The amino terminus of a calcium channel beta subunit sets rates of channel inactivation independently of the subunit's effect on activation

Asymmetric contribution of a selectivity filter gate in triggering inactivation of CaV1.3 channels

Voltage-dependent and Ca2+-dependent inactivation (VDI and CDI, respectively) of CaV channels are two biologically consequential feedback mechanisms that fine-tune Ca2+ entry into neurons and cardiomyocytes. Although known to be initiated by distinct molecular events, how these processes obstruct conduction through the channel pore remains poorly defined. Here, focusing on ultrahighly conserved tryptophan residues in the interdomain interfaces near the selectivity filter of CaV1.3, we demonstrate a critical role for asymmetric conformational changes in mediating VDI and CDI. Specifically, mutagenesis of the domain III-IV interface, but not others, enhanced VDI. Molecular dynamics simulations demonstrate that mutations in distinct selectivity filter interfaces differentially impact conformational flexibility. Furthermore, mutations in distinct domains preferentially disrupt CDI mediated by the N- versus C-lobes of CaM, thus uncovering a scheme of structural bifurcation of CaM signaling. These findings highlight the fundamental importance of the asymmetric arrangement of the pseudotetrameric CaV pore domain for feedback inhibition.

Asymmetric Contribution of a Selectivity Filter Gate in Triggering Inactivation of CaV1.3 Channels

Voltage-dependent and Ca2+-dependent inactivation (VDI and CDI, respectively) of CaV channels are two biologically consequential feedback mechanisms that fine-tune Ca2+ entry into neurons and cardiomyocytes. Although known to be initiated by distinct molecular events, how these processes obstruct conduction through the channel pore remains poorly defined. Here, focusing on ultra-highly conserved tryptophan residues in the inter-domain interfaces near the selectivity filter of CaV1.3, we demonstrate a critical role for asymmetric conformational changes in mediating VDI and CDI. Specifically, mutagenesis of the domain III-IV interface, but not others, enhanced VDI. Molecular dynamics simulations demonstrate that mutations in distinct selectivity filter interfaces differentially impact conformational flexibility. Furthermore, mutations in distinct domains preferentially disrupt CDI mediated by the N- versus C-lobes of CaM, thus uncovering a scheme of structural bifurcation of CaM signaling. These findings highlight the fundamental importance of the asymmetric arrangement of the pseudo-tetrameric CaV pore domain for feedback inhibition.

Inactivation influences the extent of inhibition of voltage-gated Ca+2 channels by Gem-implications for channelopathies

Voltage-gated Ca2+ channels (VGCC) directly control muscle contraction and neurotransmitter release, and slower processes such as cell differentiation, migration, and death. They are potently inhibited by RGK GTP-ases (Rem, Rem2, Rad, and Gem/Kir), which decrease Ca2+ channel membrane expression, as well as directly inhibit membrane-resident channels. The mechanisms of membrane-resident channel inhibition are difficult to study because RGK-overexpression causes complete or near complete channel inhibition. Using titrated levels of Gem expression in Xenopus oocytes to inhibit WT P/Q-type calcium channels by ∼50%, we show that inhibition is dependent on channel inactivation. Interestingly, fast-inactivating channels, including Familial Hemiplegic Migraine mutants, are more potently inhibited than WT channels, while slow-inactivating channels, such as those expressed with the Cavβ2a auxiliary subunit, are spared. We found similar results in L-type channels, and, remarkably, Timothy Syndrome mutant channels were insensitive to Gem inhibition. Further results suggest that RGKs slow channel recovery from inactivation and further implicate RGKs as likely modulating factors in channelopathies.

A novel calcium channel Cavβ2 splice variant with unique properties predominates in the retina

Cavβ subunits are essential for surface expression of voltage-gated calcium channel complexes and crucially modulate biophysical properties like voltage-dependent inactivation. Here, we describe the discovery and characterization of a novel Cavβ2 variant with distinct features that predominates in the retina. We determined spliced exons in retinal transcripts of the Cacnb2 gene, coding for Cavβ2, by RNA-Seq data analysis and quantitative PCR. We cloned a novel Cavβ2 splice variant from mouse retina, which we are calling β2i, and investigated biophysical properties of calcium currents with this variant in a heterologous expression system as well as its intrinsic membrane interaction when expressed alone. Our data showed that β2i predominated in the retina with expression in photoreceptors and bipolar cells. Furthermore, we observed that the β2i N-terminus exhibited an extraordinary concentration of hydrophobic residues, a distinct feature not seen in canonical variants. The biophysical properties resembled known membrane-associated variants, and β2i exhibited both a strong membrane association and a propensity for clustering, which depended on hydrophobic residues in its N-terminus. We considered available Cavβ structure data to elucidate potential mechanisms underlying the observed characteristics but resolved N-terminus structures were lacking and thus, precluded clear conclusions. With this description of a novel N-terminus variant of Cavβ2, we expand the scope of functional variation through N-terminal splicing with a distinct form of membrane attachment. Further investigation of the molecular mechanisms underlying the features of β2i could provide new angles on the way Cavβ subunits modulate Ca2+ channels at the plasma membrane.

Molecular basis of the PIP2-dependent regulation of CaV2.2 channel and its modulation by CaV β subunits

High-voltage-activated Ca2+ (CaV) channels that adjust Ca2+ influx upon membrane depolarization are differentially regulated by phosphatidylinositol 4,5-bisphosphate (PIP2) in an auxiliary CaV β subunit-dependent manner. However, the molecular mechanism by which the β subunits control the PIP2 sensitivity of CaV channels remains unclear. By engineering various α1B and β constructs in tsA-201 cells, we reported that at least two PIP2-binding sites, including the polybasic residues at the C-terminal end of I–II loop and the binding pocket in S4II domain, exist in the CaV2.2 channels. Moreover, they were distinctly engaged in the regulation of channel gating depending on the coupled CaV β2 subunits. The membrane-anchored β subunit abolished the PIP2 interaction of the phospholipid-binding site in the I–II loop, leading to lower PIP2 sensitivity of CaV2.2 channels. By contrast, PIP2 interacted with the basic residues in the S4II domain of CaV2.2 channels regardless of β2 isotype. Our data demonstrated that the anchoring properties of CaV β2 subunits to the plasma membrane determine the biophysical states of CaV2.2 channels by regulating PIP2 coupling to the nonspecific phospholipid-binding site in the I–II loop.

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.

Voltage-Gated Ca2+ Channels in Dopaminergic Substantia Nigra Neurons: Therapeutic Targets for Neuroprotection in Parkinson's Disease?

The loss of dopamine (DA)-producing neurons in the substantia nigra pars compacta (SN) underlies the core motor symptoms of the progressive movement disorder Parkinson's disease (PD). To date, no treatment to prevent or slow SN DA neurodegeneration exists; thus, the identification of the underlying factors contributing to the high vulnerability of these neurons represents the basis for the development of novel therapies. Disrupted Ca2+ homeostasis and mitochondrial dysfunction seem to be key players in the pathophysiology of PD. The autonomous pacemaker activity of SN DA neurons, in combination with low cytosolic Ca2+ buffering, leads to large somatodendritic fluctuations of intracellular Ca2+ levels that are linked to elevated mitochondrial oxidant stress. L-type voltage-gated Ca2+ channels (LTCCs) contribute to these Ca2+ oscillations in dendrites, and LTCC inhibition was beneficial in cellular and in vivo animal models of PD. However, in a recently completed phase 3 clinical trial, the dihydropyridine (DHP) LTCC inhibitor isradipine failed to slow disease progression in early PD patients, questioning the feasibility of DHPs for PD therapy. Novel evidence also suggests that R- and T-type Ca2+ channels (RTCCs and TTCCs, respectively) represent potential PD drug targets. This short review aims to (re)evaluate the therapeutic potential of LTCC, RTCC, and TTCC inhibition in light of novel preclinical and clinical data and the feasibility of available Ca2+ channel blockers to modify PD disease progression. I also summarize their cell-specific roles for SN DA neuron function and describe how their gating properties allow activity (and thus their contribution to stressful Ca2+ oscillations) during pacemaking.

Cav2.3 R-type calcium channels: from its discovery to pathogenic de novo CACNA1E variants: a historical perspective

So-called pharmacoresistant (R-type) voltage-gated Ca²⁺ channels are structurally only partially characterized. Most of them are encoded by the CACNA1E gene and are expressed as different Ca_v2.3 splice variants (variant Ca_v2.3a to Ca_v2.3e or f) as the ion conducting subunit. So far, no inherited disease is known for the CACNA1E gene but recently spontaneous mutations leading to early death were identified, which will be brought into focus. In addition, a short historical overview may highlight the development to understand that upregulation during aging, easier activation by spontaneous mutations or lack of bioavailable inorganic cations (Zn²⁺ and Cu²⁺) may lead to similar pathologies caused by cellular overexcitation.

β1 subunit stabilises sodium channel Nav1.7 against mechanical stress

KEY POINTS

The voltage-gated sodium channel Nav1.7 is a key player in neuronal excitability and pain signalling. In addition to voltage sensing, the channel is also modulated by mechanical stress. Using whole-cell patch-clamp experiments, we discovered that the sodium channel subunit β1 is able to prevent the impact of mechanical stress on Nav1.7. An intramolecular disulfide bond of β1 was identified to be essential for stabilisation of inactivation, but not activation, against mechanical stress using molecular dynamics simulations, homology modelling and site-directed mutagenesis. Our results highlight the role of segment 6 of domain IV in fast inactivation. We present a candidate mechanism for sodium channel stabilisation against mechanical stress, ensuring reliable channel functionality in living systems.

ABSTRACT

Voltage-gated sodium channels are key players in neuronal excitability and pain signalling. Precise gating of these channels is crucial as even small functional alterations can lead to pathological phenotypes such as pain or heart failure. Mechanical stress has been shown to affect sodium channel activation and inactivation. This suggests that stabilising components are necessary to ensure precise channel gating in living organisms. Here, we show that mechanical shear stress affects voltage dependence of activation and fast inactivation of the Nav1.7 channel. Co-expression of the β1 subunit, however, protects both gating modes of Nav1.7 against mechanical shear stress. Using molecular dynamics simulation, homology modelling and site-directed mutagenesis, we identify an intramolecular disulfide bond of β1 (Cys21-Cys43) which is partially involved in this process: the β1-C43A mutant prevents mechanical modulation of voltage dependence of activation, but not of fast inactivation. Our data emphasise the unique role of segment 6 of domain IV for sodium channel fast inactivation and confirm previous reports that the intracellular process of fast inactivation can be modified by interfering with the extracellular end of segment 6 of domain IV. Thus, our data suggest that physiological gating of Nav1.7 may be protected against mechanical stress in a living organism by assembly with the β1 subunit.

Unravelling the complexities of vascular smooth muscle ion channels: Fine tuning of activity by ancillary subunits

Which ion channel is the most important for regulating vascular tone? Which one is responsible for controlling the resting membrane potential or repolarization? Which channels are recruited by different intracellular signalling pathways or change in certain vascular diseases? Many different ion channels have been identified in the vasculature over the years and claimed as future therapeutic targets. Unfortunately, several of these ion channels are not just found in the vasculature, with many of them also found to have prominent functional roles in different organs of the body, which then leads to off-target effects. As cardiovascular diseases are expected to increase worldwide to epidemic proportions, ion channel research and the hunt for the next major therapeutic target to treat different vascular diseases has never been more important. However, I believe that the question we should now be asking is: which ancillary subunits are involved in regulating specific ion channels in the vasculature and do they have the potential to be new therapeutic targets?

The HOOK region of voltage-gated Ca2+ channel β subunits senses and transmits PIP2 signals to the gate

The β subunit of voltage-gated Ca²⁺ (Ca_V) channels plays an important role in regulating gating of the α1 pore-forming subunit and its regulation by phosphatidylinositol 4,5-bisphosphate (PIP₂). Subcellular localization of the Ca_V β subunit is critical for this effect; N-terminal-dependent membrane targeting of the β subunit slows inactivation and decreases PIP₂ sensitivity. Here, we provide evidence that the HOOK region of the β subunit plays an important role in the regulation of Ca_V biophysics. Based on amino acid composition, we broadly divide the HOOK region into three domains: S (polyserine), A (polyacidic), and B (polybasic). We show that a β subunit containing only its A domain in the HOOK region increases inactivation kinetics and channel inhibition by PIP₂ depletion, whereas a β subunit with only a B domain decreases these responses. When both the A and B domains are deleted, or when the entire HOOK region is deleted, the responses are elevated. Using a peptide-to-liposome binding assay and confocal microscopy, we find that the B domain of the HOOK region directly interacts with anionic phospholipids via polybasic and two hydrophobic Phe residues. The β2c-short subunit, which lacks an A domain and contains fewer basic amino acids and no Phe residues in the B domain, neither associates with phospholipids nor affects channel gating dynamically. Together, our data suggest that the flexible HOOK region of the β subunit acts as an important regulator of Ca_V channel gating via dynamic electrostatic and hydrophobic interaction with the plasma membrane.

The role of auxiliary subunits for the functional diversity of voltage-gated calcium channels

Voltage-gated calcium channels (VGCCs) represent the sole mechanism to convert membrane depolarization into cellular functions like secretion, contraction, or gene regulation. VGCCs consist of a pore-forming α₁ subunit and several auxiliary channel subunits. These subunits come in multiple isoforms and splice-variants giving rise to a stunning molecular diversity of possible subunit combinations. It is generally believed that specific auxiliary subunits differentially regulate the channels and thereby contribute to the great functional diversity of VGCCs. If auxiliary subunits can associate and dissociate from pre-existing channel complexes, this would allow dynamic regulation of channel properties. However, most auxiliary subunits modulate current properties very similarly, and proof that any cellular calcium channel function is indeed modulated by the physiological exchange of auxiliary subunits is still lacking. In this review we summarize available information supporting a differential modulation of calcium channel functions by exchange of auxiliary subunits, as well as experimental evidence in support of alternative functions of the auxiliary subunits. At the heart of the discussion is the concept that, in their native environment, VGCCs function in the context of macromolecular signaling complexes and that the auxiliary subunits help to orchestrate the diverse protein–protein interactions found in these calcium channel signalosomes. Thus, in addition to a putative differential modulation of current properties, differential subcellular targeting properties and differential protein–protein interactions of the auxiliary subunits may explain the need for their vast molecular diversity. J. Cell. Physiol. 999: 00–00, 2015. © 2015 The Authors. Journal of Cellular Physiology Published by Wiley Periodicals, Inc. J. Cell. Physiol. 230: 2019–2031, 2015. © 2015 Wiley Periodicals, Inc.

Molecular characterization and functional expression of the Apis mellifera voltage-dependent Ca2+ channels

Voltage-gated Ca(2+) channels allow the influx of Ca(2+) ions from the extracellular space upon membrane depolarization and thus serve as a transducer between membrane potential and cellular events initiated by Ca(2+) transients. Most insects are predicted to possess three genes encoding Cavα, the main subunit of Ca(2+) channels, and several genes encoding the two auxiliary subunits, Cavβ and Cavα2δ; however very few of these genes have been cloned so far. Here, we cloned three full-length cDNAs encoding the three Cavα subunits (AmelCav1a, AmelCav2a and AmelCav3a), a cDNA encoding a novel variant of the Cavβ subunit (AmelCavβc), and three full-length cDNAs encoding three Cavα2δ subunits (AmelCavα2δ1 to 3) of the honeybee Apis mellifera. We identified several alternative or mutually exclusive exons in the sequence of the AmelCav2 and AmelCav3 genes. Moreover, we detected a stretch of glutamine residues in the C-terminus of the AmelCav1 subunit that is reminiscent of the motif found in the human Cav2.1 subunit of patients with Spinocerebellar Ataxia type 6. All these subunits contain structural domains that have been identified as functionally important in their mammalian homologues. For the first time, we could express three insect Cavα subunits in Xenopus oocytes and we show that AmelCav1a, 2a and 3a form Ca(2+) channels with distinctive properties. Notably, the co-expression of AmelCav1a or AmelCav2a with AmelCavβc and AmCavα2δ1 produces High Voltage-Activated Ca(2+) channels. On the other hand, expression of AmelCav3a alone leads to Low Voltage-Activated Ca(2+) channels.

Structure-function of proteins interacting with the α1 pore-forming subunit of high-voltage-activated calcium channels

Openings of high-voltage-activated (HVA) calcium channels lead to a transient increase in calcium concentration that in turn activate a plethora of cellular functions, including muscle contraction, secretion and gene transcription. To coordinate all these responses calcium channels form supramolecular assemblies containing effectors and regulatory proteins that couple calcium influx to the downstream signal cascades and to feedback elements. According to the original biochemical characterization of skeletal muscle Dihydropyridine receptors, HVA calcium channels are multi-subunit protein complexes consisting of a pore-forming subunit (α1) associated with four additional polypeptide chains β, α2, δ, and γ, often referred to as accessory subunits. Twenty-five years after the first purification of a high-voltage calcium channel, the concept of a flexible stoichiometry to expand the repertoire of mechanisms that regulate calcium channel influx has emerged. Several other proteins have been identified that associate directly with the α1-subunit, including calmodulin and multiple members of the small and large GTPase family. Some of these proteins only interact with a subset of α1-subunits and during specific stages of biogenesis. More strikingly, most of the α1-subunit interacting proteins, such as the β-subunit and small GTPases, regulate both gating and trafficking through a variety of mechanisms. Modulation of channel activity covers almost all biophysical properties of the channel. Likewise, regulation of the number of channels in the plasma membrane is performed by altering the release of the α1-subunit from the endoplasmic reticulum, by reducing its degradation or enhancing its recycling back to the cell surface. In this review, we discuss the structural basis, interplay and functional role of selected proteins that interact with the central pore-forming subunit of HVA calcium channels.

Calmodulin mutations associated with long QT syndrome prevent inactivation of cardiac L-type Ca(2+) currents and promote proarrhythmic behavior in ventricular myocytes

Recent work has identified missense mutations in calmodulin (CaM) that are associated with severe early-onset long-QT syndrome (LQTS), leading to the proposition that altered CaM function may contribute to the molecular etiology of this subset of LQTS. To date, however, no experimental evidence has established these mutations as directly causative of LQTS substrates, nor have the molecular targets of CaM mutants been identified. Here, therefore, we test whether expression of CaM mutants in adult guinea-pig ventricular myocytes (aGPVM) induces action-potential prolongation, and whether affiliated alterations in the Ca(2+) regulation of L-type Ca(2+) channels (LTCC) might contribute to such prolongation. In particular, we first overexpressed CaM mutants in aGPVMs, and observed both increased action potential duration (APD) and heightened Ca(2+) transients. Next, we demonstrated that all LQTS CaM mutants have the potential to strongly suppress Ca(2+)/CaM-dependent inactivation (CDI) of LTCCs, whether channels were heterologously expressed in HEK293 cells, or present in native form within myocytes. This attenuation of CDI is predicted to promote action-potential prolongation and boost Ca(2+) influx. Finally, we demonstrated how a small fraction of LQTS CaM mutants (as in heterozygous patients) would nonetheless suffice to substantially diminish CDI, and derange electrical and Ca(2+) profiles. In all, these results highlight LTCCs as a molecular locus for understanding and treating CaM-related LQTS in this group of patients.

The N-terminal domain tethers the voltage-gated calcium channel β2e-subunit to the plasma membrane via electrostatic and hydrophobic interactions

The β-subunit associates with the α1 pore-forming subunit of high voltage-activated calcium channels and modulates several aspects of ion conduction. Four β-subunits are encoded by four different genes with multiple splice variants. Only two members of this family, β2a and β2e, associate with the plasma membrane in the absence of the α1-subunit. Palmitoylation on a di-cysteine motif located at the N terminus of β2a promotes membrane targeting and correlates with the unique ability of this protein to slow down inactivation. In contrast, the mechanism by which β2e anchors to the plasma membrane remains elusive. Here, we identified an N-terminal segment in β2e encompassing a cluster of positively charged residues, which is strictly required for membrane anchoring, and when transferred to the cytoplasmic β1b isoform it confers membrane localization to the latter. In the presence of negatively charged phospholipid vesicles, this segment binds to acidic liposomes dependently on the ionic strength, and the intrinsic fluorescence emission maxima of its single tryptophan blue shifts considerably. Simultaneous substitution of more than two basic residues impairs membrane targeting. Coexpression of the fast inactivating R-type calcium channels with wild-type β2e, but not with a β2e membrane association-deficient mutant, slows down inactivation. We propose that a predicted α-helix within this domain orienting parallel to the membrane tethers the β2e-subunit to the lipid bilayer via electrostatic interactions. Penetration of the tryptophan side chain into the lipidic core stabilizes the membrane-bound conformation. This constitutes a new mechanism for membrane anchoring among the β-subunit family that also sustains slowed inactivation.

How "Pharmacoresistant" is Cav2.3, the Major Component of Voltage-Gated R-type Ca2+ Channels?

Membrane-bound voltage-gated Ca²⁺ channels (VGCCs) are targets for specific signaling complexes, which regulate important processes like gene expression, neurotransmitter release and neuronal excitability. It is becoming increasingly evident that the so called “resistant” (R-type) VGCC Ca_v2.3 is critical in several physiologic and pathophysiologic processes in the central nervous system, vascular system and in endocrine systems. However its eponymous attribute of pharmacologic inertness initially made in depth investigation of the channel difficult. Although the identification of SNX-482 as a fairly specific inhibitor of Ca_v2.3 in the nanomolar range has enabled insights into the channels properties, availability of other pharmacologic modulators of Ca_v2.3 with different chemical, physical and biological properties are of great importance for future investigations. Therefore the literature was screened systematically for molecules that modulate Ca_v2.3 VGCCs.

Characterization of the first honeybee Ca²⁺ channel subunit reveals two novel species- and splicing-specific modes of regulation of channel inactivation

The honeybee is a model system to study learning and memory, and Ca(2+) signals play a key role in these processes. We have cloned, expressed, and characterized the first honeybee Ca(2+) channel subunit. We identified two splice variants of the Apis CaVβ Ca(2+) channel subunit (Am-CaVβ) and demonstrated expression in muscle and neurons. Although AmCaVβ shares with vertebrate CaVβ subunits the SH3 and GK domains, it beholds a unique N terminus that is alternatively spliced in the first exon to produce a long (a) and short (b) variant. When expressed with the CaV2 channels both, AmCaVβa and AmCaVβb, increase current amplitude, shift the voltage-sensitivity of the channel, and slow channel inactivation as the vertebrate CaVβ2a subunit does. However, as opposed to CaVβ2a, slow inactivation induced by Am-CaVβa was insensitive to palmitoylation but displayed a unique PI3K sensitivity. Inactivation produced by the b variant was PI3K-insensitive but staurosporine/H89-sensitive. Deletion of the first exon suppressed the sensitivity to PI3K inhibitors, staurosporine, or H89. Recording of Ba(2+) currents in Apis neurons or muscle cells evidenced a sensitivity to PI3K inhibitors and H89, suggesting that both AmCaVβ variants may be important to couple cell signaling to Ca(2+) entry in vivo. Functional interactions with phospho-inositide and identification of phosphorylation sites in AmCaVβa and AmCaVβb N termini, respectively, suggest that AmCaVβ splicing promoted two novel and alternative modes of regulation of channel activity with specific signaling pathways. This is the first description of a splicing-dependent kinase switch in the regulation of Ca(2+) channel activity by CaVβ subunit.

Structure and function of the β subunit of voltage-gated Ca²⁺ channels

The voltage-gated Ca²⁺ channel β subunit (Ca(v)β) is a cytosolic auxiliary subunit that plays an essential role in regulating the surface expression and gating properties of high-voltage activated (HVA) Ca²⁺ channels. It is also crucial for the modulation of HVA Ca²⁺ channels by G proteins, kinases, Ras-related RGK GTPases, and other proteins. There are indications that Ca(v)β may carry out Ca²⁺ channel-independent functions. Ca(v)β knockouts are either non-viable or result in a severe pathophysiology, and mutations in Ca(v)β have been implicated in disease. In this article, we review the structure and various biological functions of Ca(v)β, as well as recent advances. This article is part of a Special Issue entitled: Calcium channels.

A short polybasic segment between the two conserved domains of the β2a-subunit modulates the rate of inactivation of R-type calcium channel

Besides opening and closing, high voltage-activated calcium channels transit to a nonconducting inactivated state from which they do not re-open unless the plasma membrane is repolarized. Inactivation is critical for temporal regulation of intracellular calcium signaling and prevention of a deleterious rise in calcium concentration. R-type high voltage-activated channels inactivate fully in a few hundred milliseconds when expressed alone. However, when co-expressed with a particular β-subunit isoform, β(2a), inactivation is partial and develops in several seconds. Palmitoylation of a unique di-cysteine motif at the N terminus anchors β(2a) to the plasma membrane. The current view is that membrane-anchored β(2a) immobilizes the channel inactivation machinery and confers slow inactivation phenotype. β-Subunits contain one Src homology 3 and one guanylate kinase domain, flanked by variable regions with unknown structures. Here, we identified a short polybasic segment at the boundary of the guanylate kinase domain that slows down channel inactivation without relocating a palmitoylation-deficient β(2a) to the plasma membrane. Substitution of the positively charged residues within this segment by alanine abolishes its slow inactivation-conferring phenotype. The linker upstream from the polybasic segment, but not the N- and C-terminal variable regions, masks the effect of this determinant. These results reveal a novel mechanism for inhibiting voltage-dependent inactivation of R-type calcium channels by the β(2a)-subunit that might involve electrostatic interactions with an unknown target on the channel's inactivation machinery or its modulatory components. They also suggest that intralinker interactions occlude the action of the polybasic segment and that its functional availability is regulated by the palmitoylated state of the β(2a)-subunit.

A quartet of leucine residues in the guanylate kinase domain of CaVβ determines the plasma membrane density of the CaV2.3 channel

Ca(V)β subunits are formed by a Src homology 3 domain and a guanylate kinase-like (GK) domain connected through a variable HOOK domain. Complete deletion of the Src homology 3 domain (75 residues) as well as deletion of the HOOK domain (47 residues) did not alter plasma membrane density of Ca(V)2.3 nor its typical activation gating. In contrast, six-residue deletions in the GK domain disrupted cell surface trafficking and functional expression of Ca(V)2.3. Mutations of residues known to carry nanomolar affinity binding in the GK domain of Ca(V)β (P175A, P179A, M195A, M196A, K198A, S295A, R302G, R307A, E339G, N340G, and A345G) did not significantly alter cell surface targeting or gating modulation of Ca(V)2.3. Nonetheless, mutations of a quartet of leucine residues (either single or multiple mutants) in the α3, α6, β10, and α9 regions of the GK domain were found to significantly impair cell surface density of Ca(V)2.3 channels. Furthermore, the normalized protein density of Ca(V)2.3 was nearly abolished with the quadruple Ca(V)β3 Leu mutant L200G/L303G/L337G/L342G. Altogether, our observations suggest that the four leucine residues in Ca(V)β3 form a hydrophobic pocket surrounding key residues in the α-interacting domain of Ca(V)2.3. This interaction appears to play an essential role in conferring Ca(V)β-induced modulation of the protein density of Ca(V)α1 subunits in Ca(V)2 channels.

Shaping a new Ca²⁺ conductance to suppress early afterdepolarizations in cardiac myocytes

Sudden cardiac death (SCD) due to ventricular fibrillation (VF) is a major world-wide health problem. A common trigger of VF involves abnormal repolarization of the cardiac action potential causing early afterdepolarizations (EADs). Here we used a hybrid biological-computational approach to investigate the dependence of EADs on the biophysical properties of the L-type Ca(2+) current (I(Ca,L)) and to explore how modifications of these properties could be designed to suppress EADs. EADs were induced in isolated rabbit ventricular myocytes by exposure to 600 μmol l(-1) H(2)O(2) (oxidative stress) or lowering the external [K(+)] from 5.4 to 2.0-2.7 mmol l(-1) (hypokalaemia). The role of I(Ca,L) in EAD formation was directly assessed using the dynamic clamp technique: the paced myocyte's V(m) was input to a myocyte model with tunable biophysical parameters, which computed a virtual I(Ca,L), which was injected into the myocyte in real time. This virtual current replaced the endogenous I(Ca,L), which was suppressed with nifedipine. Injecting a current with the biophysical properties of the native I(Ca,L) restored EAD occurrence in myocytes challenged by H(2)O(2) or hypokalaemia. A mere 5 mV depolarizing shift in the voltage dependence of activation or a hyperpolarizing shift in the steady-state inactivation curve completely abolished EADs in myocytes while maintaining a normal Ca(i) transient. We propose that modifying the biophysical properties of I(Ca,L) has potential as a powerful therapeutic strategy for suppressing EADs and EAD-mediated arrhythmias.

Progress in the structural understanding of voltage-gated calcium channel (CaV) function and modulation

Voltage-gated calcium channels (CaVs) are large, transmembrane multiprotein complexes that couple membrane depolarization to cellular calcium entry. These channels are central to cardiac action potential propagation, neurotransmitter and hormone release, muscle contraction, and calcium-dependent gene transcription. Over the past six years, the advent of high-resolution structural studies of CaV components from different isoforms and CaV modulators has begun to reveal the architecture that underlies the exceptionally rich feedback modulation that controls CaV action. These descriptions of CaV molecular anatomy have provided new, structure-based insights into the mechanisms by which particular channel elements affect voltage-dependent inactivation (VDI), calcium‑dependent inactivation (CDI), and calcium‑dependent facilitation (CDF). The initial successes have been achieved through structural studies of soluble channel domains and modulator proteins and have proven most powerful when paired with biochemical and functional studies that validate ideas inspired by the structures. Here, we review the progress in this growing area and highlight some key open challenges for future efforts.

The ß subunit of voltage-gated Ca2+ channels

Calcium regulates a wide spectrum of physiological processes such as heartbeat, muscle contraction, neuronal communication, hormone release, cell division, and gene transcription. Major entryways for Ca(2+) in excitable cells are high-voltage activated (HVA) Ca(2+) channels. These are plasma membrane proteins composed of several subunits, including α(1), α(2)δ, β, and γ. Although the principal α(1) subunit (Ca(v)α(1)) contains the channel pore, gating machinery and most drug binding sites, the cytosolic auxiliary β subunit (Ca(v)β) plays an essential role in regulating the surface expression and gating properties of HVA Ca(2+) channels. Ca(v)β is also crucial for the modulation of HVA Ca(2+) channels by G proteins, kinases, and the Ras-related RGK GTPases. New proteins have emerged in recent years that modulate HVA Ca(2+) channels by binding to Ca(v)β. There are also indications that Ca(v)β may carry out Ca(2+) channel-independent functions, including directly regulating gene transcription. All four subtypes of Ca(v)β, encoded by different genes, have a modular organization, consisting of three variable regions, a conserved guanylate kinase (GK) domain, and a conserved Src-homology 3 (SH3) domain, placing them into the membrane-associated guanylate kinase (MAGUK) protein family. Crystal structures of Ca(v)βs reveal how they interact with Ca(v)α(1), open new research avenues, and prompt new inquiries. In this article, we review the structure and various biological functions of Ca(v)β, with both a historical perspective as well as an emphasis on recent advances.

Allethrin differentially modulates voltage-gated calcium channel subtypes in rat PC12 cells

Pyrethroid insecticides are one of the most widely used classes of insecticides. Previous studies revealed that pyrethroids potently affect the insect voltage-gated sodium (Na(+)) channel (VGSC), resulting in prolonged channel open time. However, recent findings have suggested that pyrethroids may affect targets other than the VGSC. In particular, several studies have shown that pyrethroids can modulate the activity of voltage-gated calcium (Ca(2+)) channels (VGCCs). However, these studies often reported conflicting results; some studies observed stimulatory effects, whereas others observed inhibitory effects of pyrethroids on VGCCs. This study investigated whether allethrin (AL), a well-characterized type I pyrethroid, altered VGCC characteristics measured by whole-cell recording in rat pheochromocytoma cells (PC12) differentiated with nerve growth factor (NGF). AL (5 microM) increased peak, end, and tail composite VGCC current independent of its effects on VGSCs. After blocking VGCC subtype-specific current with omega-conotoxin GVIA (GVIA, an N-type VGCC antagonist) or nimodipine (NIM, an L-type VGCC antagonist), our data further suggest that AL differentially affects VGCC subtypes. Thus, AL apparently stimulated GVIA-insensitive current while inhibiting NIM-insensitive current. AL also significantly altered the voltage dependency of activation and inactivation of L-type VGCCs. The differential modulation of VGCC subtypes by AL may explain some of the conflicting observations of other studies.

The Ca2+ channel beta subunit determines whether stimulation of Gq-coupled receptors enhances or inhibits N current

In superior cervical ganglion (SCG) neurons, stimulation of M₁ receptors (M₁Rs) produces a distinct pattern of modulation of N-type calcium (N-) channel activity, enhancing currents elicited with negative test potentials and inhibiting currents elicited with positive test potentials. Exogenously applied arachidonic acid (AA) reproduces this profile of modulation, suggesting AA functions as a downstream messenger of M₁Rs. In addition, techniques that diminish AA's concentration during M₁R stimulation minimize N-current modulation. However, other studies suggest depletion of phosphatidylinositol-4,5-bisphosphate during M₁R stimulation suffices to elicit modulation. In this study, we used an expression system to examine the physiological mechanisms regulating modulation. We found the β subunit (Ca_Vβ) acts as a molecular switch regulating whether modulation results in enhancement or inhibition. In human embryonic kidney 293 cells, stimulation of M₁Rs or neurokinin-1 receptors (NK-1Rs) inhibited activity of N channels formed by Ca_V2.2 and coexpressed with Ca_Vβ1b, Ca_Vβ3, or Ca_Vβ4 but enhanced activity of N channels containing Ca_Vβ2a. Exogenously applied AA produced the same pattern of modulation. Coexpression of Ca_Vβ2a, Ca_Vβ3, and Ca_Vβ4 recapitulated the modulatory response previously seen in SCG neurons, implying heterogeneous association of Ca_Vβ with Ca_V2.2. Further experiments with mutated, chimeric Ca_Vβ subunits and free palmitic acid revealed that palmitoylation of Ca_Vβ2a is essential for loss of inhibition. The data presented here fit a model in which Ca_Vβ2a blocks inhibition, thus unmasking enhancement. Our discovery that the presence or absence of palmitoylated Ca_Vβ2a toggles M₁R- or NK-1R–mediated modulation of N current between enhancement and inhibition identifies a novel role for palmitoylation. Moreover, these findings predict that at synapses, modulation of N-channel activity by M₁Rs or NK-1Rs will fluctuate between enhancement and inhibition based on the presence of palmitoylated Ca_Vβ2a.

Engineering proteins for custom inhibition of Ca(V) channels

The influx of Ca(2+) ions through voltage-dependent calcium (Ca(V)) channels links electrical signals to physiological responses in all excitable cells. Not surprisingly, blocking Ca(V) channel activity is a powerful method to regulate the function of excitable cells, and this is exploited for both physiological and therapeutic benefit. Nevertheless, the full potential for Ca(V) channel inhibition is not being realized by currently available small-molecule blockers or second-messenger modulators due to limitations in targeting them either to defined groups of cells in an organism or to distinct subcellular regions within a single cell. Here, we review early efforts to engineer protein molecule blockers of Ca(V) channels to fill this crucial niche. This technology would greatly expand the toolbox available to physiologists studying the biology of excitable cells at the cellular and systems level.

Regulation of voltage-gated Ca2+ channels by lipids

Great skepticism has surrounded the question of whether modulation of voltage-gated Ca(2+) channels (VGCCs) by the polyunsaturated free fatty acid arachidonic acid (AA) has any physiological basis. Here we synthesize findings from studies of both native and recombinant channels where micromolar concentrations of AA consistently inhibit both native and recombinant activity by stabilizing VGCCs in one or more closed states. Structural requirements for these inhibitory actions include a chain length of at least 18 carbons and multiple double bonds located near the fatty acid's carboxy terminus. Acting at a second site, AA increases the rate of VGCC activation kinetics, and in Ca(V)2.2 channels, increases current amplitude. We present evidence that phosphatidylinositol 4,5-bisphosphate (PIP(2)), a palmitoylated accessory subunit (beta(2a)) of VGCCs and AA appear to have overlapping sites of action giving rise to complex channel behavior. Their actions converge in a physiologically relevant manner during muscarinic modulation of VGCCs. We speculate that M(1) muscarinic receptors may stimulate multiple lipases to break down the PIP(2) associated with VGCCs and leave PIP(2)'s freed fatty acid tails bound to the channels to confer modulation. This unexpectedly simple scheme gives rise to unanticipated predictions and redirects thinking about lipid regulation of VGCCs.

Disruption of the IS6-AID linker affects voltage-gated calcium channel inactivation and facilitation

Two processes dominate voltage-gated calcium channel (Ca_V) inactivation: voltage-dependent inactivation (VDI) and calcium-dependent inactivation (CDI). The Ca_Vβ/Ca_Vα₁-I-II loop and Ca²⁺/calmodulin (CaM)/Ca_Vα₁–C-terminal tail complexes have been shown to modulate each, respectively. Nevertheless, how each complex couples to the pore and whether each affects inactivation independently have remained unresolved. Here, we demonstrate that the IS6–α-interaction domain (AID) linker provides a rigid connection between the pore and Ca_Vβ/I-II loop complex by showing that IS6-AID linker polyglycine mutations accelerate Ca_V1.2 (L-type) and Ca_V2.1 (P/Q-type) VDI. Remarkably, mutations that either break the rigid IS6-AID linker connection or disrupt Ca_Vβ/I-II association sharply decelerate CDI and reduce a second Ca²⁺/CaM/Ca_Vα₁–C-terminal–mediated process known as calcium-dependent facilitation. Collectively, the data strongly suggest that components traditionally associated solely with VDI, Ca_Vβ and the IS6-AID linker, are essential for calcium-dependent modulation, and that both Ca_Vβ-dependent and CaM-dependent components couple to the pore by a common mechanism requiring Ca_Vβ and an intact IS6-AID linker.

Functional dissection of the intramolecular Src homology 3-guanylate kinase domain coupling in voltage-gated Ca2+ channel beta-subunits

The beta-subunit of voltage-gated Ca(2+) channels is essential for trafficking the channels to the plasma membrane and regulating their gating. It contains a Src homology 3 (SH3) domain and a guanylate kinase (GK) domain, which interact intramolecularly. We investigated the structural underpinnings of this intramolecular coupling and found that in addition to a previously described SH3 domain beta strand, two structural elements are crucial for maintaining a strong and yet potentially modifiable SH3-GK intramolecular coupling: an intrinsically weak SH3-GK interface and a direct connection of the SH3 and GK domains. Alterations of these elements uncouple the two functions of the beta-subunit, degrading its ability to regulate gating while leaving its chaperone effect intact.

Mutations of nonconserved residues within the calcium channel alpha1-interaction domain inhibit beta-subunit potentiation

Voltage-dependent calcium channels consist of a pore-forming subunit (Ca(V)alpha(1)) that includes all the molecular determinants of a voltage-gated channel, and several accessory subunits. The ancillary beta-subunit (Ca(V)beta) is a potent activator of voltage-dependent calcium channels, but the mechanisms and structural bases of this regulation remain elusive. Ca(V)beta binds reversibly to a conserved consensus sequence in Ca(V)alpha(1), the alpha(1)-interaction domain (AID), which forms an alpha-helix when complexed with Ca(V)beta. Conserved aromatic residues face to one side of the helix and strongly interact with a hydrophobic pocket on Ca(V)beta. Here, we studied the effect of mutating residues located opposite to the AID-Ca(V)beta contact surface in Ca(V)1.2. Substitution of AID-exposed residues by the corresponding amino acids present in other Ca(V)alpha(1) subunits (E462R, K465N, D469S, and Q473K) hinders Ca(V)beta's ability to increase ionic-current to charge-movement ratio (I/Q) without changing the apparent affinity for Ca(V)beta. At the single channel level, these Ca(V)1.2 mutants coexpressed with Ca(V)beta(2a) visit high open probability mode less frequently than wild-type channels. On the other hand, Ca(V)1.2 carrying either a mutation in the conserved tryptophan residue (W470S, which impairs Ca(V)beta binding), or a deletion of the whole AID sequence, does not exhibit Ca(V)beta-induced increase in I/Q. In addition, we observed a shift in the voltage dependence of activation by +12 mV in the AID-deleted channel in the absence of Ca(V)beta, suggesting a direct participation of these residues in the modulation of channel activation. Our results show that Ca(V)beta-dependent potentiation arises primarily from changes in the modal gating behavior. We envision that Ca(V)beta spatially reorients AID residues that influence the channel gate. These findings provide a new framework for understanding modulation of VDCC gating by Ca(V)beta.

The guanylate kinase domain of the beta-subunit of voltage-gated calcium channels suffices to modulate gating

Inactivation of voltage-gated calcium channels is crucial for the spatiotemporal coordination of calcium signals and prevention of toxic calcium buildup. Only one member of the highly conserved family of calcium channel beta-subunits--Ca(V)beta--inhibits inactivation. This unique property has been attributed to short variable regions of the protein; however, here we report that this inhibition actually is conferred by a conserved guanylate kinase (GK) domain and, moreover, that this domain alone recapitulates Ca(V)beta-mediated modulation of channel activation. We expressed and refolded the GK domain of Ca(V)beta(2a), the unique variant that inhibits inactivation, and of Ca(V)beta(1b), an isoform that facilitates it. The refolded domains of both Ca(V)beta variants were found to inhibit inactivation of Ca(V)2.3 channels expressed in Xenopus laevis oocytes. These findings suggest that the GK domain endows calcium channels with a brake restraining voltage-dependent inactivation, and thus facilitation of inactivation by full-length Ca(V)beta requires additional structural determinants to antagonize the GK effect. We found that Ca(V)beta can switch the inactivation phenotype conferred to Ca(V)2.3 from slow to fast after posttranslational modifications during channel biogenesis. Our findings provide a framework within which to understand the modulation of inactivation and a new functional map of Ca(V)beta in which the GK domain regulates channel gating and the other conserved domain (Src homology 3) may couple calcium channels to other signaling pathways.

Differential modulation of L-type calcium channel subunits by oleate

Nonesterified fatty acids such as oleate and palmitate acutely potentiate insulin secretion from pancreatic islets in a glucose-dependent manner. In addition, recent studies show that fatty acids elevate intracellular free Ca(2+) and increase voltage-gated Ca(2+) current in mouse beta-cells, although the mechanisms involved are poorly understood. Here we utilized a heterologous system to express subunit-defined voltage-dependent L-type Ca(2+) channels (LTCC) and demonstrate that beta-cell calcium may increase in part from an interaction between fatty acid and specific calcium channel subunits. Distinct functional LTCC were assembled in both COS-7 and HEK-293 cells by expressing either one of the EYFP-tagged L-type alpha(1)-subunits (beta-cell Cav1.3 or lung Cav1.2) and ERFP-tagged islet beta-subunits (ibeta(2a) or ibeta(3)). In COS-7 cells, elevations in intracellular Ca(2+) mediated by LTCC were enhanced by an oleate-BSA complex. To extend these findings, Ca(2+) current was measured in LTCC-expressing HEK-293 cells that revealed an increase in peak Ca(2+) current within 2 min after addition of the oleate complex, with maximal potentiation occurring at voltages <0 mV. Both Cav1.3 and Cav1.2 were modulated by oleate, and the presence of different auxiliary beta-subunits resulted in differential augmentation. The potentiating effect of oleate on Cav1.2 was abolished by the pretreatment of cells with triacsin C, suggesting that long-chain CoA synthesis is necessary for Ca(2+) channel modulation. These results show for the first time that two L-type Ca(2+) channels expressed in beta-cells (Cav1.3 and Cav1.2) appear to be targeted by nonesterified fatty acids. This effect may account in part for the acute potentiation of glucose-dependent insulin secretion by fatty acids.

Atypical properties of a conventional calcium channel beta subunit from the platyhelminth Schistosoma mansoni

Background

The function of voltage-gated calcium (Ca_v) channels greatly depends on coupling to cytoplasmic accessory β subunits, which not only promote surface expression, but also modulate gating and kinetic properties of the α₁ subunit. Schistosomes, parasitic platyhelminths that cause schistosomiasis, express two β subunit subtypes: a structurally conventional β subunit and a variant β subunit with unusual functional properties. We have previously characterized the functional properties of the variant Ca_vβ subunit. Here, we focus on the modulatory phenotype of the conventional Ca_vβ subunit (SmCa_vβ) using the human Ca_v2.3 channel as the substrate for SmCa_vβ and the whole-cell patch-clamp technique.

Results

The conventional Schistosoma mansoni Ca_vβ subunit markedly increases Ca_v2.3 currents, slows macroscopic inactivation and shifts steady state inactivation in the hyperpolarizing direction. However, currents produced by Ca_v2.3 in the presence of SmCa_vβ run-down to approximately 75% of their initial amplitudes within two minutes of establishing the whole-cell configuration. This suppressive effect was independent of Ca²⁺, but dependent on intracellular Mg²⁺-ATP. Additional experiments revealed that SmCa_vβ lends the Ca_v2.3/SmCa_vβ complex sensitivity to Na⁺ ions. A mutant version of the Ca_vβ subunit lacking the first forty-six amino acids, including a string of twenty-two acidic residues, no longer conferred sensitivity to intracellular Mg²⁺-ATP and Na⁺ ions, while continuing to show wild type modulation of current amplitude and inactivation of Ca_v2.3.

Conclusion

The data presented in this article provide insights into novel mechanisms employed by platyhelminth Ca_vβ subunits to modulate voltage-gated Ca²⁺ currents that indicate interactions between the Ca²⁺ channel complex and chelated forms of ATP as well as Na⁺ ions. These results have potentially important implications for understanding previously unknown mechanisms by which platyhelminths and perhaps other organisms modulate Ca²⁺ currents in excitable cells.

N- and P/Q-type Ca2+ channels in adrenal chromaffin cells

Ca2+ is the most ubiquitous second messenger found in all cells. Alterations in [Ca2+]i contribute to a wide variety of cellular responses including neurotransmitter release, muscle contraction, synaptogenesis and gene expression. Voltage-dependent Ca2+ channels, found in all excitable cells (Hille 1992), mediate the entry of Ca2+ into cells following depolarization. Ca2+ channels are composed of a large pore-forming subunit, called the alpha1 subunit, and several accessory subunits. Ten different alpha1 subunit genes have been identified and classified into three families, Ca(v1-3) (Dunlap et al. 1995, Catterall 2000). Each alpha1 gene produces a unique Ca2+ channel. Although chromaffin cells express several different types of Ca2+ channels, this review will focus on the Cav(2.1) and Cav(2.2) channels, also known as P/Q- and N-type respectively (Nowycky et al. 1985, Llinas et al. 1989b, Wheeler et al. 1994). These channels exhibit physiological and pharmacological properties similar to their neuronal counterparts. N-, P/Q and to a lesser extent R-type Ca2+ channels are known to regulate neurotransmitter release (Hirning et al. 1988, Horne & Kemp 1991, Uchitel et al. 1992, Luebke et al. 1993, Takahashi & Momiyama 1993, Turner et al. 1993, Regehr & Mintz 1994, Wheeler et al. 1994, Wu & Saggau 1994, Waterman 1996, Wright & Angus 1996, Reid et al. 1997). N- and P/Q-type Ca2+ channels are abundant in nerve terminals where they colocalize with synaptic vesicles. Similarly, these channels play a role in neurotransmitter release in chromaffin cells (Garcia et al. 2006). N- and P/Q-type channels are subject to many forms of regulation (Ikeda & Dunlap 1999). This review pays particular attention to the regulation of N- and P/Q-type channels by heterotrimeric G-proteins, interaction with SNARE proteins, and channel inactivation in the context of stimulus-secretion coupling in adrenal chromaffin cells.

Block of CaV1.2 channels by Gd3+ reveals preopening transitions in the selectivity filter

Using the lanthanide gadolinium (Gd³⁺) as a Ca²⁺ replacing probe, we investigated the voltage dependence of pore blockage of Ca_V1.2 channels. Gd⁺³ reduces peak currents (tonic block) and accelerates decay of ionic current during depolarization (use-dependent block). Because diffusion of Gd³⁺ at concentrations used (<1 μM) is much slower than activation of the channel, the tonic effect is likely to be due to the blockage that occurred in closed channels before depolarization. We found that the dose–response curves for the two blocking effects of Gd³⁺ shifted in parallel for Ba²⁺, Sr²⁺, and Ca²⁺ currents through the wild-type channel, and for Ca²⁺ currents through the selectivity filter mutation EEQE that lowers the blocking potency of Gd³⁺. The correlation indicates that Gd³⁺ binding to the same site causes both tonic and use-dependent blocking effects. The apparent on-rate for the tonic block increases with the prepulse voltage in the range −60 to −45 mV, where significant gating current but no ionic current occurs. When plotted together against voltage, the on-rates of tonic block (−100 to −45 mV) and of use-dependent block (−40 to 40 mV) fall on a single sigmoid that parallels the voltage dependence of the gating charge. The on-rate of tonic block by Gd³⁺ decreases with concentration of Ba²⁺, indicating that the apparent affinity of the site to permeant ions is about 1 mM in closed channels. Therefore, we propose that at submicromolar concentrations, Gd³⁺ binds at the entry to the selectivity locus and that the affinity of the site for permeant ions decreases during preopening transitions of the channel.

Influence of channel subunit composition on L-type Ca2+ current kinetics and cardiac wave stability

Previous studies have demonstrated that the slope of the function relating the action potential duration (APD) and the diastolic interval, known as the APD restitution curve, plays an important role in the initiation and maintenance of ventricular fibrillation. Since the APD restitution slope critically depends on the kinetics of the L-type Ca(2+) current, we hypothesized that manipulation of the subunit composition of these channels may represent a powerful strategy to control cardiac arrhythmias. We studied the kinetic properties of the human L-type Ca(2+) channel (Ca(v)1.2) coexpressed with the alpha(2)delta-subunit alone (alpha(1C) + alpha(2)delta) or in combination with beta(2a), beta(2b), or beta(3) subunits (alpha(1C) + alpha(2)delta + beta), using Ca(2+) as the charge carrier. We then incorporated the kinetic properties observed experimentally into the L-type Ca(2+) current mathematical model of the cardiac action potential to demonstrate that the APD restitution slope can be selectively controlled by altering the subunit composition of the Ca(2+) channel. Assuming that beta(2b) most closely resembles the native cardiac L-type Ca(2+) current, the absence of beta, as well as the coexpression of beta(2a), was found to flatten restitution slope and stabilize spiral waves. These results imply that subunit modification of L-type Ca(2+) channels can potentially be used as an antifibrillatory strategy.

Functional modularity of the beta-subunit of voltage-gated Ca2+ channels

The beta-subunit of voltage-gated Ca(2+) channels plays a dual role in chaperoning the channels to the plasma membrane and modulating their gating. It contains five distinct modular domains/regions, including the variable N- and C-terminus, a conserved Src homology 3 (SH3) domain, a conserved guanylate kinase (GK) domain, and a connecting variable and flexible HOOK region. Recent crystallographic studies revealed a highly conserved interaction between the GK domain and alpha interaction domain (AID), the high-affinity binding site in the pore-forming alpha(1) subunit. Here we show that the AID-GK domain interaction is necessary for beta-subunit-stimulated Ca(2+) channel surface expression and that the GK domain alone can carry out this function. We also examined the role of each region of all four beta-subunit subfamilies in modulating P/Q-type Ca(2+) channel gating and demonstrate that the beta-subunit functions modularly. Our results support a model that the conserved AID-GK domain interaction anchors the beta-subunit to the alpha(1) subunit, enabling alpha(1)-beta pair-specific low-affinity interactions involving the N-terminus and the HOOK region, which confer on each of the four beta-subunit subfamilies its distinctive modulatory properties.

Chronic verapamil treatment remodelsICa,Lin mouse ventricle

In this study we tested the hypothesis that ventricular homeostasis of L-type Ca2+current ( ICa,L) minimally involves regulation of the main pore-forming α-subunit (CaV1.2) and auxiliary proteins that serve as positive or negative regulators of ICa,L. We treated animals for 24 h with verapamil (Ver, 3.6 mg·kg−1·day−1), isoproterenol (Iso, 30 mg·kg−1·day−1), or Iso + Ver via osmotic minipumps. To test for alterations of Ca2+channel complex components we performed real-time PCR and Western blot analysis on ventricle. In addition, cardiac myocytes (CMs) were dispersed and current was recorded in the whole cell configuration to evaluate ICa,L. Surprisingly, 24- to 48-h Ver increased CaV1.2 mRNA and protein and ICa,Lcurrent (Ver 11 ± 1pA/pF vs. control 7 ± 0.5pA/pF; P < 0.01). ICa,Lfrom CMs in Ver mice showed no change in whole cell capacitance. To examine the in vivo effects of a physiologically relevant Ca2+channel agonist, we treated mice with Iso. Twenty-four-hour Iso infusion increased heart rate; CaV1.2- and CaVβ2mRNA levels were constant, but the Ca2+channel subunit mRNA Rem was increased twofold. Cells isolated from 24-h Iso hearts showed no change in basal ICa,Ldensity and diminished responsiveness to acute 1 μM Iso. To further examine the homeostatic regulation of the Ca2+channel, we treated animals for 24 h with Iso + Ver. The influence of Iso + Ver was similar that of to Iso alone on Ca2+channel mRNAs and ICa,L, with the exception that it prevented the increase in Rem seen with Iso treatment. Long-term Ca2+channel blockade induces an increase of CaV1.2 mRNA and protein and significantly increases ICa,L.

Mechanism of Ca(v)1.2 channel modulation by the amino terminus of cardiac beta2-subunits

L-type calcium channels are composed of a pore, alpha1c (Ca(V)1.2), and accessory beta- and alpha2delta-subunits. The beta-subunit core structure was recently resolved at high resolution, providing important information on many functional aspects of channel modulation. In this study we reveal differential novel effects of five beta2-subunits isoforms expressed in human heart (beta(2a-e)) on the single L-type calcium channel current. These splice variants differ only by amino-terminal length and amino acid composition. Single-channel modulation by beta2-subunit isoforms was investigated in HEK293 cells expressing the recombinant L-type ion conducting pore. All beta2-subunits increased open probability, availability, and peak current with a highly consistent rank order (beta2a approximately = beta2b > beta2e approximately = beta2c > beta2d). We show graded modulation of some transition rates within and between deep-closed and inactivated states. The extent of modulation correlates strongly with the length of amino-terminal domains. Two mutant beta2-subunits that imitate the natural span related to length confirm this conclusion. The data show that the length of amino termini is a relevant physiological mechanism for channel closure and inactivation, and that natural alternative splicing exploits this principle for modulation of the gating properties of calcium channels.

Importance of voltage-dependent inactivation in N-type calcium channel regulation by G-proteins

Direct regulation of N-type calcium channels by G-proteins is essential to control neuronal excitability and neurotransmitter release. Binding of the G(betagamma) dimer directly onto the channel is characterized by a marked current inhibition ("ON" effect), whereas the pore opening- and time-dependent dissociation of this complex from the channel produce a characteristic set of biophysical modifications ("OFF" effects). Although G-protein dissociation is linked to channel opening, the contribution of channel inactivation to G-protein regulation has been poorly studied. Here, the role of channel inactivation was assessed by examining time-dependent G-protein de-inhibition of Ca(v)2.2 channels in the presence of various inactivation-altering beta subunit constructs. G-protein activation was produced via mu-opioid receptor activation using the DAMGO agonist. Whereas the "ON" effect of G-protein regulation is independent of the type of beta subunit, the "OFF" effects were critically affected by channel inactivation. Channel inactivation acts as a synergistic factor to channel activation for the speed of G-protein dissociation. However, fast inactivating channels also reduce the temporal window of opportunity for G-protein dissociation, resulting in a reduced extent of current recovery, whereas slow inactivating channels undergo a far more complete recovery from inhibition. Taken together, these results provide novel insights on the role of channel inactivation in N-type channel regulation by G-proteins and contribute to the understanding of the physiological consequence of channel inactivation in the modulation of synaptic activity by G-protein coupled receptors.

Regulation of maximal open probability is a separable function of Ca(v)beta subunit in L-type Ca2+ channel, dependent on NH2 terminus of alpha1C (Ca(v)1.2alpha)

β subunits (Ca_vβ) increase macroscopic currents of voltage-dependent Ca²⁺ channels (VDCC) by increasing surface expression and modulating their gating, causing a leftward shift in conductance–voltage (G-V) curve and increasing the maximal open probability, P_o,max. In L-type Ca_v1.2 channels, the Ca_vβ-induced increase in macroscopic current crucially depends on the initial segment of the cytosolic NH₂ terminus (NT) of the Ca_v1.2α (α_1C) subunit. This segment, which we term the “NT inhibitory (NTI) module,” potently inhibits long-NT (cardiac) isoform of α_1C that features an initial segment of 46 amino acid residues (aa); removal of NTI module greatly increases macroscopic currents. It is not known whether an NTI module exists in the short-NT (smooth muscle/brain type) α_1C isoform with a 16-aa initial segment. We addressed this question, and the molecular mechanism of NTI module action, by expressing subunits of Ca_v1.2 in Xenopus oocytes. NT deletions and chimeras identified aa 1–20 of the long-NT as necessary and sufficient to perform NTI module functions. Coexpression of β_2b subunit reproducibly modulated function and surface expression of α_1C, despite the presence of measurable amounts of an endogenous Ca_vβ in Xenopus oocytes. Coexpressed β_2b increased surface expression of α_1C approximately twofold (as demonstrated by two independent immunohistochemical methods), shifted the G-V curve by ∼14 mV, and increased P_o,max 2.8–3.8-fold. Neither the surface expression of the channel without Ca_vβ nor β_2b-induced increase in surface expression or the shift in G-V curve depended on the presence of the NTI module. In contrast, the increase in P_o,max was completely absent in the short-NT isoform and in mutants of long-NT α_1C lacking the NTI module. We conclude that regulation of P_o,max is a discrete, separable function of Ca_vβ. In Ca_v1.2, this action of Ca_vβ depends on NT of α_1C and is α_1C isoform specific.

The alpha1-beta-subunit interaction that modulates calcium channel activity is reversible and requires a competent alpha-interaction domain

High voltage-gated calcium channels consist of a pore-forming subunit (alpha(1)) and three nonhomologous subunits (alpha(2)/delta, beta, and gamma). Although it is well established that the beta-subunit promotes traffic of channels to the plasma membrane and modifies their activity, the reversible nature of the interaction with the alpha(1)-subunit remains controversial. Here, we address this issue by examining the effect of purified beta(2a) protein on Ca(V)1.2 and Ca(V)2.3 channels expressed in Xenopus oocytes. The beta(2a)-subunit binds to the alpha(1)-interaction domain (AID) in vitro, and when injected into oocytes, it shifts the voltage dependence of activation and increases charge movement to ionic current coupling of Ca(V)1.2 channels. This increase depended on the integrity of AID but was not abolished by bafilomycin, demonstrating that the alpha(1)-beta interaction through the AID site can take place at the plasma membrane. Furthermore, injection of beta(2a) protein inhibited inactivation of Ca(V)2.3 channels and converted fast inactivating Ca(V)2.3/beta(1b) channels to slow inactivating channels. Inhibition of inactivation required larger concentration of beta(2a) in oocytes expressing Ca(V)2.3/beta(1b) channels than expressing Ca(V)2.3 alone but reached the same maximal level as expected for a competitive interaction through a single binding site. Together, our data show that the alpha(1)-beta interaction is reversible in intact cells and defines calcium channels beta-subunits as regulatory proteins rather than stoichiometric subunits.

Two PEST-like motifs regulate Ca2+/calpain-mediated cleavage of the CaVbeta3 subunit and provide important determinants for neuronal Ca2+ channel activity

An increase in intracellular Ca2+ due to voltage-gated Ca2+ (CaV) channel opening represents an important trigger for a number of second-messenger-mediated effects ranging from neurotransmitter release to gene activation. Ca2+ entry occurs through the principal pore-forming protein but several ancillary subunits are known to more precisely tune ion influx. Among them, the CaVbeta subunits are perhaps the most important, given that they largely influence the biophysical and pharmacological properties of the channel. Notably, several functional features may be associated with specific structural regions of the CaVbeta subunits emphasizing the relevance of intramolecular domains in the physiology of these proteins. In the current report, we show that CaVbeta3 contains two PEST motifs and undergoes Ca2+ -dependent degradation which can be prevented by the specific calpain inhibitor calpeptin. Using mutant constructs lacking the PEST motifs, we present evidence that they are necessary for the cleavage of CaVbeta3 by calpain. Furthermore, the deletion of the PEST sequences did not affect the binding of CaVbeta3 to the ion-conducting CaV2.2 subunit and, when expressed in human embryonic kidney-293 cells, the PEST motif-deleted CaVbeta3 significantly increased whole-cell current density and retarded channel inactivation. Consistent with this observation, calpeptin treatment of human embryonic kidney-293 cells expressing wild-type CaVbeta3 resulted in an increase in current amplitude. Together, these findings suggest that calpain-mediated CaVbeta3 proteolysis may be an essential process for Ca2+ channel functional regulation.

A novel molecular inactivation determinant of voltage-gated CaV1.2 L-type Ca2+ channel

The inactivation of voltage-gated L-type Ca(2+) channels (Ca(V)1) regulates Ca(2+) entry and controls intracellular Ca(2+) levels that are essential for cellular activity. The molecular entities implicated in L-channel (Ca(V)1.2) inactivation are not fully identified. Here we show for the first time the functional impact of one of the two highly conserved clusters of six negatively charged glutamates and aspartate (802-807; poly ED motif) at the II-III loop of the alpha 1 subunits of rabbit of Ca(v)1.2, alpha(1)1.2 and alpha(1)1.2 DeltaN60-Delta1733) on voltage-dependent inactivation. Mutation of the poly ED motif to alanine or glutamine/asparagine greatly enhanced voltage-dependent inactivation, shifting the voltage dependence to negative potentials by >50 mV and conferring a neuronal like inactivation kinetics onto Ca(V)1.2. The large shift in the midpoint of inactivation of the steady-state inactivation kinetics was observed also in Ca(2+) or Ba(2+) and was not altered by the beta2A subunit. Missing from the fast inactivating neuronal P/Q (Ca(V)2.1)-, N (Ca(V)2.2)- or R (Ca(V)2.3)-type channels and modulating Ca(V)1.2 inactivation kinetics, the poly ED motif is likely to be a specific L-type Ca(2+) channels inactivating domain. Our results fit a model in which the poly ED either by itself or as part of a larger inactivating motif acts as Ca(V)1.2 specific built-in "stopper." In this model, Ca(V)1 accomplishes a large Ca(2+) influx during depolarization, possibly by the poly ED hindering occlusion at the pore. Furthermore, the selective designed poly ED perhaps clarifies major inactivation differences between L- and non-L-type calcium channels.