The T-type calcium channelosome

T-type calcium channels perform crucial physiological roles across a wide spectrum of tissues, spanning both neuronal and non-neuronal system. For instance, they serve as pivotal regulators of neuronal excitability, contribute to cardiac pacemaking, and mediate the secretion of hormones. These functions significantly hinge upon the intricate interplay of T-type channels with interacting proteins that modulate their expression and function at the plasma membrane. In this review, we offer a panoramic exploration of the current knowledge surrounding these T-type channel interactors, and spotlight certain aspects of their potential for drug-based therapeutic intervention.

Secretory carrier-associated membrane protein 5 regulates cell-surface targeting of T-type calcium channels

Missense mutations in the human secretary carrier-associated membrane protein 5 (SCAMP5) cause a variety of neurological disorders including neurodevelopmental delay, epilepsy, and Parkinson's disease. We recently documented the importance of SCAMP2 in the regulation of T-type calcium channel expression in the plasma membrane. Here, we show that similar to SCAMP2, the co-expression of SCAMP5 in tsA-201 cells expressing recombinant Cav3.1, Cav3.2, and Cav3.3 channels nearly abolished whole-cell T-type currents. Recording of intramembrane charge movements revealed that SCAMP5-induced inhibition of T-type currents is primarily caused by the reduced expression of functional channels in the plasma membrane. Moreover, we show that SCAMP5-mediated downregulation of Cav3.2 channels is essentially preserved with disease-causing SCAMP5 R91W and G180W mutations. Hence, this study extends our previous findings with SCAMP2 and indicates that SCAMP5 also contributes to repressing the expression of T-type channels in the plasma membrane.

Electrophysiological characterization of a Cav3.2 calcium channel missense variant associated with epilepsy and hearing loss

T-type calcium channelopathies encompass a group of human disorders either caused or exacerbated by mutations in the genes encoding different T-type calcium channels. Recently, a new heterozygous missense mutation in the CACNA1H gene that encodes the Cav3.2 T-type calcium channel was reported in a patient presenting with epilepsy and hearing loss-apparently the first CACNA1H mutation to be associated with a sensorineural hearing condition. This mutation leads to the substitution of an arginine at position 132 with a histidine (R132H) in the proximal extracellular end of the second transmembrane helix of Cav3.2. In this study, we report the electrophysiological characterization of this new variant using whole-cell patch clamp recordings in tsA-201 cells. Our data reveal minor gating alterations of the channel evidenced by a mild increase of the T-type current density and slower recovery from inactivation, as well as an enhanced sensitivity of the channel to external pH change. To what extend these biophysical changes and pH sensitivity alterations induced by the R132H mutation contribute to the observed pathogenicity remains an open question that will necessitate the analysis of additional CACNA1H variants associated with the same pathologies.

Ion channel chameleons: Switching ion selectivity by alternative splicing

Voltage-gated sodium and calcium channels are distinct, evolutionarily related ion channels that achieve remarkable ion selectivity despite sharing an overall similar structure. Classical studies have shown that ion selectivity is determined by specific binding of ions to the channel pore, enabled by signature amino acid sequences within the selectivity filter (SF). By studying ancestral channels in the pond snail (Lymnaea stagnalis), Guan et al. showed in a recent JBC article that this well-established mechanism can be tuned by alternative splicing, allowing a single Ca_V3 gene to encode both a Ca²⁺-permeable and an Na⁺-permeable channel depending on the cellular context. These findings shed light on mechanisms that tune ion selectivity in physiology and on the evolutionary basis of ion selectivity.

Secretory carrier-associated membrane protein 2 (SCAMP2) regulates cell surface expression of T-type calcium channels

Low-voltage-activated T-type Ca²⁺ channels are key regulators of neuronal excitability both in the central and peripheral nervous systems. Therefore, their recruitment at the plasma membrane is critical in determining firing activity patterns of nerve cells. In this study, we report the importance of secretory carrier-associated membrane proteins (SCAMPs) in the trafficking regulation of T-type channels. We identified SCAMP2 as a novel Ca_v3.2-interacting protein. In addition, we show that co-expression of SCAMP2 in mammalian cells expressing recombinant Ca_v3.2 channels caused an almost complete drop of the whole cell T-type current, an effect partly reversed by single amino acid mutations within the conserved cytoplasmic E peptide of SCAMP2. SCAMP2-induced downregulation of T-type currents was also observed in cells expressing Ca_v3.1 and Ca_v3.3 channel isoforms. Finally, we show that SCAMP2-mediated knockdown of the T-type conductance is caused by the lack of Ca_v3.2 expression at the cell surface as evidenced by the concomitant loss of intramembrane charge movement without decrease of total Ca_v3.2 protein level. Taken together, our results indicate that SCAMP2 plays an important role in the trafficking of Ca_v3.2 channels at the plasma membrane.

Peripheral Inflammation Results in Increased Excitability of Capsaicin-Insensitive Nociceptive DRG Neurons Mediated by Upregulation of ASICs and Voltage-Gated Ion Channels

Previously, we have characterized the capsaicin-insensitive low pH-sensitive (caps⁻lpH⁺) subtype of small-sized nociceptive dorsal root ganglion (DRG) neurons that express acid-sensing ion channels, T-type Ca²⁺ channels, and have isolectin B4-negative phenotype. These neurons demonstrated increased excitability in a model of long-term diabetes, contributing to chronic pain sensation. Here we studied changes in the excitability of the caps⁻lpH⁺ neurons and underlying changes in the functional expression and gating properties of ion channels under complete Freund's adjuvant (CFA)-induced peripheral inflammation. We have found that, under these pathological conditions, the functional expression of the acid-sensing ion channels (ASICs) and voltage-gated Na⁺ channels, was increased. In addition, T-type Ca²⁺ current was significantly increased in the neurons at the membrane potentials close to its resting value. Altogether, the observed changes in the channel functioning shifted a pH level evoking an action potential (AP) toward its physiological value and led to an increase of evoked and spontaneous excitability of the caps⁻lpH⁺ neurons that may contribute to hyperalgesia and chronic inflammatory pain.

Regulation of CaV3.2 channels by the receptor for activated C kinase 1 (Rack-1)

This study describes the interaction between Ca_V3.2 calcium channels and the receptor for activated C kinase 1 (Rack-1), a scaffold protein which has recently been implicated in neuropathic pain. The coexpression of Ca_V3.2 and Rack-1 in tsA-201 cells led to a reduction in the magnitude of whole-cell Ca_V3.2 currents and Ca_V3.2 channel expression at the plasma membrane. Co-immunoprecipitations from transfected cells show the formation of a molecular protein complex between Cav3.2 channels and Rack-1. We determined that the interaction of Rack-1 occurs at the intracellular II-III loop and the C-terminus of the channel. Finally, the coexpression of PKCβII abolished the effect of Rack-1 on current densities. Altogether, our findings show that Rack-1 regulates Ca_V3.2-mediated calcium entry in a PKC-dependent manner.

Simultaneous administration of cocaine and caffeine dysregulates HCN and T-type channels

RATIONALE

The abuse of psychostimulants has adverse consequences on the physiology of the central nervous system. In Argentina, and other South American countries, coca paste or "PACO" (cocaine and caffeine are its major components) is massively consumed with deleterious clinical consequences for the health and well-being of the general population. A scant number of studies have addressed the consequences of stimulant combination of cocaine and caffeine on the physiology of the somatosensory thalamocortical (ThCo) system.

OBJECTIVES

Our aim was to study ion conductances that have important implications regulating sleep-wake states 24-h after an acute or chronic binge-like administration of a cocaine and caffeine mixture following previously analyzed pasta base samples ("PACO"-like binge") using mice.

METHODS

We randomly injected (i.p.) male C57BL/6JFcen mice with a binge-like psychostimulants regimen during either 1 day (acute) or 1 day on/1 day off during 13 days for a total of 7 binges (chronic). Single-cell patch-clamp recordings of VB neurons were performed in thalamocortical slices 24 h after the last psychostimulant injection. We also recorded EEG/EMG from mice 24 h after being systemically treated with chronic administration of cocaine + caffeine versus saline, vehicle.

RESULTS

Our results showed notorious changes in the intrinsic properties of the VB nucleus neurons that persist after 24-h of either acute or chronic binge administrations of combined cocaine and caffeine ("PACO"-like binge). Functional dysregulation of HCN (hyperpolarization-activated cyclic nucleotide-gated) and T-type VGC (voltage-gated calcium) channels was described 24-h after acute/chronic "PACO"-like administrations. Furthermore, intracellular basal [Ca²⁺] disturbances resulted a key factor that modulated the availability and the activation of T-type channels, altering T-type "window currents." As a result, all these changes ultimately shaped the low-threshold spikes (LTS)-associated Ca²⁺ transients, regulated the membrane excitability, and altered sleep-wake transitions.

CONCLUSION

Our results suggest that deleterious consequences of stimulants cocaine and caffeine combination on the thalamocortical physiology as a whole might be related to potential neurotoxic effects of soaring intracellular [Ca²⁺].

The rise of T-type channels in melanoma progression and chemotherapeutic resistance

Hyperactivation of the Mitogen Activated Protein Kinase (MAPK) pathway is prevalent in melanoma, principally due to mutations in the BRAF and NRAS genes. MAPK inhibitors are effective only short-term, and recurrence occurs due to functional redundancies or intertwined pathways. The remodeling of Ca²⁺ signaling is also common in melanoma cells, partly through the increased expression of T-type channels (TTCCs). Here we summarize current knowledge about the prognostic value and molecular targeting of TTCCs. Furthermore, we discuss recent evidence pointing to TTCCs as molecular switches for melanoma chemoresistance, which set the grounds for novel combined therapies against the advanced disease.

Identification of a molecular gating determinant within the carboxy terminal region of Cav3.3 T-type channels

The physiological functions controlled by T-type channels are intrinsically dependent on their gating properties, and alteration of T-type channel activity is linked to several human disorders. Therefore, it is essential to develop a clear understanding of the structural determinants responsible for the unique gating features of T-type channels. Here, we have investigated the specific role of the carboxy terminal region by creating a series a deletion constructs expressed in tsA-201 cells and analyzing them by patch clamp electrophysiology. Our data reveal that the proximal region of the carboxy terminus contains a structural determinant essential for shaping several gating aspects of Cav3.3 channels, including voltage-dependence of activation and inactivation, inactivation kinetics, and coupling between the voltage sensing and the pore opening of the channel. Altogether, our data are consistent with a model in which the carboxy terminus stabilizes the channel in a closed state.

T-Type Channel Druggability at a Crossroads

Low-voltage activated T-type calcium channels mediate essential functions in the nervous system, and alteration of channel activity is causally linked to a number of neurological conditions. Therefore, T-type channels hold great promise as pharmacological targets for new medicines. In this Viewpoint, we discuss the potential of T-type channels as druggable targets and reevaluate the strategies available for developing therapeutically efficient and specific modulators of this channel.

T-type calcium channels functionally interact with spectrin (α/β) and ankyrin B

This study describes the functional interaction between the Cav3.1 and Cav3.2 T-type calcium channels and cytoskeletal spectrin (α/β) and ankyrin B proteins. The interactions were identified utilizing a proteomic approach to identify proteins that interact with a conserved negatively charged cytosolic region present in the carboxy-terminus of T-type calcium channels. Deletion of this stretch of amino acids decreased binding of Cav3.1 and Cav3.2 calcium channels to spectrin (α/β) and ankyrin B and notably also reduced T-type whole cell current densities in expression systems. Furthermore, fluorescence recovery after photobleaching analysis of mutant channels lacking the proximal C-terminus region revealed reduced recovery of both Cav3.1 and Cav3.2 mutant channels in hippocampal neurons. Knockdown of spectrin α and ankyrin B decreased the density of endogenous Cav3.2 in hippocampal neurons. These findings reveal spectrin (α/β) / ankyrin B cytoskeletal and signaling proteins as key regulators of T-type calcium channels expressed in the nervous system.

Dexamethasone-induced upregulation of CaV3.2 T-type Ca2+ channels in rat cardiac myocytes

Glucocorticoids are widely used to treat acute and chronic diseases. Unfortunately, their therapeutic use is associated with severe side effects. Glucocorticoids are known to regulate several ion channels in cardiac myocytes, including voltage-dependent Ca²⁺ channels. Low-voltage-activated T-type Ca²⁺ channels are expressed in ventricular myocytes during the fetal and perinatal period, but are practically absent in the adult. However, these channels can be re-expressed in adult cardiomyocytes under some pathological conditions. We have investigated the glucocorticoid regulation of T-type Ca²⁺ channels in rat cardiomyocytes. Molecular studies revealed that dexamethasone induces the upregulation of Ca_V3.2 mRNA in neonatal rat ventricular myocytes, whereas Ca_V3.1 mRNA is only slightly affected. Patch-clamp recordings confirmed that T-type Ca²⁺ channel currents were upregulated in dexamethasone treated cardiomyocytes, and the addition of 50 μmol/L NiCl₂ demonstrated that the Ca_V3.2 channel is responsible for this upregulation. The effect of dexamethasone on Ca_V3.2 is mediated by the activation and translocation to the cell nucleus of the glucocorticoid receptor (GR). We have isolated the upstream promoter of the Cacna1h gene and tested its activity in transfected ventricular myocytes. The initial in silico analysis of Cacna1h promoter revealed putative glucocorticoid response elements (GREs). Transcriptional activity assays combined with deletion analyses and chromatin immunoprecipitation assays demonstrated that GR binds to a region a GRE located in -1006/-985 bp of Cacna1h promoter. Importantly, upregulation of the Ca_V3.2 channel is also observed in vitro in adult rat ventricular myocytes, and in vivo in a rat model of excess of glucocorticoids.

A cell-permeant peptide corresponding to the cUBP domain of USP5 reverses inflammatory and neuropathic pain

Background

Cav3.2 T-type calcium currents in primary afferents are enhanced in various painful pathological conditions, whereas inhibiting Cav3.2 activity or expression offers a strategy for combating the development of pain hypersensitivity. We have shown that Cav3.2 channel surface density is strongly regulated by the ubiquitination machinery and we identified the deubiquitinase USP5 as a Cav3.2 channel interacting protein and regulator of its cell surface expression. We also reported that USP5 is upregulated in chronic pain conditions. Conversely, preventing its binding to the channel in vivo mediates analgesia in inflammatory and neuropathic pain models.

Results

To identify which USP5 domain is responsible for the interaction, we used a series of USP5-derived peptides corresponding to different regions in nUBP, cUBP, UBA1, and UBA2 domains to outcompete full length USP5. We identified a stretch of amino acid residues within the cUBP domain of USP5 as responsible for binding to Cav3.2 calcium channels. Based on this information, we generated a TAT-cUBP1-USP5 peptide that could disrupt the Cav3.2/USP5 interaction in vitro and tested its physiological effect in well-established models of persistent inflammatory pain (CFA test) and chronic mononeuropathy and polyneuropathy in mice (partial sciatic nerve injury and the (ob/ob) diabetic spontaneous neuropathic mice). Our results reveal that the TAT-cUBP1-USP5 peptide attenuated mechanical hyperalgesia induced by both Complete Freund’s Adjuvant and partial sciatic nerve injury, and thermal hyperalgesia in diabetic neuropathic animals. In contrast, Cav3.2 null mice were not affected by the peptide in the partial sciatic nerve injury model. Cav3.2 calcium channel levels in diabetic mice were reduced following the administration of the TAT-cUBP1-USP5 peptide.

Conclusions

Our findings reveal a crucial region in the cUBP domain of USP5 that is important for substrate recognition and binding to the III-IV linker of Cav3.2 channels. Targeting the interaction of this region with the Cav3.2 channel can be exploited as the basis for therapeutic intervention into inflammatory and neuropathic pain.

Painful Diabetic Neuropathy: Prevention or Suppression?

Pain-sensing sensory neurons (nociceptors) of the dorsal root ganglia (DRG) and dorsal horn (DH) can become sensitized (hyperexcitable) in response to pathological conditions such as diabetes, which in turn may lead to the development of painful peripheral diabetic neuropathy (PDN). Because of incomplete knowledge about the mechanisms underlying painful PDN, current treatment for painful PDN has been limited to somewhat nonspecific systemic drugs that have significant side effects or potential for abuse. Recent studies have established that several ion channels in DRG and DH neurons are dysregulated and make a previously unrecognized contribution to sensitization of pain responses by enhancing excitability of nociceptors in animal models of type 1 and type 2 PDN. Furthermore, it has been reported that targeting posttranslational modification of nociceptive ion channels such as glycosylation and methylglyoxal metabolism can completely reverse mechanical and thermal hyperalgesia in diabetic animals with PDN in vivo. Understanding details of posttranslational regulation of nociceptive channel activity may facilitate development of novel therapies for treatment of painful PDN. We argue that pharmacological targeting of the specific pathogenic mechanism rather than of the channel per se may cause fewer side effects and reduce the potential for drug abuse in patients with diabetes.

1,4-dihydropyridines: the multiple personalities of a blockbuster drug family

More than 40 years after their introduction in therapy, 1,4-dihydropyridines (DHPs) are still amongst the most prescribed drugs in the world. Though they all share a similar mechanism of action blocking L-type voltage-gated Ca(2+) channels, DHPs differ in crucial pharmacological properties like tissue selectivity and cardiodepressant activity. This review examines how changes in the DHP structure can modify the pharmacological properties of these drugs and how some of these chemical manipulations have been exploited to obtain clinically more effective molecules. Special emphasis is given to the evidence that L-type Ca(2+) channels are an heterogeneous family and that DHPs with different pharmacological properties differ in their affinity for the different isoforms of this class of channels. Data showing that DHP pharmacological heterogeneity could be in part dependent on the interaction of some of these molecules with ion channels different from the L-type Ca(2+) channels is reviewed as well.