Alternative splicing at the C-terminal but not at the N-terminal domain of the NMDA receptor NR1 is altered in the kindled hippocampus

■ REVIEW : Long-Term Modulation of Gene Expression in Epilepsy

Molecular genetics has led to major advances in the study of neurological disease over the last 2 decades. Initial advances were made in understanding specific mutations that were associated with disease, such as epilepsy and other neurological conditions. In addition to specific mutations, recent research has focused on long-lasting or permanent changes in genetic expression as an underlying substrate of acquired diseases such as epilepsy. In symptomatic epilepsy, normal brain tissue is permanently altered and develops spon taneous recurrent seizures. Evidence indicates that long-lasting changes in gene expression at both tran scriptional and post-transcriptional levels are associated with epileptogenesis. The expression of transcription factors and other regulatory proteins represent a molecular mechanism for mediating these changes. Understanding the effects of severe environmental stresses on the multiple sites of transcriptional and post-transcriptional regulation of gene expression is likely to provide important insights into the devel opment of altered neuronal function in a number of important disease states, including epilepsy. NEURO SCIENTIST 5:86-99, 1999

Emerging mechanisms and consequences of calcium regulation of alternative splicing in neurons and endocrine cells

Alternative splicing contributes greatly to proteomic complexity. How it is regulated by external stimuli to sculpt cellular properties, particularly the highly diverse and malleable neuronal properties, is an underdeveloped area of emerging interest. A number of recent studies in neurons and endocrine cells have begun to shed light on its regulation by calcium signals. Some mechanisms include changes in the trans-acting splicing factors by phosphorylation, protein level, alternative pre-mRNA splicing, and nucleocytoplasmic redistribution of proteins to alter protein-RNA or protein-protein interactions, as well as modulation of chromatin states. Importantly, functional analyses of the control of specific exons/splicing factors in the brain point to a crucial role of this regulation in synaptic maturation, maintenance, and transmission. Furthermore, its deregulation has been implicated in the pathogenesis of neurological disorders, particularly epilepsy/seizure. Together, these studies have not only provided mechanistic insights into the regulation of alternative splicing by calcium signaling but also demonstrated its impact on neuron differentiation, function, and disease. This may also help our understanding of similar regulations in other types of cells.

Development of post-traumatic epilepsy after controlled cortical impact and lateral fluid-percussion-induced brain injury in the mouse

The present study investigated the development of hyperexcitability and epilepsy in mice with traumatic brain injury (TBI) induced by controlled cortical impact (CCI) or lateral fluid-percussion injury (FPI), which are the two most commonly used experimental models of human TBI in rodents. TBI was induced with CCI to 50 (14 controls) and with lateral FPI to 45 (15 controls) C57BL/6S adult male mice. The animals were followed-up for 9 months, including three 2-week periods of continuous video-electroencephalographic (EEG) monitoring, and a seizure susceptibility test with pentylenetetrazol (PTZ). In the end, the animals were perfusion-fixed for histology. The experiment included two independent cohorts of animals. Late post-traumatic spontaneous electrographic seizures were detected in 9% of mice after CCI and 3% after lateral FPI. Eighty-two percent of mice after CCI and 71% after lateral FPI had spontaneous epileptiform spiking on EEG. In addition, 58% of mice with lateral FPI showed spontaneous epileptiform discharges. A PTZ test demonstrated increased seizure susceptibility in the majority of mice in both models, compared to control mice. There was no further progression in the occurrence of epilepsy or epileptiform spiking when follow-up was extended from 6 to 9 months. The severity of cortical or hippocampal damage did not differentiate mice with or without epileptiform activity in either model. Finally, two independent series of experiments in both injury models provided comparable data demonstrating reproducibility of the modeling. These data show that different types of impact can trigger epileptogenesis in mice. Even though the frequency of spontaneous seizures in C57BL/6S mice is low, a large majority of animals develop hyperexcitability.

The NMDA receptor complex as a therapeutic target in epilepsy: a review

A substantial amount of research has shown that N-methyl-D-aspartate receptors (NMDARs) may play a key role in the pathophysiology of several neurological diseases, including epilepsy. Animal models of epilepsy and clinical studies demonstrate that NMDAR activity and expression can be altered in association with epilepsy and particularly in some specific seizure types. NMDAR antagonists have been shown to have antiepileptic effects in both clinical and preclinical studies. There is some evidence that conventional antiepileptic drugs may also affect NMDAR function. In this review, we describe the evidence for the involvement of NMDARs in the pathophysiology of epilepsy and provide an overview of NMDAR antagonists that have been investigated in clinical trials and animal models of epilepsy.

Generation of neuronal variability and complexity

The production of specialized differentiated neurons derived from stem cells has been proposed as a revolutionary technology for regenerative medicine. However, few examples of specific neuronal cell differentiation have been described so far. Although stem-cell tissue replacement might be seemingly straightforward in other cases, the high degree of complexity of the nervous system raises the challenge of tissue replacement substantially. Understanding mechanisms of neuronal diversification will not only be relevant for therapeutic purposes but might also shed light on the differences in cognitive abilities, personality traits and psychiatric conditions observed in humans.

Ischemia induces a translocation of the splicing factor tra2-beta 1 and changes alternative splicing patterns in the brain

Alternative splice-site selection is regulated by the relative concentration of individual members of the serine-arginine family of proteins and heterogeneous nuclear ribonucleoproteins. Most of these proteins accumulate predominantly in the nucleus, and a subset of them shuttles continuously between nucleus and cytosol. We demonstrate that in primary neuronal cultures, a rise in intracellular calcium concentration induced by thapsigargin leads to a translocation of the splicing regulatory protein tra2-beta1 and a consequent change in splice-site selection. To investigate this phenomenon under physiological conditions, we used an ischemia model. Ischemia induced in the brain causes a cytoplasmic accumulation and hyperphosphorylation of tra2-beta1. In addition, several of the proteins binding to tra2-beta1, such as src associated in mitosis 68 and serine/arginine-rich proteins, accumulate in the cytosol. Concomitant with this subcellular relocalization, we observed a change in alternative splice-site usage of the ICH-1 gene. The increased usage of its alternative exons is in agreement with previous studies demonstrating its repression by a high concentration of proteins with serine/arginine-rich domains. Our findings suggest that a change in the calcium concentration associated with ischemia is part of a signaling event, which changes pre-mRNA splicing pathways by causing relocalization of proteins that regulate splice-site selection.

Differential effects of chronic ethanol treatment on N-methyl-D-aspartate R1 splice variants in fetal cortical neurons

Functional N-methyl-D-aspartate receptors consisting of NR1 and NR2 subunits are an important site of action of ethanol. Chronic ethanol treatment increases the NR1 polypeptide levels in vivo and in vitro. Chronic ethanol treatment in vitro does not significantly alter the NR1 mRNA levels, even though under similar culture conditions ethanol (50 mm, 5 days) enhances the half-life of NR1 mRNA in fetal cortical neurons. To address this phenomenon, we determined by reverse transcription-polymerase chain reaction and Western blotting whether ethanol (50 mm, 5 days) has a splice variant-specific effect on the expression of the NR1 subunit in mouse fetal cortical neurons. This report analyzes for the first time the distribution of all NR1 splice variants in these neurons. Our data indicate the presence of NR1-3a,b and NR1-4a,b splice variants in cortical neurons. Chronic ethanol treatment significantly decreased the mRNA levels of exon 5-containing NR1 splice variants (NR1-3b and NR1-4b) (-E5/+E5 = 4.6 in untreated neurons and 6.1 in ethanol-treated neurons) and had no effect on the mRNA levels of NR1-3 (+E21/-E22) and NR1-4 (-E21/-E22) splice variants. At the polypeptide level, chronic ethanol treatment significantly reduced exon 5-containing splice variants (NR1-3b and NR1-4b). However, ethanol (50 mm, 5 days) induced a significant increase in polypeptide levels of NR1-4 (-E21/-E22), without any effect on NR1-3 (+E21/-E22) polypeptide levels. These results demonstrate that chronic ethanol treatment has a selective effect on the expression of NR1 splice variants at both the mRNA and polypeptide levels in mouse fetal cortical neurons.

RNA editing at the Q/R site for the glutamate receptor subunits GLUR2, GLUR5, and GLUR6 in hippocampus and temporal cortex from epileptic patients

Posttranscriptional editing of mRNA is a phenomenon that generates molecular heterogeneity and functional variety. With the intention to test if RNA editing plays a role in pathological processes, which contribute to seizure maintenance, we examined the ratio of the unedited (Q) to edited (R) form of the AMPA receptor subunit GluR2 and kainate receptor subunits GluR5 and GluR6 in the hippocampus and temporal cerebral cortex, both excised from patients with pharmacoresistant temporal lobe epilepsies. We compared the data with samples from nonepileptic human control tissue (autopsy tissue). The ratio of Q/R editing was analyzed by means of reverse transcription-polymerase chain reaction followed by a restriction enzyme assay. We found that the editing efficiency for the kainate receptor subunits GluR5 and GluR6 was significantly higher in temporal cortex than in normal controls. The alteration in GluR5 and GluR6 mRNA editing in the neocortical tissue may reflect an adaptive reaction of ongoing seizure activity to prevent excessive Ca(2+) influx.

The splice of life: alternative splicing and neurological disease

Splicing of pre-messenger RNA is regulated differently in the brain compared with other tissues. Recognition of aberrations in splicing events that are associated with neurological disease has contributed to our understanding of disease pathogenesis in some cases. Neuron-specific proteins involved in RNA splicing and metabolism are also affected in several neurological disorders. These findings have begun to bridge what we know about the mechanisms regulating neuron-specific splicing and our understanding of neural function and disease.

Alternative splicing of the NMDAR1 glutamate receptor subunit in human temporal lobe epilepsy

It has been demonstrated in animal models that chronic epilepsy is associated with increased excitability which may result from abnormal glutamatergic transmission involving altered properties of N-methyl-D-aspartate (NMDA) receptors. We have investigated whether human temporal lobe epilepsy is associated with changes in the NMDA receptor at the molecular level by assessing the relative expression of mRNAs of the different splice variants at the N-terminal (exon 5) and C-terminal (exon 21) position for the NMDAR1 subunit. Specimens of hippocampus and temporal lobe cortex from patients with refractory epilepsy were obtained during neurosurgical operations and analyzed by means of the reverse transcription reaction followed by polymerase chain reaction. Non-epileptic control specimens obtained at autopsy exhibited a relatively high level in expression of exon 5-lacking (hippocampus: 0.87; cortex: 0.81) and exon 21-containing (hippocampus: 0.95; cortex: 0.93) transcripts. The ratio for these alternatively spliced transcripts was not significantly changed in epileptic hippocampal and cortical tissues relative to the corresponding non-epileptic samples. These results did not support a potential role for NMDAR1 splice variants in the pathophysiology of epilepsy.

Upregulation of NMDA receptors in hippocampus and cortex in the pentylenetetrazol-induced "kindling" model of epilepsy

"Kindling" is a phenomenon of epileptogenesis, which has been widely used as an experimental model of temporal lobe epilepsy. At the present work we investigated the contribution of NMDA receptors in the Pentylenetetrazol-induced "kindling" model in the mouse brain, by using quantitative autoradiography and the radioactive ligands [3H]MK801 and [3H]L-glutamate (NMDA-sensitive component). One week after establishment of kindling, a small but significant increase in [3H]MK801 as well as NMDA-sensitive [3H]glutamate binding was seen, being restricted to the molecular layer (ML) of the dentate gyrus (DG) and the CA3 region of the hippocampus. These binding augmentations persisted one month after establishment of kindling. A significant increase of NMDA receptor binding was also observed in the cortex-somatosensory and temporal one week after acquisition of the kindled state. The upregulation of NMDA receptors seen in DG and CA3 region of the hippocampus could be associated with the kindling process of this model especially with its maintenance phase, since it persists at long term, is area-specific and consistent with electrophysiological data. The increase of NMDA receptors seen in the cortex of the kindled animals could underlie the hyperexcitability detected by electrophysiological studies in this area.

Glutamate receptors and transporters in genetic and acquired models of epilepsy

Glutamate, the principal excitatory neurotransmitter in the brain, acts on three families of ionotropic receptor--AMPA (alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid), kainate and NMDA (N-methyl-D-aspartate) receptors and three families of metabotropic receptor (Group I: mGlu1 and mGlu5; Group II: mGlu2 and mGlu3; Group III: mGlu4, mGlu6, mGlu7 and mGlu8). Glutamate is removed from the synaptic cleft and the extracellular space by Na+-dependent transporters (GLAST/EAAT1, GLT/EAAT2, EAAC/EAAT3, EAAT4, EAAT5). In rodents, genetic manipulations relating to the expression or function of glutamate receptor proteins can induce epilepsy syndromes or raise seizure threshold. Decreased expression of glutamate transporters (EAAC knockdown, GLT knockout) can lead to seizures. In acquired epilepsy syndromes, a wide variety of changes in receptors and transporters have been described. Electrically-induced kindling in the rat is associated with functional potentiation of NMDA receptor-mediated responses at various limbic sites. Group I metabotropic responses are enhanced in the amygdala. To date, no genetic epilepsy in man has been identified in which the primary genetic defect involves glutamate receptors or transporters. Changes are found in some acquired syndromes, including enhanced NMDA receptor responses in dentate granule cells in patients with hippocampal sclerosis.

Molecular neuropathology of human mesial temporal lobe epilepsy

With the recent progress in surgical treatment modalities, human brain tissue from patients with intractable focal epilepsies will increasingly become available for studies on the molecular pathology, electrophysiological changes and pathogenesis of human focal epilepsies. An inherent problem for studies on human temporal lobe epilepsy (TLE) is the lack of suitable controls. Strategies to alleviate this obstacle include the use of human post mortem samples, hippocampus from experimental animals and, in particular, the comparative analysis of surgical specimens from patients with Ammon's horn sclerosis (AHS) and with focal temporal lesions but anatomically preserved hippocampal structures. In this review we focus on selected aspects of the molecular neuropathology of TLE: (1) the potential impact of persisting calretinin-immunoreactive neurons with Cajal-Retzius cell morphology, (2) astrocytic tenascin-C induction and redistribution as potential regulator of aberrant axonal sprouting and (3) alterations of Ca2+ -mediated hippocampal signalling pathways. The diverse and complex changes described so far in human TLE specimens require a systematic interdisciplinary approach to distinguish primary, epileptogenic alterations and secondary, compensatory mechanisms in the pathogenesis of human temporal lobe epilepsies.

ZFM1/SF1 mRNA in rat and gerbil brain after global ischaemia

Cerebral ischaemia results in significant brain damage, but the molecular mechanisms associated with ischaemia-induced brain injury are not well defined. We have adopted an improved differential-display method to search for new ischaemia-related genes. Among the different cDNAs isolated following transient forebrain ischaemia in rat, PH3.3 was selected for further studies. The search for homologies revealed that it is the rat homologue to human zinc finger motif 1 (ZFM1), also called mammalian splicing factor 1 (SF1). With Northern blot, PH3.3 hybridized with three mRNA species of 2.3, 2.9 and 3.6 kb, significantly increased at 6 h and 5 days after the ischaemic insult. These findings were extended also to another animal model. In situ hybridization in ischaemic gerbils showed that PH3.3 mRNA was induced in the dentate gyrus as early as 4 h post-ischaemia. Expression peaked at 2 days in the whole hippocampus and cortex, and then progressively decreased towards sham levels. By day 4, expression had disappeared almost entirely from the cells in the CA1 region of the hippocampus, concomitant with the degeneration of pyramidal neurons. Interestingly, ZFM1/SF1 has been recently identified as activated following p53-induced apoptosis. Several lines of evidence suggest that p53 may play two roles in the post-ischaemic brain. The primary role of p53 is to activate DNA repair processes, but if repair fails, apoptosis will be initiated. Thus, ZFM1/SF1 may represent a relevant link between p53 and the neuroprotective/neurodegenerative processes which follow cerebral ischaemia.

Activity-dependent regulation of alternative splicing patterns in the rat brain

Alternative splicing plays an important role in the expression of genetic information. Among the best understood alternative splicing factors are transformer and transformer-2, which regulate sexual differentiation in Drosophila. Like the Drosophila genes, the recently identified mammalian homologues are subject to alternative splicing. Using an antibody directed against the major human transformer-2 beta isoform, we show that it has a widespread expression in the rat brain. Pilocarpine-induced neuronal activity changes the alternative splicing pattern of the human transformer-2-beta gene in the brain. After neuronal stimulation, a variant bearing high similarity to a male-specific Drosophila tra-2179 isoform is switched off in the hippocampus and is detectable in the cortex. In addition, the ratio of another short RNA isoform (htra2-beta2) to htra2-beta1 is changed. Htra2-beta2 is not translated into protein, and probably helps to regulate the relative amounts of htra2-beta1 to beta3. We also observe activity-dependent changes in alternative splicing of the clathrin light chain B, c-src and NMDAR1 genes, indicating that the coordinated change of alternative splicing patterns might contribute to molecular plasticity in the brain.

Suppression of epileptogenesis by modification of N-methyl-D-aspartate receptor subunit composition

The effects of altered N-methyl-D-aspartate (NMDA) receptor subunit composition on seizure development in kindling epilepsy were assessed in transgenic mice expressing high neuronal levels of NR2D under control of the calcium/calmodulin kinase II alpha subunit (alphaCaMKII) promoter. The NR2D subunit is normally present at very low levels in the mature forebrain. Transgenic mice showed a marked reduction of amygdala kindling development. Spread of epileptic activity was retarded and generalized seizures appeared later in animals overexpressing NR2D compared with wild-type mice. The progressive lengthening of epileptiform activity, which normally occurs in kindling, was also dampened in transgenic animals. We conclude that NMDA receptor subunit composition determines the progression of experimental epilepsy.

Glutamate receptors in epilepsy

Glutamatergic synapses play a critical role in all epileptic phenomena. Broadly enhanced activation of post-synaptic glutamate receptors (ionotropic and metabotropic) is proconvulsant. Antagonists of NMDA receptors and AMPA receptors are powerful anticonvulsants in many animal models of epilepsy. A clinical application of pure specific glutamate antagonists has not yet been established. Many different alterations in glutamate receptors or transporters can potentially contribute to epileptogenesis. Several genetic alterations have been shown to be epileptogenic in animal models but no specific mutation relating to glutamatergic function has yet been linked to a human epilepsy syndrome. There is clear evidence for altered NMDA receptor function in acquired epilepsy in animal models and in man. Changes in metabotropic receptor function may also play a key role in epileptogenesis.

Regulation of the neuron-specific exon of clathrin light chain B

Clathrin light chain B (LCB) is a major component of clathrin coated vesicles, which are structures involved in intracellular transport. A neuron-specific isoform of LCB is generated by incorporation of a single exon (EN) using an alternative splicing mechanism that reflects the special demands of neurons, such as axonal transport and synaptic neurotransmission. Here, we demonstrate that this neuron-specific exon is developmentally regulated and is excluded in non-neuronal cells because its 5' and 3' splice sites deviate from the mammalian consensus sequences. A gel retardation assay indicated the presence of a developmentally regulated factor in brain that binds to the neuronal exon. In addition, EN usage is repressed by increasing the concentration of htra2-beta1, a splice factor whose isoform expression is influenced by neuronal activity. We propose that a brain-specific factor is involved in EN recognition during development and adulthood. In addition, ubiquitously expressed splicing factors such as htra2-beta1 are involved in regulating EN expression in the adult brain.

Measurement of NMDA receptor protein subunits in discrete hippocampal regions of kindled animals

Kindling refers to a phenomenon in which repeated application of initially subconvulsive electrical stimulations produces limbic and clonic motor seizures of progressively increasing severity. Once established, the increased excitability is lifelong. A diversity of studies demonstrate that kindling results in long lasting (28 days) alterations of the functional and pharmacologic properties of NMDA receptors, indicating that kindling may cause changes intrinsic to the NMDA receptor itself. Our previous studies disclosed no differences in NMDA receptor subunit gene or splice isoform mRNA expression between control and kindled animals 28 days after the last kindled seizure. Here, we extend those earlier studies by measuring levels of subunit protein for NMDAR1, NR2A, and NR2B in the hippocampus of control and kindled animals, 28 days after the last kindled seizure. We report that kindling does not effect long-lasting changes in the levels of NMDA receptor subunit protein. Together these findings support the idea that alterations in NMDA receptor protein expression do not contribute to the novel properties of NMDA receptors induced by kindling.

Differential effects of electroconvulsive shock on the glutamate receptor mRNAs for NR2A, NR2B and mGluR5b

We have studied the effects of single and repeated electroconvulsive shock (ECS) treatment on the mRNA levels of several glutamate receptors in the dentate gyrus and CA1 regions of the rat brain. In the dentate gyrus, such treatment elevated the mRNAs for the NMDA subunits NR2A and NR2B, but it reduced the mRNA for the metabotropic glutamate receptor mGlu5b. With the exception of NR2A, this effect was specific to the dentate gyrus. The changes in NR2B mRNA lasted the longest, but all changes had returned to control values after 48 h. The possible significance of such changes to the antidepressant effect of ECT is discussed.

Axonal injury alters alternative splicing of the retinal NR1 receptor: the preferential expression of the NR1b isoforms is crucial for retinal ganglion cell survival

Cellular-specific splicing of the retinal NMDAR1 receptor (NR1) and expression of NMDAR2 receptor (NR2) subunits in response to optic nerve injury was investigated by in situ hybridization in adult rats. A controlled optic nerve crush led to a clear alteration in the expression of alternatively spliced NR1 variants in the retinal ganglion cell layer (GCL). The NR1-2b and NR1-4b isoforms were preferentially expressed between 2 d and 1 week after injury, whereas expression for all other isoforms remained either unchanged or decreased to barely detectable levels within 4 weeks. Cellular silver grain density for NR2 subunits also declined in the GCL after trauma. To directly test the hypothesis that NR1b expression is crucial for cell survival after axonal trauma, we administered intraocularly an antisense oligonucleotide against the NR1b isoform 2 and 3 d after injury. This led to a drastic loss of retrogradely labeled retinal ganglion cells (RGCs). Antisense targeting clearly reduced retinal NR1 protein levels, as judged by Western blot analysis, but had no effect on the cell number in control retinas. These findings point toward injury-specific changes in alternative splicing of the NR1 receptor, which are crucial for the survival of RGCs after partial axonal trauma. We therefore propose that this reflects an adaptive, rather than a pathogenic, cellular response to neurotrauma.

Ion channels in epilepsy

There are specific alterations in the structure or function of ion channels in the epileptic brain. Some of these alterations may promote hyperexcitability, whereas others may protect neurons from the deleterious effects of epileptic discharges. With the use of human tissue resected from epilepsy patients and the comparison of cellular properties to those found in well-defined experimental models, we will continue to gain insight into the specific ion channel changes associated with epilepsies. Further genetic studies will help to elucidate the altered molecular mechanisms underlying ion channel changes in this devastating neurological disorder (Noebels, 1996). Whether it is a change in structure, function, or both, the study of ion channels in epilepsies will soon reveal specific characteristics of ion channels found only in epileptic tissue. If the altered properties of such ion channels cannot be found in control (nonepileptic) neurons, these channels might be called "epileptic" ion channels. An understanding of the specific structure, function, and pharmacology of these "epileptic" channels will yield important clues for future therapeutical approaches aimed at preventing epileptogenesis, and insight into the processes whereby ion channels become "epileptic" may finally open the way to prophylactic treatments of the epilepsies.

Long-lasting enhanced expression in the rat hippocampus of NMDAR1 splice variants in a kainate model of epilepsy

Chronic epilepsy is associated with increased excitability which may result from abnormal glutamatergic synaptic transmission involving altered properties of N-methyl-D-aspartate (NMDA) receptors. To date two gene families encoding NMDA receptor subunits have been cloned, NR1 and NR2. Eight NR1 mRNAs are generated by alternative splicing of exons 5, 21 and 22; the NR1-1 to NR1-4 C-terminal variants exist in the a or b version depending on the presence or absence of the domain encoded by exon 5. Epilepsy was induced in rats by unilateral intra-amygdalar injection of kainate and animals were killed from 6 h to 4 months following the injection. Increased NR1 mRNA levels were observed during status epilepticus (6-24 h after the injection), both psilateral and contralateral, while a second wave of NMDAR1 mRNA increase occurred in chronic epileptic animals, between 21 days and 4 months following kainate injection. Our data show: (i) a permanent increase of the NR1-2a and NR1-2b mRNA species (containing exon 22) in all hippocampal fields, both ipsilateral and contralateral, and (ii) an increase of the NR1-3 (a and b) mRNAs (containing exon 21) in the ipsilateral CA1, and NR1-3a mRNA in the ipsilateral dentate gyrus. No long-term changes were observed for the NR1-1 and NR14 splice variants. In the ipsilateral CA3 area a globally decreased mRNA expression was associated with neuronal loss. A possible contribution to the maintenance of the epileptic state by an increased expression of NMDA receptors is discussed.

Pharmacological characterization of N-methyl-D-aspartate receptors in spinal cord of rats with a chronic peripheral mononeuropathy

N-Methyl-D-aspartate (NMDA) receptor antagonists, acting in the spinal cord, are analgesic. However, the clinical utility of these antagonists is diminished by their adverse effects on cognition and behavior. To facilitate the development of spinal cord-selective NMDA receptor antagonists, we characterized ligand interactions at NMDA receptors in spinal cord of normal rats and rats with a chronic peripheral neuropathy. NMDA receptors in spinal cord were distinguished from those in cerebral cortex on the basis of differences in the potencies of competitive and noncompetitive antagonists and on the basis of differences in their response to spermidine. D(-)-2-Amino-5-phosphonopentanoic acid (AP-5) and (+)-(1-hydroxy-3-aminopyrrolidine-2-one) (HA-966) were more potent in inhibiting NMDA-dependent [3H]TCP binding in spinal cord while, conversely, MK-801 was more potent in inhibiting [3H]TCP binding to NMDA receptors in cerebral cortex. Spermidine increased [3H]TCP binding to NMDA receptors in cerebral cortex (39+/-8%) but not spinal cord (2+/-1%). Based on these properties, NMDA receptors in spinal cord more closely resembled those in cerebellum than those in cerebral cortex. Generation of a chronic neuropathy had no effect on the density of NMDA receptors in lumbar spinal cord. There were also no major changes in the potencies of competitive antagonists or channel blocking ligands, although there was a trend for kynurenic acid and D-CPP to be more potent in the spinal cords of neuropathic animals. These findings indicate that, in both normal and neuropathic pain states, NMDA receptors in spinal cord can be distinguished pharmacologically from those in cerebral cortex. These findings underscore the feasibility of developing spinal cord-selective NMDA receptor antagonists as novel analgesics.

Anticonvulsant effect of reduced NMDA receptor expression in audiogenic DBA/2 mice

Pretreatment of DBA/2 mice (n = 14-15 per group) with an 18-mer antisense probe to the NMDA-receptor submit NR1 (2 x 1 micrograms, or 2 x 83 pmol, NR1 antisense probe intracerebroventricularly, -29 and -7 h before testing for seizure response) resulted in almost complete suppression of sound-induced clonic seizures. A saline-treated group gave a 100% seizures response, while the group treated with NR1 antisense probe gave a 7% seizure response to the sound stimulus. The group treated with NR1 nonsense-probe showed no anticonvulsant protection (93% seizure response). The anticonvulsant protection observed following NR1 antisense administration was of relatively short duration, with seizure response gradually returning to control levels 12 to 24 h following the termination of antisense administration. When NR1 receptor levels were assessed by receptor autoradiography ([3H]-MK 801 and -CGP 39653 binding) in the same groups of mice, significant (20%) reductions in NR1 levels were observed in the retrosplenial cortex and the overall cortex. The seizure-induced expression of c-fos and NGFI-A in thalamus, hypothalamus, inferior colliculus and medical geniculate seen in vehicle- and NR1 nonsense-treated mice was completely blocked by NR1 antisense pretreatment.

Change in plasma membrane Ca2(+)-ATPase splice-variant expression in response to a rise in intracellular Ca2+

BACKGROUND

Most eukaryotic genes are divided into introns and exons. Upon transcription, the intronic segments are eliminated and the exonic sequences spliced together through a series of complex processing events. Alternative splicing refers to the optional inclusion or exclusion of specific exons in transcripts derived from a single gene, which leads to structural and functional changes in the encoded proteins. Although many components of the machinery directing the physical excision of introns and joining of exons have been elucidated in recent years, the signaling pathways regulating the activity of the machinery remain largely unexplored.

RESULTS

A calcium-mediated signaling pathway regulates alternative splicing at a specific site of human plasma membrane calcium pump-2 transcripts. This site consists of three exons, which are differentially used in a tissue-specific manner. In IMR32 neuroblastoma cells, a transient elevation of intracellular calcium changed the predominant pattern from one in which all three exons are included to the coexpression of a variant including only the third exon. Western-blot analysis demonstrated that the newly expressed mRNAs are faithfully translated. Once induced, the new splicing pattern was maintained over multiple cell divisions. Protein synthesis was not required to induce the alternative splice change, indicating that all components necessary for a rapid cellular response are present in the cells.

CONCLUSIONS

Calcium signaling exerts a direct influence on the regulation of alternative splicing. Notably, a calcium-mediated change in the expression of alternatively spliced variants of a calcium regulatory protein was discovered. The change in splicing occurs quickly, is persistent but reversible and leads to a corresponding change in protein expression. The specific nature in which differently spliced protein variants are expressed, and now the fact that their expression can be regulated by distinct intracellular signaling pathways, suggests that the regulation of alternative splicing by physiological stimuli is a widespread regulatory mechanism by which a cell may coordinate its responses to environmental cues.