The alpha 2/delta subunit of voltage sensitive Ca2+ channels is a single transmembrane extracellular protein which is involved in regulated secretion

Autism associated mutations in β2 subunit of voltage-gated calcium channels constitutively activate gene expression

Membrane depolarization triggers gene expression through voltage-gated calcium channels (VGCC) in a process called Excitation-transcription (ET) coupling. Mutations in the channel subunits α₁1.2, or β_2d, are associated with neurodevelopmental disorders such as ASD. Here, we found that two mutations S143F and G113S within the rat Cavβ_2a corresponding to autistic related mutations Cavβ_2d^S197F and Cavβ_2d^G167S in the human Cavβ_2d, activate ET-coupling via the RAS/ERK/CREB pathway. Membrane depolarization of HEK293 cells co-expressing α₁1.2 and α_2δ with Cavβ_2a^S143F or Cavβ_2a^G113S triggers constitutive transcriptional activation, which is correlated with facilitated channel activity. Similar to the Timothy-associated autistic mutation α₁1.2^G406R, constitutive gene activation is attributed to a hyperpolarizing shift in the activation kinetics of Cav1.2. Pulldown of RasGRF2 and RhoGEF by wt and the Cavβ_2a autistic mutants is consistent with Cavβ₂/Ras activation in ET coupling and implicates Rho signaling as yet another molecular pathway activated by Cavα₁1.2/Cavβ2 . Facilitated spontaneous channel activity preceding enhanced gene activation via the Ras/ERK/CREB pathway, appears a general molecular mechanism for Ca²⁺ channel mediated ASD and other neurodevelopmental disorders.

Distribution of thrombospondins and their neuronal receptor α2δ1 in the rat retina

The role of the extracellular matrix protein thrombospondins (TSPs) in promoting synaptogenesis is gaining more and more attention. The binding of TSP1 and TSP2 to their neuronal receptor α2δ1 stimulates excitatory synaptogenesis in the development and injury of the central nervous system; however, the specific cellular localization and expression of TSP1/2 and α2δ1 in healthy and damaged retinas is unknown. This, to a certain extent, has restricted the progress of research on the molecular mechanisms triggering synaptic plasticity after retinal injury. Here, the cellular localization and expression of TSP1/2 and their receptor α2δ1 was studied in healthy and damaged adult retina induced by elevated intraocular pressure (IOP) using double immunofluorescence labeling and confocal scanning microscopy. We showed the apparent differential distribution of TSP1 and TSP2 in the adult rat retina. TSP1 was confined to the ganglion cell layer and inner nuclear layer, in which it was preferentially expressed by ganglion cells, bipolar cells and horizontal cells but rarely expressed by glial cells. TSP2 staining was diffusely distributed in GFAP- and GS-immunopositive glial cells and processes in the inner retina. In rat retinas, α2δ1 staining was present in ganglion cells, bipolar cells, partial horizontal cells and amacrine cells and the presynaptic terminals. Müller cells and a minority of astrocytes also expressed α2δ1. On the seventh day of elevated IOP, TSP2 immunoreactivity was greatly increased, and immunopositive processes extended throughout the retinal layer and co-localized with GFAP- and GS-positive glial cells. TSP1 distribution in the retina, however, did not change distinctly. α2δ1-immunopositive processes were also increased on the seventh day after elevated IOP. Our study suggested that in the adult rat retina, TSP2, but not TSP1, secreted by glial cells may be involved in the synaptic plastic process after retinal injury through binding to its neuronal receptor α2δ1.

Gabapentin and (S)-pregabalin decrease intracellular D-serine concentrations in PC-12 cells

The effects of gabapentin (GBP) and (S)-pregabalin (PGB) on the intracellular concentrations of d-serine and the expression of serine racemase (SR) in PC-12 cells were determined. Intracellular d-serine concentrations were determined using an enantioselective capillary electrophoresis assay with laser-induced fluorescence detection. Increasing concentrations of GBP, 0.1-20μM, produced a significant decrease in d-serine concentration relative to control, 22.9±6.7% at 20μM (*p<0.05), with an IC(50) value of 3.40±0.29μM. Increasing concentrations of PGB, 0.1-10μM, produced a significant decrease in d-serine concentration relative to control, 25.3±7.6% at 10μM (*p<0.05), with an IC(50) value of 3.38±0.21μM. The compounds had no effect on the expression of monomeric-SR or dimeric-SR as determined by Western blotting. The results suggest that incubation of PC-12 cells with GBP and PGB reduced the basal activity of SR, which is most likely a result of the decreased Ca(2+) flux produced via interaction of the drugs with the α(2)-δ subunit of voltage-gated calcium channels. d-Serine is a co-agonist of the N-methyl d-aspartate receptor (NMDAR) and reduced d-serine concentrations have been associated with reduced NMDAR activity. Thus, GBP and PGB may act as indirect antagonists of NMDAR, a mechanism that may contribute to the clinical effects of the drugs in neuropathic pain.

Hydrogen sulfide inhibits L-type calcium currents depending upon the protein sulfhydryl state in rat cardiomyocytes

Hydrogen sulfide (H₂S) is a novel gasotransmitter that inhibits L-type calcium currents (I _{Ca, L}). However, the underlying molecular mechanisms are unclear. In particular, the targeting site in the L-type calcium channel where H₂S functions remains unknown. The study was designed to investigate if the sulfhydryl group could be the possible targeting site in the L-type calcium channel in rat cardiomyocytes. Cardiac function was measured in isolated perfused rat hearts. The L-type calcium currents were recorded by using a whole cell voltage clamp technique on the isolated cardiomyocytes. The L-type calcium channel containing free sulfhydryl groups in H9C2 cells were measured by using Western blot. The results showed that sodium hydrosulfide (NaHS, an H₂S donor) produced a negative inotropic effect on cardiac function, which could be partly inhibited by the oxidant sulfhydryl modifier diamide (DM). H₂S donor inhibited the peak amplitude of I _{Ca, L} in a concentration-dependent manner. However, dithiothreitol (DTT), a reducing sulfhydryl modifier markedly reversed the H₂S donor-induced inhibition of I _{Ca, L} in cardiomyocytes. In contrast, in the presence of DM, H₂S donor could not alter cardiac function and L type calcium currents. After the isolated rat heart or the cardiomyocytes were treated with DTT, NaHS could markedly alter cardiac function and L-type calcium currents in cardiomyocytes. Furthermore, NaHS could decrease the functional free sulfhydryl group in the L-type Ca²⁺ channel, which could be reversed by thiol reductant, either DTT or reduced glutathione. Therefore, our results suggest that H₂S might inhibit L-type calcium currents depending on the sulfhydryl group in rat cardiomyocytes.

Identification of a disulfide bridge essential for structure and function of the voltage-gated Ca(2+) channel α(2)δ-1 auxiliary subunit

Voltage-gated calcium (Ca(V)) channels are transmembrane proteins that form Ca(2+)-selective pores gated by depolarization and are essential regulators of the intracellular Ca(2+) concentration. By providing a pathway for rapid Ca(2+) influx, Ca(V) channels couple membrane depolarization to a wide array of cellular responses including neurotransmission, muscle contraction and gene expression. Ca(V) channels fall into two major classes, low voltage-activated (LVA) and high voltage-activated (HVA). The ion-conducting pathway of HVA channels is the α(1) subunit, which typically contains associated β and α(2)δ ancillary subunits that regulate the properties of the channel. Although it is widely acknowledged that α(2)δ-1 is post-translationally cleaved into an extracellular α(2) polypeptide and a membrane-anchored δ protein that remain covalently linked by disulfide bonds, to date the contribution of different cysteine (Cys) residues to the formation of disulfide bridges between these proteins has not been investigated. In the present report, by predicting disulfide connectivity with bioinformatics, molecular modeling and protein biochemistry experiments we have identified two Cys residues involved in the formation of an intermolecular disulfide bond of critical importance for the structure and function of the α(2)δ-1 subunit. Site directed-mutagenesis of Cys404 (located in the von Willebrand factor-A region of α(2)) and Cys1047 (in the extracellular domain of δ) prevented the association of the α(2) and δ peptides upon proteolysis, suggesting that the mature protein is linked by a single intermolecular disulfide bridge. Furthermore, co-expression of mutant forms of α(2)δ-1 Cys404Ser and Cys1047Ser with recombinant neuronal N-type (Ca(V)2.2α(1)/β(3)) channels, showed decreased whole-cell patch-clamp currents indicating that the disulfide bond between these residues is required for α(2)δ-1 function.

Targeting of voltage-gated calcium channel α2δ-1 subunit to lipid rafts is independent from a GPI-anchoring motif

Voltage-gated calcium channels (Ca_v) exist as heteromultimers comprising a pore-forming α₁ with accessory β and α₂δ subunits which modify channel trafficking and function. We previously showed that α₂δ-1 (and likely the other mammalian α₂δ isoforms - α₂δ-2, 3 and 4) is required for targeting Ca_vs to lipid rafts, although the mechanism remains unclear. Whilst originally understood to have a classical type I transmembrane (TM) topology, recent evidence suggests the α₂δ subunit contains a glycosylphosphatidylinositol (GPI)-anchor that mediates its association with lipid rafts. To test this notion, we have used a strategy based on the expression of chimera, where the reported GPI-anchoring sequences in the gabapentinoid-sensitive α₂δ-1 subunit have been substituted with those of a functionally inert Type I TM-spanning protein – PIN-G. Using imaging, electrophysiology and biochemistry, we find that lipid raft association of PIN-α₂δ is unaffected by substitution of the GPI motif with the TM domain of PIN-G. Moreover, the presence of the GPI motif alone is not sufficient for raft localisation, suggesting that upstream residues are required. GPI-anchoring is susceptible to phosphatidylinositol-phospholipase C (PI-PLC) cleavage. However, whilst raft localisation of PIN-α₂δ is disrupted by PI-PLC treatment, this is assay-dependent and non-specific effects of PI-PLC are observed on the distribution of the endogenous raft marker, caveolin, but not flotillin. Taken together, these data are most consistent with a model where α₂δ-1 retains its type I transmembrane topology and its targeting to lipid rafts is governed by sequences upstream of the putative GPI anchor, that promote protein-protein, rather than lipid-lipid interactions.

Presynaptic alpha2delta-3 is required for synaptic morphogenesis independent of its Ca2+-channel functions

Synaptogenesis involves the transformation of a growth cone into synaptic boutons specialized for transmitter release. In Drosophila embryos lacking the alpha(2)delta-3 subunit of presynaptic, voltage-dependent Ca(2+) channels, we found that motor neuron terminals failed to develop synaptic boutons and cytoskeletal abnormalities arose, including the loss of ankyrin2. Nevertheless, functional presynaptic specializations were present and apposed to clusters of postsynaptic glutamate receptors. The alpha(2)delta-3 protein has been thought to function strictly as an auxiliary subunit of the Ca(2+) channel, but the phenotype of alpha(2)delta-3 (also known as stj) mutations cannot be explained by a channel defect; embryos lacking the pore-forming alpha(1) subunit cacophony formed boutons. The synaptogenic function of alpha(2)delta-3 required only the alpha(2) peptide, whose expression sufficed to rescue bouton formation. Our results indicate that alpha(2)delta proteins have functions that are independent of their roles in the biophysics and localization of Ca(2+) channels and that synaptic architecture depends on these functions.

Targeting voltage-gated calcium channels for neuropathic pain management

Voltage-gated calcium channels (VGCC) play obligatory roles in diverse physiological functions. Pathological conditions leading to changes in their biophysical properties and expression levels may cause malfunctions of VGCC-mediated activities, resulting in disease states. It is believed that changes in VGCC properties under pain-inducing conditions may play a causal role in the development of chronic pain, including nerve injury-induced pain or neuropathic pain. For the past several decades, preclinical and clinical research in developing VGCC blockers or modulators for chronic pain management has been fruitful, leading to some U.S. Food and Drug Administration-approved drugs currently available for chronic pain management. However, their efficacy in pain relief is limited in some patients, and their long-term use is limited by their side-effect profiles. Certainly, there is room for improvement in developing more subtype-specific VGCC blockers or modulators for chronic pain conditions. In this review, we summarized the most recent preclinical and clinical studies related to chronic pain medications acting on the VGCC. We also included clinical trials aiming to expand the application of approved VGCC drugs to different pain states derived from various pathological conditions, as well as drug combination therapies trying to improve the efficacies and side-effect profiles of current pain medications.

Enhanced pre-synaptic glutamate release in deep-dorsal horn contributes to calcium channel alpha-2-delta-1 protein-mediated spinal sensitization and behavioral hypersensitivity

Nerve injury-induced expression of the spinal calcium channel alpha-2-delta-1 subunit (Ca_vα₂δ₁) has been shown to mediate behavioral hypersensitivity through a yet identified mechanism. We examined if this neuroplasticity modulates behavioral hypersensitivity by regulating spinal glutamatergic neurotransmission in injury-free transgenic mice overexpressing the Ca_vα₂δ₁ proteins in neuronal tissues. The transgenic mice exhibited hypersensitivity to mechanical stimulation (allodynia) similar to the spinal nerve ligation injury model. Intrathecally delivered antagonists for N-methyl-D-aspartate (NMDA) and α-amino-3-hydroxyl-5-methylisoxazole-4-propionic acid (AMPA)/kainate receptors, but not for the metabotropic glutamate receptors, caused a dose-dependent allodynia reversal in the transgenic mice without changing the behavioral sensitivity in wild-type mice. This suggests that elevated spinal Ca_vα₂δ₁ mediates allodynia through a pathway involving activation of selective glutamate receptors. To determine if this is mediated by enhanced spinal neuronal excitability or pre-synaptic glutamate release in deep-dorsal horn, we examined wide-dynamic-range (WDR) neuron excitability with extracellular recording and glutamate-mediated excitatory postsynaptic currents with whole-cell patch recording in deep-dorsal horn of the Ca_vα₂δ₁ transgenic mice. Our data indicated that overexpression of Ca_vα₂δ₁ in neuronal tissues led to increased frequency, but not amplitude, of miniature excitatory post synaptic currents mediated mainly by AMPA/kainate receptors at physiological membrane potentials, and also by NMDA receptors upon depolarization, without changing the excitability of WDR neurons to high intensity stimulation. Together, these findings support a mechanism of Ca_vα₂δ₁-mediated spinal sensitization in which elevated Ca_vα₂δ₁ causes increased pre-synaptic glutamate release that leads to reduced excitation thresholds of post-synaptic dorsal horn neurons to innocuous stimuli. This spinal sensitization mechanism may mediate at least partially the neuropathic pain states derived from increased pre-synaptic Ca_vα₂δ₁ expression.

Drugability of extracellular targets: discovery of small molecule drugs targeting allosteric, functional, and subunit-selective sites on GPCRs and ion channels

Beginning with the discovery of the structure of deoxyribose nucleic acid in 1953, by James Watson and Francis Crick, the sequencing of the entire human genome some 50 years later, has begun to quantify the classes and types of proteins that may have relevance to human disease with the promise of rapidly identifying compounds that can modulate these proteins so as to have a beneficial and therapeutic outcome. This so called 'drugable space' involves a variety of membrane-bound proteins including the superfamily of G-protein-coupled receptors (GPCRs), ion channels, and transporters among others. The recent number of novel therapeutics targeting membrane-bound extracellular proteins that have reached the market in the past 20 years however pales in magnitude when compared, during the same timeframe, to the advancements made in the technologies available to aid in the discovery of these novel therapeutics. This review will consider select examples of extracellular drugable targets and focus on the GPCRs and ion channels highlighting the corticotropin releasing factor (CRF) type 1 and gamma-aminobutyric acid receptors, and the Ca(V)2.2 voltage-gated ion channel. These examples will elaborate current technological advancements in drug discovery and provide a prospective framework for future drug development.

Injury discharges regulate calcium channel alpha-2-delta-1 subunit upregulation in the dorsal horn that contributes to initiation of neuropathic pain

Previous studies have shown that peripheral nerve injury in rats induces increased expression of the voltage gated calcium channel (VGCC) alpha-2-delta-1 subunit (Ca v alpha2 delta1) in spinal dorsal horn and sensory neurons in dorsal root ganglia (DRG) that correlates to established neuropathic pain states. To determine if injury discharges trigger Ca v alpha2 delta1 induction that contributes to neuropathic pain initiation, we examined allodynia onset and Ca v alpha2 delta1 levels in DRG and spinal dorsal horn of spinal nerve ligated rats after blocking injury induced neural activity with a local brief application of lidocaine on spinal nerves before the ligation. The lidocaine pretreatment blocked ligation-induced discharges in a dose-dependent manner. Similar pretreatment with the effective concentration of lidocaine diminished injury-induced increases of the Ca v alpha2 delta1 in DRG and abolished that in spinal dorsal horn specifically, and resulted in a delayed onset of tactile allodynia post-injury. Both dorsal horn Ca v alpha2 delta1 upregulation and tactile allodynia in the lidocaine pretreated rats returned to levels similar to that in saline pretreated controls 2 weeks post the ligation injury. In addition, preemptive intrathecal Ca v alpha2 delta1 antisense treatments blocked concurrently injury-induced allodynia onset and Ca v alpha2 delta1 upregulation in dorsal spinal cord. These findings indicate that injury induced discharges regulate Ca v alpha2 delta1 expression in the spinal dorsal horn that is critical for neuropathic allodynia initiation. Thus, preemptive blockade of injury-induced neural activity or Ca v alpha2 delta1 upregulation may be a beneficial option in neuropathic pain management.

Mutations in a Drosophila alpha2delta voltage-gated calcium channel subunit reveal a crucial synaptic function

Voltage-dependent calcium channels regulate many aspects of neuronal biology, including synaptic transmission. In addition to their alpha1 subunit, which encodes the essential voltage gate and selective pore, calcium channels also contain auxiliary alpha2delta, beta, and gamma subunits. Despite progress in understanding the biophysical properties of calcium channels, the in vivo functions of these auxiliary subunits remain unclear. We have isolated mutations in the gene encoding an alpha2delta calcium channel subunit (d alpha2delta-3) using a forward genetic screen in Drosophila. Null mutations in this gene are embryonic lethal and can be rescued by expression in the nervous system, demonstrating that the essential function of this subunit is neuronal. The photoreceptor phenotype of d alpha2delta-3 mutants resembles that of the calcium channel alpha1 mutant cacophony (cac), suggesting shared functions. We have examined in detail genotypes that survive to the third-instar stage. Electrophysiological recordings demonstrate that synaptic transmission is severely impaired in these mutants. Thus the alpha2delta calcium channel subunit is critical for calcium-dependent synaptic function. As such, this Drosophila isoform is the likely partner to the presynaptic calcium channel alpha1 subunit encoded by the cac locus. Consistent with this hypothesis, cacGFP fluorescence at the neuromuscular junction is reduced in d alpha2delta-3 mutants. This is the first characterization of an alpha2delta-3 mutant in any organism and indicates a necessary role for alpha2delta-3 in presynaptic vesicle release and calcium channel expression at active zones.

The P/Q-type voltage-dependent calcium channel: a therapeutic target in spinocerebellar ataxia type 6

BACKGROUND

Voltage-dependent calcium channels (VDCCs) are heteromultimeric complexes that mediate calcium influx into cells; the alpha 1A subunit is the pore-forming subunit specific to the neuronal P/Q-type VDCCs. Spinocerebellar ataxia type 6 (SCA 6) is caused by an abnormal expansion of a CAG repeat in CACNA1A, which encodes the alpha 1A subunit. Heterologous expression of mutated alpha 1A subunits resulted in increased channel inactivation in electrophysiological tests. Gabapentin and pregabalin interact with the alpha 2 delta subunit of the VDCCs and improved ataxia in cases of cortical cerebellar atrophy (CCA) and ataxia-telangiectasia.

MATERIALS AND METHODS

A bibliographical review was performed in order to find out if gabapentin and pregabalin could prove useful in the treatment of SCA 6.

RESULTS

Gabapentin and pregabalin slowed the rate of inactivation in recombinant P/Q-type VDCCs. SCA 6 shares neuropathological findings with CCA.

CONCLUSIONS

On the basis of the neuropathological identity of SCA 6 with CCA, and of the effect of gabapentin and pregabalin on recombinant VDCCs the authors put forward the hypothesis that these drugs might prove beneficial in SCA 6, as the ataxia would be expected to improve. The authors hope that researchers working with this illness will be encouraged to undertake the appropriate clinical and experimental work.

Functional biology of the alpha(2)delta subunits of voltage-gated calcium channels

In this review, we examine what is known about the mechanism of action of the auxiliary alpha2delta subunits of voltage-gated Ca(2+) (Ca(v)) channels. First, to provide some background on the alpha2delta proteins, we discuss the genes encoding these channels, in addition to the topology and predicted structure of the alpha2delta subunits. We then describe the effects of alpha2delta subunits on the biophysical properties of Ca(v) channels and their physiological function. All alpha2delta subunits increase the density at the plasma membrane of Ca(2+) channels activated by high voltage, and we discuss what is known about the mechanism underlying this trafficking. Finally, we consider the link between alpha2delta subunits and disease, both in terms of spontaneous and engineered mouse mutants that show cerebellar ataxia and spike-wave epilepsy, and in terms of neuropathic pain and the mechanism of action of the gabapentinoid drugs - small-molecule ligands of the alpha2delta-1 and alpha2delta-2 subunits.

Kinetics of internalization and degradation of N-type voltage-gated calcium channels: Role of the α2/δ subunit

The contribution of voltage-gated calcium channels to excitable cell function depends, critically, upon the mechanisms that control their expression at the cell surface. While co-assembly of the pore forming alpha(1) and auxiliary beta subunits enhances channel surface expression, the levels are still only 30-40% of those seen with the core alpha(1B)/beta(1b)/alpha(2)delta calcium channel complex. To rationalize this observation, it has been suggested that the alpha(2)/delta subunit might stabilize calcium channel expression at the cell surface. To test this notion, we have resolved the effect of the alpha(2)/delta subunit on the rates of binding, internalization and degradation of defined N-type calcium channel surface complexes expressed in HEK293 cells, through pulse-labeling with the selective, cell impermeable, radioligand [(125)I]-omega-CgTx. Through detailed kinetic and sensitivity analysis we show that alpha(1B)/beta(1b)/alpha(2)delta complexes are internalized slowly (k(int) 0.4/h), whereupon, most become degraded (k(deg) 0.02/h). In contrast, alpha(1B)/beta(1b) complexes are internalized more rapidly (k(int) 0.8/h), following which they are either quickly degraded (k(deg) 0.1/h) or are sequestered slowly (k(tra) 0.1/h) to a pool that is metabolically stable within the time-frame of our experiments (24h). In neither case did we find evidence for recycling via the cell surface. Thus, our data argue for a novel mechanism where complexes lacking an alpha(2)/delta subunit are cleared from the cell surface and are rapidly degraded or stored, possibly for further attempts at complexation as new alpha(2)/delta subunits become available. The slower rate of internalization of complexes containing the alpha(2)/delta subunit rationalizes the stabilizing effect this subunit has upon calcium channel surface expression and suggests a mechanism by which alpha(2)delta mutations may cause severe neurological deficits.

The P/Q-type voltage-dependent calcium channel as pharmacological target in spinocerebellar ataxia type 6: Gabapentin and pregabalin may be of therapeutic benefit

Voltage-dependent calcium channels (VDCCs) are heteromultimeric complexes that mediate calcium influx into cells in response to changes in membrane potential. The alpha1A subunit, encoded by the CACNA1A gene, is the pore-forming subunit specific to the neuronal P/Q-type VDCCs. These are implicated in fast excitatory and inhibitory neurotransmission. Their highest levels of expression are found in the Purkinje cell layer of the cerebellum, and in the hippocampus. Spinocerebellar ataxia type 6 (SCA 6) is an autosomal dominant cerebellar degeneration that shares neuropathological findings with late-onset cortical cerebellar atrophy (CCA). It is caused by an abnormal expansion of a trinucleotide (CAG) repeat in exon 47 of CACNA1A, on chromosome 19p13. This translates into a polyglutamine (polyQ) tract of prolonged length in the carboxyl terminal of the alpha1A subunit. Heterologous expression of mutated alpha1A subunits results in increased channel inactivation in electrophysiological tests. No treatment is known to improve SCA 6 at present, as none of the available drugs is able to reverse alpha1A dysregulation, nor disturbed protein aggregation, transport and localization in this disease. The drugs gabapentin and pregabalin interact with the alpha2delta subunit of the P/Q-type VDCCs. Gabapentin and pregabalin slow the rate of inactivation in recombinant P/Q-type VDCCs, expressed in Xenopus oocytes. These drugs improve ataxia in cases of CCA, olivopontocerebellar atrophy and ataxia-telangiectasia. On the basis of the neuropathological identity of SCA 6 with CCA, and given the capacity of gabapentin and pregabalin to decrease P/Q-type VDCCs inactivation, in this paper the authors put forward the hypothesis that the administration of gabapentin and pregabalin might prove beneficial in SCA 6 as the ataxia caused by this disease would be expected to improve. The authors hope that researchers working with this illness will be inspired and encouraged to undertake the appropriate clinical and experimental work.

Molecular physiology of cardiac repolarization

The heart is a rhythmic electromechanical pump, the functioning of which depends on action potential generation and propagation, followed by relaxation and a period of refractoriness until the next impulse is generated. Myocardial action potentials reflect the sequential activation and inactivation of inward (Na(+) and Ca(2+)) and outward (K(+)) current carrying ion channels. In different regions of the heart, action potential waveforms are distinct, owing to differences in Na(+), Ca(2+), and K(+) channel expression, and these differences contribute to the normal, unidirectional propagation of activity and to the generation of normal cardiac rhythms. Changes in channel functioning, resulting from inherited or acquired disease, affect action potential repolarization and can lead to the generation of life-threatening arrhythmias. There is, therefore, considerable interest in understanding the mechanisms that control cardiac repolarization and rhythm generation. Electrophysiological studies have detailed the properties of the Na(+), Ca(2+), and K(+) currents that generate cardiac action potentials, and molecular cloning has revealed a large number of pore forming (alpha) and accessory (beta, delta, and gamma) subunits thought to contribute to the formation of these channels. Considerable progress has been made in defining the functional roles of the various channels and in identifying the alpha-subunits encoding these channels. Much less is known, however, about the functioning of channel accessory subunits and/or posttranslational processing of the channel proteins. It has also become clear that cardiac ion channels function as components of macromolecular complexes, comprising the alpha-subunits, one or more accessory subunit, and a variety of other regulatory proteins. In addition, these macromolecular channel protein complexes appear to interact with the actin cytoskeleton and/or the extracellular matrix, suggesting important functional links between channel complexes, as well as between cardiac structure and electrical functioning. Important areas of future research will be the identification of (all of) the molecular components of functional cardiac ion channels and delineation of the molecular mechanisms involved in regulating the expression and the functioning of these channels in the normal and the diseased myocardium.

Use-dependent blockade of Cav2.2 voltage-gated calcium channels for neuropathic pain

The translocation of extracellular calcium (Ca(2+)) via voltage-gated Ca(2+) channels (VGCCs) in neurons is involved in triggering multiple physiological cell functions but also the abnormal, pathophysiological responses that develop as a consequence of injury. In conditions of neuropathic pain, VGCCs are involved in supplying the signal Ca(2+) important for the sustained neuronal firing and neurotransmitter release characteristic of these syndromes. Preclinical data have identified N-type VGCCs (Ca(v)2.2) as key participants in contributing to these Ca(2+) signaling events and clinical data with the peptide blocker Prialt have now validated Ca(v)2.2 as a bona fide target for future drug discovery efforts to identify new and novel therapeutics for neuropathic pain. Imperative for the success of such an endeavor will be the ability to identify compounds selective for Ca(v)2.2, versus other VGCCs, but also compounds which demonstrate effective blockade during the pathophysiological states of neuropathic pain without compromising channel activity associated with sustaining normal housekeeping cellular functions. An approach to obtain this research target profile is to identify compounds, which are more potent in blocking Ca(v)2.2 during higher frequencies of firing as compared to the slower more physiologically-relevant frequencies. This may be achieved by identifying compounds with enhanced potency for the inactivated state of Ca(v)2.2. This commentary explores the rationale and options for engineering a use-dependent blocker of Ca(v)2.2. It is anticipated that this use-dependent profile of channel blockade will result in new chemical entities with an improved therapeutic ratio for neuropathic pain.

Targeting mechanisms of high voltage-activated Ca2+ channels

Functional voltage-dependent Ca2+ channel complexes are assembled by three to four subunits: alpha1, beta, alpha2delta subunits (C. Leveque et al., 1994, J. Biol Chem. 269, 6306-6312; M. W. McEnery et al., 1991, Proc. Natl. Acad. Sci. U.S.A. 88, 11095-11099) and at least in muscle cells also y subunits (B. M. Curtis and W. A. Catterall, 1984, Biochemistry 23, 2113-2118). Ca2+ channels mediate the voltage-dependent Ca2+ influx in subcellular compartments, triggering such diverse processes as neurotransmitter release, dendritic action potentials, excitation-contraction, and excitation-transcription coupling. The targeting of biophysically defined Ca2+ channel complexes to the correct subcellular structures is, thus, critical to proper cell and physiological functioning. Despite their importance, surprisingly little is known about the targeting mechanisms by which Ca2+ channel complexes are transported to their site of function. Here we summarize what we know about the targeting of Ca2+ channel complexes through the cell to the plasma membrane and subcellular structures.

Molecular cloning and characterization of the human voltage-gated calcium channel alpha(2)delta-4 subunit

The voltage-gated calcium channel is composed of a pore-forming alpha(1) subunit and several regulatory subunits: alpha(2)delta, beta, and gamma. We report here the identification of a novel alpha(2)delta subunit, alpha(2)delta-4, from the expressed sequence tag database followed by its cloning and characterization. The novel alpha(2)delta-4 subunit gene contains 39 exons spanning about 130 kilobases and is co-localized with the CHCNA1C gene (alpha(1C) subunit) on human chromosome 12p13.3. Alternative splicing of the alpha(2)delta-4 gene gives rise to four potential variants, a through d. The open reading frame of human alpha(2)delta-4a is composed of 3363 base pairs encoding a protein with 1120 residues and a calculated molecular mass of 126 kDa. The alpha(2)delta-4a subunit shares 30, 32, and 61% identity with the human calcium channel alpha(2)delta-1, alpha(2)delta-2, and alpha(2)delta-3 subunits, respectively. Primary sequence comparison suggests that alpha(2)delta-4 lacks the gabapentin binding motifs characterized for alpha(2)delta-1 and alpha(2)delta-2; this was confirmed by a [(3)H]gabapentin-binding assay. In human embryonic kidney 293 cells, the alpha(2)delta-4 subunit associated with Ca(V)1.2 and beta(3) subunits and significantly increased Ca(V)1.2/beta(3)-mediated Ca(2+) influx. Immunohistochemical study revealed that the alpha(2)delta-4 subunit has limited distribution in special cell types of the pituitary, adrenal gland, colon, and fetal liver. Whether the alpha(2)delta-4 subunit plays a distinct physiological role in select endocrine tissues remains to be demonstrated.

Upregulation of dorsal root ganglion (alpha)2(delta) calcium channel subunit and its correlation with allodynia in spinal nerve-injured rats

Peripheral nerve injury can lead to a persistent neuropathic pain state in which innocuous tactile stimulation elicits pain behavior (tactile allodynia). Spinal administration of the anticonvulsant gabapentin suppresses allodynia by an unknown mechanism. In vitro studies indicate that gabapentin binds to the alpha(2)delta-1 (hereafter referred to as alpha(2)delta) subunit of voltage-gated calcium channels. We hypothesized that nerve injury may result in altered alpha(2)delta subunit expression in spinal cord and dorsal root ganglia (DRGs) and that this change may play a role in neuropathic pain processing. Using a rat neuropathic pain model in which gabapentin-sensitive tactile allodynia develops after tight ligation of the left fifth and sixth lumbar spinal nerves, we found a >17-fold, time-dependent increase in alpha(2)delta subunit expression in DRGs ipsilateral to the nerve injury. Marked alpha(2)delta subunit upregulation was also evident in rats with unilateral sciatic nerve crush, but not dorsal rhizotomy, indicating a peripheral origin of the expression regulation. The increased alpha(2)delta subunit expression preceded the allodynia onset and diminished in rats recovering from tactile allodynia. RNase protection experiments indicated that the DRG alpha(2)delta regulation was at the mRNA level. In contrast, calcium channel alpha(1B) and beta(3) subunit expression was not co-upregulated with the alpha(2)delta subunit after nerve injury. These data suggest that DRG alpha(2)delta regulation may play an unique role in neuroplasticity after peripheral nerve injury that may contribute to allodynia development.

Calcium channel alpha(2)delta subunits-structure and Gabapentin binding

High-voltage activated calcium channels are modulated by a series of auxiliary proteins, including those of the alpha(2)delta family. Until recently, only a single alpha(2)delta subunit was known, but two further members, alpha(2)delta-2 and -3, have since been identified. In this study, the structure of these two novel subunits has been characterized and binding of the antiepileptic drug gabapentin investigated. Using antibodies directed against the amino terminal portion of the proteins, the gross structure of the subunits could be analyzed by Western blotting. Similar to alpha(2)delta-1, both alpha(2)delta-2 and -3 subunits consist of two proteins-a larger alpha(2) and a smaller delta that can be separated by reduction. The subunits are also highly N-glycosylated with approximately 30 kDa of their mass consisting of oligosaccharides. alpha(2)delta-1 was detected in all mouse tissues studied, whereas alpha(2)delta-2 was found at high levels in brain and heart. The alpha(2)delta-3 subunit was observed only in brain. alpha(2)delta-1 and alpha(2)delta-2, but not alpha(2)delta-3, were found to bind gabapentin. The K(d) value of gabapentin binding to alpha(2)delta-2 was 153 nM compared with the higher affinity binding to alpha(2)delta-1 (K(d) = 59 nM).

Asymmetric arrangement of auxiliary subunits of skeletal muscle voltage-gated l-type Ca(2+) channel

Highly purified L-type Ca(2+) channel complexes containing all five subunits (alpha(1), alpha(2), beta, gamma, and delta) and complexes of alpha(1)-beta subunits were obtained from skeletal muscle triad membranes by three-step purification and by 1% Triton X-100 treatment, respectively. Their structures and the subunit arrangements were analyzed by electron microscopy. Projection images of negatively stained Ca(2+) channels and alpha(1)-beta complexes were aligned, classified and averaged. The alpha(1)-beta complex showed a hollow trapezoid shape of 12 nm height. In top view, four asymmetric domains surrounded a central depression predicted to form the channel pore. The complete Ca(2+) channel complex exhibited the cylindrical shape of 20 nm in height binding a spherical domain on one edge. Further image analysis of higher complexes of the Ca(2+) channel using a monoclonal antibody against the beta subunit showed that the alpha(1)-beta complex forms the non-decorated side of the cylinder, which can traverse the membrane from outside the cell to the cytoplasm. Based on these results, we propose that the Ca(2+) channel exhibits an asymmetric arrangement of auxiliary subunits.

Cysteine-string protein increases the calcium sensitivity of neurotransmitter exocytosis in Drosophila

Previous studies suggest that the vesicular cysteine-string protein (CSP) may modulate presynaptic Ca(2+) channel activity in fast neurotransmitter release. To test this hypothesis, we analyzed the dynamics of presynaptic Ca(2+) ion influx with the Ca(2+) indicator fluo-4 AM at csp mutant neuromuscular junctions of Drosophila. From 24 to 30 degrees C, stimulus-evoked, relative presynaptic Ca(2+) signals were increasingly larger in csp mutant boutons than in controls. Above 30 degrees C, Ca(2+) signals declined and were similar to controls at 34 degrees C. A prolonged decay of Ca(2+) signals in mutant boutons at high temperatures indicated abnormally slow Ca(2+) clearance. Cytosolic Ca(2+) at rest was determined with the ratiometric Ca(2+) indicator fura-2 AM and was similar in mutant and control boutons at 24 degrees C but higher in mutant boutons at 34 degrees C. Despite larger Ca(2+) signals in mutant boutons, evoked neurotransmitter release was always reduced in csp mutants and exhibited pronounced facilitation. Thus, a lack of Ca(2+) entry cannot explain the reduction of neurotransmitter release in csp mutants. At all temperatures tested, raising extracellular Ca(2+) increased transmitter release elicited by single stimuli in csp mutants. Collectively, these data suggest multiple functions for CSP at synaptic terminals. Increased Ca(2+) signals coupled with reduced release suggest a direct function of CSP in exocytosis downstream from Ca(2+) entry. Because the reduction of evoked release in csp mutants is counteracted by increased Ca(2+) levels, we suggest that CSP primarily increases the Ca(2+) sensitivity of the exocytotic machinery.

Different types of calcium channels and secretion from bovine chromaffin cells

Bovine chromaffin cells possess several types of Ca2+ channels, and influx of Ca2+ is known to trigger secretion. However, discrepant information about the relative importance of the individual subtypes in secretion has been reported. We used whole-cell patch-clamp measurements in isolated cells in culture combined with fura-2 microfluorimetry and pharmacological manipulation to determine the dependence of secretion on different types of Ca2+ channels. We stimulated cells with relatively long depolarizing voltage-clamp pulses in a medium containing 60 mM CaCl2. We found that, within a certain range of pulse parameters, secretion as measured by membrane capacitance changes was mainly determined by the total cumulative charge of Ca2+ inflow and the basal [Ca2+] level preceding a stimulus. Blocking or reducing the contribution of specific types of Ca2+ channels using either 20 microM nifedipine plus 10 microM nimodipine or 1 microM omegaCTxGVIA (omega-conotoxin GVIA) or 2 microM omegaCTxMVIIC (omega-conotoxin MVIIC) reduced secretion in proportion to Ca2+ charge, irrespective of the toxin used. We conclude that for long-duration stimuli, which release a large fraction of the readily releasable pool of vesicles, it is not so important through which type of channels Ca2+ enters the cell. Release is determined by the total amount of Ca2+ entering and by the filling state of the readily releasable pool, which depends on basal [Ca2+] before the stimulus. This result does not preclude that other stimulation patterns may lead to responses in which subtype specificity of Ca2+ channels matters.

Ion channels in presynaptic nerve terminals and control of transmitter release

The primary function of the presynaptic nerve terminal is to release transmitter quanta and thus activate the postsynaptic target cell. In almost every step leading to the release of transmitter quanta, there is a substantial involvement of ion channels. In this review, the multitude of ion channels in the presynaptic terminal are surveyed. There are at least 12 different major categories of ion channels representing several tens of different ion channel types; the number of different ion channel molecules at presynaptic nerve terminals is many hundreds. We describe the different ion channel molecules at the surface membrane and inside the nerve terminal in the context of their possible role in the process of transmitter release. Frequently, a number of different ion channel molecules, with the same basic function, are present at the same nerve terminal. This is especially evident in the cases of calcium channels and potassium channels. This abundance of ion channels allows for a physiological and pharmacological fine tuning of the process of transmitter release and thus of synaptic transmission.

Molecular diversity of the calcium channel alpha2delta subunit

Sequence database searches with the alpha2delta subunit as probe led to the identification of two new genes encoding proteins with the essential properties of this calcium channel subunit. Primary structure comparisons revealed that the novel alpha2delta-2 and alpha2delta-3 subunits share 55.6 and 30.3% identity with the alpha2delta-1 subunit, respectively. The number of putative glycosylation sites and cysteine residues, hydropathicity profiles, and electrophysiological character of the alpha2delta-3 subunit indicates that these proteins are functional calcium channel subunits. Coexpression of alpha2delta-3 with alpha1C and cardiac beta2a or alpha1E and beta3 subunits shifted the voltage dependence of channel activation and inactivation in a hyperpolarizing direction and accelerated the kinetics of current inactivation. The kinetics of current activation were altered only when alpha2delta-1 or alpha2delta-3 was expressed with alpha1C. The effects of alpha2delta-3 on alpha1C but not alpha1E are indistinguishable from the effects of alpha2delta-1. Using Northern blot analysis, it was shown that alpha2delta-3 is expressed exclusively in brain, whereas alpha2delta-2 is found in several tissues. In situ hybridization of mouse brain sections showed mRNA expression of alpha2delta-1 and alpha2delta-3 in the hippocampus, cerebellum, and cortex, with alpha2delta-1 strongly detected in the olfactory bulb and alpha2delta-3 in the caudate putamen.

Voltage-dependent anion channel proteins in synaptosomes of the torpedo electric organ: immunolocalization, purification, and characterization

In this study, we purified and characterized the voltage-dependent anion channel (VDAC) from the Torpedo electric organ. Using immunogold labeling, VDAC was colocalized with the voltage-gated Ca2+ channel in the synaptic plasma membrane. By immunoblot analysis, five protein bands in synaptosomes isolated from the Torpedo electric organ cross reacted with two monoclonal anti-VDAC antibody. No more than about 7 to 10% mitochondrial contains could be detected in any synaptosomal membrane preparation tested. This was estimated by comparing the specific activity in mitochondria and synaptosomes of succinate-cytochrome-c oxidoreductase and antimycin-insensitive NADH-cytochrome-c oxidoreductase activities; mitochondrial inner and outer membrane marker enzymes, respectively. [14C]DCCD (dicyclohexylcarbodiimide), which specifically label mitochondrial VDAC, labeled four 30-35 kDa protein bands that were found to interact with the anti-VDAC antibody. The distribution of the Torpedo VDAC protein bands was different among membranes isolated from various tissues. VDAC was purified from synaptosomes and a separation between two of the proteins was obtained. The two purified proteins were characterized by their single channel activity and partial amino acid sequences. Upon reconstitution into a planar lipid bilayer, the purified VDACs showed voltage-dependent channel activity with properties similar to those of purified mitochondrial VDAC. Amino acid sequence of four peptides, derived from VDAC band II, exhibited high homology to sequences present in human VDACI (98%), VDAC2 (91.8%), and VDAC3 (90%), while another peptide, derived from VDAC band III, showed lower homology to either VDAC1 (88.4%) or VDAC2 (79%). Two more peptides show high homology to the sequence present in mouse brain VDAC3 (100 and 78%). In addition, we demonstrate the translocation of ATP into synaptosomes, which is inhibited by DCCD and by the anion transport inhibitor DIDS. The possible function of VDAC in the synaptic plasma membrane is discussed.

Cloning and deletion mutagenesis of the alpha2 delta calcium channel subunit from porcine cerebral cortex. Expression of a soluble form of the protein that retains [3H]gabapentin binding activity

The anti-epileptic, anti-hyperalgesic, and anxiolytic agent gabapentin (1-(aminomethyl)-cyclohexane acetic acid or Neurontin) has previously been shown to bind with high affinity to the alpha2delta subunit of voltage-dependent calcium channels (Gee, N. S. , Brown, J. P., Dissanayake, V. U. K., Offord, J., Thurlow, R., and Woodruff, G.N. (1996) J. Biol. Chem. 271, 5768-5776). We report here the cloning, sequencing, and deletion mutagenesis of the alpha2delta subunit from porcine brain. The deduced protein sequence has a 95.9 and 98.2% identity to the rat and human neuronal alpha2 delta sequences, respectively. [3H]Gabapentin binds with a KD of 37.5 +/- 10.4 nM to membranes prepared from COS-7 cells transfected with wild-type porcine alpha2 delta cDNA. Six deletion mutants (B-G) that lack the delta polypeptide, together with varying amounts of the alpha2 component, failed to bind [3H]gabapentin. C-terminal deletion mutagenesis of the delta polypeptide identified a segment (residues 960-994) required for correct assembly of the [3H]gabapentin binding pocket. Mutant L, which lacks the putative membrane anchor in the delta sequence, was found in both membrane-associated and soluble secreted forms. The soluble form was not proteolytically cleaved into separate alpha2 and delta chains but still retained a high affinity (KD = 30.7 +/- 8.1 nM) for [3H]gabapentin. The production of a soluble alpha2delta mutant supports the single transmembrane model of the alpha2 delta subunit and is an important step toward the large-scale recombinant expression of the protein.

Modulation of human neuronal alpha 1E-type calcium channel by alpha 2 delta-subunit

Calcium channels are composed of a pore-forming subunit, alpha 1, and at least two auxiliary subunits, beta- and alpha 2 delta-subunits. It is well known that beta-subunits regulate most of the properties of the channel. The function of alpha 2 delta-subunit is less understood. In this study, the effects of the calcium channel alpha 2 delta-subunit on the neuronal alpha 1E voltage-gated calcium channel expressed in Xenopus oocytes was investigated without and with simultaneous coexpression of either the beta 1b- or the beta 2a-subunit. Most aspects of alpha 1E function were affected by alpha 2 delta. Thus alpha 2 delta caused a shift in the current-voltage and conductance-voltage curves toward more positive potentials and accelerated activation, deactivation, and the installation of the inactivation process. In addition, the efficiency with which charge movement is coupled to pore opening assessed by determining ratios of limiting conductance to limiting charge movement was decreased by alpha 2 delta by factors that ranged from 1.6 (P < 0.01) for alpha 1E-channels to 3.0 (P < 0.005) for alpha 1E beta 1b-channels. These results indicate that alpha 2 delta facilitates the expression and the maturation of alpha 1E-channels and converts these channels into molecules responding more rapidly to voltage.

Subunit interaction sites in voltage-dependent Ca2+ channels: role in channel function

Voltage-dependent Ca2+ channels are heteromeric complexes found in the plasma membrane of virtually all cell types and show a high level of electrophysiological and pharmacological diversity. Associated with the pore-forming alpha 1 subunit are the membrane anchored, largely extracellular alpha2-delta, the cytoplasmic beta and sometimes a transmembrane gamma subunit; these subunits dramatically influence the properties and surface expression of these channels. Effects vary depending on subunit isoforms, suggesting that functional diversity of native channels reflects heterogeneity of combinations. Interaction sites between subunits have been identified and advances have been made in our understanding of the molecular basis of functional effects of the auxiliary subunits, their capacity to be regulated by G proteins, and their interaction with related cellular systems.

Dissection of functional domains of the voltage-dependent Ca2+ channel alpha2delta subunit

Coexpression of the cloned voltage-dependent Ca2+ channel alpha2delta subunit with the pore-forming alpha1 subunit results in a significant increase in macroscopic current amplitude. To gain insight into the mechanism underlying this interaction, we have examined the regulatory effect of either the alpha2delta complex or the delta subunit on the Ca2+ channel alpha1 subunit. Transient transfection of tsA201 cells with the cardiac L-type alpha1C subunit alone resulted in the expression of inward voltage-activated currents as well as measurable [3H]-PN200-110 binding to membranes from transfected cells. Coexpression of the alpha2delta subunit significantly increased the macroscopic current amplitude, altered the voltage dependence and the kinetics of the current, and enhanced [3H]-PN200-110 binding. Except for the increase in amplitude, coexpression of the delta subunit reproduced entirely the effects of the full-length alpha2delta subunit on the biophysical properties of the alpha1C currents. However, no effect on specific [3H]-PN200-110 binding was observed on delta subunit coexpression. Likewise, profound effects on current kinetics of the neuronal alpha1A subunit were observed on coexpression of the alpha2delta complex in Xenopus oocytes. Furthermore, by using a chimeric strategy, we localized the region involved in this regulation to the transmembrane domain of the delta subunit. These data strongly suggest that the molecular determinants involved in alpha2delta regulation are conserved across L-type and non-L type Ca2+ channels. Taken together, our results indicate that the region of the alpha2delta subunit involved in the modulation of the gating properties of the high voltage-activated calcium channels is localized in the delta domain of the protein. In contrast, the level of membrane expression of functional channels relies on the presence of the alpha2 domain of the alpha2delta complex.

Extracellular interaction of the voltage-dependent Ca2+ channel alpha2delta and alpha1 subunits

The role of the extracellular domain of the voltage-dependent Ca2+ channel alpha2delta subunit in assembly with the alpha1C subunit was investigated. Transiently transfected tsA201 cells processed the alpha2delta subunit properly as disulfide linkages and cleavage sites between the alpha2 and delta subunits were shown to be similar to native channel protein. Coimmunoprecipitation experiments demonstrated that in the absence of delta subunits, alpha2 subunits do not assemble with alpha1 subunits. Furthermore, the transmembrane and cytoplasmic sequences in delta can be exchanged with those of an unrelated protein without any effect on the association between the alpha2delta and alpha1 proteins. Extracellular domains of the alpha2delta subunit are also shown to be responsible for increasing the binding affinity of [3H]PN200-110 (isopropyl-4-(2,1, 3-benzoxadiazol-4-yl)-1,4-dihydro-2, 6-dimethyl-5-([3H]methoxycarbonyl)-pyridine-3-carboxylate) for the alpha1C subunit. Investigation of the corresponding interaction site on the alpha1 subunit revealed that although tryptic peptides containing repeat III of native alpha1S subunit remain in association with the alpha2delta subunit during wheat germ agglutinin chromatography, repeat III by itself is not sufficient for assembly with the alpha2delta subunit. Our results suggest that the alpha2delta subunit likely interacts with more than one extracellular loop of the alpha1 subunit.

Mutation of the Ca2+ channel beta subunit gene Cchb4 is associated with ataxia and seizures in the lethargic (lh) mouse

Ca2+ channel beta subunits regulate voltage-dependent calcium currents through direct interaction with alpha 1 subunits. The beta- and alpha 1-binding motifs are conserved, and all beta subunits can stimulate current amplitude, voltage dependence, and kinetics when coexpressed with various alpha 1 subunits. We used a positional candidate approach to determine that the ataxia and seizures in the lethargic (lh) mouse arise from mutation of the beta-subunit gene Cchb4 on mouse chromosome 2. A four-nucleotide insertion into a splice donor site results in exon skipping, translational frameshift, and protein truncation with loss of the alpha 1-binding site. The lethargic phenotype is the first example of a mammalian neurological disease caused by an inherited defect in a non-pore-forming subunit of a voltage-gated ion channel.

Tissue-specific expression of splice variants of the mouse voltage-gated calcium channel alpha2/delta subunit

Five different splice variants of mouse alpha2/delta subunit isoforms (alpha2a-e), which arose from various combinations of three alternatively spliced regions, were cloned with a combination of cDNA library screening and RT-PCR. Expression patterns and relative abundance of the various isoforms in mouse tissues were determined with an RNAse protection assay. Skeletal muscle and brain expressed single isoforms, alpha2a and alpha2b, respectively; however, the cardiovascular system expressed all five isoforms. Heart expressed mainly isoforms alpha2c and alpha2d while, in contrast to other species, aorta expressed predominantly alpha2a, the 'skeletal muscle' isoform. Smooth muscle-containing tissues expressed alpha2d and alpha2e. Thus, alpha2/delta isoforms are restricted in their tissue expression, suggesting an important functional role for the differentially spliced variants.