A novel Nav1.8-FLPo driver mouse for intersectional genetics to uncover the functional significance of primary sensory neuron diversity

The recent development of single-cell and single-nucleus RNA sequencing has highlighted the extraordinary diversity of dorsal root ganglia neurons. However, the few available genetic tools limit our understanding of the functional significance of this heterogeneity. We generated a new mouse line expressing the flippase recombinase from the scn10a locus. By crossing Nav1.8Ires-FLPo mice with the AdvillinCre and RC::FL-hM3Dq mouse lines in an intersectional genetics approach, we were able to obtain somatodendritic expression of hM3Dq-mCherry selectively in the Nav1.8 lineage. The bath application of clozapine N-oxide triggered strong calcium responses selectively in mCherry+ neurons. The intraplantar injection of CNO caused robust flinching, shaking, and biting responses accompanied by strong cFos activation in the ipsilateral lumbar spinal cord. The Nav1.8Ires-FLPo mouse model will be a valuable tool for extending our understanding of the in vivo functional specialization of neuronal subsets of the Nav1.8 lineage for which inducible Cre lines are available.

Voltage tunes mGlu5 receptor function, impacting synaptic transmission

BACKGROUND AND PURPOSE

Voltage sensitivity is a common feature of many membrane proteins, including some G-protein coupled receptors (GPCRs). However, the functional consequences of voltage sensitivity in GPCRs are not well understood.

EXPERIMENTAL APPROACH

In this study, we investigated the voltage sensitivity of the post-synaptic metabotropic glutamate receptor mGlu5 and its impact on synaptic transmission. Using biosensors and electrophysiological recordings in non-excitable HEK293T cells or neurons.

KEY RESULTS

We found that mGlu5 receptor function is optimal at resting membrane potentials. We observed that membrane depolarization significantly reduced mGlu5 receptor activation, Gq-PLC/PKC stimulation, Ca2+ release and mGlu5 receptor-gated currents through transient receptor potential canonical, TRPC6, channels or glutamate ionotropic NMDA receptors. Notably, we report a previously unknown activity of the NMDA receptor at the resting potential of neurons, enabled by mGlu5 .

CONCLUSIONS AND IMPLICATIONS

Our findings suggest that mGlu5 receptor activity is directly regulated by membrane voltage which may have a significant impact on synaptic processes and pathophysiological functions.

Selective blockade of Cav1.2 (α1C) versus Cav1.3 (α1D) L-type calcium channels by the black mamba toxin calciseptine

L-type voltage-gated calcium channels are involved in multiple physiological functions. Currently available antagonists do not discriminate between L-type channel isoforms. Importantly, no selective blocker is available to dissect the role of L-type isoforms Cav1.2 and Cav1.3 that are concomitantly co-expressed in the heart, neuroendocrine and neuronal cells. Here we show that calciseptine, a snake toxin purified from mamba venom, selectively blocks Cav1.2 -mediated L-type calcium currents (ICaL) at concentrations leaving Cav1.3-mediated ICaL unaffected in both native cardiac myocytes and HEK-293T cells expressing recombinant Cav1.2 and Cav1.3 channels. Functionally, calciseptine potently inhibits cardiac contraction without altering the pacemaker activity in sino-atrial node cells, underscoring differential roles of Cav1.2- and Cav1.3 in cardiac contractility and automaticity. In summary, calciseptine is a selective L-type Cav1.2 Ca2+ channel blocker and should be a valuable tool to dissect the role of these L-channel isoforms.

Pmu1a, a novel spider toxin with dual inhibitory activity at pain targets hNaV 1.7 and hCaV 3 voltage-gated channels

Venom-derived peptides targeting ion channels involved in pain are regarded as a promising alternative to current, and often ineffective, chronic pain treatments. Many peptide toxins are known to specifically and potently block established therapeutic targets, among which the voltage-gated sodium and calcium channels are major contributors. Here, we report on the discovery and characterization of a novel spider toxin isolated from the crude venom of Pterinochilus murinus that shows inhibitory activity at both hNa_V 1.7 and hCa_V 3.2 channels, two therapeutic targets implicated in pain pathways. Bioassay-guided HPLC fractionation revealed a 36-amino acid peptide with three disulfide bridges named μ/ω-theraphotoxin-Pmu1a (Pmu1a). Following isolation and characterization, the toxin was chemically synthesized and its biological activity was further assessed using electrophysiology, revealing Pmu1a to be a toxin that potently blocks both hNa_V 1.7 and hCa_V 3. Nuclear magnetic resonance structure determination of Pmu1a shows an inhibitor cystine knot fold that is the characteristic of many spider peptides. Combined, these data show the potential of Pmu1a as a basis for the design of compounds with dual activity at the therapeutically relevant hCa_V 3.2 and hNa_V 1.7 voltage-gated channels.

The impact of C-tactile low-threshold mechanoreceptors on affective touch and social interactions in mice

Affective touch is necessary for proper neurodevelopment and sociability. However, it remains unclear how the neurons innervating the skin detect affective and social behaviors. The C low-threshold mechanoreceptors (C-LTMRs), a specific population of somatosensory neurons in mice, appear particularly well suited, physiologically and anatomically, to perceive affective and social touch. However, their contribution to sociability has not been resolved yet. Our observations revealed that C-LTMR functional deficiency induced social isolation and reduced tactile interactions in adulthood. Conversely, transient increase in C-LTMR excitability in adults, using chemogenetics, was rewarding, promoted touch-seeking behaviors, and had prosocial influences on group dynamics. This work provides the first empirical evidence that specific peripheral inputs alone can drive complex social behaviors. It demonstrates the existence of a specialized neuronal circuit, originating in the skin, wired to promote interactions with other individuals.

Calmodulin regulates Cav3 T-type channels at their gating brake

Calcium (Cav1 and Cav2) and sodium channels possess homologous CaM-binding motifs, known as IQ motifs in their C termini, which associate with calmodulin (CaM), a universal calcium sensor. Cav3 T-type channels, which serve as pacemakers of the mammalian brain and heart, lack a C-terminal IQ motif. We illustrate that T-type channels associate with CaM using co-immunoprecipitation experiments and single particle cryo-electron microscopy. We demonstrate that protostome invertebrate (LCav3) and human Cav3.1, Cav3.2, and Cav3.3 T-type channels specifically associate with CaM at helix 2 of the gating brake in the I-II linker of the channels. Isothermal titration calorimetry results revealed that the gating brake and CaM bind each other with high-nanomolar affinity. We show that the gating brake assumes a helical conformation upon binding CaM, with associated conformational changes to both CaM lobes as indicated by amide chemical shifts of the amino acids of CaM in 1H-15N HSQC NMR spectra. Intact Ca2+-binding sites on CaM and an intact gating brake sequence (first 39 amino acids of the I-II linker) were required in Cav3.2 channels to prevent the runaway gating phenotype, a hyperpolarizing shift in voltage sensitivities and faster gating kinetics. We conclude that the presence of high-nanomolar affinity binding sites for CaM at its universal gating brake and its unique form of regulation via the tuning of the voltage range of activity could influence the participation of Cav3 T-type channels in heart and brain rhythms. Our findings may have implications for arrhythmia disorders arising from mutations in the gating brake or CaM.

Activity-dependent regulation of T-type calcium channels by submembrane calcium ions

Voltage-gated Ca²⁺ channels are involved in numerous physiological functions and various mechanisms finely tune their activity, including the Ca²⁺ ion itself. This is well exemplified by the Ca²⁺-dependent inactivation of L-type Ca²⁺ channels, whose alteration contributes to the dramatic disease Timothy Syndrome. For T-type Ca²⁺ channels, a long-held view is that they are not regulated by intracellular Ca²⁺. Here we challenge this notion by using dedicated electrophysiological protocols on both native and expressed T-type Ca²⁺ channels. We demonstrate that a rise in submembrane Ca²⁺ induces a large decrease in T-type current amplitude due to a hyperpolarizing shift in the steady-state inactivation. Activation of most representative Ca²⁺-permeable ionotropic receptors similarly regulate T-type current properties. Altogether, our data clearly establish that Ca²⁺ entry exerts a feedback control on T-type channel activity, by modulating the channel availability, a mechanism that critically links cellular properties of T-type Ca²⁺ channels to their physiological roles.

DOI:http://dx.doi.org/10.7554/eLife.22331.001

Neurons, muscle cells and many other types of cells use electrical signals to exchange information and coordinate their behavior. Proteins known as calcium channels sit in the membrane that surrounds the cell and can generate electrical signals by allowing calcium ions to cross the membrane and enter the cell during electrical activities. Although calcium ions are needed to generate these electrical signals, and for many other processes in cells, if the levels of calcium ions inside cells become too high they can be harmful and cause disease.

Cells have a “feedback” mechanism that prevents calcium ion levels from becoming too high. This mechanism relies on the calcium ions that are already in the cell being able to close the calcium channels. This feedback mechanism has been extensively studied in two types of calcium channel, but it is not known whether a third group of channels – known as Cav3 channels – are also regulated in this way.

Cav3 channels are important in electrical signaling in neurons and have been linked with epilepsy, chronic pain and various other conditions in humans. Cazade et al. investigated whether calcium ions can regulate the activity of human Cav3 channels. The experiments show that these channels are indeed regulated by calcium ions, but using a distinct mechanism to other types of calcium channels. For the Cav3 channels, calcium ions alter the gating properties of the channels so that they are less easily activated . As a result, fewer Cav3 channels are “available” to provide calcium ions with a route into the cell.

The next steps following on from this work will be to identify the molecular mechanisms underlying this new feedback mechanism. Another challenge will be to find out what role this calcium ion-driven feedback plays in neurological disorders that are linked with altered Cav3 channel activity.

DOI:http://dx.doi.org/10.7554/eLife.22331.002

CACNA1H Mutations Are Associated With Different Forms of Primary Aldosteronism

Primary aldosteronism (PA) is the most common form of secondary hypertension. Mutations in KCNJ5, ATP1A1, ATP2B3 and CACNA1D are found in aldosterone producing adenoma (APA) and familial hyperaldosteronism (FH). A recurrent mutation in CACNA1H (coding for Cav3.2) was identified in a familial form of early onset PA. Here we performed whole exome sequencing (WES) in patients with different types of PA to identify new susceptibility genes. Four different heterozygous germline CACNA1H variants were identified. A de novo Cav3.2 p.Met1549Ile variant was found in early onset PA and multiplex developmental disorder. Cav3.2 p.Ser196Leu and p.Pro2083Leu were found in two patients with FH, and p.Val1951Glu was identified in one patient with APA. Electrophysiological analysis of mutant Cav3.2 channels revealed significant changes in the Ca²⁺ current properties for all mutants, suggesting a gain of function phenotype. Transfections of mutant Cav3.2 in H295R-S2 cells led to increased aldosterone production and/or expression of genes coding for steroidogenic enzymes after K⁺ stimulation. Identification of CACNA1H mutations associated with early onset PA, FH, and APA suggests that CACNA1H might be a susceptibility gene predisposing to PA with different phenotypic presentations, opening new perspectives for genetic diagnosis and management of patients with PA.

Peptidomimetic Targeting of Cavβ2 Overcomes Dysregulation of the L-Type Calcium Channel Density and Recovers Cardiac Function

BACKGROUND

L-type calcium channels (LTCCs) play important roles in regulating cardiomyocyte physiology, which is governed by appropriate LTCC trafficking to and density at the cell surface. Factors influencing the expression, half-life, subcellular trafficking, and gating of LTCCs are therefore critically involved in conditions of cardiac physiology and disease.

METHODS

Yeast 2-hybrid screenings, biochemical and molecular evaluations, protein interaction assays, fluorescence microscopy, structural molecular modeling, and functional studies were used to investigate the molecular mechanisms through which the LTCC Cavβ2 chaperone regulates channel density at the plasma membrane.

RESULTS

On the basis of our previous results, we found a direct linear correlation between the total amount of the LTCC pore-forming Cavα1.2 and the Akt-dependent phosphorylation status of Cavβ2 both in a mouse model of diabetic cardiac disease and in 6 diabetic and 7 nondiabetic cardiomyopathy patients with aortic stenosis undergoing aortic valve replacement. Mechanistically, we demonstrate that a conformational change in Cavβ2 triggered by Akt phosphorylation increases LTCC density at the cardiac plasma membrane, and thus the inward calcium current, through a complex pathway involving reduction of Cavα1.2 retrograde trafficking and protein degradation through the prevention of dynamin-mediated LTCC endocytosis; promotion of Cavα1.2 anterograde trafficking by blocking Kir/Gem-dependent sequestration of Cavβ2, thus facilitating the chaperoning of Cavα1.2; and promotion of Cavα1.2 transcription by the prevention of Kir/Gem-mediated shuttling of Cavβ2 to the nucleus, where it limits the transcription of Cavα1.2 through recruitment of the heterochromatin protein 1γ epigenetic repressor to the Cacna1c promoter. On the basis of this mechanism, we developed a novel mimetic peptide that, through targeting of Cavβ2, corrects LTCC life-cycle alterations, facilitating the proper function of cardiac cells. Delivery of mimetic peptide into a mouse model of diabetic cardiac disease associated with LTCC abnormalities restored impaired calcium balance and recovered cardiac function.

CONCLUSIONS

We have uncovered novel mechanisms modulating LTCC trafficking and life cycle and provide proof of concept for the use of Cavβ2 mimetic peptide as a novel therapeutic tool for the improvement of cardiac conditions correlated with alterations in LTCC levels and function.

Blockade of T-type calcium channels prevents tonic-clonic seizures in a maximal electroshock seizure model

T-type (Cav3) calcium channels play important roles in neuronal excitability, both in normal and pathological activities of the brain. In particular, they contribute to hyper-excitability disorders such as epilepsy. Here we have characterized the anticonvulsant properties of TTA-A2, a selective T-type channel blocker, in mouse. Using the maximal electroshock seizure (MES) as a model of tonic-clonic generalized seizures, we report that mice treated with TTA-A2 (0.3 mg/kg and higher doses) were significantly protected against tonic seizures. Although no major change in Local Field Potential (LFP) pattern was observed during the MES seizure, analysis of the late post-ictal period revealed a significant increase in the delta frequency power in animals treated with TTA-A2. Similar results were obtained for Cav3.1-/- mice, which were less prone to develop tonic seizures in the MES test, but not for Cav3.2-/- mice. Analysis of extracellular signal-regulated kinase 1/2 (ERK) phosphorylation and c-Fos expression revealed a rapid and elevated neuronal activation in the hippocampus following MES clonic seizures, which was unchanged in TTA-A2 treated animals. Overall, our data indicate that TTA-A2 is a potent anticonvulsant and that the Cav3.1 isoform plays a prominent role in mediating TTA-A2 tonic seizure protection.

Abstract 182: Mimetic peptide overcomes dysregulated L-Type Calcium Channel density and recovers myocardial function

Voltage dependent L-Type calcium-channels (LTCCs) are located on the cardiomyocyte membrane and regulate cardiac contraction and rhythmicity. In human pathologies, such as heart failure (HF), decreased inward calcium current (I Ca ) is frequently observed. Here, we generated a mimetic peptide (MP) that targets LTCCs and restores impaired intracellular calcium homeostasis through a novel mechanism. Effective delivery of MP, fused with a cell penetrating peptide, was found to correct Ca2+ alterations in a mouse model of HF, in human cardiomyocytes derived from induced pluripotent stem-cells. These data provide a proof-of-concept supporting a therapeutic role for MP to treat human diseases related to LTCC abnormalities.

Category: heart failure biology

Gd3+ and calcium sensitive, sodium leak currents are features of weak membrane-glass seals in patch clamp recordings

The properties of leaky patch currents in whole cell recording of HEK-293T cells were examined as a means to separate these control currents from expressed sodium and calcium leak channel currents from snail NALCN leak channels possessing both sodium (EKEE) and calcium (EEEE) selectivity filters. Leak currents were generated by the weakening of gigaohm patch seals by artificial membrane rupture using the ZAP function on the patch clamp amplifier. Surprisingly, we found that leak currents generated from the weakened membrane/glass seal can be surprisingly stable and exhibit behavior that is consistent with a sodium leak current derived from an expressible channel. Leaky patch currents differing by 10 fold in size were similarly reduced in size when external sodium ions were replaced with the large monovalent ion NMDG⁺. Leaky patch currents increased when external Ca²⁺ (1.2 mM) was lowered to 0.1 mM and were inhibited (>40% to >90%) with 10 µM Gd³⁺, 100 µM La³⁺, 1 mM Co²⁺ or 1 mM Cd²⁺. Leaky patch currents were relatively insensitive (<30%) to 1 mM Ni²⁺ and exhibited a variable amount of block with 1 mM verapamil and were insensitive to 100 µM mibefradil or 100 µM nifedipine. We hypothesize that the rapid changes in leak current size in response to changing external cations or drugs relates to their influences on the membrane seal adherence and the electro-osmotic flow of mobile cations channeling in crevices of a particular pore size in the interface between the negatively charged patch electrode and the lipid membrane. Observed sodium leak conductance currents in weak patch seals are reproducible between the electrode glass interface with cell membranes, artificial lipid or Sylgard rubber.

Modulation of T-type calcium channels by bioactive lipids

T-type calcium channels (T-channels/CaV3) have unique biophysical properties allowing a calcium influx at resting membrane potential of most cells. T-channels are ubiquitously expressed in many tissues and contribute to low-threshold spikes and burst firing in central neurons as well as to pacemaker activities in cardiac cells. They also emerged as potential targets to treat cancer and hypertension. Regulation of these channels appears complex, and several studies have indicated that CaV3.1, CaV3.2, and CaV3.3 currents are directly inhibited by multiple endogenous lipids independently of membrane receptors or intracellular pathways. These bioactive lipids include arachidonic acid and ω3 poly-unsaturated fatty acids; the endocannabinoid anandamide and other N-acylethanolamides; the lipoamino-acids and lipo-neurotransmitters; the P450 epoxygenase metabolite 5,6-epoxyeicosatrienoic acid; as well as similar molecules with 18-22 carbons in the alkyl chain. In this review, we summarize evidence for direct effects of these signaling molecules, the molecular mechanisms underlying the current inhibition, and the involved chemical features. The impact of this modulation in physiology and pathophysiology is discussed with a special emphasis on pain aspects and vasodilation. Overall, these data clearly indicate that T-current inhibition is an important mechanism by which bioactive lipids mediate their physiological functions.

Ca(v)3.2 calcium channels: the key protagonist in the supraspinal effect of paracetamol

To exert its analgesic action, paracetamol requires complex metabolism to produce a brain-specific lipoamino acid compound, AM404, which targets central transient receptor potential vanilloid receptors (TRPV1). Lipoamino acids are also known to induce analgesia through T-type calcium-channel inhibition (Ca(v)3.2). In this study we show that the antinociceptive effect of paracetamol in mice is lost when supraspinal Ca(v)3.2 channels are inhibited. Therefore, we hypothesized a relationship between supraspinal Ca(v)3.2 and TRPV1, via AM404, which mediates the analgesic effect of paracetamol. AM404 is able to activate TRPV1 and weakly inhibits Ca(v)3.2. Interestingly, activation of TRPV1 induces a strong inhibition of Ca(v)3.2 current. Supporting this, intracerebroventricular administration of AM404 or capsaicin produces antinociception that is lost in Ca(v)3.2(-/-) mice. Our study, for the first time, (1) provides a molecular mechanism for the supraspinal antinociceptive effect of paracetamol; (2) identifies the relationship between TRPV1 and the Ca(v)3.2 channel; and (3) suggests supraspinal Ca(v)3.2 inhibition as a potential pharmacological strategy to alleviate pain.

Cross-modulation and molecular interaction at the Cav3.3 protein between the endogenous lipids and the T-type calcium channel antagonist TTA-A2

T-type calcium channels (T/Ca(v)3-channels) are implicated in various physiologic and pathophysiologic processes such as epilepsy, sleep disorders, hypertension, and cancer. T-channels are the target of endogenous signaling lipids including the endocannabinoid anandamide, the ω3-fatty acids, and the lipoamino-acids. However, the precise molecular mechanism by which these molecules inhibit T-current is unknown. In this study, we provided a detailed electrophysiologic and pharmacologic analysis indicating that the effects of the major N-acyl derivatives on the Ca(v)3.3 current share many similarities with those of TTA-A2 [(R)-2-(4-cyclopropylphenyl)-N-(1-(5-(2,2,2-trifluoroethoxy)pyridin-2-yl)ethyl)acetamide], a synthetic T-channel inhibitor. Using radioactive binding assays with the TTA-A2 derivative [(3)H]TTA-A1 [(R)-2-(4-(tert-butyl)phenyl)-N-(1-(5-methoxypyridin-2-yl)ethyl)acetamide], we demonstrated that polyunsaturated lipids, which inhibit the Ca(v)3.3 current, as NAGly (N-arachidonoyl glycine), NASer (N-arachidonoyl-l-serine), anandamide, NADA (N-arachidonoyl dopamine), NATau (N-arachidonoyl taurine), and NA-5HT (N-arachidonoyl serotonin), all displaced [(3)H]TTA-A1 binding to membranes prepared from cells expressing Ca(v)3.3, with Ki in a micromolar or submicromolar range. In contrast, lipids with a saturated alkyl chain, as N-arachidoyl glycine and N-arachidoyl ethanolamine, which did not inhibit the Ca(v)3.3 current, had no effect on [(3)H]TTA-A1 binding. Accordingly, bio-active lipids occluded TTA-A2 effect on Ca(v)3.3 current. In addition, TTA-Q4 [(S)-4-(6-chloro-4-cyclopropyl-3-(2,2-difluoroethyl)-2-oxo-1,2,3,4-tetrahydroquinazolin-4-yl)benzonitrile], a positive allosteric modulator of [(3)H]TTA-A1 binding and TTA-A2 functional inhibition, acted in a synergistic manner to increase lipid-induced inhibition of the Ca(v)3.3 current. Overall, our results demonstrate a common molecular mechanism for the synthetic T-channel inhibitors and the endogenous lipids, and indicate that TTA-A2 and TTA-Q4 could be important pharmacologic tools to dissect the involvement of T-current in the physiologic effects of endogenous lipids.

Abstract P131: An Akt-Phosphomimetic Sequence of the Cavb2 C-Terminal Region Protects L-Type Calcium Channels from Protein Degradation

Alteration in the density or function of L-Type Calcium Channels (LTCCs) has been related to cardiovascular diseases such as heart failure and diabetic cardiomyopathy. It could therefore be envisioned that increasing LTCC density may improve cardiac function in heart failure. Recently, we determined that the Ser-Thr kinase Akt plays a key role in regulating cardiac inotropism through the modulation of LTCC density and function. Specifically, we found that the LTCC pore-forming channel subunit Cava1.2 contains highly evolutionary conserved PEST sequences (signals for rapid proteolytic degradation) that are responsible for direct Cava1.2 protein degradation. Phosphorylation of the C-terminal coiled coil of the Cavb2 chaperone subunit enhances LTCC protein stability by preventing PEST-mediated Cava1.2 degradation.

The aim of this study was to further dissect this Akt-dependent fine-tuning mechanism regulating LTCC density, searching for potential Akt-phosphomimetic (APM) molecules that could enhance LTCC density.

Using yeast two-hybrid screening, we found that APM Cavb2 sequences interact with the globular domain of Cavb2 itself in a solvent-exposed region that we named Tail Interacting Domain (TID). Biochemical and functional assays as well as site-specific mutagenesis in TID identified the minimal aminoacid sequence responsible for the TID-tail interaction.

In addition, through an approach comprising western blot analyses, fluorescent-based calcium assays, calcium current (ICaL) measurements, molecular modeling and peptide arrays, we identified the minimal APMs that efficiently protects Cava1.2 from protein degradation.

Based on our in vitro results, we suggest that the identified APM sequence/peptide could be used as a “therapeutic approach”; for increasing or reestablishing impaired cardiac contractility in mouse models of cardiomyopathy in which the LTCC density is altered, thus enhancing inotropism.

The NALCN ion channel is activated by M3 muscarinic receptors in a pancreatic beta-cell line

A previously uncharacterized putative ion channel, NALCN (sodium leak channel, non-selective), has been recently shown to be responsible for the tetrodotoxin (TTX)-resistant sodium leak current implicated in the regulation of neuronal excitability. Here, we show that NALCN encodes a current that is activated by M3 muscarinic receptors (M3R) in a pancreatic β-cell line. This current is primarily permeant to sodium ions, independent of intracellular calcium stores and G proteins but dependent on Src activation, and resistant to TTX. The current is recapitulated by co-expression of NALCN and M3R in human embryonic kidney-293 cells and in Xenopus oocytes. We also show that NALCN and M3R belong to the same protein complex, involving the intracellular I–II loop of NALCN and the intracellular i3 loop of M3R. Taken together, our data show the molecular basis of a muscarinic-activated inward sodium current that is independent of G-protein activation, and provide new insights into the properties of NALCN channels.

Akt regulates L-type Ca2+ channel activity by modulating Cavalpha1 protein stability

The insulin IGF-1–PI3K–Akt signaling pathway has been suggested to improve cardiac inotropism and increase Ca²⁺ handling through the effects of the protein kinase Akt. However, the underlying molecular mechanisms remain largely unknown. In this study, we provide evidence for an unanticipated regulatory function of Akt controlling L-type Ca²⁺ channel (LTCC) protein density. The pore-forming channel subunit Ca_vα1 contains highly conserved PEST sequences (signals for rapid protein degradation), and in-frame deletion of these PEST sequences results in increased Ca_vα1 protein levels. Our findings show that Akt-dependent phosphorylation of Ca_vβ2, the LTCC chaperone for Ca_vα1, antagonizes Ca_vα1 protein degradation by preventing Ca_vα1 PEST sequence recognition, leading to increased LTCC density and the consequent modulation of Ca²⁺ channel function. This novel mechanism by which Akt modulates LTCC stability could profoundly influence cardiac myocyte Ca²⁺ entry, Ca²⁺ handling, and contractility.

Temperature-dependent modulation of CaV3 T-type calcium channels by protein kinases C and A in mammalian cells

Modulation of low voltage-activated Ca(V)3 T-type calcium channels remains poorly characterized compared with high voltage-activated Ca(V)1 and Ca(V)2 calcium channels. Notably, it is yet unresolved whether Ca(V)3 channels are modulated by protein kinases in mammalian cells. In this study, we demonstrate that protein kinase A (PKA) and PKC (but not PKG) activation induces a potent increase in Ca(V)3.1, Ca(V)3.2, and Ca(V)3.3 currents in various mammalian cell lines. Notably, we show that protein kinase effects occur at physiological temperature ( approximately 30-37 degrees C) but not at room temperature ( approximately 22-27 degrees C). This temperature dependence could involve kinase translocation, which is impaired at room temperature. A similar temperature dependence was observed for PKC-mediated increase in high voltage-activated Ca(V)2.3 currents. We also report that neither Ca(V)3 surface expression nor T-current macroscopic properties are modified upon kinase activation. In addition, we provide evidence for the direct phosphorylation of Ca(V)3.2 channels by PKA in in vitro assays. Overall, our results clearly establish the role of PKA and PKC in the modulation of Ca(V)3 T-channels and further highlight the key role of the physiological temperature in the effects described.

Pharmacological characterization and molecular determinants of the activation of transient receptor potential V2 channel orthologs by 2-aminoethoxydiphenyl borate

Despite its expression in different cell types, transient receptor potential V2 (TRPV2) is still the most cryptic members of the TRPV channel family. 2-Aminoethoxydiphenyl borate (2APB) has been shown to be a common activator of TRPV1, TRPV2, and TRPV3, but 2APB-triggered TRPV2 activation remains to be thoroughly characterized. In this study, we have developed an assay based on cell lines stably expressing mouse TRPV2 channels and intracellular calcium measurements to perform a pharmacological profiling of the channel. Phenyl borate derivatives were found to activate mouse TRPV2 with similar potencies and thus were used to screen a panel of channel blockers. Besides the classic TRP inhibitors ruthenium red (RR) and 1-(beta-[3-(4-methoxyphenyl) propoxy]-4-methoxyphenethyl)-1H-imidazole hydrochloride (SKF96365), two potassium channel blockers, tetraethylammonium (TEA) and 4-aminopyridine, and an inhibitor of capacitative calcium entry, 1-(2-(trifluoromethyl) phenyl) imidazole (TRIM), were found to inhibit TRPV2 activation by 100 microM 2APB. Activation by 300 microM 2APB, however, could only be inhibited by RR and TRIM. Electrophysiological recordings demonstrated that TEA inhibition was use-dependent, suggesting that high concentrations of 2APB might induce a progressive conformational change of the channel. Comparison of TRPV2 orthologs revealed that the human channel was insensitive to 2APB. Analysis of chimeric constructs of mouse and human TRPV2 channels showed that the molecular determinants of 2APB sensitivity could be localized to the intracellular amino and carboxyl domains. Finally, using lentiviral-driven expression, we demonstrate that hTRPV2 exerts a dominant-negative effect on 2APB activation of native rodent TRPV2 channels and thus may provide an interesting tool to investigate cellular functions of TRPV2 channels.

Towards the discovery of novel T-type calcium channel blockers

Despite their presence in many tissues and their potential implication in various disease states, low-voltage activated T-type calcium channels (T-channels) have only recently become targets of interest. Unfortunately, the lack of selective T-channel blockers has hampered further characterisation of these channels. The recent availability of cloned T-channels, the Ca(V)3 proteins, facilitates identification of novel T-channel blockers. Also, studies performed in knockout animals have fostered novel interest. Selective inhibition of T-channels may have clinical importance in cardiovascular diseases, some forms of epilepsy, sleep disorders, pain and possibly cancer. This review focuses on novel research approaches to discover potent and selective T-channel modulators. These molecules may be potential drugs for treating human diseases, as well as important tools to decipher the physiological role of these channels.

Up- and down-regulation of the mechano-gated K(2P) channel TREK-1 by PIP (2) and other membrane phospholipids

TREK-1 is an unconventional K(+) channel that is activated by both physical and chemical stimuli. In this study, we show that the inner leaflet membrane phospholipids, including PIP(2), exert a mixed stimulatory and inhibitory effect on TREK-1. Intra-cellular phospholipids inhibit basal channel activity and activation by membrane stretch, intra-cellular acidosis and arachidonic acid. However, binding of endogenous negative inner leaflet phospholipids with poly-lysine reduces inhibition and reveals channel stimulation by exogenous intra-cellular phospholipids. A similar effect is observed with PI, PE, PS and PA, unlike DG, demonstrating that the phosphate at position 3 is required although the global charge of the molecule is not critical. Inhibition depends on the distal C-terminal domain that conditions channel mechano-sensitivity, but is independent of the positively charged PIP(2) stimulatory site in the proximal C-terminal domain. This is, to our knowledge, the first report of an ion channel dually regulated by membrane phospholipids.

Chemical determinants involved in anandamide-induced inhibition of T-type calcium channels

Anandamide, originally described as an endocannabinoid, is the main representative molecule of a new class of signaling lipids including endocannabinoids and N-acyl-related molecules, eicosanoids, and fatty acids. Bioactive lipids regulate neuronal excitability by acting on G-protein-coupled receptors (such as CB1) but also directly modulate various ionic conductances including voltage-activated T-type calcium channels (T-channels). However, little is known about the properties and the specificity of this new class of molecules on their various targets. In this study, we have investigated the chemical determinants involved in anandamide-induced inhibition of the three cloned T-channels: Ca(V)3.1, Ca(V)3.2, and Ca(V)3.3. We show that both the hydroxyl group and the alkyl chain of anandamide are key determinants of its effects on T-currents. As follows, T-currents are also inhibited by fatty acids. Inhibition of the three Ca(V)3 currents by anandamide and arachidonic acid does not involve enzymatic metabolism and occurs in cell-free inside-out patches. Inhibition of T-currents by fatty acids and N-acyl ethanolamides depends on the degree of unsaturation but not on the alkyl chain length and consequently is not restricted to eicosanoids. Inhibition increases for polyunsaturated fatty acids comprising 18-22 carbons when cis-double bonds are close to the carboxyl group. Therefore the major natural (food-supplied) and mammalian endogenous fatty acids including gamma-linolenic acid, mead acid, and arachidonic acid as well as the fully polyunsaturated omega3-fatty acids that are enriched in fish oil eicosapentaenoic and docosahexaenoic acids are potent inhibitors of T-currents, which possibly contribute to their physiological functions.

Subunit-specific modulation of T-type calcium channels by zinc

Zinc (Zn2+) functions as a signalling molecule in the nervous system and modulates many ionic channels. In this study, we have explored the effects of Zn2+ on recombinant T-type calcium channels (CaV3.1, CaV3.2 and CaV3.3). Using tsA-201 cells, we demonstrate that CaV3.2 current (IC50, 0.8 microm) is significantly more sensitive to Zn2+ than are CaV3.1 and CaV3.3 currents (IC50, 80 microm and approximately 160 microm, respectively). This inhibition of CaV3 currents is associated with a shift to more negative membrane potentials of both steady-state inactivation for CaV3.1, CaV3.2 and CaV3.3 and steady-state activation for CaV3.1 and CaV3.3 currents. We also document changes in kinetics, especially a significant slowing of the inactivation kinetics for CaV3.1 and CaV3.3, but not for CaV3.2 currents. Notably, deactivation kinetics are significantly slowed for CaV3.3 current (approximately 100-fold), but not for CaV3.1 and CaV3.2 currents. Consequently, application of Zn2+ results in a significant increase in CaV3.3 current in action potential clamp experiments, while CaV3.1 and CaV3.2 currents are significantly reduced. In neuroblastoma NG 108-15 cells, the duration of CaV3.3-mediated action potentials is increased upon Zn2+ application, indicating further that Zn2+ behaves as a CaV3.3 channel opener. These results demonstrate that Zn2+ exhibits differential modulatory effects on T-type calcium channels, which may partly explain the complex features of Zn2+ modulation of the neuronal excitability in normal and disease states.

Molecular pathways underlying the modulation of T-type calcium channels by neurotransmitters and hormones

Low-voltage-activated T-type calcium channels are expressed in various tissues, especially in the brain, where they promote neuronal firing and are involved in slow wave sleep and absence epilepsy. While the transduction pathways by which hormones and neurotransmitters modulate high-voltage-activated calcium channels are beginning to be unraveled, those implicated in T-type calcium channel regulation remain obscure. Several neurotransmitters and hormones regulate native T-type calcium channels, although some contradictory data have been reported depending on the cell type studied. This review focuses on the short-term (minutes range) modulation of T-type calcium channels by neurotransmitters and hormones and on the roles of G proteins and protein kinases in these modulatory effects. Results obtained in different native tissues are discussed and compared with the more recent studies of the three cloned T-type calcium channels CaV3.1, CaV3.2 and CaV3.3 in expression systems.

T-type calcium channels in differentiation and proliferation

Low-voltage activated, T-type calcium channels (T-channels) are expressed in many developing tissues and may be important in regulating important cellular phenotype transitions leading to cell proliferation, differentiation, growth and death. The purpose of this review is to relate and delineate the current data on the involvement of T-channels in differentiation and proliferation. Owing to the recent cloning of the CaV3.1, CaV3.2 and CaV3.3 subunits coding for T-channels, classical electrophysiological and pharmacological approaches are now being supported by molecular investigations. As T-channels are expressed in early development as well as re-expressed in several disease-states, our goal is to provide a comprehensive scheme of the current hypothesis connecting the activity of T-channels to cell differentiation and proliferation, as well as the potential physiological and pathophysiological implications.

TREK-1, a K+ channel involved in polymodal pain perception

The TREK-1 channel is a temperature-sensitive, osmosensitive and mechano-gated K+ channel with a regulation by Gs and Gq coupled receptors. This paper demonstrates that TREK-1 qualifies as one of the molecular sensors involved in pain perception. TREK-1 is highly expressed in small sensory neurons, is present in both peptidergic and nonpeptidergic neurons and is extensively colocalized with TRPV1, the capsaicin-activated nonselective ion channel. Mice with a disrupted TREK-1 gene are more sensitive to painful heat sensations near the threshold between anoxious warmth and painful heat. This phenotype is associated with the primary sensory neuron, as polymodal C-fibers were found to be more sensitive to heat in single fiber experiments. Knockout animals are more sensitive to low threshold mechanical stimuli and display an increased thermal and mechanical hyperalgesia in conditions of inflammation. They display a largely decreased pain response induced by osmotic changes particularly in prostaglandin E2-sensitized animals. TREK-1 appears as an important ion channel for polymodal pain perception and as an attractive target for the development of new analgesics.

Desensitization of mechano-gated K2P channels

The neuronal mechano-gated K2P channels TREK-1 and TRAAK show pronounced desensitization within 100 ms of membrane stretch. Desensitization persists in the presence of cytoskeleton disrupting agents, upon patch excision, and when channels are expressed in membrane blebs. Mechanosensitive currents evoked with a variety of complex stimulus protocols were globally fit to a four-state cyclic kinetic model in detailed balance, without the need to introduce adaptation of the stimulus. However, we show that patch stress can be a complex function of time and stimulation history. The kinetic model couples desensitization to activation, so that gentle conditioning stimuli do not cause desensitization. Prestressing the channels with pressure, amphipaths, intracellular acidosis, or the E306A mutation reduces the peak-to-steady-state ratio by changing the preexponential terms of the rate constants, increasing the steady-state current amplitude. The mechanical responsivity can be accounted for by a change of in-plane area of approximately 2 nm2 between the closed and open conformations. Desensitization and its regulation by chemical messengers is predicted to condition the physiological role of K2P channels.

PI3-kinase promotes TRPV2 activity independently of channel translocation to the plasma membrane

Cellular or chemical activators for most transient receptor potential channels of the vanilloid subfamily (TRPV) have been identified in recent years. A remarkable exception to this is TRPV2, for which cellular events leading to channel activation are still a matter of debate. Diverse stimuli such as extreme heat or phosphatidylinositol-3 kinase (PI3-kinase) regulated membrane insertion have been shown to promote TRPV2 channel activity. However, some of these results have proved difficult to reproduce and may underlie different gating mechanisms depending on the cell type in which TRPV2 channels are expressed. Here, we show that expression of recombinant TRPV2 can induce cytotoxicity that is directly related to channel activity since it can be prevented by introducing a charge substitution in the pore-forming domain of the channel, or by reducing extracellular calcium. In stably transfected cells, TRPV2 expression results in an outwardly rectifying current that can be recorded at all potentials, and in an increase of resting intracellular calcium concentration that can be partly prevented by serum starvation. Using cytotoxicity as a read-out of channel activity and direct measurements of cell surface expression of TRPV2, we show that inhibition of the PI3-kinase decreases TRPV2 channel activity but does not affect the trafficking of the channel to the plasma membrane. It is concluded that PI3-kinase induces or modulates the activity of recombinant TRPV2 channels; in contrast to the previously proposed mechanism, activation of TRPV2 channels by PI3-kinase is not due to channel translocation to the plasma membrane.

T-Type Calcium Channels Are Inhibited by Fluoxetine and Its Metabolite Norfluoxetine

Fluoxetine, a widely used antidepressant that primarily acts as a selective serotonin reuptake inhibitor, also inhibits various neuronal ion channels. Using the whole-cell patch-clamp technique, we have examined the effects of fluoxetine and norfluoxetine, its major active metabolite, on cloned low-voltage-activated T-type calcium channels (T channels) expressed in tsA 201 cells. Fluoxetine inhibited the three T channels Ca(V)3.1, Ca(V)3.2, and Ca(V)3.3 in a concentration-dependent manner (IC(50) = 14, 16, and 30 microM, respectively). Norfluoxetine was a more potent inhibitor than fluoxetine, especially on the Ca(V)3.3 T current (IC(50) = 5 microM). The fluoxetine block of T channels was voltage-dependent because it was significantly enhanced for T channels in the inactivated state. Fluoxetine caused a hyperpolarizing shift in steady-state inactivation, with a slower rate of recovery from the inactivated state. These results indicated a tighter binding of fluoxetine to the inactivated state than to the resting state of T channels, suggesting a more potent inhibition of T channels at physiological resting membrane potential. Indeed, fluoxetine and norfluoxetine at 1 microM strongly inhibited cloned T currents (approximately 50 and approximately 75%, respectively) in action potential clamp experiments performed with firing activities of thalamocortical relay neurons. Altogether, these data demonstrate that clinically relevant concentrations of fluoxetine exert a voltage-dependent block of T channels that may contribute to this antidepressant's pharmacological effects.

Cross-talk between the mechano-gated K2P channel TREK-1 and the actin cytoskeleton

TREK-1 (KCNK2) is a K(2P) channel that is highly expressed in fetal neurons. This K(+) channel is opened by a variety of stimuli, including membrane stretch and cellular lipids. Here, we show that the expression of TREK-1 markedly alters the cytoskeletal network and induces the formation of actin- and ezrin-rich membrane protrusions. The genetic inactivation of TREK-1 significantly alters the growth cone morphology of cultured embryonic striatal neurons. Cytoskeleton remodelling is crucially dependent on the protein kinase A phosphorylation site S333 and the interactive proton sensor E306, but is independent of channel permeation. Conversely, the actin cytoskeleton tonically represses TREK-1 mechano-sensitivity. Thus, the dialogue between TREK-1 and the actin cytoskeleton might influence both synaptogenesis and neuronal electrogenesis.

Lysophosphatidic Acid-operated K+ Channels

Lysophosphatidic acid (LPA) is an abundant cellular lipid with a myriad of biological effects. It plays an important role in both inter- and intracellular signaling. Activation of the LPA1-3 G-protein-coupled receptors explains many of the extracellular effects of LPA, including cell growth, differentiation, survival, and motility. However, LPA also acts intracellularly, activating the nuclear hormone receptor peroxisome proliferator-activated receptor-gamma that regulates gene transcription. This study shows that the novel subfamily of mechano-gated K2P channels comprising TREK-1, TREK-2, and TRAAK is strongly activated by intracellular LPA. The LPA-activated 2P domain K+ channels are intracellular ligand-gated K+ channels such as the Ca2+- or the ATP-sensitive K+ channels. LPA reversibly converts these mechano-gated, pH- and voltage-sensitive channels into leak conductances. Gating conversion of the 2P domain K+ channels by intracellular LPA represents a novel form of ion channel regulation. Thus, the TREK and TRAAK channels should be included in the LPA-associated physiological and disease states.

A phospholipid sensor controls mechanogating of the K+ channel TREK-1

TREK-1 (KCNK2 or K(2P)2.1) is a mechanosensitive K(2P) channel that is opened by membrane stretch as well as cell swelling. Here, we demonstrate that membrane phospholipids, including PIP(2), control channel gating and transform TREK-1 into a leak K(+) conductance. A carboxy-terminal positively charged cluster is the phospholipid-sensing domain that interacts with the plasma membrane. This region also encompasses the proton sensor E306 that is required for activation of TREK-1 by cytosolic acidosis. Protonation of E306 drastically tightens channel-phospholipid interaction and leads to TREK-1 opening at atmospheric pressure. The TREK-1-phospholipid interaction is critical for channel mechano-, pH(i)- and voltage-dependent gating.

Ca(v)3.2 calcium channels control an autocrine mechanism that promotes neuroblastoma cell differentiation

Calcium influx via low-voltage activated alpha(1H) (Ca(v)3.2) T-currents participates in the morphological and electrical differentiation of neuroblastoma NG108-15 cells. We investigated whether an autocrine mechanism could contribute to this differentiation process. The presence of factors secreted by NG108-15 cells was identified through the use of conditioned media (CM) obtained from differentiated cells. These CM significantly increased neuritogenesis with no change in the HVA calcium channel expression. CM-induced neuritogenesis persists during alpha(1H) current block, whereas CM obtained from cells transfected with an alpha(1H) antisense did not induce neuritogenesis. These data indicate that morphological differentiation of NG108-15 cells depends on an autocrine mechanism, which is controlled by alpha(1H) currents. Such a mechanism is likely to play a role in the various differentiation processes that imply alpha(1H) T-type Ca(2+) channels.

Mechanisms underlying excitatory effects of group I metabotropic glutamate receptors via inhibition of 2P domain K+ channels

Group I metabotropic glutamate receptors (mGluRs) are implicated in diverse processes such as learning, memory, epilepsy, pain and neuronal death. By inhibiting background K(+) channels, group I mGluRs mediate slow and long-lasting excitation. The main neuronal representatives of this K(+) channel family (K(2P) or KCNK) are TASK and TREK. Here, we show that in cerebellar granule cells and in heterologous expression systems, activation of group I mGluRs inhibits TASK and TREK channels. D-myo-inositol-1,4,5-triphosphate and phosphatidyl-4,5-inositol-biphosphate depletion are involved in TASK channel inhibition, whereas diacylglycerols and phosphatidic acids directly inhibit TREK channels. Mechanisms described here with group I mGluRs will also probably stand for many other receptors of hormones and neurotransmitters.

[Functional specificity of T-type calcium channels and their roles in neuronal differentiation]

Calcium plays a central role in cell signaling and T-type calcium channels constitute a unique route for the entry of calcium ions in excitable cells. The genuine electrophysiological properties of T-type calcium channels include activation at low voltages and window currents in the range of cell membrane resting potential. T-type channels therefore generate a specific calcium influx likely to play an important role during early stages of development, in various cellular functions including cell proliferation, cell differentiation, gene transcription and hormone secretion. Such T-channel activities are also associated with several pathological situations. With the recent cloning of three T-type pore channel subunits, alpha 1G, alpha 1H and alpha 1I (also called Cav3.1, Cav3.2 and Cav3.3, respectively), it has become possible to investigate further the role of T-type channels in various cellular functions, including neuronal differentiation. Here we describe recent data obtained in our laboratory demonstrating how T-type channels generated by the alpha 1H subunit contribute to neuronal differentiation.

Specific contribution of human T-type calcium channel isotypes (alpha(1G), alpha(1H) and alpha(1I)) to neuronal excitability

In several types of neurons, firing is an intrinsic property produced by specific classes of ion channels. Low-voltage-activated T-type calcium channels (T-channels), which activate with small membrane depolarizations, can generate burst firing and pacemaker activity. Here we have investigated the specific contribution to neuronal excitability of cloned human T-channel subunits. Using HEK-293 cells transiently transfected with the human alpha(1G) (Ca(V)3.1), alpha(1H) (Ca(V)3.2) and alpha(1I) (Ca(V)3.3) subunits, we describe significant differences among these isotypes in their biophysical properties, which are highlighted in action potential clamp studies. Firing activities occurring in cerebellar Purkinje neurons and in thalamocortical relay neurons used as voltage clamp waveforms revealed that alpha(1G) channels and, to a lesser extent, alpha(1H) channels produced large and transient currents, while currents related to alpha(1I) channels exhibited facilitation and produced a sustained calcium entry associated with the depolarizing after-potential interval. Using simulations of reticular and relay thalamic neuron activities, we show that alpha(1I) currents contributed to sustained electrical activities, while alpha(1G) and alpha(1H) currents generated short burst firing. Modelling experiments with the NEURON model further revealed that the alpha(1G) channel and alpha(1I) channel parameters best accounted for T-channel activities described in thalamocortical relay neurons and in reticular neurons, respectively. Altogether, the data provide evidence for a role of alpha(1I) channel in pacemaker activity and further demonstrate that each T-channel pore-forming subunit displays specific gating properties that account for its unique contribution to neuronal firing.

Direct inhibition of T-type calcium channels by the endogenous cannabinoid anandamide

Low-voltage-activated or T-type Ca(2+) channels (T-channels) are widely expressed, especially in the central nervous system where they contribute to pacemaker activities and are involved in the pathogenesis of epilepsy. Proper elucidation of their cellular functions has been hampered by the lack of selective pharmacology as well as the absence of generic endogenous regulations. We report here that both cloned (alpha(1G), alpha(1H) and alpha(1I) subunits) and native T-channels are blocked by the endogenous cannabinoid, anandamide. Anandamide, known to exert its physiological effects through cannabinoid receptors, inhibits T-currents independently from the activation of CB1/CB2 receptors, G-proteins, phospholipases and protein kinase pathways. Anandamide appears to be the first endogenous ligand acting directly on T-channels at submicromolar concentrations. Block of anandamide membrane transport by AM404 prevents T-current inhibition, suggesting that anandamide acts intracellularly. Anandamide preferentially binds and stabilizes T-channels in the inactivated state and is responsible for a significant decrease of T-currents associated with neuronal firing activities. Our data demonstrate that anandamide inhibition of T-channels can regulate neuronal excitability and account for CB receptor-independent effects of this signaling molecule.

The alpha1I T-type calcium channel exhibits faster gating properties when overexpressed in neuroblastoma/glioma NG 108-15 cells

The recently cloned T-type calcium channel alpha1I (Cav3.3) displays atypically slow kinetics when compared to native T-channels. Possible explanations might involve alternative splicing of the alpha1I subunit, or the use of expression systems that do not provide a suitable environment (auxiliary subunit, phosphorylation, glycosylation...). In this study, two human alpha1I splice variants, the alpha1I-a and alpha1I-b isoforms that harbour distinct carboxy-terminal regions were studied using various expression systems. As the localization of the alpha1I subunit is primarily restricted to neuronal tissues, its functional expression was conducted in the neuroblastoma/glioma cell line NG 108-15, and the results compared to those obtained in HEK-293 cells and Xenopus oocytes. In Xenopus oocytes, both isoforms exhibited very slow current kinetics compared to those obtained in HEK-293 cells, but the alpha1I-b isoform generated faster currents than the alpha1I-a isoform. Both activation and inactivation kinetics of alpha1I currents were significantly faster in NG 108-15 cells, while deactivating tail currents were two times slower, compared to those obtained in HEK-293 cells. Moreover, the alpha1-b isoform showed significantly slower deactivation kinetics both in NG 1080-15 and in HEK-293 cells. Altogether, these data emphasize the advantage of combining several expression systems to reveal subtle differences in channel properties and further indicate that the major functional differences between both human alpha1I isoforms are related to current kinetics. More importantly, these data suggest that the expression of the alpha1I subunit in neuronal cells contributes to the "normalization" of current kinetics to the more classical, fast-gated T-type Ca2+ current.

Alternatively spliced alpha(1G) (Ca(V)3.1) intracellular loops promote specific T-type Ca(2+) channel gating properties

At least three genes encode T-type calcium channel alpha(1) subunits, and identification of cDNA transcripts provided evidence that molecular diversity of these channels can be further enhanced by alternative splicing mechanisms, especially for the alpha(1G) subunit (Ca(V)3.1). Using whole-cell patch-clamp procedures, we have investigated the electrophysiological properties of five isoforms of the human alpha(1G) subunit that display a distinct III-IV linker, namely, alpha(1G-a), alpha(1G-b), and alpha(1G-bc), as well as a distinct II-III linker, namely, alpha(1G-ae), alpha(1G-be), as expressed in HEK-293 cells. We report that insertion e within the II-III linker specifically modulates inactivation, steady-state kinetics, and modestly recovery from inactivation, whereas alternative splicing within the III-IV linker affects preferentially kinetics and voltage dependence of activation, as well as deactivation and inactivation. By using voltage-clamp protocols mimicking neuronal activities, such as cerebellar train of action potentials and thalamic low-threshold spike, we describe that inactivation properties of alpha(1G-a) and alpha(1G-ae) isoforms can support channel behaviors reminiscent to those described in native neurons. Altogether, these data demonstrate that expression of distinct variants for the T-type alpha(1G) subunit can account for specific low-voltage-activated currents observed in neuronal tissues.