Characterization of RNA editing of the glutamate-receptor subunits GluR5 and GluR6 in granule cells during cerebellar development

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.

AMPAR/TARP stoichiometry differentially modulates channel properties

AMPARs control fast synaptic communication between neurons and their function relies on auxiliary subunits, which importantly modulate channel properties. Although it has been suggested that AMPARs can bind to TARPs with variable stoichiometry, little is known about the effect that this stoichiometry exerts on certain AMPAR properties. Here we have found that AMPARs show a clear stoichiometry-dependent modulation by the prototypical TARP γ2 although the receptor still needs to be fully saturated with γ2 to show some typical TARP-induced characteristics (i.e. an increase in channel conductance). We also uncovered important differences in the stoichiometric modulation between calcium-permeable and calcium-impermeable AMPARs. Moreover, in heteromeric AMPARs, γ2 positioning in the complex is important to exert certain TARP-dependent features. Finally, by comparing data from recombinant receptors with endogenous AMPAR currents from mouse cerebellar granule cells, we have determined a likely presence of two γ2 molecules at somatic receptors in this cell type.

Kainate receptor RNA editing is markedly altered by acute spinal cord injury

We have previously observed changes in the RNA editing of AMPA receptors after acute spinal cord injury (SCI); this implies that post-transcriptional modifications are capable of affecting the physiological properties of glutamate receptor channels and related signal transduction in this neurodegenerative condition. Here, we report that the editing of the ionotropic KAR is markedly decreased at both GluK1 and GluK2 Q/R sites in the epicenter of the lesion and with distinct magnitude and kinetics also in the caudal and rostral portions of the injured cord. These effects are persistent, being observed as late as 30 days after lesioning. In addition, also the I/V and Y/C sites of GluK2 were severely affected after SCI. These findings add novel information to the relevance of editing of glutamate receptors following acute SCI, thus expanding the recently emerged role of post-transcriptional mechanisms under these experimental conditions.

Comparative RNA editing in autistic and neurotypical cerebella

Adenosine-to-inosine (A-to-I) RNA editing is a neurodevelopmentally regulated epigenetic modification shown to modulate complex behavior in animals. Little is known about human A-to-I editing, but it is thought to constitute one of many molecular mechanisms connecting environmental stimuli and behavioral outputs. Thus, comprehensive exploration of A-to-I RNA editing in human brains may shed light on gene-environment interactions underlying complex behavior in health and disease. Synaptic function is a main target of A-to-I editing, which can selectively recode key amino acids in synaptic genes, directly altering synaptic strength and duration in response to environmental signals. Here, we performed a high-resolution survey of synaptic A-to-I RNA editing in a human population, and examined how it varies in autism, a neurodevelopmental disorder in which synaptic abnormalities are a common finding. Using ultra-deep (>1000 × ) sequencing, we quantified the levels of A-to-I editing of 10 synaptic genes in postmortem cerebella from 14 neurotypical and 11 autistic individuals. A high dynamic range of editing levels was detected across individuals and editing sites, from 99.6% to below detection limits. In most sites, the extreme ends of the population editing distributions were individuals with autism. Editing was correlated with isoform usage, clusters of correlated sites were identified, and differential editing patterns examined. Finally, a dysfunctional form of the editing enzyme adenosine deaminase acting on RNA B1 was found more commonly in postmortem cerebella from individuals with autism. These results provide a population-level, high-resolution view of A-to-I RNA editing in human cerebella and suggest that A-to-I editing of synaptic genes may be informative for assessing the epigenetic risk for autism.

Glutamate receptor RNA editing in health and disease

RNA editing is a post-transcriptional process with an important role in gene modification. This editing process involves site-selective deamination of adenosine into inosine in the pre-mRNA, leading to the alteration of translation codons and splicing sites in nuclear transcripts, thereby enabling functionally distinct proteins to arise from a single gene. One important instance is the neuron editing of the ionotropic glutamate receptors (iGluRs). GluRs play a key role in excitatory synaptic transmission and plasticity in the central nervous system (CNS); their channel properties are largely dictated by the subunit composition of the tetrameric receptors. AMPA/kainate channels are assembled from GluA1-4 AMPA or GluK1-5 kainate receptor subunits. In particular, three of the four AMPA and two of the five kainate receptor subunits are subject to RNA editing. The editing positions have been named on the basis of the amino acid substitutions, such as the Q/R site in AMPA GluA2; the Q/R site in GluK1 and GluK2; the R/G site in GluA2, GluA3, and GluA4; and the I/V and Y/C sites in GluK2. These amino acid changes lead to profound alterations of the channel properties. This paper reviews the most relevant data showing the importance of glutamate receptor RNA editing in finely tuning glutamatergic neurotransmission in the normal CNS and following alterations of the editing process in association with disease phenotypes. Overall, these data indicate that a highly regulated process of glutamate receptor editing is of key importance in the proper function of neuronal cells and in their ability to adapt and modulate synaptic function.

Modulation of neurite outgrowth by activation of calcium-permeable kainate receptors expressed by rat nociceptive-like dorsal root ganglion neurons

Neurite outgrowth is a fundamental step in establishing proper neuronal connections in the developing central nervous system. Dynamic control of outgrowth has been attributed to changes in growth cone Ca2+ levels in response to extracellular cues. Here we have investigated a possible role for Ca2+ permeable kainate (KA) receptors in regulating neurite outgrowth of nociceptive-like dorsal root ganglion (DRG) neurons. To identify KA receptor subunits likely to be involved, we used quantitative RT-PCR on acutely dissociated DRG and dorsal horn neurons. DRG neurons expressed more GluK1, particularly the GluK1b spice variant, than dorsal horn neurons. Conversely, dorsal horn neurons expressed more GluK2, particularly GluK2a, than DRG neurons. Further, an RNA editing assay indicated that the majority of GluK1 and GluK2 mRNA transcripts in DRG were unedited. Imaging Ca2+ transients following application of a KA receptor agonist to DRG and dorsal horn co-cultures revealed increases in intracellular Ca2+ in the growth cones of DRG neurons. In the majority of cases, this increase in Ca2+ was partly or completely blocked by Joro spider toxin (JSTX), an antagonist for Ca2+-permeable AMPA and KA receptors. Treatment of DRG/dorsal horn co-cultures with KA for 18 hours suppressed neurite outgrowth while application of the rapidly desensitizing KA receptor agonist SYM 2081, the competitive AMPA/KA receptor antagonist, CNQX, and JSTX or philanthotoxin enhanced neurite outgrowth and prevented KA effects on neurite outgrowth. Thus, Ca2+ entry through KA receptors at the growth cone of DRG neurons may be an important regulator of neurite outgrowth.

Neto2 modulation of kainate receptors with different subunit compositions

Kainate receptors are less well understood than other glutamate receptors, and synaptic kainate receptors display properties that differ from recombinant receptors. In particular, the slow decay of kainate receptor synaptic currents contrasts with the rapid deactivation and desensitization of receptors expressed in heterologous cells. We recently identified Neuropilin and Tolloid like-2 (Neto2) as a novel accessory subunit of kainate receptors and showed that Neto2 modulates the gating kinetics of GluK2 receptors. However, the kainate receptor family consists of five different subunits (GluK1-5) that can form homomeric and heteromeric receptors with different functional properties. Here, we tested whether Neto2 modulation varies with subunit composition. Rapid application techniques were used to apply glutamate to outside-out patches that contained GluK1, GluK1/5, or GluK2/5 kainate receptors. Coexpression of Neto2 slowed desensitization to varying degrees. Responses to 1 ms pulses of glutamate were also slowed by Neto2, especially for receptors containing GluK5, as were postsynaptic currents in neurons expressing recombinant kainate receptors. In addition, Neto2 markedly increased the rate at which some receptors recovered from desensitization. These results suggest that Neto2 modulates the function of most kainate receptors.

Assembly and intracellular distribution of kainate receptors is determined by RNA editing and subunit composition

Kainate receptors (KARs) modulate neuronal network activity. The molecular mechanisms that control the assembly and trafficking of KARs are unclear. Here, we examined the role of Q/R editing and subunit composition on KAR subunit assembly and subcellular distribution. The majority of GluK2 subunits undergo editing at the Q/R site in the channel pore loop. Cell surface biotinylation, cross-linking, Endoglycosidase-H analysis and gradient separation of KAR subunit assembly states revealed that Q/R editing reduces oligomerization, endoplasmic reticulum (ER) export, plasma membrane expression and stability of homomeric GluK2-containing KARs. These results indicate that Q/R editing of GluK2 may orchestrate channel subunit composition during KAR assembly in the ER. GluK2/GluK5 heteromers are the most abundant KAR subtype in the brain. While subcellular fractionation of brain tissue confirmed that both GluK2/3 and GluK5 are present in synaptosomes and tightly associated with post-synaptic density fractions, biochemical analysis revealed that endogenous GluK2/3 subunits show less complete assembly and trafficking compared with GluK5. In transgenic mice, the loss of the key assembly partner GluK2 leads to dramatic reduction in GluK5 expression. These results support the idea that the assembly and intracellular distribution of KARs is determined by RNA editing at the Q/R site and subunit composition.

Inhibition of presynaptic release-facilitatory kainate autoreceptors by extracellular cyclic GMP

We found that both alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and kainate autoreceptors were present on the glutamate-releasing terminals of cerebellar parallel/climbing fibers and that they functioned as facilitatory autoreceptors. Extracellular cGMP inhibited the neurotransmitter release evoked by presynaptic kainate receptor activation; the inhibitory effect of extracellular cGMP was selective for the kainate autoreceptor-mediated response and did not affect the AMPA autoreceptor-mediated response. Endogenously synthesized cGMP might be the physiological source for the extracellular cGMP modulating the response to kainate autoreceptor activation.

Expression of GluR6 kainate receptor subunit in granular layer of weaver mouse cerebellum

The Girk2 ( wv ) (weaver) mutation impairs migration of cerebellar granule cells from external to internal granular layer and induces neuronal death during the first 2 weeks of postnatal life. Kainate receptors are heteromeric ionotropic receptors of glutamate consisting of five subunits termed GluR5, GluR6, GluR7, KA1 and KA2. In order to investigate whether the weaver gene affects the expression of kainate receptors in weaver cerebellum, we determined mRNA expression levels of GluR6 kainate receptor subunit and [(3)H]kainic acid specific binding in the developing cerebellum, using in situ hybridization and receptor film autoradiography, respectively. In the weaver postnatal day 10 (P10) cerebellum, our data indicated lower levels of GluR6 mRNA expression and lower [(3)H]kainic acid specific binding in external granular layer (EGL) compared to normal EGL. Our results are indicative of either down-regulation of kainate receptors or modulation of their functional characteristics in weaver granule cells.

The effect of the ketogenic diet on hippocampal GluR5 and Glu(6 mRNA expression and Q/R site editing in the kainate-induced epilepsy model

Treatment with the ketogenic diet has been used to control refractory epilepsy for many years, although its anticonvulsant mechanism is unknown. By modulating synaptic transmission in the hippocampus, kainate receptors and their RNA editing might play a role in the antiseizure action of the treatment. To investigate the potential effect of the ketogenic diet on GluR(5) mRNA and GluR(6) mRNA expression and Q/R site editing, we used the kainate-induced epilepsy model in the present study. Reverse transcription polymerase chain reaction was performed to determine GluR(5) and GluR(6) mRNA expression, and RNA editing was analyzed with the BbvIota restriction enzyme assay. The results demonstrated that expression of GluR(6) mRNA, but not GluR(5) mRNA, was significantly increased after 8 weeks of dietary treatment. Neither the GluR(5) nor the GluR(6) RNA editing rate at the Q/R site was significantly changed by dietary treatment. These data indicate that GluR(6) may be involved in the anticonvulsant mechanism of ketogenic diet treatment.

A real-time PCR method for the quantitative analysis of RNA editing at specific sites

In this study, a quantitative PCR (qPCR) method was developed to determine the A-to-I RNA editing frequencies at specific sites. The A-to-I RNA editing of nuclear transcripts exerts profound effects on the biological activities of gene products. RNA editing of nuclear gene transcripts have been shown to be developmentally regulated and tissue specific, and alternations of RNA editing activities have been observed under pathological conditions. Two sites of ionotropic glutamate receptor subunits, the Q/R site of zebrafish gria2alpha and the Y/C site of grik2alpha, were chosen in this study to demonstrate the applicability of the SYBR Green detection-based real-time PCR method to measure RNA editing activities during zebrafish development. The results obtained by qPCR were consistent with those obtained by the limited primer extension. However, the qPCR method has the advantages of easy handling and low cost.

Kainate receptors and RNA editing in cholinergic neurons

Parasympathetic ganglia are considered simple relay systems that have cholinergic input and output, with modulation occurring centrally. Greater complexity is suggested, however, by our showing here that avian ciliary ganglion (CG) neurons also express a different excitatory receptor type--ionotropic glutamate receptors of the kainate subtype (KARs). This is the first report of glutamate receptor expression in the CG and KAR expression in any cholinergic neuron. We show that KARs form functional channels on CG neurons. KARs localize to CG neuron axons and somata as well as axons and terminals of pre-synaptic inputs to the CG. Glutamate transporters are expressed on Schwann cells that surround synapses on neuronal somata, and may provide a local source of glutamate. CG neurons express multiple KAR subunit mRNAs (GluR5, GluR7, and KA1), and their relative levels change dramatically during axon outgrowth and synaptic differentiation. The developmental role for KARs may depend upon their calcium permeability, a property regulated by mRNA editing. We show GluR5 editing increases predominantly at the time CG axons contact peripheral targets. Our data suggest that glutamatergic signaling may function as a local circuit mechanism to modulate excitability and calcium signaling during synapse formation and maturation in the CG in vivo.

Glutamate receptors and pain

Pain is an important survival and protection mechanism for animals. However, chronic/persistent pain may be differentiated from normal physiological pain in that it confers no obvious advantage. An accumulating body of pharmacological, electrophysiological, and behavioral evidence is emerging in support of the notion that glutamate receptors play a crucial role in pain pathways and that modulation of glutamate receptors may have potential for therapeutic utility in several categories of persistent pain, including neuropathic pain resulting from injury and/or disease of central (e.g., spinal cord injury) or peripheral nerves (e.g., diabetic neuropathy, radiculopathy) and inflammatory or joint-related pain (e.g., rheumatoid arthritis, osteoarthritis). This review focuses on the role of glutamate receptors, including both ionotropic (AMPA, NMDA and kainate) and metabotropic (mGlu1-8) receptors in persistent pain states with particular emphasis on their expression patterns in nociceptive pathways and their potential as targets for pharmacological intervention strategies.

AMPA/kainate receptor-mediated up-regulation of GABAA receptor δ subunit mRNA expression in cultured rat cerebellar granule cells is dependent on NMDA receptor activation

We have studied the effects of AMPA/kainate receptor agonists on GABA(A) receptor subunit mRNA expression in vitro in cultured rat cerebellar granule cells (CGCs). Kainate (KA) (100 microM) and high K(+) (25 mM) dramatically up-regulated delta subunit mRNA expression to 500-700% of that in control cells grown in low K(+) (5 mM). KA or high K(+) had no effect on the expression of the other major GABA(A) receptor subunits alpha1, alpha6, beta2, beta3 or gamma2. Up-regulation of delta mRNA was also detected with the AMPA receptor-selective agonist CPW-399 and to a lesser extent with the KA receptor-selective agonist ATPA. AMPA/kainate receptor-selective antagonist DNQX completely inhibited KA-, CPW-399- and ATPA-induced delta mRNA up-regulation indicating that the effects were mediated via AMPA and KA receptor activation. NMDA receptor-selective antagonist MK-801 inhibited 76% of the KA- and 57% of the CPW-399-induced delta up-regulation suggesting that KA and CPW-399 treatments may induce glutamate release resulting in NMDA receptor activation, and subsequently to delta mRNA up-regulation. In CGCs, delta subunit is a component of extrasynaptic alpha6betadelta receptors that mediate tonic inhibition. Up-regulation of delta during prolonged glutamate receptor activation or cell membrane depolarization may be a mechanism to increase tonic inhibition to counteract excessive excitation.

Detection of Purkinje cell loss following drug exposures to developing rat pups using reverse transcriptase-polymerase chain reaction (RT-PCR) analysis for calbindin-D28k mRNA expression

A technique is described that allows for the identification and quantification of Purkinje cell loss in cerebellum subsequent to developmental toxic exposures. This technique relies upon the extensively validated findings that the Purkinje cell is the only site of expression in the cerebellum of the calcium binding protein calbindin-D28k. Thus, analysis of mRNA expression specific to this protein by comparison to matched controls provides a reliable means of determining whether cell loss has occurred. Purkinje cell loss was induced in rat pups by ethanol exposure on postnatal day (PN) 4 or valproic acid administration to pregnant dams on gestational day 13. Analysis was conducted on PN5 or PN10 and the results compared to parallel groups of pups where the Purkinje cells were counted by traditional means. When compared to matched control rat pups the decrease in calbindin-D28k mRNA expression indicates Purkinje cell loss regardless of whether the cell loss was induced by prenatal valproic acid or postnatal ethanol exposure. The availability of a biochemical alternative to histological cell counting allows for more detailed analyses of the mechanisms of Purkinje cell death induced by these two toxicants, including analyses of the early alterations in signal transduction proteins.

Kainate down-regulates a subset of GABAA receptor subunits expressed in cultured mouse cerebellar granule cells

The effect of kainate, an agonist selective for ionotropic AMPA/kainate type of glutamate receptors, on GABAA receptor subunit expression in cultured mouse cerebellar granule cells was studied using quantitative RT-PCR, ligand binding and electrophysiology. Chronic kainate treatment, without producing excitotoxicity, resulted in preferential, dose- and time-dependent down-regulation of alpha1, alpha6 and beta2 subunit mRNA expression, the expression of beta3, gamma2 and delta subunit mRNAs being less affected. The down-regulation was reversed by DNQX, an AMPA/kainate-selective glutamate receptor antagonist. A 14-day kainate treatment resulted in 46% decrease of total [3H]Ro 15-4513 binding to the benzodiazepine sites. Diazepam-insensitive [3H]Ro 15-4513 binding was decreased by 89% in accordance with very low amount of alpha6 subunit mRNA present. Diazepam-sensitive [3H]Ro 154513 binding was decreased only by 40%, contrasting >90% decrease in alpha1 subunit mRNA expression. However, this was consistent with lower potentiation of GABA-evoked currents in kainate-treated than control cells by the alpha1-selective benzodiazepine site ligand zolpidem, suggesting compensatory expression of alpha5 (and/or alpha2 or alpha3) subunits producing diazepam-sensitive but zolpidem-insensitive receptor subtypes. In conclusion, chronic kainate treatment of cerebellar granule cells selectively down-regulates oil, alpha6 and beta2 subunits resulting in altered GABAA receptor pharmacology.

Kainate receptors and synaptic transmission

Excitatory glutamatergic transmission involves a variety of different receptor types, each with distinct properties and functions. Physiological studies have identified both post- and presynaptic roles for kainate receptors, which are a subtype of the ionotropic glutamate receptors. Kainate receptors contribute to excitatory postsynaptic currents in many regions of the central nervous system including hippocampus, cortex, spinal cord and retina. In some cases, postsynaptic kainate receptors are co-distributed with alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-methyl-D-aspartate (NMDA) receptors, but there are also synapses where transmission is mediated exclusively by postsynaptic kainate receptors: for example, in the retina at connections made by cones onto off bipolar cells. Modulation of transmitter release by presynaptic kainate receptors can occur at both excitatory and inhibitory synapses. The depolarization of nerve terminals by current flow through ionotropic kainate receptors appears sufficient to account for most examples of presynaptic regulation; however, a number of studies have provided evidence for metabotropic effects on transmitter release that can be initiated by activation of kainate receptors. Recent analysis of knockout mice lacking one or more of the subunits that contribute to kainate receptors, as well as studies with subunit-selective agonists and antagonists, have revealed the important roles that kainate receptors play in short- and long-term synaptic plasticity. This review briefly addresses the properties of kainate receptors and considers in greater detail the physiological analysis of their contributions to synaptic transmission.

Activation of spinal kainate receptors after inflammation: behavioral hyperalgesia and subunit gene expression

We determined whether neural responses to inflammation and hyperalgesia involve activation of kainate receptors, a subgroup of glutamate receptors. Inflammation was introduced into the hind paw by intraplantar injection of complete Freund's adjuvant. The inflammation-induced thermal hyperalgesia was attenuated by intrathecal administration of a non-selective alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate receptor antagonist, 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo[f]quinoxaline-7-sulfonamide disodium (NBQX), as well as by selective kainate receptor antagonists, 6,7,8,9-tetrohydro-5-nitro-1H-benz[g]indole-2,3-dione 3-oxime (NS-102) and 3S,4aR,6S,8aR-6-(4-carboxyphenyl)methyl-1,2,3,4,4a,5,6,7,8,8a-deca-hydroisoquinoline-3-carboxylic acid (LY382884). Reverse transcription-polymerase chain reaction (RT-PCR) indicated that the GluR5 and GluR6, but not the GluR7, KA1 and KA2 subunits, exhibited increased mRNA expression at 2 h to 3 days following inflammation (P<0.05). Western blot showed an increase in GluR6 protein levels (P<0.01) with a time course consistent with the changes in its mRNA levels. cDNA sequence and BbvI endonuclease digestion of the GluR6 PCR product revealed that the upregulated GluR6 mRNAs were predominantly the unedited form (Q). These results suggest that a selective upregulation of kainate receptor subunit expression contributes to inflammatory hyperalgesia.

RNA editing of neurotransmitter receptors in the mammalian brain

RNA editing refers to various posttranscriptional mechanisms that alter the nucleotide sequence of RNA. In the mammalian brain, RNA editing results in significant changes in the functional properties of receptors for the important neurotransmitters glutamate and serotonin. These changes result from site-specific deamination of single adenosines in the pre-messenger RNA encoding these receptors. Here, we review what is known about the mechanisms underlying this editing, the consequences of RNA editing for glutamate and serotonin receptor function, and recent studies on transgenic mice and human post-mortem tissue that have begun to elucidate the role of RNA editing in the intact mammalian brain.

Excitotoxic profiles of novel, low-affinity kainate receptor agonists in primary cultures of murine cerebellar granule cells

The involvement of low-affinity kainate (KA) receptors in neuronal injury was investigated by employing a variety of agonists active at GluR5-7. Their excitotoxic profiles were determined in primary cultures of cerebellar granule cells, which abundantly expressed low-affinity KA receptors, and in the absence of any AMPA receptor-mediated neurotoxicity. Neurotoxicity induced by these compounds was analysed by phase contrast microscopy, a cell viability assay, the TUNEL technique (apoptosis), and by employing propidium iodide (PI; necrosis). All agonists induced concentration-dependent neurotoxicity, with rank order (EC(50) values; microM): (S)-iodowillardiine (IW) 0.2>(2S,4R)-4-methylglutamate (4-MG) 36>(2S,4R,6E)-2-amino-4-carboxy-7-(2-naphthyl)hept-6-enoic acid (LY339434) 46>KA 74>(RS)-2-amino-3-(hydroxy-5-tert-butylisoxazol-4yl)propanoic acid (ATPA) 88. IW exposure resulted in apoptosis at lower concentrations (<30 microM) and necrosis at higher concentrations, both of which were attenuated by CNQX (50 microM), but not MK-801 (10 microM). ATPA-mediated neurotoxicity was purely apoptotic and was attenuated by the non-NMDA receptor antagonists. Both IW and ATPA induced injury with the morphological characteristics of apoptosis shown by the presence of TUNEL-positive neurones. LY339434-mediated neuronal injury was only attenuated by MK-801 and was necrotic in nature. Similarly, 4-MG (>30 microM) exposure caused necrosis that was partially attenuated by MK-801 (10 microM) and CNQX (50 microM). The patterns of neurotoxicity possessed a complex pharmacological profile, demonstrated an apoptotic-necrotic continuum and were inconsistent with past findings, further outlining the importance of characterizing novel compounds at native receptors. ATPA and to a lesser extent IW appear to be suitable drugs for low-affinity KA receptors. Since toxicity-mediated by low-affinity KA receptors seem likely to contribute to neurodegenerative conditions, our study importantly examines the excitotoxic profile of these novel agonists.

Molecular physiology of kainate receptors

A decade ago, our understanding of the molecular properties of kainate receptors and their involvement in synaptic physiology was essentially null. A plethora of recent studies has altered this situation profoundly such that kainate receptors are now regarded as key players in the modulation of transmitter release, as important mediators of the postsynaptic actions of glutamate, and as possible targets for the development of antiepileptic and analgesic drugs. In this review, we summarize our current knowledge of the properties of kainate receptors focusing on four key issues: 1) their structural and biophysical features, 2) the important progress in their pharmacological characterization, 3) their pre- and postsynaptic mechanisms of action, and 4) their involvement in a series of physiological and pathological processes. Finally, although significant progress has been made toward the elucidation of their importance for brain function, kainate receptors remain largely an enigma and, therefore, we propose some new roads that should be explored to obtain a deeper understanding of this young, but intriguing, class of proteins.

Glutamate receptor subunit expression in primary neuronal and secondary glial cultures

We report on the expression of ionotropic glutamate receptor subunits in primary neuronal cultures from rat cortex, hippocampus and cerebellum and of metabotropic glutamate (mGlu) receptor subtypes in these neuronal cultures as well as in cortical astroglial cultures. We found that the NMDA receptor (NR) subunits NR1, NR2A and NR2B were expressed in all three cultures. Each of the three cultures showed also expression of the four AMPA receptor subunits. Although RT-PCR detected mRNA of all kainate (KA) subunits in the three cultures, western blot showed only expression of Glu6 and KA2 receptor subunits. The expression analysis of mGlu receptors indicated the presence of all mGlu receptor subtype mRNAs in the three neuronal cultures, except for mGlu2 receptor mRNA, which was not detected in the cortical and cerebellar culture. mGlu1a/alpha, -2/3 and -5 receptor proteins were present in all three cultures, whereas mGlu4a and mGlu8a receptor proteins were not detected. Astroglial cultures were grown in either serum-containing or chemically defined medium. Only mGlu5 receptor protein was found in astroglial cultures grown in serum-containing medium. When astrocytes were cultured in chemically defined medium, mGlu3, -5 and -8 receptor mRNAs were detected, but at the protein level, still only mGlu5 receptor was found.

Larger intercellular variation in (Q/R) editing of GluR6 than GluR5 revealed by single cell RT-PCR

RNA editing of the pre-mRNA encoding the kainate receptor subtypes determines the Ca2+ permeability and the rectifying properties of the receptors in which these are assembled. GluR6 pre-mRNA contains three characterized editing sites: Q/R, IN and the Y/C, whereas GluR5 pre-mRNA contains only the (Q/R) site. Single cell RT-PCR was used on cultured cortical neurons to determine the relative expression and editing levels of the kainate receptor subunits encoding mRNA. The analysis showed a large intercellular variation in editing efficiency. The overall lower level of GluR5 editing, in the culture, compared to GluR6 editing is a result of an approximately 60% lower editing efficiency of GluR5 pre-mRNA, within single cells, compared with GluR6.

Concentration-dependent substate behavior of native AMPA receptors

AMPA-type glutamate receptors mediate most excitatory postsynaptic currents (EPSCs) at central synapses, and their conductance determines in part the size of EPSCs. The conductance of a recombinant AMPA receptor depends on the number of agonist molecules bound to the channel. Here we tested whether native AMPA and kainate receptors show this behavior in outside-out patches from neurons in situ by measuring conductance levels of single channels over a wide range of agonist concentrations. We found that the conductance of AMPA, but not kainate, receptors depended strongly on agonist concentration. Our results suggest that alterations in the glutamate concentration in the synaptic cleft may change the apparent unitary conductance of postsynaptic AMPA receptors.

Kainate receptor-mediated synaptic currents in cerebellar Golgi cells are not shaped by diffusion of glutamate

We report the presence of kainate receptors (KARs) in cerebellar Golgi cells of wild-type but not GluR6-deficient mice. Parallel fiber stimulation activates KAR-mediated synaptic currents [KAR-excitatory postsynaptic currents (EPSCs)] of small amplitude. KAR-EPSCs greatly differ from synaptic currents mediated by alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptors (AMPAR-EPSCs) at the same synapse. KAR-EPSCs display slow rise and decay time and summate in response to a train of stimulations. By using PDA, a low-affinity competitive antagonist and agents that modify the clearance of glutamate, we show that these properties cannot be explained by diffusion of glutamate outside of the synaptic cleft and activation of extrasynaptic KARs. These data suggest that the slow kinetic of KAR-EPSCs is due to intrinsic properties of KARs being localized at postsynaptic sites. The contrasting properties of KAR- and AMPAR-EPSCs in terms of kinetics and summation offer the possibility for a glutamatergic synapse to integrate excitatory inputs over two different time scales.

Heteromeric kainate receptors formed by the coassembly of GluR5, GluR6, and GluR7

In the CNS kainate subtype glutamate receptors (GluRs) are likely to be heteromeric assemblies containing multiple gene products. However, although recombinant kainate receptors from the GluR5-GluR7 gene family have been studied extensively in their homomeric forms, there have been no tests to determine whether these subunits can coassemble with each other. We used the GluR5 selective agonists (RS)-2-amino-3-(3-hydroxy-5-tertbutylisoxazol-4-yl)propanoic acid (ATPA) and (S)-5-iodowillardiine (I-will) to test for the coassembly of GluR5 with GluR6 and GluR7 by measuring changes in rectification that occur for heteromeric receptors containing both edited and unedited Q/R site subunits. Birectifying ATPA and I-will responses resulting from polyamine block for homomeric GluR5(Q) became outwardly rectifying when GluR6(R) was coexpressed with GluR5(Q), although GluR6 was not activated by ATPA or I-will, indicating the formation of heteromeric receptors. Similar approaches showed the coassembly of GluR7 with GluR6 and GluR5. Heteromeric kainate receptors containing both GluR5 and GluR6 subunits exhibited novel functional properties, including reduced desensitization and faster recovery from desensitization than those recorded for homomeric GluR5. Coexpression of GluR6 with GluR5 also enhanced the magnitude of responses to GluR5 selective agonists. In contrast, the coassembly of GluR7 with GluR6 markedly decreased the amplitude of agonist responses. Our results indicate that, similar to AMPA receptors, the kainate receptor subunits GluR5-GluR7 exhibit promiscuous coassembly. The formation of heteromeric kainate receptors may help to explain why the functional properties of native kainate receptors differ from those that have been reported for recombinant kainate receptors.

Regulated expression of estrogen receptor alpha and beta mRNA in granule cells during development of the rat cerebellum

A semi-quantitative RT-PCR approach was used to characterize expression of the mRNA encoding estrogen receptors in developing cerebellar granule cells of the rat. Evidence is presented for expression of both ERalpha and ERbeta transcripts in granule cells throughout the first 15 postnatal days. While transcripts encoding both ERalpha and ERbeta were expressed in granule cells, the relative levels of expression varied significantly during the first two postnatal weeks of cerebellar development. The ERalpha mRNA was expressed at the lowest level on the first day following birth; whereas expression of ERbeta was highest on that day. On the fourth postnatal day the expression of ERalpha increased, while there was a significant decrease in ERbeta expression. Between postnatal day 4 and 15, the expression of the mRNA of each receptor varied in a similar fashion; expression decreased slightly between days 4 and 10 and then increased significantly on day 15. Alternative splicing of the ERbeta transcripts was also investigated and was likewise found to vary during granule cell development. Initially, the mRNA encoding the beta1 isoform was predominant, but by day 4, the beta2 isoform was the major isoform expressed. On postnatal days 7 and 10, there was not a significant difference between the level of beta1 and beta2 expressed. By day 15, beta1 was again the predominant ERbeta isoform accounting for nearly 90% of all ERbeta transcripts expressed.

Distinct kainate receptor phenotypes in immature and mature mouse cerebellar granule cells

1. Although glutamate receptors have been shown to be involved in neuronal maturation, a developmental role for kainate-type receptors has not been described. In addition, the single-channel properties of native kainate receptors have not been studied in situ. We have characterized the electrophysiological properties of native kainate receptors of granule cell neurons at two distinct stages in postnatal development, using whole-cell and outside-out patch-clamp recordings in acute cerebellar slices. 2. Kainate-type currents were detected in both immature and mature granule cells. However, noise analysis showed that the apparent unitary conductance of kainate-type channels is significantly higher in proliferating than post-migratory granule cells. The conductance and rectification behaviour of the channels in immature granule cells indicate that they contain unedited GluR5 and GluR6 subunits and are likely to be calcium permeable. 3. Single-channel kainate-type currents were observed in outside-out patches from proliferating granule cells in the external germinal layer. The kinetic behaviour of kainate receptors in immature cells was complex. Openings to multiple conductance levels were observed, although our analysis indicates that the channels spend most of their open time in a 4 pS state.

RNA editing of a Drosophila sodium channel gene

Extensive analysis of cDNAs from the para locus in D. melanogaster reveals posttranscriptional modifications indicative of adenosine-to-inosine RNA editing. Most of these edits occur in highly conserved regions of the Na+ channel, and they occur in distant relatives of D. melanogaster as well. Sequence comparison between species has identified putative cis-acting elements important for each RNA editing site. Double-stranded RNA secondary structures with striking similarity to known RNA editing sites were generated based on these data. In addition, the RNA editing sites appear to be developmentally regulated. We have cloned a potential RNA editase, DRED, with a high degree of homology to the mammalian RED1,2 genes. The DRED locus itself is highly regulated by transcription from alternative promoters and alternative splicings.

Editing of glutamate receptor subunit B pre-mRNA by splice-site variants of interferon-inducible double-stranded RNA-specific adenosine deaminase ADAR1

The interferon-inducible RNA-specific adenosine deaminase (ADAR1) is an RNA-editing enzyme that catalyzes the deamination of adenosine in double-stranded RNA structures. Three alternative splice-site variants of ADAR1 (ADAR1-a, -b, and -c) occur that possess functionally distinct double-stranded RNA-binding motifs as measured with synthetic double-stranded RNA substrates. The pre-mRNA transcript encoding the B subunit of glutamate receptor (GluR-B) has two functionally important editing sites (Q/R and R/G sites) that undergo selective A-to-I conversions. We have examined the ability of the three ADAR1 splice-site variants to catalyze the editing of GluR-B pre-mRNA at the Q/R and R/G sites as well as an intron hotspot (+60) of unknown function. Measurement of GluR-B pre-mRNA editing in vitro revealed different site-specific deamination catalyzed by the three ADAR1 variants. The ADAR1-a, -b, and -c splice variants all efficiently edited the R/G site and the intron +60 hotspot but exhibited little editing activity at the Q/R site. ADAR1-b and -c showed higher editing activity than ADAR1-a for the R/G site, whereas the intron +60 site was edited with comparable efficiency by all three ADAR1 splice variants. Mutational analysis revealed that the functional importance of each of the three RNA-binding motifs of ADAR1 varied with the specific target editing site in GluR-B RNA. Quantitative reverse transcription-polymerase chain reaction analyses of GluR-B RNA from dissected regions of rat brain showed significant expression and editing at the R/G site in all brain regions examined except the choroid plexus. The relative levels of the alternatively spliced flip and flop isoforms of GluR-B RNA varied among the choroid plexus, cortex, hippocampus, olfactory bulb, and striatum, but in all regions of rat brain the editing of the flip isoform was greater than that of the flop isoform.

High-affinity kainate-type ion channels in rat cerebellar granule cells

1. Patch-clamp recordings were made from rat cerebellar granule cells in primary culture. In cells pre-exposed to concanavalin A (ConA) to remove kainate receptor desensitization, concentration-response data for kainate showed two components. The EC50 value for the high-affinity component (4 microM) was consistent with activation of kainate-type channels. ConA enhanced the apparent potency of the kainate receptor ligand SYM 2081 by 100-fold. 2. In ConA-treated granule cells, currents evoked by 10 microM kainate were not significantly reduced by the AMPA receptor antagonist GYKI 53655, nor were these currents significantly reduced by the co-application of 100 microM AMPA. Currents activated by low concentrations of kainate in the presence of AMPA were completely inhibited by 10 microM La3+. 3. Single-cell reverse transcriptase-polymerase chain reaction (RT-PCR) analysis indicated that granule cells express both unedited (Q) and edited (R) versions of GluR5, with the majority of the GluR5 transcripts being unedited. In contrast, BluR6(R) was detected in seven cells and GluR6(Q) was detected in one granule cell. 4. Whole-cell current-voltage curves for kainate-type currents in granule cells were measured and the ratio of the slope conductances at +40 MV and -40 mV was used as an index of rectification. The mean +40 mV/-40 mV ratio determined from thirty-six granule cells was 1.3 +/- 0.1. Spectral density analysis of kainate-evoked whole-cell current noise gave values for the apparent single-channel conductance, gamma(noise), that were on average about 1 pS. 5. To compare further the properties of recombinant kainate channels with the native kainate-type channels in granule cells, we determined EC50 and gamma(noise) values for SYM 2081 in stable cell lines expressing either (GluR6(R) or GluR6(R) and KA2. Co-expression of KA2 with GluR6(R) shifts the EC50 and gamma(noise) values determined for SYM 2081 closer to the values typically found for native kainate-type channels in granule cells. 6. The results demonstrate that cerebellar granule cells in culture express functional kainate-type channels and that in most cells these channels show properties that are similar to those determined for heteromeric channels formed from GluR6(R) and KA2. However, the results also suggest that different granule cells express different repertoires of kainate-type channels with different, and perhaps variable, subunit composition.