Importance of the different beta subunits in the membrane expression of the alpha1A and alpha2 calcium channel subunits: studies using a depolarization-sensitive alpha1A antibody

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.

CaVβ subunit-mediated up-regulation of CaV2.2 currents triggered by D2 dopamine receptor activation

Voltage-dependent Ca(2+) channels (VDCCs) are subject to modulation by a number of pathways, including membrane-delimited inhibition by heterotrimeric G-proteins and modulation through phosphorylation by diverse kinases. Here we report that in the Xenopus oocyte expression system Ca(V)2.2 channels undergo a sustained, linear and irreversible run-up lasting up to 30 min, which is potentiated during G-protein-mediated inhibition by activation of co-expressed G-protein coupled receptors (GPCRs). This up-regulation is not a result of receptor desensitization, but is associated with a hyperpolarization of the voltage for activation and depends on the presence of accessory subunits such that beta subunits promote, and alpha2delta subunits oppose the current increase. We have investigated the involvement of G-proteins and found that over-expression of Galpha(o) subunits or Galpha-transducin reduced the amount of agonist-mediated up-regulation. However, we have found no evidence for the involvement of any second messenger pathways in the increase of current run-up in the presence of a GPCR agonist. Taken together, our data suggest that the effect reported herein involves an enhancement of the GTPase activity of endogenous Galpha subunits, which is triggered by GPCR activation and mediated by accessory Ca(V)beta subunits. It may involve an increased association of Ca(V)beta subunits with alpha1 subunits in the plasma membrane or trafficking of channels to the plasma membrane.

The voltage-dependent calcium channel beta subunit contains two stable interacting domains

Voltage-dependent calcium channels selectively enable Ca2+ ion movement through cellular membranes. These multiprotein complexes are involved in a wide spectrum of biological processes such as signal transduction and cellular homeostasis. alpha1 is the membrane pore-forming subunit, whereas beta is an intracellular subunit that binds to alpha1, facilitating and modulating channel function. We have expressed, purified, and characterized recombinant beta3 and beta2a using both biochemical and biophysical methods, including electrophysiology, to better understand the beta family's protein structural and functional correlates. Our results indicate that the beta protein is composed of two distinct domains that associate with one another in a stable manner. The data also suggest that the polypeptide regions outside these domains are not structured when beta is not in complex with the channel. In addition, the beta structural core, comprised of just these two domains without other sequences, binds tightly to the alpha interaction domain (AID) motif, a sequence derived from the alpha1 subunit and the principal anchor site of beta. Domain II is responsible for this binding, but domain I enhances it.

Human neuronal stargazin-like proteins, gamma2, gamma3 and gamma4; an investigation of their specific localization in human brain and their influence on CaV2.1 voltage-dependent calcium channels expressed in Xenopus oocytes

BACKGROUND

Stargazin (gamma2) and the closely related gamma3, and gamma4 transmembrane proteins are part of a family of proteins that may act as both neuronal voltage-dependent calcium channel (VDCC) gamma subunits and transmembrane alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproponinc (AMPA) receptor regulatory proteins (TARPs). In this investigation, we examined the distribution patterns of the stargazin-like proteins gamma2, gamma3, and gamma4 in the human central nervous system (CNS). In addition, we investigated whether human gamma2 or gamma4 could modulate the electrophysiological properties of a neuronal VDCC complex transiently expressed in Xenopus oocytes.

RESULTS

The mRNA encoding human gamma2 is highly expressed in cerebellum, cerebral cortex, hippocampus and thalamus, whereas gamma3 is abundant in cerebral cortex and amygdala and gamma4 in the basal ganglia. Immunohistochemical analysis of the cerebellum determined that both gamma2 and gamma4 are present in the molecular layer, particularly in Purkinje cell bodies and dendrites, but have an inverse expression pattern to one another in the dentate cerebellar nucleus. They are also detected in the interneurons of the granule cell layer though only gamma2 is clearly detected in granule cells. The hippocampus stains for gamma2 and gamma4 throughout the layers of the every CA region and the dentate gyrus, whilst gamma3 appears to be localized particularly to the pyramidal and granule cell bodies. When co-expressed in Xenopus oocytes with a CaV2.1/beta4 VDCC complex, either in the absence or presence of an alpha2delta2 subunit, neither gamma2 nor gamma4 significantly modulated the VDCC peak current amplitude, voltage-dependence of activation or voltage-dependence of steady-state inactivation.

CONCLUSION

The human gamma2, gamma3 and gamma4 stargazin-like proteins are detected only in the CNS and display differential distributions among brain regions and several cell types in found in the cerebellum and hippocampus. These distribution patterns closely resemble those reported by other laboratories for the rodent orthologues of each protein. Whilst the fact that neither gamma2 nor gamma4 modulated the properties of a VDCC complex with which they could associate in vivo in Purkinje cells adds weight to the hypothesis that the principal role of these proteins is not as auxiliary subunits of VDCCs, it does not exclude the possibility that they play another role in VDCC function.

Differential modulation of cardiac Ca2+ channel gating by beta-subunits

To investigate the mechanisms that increase ionic currents when Ca(2+) channels' alpha(1) subunits are co-expressed with the beta-subunits, we compared channel activity of Ca(V)1.2 (alpha(1C)) co-expressed with beta(1a) and beta(2a) in Xenopus oocytes. Normalized by charge movement, ionic currents were near threefold larger with beta(2a) than with beta(1a). At the single-channel level, the open probability (P(o)) was over threefold larger with beta(2a), and traces with high P(o) were more frequent. Among traces with P(o) > 0.1, the mean duration of burst of openings (MBD) were nearly twice as long for alpha(1C)beta(2a) (15.1 +/- 0.7 ms) than for alpha(1C)beta(1a) (8.4 +/- 0.5 ms). Contribution of endogenous beta(3xo) was ruled out by comparing MBDs with alpha(1C)-cRNA alone (4.7 +/- 0.1 ms) with beta(3xo) (14.3 +/- 1.1 ms), and with beta(1b) (8.2 +/- 0.5 ms). Open-channel current amplitude distributions were indistinguishable for alpha(1C)beta(1a) and alpha(1C)beta(2a), indicating that opening and closing kinetics are similar with both subunits. Simulations with constant opening and closing rates reproduced the microscopic kinetics accurately, and therefore we conclude that the conformational change-limiting MBD is differentially regulated by the beta-subunits and contributes to the larger ionic currents associated with beta(2a), whereas closing and opening rates do not change, which should reflect the activity of a separate gate.

Distinctive modulatory effects of five human auxiliary beta2 subunit splice variants on L-type calcium channel gating

Sequence analysis of the human genome permitted cloning of five Ca(2+)-channel beta(2) splice variants (beta(2a)-beta(2e)) that differed only in their proximal amino-termini. The functional consequences of such beta(2)-subunit diversity were explored in recombinant L-type channels reconstituted in HEK 293 cells. Beta(2a) and beta(2e) targeted autonomously to the plasma membrane, whereas beta(2b)-beta(2d) localized to the cytosol when expressed in HEK 293 cells. The pattern of modulation of L-type channel voltage-dependent inactivation gating correlated with the subcellular localization of the component beta(2) variant-membrane-bound beta(2a) and beta(2e) subunits conferred slow(er) channel inactivation kinetics and displayed a smaller fraction of channels recovering from inactivation with fast kinetics, compared to beta(2b)-beta(2d) channels. The varying effects of beta(2) subunits on inactivation gating were accounted for by a quantitative model in which L-type channels reversibly distributed between fast and slow forms of voltage-dependent inactivation-membrane-bound beta(2) subunits substantially decreased the steady-state fraction of fast inactivating channels. Finally, the beta(2) variants also had distinctive effects on L-type channel steady-state activation gating, as revealed by differences in the waveforms of tail-activation (G-V) curves, and conferred differing degrees of prepulse facilitation to the channel. Our results predict important physiological consequences arising from subtle changes in Ca(2+)-channel beta(2)-subunit structure due to alternative splicing and emphasize the utility of splice variants in probing structure-function mechanisms.

Voltage-gated Ca2+ channel Ca(V)1.3 subunit expressed in the hair cell epithelium of the sacculus of the trout Oncorhynchus mykiss: cloning and comparison across vertebrate classes

Full-length sequence (>6.5 kb) has been determined for the Ca(V)1.3 pore-forming subunit of the voltage-gated Ca(2+) channel from the saccular hair cells of the rainbow trout (Oncorhynchus mykiss). Primary structure was obtained from overlapping PCR and cloned fragments, amplified by primers based on teleost, avian, and mammalian sources. Trout saccular Ca(V)1.3 was localized to hair cells, as evidenced by its isolation from an epithelial layer in which the hair cell is the only intact cell type. The predicted amino acid sequence of the trout hair cell Ca(V)1.3 is approximately 70% identical to the sequences of avian and mammalian Ca(V)1.3 subunits and shows L-type characteristics. The trout hair cell Ca(V)1.3 expresses a 26-aa insert in the I-II cytoplasmic loop (exon 9a) and a 10-aa insert in the IVS2-IVS3 cytoplasmic loop (exon 30a), neither of which is appreciably represented in trout brain. The exon 9a insert also occurs in hair cell organs of chick and rat, and appears as an exon in human genomic Ca(V)1.3 sequence (but not in the Ca(V)1.3 coding sequence expressed in human brain or pancreas). The exon 30a insert, although expressed in hair cells of chick as well as trout, does not appear in comparable rat or human tissues. Further, the IIIS2 region shows a splice choice (exon 22a) that is associated with the hair cell organs of trout, chick, and rat, but is not found in human genomic sequence. The elucidation of the primary structure of the voltage-gated Ca(2+) channel Ca(V)1.3 subunit from hair cells of the teleost, representing the lowest of the vertebrate classes, suggests a generality of sensory mechanism for Ca(V)1.3 across hair cell systems. In particular, the exon 9a insert of this channel appears to be the molecular feature most consistently associated with hair cells from fish to mammal, consonant with the hypothesis that the latter region may be a signature for the hair cell.

3D structure of the skeletal muscle dihydropyridine receptor

The dihydropyridine receptors (DHPR) are L-type voltage-gated calcium channels that regulate the flux of calcium ions across the cell membrane. Here we present the three-dimensional (3D) structure at approximately 27A resolution of purified skeletal muscle DHPR, as determined by electron microscopy and single particle analysis. Here both biochemical and 3D structural data indicate that DHPR is dimeric. DHPR dimers are composed of two arch-shaped monomers approximately 210A across and approximately 75A thick, that interact very tightly at each end of the arch. The roughly toroidal structure of the two monomers encloses a cylindrical space of approximately 80A diameter, which is then closed on each side by two dome-shaped protein densities reaching over from each monomer arch. The dome-shaped domains have a length of approximately 80-90A and a maximum height of approximately 45A. Small orifices punctuate their exterior surface. The 3D structure disclosed here may have important implications for the understanding of DHPR Ca(2+) channel function. We also propose a model for its in vivo interactions with the calcium release channel at the junctional sarcoplasmic recticulum.

Mutations in high-voltage-activated calcium channel genes stimulate low-voltage-activated currents in mouse thalamic relay neurons

Ca2+ currents, especially those activated at low voltages (LVA), influence burst generation in thalamocortical circuitry and enhance the abnormal rhythmicity associated with absence epilepsy. Mutations in several genes for high-voltage-activated (HVA) Ca2+ channel subunits are linked to spike-wave seizure phenotypes in mice; however, none of these mutations are predicted to increase intrinsic membrane excitability or directly enhance LVA currents. We examined biophysical properties of both LVA and HVA Ca2+ currents in thalamic cells of tottering (tg; Cav2.1/alpha1A subunit), lethargic (lh; beta4 subunit), and stargazer (stg; gamma2 subunit) brain slices. We observed 46, 51, and 45% increases in peak current densities of LVA Ca2+ currents evoked at -50 mV from -110 mV in tg, lh, and stg mice, respectively, compared with wild type. The half-maximal voltages for steady-state inactivation of LVA currents were shifted in a depolarized direction by 7.5-13.5 mV in all three mutants, although no alterations in the time-constant for recovery from inactivation of LVA currents were found. HVA peak current densities in tg and stg were increased by 22 and 45%, respectively, and a 5 mV depolarizing shift of the activation curve was observed in lh. Despite elevated LVA amplitudes, no alterations in mRNA expression of the genes mediating T-type subunits, Cav3.1/alpha1G, Cav3.2/alpha1H, or Cav3.3/alpha1I, were detected in the three mutants. Our data demonstrate that mutation of Cav2.1 or regulatory subunit genes increases intrinsic membrane excitability in thalamic neurons by potentiating LVA Ca2+ currents. These alterations increase the probability for abnormal thalamocortical synchronization and absence epilepsy in tg, lh, and stg mice.

Multiple determinants in voltage-dependent P/Q calcium channels control their retention in the endoplasmic reticulum

Surface expression level of voltage-dependent calcium channels is tightly controlled in neurons to avoid the resulting cell toxicity generally associated with excessive calcium entry. Cell surface expression of high voltage-activated calcium channels requires the association of the pore-forming subunit, Cavalpha, with the auxiliary subunit, Cavbeta. In the absence of this auxiliary subunit, Cavalpha is retained in the endoplasmic reticulum (ER) through mechanisms that are still poorly understood. Here, we have investigated, by a quantitative method based on the use of CD8 alpha chimeras, the molecular determinants of Cavalpha2.1 that are responsible for the retention, in the absence of auxiliary subunits, of P/Q calcium channels in the ER (referred to here as 'ER retention'). This study demonstrates that the I-II loop of Cavalpha2.1 contains multiple ER-retention determinants beside the beta subunit association domain. In addition, the I-II loop is not the sole domain of calcium channel retention as two regions identified for their ability to interact with the I-II loop, the N- and C-termini of Cavalpha2.1, also produce ER retention. It is also not an obligatory determinant as, similarly to low-threshold calcium channels, the I-II loop of Cavalpha1.1 does not produce ER retention in COS7 cells. The data presented here suggests that ER retention is suppressed by sequential molecular events that include: (i). a correct folding of Cavalpha in order to mask several internal ER-retention determinants and (ii). the association of other proteins, including the Cavbeta subunit, to suppress the remaining ER-retention determinants.

A specific tryptophan in the I-II linker is a key determinant of beta-subunit binding and modulation in Ca(V)2.3 calcium channels

The ancillary beta subunits modulate the activation and inactivation properties of high-voltage activated (HVA) Ca(2+) channels in an isoform-specific manner. The beta subunits bind to a high-affinity interaction site, alpha-interaction domain (AID), located in the I-II linker of HVA alpha1 subunits. Nine residues in the AID motif are absolutely conserved in all HVA channels (QQxExxLxGYxxWIxxxE), but their contribution to beta-subunit binding and modulation remains to be established in Ca(V)2.3. Mutations of W386 to either A, G, Q, R, E, F, or Y in Ca(V)2.3 disrupted [(35)S]beta3-subunit overlay binding to glutathione S-transferase fusion proteins containing the mutated I-II linker, whereas mutations (single or multiple) of nonconserved residues did not affect the protein-protein interaction with beta3. The tryptophan residue at position 386 appears to be an essential determinant as substitutions with hydrophobic (A and G), hydrophilic (Q, R, and E), or aromatic (F and Y) residues yielded the same results. beta-Subunit modulation of W386 (A, G, Q, R, E, F, and Y) and Y383 (A and S) mutants was investigated after heterologous expression in Xenopus oocytes. All mutant channels expressed large inward Ba(2+) currents with typical current-voltage properties. Nonetheless, the typical hallmarks of beta-subunit modulation, namely the increase in peak currents, the hyperpolarization of peak voltages, and the modulation of the kinetics and voltage dependence of inactivation, were eliminated in all W386 mutants, although they were preserved in part in Y383 (A and S) mutants. Altogether these results suggest that W386 is critical for beta-subunit binding and modulation of HVA Ca(2+) channels.

The presence of Ca2+ channel beta subunit is required for mitogen-activated protein kinase (MAPK)-dependent modulation of alpha1B Ca2+ channels in COS-7 cells

In rat sensory neurones, voltage-dependent calcium channels (VDCCs), including the N-type, are tonically up-regulated via Ras/mitogen-activated protein kinase (MAPK) signalling. To determine whether VDCC beta subunit is involved in this process, the role of the four neuronal betas (beta1b, beta2a, beta3, beta4) in MAPK-dependent modulation of alpha1B (Ca(v)2.2, N-type) Ca(2+) channels has been examined in COS-7 cells. MAPK is exclusively activated by MAPK kinase (MEK), and here, acute application of a MEK-specific inhibitor UO126, significantly inhibited peak alpha1B Ca2+ channel current (I(max)) within a period of 5-10 min, regardless of which beta subunit was co-expressed (25-50 %, P < 0.01). With beta2a however, the percentage inhibition of I(max) was less than that observed with any other beta (ANOVA: F(3,34) = 6.48, P < 0.01). UO126 also caused a hyperpolarising shift (6 +/- 1 mV, P < 0.001) in the voltage dependence of beta2a current activation, such that inhibition occurred only at depolarised potentials (> +5 mV) whereas at more negative potentials the current amplitude was enhanced. A marked change in beta2a current kinetics, perceived either as decreased activation or increased inactivation, was also associated with UO126 application. A similar effect of UO126 on beta4 current kinetics was also observed. The beta2a-specific effects of UO126 on current inhibition and voltage dependence of activation were abolished when alpha1B was co-expressed with de-palmitoylated beta2a(C3,4S), in which amino terminal cysteines 3 and 4 had been mutated to serines. In the absence of beta subunit, UO126 had no effect on alpha1B Ca2+ channel current. Together, these data suggest an absolute requirement for beta in MAPK-dependent modulation of these channels. Since beta subunits vary both in their temporal expression and localisation within neurones, beta subunit-dependent modulation of N-type Ca2+ channels via MAPK could provide an important new mechanism by which to fine-tune neurotransmitter release.

A mutation in the beta interaction domain of the Ca(2+) channel alpha(1C) subunit reduces the affinity of the (+)-[(3)H]isradipine binding site

The molecular mechanisms of how alpha(1) and beta subunits of voltage-gated Ca(2+) channels interact with one another are still controversial. Here we show that despite a mutation in the beta interaction domain that has previously been shown to disrupt binding, alpha(1C)Y467S and beta(1a-myc) still formed immunoprecipitable complexes when coexpressed in tsA201 cells. However, the alpha(1C)Y467S-beta(1a-myc) complexes had a decreased affinity to (+)-[(3)H]isradipine. This indicates that the beta interaction domain in the I-II loop of the alpha(1) subunit is not merely an anchor required for the functional interaction of the two Ca(2+) channel subunits but is itself part of the effector pathway for beta-induced channel modulation.

Novel functional properties of Ca(2+) channel beta subunits revealed by their expression in adult rat heart cells

Recombinant adenoviruses were used to overexpress green fluorescent protein (GFP)-fused auxiliary Ca(2+) channel beta subunits (beta(1)-beta(4)) in cultured adult rat heart cells, to explore new dimensions of beta subunit functions in vivo. Distinct beta-GFP subunits distributed differentially between the surface sarcolemma, transverse elements, and nucleus in single heart cells. All beta-GFP subunits increased the native cardiac whole-cell L-type Ca(2+) channel current density, but produced distinctive effects on channel inactivation kinetics. The degree of enhancement of whole-cell current density was non-uniform between beta subunits, with a rank order of potency beta(2a) approximately equal to beta(4) > beta(1b) > beta(3). For each beta subunit, the increase in L-type current density was accompanied by a correlative increase in the maximal gating charge (Q(max)) moved with depolarization. However, beta subunits produced characteristic effects on single L-type channel gating, resulting in divergent effects on channel open probability (P(o)). Quantitative analysis and modelling of single-channel data provided a kinetic signature for each channel type. Spurred on by ambiguities regarding the molecular identity of the actual endogenous cardiac L-type channel beta subunit, we cloned a new rat beta(2) splice variant, beta(2b), from heart using 5' rapid amplification of cDNA ends (RACE) PCR. By contrast with beta(2a), expression of beta(2b) in heart cells yielded channels with a microscopic gating signature virtually identical to that of native unmodified channels. Our results provide novel insights into beta subunit functions that are unattainable in traditional heterologous expression studies, and also provide new perspectives on the molecular identity of the beta subunit component of cardiac L-type Ca(2+) channels. Overall, the work establishes a powerful experimental paradigm to explore novel functions of ion channel subunits in their native environments.

The novel product of a five-exon stargazin-related gene abolishes Ca(V)2.2 calcium channel expression

We have cloned and characterized a new member of the voltage-dependent Ca(2+) channel gamma subunit family, with a novel gene structure and striking properties. Unlike the genes of other potential gamma subunits identified by their homology to the stargazin gene, CACNG7 is a five-, and not four-exon gene whose mRNA encodes a protein we have designated gamma(7). Expression of human gamma(7) has been localized specifically to brain. N-type current through Ca(V)2.2 channels was almost abolished when co-expressed transiently with gamma(7) in either Xenopus oocytes or COS-7 cells. Furthermore, immunocytochemistry and western blots show that gamma(7) has this effect by causing a large reduction in expression of Ca(V)2.2 rather than by interfering with trafficking or biophysical properties of the channel. No effect of transiently expressed gamma(7) was observed on pre-existing endogenous N-type calcium channels in sympathetic neurones. Low homology to the stargazin-like gamma subunits, different gene structure and the unique functional properties of gamma(7) imply that it represents a distinct subdivision of the family of proteins identified by their structural and sequence homology to stargazin.

The ducky mutation in Cacna2d2 results in altered Purkinje cell morphology and is associated with the expression of a truncated alpha 2 delta-2 protein with abnormal function

The mouse mutant ducky, a model for absence epilepsy, is characterized by spike-wave seizures and cerebellar ataxia. A mutation in Cacna2d2, the gene encoding the alpha 2 delta-2 voltage-dependent calcium channel accessory subunit, has been found to underlie the ducky phenotype. The alpha 2 delta-2 mRNA is strongly expressed in cerebellar Purkinje cells. We show that du/du mice have abnormalities in their Purkinje cell dendritic tree. The mutation in alpha 2 delta-2 results in the introduction of a premature stop codon and predicts the expression of a truncated protein encoded by the first three exons of Cacna2d2, followed by 8 novel amino acids. We show that both mRNA and protein corresponding to this predicted transcript are expressed in du/du cerebellum and present in Purkinje cells. Whereas the alpha 2 delta-2 subunit increased the peak current density of the Ca(V)2.1/beta(4) channel combination when co-expressed in vitro, co-expression with the truncated mutant alpha 2 delta-2 protein reduced current density, indicating that it may contribute to the du phenotype.

Structure and function of voltage-dependent ion channel regulatory beta subunits

Voltage-dependent K(+), Ca(2+), and Na(+) channels play vital roles in basic physiological processes, including management of the action potential, signal transduction, and secretion. They share the common function of passively transporting ions across cell membranes; thus, it would not be surprising if they should exhibit similarities of both structure and mechanism. Indeed, the principal pore-forming (alpha) subunits of each show either exact or approximate 4-fold symmetry and share a similar transmembrane topology, and all are gated by changes in membrane potential. Furthermore, these channels all possess an auxiliary polypeptide, designated the beta subunit, which plays an important role in their regulation. Despite considerable functional semblences and abilities to interact with structurally similar alpha subunits, however, there is considerable structural diversity among the beta subunits. In this review, we discuss the similarities and differences in the structures and functions of the beta subunits of the voltage-dependent K(+), Ca(2+), and Na(+) channels.

Dominant-negative synthesis suppression of voltage-gated calcium channel Cav2.2 induced by truncated constructs

Voltage-gated calcium channel alpha1 subunits consist of four domains (I-IV), each with six transmembrane segments. A number of truncated isoforms have been identified to occur as a result of alternative splicing or mutation. We have examined the functional consequences for expression of full-length Ca(v)2.2 (alpha1B) of its coexpression with truncated constructs of Ca(v)2.2. Domains I-II or domains III-IV, when expressed individually, together with the accessory subunits beta1b and alpha2delta-1, did not form functional channels. When they were coexpressed, low-density whole-cell currents and functional channels with properties similar to wild-type channels were observed. However, when domain I-II, domain III-IV, or domain I alone were coexpressed with full-length Ca(v)2.2, they markedly suppressed its functional expression, although at the single channel level, when channels were recorded, there were no differences in their biophysical properties. Furthermore, when it was coexpressed with either domain I-II or domain I, the fluorescence of green fluorescent protein (GFP)-Ca(v)2.2 and expression of Ca(v)2.2 protein was almost abolished. Suppression does not involve sequestration of the Ca(v)beta subunit, because loss of GFP-Ca(v)2.2 expression also occurred in the absence of beta subunit, and the effect of domain I-II or domain I could not be mimicked by the cytoplasmic I-II loop of Ca(v)2.2. It requires transmembrane segments, because the isolated Ca(v)2.2 N terminus did not have any effect. Our results indicate that the mechanism of suppression of Ca(v)2.2 by truncated constructs containing domain I involves inhibition of channel synthesis, which may represent a role of endogenously expressed truncated Ca(v) isoforms.

Evidence for two concentration-dependent processes for beta-subunit effects on alpha1B calcium channels

beta-Subunits of voltage-dependent Ca(2+) channels regulate both their expression and biophysical properties. We have injected a range of concentrations of beta3-cDNA into Xenopus oocytes, with a fixed concentration of alpha1B (Ca(V)2.2) cDNA, and have quantified the corresponding linear increase of beta3 protein. The concentration dependence of a number of beta3-dependent processes has been studied. First, the dependence of the a1B maximum conductance on beta3-protein occurs with a midpoint around the endogenous concentration of beta3 (approximately 17 nM). This may represent the interaction of the beta-subunit, responsible for trafficking, with the I-II linker of the nascent channel. Second, the effect of beta3-subunits on the voltage dependence of steady-state inactivation provides evidence for two channel populations, interpreted as representing alpha1B without or with a beta3-subunit, bound with a lower affinity of 120 nM. Third, the effect of beta3 on the facilitation rate of G-protein-modulated alpha1B currents during a depolarizing prepulse to +100 mV provides evidence for the same two populations, with the rapid facilitation rate being attributed to Gbetagamma dissociation from the beta-subunit-bound alpha1B channels. The data are discussed in terms of two hypotheses, either binding of two beta-subunits to the alpha1B channel or a state-dependent alteration in affinity of the channel for the beta-subunit.

Regulation of Ca2+ channel expression at the cell surface by the small G-protein kir/Gem

Voltage-dependent calcium (Ca2+) channels are involved in many specialized cellular functions, and are controlled by intracellular signals such as heterotrimeric G-proteins, protein kinases and calmodulin (CaM). However, the direct role of small G-proteins in the regulation of Ca2+ channels is unclear. We report here that the GTP-bound form of kir/Gem, identified originally as a Ras-related small G-protein that binds CaM, inhibits high-voltage-activated Ca2+ channel activities by interacting directly with the beta-subunit. The reduced channel activities are due to a decrease in alpha1-subunit expression at the plasma membrane. The binding of Ca2+/CaM to kir/Gem is required for this inhibitory effect by promoting the cytoplasmic localization of kir/Gem. Inhibition of L-type Ca2+ channels by kir/Gem prevents Ca2+-triggered exocytosis in hormone-secreting cells. We propose that the small G-protein kir/Gem, interacting with beta-subunits, regulates Ca2+ channel expression at the cell surface.

Different localizations and functions of L-type and N-type calcium channels during development of hippocampal neurons

Using immunocytochemical assays and patch-clamp and calcium-imaging recordings, we demonstrate that L-type and N-type calcium channels have distinct patterns of expression and distribution and play different functional roles during hippocampal neuron differentiation. L-type channels, which support the depolarization-induced calcium influx in neurons from the very early developmental stages, are functionally restricted to the somatodendritic compartment throughout neuronal development and play a crucial role in supporting neurite outgrowth at early developmental stages. N-type channels, which start contributing at later neuronal differentiation stages (3-4 DIV), are also functionally expressed in the axons of immature neurons. At this developmental stage preceding synaptogenesis, N-type (but not L-type) channels are involved in controlling synaptic vesicle recycling. It is only at later developmental stages (10-12 DIV), when the neurons have established a clear axodendritic polarity and form synaptic contacts, that N-type channels are progressively excluded from the axon. Electrophysiological recordings of single neurons growing in microislands revealed that synaptic maturation coincides with a progressive increase in N-type channels in the somatodendritic region and a progressive decrease in the N-type channels supporting glutamate release from the presynaptic terminal. These results indicate that L-type and N-type calcium channels undergo dynamic, developmentally regulated rearrangements in regional distribution and function and also suggest that different mechanisms may be involved in the sorting and/or stabilization of these two types of channels in different plasma membrane domains during neuronal differentiation.

Calcium channel beta subunits differentially modulate recovery of the channel from inactivation

We examined the effects of calcium channel beta subunits upon the recovery from inactivation of alpha(1) subunits expressed in Xenopus oocytes. Recovery of the current carried by the L-type alpha(1) subunit (cyCa(v)1) from the jellyfish Cyanea capillata was accelerated by coexpression of any beta subunit, but the degree of potentiation differed according to which beta isoform was coexpressed. The Cyanea beta subunit was most effective, followed by the mammalian b(3), b(4), and beta(2a) subtypes. Recovery of the human Ca(v)2.3 subunit was also modulated by beta subunits, but was slowed instead. beta(3) was the most potent subunit tested, followed by beta(4), then beta(2a), which had virtually no effect. These results demonstrate that different beta subunit isoforms can affect recovery of the channel to varying degrees, and provide an additional mechanism by which beta subunits can differentially regulate alpha(1) subunits.

The triad targeting signal of the skeletal muscle calcium channel is localized in the COOH terminus of the alpha(1S) subunit

The specific localization of L-type Ca(2+) channels in skeletal muscle triads is critical for their normal function in excitation-contraction (EC) coupling. Reconstitution of dysgenic myotubes with the skeletal muscle Ca(2+) channel alpha(1S) subunit restores Ca(2+) currents, EC coupling, and the normal localization of alpha(1S) in the triads. In contrast, expression of the neuronal alpha(1A) subunit gives rise to robust Ca(2+) currents but not to triad localization. To identify regions in the primary structure of alpha(1S) involved in the targeting of the Ca(2+) channel into the triads, chimeras of alpha(1S) and alpha(1A) were constructed, expressed in dysgenic myotubes, and their subcellular distribution was analyzed with double immunofluorescence labeling of the alpha(1S)/alpha(1A) chimeras and the ryanodine receptor. Whereas chimeras containing the COOH terminus of alpha(1A) were not incorporated into triads, chimeras containing the COOH terminus of alpha(1S) were correctly targeted. Mapping of the COOH terminus revealed a triad-targeting signal contained in the 55 amino-acid sequence (1607-1661) proximal to the putative clipping site of alpha(1S). Transferring this triad targeting signal to alpha(1A) was sufficient for targeting and clustering the neuronal isoform into skeletal muscle triads and caused a marked restoration of Ca(2+)-dependent EC coupling.

Interaction between G proteins and accessory subunits in the regulation of 1B calcium channels in Xenopus oocytes

The accessory beta subunits of voltage-dependent Ca2+ channels (VDCCs) have been shown to regulate their biophysical properties and have also been suggested to antagonise the G protein inhibition of N-type (alpha1B), P/Q-type (alpha1A) and alpha1E channels. Here we have examined the voltage-dependent involvement of the four neuronal isoforms (beta1b, beta2a, beta3 and beta4) in the process of G protein modulation of alpha1B Ca2+ channels. All beta subunits hyperpolarized alpha1B current activation, and all antagonised the G protein-mediated depolarisation of current activation. However, except in the case of beta2a, there was no generalised reduction by beta subunits in the maximal extent of receptor-mediated inhibition of alpha1B current. In addition, all VDCC beta subunits enhanced the rate of current facilitation at +100 mV, for both receptor-mediated and tonic modulation. The rank order for enhancement of facilitation rate was beta3 > beta4 > beta1b > beta2a. In contrast, the amount of voltage-dependent facilitation during tonic modulation was reduced by beta subunit co-expression, despite the fact that the apparent Gbetagamma dissociation rate at +100 mV was enhanced by beta subunits to a similar level as for agonist-induced modulation. Our data provide evidence that G protein activation antagonises Ca2+-channel beta subunit-induced hyperpolarisation of current activation. Conversely, co-expression of all beta subunits increases the apparent Gbetagamma dimer dissociation rate during a depolarising prepulse. This latter feature suggests the co-existence of bound Ca2+-channel beta subunits and Gbetagamma dimers on the alpha1B subunits. Future work will determine how the interaction between Gbetagamma dimers and Ca2+-channel beta subunits with alpha1B results in a functional antagonism at the molecular level.

Calcium channel beta subunit promotes voltage-dependent modulation of alpha 1 B by G beta gamma

Voltage-dependent calcium channels (VDCCs) are heteromultimers composed of a pore-forming alpha1 subunit and auxiliary subunits, including the intracellular beta subunit, which has a strong influence on the channel properties. Voltage-dependent inhibitory modulation of neuronal VDCCs occurs primarily by activation of G-proteins and elevation of the free G beta gamma dimer concentration. Here we have examined the interaction between the regulation of N-type (alpha 1 B) channels by their beta subunits and by G beta gamma dimers, heterologously expressed in COS-7 cells. In contrast to previous studies suggesting antagonism of G protein inhibition by the VDCC beta subunit, we found a significantly larger G beta gamma-dependent inhibition of alpha 1 B channel activation when the VDCC alpha 1 B and beta subunits were coexpressed. In the absence of coexpressed VDCC beta subunit, the G beta gamma dimers, either expressed tonically or elevated via receptor activation, did not produce the expected features of voltage-dependent G protein modulation of N-type channels, including slowed activation and prepulse facilitation, while VDCC beta subunit coexpression restored all of the hallmarks of G beta gamma modulation. These results suggest that the VDCC beta subunit must be present for G beta gamma to induce voltage-dependent modulation of N-type calcium channels.

Abnormal intracellular ca(2+)homeostasis and disease

A whole range of cell functions are regulated by the free cytosolic Ca(2+)concentration. Activator Ca(2+)from the extracellular space enters the cell through various types of Ca(2+)channels and sometimes the Na(+)/Ca(2+)-exchanger, and is actively extruded from the cell by Ca(2+)pumps and Na(+)/Ca(2+)-exchangers. Activator Ca(2+)can also be released from internal Ca(2+)stores through inositol trisphosphate or ryanodine receptors and is taken up into these organelles by means of Ca(2+)pumps. The resulting Ca(2+)signal is highly organized in space, frequency and amplitude because the localization and the integrated free cytosolic Ca(2+)concentration over time contain specific information. Mutations or functional abnormalities in the various Ca(2+)transporters, which in vitro seem to induce trivial functional alterations, therefore, often lead to a plethora of diseases. Skeletal-muscle pathology can be caused by mutations in ryanodine receptors (malignant hyperthermia, porcine stress syndrome, central-core disease), dihydropyridine receptors (familial hypokalemic periodic paralysis, malignant hyperthermia, muscular dysgenesis) or Ca(2+)pumps (Brody disease). Ca(2+)-pump mutations in cutaneous epidermal keratinocytes and cochlear hair cells lead to, skin diseases (Darier and Hailey-Hailey) and hearing/vestibular problems respectively. Mutated Ca(2+)channels in the photoreceptor plasma membrane cause vision problems. Hemiplegic migraine, spinocerebellar ataxia type-6, one form of episodic ataxia and some forms of epilepsy can be due to mutations in plasma-membrane Ca(2+)channels, while antibodies against these channels play a pathogenic role in all patients with the Lambert-Eaton myasthenic syndrome and may be of significance in sporadic amyotrophic lateral sclerosis. Brain inositol trisphosphate receptors have been hypothesized to contribute to the pathology in opisthotonos mice, manic-depressive illness and perhaps Alzheimer's disease. Various abnormalities in Ca(2+)-handling proteins have been described in heart during aging, hypertrophy, heart failure and during treatment with immunosuppressive drugs and in diabetes mellitus. In some instances, disease-causing mutations or abnormalities provide us with new insights into the cell biology of the various Ca(2+)transporters.

The alpha1B Ca2+ channel amino terminus contributes determinants for beta subunit-mediated voltage-dependent inactivation properties

Co-expression of auxiliary beta subunits with the alpha1B Ca2+ channel subunit in COS-7 cells resulted in an increase in current density and a hyperpolarising shift in the mid-point of activation. Amongst the beta subunits, beta2a in particular, but also beta4 and beta1b caused a significant retardation of the voltage-dependent inactivation compared to currents with alpha1B alone, whilst no significant changes in inactivation properties were seen for the beta3 subunit in this system. Prevention of beta2a palmitoylation, by introducing cysteine to serine mutations (beta2a(C3,4S)), greatly reduced the ability of beta2a to retard voltage-dependent inactivation. Deletion of the proximal half of the alpha1B cytoplasmic amino terminus (alpha1BDelta1-55) differentially affected beta subunit-mediated voltage-dependent inactivation properties. These effects were prominent with the beta2a subunit and, to a lesser extent, with beta1b. For beta2a, the major effects of this deletion were a partial reversal of beta2a-mediated retardation of inactivation and the introduction of a fast component of inactivation, not seen with full-length alpha1B. Deletion of the amino terminus had no other major effects on the measured biophysical properties of alpha1B when co-expressed with beta subunits. Transfer of the whole alpha1B amino terminus into alpha1C (alpha1bCCCC) conferred a similar retardation of inactivation on alpha1C when co-expressed with beta2a to that seen in parental alpha1B. Individual (alpha1B(Q47A) and alpha1B(R52A)) and double (alpha1B(R52,54A)) point mutations within the amino terminus of alpha1B also opposed the beta2a-mediated retardation of alpha1B inactivation kinetics. These results indicate that the alpha1B amino terminus contains determinants for beta subunit-mediated voltage-dependent inactivation properties. Furthermore, effects were beta subunit selective. As deletion of the alpha1B amino terminus only partially opposed beta subunit-mediated changes in inactivation properties, the amino terminus is likely to contribute to a complex site necessary for complete beta subunit function.

Acidic motif responsible for plasma membrane association of the voltage-dependent calcium channel beta1b subunit

Voltage-dependent calcium channels consist of a pore-forming transmembrane alpha1-subunit, which is known to associate with a number of accessory subunits, including alpha2-delta- and beta-subunits. The beta-subunits, of which four have been identified (beta1-4), are intracellular proteins that have marked effects on calcium channel trafficking and function. In a previous study, we observed that the beta1b-subunit showed selective plasma membrane association when expressed alone in COS7 cells, whereas beta3 and beta4 did not. In this study, we have examined the basis for this, and have identified, by making chimeric beta-subunits, that the C-terminal region, which shows most diversity between beta-subunits, of beta1b is responsible for its plasma membrane association. Furthermore we have identified, by deletion mutations, an 11-amino acid motif present in the C terminus of beta1b but not in beta3 (amino acids 547-556 of beta1b, WEEEEDYEEE), which when deleted, reduces membrane association of beta1b. Future research aims to identify what is binding to this sequence in beta1b to promote membrane association of this calcium channel subunit. It is possible that such membrane association is important for the selective localization or clustering of particular calcium channels with which beta1b is associated.

Ca(2+) channel modulation by recombinant auxiliary beta subunits expressed in young adult heart cells

L-type Ca(2+) channels contribute importantly to the normal excitation-contraction coupling of physiological hearts, and to the functional derangement seen in heart failure. Although Ca(2+) channel auxiliary beta(1-4) subunits are among the strongest modulators of channel properties, little is known about their role in regulating channel behavior in actual heart cells. Current understanding draws almost exclusively from heterologous expression of recombinant subunits in model systems, which may differ from cardiocytes. To study beta-subunit effects in the cardiac setting, we here used an adenoviral-component gene-delivery strategy to express recombinant beta subunits in young adult ventricular myocytes cultured from 4- to 6-week-old rats. The main results were the following. (1) A component system of replication-deficient adenovirus, poly-L-lysine, and expression plasmids encoding beta subunits could be optimized to transfect young adult myocytes with 1% to 10% efficiency. (2) A reporter gene strategy based on green fluorescent protein (GFP) could be used to identify successfully transfected cells. Because fusion of GFP to beta subunits altered intrinsic beta-subunit properties, we favored the use of a bicistronic expression plasmid encoding both GFP and a beta subunit. (3) Despite the heteromultimeric composition of L-type channels (composed of alpha(1C), beta, and alpha(2)delta), expression of recombinant beta subunits alone enhanced Ca(2+) channel current density up to 3- to 4-fold, which argues that beta subunits are "rate limiting" for expression of current in heart. (4) Overexpression of the putative "cardiac" beta(2a) subunit more than halved the rate of voltage-dependent inactivation at +10 mV. This result demonstrates that beta subunits can tune inactivation in the myocardium and suggests that other beta subunits may be functionally dominant in the heart. Overall, this study points to the possible therapeutic potential of beta subunits to ameliorate contractile dysfunction and excitability in heart failure.

Cloning of the beta(2a) subunit of the voltage-dependent calcium channel from human heart: cooperative effect of alpha(2)/delta and beta(2a) on the membrane expression of the alpha(1C) subunit

Expression and membrane localization of an epitope-tagged human Ca(2+) channel alpha(1C) subunit were monitored in Xenopus oocytes by confocal microscopy and electrophysiological recording. When alpha(2)/delta and beta(2a) were separately coexpressed with the alpha(1C) subunit, assessment by confocal microscopy showed an 86 and 225% increase of the channel density, respectively. Simultaneous coexpression of alpha(2)/delta and beta(2a) subunits resulted in a cooperative (470%) increase. Electrophysiological measurements performed in parallel revealed that the current augmentation by the alpha(2)/delta subunit is totally attributable to an increase in channel density, whereas the beta(2a) subunit, in addition to increasing channel density, also facilitates channel opening.

The I-II loop of the Ca2+ channel alpha1 subunit contains an endoplasmic reticulum retention signal antagonized by the beta subunit

The auxiliary beta subunit is essential for functional expression of high voltage-activated Ca2+ channels. This effect is partly mediated by a facilitation of the intracellular trafficking of alpha1 subunit toward the plasma membrane. Here, we demonstrate that the I-II loop of the alpha1 subunit contains an endoplasmic reticulum (ER) retention signal that severely restricts the plasma membrane incorporation of alpha1 subunit. Coimmunolabeling reveals that the I-II loop restricts expression of a chimera CD8-I-II protein to the ER. The beta subunit reverses the inhibition imposed by the retention signal. Extensive deletion of this retention signal in full-length alpha1 subunit facilitates the cell surface expression of the channel in the absence of beta subunit. Our data suggest that the beta subunit favors Ca2+ channel plasma membrane expression by inhibiting an expression brake contained in beta-binding alpha1 sequences.

Voltage-gated ion channels and hereditary disease

By the introduction of technological advancement in methods of structural analysis, electronics, and recombinant DNA techniques, research in physiology has become molecular. Additionally, focus of interest has been moving away from classical physiology to become increasingly centered on mechanisms of disease. A wonderful example for this development, as evident by this review, is the field of ion channel research which would not be nearly as advanced had it not been for human diseases to clarify. It is for this reason that structure-function relationships and ion channel electrophysiology cannot be separated from the genetic and clinical description of ion channelopathies. Unique among reviews of this topic is that all known human hereditary diseases of voltage-gated ion channels are described covering various fields of medicine such as neurology (nocturnal frontal lobe epilepsy, benign neonatal convulsions, episodic ataxia, hemiplegic migraine, deafness, stationary night blindness), nephrology (X-linked recessive nephrolithiasis, Bartter), myology (hypokalemic and hyperkalemic periodic paralysis, myotonia congenita, paramyotonia, malignant hyperthermia), cardiology (LQT syndrome), and interesting parallels in mechanisms of disease emphasized. Likewise, all types of voltage-gated ion channels for cations (sodium, calcium, and potassium channels) and anions (chloride channels) are described together with all knowledge about pharmacology, structure, expression, isoforms, and encoding genes.

Properties of Q-type calcium channels in neostriatal and cortical neurons are correlated with beta subunit expression

In brain neurons, P- and Q-type Ca(2+) channels both appear to include a class A alpha1 subunit. In spite of this similarity, these channels differ pharmacologically and biophysically, particularly in inactivation kinetics. The molecular basis for this difference is unclear. In heterologous systems, alternative splicing and ancillary beta subunits have been shown to alter biophysical properties of channels containing a class A alpha1 subunit. To test the hypothesis that similar mechanisms are at work in native systems, P- and Q-type currents were characterized in acutely isolated rat neostriatal, medium spiny neurons and cortical pyramidal neurons using whole-cell voltage-clamp techniques. Cells were subsequently aspirated and subjected to single-cell RT-PCR (scRT-PCR) analysis of calcium channel alpha(1) and beta (beta(1-4)) subunit expression. In both cortical and neostriatal neurons, P- and Q-type currents were found in cells expressing class A alpha(1) subunit mRNA. Although P-type currents in cortical and neostriatal neurons were similar, Q-type currents differed significantly in inactivation kinetics. Notably, Q-type currents in neostriatal neurons were similar to P-type currents in inactivation rate. The variation in Q-type channel biophysics was correlated with beta subunit expression. Neostriatal neurons expressed significantly higher levels of beta(2a) mRNA and lower levels of beta(1b) mRNA than cortical neurons. These findings are consistent with the association of beta(2a) and beta(1b) subunits with slow and fast inactivation, respectively. Analysis of alpha(1A) splice variants in the linker between domains I and II failed to provide an alternative explanation for the differences in inactivation rates. These findings are consistent with the hypothesis that the biophysical properties of Q-type channels are governed by beta subunit isoforms and are separable from toxin sensitivity.

Single gene defects in mice: the role of voltage-dependent calcium channels in absence models

Nineteen genes encoding alpha1, beta, gamma, or alpha2delta voltage-dependent calcium channel subunits have been identified to date. Recent studies have found that three of these genes are mutated in mice with generalised cortical spike-wave discharges (models of human absence epilepsy), emphasising the importance of calcium channels in regulating the expression of this inherited seizure phenotype. The tottering (tg) locus encodes the calcium channel alpha1 subunit gene Cacna1a, lethargic (lh) encodes the beta subunit gene Cacnb4, and stargazer (stg) encodes the gamma subunit gene Cacng2. These calcium channel mutants should provide important insights into the basic mechanisms of neuronal synchronisation, and the genes may be considered candidates for involvement in similar human disorders. The mutant models offer an important opportunity to elucidate the molecular, developmental, and physiological mechanisms underlying one subtype of absence epilepsy. Since calcium channels are involved in numerous cellular functions, including proliferation and differentiation, membrane excitability, neurite outgrowth and synaptogenesis, signal transduction, and gene expression, their role in generating the absence epilepsy phenotype may be complex. A comparative analysis of channel function and neural excitability patterns in tottering, lethargic, and stargazer brain should be useful in identifying the common elements of calcium channel involvement in these absence models.

Identification of residues in the N terminus of alpha1B critical for inhibition of the voltage-dependent calcium channel by Gbeta gamma

To examine the role of the intracellular N terminus in the G-protein modulation of the neuronal voltage-dependent calcium channel (VDCC) alpha1B, we have pursued two routes of investigation. First, we made chimeric channels between alpha1B and alpha1C, the latter not being modulated by Gbeta gamma subunits. VDCC alpha1 subunit constructs were coexpressed with accessory alpha2delta and beta2a subunits in Xenopus oocytes and mammalian (COS-7) cells. G-protein modulation of expressed alpha1 subunits was induced by activation of coexpressed dopamine (D2) receptors with quinpirole in oocytes, or by cotransfection of Gbeta1gamma2 subunits in COS-7 cells. For the chimeric channels, only those with the N terminus of alpha1B showed any G-protein modulation; further addition of the first transmembrane domain and I-II intracellular linker of alpha1B increased the degree of modulation. To determine the amino acids within the alpha1B N terminus, essential for G-protein modulation, we made mutations of this sequence and identified three amino acids (S48, R52, and R54) within an 11 amino acid sequence as being critical for G-protein modulation, with I49 being involved to a lesser extent. This sequence may comprise an essential part of a complex Gbeta gamma-binding site or be involved in its subsequent action.

Distribution of voltage-dependent calcium channel beta subunits in the hippocampus of patients with temporal lobe epilepsy

Voltage-dependent Ca2+ channels constitute a major class of plasma membrane channels through which a significant amount of extracellular Ca2+ enters neuronal cells. Their pore-forming alpha1 subunits are associated with cytoplasmic regulatory beta subunits, which modify the distinct biophysical and pharmacological properties of the alpha1 subunits. Studies in animal models indicate altered expression of alpha1 and/or beta subunits in epilepsy. We have focused on the regulatory beta subunits and have analysed the immunoreactivity patterns of the beta1, beta2, beta3 and beta4 subunits in the hippocampus of patients with temporal lobe epilepsy (n = 18) compared to control specimens (n = 2). Temporal lobe epilepsy specimens were classified as Ammon's horn sclerosis (n = 9) or focal lesions without alteration of hippocampal cytoarchitecture (n = 9). Immunoreactivity for the beta subunits was observed in neuronal cell bodies, dendrites and neuropil. The beta1, beta2 and beta3 subunits were found mainly in cell bodies while the beta4 subunit was primarily localized to dendrites. Compared to the control specimens, epilepsy specimens of the Ammon's horn sclerosis and of the lesion group showed a similar beta subunit distribution, except for beta1 and beta2 staining in the Ammon's horn sclerosis group: in the severely sclerotic hippocampal subfields of these specimens, beta1 and beta2 immunoreactivity was enhanced in some of the remaining neuronal cell bodies and, in addition, strongly marked dendrites. Thus, hippocampal neurons apparently express multiple classes of beta subunits which segregate into particular subcellular domains. In addition, the enhancement of beta1 and beta2 immunoreactivity in neuronal cell bodies and the additional shift of the beta1 and beta2 subunits into the dendritic compartment in severely sclerotic hippocampal regions indicate specific changes in Ammon's horn sclerosis. Altered expression of these beta subunits may lead to increased currents carried by voltage-dependent calcium channels and to enhanced synaptic excitability.

The effect of alpha2-delta and other accessory subunits on expression and properties of the calcium channel alpha1G

1. The effect has been examined of the accessory alpha2-delta and beta subunits on the properties of alpha1G currents expressed in monkey COS-7 cells and Xenopus oocytes. 2. In immunocytochemical experiments, the co-expression of alpha2-delta increased plasma membrane localization of expressed alpha1G and conversely, the heterologous expression of alpha1G increased immunostaining for endogenous alpha2-delta, suggesting an interaction between the two subunits. 3. Heterologous expression of alpha2-delta together with alpha1G in COS-7 cells increased the amplitude of expressed alpha1G currents by about 2-fold. This finding was confirmed in the Xenopus oocyte expression system. The truncated delta construct did not increase alpha1G current amplitude, or increase its plasma membrane expression. This indicates that it is the exofacial alpha2 domain that is involved in the enhancement by alpha2-delta. 4. Beta1b also produced an increase of functional expression of alpha1G, either in the absence or the presence of heterologously expressed alpha2-delta, whereas the other beta subunits had much smaller effects. 5. None of the accessory subunits had any marked influence on the voltage dependence or kinetics of the expressed alpha1G currents. These results therefore suggest that alpha2-delta and beta1b interact with alpha1G to increase trafficking of, or stabilize, functional alpha1G channels expressed at the plasma membrane.

Current modulation and membrane targeting of the calcium channel alpha1C subunit are independent functions of the beta subunit

1. The beta subunits of voltage-sensitive calcium channels facilitate the incorporation of channels into the plasma membrane and modulate calcium currents. In order to determine whether these two effects of the beta subunit are interdependent or independent of each other we studied plasma membrane incorporation of the channel subunits with green fluorescent protein and immunofluorescence labelling, and current modulation with whole-cell and single-channel patch-clamp recordings in transiently transfected human embryonic kidney tsA201 cells. 2. Coexpression of rabbit cardiac muscle alpha1C with rabbit skeletal muscle beta1a, rabbit heart/brain beta2a or rat brain beta3 subunits resulted in the colocalization of alpha1C with beta and in a marked translocation of the channel complexes into the plasma membrane. In parallel, the whole-cell current density and single-channel open probability were increased. Furthermore, the beta2a isoform specifically altered the voltage dependence of current activation and the inactivation kinetics. 3. A single amino acid substitution in the beta subunit interaction domain of alpha1C (alpha1CY467S) disrupted the colocalization and plasma membrane targeting of both subunits without affecting the beta subunit-induced modulation of whole-cell currents and single-channel properties. 4. These results show that the modulation of calcium currents by beta subunits can be explained by beta subunit-induced changes of single-channel properties, but the formation of stable alpha1C-beta complexes and their increased incorporation into the plasma membrane appear not to be necessary for functional modulation.

Decreased (45)Ca(2)(+) uptake in P/Q-type calcium channels in homozygous lethargic (Cacnb4lh) mice is associated with increased beta3 and decreased beta4 calcium channel subunit mRNA expression

The mutated gene in the lethargic (Cacnb4lh) mouse model of absence seizures encodes the beta4 subunit of voltage-gated calcium channels (VGCCs), leading to decreased mRNA expression of a beta4 subunit that is truncated and cannot bind to alpha1 subunits of VGCCs. In this study we accomplished two goals. First, we studied the functional consequence of altered VGCCs by examining the effects of a selective P/Q-type channel antagonist on KCl-induced (45)Ca(2)(+) uptake in brain synaptosomes from Cacnb4lh homozygotes and non-epileptic controls (designated by +/+). We found that depolarization-induced (45)Ca(2)(+) uptake was significantly reduced in the brains of Cacnb4lh homozygotes, and that the reduced uptake was completely accounted for by reduced function of P/Q-type calcium channel. Second, we examined VGCC subunit composition to determine if other subunits were altered in addition to the mutation affecting beta4 subunits in Cacnb4lh homozygotes; when alterations were found, we determined if they were regional or global. We used in situ hybridization histochemistry (ISHH) to analyze the neuro-anatomic distribution of beta4, beta1b, beta2, beta3, alpha1A, alpha1B, alpha1C, alpha1E, and alpha1G subunit mRNAs in brain sections from matched Cacnb4lh homozygotes and +/+ controls. Our results indicated that expression of beta4 subunit mRNA is globally reduced throughout the brains of Cacnb4lh homozygotes, in contrast to a small but significant global increase in the expression of beta3 subunit mRNA. There were no significant differences in expression of the other VGCC subunit mRNAs examined. Together, these findings indicate that a host of changes in VGCC subunit composition accompany reduced function of P/Q-type channels in homozygous lethargic mice.

N-type calcium channel inactivation probed by gating-current analysis

N-type calcium channels inactivate most rapidly in response to moderate, not extreme depolarization. This behavior reflects an inactivation rate that bears a U-shaped dependence on voltage. Despite this apparent similarity to calcium-dependent inactivation, N-type channel inactivation is insensitive to the identity of divalent charge carrier and, in some reports, to the level of internal buffering of divalent cations. Hence, the inactivation of N-type channels fits poorly with the "classic" profile for either voltage-dependent or calcium-dependent inactivation. To investigate this unusual inactivation behavior, we expressed recombinant N-type calcium channels in mammalian HEK 293 cells, permitting in-depth correlation of ionic current inactivation with potential alterations of gating current properties. Such correlative measurements have been particularly useful in distinguishing among various inactivation mechanisms in other voltage-gated channels. Our main results are the following: 1) The degree of gating charge immobilization was unchanged by the block of ionic current and precisely matched by the extent of ionic current inactivation. These results argue for a purely voltage-dependent mechanism of inactivation. 2) The inactivation rate was fastest at a voltage where only approximately (1)/(3) of the total gating charge had moved. This unusual experimental finding implies that inactivation occurs most rapidly from intermediate closed conformations along the activation pathway, as we demonstrate with novel analytic arguments applied to coupled-inactivation schemes. These results provide strong, complementary support for a "preferential closed-state" inactivation mechanism, recently proposed on the basis of ionic current measurements of recombinant N-type channels (Patil et al., . Neuron. 20:1027-1038).

N-type calcium channel/syntaxin/SNAP-25 complex probed by antibodies to II-III intracellular loop of the alpha1B subunit

Neuronal voltage-dependent calcium channels are integral components of cellular excitation and neurosecretion. In addition to mediating the entry of calcium across the plasma membrane, both N-type and P/Q-type voltage-dependent calcium channels have been shown to form stable complexes with synaptic vesicle and presynaptic membrane proteins, indicating a structural role for the voltage-dependent calcium channels in secretion. Recently, detailed structural analyses of N-type calcium channels have identified residues amino acids 718-963 as the site in the rat alpha1B subunit that mediates binding to syntaxin, synaptosome-associated protein of 25,000 mol. wt and synaptotagmin [Sheng et al. (1996) Nature 379, 451-454]. The purpose of this study was to employ site-directed antibodies to target domains within and outside of the interaction site on the rat alpha1B to probe potential binding sites for syntaxin/SNAP-25/synaptotagmin. Our results demonstrate that both antibodies employed in this study have access to their epitopes on the alpha1B as evidenced by equivalent immunoprecipitation of native [125I]omega-conotoxin GVIA-labeled alpha1B protein from CHAPS-solubilized preparations. The N-type voltage-dependent calcium channel immunoprecipitated by Ab CW14, the antibody directed to a domain outside of the synprint site, is associated with syntaxin and SNAP-25 with the recovery of these proteins, increasing in parallel to the recovery of alpha1B. However, when we used the antibody raised to an epitope within the synprint site (Ab CW8) to immunoprecipitate N-type calcium channels, the alpha1B was depleted of more than 65% of syntaxin and 80% of SNAP-25 when compared to the recovery of these proteins using Ab CW14. This is the first report of a defined epitope on the alpha1B subunit II-III loop (amino acids 863-875) whose perturbation by a site-directed antibody influences the dissociation of SNAP-25 and syntaxin.

A new beta subtype-specific interaction in alpha1A subunit controls P/Q-type Ca2+ channel activation

The cytoplasmic beta subunit of voltage-dependent calcium channels modulates channel properties in a subtype-specific manner and is important in channel targeting. A high affinity interaction site between the alpha1 interaction domain (AID) in the I-II cytoplasmic loop of alpha1 and the beta interaction domain (BID) of the beta subunit is highly conserved among subunit subtypes. We describe a new subtype-specific interaction (Ss1) between the amino-terminal cytoplasmic domain of alpha1A (BI-2) and the carboxyl terminus of beta4. Like the interaction identified previously () between the carboxyl termini of alpha1A and beta4 (Ss2), the affinity of this interaction is lower than AID-BID, suggesting that these are secondary interactions. Ss1 and Ss2 involve overlapping sites on beta4 and are competitive, but neither inhibits the interaction with AID. The interaction with the amino terminus of alpha1 is isoform-dependent, suggesting a role in the specificity of alpha1-beta pairing. Coexpression of beta4 in Xenopus oocytes produces a reduced hyperpolarizing shift in the I-V curve of the alpha1A channel compared with beta3 (not exhibiting this interaction). Replacing the amino terminus of alpha1A with that of alpha1C abolishes this difference. Our data contribute to our understanding of the molecular organization of calcium channels, providing a functional basis for variation in subunit composition of native P/Q-type channels.

Differential plasma membrane targeting of voltage-dependent calcium channel subunits expressed in a polarized epithelial cell line

1. Voltage-dependent calcium channels (VDCCs) show a highly non-uniform distribution in many cell types, including neurons and other polarized secretory cells. We have examined whether this can be mimicked in a polarized epithelial cell line (Madin-Darby canine kidney), which has been used extensively to study the targeting of proteins. 2. We expressed the VDCC alpha1A, alpha1B or alpha1C subunits either alone or in combination with accessory subunits alpha2-delta and the different beta subunits, and examined their localization immunocytochemically. An alpha1 subunit was only targeted to the plasma membrane if co-expressed with the accessory subunits. 3. The combination alpha1C/alpha2-delta and all beta subunits was always localized predominantly to the basolateral membrane. It has been suggested that this is equivalent to somatodendritic targeting in neurons. 4. In contrast, the alpha1B subunit was expressed at the apical membrane with all the accessory subunit combinations, by 24 h after microinjection. This membrane destination shows some parallels with axonal targeting in neurons. 5. The alpha1A subunit was consistently observed at the apical membrane in the combinations alpha1A/alpha2-delta/beta1b or beta4. In contrast, when co-expressed with alpha2-delta/beta2a, alpha1A was clearly targeted to the basolateral membrane. 6. In conclusion, the VDCC alpha1 subunit appears to be the primary determinant for targeting the VDCC complex, but the beta subunit can modify this destination, particularly for alpha1A.

Modelling of a voltage-dependent Ca2+ channel beta subunit as a basis for understanding its functional properties

Structure prediction methods have been used to establish a domain structure for the voltage-dependent calcium channel beta subunit, beta1b. One domain was identified from homology searches as an SH3 domain, whilst another was shown, using threading algorithms, to be similar to yeast guanylate kinase. This domain structure suggested relatedness to the membrane-associated guanylate kinase protein family, and that the N-terminal domain of the beta subunit might be similar to a PDZ domain. Three-dimensional model structures have been constructed for these three domains. The extents of the domains are consistent with functional properties and mutational assays of the subunit, and provide a basis for understanding its modulatory function.

Complexes of the alpha1C and beta subunits generate the necessary signal for membrane targeting of class C L-type calcium channels

In the present study, we investigated the role of channel subunits in the membrane targeting of voltage-dependent L-type calcium channel complexes. We co-expressed the calcium channel pore-forming alpha1C subunit with different accessory beta subunits in HEK-tsA201 cells and examined the subcellular localization of the channel subunits by immunohistochemistry using confocal microscopy and whole-cell radioligand binding studies. While the pore-forming alpha1C subunit exhibited perinuclear staining when expressed alone, and several of the wild-type and mutant beta subunits also exhibited intracellular staining, co-expression of the alpha1C subunit with either the wild-type beta2a subunit, a palmitoylation-deficient beta2a(C3S/C4S) mutant or three other nonpalmitoylated beta isoforms (beta1b, beta3, and beta4 subunits) resulted in the redistribution of both the alpha1C and beta subunits into clusters along the cell surface. Furthermore, the redistribution of calcium channel complexes to the plasma membrane was observed when alpha1C was co-expressed with an N- and C-terminal truncated mutant beta2a containing only the central conserved regions. However, when the alpha1C subunit was co-expressed with an alpha1 beta interaction-deficient mutant, beta2aBID-, we did not observe formation of the channels at the plasma membrane. In addition, an Src homology 3 motif mutant of beta2a that was unable to interact with the alpha1C subunit also failed to target channel complexes to the plasma membrane. Interestingly, co-expression of the pore-forming alpha1C subunit with the largely peripheral accessory alpha2 delta subunit was ineffective in recruiting alpha1C to the plasma membrane, while co-distribution of all three subunits was observed when beta2a was co-expressed with the alpha1C and alpha2 delta subunits. Taken together, our results suggested that the signal necessary for correct plasma membrane targeting of the class C L-type calcium channel complexes is generated as a result of a functional interaction between the alpha1 and beta subunits.

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.

Membrane targeting of L-type calcium channels. Role of palmitoylation in the subcellular localization of the beta2a subunit

In this study, we report that palmitoylation was a critical determinant of the subcellular localization of the rat beta2a subunit of voltage-dependent calcium channels. Immunohistochemical staining of transfected cells revealed that a palmitoylation-deficient beta2a subunit exhibited a diffuse intracellular staining pattern, in contrast to the plasma membrane distribution seen with the wild-type beta2a subunit. Unexpectedly, mutations in regions distal to the palmitoylation sites at Cys3 and Cys4 affected palmitoylation of the beta2a protein. Mutations in an src homology 3 motif of the beta2a subunit affected both palmitoylation and subcellular localization of the beta2a protein. A mutation in the beta interaction domain, which disrupted interactions between the expressed alpha1 and beta subunits, also resulted in a decreased palmitoylation and diffuse intracellular localization of the beta2a protein. Studies of chimeric proteins revealed that the 16-amino acid N terminus of the beta2a subunit was sufficient to confer palmitoylation to the nonpalmitoylated beta1b and beta3 isoforms. However, palmitoylation of chimeric beta subunits was by itself insufficient to restore the plasma membrane localization observed with the wild-type beta2a protein. Treatment of transfected cells with brefeldin A increased the amount of palmitic acid incorporated in the beta2a protein, suggesting that palmitoylation of beta2a occurs during or shortly after protein synthesis. Two other beta2 variants, the rabbit beta2a and beta2b, which lack the palmitoylation sties at Cys3 and Cys4, exhibited a diffuse intracellular staining pattern and were not palmitoylated.

Altered expression and assembly of N-type calcium channel alpha1B and beta subunits in epileptic lethargic (lh/lh) mouse

Voltage-dependent calcium channels (VDCC) are multisubunit complexes whose expression and targeting require the assembly of the pore-forming alpha1 with auxiliary beta and alpha2/delta subunits. The developmentally regulated expression and differential assembly of beta isoforms with the alpha1B subunit to form N-type VDCC suggested a unique role for the beta4 isoform in VDCC maturation (Vance, C. L., Begg, C. M., Lee, W.-L., Haase, H., Copeland, T. D., and McEnery, M. W. (1998) J. Biol. Chem. 273, 14495-14502). The focus of this study is the expression and assembly of alpha1B and beta isoforms in the epileptic mouse, lethargic (lh/lh), a mutant anticipated to produce a truncated beta4 subunit (Burgess, D. L., Jones, J. M., Meisler, M. H., and Noebels, J. L. (1997) Cell 88, 385-392). In this report, we demonstrate that neither full-length nor truncated beta4 protein is expressed in lh/lh mice. The absence of beta4 in lh/lh mice is associated with decreased expression of N-type VDCC in forebrain and cerebellum. The most surprising characteristic of the lh/lh mouse is increased expression of beta1b protein. This result suggests a previously unidentified cellular mechanism wherein expression of the total pool of available beta subunits is under tight metabolic regulation. As a consequence of increased beta1b expression, the beta1b is increased in its incorporation into alpha1B/beta complexes relative to wild type. Thus, in striking similarity to the population of N-type VDCC present in immature rat brain, the population of N-type VDCC present in adult lh/lh mice is characterized by the absence of beta4 with increased beta1b expression and assembly into N-type VDCC. It is intriguing to speculate that the increased excitability and susceptibility to seizures observed in the lh/lh mouse arises from the inappropriate expression of an immature population of N-type VDCC throughout neuronal development.