AMPA and NMDA receptors, however, have an equilibrium potential near 0 mV. The prevention of calcium entry into the cell on activation of GluA2-containing AMPARs is proposed to guard against excitotoxicity.
The subunit composition of the AMPAR is also important for the way this receptor is modulated. If an AMPAR lacks GluA2 subunits, then it is susceptible to being blocked in a voltage-dependent manner by a class of molecules called polyamines. Thus, when the neuron is at a depolarized membrane potential, polyamines will block the AMPAR channel more strongly, preventing the flux of potassium ions through the channel pore. GluA2-lacking AMPARs are, thus, said to have an inwardly rectifying I/V curve, which means that they pass less outward current than inward current at equivalent distance from the reversal potential. Calcium permeable AMPARs are found typically early during postnatal development on neocortical pyramidal neurons, some interneurons, or in dopamine neurons of the ventral tegmental area after the exposure to an addictive drug.
Alongside RNA editing, alternative splicing allows a range of functional AMPA receptor subunits beyond what is encoded in the genome. In other words, although one gene is encoded for each subunit, splicing after transcription from DNA allows some exons to be translated interchangeably, leading to several functionally different subunits from each gene.
The flip/flop sequence is one such interchangeable exon. A 38-amino acid sequence found prior to the fourth membranous domain in all four AMPAR subunits, it determines the speed of desensitization of the receptor and also the speed at which the receptor is resensitized and the rate of channel closing. The flip form is present in prenatal AMPA receptors and gives a sustained current in response to glutamate activation.

Synaptic plasticity

AMPA receptors are both glutamate receptors and cation channels that are integral to plasticity and synaptic transmission at many postsynaptic membranes. One of the most widely and thoroughly investigated forms of plasticity in the nervous system is known as long-term potentiation. There are two necessary components of LTP: presynaptic glutamate release and postsynaptic depolarization. Therefore, LTP can be induced experimentally in a paired electrophysiological recording when a presynaptic cell is stimulated to release glutamate on a postsynaptic cell that is depolarized. The typical LTP induction protocol involves a "tetanus" stimulation, which is a 100-Hz stimulation for 1 second. When one applies this protocol to a pair of cells, one will see a sustained increase of the amplitude of the excitatory postsynaptic potential following tetanus. This response is interesting because it is thought to be the physiological correlation for learning and memory in the cell. In fact, it has been shown that, following a single paired-avoidance paradigm in mice, LTP can be recorded in some hippocampal synapses in vivo.
The molecular basis for LTP has been extensively studied, and AMPARs have been shown to play an integral role in the process.
Both GluR1 and GluR2 play an important role in synaptic plasticity. It is now known that the underlying physiological correlation for the increase in EPSP size is a postsynaptic upregulation of AMPARs at the membrane, which is accomplished through the interactions of AMPARs with many cellular proteins.
The simplest explanation for LTP is as follows. Glutamate binds to postsynaptic AMPARs and another glutamate receptor, the NMDA receptor. Ligand binding causes the AMPARs to open, and Na⁺ flows into the postsynaptic cell, resulting in a depolarization. NMDARs, on the other hand, do not open directly because their pores are occluded at resting membrane potential by Mg²⁺ ions. NMDARs can open only when a depolarization from the AMPAR activation leads to repulsion of the Mg²⁺ cation out into the extracellular space, allowing the pore to pass current. Unlike AMPARs, however, NMDARs are permeable to both Na⁺ and Ca²⁺. The Ca²⁺ that enters the cell triggers the upregulation of AMPARs to the membrane, which results in a long-lasting increase in EPSP size underlying LTP. The calcium entry also phosphorylates CaMKII, which phosphorylates AMPARs, increasing their single-channel conductance.