C-Terminal alternative splicing changes the gating properties of a human spinal cord calcium channel alpha 1A subunit

More than a pore: How voltage-gated calcium channels act on different levels of neuronal communication regulation

Voltage-gated calcium channels (VGCCs) represent key regulators of the calcium influx through the plasma membrane of excitable cells, like neurons. Activated by the depolarization of the membrane, the opening of VGCCs induces very transient and local changes in the intracellular calcium concentration, known as calcium nanodomains, that in turn trigger calcium-dependent signaling cascades and the release of chemical neurotransmitters. Based on their central importance as concierges of excitation-secretion coupling and therefore neuronal communication, VGCCs have been studied in multiple aspects of neuronal function and malfunction. However, studies on molecular interaction partners and recent progress in omics technologies have extended the actual concept of these molecules. With this review, we want to illustrate some new perspectives of VGCCs reaching beyond their function as calcium-permeable pores in the plasma membrane. Therefore, we will discuss the relevance of VGCCs as voltage sensors in functional complexes with ryanodine receptors, channel-independent actions of auxiliary VGCC subunits, and provide an insight into how VGCCs even directly participate in gene regulation. Furthermore, we will illustrate how structural changes in the intracellular C-terminus of VGCCs generated by alternative splicing events might not only affect the biophysical channel characteristics but rather determine their molecular environment and downstream signaling pathways.

Calcium Channels Genes and Their Epilepsy Phenotypes

Calcium (Ca2+) channel gene mutations play an important role in the pathogenesis of neurological episodic disorders like epilepsy. CACNA1A and CACNA1H genes are involved in the synthesis of calcium channels. Mutations in the α1A subunit of the P/Q type voltage-gated calcium channel gene (CACNA1A) located in 19p13.13, which encodes for the transmembrane pore-forming subunit of CAV2.1 voltage-dependent calcium channel, have been correlated to a large clinical spectrum of epilepsy such as idiopathic genetic epilepsy, early infantile epilepsy, and febrile seizures. Moreover, CACNA1A mutations have been demonstrated to be involved in spinocerebellar ataxia type 6, familiar hemiplegic migraine, episodic ataxia type 2, early-onset encephalopathy, and hemiconvulsion–hemiplegia epilepsy syndrome. This wide phenotype heterogeneity associated with CACNA1A mutations is correlated to different clinical and electrophysiological manifestations. CACNA1H gene, located in 16p13.3, encodes the α1H subunit of T-type calcium channel, expressing the transmembrane pore-forming subunit Cav3.2. Despite data still remain controversial, it has been identified as an important gene whose mutations seem strictly related to the pathogenesis of childhood absence epilepsy and other generalized epilepsies. The studied variants are mainly gain-of-function, hence responsible for an increase in neuronal susceptibility to seizures. CACNA1H mutations have also been associated with autism spectrum disorder and other behavior disorders. More recently, also amyotrophic lateral sclerosis has been related to CACNA1H alterations. The aim of this review, other than describe the CACNA1A and CACNA1H gene functions, is to identify mutations reported in literature and to analyze their possible correlations with specific epileptic disorders, purposing to guide an appropriate medical treatment recommendation.

Alternatively Spliced Human TREK-1 Variants Alter TREK-1 Channel Function and Localization

TREK-1, an outward-rectifying potassium channel activated by stretch, is found in the myometrium of pregnant women. Decreased expression of TREK-1 near term suggests that TREK-1 may contribute to uterine quiescence during gestation. Five alternatively spliced TREK-1 variants were identified in the myometrium of mothers who delivered spontaneously preterm (<37 wk), leading to the hypothesis that these TREK-1 variants could interfere with TREK-1 function or expression. To investigate a potential role for these variants, immunofluorescence, cell surface assays, Western blots, and patch clamp were employed to study TREK-1 and TREK-1 variants expressed in HEK293T cells. The results of this study demonstrate that coexpression of TREK-1 with TREK-1 variants alters TREK-1 expression and suppresses channel function. Each variant affected TREK-1 in a disparate manner. In HEK293T cells coexpressing TREK-1 and each variant, TREK-1 membrane expression was diminished with compartmentalization inside the cell. When expressed alone, individual variants displayed channel properties that were significantly decreased compared to full-length TREK-1. In coexpression studies using patch clamp, basal TREK-1 currents were reduced by ∼64% (4.3 vs. 12.0 pA/pF) on average at 0 mV when coexpressed with each variant. TREK-1 currents that were activated by intracellular acidosis were reduced an average of ∼77% (21.4 vs. 94.5 pA/pF) at 0 mV when cells were transfected with TREK-1 and any one of the splice variants. These data correlate the presence of TREK-1 variants to reduced TREK-1 activity, suggesting a pathological role for TREK-1 variants in preterm labor.

Compensatory regulation of Cav2.1 Ca2+ channels in cerebellar Purkinje neurons lacking parvalbumin and calbindin D-28k

Ca(v)2.1 channels regulate Ca(2+) signaling and excitability of cerebellar Purkinje neurons. These channels undergo a dual feedback regulation by incoming Ca(2+) ions, Ca(2+)-dependent facilitation and inactivation. Endogenous Ca(2+)-buffering proteins, such as parvalbumin (PV) and calbindin D-28k (CB), are highly expressed in Purkinje neurons and therefore may influence Ca(v)2.1 regulation by Ca(2+). To test this, we compared Ca(v)2.1 properties in dissociated Purkinje neurons from wild-type (WT) mice and those lacking both PV and CB (PV/CB(-/-)). Unexpectedly, P-type currents in WT and PV/CB(-/-) neurons differed in a way that was inconsistent with a role of PV and CB in acute modulation of Ca(2+) feedback to Ca(v)2.1. Ca(v)2.1 currents in PV/CB(-/-) neurons exhibited increased voltage-dependent inactivation, which could be traced to decreased expression of the auxiliary Ca(v)beta(2a) subunit compared with WT neurons. Although Ca(v)2.1 channels are required for normal pacemaking of Purkinje neurons, spontaneous action potentials were not different in WT and PV/CB(-/-) neurons. Increased inactivation due to molecular switching of Ca(v)2.1 beta-subunits may preserve normal activity-dependent Ca(2+) signals in the absence of Ca(2+)-buffering proteins in PV/CB(-/-) Purkinje neurons.

Novel splice variants of rat CaV2.1 that lack much of the synaptic protein interaction site are expressed in neuroendocrine cells

Voltage-gated Ca(2+) channels are responsible for the activation of the Ca(2+) influx that triggers exocytotic secretion. The synaptic protein interaction (synprint) site found in the II-III loop of Ca(V)2.1 and Ca(V)2.2 mediates a physical association with synaptic proteins that may be crucial for fast neurotransmission and axonal targeting. We report here the use of nested PCR to identify two novel splice variants of rat Ca(V)2.1 that lack much of the synprint site. Furthermore, we compare immunofluorescence data derived from antibodies directed against sequences in the Ca(V)2.1 synprint site and carboxyl terminus to show that channel variants lacking a portion of the synprint site are expressed in two types of neuroendocrine cells. Immunofluorescence data also suggest that such variants are properly targeted to neuroendocrine terminals. When expressed in a mammalian cell line, both splice variants yielded Ca(2+) currents, but the variant containing the larger of the two deletions displayed a reduced current density and a marked shift in the voltage dependence of inactivation. These results have important implications for Ca(V)2.1 function and for the mechanisms of Ca(V)2.1 targeting in neurons and neuroendocrine cells.

Human heart failure is associated with abnormal C-terminal splicing variants in the cardiac sodium channel

Heart failure (HF) is associated with reduced cardiac Na+ channel (SCN5A) current. We hypothesized that abnormal transcriptional regulation of this ion channel during HF could help explain the reduced current. Using human hearts explanted at the transplantation, we have identified 3 human C-terminal SCN5A mRNA splicing variants predicted to result in truncated, nonfunctional channels. As compared with normal hearts, the explanted ventricles showed an upregulation of 2 of the variants and a downregulation of the full-length mRNA transcript such that the E28A transcript represented only 48.5% (P<0.01) of the total SCN5A mRNA. This correlated with a 62.8% (P<0.01) reduction in Na+ channel protein. Lymphoblasts and skeletal muscle expressing SCN5A also showed identical C-terminal splicing variants. Variants showed reduced membrane protein and no functional current. Transfection of truncation variants into a cell line stably transfected with the full-length Na+ channel resulted in dose-dependent reductions in channel mRNA and current. Introduction of a premature truncation in the C-terminal region in a single allele of the mouse SCN5A resulted in embryonic lethality. Embryonic stem cell-derived cardiomyocytes expressing the construct showed reductions in Na+ channel-dependent electrophysiological parameters, suggesting that the presence of truncated Na+ channel mRNA at levels seen in HF is likely to be physiologically significant. In summary, chronic HF was associated with an increase in 2 truncated SCN5A variants and a decrease in the native mRNA. These splice variations may help explain a loss of Na+ channel protein and may contribute to the increased arrhythmic risk in clinical HF.

Molecular pathogenesis of spinocerebellar ataxia type 6

Spinocerebellar ataxia type 6 (SCA6) is a neurodegenerative disorder caused by abnormal expansions of a trinucleotide CAG repeat in exon 47 of the CACNA1A gene, which encodes the alpha1A subunit of the P/Q-type voltage-gated calcium channel. The CAG repeat expansion is translated into an elongated polyglutamine tract in the carboxyl terminus of the alpha1A subunit. The alpha1A subunit is the main pore-forming subunit of the P/Q-type calcium channel. Patients with SCA6 suffer from a severe form of progressive ataxia and cerebellar dysfunction. Design of treatments for this disorder will depend on better definition of the mechanism of disease. As a disease arising from a mutation in an ion channel gene, SCA6 may behave as an ion channelopathy, and may respond to attempts to modulate or correct ion channel function. Alternatively, as a disease in which the mutant protein contains an expanded polyglutamine tract, SCA6 may respond to the targets of drug therapies developed for Huntington's disease and other polyglutamine disorders. In this review we will compare SCA6 to other polyglutamine diseases and channelopathies, and we will highlight recent advances in our understanding of alpha1A subunits and SCA6 pathology. We also propose a mechanism for how two seemingly divergent hypotheses can be combined into a cohesive model for disease progression.

Altered frequency-dependent inactivation and steady-state inactivation of polyglutamine-expanded α1A in SCA6

Spinocerebellar ataxia type 6 (SCA6) is a neurodegenerative disease of the cerebellum and inferior olives characterized by a late-onset cerebellar ataxia and selective loss of Purkinje neurons ( 15 , 16 ). SCA6 arises from an expansion of the polyglutamine tract located in exon 47 of the α1A (P/Q-type calcium channel) gene from a nonpathogenic size of 4 to 18 glutamines (CAG4–18) to CAG19–33 in SCA6. The molecular basis of SCA6 is poorly understood. To date, the biophysical properties studied in heterologous systems support both a gain and a loss of channel function in SCA6. We studied the behavior of the human α1A isoform, previously found to elicit a gain of function in disease ( 41 ), focusing on properties in which the COOH terminus of the channel is critical for function: we analyzed the current properties in the presence of β4- and β2a-subunits (both known to interact with the α1A COOH terminus), current kinetics of activation and inactivation, calcium-dependent inactivation and facilitation, voltage-dependent inactivation, frequency dependence, and steady-state activation and inactivation properties. We found that SCA6 channels have decreased activity-dependent inactivation and a depolarizing shift (+6 mV) in steady-state inactivation properties consistent with a gain of function.

Alternative splicing in the C-terminus of CaV2.2 controls expression and gating of N-type calcium channels

N-type Ca(V)2.2 calcium channels localize to presynaptic nerve terminals of nociceptors where they control neurotransmitter release. Nociceptive neurons express a unique set of ion channels and receptors important for optimizing their role in transmission of noxious stimuli. Included among these is a structurally and functionally distinct N-type calcium channel splice isoform, Ca(V)2.2e[37a], expressed in a subset of nociceptors and with limited expression in other parts of the nervous system. Ca(V)2.2[e37a] arises from the mutually exclusive replacement of e37a for e37b in the C-terminus of Ca(V)2.2 mRNA. N-type current densities in nociceptors that express a combination of Ca(V)2.2e[37a] and Ca(V)2.2e[37b] mRNAs are significantly larger compared to cells that express only Ca(V)2.2e[37b]. Here we show that e37a supports increased expression of functional N-type channels and an increase in channel open time as compared to Ca(V)2.2 channels that contain e37b. To understand how e37a affects N-type currents we compared macroscopic and single-channel ionic currents as well as gating currents in tsA201 cells expressing Ca(V)2.2e[37a] and Ca(V)2.2e[37b]. When activated, Ca(V)2.2e[37a] channels remain open for longer and are expressed at higher density than Ca(V)2.2e[37b] channels. These unique features of the Ca(V)2.2e[37a] isoform combine to augment substantially the amount of calcium that enters cells in response to action potentials. Our studies of the e37a/e37b splice site reveal a multifunctional domain in the C-terminus of Ca(V)2.2 that regulates the overall activity of N-type calcium channels in nociceptors.

Bovine CACNA1A gene and comparative analysis of the CAG repeats associated to human spinocerebellar ataxia type-6

A small expansion of a CAG repeat domain in exon 47 of the human CACNA1A gene, which codes for the pore-forming alpha1A subunit of P/Q-type Ca2+ channels, causes spinocerebellar ataxia type-6. Only the human alpha1A protein has been demonstrated to contain the poly(Q) tract, although this locus has also recently been detected in ape genomes. To our knowledge, no further information has been published on other mammal species. Here, we have cloned the full-length alpha1A subunit in a non-primate species, the cow. The results have made it possible to explore the exon organization of the bovine CACNA1A gene as well as the splice alpha1A isoforms expressed by bovine chromaffin cells. We found a splice variant of the protein that, as in humans, also contains a polymorphic poly(Q) tract. Based on this result and using data from different Genome Databases, we performed an interspecies comparison of exon 47 and discovered that the poly(Q) tract is present in all the species studied, with the exception of primitive fish and rodents. Our results provide insight into the evolution of the CAG repeat tract at the C-terminus coding region of the CACNA1A gene.

Cav3.1 splice variant expression during neuronal differentiation of Y-79 retinoblastoma cells

A decrease in transient-type calcium channel current, Ca(v)3.1 protein and the mRNA encoding these channels has been reported during differentiation of human retinoblastoma cells. In this study, we examined splice variants of Ca(v)3.1 before and after neuronal differentiation of the Y-79 retinoblastoma cell line to investigate the potential contribution of Ca(v)3.1 to Y-79 differentiation. In Ca(v)3.1, alternative splicing induces variations in three cytoplasmic regions, e.g. the link between domains II and III (producing isoforms e+ and e-), the link between domains III and IV (producing isoforms a, b, ac and bc) and the carboxy terminal region (producing isoforms f and d). Our results demonstrate that Ca(v)3.1e was not expressed in either undifferentiated or differentiated retinoblastoma cells. Splice variants Ca(v)3.1ac; Ca(v)3.1bc and Ca(v)3.1b were all identified in undifferentiated retinoblastoma cells, while expression of these variants in differentiated cells was restricted to the Ca(v)3.1bc isoform. The carboxy terminal variant Ca(v)3.1f is expressed independently of the differentiation status of retinoblastoma cells with or without Ca(v)3.1d. Examination of the functional contribution of Ca(v)3.1 protein to Y-79 cell differentiation revealed that in Y-79 cells transfected with Ca(v)3.1 antisense oligodeoxynucleotides, knockdown of Ca(v)3.1 did not alter the time-course of differentiation or neuritogenesis. The changes in Ca(v)3.1 splice variants were not required for the initiation of differentiation but may be associated with tissue-specific expression or localized alterations in Ca(2+) signaling that are essential for establishment of the mature differentiated phenotype.

Alternative splicing generates a smaller assortment of CaV2.1 transcripts in cerebellar Purkinje cells than in the cerebellum

P/Q-type calcium channels control many calcium-driven functions in the brain. The CACNA1A gene encoding the pore-forming CaV2.1 (alpha1A) subunit of P/Q-type channels undergoes alternative splicing at multiple loci. This results in channel variants with different phenotypes. However, the combinatorial patterns of alternative splice events at two or more loci, and hence the diversity of CaV2.1 transcripts, are incompletely defined for specific brain regions and types of brain neurons. Using RT-PCR and splice variant-specific primers, we have identified multiple CaV2.1 transcript variants defined by different pairs of splice events in the cerebellum of adult rat. We have uncovered new splice variations between exons 28 and 34 (some of which predict a premature stop codon) and a new variation in exon 47 (which predicts a novel extended COOH-terminus). Single cell RT-PCR reveals that each individual cerebellar Purkinje neuron also expresses multiple alternative CaV2.1 transcripts, but the assortment is smaller than in the cerebellum. Two of these variants encode different extended COOH-termini which are not the same as those previously reported in Purkinje cells of the mouse. Our patch-clamp recordings show that calcium channel currents in the soma and dendrites of Purkinje cells are largely inhibited by a concentration of omega-agatoxin IVA selective for P-type over Q-type channels, suggesting that the different transcripts may form phenotypic variants of P-type calcium channels in Purkinje cells. These results expand the known diversity of CaV2.1 transcripts in cerebellar Purkinje cells, and propose the selective expression of distinct assortments of CaV2.1 transcripts in different brain neurons and species.

Modal gating of human CaV2.1 (P/Q-type) calcium channels: I. The slow and the fast gating modes and their modulation by beta subunits

The single channel gating properties of human Ca_V2.1 (P/Q-type) calcium channels and their modulation by the auxiliary β_1b, β_2e, β_3a, and β_4a subunits were investigated with cell-attached patch-clamp recordings on HEK293 cells stably expressing human Ca_V2.1 channels. These calcium channels showed a complex modal gating, which is described in this and the following paper (Fellin, T., S. Luvisetto, M. Spagnolo, and D. Pietrobon. 2004. J. Gen. Physiol. 124:463–474). Here, we report the characterization of two modes of gating of human Ca_V2.1 channels, the slow mode and the fast mode. A channel in the two gating modes differs in mean closed times and latency to first opening (both longer in the slow mode), in voltage dependence of the open probability (larger depolarizations are necessary to open the channel in the slow mode), in kinetics of inactivation (slower in the slow mode), and voltage dependence of steady-state inactivation (occurring at less negative voltages in the slow mode). Ca_V2.1 channels containing any of the four β subtypes can gate in either the slow or the fast mode, with only minor differences in the rate constants of the transitions between closed and open states within each mode. In both modes, Ca_V2.1 channels display different rates of inactivation and different steady-state inactivation depending on the β subtype. The type of β subunit also modulates the relative occurrence of the slow and the fast gating mode of Ca_V2.1 channels; β_3a promotes the fast mode, whereas β_4a promotes the slow mode. The prevailing mode of gating of Ca_V2.1 channels lacking a β subunit is a gating mode in which the channel shows shorter mean open times, longer mean closed times, longer first latency, a much larger fraction of nulls, and activates at more positive voltages than in either the fast or slow mode.

Presynaptic Ca2+ channels compete for channel type-preferring slots in altered neurotransmission arising from Ca2+ channelopathy

Several human channelopathies result from mutations in alpha1A, the pore-forming subunit of P/Q-type Ca2+ channels, conduits of presynaptic Ca2+ entry for evoked neurotransmission. We found that wild-type human alpha1A subunits supported transmission between cultured mouse hippocampal neurons equally well as endogenous mouse alpha1A, whereas introduction of impermeant human alpha1A hampered the effect of endogenous subunits. Thus, presynaptic P/Q-type channels may compete for channel type-preferring "slots" that limit their synaptic effectiveness. The existence of slots generates predictions for how neurotransmission might be affected by changes in Ca2+ channel properties, which we tested by studying alpha1A mutations that are associated with familial hemiplegic migraine type 1 (FHM1). Mutant human P/Q-type channels were impaired in contributing to neurotransmission in precise accord with their deficiency in supporting whole-cell Ca2+ channel activity. Expression of mutant channels in wild-type neurons reduced the synaptic contribution of P/Q-type channels, suggesting that competition for type-preferring slots might support the dominant inheritance of FHM1.

Alternative splicing in the voltage-sensing region of N-Type CaV2.2 channels modulates channel kinetics

The CaV2.2 gene encodes the functional core of the N-type calcium channel. This gene has the potential to generate thousands of CaV2.2 splice isoforms with different properties. However, the functional significance of most sites of alternative splicing is not established. The IVS3-IVS4 region contains an alternative splice site that is conserved evolutionarily among CaValpha1 genes from Drosophila to human. In CaV2.2, inclusion of exon 31a in the IVS3-IVS4 region is restricted to the peripheral nervous system, and its inclusion slows the speed of channel activation. To investigate the effects of exon 31a in more detail, we generated four tsA201 cell lines stably expressing CaV2.2 splice isoforms. Coexpression of auxiliary CaVbeta and CaValpha2delta subunits was required to reconstitute currents with the kinetics of N-type channels from neurons. Channels including exon 31a activated and deactivated more slowly at all voltages. Current densities were high enough in the stable cell lines co-expressing CaValpha2delta to resolve gating currents. The steady-state voltage dependence of charge movement was not consistently different between splice isoforms, but on gating currents from the exon 31a-containing CaV2.2 isoform decayed with a slower time course, corresponding to slower movement of the charge sensor. Exon 31a-containing CaV2.2 is restricted to peripheral ganglia; and the slower gating kinetics of CaV2.2 splice isoforms containing exon 31a correlated reasonably well with the properties of native N-type currents in sympathetic neurons. Our results suggest that alternative splicing in the S3-S4 linker influences the kinetics but not the voltage dependence of N-type channel gating.

The impact of splice isoforms on voltage-gated calcium channel alpha1 subunits

Semi-conserved exon boundaries in members of the CACNA1 gene family result in recurring pre-mRNA splicing patterns. The resulting variations in the encoded pore-forming subunit of the voltage-gated calcium channel affect functionally significant regions, such as the vicinity of the voltage-sensing S4 segments or the intracellular loops that are important for protein interaction. In addition to generating functional diversity, RNA splicing regulates the quantitative expression of other splice isoforms of the same gene by producing transcripts with premature stop codons which encode two-domain or three-domain channels. An overview of some of the known splice isoforms of the alpha(1) calcium channel subunits and their significance is given.

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.

Systematic identification of splice variants in human P/Q-type channel alpha1(2.1) subunits: implications for current density and Ca2+-dependent inactivation

P/Q-type (Ca(v)2.1) calcium channels support a host of Ca2+-driven neuronal functions in the mammalian brain. Alternative splicing of the main alpha1A (alpha1(2.1)) subunit of these channels may thereby represent a rich strategy for tuning the functional profile of diverse neurobiological processes. Here, we applied a recently developed "transcript-scanning" method for systematic determination of splice variant transcripts of the human alpha1(2.1) gene. This screen identified seven loci of variation, which together have never been fully defined in humans. Genomic sequence analysis clarified the splicing mechanisms underlying the observed variation. Electrophysiological characterization and a novel analytical paradigm, termed strength-current analysis, revealed that one focus of variation, involving combinatorial inclusion and exclusion of exons 43 and 44, exerted a primary effect on current amplitude and a corollary effect on Ca2+-dependent channel inactivation. These findings significantly expand the anticipated scope of functional diversity produced by splice variation of P/Q-type channels.

The alpha(1A) subunits of rat brain calcium channels are developmentally regulated by alternative RNA splicing

Calcium influx through voltage-gated calcium channels governs important aspects of CNS development. Multiple alternative splicings of the pore-forming alpha(1) subunits have been evidenced in adult brain but little information about their expression during ontogenesis is presently available. The aim of this study was to focus on the expression of three rat voltage-gated calcium channel alpha(1A) splice variants (alpha(1A-a), alpha(1A-b) and alpha(1A-EFe)) during brain ontogenesis in vivo. Using a reverse transcription-polymerase chain reaction strategy, we found that the three isoforms have different timings of development throughout the brain: alpha(1A-b) is expressed from embryonic to the adult stage, alpha(1A--EFe) is restricted to the embryonic period whereas alpha(1A-a) is expressed only postnatally. In situ hybridization indicated that alpha(1A-a) and alpha(1A-b) isoforms develop with different regional and cellular patterns. In hippocampus and cerebellum, alpha(1A-b) represented the predominant isoform at all developmental stages. Taken together, these data reveal that alternative RNA splicing may modulate the alpha(1A) calcium channel properties during development.

Novel Cav2.1 splice variants isolated from Purkinje cells do not generate P-type Ca2+ current

The alpha(1)2.1 (alpha(1A)) subunits of P-type and Q-type Ca(2+) channels are encoded by a single gene, Cacna1a. Although these channels differ in the inactivation kinetics and sensitivity to omega-agatoxin IVA, the mechanism underlying these differences remains to be clarified. Alternative splicings of the Cacna1a transcript have been postulated to contribute to the respective properties, however, the splice variants responsible for P-type Ca(2+) channels have not been identified. To explore P-type-specific splice variants, we aimed at cloning alpha(1)2.1 from isolated mouse Purkinje cells using single-cell reverse transcription-PCR, because in Purkinje cells P-type currents dominate over the whole currents (>95%) with Q-type currents undetected. As a result, two novel splice variants were cloned. Compared with the previously cloned mouse alpha(1)2.1, two novel variants had additional 48 amino acids at the amino termini, six single amino acid changes, and splicing variations at the exon 46/47 boundary, which produced different carboxyl termini. Furthermore, one variant had one RNA editing site. However, electrophysiological and pharmacological studies indicated that these variants did not generate P-type current in cultured cells. These results suggest that P-type-specific splice variants may exist but that post-translational processing or modification by uncharacterized interacting proteins is also required for generating the P-type current.

Alternative splicing of the beta 4 subunit has alpha1 subunit subtype-specific effects on Ca2+ channel gating

Ca2+ channel beta subunits are important molecular determinants of the kinetics and voltage dependence of Ca2+ channel gating. Through direct interactions with channel-forming alpha1 subunits, beta subunits enhance expression levels, accelerate activation, and have variable effects on inactivation. Four distinct beta subunit genes each encode five homologous sequence domains (D1-5), three of which (D1, D3, and D5) undergo alternative splicing. We have isolated from human spinal cord a novel alternatively spliced beta4 subunit containing a short form of domain D1 (beta4a) that is highly homologous to N termini of Xenopus and rat beta3 subunits. The purpose of this study was to compare the gating properties of various alpha1 subunit complexes containing beta4a with those of complexes containing a beta4 subunit with a longer form of domain D1, beta4b. Expression in Xenopus oocytes revealed that, relative to alpha1A and alpha1B complexes containing beta4b, the voltage dependence of activation and inactivation of complexes containing beta4a were shifted to more depolarized potentials. Moreover, alpha1A and alpha1B complexes containing beta4a inactivated at a faster rate. Interestingly, beta4 subunit alternative splicing did not influence the gating properties of alpha1C and alpha1E subunits. Experiments with beta4 deletion mutants revealed that both the N and C termini of the beta4 subunit play critical roles in setting voltage-dependent gating parameters and that their effects are alpha1 subunit specific. Our data are best explained by a model in which distinct modes of activation and inactivation result from beta-subunit splice variant-specific interactions with an alpha1 subunit gating structure.

Increased expression of alpha 1A Ca2+ channel currents arising from expanded trinucleotide repeats in spinocerebellar ataxia type 6

The expansion of polyglutamine tracts encoded by CAG trinucleotide repeats is a common mutational mechanism in inherited neurodegenerative diseases. Spinocerebellar ataxia type 6 (SCA6), an autosomal dominant, progressive disease, arises from trinucleotide repeat expansions present in the coding region of CACNA1A (chromosome 19p13). This gene encodes alpha(1A), the principal subunit of P/Q-type Ca(2+) channels, which are abundant in the CNS, particularly in cerebellar Purkinje and granule neurons. We assayed ion channel function by introduction of human alpha(1A) cDNAs in human embryonic kidney 293 cells that stably coexpressed beta(1) and alpha(2)delta subunits. Immunocytochemical analysis showed a rise in intracellular and surface expression of alpha(1A) protein when CAG repeat lengths reached or exceeded the pathogenic range for SCA6. This gain at the protein level was not a consequence of changes in RNA stability, as indicated by Northern blot analysis. The electrophysiological behavior of alpha(1A) subunits containing expanded (EXP) numbers of CAG repeats (23, 27, and 72) was compared against that of wild-type subunits (WT) (4 and 11 repeats) using standard whole-cell patch-clamp recording conditions. The EXP alpha(1A) subunits yielded functional ion channels that supported inward Ca(2+) channel currents, with a sharp increase in P/Q Ca(2+) channel current density relative to WT. Our results showed that Ca(2+) channels from SCA6 patients display near-normal biophysical properties but increased current density attributable to elevated protein expression at the cell surface.