Facilitation of currents through rat Ca2+-permeable AMPA receptor channels by activity-dependent relief from polyamine block

Enhanced functional detection of synaptic calcium-permeable AMPA receptors using intracellular NASPM

Calcium-permeable AMPA-type glutamate receptors (CP-AMPARs) contribute to many forms of synaptic plasticity and pathology. They can be distinguished from GluA2-containing calcium-impermeable AMPARs by the inward rectification of their currents, which reflects voltage-dependent channel block by intracellular spermine. However, the efficacy of this weakly permeant blocker is differentially altered by the presence of AMPAR auxiliary subunits - including transmembrane AMPAR regulatory proteins, cornichons, and GSG1L - which are widely expressed in neurons and glia. This complicates the interpretation of rectification as a measure of CP-AMPAR expression. Here, we show that the inclusion of the spider toxin analog 1-naphthylacetyl spermine (NASPM) in the intracellular solution results in a complete block of GluA1-mediated outward currents irrespective of the type of associated auxiliary subunit. In neurons from GluA2-knockout mice expressing only CP-AMPARs, intracellular NASPM, unlike spermine, completely blocks outward synaptic currents. Thus, our results identify a functional measure of CP-AMPARs, that is unaffected by their auxiliary subunit content.

Structure, Function, and Pharmacology of Glutamate Receptor Ion Channels

Many physiologic effects of l-glutamate, the major excitatory neurotransmitter in the mammalian central nervous system, are mediated via signaling by ionotropic glutamate receptors (iGluRs). These ligand-gated ion channels are critical to brain function and are centrally implicated in numerous psychiatric and neurologic disorders. There are different classes of iGluRs with a variety of receptor subtypes in each class that play distinct roles in neuronal functions. The diversity in iGluR subtypes, with their unique functional properties and physiologic roles, has motivated a large number of studies. Our understanding of receptor subtypes has advanced considerably since the first iGluR subunit gene was cloned in 1989, and the research focus has expanded to encompass facets of biology that have been recently discovered and to exploit experimental paradigms made possible by technological advances. Here, we review insights from more than 3 decades of iGluR studies with an emphasis on the progress that has occurred in the past decade. We cover structure, function, pharmacology, roles in neurophysiology, and therapeutic implications for all classes of receptors assembled from the subunits encoded by the 18 ionotropic glutamate receptor genes. SIGNIFICANCE STATEMENT: Glutamate receptors play important roles in virtually all aspects of brain function and are either involved in mediating some clinical features of neurological disease or represent a therapeutic target for treatment. Therefore, understanding the structure, function, and pharmacology of this class of receptors will advance our understanding of many aspects of brain function at molecular, cellular, and system levels and provide new opportunities to treat patients.

Interplay between Gating and Block of Ligand-Gated Ion Channels

Drugs that inhibit ion channel function by binding in the channel and preventing current flow, known as channel blockers, can be used as powerful tools for analysis of channel properties. Channel blockers are used to probe both the sophisticated structure and basic biophysical properties of ion channels. Gating, the mechanism that controls the opening and closing of ion channels, can be profoundly influenced by channel blocking drugs. Channel block and gating are reciprocally connected; gating controls access of channel blockers to their binding sites, and channel-blocking drugs can have profound and diverse effects on the rates of gating transitions and on the stability of channel open and closed states. This review synthesizes knowledge of the inherent intertwining of block and gating of excitatory ligand-gated ion channels, with a focus on the utility of channel blockers as analytic probes of ionotropic glutamate receptor channel function.

Evaluation of Altered Glutamatergic Activity in a Piglet Model of Hypoxic-Ischemic Brain Damage Using 1H-MRS

Methods

Twenty-five newborn piglets were selected and then randomly assigned to the control group (n = 5) and the model group (n = 20) subjected to HI. HI was induced by blocking bilateral carotid blood flow under simultaneous inhalation of a 6% oxygen mixture. ¹H-MRS data were acquired from the basal ganglia at the following time points after HI: 6, 12, 24, and 72 h. Changes in protein levels of EAAT2 and GluR2 were determined by immunohistochemical analysis. Correlations among metabolite concentrations, metabolite ratios, and the protein levels of EAAT2 and GluR2 were investigated.

Results

The Glu level sharply increased after HI, reached a transient low level of depletion that approached the normal level in the control group, and subsequently increased again. Negative correlations were found between concentrations of Glu and EAAT2 protein levels (R _s = -0.662, P < 0.001) and between the Glu/creatine (Cr) ratio and EAAT2 protein level (R _s = -0.664, P < 0.001). Moreover, changes in GluR2 protein level were significantly and negatively correlated with those in Glu level (the absolute Glu concentration, R _s = -0.797, P < 0.001; Glu/Cr, R _s = -0.567, P = 0.003).

Conclusions

Changes in Glu level measured by ¹H-MRS were inversely correlated with those in EAAT2 and GluR2 protein levels following HI, and the results demonstrated that ¹H-MRS can reflect the early changes of glutamatergic activity in vivo.

In Vivo Regulation of Oligodendrocyte Precursor Cell Proliferation and Differentiation by the AMPA-Receptor Subunit GluA2

The functional role of AMPA receptor (AMPAR)-mediated synaptic signaling between neurons and oligodendrocyte precursor cells (OPCs) remains enigmatic. We modified the properties of AMPARs at axon-OPC synapses in the mouse corpus callosum in vivo during the peak of myelination by targeting the GluA2 subunit. Expression of the unedited (Ca²⁺ permeable) or the pore-dead GluA2 subunit of AMPARs triggered proliferation of OPCs and reduced their differentiation into oligodendrocytes. Expression of the cytoplasmic C-terminal (GluA2(813-862)) of the GluA2 subunit (C-tail), a modification designed to affect the interaction between GluA2 and AMPAR-binding proteins and to perturb trafficking of GluA2-containing AMPARs, decreased the differentiation of OPCs without affecting their proliferation. These findings suggest that ionotropic and non-ionotropic properties of AMPARs in OPCs, as well as specific aspects of AMPAR-mediated signaling at axon-OPC synapses in the mouse corpus callosum, are important for balancing the response of OPCs to proliferation and differentiation cues.

Polyamine-mediated channel block of ionotropic glutamate receptors and its regulation by auxiliary proteins

Most excitatory neurotransmission in the mammalian brain is mediated by a family of plasma membrane-bound signaling proteins called ionotropic glutamate receptors (iGluRs). iGluRs assemble at central synapses as tetramers, forming a central ion-channel pore whose primary function is to rapidly transport Na⁺ and Ca²⁺ in response to binding the neurotransmitter l-glutamic acid. The pore of iGluRs is also accessible to bulkier cytoplasmic cations, such as the polyamines spermine, spermidine, and putrescine, which are drawn into the permeation pathway, but get stuck and block the movement of other ions. The degree of this polyamine-mediated channel block is highly regulated by processes that control the free cytoplasmic polyamine concentration, the membrane potential, or the iGluR subunit composition. Recently, an additional regulation by auxiliary proteins, most notably transmembrane AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptor regulatory proteins (TARPs), cornichons, and neuropilin and tolloid-like proteins (NETOs), has been identified. Here, I review what we have learned of polyamine block of iGluRs and its regulation by auxiliary subunits. TARPs, cornichons, and NETOs attenuate the channel block by enabling polyamines to exit the pore. As a result, polyamine permeation occurs at more negative and physiologically relevant membrane potentials. The structural basis for enhanced polyamine transport remains unresolved, although alterations in both channel architecture and charge-screening mechanisms have been proposed. That auxiliary subunits can attenuate the polyamine block reveals an unappreciated impact of polyamine permeation in shaping the signaling properties of neuronal AMPA- and kainate-type iGluRs. Moreover, enhanced polyamine transport through iGluRs may have a role in regulating cellular polyamine levels.

The Role of Polyamine-Dependent Facilitation of Calcium Permeable AMPARs in Short-Term Synaptic Enhancement

Depending on subunit composition AMPA receptor channels can be subdivided into two groups: GluA2-containing calcium impermeable AMPARs, and GluA2-lacking calcium permeable, AMPARs. These two groups differ in a number of biophysical properties and, most likely, in their functional role at glutamatergic synapses. GluA2-lacking channels have received a lot of attention over the last two decades mainly due to high calcium permeability, which was suggested to play a significant role in the induction of long-term synaptic plasticity in healthy tissue and neuronal death under neuropathological conditions. However, calcium permeable AMPARs possess another property that can contribute substantially to frequency dependent dynamics of synaptic efficacy. In the closed state calcium permeable AMPARs are blocked by endogenous polyamines, however, repetitive activation leads to progressive relief from the block and to the facilitation of ion flux through these channels. Polyamine-dependent facilitation of AMPARs can contribute to short-term plasticity at synapses that have high initial release probability and express calcium permeable AMPARs. During synaptic transmission activity-dependent relief from polyamine block of postsynaptic calcium-permeable AMPARs either counteracts presynaptic short-term depression in a frequency-dependent manner or, under specific stimulation conditions, induces facilitation of a synaptic response. Taking into account the fact that expression of calcium permeable AMPARs is developmentally regulated, depends on network activity and increases in diseased brain states, polyamine-dependent facilitation of calcium permeable AMPARs is an important, entirely postsynaptic mechanism of synaptic gain regulation.

R/G editing in GluA2Rflop modulates the functional difference between GluA1 flip and flop variants in GluA1/2R heteromeric channels

In α-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA) receptors, RNA editing and alternative splicing generate sequence variants, and those variants, as in GluA2-4 AMPA receptor subunits, generally show different properties. Yet, earlier studies have shown that the alternatively spliced, flip and flop variants of GluA1 AMPA receptor subunit exhibit no functional difference in homomeric channel form. Using a laser-pulse photolysis technique, combined with whole-cell recording, we measured the rate of channel opening, among other kinetic properties, for a series of AMPA channels with different arginine/glycine (R/G) editing and flip/flop status. We find that R/G editing in the GluA2 subunit modulates the channel properties in both homomeric (GluA2Q) and complex (GluA2Q/2R and GluA1/2R) channel forms. However, R/G editing is only effective in flop channels. Specifically, editing at the R/G site on the GluA2R flop isoform accelerates the rate of channel opening and desensitization for GluA1/2R channels more pronouncedly with the GluA1 being in the flop form than in the flip form; yet R/G editing has no effect on either channel-closing rate or EC₅₀. Our results suggest R/G editing via GluA2R serve as a regulatory mechanism to modulate the function of GluA2R-containing, native receptors involved in fast excitatory synaptic transmission.

Role of BACE1 in Alzheimer's synaptic function

Alzheimer's disease (AD) is the most common age-dependent disease of dementia, and there is currently no cure available. This hallmark pathologies of AD are the presence of amyloid plaques and neurofibrillary tangles. Although the exact etiology of AD remains a mystery, studies over the past 30 have shown that abnormal generation or accumulation of β-amyloid peptides (Aβ) is likely to be a predominant early event in AD pathological development. Aβ is generated from amyloid precursor protein (APP) via proteolytic cleavage by β-site APP cleaving enzyme 1 (BACE1). Chemical inhibition of BACE1 has been shown to reduce Aβ in animal studies and in human trials. While BACE1 inhibitors are currently being tested in clinical trials to treat AD patients, it is highly important to understand whether BACE1 inhibition will significantly impact cognitive functions in AD patients. This review summarizes the recent studies on BACE1 synaptic functions. This knowledge will help to guide the proper use of BACE1 inhibitors in AD therapy.

Identification and characterization of RNA aptamers: A long aptamer blocks the AMPA receptor and a short aptamer blocks both AMPA and kainate receptors

AMPA and kainate receptors, along with NMDA receptors, represent different subtypes of glutamate ion channels. AMPA and kainate receptors share a high degree of sequence and structural similarities, and excessive activity of these receptors has been implicated in neurological diseases such as epilepsy. Therefore, blocking detrimental activity of both receptor types could be therapeutically beneficial. Here, we report the use of an in vitro evolution approach involving systematic evolution of ligands by exponential enrichment with a single AMPA receptor target (i.e. GluA1/2R) to isolate RNA aptamers that can potentially inhibit both AMPA and kainate receptors. A full-length or 101-nucleotide (nt) aptamer selectively inhibited GluA1/2R with a KI of ∼5 μm, along with GluA1 and GluA2 AMPA receptor subunits. Of note, its shorter version (55 nt) inhibited both AMPA and kainate receptors. In particular, this shorter aptamer blocked equally potently the activity of both the GluK1 and GluK2 kainate receptors. Using homologous binding and whole-cell recording assays, we found that an RNA aptamer most likely binds to the receptor's regulatory site and inhibits it noncompetitively. Our results suggest the potential of using a single receptor target to develop RNA aptamers with dual activity for effectively blocking both AMPA and kainate receptors.

An evolving view of retinogeniculate transmission

The thalamocortical (TC) relay neuron of the dorsoLateral Geniculate Nucleus (dLGN) has borne its imprecise label for many decades in spite of strong evidence that its role in visual processing transcends the implied simplicity of the term "relay". The retinogeniculate synapse is the site of communication between a retinal ganglion cell and a TC neuron of the dLGN. Activation of retinal fibers in the optic tract causes reliable, rapid, and robust postsynaptic potentials that drive postsynaptics spikes in a TC neuron. Cortical and subcortical modulatory systems have been known for decades to regulate retinogeniculate transmission. The dynamic properties that the retinogeniculate synapse itself exhibits during and after developmental refinement further enrich the role of the dLGN in the transmission of the retinal signal. Here we consider the structural and functional substrates for retinogeniculate synaptic transmission and plasticity, and reflect on how the complexity of the retinogeniculate synapse imparts a novel dynamic and influential capacity to subcortical processing of visual information.

Diverse roles for ionotropic glutamate receptors on inhibitory interneurons in developing and adult brain

Glutamate receptor-mediated recruitment of GABAergic inhibitory interneurons is a critical determinant of network processing. Early studies observed that many, but not all, interneuron glutamatergic synapses contain AMPA receptors that are GluA2-subunit lacking and Ca(2+) permeable, making them distinct from AMPA receptors at most principal cell synapses. Subsequent studies demonstrated considerable alignment of synaptic AMPA and NMDA receptor subunit composition within specific subtypes of interneurons, suggesting that both receptor expression profiles are developmentally and functionally linked. Indeed glutamate receptor expression profiles are largely predicted by the embryonic origins of cortical interneurons within the medial and caudal ganglionic eminences of the developing telencephalon. Distinct complements of AMPA and NMDA receptors within different interneuron subpopulations contribute to the differential recruitment of functionally divergent interneuron subtypes by common afferent inputs for appropriate feed-forward and feedback inhibitory drive and network entrainment. In contrast, the lesser-studied kainate receptors, which are often present at both pre- and postsynaptic sites, appear to follow an independent developmental expression profile. Loss of specific ionotropic glutamate receptor (iGluR) subunits during interneuron development has dramatic consequences for both cellular and network function, often precipitating circuit inhibition-excitation imbalances and in some cases lethality. Here we briefly review recent findings highlighting the roles of iGluRs in interneuron development.

NMDA Receptor-Dependent LTD Requires Transient Synaptic Incorporation of Ca²⁺-Permeable AMPARs Mediated by AKAP150-Anchored PKA and Calcineurin

Information processing in the brain requires multiple forms of synaptic plasticity that converge on regulation of NMDA and AMPA-type glutamate receptors (NMDAR, AMPAR), including long-term potentiation (LTP) and long-term depression (LTD) and homeostatic scaling. In some cases, LTP and homeostatic plasticity regulate synaptic AMPAR subunit composition to increase the contribution of Ca(2+)-permeable receptors (CP-AMPARs) containing GluA1 but lacking GluA2 subunits. Here, we show that PKA anchored to the scaffold protein AKAP150 regulates GluA1 phosphorylation and plays a novel role controlling CP-AMPAR synaptic incorporation during NMDAR-dependent LTD. Using knockin mice that are deficient in AKAP-anchoring of either PKA or the opposing phosphatase calcineurin, we found that CP-AMPARs are recruited to hippocampal synapses by anchored PKA during LTD induction but are then rapidly removed by anchored calcineurin. Importantly, blocking CP-AMPAR recruitment, removal, or activity interferes with LTD. Thus, CP-AMPAR synaptic recruitment is required to transiently augment NMDAR Ca(2+) signaling during LTD induction.

Superactivation of AMPA receptors by auxiliary proteins

Glutamate receptors form complexes in the brain with auxiliary proteins, which control their activity during fast synaptic transmission through a seemingly bewildering array of effects. Here we devise a way to isolate the activation of complexes using polyamines, which enables us to show that transmembrane AMPA receptor regulatory proteins (TARPs) exert their effects principally on the channel opening reaction. A thermodynamic argument suggests that because TARPs promote channel opening, receptor activation promotes AMPAR-TARP complexes into a superactive state with high open probability. A simple model based on this idea predicts all known effects of TARPs on AMPA receptor function. This model also predicts unexpected phenomena including massive potentiation in the absence of desensitization and supramaximal recovery that we subsequently detected in electrophysiological recordings. This transient positive feedback mechanism has implications for information processing in the brain, because it should allow activity-dependent facilitation of excitatory synaptic transmission through a postsynaptic mechanism.

Transmembrane AMPA receptor regulatory proteins (TARPs) exert a number of effects on fast glutamatergic synaptic transmission. Here, the authors model the interactions of two such proteins, Stargazin and γ-8, and propose a simple kinetic mechanism by which TARPs modulate channel opening reaction.

Enhanced long term potentiation and decreased AMPA receptor desensitization in the acute period following a single kainate induced early life seizure

Neonatal seizures are associated with long term disabilities including epilepsy and cognitive deficits. Using a neonatal seizure rat model that does not develop epilepsy, but develops a phenotype consistent with other models of intellectual disability (ID) and autism spectrum disorders (ASD), we sought to isolate the acute effects of a single episode of early life seizure on hippocampal CA1 synaptic development and plasticity. We have previously shown chronic changes in glutamatergic synapses, loss of long term potentiation (LTP) and enhanced long term depression (LTD), in the adult male rat ~50days following kainic acid (KA) induced early life seizure (KA-ELS) in post-natal (P) 7day old male Sprague-Dawley rats. In the present work, we examined the electrophysiological properties and expression levels of glutamate receptors in the acute period, 2 and 7days, post KA-ELS. Our results show for the first time enhanced LTP 7days after KA-ELS, but no change 2days post KA-ELS. Additionally, we report that ionotropic α-amino-3-hydroxy-5-methyl-isoxazole-propionic acid type glutamate receptor (AMPAR) desensitization is decreased in the same time frame, with no changes in AMPAR expression, phosphorylation, or membrane insertion. Inappropriate enhancement of the synaptic connections in the acute period after the seizure could alter the normal patterning of synaptic development in the hippocampus during this critical period and contribute to learning deficits. Thus, this study demonstrates a novel mechanism by which KA-ELS alters early network properties that potentially lead to adverse outcomes.

Glutamate receptor pores

Glutamate receptors are ligand-gated ion channels that mediate fast excitatory synaptic transmission throughout the central nervous system. Functional receptors are homo- or heteromeric tetramers with each subunit contributing a re-entrant pore loop that dips into the membrane from the cytoplasmic side. The pore loops form a narrow constriction near their apex with a wide vestibule toward the cytoplasm and an aqueous central cavity facing the extracellular solution. This article focuses on the pore region, reviewing how structural differences among glutamate receptor subtypes determine their distinct functional properties.

Evidence of calcium-permeable AMPA receptors in dendritic spines of CA1 pyramidal neurons

GluA2-lacking, calcium-permeable α-amino-3-hydroxy-5-methylisoxazole-4-propionate receptors (AMPARs) have unique properties, but their presence at excitatory synapses in pyramidal cells is controversial. We have tested certain predictions of the model that such receptors are present in CA1 cells and show here that the polyamine spermine, but not philanthotoxin, causes use-dependent inhibition of synaptically evoked excitatory responses in stratum radiatum, but not s. oriens, in cultured and acute hippocampal slices. Stimulation of single dendritic spines by photolytic release of caged glutamate induced an N-methyl-d-aspartate receptor-independent, use- and spermine-sensitive calcium influx only at apical spines in cultured slices. Bath application of glutamate also triggered a spermine-sensitive influx of cobalt into CA1 cell dendrites in s. radiatum. Responses of single apical, but not basal, spines to photostimulation displayed prominent paired-pulse facilitation (PPF) consistent with use-dependent relief of cytoplasmic polyamine block. Responses at apical dendrites were diminished, and PPF was increased, by spermine. Intracellular application of pep2m, which inhibits recycling of GluA2-containing AMPARs, reduced apical spine responses and increased PPF. We conclude that some calcium-permeable, polyamine-sensitive AMPARs, perhaps lacking GluA2 subunits, are present at synapses on apical dendrites of CA1 pyramidal cells, which may allow distinct forms of synaptic plasticity and computation at different sets of excitatory inputs.

Postsynaptic target specific synaptic dysfunctions in the CA3 area of BACE1 knockout mice

Beta-amyloid precursor protein cleaving enzyme 1 (BACE1), a major neuronal β-secretase critical for the formation of β-amyloid (Aβ) peptide, is considered one of the key therapeutic targets that can prevent the progression of Alzheimer's disease (AD). Although a complete ablation of BACE1 gene prevents Aβ formation, we previously reported that BACE1 knockouts (KOs) display presynaptic deficits, especially at the mossy fiber (MF) to CA3 synapses. Whether the defect is specific to certain inputs or postsynaptic targets in CA3 is unknown. To determine this, we performed whole-cell recording from pyramidal cells (PYR) and the stratum lucidum (SL) interneurons in the CA3, both of which receive excitatory MF terminals with high levels of BACE1 expression. BACE1 KOs displayed an enhancement of paired-pulse facilitation at the MF inputs to CA3 PYRs without changes at the MF inputs to SL interneurons, which suggests postsynaptic target specific regulation. The synaptic dysfunction in CA3 PYRs was not restricted to excitatory synapses, as seen by an increase in the paired-pulse ratio of evoked inhibitory postsynaptic currents from SL to CA3 PYRs. In addition to the changes in evoked synaptic transmission, BACE1 KOs displayed a reduction in the frequency of miniature excitatory and inhibitory postsynaptic currents (mEPSCs and mIPSCs) in CA3 PYRs without alteration in mEPSCs recorded from SL interneurons. This suggests that the impairment may be more global across diverse inputs to CA3 PYRs. Our results indicate that the synaptic dysfunctions seen in BACE1 KOs are specific to the postsynaptic target, the CA3 PYRs, independent of the input type.

Acute synthesis of CPEB is required for plasticity of visual avoidance behavior in Xenopus

Neural plasticity requires protein synthesis, but the identity of newly synthesized proteins generated in response to plasticity-inducing stimuli remains unclear. We used in vivo bio-orthogonal noncanonical amino acid tagging (BONCAT) with the methionine analog azidohomoalanine (AHA) combined with the multidimensional protein identification technique (MudPIT) to identify proteins that are synthesized in the tadpole brain over 24 hr. We induced conditioning-dependent plasticity of visual avoidance behavior, which required N-methyl-D-aspartate (NMDA) and Ca(2+)-permeable α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, αCaMKII, and rapid protein synthesis. Combining BONCAT with western blots revealed that proteins including αCaMKII, MEK1, CPEB, and GAD65 are synthesized during conditioning. Acute synthesis of CPEB during conditioning is required for behavioral plasticity as well as conditioning-induced synaptic and structural plasticity in the tectal circuit. We outline a signaling pathway that regulates protein-synthesis-dependent behavioral plasticity in intact animals, identify newly synthesized proteins induced by visual experience, and demonstrate a requirement for acute synthesis of CPEB in plasticity.

Q/R site interactions with the M3 helix in GluK2 kainate receptor channels revealed by thermodynamic mutant cycles

RNA editing at the Q/R site near the apex of the pore loop of AMPA and kainate receptors controls a diverse array of channel properties, including ion selectivity and unitary conductance and susceptibility to inhibition by polyamines and cis-unsaturated fatty acids, as well as subunit assembly into tetramers and regulation by auxiliary subunits. How these different aspects of channel function are all determined by a single amino acid substitution remains poorly understood; however, several lines of evidence suggest that interaction between the pore helix (M2) and adjacent segments of the transmembrane inner (M3) and outer (M1) helices may be involved. In the present study, we have used double mutant cycle analysis to test for energetic coupling between the Q/R site residue and amino acid side chains along the M3 helix. Our results demonstrate interaction with several M3 locations and particularly strong coupling to substitution for L614 at the level of the central cavity. In this location, replacement with smaller side chains completely and selectively reverses the effect of fatty acids on gating of edited channels, converting strong inhibition of wild-type GluK2(R) to nearly 10-fold potentiation of GluK2(R) L614A.

Experience-dependent homeostatic synaptic plasticity in neocortex

The organism's ability to adapt to the changing sensory environment is due in part to the ability of the nervous system to change with experience. Input and synapse specific Hebbian plasticity, such as long-term potentiation (LTP) and long-term depression (LTD), are critical for sculpting the nervous system to wire its circuit in tune with the environment and for storing memories. However, these synaptic plasticity mechanisms are innately unstable and require another mode of plasticity that maintains homeostasis to allow neurons to function within a desired dynamic range. Several modes of homeostatic adaptation are known, some of which work at the synaptic level. This review will focus on the known mechanisms of experience-induced homeostatic synaptic plasticity in the neocortex and their potential function in sensory cortex plasticity. This article is part of the Special Issue entitled 'Homeostatic Synaptic Plasticity'.

AKAP150-anchored calcineurin regulates synaptic plasticity by limiting synaptic incorporation of Ca2+-permeable AMPA receptors

AMPA receptors (AMPARs) are tetrameric ion channels assembled from GluA1-GluA4 subunits that mediate the majority of fast excitatory synaptic transmission in the brain. In the hippocampus, most synaptic AMPARs are composed of GluA1/2 or GluA2/3 with the GluA2 subunit preventing Ca(2+) influx. However, a small number of Ca(2+)-permeable GluA1 homomeric receptors reside in extrasynaptic locations where they can be rapidly recruited to synapses during synaptic plasticity. Phosphorylation of GluA1 S845 by the cAMP-dependent protein kinase (PKA) primes extrasynaptic receptors for synaptic insertion in response to NMDA receptor Ca(2+) signaling during long-term potentiation (LTP), while phosphatases dephosphorylate S845 and remove synaptic and extrasynaptic GluA1 during long-term depression (LTD). PKA and the Ca(2+)-activated phosphatase calcineurin (CaN) are targeted to GluA1 through binding to A-kinase anchoring protein 150 (AKAP150) in a complex with PSD-95, but we do not understand how the opposing activities of these enzymes are balanced to control plasticity. Here, we generated AKAP150ΔPIX knock-in mice to selectively disrupt CaN anchoring in vivo. We found that AKAP150ΔPIX mice lack LTD but express enhanced LTP at CA1 synapses. Accordingly, basal GluA1 S845 phosphorylation is elevated in AKAP150ΔPIX hippocampus, and LTD-induced dephosphorylation and removal of GluA1, AKAP150, and PSD-95 from synapses are impaired. In addition, basal synaptic activity of GluA2-lacking AMPARs is increased in AKAP150ΔPIX mice and pharmacologic antagonism of these receptors restores normal LTD and inhibits the enhanced LTP. Thus, AKAP150-anchored CaN opposes PKA phosphorylation of GluA1 to restrict synaptic incorporation of Ca(2+)-permeable AMPARs both basally and during LTP and LTD.

The auxiliary subunits Neto1 and Neto2 reduce voltage-dependent inhibition of recombinant kainate receptors

Kainate receptors can be subject to voltage-dependent block by intracellular polyamines, which causes inward rectification of the current-voltage relationship. Sensitivity to polyamine block is largely determined by the identity of a residue within the pore domain that can be altered through RNA editing. This process causes replacement of the encoded glutamine(Q) with a positively charged arginine(R), eliminating polyamine inhibition and thus inward rectification. In neurons, kainate receptors can associate with the auxiliary subunits Neto1 or Neto2. These transmembrane proteins alter the trafficking, channel kinetics, and pharmacology of the receptors in a subunit-dependent manner. We found that coexpression of Neto subunits with recombinant GluK2(Q) kainate receptors greatly reduced inward rectification without altering calcium permeability. This effect was separate from modulation of channel kinetics, as mutations within the extracellular LDLa domain of the Neto proteins completely eliminated their effects on desensitization but only reduced their effects on rectification. Conversely, deletion of the intracellular C-terminal domain of Neto1 or Neto2 or neutralization of positively charged residues within this domain prevented the reduction in rectification but did not alter effects on channel kinetics. These results demonstrate new roles for Neto1 and Neto2 in regulating kainate receptor function and identify domains within these auxiliary subunits important for mediating their effects.

Consequences of inhibiting amyloid precursor protein processing enzymes on synaptic function and plasticity

Alzheimer's disease (AD) is a neurodegenerative disease, one of whose major pathological hallmarks is the accumulation of amyloid plaques comprised of aggregated β-amyloid (Aβ) peptides. It is now recognized that soluble Aβ oligomers may lead to synaptic dysfunctions early in AD pathology preceding plaque deposition. Aβ is produced by a sequential cleavage of amyloid precursor protein (APP) by the activity of β- and γ-secretases, which have been identified as major candidate therapeutic targets of AD. This paper focuses on how Aβ alters synaptic function and the functional consequences of inhibiting the activity of the two secretases responsible for Aβ generation. Abnormalities in synaptic function resulting from the absence or inhibition of the Aβ-producing enzymes suggest that Aβ itself may have normal physiological functions which are disrupted by abnormal accumulation of Aβ during AD pathology. This interpretation suggests that AD therapeutics targeting the β- and γ-secretases should be developed to restore normal levels of Aβ or combined with measures to circumvent the associated synaptic dysfunction(s) in order to have minimal impact on normal synaptic function.

AMPA receptor assembly: atomic determinants and built-in modulators

Glutamate-gated ion channels (iGluRs) predominantly operate as heterotetramers to mediate excitatory neurotransmission at glutamatergic synapses. The subunit composition of the receptors determines their targeting to synaptic sites and signalling properties and is therefore a fundamental parameter for neuronal computations. iGluRs assemble as obligatory or preferential heteromers; the mechanisms underlying this selective assembly are only starting to emerge. Here we review recent work in the field and provide an in-depth update on atomic determinants in the assembly domains, which have been facilitated by recent advances in iGluR structural biology. We also discuss the role of alternative RNA processing in the ligand-binding domain, which modulates a central subunit interface and has the capacity to modulate receptor formation in response to external cues. Finally, we review the emerging physiological significance of signalling via distinct iGluR heterotetramers and provide examples of how recruitment of functionally diverse receptors modulates excitatory neurotransmission under physiological and pathological conditions.

GluA2-lacking AMPA receptors in hippocampal CA1 cell synapses: evidence from gene-targeted mice

The GluA2 subunit in heteromeric alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor channels restricts Ca²⁺ permeability and block by polyamines, rendering linear the current-voltage relationship of these glutamate-gated cation channels. Although GluA2-lacking synaptic AMPA receptors occur in GABA-ergic inhibitory neurons, hippocampal CA1 pyramidal cell synapses are widely held to feature only GluA2 containing AMPA receptors. A controversy has arisen from reports of GluA2-lacking AMPA receptors at hippocampal CA3-to-CA1 cell synapses and a study contesting these findings. Here we sought independent evidence for the presence of GluA2-lacking AMPA receptors in CA1 pyramidal cell synapses by probing the sensitivity of their gated cation channels in wild-type (WT) mice and gene-targeted mouse mutants to philanthotoxin, a specific blocker of GluA2-lacking AMPA receptors. The mutants either lacked GluA2 for maximal philanthotoxin sensitivity, or, for minimal sensitivity, expressed GluA1 solely in a Q/R site-edited version or not at all. Our comparative electrophysiological analyses provide incontrovertible evidence for the presence in wild-type CA1 pyramidal cell synapses of GluA2-less AMPA receptor channels. This article is part of a Special Issue entitled “Calcium permeable AMPARs in synaptic plasticity and disease.”

Ca-permeable AMPA receptors in homeostatic synaptic plasticity

Neurons possess diverse mechanisms of homeostatic adaptation to overall changes in neural and synaptic activity, which are critical for proper brain functions. Homeostatic regulation of excitatory synapses has been studied in the context of synaptic scaling, which allows neurons to adjust their excitatory synaptic gain to maintain their activity within a dynamic range. Recent evidence suggests that one of the main mechanisms underlying synaptic scaling is by altering the function of postsynaptic α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs), including synaptic expression of Ca²⁺-permeable (CP-) AMPARs. CP-AMPARs endow synapses with unique properties, which may benefit adaptation of neurons to periods of inactivity as would occur when a major input is lost. This review will summarize how synaptic expression of CP-AMPARs is regulated during homeostatic synaptic plasticity in the context of synaptic scaling, and will address the potential functional consequences of altering synaptic CP-AMPAR content.

Remodeling of synaptic AMPA receptor subtype alters the probability and pattern of action potential firing

Changes in the subunit composition of postsynaptic AMPA-type glutamate receptors can be induced at CNS synapses by neural activity and under certain pathological conditions. Fear-induced incorporation of GluR2-containing receptors at cerebellar synapses selectively prolongs the decay time of synaptic currents, whereas a switch from GluR2-lacking to GluR2-containing receptors induced by parallel fiber stimulation reduces the amplitude in addition to lengthening the duration of EPSCs. Although it is often assumed that these two forms of synaptic plasticity will alter action potential (AP) firing in the postsynaptic neuron, this has not been directly tested. Using a dynamic current-clamp approach, we now show that the fear-induced increase in EPSC duration increases the size of EPSPs and thereby markedly enhances the AP firing probability. In contrast, the parallel fiber stimulation-triggered switch in GluR2 expression reduces the EPSP-AP coupling because of the decrease in the synaptic current amplitude. The switch also abolished the paired-pulse facilitation that arose from an activity and spermine-dependent unblock of GluR2-lacking receptors and hence reduced the ability of paired stimuli to evoke two consecutive APs. Therefore, fear-induced incorporation of GluR2 receptors enhances the EPSP-AP coupling, but the parallel fiber stimulation-triggered switch reduces both the EPSP-AP coupling and evoked AP doublets. In contrast to long-term potentiation and depression, which modify the amplitude of synaptic currents, this activity-induced change in AMPA receptor phenotype alters synaptic conductance waveform and postsynaptic short-term plasticity. These changes modulate both the probability and pattern of evoked AP firing via a fundamentally different mechanism from long-term potentiation and long-term depression.

Voltage-dependent and -independent block of α-amino-3-hydroxy-5-methylisoxazole-4-propionate receptor channels

Polyamine-containing toxins and synthetic dicationic derivatives of adamantane and phenylcyclohexyl selectively antagonize Ca(2+)-permeable α-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptor channels. These compounds demonstrate voltage-dependent open-channel block and are trapped by closed channels. In this study, we describe an alternative mechanism of non-competitive AMPA receptor inhibition caused by 9-aminoacridine and some of its derivatives. These compounds exhibit similar potency against Ca(2+)-permeable and Ca(2+)-impermeable AMPA receptors. The inhibition is largely voltage-independent, binding and unbinding do not require presence of agonist. We conclude that 9-aminoacridine binds to a shallow site in the AMPA receptor, which is located above the activation gate. A comparison of three-dimensional structures of the antagonists suggests that the 'V-like' shape of the hydrophobic headgroup favors voltage-dependent binding to the deep site in the channel pore, whereas the compounds possessing flat aromatic headgroups preferably bind to the shallow site. The characterization of the novel mechanism of AMPA receptor channel antagonism opens a way to develop a new family of pharmacological agents, which can be of scientific and practical importance.

Glutamate receptor ion channels: structure, regulation, and function

The mammalian ionotropic glutamate receptor family encodes 18 gene products that coassemble to form ligand-gated ion channels containing an agonist recognition site, a transmembrane ion permeation pathway, and gating elements that couple agonist-induced conformational changes to the opening or closing of the permeation pore. Glutamate receptors mediate fast excitatory synaptic transmission in the central nervous system and are localized on neuronal and non-neuronal cells. These receptors regulate a broad spectrum of processes in the brain, spinal cord, retina, and peripheral nervous system. Glutamate receptors are postulated to play important roles in numerous neurological diseases and have attracted intense scrutiny. The description of glutamate receptor structure, including its transmembrane elements, reveals a complex assembly of multiple semiautonomous extracellular domains linked to a pore-forming element with striking resemblance to an inverted potassium channel. In this review we discuss International Union of Basic and Clinical Pharmacology glutamate receptor nomenclature, structure, assembly, accessory subunits, interacting proteins, gene expression and translation, post-translational modifications, agonist and antagonist pharmacology, allosteric modulation, mechanisms of gating and permeation, roles in normal physiological function, as well as the potential therapeutic use of pharmacological agents acting at glutamate receptors.

Fatty acid modulation and polyamine block of GluK2 kainate receptors analyzed by scanning mutagenesis

RNA editing of kainate receptor subunits at the Q/R site determines their susceptibility to inhibition by cis-unsaturated fatty acids as well as block by cytoplasmic polyamines. Channels comprised of unedited (Q) subunits are strongly blocked by polyamines, but insensitive to fatty acids, such as arachidonic acid (AA) and docosahexaenoic acid (DHA), whereas homomeric edited (R) channels resist polyamine block but are inhibited by AA and DHA. In the present study, we have analyzed fatty acid modulation of whole-cell currents mediated by homomeric recombinant GluK2 (formerly GluR6) channels with individual residues in the pore-loop, M1 and M3 transmembrane helices replaced by scanning mutagenesis. Our results define three abutting surfaces along the M1, M2, and M3 helices where gain-of-function substitutions render GluK2(Q) channels susceptible to fatty acid inhibition. In addition, we identify four locations in the M3 helix (F611, L614, S618, and T621) at the level of the central cavity where Arg substitution increases relative permeability to chloride and eliminates polyamine block. Remarkably, for two of these positions, L614R and S618R, exposure to fatty acids reduces the apparent chloride permeability and potentiates whole-cell currents approximately 5 and 2.5-fold, respectively. Together, our results suggest that AA and DHA alter the orientation of M3 in the open state, depending on contacts at the interface between M1, M2, and M3. Moreover, our results demonstrate the importance of side chains within the central cavity in determining ionic selectivity and block by cytoplasmic polyamines despite the inverted orientation of GluK2 as compared with potassium channels and other pore-loop family members.

Distinct AMPA-type glutamatergic synapses in developing rat CA1 hippocampus

We assessed synaptic α-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptor (AMPAR) properties during synaptogenesis to describe the development of individual glutamatergic synapses on rat hippocampal CA1 principal neurons. Pharmacologically isolated AMPAR-mediated glutamatergic synaptic currents [evoked by stimulation of the Schaffer Collateral pathway, excitatory postsynaptic currents (EPSCs)], had significantly greater inward-rectification at ages P5-7 compared with P8-18. These inward rectifying EPSCs demonstrated paired-pulse dependent unblocking at positive holding potentials, consistent with voltage-dependent internal polyamine block. Measurements of paired-pulse facilitation did not support altered presynaptic properties associated with inward rectification. Using asynchronous EPSCs (aEPSCs) to analyze populations of individual synapses, we found that quantal amplitudes (Q) increased across early postnatal development (P5-P18) and were directly modulated by increases in the number of activated receptors. Quantal AMPAR decay kinetics (aEPSC τ(decay)s) exhibited the highest coefficient of variation (CV) from P5 to 7 and became markedly less variable at P8-18. At P5-7, faster quantal kinetics coexisted with much slower kinetics; only slower quantal kinetics were found at P8-18. This supports diverse quantal synaptic properties limited to P5-7. Multivariate cluster analysis of Q, CV(τ decay), and median τ(decay) supported a segregation of neurons into two distinct age groups of P5-7 and P8-18, similar to the age-related segregation suggested by inward rectification. Taken together, these findings support synaptic, calcium permeable AMPARs at a subset of synapses onto CA1 pyramidal neurons exclusively at P5-7. These distinct synapses coexist with those sharing the properties of more mature synapses. These synapses disappear after P7 as activated receptor numbers increase with age.

Development of calcium-permeable AMPA receptors and their correlation with NMDA receptors in fast-spiking interneurons of rat prefrontal cortex

Abnormal influx of Ca(2+) is thought to contribute to the neuronal injury associated with a number of brain disorders, and Ca(2+)-permeable AMPA receptors (CP-AMPARs) play a critical role in the pathological process. Despite the apparent vulnerability of fast-spiking (FS) interneurons in neurological disorders, little is known about the CP-AMPARs expressed by functionally identified FS interneurons in the developing prefrontal cortex (PFC). We investigated the development of inwardly rectifying AMPA receptor-mediated currents and their correlation with NMDA receptor-mediated currents in FS interneurons in the rat PFC. We found that 78% of the FS interneurons expressed a low rectification index, presumably Ca(2+)-permeable AMPARs, with only 22% exhibiting AMPARs with a high rectification index, probably Ca(2+) impermeable (CI). FS interneurons with CP-AMPARs exhibited properties distinct from those expressing CI-AMPARs, although both displayed similar morphologies, passive membrane properties and AMPA currents at resting membrane potentials. The AMPA receptors also exhibited dramatic changes during cortical development with significantly more FS interneurons with CP-AMPARs and a clearly decreased rectification index during adolescence. In addition, FS interneurons with CP-AMPARs exhibited few or no NMDA currents, distinct frequency-dependent synaptic facilitation, and protracted maturation in short-term plasticity. These data suggest that CP-AMPARs in FS interneurons may play a critical role in neuronal integration and that their characteristic properties may make these cells particularly vulnerable to disruptive influences in the PFC, thus contributing to the onset of many psychiatric disorders.

Gating modes in AMPA receptors

AMPA receptors are ligand-gated ion channels that show multiple conductance levels, indicating that gating of individual AMPA subunits is to some extent independent of the other subunits. To study AMPAR subunit interactions during activation gating, we recorded from single channels in the absence of channel block and desensitization and at negative and positive membrane potentials. In saturating glutamate, the relative occupancies of the various conductance levels are consistent with complete subunit independence. In contrast, the relative occupancies in subsaturating glutamate indicate that the channel switches between a low open probability mode and a high open probability mode in which the behavior of the channel is identical to that in saturating glutamate. These gating modes occur at both negative and positive potentials, with the high open probability mode becoming more prominent at positive potentials. The switch between gating modes and its modulation by voltage and other factors may constitute a novel mechanism regulating AMPAR-mediated synaptic activity.

Learning to see: patterned visual activity and the development of visual function

To successfully interact with their environments, developing organisms need to correctly process sensory information and generate motor outputs appropriate to their size and structure. Patterned sensory experience has long been known to induce various forms of developmental plasticity that ultimately shape mature neural circuits. These same types of plasticity also allow developing organisms to respond appropriately to the external world by dynamically adapting neural circuit function to ongoing changes in brain circuitry and sensory input. Recent work on the visual systems of frogs and fish has provided an unprecedented view into how visual experience dynamically affects circuit function at many levels, ranging from gene expression to network function, ultimately leading to system-wide functional adaptations.

Subunit composition of synaptic AMPA receptors revealed by a single-cell genetic approach

The precise subunit composition of synaptic ionotropic receptors in the brain is poorly understood. This information is of particular importance with regard to AMPA-type glutamate receptors, the multimeric complexes assembled from GluA1-A4 subunits, as the trafficking of these receptors into and out of synapses is proposed to depend upon the subunit composition of the receptor. We report a molecular quantification of synaptic AMPA receptors (AMPARs) by employing a single-cell genetic approach coupled with electrophysiology in hippocampal CA1 pyramidal neurons. In contrast to prevailing views, we find that GluA1A2 heteromers are the dominant AMPARs at CA1 cell synapses (approximately 80%). In cells lacking GluA1, -A2, and -A3, synapses are devoid of AMPARs, yet synaptic NMDA receptors (NMDARs) and dendritic morphology remain unchanged. These data demonstrate a functional dissociation of AMPARs from trafficking of NMDARs and neuronal morphogenesis. This study provides a functional quantification of the subunit composition of AMPARs in the CNS and suggests novel roles for AMPAR subunits in receptor trafficking.

Engineering a high-affinity allosteric binding site for divalent cations in kainate receptors

Kainate receptors are allosterically regulated by sodium ions. Removal of Na+ from the extracellular solution, or replacement of Na+ by larger monovalent cations, inhibits kainate receptor activity. Sodium binds at a negatively charged cavity in the extracellular neurotransmitter binding domain that is capped by a small hydrophobic residue. Prior work revealed that introduction of aspartic acid at this site strongly reduces GluK2 sensitivity to monovalent cations of different size. We found that the GluK2 M739D mutant is also insensitive to substitution of Na+ by the large organic cations Tris and NMDG. Because these are excluded from the Na+ binding site by steric hindrance, we investigated the possibility that divalent cations can substitute for Na+. We show that in Na+ free solutions with low concentrations of Ca2+ and Mg2+ the GluK2 M739D mutant is inhibited by EDTA; that divalent ions in the micromolar concentration range act as positive allosteric modulators; and that the chemistry of the mutant cation binding site is typical of Ca2+ and Mg2+ binding sites found in protein crystal structures. Hence, the apparent insensitivity of the M739D mutant to monovalent cations is due to the adventitious allosteric effects of divalent ions at physiological concentrations and below.

Roles of distinct glutamate receptors in induction of anti-Hebbian long-term potentiation

Many glutamatergic synapses on interneurons involved in feedback inhibition in the CA1 region of the hippocampus exhibit an unusual form of long-term potentiation (LTP) that is induced only if presynaptic glutamate release occurs when the postsynaptic membrane potential is relatively hyperpolarized. We have named this phenomenon 'anti-Hebbian' LTP because it is prevented by postsynaptic depolarization during afferent activity, and hence its induction requirements are opposite to those of Hebbian NMDA receptor-dependent LTP. This symposium report addresses the roles of distinct glutamate receptors in the induction of anti-Hebbian LTP. Inwardly rectifying Ca(2+)-permeable AMPA receptors mediate fast glutamatergic signalling at synapses that exhibit this form of LTP, and they are highly likely to mediate the instructive signal that triggers the cascade leading to synapse strengthening. NMDA receptors, on the other hand, play no role, nor do they contribute substantially to synaptic transmission at synapses that exhibit anti-Hebbian LTP. Both kainate and group I metabotropic glutamate receptors are abundant in at least some interneurons in the feedback inhibitory circuit. Delineating the roles of kainate receptors has been hampered by sub-optimal pharmacological tools. As for group I metabotropic glutamate receptors, their role in anti-Hebbian LTP is permissive at the very least in some interneuron types, although an instructive role has been suggested in other forms of activity-dependent plasticity.

Differential mechanisms of transmission and plasticity at mossy fiber synapses

The last few decades have seen the hippocampal formation at front and center in the field of synaptic transmission. However, much of what we know about hippocampal short- and long-term plasticity has been obtained from research at one particular synapse; the Schaffer collateral input onto principal cells of the CA1 subfield. A number of recent studies, however, have demonstrated that there is much to be learned about target-specific mechanisms of synaptic transmission by study of the lesser known synapse made between the granule cells of the dentate gyrus; the so-called mossy fiber synapse, and its targets both within the hilar region and the CA3 hippocampus proper. Indeed investigation of this synapse has provided an embarrassment of riches concerning mechanisms of transmission associated with feedforward excitatory and inhibitory control of the CA3 hippocampus. Importantly, work from a number of labs has revealed that mossy fiber synapses possess unique properties at both the level of their anatomy and physiology, and serve as an outstanding example of a synapse designed for target-specific compartmentalization of synaptic transmission. The purpose of the present review is to highlight several aspects of this synapse as they pertain to a novel mechanism of bidirectional control of synaptic plasticity at mossy fiber synapses made onto hippocampal stratum lucidum interneurons. It is not my intention to pour over all that is known regarding the mossy fiber synapse since many have explored this topic exhaustively in the past and interested readers are directed to other fine reviews (Henze et al., 2000; Urban et al., 2001; Lawrence and McBain, 2003; Bischofberger et al., 2006; Nicoll and Schmitz, 2005).

Developmental expression of Ca2+-permeable AMPA receptors underlies depolarization-induced long-term depression at mossy fiber CA3 pyramid synapses

Many central excitatory synapses undergo developmental alterations in the molecular and biophysical characteristics of postsynaptic ionotropic glutamate receptors via changes in subunit composition. Concerning AMPA receptors (AMPARs), glutamate receptor 2 subunit (GluR2)-containing, Ca2+-impermeable AMPARs (CI-AMPARs) prevail at synapses between mature principal neurons; however, accumulating evidence indicates that GluR2-lacking, Ca2+-permeable AMPARs (CP-AMPARs) contribute at these synapses early in development. Here, we used a combination of imaging and electrophysiological recording techniques to investigate potential roles for CP-AMPARs at developing hippocampal mossy fiber-CA3 pyramidal cell (MF-PYR) synapses. We found that transmission at nascent MF-PYR synapses is mediated by a mixed population of CP- and CI-AMPARs as evidenced by polyamine-dependent inwardly rectifying current-voltage (I-V) relationships, and partial philanthotoxin sensitivity of synaptic events. CP-AMPAR expression at MF-PYR synapses is transient, being limited to the first 3 postnatal weeks. Moreover, the expression of CP-AMPARs is regulated by the PDZ (postsynaptic density-95/Discs large/zona occludens-1) domain-containing protein interacting with C kinase 1 (PICK1), because MF-PYR synapses in young PICK1 knock-out mice are philanthotoxin insensitive with linear I-V relationships. Strikingly, MF-PYR transmission via CP-AMPARs is selectively depressed during depolarization-induced long-term depression (DiLTD), a postsynaptic form of MF-PYR plasticity observed only at young MF-PYR synapses. The selective depression of CP-AMPARs during DiLTD was evident as a loss of postsynaptic CP-AMPAR-mediated Ca2+ transients in PYR spines and reduced rectification of MF-PYR synaptic currents. Preferential targeting of CP-AMPARs during DiLTD is further supported by a lack of DiLTD in young PICK1 knock-out mice. Together, these findings indicate that the transient participation of CP-AMPARs at young MF-PYR synapses dictates the developmental window to observe DiLTD.

Stargazin attenuates intracellular polyamine block of calcium-permeable AMPA receptors

Endogenous polyamines profoundly affect the activity of various ion channels, including that of calcium-permeable AMPA-type glutamate receptors (CP-AMPARs). Here we show that stargazin, a transmembrane AMPAR regulatory protein (TARP) known to influence transport, gating and desensitization of AMPARs, greatly reduces block of CP-AMPARs by intracellular polyamines. By decreasing CP-AMPAR affinity for cytoplasmic polyamines, stargazin enhances the charge transfer following single glutamate applications and eliminates the frequency-dependent facilitation seen with repeated applications. In cerebellar stellate cells, which express both synaptic CP-AMPARs and stargazin, we found that the rectification and unitary conductance of channels underlying excitatory postsynaptic currents were matched by those of recombinant AMPARs only when the latter were associated with stargazin. Taken together, our observations establish modulatory actions of stargazin that are specific to CP-AMPARs, and suggest that during synaptic transmission the activity of such receptors, and thus calcium influx, is fundamentally changed by TARPs.

GluR2-free ?-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptors intensify demyelination in experimental autoimmune encephalomyelitis

We adopted a genetic approach to test the importance of edited GluR2-free, Ca(2+)-permeable, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors in the pathophysiology of experimental autoimmune encephalomyelitis, an inflammatory demyelinative disorder resembling multiple sclerosis. Initial studies showed that oligodendroglial lineage cells from mice lacking functional copies of the gene encoding the GluR3 AMPA receptor subunit (Gria3) had a diminished capacity to assemble edited GluR2-free AMPA receptors, and were resistant to excitotoxicity in vitro. Neurological deficits and spinal cord demyelination elicited by immunization with myelin oligodendrocyte glycoprotein peptide were substantially milder in these Gria3 mutant mice than in their wild-type littermates. These results support the hypothesis that oligodendroglial excitotoxicity mediated by AMPA receptors that do not contain edited GluR2 subunits contributes to demyelination in experimental autoimmune encephalomyelitis, and suggest that inhibiting these Ca(2+)-permeable AMPA receptors would be therapeutic in multiple sclerosis.

The Role of the GluR2 Subunit in AMPA Receptor Function and Synaptic Plasticity

The AMPA receptor (AMPAR) GluR2 subunit dictates the critical biophysical properties of the receptor, strongly influences receptor assembly and trafficking, and plays pivotal roles in a number of forms of long-term synaptic plasticity. Most neuronal AMPARs contain this critical subunit; however, in certain restricted neuronal populations and under certain physiological or pathological conditions, AMPARs that lack this subunit are expressed. There is a current surge of interest in such GluR2-lacking Ca2+-permeable AMPARs in how they affect the regulation of synaptic transmission. Here, we bring together recent data highlighting the novel and important roles of GluR2 in synaptic function and plasticity.

Inhibition of arginase I activity by RNA interference attenuates IL-13-induced airways hyperresponsiveness

Increased arginase I activity is associated with allergic disorders such as asthma. How arginase I contributes to and is regulated by allergic inflammatory processes remains unknown. CD4+ Th2 lymphocytes (Th2 cells) and IL-13 are two crucial immune regulators that use STAT6-dependent pathways to induce allergic airways inflammation and enhanced airways responsiveness to spasmogens (airways hyperresponsiveness (AHR)). This pathway is also used to activate arginase I in isolated cells and in hepatic infection with helminths. In the present study, we show that arginase I expression is also regulated in the lung in a STAT6-dependent manner by Th2-induced allergic inflammation or by IL-13 alone. IL-13-induced expression of arginase I correlated directly with increased synthesis of urea and with reduced synthesis of NO. Expression of arginase I, but not eosinophilia or mucus hypersecretion, temporally correlated with the development, persistence, and resolution of IL-13-induced AHR. Pharmacological supplementation with l-arginine or with NO donors amplified or attenuated IL-13-induced AHR, respectively. Moreover, inducing loss of function of arginase I specifically in the lung by using RNA interference abrogated the development of IL-13-induced AHR. These data suggest an important role for metabolism of l-arginine by arginase I in the modulation of IL-13-induced AHR and identify a potential pathway distal to cytokine receptor interactions for the control of IL-13-mediated bronchoconstriction in asthma.

NMDA receptors trigger neurosecretion of 5-HT within dorsal raphe nucleus of the rat in the absence of action potential firing

Activity and calcium-dependent release of neurotransmitters from the somatodendritic compartment is an important signalling mechanism between neurones throughout the brain. NMDA receptors and vesicles filled with neurotransmitters occur in close proximity in many brain areas. It is unknown whether calcium influx through these receptors can trigger the release of somatodendritic vesicles directly, or whether postsynaptic action potential firing is necessary for release of these vesicles. Here we addressed this question by studying local release of serotonin (5-HT) from dorsal raphé nucleus (DRN) neurones. We performed capacitance measurements to monitor the secretion of vesicles in giant soma patches, in response to short depolarizations and action potential waveforms. Amperometric measurements confirmed that secreted vesicles contained 5-HT. Surprisingly, two-photon imaging of DRN neurones in slices revealed that dendritic calcium concentration changes in response to somatic firing were restricted to proximal dendritic areas. This implied that alternative calcium entry pathways may dominate the induction of vesicle secretion from distal dendrites. In line with this, transient NMDA receptor activation, in the absence of action potential firing, was sufficient to induce capacitance changes. By monitoring GABAergic transmission onto DRN 5-HT neurones in slices, we show that endogenous NMDA receptor activation, in the absence of postsynaptic firing, induced release of 5-HT, which in turn increased the frequency of GABAergic inputs through activation of 5-HT(2) receptors. We propose here that calcium influx through NMDA receptors can directly induce postsynaptic 5-HT release from DRN neurones, which in turn may facilitate GABAergic input onto these cells.

Regulation of Ca2+-permeable AMPA receptors: synaptic plasticity and beyond

AMPA-type glutamate receptors (AMPARs) mediate most fast excitatory synaptic transmission in the brain. Diversity in excitatory signalling arises, in part, from functional differences among AMPAR subtypes. Although the rapid insertion or deletion of AMPARs is recognised as important for the expression of conventional forms of long-term synaptic plasticity--triggered, for example, by Ca2+ entry through NMDA-type glutamate receptors--only recently has attention focused on novel forms of plasticity that are regulated by, or alter the expression of, Ca2+-permeable AMPARs. The dynamic regulation of these receptors is important for normal synaptic function and in disease states.

BACE1, a major determinant of selective vulnerability of the brain to amyloid-beta amyloidogenesis, is essential for cognitive, emotional, and synaptic functions

A transmembrane aspartyl protease termed beta-site APP cleavage enzyme 1 (BACE1) that cleaves the amyloid-beta precursor protein (APP), which is abundant in neurons, is required for the generation of amyloid-beta (Abeta) peptides implicated in the pathogenesis of Alzheimer's disease (AD). We now demonstrate that BACE1, enriched in neurons of the CNS, is a major determinant that predisposes the brain to Abeta amyloidogenesis. The physiologically high levels of BACE1 activity coupled with low levels of BACE2 and alpha-secretase anti-amyloidogenic activities in neurons is a major contributor to the accumulation of Abeta in the CNS, whereas other organs are spared. Significantly, deletion of BACE1 in APPswe;PS1DeltaE9 mice prevents both Abeta deposition and age-associated cognitive abnormalities that occur in this model of Abeta amyloidosis. Moreover, Abeta deposits are sensitive to BACE1 dosage and can be efficiently cleared from the CNS when BACE1 is silenced. However, BACE1 null mice manifest alterations in hippocampal synaptic plasticity as well as in performance on tests of cognition and emotion. Importantly, memory deficits but not emotional alterations in BACE1(-/-) mice are prevented by coexpressing APPswe;PS1DeltaE9 transgenes, indicating that other potential substrates of BACE1 may affect neural circuits related to emotion. Our results establish BACE1 and APP processing pathways as critical for cognitive, emotional, and synaptic functions, and future studies should be alert to potential mechanism-based side effects that may occur with BACE1 inhibitors designed to ameliorate Abeta amyloidosis in AD.