Ca2+ channel beta-subunits: structural insights AID our understanding

A peptidomimetic modulator of the CaV2.2 N-type calcium channel for chronic pain

Transmembrane Cav2.2 (N-type) voltage-gated calcium channels are genetically and pharmacologically validated, clinically relevant pain targets. Clinical block of Cav2.2 (e.g., with Prialt/Ziconotide) or indirect modulation [e.g., with gabapentinoids such as Gabapentin (GBP)] mitigates chronic pain but is encumbered by side effects and abuse liability. The cytosolic auxiliary subunit collapsin response mediator protein 2 (CRMP2) targets Cav2.2 to the sensory neuron membrane and regulates their function via an intrinsically disordered motif. A CRMP2-derived peptide (CBD3) uncouples the Cav2.2-CRMP2 interaction to inhibit calcium influx, transmitter release, and pain. We developed and applied a molecular dynamics approach to identify the A1R2 dipeptide in CBD3 as the anchoring Cav2.2 motif and designed pharmacophore models to screen 27 million compounds on the open-access server ZincPharmer. Of 200 curated hits, 77 compounds were assessed using depolarization-evoked calcium influx in rat dorsal root ganglion neurons. Nine small molecules were tested electrophysiologically, while one (CBD3063) was also evaluated biochemically and behaviorally. CBD3063 uncoupled Cav2.2 from CRMP2, reduced membrane Cav2.2 expression and Ca2+ currents, decreased neurotransmission, reduced fiber photometry-based calcium responses in response to mechanical stimulation, and reversed neuropathic and inflammatory pain across sexes in two different species without changes in sensory, sedative, depressive, and cognitive behaviors. CBD3063 is a selective, first-in-class, CRMP2-based peptidomimetic small molecule, which allosterically regulates Cav2.2 to achieve analgesia and pain relief without negative side effect profiles. In summary, CBD3063 could potentially be a more effective alternative to GBP for pain relief.

Modulation of L-type calcium channels in Alzheimer's disease: A potential therapeutic target

Calcium plays a fundamental role in various signaling pathways and cellular processes in the human organism. In the nervous system, voltage-gated calcium channels such as L-type calcium channels (LTCCs) are critical elements in mediating neurotransmitter release, synaptic integration and plasticity. Dysfunction of LTCCs has been implicated in both aging and Alzheimer's Disease (AD), constituting a key component of calcium hypothesis of AD. As such, LTCCs are a promising drug target in AD. However, due to their structural and functional complexity, the mechanisms by which LTCCs contribute to AD are still unclear. In this review, we briefly summarize the structure, function, and modulation of LTCCs that are the backbone for understanding pathological processes involving LTCCs. We suggest targeting molecular pathways up-regulating LTCCs in AD may be a more promising approach, given the diverse physiological functions of LTCCs and the ineffectiveness of LTCC blockers in clinical studies.

Comparison of quinazoline and benzoylpyrazoline chemotypes targeting the CaVα-β interaction as antagonists of the N-type CaV2.2 channel

Structural studies with an α subunit fragment of voltage-gated calcium (CaV) channels in complex with the CaVβ subunits revealed a high homology between the various CaVα-β subunits, predicting that targeting of this interface would result in nonselective compounds. Despite this likelihood, my laboratory initiated a rational structure-based screening campaign focusing on "hot spots" on the alpha interacting domain (AID) of the CaVβ2a subunits and identified the small molecule 2-(3,5-dimethylisoxazol-4-yl)-N-((4-((3-phenylpropyl)amino)quinazolin-2-yl)methyl)acetamide ( IPPQ ) which selectively targeted the interface between the N-type calcium (CaV2.2) channel and CaVβ. IPPQ (i) specifically bound to CaVβ2a; (ii) inhibited CaVβ2 's interaction with CaV.2-AID; (iii) inhibited CaV2.2 currents in sensory neurons; (iv) inhibited pre-synaptic localization of CaV2.2 in vivo; and (v) inhibited spinal neurotransmission, which resulted in decreased neurotransmitter release. IPPQ was anti-nociceptive in naïve rats and reversed mechanical allodynia and thermal hyperalgesia in rodent models of acute, neuropathic, and genetic pain. In structure-activity relationship (SAR) studies focused on improving binding affinity of IPPQ , another compound (BTT-369), a benzoyl-3,4-dihydro-1'H,2 H-3,4'-bipyrazole class of compounds, was reported by Chen and colleagues, based on work conducted in my laboratory beginning in 2008. BTT-369 contains tetraaryldihydrobipyrazole scaffold - a chemotype featuring phenyl groups known to be significantly metabolized, lower the systemic half-life, and increase the potential for toxicity. Furthermore, the benzoylpyrazoline skeleton in BTT-369 is patented across multiple therapeutic indications. Prior to embarking on an extensive optimization campaign of IPPQ , we performed a head-to-head comparison of the two compounds. We conclude that IPPQ is superior to BTT-369 for on-target efficacy, setting the stage for SAR studies to improve on IPPQ for the development of novel pain therapeutics.

Voltage-dependent calcium channel β subunit-derived peptides reduce excitatory neurotransmission and arterial blood pressure

AIMS

Voltage-dependent calcium channels (VDCCs) play an important role in various physiological functions in the nervous system and the cardiovascular system. In L-, N-, P/Q-, and R-type VDCCs, β subunit assists the channels for membrane targeting and modulates channel properties. In this study, we investigated whether an inhibition of the β subunit binding to α subunit, the pore-forming main subunit of VDCCs, have any effect on channel activation and physiological functions.

MAIN METHODS

Peptides derived from the specific regions of β subunit that bind to the α-interaction domain in I-II linker of α subunit were manufactured, presuming that the peptides interrupt α-β subunit interaction in the channel complex. Then, they were tested on voltage-activated Ca²⁺ currents recorded in acutely isolated trigeminal ganglion (TG) neurons, excitatory postsynaptic currents (EPSCs) in the spinal dorsal horn neurons, and arterial blood pressure (BP) recorded from the rat femoral artery.

KEY FINDINGS

When applied internally through patch pipettes, the peptides decreased the peak amplitudes of the voltage-activated Ca²⁺ currents. After fusing with HIV transactivator of transcription (TAT) sequence to penetrate cell membrane, the peptides significantly decreased the peak amplitudes of Ca²⁺ currents and the peak amplitudes of EPSCs upon the external application through bath solution. Furthermore, the TAT-fused peptides dose dependently reduced the rat BP when administered intravenously.

SIGNIFICANCE

These data suggest that an interruption of α-β subunit association in VDCC complex inhibits channel activation, thereby reducing VDCC-mediated physiological functions such as excitatory neurotransmission and arterial BP.

Targeting the CaVα-CaVβ interaction yields an antagonist of the N-type CaV2.2 channel with broad antinociceptive efficacy

Inhibition of voltage-gated calcium (CaV) channels is a potential therapy for many neurological diseases including chronic pain. Neuronal CaV1/CaV2 channels are composed of α, β, γ and α2δ subunits. The β subunits of CaV channels are cytoplasmic proteins that increase the surface expression of the pore-forming α subunit of CaV. We targeted the high-affinity protein-protein interface of CaVβ's pocket within the CaVα subunit. Structure-based virtual screening of 50,000 small molecule library docked to the β subunit led to the identification of 2-(3,5-dimethylisoxazol-4-yl)-N-((4-((3-phenylpropyl)amino)quinazolin-2-yl)methyl)acetamide (IPPQ). This small molecule bound to CaVβ and inhibited its coupling with N-type voltage-gated calcium (CaV2.2) channels, leading to a reduction in CaV2.2 currents in rat dorsal root ganglion sensory neurons, decreased presynaptic localization of CaV2.2 in vivo, decreased frequency of spontaneous excitatory postsynaptic potentials and miniature excitatory postsynaptic potentials, and inhibited release of the nociceptive neurotransmitter calcitonin gene-related peptide from spinal cord. IPPQ did not target opioid receptors nor did it engage inhibitory G protein-coupled receptor signaling. IPPQ was antinociceptive in naive animals and reversed allodynia and hyperalgesia in models of acute (postsurgical) and neuropathic (spinal nerve ligation, chemotherapy- and gp120-induced peripheral neuropathy, and genome-edited neuropathy) pain. IPPQ did not cause akinesia or motor impairment, a common adverse effect of CaV2.2 targeting drugs, when injected into the brain. IPPQ, a quinazoline analog, represents a novel class of CaV2.2-targeting compounds that may serve as probes to interrogate CaVα-CaVβ function and ultimately be developed as a nonopioid therapeutic for chronic pain.

A Family of Auxiliary Subunits of the TRP Cation Channel Encoded by the Complex inaF Locus

TRP channels function in many types of sensory receptor cells. Despite extensive analyses, an open question is whether there exists a family of auxiliary subunits, which could influence localization, trafficking, and function of TRP channels. Here, using Drosophila melanogaster, we reveal a previously unknown TRP interacting protein, INAF-C, which is expressed exclusively in the ultraviolet-sensing R7 photoreceptor cells. INAF-C is encoded by an unusual locus comprised of four distinct coding regions, which give rise to four unique single-transmembrane-containing proteins. With the exception of INAF-B, roles for the other INAF proteins were unknown. We found that both INAF-B and INAF-C are required for TRP stability and localization in R7 cells. Conversely, loss of just INAF-B greatly reduced TRP from other types of photoreceptor cells, but not R7. The requirements for TRP and INAF are reciprocal, since loss of TRP decreased the concentrations of both INAF-B and INAF-C. INAF-A, which is not normally expressed in photoreceptor cells, can functionally substitute for INAF-B, indicating that it is a third TRP auxiliary protein. Reminiscent of the structural requirements between K_v channels and KCNE auxiliary subunits, the codependencies of TRP and INAF depended on several transmembrane domains (TMDs) in TRP, and the TMD and the C-terminus of INAF-B. Our studies support a model in which the inaF locus encodes a family of at least three TRP auxiliary subunits.

Excitation-contraction coupling in skeletal muscle: recent progress and unanswered questions

Excitation-contraction coupling (ECC) is a physiological process that links excitation of muscles by the nervous system to their mechanical contraction. In skeletal muscle, ECC is initiated with an action potential, generated by the somatic nervous system, which causes a depolarisation of the muscle fibre membrane (sarcolemma). This leads to a rapid change in the transmembrane potential, which is detected by the voltage-gated Ca²⁺ channel dihydropyridine receptor (DHPR) embedded in the sarcolemma. DHPR transmits the contractile signal to another Ca²⁺ channel, ryanodine receptor (RyR1), embedded in the membrane of the sarcoplasmic reticulum (SR), which releases a large amount of Ca²⁺ ions from the SR that initiate muscle contraction. Despite the fundamental role of ECC in skeletal muscle function of all vertebrate species, the molecular mechanism underpinning the communication between the two key proteins involved in the process (DHPR and RyR1) is still largely unknown. The goal of this work is to review the recent progress in our understanding of ECC in skeletal muscle from the point of view of the structure and interactions of proteins involved in the process, and to highlight the unanswered questions in the field.

Small-molecule CaVα1⋅CaVβ antagonist suppresses neuronal voltage-gated calcium-channel trafficking

Extracellular calcium flow through neuronal voltage-gated Ca_V2.2 calcium channels converts action potential-encoded information to the release of pronociceptive neurotransmitters in the dorsal horn of the spinal cord, culminating in excitation of the postsynaptic central nociceptive neurons. The Ca_V2.2 channel is composed of a pore-forming α₁ subunit (Ca_Vα₁) that is engaged in protein-protein interactions with auxiliary α₂/δ and β subunits. The high-affinity Ca_V2.2α₁⋅Ca_Vβ₃ protein-protein interaction is essential for proper trafficking of Ca_V2.2 channels to the plasma membrane. Here, structure-based computational screening led to small molecules that disrupt the Ca_V2.2α₁⋅Ca_Vβ₃ protein-protein interaction. The binding mode of these compounds reveals that three substituents closely mimic the side chains of hot-spot residues located on the α-helix of Ca_V2.2α₁ Site-directed mutagenesis confirmed the critical nature of a salt-bridge interaction between the compounds and Ca_Vβ₃ Arg-307. In cells, compounds decreased trafficking of Ca_V2.2 channels to the plasma membrane and modulated the functions of the channel. In a rodent neuropathic pain model, the compounds suppressed pain responses. Small-molecule α-helical mimetics targeting ion channel protein-protein interactions may represent a strategy for developing nonopioid analgesia and for treatment of other neurological disorders associated with calcium-channel trafficking.

Voltage-gated calcium channels: their discovery, function and importance as drug targets

This review will first describe the importance of Ca²⁺ entry for function of excitable cells, and the subsequent discovery of voltage-activated calcium conductances in these cells. This finding was rapidly followed by the identification of multiple subtypes of calcium conductance in different tissues. These were initially termed low- and high-voltage activated currents, but were then further subdivided into L-, N-, PQ-, R- and T-type calcium currents on the basis of differing pharmacology, voltage-dependent and kinetic properties, and single channel conductance. Purification of skeletal muscle calcium channels allowed the molecular identification of the pore-forming and auxiliary α₂δ, β and ϒ subunits present in these calcium channel complexes. These advances then led to the cloning of the different subunits, which permitted molecular characterisation, to match the cloned channels with physiological function. Studies with knockout and other mutant mice then allowed further investigation of physiological and pathophysiological roles of calcium channels. In terms of pharmacology, cardiovascular L-type channels are targets for the widely used antihypertensive 1,4-dihydropyridines and other calcium channel blockers, N-type channels are a drug target in pain, and α₂δ-1 is the therapeutic target of the gabapentinoid drugs, used in neuropathic pain. Recent structural advances have allowed a deeper understanding of Ca²⁺ permeation through the channel pore and the structure of both the pore-forming and auxiliary subunits. Voltage-gated calcium channels are subject to multiple pathways of modulation by G-protein and second messenger regulation. Furthermore, their trafficking pathways, subcellular localisation and functional specificity are the subjects of active investigation.

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.

Structural and biophysical analyses of the skeletal dihydropyridine receptor β subunit β1a reveal critical roles of domain interactions for stability

Excitation-contraction (EC) coupling in skeletal muscle requires a physical interaction between the voltage-gated calcium channel dihydropyridine receptor (DHPR) and the ryanodine receptor Ca²⁺ release channel. Although the exact molecular mechanism that initiates skeletal EC coupling is unresolved, it is clear that both the α₁ and β subunits of DHPR are essential for this process. Here, we employed a series of techniques, including size-exclusion chromatography-multi-angle light scattering, differential scanning fluorimetry, and isothermal calorimetry, to characterize various biophysical properties of the skeletal DHPR β subunit β_1a Removal of the intrinsically disordered N and C termini and the hook region of β_1a prevented oligomerization, allowing for its structural determination by X-ray crystallography. The structure had a topology similar to that of previously determined β isoforms, which consist of SH3 and guanylate kinase domains. However, transition melting temperatures derived from the differential scanning fluorimetry experiments indicated a significant difference in stability of ∼2-3 °C between the β_1a and β_2a constructs, and the addition of the DHPR α_1s I-II loop (α-interaction domain) peptide stabilized both β isoforms by ∼6-8 °C. Similar to other β isoforms, β_1a bound with nanomolar affinity to the α-interaction domain, but binding affinities were influenced by amino acid substitutions in the adjacent SH3 domain. These results suggest that intramolecular interactions between the SH3 and guanylate kinase domains play a role in the stability of β_1a while also providing a conduit for allosteric signaling events.

Evolutionary insights into T-type Ca2+ channel structure, function, and ion selectivity from the Trichoplax adhaerens homologue

The role of T-type calcium channels in animals without nervous systems is unknown. Smith et al. characterize TCa_v3 from Trichoplax adhaerens, finding expression in neurosecretory-like cells and preference for Ca²⁺ over Na⁺ via strong extracellular Ca²⁺ block, despite low selectivity for Ca²⁺ in the pore.

Four-domain voltage-gated Ca²⁺ (Ca_v) channels play fundamental roles in the nervous system, but little is known about when or how their unique properties and cellular roles evolved. Of the three types of metazoan Ca_v channels, Ca_v1 (L-type), Ca_v2 (P/Q-, N- and R-type) and Ca_v3 (T-type), Ca_v3 channels are optimized for regulating cellular excitability because of their fast kinetics and low activation voltages. These same properties permit Ca_v3 channels to drive low-threshold exocytosis in select neurons and neurosecretory cells. Here, we characterize the single T-type calcium channel from Trichoplax adhaerens (TCa_v3), an early diverging animal that lacks muscle, neurons, and synapses. Co-immunolocalization using antibodies against TCa_v3 and neurosecretory cell marker complexin labeled gland cells, which are hypothesized to play roles in paracrine signaling. Cloning and in vitro expression of TCa_v3 reveals that, despite roughly 600 million years of divergence from other T-type channels, it bears the defining structural and biophysical features of the Ca_v3 family. We also characterize the channel’s cation permeation properties and find that its pore is less selective for Ca²⁺ over Na⁺ compared with the human homologue Ca_v3.1, yet it exhibits a similar potent block of inward Na⁺ current by low external Ca²⁺ concentrations (i.e., the Ca²⁺ block effect). A comparison of the permeability features of TCa_v3 with other cloned channels suggests that Ca²⁺ block is a locus of evolutionary change in T-type channel cation permeation properties and that mammalian channels distinguish themselves from invertebrate ones by bearing both stronger Ca²⁺ block and higher Ca²⁺ selectivity. TCa_v3 is the most divergent metazoan T-type calcium channel and thus provides an evolutionary perspective on Ca_v3 channel structure–function properties, ion selectivity, and cellular physiology.

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.

L-type calcium channels as drug targets in CNS disorders

L-type calcium channels are present in most electrically excitable cells and are needed for proper brain, muscle, endocrine and sensory function. There is accumulating evidence for their involvement in brain diseases such as Parkinson disease, febrile seizures and neuropsychiatric disorders. Pharmacological inhibition of brain L-type channel isoforms, Cav1.2 and Cav1.3, may therefore be of therapeutic value. Organic calcium channels blockers are clinically used since decades for the treatment of hypertension, cardiac ischemia, and arrhythmias with a well-known and excellent safety profile. This pharmacological benefit is mainly mediated by the inhibition of Cav1.2 channels in the cardiovascular system. Despite their different biophysical properties and physiological functions, both brain channel isoforms are similarly inhibited by existing calcium channel blockers. In this review we will discuss evidence for altered L-type channel activity in human brain pathologies, new therapeutic implications of existing blockers and the rationale and current efforts to develop Cav1.3-selective compounds.

BARP suppresses voltage-gated calcium channel activity and Ca2+-evoked exocytosis

Voltage-gated calcium channels (VGCCs) are key regulators of cell signaling and Ca(2+)-dependent release of neurotransmitters and hormones. Understanding the mechanisms that inactivate VGCCs to prevent intracellular Ca(2+) overload and govern their specific subcellular localization is of critical importance. We report the identification and functional characterization of VGCC β-anchoring and -regulatory protein (BARP), a previously uncharacterized integral membrane glycoprotein expressed in neuroendocrine cells and neurons. BARP interacts via two cytosolic domains (I and II) with all Cavβ subunit isoforms, affecting their subcellular localization and suppressing VGCC activity. Domain I interacts at the α1 interaction domain-binding pocket in Cavβ and interferes with the association between Cavβ and Cavα1. In the absence of domain I binding, BARP can form a ternary complex with Cavα1 and Cavβ via domain II. BARP does not affect cell surface expression of Cavα1 but inhibits Ca(2+) channel activity at the plasma membrane, resulting in the inhibition of Ca(2+)-evoked exocytosis. Thus, BARP can modulate the localization of Cavβ and its association with the Cavα1 subunit to negatively regulate VGCC activity.

Differential calcium signaling mediated by voltage-gated calcium channels in rat retinal ganglion cells and their unmyelinated axons

Aberrant calcium regulation has been implicated as a causative factor in the degeneration of retinal ganglion cells (RGCs) in numerous injury models of optic neuropathy. Since calcium has dual roles in maintaining homeostasis and triggering apoptotic pathways in healthy and injured cells, respectively, investigation of voltage-gated Ca channel (VGCC) regulation as a potential strategy to reduce the loss of RGCs is warranted. The accessibility and structure of the retina provide advantages for the investigation of the mechanisms of calcium signalling in both the somata of ganglion cells as well as their unmyelinated axons. The goal of the present study was to determine the distribution of VGCC subtypes in the cell bodies and axons of ganglion cells in the normal retina and to define their contribution to calcium signals in these cellular compartments. We report L-type Ca channel α1C and α1D subunit immunoreactivity in rat RGC somata and axons. The N-type Ca channel α1B subunit was in RGC somata and axons, while the P/Q-type Ca channel α1A subunit was only in the RGC somata. We patch clamped isolated ganglion cells and biophysically identified T-type Ca channels. Calcium imaging studies of RGCs in wholemounted retinas showed that selective Ca channel antagonists reduced depolarization-evoked calcium signals mediated by L-, N-, P/Q- and T-type Ca channels in the cell bodies but only by L-type Ca channels in the axons. This differential contribution of VGCC subtypes to calcium signals in RGC somata and their axons may provide insight into the development of target-specific strategies to spare the loss of RGCs and their axons following injury.

What do we really know and what do we need to know: some controversies, perspectives, and surprises

TRP channels comprise one of the most rapid growing research topics in ion channel research, in fields related to ion channels including channelopathies and translational medicine. We provide here a critical survey on our current knowledge of TRP channels and highlight some of the still open or controversial questions. This comprises questions related to evolution of TRP channels; biophysics, i.e., permeation; pore properties and gating; modulation; the still-elusive 3D structure; and channel subunits but also their role as general sensory channels and in human diseases. We will conclude that our knowledge on TRP channels is still at the very beginning of an exciting research journey.

Protein kinase C-dependent activation of CaV1.2 channels selectively controls human TH2-lymphocyte functions

BACKGROUND

In addition to calcium release-activated calcium channel/ORAI calcium channels, the role of voltage-gated calcium (Cav1) channels in T-cell calcium signaling is emerging. Cav1 channels are formed by α1 (CaV1.1 to CaV1.4) and auxiliary subunits. We previously demonstrated that mouse TH2 cells selectively overexpressed CaV1.2 and CaV1.3 channels. Knocking down these channels with Cav1 antisense (AS) oligonucleotides inhibited TH2 functions and experimental asthma.

OBJECTIVE

We investigated the expression profile and role of Cav1 channels in human T-cell subsets, with a focus on TH2 cells.

METHODS

We compared the profile of CaV1 channel subunit expression in T-cell subsets isolated ex vivo from the blood of healthy donors, as well as in vitro-polarized T-cell subsets, and tested the effect of the Cav1 inhibitors nicardipine and Cav1.2AS on their functions.

RESULTS

CaV1.4 expression was detectable in CD4(+) T cells, ex vivo TH1 cells, and TH17 cells, whereas Cav1.2 channels predominated in TH2 cells only. T-cell activation resulted in Cav1.4 downregulation, whereas Cav1.2 expression was selectively maintained in polarized TH2 cells and absent in TH1 or TH9 cells. Nicardipine and CaV1.2AS decreased Ca(2+) and cytokine responses in TH2, but not TH1, cells. Protein kinase C (PKC) α/β inhibition decreased Ca(2+) and cytokine responses, whereas both calcium and cytokine responses induced by PKC activation were inhibited by nicardipine or Cav1.2AS in TH2 cells.

CONCLUSION

This study highlights the selective expression of Cav1.2 channels in human TH2 cells and the role of PKC-dependent Cav1.2 channel activation in TH2 cell function. Blocking PKC or Cav1.2 channel activation in TH2 cells might represent new strategies to treat allergic diseases in human subjects.

trans-Resveratrol protects ischemic PC12 Cells by inhibiting the hypoxia associated transcription factors and increasing the levels of antioxidant defense enzymes

An in vitro model of ischemic cerebral stroke [oxygen-glucose deprivation (OGD) for 6 h followed by 24 h reoxygenation (R)] with PC12 cells increases Ca(2+) influx by upregulating native L-type Ca(2+) channels and reactive oxygen species (ROS) generation. This reactive oxygen species generation and increase in intracellular Ca(2+) triggers the expression of hypoxic homeostasis transcription factors such as hypoxia induced factor-1 alpha (HIF-1α), Cav-beta 3 (Cav β3), signal transducer and activator of transcription 3 (STAT3), heat shock protein 27 (hsp-27), and cationic channel transient receptor potential melastatin 7 (TRPM7). OGD insulted PC12 cells were subjected to biologically safe doses (5, 10, and 25 μM) of trans-resveratrol in three different treatment groups: 24 h prior to OGD (pre-treatment); 24 h post OGD (post-treatment); and from 24 h before OGD to end of reoxygenation period (whole-treatment). Here, we demonstrated that OGD-R-induced neuronal injury/death is by reactive oxygen species generation, increase in intracellular calcium levels, and decrease in antioxidant defense enzymes. trans-Resveratrol increases the viability of OGD-R insulted PC12 cells, which was assessed by using MTT, NRU, and LDH release assay. In addition, trans-resveratrol significantly decreases reactive oxygen species generation, intracellular Ca(2+) levels, and hypoxia associated transcription factors and also increases the level of antioxidant defense enzymes. Our data shows that the whole-treatment group of trans-resveratrol is most efficient in decreasing hypoxia induced cell death through its antioxidant properties.

Timothy syndrome is associated with activity-dependent dendritic retraction in rodent and human neurons

L-type voltage gated calcium channels have an important role in neuronal development by promoting dendritic growth and arborization. A point mutation in the gene encoding Ca(V)1.2 causes Timothy syndrome, a neurodevelopmental disorder associated with autism spectrum disorders (ASDs). We report that channels with the Timothy syndrome alteration cause activity-dependent dendrite retraction in rat and mouse neurons and in induced pluripotent stem cell (iPSC)-derived neurons from individuals with Timothy syndrome. Dendrite retraction was independent of calcium permeation through the mutant channel, was associated with ectopic activation of RhoA and was inhibited by overexpression of the channel-associated GTPase Gem. These results suggest that Ca(V)1.2 can activate RhoA signaling independently of Ca(2+) and provide insights into the cellular basis of Timothy syndrome and other ASDs.

Anion-sensitive fluorophore identifies the Drosophila swell-activated chloride channel in a genome-wide RNA interference screen

When cells swell in hypo-osmotic solutions, chloride-selective ion channels (Cl(swell)) activate to reduce intracellular osmolality and prevent catastrophic cell rupture. Despite intensive efforts to assign a molecular identity to the mammalian Cl(swell) channel, it remains unknown. In an unbiased genome-wide RNA interference (RNAi) screen of Drosophila cells stably expressing an anion-sensitive fluorescent indicator, we identify Bestrophin 1 (dBest1) as the Drosophila Cl(swell) channel. Of the 23 screen hits with mammalian homologs and predicted transmembrane domains, only RNAi specifically targeting dBest1 eliminated the Cl(swell) current (I(Clswell)). We further demonstrate the essential contribution of dBest1 to Drosophila I(Clswell) with the introduction of a human Bestrophin disease-associated mutation (W94C). Overexpression of the W94C construct in Drosophila cells significantly reduced the endogenous I(Clswell). We confirm that exogenous expression of dBest1 alone in human embryonic kidney (HEK293) cells creates a clearly identifiable Drosophila-like I(Clswell). In contrast, activation of mouse Bestrophin 2 (mBest2), the closest mammalian ortholog of dBest1, is swell-insensitive. The first 64 residues of dBest1 conferred swell activation to mBest2. The chimera, however, maintains mBest2-like pore properties, strongly indicating that the Bestrophin protein forms the Cl(swell) channel itself rather than functioning as an essential auxiliary subunit. dBest1 is an anion channel clearly responsive to swell; this activation depends upon its N-terminus.

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.

Calcium channel auxiliary α2δ and β subunits: trafficking and one step beyond

The voltage-gated calcium channel α(2)δ and β subunits are traditionally considered to be auxiliary subunits that enhance channel trafficking, increase the expression of functional calcium channels at the plasma membrane and influence the channels' biophysical properties. Accumulating evidence indicates that these subunits may also have roles in the nervous system that are not directly linked to calcium channel function. For example, β subunits may act as transcriptional regulators, and certain α(2)δ subunits may function in synaptogenesis. The aim of this Review is to examine both the classic and novel roles for these auxiliary subunits in voltage-gated calcium channel function and beyond.

Trafficking and stability of voltage-gated calcium channels

Voltage-gated calcium channels are important mediators of calcium influx into electrically excitable cells. The amount of calcium entering through this family of channel proteins is not only determined by the functional properties of channels embedded in the plasma membrane but also by the numbers of channels that are expressed at the cell surface. The trafficking of channels is controlled by numerous processes, including co-assembly with ancillary calcium channel subunits, ubiquitin ligases, and interactions with other membrane proteins such as G protein coupled receptors. Here we provide an overview about the current state of knowledge of calcium channel trafficking to the cell membrane, and of the mechanisms regulating the stability and internalization of this important ion channel family.

NAADP on target

Nicotinic acid adenine dinucleotide phosphate (NAADP) is a potent intracellular Ca(2+)-mobilising messenger. Much evidence indicates that NAADP targets novel Ca(2+) channels located on acidic organelles but the identity of these channels has remained obscure. Recent studies have converged on a novel class of ion channels, the two-pore channels (TPCs) as likely molecular targets. The location of these channels to the endo-lysosomal system and their sensitivity to NAADP match closely those of endogenous NAADP-sensitive channels in both mammalian cells and sea urchin eggs, where the effects of NAADP were discovered. Moreover, the functional coupling of TPCs to archetypal endoplasmic reticulum (ER) Ca(2+) channels is also matched. Biophysical analysis in conjunction with site-directed mutagenesis demonstrates that TPCs are pore-forming subunits of NAADP-gated ion channels. TPCs have a unique two-repeat structure, are regulated by N-linked glycosylation and harbor an endo-lysosomal targeting motif in their N-terminus. Knockdown studies have shown TPCs to regulate smooth muscle contraction, differentiation and endothelial cell activation consistent with previous studies implicating NAADP in these processes. Thus multiple lines of evidence indicate that TPCs are the likely long sought targets for NAADP.

New insight into praziquantel against various developmental stages of schistosomes

Praziquantel, due to high efficacy, excellent tolerability, few and transient side effects, simple administration, and competitive cost, is virtually the only drug of choice for treatment of human schistosomiasis. Treatment of schistosomiasis has shown great advances with the introduction of the drug into the therapeutic arsenal in areas that are endemic for the parasite. However, the drug presents various efficacies against different developmental stages of schistosomes, appearing an oddity intermitted mode. The present review article reviews the effects and mechanism of action of praziquantel against schistosomes briefly and suggests the research on this oddity phenomenon.

Distribution of voltage gated calcium channel β subunits in the mouse retina

Voltage gated calcium channels (VGCCs) are essential to neuronal excitation and signal transduction. They are multimeric in structure and comprised of an alpha subunit that functions as a calcium pore and two additional subunits: an alpha2delta subunit and a cytoplasmic beta subunit. To better understand the role of VGCCs in the retina we used immunohistochemical methods to determine the distribution of VGCC β subunits in normal and mutant mice. To verify the specificity of each antibody and to examine the potential for subunit redistribution when beta subunit expression is perturbed, we used 4 mutant mouse lines that each lack a specific β subunit isoform (β(1)-β(4)). We found the β(1) subunit distributed on cell bodies in the inner nuclear layer (INL) and on processes within both the inner and outer limiting membrane; the β(2) subunit localized to the outer plexiform layer (OPL) and inner plexiform layer (IPL); the β(3) subunit was localized to three narrow and distinct bands within the IPL; the β(4) subunit was localized to three diffuse bands within the IPL. Loss of one β subunit affected labeling intensity but not general distribution patterns of other β subunits. It is likely that VGCCs critical for retinal signal transmission are comprised of the β(2) subunit in the OPL and any of the 4 β subunits in the IPL. Our results suggest that within the OPL the α(1F) subunit pairs predominantly with the β(2) subunit while within the IPL it may pair with either any β subunit.

Inhibition of synaptic transmission and G protein modulation by synthetic CaV2.2 Ca²+ channel peptides

Modulation of presynaptic voltage-dependent Ca2+ channels is a major means of controlling neurotransmitter release. The CaV2.2Ca2+ channel subunit contains several inhibitory interaction sites for Gβγ subunits, including the amino terminal (NT) and I-II loop. The NT and I-II loop have also been proposed to undergo a G protein-gated inhibitory interaction, whilst the NT itself has also been proposed to suppress CaV2 channel activity. Here, we investigate the effects of an amino terminal (CaV2.2[45-55]) 'NT peptide' and a I-II loop alpha interaction domain (CaV2.2[377-393]) 'AID peptide' on synaptic transmission, Ca2+ channel activity and G protein modulation in superior cervical ganglion neurones (SCGNs). Presynaptic injection of NT or AID peptide into SCGN synapses inhibited synaptic transmission and also attenuated noradrenaline-induced G protein modulation. In isolated SCGNs, NT and AID peptides reduced whole-cell Ca2+ current amplitude, modified voltage dependence of Ca2+ channel activation and attenuated noradrenaline-induced G protein modulation. Co-application of NT and AID peptide negated inhibitory actions. Together, these data favour direct peptide interaction with presynaptic Ca2+ channels, with effects on current amplitude and gating representing likely mechanisms responsible for inhibition of synaptic transmission. Mutations to residues reported as determinants of Ca2+ channel function within the NT peptide negated inhibitory effects on synaptic transmission, Ca2+ current amplitude and gating and G protein modulation. A mutation within the proposed QXXER motif for G protein modulation did not abolish inhibitory effects of the AID peptide. This study suggests that the CaV2.2 amino terminal and I-II loop contribute molecular determinants for Ca2+ channel function; the data favour a direct interaction of peptides with Ca2+ channels to inhibit synaptic transmission and attenuate G protein modulation.

Interaction of bestrophin-1 and Ca2+ channel β-subunits: identification of new binding domains on the bestrophin-1 C-terminus

Bestrophin-1 modulates currents through voltage-dependent L-type Ca²⁺ channels by physically interacting with the β-subunits of Ca²⁺ channels. The main function of β-subunits is to regulate the number of pore-forming Ca_V-subunits in the cell membrane and modulate Ca²⁺ channel currents. To understand the influence of full-length bestrophin-1 on β-subunit function, we studied binding and localization of bestrophin-1 and Ca²⁺ channel subunits, together with modulation of Ca_V1.3 Ca²⁺ channels currents. In heterologeous expression, bestrophin-1 showed co-immunoprecipitation with either, β3-, or β4-subunits. We identified a new highly conserved cluster of proline-rich motifs on the bestrophin-1 C-terminus between amino acid position 468 and 486, which enables possible binding to SH3-domains of β-subunits. A bestrophin-1 that lacks these proline-rich motifs (ΔCT-PxxP bestrophin-1) showed reduced efficiency to co-immunoprecipitate with β3 and β4-subunits. In the presence of ΔCT-PxxP bestrophin-1, β4-subunits and Ca_V1.3 subunits partly lost membrane localization. Currents from Ca_V1.3 subunits were modified in the presence of β4-subunit and wild-type bestrophin-1: accelerated time-dependent activation and reduced current density. With ΔCTPxxP bestrophin-1, currents showed the same time-dependent activation as with wild-type bestrophin-1, but the current density was further reduced due to decreased number of Ca²⁺ channels proteins in the cell membrane. In summary, we described new proline-rich motifs on bestrophin-1 C-terminus, which help to maintain the ability of β-subunits to regulate surface expression of pore-forming Ca_V Ca²⁺-channel subunits.

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

The β-subunits of voltage-gated calcium channels regulate their functional expression and properties. Two mechanisms have been proposed for this, an effect on gating and an enhancement of expression. With respect to the effect on expression, β-subunits have been suggested to enhance trafficking by masking an unidentified endoplasmic reticulum (ER) retention signal. Here we have investigated whether, and how, β-subunits affect the level of Ca(V)2.2 channels within somata and neurites of cultured sympathetic neurons. We have used YFP-Ca(V)2.2 containing a mutation (W391A), that prevents binding of β-subunits to its I-II linker and found that expression of this channel was much reduced compared with WT CFP-Ca(V)2.2 when both were expressed in the same neuron. This effect was particularly evident in neurites and growth cones. The difference between the levels of YFP-Ca(V)2.2(W391A) and CFP-Ca(V)2.2(WT) was lost in the absence of co-expressed β-subunits. Furthermore, the relative reduction of expression of Ca(V)2.2(W391A) compared with the WT channel was reversed by exposure to two proteasome inhibitors, MG132 and lactacystin, particularly in the somata. In further experiments in tsA-201 cells, we found that proteasome inhibition did not augment the cell surface Ca(V)2.2(W391A) level but resulted in the observation of increased ubiquitination, particularly of mutant channels. In contrast, we found no evidence for selective retention of Ca(V)2.2(W391A) in the ER, in either the soma or growth cones. In conclusion, there is a marked effect of β-subunits on Ca(V)2.2 expression, particularly in neurites, but our results point to protection from proteasomal degradation rather than masking of an ER retention signal.

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.

Molecular mechanism of calcium channel regulation in the fight-or-flight response

During the fight-or-flight response, the sympathetic nervous system stimulates L-type calcium ion (Ca2+) currents conducted by Ca(V)1 channels through activation of β-adrenergic receptors, adenylyl cyclase, and phosphorylation by adenosine 3',5'-monophosphate-dependent protein kinase [also known as protein kinase A (PKA)], increasing contractility of skeletal and cardiac muscles. We reconstituted this regulation of cardiac Ca(V)1.2 channels in non-muscle cells by forming an autoinhibitory signaling complex composed of Ca(V)1.2Δ1800 (a form of the channel truncated at the in vivo site of proteolytic processing), its noncovalently associated distal carboxyl-terminal domain, the auxiliary α₂δ₁ and β(2b) subunits, and A-kinase anchoring protein 15 (AKAP15). A factor of 3.6 range of Ca(V)1.2 channel activity was observed from a minimum in the presence of protein kinase inhibitors to a maximum upon activation of adenylyl cyclase. Basal Ca(V)1.2 channel activity in unstimulated cells was regulated by phosphorylation of serine-1700 and threonine-1704, two residues located at the interface between the distal and the proximal carboxyl-terminal regulatory domains, whereas further stimulation of channel activity through the PKA signaling pathway only required phosphorylation of serine-1700. Our results define a conceptual framework for Ca(V)1.2 channel regulation and identify sites of phosphorylation that regulate channel activity.

The N terminus of a schistosome beta subunit regulates inactivation and current density of a Cav2 channel

The β subunit of high voltage-activated Ca(2+) (Ca(v)) channels targets the pore-forming α(1) subunit to the plasma membrane and tunes the biophysical phenotype of the Ca(v) channel complex. We used a combination of molecular biology and whole-cell patch clamp to investigate the functional role of a long N-terminal polyacidic motif (NPAM) in a Ca(v)β subunit of the human parasite Schistosoma mansoni (β(Sm)), a motif that does not occur in other known Ca(v)β subunits. When expressed in human embryonic kidney cells stably expressing Ca(v)2.3, β(Sm) accelerates Ca(2+)/calmodulin-independent inactivation of Ca(v)2.3. Deleting the first 44 amino acids of β(Sm), a region that includes NPAM, significantly slows the predominant time constant of inactivation (τ(fast)) under conditions that prevent Ca(2+)/CaM-dependent inactivation (β(Sm): τ(fast) = 66 ms; β(SmΔ2-44): τ(fast) = 111 ms, p < 0.01). Interestingly, deleting the amino acids that are N-terminal to NPAM (2-24 or 2-17) results in faster inactivation than with an intact N terminus (τ(fast) = 42 ms with β(SmΔ2-17); τ(fast) = 40 ms with β(SmΔ2-24), p < 0.01). This suggests that NPAM is the structural determinant for accelerating Ca(2+)/calmodulin-independent inactivation. We also created three chimeric subunits that contain the first 44 amino acids of β(Sm) attached to mammalian β(1b), β(2a), and β(3) subunits. For any given mammalian β subunit, inactivation was faster if it contained the N terminus of β(Sm) than if it did not. Co-expression of the mammalian α(2)δ-1 subunit resulted in doubling of the inactivation rate, but the effects of NPAM persisted. Thus, it appears that the schistosome Ca(v) channel complex has acquired a new function that likely contributes to reducing the amount of Ca(2+) that enters the cells in vivo. This feature is of potential interest as a target for new antihelminthics.

Interaction of T-type calcium channel Ca(V)3.3 with the β-subunit

The β-subunit of high-voltage-activated (HVA) calcium channels is essential for the regulation of expression and gating. On the other hand, various reports have suggested that β subunits play no role in the regulation of low-voltage-activated T-type channels. In addition there has been no clear demonstration of a physical interaction between the α-subunit of T-type channel with β-subunit. In this study, we systematically investigated the interaction between Ca(V)α and Ca(V)β. The four Ca(V)β isoforms were expressed in a bacterial system and purified into homogeneity, whereas the ten types of Ca(V)α alpha interaction domain (AID) peptides were chemically synthesized. All possible combinations of Ca(V)α and Ca(V)β were then tested for by in vitro immunoassays. We describe here the identification of a new interaction between Ca(V)3.3 and Ca(V)β proteins. This interaction is of low affinity compared to that between the AID of the HVA α-subunit and the alpha-binding pocket (ABP) site of the β-subunit. The AID peptide of HVA channel exerted no effect on the Ca(V)3.3-Ca(V)β interaction, thus demonstrating that another site not in the ABP of Ca(V)β protein played a role in binding with Ca(V)3.3. This is the first demonstration of an α-β subunit interaction in a T-type calcium channel.

Rem GTPase interacts with the proximal CaV1.2 C-terminus and modulates calcium-dependent channel inactivation

The Rem, Rem2, Rad, and Gem/Kir (RGK) GTPases, comprise a subfamily of small Ras-related GTP-binding proteins, and have been shown to potently inhibit high voltage-activated Ca(2+) channel current following overexpression. Although the molecular mechanisms underlying RGK-mediated Ca(2+) channel regulation remains controversial, recent studies suggest that RGK proteins inhibit Ca(2+) channel currents at the plasma membrane in part by interactions with accessory channel β subunits. In this paper, we extend our understanding of the molecular determinants required for RGK-mediated channel regulation by demonstrating a direct interaction between Rem and the proximal C-terminus of Ca(V)1.2 (PCT), including the CB/IQ domain known to contribute to Ca(2+)/calmodulin (CaM)-mediated channel regulation. The Rem2 and Rad GTPases display similar patterns of PCT binding, suggesting that the Ca(V)1.2 C-terminus represents a common binding partner for all RGK proteins. In vitro Rem:PCT binding is disrupted by Ca(2+)/CaM, and this effect is not due to Ca(2+)/CaM binding to the Rem C-terminus. In addition, co-overexpression of CaM partially relieves Rem-mediated L-type Ca(2+) channel inhibition and slows the kinetics of Ca(2+)-dependent channel inactivation. Taken together, these results suggest that the association of Rem with the PCT represents a crucial molecular determinant in RGK-mediated Ca(2+) channel regulation and that the physiological function of the RGK GTPases must be re-evaluated. Rather than serving as endogenous inhibitors of Ca(2+) channel activity, these studies indicate that RGK proteins may play a more nuanced role, regulating Ca(2+) currents via modulation of Ca(2+)/CaM-mediated channel inactivation kinetics.

Scanning mutagenesis of the I-II loop of the Cav2.2 calcium channel identifies residues Arginine 376 and Valine 416 as molecular determinants of voltage dependent G protein inhibition

Direct interaction with the beta subunit of the heterotrimeric G protein complex causes voltage-dependent inhibition of N-type calcium channels. To further characterize the molecular determinants of this interaction, we performed scanning mutagenesis of residues 372-387 and 410-428 of the N-type channel alpha1 subunit, in which individual residues were replaced by either alanine or cysteine. We coexpressed wild type Gbeta1gamma2 subunits with either wild type or point mutant N-type calcium channels, and voltage-dependent, G protein-mediated inhibition of the channels (VDI) was assessed using patch clamp recordings. The resulting data indicate that Arg376 and Val416 of the alpha1 subunit, residues which are surface-exposed in the presence of the calcium channel beta subunit, contribute significantly to the functional inhibition by Gbeta1. To further characterize the roles of Arg376 and Val416 in this interaction, we performed secondary mutagenesis of these residues, coexpressing the resulting mutants with wild type Gbeta1gamma2 subunits and with several isoforms of the auxiliary beta subunit of the N-type channel, again assessing VDI using patch clamp recordings. The results confirm the importance of Arg376 for G protein-mediated inhibition and show that a single amino acid substitution to phenylalanine drastically alters the abilities of auxiliary calcium channel subunits to regulate G protein inhibition of the channel.

Role of glycine residues highly conserved in the S2-S3 linkers of domains I and II of voltage-gated calcium channel alpha(1) subunits

The pore-forming component of voltage-gated calcium channels, alpha(1) subunit, contains four structurally conserved domains (I-IV), each of which contains six transmembrane segments (S1-S6). We have shown previously that a Gly residue in the S2-S3 linker of domain III is completely conserved from yeasts to humans and important for channel activity. The Gly residues in the S2-S3 linkers of domains I and II, which correspond positionally to the Gly in the S2-S3 linker of domain III, are also highly conserved. Here, we investigated the role of the Gly residues in the S2-S3 linkers of domains I and II of Ca(v)1.2. Each of the Gly residues was replaced with Glu or Gln to produce mutant Ca(v)1.2s; G182E, G182Q, G579E, G579Q, and the resulting mutants were transfected into BHK6 cells. Whole-cell patch-clamp recordings showed that current-voltage relationships of the four mutants were the same as those of wild-type Ca(v)1.2. However, G182E and G182Q showed significantly smaller current densities because of mislocalization of the mutant proteins, suggesting that Gly(182) in domain I is involved in the membrane trafficking or surface expression of alpha(1) subunit. On the other hand, G579E showed a slower voltage-dependent current inactivation (VDI) compared to Ca(v)1.2, although G579Q showed a normal VDI, implying that Gly(579) in domain II is involved in the regulation of VDI and that the incorporation of a negative charge alters the VDI kinetics. Our findings indicate that the two conserved Gly residues are important for alpha(1) subunit to become functional.

Transient and big are key features of an invertebrate T-type channel (LCav3) from the central nervous system of Lymnaea stagnalis

Here we describe features of the first non-mammalian T-type calcium channel (LCa(v)3) expressed in vitro. This molluscan channel possesses combined biophysical properties that are reminiscent of all mammalian T-type channels. It exhibits T-type features such as "transient" kinetics, but the "tiny" label, usually associated with Ba(2+) conductance, is hard to reconcile with the "bigness" of this channel in many respects. LCa(v)3 is 25% larger than any voltage-gated ion channel expressed to date. It codes for a massive, 322-kDa protein that conducts large macroscopic currents in vitro. LCa(v)3 is also the most abundant Ca(2+) channel transcript in the snail nervous system. A window current at typical resting potentials appears to be at least as large as that reported for mammalian channels. This distant gene provides a unique perspective to analyze the structural, functional, drug binding, and evolutionary aspects of T-type channels.

Anion-sensitive regions of L-type CaV1.2 calcium channels expressed in HEK293 cells

L-type calcium currents (I_Ca) are influenced by changes in extracellular chloride, but sites of anion effects have not been identified. Our experiments showed that CaV1.2 currents expressed in HEK293 cells are strongly inhibited by replacing extracellular chloride with gluconate or perchlorate. Variance-mean analysis of I_Ca and cell-attached patch single channel recordings indicate that gluconate-induced inhibition is due to intracellular anion effects on Ca²⁺ channel open probability, not conductance. Inhibition of CaV1.2 currents produced by replacing chloride with gluconate was reduced from ∼75%–80% to ∼50% by omitting β subunits but unaffected by omitting α₂δ subunits. Similarly, gluconate inhibition was reduced to ∼50% by deleting an α1 subunit N-terminal region of 15 residues critical for β subunit interactions regulating open probability. Omitting β subunits with this mutant α1 subunit did not further diminish inhibition. Gluconate inhibition was unchanged with expression of different β subunits. Truncating the C terminus at AA1665 reduced gluconate inhibition from ∼75%–80% to ∼50% whereas truncating it at AA1700 had no effect. Neutralizing arginines at AA1696 and 1697 by replacement with glutamines reduced gluconate inhibition to ∼60% indicating these residues are particularly important for anion effects. Expressing CaV1.2 channels that lacked both N and C termini reduced gluconate inhibition to ∼25% consistent with additive interactions between the two tail regions. Our results suggest that modest changes in intracellular anion concentration can produce significant effects on CaV1.2 currents mediated by changes in channel open probability involving β subunit interactions with the N terminus and a short C terminal region.

The voltage-gated calcium-channel beta subunit: more than just an accessory

Voltage-gated Ca(2+) channels (VGCCs) are involved in a number of excitatory processes in the cell that regulate muscle contraction, neurotransmitter release, gene regulation, and neuronal migration. They consist of a central pore-forming alpha(1) subunit together with a number of associated auxiliary subunits including a cytoplasmic beta subunit. With the aid of X-ray crystallography, it has been found that the beta subunits of VGCCs (beta(2a), beta(3), and beta(4)) interact strongly with the I-II loop of the pore-forming alpha(1) subunit. Here we discuss the potential interaction sites of beta(1a) with its alpha(1) subunit as well as the skeletal ryanodine receptor. We suggest that not only can beta(1a) interact with the alpha(1) subunit I-II loop, but more subtle interactions may be possible through the II-III loop via the beta(1a) SH3 domain. Such findings could have important implications with respect to EC coupling.

Localization and targeting of voltage-dependent ion channels in mammalian central neurons

The intrinsic electrical properties and the synaptic input-output relationships of neurons are governed by the action of voltage-dependent ion channels. The localization of specific populations of ion channels with distinct functional properties at discrete sites in neurons dramatically impacts excitability and synaptic transmission. Molecular cloning studies have revealed a large family of genes encoding voltage-dependent ion channel principal and auxiliary subunits, most of which are expressed in mammalian central neurons. Much recent effort has focused on determining which of these subunits coassemble into native neuronal channel complexes, and the cellular and subcellular distributions of these complexes, as a crucial step in understanding the contribution of these channels to specific aspects of neuronal function. Here we review progress made on recent studies aimed to determine the cellular and subcellular distribution of specific ion channel subunits in mammalian brain neurons using in situ hybridization and immunohistochemistry. We also discuss the repertoire of ion channel subunits in specific neuronal compartments and implications for neuronal physiology. Finally, we discuss the emerging mechanisms for determining the discrete subcellular distributions observed for many neuronal ion channels.

Orientation of the calcium channel beta relative to the alpha(1)2.2 subunit is critical for its regulation of channel activity

BACKGROUND

The Ca(v)beta subunits of high voltage-activated Ca(2+) channels control the trafficking and biophysical properties of the alpha(1) subunit. The Ca(v)beta-alpha(1) interaction site has been mapped by crystallographic studies. Nevertheless, how this interaction leads to channel regulation has not been determined. One hypothesis is that betas regulate channel gating by modulating movements of IS6. A key requirement for this direct-coupling model is that the linker connecting IS6 to the alpha-interaction domain (AID) be a rigid structure.

METHODOLOGY/PRINCIPAL FINDINGS

The present study tests this hypothesis by altering the flexibility and orientation of this region in alpha(1)2.2, then testing for Ca(v)beta regulation using whole cell patch clamp electrophysiology. Flexibility was induced by replacement of the middle six amino acids of the IS6-AID linker with glycine (PG6). This mutation abolished beta2a and beta3 subunits ability to shift the voltage dependence of activation and inactivation, and the ability of beta2a to produce non-inactivating currents. Orientation of Ca(v)beta with respect to alpha(1)2.2 was altered by deletion of 1, 2, or 3 amino acids from the IS6-AID linker (Bdel1, Bdel2, Bdel3, respectively). Again, the ability of Ca(v)beta subunits to regulate these biophysical properties were totally abolished in the Bdel1 and Bdel3 mutants. Functional regulation by Ca(v)beta subunits was rescued in the Bdel2 mutant, indicating that this part of the linker forms beta-sheet. The orientation of beta with respect to alpha was confirmed by the bimolecular fluorescence complementation assay.

CONCLUSIONS/SIGNIFICANCE

These results show that the orientation of the Ca(v)beta subunit relative to the alpha(1)2.2 subunit is critical, and suggests additional points of contact between these subunits are required for Ca(v)beta to regulate channel activity.

Alternative splicing within the I-II loop controls surface expression of T-type Ca(v)3.1 calcium channels

Molecular diversity of T-type/Ca(v)3 Ca2+ channels is created by expression of three genes and alternative splicing of those genes. Prompted by the important role of the I-II linker in gating and surface expression of Ca(v)3 channels, we describe here the properties of a novel variant that partially deletes this loop. The variant is abundantly expressed in rat brain, even exceeding transcripts with the complete exon 8. Electrophysiological analysis of the Delta8b variant revealed enhanced current density compared to Ca(v)3.1a, but similar gating. Luminometry experiments revealed an increase in the expression of Delta8b channels at the plasma membrane. We conclude that alternative splicing of Ca(v)3 channels regulates surface expression and may underlie disease states in which T-channel current density is increased.

I-II loop structural determinants in the gating and surface expression of low voltage-activated calcium channels

The intracellular loops that interlink the four transmembrane domains of Ca²⁺- and Na⁺-channels (Ca_v, Na_v) have critical roles in numerous forms of channel regulation. In particular, the intracellular loop that joins repeats I and II (I–II loop) in high voltage-activated (HVA) Ca²⁺ channels possesses the binding site for Ca_vβ subunits and plays significant roles in channel function, including trafficking the α₁ subunits of HVA channels to the plasma membrane and channel gating. Although there is considerable divergence in the primary sequence of the I–II loop of Ca_v1/Ca_v2 HVA channels and Ca_v3 LVA/T-type channels, evidence for a regulatory role of the I–II loop in T-channel function has recently emerged for Ca_v3.2 channels. In order to provide a comprehensive view of the role this intracellular region may play in the gating and surface expression in Ca_v3 channels, we have performed a structure-function analysis of the I–II loop in Ca_v3.1 and Ca_v3.3 channels using selective deletion mutants. Here we show the first 60 amino acids of the loop (post IS6) are involved in Ca_v3.1 and Ca_v3.3 channel gating and kinetics, which establishes a conserved property of this locus for all Ca_v3 channels. In contrast to findings in Ca_v3.2, deletion of the central region of the I–II loop in Ca_v3.1 and Ca_v3.3 yielded a modest increase (+30%) and a reduction (−30%) in current density and surface expression, respectively. These experiments enrich our understanding of the structural determinants involved in Ca_v3 function by highlighting the unique role played by the intracellular I–II loop in Ca_v3.2 channel trafficking, and illustrating the prominent role of the gating brake in setting the slow and distinctive slow activation kinetics of Ca_v3.3.

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

Ca_Vβ subunits of voltage-gated calcium channels contain two conserved domains, a src-homology-3 (SH3) domain and a guanylate kinase-like (GK) domain with an intervening HOOK domain. We have shown in a previous study that, although Gβγ-mediated inhibitory modulation of Ca_V2.2 channels did not require the interaction of a Ca_Vβ subunit with the Ca_Vα1 subunit, when such interaction was prevented by a mutation in the α1 subunit, G protein modulation could not be removed by a large depolarization and showed voltage-independent properties (Leroy et al., J Neurosci 25:6984–6996, 2005). In this study, we have investigated the ability of mutant and truncated Ca_Vβ subunits to support voltage-dependent G protein modulation in order to determine the minimal domain of the Ca_Vβ subunit that is required for this process. We have coexpressed the Ca_Vβ subunit constructs with Ca_V2.2 and α₂δ-2, studied modulation by the activation of the dopamine D2 receptor, and also examined basal tonic modulation. Our main finding is that the Ca_Vβ subunit GK domains, from either β1b or β2, are sufficient to restore voltage dependence to G protein modulation. We also found that the removal of the variable HOOK region from β2a promotes tonic voltage-dependent G protein modulation. We propose that the absence of the HOOK region enhances Gβγ binding affinity, leading to greater tonic modulation by basal levels of Gβγ. This tonic modulation requires the presence of an SH3 domain, as tonic modulation is not supported by any of the Ca_Vβ subunit GK domains alone.

Novel CaV2.1 clone replicates many properties of Purkinje cell CaV2.1 current

The P-type calcium current is mediated by a voltage-sensing CaV2.1 alpha subunit in combination with modulatory auxiliary subunits. In Purkinje neurones, this current has distinctively slow inactivation kinetics that may depend on alternative splicing of the alpha subunit and/or association with different CaVbeta subunits. To better understand the molecular components of P-type calcium current, we cloned a CaV2.1 cDNA from total mouse brain. The full-length CaV2.1 isoform that we isolated (GenBank AY714490) contains sequences recently shown to be present in Purkinje neurones. In agreement with previously published work, the alternatively spliced amino acid V421, implicated in slow inactivation, was not encoded in AY714490 and was absent from reverse transcription-polymerase chain reaction products generated from single Purkinje cells. Next, we studied the expression of the four known mouse auxiliary CaVbeta2 isoforms in Purkinje neurones. Confirmation of the presence of CaVbeta2a in Purkinje cells, previously shown by others to slow CaV2.1 kinetics, led us to characterize its influence on current dynamics. We studied currents generated by the clone AY714490 coexpressed in tsA201 cells with four different CaVbeta subunits. In addition to the well-documented slowing of open-state inactivation kinetics, coexpression with the CaVbeta2a subunit also protected CaV2.1 channels from closed-state inactivation and prevented the channel from inactivating during physiological trains of action potential-like stimuli. This strong resistance to inactivation parallels the property of Purkinje neurone P-type currents and is suggestive of a role for CaVbeta2a in modulating the inactivation properties of P-type calcium currents in Purkinje neurones.

The RGK family of GTP-binding proteins: regulators of voltage-dependent calcium channels and cytoskeleton remodeling

RGK proteins constitute a novel subfamily of small Ras-related proteins that function as potent inhibitors of voltage-dependent (VDCC) Ca(2+) channels and regulators of actin cytoskeletal dynamics. Within the larger Ras superfamily, RGK proteins have distinct regulatory and structural characteristics, including nonconservative amino acid substitutions within regions known to participate in nucleotide binding and hydrolysis and a C-terminal extension that contains conserved regulatory sites which control both subcellular localization and function. RGK GTPases interact with the VDCC beta-subunit (Ca(V)beta) and inhibit Rho/Rho kinase signaling to regulate VDCC activity and the cytoskeleton respectively. Binding of both calmodulin and 14-3-3 to RGK proteins, and regulation by phosphorylation controls cellular trafficking and the downstream signaling of RGK proteins, suggesting that a complex interplay between interacting protein factors and trafficking contribute to their regulation.

Analysis of the Rem2 - voltage dependant calcium channel beta subunit interaction and Rem2 interaction with phosphorylated phosphatidylinositide lipids

Voltage dependant calcium channels (VDCC) play a critical role in coupling electrical excitability to important physiological events such as secretion by neuronal and endocrine cells. Rem2, a GTPase restricted to neuroendocrine cell types, regulates VDCC activity by a mechanism that involves interaction with the VDCC beta subunit (Ca(V)beta). Mapping studies reveal that Rem2 binds to the guanylate kinase domain (GK) of the Ca(V)beta subunit that also contains the high affinity binding site for the pore forming and voltage sensing VDCC alpha subunit (Ca(V)alpha) interaction domain (AID). Moreover, fine mapping indicates that Rem2 binds to the GK domain in a region distinct from the AID interaction site, and competitive inhibition studies reveal that Rem2 does not disrupt Ca(V)alpha - Ca(V)beta binding. Instead, the Ca(V)beta subunit appears to serve a scaffolding function, simultaneously binding both Rem2 and AID. Previous studies have found that in addition to Ca(V)beta binding, Rem2 must be localized to the plasma membrane to inhibit VDCC function. Plasma membrane localization requires the C-terminus of Rem2 and binding studies indicate that this domain directs phosphorylated phosphatidylinositide (PIP) lipids association. Plasma membrane localization may provide a unique point of regulation since the ability of Rem2 to bind PIP lipids is inhibited by the phosphoserine dependant binding of 14-3-3 proteins. Thus, in addition to Ca(V)beta binding, VDCC blockade by Rem2 is likely to be controlled by both the localized concentration of membrane PIP lipids and direct 14-3-3 binding to the Rem2 C-terminus.