Neuronal Voltage-gated Calcium Channels Research Articles

Chapter 8 - The episodic ataxias

The primary episodic ataxias (EAs) are a group of autosomal-dominant disorders characterized by transient recurrent incoordination and truncal instability, often triggered by physical exertion or emotional stress and variably associated with progressive baseline ataxia. There are now nine designated subtypes EA1-9 (OMIM) and late onset cerebellar ataxia with episodic features as newly designated SCA27B, based largely on genetic loci. Mutations have been identified in multiple individuals and families in 4 of the 9 EA subtypes, mostly with the onset before adulthood. This chapter focuses on the clinical assessment and management of EA, genetic diagnosis, and neurophysiologic consequences of the causative mutations in the best characterized EA syndromes: EA1 caused by mutations in KCNA1 encoding a neuronal voltage-gated potassium channel, EA2 caused by mutations in CACNA1A encoding a neuronal voltage-gated calcium channel, EA6 caused by mutations in SLC1A3 encoding a glutamate transporter that is also an anion channel, and SCA27B with late onset episodic ataxia caused by an intronic trinucleotide repeat in FGF14 encoding fibroblast growth factor 14 important in regulating the distribution of voltage-gated sodium channels in the cerebellar Purkinje and granule cells. The study of EA has illuminated previously unrecognized but important roles of ion channels and transporters in brain function with shared mechanisms underlying cerebellar ataxia, migraine, and epilepsy.

Disease-associated calmodulin variants impair regulation of neuronal voltage-gated calcium channels.

Calmodulin (CaM) is a regulatory protein that associates with numerous binding partners, and it participates in key mechanisms by which multiple types of excitable cells sense and respond to changing levels of calcium. In neurons and cardiomyocytes, calcium ions flux into the cytosol through voltage-gated calcium channels (VGCCs) and bind to CaM. These binding events trigger an array of important feedback processes, including calcium-dependent inactivation (CDI) and calcium-dependent facilitation (CDF) of multiple VGCC subtypes. In this way, CaM modulates VGCC activity and participates in calcium signaling cascades. Calmodulinopathies are a group of related disorders that arise from pathogenic mutations in CaM. These mutations alter cardiac calcium dynamics in a way that leads to debilitating disorders, including long-QT syndrome (LQTS). LQTS in calmodulinopathy patients has been shown to arise from the mutant forms of CaM preventing the protein from mediating proper CDI of cardiac CaV1.2 L-type channels. However, CaM regulation of VGCCs is not exclusive to the heart, and disruption of these processes in neuronal VGCCs may contribute to the comorbid neurological symptoms frequently observed in calmodulinopathy patients, including autism spectrum disorder, developmental delay, and seizures. We investigated the impacts of calmodulinopathic mutations on VGCCs known to be important for neuronal function. We found that CaM mutants caused diminished CDI in CaV1.3 channels similar to the deficit of CDI seen in CaV1.2 channels, as well as disruption of CDF in CaV2.1 channels. Interestingly, the pattern of CaV2.1 CDF disruptions by certain mutations does not entirely match the pattern seen in CaV1.2 and CaV1.3, indicating a differential effect of select mutations in this neuronal channel. Overall, our results suggest that calmodulinopathic mutations may have consequences to calcium dynamics that extend into neural tissues, in addition to their well-described impacts on cardiac health.

The role of voltage-gated calcium channels in the mechanisms of anesthesia and perioperative analgesia.

A family of neuronal voltage-gated calcium channels (VGCCs) have received only recently a significant consideration regarding the mechanisms of anesthesia because VGCC inhibition may be important in anesthetic action by decreasing neuronal excitability and presynaptic excitatory transmission. The T-type VGCCs channels (T-channels), although rarely involved in synaptic neurotransmitter release, play an important role in controlling neuronal excitability and in generating spontaneous oscillatory bursting of groups of neurons in the thalamus thought to be involved in regulating the state of arousal and sleep. Furthermore, these channels are important regulators of neuronal excitability in pain pathway. This review will provide an overview of historic perspective and the recent literature on the role of VGCCs and T-channel inhibition in particular in the mechanisms of action of anesthetics and analgesics. Recent research in the field of novel mechanisms of hypnotic action of anesthetics revealed significant contribution of the Ca V 3.1 isoform of T-channels expressed in the thalamus. Furthermore, perioperative analgesia can be achieved by targeting Ca V 3.2 isoform of these channels that is abundantly expressed in pain pathways. The review summarizes current knowledge regarding the contribution of T-channels in hypnosis and analgesia. Further preclinical and clinical studies are needed to validate their potential for developing novel anesthetics and new perioperative pain therapies.

Brain-penetrant calcium channel blockers are associated with a reduced incidence of neuropsychiatric disorders

Calcium channel blockers (CCBs) differ in their ability to penetrate into the brain. Pharmacoepidemiological studies suggest that CCBs as a class may have beneficial effects on the risks and outcomes of some psychiatric and neurological disorders. It is plausible but unknown whether this effect relates to their brain penetrance. To address this, we used the TriNetX electronic health records network to identify people prescribed a brain-penetrant CCB (BP-CCB), or those given amlodipine, a CCB with low brain penetrability. We created cohorts of patients who, prior to first CCB exposure, either had to have, or could not have had, a recorded ICD-10 diagnosis in any of the following categories: psychotic disorder; affective disorder (including bipolar disorder and major depressive disorder); anxiety disorder; substance use disorder; sleep disorder; delirium; dementia, or movement disorder. Cohort pairs were propensity score matched for age, sex, race, blood pressure, body mass index, and a range of other variables. The outcomes were the incidence of these disorders measured over a two-year exposure period. Matched cohort sizes ranged from 17,896 to 49,987. In people with no prior history of psychiatric or neurodegenerative disorder, there was a significantly lower incidence of most disorders with BP-CCBs compared to amlodipine, with risk ratios ranging from 0.64 to 0.88 and an overall risk ratio of 0.88, i.e. a risk reduction of 12%. In people who did have a prior psychiatric or neurodegenerative diagnosis, differences were much smaller, but again showed lower risks for several disorders with BP-CCBs compared to amlodipine. The differences were somewhat more marked in women and in people less than 60 years old. Results were similar when comparing BP-CCBs with verapamil and diltiazem. We also compared BP-CCBs with angiotensin receptor blockers, and found an overall risk ratio of 0.94 in favour of BP-CCBs, but with differential effects across disorders including a higher risk of psychotic disorder and dementia, but a lower risk for anxiety and sleep disorders. In some analyses, there was evidence of residual confounding even after the extensive matching, in that negative control outcomes showed a reduced incidence with BP-CCBs relative to the comparator cohort. In summary, CCBs that readily penetrate the brain are associated with a lower incidence of neuropsychiatric disorders, especially first diagnoses, compared to CCBs which do not. This may reflect their blockade of neuronal voltage-gated calcium channels. The findings encourage repurposing trials using existing BP-CCBs, and suggest that novel BP-CCBs with enhanced and more selective central actions might have greater therapeutic potential for psychiatric and neurodegenerative disorders.

Further Evidence that Inhibition of Neuronal Voltage-Gated Calcium Channels Contributes to the Hypnotic Effect of Neurosteroid Analogue, 3β-OH.

We recently reported that a neurosteroid analogue with T-channel-blocking properties (3β,5β,17β)-3-hydroxyandrostane-17-carbonitrile (3β-OH), induced hypnosis in rat pups without triggering neuronal apoptosis. Furthermore, we found that the inhibition of the CaV3.1 isoform of T-channels contributes to the hypnotic properties of 3β-OH in adult mice. However, the specific mechanisms underlying the role of other subtypes of voltage-gated calcium channels in thalamocortical excitability and oscillations in vivo during 3β-OH-induced hypnosis are largely unknown. Here, we used patch-clamp recordings from acute brain slices, in vivo electroencephalogram (EEG) recordings, and mouse genetics with wild-type (WT) and CaV2.3 knock-out (KO) mice to further investigate the molecular mechanisms of neurosteroid-induced hypnosis. Our voltage-clamp recordings showed that 3β-OH inhibited recombinant CaV2.3 currents. In subsequent current-clamp recordings in thalamic slices ex vivo, we found that selective CaV2.3 channel blocker (SNX-482) inhibited stimulated tonic firing and increased the threshold for rebound burst firing in WT animals. Additionally, in thalamic slices we found that 3β-OH inhibited spike-firing more profoundly in WT than in mutant mice. Furthermore, 3β-OH reduced bursting frequencies in WT but not mutant animals. In ensuing in vivo experiments, we found that intra-peritoneal injections of 3β-OH were less effective in inducing LORR in the mutant mice than in the WT mice, with expected sex differences. Furthermore, the reduction in total α, β, and low γ EEG power was more profound in WT than in CaV2.3 KO females over time, while at 60 min after injections of 3β-OH, the increase in relative β power was higher in mutant females. In addition, 3β-OH depressed EEG power more strongly in the male WT than in the mutant mice and significantly increased the relative δ power oscillations in WT male mice in comparison to the mutant male animals. Our results demonstrate for the first time the importance of the CaV2.3 subtype of voltage-gated calcium channels in thalamocortical excitability and the oscillations that underlie neurosteroid-induced hypnosis.

Analysing an allelic series of rare missense variants of CACNA1I in a Swedish schizophrenia cohort.

CACNA1I is implicated in the susceptibility to schizophrenia by large-scale genetic association studies of single nucleotide polymorphisms. However, the channelopathy of CACNA1I in schizophrenia is unknown. CACNA1I encodes CaV3.3, a neuronal voltage-gated calcium channel that underlies a subtype of T-type current that is important for neuronal excitability in the thalamic reticular nucleus and other regions of the brain. Here, we present an extensive functional characterization of 57 naturally occurring rare and common missense variants of CACNA1I derived from a Swedish schizophrenia cohort of more than 10 000 individuals. Our analysis of this allelic series of coding CACNA1I variants revealed that reduced CaV3.3 channel current density was the dominant phenotype associated with rare CACNA1I coding alleles derived from control subjects, whereas rare CACNA1I alleles from schizophrenia patients encoded CaV3.3 channels with altered responses to voltages. CACNA1I variants associated with altered current density primarily impact the ionic channel pore and those associated with altered responses to voltage impact the voltage-sensing domain. CaV3.3 variants associated with altered voltage dependence of the CaV3.3 channel and those associated with peak current density deficits were significantly segregated across affected and unaffected groups (Fisher’s exact test, P = 0.034). Our results, together with recent data from the SCHEMA (Schizophrenia Exome Sequencing Meta-Analysis) cohort, suggest that reduced CaV3.3 function may protect against schizophrenia risk in rare cases. We subsequently modelled the effect of the biophysical properties of CaV3.3 channel variants on thalamic reticular nucleus excitability and found that compared with common variants, ultrarare CaV3.3-coding variants derived from control subjects significantly decreased thalamic reticular nucleus excitability (P = 0.011). When all rare variants were analysed, there was a non-significant trend between variants that reduced thalamic reticular nucleus excitability and variants that either had no effect or increased thalamic reticular nucleus excitability across disease status. Taken together, the results of our functional analysis of an allelic series of >50 CACNA1I variants in a schizophrenia cohort reveal that loss of function of CaV3.3 is a molecular phenotype associated with reduced disease risk burden, and our approach may serve as a template strategy for channelopathies in polygenic disorders.

Opioid Receptor Regulation of Neuronal Voltage-Gated Calcium Channels.

Neuronal voltage-gated calcium channels play a pivotal role in the conversion of electrical signals into calcium entry into nerve endings that is required for the release of neurotransmitters. They are under the control of a number of cellular signaling pathways that serve to fine tune synaptic activities, including G-protein coupled receptors (GPCRs) and the opioid system. Besides modulating channel activity via activation of second messengers, GPCRs also physically associate with calcium channels to regulate their function and expression at the plasma membrane. In this mini review, we discuss the mechanisms by which calcium channels are regulated by classical opioid and nociceptin receptors. We highlight the importance of this regulation in the control of neuronal functions and their implication in the development of disease conditions. Finally, we present recent literature concerning the use of novel μ-opioid receptor/nociceptin receptor modulators and discuss their use as potential drug candidates for the treatment of pain.

FMRP regulates presynaptic localization of neuronal voltage gated calcium channels

Fragile X syndrome (FXS), the most common form of inherited intellectual disability and autism, results from the loss of fragile X mental retardation protein (FMRP). We have recently identified a direct interaction of FMRP with voltage-gated Ca2+ channels that modulates neurotransmitter release. In the present study we used a combination of optophysiological tools to investigate the impact of FMRP on the targeting of voltage-gated Ca2+ channels to the active zones in neuronal presynaptic terminals. We monitored Ca2+ transients at synaptic boutons of dorsal root ganglion (DRG) neurons using the genetically-encoded Ca2+ indicator GCaMP6f tagged to synaptophysin. We show that knock-down of FMRP induces an increase of the amplitude of the Ca2+ transient in functionally-releasing presynaptic terminals, and that this effect is due to an increase of N-type Ca2+ channel contribution to the total Ca2+ transient. Dynamic regulation of CaV2.2 channel trafficking is key to the function of these channels in neurons. Using a CaV2.2 construct with an α-bungarotoxin binding site tag, we further investigate the impact of FMRP on the trafficking of CaV2.2 channels. We show that forward trafficking of CaV2.2 channels from the endoplasmic reticulum to the plasma membrane is reduced when co-expressed with FMRP. Altogether our data reveal a critical role of FMRP on localization of CaV channels to the presynaptic terminals and how its defect in a context of FXS can profoundly affect synaptic transmission.

Gabapentinoid prescription in oral medicine and oral surgery practice. Part I − Experience from a French Orofacial pain clinic

Gabapentinoids, pregabalin and gabapentin, are neuronal voltage-gated calcium channel inhibitors mainly prescribed for the treatment of partial epilepsy and neuropathic pain. Although their efficacy as first-line treatments for painful neuropathic conditions (and several non-neuropathic painful conditions) has been well established in general medical practice, their efficacy and prescription in Oral Medicine and Oral Surgery practice has received little attention so far. This didactic article, the first of a two-part series, aims to present the experience of a French tertiary orofacial pain clinic regarding the prescription of gabapentinoids in Orofacial pain conditions that fall within the scope of the oral surgeon's practice.

Gabapentinoid prescription in Oral Medicine and Oral Surgery practice. Part II − a systematic scoping review of the literature

Introduction: Gabapentinoids, pregabalin and gabapentin, neuronal voltage-gated calcium channel inhibitors are first-line treatments for painful neuropathic conditions (and several non-neuropathic painful conditions). Nevertheless, their efficacy and prescription in Oral Medicine and Oral Surgery practice has received little attention so far. A previous article, the first of a two-part series, presented the experience of a French tertiary orofacial pain clinic regarding the prescription of gabapentinoids in orofacial conditions. This second article aimed to explore the scientific literature on the subject. Material and methods: A systematic scoping review was conducted on multiple relevant databases (MEDLINE®, Cochrane®, Agence Nationale de Sécurité du Médicament et des produits de santé, Haute Autorité de Santé) and journal archives (JOMOS, JSOMFS) to assess the indications, non-indications and contraindications of gabapentinoids in an Oral Medicine/Oral Surgery context. Results: Out of 131 records selected during the initial screening, 34 matched the inclusion criteria and were used for subsequent analyses. Gabapentinoids were prescribed in three clinical contexts: orofacial pain management (32 studies), anxiolysis (1 study) and prevention of postoperative nausea/vomiting (1 study), with variable quality of evidence: high (6 studies), moderate (3 studies), low (5 studies) and very low (20 studies) quality studies (GRADE scale). Untoward effects of gabapentinoids were reported in 16 studies, mainly neurological (vertigo, drowsiness, sedation) and gastro-intestinal (nausea, vomiting, diarrhea, constipation). Gabapentinoids were ineffective in preemptive and postoperative analgesia and for the management of mucositis-related pain. Discussion: There is some evidence supporting the use of gabapentinoids in Oral Medicine/Oral Surgery in adherence with current practices observed in France and other countries (practices often extrapolated from their use in other non-orofacial painful conditions). The methodological quality of the studies included in this scoping review is often poor and publication bias is most probable in this field. Therefore, any conclusion drawn from such studies must be subject to circumspection. Conclusion: Data obtained from the present scoping review suggests the potential use of gabapentinoids as second-line treatments for anxiolysis, prevention of postoperative nausea/vomiting and the management of trigeminal neuralgia and masticatory myalgia. Other potential indications of gabapentinoids in Oral Medicine/Oral Surgery practice include cranial neuralgias, post-traumatic trigeminal neuropathies, first bite syndrome, burning mouth syndrome and migraine prophylaxis, when other treatment options are inefficient or unavailable.

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

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

Retinoid receptor-based signaling plays a role in voltage-dependent inhibition of invertebrate voltage-gated Ca2+ channels

The retinoic acid receptor (RAR) and retinoid X receptor (RXR) mediate the cellular effects of retinoids (derivatives of vitamin A). Both RAR and RXR signaling events are implicated in hippocampal synaptic plasticity. Furthermore, retinoids can interact with calcium signaling during homeostatic plasticity. We recently provided evidence that retinoids attenuate calcium current (ICa) through neuronal voltage-gated calcium channels (VGCCs). We now examined the possibility that constitutive activity of neuronal RXR and/or RAR alters calcium influx via the VGCCs. We found that in neurons of the mollusk Lymnaea stagnalis, two different RXR antagonists (PA452 and HX531) had independent and opposing effects on ICa that were also time-dependent; whereas the RXR pan-antagonist PA452 enhanced ICa, HX531 reduced ICa Interestingly, this effect of HX531 occurred through voltage-dependent inhibition of VGCCs, a phenomenon known to influence neurotransmitter release from neurons. This inhibition appeared to be independent of G proteins and was largely restricted to Cav2 Ca2+ channels. Of note, an RAR pan-antagonist, LE540, also inhibited ICa but produced G protein-dependent, voltage-dependent inhibition of VGCCs. These findings provide evidence that retinoid receptors interact with G proteins in neurons and suggest mechanisms by which retinoids might affect synaptic calcium signaling.

Bi-allelic Loss-of-Function CACNA1B Mutations in Progressive Epilepsy-Dyskinesia

The occurrence of non-epileptic hyperkinetic movements in the context of developmental epileptic encephalopathies is an increasingly recognized phenomenon. Identification of causative mutations provides an important insight into common pathogenic mechanisms that cause both seizures and abnormal motor control. We report bi-allelic loss-of-function CACNA1B variants in six children from three unrelated families whose affected members present with a complex and progressive neurological syndrome. All affected individuals presented with epileptic encephalopathy, severe neurodevelopmental delay (often with regression), and a hyperkinetic movement disorder. Additional neurological features included postnatal microcephaly and hypotonia. Five children died in childhood or adolescence (mean age of death: 9 years), mainly as a result of secondary respiratory complications. CACNA1B encodes the pore-forming subunit of the pre-synaptic neuronal voltage-gated calcium channel Cav2.2/N-type, crucial for SNARE-mediated neurotransmission, particularly in theearly postnatal period. Bi-allelic loss-of-function variants in CACNA1B are predicted to cause disruption of Ca2+ influx, leading to impaired synaptic neurotransmission. The resultant effect on neuronal function is likely to be important in the development of involuntary movements and epilepsy. Overall, our findings provide further evidence for the key role of Cav2.2 in normal human neurodevelopment.

(344) A Photoswitchable GPCR-Based Opsin for Synaptic Inhibition

The development of diverse molecular tools to manipulate the activity of defined cell types has greatly accelerated our understanding of the neural circuits underlying complex behaviors. The most commonly used opsins are light-gated ion channels or pumps to control activity on millisecond timescales. While optogenetic activation can be precisely tuned across a range of firing frequencies, neuronal inhibition has proven to be more problematic. The most commonly used inhibitory opsins are chloride or proton pumps requiring constant illumination, and biophysical constraints due to different subcellular ion gradients can trigger unwanted excitation. The development of opto-GPCR chimeras grants spatial and temporal control of distinct G-protein coupled intracellular signaling cascades, and in contrast to binary on/off manipulations, modulating endogenous activity patterns may more accurately reflect circuit dynamics. However, these rhodopsin-based approaches possess several limitations, including high photosensitivity and irreversible activation. Here we have identified a novel photoswitchable GPCR-based opsin that engages endogenous inhibitory signaling cascades to silence synaptic transmission. This UV/blue light-sensitive opsin couples to G-proteins to reversibly inhibit neuronal voltage-gated calcium channel function, with similar efficacy to endogenous GABABRs. Long-term optical inhibition can be achieved with pulsed light, does not desensitize, and most importantly, inhibition is rapidly reversed by illumination with amber light. This opsin can also be stimulated by 2-photon excitation, permitting subcellular activation of G-protein subunits, which may allow for precise patterns of inhibition deep within tissues. Lastly, we found that optical stimulation in vivo inhibits dopamine neurons to reduce motivated behaviors. We are now adapting this GPCR-based opsin to serve as a novel scaffold for next-generation opto-XR chimeras with photoswitchable control of GPCR signaling cascades to better understand the roles of these important therapeutic targets in different pain circuits.

Increased Calcium Influx through L-type Calcium Channels in Human and Mouse Neural Progenitors Lacking Fragile X Mental Retardation Protein.

SummaryThe absence of FMR1 protein (FMRP) causes fragile X syndrome (FXS) and disturbed FMRP function is implicated in several forms of human psychopathology. We show that intracellular calcium responses to depolarization are augmented in neural progenitors derived from human induced pluripotent stem cells and mouse brain with FXS. Increased calcium influx via nifedipine-sensitive voltage-gated calcium (Cav) channels contributes to the exaggerated responses to depolarization and type 1 metabotropic glutamate receptor activation. The ratio of L-type/T-type Cav channel expression is increased in FXS progenitors and correlates with enhanced progenitor differentiation to glutamate-responsive cells. Genetic reduction of brain-derived neurotrophic factor in FXS mouse progenitors diminishes the expression of Cav channels and activity-dependent responses, which are associated with increased phosphorylation of the phospholipase C-γ1 site within TrkB receptors and changes of differentiating progenitor subpopulations. Our results show developmental effects of increased calcium influx via L-type Cav channels in FXS neural progenitors.

Retinoic acid inhibits neuronal voltage-gated calcium channels

Retinoic acid is the active metabolite of vitamin A and regulates several important cellular processes by activating retinoic acid receptors (RAR) and retinoid X receptors (RXR). These receptors generally act as transcription factors, though non-genomic actions of both retinoic acid and the receptors have also been reported. One such nongenomic effect includes the modulation of Ca2+ levels during homeostatic synaptic plasticity in the hippocampus. Retinoic acid can thus affect Ca2+ signaling and can potentially control both synaptic plasticity and neuronal firing. However, whether retinoic acid can regulate voltage-gated Ca2+ channels (either via genomic or nongenomic actions), which are fundamental to these processes, has not yet been studied in detail.Here we demonstrate the effects of retinoic acid on the biophysical properties of voltage-gated Ca2+ channels in cultured invertebrate motorneurons. Overnight exposure to physiological concentrations of retinoic acid significantly inhibited the voltage-gated Ca2+ current (ICa) in an isomer-dependent manner. Specifically, all-trans retinoic acid (atRA), but not 9-cis RA (9cRA), depolarized the voltage of half-maximal activation of ICa. AtRA also reduced the rate of channel activation and delayed recovery from inactivation. We provide evidence that both L-type and non-L-type voltage-gated Ca2+ channels are affected by atRA, as both nifedipine-sensitive and nifedipine-resistant ICa were inhibited in these neurons. These effects of retinoic acid are thought to be at least partially mediated by the retinoid receptors, as treatment of the neurons with synthetic RAR and RXR agonists produced a similar inhibition of ICa.

Trafficking of neuronal calcium channels.

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

Melanocortin 4 receptor constitutive activity inhibits L-type voltage-gated calcium channels in neurons

The melanocortin 4 receptor (MC4R) is a G protein-coupled receptor (GPCR) that is expressed in several brain nuclei playing a crucial role in the regulation of energy balance controlling the homeostasis of the organism. It displays both agonist-evoked and constitutive activity, and moreover, it can couple to different G proteins. Most of the research on MC4R has been focused on agonist-induced activity, while the molecular and cellular basis of MC4R constitutive activity remains scarcely studied.We have previously shown that neuronal N-type voltage-gated calcium channels (CaV2.2) are inhibited by MC4R agonist-dependent activation, while the CaV subtypes that carry L- and P/Q-type current are not. Here, we tested the hypothesis that MC4R constitutive activity can affect CaV, with focus on the channel subtypes that can control transcriptional activity coupled to depolarization (L-type, CaV1.2/1.3) and neurotransmitter release (N- and P/Q-type, CaV2.2 and CaV2.1). We found that MC4R constitutive activity inhibits specifically CaV1.2/1.3 and CaV2.1 subtypes of CaV. We also explored the signaling pathways mediating this inhibition, and thus propose that agonist-dependent and basal MC4R activation modes signal differentially through Gs and Gi/o pathways to impact on different CaV subtypes. In addition, we found that chronic incubation with MC4R endogenous inverse agonist, agouti and agouti-related peptide (AgRP), occludes CaV inhibition in a cell line and in amygdaloid complex cultured neurons as well. Thus, we define new mechanisms of control of the main mediators of depolarization-induced calcium entry into neurons by a GPCR that displays constitutive activity.

Frequency of Synaptic Autoantibody Accompaniments and Neurological Manifestations of Thymoma.

Thymoma is commonly recognized in association with paraneoplastic autoimmune myasthenia gravis (MG), an IgG-mediated impairment of synaptic transmission targeting the nicotinic acetylcholine receptor of muscle. Newly identified synaptic autoantibodies may expand the serological profile of thymoma. To investigate the frequency of potentially pathogenic neural synaptic autoantibodies in patients with thymoma. We retrospectively identified patients with histopathologically confirmed thymoma and serum available to test for synaptic autoantibodies (collected 1986-2014) at the Mayo Clinic Neuroimmunology Laboratory. We identified and classified 193 patients with thymoma into 4 groups: (1) lacking neurological autoimmunity (n = 43); (2) isolated MG (n = 98); (3) MG plus additional autoimmune neurological manifestations (n = 26); and (4) neurological autoimmunity other than MG (n = 26). Clinical presentation and serum profile of autoantibodies reactive with molecularly defined synaptic plasma membrane proteins of muscle, peripheral, and central nervous systems. Of the 193 patients with thymoma, mean patient age was 52 years and did not significantly differ by sex (106 women) or group. Myasthenia gravis was the most prevalent clinical manifestation (64%) followed by dysautonomia (16 patients [8%]) and encephalopathy (15 patients [8%]); 164 patients (85%) had at least 1 synaptic autoantibody, and 63 of these patients (38%) had at least 1 more. Muscle acetylcholine receptor was most frequent (78%), followed by ganglionic acetylcholine receptor (20%), voltage-gated Kv1 potassium channel-complex (13%), and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (5%). Less frequent were aquaporin-4, voltage-gated Kv1 potassium channel-complex related proteins (leucine-rich glioma-inactivated 1 and contactin-associated protein-like 2), glycine receptor, and γ-aminobutyric acid-A receptor. Synaptic autoantibodies were significantly more frequent in patients with neurological autoimmunity than in those without and were most frequent in patients with neurological manifestations other than or in addition to MG. Synaptic autoantibodies, particularly those reactive with ion channels of the ligand-gated nicotinic acetylcholine receptor superfamily (namely α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, glycine, and γ-aminobutyric acid-A receptors), were prevalent in patients with thymoma. Autoantibodies of this extended spectrum may enhance autoimmune serological testing as an aid to preoperative thymoma diagnosis. Detection of currently known synaptic autoantibody specificities absent from this profile have potential algorithmic usefulness as negative predictors for thymoma (as recognized for neuronal voltage-gated calcium channel autoantibodies).

ATP1A2 Mutations in Migraine: Seeing through the Facets of an Ion Pump onto the Neurobiology of Disease.

Mutations in four genes have been identified in familial hemiplegic migraine (FHM), from which CACNA1A (FHM type 1) and SCN1A (FHM type 3) code for neuronal voltage-gated calcium or sodium channels, respectively, while ATP1A2 (FHM type 2) encodes the α2 isoform of the Na+,K+-ATPase's catalytic subunit, thus classifying FHM primarily as an ion channel/ion transporter pathology. FHM type 4 is attributed to mutations in the PRRT2 gene, which encodes a proline-rich transmembrane protein of as yet unknown function. The Na+,K+-ATPase maintains the physiological gradients for Na+ and K+ ions and is, therefore, critical for the activity of ion channels and transporters involved neuronal excitability, neurotransmitter uptake or Ca2+ signaling. Strikingly diverse functional abnormalities have been identified for disease-linked ATP1A2 mutations which frequently lead to changes in the enzyme's voltage-dependent properties, kinetics, or apparent cation affinities, but some mutations are truly deleterious for enzyme function and thus cause full haploinsufficiency. Here, we summarize structural and functional data about the Na+,K+-ATPase available to date and an overview is provided about the particular properties of the α2 isoform that explain its physiological relevance in electrically excitable tissues. In addition, current concepts about the neurobiology of migraine, the correlations between primary brain dysfunction and mechanisms of headache pain generation are described, together with insights gained recently from modeling approaches in computational neuroscience. Then, a survey is given about ATP1A2 mutations implicated in migraine cases as documented in the literature with focus on mutations that were described to completely destroy enzyme function, or lead to misfolded or mistargeted protein in particular model cell lines. We also discuss whether or not there are correlations between these most severe mutational effects and clinical phenotypes. Finally, perspectives for future research on the implications of Na+,K+-ATPase mutations in human pathologies are presented.