Biphasic, opposing modulation of cloned neuronal alpha1E Ca channels by distinct signaling pathways coupled to M2 muscarinic acetylcholine receptors

Epilepsy-linked kinase CDKL5 phosphorylates voltage-gated calcium channel Cav2.3, altering inactivation kinetics and neuronal excitability

Developmental and epileptic encephalopathies (DEEs) are a group of rare childhood disorders characterized by severe epilepsy and cognitive deficits. Numerous DEE genes have been discovered thanks to advances in genomic diagnosis, yet putative molecular links between these disorders are unknown. CDKL5 deficiency disorder (CDD, DEE2), one of the most common genetic epilepsies, is caused by loss-of-function mutations in the brain-enriched kinase CDKL5. To elucidate CDKL5 function, we looked for CDKL5 substrates using a SILAC-based phosphoproteomic screen. We identified the voltage-gated Ca2+ channel Cav2.3 (encoded by CACNA1E) as a physiological target of CDKL5 in mice and humans. Recombinant channel electrophysiology and interdisciplinary characterization of Cav2.3 phosphomutant mice revealed that loss of Cav2.3 phosphorylation leads to channel gain-of-function via slower inactivation and enhanced cholinergic stimulation, resulting in increased neuronal excitability. Our results thus show that CDD is partly a channelopathy. The properties of unphosphorylated Cav2.3 closely resemble those described for CACNA1E gain-of-function mutations causing DEE69, a disorder sharing clinical features with CDD. We show that these two single-gene diseases are mechanistically related and could be ameliorated with Cav2.3 inhibitors.

Cav3 T-Type Voltage-Gated Ca2+ Channels and the Amyloidogenic Environment: Pathophysiology and Implications on Pharmacotherapy and Pharmacovigilance

Voltage-gated Ca2+ channels (VGCCs) were reported to play a crucial role in neurotransmitter release, dendritic resonance phenomena and integration, and the regulation of gene expression. In the septohippocampal system, high- and low-voltage-activated (HVA, LVA) Ca2+ channels were shown to be involved in theta genesis, learning, and memory processes. In particular, HVA Cav2.3 R-type and LVA Cav3 T-type Ca2+ channels are expressed in the medial septum-diagonal band of Broca (MS-DBB), hippocampal interneurons, and pyramidal cells, and ablation of both channels was proven to severely modulate theta activity. Importantly, Cav3 Ca2+ channels contribute to rebound burst firing in septal interneurons. Consequently, functional impairment of T-type Ca2+ channels, e.g., in null mutant mouse models, caused tonic disinhibition of the septohippocampal pathway and subsequent enhancement of hippocampal theta activity. In addition, impairment of GABA A/B receptor transcription, trafficking, and membrane translocation was observed within the septohippocampal system. Given the recent findings that amyloid precursor protein (APP) forms complexes with GABA B receptors (GBRs), it is hypothesized that T-type Ca2+ current reduction, decrease in GABA receptors, and APP destabilization generate complex functional interdependence that can constitute a sophisticated proamyloidogenic environment, which could be of potential relevance in the etiopathogenesis of Alzheimer's disease (AD). The age-related downregulation of T-type Ca2+ channels in humans goes together with increased Aβ levels that could further inhibit T-type channels and aggravate the proamyloidogenic environment. The mechanistic model presented here sheds new light on recent reports about the potential risks of T-type Ca2+ channel blockers (CCBs) in dementia, as observed upon antiepileptic drug application in the elderly.

Enhanced hippocampal type II theta activity AND altered theta architecture in mice lacking the Cav3.2 T-type voltage-gated calcium channel

T-type Ca²⁺ channels are assumed to contribute to hippocampal theta oscillations. We used implantable video-EEG radiotelemetry and qPCR to unravel the role of Ca_v3.2 Ca²⁺ channels in hippocampal theta genesis. Frequency analysis of spontaneous long-term recordings in controls and Ca_v3.2^-/- mice revealed robust increase in relative power in the theta (4-8 Hz) and theta-alpha (4-12 Hz) ranges, which was most prominent during the inactive stages of the dark cycles. Urethane injection experiments also showed enhanced type II theta activity and altered theta architecture following Ca_v3.2 ablation. Next, gene candidates from hippocampal transcriptome analysis of control and Ca_v3.2^-/- mice were evaluated using qPCR. Dynein light chain Tctex-Type 1 (Dynlt1b) was significantly reduced in Ca_v3.2^-/- mice. Furthermore, a significant reduction of GABA A receptor δ subunits and GABA B1 receptor subunits was observed in the septohippocampal GABAergic system. Our results demonstrate that ablation of Ca_v3.2 significantly alters type II theta activity and theta architecture. Transcriptional changes in synaptic transporter proteins and GABA receptors might be functionally linked to the electrophysiological phenotype.

Review: Cav2.3 R-type Voltage-Gated Ca2+ Channels - Functional Implications in Convulsive and Non-convulsive Seizure Activity

Background:

Researchers have gained substantial insight into mechanisms of synaptic transmission, hyperexcitability, excitotoxicity and neurodegeneration within the last decades. Voltage-gated Ca²⁺ channels are of central relevance in these processes. In particular, they are key elements in the etiopathogenesis of numerous seizure types and epilepsies. Earlier studies predominantly targeted on Ca_v2.1 P/Q-type and Ca_v3.2 T-type Ca²⁺ channels relevant for absence epileptogenesis. Recent findings bring other channels entities more into focus such as the Ca_v2.3 R-type Ca²⁺ channel which exhibits an intriguing role in ictogenesis and seizure propagation. Ca_v2.3 R-type voltage gated Ca²⁺ channels (VGCC) emerged to be important factors in the pathogenesis of absence epilepsy, human juvenile myoclonic epilepsy (JME), and cellular epileptiform activity, e.g. in CA1 neurons. They also serve as potential target for various antiepileptic drugs, such as lamotrigine and topiramate.

Objective:

This review provides a summary of structure, function and pharmacology of VGCCs and their fundamental role in cellular Ca²⁺ homeostasis. We elaborate the unique modulatory properties of Ca_v2.3 R-type Ca²⁺ channels and point to recent findings in the proictogenic and proneuroapoptotic role of Ca_v2.3 R-type VGCCs in generalized convulsive tonic–clonic and complex-partial hippocampal seizures and its role in non-convulsive absence like seizure activity.

Conclusion:

Development of novel Ca_v2.3 specific modulators can be effective in the pharmacological treatment of epilepsies and other neurological disorders.

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

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

Pharmacoresistant Cav 2·3 (E-type/R-type) voltage-gated calcium channels influence heart rate dynamics and may contribute to cardiac impulse conduction

Voltage-gated Ca(2+) channels regulate cardiac automaticity, rhythmicity and excitation-contraction coupling. Whereas L-type (Cav 1·2, Cav 1·3) and T-type (Cav 3·1, Cav 3·2) channels are widely accepted for their functional relevance in the heart, the role of Cav 2·3 Ca(2+) channels expressing R-type currents remains to be elucidated. We have investigated heart rate dynamics in control and Cav 2·3-deficient mice using implantable electrocardiogram radiotelemetry and pharmacological injection experiments. Autonomic block revealed that the intrinsic heart rate does not differ between both genotypes. Systemic administration of isoproterenol resulted in a significant reduction in interbeat interval in both genotypes. It remained unaffected after administering propranolol in Cav 2·3(-|-) mice. Heart rate from isolated hearts as well as atrioventricular conduction for both genotypes differed significantly. Additionally, we identified and analysed the developmental expression of two splice variants, i.e. Cav 2·3c and Cav 2·3e. Using patch clamp technology, R-type currents could be detected in isolated prenatal cardiomyocytes and be related to R-type Ca(2+) channels. Our results indicate that on the systemic level, the pharmacologically inducible heart rate range and heart rate reserve are impaired in Cav 2·3 (-|-) mice. In addition, experiments on Langendorff perfused hearts elucidate differences in basic properties between both genotypes. Thus, Cav 2·3 does not only contribute to the cardiac autonomous nervous system but also to intrinsic rhythm propagation.

Hippocampal Seizure Resistance and Reduced Neuronal Excitotoxicity in Mice Lacking the Cav2.3 E/R-Type Voltage-Gated Calcium Channel

Voltage-gated calcium channels are key components in the etiology and pathogenesis of epilepsies. Former studies mainly focused on P/Q-type Cav2.1 and T-type Cav3.2 Ca2+ channels involved in absence epileptogenesis, but recent findings also point to an intriguing role of the Cav2.3 E/R-type Ca2+ channel in ictogenesis and seizure propagation. Based on the observation that Cav2.3 is thought to induce plateau potentials in CA1 pyramidal cells, which can trigger epileptiform activity, our recent investigation revealed reduced PTZ-seizure susceptibility and altered seizure architecture in Cav2.3−/− mice compared with controls. In the present study we tested hippocampal seizure susceptibility in Cav2.3-deficient mice using surface and deep intrahippocampal telemetric EEG recordings as well as phenotypic seizure video analysis. Administration of kainic acid (30 mg/kg ip) revealed clear alteration in behavioral seizure architecture and dramatic resistance to limbic seizures in Cav2.3−/− mice compared with controls, whereas no difference in hippocampal EEG seizure activity between both genotypes could be detected at this suprathreshold dosage. The same tendency was observed for NMDA seizure susceptibility (150 mg/kg ip) approaching the level of significance. In addition, histochemical analysis within the hippocampus revealed that excitotoxic effects after kainic acid administration are absent in Cav2.3−/− mice, whereas Cav2.3+/+ animals exhibited clear and typical signs of excitotoxic cell death. These findings clearly indicate that the Cav2.3 voltage-gated calcium channel plays a crucial role in both hippocampal ictogenesis and seizure generalization and is of central importance in neuronal degeneration after excitotoxic events.

Neurokinin 1 receptors trigger overlapping stimulation and inhibition of CaV2.3 (R-type) calcium channels

Neurokinin (NK) 1 receptors and CaV2.3 calcium channels are both expressed in nociceptive neurons, and mice lacking either protein display altered responses to noxious stimuli. Here, we examined modulation of CaV2.3 through NK1 receptors expressed in human embryonic kidney 293 cells. We find that NK1 receptors generate complex modulation of CaV2.3. In particular, weak activation of these receptors evokes mainly stimulation of CaV2.3, whereas strong receptor activation elicits profound inhibition that overlaps with channel stimulation. Unlike R-type channels encoded by CaV2.3, L-type (CaV1.3), N-type (CaV2.2), and P/Q-type (CaV2.1) channels are inhibited, but not stimulated, through NK1 receptors. Pharmacological experiments show that protein kinase C (PKC) mediates stimulation of CaV2.3 through NK1 receptors. The signaling mechanisms underlying inhibition were explored by expressing proteins that buffer either Galpha(q/11) (regulator of G protein signaling protein 3T and carboxyl-terminal region of phospholipase C-beta1) or Gbeta gamma subunits (transducin and the carboxyl-terminal region of bovine G-protein-coupled receptor kinase). A fast component of inhibition was attenuated by buffering Gbeta gamma, whereas a slow component of inhibition was reduced by buffering Galpha(q/11). When both Gbeta gamma and Galpha(q/11) were simultaneously buffered in the same cells, inhibition was virtually eliminated, but receptor activation still triggered substantial stimulation of CaV2.3. We also report that NK1 receptors accelerate the inactivation kinetics of CaV2.3 currents. Altogether, our results indicate that NK1 receptors modulate CaV2.3 using three different signaling mechanisms: a fast inhibition mediated by Gbeta gamma, a slow inhibition mediated by Galpha(q/11), and a slow stimulation mediated by PKC. This new information concerning R-type calcium channels and NK1 receptors may help in understanding nociception, synaptic plasticity, and other physiological processes.

Transmitter modulation of spike-evoked calcium transients in arousal related neurons: muscarinic inhibition of SNX-482-sensitive calcium influx

Nitric oxide synthase (NOS)-containing cholinergic neurons in the laterodorsal tegmentum (LDT) influence behavioral and motivational states through their projections to the thalamus, ventral tegmental area and a brainstem 'rapid eye movement (REM)-induction' site. Action potential-evoked intracellular calcium transients dampen excitability and stimulate NO production in these neurons. In this study, we investigated the action of several arousal-related neurotransmitters and the role of specific calcium channels in these LDT Ca(2+)-transients by simultaneous whole-cell recording and calcium imaging in mouse (P14-P30) brain slices. Carbachol, noradrenaline and adenosine inhibited spike-evoked Ca(2+)-transients, while histamine, t-ACPD, a metabotropic glutamate receptor agonist, and orexin-A did not. Carbachol inhibition was blocked by atropine, was insensitive to blockade of G-protein-coupled inward rectifier (GIRK) channels and was not inhibited by nifedipine, omega-conotoxin GVIA or omega-agatoxin IVA, which block L-, N- and P/Q-type calcium channels, respectively. In contrast, SNX-482 (100 nm), a selective antagonist of R-type calcium channels containing the alpha1E (Cav2.3) subunit, attenuated carbachol inhibition of the somatic spike-evoked calcium transient. To our knowledge, this is the first demonstration of muscarinic inhibition of native SNX-482-sensitive R-channels. Our findings indicate that muscarinic modulation of these channels plays an important role in the feedback control of cholinergic LDT neurons and that inhibition of spike-evoked Ca(2+)-transients is a common action of neurotransmitters that also activate GIRK channels in these neurons. Because spike-evoked calcium influx dampens excitability, our findings suggest that these 'inhibitory' transmitters could boost firing rate and enhance responsiveness to excitatory inputs during states of high firing, such as waking and REM sleep.

An M2-like muscarinic receptor enhances a delayed rectifier K+ current in rat sympathetic neurones

BACKGROUND AND PURPOSE

Resting superior cervical ganglion (SCG) neurones are phasic cells that switch to a tonic mode of firing upon muscarinic receptor stimulation. This effect is partially due to the muscarinic inhibition of the M-current. Because delayed rectifier K+ channels are essential to sustain tonic firing in central neurones, we asked whether the delayed rectifier current IKV in SCG neurones was modulated by the muscarinic receptors expressed in these cells.

EXPERIMENTAL APPROACH

Whole-cell patch-clamp records of M-current and IKV were done in cultured or acutely dissociated rat SCG neurones. To characterize the receptor that regulates IKV, cells were bathed with muscarinic agonists and antagonists, relatively specific for receptor subtypes.

KEY RESULTS

The muscarinic agonist oxotremorine-M (Oxo-M) enhanced IKV by approximately 46% relative to its basal value. This effect remained unaltered when M-current was suppressed by linopirdine or Ba2+. Enhancement of IKV was insensitive to the M1-antagonist pirenzepine, whereas it was inhibited (approximately 60%) by the M2/4-antagonist himbacine. Further, the relatively specific M2-agonist bethanechol was as potent as Oxo-M in enhancing IKV. The modulation of IKV was insensitive to pertussis toxin (PTX), but was severely attenuated when internal ATP was replaced by its non-hydrolysable analogue AMP-PNP.

CONCLUSIONS AND IMPLICATIONS

These results suggest that an M2-like muscarinic receptor couples to a PTX-insensitive G-protein and to an ATP-dependent pathway to enhance IKV. Modulation of IKV must be taken into consideration in order to understand more precisely how muscarinic receptors acting on different ion channels regulate sympathetic excitability.

Muscarinic modulation of Cav2.3 (R-type) calcium channels is antagonized by RGS3 and RGS3T

Ca(2+) influx through voltage-gated R-type (Ca(V)2.3) Ca(2+) channels is important for hormone and neurotransmitter secretion and other cellular events. Previous studies have shown that Ca(V)2.3 is both inhibited and stimulated through signaling mechanisms coupled to muscarinic ACh receptors. We previously demonstrated that muscarinic stimulation of Ca(V)2.3 is blocked by regulator of G protein signaling (RGS) 2. Here we investigated whether muscarinic inhibition of Ca(V)2.3 is antagonized by RGS3. RGS3 is particularly interesting because it contains a lengthy ( approximately 380 residue) amino-terminal domain of uncertain physiological function. Ca(V)2.3, M(2) muscarinic ACh receptors (M(2)R), and various deletion mutants of RGS3, including its native isoform RGS3T, were expressed in HEK293 cells, and agonist-dependent inhibition of Ca(V)2.3 was quantified using whole cell patch-clamp recordings. Full-length RGS3, RGS3T, and the core domain of RGS3 were equally effective in antagonizing inhibition of Ca(V)2.3 through M(2)R. These results identify RGS3 and RGS3T as potential physiological regulators of R-type Ca(2+) channels. Furthermore, they suggest that the signaling activity of RGS3 is unaffected by its extended amino-terminal domain. Confocal microscopy was used to examine the intracellular locations of four RGS3-enhanced green fluorescent protein fusion proteins. The RGS3 core domain was uniformly distributed throughout both cytoplasm and nucleus. By contrast, full-length RGS3, RGS3T, and the amino-terminal domain of RGS3 were restricted to the cytoplasm. These observations suggest that the amino terminus of RGS3 may serve to confine it to the cytoplasmic compartment where it can interact with cell surface receptors, heterotrimeric G proteins, and other signaling proteins.

The GAR-3 muscarinic receptor cooperates with calcium signals to regulate muscle contraction in the Caenorhabditis elegans pharynx

Muscarinic acetylcholine receptors regulate the activity of neurons and muscle cells through G-protein-coupled cascades. Here, we identify a pathway through which the GAR-3 muscarinic receptor regulates both membrane potential and excitation-contraction coupling in the Caenorhabditis elegans pharyngeal muscle. GAR-3 signaling is enhanced in worms overexpressing gar-3 or lacking GPB-2, a G-protein beta-subunit involved in RGS-mediated inhibition of G(o)alpha- and G(q)alpha-linked pathways. High levels of signaling through GAR-3 inhibit pharyngeal muscle relaxation and impair feeding--but do not block muscle repolarization--when worms are exposed to arecoline, a muscarinic agonist. Loss of gar-3 function results in shortened action potentials and brief muscle contractions in the pharyngeal terminal bulb. High levels of calcium entry through voltage-gated channels also impair terminal bulb relaxation and sensitize worms to the toxic effects of arecoline. Mutation of gar-3 reverses this sensitivity, suggesting that GAR-3 regulates calcium influx or calcium-dependent processes. Because the effects of GAR-3 signaling on membrane depolarization and muscle contraction can be separated, we conclude that GAR-3 regulates multiple calcium-dependent processes in the C. elegans pharyngeal muscle.

Ablation of Cav2.3 / E?type voltage?gated calcium channel results in cardiac arrhythmia and altered autonomic control within the murine cardiovascular system

Voltage-gated calcium channels are key components in cardiac electrophysiology. We demonstrate that Ca(v)2.3 is expressed in mouse and human heart and that mice lacking the Ca(v)2.3 voltage-gated calcium channel exhibit severe alterations in cardiac function. Amplified cDNA fragments from murine heart and single cardiomyocytes reveal the expression of three different Ca(v)2.3 splice variants. The ablation of Ca(v)2.3 was found to be accompanied by a compensatory upregulation of the Ca(v)3.1 T-type calcium channel, while other voltage-gated calcium channels remained unaffected. Telemetric ECG recordings from Ca(v)2.3 deficient mice displayed subsidiary escape rhythm, altered atrial activation patterns, atrioventricular conduction disturbances and alteration in QRS-morphology. Furthermore, time domain analysis of heart rate variability (HRV) in Ca(v)2.3(-/-) mice exhibited a significant increase in heart rate as well as in the coefficient of variance (CV) compared to control mice. Administration of atropin/propranolol revealed that increased heart rate was due to enhanced sympathetic tonus and that partial decrease of CV in Ca(v)2.3(-/-) mice after autonomic block was in accordance with a complete abolishment of 2(nd) degree atrioventricular block. However, escape rhythms, atrial activation disturbances and QRS-dysmorphology remained unaffected, indicating that these are intrinsic cardiac features in Ca(v)2.3(-/-) mice. We conclude that the expression of Ca(v)2.3 is essential for normal impulse generation and conduction in murine heart.

Differential Modulation of CaV2.3 Ca2+Channels by Gαq/11-Coupled Muscarinic Receptors

CaV2.3 subunits are expressed in neuronal and neuroendocrine cells where they are believed to form native R-type Ca2+ channels. Although R-type currents are involved in triggering neurotransmitter and hormone secretion, little is known about their modulation. Previous studies have shown that muscarinic acetylcholine receptors evoke both inhibition and stimulation of CaV2.3. Muscarinic inhibition of CaV2.3 is mediated by Gbetagamma subunits, whereas stimulation is mediated by pertussis toxin-insensitive Galpha subunits. In the present study, we compared modulation of CaV2.3 by the three Galphaq/11-coupled muscarinic receptors (M1, M3, and M5). Our data indicate that these receptors trigger comparable stimulation of CaV2.3. The signaling pathway that mediates stimulation was meticulously analyzed for M1 receptors. Stimulation is blocked by neutralizing antibodies directed against Galphaq/11, coexpression of the regulatory domain of protein kinase Cdelta (PKCdelta), preactivating PKC with phorbol ester, or pharmacological suppression of PKC with bisindolylmaleimide I. Stimulation of CaV2.3 is Ca(2+)-independent and insensitive to 12-(2-cyanoethyl)-6,7,12,13-tetrahydro-13-methyl-5-oxo-5H-indolo(2,3-a)pyrrolo(3,4-c)-carbazole (Gö 6976), a specific inhibitor of Ca(2+)-dependent PKC isozymes. These results indicate that muscarinic stimulation of CaV2.3 involves signaling by Galphaq/11, diacylglycerol, and a Ca(2+)-independent PKC. In contrast to stimulation, the magnitude of CaV2.3 inhibition depended on receptor subtype, with M3 and M5 receptors producing much larger CaV2.3 inhibition than M1 receptors. Interestingly, muscarinic inhibition of CaV2.3 was notably enhanced during pharmacological suppression of PKC, suggesting the presence of cross-talk between Gbetagamma-mediated inhibition and PKC-mediated stimulation of R-type channels similar to that described previously for N-type channels.

Arachidonic acid mediates muscarinic inhibition and enhancement of N-type Ca2+ current in sympathetic neurons

N-type Ca(2+) channels participate in acute activity-dependent processes such as regulation of Ca(2+)-activated K(+) channels and in more prolonged events such as gene transcription and long-term depression. A slow postsynaptic M(1) muscarinic receptor-mediated modulation of N-type current in superior cervical ganglion neurons may be important in regulating these processes. This slow pathway inhibits N-type current by using a diffusible second messenger that has remained unidentified for more than a decade. Using whole-cell patch-clamp techniques, which isolate the slow pathway, we found that the muscarinic agonist oxotremorine methiodide not only inhibits currents at positive potentials but enhances N-type current at negative potentials. Enhancement was also observed in cell-attached patches. These findings provide evidence for N-type Ca(2+)-current enhancement by a classical neurotransmitter. Moreover, enhancement and inhibition of current by oxotremorine methiodide mimics modulation observed with direct application of a low concentration of arachidonic acid (AA). Although no transmitter has been reported to use AA as a second messenger to modulate any Ca(2+) current in either neuronal or nonneuronal cells, we nevertheless tested whether a fatty acid signaling cascade was involved. Blocking phospholipase C, phospholipase A(2), or AA but not AA metabolism minimized muscarinic modulation of N-type current, supporting the participation of these molecules in the slow pathway. A role for the G protein G(q) was also confirmed by blocking muscarinic modulation of Ca(2+) currents with anti-G(qalpha) antibody. Our finding that AA participates in the slow pathway strongly suggests that it may be the previously unknown diffusible second messenger.

Reconstituted slow muscarinic inhibition of neuronal (Ca(v)1.2c) L-type Ca2+ channels

Ca(2+) influx through L-type channels is critical for numerous physiological functions. Relatively little is known about modulation of neuronal L-type Ca(2+) channels. We studied modulation of neuronal Ca(V)1.2c channels heterologously expressed in HEK293 cells with each of the known muscarinic acetylcholine receptor subtypes. Galphaq/11-coupled M1, M3, and M5 receptors each produced robust inhibition of Ca(V)1.2c, whereas Galphai/o-coupled M2 and M4 receptors were ineffective. Channel inhibition through M1 receptors was studied in detail and was found to be kinetically slow, voltage-independent, and pertussis toxin-insensitive. Slow inhibition of Ca(V)1.2c was blocked by coexpressing RGS2 or RGS3T or by intracellular dialysis with antibodies directed against Galphaq/11. In contrast, inhibition was not reduced by coexpressing betaARK1ct or Galphat. These results indicate that slow inhibition required signaling by Galphaq/11, but not Gbetagamma, subunits. Slow inhibition did not require Ca(2+) transients or Ca(2+) influx through Ca(V)1.2c channels. Additionally, slow inhibition was insensitive to pharmacological inhibitors of phospholipases, protein kinases, and protein phosphatases. Intracellular BAPTA prevented slow inhibition via a mechanism other than Ca(2+) chelation. The cardiac splice-variant of Ca(V)1.2 (Ca(V)1.2a) and a splice-variant of the neuronal/neuroendocrine Ca(V)1.3 channel also appeared to undergo slow muscarinic inhibition. Thus, slow muscarinic inhibition may be a general characteristic of L-type channels having widespread physiological significance.

Regulation of ion channels by protein tyrosine phosphorylation

Ion channels are regulated by protein phosphorylation and dephosphorylation of serine, threonine, and tyrosine residues. Evidence for the latter process, tyrosine phosphorylation, has increased substantially since this topic was last reviewed. In this review, we present a comprehensive summary and synthesis of the literature regarding the mechanism and function of ion channel regulation by protein tyrosine kinases and phosphatases. Coverage includes the majority of voltage-gated, ligand-gated, and second messenger-gated channels as well as several types of channels that have not yet been cloned, including store-operated Ca2+ channels, nonselective cation channels, and epithelial Na+ and Cl- channels. Additionally, we discuss the critical roles that channel-associated scaffolding proteins may play in localizing protein tyrosine kinases and phosphatases to the vicinity of ion channels.

Functional coupling between 'R-type' Ca2+ channels and insulin secretion in the insulinoma cell line INS-1

Among voltage-gated Ca2+ channels the non-dihydropyridine-sensitive alpha1E subunit is functionally less well characterized than the structurally related alpha1A (omega-agatoxin-IVA sensitive, P- /Q-type) and alpha1B (omega-conotoxin-GVIA sensitive, N-type) subunits. In the rat insulinoma cell line, INS-1, a tissue-specific splice variant of alpha1E (alpha1Ee) has been characterized at the mRNA and protein levels, suggesting that INS-1 cells are a suitable model for investigating the function of alpha1Ee. In alpha1E-transfected human embryonic kidney (HEK-293) cells the alpha1E-selective peptide antagonist SNX-482 (100 nM) reduces alpha1Ed- and alpha1Ee-induced Ba2+ inward currents in the absence and presence of the auxiliary subunits beta3 and alpha2delta-2 by more than 80%. The inhibition is fast and only partially reversible. No effect of SNX-482 was detected on the recombinant T-type Ca2+ channel subunits alpha1G, alpha1H, and alpha1I showing that the toxin from the venom of Hysterocrates gigas is useful as an alpha1E-selective antagonist. After blocking known components of Ca2+ channel inward current in INS-1 cells by 2 microM (+/-)-isradipine plus 0.5 microM omega-conotoxin-MVIIC, the remaining current is reduced by 100 nM SNX-482 from -12.4 +/- 1.2 pA/pF to -7.6 +/- 0.5 pA/pF (n = 9). Furthermore, in INS-1 cells, glucose- and KCl-induced insulin release are reduced by SNX-482 in a dose-dependent manner leading to the conclusion that alpha1E, in addition to L-type and non-L-type (alpha1A-mediated) Ca2+ currents, is involved in Ca2+ dependent insulin secretion of INS-1 cells.

Properties of voltage-gated Ca(2+) channels in rabbit ventricular myocytes expressing Ca(2+) channel alpha(1E) cDNA

Using the whole-cell patch-clamp technique, we have studied the properties of alpha(1E) Ca(2+) channel transfected in cardiac myocytes. We have also investigated the effect of foreign gene expression on the intrinsic L-type current (I(Ca,L)). Expression of green fluorescent protein significantly decreased the I(Ca,L). By contrast, expression of alpha(1E) with beta(2b) and alpha(2)/delta significantly increased the total Ca(2+) current, and in these cells a Ca(2+) antagonist, PN-200-110 (PN), only partially blocked the current. The remaining PN-resistant current was abolished by the application of a low concentration of Ni(2+) and was little affected by changing the charge carrier from Ca(2+) to Ba(2+) or by beta-adrenergic stimulation. On the basis of its voltage range for activation, this channel was classified as a high-voltage activated channel. Thus the expression of alpha(1E) did not generate T-like current in cardiac myocytes. On the other hand, expression of alpha(1E) decreased I(Ca,L) and slowed the I(Ca,L) inactivation. This inactivation slowing was attenuated by the beta(2b) coexpression, suggesting that the alpha(1E) may slow the inactivation of I(Ca,L) by scrambling with alpha(1C) for intrinsic auxiliary beta.

Muscarinic stimulation of alpha1E Ca channels is selectively blocked by the effector antagonist function of RGS2 and phospholipase C-beta1

Neuronal alpha1E Ca channel subunits are widely expressed in mammalian brain, where they are thought to form R-type Ca channels. Recent studies have demonstrated that R-type channels contribute to neurosecretion and dendritic Ca influx, but little is known concerning their modulation. Here we show that alpha1E channels are strongly stimulated, and only weakly inhibited, through M1 muscarinic acetylcholine receptors. Both forms of channel modulation are mediated by pertussis toxin-insensitive G-proteins. Channel stimulation is blocked by regulator of G-protein signaling 2 (RGS2) or the C-terminal region of phospholipase C-beta1 (PLCbeta1ct), which have been previously shown to function as GTPase-activating proteins for Galphaq. In contrast, RGS2 and PLCbeta1ct do not block inhibition of alpha1E through M1 receptors. Inhibition is prevented, however, by the C-terminal region of beta-adrenergic receptor kinase 1, which sequesters Gbetagamma dimers. Thus, stimulation of alpha1E is mediated by a pertussis toxin-insensitive Galpha subunit (e.g., Galphaq), whereas inhibition is mediated by Gbetagamma. The ability of RGS2 and PLCbeta1ct to selectively block stimulation indicates these proteins functioned primarily as effector antagonists. In support of this interpretation, RGS2 prevented stimulation of alpha1E with non-hydrolyzable guanosine 5'-0-(3-thiotriphosphate). We also report strong muscarinic stimulation of rbE-II, a variant alpha1E Ca channel that is insensitive to voltage-dependent inhibition. Our results predict that Galphaq-coupled receptors predominantly stimulate native R-type Ca channels. Receptor-mediated enhancement of R-type Ca currents may have important consequences for neurosecretion, dendritic excitability, gene expression, or other neuronal functions.

Heteromeric assembly of GABA(B)R1 and GABA(B)R2 receptor subunits inhibits Ca(2+) current in sympathetic neurons

Neuronal GABA(B) receptors regulate calcium and potassium currents via G-protein-coupled mechanisms and play a critical role in long-term inhibition of synaptic transmission in the CNS. Recent studies have demonstrated that assembly of GABA(B) receptor GABA(B)R1 and GABA(B)R2 subunits into functional heterodimers is required for coupling to potassium channels in heterologous systems. However whether heterodimerization is required for the coupling of GABA(B) receptors to effector systems in neurons remains to be established. To address this issue, we have studied the coupling of recombinant GABA(B) receptors to endogenous Ca(2+) channels in superior cervical ganglion (SCG) neurons using nuclear microinjection to introduce both sense and antisense expression constructs. Patch-clamp recording from neurons injected with both GABA(B)R1a/1b and GABA(B)R2 cDNAs or with GABA(B)R2 alone produced marked baclofen-mediated inhibition of Ca(2+) channel currents via a pertussis toxin-sensitive mechanism. The actions of baclofen were blocked by CGP62349, a specific GABA(B) antagonist, and were voltage dependent. Interestingly, SCGs were found to express abundantly GABA(B)R1 but not GABA(B)R2 at the protein level. To determine whether heterodimerization of GABA(B)R1 and GABA(B)R2 subunits was required for Ca(2+) inhibition, the GABA(B)R2 expression construct was microinjected with a GABA(B)R1 antisense construct. This resulted in a dramatic decrease in the levels of the endogenous GABA(B)R1 protein and a marked reduction in the inhibitory effects of baclofen on Ca(2+) currents. Therefore our results suggest that in neurons heteromeric assemblies of GABA(B)R1 and GABA(B)R2 are essential to mediate GABAergic inhibition of Ca(2+) channel currents.