Negatively charged residues in the first extracellular loop of the L-type CaV1.2 channel anchor the interaction with the CaVα2δ1 auxiliary subunit

Investigating the Impact of Electrostatic Interactions on Calmodulin Binding and Ca2+-Dependent Activation of the Calcium-Gated Potassium SK4 Channel

Ca2+ binding to the ubiquitous Ca2+ sensing protein calmodulin (CaM) activates the intermediate conductance Ca2+-activated SK4 channel. Potential hydrophilic pockets for CaM binding have been identified at the intracellular HA and HB helices in the C-terminal of SK4 from the three published cryo-EM structures of SK4. Single charge reversal substitutions at either site, significantly weakened the pull-down of SK4 by CaM wild-type (CaM), and decreased the TRAM-34 sensitive outward K+ current densities in native HEK293T cells when compared with SK4 WT measured under the same conditions. Only the doubly substituted SK4 R352D/R355D (HB helix) obliterated the CaM-mediated pull-down and thwarted outward K+ currents. However, overexpression of CaM E84K/E87K, which had been predicted to face the arginine doublet, restored the CaM-mediated pull-down of SK4 R352D/R355D and normalized its whole-cell current density. Virtual analysis of the putative salt bridges supports a unique role for the positively charged arginine doublet at the HB helix into anchoring the interaction with the negatively charged CaM glutamate 84 and 87 CaM. Our findings underscore the unique contribution of electrostatic interactions in carrying CaM binding onto SK4 and support the role of the C-terminal HB helix to the Ca2+-dependent gating process.

EMC chaperone-CaV structure reveals an ion channel assembly intermediate

Voltage-gated ion channels (VGICs) comprise multiple structural units, the assembly of which is required for function1,2. Structural understanding of how VGIC subunits assemble and whether chaperone proteins are required is lacking. High-voltage-activated calcium channels (CaVs)3,4 are paradigmatic multisubunit VGICs whose function and trafficking are powerfully shaped by interactions between pore-forming CaV1 or CaV2 CaVα1 (ref. 3), and the auxiliary CaVβ5 and CaVα2δ subunits6,7. Here we present cryo-electron microscopy structures of human brain and cardiac CaV1.2 bound with CaVβ3 to a chaperone-the endoplasmic reticulum membrane protein complex (EMC)8,9-and of the assembled CaV1.2-CaVβ3-CaVα2δ-1 channel. These structures provide a view of an EMC-client complex and define EMC sites-the transmembrane (TM) and cytoplasmic (Cyto) docks; interaction between these sites and the client channel causes partial extraction of a pore subunit and splays open the CaVα2δ-interaction site. The structures identify the CaVα2δ-binding site for gabapentinoid anti-pain and anti-anxiety drugs6, show that EMC and CaVα2δ interactions with the channel are mutually exclusive, and indicate that EMC-to-CaVα2δ hand-off involves a divalent ion-dependent step and CaV1.2 element ordering. Disruption of the EMC-CaV complex compromises CaV function, suggesting that the EMC functions as a channel holdase that facilitates channel assembly. Together, the structures reveal a CaV assembly intermediate and EMC client-binding sites that could have wide-ranging implications for the biogenesis of VGICs and other membrane proteins.

A CACNA1C variant associated with cardiac arrhythmias provides mechanistic insights in the calmodulation of L-type Ca2+ channels

We recently reported the identification of a de novo single nucleotide variant in exon 9 of CACNA1C associated with prolonged repolarization interval. Recombinant expression of the glycine to arginine variant at position 419 produced a gain in the function of the L-type Ca_V1.2 channel with increased peak current density and activation gating but without significant decrease in the inactivation kinetics. We herein reveal that these properties are replicated by overexpressing calmodulin (CaM) with Ca_V1.2 WT and are reversed by exposure to the CaM antagonist W-13. Phosphomimetic (T79D or S81D), but not phosphoresistant (T79A or S81A), CaM surrogates reproduced the impact of CaM WT on the function of Ca_V1.2 WT. The increased channel activity of Ca_V1.2 WT following overexpression of CaM was found to arise in part from enhanced cell surface expression. In contrast, the properties of the variant remained unaffected by any of these treatments. Ca_V1.2 substituted with the α-helix breaking proline residue were more reluctant to open than Ca_V1.2 WT but were upregulated by phosphomimetic CaM surrogates. Our results indicate that (1) CaM and its phosphomimetic analogs promote a gain in the function of Ca_V1.2 and (2) the structural properties of the first intracellular linker of Ca_V1.2 contribute to its CaM-induced modulation. We conclude that the CACNA1C clinical variant mimics the increased activity associated with the upregulation of Ca_V1.2 by Ca²⁺-CaM, thus maintaining a majority of channels in a constitutively active mode that could ultimately promote ventricular arrhythmias.

Divergent Ca2+/calmodulin feedback regulation of CaV1 and CaV2 voltage-gated calcium channels evolved in the common ancestor of Placozoa and Bilateria

Ca_V1 and Ca_V2 voltage-gated calcium channels evolved from an ancestral Ca_V1/2 channel via gene duplication somewhere near the stem animal lineage. The divergence of these channel types led to distinguishing functional properties that are conserved among vertebrates and bilaterian invertebrates and contribute to their unique cellular roles. One key difference pertains to their regulation by calmodulin (CaM), wherein bilaterian Ca_V1 channels are uniquely subject to pronounced, buffer-resistant Ca²⁺/CaM-dependent inactivation, permitting negative feedback regulation of calcium influx in response to local cytoplasmic Ca²⁺ rises. Early diverging, nonbilaterian invertebrates also possess Ca_V1 and Ca_V2 channels, but it is unclear whether they share these conserved functional features. The most divergent animals to possess both Ca_V1 and Ca_V2 channels are placozoans such as Trichoplax adhaerens, which separated from other animals over 600 million years ago shortly after their emergence. Hence, placozoans can provide important insights into the early evolution of Ca_V1 and Ca_V2 channels. Here, we build upon previous characterization of Trichoplax Ca_V channels by determining the cellular expression and ion-conducting properties of the Ca_V1 channel orthologue, TCa_V1. We show that TCa_V1 is expressed in neuroendocrine-like gland cells and contractile dorsal epithelial cells. In vitro, this channel conducts dihydropyridine-insensitive, high-voltage–activated Ca²⁺ currents with kinetics resembling those of rat Ca_V1.2 but with left-shifted voltage sensitivity for activation and inactivation. Interestingly, TCa_V1, but not TCa_V2, exhibits buffer-resistant Ca²⁺/CaM-dependent inactivation, indicating that this functional divergence evolved prior to the emergence of bilaterian animals and may have contributed to their unique adaptation for cytoplasmic Ca²⁺ signaling within various cellular contexts.

Sympathetic Stimulation Upregulates the Ca2+ Channel Subunit, CaVα2δ1, via the β1 and ERK 1/2 Pathway in Neonatal Ventricular Cardiomyocytes

Intracellular Ca2+ overload secondary to chronic hemodynamic stimuli promotes the recruitment of Ca2+-dependent signaling implicated in cardiomyocyte hypertrophy. The present study tested the hypothesis that sympathetic-mediated hypertrophy of neonatal rat ventricular cardiomyocytes (NRVMs) translated to an increase in calcium influx secondary to the upregulation of CaV1.2 channel subunits. Confocal imaging of norepinephrine (NE)-treated NRVMs revealed a hypertrophic response compared to untreated NRVMs. L-type CaV1.2 peak current density was increased 4-fold following a 24-h stimulation with NE. NE-treated NRVMs exhibited a significant upregulation of CaVα2δ1 and CaVβ3 protein levels without significant changes of CaVα1C and CaVβ2 protein levels. Pre-treatment with the β1-blocker metoprolol failed to inhibit hypertrophy or CaVβ3 upregulation whereas CaVα2δ1 protein levels were significantly reduced. NE promoted the phosphorylation of ERK 1/2, and the response was attenuated by the β1-blocker. U0126 pre-treatment suppressed NE-induced ERK1/2 phosphorylation but failed to attenuate hypertrophy. U0126 inhibition of ERK1/2 phosphorylation prevented NE-mediated upregulation of CaVα2δ1, whereas CaVβ3 protein levels remained elevated. Thus, β1-adrenergic receptor-mediated recruitment of the ERK1/2 plays a seminal role in the upregulation of CaVα2δ1 in NRVMs independent of the concomitant hypertrophic response. However, the upregulation of CaVβ3 protein levels may be directly dependent on the hypertrophic response of NRVMs.

Role of High Voltage-Gated Ca2+ Channel Subunits in Pancreatic β-Cell Insulin Release. From Structure to Function

The pancreatic islets of Langerhans secrete several hormones critical for glucose homeostasis. The β-cells, the major cellular component of the pancreatic islets, secrete insulin, the only hormone capable of lowering the plasma glucose concentration. The counter-regulatory hormone glucagon is secreted by the α-cells while δ-cells secrete somatostatin that via paracrine mechanisms regulates the α- and β-cell activity. These three peptide hormones are packed into secretory granules that are released through exocytosis following a local increase in intracellular Ca2+ concentration. The high voltage-gated Ca2+ channels (HVCCs) occupy a central role in pancreatic hormone release both as a source of Ca2+ required for excitation-secretion coupling as well as a scaffold for the release machinery. HVCCs are multi-protein complexes composed of the main pore-forming transmembrane α1 and the auxiliary intracellular β, extracellular α2δ, and transmembrane γ subunits. Here, we review the current understanding regarding the role of all HVCC subunits expressed in pancreatic β-cell on electrical activity, excitation-secretion coupling, and β-cell mass. The evidence we review was obtained from many seminal studies employing pharmacological approaches as well as genetically modified mouse models. The significance for diabetes in humans is discussed in the context of genetic variations in the genes encoding for the HVCC subunits.

An ancestral MAGUK protein supports the modulation of mammalian voltage-gated Ca2+ channels through a conserved CaVβ-like interface

Eukaryote voltage-gated Ca²⁺ channels of the Ca_V2 channel family are hetero-oligomers formed by the pore-forming Ca_Vα1 protein assembled with auxiliary Ca_Vα2δ and Ca_Vβ subunits. Ca_Vβ subunits are formed by a Src homology 3 (SH3) domain and a guanylate kinase (GK) domain connected through a HOOK domain. The GK domain binds a conserved cytoplasmic region of the pore-forming Ca_Vα1 subunit referred as the "AID". Herein we explored the phylogenetic and functional relationship between Ca_V channel subunits in distant eukaryotic organisms by investigating the function of a MAGUK protein (XM_004990081) cloned from the choanoflagellate Salpingoeca rosetta (Sro). This MAGUK protein (Sroβ) features SH3 and GK structural domains with a 25% primary sequence identity to mammalian Ca_Vβ. Recombinant expression of its cDNA with mammalian high-voltage activated Ca²⁺ channel Ca_V2.3 in mammalian HEK cells produced robust voltage-gated inward Ca²⁺ currents with typical activation and inactivation properties. Like Ca_Vβ, Sroβ prevents fast degradation of total Ca_V2.3 proteins in cycloheximide assays. The three-dimensional homology model predicts an interaction between the GK domain of Sroβ and the AID motif of the pore-forming Ca_Vα1 protein. Substitution of AID residues Trp (W386A) and Tyr (Y383A) significantly impaired co-immunoprecipitation of Ca_V2.3 with Sroβ and functional upregulation of Ca_V2.3 currents. Likewise, a 6-residue deletion within the GK domain of Sroβ, similar to the locus found in mammalian Ca_Vβ, significantly reduced peak current density. Altogether our data demonstrate that an ancestor MAGUK protein reconstitutes the biophysical and molecular features responsible for channel upregulation by mammalian Ca_Vβ through a minimally conserved molecular interface.

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 Ca_V2.1 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 Ca_V2.2 selectivity filter mutants are still influenced by auxiliary α₂δ subunits and, albeit to a reduced extent, by β subunits, indicating the channels are not grossly misfolded. Our results indicate that Ca²⁺ binding in the pore of Ca_V2 channels may promote their correct trafficking, in combination with auxiliary subunits. Furthermore, physiological studies utilizing selectivity filter mutant Ca_V channels should be interpreted with caution.

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Selectivity filter mutations in Ca_V2 channels block inward Ba²⁺ currents

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Surprisingly, these mutations severely reduce trafficking of the Ca_V2 channels

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Pore mutant N-type channels show reduced expression in presynaptic terminals

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Pore mutant channels still require β and α₂δ and thus are not grossly misfolded

Highly conserved extracellular residues mediate interactions between pore-forming and regulatory subunits of the yeast Ca2+ channel related to the animal VGCC/NALCN family

Yeasts and fungi generate Ca²⁺ signals in response to environmental stresses through Ca²⁺ channels essentially composed of Cch1 and Mid1. Cch1 is homologous to the pore-forming α₁ subunit of animal voltage-gated Ca²⁺ channels (VGCCs) and sodium leak channels nonselective (NALCNs), whereas Mid1 is a membrane-associated protein similar to the regulatory α₂/δ subunit of VGCCs and the regulatory subunit of NALCNs. Although the physiological roles of Cch1/Mid1 channels are known, their molecular regulation remains elusive, including subunit interactions regulating channel functionality. Herein, we identify amino acid residues involved in interactions between the pore-forming Cch1 subunit and the essential regulatory Mid1 subunit of Saccharomyces cerevisiae. In vitro mutagenesis followed by functional assays and co-immunoprecipitation experiments reveal that three residues present in a specific extracellular loop in the repeat III region of Cch1 are required for interaction with Mid1, and that one essential Mid1 residue is required for interaction with Cch1. Importantly, these residues are necessary for Ca²⁺ channel activity and are highly conserved in fungal and animal counterparts. We discuss that this unique subunit interaction-based regulatory mechanism for Cch1 differs from that of VGCCs/NALCNs.

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 (Ca_V2.2) channels, which are important in neurotransmission. Previous structural studies show the α2δ-1 VWA domain interacting with the first loop in Ca_V1.1 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 Ca_V2.2 currents substantially (although less than α2δ-1) and increases Ca_V2.2 cell surface expression by reducing endocytosis. Although the effects of α2δ-1 are abolished by mutation of Asp122 in Ca_V2.2 domain-I, which mediates interaction with its VWA domain, the Cachd1 responses are unaffected. Furthermore, Cachd1 co-immunoprecipitates with Ca_V2.2 and inhibits co-immunoprecipitation of α2δ-1 by Ca_V2.2. Cachd1 also competes with α2δ-1 for effects on trafficking. Thus, Cachd1 influences both Ca_V2.2 trafficking and function and can inhibit responses to α2δ-1.

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Cachd1 enhances Ca_V2.2 currents and increases Ca_V2.2 surface expression

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Effects of Cachd1 are not prevented by mutation in Ca_V2.2 VWA interaction site

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The effects of α₂δ-1 are prevented by the same mutation in Ca_V2.2

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Cachd1 competes with α₂δ-1 for its effects on Ca_V2.2

Voltage-gated calcium channel α 2δ subunits: an assessment of proposed novel roles

Voltage-gated calcium (Ca_V) channels are associated with β and α₂δ auxiliary subunits. This review will concentrate on the function of the α₂δ protein family, which has four members. The canonical role for α₂δ subunits is to convey a variety of properties on the Ca_V1 and Ca_V2 channels, increasing the density of these channels in the plasma membrane and also enhancing their function. More recently, a diverse spectrum of non-canonical interactions for α₂δ proteins has been proposed, some of which involve competition with calcium channels for α₂δ or increase α₂δ trafficking and others which mediate roles completely unrelated to their calcium channel function. The novel roles for α₂δ proteins which will be discussed here include association with low-density lipoprotein receptor-related protein 1 (LRP1), thrombospondins, α-neurexins, prion proteins, large conductance (big) potassium (BK) channels, and N-methyl-d-aspartate (NMDA) receptors.

Calcium channel gating

Tuned calcium entry through voltage-gated calcium channels is a key requirement for many cellular functions. This is ensured by channel gates which open during membrane depolarizations and seal the pore at rest. The gating process is determined by distinct sub-processes: movement of voltage-sensing domains (charged S4 segments) as well as opening and closure of S6 gates. Neutralization of S4 charges revealed that pore opening of CaV1.2 is triggered by a "gate releasing" movement of all four S4 segments with activation of IS4 (and IIIS4) being a rate-limiting stage. Segment IS4 additionally plays a crucial role in channel inactivation. Remarkably, S4 segments carrying only a single charged residue efficiently participate in gating. However, the complete set of S4 charges is required for stabilization of the open state. Voltage clamp fluorometry, the cryo-EM structure of a mammalian calcium channel, biophysical and pharmacological studies, and mathematical simulations have all contributed to a novel interpretation of the role of voltage sensors in channel opening, closure, and inactivation. We illustrate the role of the different methodologies in gating studies and discuss the key molecular events leading CaV channels to open and to close.

Proteolytic maturation of α2δ controls the probability of synaptic vesicular release

Auxiliary α2δ subunits are important proteins for trafficking of voltage-gated calcium channels (CaV) at the active zones of synapses. We have previously shown that the post-translational proteolytic cleavage of α2δ is essential for their modulatory effects on the trafficking of N-type (CaV2.2) calcium channels (Kadurin et al., 2016). We extend these results here by showing that the probability of presynaptic vesicular release is reduced when an uncleaved α2δ is expressed in rat neurons and that this inhibitory effect is reversed when cleavage of α2δ is restored. We also show that asynchronous release is influenced by the maturation of α2δ-1, highlighting the role of CaV channels in this component of vesicular release. We present additional evidence that CaV2.2 co-immunoprecipitates preferentially with cleaved wild-type α2δ. Our data indicate that the proteolytic maturation increases the association of α2δ-1 with CaV channel complex and is essential for its function on synaptic release.

A three-way inter-molecular network accounts for the CaVα2δ1-induced functional modulation of the pore-forming CaV1.2 subunit

L-type Ca_V1.2 channels are essential for the excitation-contraction coupling in cardiomyocytes and are hetero-oligomers of a pore-forming Ca_Vα1C assembled with Ca_Vβ and Ca_Vα2δ1 subunits. A direct interaction between Ca_Vα2δ1 and Asp-181 in the first extracellular loop of Ca_Vα1 reproduces the native properties of the channel. A 3D model of the von Willebrand factor type A (VWA) domain of Ca_Vα2δ1 complexed with the voltage sensor domain of Ca_Vα1C suggests that Ser-261 and Ser-263 residues in the metal ion-dependent adhesion site (MIDAS) motif are determinant in this interaction, but this hypothesis is untested. Here, coimmunoprecipitation assays and patch-clamp experiments of single-substitution variants revealed that Ca_Vα2δ1 Asp-259 and Ser-261 are the two most important residues in regard to protein interactions and modulation of Ca_V1.2 currents. In contrast, mutating the side chains of Ca_Vα2δ1 Ser-263, Thr-331, and Asp-363 with alanine did not completely prevent channel function. Molecular dynamics simulations indicated that the carboxylate side chain of Ca_Vα2δ1 Asp-259 coordinates the divalent cation that is further stabilized by the oxygen atoms from the hydroxyl side chain of Ca_Vα2δ1 Ser-261 and the carboxylate group of Ca_Vα1C Asp-181. In return, the hydrogen atoms contributed by the side chain of Ser-261 and the main chain of Ser-263 bonded the oxygen atoms of Ca_V1.2 Asp-181. We propose that Ca_Vα2δ1 Asp-259 promotes Ca²⁺ binding necessary to produce the conformation of the VWA domain that locks Ca_Vα2δ1 Ser-261 and Ser-263 within atomic distance of Ca_Vα1C Asp-181. This three-way network appears to account for the Ca_Vα2δ1-induced modulation of Ca_V1.2 currents.