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.

A Crash Course in Calcium Channels

Much progress has been made in understanding the molecular physiology and pharmacology of calcium channels. Recently, there have been tremendous advances in learning about calcium channel structure and function through crystallography and cryo-electron microscopy studies. Here, I will give an overview of our knowledge about calcium channels, and highlight two recent studies that give important insights into calcium channel structure.

The Cacna1h mutation in the GAERS model of absence epilepsy enhances T-type Ca2+ currents by altering calnexin-dependent trafficking of Cav3.2 channels

Low-voltage-activated T-type calcium channels are essential contributors to the functioning of thalamocortical neurons by supporting burst-firing mode of action potentials. Enhanced T-type calcium conductance has been reported in the Genetic Absence Epilepsy Rat from Strasbourg (GAERS) and proposed to be causally related to the overall development of absence seizure activity. Here, we show that calnexin, an endoplasmic reticulum integral membrane protein, interacts with the III-IV linker region of the Ca_v3.2 channel to modulate the sorting of the channel to the cell surface. We demonstrate that the GAERS missense mutation located in the Ca_v3.2 III-IV linker alters the Ca_v3.2/calnexin interaction, resulting in an increased surface expression of the channel and a concomitant elevation in calcium influx. Our study reveals a novel mechanism that controls the expression of T-type channels, and provides a molecular explanation for the enhancement of T-type calcium conductance in GAERS.

Discovery and mode of action of a novel analgesic β-toxin from the African spider Ceratogyrus darlingi

Spider venoms are rich sources of peptidic ion channel modulators with important therapeutical potential. We screened a panel of 60 spider venoms to find modulators of ion channels involved in pain transmission. We isolated, synthesized and pharmacologically characterized Cd1a, a novel peptide from the venom of the spider Ceratogyrus darlingi. Cd1a reversibly paralysed sheep blowflies (PD₅₀ of 1318 pmol/g) and inhibited human Ca_v2.2 (IC₅₀ 2.6 μM) but not Ca_v1.3 or Ca_v3.1 (IC₅₀ > 30 μM) in fluorimetric assays. In patch-clamp electrophysiological assays Cd1a inhibited rat Ca_v2.2 with similar potency (IC₅₀ 3 μM) without influencing the voltage dependence of Ca_v2.2 activation gating, suggesting that Cd1a doesn’t act on Ca_v2.2 as a classical gating modifier toxin. The Cd1a binding site on Ca_v2.2 did not overlap with that of the pore blocker ω-conotoxin GVIA, but its activity at Ca_v2.2-mutant indicated that Cd1a shares some molecular determinants with GVIA and MVIIA, localized near the pore region. Cd1a also inhibited human Na_v1.1–1.2 and Na_v1.7–1.8 (IC₅₀ 0.1–6.9 μM) but not Na_v1.3–1.6 (IC₅₀ > 30 μM) in fluorimetric assays. In patch-clamp assays, Cd1a strongly inhibited human Na_v1.7 (IC₅₀ 16 nM) and produced a 29 mV depolarising shift in Na_v1.7 voltage dependence of activation. Cd1a (400 pmol) fully reversed Na_v1.7-evoked pain behaviours in mice without producing side effects. In conclusion, Cd1a inhibited two anti-nociceptive targets, appearing to interfere with Ca_v2.2 inactivation gating, associated with the Ca_v2.2 α-subunit pore, while altering the activation gating of Na_v1.7. Cd1a was inactive at some of the Na_v and Ca_v channels expressed in skeletal and cardiac muscles and nodes of Ranvier, apparently contributing to the lack of side effects at efficacious doses, and suggesting potential as a lead for development of peripheral pain treatments.

A T-type channel-calmodulin complex triggers αCaMKII activation

Calmodulin (CaM) is an important signaling molecule that regulates a vast array of cellular functions by activating second messengers involved in cell function and plasticity. Low voltage-activated calcium channels of the Cav3 family have the important role of mediating low threshold calcium influx, but were not believed to interact with CaM. We find a constitutive association between CaM and the Cav3.1 channel at rest that is lost through an activity-dependent and Cav3.1 calcium-dependent CaM dissociation. Moreover, Cav3 calcium influx is sufficient to activate αCaMKII in the cytoplasm in a manner that depends on an intact Cav3.1 C-terminus needed to support the CaM interaction. Our findings thus establish that T-type channel calcium influx invokes a novel dynamic interaction between CaM and Cav3.1 channels to trigger a signaling cascade that leads to αCaMKII activation.

Down-regulation of T-type Cav3.2 channels by hyperpolarization-activated cyclic nucleotide-gated channel 1 (HCN1): Evidence of a signaling complex

Formation of complexes between ion channels is important for signal processing in the brain. Here we investigate the biochemical and biophysical interactions between HCN1 channels and Cav3.2 T-type channels. We found that HCN1 co-immunoprecipitated with Cav3.2 from lysates of either mouse brain or tsA-201 cells, with the HCN1 N-terminus associating with the Cav3.2 N-terminus. Cav3.2 channel activity appeared to be functionally regulated by HCN1. The expression of HCN1 induced a decrease in Cav3.2 Ba²⁺ influx (I_Ba²⁺) along with altered channel kinetics and a depolarizing shift in activation gating. However, a reciprocal regulation of HCN1 by Cav3.2 was not observed. This study highlights a regulatory role of HCN1 on Cav3.2 voltage-dependent properties, which are expected to affect physiologic functions such as synaptic transmission and cellular excitability.

Synthesis of new N3-substituted dihydropyrimidine derivatives as L-/T- type calcium channel blockers

Cardiovascular diseases (CVDs) are the main cause of deaths worldwide. Up-to-date, hypertension is the most significant contributing factor to CVDs. Recent clinical studies recommend calcium channel blockers (CCBs) as effective treatment alone or in combination with other medications. Being the most clinically useful CCBs, 1,4-dihydropyridines (DHPs) attracted great interest in improving potency and selectivity. However, the short plasma half-life which may be attributed to the metabolic oxidation to the pyridine-counterparts is considered as a major limitation for this class. Among the most efficient modifications of the DHP scaffold, is the introduction of biologically active N3-substituted dihydropyrimidine mimics (DHPMs). Again, some potent DHPMs showed only in vitro activity due to first pass effect through hydrolysis and removal of the N3-substitutions. Herein, the synthesis of new N3-substituted DHPMs with various functionalities linked to the DHPM core via two-carbon spacer to guard against possible metabolic inactivation is described. It was designed to keep close structural similarities to clinically efficient DHPs and the reported lead DHPMs analogues, while attempting to improve the pharmacokinetic properties through better metabolic stability. Applying whole batch clamp technique, five compounds showed promising L- and T- type calcium channel blocking activity and were identified as lead compounds. Structure requirements for selectivity against Ca_v1.2 as well against Ca_v3.2 are described.

Carisbamate blockade of T-type voltage-gated calcium channels

OBJECTIVES

Carisbamate (CRS) is a novel monocarbamate compound that possesses antiseizure and neuroprotective properties. However, the mechanisms underlying these actions remain unclear. Here, we tested both direct and indirect effects of CRS on several cellular systems that regulate intracellular calcium concentration [Ca²⁺ ]_i .

METHODS

We used a combination of cellular electrophysiologic techniques, as well as cell viability, Store Overload-Induced Calcium Release (SOICR), and mitochondrial functional assays to determine whether CRS might affect [Ca²⁺ ]_i levels through actions on the endoplasmic reticulum (ER), mitochondria, and/or T-type voltage-gated Ca²⁺ channels.

RESULTS

In CA3 pyramidal neurons, kainic acid induced significant elevations in [Ca²⁺ ]_i and long-lasting neuronal hyperexcitability, both of which were reversed in a dose-dependent manner by CRS. Similarly, CRS suppressed spontaneous rhythmic epileptiform activity in hippocampal slices exposed to zero-Mg²⁺ or 4-aminopyridine. Treatment with CRS also protected murine hippocampal HT-22 cells against excitotoxic injury with glutamate, and this was accompanied by a reduction in [Ca²⁺ ]_i . Neither kainic acid nor CRS alone altered the mitochondrial membrane potential (ΔΨ) in intact, acutely isolated mitochondria. In addition, CRS did not affect mitochondrial respiratory chain activity, Ca²⁺ -induced mitochondrial permeability transition, and Ca²⁺ release from the ER. However, CRS significantly decreased Ca²⁺ flux in human embryonic kidney tsA-201 cells transfected with Ca_v 3.1 (voltage-dependent T-type Ca²⁺ ) channels.

SIGNIFICANCE

Our data indicate that the neuroprotective and antiseizure activity of CRS likely results in part from decreased [Ca²⁺ ]_i accumulation through blockade of T-type Ca²⁺ channels.

Blocking microglial pannexin-1 channels alleviates morphine withdrawal in rodents

Opiates are essential for treating pain, but termination of opiate therapy can cause a debilitating withdrawal syndrome in chronic users. To alleviate or avoid the aversive symptoms of withdrawal, many of these individuals continue to use opiates. Withdrawal is therefore a key determinant of opiate use in dependent individuals, yet its underlying mechanisms are poorly understood and effective therapies are lacking. Here, we identify the pannexin-1 (Panx1) channel as a therapeutic target in opiate withdrawal. We show that withdrawal from morphine induces long-term synaptic facilitation in lamina I and II neurons within the rodent spinal dorsal horn, a principal site of action for opiate analgesia. Genetic ablation of Panx1 in microglia abolished the spinal synaptic facilitation and ameliorated the sequelae of morphine withdrawal. Panx1 is unique in its permeability to molecules up to 1 kDa in size and its release of ATP. We show that Panx1 activation drives ATP release from microglia during morphine withdrawal and that degrading endogenous spinal ATP by administering apyrase produces a reduction in withdrawal behaviors. Conversely, we found that pharmacological inhibition of ATP breakdown exacerbates withdrawal. Treatment with a Panx1-blocking peptide (¹⁰panx) or the clinically used broad-spectrum Panx1 blockers, mefloquine or probenecid, suppressed ATP release and reduced withdrawal severity. Our results demonstrate that Panx1-mediated ATP release from microglia is required for morphine withdrawal in rodents and that blocking Panx1 alleviates the severity of withdrawal without affecting opiate analgesia.

Regulation of voltage gated calcium channels by GPCRs and post-translational modification

Calcium entry via voltage gated calcium channels mediates a wide range of physiological functions, whereas calcium channel dysregulation has been associated with numerous pathophysiological conditions. There are myriad cell signaling pathways that act on voltage gated calcium channels to fine tune their activities and to regulate their cell surface expression. These regulatory mechanisms include the activation of G protein-coupled receptors and downstream phosphorylation events, and their control over calcium channel trafficking through direct physical interactions. Calcium channels also undergo post-translational modifications that alter both function and density of the channels in the plasma membrane. Here we focus on select aspects of these regulatory mechanisms and highlight recent developments.

Block of voltage-gated calcium channels by peptide toxins

Venoms from various predatory species, such as fish hunting molluscs scorpions, snakes and arachnids contain a large spectrum of toxins that include blockers of voltage-gated calcium channels. These peptide blockers act by two principal manners - physical occlusion of the pore and prevention of activation gating. Many of the calcium channel-blocking peptides have evolved to tightly occupy their binding pocket on the principal pore forming subunit of the channel, often rendering block poorly reversible. Moreover, several of the best characterized blocking peptides have developed a high degree of channel subtype selectivity. Here we give an overview of different types of calcium channel-blocking toxins, their mechanism of action, channel subtype specificity, and potential use as therapeutic agents. This article is part of the Special Issue entitled 'Venom-derived Peptides as Pharmacological Tools.'

Anticonvulsant mechanisms of piperine, a piperidine alkaloid

Piperine, a natural compound isolated from the fruits of Piper, is known to modulate several neurotransmitter systems such as serotonin, norepinephrine, and GABA, all of which have been linked to the development of convulsions. Fruits of Piper species have been suggested as means for managing seizure disorders. The present study was designed to elucidate the anticonvulsant effect of piperine and its mechanisms of action using in-silico, in-vivo and in-vitro techniques.PASS software was used to determine its possible activity and mechanisms. Furthermore the latency for development of convulsions and mortality rate was recorded in different experimental mouse models of epilepsy (pentylenetetrazole, maximal electroshock, NMDA, picrotoxin, bicuculline, BAYK-8644, strychnine-induced convulsions) after administration of various doses of piperine (5, 10 and 20 mg/kg, i.p.). Finally, the effect of piperine on Na(+) and Ca(2+) channels were evaluated using the whole cell patch clamp techniqueOur results revealed that piperine decreased mortality in the MES-induced seizure model. Moreover, piperine (10 mg/kg) delayed the onset of tonic clonic convulsions in the pentylenetetrazole test and reduced associated mortality. Furthermore, an anticonvulsant dose of piperine also delayed the onset of tonic clonic seizures in strychnine, picrotoxin and BAY K-8644. Complete protection against mortality was observed in BAYK-8644 induced convulsions. Finally, whole cell patch clamp analysis suggested an inhibitory effect of piperine on Na(+) channels. Together, our data suggest Na(+) channel antagonist activity as a contributor to the complex anticonvulsant mechanisms of piperine.

Calcium Channel Signaling Complexes with Receptors and Channels

Voltage-gated calcium channels are not only mediators of cell signalling events, but also are recipients of signalling inputs from G protein coupled receptors (GPCRs) and their associated second messenger pathways. The coupling of GPCRs to calcium channels is optimized through the formation of receptor-channel complexes. In addition, this provides a mechanism for receptorchannel co-trafficking to and from the plasma membrane. On the other hand, voltage-gated calcium channel activity affects other types of ion channels such as voltage-and calcium-activated potassium channels. Coupling efficiency between these two families of channels is also enhanced through the formation of channel-channel complexes. This review provides a concise overview of the current state of knowledge on the physical interactions between voltage-gated calcium channels and members of the GPCR family, and with other types of ion channels.

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.

Synthesis and characterization of a disubstituted piperazine derivative with T-type channel blocking action and analgesic properties

Background

T-type calcium channels are important contributors to signaling in the primary afferent pain pathway and are thus important targets for the development of analgesics. It has been previously reported that certain piperazine-based compounds such as flunarizine are able to inhibit T-type calcium channels. Thus, we hypothesized that novel piperazine compounds could potentially act as analgesics.

Results

Here, we have created a series of 14 compound derivatives around a diphenyl methyl-piperazine core pharmacophore. Testing their effects on transiently expressed Cav3.2 calcium channels revealed one derivative (3-((4-(bis(4-fluorophenyl)methyl)piperazin-1-yl)methyl)-4-(2-methoxyphenyl)-1,2,5-oxadiazole 2-oxide, compound 10e) as a potent blocker. 10e mediate tonic block of these channels with an IC50 of around 4 micromolar. 10e also blocked Cav3.1 and Cav3.3 channels, but only weakly affected high-voltage-activated Cav1.2 and Cav2.2 channels. Intrathecal delivery of 10e mediated relief from formalin and complete Freund’s adjuvant induced inflammatory pain that was ablated by genetic knockout of Cav3.2 channels.

Conclusions

Altogether, our data identify a novel T-type calcium channel blocker with tight structure activity relationship (SAR) and relevant in vivo efficacy in inflammatory pain conditions.

Reduced Hyperpolarization-Activated Current Contributes to Enhanced Intrinsic Excitability in Cultured Hippocampal Neurons from PrP(-/-) Mice

Genetic ablation of cellular prion protein (PrP^C) has been linked to increased neuronal excitability and synaptic activity in the hippocampus. We have previously shown that synaptic activity in hippocampi of PrP-null mice is increased due to enhanced N-methyl-D-aspartate receptor (NMDAR) function. Here, we focused on the effect of PRNP gene knock-out (KO) on intrinsic neuronal excitability, and in particular, the underlying ionic mechanism in hippocampal neurons cultured from P0 mouse pups. We found that the absence of PrP^C profoundly affected the firing properties of cultured hippocampal neurons in the presence of synaptic blockers. The membrane impedance was greater in PrP-null neurons, and this difference was abolished by the hyperpolarization-activated cyclic nucleotide-gated (HCN) channel blocker ZD7288 (100 μM). HCN channel activity appeared to be functionally regulated by PrP^C. The amplitude of voltage sag, a characteristic of activating HCN channel current (I_h), was decreased in null mice. Moreover, I_h peak current was reduced, along with a hyperpolarizing shift in activation gating and slower kinetics. However, neither HCN1 nor HCN2 formed a biochemical complex with PrP^C. These results suggest that the absence of PrP downregulates the activity of HCN channels through activation of a cell signaling pathway rather than through direct interactions. This in turn contributes to an increase in membrane impedance to potentiate neuronal excitability.

Assessing the role of IKCa channels in generating the sAHP of CA1 hippocampal pyramidal cells

Our previous work reported that KCa3.1 (IKCa) channels are expressed in CA1 hippocampal pyramidal cells and contribute to the slow afterhyperpolarization that regulates spike accommodation in these cells. The current report presents data from single cell RT-PCR that further reveals mRNA in CA1 cells that corresponds to the sequence of an IKCa channel from transmembrane segments 5 through 6 including the pore region, revealing the established binding sites for 4 different IKCa channel blockers. A comparison of methods to internally apply the IKCa channel blocker TRAM-34 shows that including the drug in an electrode from the onset of an experiment is unviable given the speed of drug action upon gaining access for whole-cell recordings. Together the data firmly establish IKCa channel expression in CA1 neurons and clarify methodological requirements to obtain a block of IKCa channel activity through internal application of TRAM-34.

IKCa channels are a critical determinant of the slow AHP in CA1 pyramidal neurons

Control over the frequency and pattern of neuronal spike discharge depends on Ca2+-gated K+ channels that reduce cell excitability by hyperpolarizing the membrane potential. The Ca2+-dependent slow afterhyperpolarization (sAHP) is one of the most prominent inhibitory responses in the brain, with sAHP amplitude linked to a host of circuit and behavioral functions, yet the channel that underlies the sAHP has defied identification for decades. Here, we show that intermediate-conductance Ca2+-dependent K+ (IKCa) channels underlie the sAHP generated by trains of synaptic input or postsynaptic stimuli in CA1 hippocampal pyramidal cells. These findings are significant in providing a molecular identity for the sAHP of central neurons that will identify pharmacological tools capable of potentially modifying the several behavioral or disease states associated with the sAHP.

Protein interactome mining defines melatonin MT1 receptors as integral component of presynaptic protein complexes of neurons

In mammals, the hormone melatonin is mainly produced by the pineal gland with nocturnal peak levels. Its peripheral and central actions rely either on its intrinsic antioxidant properties or on binding to melatonin MT1 and MT2 receptors, belonging to the G protein-coupled receptor (GPCR) super-family. Melatonin has been reported to be involved in many functions of the central nervous system such as circadian rhythm regulation, neurotransmission, synaptic plasticity, memory, sleep, and also in Alzheimer's disease and depression. However, little is known about the subcellular localization of melatonin receptors and the molecular aspects involved in neuronal functions of melatonin. Identification of protein complexes associated with GPCRs has been shown to be a valid approach to improve our understanding of their function. By combining proteomic and genomic approaches we built an interactome of MT1 and MT2 receptors, which comprises 378 individual proteins. Among the proteins interacting with MT1 , but not with MT2 , we identified several presynaptic proteins, suggesting a potential role of MT1 in neurotransmission. Presynaptic localization of MT1 receptors in the hypothalamus, striatum, and cortex was confirmed by subcellular fractionation experiments and immunofluorescence microscopy. MT1 physically interacts with the voltage-gated calcium channel Cav 2.2 and inhibits Cav 2.2-promoted Ca(2+) entry in an agonist-independent manner. In conclusion, we show that MT1 is part of the presynaptic protein network and negatively regulates Cav 2.2 activity, providing a first hint for potential synaptic functions of MT1.