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

Genomic evidence for the suitability of Göttingen Minipigs with a rare seizure phenotype as a model for human epilepsy

Epilepsy is a complex genetic disorder that affects about 2% of the global population. Although the frequency and severity of epileptic seizures can be reduced by a range of pharmacological interventions, there are no disease-modifying treatments for epilepsy. The development of new and more effective drugs is hindered by a lack of suitable animal models. Available rodent models may not recapitulate all key aspects of the disease. Spontaneous epileptic convulsions were observed in few Göttingen Minipigs (GMPs), which may provide a valuable alternative animal model for the characterisation of epilepsy-type diseases and for testing new treatments. We have characterised affected GMPs at the genome level and have taken advantage of primary fibroblast cultures to validate the functional impact of fixed genetic variants on the transcriptome level. We found numerous genes connected to calcium metabolism that have not been associated with epilepsy before, such as ADORA2B, CAMK1D, ITPKB, MCOLN2, MYLK, NFATC3, PDGFD, and PHKB. Our results have identified two transcription factor genes, EGR3 and HOXB6, as potential key regulators of CACNA1H, which was previously linked to epilepsy-type disorders in humans. Our findings provide the first set of conclusive results to support the use of affected subsets of GMPs as an alternative and more reliable model system to study human epilepsy. Further neurological and pharmacological validation of the suitability of GMPs as an epilepsy model is therefore warranted.

Pathophysiology of ion channels in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) stands as the most prevalent and severe form of motor neuron disease, affecting an estimated 2 in 100,000 individuals worldwide. It is characterized by the progressive loss of cortical, brainstem, and spinal motor neurons, ultimately resulting in muscle weakness and death. Although the etiology of ALS remains poorly understood in most cases, the remodelling of ion channels and alteration in neuronal excitability represent a hallmark of the disease, manifesting not only during the symptomatic period but also in the early pre-symptomatic stages. In this review, we delve into these alterations observed in ALS patients and preclinical disease models, and explore their consequences on neuronal activities. Furthermore, we discuss the potential of ion channels as therapeutic targets in the context of ALS.

T-type Ca2+ and persistent Na+ currents synergistically elevate ventral, not dorsal, entorhinal cortical stellate cell excitability

Dorsal and ventral medial entorhinal cortex (mEC) regions have distinct neural network firing patterns to differentially support functions such as spatial memory. Accordingly, mEC layer II dorsal stellate neurons are less excitable than ventral neurons. This is partly because the densities of inhibitory conductances are higher in dorsal than ventral neurons. Here, we report that T-type Ca2+ currents increase 3-fold along the dorsal-ventral axis in mEC layer II stellate neurons, with twice as much CaV3.2 mRNA in ventral mEC compared with dorsal mEC. Long depolarizing stimuli trigger T-type Ca2+ currents, which interact with persistent Na+ currents to elevate the membrane voltage and spike firing in ventral, not dorsal, neurons. T-type Ca2+ currents themselves prolong excitatory postsynaptic potentials (EPSPs) to enhance their summation and spike coupling in ventral neurons only. These findings indicate that T-type Ca2+ currents critically influence the dorsal-ventral mEC stellate neuron excitability gradient and, thereby, mEC dorsal-ventral circuit activity.

Whole Exome Sequencing of Hemiplegic Migraine Patients Shows an Increased Burden of Missense Variants in CACNA1H and CACNA1I Genes

Hemiplegic migraine (HM) is a rare subtype of migraine with aura. Given that causal missense mutations in the voltage-gated calcium channel α1A subunit gene CACNA1A have been identified in a subset of HM patients, we investigated whether HM patients without a mutation have an increased burden of such variants in the "CACNA1x gene family". Whole exome sequencing data of an Australian cohort of unrelated HM patients (n = 184), along with public data from gnomAD, as controls, was used to assess the burden of missense variants in CACNA1x genes. We performed both a variant and a subject burden test. We found a significant burden for the number of variants in CACNA1E (p = 1.3 × 10^-4), CACNA1H (p < 2.2 × 10^-16) and CACNA1I (p < 2.2 × 10^-16). There was also a significant burden of subjects with missense variants in CACNA1E (p = 6.2 × 10^-3), CACNA1H (p < 2.2 × 10^-16) and CACNA1I (p < 2.2 × 10^-16). Both the number of variants and number of subjects were replicated for CACNA1H (p = 3.5 × 10^-8; p = 0.012) and CACNA1I (p = 0.019, p = 0.044), respectively, in a Dutch clinical HM cohort (n = 32), albeit that CACNA1I did not remain significant after multiple testing correction. Our data suggest that HM, in the absence of a single causal mutation, is a complex trait, in which an increased burden of missense variants in CACNA1H and CACNA1I may contribute to the risk of disease.

Constitutive activity of the dopamine (D5 ) receptor, highly expressed in CA1 hippocampal neurons, selectively reduces CaV 3.2 and CaV 3.3 currents

BACKGROUND AND PURPOSE

Ca_V 3.1-3 currents differentially contribute to neuronal firing patterns. Ca_V 3 are regulated by G protein-coupled receptors (GPCRs) activity, but information about Ca_V 3 as targets of the constitutive activity of GPCRs is scarce. We investigate the impact of D₅ recpetor constitutive activity, a GPCR with high levels of basal activity, on Ca_V 3 functionality. D₅ recpetor and Ca_V 3 are expressed in the hippocampus and have been independently linked to pathophysiological states associated with epilepsy.

EXPERIMENTAL APPROACH

Our study models were HEK293T cells heterologously expressing D₁ or D₅ receptor and Ca_V 3.1-3, and mouse brain slices containing the hippocampus. We used chlorpromazine (D₁ /D₅ inverse agonist) and a D₅ receptor mutant lacking constitutive activity as experimental tools. We measured Ca_V 3 currents and excitability parameters using the patch-clamp technique. We completed our study with computational modelling and imaging technique.

KEY RESULTS

We found a higher sensitivity to TTA-P2 (Ca_V 3 blocker) in CA1 pyramidal neurons obtained from chlorpromazine-treated animals compared with vehicle-treated animals. We found that Ca_V 3.2 and Ca_V 3.3-but not Ca_V 3.1-are targets of D₅ receptor constitutive activity in HEK293T cells. Finally, we found an increased firing rate in CA1 pyramidal neurons from chlorpromazine-treated animals in comparison with vehicle-treated animals. Similar changes in firing rate were observed on a neuronal model with controlled Ca_V 3 currents levels.

CONCLUSIONS AND IMPLICATIONS

Native hippocampal Ca_V 3 and recombinant Ca_V 3.2-3 are sensitive to D₅ receptor constitutive activity. Manipulation of D₅ receptor constitutive activity could be a valuable strategy to control neuronal excitability, especially in exacerbated conditions such as epilepsy.

Intercommunication between Voltage-Gated Calcium Channels and Estrogen Receptor/Estrogen Signaling: Insights into Physiological and Pathological Conditions

Voltage-gated calcium channels (VGCCs) and estrogen receptors are important cellular proteins that have been shown to interact with each other across varied cells and tissues. Estrogen hormone, the ligand for estrogen receptors, can also exert its effects independent of estrogen receptors that collectively constitute non-genomic mechanisms. Here, we provide insights into the VGCC regulation by estrogen and the possible mechanisms involved therein across several cell types. Notably, most of the interaction is described in neuronal and cardiovascular tissues given the importance of VGCCs in these electrically excitable tissues. We describe the modulation of various VGCCs by estrogen known so far in physiological conditions and pathological conditions. We observed that in most in vitro studies higher concentrations of estrogen were used while a handful of in vivo studies used meager concentrations resulting in inhibition or upregulation of VGCCs, respectively. There is a need for more relevant physiological assays to study the regulation of VGCCs by estrogen. Additionally, other interacting receptors and partners need to be identified that may be involved in exerting estrogen receptor-independent effects of estrogen.

Stereospecific Effects of Benzimidazolonepiperidine Compounds on T-Type Ca2+ Channels and Pain

T-type calcium channels activate in response to subthreshold membrane depolarizations and represent an important source of Ca²⁺ influx near the resting membrane potential. These channels regulate neuronal excitability and have been linked to pain. For this reason, T-type calcium channels are suitable molecular targets for the development of new non-opioid analgesics. Our previous work identified an analogue of benzimidazolonepiperidine, 5bk, that preferentially inhibited Ca_V3.2 channels and reversed mechanical allodynia. In this study, we synthesized and screened a small library of 47 compounds derived from 5bk. We found several compounds that inhibited the Ca²⁺ influx in DRG neurons of all sizes. After separating the enantiomers of each active compound, we found two compounds, 3-25-R and 3-14-3-S, that potently inhibited the Ca²⁺ influx. Whole-cell patch clamp recordings from small- to medium-sized DRG neurons revealed that both compounds decreased total Ca²⁺. Application of 3-14-3-S (but not 3-25-R) blocked transiently expressed Ca_V3.1-3.3 channels with a similar IC₅₀ value. 3-14-3-S decreased T-type, but not N-type, Ca²⁺ currents in DRG neurons. Furthermore, intrathecal delivery of 3-14-3-S relieved tonic, neuropathic, and inflammatory pain in preclinical models. 3-14-3-S did not exhibit any activity against G protein-coupled opioid receptors. Preliminary docking studies also suggest that 3-14-3-S can bind to the central pore domain of T-type channels. Together, our chemical characterization and functional and behavioral data identify a novel T-type calcium channel blocker with in vivo efficacy in experimental models of tonic, neuropathic, and inflammatory pain.

T-type channels in cancer cells: Driving in reverse

In the strongly polarized membranes of excitable cells, activation of T-type Ca²⁺ channels (TTCCs) by weak depolarizing stimuli allows the influx of Ca²⁺ which further amplifies membrane depolarization, thus "recruiting" higher threshold voltage-gated channels to promote action potential firing. Nonetheless, TTCCs perform other functions in the plasma membrane of both excitable and non-excitable cells, in which they regulate a number of biochemical pathways relevant for cell cycle and cell fate. Furthermore, data obtained in the last 20 years have shown the involvement of TTCCs in tumor biology, designating them as promising chemotherapeutic targets. However, their activity in the steadily-depolarized membranes of cancer cells, in which most voltage-gated channels are in the inactivated (nonconducting) state, is counter-intuitive. Here we discuss that in cancer cells weak hyperpolarizing stimuli increase the fraction of open TTCCs which, in association with Ca²⁺-dependent K⁺ channels, may critically boost membrane hyperpolarization and driving force for Ca²⁺ entry through different voltage-independent Ca²⁺ channels. Available evidence also shows that TTCCs participate in positive feedback circuits with signaling effectors, which may warrant a switch-like activation of pro-proliferative and pro-survival pathways in spite of their low availability. Unravelling TTCC modus operandi in the context of non-excitable membranes may facilitate the development of novel anticancer approaches.

Neocortical localization and thalamocortical modulation of neuronal hyperexcitability contribute to Fragile X Syndrome

Fragile X Syndrome (FXS) is a monogenetic form of intellectual disability and autism in which well-established knockout (KO) animal models point to neuronal hyperexcitability and abnormal gamma-frequency physiology as a basis for key disorder features. Translating these findings into patients may identify tractable treatment targets. Using source modeling of resting-state electroencephalography data, we report findings in FXS, including 1) increases in localized gamma activity, 2) pervasive changes of theta/alpha activity, indicative of disrupted thalamocortical modulation coupled with elevated gamma power, 3) stepwise moderation of low and high-frequency abnormalities based on female sex, and 4) relationship of this physiology to intellectual disability and neuropsychiatric symptoms. Our observations extend findings in Fmr1-/- KO mice to patients with FXS and raise a key role for disrupted thalamocortical modulation in local hyperexcitability. This systems-level mechanism has received limited preclinical attention but has implications for understanding fundamental disease mechanisms.

Central and peripheral contributions of T-type calcium channels in pain

Chronic pain is a severely debilitating condition that reflects a long-term sensitization of signal transduction in the afferent pain pathway. Among the key players in this pathway are T-type calcium channels, in particular the Cav3.2 isoform. Because of their biophysical characteristics, these channels are ideally suited towards regulating neuronal excitability. Recent evidence suggests that T-type channels contribute to excitability of neurons all along the ascending and descending pain pathways, within primary afferent neurons, spinal dorsal horn neurons, and within pain-processing neurons in the midbrain and cortex. Here we review the contribution of T-type channels to neuronal excitability and function in each of these neuronal populations and how they are dysregulated in chronic pain conditions. Finally, we discuss their molecular pharmacology and the potential role of these channels as therapeutic targets for chronic pain.

Modulation of Recombinant Human T-Type Calcium Channels by Δ9-Tetrahydrocannabinolic Acid In Vitro

Introduction: Low voltage-activated T-type calcium channels (T-type I_Ca), Ca_V3.1, Ca_V3.2, and Ca_V3.3, are opened by small depolarizations from the resting membrane potential in many cells and have been associated with neurological disorders, including absence epilepsy and pain. Δ⁹-tetrahydrocannabinol (THC) is the principal psychoactive compound in Cannabis and also directly modulates T-type I_Ca; however, there is no information about functional activity of most phytocannabinoids on T-type calcium channels, including Δ⁹-tetrahydrocannabinolic acid (THCA), the natural nonpsychoactive precursor of THC. The aim of this work was to characterize THCA effects on T-type calcium channels. Materials and Methods: We used HEK293 Flp-In-TREx cells stably expressing Ca_V3.1, 3.2, or 3.3. Whole-cell patch clamp recordings were made to investigate cannabinoid modulation of I_Ca. Results: THCA and THC inhibited the peak current amplitude Ca_V3.1 with pEC₅₀s of 6.0±0.7 and 5.6±0.4, respectively. THC (1 μM) or THC produced a significant negative shift in half activation and inactivation of Ca_V3.1, and both drugs prolonged Ca_V3.1 deactivation kinetics. THCA (10 μM) inhibited Ca_V3.2 by 53%±4%, and both THCA and THC produced a substantial negative shift in the voltage for half inactivation and modest negative shift in half activation of Ca_V3.2. THC prolonged the deactivation time of Ca_V3.2, while THCA did not. THCA inhibited the peak current of Ca_V3.3 by 43%±2% (10 μM) but did not notably affect Ca_V3.3 channel activation or inactivation; however, THC caused significant hyperpolarizing shift in Ca_V3.3 steady-state inactivation. Discussion: THCA modulated T-type I_Ca currents in vitro, with significant modulation of kinetics and voltage dependence at low μM concentrations. This study suggests that THCA may have potential for therapeutic use in pain and epilepsy through T-type calcium channel modulation without the unwanted psychoactive effects associated with THC.

Change in Cav3.2 T-Type Calcium Channel Induced by Varicella-Zoster Virus Participates in the Maintenance of Herpetic Neuralgia

Pain, as the most prevalent neurological complication of herpes zoster (HZ), may occur before or during the rash onset or even after the rash has recovered. Particularly, postherpetic neuralgia (PHN) is a refractory chronic condition, usually defined as pain persisting for 3 months or longer from the onset of HZ. Pain evoked by HZ impairs the normal physical and emotional functions of the patients, severely reducing their quality of life. However, how zoster-associated pain occurs and develops into PHN are elusive, making PHN difficult to predict. Uncovering the pathogenesis of zoster-associated pain (or HN) helps us to better understand the onset of PHN and supports developing more effective treatments. In this study, we successfully constructed a model for zoster-associated pain through varicella-zoster virus (VZV) infections of mouse footpads and pain behavior assessments. Next, we used the Kyoto Encyclopedia of Genes and Genomes (KEGG) and the Gene Ontology (GO) to analyze PHN rodent dorsal root ganglion (DRG) gene microarray data and found that calcium signal disorder might be involved in the onset of PHN. By using reverse transcription real-time fluorescent quantitative PCR (RT-qPCR) and Western blotting, we confirmed that VZV infection could significantly upregulate the expression of T-type calcium channel Cav3.2 in DRG and spinal dorsal horn (SDH). Intrathecal administration of Cav3.2 blocker (2R/S)-6-prenylnaringenin (6-PNG) relieved mechanical and thermal hyperalgesia induced by VZV. Taken together, our data indicated that VZV might participate in the occurrence and development of HN by upregulating the expression of Cav3.2 in DRG and SDH. These findings will help to reveal the underlying mechanisms on long-lasting pain and PHN formation, providing a new insight that Cav3.2 can be the promising drug target for remitting PHN.

Painful diabetic neuropathy leads to functional CaV3.2 expression and spontaneous activity in skin nociceptors of mice

Painful diabetic neuropathy occurs in approximately 20% of diabetic patients with underlying pathomechanisms not fully understood. We evaluated the contribution of the CaV3.2 isoform of T-type calcium channel to hyperglycemia-induced changes in cutaneous sensory C-fiber functions and neuropeptide release employing the streptozotocin (STZ) diabetes model in congenic mouse strains including global knockouts (KOs). Hyperglycemia established for 3-5 weeks in male C57BL/6J mice led to major reorganizations in peripheral C-fiber functions. Unbiased electrophysiological screening of mechanosensitive single-fibers in isolated hairy hindpaw skin revealed a relative loss of (polymodal) heat sensing in favor of cold sensing. In healthy CaV3.2 KO mice both heat and cold sensitivity among the C-fibers seemed underrepresented in favor of exclusive mechanosensitivity, low-threshold in particular, which deficit became significant in the diabetic KOs. Diabetes also led to a marked increase in the incidence of spontaneous discharge activity among the C-fibers of wildtype mice, which was reduced by the specific CaV3.2 blocker TTA-P2 and largely absent in the KOs. Evaluation restricted to the peptidergic class of nerve fibers - measuring KCl-stimulated CGRP release - revealed a marked reduction in the sciatic nerve by TTA-P2 in healthy but not diabetic wildtypes, the latter showing CGRP release that was as much reduced as in healthy and, to the same extent, in diabetic CaV3.2 KOs. These data suggest that diabetes abrogates all CaV3.2 functionality in the peripheral nerve axons. In striking contrast, diabetes markedly increased the KCl-stimulated CGRP release from isolated hairy skin of wildtypes but not KO mice, and TTA-P2 reversed this increase, strongly suggesting a de novo expression of CaV3.2 in peptidergic cutaneous nerve endings which may contribute to the enhanced spontaneous activity. De-glycosylation by neuraminidase showed clear desensitizing effects, both in regard to spontaneous activity and stimulated CGRP release, but included actions independent of CaV3.2. However, as diabetes-enhanced glycosylation is decisive for intra-axonal trafficking, it may account for the substantial reorganizations of the CaV3.2 distribution. The results may strengthen the validation of CaV3.2 channel as a therapeutic target of treating painful diabetic neuropathy.

CACNA1I gain-of-function mutations differentially affect channel gating and cause neurodevelopmental disorders

T-type calcium channels (Cav3.1 to Cav3.3) regulate low-threshold calcium spikes, burst firing and rhythmic oscillations of neurons and are involved in sensory processing, sleep, and hormone and neurotransmitter release. Here, we examined four heterozygous missense variants in CACNA1I, encoding the Cav3.3 channel, in patients with variable neurodevelopmental phenotypes. The p.(Ile860Met) variant, affecting a residue in the putative channel gate at the cytoplasmic end of the IIS6 segment, was identified in three family members with variable cognitive impairment. The de novo p.(Ile860Asn) variant, changing the same amino acid residue, was detected in a patient with severe developmental delay and seizures. In two additional individuals with global developmental delay, hypotonia, and epilepsy, the variants p.(Ile1306Thr) and p.(Met1425Ile), substituting residues at the cytoplasmic ends of IIIS5 and IIIS6, respectively, were found. Because structure modelling indicated that the amino acid substitutions differentially affect the mobility of the channel gate, we analysed possible effects on Cav3.3 channel function using patch-clamp analysis in HEK293T cells. The mutations resulted in slowed kinetics of current activation, inactivation, and deactivation, and in hyperpolarizing shifts of the voltage-dependence of activation and inactivation, with Cav3.3-I860N showing the strongest and Cav3.3-I860M the weakest effect. Structure modelling suggests that by introducing stabilizing hydrogen bonds the mutations slow the kinetics of the channel gate and cause the gain-of-function effect in Cav3.3 channels. The gating defects left-shifted and increased the window currents, resulting in increased calcium influx during repetitive action potentials and even at resting membrane potentials. Thus, calcium toxicity in neurons expressing the Cav3.3 variants is one likely cause of the neurodevelopmental phenotype. Computer modelling of thalamic reticular nuclei neurons indicated that the altered gating properties of the Cav3.3 disease variants lower the threshold and increase the duration and frequency of action potential firing. Expressing the Cav3.3-I860N/M mutants in mouse chromaffin cells shifted the mode of firing from low-threshold spikes and rebound burst firing with wild-type Cav3.3 to slow oscillations with Cav3.3-I860N and an intermediate firing mode with Cav3.3-I860M, respectively. Such neuronal hyper-excitability could explain seizures in the patient with the p.(Ile860Asn) mutation. Thus, our study implicates CACNA1I gain-of-function mutations in neurodevelopmental disorders, with a phenotypic spectrum ranging from borderline intellectual functioning to a severe neurodevelopmental disorder with epilepsy.

Targeting T-type/CaV3.2 channels for chronic pain

T-type calcium channels regulate neuronal excitability and are important contributors of pain processing. CaV3.2 channels are the major isoform expressed in nonpeptidergic and peptidergic nociceptive neurons and are emerging as promising targets for pain treatment. Numerous studies have shown that CaV3.2 expression and/or activity are significantly increased in spinal dorsal horn and in dorsal root ganglia neurons in different inflammatory and neuropathic pain models. Pharmacological campaigns to inhibit the functional expression of CaV3.2 for treatment of pain have focused on the development of direct channel blockers, but none have produced lead candidates. Targeting the proteins that regulate the trafficking or transcription, and the ones that modify the channels via post-translational modifications are alternative means to regulate expression and function of CaV3.2 channels and hence to develop new drugs to control pain. Here we synthesize data supporting a role for CaV3.2 in numerous pain modalities and then discuss emerging opportunities for the indirect targeting of CaV3.2 channels.

Modeling temperature- and Cav3 subtype-dependent alterations in T-type calcium channel mediated burst firing

T-type calcium channels are important regulators of neuronal excitability. The mammalian brain expresses three T-type channel isoforms (Cav3.1, Cav3.2 and Cav3.3) with distinct biophysical properties that are critically regulated by temperature. Here, we test the effects of how temperature affects spike output in a reduced firing neuron model expressing specific Cav3 channel isoforms. The modeling data revealed only a minimal effect on baseline spontaneous firing near rest, but a dramatic increase in rebound burst discharge frequency for Cav3.1 compared to Cav3.2 or Cav3.3 due to differences in window current or activation/recovery time constants. The reduced response by Cav3.2 could optimize its activity where it is expressed in peripheral tissues more subject to temperature variations than Cav3.1 or Cav3.3 channels expressed prominently in the brain. These tests thus reveal that aspects of neuronal firing behavior are critically dependent on both temperature and T-type calcium channel subtype.

Robust switches in thalamic network activity require a timescale separation between sodium and T-type calcium channel activations

Switches in brain states, synaptic plasticity and neuromodulation are fundamental processes in our brain that take place concomitantly across several spatial and timescales. All these processes target neuron intrinsic properties and connectivity to achieve specific physiological goals, raising the question of how they can operate without interfering with each other. Here, we highlight the central importance of a timescale separation in the activation of sodium and T-type calcium channels to sustain robust switches in brain states in thalamic neurons that are compatible with synaptic plasticity and neuromodulation. We quantify the role of this timescale separation by comparing the robustness of rhythms of six published conductance-based models at the cellular, circuit and network levels. We show that robust rhythm generation requires a T-type calcium channel activation whose kinetics are situated between sodium channel activation and T-type calcium channel inactivation in all models despite their quantitative differences.

Our brain is constantly processing information either from the environment to quickly react to incoming events or learning from experience to shape our memory. These brain states translate a collective activity of neurons interconnected via synaptic connections. Here, we focus on the thalamic network showing a transition from an active to an oscillatory mode at the population level, reverberating a switch from tonic to bursting mode at the cellular level. We are questioning how these activity fluctuations can be robustly modeled despite synaptic plasticity affecting the network configuration and the presence of neuromodulators affecting neuron intrinsic properties. To do so, we investigate six conductance-based models and their ability to reproduce activity switches at the cellular, circuit and population levels. We highlight that the robustness requires the timescale separation between the fast activation of sodium channels compared to the slow activation of T-type calcium channels. Our results show that this kinetics difference is not a computational detail but rather makes a model suitable and robust to study the interaction between switches in brain states, synaptic plasticity and neuromodulation.

Functional Microstructure of CaV-Mediated Calcium Signaling in the Axon Initial Segment

The axon initial segment (AIS) is a specialized neuronal compartment in which synaptic input is converted into action potential (AP) output. This process is supported by a diverse complement of sodium, potassium, and calcium channels (CaV). Different classes of sodium and potassium channels are scaffolded at specific sites within the AIS, conferring unique functions, but how calcium channels are functionally distributed within the AIS is unclear. Here, we use conventional two-photon laser scanning and diffraction-limited, high-speed spot two-photon imaging to resolve AP-evoked calcium dynamics in the AIS with high spatiotemporal resolution. In mouse layer 5 prefrontal pyramidal neurons, calcium influx was mediated by a mix of CaV2 and CaV3 channels that differentially localized to discrete regions. CaV3 functionally localized to produce nanodomain hotspots of calcium influx that coupled to ryanodine-sensitive stores, whereas CaV2 localized to non-hotspot regions. Thus, different pools of CaVs appear to play distinct roles in AIS function.SIGNIFICANCE STATEMENT The axon initial segment (AIS) is the site where synaptic input is transformed into action potential (AP) output. It achieves this function through a diverse complement of sodium, potassium, and calcium channels (CaV). While the localization and function of sodium channels and potassium channels at the AIS is well described, less is known about the functional distribution of CaVs. We used high-speed two-photon imaging to understand activity-dependent calcium dynamics in the AIS of mouse neocortical pyramidal neurons. Surprisingly, we found that calcium influx occurred in two distinct domains: CaV3 generates hotspot regions of calcium influx coupled to calcium stores, whereas CaV2 channels underlie diffuse calcium influx between hotspots. Therefore, different CaV classes localize to distinct AIS subdomains, possibly regulating distinct cellular processes.

Inhibitory Mechanism of the Isoflavone Derivative Genistein in the Human CaV3.3 Channel

Regulation of cellular excitability and oscillatory behavior of resting membrane potential in nerve cells are largely mediated by the low-voltage activated T-type calcium channels. This calcium channel family is constituted by three isoforms, namely, Ca_V3.1, Ca_V3.2, and Ca_V3.3, that are largely distributed in the nervous system and other parts of the body. Dysfunction of T-type calcium channels is associated with a wide range of pathophysiologies including epilepsy, neuropathic pain, cardiac problems, and major depressive disorders. Due to their pharmacological relevance, finding molecular agents able to modulate the channel's function may provide therapeutic means to ameliorate their related disorders. Here we used electrophysiological experiments to show that genistein, a canonical tyrosine kinase inhibitor, reduces the activity of the human Ca_V3.3 channel in a concentration-dependent manner. The inhibitory effect of genistein is independent of tyrosine kinase modulation and does not affect the voltage-dependent gating of the channel. Subsequently, we used computational methods to identify plausible molecular poses for the interaction of genistein and the Ca_V3.3 channel. Starting from different molecular poses, we carried out all-atom molecular dynamics (MD) simulations to identify the interacting determinants for the Ca_V3.3/genistein complex formation. Our extensive (microsecond-length) simulations suggest specific binding interactions that seem to stabilize the protein/inhibitor complex. Furthermore, our results from the unbiased MD simulations are in good agreement with the recently solved cryoelectron microscopy structure of the Ca_V3.1/Z944 complex in terms of both the location of the ligand binding site and the role of several equivalent amino acid residues. Proposed interacting complex loci were subsequently tested and corroborated by electrophysiological experiments using another naturally occurring isoflavone derivative, daidzein. Thus, by using a combination of in vitro and in silico techniques, we have identified interacting determinants relevant to the Ca_V3.3/genistein complex formation and propose that genistein directly blocks the function of the human Ca_V3.3 channel as a result of such interaction. Specifically, we proposed that a combination of polar interactions involving the three hydroxyl groups of genistein and an aromatic interaction with the fused rings are the main binding interactions in the complex formation. Our results pave the way for the rational development of improved and novel low-voltage activated T-type calcium channel inhibitors.

Expression and functions of N-type Cav2.2 and T-type Cav3.1 channels in rat vasopressin neurons under normotonic conditions

Arginine vasopressin (AVP) neurons play essential roles in sensing the change in systemic osmolarity and regulating AVP release from their neuronal terminals to maintain the plasma osmolarity. AVP exocytosis depends on the Ca2+ entry via voltage-gated Ca2+ channels (VGCCs) in AVP neurons. In this study, suppression by siRNA-mediated knockdown and pharmacological sensitivity of VGCC currents evidenced molecular and functional expression of N-type Cav2.2 and T-type Cav3.1 in AVP neurons under normotonic conditions. Also, both the Cav2.2 and Cav3.1 currents were found to be sensitive to flufenamic acid (FFA). TTX-insensitive spontaneous action potentials were suppressed by FFA and T-type VGCC blocker Ni2+. However, Cav2.2-selective ω-conotoxin GVIA failed to suppress the firing activity. Taken together, it is concluded that Cav2.2 and Cav3.1 are molecularly and functionally expressed and both are sensitive to FFA in unstimulated rat AVP neurons. Also, it is suggested that Cav3.1 is primarily involved in their action potential generation.

Modified parylene-N films as chemical microenvironments for differentiation and spheroid formation of osteoblast cells

In this work, the influence of parylene N film on the spheroid formation of osteoblast-like cells (MG-63) was determined and compared with that of high-hydrophilicity microenvironments, such as hydrophilic culture matrix and ultraviolet-treated parylene N film. To elucidate the change in cell properties due to the microenvironment of parylene N film, global gene expression profiles of MG-63 cells on parylene N film were analyzed. We confirmed the upregulated expression of osteoblast differentiation- and proliferation-related genes, such as Runx2, ALPL, and BGLAP and MKi67 and PCNA, respectively, using the real-time polymerase chain reaction. In addition, the differentiation and proliferation of osteoblast cells cultured on parylene N film were validated using immunostaining. Finally, the formation of spheroids and regulation of differentiation in human mesenchymal stem cells (MSCs) on parylene N film was demonstrated. The results of this study confirm that the microenvironment with the controlled hydrophobic property of parylene N film could effectively trigger the bone differentiation and maintains the proliferation of MSCs, similar to MG-63 cells without any scaffold structures or physical treatments.

Neuronal Cav3 channelopathies: recent progress and perspectives

T-type, low-voltage activated, calcium channels, now designated Cav3 channels, are involved in a wide variety of physiological functions, especially in nervous systems. Their unique electrophysiological properties allow them to finely regulate neuronal excitability and to contribute to sensory processing, sleep, and hormone and neurotransmitter release. In the last two decades, genetic studies, including exploration of knock-out mouse models, have greatly contributed to elucidate the role of Cav3 channels in normal physiology, their regulation, and their implication in diseases. Mutations in genes encoding Cav3 channels (CACNA1G, CACNA1H, and CACNA1I) have been linked to a variety of neurodevelopmental, neurological, and psychiatric diseases designated here as neuronal Cav3 channelopathies. In this review, we describe and discuss the clinical findings and supporting in vitro and in vivo studies of the mutant channels, with a focus on de novo, gain-of-function missense mutations recently discovered in CACNA1G and CACNA1H. Overall, the studies of the Cav3 channelopathies help deciphering the pathogenic mechanisms of corresponding diseases and better delineate the properties and physiological roles Cav3 channels.

Low voltage-activated channels in rat pulmonary vein cardiomyocytes: coexistence of a non-selective cationic channel and of T-type Ca channels

In rat pulmonary vein (PV) cardiomyocytes (CM), norepinephrine (NE) induces an automatic activity consisting of bursts of slow action potentials which depend on Ca²⁺ (upstroke) and Na⁺ (inter-burst) channels. Our objective was to characterize low voltage-activated (LVA) currents in rat PVCM susceptible to trigger this activity. Whole-cell I_Ca (5 mM Ca²⁺) was recorded from - 100 mV with classical Na⁺- and K⁺-free solutions. A fast LVA I_Ca (FLVA-I_Ca), present in ≈ 56% of PVCM between ~ - 50 to - 20 mV, was blocked by 10 μM TTX and markedly increased by addition of NaCl (1 or 3 mM) or KCl (5 or 10 mM). Permeability ratios P'_Ca/P_Na and P'_Ca/P_K calculated for bi-ionic conditions were respectively 2.25 ± 0.51 and 1.88 ± 0.25, and not different from a value of 2. FLVA-I_Ca was increased by 10 μM NE and 300 nM BayK8644, decreased by 5 μM nifedipine but not blocked by ranolazine (10 μM). NiCl₂ (40 μM) and TTA-A2 (10 or 100 nM) increased FLVA-I_Ca. Similar results were obtained in left atrial (LA) CM. Neither Ba²⁺ nor Sr²⁺ alone could permeate the FLVA channel or block Ca²⁺ influx but revealed a large slower activating and inactivating LVA Ca²⁺ current (SLVA-I_Ca), present in 10 out of 80 PVCM, absent in LACM, and partially inhibited by 100 nM TTA-A2. Therefore, the ionic channel underlying FLVA-I_Ca is likely a fast voltage-gated non-selective channel with a dihydropyridine binding site. SLVA-I_Ca might correspond to Ca²⁺ influx through Ca_v3.x channels and contribute to triggering NE-induced automatic activity in the PV myocardial sleeve.

Cdk5-Dependent Phosphorylation of CaV3.2 T-Type Channels: Possible Role in Nerve Ligation-Induced Neuropathic Allodynia and the Compound Action Potential in Primary Afferent C Fibers

Voltage-gated T-type Ca²⁺ (Ca_V3) channels regulate diverse physiological events, including neuronal excitability, and have been linked to several pathological conditions such as absence epilepsy, cardiovascular diseases, and neuropathic pain. It is also acknowledged that calcium/calmodulin-dependent protein kinase II and protein kinases A and C regulate the activity of T-type channels. Interestingly, peripheral nerve injury induces tactile allodynia and upregulates Ca_V3.2 channels and cyclin-dependent kinase 5 (Cdk5) in dorsal root ganglia (DRG) and spinal dorsal horn. Here, we report that recombinant Ca_V3.2 channels expressed in HEK293 cells are regulatory targets of Cdk5. Site-directed mutagenesis showed that the relevant sites for this regulation are residues S561 and S1987. We also found that Cdk5 may regulate Ca_V3.2 channel functional expression in rats with mechanical allodynia induced by spinal nerve ligation (SNL). Consequently, the Cdk5 inhibitor olomoucine affected the compound action potential recorded in the spinal nerves, as well as the paw withdrawal threshold. Likewise, Cdk5 expression was upregulated after SNL in the DRG. These findings unveil a novel mechanism for how phosphorylation may regulate Ca_V3.2 channels and suggest that increased channel activity by Cdk5-mediated phosphorylation after SNL contributes nerve injury-induced tactile allodynia.SIGNIFICANCE STATEMENT Neuropathic pain is a current public health challenge. It can develop as a result of injury or nerve illness. It is acknowledged that the expression of various ion channels can be altered in neuropathic pain, including T-type Ca²⁺ channels that are expressed in sensory neurons, where they play a role in the regulation of cellular excitability. The present work shows that the exacerbated expression of Cdk5 in a preclinical model of neuropathic pain increases the functional expression of Ca_V3.2 channels. This finding is relevant for the understanding of the molecular pathophysiology of the disease. Additionally, this work may have a substantial translational impact, since it describes a novel molecular pathway that could represent an interesting therapeutic alternative for neuropathic pain.

A novel phospho-modulatory mechanism contributes to the calcium-dependent regulation of T-type Ca2+ channels

Ca_v3 / T-type Ca²⁺ channels are dynamically regulated by intracellular Ca²⁺ ions, which inhibit Ca_v3 availability. Here, we demonstrate that this inhibition becomes irreversible in the presence of non-hydrolysable ATP analogs, resulting in a strong hyperpolarizing shift in the steady-state inactivation of the residual Ca_v3 current. Importantly, the effect of these ATP analogs was prevented in the presence of intracellular BAPTA. Additional findings obtained using intracellular dialysis of inorganic phosphate and alkaline phosphatase or NaN₃ treatment further support the involvement of a phosphorylation mechanism. Contrasting with Ca_v1 and Ca_v2 Ca²⁺ channels, the Ca²⁺-dependent modulation of Ca_v3 channels appears to be independent of calmodulin, calcineurin and endocytic pathways. Similar findings were obtained for the native T-type Ca²⁺ current recorded in rat thalamic neurons of the central medial nucleus. Overall, our data reveal a new Ca²⁺ sensitive phosphorylation-dependent mechanism regulating Ca_v3 channels, with potentially important physiological implications for the multiple cell functions controlled by T-type Ca²⁺ channels.

Exploring the molecular basis of neuronal excitability in a vocal learner

BACKGROUND

Vocal learning, the ability to learn to produce vocalizations through imitation, relies on specialized brain circuitry known in songbirds as the song system. While the connectivity and various physiological properties of this system have been characterized, the molecular genetic basis of neuronal excitability in song nuclei remains understudied. We have focused our efforts on examining voltage-gated ion channels to gain insight into electrophysiological and functional features of vocal nuclei. A previous investigation of potassium channel genes in zebra finches (Taeniopygia guttata) revealed evolutionary modifications unique to songbirds, as well as transcriptional specializations in the song system [Lovell PV, Carleton JB, Mello CV. BMC Genomics 14:470 2013]. Here, we expand this approach to sodium, calcium, and chloride channels along with their modulatory subunits using comparative genomics and gene expression analysis encompassing microarrays and in situ hybridization.

RESULTS

We found 23 sodium, 38 calcium, and 33 chloride channel genes (HGNC-based classification) in the zebra finch genome, several of which were previously unannotated. We determined 15 genes are missing relative to mammals, including several genes (CLCAs, BEST2) linked to olfactory transduction. The majority of sodium and calcium but few chloride channels showed differential expression in the song system, among them SCN8A and CACNA1E in the direct motor pathway, and CACNG4 and RYR2 in the anterior forebrain pathway. In several cases, we noted a seemingly coordinated pattern across multiple nuclei (SCN1B, SCN3B, SCN4B, CACNB4) or sparse expression (SCN1A, CACNG5, CACNA1B).

CONCLUSION

The gene families examined are highly conserved between avian and mammalian lineages. Several cases of differential expression likely support high-frequency and burst firing in specific song nuclei, whereas cases of sparse patterns of expression may contribute to the unique electrophysiological signatures of distinct cell populations. These observations lay the groundwork for manipulations to determine how ion channels contribute to the neuronal excitability properties of vocal learning systems.

Activation of somatostatin receptor 5 suppresses T-type Ca2+ channels through NO/cGMP/PKG signaling pathway in rat retinal ganglion cells

Somatostatin has been shown to modulate a variety of neuronal functions by activating the five specific G-protein coupled receptors (sst₁-sst₅). Here, effects of sst₅ receptor activation on T-type Ca²⁺ channels in acutely isolated retinal ganglion cells (RGCs) of rats were investigated using whole-cell patch-clamp techniques. The sst₅ receptor specific agonist L-817,818 significantly and reversibly suppressed T-type Ca²⁺ currents, and shifted inactivation curve of the channels toward hyperpolarization direction. The effect of L-817,818 was in a dose-dependent manner, with an IC₅₀ being 8.8 μM. Pertussis toxin-sensitive G_i/o protein mediated intracellular nitric oxide (NO)/cGMP/protein kinase G (PKG) signaling cascade was involved in the L-817,818 effect on Ca²⁺ currents because pharmacological interference of each of these signaling molecules abolished the L-817,818 effect. In contrast, neither phospholipase C/protein kinase C nor cAMP/protein kinase A signal pathways seemed likely to be involved because the L-817,818 effect persisted when these signaling pathways were blocked by U73122, bisindolylmaleimide IV, chelerythrine chloride, and Rp-cAMP, respectively. These results suggest that activation of sst₅ receptors suppresses T-type Ca²⁺ currents in rat RGCs through intracellular NO/cGMP/PKG signaling pathway, which may provide a potential mechanism for protecting RGCs against injury.

Regulation of T-type Ca2+ channel expression by interleukin-6 in sensory-like ND7/23 cells post-herpes simplex virus (HSV-1) infection

Herpes simplex virus-type 1 (HSV-1) infection of sensory neurons may lead to a significant reduction in the expression of voltage-activated Na⁺ and Ca²⁺ channels, which can disrupt the transmission of pain information. Viral infection also results in the secretion of various pro-inflammatory cytokines, including interleukin (IL)-6. In this work, we tested whether IL-6 regulates the expression of Na⁺ and Ca²⁺ channels post-HSV-1 infection in ND7/23 sensory-like neurons. Our results demonstrate that HSV-1 infection causes a significant decrease in the protein expression of the Cav3.2 T-type Ca²⁺ channel subunit, despite increasing Cav3.2 mRNA synthesis. Neither Cav3.2 mRNA nor total protein content was affected by IL-6 treatment post-HSV-1 infection. In ND7/23 cells, HSV-1 infection caused a significant reduction in the expression of Na⁺ and T-type Ca²⁺ channels within 48 h. Exposure of ND7/23 cells to IL-6 for 24 h post-infection reverses the effect of HSV-1, resulting in a significant increase in T-type Ca²⁺ current density. However, Na⁺ currents were not restored by 24-h treatment with IL-6 post-HSV-1 infection of ND7/23 cells. The ability of IL-6 to increase the functional expression of T-type Ca²⁺ channels on the membrane was blocked by the inhibition of protein trafficking with brefeldin-A and ERK1/2 activation. These results indicate that IL-6 release following HSV-1 infection regulates the expression of T-type Ca²⁺ channels, which may alter the transmission of pain information.

The Hard-To-Close Window of T-Type Calcium Channels

T-Type calcium channels (TTCCs) are key regulators of membrane excitability, which is the reason why TTCC pharmacology is subject to intensive research in the neurological and cardiovascular fields. TTCCs also play a role in cancer physiology, and pharmacological blockers such as tetralols and dihydroquinazolines (DHQs) reduce the viability of cancer cells in vitro and slow tumor growth in murine xenografts. However, the available compounds are better suited to blocking TTCCs in excitable membranes rather than TTCCs contributing window currents at steady potentials. Consistently, tetralols and dihydroquinazolines exhibit cytostatic/cytotoxic activities at higher concentrations than those required for TTCC blockade, which may involve off-target effects. Gene silencing experiments highlight the targetability of TTCCs, but further pharmacological research is required for TTCC blockade to become a chemotherapeutic option.

Adrenergic agonist induces rhythmic firing in quiescent cardiac preganglionic neurons in nucleus ambiguous via activation of intrinsic membrane excitability

Cholinergic vagal nerves projecting from neurons in the brain stem nucleus ambiguus (NAm) play a predominant role in cardiac parasympathetic pacemaking control. Central adrenergic signaling modulates the tone of this vagal output; however, the exact excitability mechanisms are not fully understood. We investigated responses of NAm neurons to adrenergic agonists using in vitro mouse brain stem slices. Preganglionic NAm neurons were identified by ChAT-tdTomato fluorescence in young adult transgenic mice, and their cardiac projection was confirmed by retrograde dye tracing. Juxtacellular recordings detected sparse or absent spontaneous action potentials (AP) in NAm neurons. However, bath application of epinephrine or norepinephrine strongly and reversibly activated most NAm neurons regardless of their basal firing rate. Epinephrine was more potent than norepinephrine, and this activation largely depends on α₁-adrenoceptors. Interestingly, adrenergic activation of NAm neurons does not require an ionotropic synaptic mechanism, because postsynaptic excitatory or inhibitory receptor blockade did not occlude the excitatory effect, and bath-applied adrenergic agonists did not alter excitatory or inhibitory synaptic transmission. Instead, adrenergic agonists significantly elevated intrinsic membrane excitability to facilitate generation of recurrent action potentials. T-type calcium current and hyperpolarization-activated current are involved in this excitation pattern, although not required for spontaneous AP induction by epinephrine. In contrast, pharmacological blockade of persistent sodium current significantly inhibited the adrenergic effects. Our results demonstrate that central adrenergic signaling enhances the intrinsic excitability of NAm neurons and that persistent sodium current is required for this effect. This central balancing mechanism may counteract excessive peripheral cardiac excitation during increased sympathetic tone.

NEW & NOTEWORTHY Cardiac preganglionic cholinergic neurons in the nucleus ambiguus (NAm) are responsible for slowing cardiac pacemaking. This study identified that adrenergic agonists can induce rhythmic action potentials in otherwise quiescent cholinergic NAm preganglionic neurons in brain stem slice preparation. The modulatory influence of adrenaline on central parasympathetic outflow may contribute to both physiological and deleterious cardiovascular regulation.

Cell-Type Specific Distribution of T-Type Calcium Currents in Lamina II Neurons of the Rat Spinal Cord

Spinal lamina II (substantia gelatinosa, SG) neurons integrate nociceptive information from the primary afferents and are classified according to electrophysiological (tonic firing, delayed firing, single spike, initial burst, phasic firing, gap firing and reluctant firing) or morphological (islet, central, vertical, radial and unclassified) criteria. T-type calcium (Cav3) channels play an essential role in the central mechanism of pathological pain, but the electrophysiological properties and the cell-type specific distribution of T-type channels in SG neurons have not been fully elucidated. To investigate the electrophysiological and morphological features of T-type channel-expressing or -lacking neurons, voltage- and current-clamp recordings were performed on either transverse or parasagittal spinal cord slices. Recording made in transverse spinal cord slices showed that an inward current (I _T) was observed in 44.5% of the SG neurons that was fully blocked by Ni²⁺ and TTA-A2. The amplitude of I _T depended on the magnitude and the duration of hyperpolarization pre-pulse. The voltage for eliciting and maximizing I _T were -70 mV and -35 mV, respectively. In addition, we found that most of the I _T-expressing neurons are tonic firing neurons and exhibit more negative action potential (AP) threshold and smaller difference of AP threshold and resting membrane potential (RMP) than those neurons lacking I _T. Consistently, a specific T-type calcium channel blocker TTA-P2 increased the AP threshold and enlarged the difference between AP threshold and membrane potential (I_hold = 0). Meanwhile, the morphological analysis indicated that most of the I _T-expressing neurons are islet neurons. In conclusion, we identify a cell-type specific distribution and the function of T-type channels in SG neurons. These findings might provide new insights into the mechanisms underlying the contribution of T-type channels in sensory transmission.

Effect of 17β-oestradiol on T-type calcium channels in the lateral habenula

T-type calcium channels (T-channels) are critical for regulating neuronal excitability. Oestrogen alters neuronal excitability by modulating the expression of T-channels. The lateral habenula (LHb), as a link between the limbic system and midbrain structures, expresses T-channels and ERs. However, little is known about the role of oestrogen with respect to modulating T-channels in the LHb. In the present study, we investigated the distribution of T-channels in 3 LHb subregions (rostral, middle and caudal) in normal female rats. Next, we analysed the influence of 17β-oestradiol (E₂ ) on T-channels in the LHb in ovariectomised (OVX) rats (oil and E₂ groups) using whole-cell patch clamp recording and a real-time polymerase chain reaction (PCR). In normal rats, the results obtained showed that the peak of T-type calcium current (I_T ) was -474.61 ± 48.33 pA and I_T density was -29.11 ± 1.93 pA/pF. The I_T peak and I_T density on LHb neurones gradually decreased across the rostrocaudal axis. The neuronal firing pattern varied depending on the location: burst firing was dominant (53.85%) in the rostral LHb, whereas tonic firing was dominant (79.31%) in the caudal LHb. In OVX rats, real-time PCR analysis revealed that E₂ treatment decreased Cav3.3 mRNA expression in the caudal LHb. Patch clamp recording showed that E₂ treatment decreased the peak I_T and also reduced the low-threshold spikes (LTS) number, amplitude and width of LTS in the caudal LHb. Taken together, the results obtained in the present study suggest that E₂ may inhibit T-channel activity by selectively down-regulating Cav3.3 calcium channel in the caudal LHb, leading to reduced the possibility of burst firing.

CaV 3.2 drives sustained burst-firing, which is critical for absence seizure propagation in reticular thalamic neurons

OBJECTIVE

Genetic alterations have been identified in the CACNA1H gene, encoding the CaV 3.2 T-type calcium channel in patients with absence epilepsy, yet the precise mechanisms relating to seizure propagation and spike-wave-discharge (SWD) pacemaking remain unknown. Neurons of the thalamic reticular nucleus (TRN) express high levels of CaV 3.2 calcium channels, and we investigated whether a gain-of-function mutation in the Cacna1h gene in Genetic Absence Epilepsy Rats from Strasbourg (GAERS) contributes to seizure propagation and pacemaking in the TRN.

METHODS

Pathophysiological contributions of CaV 3.2 calcium channels to burst firing and absence seizures were assessed in vitro using acute brain slice electrophysiology and quantitative real-time polymerase chain reaction (PCR) and in vivo using free-moving electrocorticography recordings.

RESULTS

TRN neurons from GAERS display sustained oscillatory burst-firing that is both age- and frequency-dependent, occurring only in the frequencies overlapping with GAERS SWDs and correlating with the expression of a CaV 3.2 mutation-sensitive splice variant. In vivo knock-down of CaV 3.2 using direct thalamic injection of lipid nanoparticles containing CaV 3.2 dicer small interfering (Dsi) RNA normalized TRN burst-firing, and in free-moving GAERS significantly shortened seizures.

SIGNIFICANCE

This supports a role for TRN CaV 3.2 T-type channels in propagating thalamocortical network seizures and setting the pacemaking frequency of SWDs.

Cytosolic ATP Relieves Voltage-Dependent Inactivation of T-Type Calcium Channels and Facilitates Excitability of Neurons in the Rat Central Medial Thalamus

The central medial nucleus (CeM) is a part of the intralaminar thalamus, which is involved in the control of arousal and sensory processing. However, ionic conductances and mechanisms that regulate the activity of the CeM are not well studied. Here, we used in vitro electrophysiology in acute brain slices from adolescent rats to demonstrate that T-type calcium currents (T-currents) are prominent in the majority of the studied CeM neurons and are critical determinants of low-threshold calcium spikes (LTSs), which in turn regulate excitability of these neurons. Using an ATP-free internal solution decreased T-current density and induced a profound hyperpolarizing shift in steady-state inactivation curves while voltage-dependent activation kinetics were spared. Furthermore, selective pharmacological blockade of T-channels or use of an ATP-free solution reduced both tonic action potential (AP) frequency and rebound burst firing in CeM neurons. Our results indicate that T-channels are critical regulators of a thalamocortical circuit output and suggest that cytosolic ATP could be an endogenous regulatory mechanism in which T-channels may functionally gate sensory transmission and arousal in vivo.

A computational model for how the fast afterhyperpolarization paradoxically increases gain in regularly firing neurons

The gain of a neuron, the number and frequency of action potentials triggered in response to a given amount of depolarizing injection, is an important behavior underlying a neuron's function. Variations in action potential waveform can influence neuronal discharges by the differential activation of voltage- and ion-gated channels long after the end of a spike. One component of the action potential waveform, the afterhyperpolarization (AHP), is generally considered an inhibitory mechanism for limiting firing rates. In dentate gyrus granule cells (DGCs) expressing fast-gated BK channels, large fast AHPs (fAHP) are paradoxically associated with increased gain. In this article, we describe a mechanism for this behavior using a computational model. Hyperpolarization provided by the fAHP enhances activation of a dendritic inward current (a T-type Ca²⁺ channel is suggested) that, in turn, boosts rebound depolarization at the soma. The model suggests that the fAHP may both reduce Ca²⁺ channel inactivation and, counterintuitively, enhance its activation. The magnitude of the rebound depolarization, in turn, determines the activation of a subsequent, slower inward current (a persistent Na⁺ current is suggested) limiting the interspike interval. Simulations also show that the effect of AHP on gain is also effective for physiologically relevant stimulation; varying AHP amplitude affects interspike interval across a range of "noisy" stimulus frequency and amplitudes. The mechanism proposed suggests that small fAHPs in DGCs may contribute to their limited excitability. NEW & NOTEWORTHY The afterhyperpolarization (AHP) is canonically viewed as a major factor underlying the refractory period, serving to limit neuronal firing rate. We recently reported that enhancing the amplitude of the fast AHP (fAHP) in a relatively slowly firing neuron (vs. fast spiking neurons) expressing fast-gated BK channels augments neuronal excitability. In this computational study, we present a novel, quantitative hypothesis for how varying the amplitude of the fAHP can, paradoxically, influence a subsequent spike tens of milliseconds later.

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.

Regulation of the CaV3.2 calcium channels in health and disease Regulácia CaV3.2 vápnikových kanálov v zdraví a chorobe

Family of T-type or low-voltage activated calcium channels consists of three members: CaV3.1, CaV3.2, and CaV3.3. CaV3.2 channel has almost identical biophysical properties as the CaV3.1 channel, but is distinguished by a specific tissue expression profile and a prominent role in several pathologies, including neuropathic pain, epilepsy, and dysregulation of cardiac rhythm. Further, it may be involved in phenotype of autism spectrum disorders, and amyotrophic lateral sclerosis. It represents a promising target for future pharmacotherapies.

Tonotopic Variation of the T-Type Ca2+ Current in Avian Auditory Coincidence Detector Neurons

Neurons in avian nucleus laminaris (NL) are binaural coincidence detectors for sound localization and are characterized by striking structural variations in dendrites and axon initial segment (AIS) according to their acoustic tuning [characteristic frequency (CF)]. T-type Ca²⁺ (CaT) channels regulate synaptic integration and firing behavior at these neuronal structures. However, whether or how CaT channels contribute to the signal processing in NL neurons is not known. In this study, we addressed this issue with whole-cell recording and two-photon Ca²⁺ imaging in brain slices of posthatch chicks of both sexes. We found that the CaT current was prominent in low-CF neurons, whereas it was almost absent in higher-CF neurons. In addition, a large Ca²⁺ transient occurred at the dendrites and the AIS of low-CF neurons, indicating a localization of CaT channels at these structures in the neurons. Because low-CF neurons have long dendrites, dendritic CaT channels may compensate for the attenuation of EPSPs at dendrites. Furthermore, the short distance of AIS from the soma may accelerate activation of axonal CaT current in the neurons and help EPSPs reach spike threshold. Indeed, the CaT current was activated by EPSPs and augmented the synaptic response and spike generation of the neurons. Notably, the CaT current was inactivated during repetitive inputs, and these augmenting effects predominated at the initial phase of synaptic activity. These results suggested that dendritic and axonal CaT channels increase the sensitivity to sound at its onset, which may expand the dynamic range for binaural computation in low-CF NL neurons.SIGNIFICANCE STATEMENT Neurons in nucleus laminaris are binaural coincidence detectors for sound localization. We report that T-type Ca²⁺ (CaT) current was prominent at dendrites and the axonal trigger zone in neurons tuned to low-frequency sound. Because these neurons have long dendrites and a closer trigger zone compared with those tuned to higher-frequency sound, the CaT current augmented EPSPs at dendrites and accelerated spike triggers in the neurons, implying a strategic arrangement of the current within the nucleus. This effect was limited to the onset of repetitive inputs due to progressive inactivation of CaT current. The results suggested that the CaT current increases the sensitivity to sound at its onset, which may expand the dynamic range for binaural computation of low-frequency sound.

GABAB receptors suppress burst-firing in reticular thalamic neurons

Burst-firing in thalamic neurons is known to play a key role in mediating thalamocortical (TC) oscillations that are associated with non-REM sleep and some types of epileptic seizure. Within the TC system the primary output of GABAergic neurons in the reticular thalamic nucleus (RTN) is thought to induce the de-inactivation of T-type calcium channels in thalamic relay (TR) neurons, promoting burst-firing drive to the cortex and the propagation of TC network activity. However, RTN neurons also project back onto other neurons within the RTN. The role of this putative negative feedback upon the RTN itself is less well understood, although is hypothesized to induce de-synchronization of RTN neuron firing leading to the suppression of TC oscillations. Here we tested two hypotheses concerning possible mechanisms underlying TC oscillation modulation. Firstly, we assessed the burst-firing behavior of RTN neurons in response to GABA_B receptor activation using acute brain slices. The selective GABA_B receptor agonist baclofen was found to induce suppression of burst-firing concurrent with effects on membrane input resistance. Secondly, RTN neurons express Ca_V3.2 and Ca_V3.3 T-type calcium channel isoforms known to contribute toward TC burst-firing and we examined the modulation of these channels by GABA_B receptor activation. Utilizing exogenously expressed T-type channels we assessed whether GABA_B receptor activation could directly alter T-type calcium channel properties. Overall, GABA_B receptor activation had only modest effects on Ca_V3.2 and Ca_V3.3 isoforms. The only effect that could be predicted to suppress burst-firing was a hyperpolarized shift in the voltage-dependence of inactivation, potentially causing lower channel availability at membrane potentials critical for burst-firing. Conversely, other effects observed such as a hyperpolarized shift in the voltage-dependence of activation of both Ca_V3.2 and Ca_V3.3 as well as increased time constant of activation of the Ca_V3.3 isoform would be expected to enhance burst-firing. Together, we hypothesize that GABA_B receptor activation mediates multiple downstream effectors that combined act to suppress burst-firing within the RTN. It appears unlikely that direct GABA_B receptor-mediated modulation of T-type calcium channels is the major mechanistic contributor to this suppression.

Modulation of T-type Ca2+ channels by Lavender and Rosemary extracts

Medicinal plants represent a significant reservoir of unexplored substances for early-stage drug discovery. Of interest, two flowering Mediterranean plants have been used for thousands of years for their beneficial effects on nervous disorders, including anxiety and mood. However, the therapeutic potential of these plants regarding their ability to target ion channels and neuronal excitability remains largely unknown. Towards this goal, we have investigated the ability of Lavender and Rosemary to modulate T-type calcium channels (TTCCs). TTCCs play important roles in neuronal excitability, neuroprotection, sensory processes and sleep. These channels are also involved in epilepsy and pain. Using the whole-cell patch-clamp technique, we have characterized how Lavender and Rosemary extracts, as well as their major active compounds Linalool and Rosmarinic acid, modulate the electrophysiological properties of recombinant TTCCs (CaV3.2) expressed in HEK-293T cells. Both the methanolic and essential oil extracts as well as the active compounds of these plants inhibit Cav3.2 current in a concentration-dependent manner. In addition, these products also induce a negative shift of the steady-state inactivation of CaV3.2 current with no change in the activation properties. Taken together, our findings reveal that TTCCs are a molecular target of the Lavender and Rosemary compounds, suggesting that inhibition of TTCCs could contribute to the anxiolytic and the neuroprotective effects of these plants.

The Potential Role of Gap Junctional Plasticity in the Regulation of State

Electrical synapses are the functional correlate of gap junctions and allow transmission of small molecules and electrical current between coupled neurons. Instead of static pores, electrical synapses are actually plastic, similar to chemical synapses. In the thalamocortical system, gap junctions couple inhibitory neurons that are similar in their biochemical profile, morphology, and electrophysiological properties. We postulate that electrical synaptic plasticity among inhibitory neurons directly interacts with the switching between different firing patterns in a state-dependent and type-dependent manner. In neuronal networks, electrical synapses may function as a modifiable resonance feedback system that enables stable oscillations. Furthermore, the plasticity of electrical synapses may play an important role in regulation of state, synchrony, and rhythmogenesis in the mammalian thalamocortical system, similar to chemical synaptic plasticity. Based on their plasticity, rich diversity, and specificity, electrical synapses are thus likely to participate in the control of consciousness and attention.

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

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

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

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

T-type calcium channels in synaptic plasticity

The role of T-type calcium currents is rarely considered in the extensive literature covering the mechanisms of long-term synaptic plasticity. This situation reflects the lack of suitable T-type channel antagonists that till recently has hampered investigations of the functional roles of these channels. However, with the development of new pharmacological and genetic tools, a clear involvement of T-type channels in synaptic plasticity is starting to emerge. Here, we review a number of studies showing that T-type channels participate to numerous homo- and hetero-synaptic plasticity mechanisms that involve different molecular partners and both pre- and post-synaptic modifications. The existence of T-channel dependent and independent plasticity at the same synapse strongly suggests a subcellular localization of these channels and their partners that allows specific interactions. Moreover, we illustrate the functional importance of T-channel dependent synaptic plasticity in neocortex and thalamus.

Ontogenic Changes and Differential Localization of T-type Ca(2+) Channel Subunits Cav3.1 and Cav3.2 in Mouse Hippocampus and Cerebellum

T-type calcium (Ca(2+)) channels play a central role in regulating membrane excitability in the brain. Although the contributions of T-type current to neuron output is often proposed to reflect a differential distribution of T-type channel subtypes to somato-dendritic compartments, their precise subcellular distributions in central neurons are not fully determined. Using histoblot and high-resolution immunoelectron microscopic techniques, we have investigated the expression, regional distribution and subcellular localization of T-type Cav3.1 and Cav3.2 channel subunits in the adult brain, as well as the ontogeny of expression during postnatal development. Histoblot analysis showed that Cav3.1 and Cav3.2 proteins were widely expressed in the brain, with mostly non-overlapping patterns. Cav3.1 showed the highest expression level in the molecular layer (ml) of the cerebellum (Cb), and Cav3.2 in the hippocampus (Hp) and the ml of Cb. During development, levels of Cav3.1 and Cav3.2 increased with age, although there were marked region- and developmental stage-specific differences in their expression. At the cellular and subcellular level, immunoelectron microscopy showed that labeling for Cav3.1 was present in somato-dendritic domains of hippocampal interneurons and Purkinje cells (PCs), while Cav3.2 was present in somato-dendritic domains of CA1 pyramidal cells, hippocampal interneurons and PCs. Most of the immunoparticles for Cav3.1 and Cav3.2 were either associated with the plasma membrane or the intracellular membranes, with notable differences depending on the compartment. Thus, Cav3.1 was mainly located in the plasma membrane of interneurons, whereas Cav3.2 was mainly located in the plasma membrane of dendritic spines and had a major intracellular distribution in dendritic shafts. In PCs, Cav3.1 and Cav3.2 showed similar distribution patterns. In addition to its main postsynaptic distribution, Cav3.2 but not Cav3.1 was also detected in axon terminals establishing excitatory synapses. These results shed new light on the subcellular localization of T-type channel subunits and provide evidence for the non-uniform distribution of Cav3.1 and Cav3.2 subunits over the plasma membrane of central neurons, which may account for the functional heterogeneity of T-type mediated current.

Interictal Abnormalities of Gamma Band Activity in Visual Evoked Responses in Migraine: An Indication of Thalamocortical Dysrhythmia?

Between attacks, migraineurs lack habituation in standard visual evoked potentials (VEPs). Visual stimuli also evoke high-frequency oscillations in the gamma band range (GBOs, 20–35 Hz) assumed to be generated both at subcortical (early GBOs) and cortical levels (late GBOs). The consecutive peaks of GBOs were analysed regarding amplitude and habituation in six successive blocks of 100 averaged pattern reversal (PR)-VEPs in healthy volunteers and interictally in migraine with (MA) or without aura patients. Amplitude of the two early GBO components in the first PR-VEP block was significantly increased in MA patients. There was a significant habituation deficit of the late GBO peaks in migraineurs. The increased amplitude of early GBOs could be related to the increased interictal visual discomfort reported by patients. We hypothesize that the hypo-functioning serotonergic pathways may cause, in line with the thalamocortical dysrhythmia theory, a functional disconnection of the thalamus leading to decreased intracortical lateral inhibition, which can induce dishabituation.

CaV1.2/CaV3.x channels mediate divergent vasomotor responses in human cerebral arteries

The regulation of arterial tone is critical in the spatial and temporal control of cerebral blood flow. Voltage-gated Ca(2+) (CaV) channels are key regulators of excitation-contraction coupling in arterial smooth muscle, and thereby of arterial tone. Although L- and T-type CaV channels have been identified in rodent smooth muscle, little is known about the expression and function of specific CaV subtypes in human arteries. Here, we determined which CaV subtypes are present in human cerebral arteries and defined their roles in determining arterial tone. Quantitative polymerase chain reaction and Western blot analysis, respectively, identified mRNA and protein for L- and T-type channels in smooth muscle of cerebral arteries harvested from patients undergoing resection surgery. Analogous to rodents, CaV1.2 (L-type) and CaV3.2 (T-type) α1 subunits were expressed in human cerebral arterial smooth muscle; intriguingly, the CaV3.1 (T-type) subtype present in rodents was replaced with a different T-type isoform, CaV3.3, in humans. Using established pharmacological and electrophysiological tools, we separated and characterized the unique profiles of Ca(2+) channel subtypes. Pressurized vessel myography identified a key role for CaV1.2 and CaV3.3 channels in mediating cerebral arterial constriction, with the former and latter predominating at higher and lower intraluminal pressures, respectively. In contrast, CaV3.2 antagonized arterial tone through downstream regulation of the large-conductance Ca(2+)-activated K(+) channel. Computational analysis indicated that each Ca(2+) channel subtype will uniquely contribute to the dynamic regulation of cerebral blood flow. In conclusion, this study documents the expression of three distinct Ca(2+) channel subtypes in human cerebral arteries and further shows how they act together to orchestrate arterial tone.

The Role of Calcium Channels in Epilepsy

A central theme in the quest to unravel the genetic basis of epilepsy has been the effort to elucidate the roles played by inherited defects in ion channels. The ubiquitous expression of voltage-gated calcium channels (VGCCs) throughout the central nervous system (CNS), along with their involvement in fundamental processes, such as neuronal excitability and synaptic transmission, has made them attractive candidates. Recent insights provided by the identification of mutations in the P/Q-type calcium channel in humans and rodents with epilepsy and the finding of thalamic T-type calcium channel dysfunction in the absence of seizures have raised expectations of a causal role of calcium channels in the polygenic inheritance of idiopathic epilepsy. In this review, we consider how genetic variation in neuronal VGCCs may influence the development of epilepsy.