Desensitization and resensitization are independently regulated in human recombinant GluR subunit coassemblies

Genome- and transcriptome-wide splicing associations with alcohol use disorder

Genetic mechanisms of alternative mRNA splicing have been shown in the brain for a variety of neuropsychiatric traits, but not substance use disorders. Our study utilized RNA-sequencing data on alcohol use disorder (AUD) in four brain regions (n = 56; ages 40-73; 100% 'Caucasian'; PFC, NAc, BLA and CEA) and genome-wide association data on AUD (n = 435,563, ages 22-90; 100% European-American). Polygenic scores of AUD were associated with AUD-related alternative mRNA splicing in the brain. We identified 714 differentially spliced genes between AUD vs controls, which included both putative addiction genes and novel gene targets. We found 6463 splicing quantitative trait loci (sQTLs) that linked to the AUD differentially spliced genes. sQTLs were enriched in loose chromatin genomic regions and downstream gene targets. Additionally, the heritability of AUD was enriched for DNA variants in and around differentially spliced genes associated with AUD. Our study also performed splicing transcriptome-wide association studies (TWASs) of AUD and other drug use traits that unveiled specific genes for follow-up and splicing correlations across SUDs. Finally, we showed that differentially spliced genes between AUD vs control were also associated with primate models of chronic alcohol consumption in similar brain regions. Our study found substantial genetic contributions of alternative mRNA splicing in AUD.

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.

Pharmacological characterization of N-[(2S)-5-(6-fluoro-3-pyridinyl)-2, 3-dihydro-1H-inden-2-yl]-2-propanesulfonamide: a novel, clinical AMPA receptor positive allosteric modulator

BACKGROUND AND PURPOSE

AMPA receptor positive allosteric modulators represent a potential therapeutic strategy to improve cognition in people with schizophrenia. These studies collectively constitute the preclinical pharmacology data package used to build confidence in the pharmacology of this molecule and enable a clinical trial application.

EXPERIMENTAL APPROACH

[N-[(2S)-5-(6-fluoro-3-pyridinyl)-2,3-dihydro 1H-inden-2-yl]-2-propanesulfonamide] (UoS12258) was profiled in a number of in vitro and in vivo studies to highlight its suitability as a novel therapeutic agent.

KEY RESULTS

We demonstrated that UoS12258 is a selective, positive allosteric modulator of the AMPA receptor. At rat native hetero-oligomeric AMPA receptors, UoS12258 displayed a minimum effective concentration of approximately 10 nM in vitro and enhanced AMPA receptor-mediated synaptic transmission at an estimated free brain concentration of approximately 15 nM in vivo. UoS12258 reversed a delay-induced deficit in novel object recognition in rats after both acute and sub-chronic dosing. Sub-chronic dosing reduced the minimum effective dose from 0.3 to 0.03 mg·kg^-1 . UoS12258 was also effective at improving performance in two other cognition models, passive avoidance in scopolamine-impaired rats and water maze learning and retention in aged rats. In side-effect profiling studies, UoS12258 did not produce significant changes in the maximal electroshock threshold test at doses below 10 mg·kg^-1 .

CONCLUSION AND IMPLICATIONS

We conclude that UoS12258 is a potent and selective AMPA receptor modulator exhibiting cognition enhancing properties in several rat behavioural models superior to other molecules that have previously entered clinical evaluation.

Function of alternative splicing

Almost all polymerase II transcripts undergo alternative pre-mRNA splicing. Here, we review the functions of alternative splicing events that have been experimentally determined. The overall function of alternative splicing is to increase the diversity of mRNAs expressed from the genome. Alternative splicing changes proteins encoded by mRNAs, which has profound functional effects. Experimental analysis of these protein isoforms showed that alternative splicing regulates binding between proteins, between proteins and nucleic acids as well as between proteins and membranes. Alternative splicing regulates the localization of proteins, their enzymatic properties and their interaction with ligands. In most cases, changes caused by individual splicing isoforms are small. However, cells typically coordinate numerous changes in 'splicing programs', which can have strong effects on cell proliferation, cell survival and properties of the nervous system. Due to its widespread usage and molecular versatility, alternative splicing emerges as a central element in gene regulation that interferes with almost every biological function analyzed.

Biophysical mechanisms regulating AMPA receptor accumulation at synapses

Controlling the number of AMPA receptors at synapses is fundamental for fast synaptic transmission as well as for long term adaptations in synaptic strength. In this review, we examine the biophysical mechanisms implicated in regulating AMPAR levels at the cell surface and at synapses. We first describe the structure and function of AMPARs, as well as their interactions with various proteins regulating their traffic and function. Second we review the vesicular trafficking mechanism involving exocytosis and endocytosis, by which AMPARs reach the cell surface and are internalized, respectively. Third, we examine the properties of lateral diffusion of AMPARs and their trapping at post-synaptic densities. Finally, we discuss how these two parallel mechanisms are integrated in time and space to control changes in synaptic AMPAR levels in response to plasticity protocols. This review highlights the important role of the extra-synaptic AMPAR pool, which makes an obligatory link between vesicular trafficking and trapping or release at synapses.

Optogenetic stimulation of the corticothalamic pathway affects relay cells and GABAergic neurons differently in the mouse visual thalamus

The dorsal lateral geniculate nucleus (dLGN) serves as the primary conduit of retinal information to visual cortex. In addition to retinal input, dLGN receives a large feedback projection from layer VI of visual cortex. Such input modulates thalamic signal transmission in different ways that range from gain control to synchronizing network activity in a stimulus-specific manner. However, the mechanisms underlying such modulation have been difficult to study, in part because of the complex circuitry and diverse cell types this pathway innervates. To address this and overcome some of the technical limitations inherent in studying the corticothalamic (CT) pathway, we adopted a slice preparation in which we were able to stimulate CT terminal arbors in the visual thalamus of the mouse with blue light by using an adeno-associated virus to express the light-gated ion channel, ChIEF, in layer VI neurons. To examine the postsynaptic responses evoked by repetitive CT stimulation, we recorded from identified relay cells in dLGN, as well as GFP expressing GABAergic neurons in the thalamic reticular nucleus (TRN) and intrinsic interneurons of dLGN. Relay neurons exhibited large glutamatergic responses that continued to increase in amplitude with each successive stimulus pulse. While excitatory responses were apparent at postnatal day 10, the strong facilitation noted in adult was not observed until postnatal day 21. GABAergic neurons in TRN exhibited large initial excitatory responses that quickly plateaued during repetitive stimulation, indicating that the degree of facilitation was much larger for relay cells than for TRN neurons. The responses of intrinsic interneurons were smaller and took the form of a slow depolarization. These differences in the pattern of excitation for different thalamic cell types should help provide a framework for understanding how CT feedback alters the activity of visual thalamic circuitry during sensory processing as well as different behavioral or pathophysiological states.

Focal cortical infarcts alter intrinsic excitability and synaptic excitation in the reticular thalamic nucleus

Focal cortical injuries result in death of cortical neurons and their efferents and ultimately in death or damage of thalamocortical relay (TCR) neurons that project to the affected cortical area. Neurons of the inhibitory reticular thalamic nucleus (nRT) receive excitatory inputs from corticothalamic and thalamocortical axons and are thus denervated by such injuries, yet nRT cells generally survive these insults to a greater degree than TCR cells. nRT cells inhibit TCR cells, regulate thalamocortical transmission, and generate cerebral rhythms including those involved in thalamocortical epilepsies. The survival and reorganization of nRT after cortical injury would determine recovery of thalamocortical circuits after injury. However, the physiological properties and connectivity of the survivors remain unknown. To study possible alterations in nRT neurons, we used the rat photothrombosis model of cortical stroke. Using in vitro patch-clamp recordings at various times after the photothrombotic injury, we show that localized strokes in the somatosensory cortex induce long-term reductions in intrinsic excitability and evoked synaptic excitation of nRT cells by the end of the first week after the injury. We find that nRT neurons in injured rats show (1) decreased membrane input resistance, (2) reduced low-threshold calcium burst responses, and (3) weaker evoked excitatory synaptic responses. Such alterations in nRT cellular excitability could lead to loss of nRT-mediated inhibition in relay nuclei, increased output of surviving TCR cells, and enhanced thalamocortical excitation, which may facilitate recovery of thalamic and cortical sensory circuits. In addition, such changes could be maladaptive, leading to injury-induced epilepsy.

Fast vesicle replenishment and rapid recovery from desensitization at a single synaptic release site

When the synaptic connection between two neurons consists of a small number of release sites, the ability to maintain transmission at high frequencies is limited by vesicle mobilization and by the response of postsynaptic receptors. These two properties were examined at single release sites between granule cells and stellate cells by triggering bursts of quantal events either with alpha-latrotoxin or with high-frequency trains of presynaptic activity. Bursts and evoked responses consisted of tens to hundreds of events with frequencies of up to hundreds per second. This indicates that single release sites can rapidly supply vesicles from a reserve pool to a release-ready pool. In addition, postsynaptic AMPA receptors recover from desensitization with a time constant of approximately 5 ms. Thus, even for synapses composed of a single release site, granule cells can effectively activate stellate cells during sustained high-frequency transmission because of rapid vesicle mobilization and fast recovery of AMPA receptors from desensitization.

Differential response dynamics of corticothalamic glutamatergic synapses in the lateral geniculate nucleus and thalamic reticular nucleus

The corticothalamic feedback pathway provides excitatory synaptic input to both the thalamic reticular nucleus and the lateral geniculate nucleus. We studied excitatory postsynaptic currents elicited from corticothalamic stimulation in the visual sector of the thalamic reticular nucleus and the lateral geniculate nucleus to compare the response of these neurons to stimulation of their common input pathway. Using whole cell patch clamp recordings in ferret thalamic slices, we compared single excitatory postsynaptic current decay kinetics, presynaptic glutamate release dynamics through paired pulse facilitation and responses to corticothalamic train stimulation. We found that single thalamic reticular nucleus excitatory postsynaptic currents were significantly sharper than lateral geniculate nucleus responses. The mean thalamic reticular nucleus excitatory postsynaptic current decay constant (tau) was 4.9+/-0.5 ms, while the mean lateral geniculate nucleus excitatory postsynaptic current tau value was 11.8+/-0.8 ms. Presynaptic release dynamics as measured by responses to paired stimuli were conserved between the thalamic reticular nucleus and lateral geniculate nucleus. However, facilitating responses to train stimulation were markedly different between nuclei. Lateral geniculate nucleus responses showed proportionately larger facilitation (reaching 842.9 +/- 76.4% of excitatory postsynaptic current 1 amplitude) than thalamic reticular nucleus responses (reaching 223.1 +/- 44.0% of excitatory postsynaptic current 1 amplitude). These data indicate that while the corticothalamic pathway produces excitatory postsynaptic currents in both the thalamic reticular nucleus and lateral geniculate nucleus, other factors uniquely affect the functional integration of the inputs in each nucleus.

Molecular analysis of the interaction of the pyrazine derivatives RPR119990 and RPR117824 with human AMPA-type glutamate receptor channels

Antagonizing glutamatergic neurotransmission by blockade of AMPA-type glutamate receptors is a promising pharmacological strategy in the treatment of neurodegenerative diseases. We investigated the interaction of two new pyrazine derivatives (RPR119990 and RPR117824) with recombinant AMPA-type glutamate receptors. Recombinant homooligomeric GluR1flop, GluR2flip, GluR2flop, GluR6, non-desensitizing GluR2 L504Y channels and heterooligomeric GluR1/2 channels were expressed in HEK293 cells. AMPA-type channels were competitively inhibited by RPR119990 or RPR117824 with an IC(50) around 10 nM, at GluR6 channels the dose-response relation of the inhibition was shifted to higher concentrations. Non-desensitizing GluR2 L504Y channels were used to further characterize the inhibition. After equilibration with the agonist a marked dose-dependent current decay upon coapplication of glutamate and RPR119990 and a dose-independent time course of recovery from block was observed. The extents of current inhibition as well as the time constant of current decay upon addition of the blocker to the test solution were dependent on agonist concentration, pointing to a competitive inhibition. Quantitative analysis of the experimental data using computerized simulations are compatible with a competitive block mechanism and provides hints to binding sites at unliganded and liganded closed states of the receptor.

Two mechanisms of action of the adamantane derivative IEM-1460 at human AMPA-type glutamate receptors

1. Antagonizing glutamatergic neurotransmission by blockade of AMPA-type glutamate receptors (GluR) is a promising pharmacological strategy for neuroprotection in neurodegenerative diseases and acute treatment of stroke. 2. We investigated the interaction of the adamantane derivative IEM-1460 with human wild-type and mutant AMPA-type GluR channels. Different recombinant homooligomeric human AMPA-type GluR channels and a rat nondesensitizing mutant GluR (GluR2 L504Y) channel were expressed in HEK293 cells and investigated using the patch-clamp technique in combination with ultrafast agonist application. 3. When IEM-1460 was coapplied with glutamate, an open channel block mechanism was observed at slow desensitizing GluR2 flip (>/=0.1 mM IEM-1460) and nondesensitizing GluR2 L504Y channels (>/=1 microM IEM-1460). 4. A competitive block of AMPA-type channels was observed with IC(50) values for the dose block curves of 0.1 mM IEM-1460 at human unmutated and 10 microM IEM-1460 at mutant GluR channels. 5. Nondesensitizing GluR2 L504Y channels were used to further characterize the block mechanism. After equilibration with the agonist, a current decay upon coapplication of glutamate and IEM-1460 was observed. The recovery from block was independent of the glutamate and IEM-1460 concentration. The extent of current inhibition as well as the time constant of current decay upon addition of the blocker to the test solution were dependent on agonist concentration; this strongly points to an additional competitive-like block mechanism of IEM-1460 at human AMPA-type GluR channels. 6. The data were interpreted in the frame of a molecular scheme with two binding sites of IEM-1460 at the receptor, one at the unliganded resting and the other at the fully liganded open state of the channels.