•The molecular composition of native GABAAR complexes was recently identified.•Recent advances have been made regarding the assembly of this complex.•Surface expression of GABAARs is independent of ...GARLHs.•GARLHs control synaptic clustering of the GABAAR complexes.
The ionotropic GABA receptor (GABAAR) mediates fast inhibition in the brain. The GABAAR pore-forming (α, β, and non-α/β) subunits were isolated approximately 30 years ago and have since been the focus of extensive studies. As a result, many properties of GABAARs, including subunit assembly and channel and pharmacological properties, have been discovered. However, several of the underlying mechanisms such as the process for the synaptic localization of GABAARs remain unsolved. A reinvestigation of native GABAAR complexes in the brain and primary neurons identified two major molecular constituents, namely, the transmembrane GARLH/LHFPL protein family and the inhibitory synaptic protein neuroligin 2. This identification of the principal components of native receptor complexes may provide new mechanistic insight on receptor regulation.
Glutamate, the major excitatory neurotransmitter in the brain, acts primarily on two types of ionotropic receptors: alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors and ...N-methyl-D-aspartate (NMDA) receptors. Work over the past decade indicates that regulated changes in the number of synaptic AMPA receptors may serve as a mechanism for information storage. Recent studies demonstrate that a family of small transmembrane AMPA receptor regulatory proteins (TARPs) controls both AMPA receptor trafficking and channel gating. TARPs provide the first example of auxiliary subunits of ionotropic receptors. Here we review the pivotal role that TARPs play in the life cycle of AMPA receptors.
Glutamate is a major excitatory neurotransmitter in the vertebrate brain. AMPA-type glutamate receptors mediate fast excitatory transmission. AMPA receptors assemble with transmembrane AMPA receptor ...regulatory protein (TARP) auxiliary subunits and function as native ion channels. However, the assembly and stoichiometry of AMPA receptor and TARP complexes remain unclear. Here, we developed a novel strategy to determine the assembly and stoichiometry of this protein complex and found that functional AMPA receptors indeed assembled as a tetramer in a dimer-of-dimers structure. Furthermore, we found that the AMPA receptor auxiliary subunit, TARP, had a variable stoichiometry (1-4 TARP units) on AMPA receptors and that 1 TARP unit was sufficient to modulate AMPA receptor activity. In neurons, TARP had fixed and minimum stoichiometry on AMPA receptors. This fundamental composition of the AMPA receptor/TARP complex is important for the elucidation of the molecular machinery that underlies synaptic transmission.
AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid) receptors mediate fast excitatory synaptic transmission in the brain. These ion channels rapidly deactivate and desensitize, which ...determine the time course of synaptic transmission. Here, we find that the AMPA receptor interacting protein, stargazin, not only mediates AMPA receptor trafficking but also shapes synaptic responses by slowing channel deactivation and desensitization. The cytoplasmic tail of stargazin determines receptor trafficking, whereas the ectodomain controls channel properties. Stargazin alters AMPA receptor kinetics by increasing the rate of channel opening. Disrupting the interaction of stargazin ectodomain with hippocampal AMPA receptors alters the amplitude and shape of synaptic responses, establishing a crucial function for stargazin in controlling the efficacy of synaptic transmission in the brain.
Anesthetics are generally associated with sedation, but some anesthetics can also increase brain and motor activity-a phenomenon known as paradoxical excitation. Previous studies have identified GABA
...receptors as the primary targets of most anesthetic drugs, but how these compounds produce paradoxical excitation is poorly understood. To identify and understand such compounds, we applied a behavior-based drug profiling approach. Here, we show that a subset of central nervous system depressants cause paradoxical excitation in zebrafish. Using this behavior as a readout, we screened thousands of compounds and identified dozens of hits that caused paradoxical excitation. Many hit compounds modulated human GABA
receptors, while others appeared to modulate different neuronal targets, including the human serotonin-6 receptor. Ligands at these receptors generally decreased neuronal activity, but paradoxically increased activity in the caudal hindbrain. Together, these studies identify ligands, targets, and neurons affecting sedation and paradoxical excitation in vivo in zebrafish.
Transmembrane AMPA receptor regulatory proteins (TARPs) and cornichon proteins (CNIH-2/3) independently modulate AMPA receptor trafficking and gating. However, the potential for interactions of these ...subunits within an AMPA receptor complex is unknown. Here, we find that TARPs γ-4, γ-7, and γ-8, but not γ-2, γ-3, or γ-5, cause AMPA receptors to “resensitize” upon continued glutamate application. With γ-8, resensitization occurs with all GluA subunit combinations; however, γ-8-containing hippocampal neurons do not display resensitization. In recombinant systems, CNIH-2 abrogates γ-8-mediated resensitization and modifies AMPA receptor pharmacology and gating to match that of hippocampal neurons. In hippocampus, γ-8 and CNIH-2 associate in postsynaptic densities and CNIH-2 protein levels are markedly diminished in γ-8 knockout mice. Manipulating neuronal CNIH-2 levels modulates the electrophysiological properties of extrasynaptic and synaptic γ-8-containing AMPA receptors. Thus, γ-8 and CNIH-2 functionally interact with common hippocampal AMPA receptor complexes to modulate synergistically kinetics and pharmacology.
► TARP and cornichon subunits synergistically modulate hippocampal AMPA receptors ► AMPA receptor resensitization induced by TARP subunits
Glutamate is the most abundant excitatory neurotransmitter in the brain, and distinct classes of glutamate receptors coordinate synaptic transmission and spike generation upon various levels of ...neuronal activity. However, the mechanisms remain unclear. Here, we found that loss of synaptic AMPA receptors increased kainate receptor activity in cerebellar granule cells without changing NMDA receptors. The augmentation of kainate receptor-mediated currents in the absence of AMPA receptor activity is required for spike generation and is mediated by the increased expression of the GluK5 high-affinity kainate receptor subunit. Increase in GluK5 expression is sufficient to enhance kainate receptor activity by modulating receptor channel properties, but not localization. Furthermore, we demonstrate that the combined loss of the AMPA receptor auxiliary TARPγ-2 subunit and the GluK5 subunit leads to early mouse lethality. Our findings reveal mechanisms mediated by distinct classes of postsynaptic glutamate receptors for the homeostatic maintenance of the neuronal activity.
•AMPA and kainate receptors share essential roles as postsynaptic depolarizers in vivo•Kainate receptors play distinct homeostatic roles in regulating postsynaptic strength•Activity-dependent switch in subunit compositions of kainate receptors•Mimic AMPAR- and KAR-mediated transmission with receptors expressed heterologously
Distinct classes of glutamate receptors coordinate synaptic strength upon various levels of neuronal activity. Yan et al. report that two distinct classes of glutamate receptors, AMPARs and KARs, control synaptic transmission and spike generation via a homeostatic mechanism in vivo.
Ionotropic glutamate receptors principally mediate fast excitatory transmission in the brain. Among the three classes of ionotropic glutamate receptors, kainate receptors (KARs) have a unique brain ...distribution, which has been historically defined by (3)H-radiolabeled kainate binding. Compared with recombinant KARs expressed in heterologous cells, synaptic KARs exhibit characteristically slow rise-time and decay kinetics. However, the mechanisms responsible for these distinct KAR properties remain unclear. We found that both the high-affinity binding pattern in the mouse brain and the channel properties of native KARs are determined by the KAR auxiliary subunit Neto1. Through modulation of agonist binding affinity and off-kinetics of KARs, but not trafficking of KARs, Neto1 determined both the KAR high-affinity binding pattern and the distinctively slow kinetics of postsynaptic KARs. By regulating KAR excitatory postsynaptic current kinetics, Neto1 can control synaptic temporal summation, spike generation and fidelity.
The transmembrane α-amino-3-hydroxyl-5-methyl-4-isoxazolepropionic acid (AMPA) receptor regulatory protein γ−8 (TARP γ−8) constitutes an auxiliary subunit of AMPA receptors, which mediates various ...brain functions including learning and memory. TARP γ−8 has emerged as a promising therapeutic target for central nervous system disorders. Despite considerable efforts, previously reported TARP γ−8 PET radioligands, such as
11
CTARP-1903 or the
11
CTARP-1811 series, were plagued by limited brain uptake and/or high nonspecific binding
in vivo
. Herein, we developed two novel
11
C-labeled probes,
11
C
8
and
11
C
15
(also named as
11
CTARP-2105), of which the latter exhibited reasonable brain uptake as well as specific binding towards TARP γ−8 both
in vitro
and
in vivo
, as confirmed for the TARP γ−8-rich hippocampus by blocking experiments with the commercially available TARP γ−8 inhibitor, JNJ-55511118. Overall,
11
C
15
exhibited promising tracer characteristics and proved to be a lead positron-emission tomography (PET) ligand for non-invasive quantification of TARP γ−8 in the mammalian brain.
AMPA receptor (AMPAR) complexes contain auxiliary subunits that modulate receptor trafficking and gating. In addition to the transmembrane AMPAR regulatory proteins (TARPs) and cornichons (CNIH-2/3), ...recent proteomic studies identified a diverse array of additional AMPAR-associated transmembrane and secreted partners. We systematically surveyed these and found that PORCN and ABHD6 increase GluA1 levels in transfected cells. Knockdown of PORCN in rat hippocampal neurons, which express it in high amounts, selectively reduces levels of all tested AMPAR complex components. Regulation of AMPARs is independent of PORCN’s membrane-associated O-acyl transferase activity. PORCN knockdown in hippocampal neurons decreases AMPAR currents and accelerates desensitization and leads to depletion of TARP γ-8 from AMPAR complexes. Conditional PORCN knockout mice also exhibit specific changes in AMPAR expression and gating that reduce basal synaptic transmission but leave long-term potentiation intact. These studies define additional roles for PORCN in controlling synaptic transmission by regulating the level and composition of hippocampal AMPAR complexes.
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•PORCN controls assembly and stability of hippocampal AMPARs at the level of the ER•PORCN knockout reduces basal synaptic transmission but not long-term potentiation•Functional interaction with AMPARs is independent of PORCN’s catalytic activity
Recent proteomic studies have identified an array of proteins associated with AMPA-type glutamate receptors. Erlenhardt et al. now systematically survey this collection, finding that the membrane-bound O-acyl transferase PORCN plays a crucial role in regulating basal synaptic transmission by controlling AMPAR complex composition and stability at the level of the ER.
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