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.

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.

Rocker is a new variant of the voltage-dependent calcium channel gene Cacna1a

Rocker (gene symbol rkr), a new neurological mutant phenotype, was found in descendents of a chemically mutagenized male mouse. Mutant mice display an ataxic, unstable gait accompanied by an intention tremor, typical of cerebellar dysfunction. These mice are fertile and appear to have a normal life span. Segregation analysis reveals rocker to be an autosomal recessive trait. The overall cytoarchitecture of the young adult brain appears normal, including its gross cerebellar morphology. Golgi-Cox staining, however, reveals dendritic abnormalities in the mature cerebellar cortex characterized by a reduction of branching in the Purkinje cell dendritic arbor and a "weeping willow" appearance of the secondary branches. Using simple sequence length polymorphism markers, the rocker locus was mapped to mouse chromosome 8 within 2 centimorgans of the calcium channel alpha1a subunit (Cacna1a, formerly known as tottering) locus. Complementation tests with the leaner mutant allele (Cacna1a(la)) produced mutant animals, thus identifying rocker as a new allele of Cacna1a (Cacna1a(rkr)). Sequence analysis of the cDNA revealed rocker to be a point mutation resulting in an amino acid exchange: T1310K between transmembrane regions 5 and 6 in the third homologous domain. Important distinctions between rocker and the previously characterized alleles of this locus include the absence of aberrant tyrosine hydroxylase expression in Purkinje cells and the separation of the absence seizures (spike/wave type discharges) from the paroxysmal dyskinesia phenotype. Overall these findings point to an important dissociation between the seizure phenotypes and the abnormalities in catecholamine metabolism, and they emphasize the value of allelic series in the study of gene function.

Alternative splicing of a short cassette exon in alpha1B generates functionally distinct N-type calcium channels in central and peripheral neurons

The N-type Ca channel alpha1B subunit is localized to synapses throughout the nervous system and couples excitation to release of neurotransmitters. In a previous study, two functionally distinct variants of the alpha1B subunit were identified, rnalpha1B-b and rnalpha1B-d, that differ at two loci;four amino acids [SerPheMetGly (SFMG)] in IIIS3-S4 and two amino acids [GluThr (ET)] in IVS3-S4. These variants are reciprocally expressed in rat brain and sympathetic ganglia (). We now show that the slower activation kinetics of rnalpha1B-b (DeltaSFMG/+ET) compared with rnalpha1B-d (+SFMG/DeltaET) channels are fully accounted for by the insertion of ET in IVS3-S4 and not by the lack of SFMG in IIIS3-S4. We also show that the inactivation kinetics of these two variants are indistinguishable. Through genomic analysis we identify a six-base cassette exon that encodes the ET site and with ribonuclease protection assays demonstrate that the expression of this mini-exon is essentially restricted to alpha1B RNAs of peripheral neurons. We also show evidence for regulated alternative splicing of a six-base exon encoding NP in the IVS3-S4 linker of the closely related alpha1A gene and establish that residues NP can functionally substitute for ET in domain IVS3-S4 of alpha1B. The selective expression of functionally distinct Ca channel splice variants of alpha1B and alpha1A subunits in different regions of the nervous system adds a new dimension of diversity to voltage-dependent Ca signaling in neurons that may be important for optimizing action potential-dependent transmitter release at different synapses.

Comparison of N- and P/Q-type voltage-gated calcium channel current inhibition

Activation of N- and P/Q-type voltage-gated calcium channels triggers neurotransmitter release at central and peripheral synapses. These channels are targets for regulatory mechanisms, including inhibition by G-protein-linked receptors. Inhibition of P/Q-type channels has been less well studied than the extensively characterized inhibition of N-type channels, but it is thought that they are inhibited by similar mechanisms although possibly to a lesser extent than N-type channels. The aim of this study was to compare the inhibition of the two channel types. Calcium currents were recorded from adrenal chromaffin cells and isolated by the selective blockers omega-conotoxin GVIA (1 microM) and omega-agatoxin IVA (400 nM). The inhibition was elicited by ATP (100 microM) or intracellular application of GTP-gamma-S. It was classified as voltage-sensitive (relieved by a conditioning prepulse) or voltage-insensitive (present after a conditioning prepulse). The voltage-insensitive inhibition accounted for a 20% reduction of both currents, whereas the voltage-sensitive inhibition reduced the N-type current by 45% but the P/Q-type current by 18%. However, the voltage dependence of the inhibition, the time course of relief from inhibition during a conditioning prepulse, and the time course of reinhibition after such a prepulse showed few differences between the N- and P/Q-type channels. Assuming a simple bimolecular reaction, our data suggest that changes in the kinetics of the G-protein/channel interaction alone cannot explain the differences in the inhibition of the N- and P/Q-type calcium channels. The subtle differences in inhibition may facilitate the selective regulation of neurotransmitter release.