Beta-subunits promote the expression of Ca(V)2.2 channels by reducing their proteasomal degradation

The intracellular C-terminus confers compartment-specific targeting of voltage-gated calcium channels

To achieve the functional polarization that underlies brain computation, neurons sort protein material into distinct compartments. Ion channel composition, for example, differs between axons and dendrites, but the molecular determinants for their polarized trafficking remain obscure. Here, we identify mechanisms that target voltage-gated Ca2+ channels (CaVs) to distinct subcellular compartments. In hippocampal neurons, CaV2s trigger neurotransmitter release at the presynaptic active zone, and CaV1s localize somatodendritically. After knockout of all three CaV2s, expression of CaV2.1, but not CaV1.3, restores neurotransmitter release. We find that chimeric CaV1.3s with CaV2.1 intracellular C-termini localize to the active zone, mediate synaptic vesicle exocytosis, and render release sensitive to CaV1 blockers. This dominant targeting function of the CaV2.1 C-terminus requires the first EF hand in its proximal segment, and replacement of the CaV2.1 C-terminus with that of CaV1.3 abolishes CaV2.1 active zone localization and function. We conclude that CaV intracellular C-termini mediate compartment-specific targeting.

Peptide and Peptidomimetic Inhibitors Targeting the Interaction of Collapsin Response Mediator Protein 2 with the N-Type Calcium Channel for Pain Relief

Ion channels serve pleiotropic functions. Often found in complexes, their activities and functions are sculpted by auxiliary proteins. We discovered that collapsin response mediator protein 2 (CRMP2) is a binding partner and regulator of the N-type voltage-gated calcium channel (CaV2.2), a genetically validated contributor to chronic pain. Herein, we trace the discovery of a new peptidomimetic modulator of this interaction, starting from the identification and development of CBD3, a CRMP2-derived CaV binding domain peptide. CBD3 uncouples CRMP2-CaV2.2 binding to decrease CaV2.2 surface localization and calcium currents. These changes occur at presynaptic sites of nociceptive neurons and indeed, CBD3 ameliorates chronic pain in preclinical models. In pursuit of a CBD3 peptidomimetic, we exploited a unique approach to identify a dipeptide with low conformational flexibility and high solvent accessibility that anchors binding to CaV2.2. From a pharmacophore screen, we obtained CBD3063, a small-molecule that recapitulated CBD3's activity, reversing nociceptive behaviors in rodents of both sexes without sensory, affective, or cognitive effects. By disrupting the CRMP2-CaV2.2 interaction, CBD3063 exerts these effects indirectly through modulating CaV2.2 trafficking, supporting CRMP2 as an auxiliary subunit of CaV2.2. The parent peptide CBD3 was also found by us and others to have neuroprotective properties at postsynaptic sites, through N-methyl-d-aspartate receptor and plasmalemmal Na+/Ca2+ exchanger 3, potentially acting as an auxiliary subunit for these pathways as well. Our new compound is poised to address several open questions regarding CRMP2's role in regulating the CaV2.2 pathways to treat pain with the potential added benefit of neuroprotection.

Synaptotagmin-11 facilitates assembly of a presynaptic signaling complex in post-Golgi cargo vesicles

GABAB receptors (GBRs), the G protein-coupled receptors for GABA, regulate synaptic transmission throughout the brain. A main synaptic function of GBRs is the gating of Cav2.2-type Ca2+ channels. However, the cellular compartment where stable GBR/Cav2.2 signaling complexes form remains unknown. In this study, we demonstrate that the vesicular protein synaptotagmin-11 (Syt11) binds to both the auxiliary GBR subunit KCTD16 and Cav2.2 channels. Through these dual interactions, Syt11 recruits GBRs and Cav2.2 channels to post-Golgi vesicles, thus facilitating assembly of GBR/Cav2.2 signaling complexes. In addition, Syt11 stabilizes GBRs and Cav2.2 channels at the neuronal plasma membrane by inhibiting constitutive internalization. Neurons of Syt11 knockout mice exhibit deficits in presynaptic GBRs and Cav2.2 channels, reduced neurotransmitter release, and decreased GBR-mediated presynaptic inhibition, highlighting the critical role of Syt11 in the assembly and stable expression of GBR/Cav2.2 complexes. These findings support that Syt11 acts as a vesicular scaffold protein, aiding in the assembly of signaling complexes from low-abundance components within transport vesicles. This mechanism enables insertion of pre-assembled functional signaling units into the synaptic membrane.

Active Zone Trafficking of CaV2/UNC-2 Channels Is Independent of β/CCB-1 and α2δ/UNC-36 Subunits

The CaV2 voltage-gated calcium channel is the major conduit of calcium ions necessary for neurotransmitter release at presynaptic active zones (AZs). The CaV2 channel is a multimeric complex that consists of a pore-forming α1 subunit and two auxiliary β and α2δ subunits. Although auxiliary subunits are critical for channel function, whether they are required for α1 trafficking is unresolved. Using endogenously fluorescent protein-tagged CaV2 channel subunits in Caenorhabditis elegans, we show that UNC-2/α1 localizes to AZs even in the absence of CCB-1/β or UNC-36/α2δ, albeit at low levels. When UNC-2 is manipulated to be trapped in the endoplasmic reticulum (ER), CCB-1 and UNC-36 fail to colocalize with UNC-2 in the ER, indicating that they do not coassemble with UNC-2 in the ER. Moreover, blocking ER-associated degradation does not further increase presynaptic UNC-2 channels in ccb-1 or unc-36 mutants, indicating that UNC-2 levels are not regulated in the ER. An unc-2 mutant lacking C-terminal AZ protein interaction sites with intact auxiliary subunit binding sites displays persistent presynaptic UNC-2 localization and a prominent increase of UNC-2 channels in nonsynaptic axonal regions, underscoring a protective role of auxiliary subunits against UNC-2 degradation. In the absence of UNC-2, presynaptic CCB-1 and UNC-36 are profoundly diminished to barely detectable levels, indicating that UNC-2 is required for the presynaptic localization of CCB-1 and UNC-36. Together, our findings demonstrate that although the pore-forming subunit does not require auxiliary subunits for its trafficking and transport to AZs, it recruits auxiliary subunits to stabilize and expand calcium channel signalosomes.SIGNIFICANCE STATEMENT Synaptic transmission in the neuron hinges on the coupling of synaptic vesicle exocytosis with calcium influx. This calcium influx is mediated by CaV2 voltage-gated calcium channels. These channels consist of one pore-forming α1 subunit and two auxiliary β and α2δ subunits. The auxiliary subunits enhance channel function and regulate the overall level of channels at presynaptic terminals. However, it is not settled how these auxiliary subunits regulate the overall channel level. Our study in C. elegans finds that although the auxiliary subunits do not coassemble with α1 and aid trafficking, they are recruited to α1 and stabilize the channel complex at presynaptic terminals. Our study suggests that drugs that target the auxiliary subunits can directly destabilize and have an impact on CaV2 channels.

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.

Mechanisms controlling the trafficking, localization, and abundance of presynaptic Ca2+ channels

Voltage-gated Ca2+ channels (VGCCs) mediate Ca2+ influx to trigger neurotransmitter release at specialized presynaptic sites termed active zones (AZs). The abundance of VGCCs at AZs regulates neurotransmitter release probability (Pr ), a key presynaptic determinant of synaptic strength. Given this functional significance, defining the processes that cooperate to establish AZ VGCC abundance is critical for understanding how these mechanisms set synaptic strength and how they might be regulated to control presynaptic plasticity. VGCC abundance at AZs involves multiple steps, including channel biosynthesis (transcription, translation, and trafficking through the endomembrane system), forward axonal trafficking and delivery to synaptic terminals, incorporation and retention at presynaptic sites, and protein recycling. Here we discuss mechanisms that control VGCC abundance at synapses, highlighting findings from invertebrate and vertebrate models.

Regulation of Presynaptic Calcium Channels

Voltage-gated calcium channels (VGCCs), especially Cav2.1 and Cav2.2, are the major mediators of Ca2+ influx at the presynaptic membrane in response to neuron excitation, thereby exerting a predominant control on synaptic transmission. To guarantee the timely and precise release of neurotransmitters at synapses, the activity of presynaptic VGCCs is tightly regulated by a variety of factors, including auxiliary subunits, membrane potential, G protein-coupled receptors (GPCRs), calmodulin (CaM), Ca2+-binding proteins (CaBP), protein kinases, various interacting proteins, alternative splicing events, and genetic variations.

Selective posttranslational inhibition of CaVβ1-associated voltage-dependent calcium channels with a functionalized nanobody

Ca²⁺ influx through high-voltage-activated calcium channels (HVACCs) controls diverse cellular functions. A critical feature enabling a singular signal, Ca²⁺ influx, to mediate disparate functions is diversity of HVACC pore-forming α₁ and auxiliary Ca_Vβ₁-Ca_Vβ₄ subunits. Selective Ca_Vα₁ blockers have enabled deciphering their unique physiological roles. By contrast, the capacity to post-translationally inhibit HVACCs based on Ca_Vβ isoform is non-existent. Conventional gene knockout/shRNA approaches do not adequately address this deficit owing to subunit reshuffling and partially overlapping functions of Ca_Vβ isoforms. Here, we identify a nanobody (nb.E8) that selectively binds Ca_Vβ₁ SH3 domain and inhibits Ca_Vβ₁-associated HVACCs by reducing channel surface density, decreasing open probability, and speeding inactivation. Functionalizing nb.E8 with Nedd4L HECT domain yielded Chisel-1 which eliminated current through Ca_Vβ₁-reconstituted Ca_V1/Ca_V2 and native Ca_V1.1 channels in skeletal muscle, strongly suppressed depolarization-evoked Ca²⁺ influx and excitation-transcription coupling in hippocampal neurons, but was inert against Ca_Vβ₂-associated Ca_V1.2 in cardiomyocytes. The results introduce an original method for probing distinctive functions of ion channel auxiliary subunit isoforms, reveal additional dimensions of Ca_Vβ₁ signaling in neurons, and describe a genetically-encoded HVACC inhibitor with unique properties.

The ubiquitin E3 ligase Parkin regulates neuronal CaV1.3 channel functional expression

Neuronal L-type Ca²⁺ channels of the Ca_V1.3 subclass are transmembrane protein complexes that contribute to the pacemaker activity in the adult substantia nigra dopaminergic neurons. The altered function of these channels may play a role in the development and progress of neurodegenerative mechanisms implicated in Parkinson's disease (PD). Although L-type channel expression is precisely regulated, an increased functional expression has been observed in PD. Previously, we showed that Parkin, an E3 enzyme of the ubiquitin-proteasome system (UPS) interacts with neuronal Ca_V2.2 channels promoting their ubiquitin-mediated degradation. In addition, previous studies show an increase in Ca_V1.3 channel activity in dopaminergic neurons of the SNc and that Parkin expression is reduced in PD. These findings suggest that the decrease in Parkin may affect the proteasomal degradation of Ca_V1.3, which helps explain the increase in channel activity. Therefore, the present report aims to gain insight into the degradation mechanisms of the neuronal Ca_V1.3 channel by the UPS. Immunoprecipitation assays showed the interaction between Parkin and the Ca_V1.3 channels expressed in HEK-293 cells and neural tissues. Likewise, Parkin overexpression reduced the total and membrane channel levels and decreased the current density. Consistent with this, patch-clamp recordings in the presence of an inhibitor of the UPS, MG132, prevented the effects of Parkin, suggesting enhanced channel proteasomal degradation. In addition, the half-life of the pore-forming Ca_V1.3α₁ protein was significantly reduced by Parkin overexpression. Finally, electrophysiological recordings using a PRKN knockout HEK-293 cell line generated by CRISPR/Cas9 showed increased current density. These results suggest that Parkin promotes the proteasomal degradation of Ca_V1.3, which may be a relevant aspect for the pathophysiology of PD.NEW & NOTEWORTHY The increased expression of Ca_V1.3 calcium channels is a crucial feature of Parkinson's disease (PD) pathophysiology. However, the mechanisms that determine this increase are not yet defined. Parkin, an enzyme of the ubiquitin-proteasome system, is known to interact with neuronal channels promoting their ubiquitin-mediated degradation. Interestingly, Parkin mutations also play a role in PD. Here, the degradation mechanisms of Ca_V1.3 channels and their relationship with the pathophysiology of PD are studied in detail.

Calcium Channels in Retinal Function and Disease

Voltage-gated Ca2+ (Cav) channels play pivotal roles in regulating gene transcription, neuronal excitability, and neurotransmitter release. To meet the spatial and temporal demands of visual signaling, Cav channels exhibit unusual properties in the retina compared to their counterparts in other areas of the nervous system. In this article, we review current concepts regarding the specific subtypes of Cav channels expressed in the retina, their intrinsic properties and forms of modulation, and how their dysregulation could lead to retinal disease.

The calcium channel terminator: hasta la vista pain

Chronic pain remains a major burden and is difficult to treat. N-type calcium channels may be a suitable therapeutic target for analgesics, and a new study from Colecraft and colleagues utilizes a clever new way to modulate their expression to achieve therapeutic benefits in preclinical models of neuropathic pain.

Physiological Overview of the Potential Link between the UPS and Ca2+ Signaling

The ubiquitin–proteasome system (UPS) is the main proteolytic pathway by which damaged target proteins are degraded after ubiquitination and the recruit of ubiquitinated proteins, thus regulating diverse physiological functions and the maintenance in various tissues and cells. Ca²⁺ signaling is raised by oxidative or ER stress. Although the basic function of the UPS has been extensively elucidated and has been continued to define its mechanism, the precise relationship between the UPS and Ca²⁺ signaling remains unclear. In the present review, we describe the relationship between the UPS and Ca²⁺ signaling, including Ca²⁺-associated proteins, to understand the end point of oxidative stress. The UPS modulates Ca²⁺ signaling via the degradation of Ca²⁺-related proteins, including Ca²⁺ channels and transporters. Conversely, the modulation of UPS is driven by increases in the intracellular Ca²⁺ concentration. The multifaceted relationship between the UPS and Ca²⁺ plays critical roles in different tissue systems. Thus, we highlight the potential crosstalk between the UPS and Ca²⁺ signaling by providing an overview of the UPS in different organ systems and illuminating the relationship between the UPS and autophagy.

More than a pore: How voltage-gated calcium channels act on different levels of neuronal communication regulation

Voltage-gated calcium channels (VGCCs) represent key regulators of the calcium influx through the plasma membrane of excitable cells, like neurons. Activated by the depolarization of the membrane, the opening of VGCCs induces very transient and local changes in the intracellular calcium concentration, known as calcium nanodomains, that in turn trigger calcium-dependent signaling cascades and the release of chemical neurotransmitters. Based on their central importance as concierges of excitation-secretion coupling and therefore neuronal communication, VGCCs have been studied in multiple aspects of neuronal function and malfunction. However, studies on molecular interaction partners and recent progress in omics technologies have extended the actual concept of these molecules. With this review, we want to illustrate some new perspectives of VGCCs reaching beyond their function as calcium-permeable pores in the plasma membrane. Therefore, we will discuss the relevance of VGCCs as voltage sensors in functional complexes with ryanodine receptors, channel-independent actions of auxiliary VGCC subunits, and provide an insight into how VGCCs even directly participate in gene regulation. Furthermore, we will illustrate how structural changes in the intracellular C-terminus of VGCCs generated by alternative splicing events might not only affect the biophysical channel characteristics but rather determine their molecular environment and downstream signaling pathways.

CaV β controls the endocytic turnover of CaV 1.2 L-type calcium channel

Membrane depolarization activates the multisubunit Ca_V 1.2 L-type calcium channel initiating various excitation coupling responses. Intracellular trafficking into and out of the plasma membrane regulates the channel's surface expression and stability, and thus, the strength of Ca_V 1.2-mediated Ca²⁺ signals. The mechanisms regulating the residency time of the channel at the cell membrane are unclear. Here, we coexpressed the channel core complex Ca_V 1.2α₁ pore-forming and auxiliary Ca_V β subunits and analyzed their trafficking dynamics from single-particle-tracking trajectories. Speed histograms obtained for each subunit were best fitted to a sum of diffusive and directed motion terms. The same mean speed for the highest-mobility state underlying directed motion was found for all subunits. The frequency of this component increased by covalent linkage of Ca_V β to Ca_V 1.2α₁ suggesting that high-speed transport occurs in association with Ca_V β. Selective tracking of Ca_V 1.2α₁ along the postendocytic pathway failed to show the highly mobile state, implying Ca_V β-independent retrograde transport. Retrograde speeds of Ca_V 1.2α₁ are compatible with myosin VI-mediated backward transport. Moreover, residency time at the cell surface was significantly prolonged when Ca_V 1.2α₁ was covalently linked to Ca_V β. Thus, Ca_V β promotes fast transport speed along anterograde trafficking and acts as a molecular switch controlling the endocytic turnover of L-type calcium channels.

The life cycle of voltage-gated Ca2+ channels in neurons: an update on the trafficking of neuronal calcium channels

Neuronal voltage-gated Ca2+ (CaV) channels play a critical role in cellular excitability, synaptic transmission, excitation-transcription coupling and activation of intracellular signaling pathways. CaV channels are multiprotein complexes and their functional expression in the plasma membrane involves finely tuned mechanisms, including forward trafficking from the endoplasmic reticulum (ER) to the plasma membrane, endocytosis and recycling. Whether genetic or acquired, alterations and defects in the trafficking of neuronal CaV channels can have severe physiological consequences. In this review, we address the current evidence concerning the regulatory mechanisms which underlie precise control of neuronal CaV channel trafficking and we discuss their potential as therapeutic targets.

CaV2.2 (N-type) voltage-gated calcium channels are activated by SUMOylation pathways

SUMOylation is an important post-translational modification process involving covalent attachment of SUMO (Small Ubiquitin-like MOdifier) protein to target proteins. Here, we investigated the potential for SUMO-1 protein to modulate the function of the CaV2.2 (N-type) voltage-gated calcium channel (VGCC), a protein vital for presynaptic neurotransmitter release. Co-expression of SUMO-1, but not the conjugation-deficient mutant SUMO-1ΔGG, increased heterologously-expressed CaV2.2 Ca2+ current density, an effect potentiated by the conjugating enzyme Ubc9. Expression of sentrin-specific protease (SENP)-1 or Ubc9 alone, had no effect on recombinant CaV2.2 channels. Co-expression of SUMO-1 and Ubc9 caused an increase in whole-cell maximal conductance (Gmax) and a hyperpolarizing shift in the midpoint of activation (V1/2). Mutation of all five CaV2.2 lysine residues to arginine within the five highest probability (>65 %) SUMOylation consensus motifs (SCMs) (construct CaV2.2-Δ5KR), produced a loss-of-function mutant. Mutagenesis of selected individual lysine residues identified K394, but not K951, as a key residue for SUMO-1-mediated increase in CaV2.2 Ca2+ current density. In synaptically-coupled superior cervical ganglion (SCG) neurons, SUMO-1 protein was distributed throughout the cell body, axons and dendrites and presumptive presynaptic terminals, whilst SUMO-1ΔGG protein was largely confined to the cell body, in particular, the nucleus. SUMO-1 expression caused increases in paired excitatory postsynaptic potential (EPSP) ratio at short (20-120 ms) inter-stimuli intervals in comparison to SUMO-1ΔGG, consistent with an increase in residual presynaptic Ca2+ current and an increase in release probability of synaptic vesicles. Together, these data provide evidence for CaV2.2 VGCCs as novel targets for SUMOylation pathways.

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.

Functions of Presynaptic Voltage-gated Calcium Channels

Voltage-gated calcium channels are the principal conduits for depolarization-mediated Ca²⁺ entry into excitable cells. In this review, the biophysical properties of the relevant members of this family of channels, those that are present in presynaptic terminals, will be discussed in relation to their function in mediating neurotransmitter release. Voltage-gated calcium channels have properties that ensure they are specialized for particular roles, for example, differences in their activation voltage threshold, their various kinetic properties, and their voltage-dependence of inactivation. All these attributes play into the ability of the various voltage-gated calcium channels to participate in different patterns of presynaptic vesicular release. These include synaptic transmission resulting from single action potentials, and longer-term changes mediated by bursts or trains of action potentials, as well as release resulting from graded changes in membrane potential in specialized sensory synapses.

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

New aspects in cardiac L-type Ca2+ channel regulation

Cardiac excitation-contraction coupling is initiated with the influx of Ca2+ ions across the plasma membrane through voltage-gated L-type calcium channels. This process is tightly regulated by modulation of the channel open probability and channel localization. Protein kinase A (PKA) is found in close association with the channel and is one of the main regulators of its function. Whether this kinase is modulating the channel open probability by phosphorylation of key residues or via alternative mechanisms is unclear. This review summarizes recent findings regarding the PKA-mediated channel modulation and will highlight recently discovered regulatory mechanisms that are independent of PKA activity and involve protein-protein interactions and channel localization.

Presynaptic calcium channels: specialized control of synaptic neurotransmitter release

Chemical synapses are heterogeneous junctions formed between neurons that are specialized for the conversion of electrical impulses into the exocytotic release of neurotransmitters. Voltage-gated Ca2+ channels play a pivotal role in this process as they are the major conduits for the Ca2+ ions that trigger the fusion of neurotransmitter-containing vesicles with the presynaptic membrane. Alterations in the intrinsic function of these channels and their positioning within the active zone can profoundly alter the timing and strength of synaptic output. Advances in optical and electron microscopic imaging, structural biology and molecular techniques have facilitated recent breakthroughs in our understanding of the properties of voltage-gated Ca2+ channels that support their presynaptic functions. Here we examine the nature of these channels, how they are trafficked to and anchored within presynaptic boutons, and the mechanisms that allow them to function optimally in shaping the flow of information through neural circuits.

FMRP regulates presynaptic localization of neuronal voltage gated calcium channels

Fragile X syndrome (FXS), the most common form of inherited intellectual disability and autism, results from the loss of fragile X mental retardation protein (FMRP). We have recently identified a direct interaction of FMRP with voltage-gated Ca²⁺ channels that modulates neurotransmitter release. In the present study we used a combination of optophysiological tools to investigate the impact of FMRP on the targeting of voltage-gated Ca²⁺ channels to the active zones in neuronal presynaptic terminals. We monitored Ca²⁺ transients at synaptic boutons of dorsal root ganglion (DRG) neurons using the genetically-encoded Ca²⁺ indicator GCaMP6f tagged to synaptophysin. We show that knock-down of FMRP induces an increase of the amplitude of the Ca²⁺ transient in functionally-releasing presynaptic terminals, and that this effect is due to an increase of N-type Ca²⁺ channel contribution to the total Ca²⁺ transient. Dynamic regulation of Ca_V2.2 channel trafficking is key to the function of these channels in neurons. Using a Ca_V2.2 construct with an α-bungarotoxin binding site tag, we further investigate the impact of FMRP on the trafficking of Ca_V2.2 channels. We show that forward trafficking of Ca_V2.2 channels from the endoplasmic reticulum to the plasma membrane is reduced when co-expressed with FMRP. Altogether our data reveal a critical role of FMRP on localization of Ca_V channels to the presynaptic terminals and how its defect in a context of FXS can profoundly affect synaptic transmission.

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Loss of FMRP increases presynaptic Ca²⁺ transients.

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FMRP is a negative regulator of presynaptic Ca_v2.2 channel abundance.

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FMRP reduces the forward trafficking of Ca_v2.2 channels from ER to plasma membrane.

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Distal part of FMRP carboxy terminus is key for interaction with Ca_v2.2 channels.

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

High-Voltage-Activated Calcium Channel in the Afferent Pain Pathway: An Important Target of Pain Therapies

High-voltage-activated (HVA) Ca2+ channels are widely expressed in the nervous system. They play an important role in pain conduction by participating in various physiological processes such as synaptic transmission, changes in synaptic plasticity, and neuronal excitability. Available evidence suggests that the HVA channel is an important therapeutic target for pain management. In this review, we summarize the changes in different subtypes of HVA channel during pain and present the currently available evidence from the clinical application of HVA channel blockers. We also review novel drugs in various phases of development. Moreover, we discuss the future prospects of HVA channel blockers in order to promote "bench-to-bedside" translation.

Differential regulation of Cav2.2 channel exon 37 variants by alternatively spliced μ-opioid receptors

We have examined the regulation of mutually exclusive Cav2.2 exon 37a and b variants by the mouse μ-opioid receptor (mMOR) C-terminal splice variants 1, 1C and 1O in tsA-201 cells. Electrophysiological analyses revealed that both channel isoforms exhibit DAMGO-induced voltage-dependent (Gβγ-mediated) inhibition and its recovery by voltage pre-pulses, as well as a voltage-independent component. However, the two channel isoforms differ in their relative extent of voltage-dependent and independent inhibition, with Cav2.2-37b showing significantly more voltage-dependent inhibition upon activation of the three mMOR receptors studied. In addition, coexpression of either mMOR1 or mMOR1C results in an agonist-independent reduction in the peak current density of Cav2.2-37a channels, whereas the peak current density of Cav2.2-37b does not appear to be affected. Interestingly, this decrease is not due to an effect on channel expression at the plasma membrane, as demonstrated by biotinylation experiments. We further examined the mechanism underlying the agonist-independent modulation of Cav2.2-37a by mMOR1C. Incubation of cells with pertussis toxin did not affect the mMOR1C mediated inhibition of Cav2.2-37a currents, indicating a lack of involvement of Gi/o signaling. However, when a Src tyrosine kinase inhibitor was applied, the effect of mMOR1C was lost. Moreover, when we recorded currents using a Cav2.2-37a mutant in which tyrosine 1747 was replaced with phenylalanine (Y1747F), the agonist independent effects of mMOR1C were abolished. Altogether our findings show that Cav2.2-37a and Cav2.2-37b isoforms are subject to differential regulation by C-terminal splice variants of mMORs, and that constitutive mMOR1C activity and downstream tyrosine kinase activity exert a selective inhibition of the Cav2.2-37a splice variant, an N-type channel isoform that is highly enriched in nociceptors. Our study provides new insights into the roles of the MOR full-length C-terminal variants in modulating Cav2.2 channel isoform activities.

A potent voltage-gated calcium channel inhibitor engineered from a nanobody targeted to auxiliary CaVβ subunits

Inhibiting high-voltage-activated calcium channels (HVACCs; Ca_V1/Ca_V2) is therapeutic for myriad cardiovascular and neurological diseases. For particular applications, genetically-encoded HVACC blockers may enable channel inhibition with greater tissue-specificity and versatility than is achievable with small molecules. Here, we engineered a genetically-encoded HVACC inhibitor by first isolating an immunized llama nanobody (nb.F3) that binds auxiliary HVACC Ca_Vβ subunits. Nb.F3 by itself is functionally inert, providing a convenient vehicle to target active moieties to Ca_Vβ-associated channels. Nb.F3 fused to the catalytic HECT domain of Nedd4L (Ca_V-aβlator), an E3 ubiquitin ligase, ablated currents from diverse HVACCs reconstituted in HEK293 cells, and from endogenous Ca_V1/Ca_V2 channels in mammalian cardiomyocytes, dorsal root ganglion neurons, and pancreatic β cells. In cardiomyocytes, Ca_V-aβlator redistributed Ca_V1.2 channels from dyads to Rab-7-positive late endosomes. This work introduces Ca_V-aβlator as a potent genetically-encoded HVACC inhibitor, and describes a general approach that can be broadly adapted to generate versatile modulators for macro-molecular membrane protein complexes.

Emerging roles for multifunctional ion channel auxiliary subunits in cancer

Several superfamilies of plasma membrane channels which regulate transmembrane ion flux have also been shown to regulate a multitude of cellular processes, including proliferation and migration. Ion channels are typically multimeric complexes consisting of conducting subunits and auxiliary, non-conducting subunits. Auxiliary subunits modulate the function of conducting subunits and have putative non-conducting roles, further expanding the repertoire of cellular processes governed by ion channel complexes to processes such as transcellular adhesion and gene transcription. Given this expansive influence of ion channels on cellular behaviour it is perhaps no surprise that aberrant ion channel expression is a common occurrence in cancer. This review will focus on the conducting and non-conducting roles of the auxiliary subunits of various Ca2+, K+, Na+ and Cl- channels and the burgeoning evidence linking such auxiliary subunits to cancer. Several subunits are upregulated (e.g. Cavβ, Cavγ) and downregulated (e.g. Kvβ) in cancer, while other subunits have been functionally implicated as oncogenes (e.g. Navβ1, Cavα2δ1) and tumour suppressor genes (e.g. CLCA2, KCNE2, BKγ1) based on in vivo studies. The strengthening link between ion channel auxiliary subunits and cancer has exposed these subunits as potential biomarkers and therapeutic targets. However further mechanistic understanding is required into how these subunits contribute to tumour progression before their therapeutic potential can be fully realised.

β-adrenergic-mediated dynamic augmentation of sarcolemmal CaV 1.2 clustering and co-operativity in ventricular myocytes

Key points

Prevailing dogma holds that activation of the β‐adrenergic receptor/cAMP/protein kinase A signalling pathway leads to enhanced L‐type Ca_V1.2 channel activity, resulting in increased Ca²⁺ influx into ventricular myocytes and a positive inotropic response. However, the full mechanistic and molecular details underlying this phenomenon are incompletely understood.

Ca_V1.2 channel clusters decorate T‐tubule sarcolemmas of ventricular myocytes. Within clusters, nanometer proximity between channels permits Ca²⁺‐dependent co‐operative gating behaviour mediated by physical interactions between adjacent channel C‐terminal tails.

We report that stimulation of cardiomyocytes with isoproterenol, evokes dynamic, protein kinase A‐dependent augmentation of Ca_V1.2 channel abundance along cardiomyocyte T‐tubules, resulting in the appearance of channel ‘super‐clusters’, and enhanced channel co‐operativity that amplifies Ca²⁺ influx.

On the basis of these data, we suggest a new model in which a sub‐sarcolemmal pool of pre‐synthesized Ca_V1.2 channels resides in cardiomyocytes and can be mobilized to the membrane in times of high haemodynamic or metabolic demand, to tune excitation–contraction coupling.

Abstract

Voltage‐dependent L‐type Ca_V1.2 channels play an indispensable role in cardiac excitation–contraction coupling. Activation of the β‐adrenergic receptor (βAR)/cAMP/protein kinase A (PKA) signalling pathway leads to enhanced Ca_V1.2 activity, resulting in increased Ca²⁺ influx into ventricular myocytes and a positive inotropic response. Ca_V1.2 channels exhibit a clustered distribution along the T‐tubule sarcolemma of ventricular myocytes where nanometer proximity between channels permits Ca²⁺‐dependent co‐operative gating behaviour mediated by dynamic, physical, allosteric interactions between adjacent channel C‐terminal tails. This amplifies Ca²⁺ influx and augments myocyte Ca²⁺ transient and contraction amplitudes. We investigated whether βAR signalling could alter Ca_V1.2 channel clustering to facilitate co‐operative channel interactions and elevate Ca²⁺ influx in ventricular myocytes. Bimolecular fluorescence complementation experiments reveal that the βAR agonist, isoproterenol (ISO), promotes enhanced Ca_V1.2–Ca_V1.2 physical interactions. Super‐resolution nanoscopy and dynamic channel tracking indicate that these interactions are expedited by enhanced spatial proximity between channels, resulting in the appearance of Ca_V1.2 ‘super‐clusters’ along the z‐lines of ISO‐stimulated cardiomyocytes. The mechanism that leads to super‐cluster formation involves rapid, dynamic augmentation of sarcolemmal Ca_V1.2 channel abundance after ISO application. Optical and electrophysiological single channel recordings confirm that these newly inserted channels are functional and contribute to overt co‐operative gating behaviour of Ca_V1.2 channels in ISO stimulated myocytes. The results of the present study reveal a new facet of βAR‐mediated regulation of Ca_V1.2 channels in the heart and support the novel concept that a pre‐synthesized pool of sub‐sarcolemmal Ca_V1.2 channel‐containing vesicles/endosomes resides in cardiomyocytes and can be mobilized to the sarcolemma to tune excitation–contraction coupling to meet metabolic and/or haemodynamic demands.

Prevailing dogma holds that activation of the β‐adrenergic receptor/cAMP/protein kinase A signalling pathway leads to enhanced L‐type Ca_V1.2 channel activity, resulting in increased Ca²⁺ influx into ventricular myocytes and a positive inotropic response. However, the full mechanistic and molecular details underlying this phenomenon are incompletely understood.

Ca_V1.2 channel clusters decorate T‐tubule sarcolemmas of ventricular myocytes. Within clusters, nanometer proximity between channels permits Ca²⁺‐dependent co‐operative gating behaviour mediated by physical interactions between adjacent channel C‐terminal tails.

We report that stimulation of cardiomyocytes with isoproterenol, evokes dynamic, protein kinase A‐dependent augmentation of Ca_V1.2 channel abundance along cardiomyocyte T‐tubules, resulting in the appearance of channel ‘super‐clusters’, and enhanced channel co‐operativity that amplifies Ca²⁺ influx.

On the basis of these data, we suggest a new model in which a sub‐sarcolemmal pool of pre‐synthesized Ca_V1.2 channels resides in cardiomyocytes and can be mobilized to the membrane in times of high haemodynamic or metabolic demand, to tune excitation–contraction coupling.

Cardiac CaV1.2 channels require β subunits for β-adrenergic-mediated modulation but not trafficking

Ca2+ channel β-subunit interactions with pore-forming α-subunits are long-thought to be obligatory for channel trafficking to the cell surface and for tuning of basal biophysical properties in many tissues. Unexpectedly, we demonstrate that transgenic expression of mutant α1C subunits lacking capacity to bind CaVβ can traffic to the sarcolemma in adult cardiomyocytes in vivo and sustain normal excitation-contraction coupling. However, these β-less Ca2+ channels cannot be stimulated by β-adrenergic pathway agonists, and thus adrenergic augmentation of contractility is markedly impaired in isolated cardiomyocytes and in hearts. Similarly, viral-mediated expression of a β-subunit-sequestering peptide sharply curtailed β-adrenergic stimulation of WT Ca2+ channels, identifying an approach to specifically modulate β-adrenergic regulation of cardiac contractility. Our data demonstrate that β subunits are required for β-adrenergic regulation of CaV1.2 channels and positive inotropy in the heart, but are dispensable for CaV1.2 trafficking to the adult cardiomyocyte cell surface, and for basal function and excitation-contraction coupling.

Voltage-Gated Calcium Channels: Key Players in Sensory Coding in the Retina and the Inner Ear

Calcium influx through voltage-gated Ca (Ca_V) channels is the first step in synaptic transmission. This review concerns Ca_V channels at ribbon synapses in primary sense organs and their specialization for efficient coding of stimuli in the physical environment. Specifically, we describe molecular, biochemical, and biophysical properties of the Ca_V channels in sensory receptor cells of the retina, cochlea, and vestibular apparatus, and we consider how such properties might change over the course of development and contribute to synaptic plasticity. We pay particular attention to factors affecting the spatial arrangement of Ca_V channels at presynaptic, ribbon-type active zones, because the spatial relationship between Ca_V channels and release sites has been shown to affect synapse function critically in a number of systems. Finally, we review identified synaptopathies affecting sensory systems and arising from dysfunction of L-type, Ca_V1.3, and Ca_V1.4 channels or their protein modulatory elements.

Increase of CaV3 channel activity induced by HVA β1b-subunit is not mediated by a physical interaction

Objective

Low voltage-activated (LVA) calcium channels are crucial for regulating oscillatory behavior in several types of neurons and other excitable cells. LVA channels dysfunction has been implicated in epilepsy, neuropathic pain, cancer, among other diseases. Unlike for High Voltage-Activated (HVA) channels, voltage-dependence and kinetics of currents carried by recombinant LVA, i.e., Ca_V3 channels, are quite similar to those observed in native currents. Therefore, whether these channels are regulated by HVA auxiliary subunits, remain controversial. Here, we used the α1-subunits of Ca_V3.1, Ca_V3.2, and Ca_V3.3 channels, together with HVA auxiliary β-subunits to perform electrophysiological, confocal microscopy and immunoprecipitation experiments, in order to further explore this possibility.

Results

Functional expression of Ca_V3 channels is up-regulated by all four β-subunits, although most consistent effects were observed with the β1b-subunit. The biophysical properties of Ca_V3 channels were not modified by any β-subunit. Furthermore, although β1b-subunits increased colocalization of GFP-tagged Ca_V3 channels and the plasma membrane of HEK-293 cells, western blots analysis revealed the absence of physical interaction between Ca_V3.3 and β1b-subunits as no co-immunoprecipitation was observed. These results provide solid evidence that the up-regulation of LVA channels in the presence of HVA-β1b subunit is not mediated by a high affinity interaction between both proteins.

Electronic supplementary material

The online version of this article (10.1186/s13104-018-3917-1) contains supplementary material, which is available to authorized users.

Rapid Turnover of the Cardiac L-Type CaV1.2 Channel by Endocytic Recycling Regulates Its Cell Surface Availability

Calcium entry through Ca_V1.2 L-type calcium channels regulates cardiac contractility. Here, we study the impact of exocytic and post-endocytic trafficking on cell surface channel abundance in cardiomyocytes. Single-molecule localization and confocal microscopy reveal an intracellular Ca_V1.2 pool tightly associated with microtubules from the perinuclear region to the cell periphery, and with actin filaments at the cell cortex. Channels newly inserted into the plasma membrane become internalized with an average time constant of 7.5 min and are sorted out to the Rab11a-recycling compartment. Ca_V1.2 recycling suffices for maintaining stable L-type current amplitudes over 20 hr independent of de novo channel transport along microtubules. Disruption of the actin cytoskeleton re-routes Ca_V1.2 from recycling toward lysosomal degradation. We identify endocytic recycling as essential for the homeostatic regulation of voltage-dependent calcium influx into cardiomyocytes. This mechanism provides the basis for a dynamic adjustment of the channel's surface availability and thus, of heart's contraction.

Involvement of ion channels in allergy

Allergic asthma is a complex disease, often characterized by an inappropriate Th2 response to normally harmless allergens. Epithelial cells damaged or activated by the allergen produce IL-33, TSLP and IL-25, activating ILC2 and dendritic cells. The latter migrate into lymph nodes where they induce Th2-cell commitment. Th2 and other type 2 innate inflammatory cells trigger inflammation and airway hyper-reactivity. The toolbox consisting of the ion channels varies from one cellular type to another and depends on its activation state, offering the possibility to design novel drugs in the field of allergy. We will discuss about some channels as calcium, nonselective cation, potassium and chloride channels that appear as good candidates in allergy.

The CaV2α1 EF-hand F helix tyrosine, a highly conserved locus for GPCR inhibition of CaV2 channels

The sensory neuron of Aplysia californica participates in several forms of presynaptic plasticity including homosynaptic depression, heterosynaptic depression, facilitation and the reversal of depression. The calcium channel triggering neurotransmitter release at most synapses is Ca_V2, consisting of the pore forming α1 subunit (Ca_V2α1), and auxiliary Ca_Vβ, and Ca_Vα2δ subunits. To determine the role of the Ca_V2 channel in presynaptic plasticity in Aplysia, we cloned Aplysia Ca_V2α1, Ca_Vβ, and Ca_Vα2δ and over-expressed the proteins in Aplysia sensory neurons (SN). We show expression of exogenous Ca_V2α1 in the neurites of cultured Aplysia SN. One proposed mechanism for heterosynaptic depression in Aplysia is through inhibition of Ca_V2. Here, we demonstrate that heterosynaptic depression of the Ca_V2 calcium current is inhibited when a channel with a Y-F mutation at the conserved Src phosphorylation site is expressed, showing the strong conservation of this mechanism over evolution. We also show that the Y-F mutation reduces heterosynaptic inhibition of neurotransmitter release, highlighting the physiological importance of this mechanism for the regulation of synaptic efficacy. These results also demonstrate our ability to replace endogenous Ca_V2 channels with recombinant channels allowing future examination of the structure function relationship of Ca_V2 in the regulation of transmitter release in this system.

The β and α2δ auxiliary subunits of voltage-gated calcium channel 1 (Cav1) are required for TH2 lymphocyte function and acute allergic airway inflammation

BACKGROUND

T lymphocytes express not only cell membrane ORAI calcium release-activated calcium modulator 1 but also voltage-gated calcium channel (Ca_v) 1 channels. In excitable cells these channels are composed of the ion-forming pore α1 and auxiliary subunits (β and α2δ) needed for proper trafficking and activation of the channel. Previously, we disclosed the role of Ca_v1.2 α1 in mouse and human T_H2 but not T_H1 cell functions and showed that knocking down Ca_v1 α1 prevents experimental asthma.

OBJECTIVE

We investigated the role of β and α2δ auxiliary subunits on Ca_v1 α1 function in T_H2 lymphocytes and on the development of acute allergic airway inflammation.

METHODS

We used Ca_vβ antisense oligonucleotides to knock down Ca_vβ and gabapentin, a drug that binds to and inhibits α2δ1 and α2δ2, to test their effects on T_H2 functions and their capacity to reduce allergic airway inflammation.

RESULTS

Mouse and human T_H2 cells express mainly Ca_vβ1, β3, and α2δ2 subunits. Ca_vβ antisense reduces T-cell receptor-driven calcium responses and cytokine production by mouse and human T_H2 cells with no effect on T_H1 cells. Ca_vβ is mainly involved in restraining Ca_v1.2 α1 degradation through the proteasome because a proteasome inhibitor partially restores the α1 protein level. Gabapentin impairs the T-cell receptor-driven calcium response and cytokine production associated with the loss of α2δ2 protein in T_H2 cells.

CONCLUSIONS

These results stress the role of Ca_vβ and α2δ2 auxiliary subunits in the stability and activation of Ca_v1.2 channels in T_H2 lymphocytes both in vitro and in vivo, as demonstrated by the beneficial effect of Ca_vβ antisense and gabapentin in allergic airway inflammation.

Effect of knockout of α2δ-1 on action potentials in mouse sensory neurons

Gene deletion of the voltage-gated calcium channel auxiliary subunit α₂δ-1 has been shown previously to have a cardiovascular phenotype, and a reduction in mechano- and cold sensitivity, coupled with delayed development of neuropathic allodynia. We have also previously shown that dorsal root ganglion (DRG) neuron calcium channel currents were significantly reduced in α₂δ-1 knockout mice. To extend our findings in these sensory neurons, we have examined here the properties of action potentials (APs) in DRG neurons from α₂δ-1 knockout mice in comparison to their wild-type (WT) littermates, in order to dissect how the calcium channels that are affected by α₂δ-1 knockout are involved in setting the duration of individual APs and their firing frequency. Our main findings are that there is reduced Ca²⁺ entry on single AP stimulation, particularly in the axon proximal segment, reduced AP duration and reduced firing frequency to a 400 ms stimulation in α₂δ-1 knockout neurons, consistent with the expected role of voltage-gated calcium channels in these events. Furthermore, lower intracellular Ca²⁺ buffering also resulted in reduced AP duration, and a lower frequency of AP firing in WT neurons, mimicking the effect of α₂δ-1 knockout. By contrast, we did not obtain any consistent evidence for the involvement of Ca²⁺-activation of large conductance calcium-activated potassium (BK) and small conductance calcium-activated potassium (SK) channels in these events. In conclusion, the reduced Ca²⁺ elevation as a result of single AP stimulation is likely to result from the reduced duration of the AP in α₂δ-1 knockout sensory neurons.

This article is part of the themed issue ‘Evolution brings Ca²⁺ and ATP together to control life and death’.

Constitutive activity of the Ghrelin receptor reduces surface expression of voltage-gated Ca2+ channels in a CaVβ-dependent manner

Voltage-gated Ca²⁺ (Ca_V) channels couple membrane depolarization to Ca²⁺ influx, triggering a range of Ca²⁺-dependent cellular processes. Ca_V channels are, therefore, crucial in shaping neuronal activity and function, depending on their individual temporal and spatial properties. Furthermore, many neurotransmitters and drugs that act through G protein coupled receptors (GPCRs), modulate neuronal activity by altering the expression, trafficking, or function of Ca_V channels. GPCR-dependent mechanisms that downregulate Ca_V channel expression levels are observed in many neurons but are, by comparison, less studied. Here we show that the growth hormone secretagogue receptor type 1a (GHSR), a GPCR, can inhibit the forwarding trafficking of several Ca_V subtypes, even in the absence of agonist. This constitutive form of GPCR inhibition of Ca_V channels depends on the presence of a Ca_Vβ subunit. Ca_Vβ subunits displace Ca_Vα₁ subunits from the endoplasmic reticulum. The actions of GHSR on Ca_V channels trafficking suggest a role for this signaling pathway in brain areas that control food intake, reward, and learning and memory.

Involvement of Parkin in the ubiquitin proteasome system-mediated degradation of N-type voltage-gated Ca2+ channels

N-type calcium (CaV2.2) channels are widely expressed in the brain and the peripheral nervous system, where they play important roles in the regulation of transmitter release. Although CaV2.2 channel expression levels are precisely regulated, presently little is known regarding the molecules that mediate its synthesis and degradation. Previously, by using a combination of biochemical and functional analyses, we showed that the complex formed by the light chain 1 of the microtubule-associated protein 1B (LC1-MAP1B) and the ubiquitin-proteasome system (UPS) E2 enzyme UBE2L3, may interact with the CaV2.2 channels promoting ubiquitin-mediated degradation. The present report aims to gain further insights into the possible mechanism of degradation of the neuronal CaV2.2 channel by the UPS. First, we identified the enzymes UBE3A and Parkin, members of the UPS E3 ubiquitin ligase family, as novel CaV2.2 channel binding partners, although evidence to support a direct protein-protein interaction is not yet available. Immunoprecipitation assays confirmed the interaction between UBE3A and Parkin with CaV2.2 channels heterologously expressed in HEK-293 cells and in neural tissues. Parkin, but not UBE3A, overexpression led to a reduced CaV2.2 protein level and decreased current density. Electrophysiological recordings performed in the presence of MG132 prevented the actions of Parkin suggesting enhanced channel proteasomal degradation. Together these results unveil a novel functional coupling between Parkin and the CaV2.2 channels and provide a novel insight into the basic mechanisms of CaV channels protein quality control and functional expression.

CaV2 channel subtype expression in rat sympathetic neurons is selectively regulated by α2δ subunits

Type two voltage gated calcium (Ca_V2) channels are the primary mediators of neurotransmission at neuronal presynapses, but their function at neural soma is also important in regulating excitability. ¹ Mechanisms that regulate Ca_V2 channel expression at synapses have been studied extensively, which motivated us to perform similar studies in the soma. Rat sympathetic neurons from the superior cervical ganglion (SCG) natively express Ca_V2.2 and Ca_V2.3. ² We noted previously that heterologous expression of Ca_V2.1 but not Ca_V2.2 results in increased calcium current in SCG neurons. ³ In the present study, we extended these observations to show that both Ca_V2.1 and Ca_V2.3 expression resulted in increased calcium currents while Ca_V2.2 expression did not. Further, Ca_V2.1 could displace native Ca_V2.2 channels, but Ca_V2.3 expression could not. Heterologous expression of the individual accessory subunits α₂δ-1, α₂δ-2, α₂δ-3, or β4 alone failed to increase current density, suggesting that the calcium current ceiling when Ca_V2.2 was over-expressed was not due to lack of these subunits. Interestingly, introduction of recombinant α2δ subunits produced surprising effects on displacement of native Ca_V2.2 by recombinant channels. Both α₂δ-1 and α₂δ-2 seemed to promote Ca_V2.2 displacement by recombinant channel expression, while α₂δ-3 appeared to protect Ca_V2.2 from displacement. Thus, we observe a selective prioritization of Ca_V channel functional expression in neurons by specific α₂δ subunits. These data highlight a new function for α₂δ subtypes that could shed light on subtype selectivity of Ca_V2 membrane expression.

The HOOK region of β subunits controls gating of voltage-gated Ca2+ channels by electrostatically interacting with plasma membrane

Recently, we showed that the HOOK region of the β2 subunit electrostatically interacts with the plasma membrane and regulates the current inactivation and phosphatidylinositol 4,5-bisphosphate (PIP₂) sensitivity of voltage-gated Ca²⁺ (Ca_V) 2.2 channels. Here, we report that voltage-dependent gating and current density of the Ca_V2.2 channels are also regulated by the HOOK region of the β2 subunit. The HOOK region can be divided into 3 domains: S (polyserine), A (polyacidic), and B (polybasic). We found that the A domain shifted the voltage-dependent inactivation and activation of Ca_V2.2 channels to more hyperpolarized and depolarized voltages, respectively, whereas the B domain evoked these responses in the opposite directions. In addition, the A domain decreased the current density of the Ca_V2.2 channels, while the B domain increased it. Together, our data demonstrate that the flexible HOOK region of the β2 subunit plays an important role in determining the overall Ca_V channel gating properties.

LRP1 influences trafficking of N-type calcium channels via interaction with the auxiliary α2δ-1 subunit

Voltage-gated Ca²⁺ (Ca_V) channels consist of a pore-forming α1 subunit, which determines the main functional and pharmacological attributes of the channel. The Ca_V1 and Ca_V2 channels are associated with auxiliary β- and α₂δ-subunits. The molecular mechanisms involved in α₂δ subunit trafficking, and the effect of α₂δ subunits on trafficking calcium channel complexes remain poorly understood. Here we show that α₂δ-1 is a ligand for the Low Density Lipoprotein (LDL) Receptor-related Protein-1 (LRP1), a multifunctional receptor which mediates trafficking of cargoes. This interaction with LRP1 is direct, and is modulated by the LRP chaperone, Receptor-Associated Protein (RAP). LRP1 regulates α₂δ binding to gabapentin, and influences calcium channel trafficking and function. Whereas LRP1 alone reduces α₂δ-1 trafficking to the cell-surface, the LRP1/RAP combination enhances mature glycosylation, proteolytic processing and cell-surface expression of α₂δ-1, and also increase plasma-membrane expression and function of Ca_V2.2 when co-expressed with α₂δ-1. Furthermore RAP alone produced a small increase in cell-surface expression of Ca_V2.2, α₂δ-1 and the associated calcium currents. It is likely to be interacting with an endogenous member of the LDL receptor family to have these effects. Our findings now provide a key insight and new tools to investigate the trafficking of calcium channel α₂δ subunits.

Trafficking of neuronal calcium channels

Neuronal voltage-gated calcium channels (VGCCs) serve complex yet essential physiological functions via their pivotal role in translating electrical signals into intracellular calcium elevations and associated downstream signalling pathways. There are a number of regulatory mechanisms to ensure a dynamic control of the number of channels embedded in the plasma membrane, whereas alteration of the surface expression of VGCCs has been linked to various disease conditions. Here, we provide an overview of the mechanisms that control the trafficking of VGCCs to and from the plasma membrane, and discuss their implication in pathophysiological conditions and their potential as therapeutic targets.

Ubiquitin Ligase RNF138 Promotes Episodic Ataxia Type 2-Associated Aberrant Degradation of Human Cav2.1 (P/Q-Type) Calcium Channels

Voltage-gated Ca_V2.1 channels comprise a pore-forming α_1A subunit with auxiliary α₂δ and β subunits. Ca_V2.1 channels play an essential role in regulating synaptic signaling. Mutations in the human gene encoding the Ca_V2.1 subunit are associated with the cerebellar disease episodic ataxia type 2 (EA2). Several EA2-causing mutants exhibit impaired protein stability and exert dominant-negative suppression of Ca_V2.1 wild-type (WT) protein expression via aberrant proteasomal degradation. Here, we set out to delineate the protein degradation mechanism of human Ca_V2.1 subunit by identifying RNF138, an E3 ubiquitin ligase, as a novel Ca_V2.1-binding partner. In neurons, RNF138 and Ca_V2.1 coexist in the same protein complex and display notable subcellular colocalization at presynaptic and postsynaptic regions. Overexpression of RNF138 promotes polyubiquitination and accelerates protein turnover of Ca_V2.1. Disrupting endogenous RNF138 function with a mutant (RNF138-H36E) or shRNA infection significantly upregulates the Ca_V2.1 protein level and enhances Ca_V2.1 protein stability. Disrupting endogenous RNF138 function also effectively rescues the defective protein expression of EA2 mutants, as well as fully reversing EA2 mutant-induced excessive proteasomal degradation of Ca_V2.1 WT subunits. RNF138-H36E coexpression only partially restores the dominant-negative effect of EA2 mutants on Ca_V2.1 WT functional expression, which can be attributed to defective membrane trafficking of Ca_V2.1 WT in the presence of EA2 mutants. We propose that RNF138 plays a critical role in the homeostatic regulation of Ca_V2.1 protein level and functional expression and that RNF138 serves as the primary E3 ubiquitin ligase promoting EA2-associated aberrant degradation of human Ca_V2.1 subunits.SIGNIFICANCE STATEMENT Loss-of-function mutations in the human Ca_V2.1 subunit are linked to episodic ataxia type 2 (EA2), a dominantly inherited disease characterized by paroxysmal attacks of ataxia and nystagmus. EA2-causing mutants may exert dominant-negative effects on the Ca_V2.1 wild-type subunit via aberrant proteasomal degradation. The molecular nature of the Ca_V2.1 ubiquitin-proteasome degradation pathway is currently unknown. The present study reports the first identification of an E3 ubiquitin ligase for Ca_V2.1, RNF138. Ca_V2.1 protein stability is dynamically regulated by RNF138 and auxiliary α₂δ and β subunits. We provide a proof of concept that protecting the human Ca_V2.1 subunit from excessive proteasomal degradation with specific interruption of endogenous RNF138 function may partially contribute to the future development of a novel therapeutic strategy for EA2 patients.

Glycosylation of voltage-gated calcium channels in health and disease

Voltage-gated calcium channels (VGCCs) are transmembrane proteins that translate electrical activities into intracellular calcium elevations and downstream signaling pathways. They serve essential physiological functions including communication between nerve cells, muscle contraction, cardiac activity, and release of hormones and neurotransmitters. Asparagine-linked glycosylation has emerged as an essential post-translational modification to control the number of channels embedded in the plasma membrane but also their functional gating properties. This review provides a comprehensive overview about the current state of knowledge on the role of glycosylation in the expression and functioning of VGCCs, and discusses how variations in the glycosylation of the channel proteins can contribute to pathological conditions.

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 (Ca_V2.2) 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 Ca_V2.2 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.

DOI:http://dx.doi.org/10.7554/eLife.21143.001

Regulation of voltage gated calcium channels by GPCRs and post-translational modification

Calcium entry via voltage gated calcium channels mediates a wide range of physiological functions, whereas calcium channel dysregulation has been associated with numerous pathophysiological conditions. There are myriad cell signaling pathways that act on voltage gated calcium channels to fine tune their activities and to regulate their cell surface expression. These regulatory mechanisms include the activation of G protein-coupled receptors and downstream phosphorylation events, and their control over calcium channel trafficking through direct physical interactions. Calcium channels also undergo post-translational modifications that alter both function and density of the channels in the plasma membrane. Here we focus on select aspects of these regulatory mechanisms and highlight recent developments.

The CaVβ Subunit Protects the I-II Loop of the Voltage-gated Calcium Channel CaV2.2 from Proteasomal Degradation but Not Oligoubiquitination

CaVβ subunits interact with the voltage-gated calcium channel CaV2.2 on a site in the intracellular loop between domains I and II (the I-II loop). This interaction influences the biophysical properties of the channel and leads to an increase in its trafficking to the plasma membrane. We have shown previously that a mutant CaV2.2 channel that is unable to bind CaVβ subunits (CaV2.2 W391A) was rapidly degraded (Waithe, D., Ferron, L., Page, K. M., Chaggar, K., and Dolphin, A. C. (2011) J. Biol. Chem. 286, 9598-9611). Here we show that, in the absence of CaVβ subunits, a construct consisting of the I-II loop of CaV2.2 was directly ubiquitinated and degraded by the proteasome system. Ubiquitination could be prevented by mutation of all 12 lysine residues in the I-II loop to arginines. Including a palmitoylation motif at the N terminus of CaV2.2 I-II loop was insufficient to target it to the plasma membrane in the absence of CaVβ subunits even when proteasomal degradation was inhibited with MG132 or ubiquitination was prevented by the lysine-to-arginine mutations. In the presence of CaVβ subunit, the palmitoylated CaV2.2 I-II loop was protected from degradation, although oligoubiquitination could still occur, and was efficiently trafficked to the plasma membrane. We propose that targeting to the plasma membrane requires a conformational change in the I-II loop that is induced by binding of the CaVβ subunit.

Voltage-gated calcium channels and their auxiliary subunits: physiology and pathophysiology and pharmacology

Voltage‐gated calcium channels are essential players in many physiological processes in excitable cells. There are three main subdivisions of calcium channel, defined by the pore‐forming α₁ subunit, the Ca_V1, Ca_V2 and Ca_V3 channels. For all the subtypes of voltage‐gated calcium channel, their gating properties are key for the precise control of neurotransmitter release, muscle contraction and cell excitability, among many other processes. For the Ca_V1 and Ca_V2 channels, their ability to reach their required destinations in the cell membrane, their activation and the fine tuning of their biophysical properties are all dramatically influenced by the auxiliary subunits that associate with them. Furthermore, there are many diseases, both genetic and acquired, involving voltage‐gated calcium channels. This review will provide a general introduction and then concentrate particularly on the role of auxiliary α₂δ subunits in both physiological and pathological processes involving calcium channels, and as a therapeutic target.

Molecular Basis of the Membrane Interaction of the β2e Subunit of Voltage-Gated Ca(2+) Channels

The auxiliary β subunit plays an important role in the regulation of voltage-gated calcium (CaV) channels. Recently, it was revealed that β2e associates with the plasma membrane through an electrostatic interaction between N-terminal basic residues and anionic phospholipids. However, a molecular-level understanding of β-subunit membrane recruitment in structural detail has remained elusive. In this study, using a combination of site-directed mutagenesis, liposome-binding assays, and multiscale molecular-dynamics (MD) simulation, we developed a physical model of how the β2e subunit is recruited electrostatically to the plasma membrane. In a fluorescence resonance energy transfer assay with liposomes, binding of the N-terminal peptide (23 residues) to liposome was significantly increased in the presence of phosphatidylserine (PS) and phosphatidylinositol 4,5-bisphosphate (PIP2). A mutagenesis analysis suggested that two basic residues proximal to Met-1, Lys-2 (K2) and Trp-5 (W5), are more important for membrane binding of the β2e subunit than distal residues from the N-terminus. Our MD simulations revealed that a stretched binding mode of the N-terminus to PS is required for stable membrane attachment through polar and nonpolar interactions. This mode obtained from MD simulations is consistent with experimental results showing that K2A, W5A, and K2A/W5A mutants failed to be targeted to the plasma membrane. We also investigated the effects of a mutated β2e subunit on inactivation kinetics and regulation of CaV channels by PIP2. In experiments with voltage-sensing phosphatase (VSP), a double mutation in the N-terminus of β2e (K2A/W5A) increased the PIP2 sensitivity of CaV2.2 and CaV1.3 channels by ∼3-fold compared with wild-type β2e subunit. Together, our results suggest that membrane targeting of the β2e subunit is initiated from the nonspecific electrostatic insertion of N-terminal K2 and W5 residues into the membrane. The PS-β2e interaction observed here provides a molecular insight into general principles for protein binding to the plasma membrane, as well as the regulatory roles of phospholipids in transporters and ion channels.