The Interplay Between Splicing of Two Exon Combinations Differentially Affects Membrane Targeting and Function of Human CaV2.2.

N-type calcium channels (CaV2.2) are predominantly localized in presynaptic terminals, and are particularly important for pain transmission in the spinal cord. Furthermore, they have multiple isoforms, conferred by alternatively spliced or cassette exons, which are differentially expressed. Here, we have examined alternatively spliced exon47 variants that encode a long or short C-terminus in human CaV2.2. In the Ensembl database, all short exon47-containing transcripts were associated with the absence of exon18a, therefore, we also examined the effect of inclusion or absence of exon18a, combinatorially with the exon47 splice variants. We found that long exon47, only in the additional presence of exon18a, results in CaV2.2 currents that have a 3.6-fold greater maximum conductance than the other three combinations. In contrast, cell-surface expression of CaV2.2 in both tsA-201 cells and hippocampal neurons is increased ∼4-fold by long exon47, relative to short exon47, in either the presence or the absence of exon18a. This surprising discrepancy between trafficking and function indicates that cell-surface expression is enhanced by long exon47, independently of exon18a. However, in the presence of long exon47, exon18a mediates an additional permissive effect on CaV2.2 gating. We also investigated the single-nucleotide polymorphism in exon47 that has been linked to schizophrenia and Parkinson's disease, which we found is only non-synonymous in the short exon47 C-terminal isoform, resulting in two minor alleles. This study highlights the importance of investigating the combinatorial effects of exon inclusion, rather than each in isolation, in order to increase our understanding of calcium channel function.

T-type Ca2+ and persistent Na+ currents synergistically elevate ventral, not dorsal, entorhinal cortical stellate cell excitability.

Dorsal and ventral medial entorhinal cortex (mEC) regions have distinct neural network firing patterns to differentially support functions such as spatial memory. Accordingly, mEC layer II dorsal stellate neurons are less excitable than ventral neurons. This is partly because the densities of inhibitory conductances are higher in dorsal than ventral neurons. Here, we report that T-type Ca2+ currents increase 3-fold along the dorsal-ventral axis in mEC layer II stellate neurons, with twice as much CaV3.2 mRNA in ventral mEC compared with dorsal mEC. Long depolarizing stimuli trigger T-type Ca2+ currents, which interact with persistent Na+ currents to elevate the membrane voltage and spike firing in ventral, not dorsal, neurons. T-type Ca2+ currents themselves prolong excitatory postsynaptic potentials (EPSPs) to enhance their summation and spike coupling in ventral neurons only. These findings indicate that T-type Ca2+ currents critically influence the dorsal-ventral mEC stellate neuron excitability gradient and, thereby, mEC dorsal-ventral circuit activity.

Biallelic CACNA2D1 loss-of-function variants cause early-onset developmental epileptic encephalopathy.

Voltage-gated calcium (CaV) channels form three subfamilies (CaV1-3). The CaV1 and CaV2 channels are heteromeric, consisting of an α1 pore-forming subunit, associated with auxiliary CaVß and α2δ subunits. The α2δ subunits are encoded in mammals by four genes, CACNA2D1-4. They play important roles in trafficking and function of the CaV channel complexes. Here we report biallelic variants in CACNA2D1, encoding the α2δ-1 protein, in two unrelated individuals showing a developmental and epileptic encephalopathy. Patient 1 has a homozygous frameshift variant c.818_821dup/p.(Ser275Asnfs*13) resulting in nonsense-mediated mRNA decay of the CACNA2D1 transcripts, and absence of α2δ-1 protein detected in patient-derived fibroblasts. Patient 2 is compound heterozygous for an early frameshift variant c.13_23dup/p.(Leu9Alafs*5), highly probably representing a null allele and a missense variant c.626G>A/p.(Gly209Asp). Our functional studies show that this amino-acid change severely impairs the function of α2δ-1 as a calcium channel subunit, with strongly reduced trafficking of α2δ-1G209D to the cell surface and a complete inability of α2δ-1G209D to increase the trafficking and function of CaV2 channels. Thus, biallelic loss-of-function variants in CACNA2D1 underlie the severe neurodevelopmental disorder in these two patients. Our results demonstrate the critical importance and non-interchangeability of α2δ-1 and other α2δ proteins for normal human neuronal development.

Disruption of the Key Ca2+ Binding Site in the Selectivity Filter of Neuronal Voltage-Gated Calcium Channels Inhibits Channel Trafficking.

Voltage-gated calcium channels are exquisitely Ca2+ selective, conferred primarily by four conserved pore-loop glutamate residues contributing to the selectivity filter. There has been little previous work directly measuring whether the trafficking of calcium channels requires their ability to bind Ca2+ in the selectivity filter or to conduct Ca2+. Here, we examine trafficking of neuronal CaV2.1 and 2.2 channels with mutations in their selectivity filter and find reduced trafficking to the cell surface in cell lines. Furthermore, in hippocampal neurons, there is reduced trafficking to the somatic plasma membrane, into neurites, and to presynaptic terminals. However, the CaV2.2 selectivity filter mutants are still influenced by auxiliary α2δ subunits and, albeit to a reduced extent, by ß subunits, indicating the channels are not grossly misfolded. Our results indicate that Ca2+ binding in the pore of CaV2 channels may promote their correct trafficking, in combination with auxiliary subunits. Furthermore, physiological studies utilizing selectivity filter mutant CaV channels should be interpreted with caution.

Ablation of α2δ-1 inhibits cell-surface trafficking of endogenous N-type calcium channels in the pain pathway in vivo.

The auxiliary α2δ calcium channel subunits play key roles in voltage-gated calcium channel function. Independent of this, α2δ-1 has also been suggested to be important for synaptogenesis. Using an epitope-tagged knockin mouse strategy, we examined the effect of α2δ-1 on CaV2.2 localization in the pain pathway in vivo, where CaV2.2 is important for nociceptive transmission and α2δ-1 plays a critical role in neuropathic pain. We find CaV2.2 is preferentially expressed on the plasma membrane of calcitonin gene-related peptide-positive small nociceptors. This is paralleled by strong presynaptic expression of CaV2.2 in the superficial spinal cord dorsal horn. EM-immunogold localization shows CaV2.2 predominantly in active zones of glomerular primary afferent terminals. Genetic ablation of α2δ-1 abolishes CaV2.2 cell-surface expression in dorsal root ganglion neurons and dramatically reduces dorsal horn expression. There was no effect of α2δ-1 knockout on other dorsal horn pre- and postsynaptic markers, indicating the primary afferent pathways are not otherwise affected by α2δ-1 ablation.

The α2δ-like Protein Cachd1 Increases N-type Calcium Currents and Cell Surface Expression and Competes with α2δ-1.

Voltage-gated calcium channel auxiliary α2δ subunits are important for channel trafficking and function. Here, we compare the effects of α2δ-1 and an α2δ-like protein called Cachd1 on neuronal N-type (CaV2.2) channels, which are important in neurotransmission. Previous structural studies show the α2δ-1 VWA domain interacting with the first loop in CaV1.1 domain-I via its metal ion-dependent adhesion site (MIDAS) motif and additional Cache domain interactions. Cachd1 has a disrupted MIDAS motif. However, Cachd1 increases CaV2.2 currents substantially (although less than α2δ-1) and increases CaV2.2 cell surface expression by reducing endocytosis. Although the effects of α2δ-1 are abolished by mutation of Asp122 in CaV2.2 domain-I, which mediates interaction with its VWA domain, the Cachd1 responses are unaffected. Furthermore, Cachd1 co-immunoprecipitates with CaV2.2 and inhibits co-immunoprecipitation of α2δ-1 by CaV2.2. Cachd1 also competes with α2δ-1 for effects on trafficking. Thus, Cachd1 influences both CaV2.2 trafficking and function and can inhibit responses to α2δ-1.

Proteolytic maturation of α2δ represents a checkpoint for activation and neuronal trafficking of latent calcium channels.

The auxiliary α2δ subunits of voltage-gated calcium channels are extracellular membrane-associated proteins, which are post-translationally cleaved into disulfide-linked polypeptides α2 and δ. We now show, using α2δ constructs containing artificial cleavage sites, that this processing is an essential step permitting voltage-dependent activation of plasma membrane N-type (CaV2.2) calcium channels. Indeed, uncleaved α2δ inhibits native calcium currents in mammalian neurons. By inducing acute cell-surface proteolytic cleavage of α2δ, voltage-dependent activation of channels is promoted, independent from the trafficking role of α2δ. Uncleaved α2δ does not support trafficking of CaV2.2 channel complexes into neuronal processes, and inhibits Ca2+ entry into synaptic boutons, and we can reverse this by controlled intracellular proteolytic cleavage. We propose a model whereby uncleaved α2δ subunits maintain immature calcium channels in an inhibited state. Proteolytic processing of α2δ then permits voltage-dependent activation of the channels, acting as a checkpoint allowing trafficking only of mature calcium channel complexes into neuronal processes.

A CaV2.1 N-terminal fragment relieves the dominant-negative inhibition by an Episodic ataxia 2 mutant.

Episodic ataxia 2 (EA2) is an autosomal dominant disorder caused by mutations in the gene CACNA1A that encodes the pore-forming CaV2.1 calcium channel subunit. The majority of EA2 mutations reported so far are nonsense or deletion/insertion mutations predicted to form truncated proteins. Heterologous expression of wild-type CaV2.1, together with truncated constructs that mimic EA2 mutants, significantly suppressed wild-type calcium channel function, indicating that the truncated protein produces a dominant-negative effect (Jouvenceau et al., 2001; Page et al., 2004). A similar finding has been shown for CaV2.2 (Raghib et al., 2001). We show here that a highly conserved sequence in the cytoplasmic N-terminus is involved in this process, for both CaV2.1 and CaV2.2 channels. Additionally, we were able to interfere with the suppressive effect of an EA2 construct by mutating key N-terminal residues within it. We postulate that the N-terminus of the truncated channel plays an essential part in its interaction with the full-length CaV2.1, which prevents the correct folding of the wild-type channel. In agreement with this, we were able to disrupt the interaction between EA2 and the full length channel by co-expressing a free N-terminal peptide.

Genetic disruption of voltage-gated calcium channels in psychiatric and neurological disorders.

This review summarises genetic studies in which calcium channel genes have been connected to the spectrum of neuropsychiatric syndromes, from bipolar disorder and schizophrenia to autism spectrum disorders and intellectual impairment. Among many other genes, striking numbers of the calcium channel gene superfamily have been implicated in the aetiology of these diseases by various DNA analysis techniques. We will discuss how these relate to the known monogenic disorders associated with point mutations in calcium channels. We will then examine the functional evidence for a causative link between these mutations or single nucleotide polymorphisms and the disease processes. A major challenge for the future will be to translate the expanding psychiatric genetic findings into altered physiological function, involvement in the wider pathology of the diseases, and what potential that provides for personalised and stratified treatment options for patients.

Functional exofacially tagged N-type calcium channels elucidate the interaction with auxiliary α2δ-1 subunits.

CaV1 and CaV2 voltage-gated calcium channels are associated with ß and α2δ accessory subunits. However, examination of cell surface-associated CaV2 channels has been hampered by the lack of antibodies to cell surface-accessible epitopes and of functional exofacially tagged CaV2 channels. Here we report the development of fully functional CaV2.2 constructs containing inserted surface-accessible exofacial tags, which allow visualization of only those channels at the plasma membrane, in both a neuronal cell line and neurons. We first examined the effect of the auxiliary subunits. Although α2δ subunits copurify with CaV2 channels, it has recently been suggested that this interaction is easily disrupted and nonquantitative. We have now tested whether α2δ subunits are associated with these channels at the cell surface. We found that, whereas α2δ-1 is readily observed at the plasma membrane when expressed alone, it appears absent when coexpressed with CaV2.2/ß1b, despite our finding that α2δ-1 increases plasma-membrane CaV2.2 expression. However, this was due to occlusion of the antigenic epitope by association with CaV2.2, as revealed by antigen retrieval; thus, our data provide evidence for a tight interaction between α2δ-1 and the α1 subunit at the plasma membrane. We further show that, although CaV2.2 cell-surface expression is reduced by gabapentin in the presence of wild-type α2δ-1 (but not a gabapentin-insensitive α2δ-1 mutant), the interaction between CaV2.2 and α2δ-1 is not disrupted by gabapentin. Altogether, these results demonstrate that CaV2.2 and α2δ-1 are intimately associated at the plasma membrane and allow us to infer a region of interaction.

Differential upregulation in DRG neurons of an α2δ-1 splice variant with a lower affinity for gabapentin after peripheral sensory nerve injury.

The α2δ-1 protein is an auxiliary subunit of voltage-gated calcium channels, critical for neurotransmitter release. It is upregulated in dorsal root ganglion (DRG) neurons following sensory nerve injury, and is also the therapeutic target of the gabapentinoid drugs, which are efficacious in both experimental and human neuropathic pain conditions. α2δ-1 has 3 spliced regions: A, B, and C. A and C are cassette exons, whereas B is introduced via an alternative 3' splice acceptor site. Here we have examined the presence of α2δ-1 splice variants in DRG neurons, and have found that although the main α2δ-1 splice variant in DRG is the same as that in brain (α2δ-1 ΔA+B+C), there is also another α2δ-1 splice variant (ΔA+BΔC), which is expressed in DRG neurons and is differentially upregulated compared to the main DRG splice variant α2δ-1 ΔA+B+C following spinal nerve ligation. Furthermore, this differential upregulation occurs preferentially in a small nonmyelinated DRG neuron fraction, obtained by density gradient separation. The α2δ-1 ΔA+BΔC splice variant supports CaV2 calcium currents with unaltered properties compared to α2δ-1 ΔA+B+C, but shows a significantly reduced affinity for gabapentin. This variant could therefore play a role in determining the efficacy of gabapentin in neuropathic pain.

Calcium currents are enhanced by α2δ-1 lacking its membrane anchor.

The accessory α(2)δ subunits of voltage-gated calcium channels are membrane-anchored proteins, which are highly glycosylated, possess multiple disulfide bonds, and are post-translationally cleaved into α(2) and δ. All α(2)δ subunits have a C-terminal hydrophobic, potentially trans-membrane domain and were described as type I transmembrane proteins, but we found evidence that they can be glycosylphosphatidylinositol-anchored. To probe further the function of membrane anchoring in α(2)δ subunits, we have now examined the properties of α(2)δ-1 constructs truncated at their putative glycosylphosphatidylinositol anchor site, located before the C-terminal hydrophobic domain (α(2)δ-1ΔC-term). We find that the majority of α(2)δ-1ΔC-term is soluble and secreted into the medium, but unexpectedly, some of the protein remains associated with detergent-resistant membranes, also termed lipid rafts, and is extrinsically bound to the plasma membrane. Furthermore, heterologous co-expression of α(2)δ-1ΔC-term with Ca(V)2.1/ß1b results in a substantial enhancement of the calcium channel currents, albeit less than that produced by wild-type α(2)δ-1. These results call into question the role of membrane anchoring of α(2)δ subunits for calcium current enhancement.

The alpha2delta subunits of voltage-gated calcium channels form GPI-anchored proteins, a posttranslational modification essential for function.

Voltage-gated calcium channels are thought to exist in the plasma membrane as heteromeric proteins, in which the alpha1 subunit is associated with two auxiliary subunits, the intracellular beta subunit and the alpha(2)delta subunit; both of these subunits influence the trafficking and properties of Ca(V)1 and Ca(V)2 channels. The alpha(2)delta subunits have been described as type I transmembrane proteins, because they have an N-terminal signal peptide and a C-terminal hydrophobic and potentially transmembrane region. However, because they have very short C-terminal cytoplasmic domains, we hypothesized that the alpha(2)delta proteins might be associated with the plasma membrane through a glycosylphosphatidylinositol (GPI) anchor attached to delta rather than a transmembrane domain. Here, we provide biochemical, immunocytochemical, and mutational evidence to show that all of the alpha(2)delta subunits studied, alpha(2)delta-1, alpha(2)delta-2, and alpha(2)delta-3, show all of the properties expected of GPI-anchored proteins, both when heterologously expressed and in native tissues. They are substrates for prokaryotic phosphatidylinositol-phospholipase C (PI-PLC) and trypanosomal GPI-PLC, which release the alpha(2)delta proteins from membranes and intact cells and expose a cross-reacting determinant epitope. PI-PLC does not affect control transmembrane or membrane-associated proteins. Furthermore, mutation of the predicted GPI-anchor sites markedly reduced plasma membrane and detergent-resistant membrane localization of alpha(2)delta subunits. We also show that GPI anchoring of alpha(2)delta subunits is necessary for their function to enhance calcium currents, and PI-PLC treatment only reduces calcium current density when alpha(2)delta subunits are coexpressed. In conclusion, this study redefines our understanding of alpha(2)delta subunits, both in terms of their role in calcium-channel function and other roles in synaptogenesis.

N terminus is key to the dominant negative suppression of Ca(V)2 calcium channels: implications for episodic ataxia type 2.

Expression of the calcium channels Ca(V)2.1 and Ca(V)2.2 is markedly suppressed by co-expression with truncated constructs containing Domain I. This is the basis for the phenomenon of dominant negative suppression observed for many of the episodic ataxia type 2 mutations in Ca(V)2.1 that predict truncated channels. The process of dominant negative suppression has been shown previously to stem from interaction between the full-length and truncated channels and to result in downstream consequences of the unfolded protein response and endoplasmic reticulum-associated protein degradation. We have now identified the specific domain that triggers this effect. For both Ca(V)2.1 and Ca(V)2.2, the minimum construct producing suppression was the cytoplasmic N terminus. Suppression was enhanced by tethering the N terminus to the membrane with a CAAX motif. The 11-amino acid motif (including Arg(52) and Arg(54)) within the N terminus, which we have previously shown to be required for G protein modulation, is also essential for dominant negative suppression. Suppression is prevented by addition of an N-terminal tag (XFP) to the full-length and truncated constructs. We further show that suppression of Ca(V)2.2 currents by the N terminus-CAAX construct is accompanied by a reduction in Ca(V)2.2 protein level, and this is also prevented by mutation of Arg(52) and Arg(54) to Ala in the truncated construct. Taken together, our evidence indicates that both the extreme N terminus and the Arg(52), Arg(54) motif are involved in the processes underlying dominant negative suppression.

Determinants of the voltage dependence of G protein modulation within calcium channel beta subunits.

CaVbeta subunits of voltage-gated calcium channels contain two conserved domains, a src-homology-3 (SH3) domain and a guanylate kinase-like (GK) domain with an intervening HOOK domain. We have shown in a previous study that, although Gbetagamma-mediated inhibitory modulation of CaV2.2 channels did not require the interaction of a CaVbeta subunit with the CaValpha1 subunit, when such interaction was prevented by a mutation in the alpha1 subunit, G protein modulation could not be removed by a large depolarization and showed voltage-independent properties (Leroy et al., J Neurosci 25:6984-6996, 2005). In this study, we have investigated the ability of mutant and truncated CaVbeta subunits to support voltage-dependent G protein modulation in order to determine the minimal domain of the CaVbeta subunit that is required for this process. We have coexpressed the CaVbeta subunit constructs with CaV2.2 and alpha2delta-2, studied modulation by the activation of the dopamine D2 receptor, and also examined basal tonic modulation. Our main finding is that the CaVbeta subunit GK domains, from either beta1b or beta2, are sufficient to restore voltage dependence to G protein modulation. We also found that the removal of the variable HOOK region from beta2a promotes tonic voltage-dependent G protein modulation. We propose that the absence of the HOOK region enhances Gbetagamma binding affinity, leading to greater tonic modulation by basal levels of Gbetagamma. This tonic modulation requires the presence of an SH3 domain, as tonic modulation is not supported by any of the CaVbeta subunit GK domains alone.

The stargazin-related protein gamma 7 interacts with the mRNA-binding protein heterogeneous nuclear ribonucleoprotein A2 and regulates the stability of specific mRNAs, including CaV2.2.

The role(s) of the novel stargazin-like gamma-subunit proteins remain controversial. We have shown previously that the neuron-specific gamma7 suppresses the expression of certain calcium channels, particularly Ca(V)2.2, and is therefore unlikely to operate as a calcium channel subunit. We now show that the effect of gamma7 on Ca(V)2.2 expression is via an increase in the degradation rate of Ca(V)2.2 mRNA and hence a reduction of Ca(V)2.2 protein level. Furthermore, exogenous expression of gamma7 in PC12 cells also decreased the endogenous Ca(V)2.2 mRNA level. Conversely, knockdown of endogenous gamma7 with short-hairpin RNAs produced a reciprocal enhancement of Ca(V)2.2 mRNA stability and an increase in endogenous calcium currents in PC12 cells. Moreover, both endogenous and expressed gamma7 are present on intracellular membranes, rather than the plasma membrane. The cytoplasmic C terminus of gamma7 is essential for all its effects, and we show that gamma7 binds directly via its C terminus to a heterogeneous nuclear ribonucleoprotein (hnRNP A2), which also binds to a motif in Ca(V)2.2 mRNA, and is associated with native Ca(V)2.2 mRNA in PC12 cells. The expression of hnRNP A2 enhances Ca(V)2.2 I(Ba), and this enhancement is prevented by a concentration of gamma7 that alone has no effect on I(Ba). The effect of gamma7 is selective for certain mRNAs because it had no effect on alpha2delta-2 mRNA stability, but it decreased the mRNA stability for the potassium-chloride cotransporter, KCC1, which contains a similar hnRNP A2 binding motif to that in Ca(V)2.2 mRNA. Our results indicate that gamma7 plays a role in stabilizing Ca(V)2.2 mRNA.

The HOOK-domain between the SH3 and the GK domains of Cavbeta subunits contains key determinants controlling calcium channel inactivation.

Ca(v)beta subunits of voltage-gated calcium channels contain two conserved domains, a src-homology-3 (SH3)-domain and a guanylate kinase-like (GK)-domain. The SH3-domain is split, with its final (fifth) beta-strand separated from the rest of the domain by an intervening sequence termed the HOOK-domain, whose sequence varies between Ca(v)beta subunits. Here we have been guided by the recent structural studies of Ca(v)beta subunits in the design of specific truncated constructs, with the goal of investigating the role of the HOOK-domain of Ca(v)beta subunits in the modulation of inactivation of N-type calcium channels. We have coexpressed the beta subunit constructs with Ca(v)2.2 and alpha(2)delta-2, using the N-terminally palmitoylated beta(2a) subunit, because it supports very little voltage-dependent inactivation, and made comparisons with beta(1b) domains. Deletion of the variable region of the beta(2a) HOOK-domain resulted in currents with a rapidly inactivating component, and additional mutation of the beta(2a) palmitoylation motif further enhanced inactivation. The isolated GK-domain of beta(2a) alone enhanced current amplitude, but the currents were rapidly and completely inactivating. When the beta(2a)-GK-domain construct was extended proximally, by including the HOOK-domain and the epsilon-strand of the SH3-domain, inactivation was about four-fold slower than in the absence of the HOOK domain. When the SH3-domain of beta(2a) truncated prior to the HOOK-domain was coexpressed with the (HOOK+epsilonSH3+GK)-domain of beta(2a), all the properties of beta(2a) were restored, in terms of loss of inactivation. Furthermore, removal of the HOOK sequence from the (HOOK+epsilonSH3+GK)-beta(2a) construct increased inactivation. Together, these results provide evidence that the HOOK domain is an important determinant of inactivation.

The calcium channel alpha2delta-2 subunit partitions with CaV2.1 into lipid rafts in cerebellum: implications for localization and function.

The accessory alpha2delta subunits of voltage-gated calcium channels are highly glycosylated transmembrane proteins that interact with calcium channel alpha1 subunits to enhance calcium currents. We compared the membrane localization and processing of native cerebellar alpha2delta-2 subunits with alpha2delta-2 stably expressed in tsA-201 cells. We identified that alpha2delta-2 is completely concentrated in cholesterol-rich microdomains (lipid rafts) in cerebellum, in which it substantially colocalizes with the calcium channel alpha1 subunit CaV2.1, although CaV2.1 is also present in the Triton X-100-soluble fraction. In tsA-201 cells, unlike cerebellum, alpha2delta-2 is not completely proteolytically processed into alpha2-2 and delta-2. However, this processing is more complete in the lipid raft fraction of tsA-201 cells, in which alpha2delta-2 also colocalizes with CaV2.1. Cholesterol depletion of intact cells disrupted their lipid rafts and enhanced CaV2.1/alpha2delta-2/beta4 currents. Furthermore, alpha2delta-2 coimmunoprecipitates with lipid raft-associated proteins of the stomatin family. The apparent affinity of alpha2delta-2 for its ligand gabapentin is increased markedly in the cholesterol-rich microdomain fractions, in both cerebellum and the stable alpha2delta-2 cell line. In contrast, alpha2delta-2 containing a point mutation (R282A) has a much lower affinity for gabapentin, and this is not enhanced in the lipid raft fraction. This R282A mutant alpha2delta-2 shows reduced functionality in terms of enhancement of CaV2.1/beta4 calcium currents, suggesting that the integrity of the gabapentin binding site may be important for normal functioning of alpha2delta-2. Together, these results indicate that both alpha2delta-2 and CaV2.1 are normally associated with cholesterol-rich microdomains, and this influences their functionality.

The importance of occupancy rather than affinity of CaV(beta) subunits for the calcium channel I-II linker in relation to calcium channel function.

The Ca(V)beta subunits of voltage-gated calcium channels regulate the trafficking and biophysical properties of these channels. We have taken advantage of mutations in the tyrosine residue within the alpha interaction domain (AID) in the I-II linker of Ca(V)2.2 which reduce, but do not abolish, the binding of beta1b to the AID of Ca(V)2.2. We have found that the mutation Y388S decreased the affinity of Ca(V)beta1b binding to the Ca(V)2.2 I-II linker from 14 to 329 nm. However, the Y388S mutation had no effect on current density and cell surface expression of Ca(V)2.2/alpha2delta-2/beta1b channels expressed in human embryonic kidney tsA-201 cells, when equivalent proportions of cDNA were used. Furthermore, despite the 24-fold reduced affinity of Ca(V)beta1b for the Y388S I-II linker of Ca(V)2.2, all the key features of modulation as well as trafficking by Ca(V)beta subunits remained intact. This is in contrast to the much more marked effect of the W391A mutation, which abolished interaction with the Ca(V)2.2 I-II linker, and very markedly affected the trafficking of the channels. However, using the Xenopus oocyte expression system, where expression levels can be accurately titrated, when Ca(V)beta1b cDNA was diluted 50-fold, all evidence of interaction with Ca(V)2.2 Y388S was lost, although wild-type Ca(V)2.2 was still normally modulated by the reduced concentration of beta1b. These results indicate that high affinity interaction with the alpha1 subunit is not necessary for any of the modulatory effects of Ca(V)beta subunits, but occupancy of the interaction site is important, and this will occur, despite the reduced affinity, if the Ca(V)beta subunit is present in sufficient excess.

Interaction via a key tryptophan in the I-II linker of N-type calcium channels is required for beta1 but not for palmitoylated beta2, implicating an additional binding site in the regulation of channel voltage-dependent properties.

The CaVbeta subunits of voltage-gated calcium channels regulate these channels in several ways. Here we investigate the role of these auxiliary subunits in the expression of functional N-type channels at the plasma membrane and in the modulation by G-protein-coupled receptors of this neuronal channel. To do so, we mutated tryptophan 391 to an alanine within the alpha-interacting domain (AID) in the I-II linker of CaV2.2. We showed that the mutation W391 virtually abolishes the binding of CaVbeta1b and CaVbeta2a to the CaV2.2 I-II linker and strongly reduced current density and cell surface expression of both CaV2.2/alpha2delta-2/beta1b and/beta2a channels. When associated with CaVbeta1b, the W391A mutation also prevented the CaVbeta1b-mediated hyperpolarization of CaV2.2 channel activation and steady-state inactivation. However, the mutated CaV2.2W391A/beta1b channels were still inhibited to a similar extent by activation of the D2 dopamine receptor with the agonist quinpirole. Nevertheless, key hallmarks of G-protein modulation of N-type currents, such as slowed activation kinetics and prepulse facilitation, were not observed for the mutated channel. In contrast, CaVbeta2a was still able to completely modulate the biophysical properties of CaV2.2W391A channel and allow voltage-dependent G-protein modulation of CaV2.2W391A. Additional data suggest that the concentration of CaVbeta2a in the proximity of the channel is enhanced independently of its binding to the AID by its palmitoylation. This is essentially sufficient for all of the functional effects of CaVbeta2a, which may occur via a second lower-affinity binding site, except trafficking the channel to the plasma membrane, which requires interaction with the AID region.