De novo variants in CACNA1E found in patients with intellectual disability, developmental regression and social cognition deficit but no seizures

BACKGROUND

De novo variants in the voltage-gated calcium channel subunit α1 E gene (CACNA1E) have been described as causative of epileptic encephalopathy with contractures, macrocephaly and dyskinesias.

METHODS

Following the observation of an index patient with developmental delay and autism spectrum disorder (ASD) without seizures who had a de novo deleterious CACNA1E variant, we screened GeneMatcher for other individuals with CACNA1E variants and neurodevelopmental phenotypes without epilepsy. The spectrum of pathogenic CACNA1E variants was compared to the mutational landscape of variants in the gnomAD control population database.

RESULTS

We identified seven unrelated individuals with intellectual disability, developmental regression and ASD-like behavioral profile, and notably without epilepsy, who had de novo heterozygous putatively pathogenic variants in CACNA1E. Age of onset of clinical manifestation, presence or absence of regression and degree of severity were variable, and no clear-cut genotype-phenotype association could be recognized. The analysis of disease-associated variants and their comparison to benign variants from the control population allowed for the identification of regions in the CACNA1E protein that seem to be intolerant to substitutions and thus more likely to harbor pathogenic variants. As in a few reported cases with CACNA1E variants and epilepsy, one patient showed a positive clinical behavioral response to topiramate, a specific calcium channel modulator.

LIMITATIONS

The significance of our study is limited by the absence of functional experiments of the effect of identified variants, the small sample size and the lack of systematic ASD assessment in all participants. Moreover, topiramate was given to one patient only and for a short period of time.

CONCLUSIONS

Our results indicate that CACNA1E variants may result in neurodevelopmental disorders without epilepsy and expand the mutational and phenotypic spectrum of this gene. CACNA1E deserves to be included in gene panels for non-specific developmental disorders, including ASD, and not limited to patients with seizures, to improve diagnostic recognition and explore the possible efficacy of topiramate.

Cav2.3 channels contribute to dopaminergic neuron loss in a model of Parkinson's disease

Degeneration of dopaminergic neurons in the substantia nigra causes the motor symptoms of Parkinson's disease. The mechanisms underlying this age-dependent and region-selective neurodegeneration remain unclear. Here we identify Cav2.3 channels as regulators of nigral neuronal viability. Cav2.3 transcripts were more abundant than other voltage-gated Ca2+ channels in mouse nigral neurons and upregulated during aging. Plasmalemmal Cav2.3 protein was higher than in dopaminergic neurons of the ventral tegmental area, which do not degenerate in Parkinson's disease. Cav2.3 knockout reduced activity-associated nigral somatic Ca2+ signals and Ca2+-dependent after-hyperpolarizations, and afforded full protection from degeneration in vivo in a neurotoxin Parkinson's mouse model. Cav2.3 deficiency upregulated transcripts for NCS-1, a Ca2+-binding protein implicated in neuroprotection. Conversely, NCS-1 knockout exacerbated nigral neurodegeneration and downregulated Cav2.3. Moreover, NCS-1 levels were reduced in a human iPSC-model of familial Parkinson's. Thus, Cav2.3 and NCS-1 may constitute potential therapeutic targets for combatting Ca2+-dependent neurodegeneration in Parkinson's disease.

Calcium Channel CaV2.3 Subunits Regulate Hepatic Glucose Production by Modulating Leptin-Induced Excitation of Arcuate Pro-opiomelanocortin Neurons

Leptin acts on hypothalamic pro-opiomelanocortin (POMC) neurons to regulate glucose homeostasis, but the precise mechanisms remain unclear. Here, we demonstrate that leptin-induced depolarization of POMC neurons is associated with the augmentation of a voltage-gated calcium (CaV) conductance with the properties of the "R-type" channel. Knockdown of the pore-forming subunit of the R-type (CaV2.3 or Cacna1e) conductance in hypothalamic POMC neurons prevented sustained leptin-induced depolarization. In vivo POMC-specific Cacna1e knockdown increased hepatic glucose production and insulin resistance, while body weight, feeding, or leptin-induced suppression of food intake were not changed. These findings link Cacna1e function to leptin-mediated POMC neuron excitability and glucose homeostasis and may provide a target for the treatment of diabetes.

Apamin Boosting of Synaptic Potentials in CaV2.3 R-Type Ca2+ Channel Null Mice

SK2- and KV4.2-containing K+ channels modulate evoked synaptic potentials in CA1 pyramidal neurons. Each is coupled to a distinct Ca2+ source that provides Ca2+-dependent feedback regulation to limit AMPA receptor (AMPAR)- and NMDA receptor (NMDAR)-mediated postsynaptic depolarization. SK2-containing channels are activated by Ca2+ entry through NMDARs, whereas KV4.2-containing channel availability is increased by Ca2+ entry through SNX-482 (SNX) sensitive CaV2.3 R-type Ca2+ channels. Recent studies have challenged the functional coupling between NMDARs and SK2-containing channels, suggesting that synaptic SK2-containing channels are instead activated by Ca2+ entry through R-type Ca2+ channels. Furthermore, SNX has been implicated to have off target affects, which would challenge the proposed coupling between R-type Ca2+ channels and KV4.2-containing K+ channels. To reconcile these conflicting results, we evaluated the effect of SK channel blocker apamin and R-type Ca2+ channel blocker SNX on evoked excitatory postsynaptic potentials (EPSPs) in CA1 pyramidal neurons from CaV2.3 null mice. The results show that in the absence of CaV2.3 channels, apamin application still boosted EPSPs. The boosting effect of CaV2.3 channel blockers on EPSPs observed in neurons from wild type mice was not observed in neurons from CaV2.3 null mice. These data are consistent with a model in which SK2-containing channels are functionally coupled to NMDARs and KV4.2-containing channels to CaV2.3 channels to provide negative feedback regulation of EPSPs in the spines of CA1 pyramidal neurons.

Genome-wide association study identifies QTLs for EBV of backfat thickness and average daily gain in Duroc pigs

Backfat thickness (BFT) and average daily gain (ADG) are two important economic traits in commercial swine production. Identifying QTLs and uncovering the molecular mechanism for BFT and ADG would greatly help to speed up the breeding progress. In current breeding program, EBV for these two traits are calculated and formulated a comprehensive breeding index, which then be used to improve pig performance. Using Illumina PorcineSNP60 BeadChip, a pilot genomewide association studies (GWAS) for BFT and ADG in 83 Duroc pigs were performed. A total of 31 genome-wise significant SN Ps were detected to be associated with BFT on SSC 4, 9, 11, 12 and 14, ten of which were coincident with previously reported QTL regions. There are two genome-wise loci prominently associated with ADG on SSC2 and SSC13, respectively. The two loci on SSC2 are well overlapped with the QTL regions previously reported. All the 31 significant SNPs associated with BFT are verified on 219 outbreed pigs, six SN Ps reach an extreme significant level and seven SNP reaches a significant level, CACNA1E and ACBD6 are chosen as positional candidate genes. Our findings not only confirmed previously findings, but also revealed a number of novel SNPs associated with BFT and ADG. Two positional candidate genes CACNA1E and ACBD6 were identified for further study. These results would facilitate the identification of causative genes for BFT and ADG.

Association between genetic polymorphisms in Ca(v)2.3 (R-type) Ca2+ channels and fentanyl sensitivity in patients undergoing painful cosmetic surgery

Individual differences in the sensitivity to fentanyl, a widely used opioid analgesic, lead to different proper doses of fentanyl, which can hamper effective pain treatment. Voltage-activated Ca(2+) channels (VACCs) play a crucial role in the nervous system by controlling membrane excitability and calcium signaling. Ca(v)2.3 (R-type) VACCs have been especially thought to play critical roles in pain pathways and the analgesic effects of opioids. However, unknown is whether single-nucleotide polymorphisms (SNPs) of the human CACNA1E (calcium channel, voltage-dependent, R type, alpha 1E subunit) gene that encodes Cav2.3 VACCs influence the analgesic effects of opioids. Thus, the present study examined associations between fentanyl sensitivity and SNPs in the human CACNA1E gene in 355 Japanese patients who underwent painful orofacial cosmetic surgery, including bone dissection. We first conducted linkage disequilibrium (LD) analyses of 223 SNPs in a region that contains the CACNA1E gene using genomic samples from 100 patients, and a total of 13 LD blocks with 42 Tag SNPs were observed within and around the CACNA1E gene region. In the preliminary study using the same 100 genomic samples, only the rs3845446 A/G SNP was significantly associated with perioperative fentanyl use among these 42 Tag SNPs. In a confirmatory study using the other 255 genomic samples, this SNP was also significantly associated with perioperative fentanyl use. Thus, we further analyzed associations between genotypes of this SNP and all of the clinical data using a total of 355 samples. The rs3845446 A/G SNP was associated with intraoperative fentanyl use, 24 h postoperative fentanyl requirements, and perioperative fentanyl use. Subjects who carried the minor G allele required significantly less fentanyl for pain control compared with subjects who did not carry this allele. Although further validation is needed, the present findings show the possibility of the involvement of CACNA1E gene polymorphisms in fentanyl sensitivity.

The C-terminus of human Ca(v)2.3 voltage-gated calcium channel interacts with alternatively spliced calmodulin-2 expressed in two human cell lines

Ca(v)2.3 containing voltage-activated Ca(2+) channels are expressed in excitable cells and trigger neurotransmitter and peptide-hormone release. Their expression remote from the fast release sites leads to the accumulation of presynaptic Ca(2+) which can both, facilitate and inhibit the influx of Ca(2+) ions through Ca(v)2.3. The facilitated Ca(2+) influx was recently related to hippocampal postsynaptic facilitation and long term potentiation. To analyze Ca(2+) mediated modulation of cellular processes more in detail, protein partners of the carboxy terminal tail of Ca(v)2.3 were identified by yeast-2-hybrid screening, leading in two human cell lines to the detection of a novel, extended and rarely occurring splice variant of calmodulin-2 (CaM-2), called CaM-2-extended (CaM-2-ext). CaM-2-ext interacts biochemically with the C-terminus of Ca(v)2.3 similar to the classical CaM-2 as shown by co-immunoprecipitation. Functionally, only CaM-2-ext reduces whole cell inward currents significantly. The insertion of the novel 46 nts long exon and the consecutive expression of CaM-2-ext must be dependent on a new upstream translation initiation site which is only rarely used in the tested human cell lines. The structure of the N-terminal extension is predicted to be more hydrophobic than the remaining CaM-2-ext protein, suggesting that it may help to dock it to the lipophilic membrane surrounding.

CACNA1E variants affect beta cell function in patients with newly diagnosed type 2 diabetes. the Verona newly diagnosed type 2 diabetes study (VNDS) 3

Background

Genetic variability of the major subunit (CACNA1E) of the voltage-dependent Ca²⁺ channel Ca_V2.3 is associated to risk of type 2 diabetes, insulin resistance and impaired insulin secretion in nondiabetic subjects. The aim of the study was to test whether CACNA1E common variability affects beta cell function and/or insulin sensitivity in patients with newly diagnosed type 2 diabetes.

Methodology/Principal Findings

In 595 GAD-negative, drug naïve patients (mean±SD; age: 58.5±10.2 yrs; BMI: 29.9±5 kg/m², HbA1c: 7.0±1.3) with newly diagnosed type 2 diabetes we: 1. genotyped 10 tag SNPs in CACNA1E region reportedly covering ∼93% of CACNA1E common variability: rs558994, rs679931, rs2184945, rs10797728, rs3905011, rs12071300, rs175338, rs3753737, rs2253388 and rs4652679; 2. assessed clinical phenotypes, insulin sensitivity by the euglycemic insulin clamp and beta cell function by state-of-art modelling of glucose/C-peptide curves during OGTT. Five CACNA1E tag SNPs (rs10797728, rs175338, rs2184945, rs3905011 and rs4652679) were associated with specific aspects of beta cell function (p<0.05−0.01). Both major alleles of rs2184945 and rs3905011 were each (p<0.01 and p<0.005, respectively) associated to reduced proportional control with a demonstrable additive effect (p<0.005). In contrast, only the major allele of rs2253388 was related weakly to more severe insulin resistance (p<0.05).

Conclusions/Significance

In patients with newly diagnosed type 2 diabetes CACNA1E common variability is strongly associated to beta cell function. Genotyping CACNA1E might be of help to infer the beta cell functional phenotype and to select a personalized treatment.

Channeling dysglycemia: ion-channel variations perturbing glucose homeostasis

Maintaining blood glucose homeostasis is a complex process that depends on pancreatic islet hormone secretion. Hormone secretion from islets is coupled to calcium entry which results from regenerative islet cell electrical activity. Therefore, the ionic mechanisms that regulate calcium entry into islet cells are crucial for maintaining normal glucose homeostasis. Genome-wide association studies (GWAS) have identified single-nucleotide polymorphisms (SNPs), including five located in or near ion-channel or associated subunit genes, which show an association with human diseases characterized by dysglycemia. This review focuses on polymorphisms and mutations in ion-channel genes that are associated with perturbations in human glucose homeostasis and discusses their potential roles in modulating pancreatic islet hormone secretion.

Boosting of synaptic potentials and spine Ca transients by the peptide toxin SNX-482 requires alpha-1E-encoded voltage-gated Ca channels

The majority of glutamatergic synapses formed onto principal neurons of the mammalian central nervous system are associated with dendritic spines. Spines are tiny protuberances that house the proteins that mediate the response of the postsynaptic cell to the presynaptic release of glutamate. Postsynaptic signals are regulated by an ion channel signaling cascade that is active in individual dendritic spines and involves voltage-gated calcium (Ca) channels, small conductance (SK)-type Ca-activated potassium channels, and NMDA-type glutamate receptors. Pharmacological studies using the toxin SNX-482 indicated that the voltage-gated Ca channels that signal within spines to open SK channels belong to the class Ca(V)2.3, which is encoded by the Alpha-1E pore-forming subunit. In order to specifically test this conclusion, we examined the effects of SNX-482 on synaptic signals in acute hippocampal slices from knock-out mice lacking the Alpha-1E gene. We find that in these mice, application of SNX-482 has no effect on glutamate-uncaging evoked synaptic potentials and Ca influx, indicating that that SNX-482 indeed acts via the Alpha-1E-encoded Ca(V)2.3 channel.

Arrhythmia in Isolated Prenatal Hearts after Ablation of the Cav2.3 (α1E) Subunit of Voltage-gated Ca2+ Channels

A voltage-gated calcium channel containing Cav2.3e (alpha1Ee) as the ion conducting pore has recently been detected in rat heart. Functional evidence for this Ca2+ channel to be involved in the regulation of heart beating, besides L- and T-type channels, was derived from murine embryos where the gene for Cav1.2 had been ablated. The remaining "L-type like" current component was not related to recombinant splice variants of Cav1.3 containing channels. As recombinant Cav2.3 channels from rat were reported to be weakly dihydropyridine sensitive, the spontaneous activity of the prenatal hearts from Cav2.3(-|-) mice was compared to that of Cav2.3(+|+) control animals to investigate if Cav2.3 could represent such a L-type like Ca(2+) channel. The spontaneous activity of murine embryonic hearts was recorded by using a multielectrode array. Between day 9.5 p.c. to 12.5 p.c., the beating frequency of isolated embryonic hearts from Cav2.3-deficient mice did not differ significantly from control mice but the coefficient of variation within individual episodes was more than four-fold increased in Cav2.3-deficient mice indicating arrhythmia. In isolated hearts from wild type mice, arrhythmia was induced by superfusion with a solution containing 200 nM SNX-482, a blocker of some R-type voltage gated Ca2+ channels, suggesting that R-type channels containing the splice variant Cav2.3e as ion conducting pore stabilize a more regular heart beat in prenatal mice.

Polymorphisms in the gene encoding the voltage-dependent Ca(2+) channel Ca (V)2.3 (CACNA1E) are associated with type 2 diabetes and impaired insulin secretion

AIMS/HYPOTHESIS

Glucose-stimulated insulin secretion is dependent on the electrical activity of beta cells; hence, genes encoding beta cell ion channels are potential candidate genes for type 2 diabetes. The gene encoding the voltage-dependent Ca(2+) channel Ca(V)2.3 (CACNA1E), telomeric to a region that has shown suggestive linkage to type 2 diabetes (1q21-q25), has been ascribed a role for second-phase insulin secretion.

METHODS

Based upon the genotyping of 52 haplotype tagging single nucleotide polymorphisms (SNPs) in a type 2 diabetes case-control sample (n = 1,467), we selected five SNPs that were nominally associated with type 2 diabetes and genotyped them in the following groups (1) a new case-control sample of 6,570 individuals from Sweden; (2) 2,293 individuals from the Botnia prospective cohort; and (3) 935 individuals with insulin secretion data from an IVGTT.

RESULTS

The rs679931 TT genotype was associated with (1) an increased risk of type 2 diabetes in the Botnia case-control sample [odds ratio (OR) 1.4, 95% CI 1.0-2.0, p = 0.06] and in the replication sample (OR 1.2, 95% CI 1.0-1.5, p = 0.01 one-tailed), with a combined OR of 1.3 (95% CI 1.1-1.5, p = 0.004 two-tailed); (2) reduced insulin secretion [insulinogenic index at 30 min p = 0.02, disposition index (D (I)) p = 0.03] in control participants during an OGTT; (3) reduced second-phase insulin secretion at 30 min (p = 0.04) and 60 min (p = 0.02) during an IVGTT; and (4) reduced D (I) over time in the Botnia prospective cohort (p = 0.05).

CONCLUSIONS/INTERPRETATION

We conclude that genetic variation in the CACNA1E gene contributes to an increased risk of the development of type 2 diabetes by reducing insulin secretion.

Variants in the Ca V 2.3 (alpha 1E) subunit of voltage-activated Ca2+ channels are associated with insulin resistance and type 2 diabetes in Pima Indians

OBJECTIVE

Linkage to type 2 diabetes has been reported on chromosome 1q21-25 in Pima Indians. Fine mapping identified single nucleotide polymorphisms (SNPs) near the CACNA1E gene associated with this disease. CACNA1E encodes the voltage-dependent calcium channel Ca(v)2.3 Ca(2+), and mice lacking this channel exhibit impaired glucose tolerance and insulin secretion. Therefore, CACNA1E was investigated as a positional candidate gene.

RESEARCH DESIGN AND METHODS

CACNA1E was sequenced, and 28 SNPs were genotyped in the same group of Pima subjects who had been analyzed in the linkage study. Allele-specific expression was used to functionally evaluate a variant in the 3' untranslated region (UTR).

RESULTS

A novel G/A variant in the 3'-UTR was associated with young-onset type 2 diabetes (odds ratio 2.09 per copy of the G-allele [95% CI 1.31-3.33], adjusted P = 0.001) and had an effect on the evidence for linkage at chromosome 1q21-25 (P = 0.004). Among 372 nondiabetic Pima subjects who had undergone metabolic testing, the risk allele was associated with reduced insulin action including increased fasting, 30, 60, and 120 min plasma glucose concentrations and increased fasting plasma insulin during an oral glucose tolerance test (all P < 0.01), as well as a decreased rate of insulin-stimulated glucose disposal at both physiologically and maximally stimulated insulin concentrations (both P < 0.002). Functional analysis of this variant showed that the nonrisk allele had a 2.3-fold higher expression compared with the risk allele.

CONCLUSIONS

A functional variant in CACNA1E contributes to type 2 diabetes susceptibility by affecting insulin action. This variant partially explains the linkage to type 2 diabetes on chromosome 1q21-25 in Pima Indians.

R-type Ca(2+)-channel-evoked CICR regulates glucose-induced somatostatin secretion

Pancreatic islets have a central role in blood glucose homeostasis. In addition to insulin-producing beta-cells and glucagon-secreting alpha-cells, the islets contain somatostatin-releasing delta-cells. Somatostatin is a powerful inhibitor of insulin and glucagon secretion. It is normally secreted in response to glucose and there is evidence suggesting its release becomes perturbed in diabetes. Little is known about the control of somatostatin release. Closure of ATP-regulated K(+)-channels (K(ATP)-channels) and a depolarization-evoked increase in cytoplasmic free Ca(2+) concentration ([Ca(2+)](i)) have been proposed to be essential. Here, we report that somatostatin release evoked by high glucose (>or=10 mM) is unaffected by the K(ATP)-channel activator diazoxide and proceeds normally in K(ATP)-channel-deficient islets. Glucose-induced somatostatin secretion is instead primarily dependent on Ca(2+)-induced Ca(2+)-release (CICR). This constitutes a novel mechanism for K(ATP)-channel-independent metabolic control of pancreatic hormone secretion.

Amplification and overexpression of CACNA1E correlates with relapse in favorable histology Wilms' tumors

PURPOSE

The most well established molecular markers of poor outcome in Wilms' tumor are loss of heterozygosity at chromosomes 1p and/or 16q, although to date no specific genes at these loci have been identified. We have previously shown a link between genomic gain of chromosome 1q and tumor relapse and sought to further elucidate the role of genes on 1q in treatment failure.

EXPERIMENTAL DESIGN

Microarray-based comparative genomic hybridization identified a microamplification harboring a single gene (CACNA1E) at 1q25.3 in 6 of 76 (7.9%) Wilms' tumors, correlating with a shorter relapse-free survival (P = 0.0044, log-rank test). Further characterization of this gene was carried out by measuring mRNA and protein expression as well as stable transfection of HEK293 cells.

RESULTS

Overexpression of the CACNA1E transcript was associated with DNA copy number (P = 0.0204, ANOVA) and tumor relapse (P = 0.0851, log-rank test). Immunohistochemistry against the protein product Ca(V)2.3 revealed expression localized to the apical membrane in the distal tubules of normal kidney but not to the metanephric blastemal cells of fetal kidney from which Wilms' tumors arise. Nuclear localization in 99 of 160 (61.9%) Wilms' tumor cases correlated with a reduced relapse-free survival, particularly in cases treated with preoperative chemotherapy (P = 0.009, log-rank test). Expression profiling of stably transfected HEK293 cells revealed specific up-regulation of the immediate early response genes EGR1/EGR2/EGR3 and FOS/FOSB, mediated by activation of the MEK/ERK5/Nur77 pathway.

CONCLUSIONS

These data identify a unique genetic aberration with direct clinical relevance in Wilms' tumor relapse and provide evidence for a potential novel mechanism of treatment resistance in these tumors.

Characterization of acetylcholine release and the compensatory contribution of non-Cav2.1 channels at motor nerve terminals of leaner Cav2.1-mutant mice

The severely ataxic and epileptic mouse leaner (Ln) carries a natural splice site mutation in Cacna1a, leading to a C-terminal truncation of the encoded Ca(v)2.1 alpha(1) protein. Ca(v)2.1 is a neuronal Ca(2+) channel, mediating neurotransmitter release at many central synapses and the peripheral neuromuscular junction (NMJ). With electrophysiological analyses we demonstrate severely reduced ( approximately 50%) neurotransmitter release at Ln NMJs. This equals the reduction at NMJs of Cacna1a null-mutant (Ca(v)2.1-KO) mice, which display a neurological phenotype remarkably similar to that of Ln mice. However, using selective Ca(v) channel blocking compounds we revealed a compensatory contribution profile of non-Ca(v)2.1 type channels at Ln NMJs that differs completely from that at Ca(v)2.1-KO NMJs. Our data indicate that the residual function and presence of Ln-mutated Ca(v)2.1 channels precludes presynaptic compensatory recruitment of Ca(v)1 and Ca(v)2.2 channels, and hampers that of Ca(v)2.3 channels. This is the first report directly showing at single synapses the deficits and plasticity in transmitter release resulting from the Ln mutation of Cacna1a.

Subunits of voltage-gated calcium channels in murine spiral ganglion cells

CONCLUSION

The results show that alpha1D, alpha1E, alpha2/delta, beta1, and beta3 subunits are expressed in spiral ganglion cells (SGCs), and the coexpression of alpha1D and alpha1E suggests the presence of L-type and R-type calcium channels in mammalian SGCs.

OBJECTIVE

To investigate the types of subunits of voltage-gated calcium channels in SGCs of the mouse.

MATERIALS AND METHODS

SGCs were isolated from cochleae of neonatal mice and cultured for 24 h. Total RNA was extracted from cultured cells. After reverse transcription, the resulting cDNA was amplified by PCR with primers targeted to nucleotide sequences corresponding to seven different calcium channel subunits. The types of calcium channel subunits were identified by PCR analysis and nucleotide sequencing.

RESULTS

RT-PCR showed the strong and consistent amplification of alpha1D, alpha1E, alpha2/delta, beta1, and beta3 subunits from the mRNA of SGCs, and nucleotide sequencing confirmed the identity of mouse cochlear subunit cDNAs.

Molecular basis of Ca(v)2.3 calcium channels in rat nociceptive neurons

Ca(v)2.3 calcium channels play an important role in pain transmission in peripheral sensory neurons. Six Ca(v)2.3 isoforms resulting from different combinations of three inserts (inserts I and II in the II-III loop and insert III in the carboxyl-terminal region) have been identified in different mammalian tissues. To date, however, Ca(v)2.3 isoforms unique to primary sensory neurons have not been identified. In this study, we determined Ca(v)2.3 isoforms expressed in the rat trigeminal ganglion neurons. Whole tissue reverse transcription (RT)-PCR analyses revealed that only two isoforms, Ca(v)2.3a and Ca(v)2.3e, are present in TG neurons. Using single cell RT-PCR, we found that Ca(v)2.3e is the major isoform, whereas Ca(v)2.3e expression is highly restricted to small (<16 mum) isolectin B4-negative and tyrosine kinase A-positive neurons. Ca(v)2.3e was also preferentially detected in neurons expressing the nociceptive marker, transient receptor potential vanilloid 1. Single cell RT-PCR following calcium imaging and whole-cell patch clamp recordings provided evidence of an association between an R-type calcium channel component and Ca(v)2.3e expression. Our results suggest that Ca(v)2.3e in sensory neurons may be a potential target for the treatment of pain.

Neurokinin 1 receptors trigger overlapping stimulation and inhibition of CaV2.3 (R-type) calcium channels

Neurokinin (NK) 1 receptors and CaV2.3 calcium channels are both expressed in nociceptive neurons, and mice lacking either protein display altered responses to noxious stimuli. Here, we examined modulation of CaV2.3 through NK1 receptors expressed in human embryonic kidney 293 cells. We find that NK1 receptors generate complex modulation of CaV2.3. In particular, weak activation of these receptors evokes mainly stimulation of CaV2.3, whereas strong receptor activation elicits profound inhibition that overlaps with channel stimulation. Unlike R-type channels encoded by CaV2.3, L-type (CaV1.3), N-type (CaV2.2), and P/Q-type (CaV2.1) channels are inhibited, but not stimulated, through NK1 receptors. Pharmacological experiments show that protein kinase C (PKC) mediates stimulation of CaV2.3 through NK1 receptors. The signaling mechanisms underlying inhibition were explored by expressing proteins that buffer either Galpha(q/11) (regulator of G protein signaling protein 3T and carboxyl-terminal region of phospholipase C-beta1) or Gbeta gamma subunits (transducin and the carboxyl-terminal region of bovine G-protein-coupled receptor kinase). A fast component of inhibition was attenuated by buffering Gbeta gamma, whereas a slow component of inhibition was reduced by buffering Galpha(q/11). When both Gbeta gamma and Galpha(q/11) were simultaneously buffered in the same cells, inhibition was virtually eliminated, but receptor activation still triggered substantial stimulation of CaV2.3. We also report that NK1 receptors accelerate the inactivation kinetics of CaV2.3 currents. Altogether, our results indicate that NK1 receptors modulate CaV2.3 using three different signaling mechanisms: a fast inhibition mediated by Gbeta gamma, a slow inhibition mediated by Galpha(q/11), and a slow stimulation mediated by PKC. This new information concerning R-type calcium channels and NK1 receptors may help in understanding nociception, synaptic plasticity, and other physiological processes.

Molecular basis of inherited calcium channelopathies: role of mutations in pore-forming subunits

The pore-forming alpha subunits of voltage-gated calcium channels contain the essential biophysical machinery that underlies calcium influx in response to cell depolarization. In combination with requisite auxiliary subunits, these pore subunits form calcium channel complexes that are pivotal to the physiology and pharmacology of diverse cells ranging from sperm to neurons. Not surprisingly, mutations in the pore subunits generate diverse pathologies, termed channelopathies, that range from failures in excitation-contraction coupling to night blindness. Over the last decade, major insights into the mechanisms of pathogenesis have been derived from animals showing spontaneous or induced mutations. In parallel, there has been considerable growth in our understanding of the workings of voltage-gated ion channels from a structure-function, regulation and cell biology perspective. Here we document our current understanding of the mutations underlying channelopathies involving the voltage-gated calcium channel alpha subunits in humans and other species.

Blocking the R-type (Cav2.3) Ca2+ channel enhanced morphine analgesia and reduced morphine tolerance

Morphine is the drug of choice to treat intractable pain, although prolonged administration often causes undesirable side-effects including analgesic tolerance. It is speculated that voltage-dependent Ca(2+) channels (VDCCs) play a key role in morphine analgesia and tolerance. To examine the subtype specificity of VDCCs in these processes, we analysed mice lacking N-type (Ca(v)2.2) or R-type (Ca(v)2.3) VDCCs. Systemic morphine administration or exposure to warm water swim-stress, known to induce endogenous opioid release, resulted in greater analgesia in Ca(v)2.3(-/-) mice than in controls. Moreover, Ca(v)2.3(-/-) mice showed resistance to morphine tolerance. In contrast, Ca(v)2.2(-/-) mice showed similar levels of analgesia and tolerance to control mice. Intracerebroventricular (i.c.v.) but not intrathecal (i.t.) administration of morphine reproduced the result of systemic morphine in Ca(v)2.3(-/-) mice. Furthermore, i.c.v. administration of an R-type channel blocker potentiated morphine analgesia in wild-type mice. Thus, the inhibition of R-type Ca(2+) current could lead to high-efficiency opioid therapy without tolerance.

[Subunit of voltage-dependent calcium channels in the murine spiral ganglion cells]

OBJECTIVE

To investigate subunit type of voltage-dependent calcium channels in the spiral ganglion cells of the mouse.

METHODS

The spiral ganglion cells were dissected from cochleae of neonatal mice and cultured for 24 h. Total RNA was extracted from cultured spiral ganglion cells. After reverse transcription, resulting cDNA was amplified by polymerase chain reaction (PCR) with primers targeted to nucleotide sequences corresponding to 7 different calcium channel subunits. The types of calcium channel subunits were identified by PCR analysis and nucleotide sequencing.

RESULTS

Reverse transcription (RT)-PCR products representing subunit gene expression were strongly and consistently amplified for alpha1 D, alpha1 E, alpha2/delta, beta1 and beta3. Nucleotide sequencing confirmed the identity of mouse cochlear subunit cDNAs.

CONCLUSIONS

alpha1D, alpha1E, alpha2/delta, beta1 and beta3 subunits are expressed in spiral ganglion cells. And the coexpression of alpha1D and alpha1 E demonstrate the presence of L-type and R-type calcium channels in mammalian spiral ganglion cells.

High expression of the R-type voltage-gated Ca2+ channel and its involvement in Ca2+-dependent gonadotropin-releasing hormone release in GT1-7 cells

The GT1 cell has been widely used as a model cell to study cellular functions of GnRH neurons. Despite the importance of Ca(2+) channels, little is known except for L- and T-type Ca(2+) channels in GT1 cells. Therefore, we studied the diversity of voltage-gated Ca(2+) channels in GT1-7 cells with perforated-patch clamp and RT-PCR. An R-type Ca(2+) channel blocker, SNX-482, inhibited the Ca(2+) currents by 75.6% in all cells examined (n = 9). A T-type Ca(2+) channel blocker, Ni(2+), inhibited the Ca(2+) currents by 12.6% in all cells examined (n = 9). An L-type Ca(2+) channel blocker, nimodipine, inhibited the Ca(2+) currents by 17.9% in five of 11 cells examined. When using Ba(2+) as a charge carrier, another dihydropyridine antagonist, nifedipine, clearly inhibited the currents by 12.1% in all cells examined (n = 16). An N-type Ca(2+) channel blocker, omega-conotoxin-GVIA, inhibited the Ca(2+) currents by 13.8% in three of 20 cells examined. A P/Q type Ca(2+) channel blocker, omega-agatoxin-IVA, had no effect on the currents (n = 9). RT-PCR revealed that GT1-7 cells expressed the alpha(1B), alpha(1D), alpha(1E), and alpha(1H) subunit mRNA. Furthermore, SNX-482 and nifedipine inhibited the high K(+)-induced increase in the intracellular Ca(2+) concentration and GnRH release. omega-Conotoxin-GVIA and omega-agatoxin-IVA had no effect. These results suggest that GT1-7 cells express R-, L-, N-, and T-type voltage-gated Ca(2+) channels; the R-type was a major current component, and the L-, N-, and T-types were minor ones. The R- and L-type Ca(2+) channels play a critical role in the regulation of Ca(2+)-dependent GnRH release.

Electrophysiological and molecular evidence of L-(Cav1), N- (Cav2.2), and R- (Cav2.3) type Ca2+ channels in rat cortical astrocytes

Changes in intracellular Ca2+ levels are an important signal underlying neuron-glia cross-talk, but little is known about the possible role of voltage-gated Ca2+ channels (VGCCs) in controlling glial cell Ca2+ influx. We investigated the pharmacological and biophysical features of VGCCs in cultured rat cortical astrocytes. In whole-cell patch-clamp experiments, L-channel blockade (5 microM nifedipine) reduced Ba2+ current amplitude by 28% of controls, and further decrease (32%) was produced by N-channel blockade (3 microM omega-conotoxin-GVIA). No significant additional changes were observed after P/Q channel blockade (3 microM omega-conotoxin-MVIIC). Residual current (36% of controls) amounted to roughly the same percentage (34%) that was abolished by R-channel blockade (100 nM SNX-482). Electrophysiological evidence of L-, N-, and R-channels was associated with RT-PCR detection of mRNA transcripts for VGCC subunits alpha1C (L-type), alpha1B (N-type), and alpha1E (R-type). In cell-attached recordings, single-channel properties (L-currents: amplitude, -1.21 +/- 0.02 pA at 10 mV; slope conductance, 22.0 +/- 1.1 pS; mean open time, 5.95 +/- 0.24 ms; N-currents: amplitude, -1.09 +/- 0.02 pA at 10 mV; slope conductance, 18.0 +/- 1.1 pS; mean open time, 1.14 +/- 0.02 ms; R-currents: amplitude, -0.81 +/- 0.01 pA at 20 mV; slope conductance, 10.5 +/- 0.3 pS; mean open time, 0.88 +/- 0.02 ms) resembled those of corresponding VGCCs in neurons. These novel findings indicate that VGCC expression by cortical astrocytes may be more varied than previously thought, suggesting that these channels may indeed play substantial roles in the regulation of astrocyte Ca2+ influx, which influences neuron-glia cross-talk and numerous other calcium-mediated glial-cell functions.

Overexpressed Ca(v)beta3 inhibits N-type (Cav2.2) calcium channel currents through a hyperpolarizing shift of ultra-slow and closed-state inactivation

It has been shown that β auxiliary subunits increase current amplitude in voltage-dependent calcium channels. In this study, however, we found a novel inhibitory effect of β3 subunit on macroscopic Ba²⁺ currents through recombinant N- and R-type calcium channels expressed in Xenopus oocytes. Overexpressed β3 (12.5 ng/cell cRNA) significantly suppressed N- and R-type, but not L-type, calcium channel currents at “physiological” holding potentials (HPs) of −60 and −80 mV. At a HP of −80 mV, coinjection of various concentrations (0–12.5 ng) of the β3 with Ca_v2.2α₁ and α₂δ enhanced the maximum conductance of expressed channels at lower β3 concentrations but at higher concentrations (>2.5 ng/cell) caused a marked inhibition. The β3-induced current suppression was reversed at a HP of −120 mV, suggesting that the inhibition was voltage dependent. A high concentration of Ba²⁺ (40 mM) as a charge carrier also largely diminished the effect of β3 at −80 mV. Therefore, experimental conditions (HP, divalent cation concentration, and β3 subunit concentration) approaching normal physiological conditions were critical to elucidate the full extent of this novel β3 effect. Steady-state inactivation curves revealed that N-type channels exhibited “closed-state” inactivation without β3, and that β3 caused an ∼40-mV negative shift of the inactivation, producing a second component with an inactivation midpoint of approximately −85 mV. The inactivation of N-type channels in the presence of a high concentration (12.5 ng/cell) of β3 developed slowly and the time-dependent inactivation curve was best fit by the sum of two exponential functions with time constants of 14 s and 8.8 min at −80 mV. Similar “ultra-slow” inactivation was observed for N-type channels without β3. Thus, β3 can have a profound negative regulatory effect on N-type (and also R-type) calcium channels by causing a hyperpolarizing shift of the inactivation without affecting “ultra-slow” and “closed-state” inactivation properties.

Identification of Sites Responsible for Potentiation of Type 2.3 Calcium Currents by Acetyl-β-methylcholine

To address mechanisms for the differential sensitivity of voltage-gated Ca2+ channels (Cav) to agonists, channel activity was compared in Xenopus oocytes coexpressing muscarinic M(1) receptors and different Cav alpha1 subunits, all with beta1B,alpha2/delta subunits. Acetyl-beta-methylcholine (MCh) decreased Cav 1.2c currents, did not affect 2.1 or 2.2 currents, but potentiated Cav 2.3 currents. Phorbol 12-myristate 13-acetate (PMA) did not affect Cav 1.2c or 2.1 currents but potentiated 2.2 and 2.3 currents. Comparison of the amino acid sequences of the alpha1 subunits revealed a set of potential protein kinase C phosphorylation sites in common between the 2.2 and 2.3 channels that respond to PMA and a set of potential sites unique to the alpha1 2.3 subunits that respond to MCh. Quadruple Ser --> Ala mutation of the predicted MCh sites in the alpha1 2.3 subunit (Ser-888, Ser-892, and Ser-894 in the II-III linker and Ser-1987 in the C terminus) caused loss of the MCh response but not the PMA response. Triple Ser --> Ala mutation of just the II-III linker sites gave similar results. Ser-888 or Ser-892 was sufficient for the MCh responsiveness, whereas Ser-894 required the presence of Ser-1987. Ser --> Asp substitution of Ser-888, Ser-892, Ser-1987, and Ser-892/Ser-1987 increased the basal current and decreased the MCh response but did not alter the PMA response. These results reveal that sites unique to the II-III linker of alpha1 2.3 subunits mediate the responsiveness of Cav 2.3 channels to MCh. Because Cav 2.3 channels contribute to action potential-induced Ca2+ influx, these sites may account for M1 receptor-mediated regulation of neurotransmission at some synapses.

Unified mechanisms of Ca2+ regulation across the Ca2+ channel family

L-type (CaV1.2) and P/Q-type (CaV2.1) calcium channels possess lobe-specific CaM regulation, where Ca2+ binding to one or the other lobe of CaM triggers regulation, even with inverted polarity of modulation between channels. Other major members of the CaV1-2 channel family, R-type (CaV2.3) and N-type (CaV2.2), have appeared to lack such CaM regulation. We report here that R- and N-type channels undergo Ca(2+)-dependent inactivation, which is mediated by the CaM N-terminal lobe and present only with mild Ca2+ buffering (0.5 mM EGTA) characteristic of many neurons. These features, together with the CaM regulatory profiles of L- and P/Q-type channels, are consistent with a simplifying principle for CaM signal detection in CaV1-2 channels-independent of channel context, the N- and C-terminal lobes of CaM appear invariably specialized for decoding local versus global Ca2+ activity, respectively.

Comparative effects of methylmercury and Hg(2+) on human neuronal N- and R-type high-voltage activated calcium channels transiently expressed in human embryonic kidney 293 cells

Expression cDNA clones of alpha1B-1 or alpha1E-3 subunits coding for human neuronal N-(Cav2.2) or R-subtype (Cav2.3) Ca2+ channels, respectively, was combined with alpha2-bdelta and beta3-a Ca2+ channel subunits, and transfected into human embryonic kidney cells for transient expression to determine whether specific types of neuronal voltage-sensitive Ca2+ channels are affected differentially by methylmercury (MeHg) and Hg2+. For both Ca2+ channel subtypes, MeHg (0.125-5.0 microM) or Hg2+ (0.1-5 microM) caused a time- and concentration-dependent reduction of current. MeHg caused an initial, rapid component and a subsequent more gradual component of inhibition. The rapid component of block was completed between 100 and 150 s after beginning treatment. At 0.125 to 1.25 microM, MeHg caused a more gradual decline in current. Apparent IC50 values were 1.3 and 1.1 microM for MeHg, and 2.2 and 0.7 microM for Hg2+ on N- and R-types, respectively. For N-type current, effects of Hg2+ were initially greater on the peak current than on the sustained current remaining at the end of a test pulse; subsequently, Hg2+ blocked both components of current. For R-type current, Hg2+ affected peak and sustained current approximately equally. Kinetics of inactivation also seemed to be affected by Hg2+ in cells expressing N-type but not R-type current. Washing with MeHg-free solution could not reverse effects of MeHg on either type of current. The effect of Hg2+ on N- but not R-type current was partially reversed by Hg2+-free wash solution. Therefore, different types of Ca2+ channels have differential susceptibility to neurotoxic mercurials even when expressed in the same cell type.

Functional specialization of presynaptic Cav2.3 Ca2+ channels

Ca2+ influx into presynaptic terminals via voltage-dependent Ca2+ channels triggers fast neurotransmitter release as well as different forms of synaptic plasticity. Using electrophysiological and genetic techniques we demonstrate that presynaptic Ca2+ entry through Cav2.3 subunits contributes to the induction of mossy fiber LTP and posttetanic potentiation by brief trains of presynaptic action potentials while they do not play a role in fast synaptic transmission, paired-pulse facilitation, or frequency facilitation. This functional specialization is most likely achieved by a localization remote from the release machinery and by a Cav2.3 channel-dependent facilitation of presynaptic Ca2+ influx. Thus, the presence of Cav2.3 channels boosts the accumulation of presynaptic Ca2+ triggering presynaptic LTP and posttetanic potentiation without affecting the low release probability that is a prerequisite for the enormous plasticity displayed by mossy fiber synapses.

Anesthetic sensitivities to propofol and halothane in mice lacking the R-type (Cav2.3) Ca2+ channel

Because inhibition of voltage-dependent Ca(2+) channels can be a mechanism underlying general anesthesia, we examined sensitivities to propofol and halothane in mice lacking the R-type (Ca(v)2.3) channel widely expressed in neurons. Sleep time after propofol injection (26 mg/kg IV) and halothane MAC(RR) and MAC (50% effective concentrations for the loss of the righting reflex and for the tail pinch/withdrawal response, respectively) were determined. Significantly shorter propofol-induced sleep time (291.6 +/- 16.8 s versus 344.4 +/- 12.1 s) and larger halothane MAC(RR) (1.11% +/- 0.04% versus 0.98% +/- 0.03%) were observed in Ca(v)2.3 channel knockouts (Ca(v)2.3(-/-)) than in wild-type (Ca(v)2.3(+/+)) litter mates. To investigate the basis of the decreased anesthetic sensitivities in vivo, field excitatory postsynaptic potentials and population spikes (PSs) were recorded from Schaffer collateral CA1 synapses in hippocampal slices. Propofol (10-30 micro M) inhibited PSs by potentiating gamma-aminobutyric acid-ergic inhibition, and this potentiation was markedly smaller at 30 micro M in Ca(v)2.3(-/-) mice, possibly accounting for the decreased propofol sensitivity in vivo. Halothane (1.4%-2.2%) inhibited field excitatory postsynaptic potentials similarly in both genotypes, whereas 1%-2% halothane depressed PSs more in Ca(v)2.3(-/-) mice, suggesting the postsynaptic role of the R-type channel in the propagation of excitation and other mechanisms underlying the increased halothane MAC(RR) in Ca(v)2.3(-/-) mice.

IMPLICATIONS

Because inhibition of neuronal Ca(2+) currents can be a mechanism underlying general anesthesia, we examined anesthetic sensitivities in mice lacking the R-type (Ca(v)2.3) Ca(2+) channels both in vivo and in hippocampal slices. Decreased sensitivities in mutant mice imply a possibility that agents blocking this channel may increase the requirements of anesthetics/hypnotics.

Alternative splicing of Cav2 genes and their functional significance

Alternative splicing is one of the most pharmacologically and physiologically significant mechanisms for the functional diversity of the mammalian genomes. Here I review recent results on the diversity of the Ca(v)2 subclass of voltage-dependent Ca(2+) channel gene in neurons. Although the entire picture of alternative splicing is not yet understood, emerging evidences suggest the Ca(v)2 isoforms permit optimization of Ca(2+) signaling in different regions of the brain with specific pharmacological ligands.

The Ca(V)2.3 Ca(2+) channel subunit contributes to R-type Ca(2+) currents in murine hippocampal and neocortical neurones

Different subtypes of voltage-dependent Ca(2+) currents in native neurones are essential in coupling action potential firing to Ca(2+) influx. For most of these currents, the underlying Ca(2+) channel subunits have been identified on the basis of pharmacological and biophysical similarities. In contrast, the molecular basis of R-type Ca(2+) currents remains controversial. We have therefore examined the contribution of the Ca(V)2.3 (alpha(1E)) subunits to R-type currents in different types of central neurones using wild-type mice and mice in which the Ca(V)2.3 subunit gene was deleted. In hippocampal CA1 pyramidal cells and dentate granule neurones, as well as neocortical neurones of wild-type mice, Ca(2+) current components resistant to the combined application of omega-conotoxin GVIA and MVIIC, omega-agatoxin IVa and nifedipine (I(Ca,R)) were detected that were composed of a large R-type and a smaller T-type component. In Ca(V)2.3-deficient mice, I(Ca,R) was considerably reduced in CA1 neurones (79 %) and cortical neurones (87 %), with less reduction occurring in dentate granule neurones (47 %). Analysis of tail currents revealed that the reduction of I(Ca,R) is due to a selective reduction of the rapidly deactivating R-type current component in CA1 and cortical neurones. In all cell types, I(Ca,R) was highly sensitive to Ni(2+) (100 microM: 71-86 % block). A selective antagonist of cloned Ca(V)2.3 channels, the spider toxin SNX-482, partially inhibited I(Ca,R) at concentrations up to 300 nM in dentate granule cells and cortical neurones (50 and 57 % block, EC(50) 30 and 47 nM, respectively). I(Ca,R) in CA1 neurones was significantly less sensitive to SNX-482 (27 % block, 300 nM SNX-482). Taken together, our results show clearly that Ca(V)2.3 subunits underlie a significant fraction of I(Ca,R) in different types of central neurones. They also indicate that Ca(V)2.3 subunits may give rise to Ca(2+) currents with differing pharmacological properties in native neurones.

Effects of ablation of N- and R-type Ca(2+) channels on pain transmission

Recently several mutant mouse lines lacking neuronal voltage-dependent Ca(2+) channels (VDCCs) have been established by the use of gene targeting in embryonic stem cells. Pain-related behaviors in Ca(v)2.2 (alpha(1B)) and Ca(v)2.3 (alpha(1E)) knockout mice were studied to gain further insight into the mechanism of pain transmission, where VDCCs are thought to play important roles. We review here the data from these recent studies. Ca(v)2.3-/- mice showed normal responses to acute painful stimuli, and reduced responses to the somatic inflammatory pain stimuli. Ca(v)2.3+/- mice exhibited reduced symptoms of visceral inflammatory pain. Ca(v)2.3-/- mice showed abnormal behavior related to the descending antinociceptive mechanism activated by the intraperitoneal injection of acetic acid. Ca(v)2.2-/- mice showed variable acute nociceptive responses depending on the mutant lines. However, all the lines of Ca(v)2.2-/- mice exhibited reduced responses in the phase 2 of the formalin test, suggesting a suppression of inflammatory pain. Furthermore Ca(v)2.2-/- mice showed markedly reduced neuropathic pain symptoms after spinal nerve ligation. Impaired antinociception, similar to that seen in the Ca(v)2.3-/- mice, was also observed in the Ca(v)2.2-/- mice. Therefore, it is suggested that these mutant mice could provide novel models to delineate the nociceptive and antinociceptive mechanisms.

Molecular basis of R-type calcium channels in central amygdala neurons of the mouse

R-type Ca2+ channels play a critical role in coupling excitability to dendritic Ca2+ influx and neuronal secretion. Unlike other types of voltage-sensitive Ca2+ channels (L, N, P/Q, and T type), the molecular basis for the R-type Ca2+ channel is still unclear, thereby limiting further detailed analyses of R-type Ca2+ channel physiology. The prevailing hypothesis is that alpha(1E) (Ca(V)2.3) gene encodes for R-type Ca2+ channels, but the dearth of critical evidence has rendered this hypothesis controversial. Here we generated alpha1E-deficient mice (alpha1E-/-) and examined the status of voltage-sensitive Ca2+ currents in central amygdala (CeA) neurons that exhibit abundant alpha1E expression and R-type Ca2+ currents. The majority of R-type currents in CeA neurons were eliminated in alpha1E-/- mice whereas other Ca2+ channel types were unaffected. These data clearly indicate that the expression of alpha1E gene underlies R-type Ca2+ channels in CeA neurons. Furthermore, the alpha1E-/- sign mice exhibited signs of enhanced fear as evidenced by their vigorous escaping behavior and aversion to open-field conditions. These latter findings imply a possible role of alpha1E-based R-type Ca2+ currents in amygdala physiology associated with fear.

Functional coupling between 'R-type' Ca2+ channels and insulin secretion in the insulinoma cell line INS-1

Among voltage-gated Ca2+ channels the non-dihydropyridine-sensitive alpha1E subunit is functionally less well characterized than the structurally related alpha1A (omega-agatoxin-IVA sensitive, P- /Q-type) and alpha1B (omega-conotoxin-GVIA sensitive, N-type) subunits. In the rat insulinoma cell line, INS-1, a tissue-specific splice variant of alpha1E (alpha1Ee) has been characterized at the mRNA and protein levels, suggesting that INS-1 cells are a suitable model for investigating the function of alpha1Ee. In alpha1E-transfected human embryonic kidney (HEK-293) cells the alpha1E-selective peptide antagonist SNX-482 (100 nM) reduces alpha1Ed- and alpha1Ee-induced Ba2+ inward currents in the absence and presence of the auxiliary subunits beta3 and alpha2delta-2 by more than 80%. The inhibition is fast and only partially reversible. No effect of SNX-482 was detected on the recombinant T-type Ca2+ channel subunits alpha1G, alpha1H, and alpha1I showing that the toxin from the venom of Hysterocrates gigas is useful as an alpha1E-selective antagonist. After blocking known components of Ca2+ channel inward current in INS-1 cells by 2 microM (+/-)-isradipine plus 0.5 microM omega-conotoxin-MVIIC, the remaining current is reduced by 100 nM SNX-482 from -12.4 +/- 1.2 pA/pF to -7.6 +/- 0.5 pA/pF (n = 9). Furthermore, in INS-1 cells, glucose- and KCl-induced insulin release are reduced by SNX-482 in a dose-dependent manner leading to the conclusion that alpha1E, in addition to L-type and non-L-type (alpha1A-mediated) Ca2+ currents, is involved in Ca2+ dependent insulin secretion of INS-1 cells.

The status of voltage-dependent calcium channels in alpha 1E knock-out mice

It has been hypothesized that R-type Ca currents result from the expression of the alpha(1E) gene. To test this hypothesis we examined the properties of voltage-dependent Ca channels in mice in which the alpha(1E) Ca channel subunit had been deleted. Application of omega-conotoxin GVIA, omega-agatoxin IVA, and nimodipine to cultured cerebellar granule neurons from wild-type mice inhibited components of the whole-cell Ba current, leaving a "residual" R current with an amplitude of approximately 30% of the total Ba current. A minor portion of this R current was inhibited by the alpha(1E)-selective toxin SNX-482, indicating that it resulted from the expression of alpha(1E). However, the majority of the R current was not inhibited by SNX-482. The SNX-482-sensitive portion of the granule cell R current was absent from alpha(1E) knock-out mice. We also identified a subpopulation of dorsal root ganglion (DRG) neurons from wild-type mice that expressed an SNX-482-sensitive component of the R current. However as with granule cells, most of the DRG R current was not blocked by SNX-482. We conclude that there exists a component of the R current that results from the expression of the alpha(1E) Ca channel subunit but that the majority of R currents must result from the expression of other Ca channel alpha subunits.

Unique properties of R-type calcium currents in neocortical and neostriatal neurons

Whole cell recordings from acutely dissociated neocortical pyramidal neurons and striatal medium spiny neurons exhibited a calcium-channel current resistant to known blockers of L-, N-, and P/Q-type Ca(2+) channels. These R-type currents were characterized as high-voltage-activated (HVA) by their rapid deactivation kinetics, half-activation and half-inactivation voltages, and sensitivity to depolarized holding potentials. In both cell types, the R-type current activated at potentials relatively negative to other HVA currents in the same cell type and inactivated rapidly compared with the other HVA currents. The main difference between cell types was that R-type currents in neocortical pyramidal neurons inactivated at more negative potentials than R-type currents in medium spiny neurons. Ni(2+) sensitivity was not diagnostic for R-type currents in either cell type. Single-cell RT-PCR revealed that both cell types expressed the alpha1E mRNA, consistent with this subunit being associated with the R-type current.

CaV2.2 and CaV2.3 (N- and R-type) Ca2+ channels in depolarization-evoked entry of Ca2+ into mouse sperm

As sperm prepare for fertilization, surface Ca(2+) channels must open to initiate required, Ca(2+)-mediated events. However, the molecular identity and functional properties of sperm Ca(2+) channels remain uncertain. Here, we use rapid local perfusion and single-cell photometry to examine the kinetics of calcium responses of mouse sperm to depolarizing stimuli. The linear rise of intracellular [Ca(2+)] evoked by approximately 10-s applications of an alkaline high [K(+)] medium directly reports activity of voltage-gated Ca(2+) channels. Little response occurs if external Ca(2+) is removed or if external or internal pH is elevated without depolarization. Responses are inhibited 30-40% by 30-100 micrometer Ni(2+) and more completely by 100-300 micrometer Cd(2+). They resist the dihydropyridines nitrendipine and PN200-110, but 1-10 micrometer mibefradil inhibits reversibly. They also resist the venom toxins calciseptine, omega-conotoxin MVIIC, and kurtoxin, but omega-conotoxin GVIA (5 micrometer) inhibits approximately 50%. GVIA also partially blocks transient, low voltage activated Ca(2+) currents of patch-clamped spermatids. Differential sensitivity of sperm responses to Ni(2+) and Cd(2+) and partial blockade by GVIA indicate that depolarization opens at least two types of voltage-gated Ca(2+) channels in epididymal sperm examined prior to capacitation. Involvement of a previously undetected Ca(V)2.2 (N-type) channel, suggested by the action of GVIA, is substantiated by immunodetection of Ca(2+) channel alpha(1B) subunits in sperm and sperm extracts. Resistance to dihydropyridines, calciseptine, MVIIC, and kurtoxin indicates that Ca(V)1, Ca(V)2.1, and Ca(V)3 (L-, P/Q-, and T-type) channels contribute little to this evoked response. Partial sensitivity to 1 micrometer mibefradil and an enhanced sensitivity of the GVIA-resistant component of response to Ni(2+) suggest participation of a Ca(V)2.3 (R-type) channel specified by previously found alpha(1E) subunits. Our examination of depolarization-evoked Ca(2+) entry indicates that mature sperm possess a larger palette of voltage-gated Ca(2+) channels than previously thought. Such diversity may permit specific responses to multiple cues encountered on the path to fertilization.

Altered pain responses in mice lacking alpha 1E subunit of the voltage-dependent Ca2+ channel

alpha(1) subunit of the voltage-dependent Ca(2+) channel is essential for channel function and determines the functional specificity of various channel types. alpha(1E) subunit was originally identified as a neuron-specific one, but the physiological function of the Ca(2+) channel containing this subunit (alpha(1E) Ca(2+) channel) was not clear compared with other types of Ca(2+) channels because of the limited availability of specific blockers. To clarify the physiological roles of the alpha(1E) Ca(2+) channel, we have generated alpha(1E) mutant (alpha(1E)-/-) mice by gene targeting. The lacZ gene was inserted in-frame and used as a marker for alpha(1E) subunit expression. alpha(1E)-/- mice showed reduced spontaneous locomotor activities and signs of timidness, but other general behaviors were apparently normal. As involvement of alpha(1E) in pain transmission was suggested by localization analyses with 5-bromo-4-chloro-3-indolyl beta-d-galactopyranoside staining, we conducted several pain-related behavioral tests using the mutant mice. Although alpha(1E)+/- and alpha(1E)-/- mice exhibited normal pain behaviors against acute mechanical, thermal, and chemical stimuli, they both showed reduced responses to somatic inflammatory pain. alpha(1E)+/- mice showed reduced response to visceral inflammatory pain, whereas alpha(1E)-/- mice showed apparently normal response compared with that of wild-type mice. Furthermore, alpha(1E)-/- mice that had been presensitized with a visceral noxious conditioning stimulus showed increased responses to a somatic inflammatory pain, in marked contrast with the wild-type mice in which long-lasting effects of descending antinociceptive pathway were predominant. These results suggest that the alpha(1E) Ca(2 +) channel controls pain behaviors by both spinal and supraspinal mechanisms.

alpha(1E) subunits form the pore of three cerebellar R-type calcium channels with different pharmacological and permeation properties

R-type Ca(2+) channels cooperate with P/Q- and N-type channels to control neurotransmitter release at central synapses. The leading candidate as pore-forming subunit of R-type channels is the alpha(1E) subunit. However, R-type Ca(2+) currents with permeation and/or pharmacological properties different from those of recombinant Ca(2+) channels containing alpha(1E) subunits have been described, and therefore the molecular nature of R-type Ca(2+) channels remains not completely settled. Here, we show that the R-type Ca(2+) current of rat cerebellar granule cells consists of two components inhibited with different affinity by the alpha(1E) selective antagonist SNX482 (IC(50) values of 6 and 81 nM) and a third component resistant to SNX482. The SNX482-sensitive R-type current shows the unique permeation properties of recombinant alpha(1E) channels; it is larger with Ca(2+) than with Ba(2+) as charge carrier, and it is highly sensitive to Ni(2+) block and has a voltage-dependence of activation consistent with that of G2 channels with unitary conductance of 15 pS. On the other hand, the SNX482-resistant R-type current shows permeation properties similar to those of recombinant alpha(1A) and alpha(1B) channels; it is larger with Ba(2+) than with Ca(2+) as charge carrier(,) and it has a low sensitivity to Ni(2+) block and a voltage-dependence of activation consistent with that of G3 channels with unitary conductance of 20 pS. Gene-specific knock-down by antisense oligonucleotides demonstrates that the different cerebellar R-type channels are all encoded by the alpha(1E) gene, suggesting the existence of alpha(1E) isoforms with different pore properties.