MEPIRAPIM-derived synthetic cannabinoids inhibit T-type calcium channels with divergent effects on seizures in rodent models of epilepsy

Background: T-type Ca²⁺ channels (Ca_v3) represent emerging therapeutic targets for a range of neurological disorders, including epilepsy and pain. To aid the development and optimisation of new therapeutics, there is a need to identify novel chemical entities which act at these ion channels. A number of synthetic cannabinoid receptor agonists (SCRAs) have been found to exhibit activity at T-type channels, suggesting that cannabinoids may provide convenient chemical scaffolds on which to design novel Ca_v3 inhibitors. However, activity at cannabinoid type 1 (CB₁) receptors can be problematic because of central and peripheral toxicities associated with potent SCRAs. The putative SCRA MEPIRAPIM and its analogues were recently identified as Ca_v3 inhibitors with only minimal activity at CB₁ receptors, opening the possibility that this scaffold may be exploited to develop novel, selective Ca_v3 inhibitors. Here we present the pharmacological characterisation of SB2193 and SB2193F, two novel Ca_v3 inhibitors derived from MEPIRAPIM. Methods: The potency of SB2193 and SB2193F was evaluated in vitro using a fluorometric Ca²⁺ flux assay and confirmed using whole-cell patch-clamp electrophysiology. In silico docking to the cryo-EM structure of Ca_v3.1 was also performed to elucidate structural insights into T-type channel inhibition. Next, in vivo pharmacokinetic parameters in mouse brain and plasma were determined using liquid chromatography-mass spectroscopy. Finally, anticonvulsant activity was assayed in established genetic and electrically-induced rodent seizure models. Results: Both MEPIRAPIM derivatives produced potent inhibition of Ca_v3 channels and were brain penetrant, with SB2193 exhibiting a brain/plasma ratio of 2.7. SB2193 was further examined in mouse seizure models where it acutely protected against 6 Hz-induced seizures. However, SB2193 did not reduce spontaneous seizures in the Scn1a ^+/- mouse model of Dravet syndrome, nor absence seizures in the Genetic Absence Epilepsy Rat from Strasbourg (GAERS). Surprisingly, SB2193 appeared to increase the incidence and duration of spike-and-wave discharges in GAERS animals over a 4 h recording period. Conclusion: These results show that MEPIRAPIM analogues provide novel chemical scaffolds to advance Ca_v3 inhibitors against certain seizure types.

The anticonvulsant phytocannabinoids CBGVA and CBDVA inhibit recombinant T-type channels

Introduction: Cannabidiol (CBD) has been clinically approved for intractable epilepsies, offering hope that novel anticonvulsants in the phytocannabinoid class might be developed. Looking beyond CBD, we have recently reported that a series of biosynthetic precursor molecules found in cannabis display anticonvulsant properties. However, information on the pharmacological activities of these compounds on CNS drug targets is limited. The current study aimed to fill this knowledge gap by investigating whether anticonvulsant phytocannabinoids affect T-type calcium channels, which are known to modulate neuronal excitability, and may be relevant to the anti-seizure effects of this class of compounds. Materials and methods: A fluorescence-based assay was used to screen the ability of the phytocannabinoids to inhibit human T-type calcium channels overexpressed in HEK-293 cells. A subset of compounds was further examined using patch-clamp electrophysiology. Alphascreen technology was used to characterise selected compounds against G-protein coupled-receptor 55 (GPR55) overexpressed in HEK-293 cells, as GPR55 is another target of the phytocannabinoids. Results: A single 10 µM concentration screen in the fluorescence-based assay showed that phytocannabinoids inhibited T-type channels with substantial effects on Ca_v3.1 and Ca_v3.2 channels compared to the Ca_v3.3 channel. The anticonvulsant phytocannabinoids cannabigerovarinic acid (CBGVA) and cannabidivarinic acid (CBDVA) had the greatest magnitudes of effect (≥80% inhibition against Ca_v3.1 and Ca_v3.2), so were fully characterized in concentration-response studies. CBGVA and CBDVA had IC₅₀ values of 6 μM and 2 µM on Ca_v3.1 channels; 2 μM and 11 µM on Ca_v3.2 channels, respectively. Biophysical studies at Ca_v3.1 showed that CBGVA caused a hyperpolarisation shift of steady-state inhibition. Both CBGVA and CBDVA had a use-dependent effect and preferentially inhibited Ca_v3.1 current in a slow inactivated state. CBGVA and CBDVA were also shown to antagonise GPR55. Conclusion and implications: These findings show that CBGVA and CBDVA inhibit T-type calcium channels and GPR55. These compounds should be further investigated to develop novel therapeutics for treating diseases associated with dysfunctional T-type channel activity.

Putative Synthetic Cannabinoids MEPIRAPIM, 5F-BEPIRAPIM (NNL-2), and Their Analogues Are T-Type Calcium Channel (CaV3) Inhibitors

Synthetic cannabinoid receptor agonists (SCRAs) are a large and growing class of new psychoactive substances (NPSs). Two recently identified compounds, MEPIRAPIM and 5F-BEPIRAPIM (NNL-2), have not been confirmed as agonists of either cannabinoid receptor subtype but share structural similarities with both SCRAs and a class of T-type calcium channel (Ca_V3) inhibitors under development as new treatments for epilepsy and pain. In this study, MEPIRAPIM and 5F-BEPIRAPIM and 10 systematic analogues were synthesized, analytically characterized, and pharmacologically evaluated using in vitro cannabinoid receptor and Ca_V3 assays. Several compounds showed micromolar affinities for CB₁ and/or CB₂, with several functioning as low potency agonists of CB₁ and CB₂ in a membrane potential assay. 5F-BEPIRAPIM and four other derivatives were identified as potential Ca_V3 inhibitors through a functional calcium flux assay (>70% inhibition), which was further confirmed using whole-cell patch-clamp electrophysiology. Additionally, MEPIRAPIM and 5F-BEPIRAPIM were evaluated in vivo using a cannabimimetic mouse model. Despite detections of MEPIRAPIM and 5F-BEPIRAPIM in the NPS market, only the highest MEPIRAPIM dose (30 mg/kg) elicited a mild hypothermic response in mice, with no hypothermia observed for 5F-BEPIRAPIM, suggesting minimal central CB₁ receptor activity.

Inhibition of human recombinant T-type calcium channels by phytocannabinoids in vitro

BACKGROUND AND PURPOSE

T-type Ca channels (I_Ca ) regulate neuronal excitability and contribute to neurotransmitter release. The phytocannabinoids Δ⁹ -tetrahydrocannabinol and cannabidiol effectively modulate T-type I_Ca , but effects of other biologically active phytocannabinoids on these channels are unknown. We thus investigated the modulation of T-type I_Ca by low abundance phytocannabinoids.

EXPERIMENTAL APPROACH

A fluorometric (FLIPR) assay was used to investigate modulation of human T-type I_Ca (Ca_V 3.1, 3.2 and 3.3) stably expressed in FlpIn-TREx HEK293 cells. The biophysical effects of some compounds were examined using whole-cell patch clamp recordings from the same cells.

KEY RESULTS

In the FLIPR assay, all eleven phytocannabinoids tested modulated T-type I_Ca , with most inhibiting Ca_V 3.1 and Ca_V 3.2 more effectively than Ca_V 3.3. Cannabigerolic acid was the most potent inhibitor of Ca_V 3.1 (pIC₅₀ 6.1 ± 0.6) and Ca_V 3.2 (pIC₅₀ 6.4 ± 0.4); in all cases phytocannabinoid acids were more potent than their corresponding neutral forms. In patch clamp recordings, cannabigerolic acid inhibited Ca_V 3.1 and 3.2 with similar potency to the FLIPR assay, the inhibition was associated with significant hyperpolarizing shift in activation and steady state inactivation of these channels. In contrast, cannabidiol, cannabidivarin and cannabigerol only affected channel inactivation.

CONCLUSION AND IMPLICATIONS

Modulation of T-type calcium channels is a common property of phytocannabinoids, which all increase steady state inactivation at physiological membrane potentials, with some also affecting channel activation. Thus, T-type I_Ca may be a common site of action for phytocannabinoids, and the diverse actions of phytocannabinoids on channel gating may provide insight into structural requirement for selective T-type I_Ca modulators.

A Synthetically Accessible Small-Molecule Inhibitor of USP5-Cav3.2 Calcium Channel Interactions with Analgesic Properties

Cav3.2 calcium channels are important mediators of nociceptive signaling in the primary afferent pain pathway, and their expression is increased in various rodent models of chronic pain. Previous work from our laboratory has shown that this is in part mediated by an aberrant expression of deubiquitinase USP5, which associates with these channels and increases their stability. Here, we report on a novel bioactive rhodanine compound (II-1), which was identified in compound library screens. II-1 inhibits biochemical interactions between USP5 and the Cav3.2 domain III-IV linker in a dose-dependent manner, without affecting the enzymatic activity of USP5. Molecular docking analysis reveals two potential binding pockets at the USP5-Cav3.2 interface that are distinct from the binding site of the deubiquitinase inhibitor WP1130 (a.k.a. degrasyn). With an understanding of the ability of some rhodanines to produce false positives in high-throughput screening, we have conducted several orthogonal assays to confirm the validity of this hit, including in vivo experiments. Intrathecal delivery of II-1 inhibited both phases of formalin-induced nocifensive behaviors in mice, as well as abolished thermal hyperalgesia induced by the delivery of complete Freund's adjuvant (CFA) to the hind paw. The latter effects were abolished in Cav3.2 null mice, thus confirming that Cav3.2 is required for the action of II-1. II-1 also mediated a robust inhibition of mechanical allodynia induced by injury to the sciatic nerve. Altogether, our data uncover a novel class of analgesics─well suited to rapid structure-activity relationship studies─that target the Cav3.2/USP5 interface.

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.

Modulation of human T-type calcium channels by synthetic cannabinoid receptor agonists in vitro

BACKGROUND AND PURPOSE

Consumption of Synthetic Cannabinoid Receptor agonists (SCRAs) is associated with severe adverse reactions including seizures, arrhythmias and death, but the molecular mechanisms surrounding SCRA toxicity are not yet established. These disease-like symptoms are also synonymous with altered T-type calcium channel activity which controls rhythmicity in the heart and brain. This study examined whether SCRAs alter T-type activity and whether this represents a possible mechanism of toxicity.

EXPERIMENTAL APPROACH

Fluorescence-based and electrophysiology assays were used to screen 16 structurally related synthetic cannabinoids for their ability to inhibit human T-type calcium channels expressed in HEK293 cells. The most potent compounds were then further examined using patch clamp electrophysiology.

KEY RESULTS

MDMB-CHMICA and AMB-CHMINACA potently blocked Cav3.2 with IC50 values of 1.5 and 0.74 μM respectively. Current inhibition increased from 47 to 80% and 45-87% respectively when the channel was in slow-inactivated state. Both SCRAs had little effect on steady state inactivation, however MDMB-CHMICA significantly shifted the half activation potential by -7mV. Neither drug produced frequency dependent block, in contrast to the phytocannabinoid Δ9-THC.

CONCLUSIONS AND IMPLICATIONS

SCRAs are potent agonists of CB1 receptors and can be extremely toxic, but observed toxicity also resembles symptoms associated with altered Cav3.2 activity. Many SCRAs tested were potent modulators of Cav3.2, raising the possibility that SC toxicity may be due in part to Cav3.2 modulation. This potent T-type channel modulation suggests the possibility of SCRAs as a new drug class with potential to treat diseases associated with altered T-type channel activity. This article is part of the special issue on 'Cannabinoids'.

Differential activation of G protein-mediated signaling by synthetic cannabinoid receptor agonists

Synthetic cannabinoid receptor agonists (SCRAs) are new psychoactive substances associated with acute intoxication and even death. However, the molecular mechanisms through which SCRAs may exert their toxic effects remain unclear-including the potential differential activation of G protein subtypes by cannabinoid receptor type 1 (CB1), a major target of SCRA. We measured CB1-mediated activation of Gαs and Gαi/o proteins by SCRAs by examining stimulation (pertussis toxin, PTX treated) as well as inhibition (non-PTX treated) of forskolin (FSK)-induced cyclic adenosine monophosphate (cAMP) accumulation in human embryonic kidney (HEK) cells stably expressing CB1. Real-time measurements of stimulation and inhibition of cAMP levels were made using a BRET biosensor. We found that the maximum concentration of SCRAs tested (10 µmol L-1 ), increased cAMP levels 12%-45% above that produced by FSK alone, while the phytocannabinoid THC did not significantly alter cAMP levels in PTX-treated HEK-CB1 cells. All SCRAs had greater potency to inhibit FSK-induced cAMP levels than to stimulate cAMP levels. The rank order of potencies for SCRA stimulation of cAMP (Gαs ) was PB-22 > 5F-MDMB-PICA > JWH-018 ≈ AB-FUBINACA > XLR-11. By contrast, the potency of SCRAs for inhibition of cAMP (Gαi/o ) was 5F-MDMB-PICA > AB-FUBINACA > PB-22 > JWH-018 > XLR-11. The different rank order of potency and EMax  of the SCRAs to stimulate Gαs -like signaling compared to Gαi/o signaling suggests differences in G protein preference between SCRAs. Understanding the apparent differences among these drugs may contribute to unravelling their complex effects in humans.

Differential modulation of NMDA and AMPA receptors by cellular prion protein and copper ions

N-Methyl-D-aspartate receptors (NMDARs) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) are two major types of ionotropic glutamate receptors involved in synaptic transmission. However, excessive activity of these receptors can be cytotoxic and thus their function must be precisely controlled. We have previously reported that NMDA receptor activity is dysregulated following genetic knockout of cellular prion protein (PrP^C), and that PrP^C regulation of NMDA receptors is copper-dependent. Here, we employed electrophysiological methods to study NMDAR and AMPAR currents of cultured hippocampal neurons from PrP^C overexpresser mice. We show that NMDA receptor current amplitude and kinetics are differentially modulated by overexpression of human or mouse PrP^C. By contrast, AMPA receptor activity was unaffected. Nonetheless, AMPA receptor activity was modulated by copper ions in a manner similar to what we previously reported for NMDA receptors. Taken together, our findings reveal that AMPA and NMDA receptors are differentially regulated by PrP^C, but share common modulation by copper ions.

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.

Characterization of novel cannabinoid based T-type calcium channel blockers with analgesic effects

Low-voltage-activated (T-type) calcium channels are important regulators of the transmission of nociceptive information in the primary afferent pathway and finding ligands that modulate these channels is a key focus of the drug discovery field. Recently, we characterized a set of novel compounds with mixed cannabinoid receptor/T-type channel blocking activity and examined their analgesic effects in animal models of pain. Here, we have built on these previous findings and synthesized a new series of small organic compounds. We then screened them using whole-cell voltage clamp techniques to identify the most potent T-type calcium channel inhibitors. The two most potent blockers (compounds 9 and 10) were then characterized using radioligand binding assays to determine their affinity for CB1 and CB2 receptors. The structure–activity relationship and optimization studies have led to the discovery of a new T-type calcium channel blocker, compound 9. Compound 9 was efficacious in mediating analgesia in mouse models of acute inflammatory pain and in reducing tactile allodynia in the partial nerve ligation model. This compound was shown to be ineffective in Cav3.2 T-type calcium channel null mice at therapeutically relevant concentrations, and it caused no significant motor deficits in open field tests. Taken together, our data reveal a novel class of compounds whose physiological and therapeutic actions are mediated through block of Cav3.2 calcium channels.

The deubiquitinating enzyme USP5 modulates neuropathic and inflammatory pain by enhancing Cav3.2 channel activity

T-type calcium channels are essential contributors to the transmission of nociceptive signals in the primary afferent pain pathway. Here, we show that T-type calcium channels are ubiquitinated by WWP1, a plasma-membrane-associated ubiquitin ligase that binds to the intracellular domain III-IV linker region of the Cav3.2 T-type channel and modifies specific lysine residues in this region. A proteomic screen identified the deubiquitinating enzyme USP5 as a Cav3.2 III-IV linker interacting partner. Knockdown of USP5 via shRNA increases Cav3.2 ubiquitination, decreases Cav3.2 protein levels, and reduces Cav3.2 whole-cell currents. In vivo knockdown of USP5 or uncoupling USP5 from native Cav3.2 channels via intrathecal delivery of Tat peptides mediates analgesia in both inflammatory and neuropathic mouse models of mechanical hypersensitivity. Altogether, our experiments reveal a cell signaling pathway that regulates T-type channel activity and their role in nociceptive signaling.

Block of T-type calcium channels by protoxins I and II

Background

Low-voltage-activated (T-type) calcium channels play a crucial role in a number of physiological processes, including neuronal and cardiac pacemaker activity and nociception. Therefore, finding specific modulators and/or blockers of T-type channels has become an important field of drug discovery. One characteristic of T-type calcium channels is that they share several structural similarities with voltage-gated sodium channels (VGSCs). We therefore hypothesized that binding sites for certain sodium channel blocking peptide toxins may be present in T-type calcium channels.

Findings

The sodium channel blocker ProTx I tonically blocked native and transiently expressed T-type channels in the sub- to low micro molar range with at least a ten-fold selectivity for the T-type calcium channel hCav3.1 over hCav3.3, and more than one hundred fold selectivity over hCav3.2. Using chimeras of hCav3.1 and hCav3.3, we determined that the domain IV region of hCav3.1 is a major determinant of toxin affinity, with a minor contribution from domain II. Further analysis revealed several residues in a highly conserved region between T-type and sodium channels that may correspond to toxin binding sites. Mutagenesis of several of these residues on an individual basis, however, did not alter the blocking effects of the toxin. ProTx II on the other hand preferentially blocked hCav3.2 and significantly shifted the steady state inactivation of this channel.

Conclusions

ProTx I blocks hCav3.1 both selectively and with high affinity. Domain IV appears to play a major role in this selectivity with some contribution from domain II. Given the structural similarities between sodium and T-type calcium channels and the apparent conservation in toxin binding sites, these data could provide insights into the development and synthesis of novel T-type channel antagonists.

A novel slow-inactivation-specific ion channel modulator attenuates neuropathic pain

Voltage-gated ion channels are implicated in pain sensation and transmission signaling mechanisms within both peripheral nociceptors and the spinal cord. Genetic knockdown and knockout experiments have shown that specific channel isoforms, including Na(V)1.7 and Na(V)1.8 sodium channels and Ca(V)3.2 T-type calcium channels, play distinct pronociceptive roles. We have rationally designed and synthesized a novel small organic compound (Z123212) that modulates both recombinant and native sodium and calcium channel currents by selectively stabilizing channels in their slow-inactivated state. Slow inactivation of voltage-gated channels can function as a brake during periods of neuronal hyperexcitability, and Z123212 was found to reduce the excitability of both peripheral nociceptors and lamina I/II spinal cord neurons in a state-dependent manner. In vivo experiments demonstrate that oral administration of Z123212 is efficacious in reversing thermal hyperalgesia and tactile allodynia in the rat spinal nerve ligation model of neuropathic pain and also produces acute antinociception in the hot-plate test. At therapeutically relevant concentrations, Z123212 did not cause significant motor or cardiovascular adverse effects. Taken together, the state-dependent inhibition of sodium and calcium channels in both the peripheral and central pain signaling pathways may provide a synergistic mechanism toward the development of a novel class of pain therapeutics.

Extended spectrum of idiopathic generalized epilepsies associated withCACNA1Hfunctional variants

OBJECTIVE

The relationship between genetic variation in the T-type calcium channel gene CACNA1H and childhood absence epilepsy is well established. The purpose of this study was to investigate the range of epilepsy syndromes for which CACNA1H variants may contribute to the genetic susceptibility architecture and determine the electrophysiological effects of these variants in relation to proposed mechanisms underlying seizures.

METHODS

Exons 3 to 35 of CACNA1H were screened for variants in 240 epilepsy patients (167 unrelated) and 95 control subjects by single-stranded conformation analysis followed by direct sequencing. Cascade testing of families was done by sequencing or single-stranded conformation analysis. Selected variants were introduced into the CACNA1H protein by site-directed mutagenesis. Constructs were transiently transfected into human embryo kidney cells, and electrophysiological data were acquired.

RESULTS

More than 100 variants were detected, including 19 novel variants leading to amino acid changes in subjects with phenotypes including childhood absence, juvenile absence, juvenile myoclonic and myoclonic astatic epilepsies, as well as febrile seizures and temporal lobe epilepsy. Electrophysiological analysis of 11 variants showed that 9 altered channel properties, generally in ways that would be predicted to increase calcium current.

INTERPRETATION

Variants in CACNA1H that alter channel properties are present in patients with various generalized epilepsy syndromes. We propose that these variants contribute to an individual's susceptibility to epilepsy but are not sufficient to cause epilepsy on their own. The genetic architecture is dominated by rare functional variants; therefore, CACNA1H would not be easily identified as a susceptibility gene by a genome-wide case-control study seeking a statistical association.

Functional Analysis of Cav3.2 T-type Calcium Channel Mutations Linked to Childhood Absence Epilepsy

PURPOSE

Childhood absence epilepsy (CAE) is an idiopathic form of seizure disorder that is believed to have a genetic basis.

METHODS

We examined the biophysical consequences of seven mutations in the Ca(v)3.2 T-type calcium channel gene linked to CAE.

RESULTS

Of the channel variants examined, one of the mutants, a replacement of glycine 848 in the domain II-S2 region with serine, resulted in significant slowing of the time courses of both activation and inactivation across a wide range of membrane potentials. These changes are consistent with increased channel activity in response to prolonged membrane depolarizations.

CONCLUSIONS

Taken together, these findings suggest that such little changes in channel gating may contribute to the etiology of CAE.

Molecular basis of protein kinase C-induced activation of ATP-sensitive potassium channels

Potassium channels that are inhibited by internal ATP (K(ATP) channels) provide a critical link between metabolism and cellular excitability. Protein kinase C (PKC) acts on K(ATP) channels to regulate diverse cellular processes, including cardioprotection by ischemic preconditioning and pancreatic insulin secretion. PKC action decreases the Hill coefficient of ATP binding to cardiac K(ATP) channels, thereby increasing their open probability at physiological ATP concentrations. We show that PKC similarly regulates recombinant channels from both the pancreas and heart. Surprisingly, PKC acts via phosphorylation of a specific, conserved threonine residue (T180) in the pore-forming subunit (Kir6.2). Additional PKC consensus sites exist on both Kir and the larger sulfonylurea receptor (SUR) subunits. Nonetheless, T180 controls changes in open probability induced by direct PKC action either in the absence of, or in complex with, the accessory SUR1 (pancreatic) or SUR2A (cardiac) subunits. The high degree of conservation of this site among different K(ATP) channel isoforms suggests that this pathway may have wide significance for the physiological regulation of K(ATP) channels in various tissues and organelles.

Nitric oxide regulates cyclic GMP-dependent protein kinase phosphorylation in rat brain

Nitric oxide (NO) acts via soluble guanylyl cyclase to increase cyclic GMP (cGMP), which can regulate various targets including protein kinases. Western blotting showed that type II cGMP-dependent protein kinase (cGK II) is widely expressed in various brain regions, especially in the thalamus. In thalamic extracts, the phosphorylation of several proteins, including cGK II, was increased by exogenous NO or cGMP. In vivo pretreatment with a NO synthase inhibitor reduced the phosphorylation of cGK II, and this could be reversed by exogenous NO or cGMP. Conversely, brainstem electrical stimulation, which enhances thalamic NO release, caused a NO synthase-dependent increase in the phosphorylation of thalamic cGK II. These results indicate that endogenous NO regulates cGMP-dependent protein phosphorylation in the thalamus. The activation of cGKII by NO may play a role in thalamic mechanisms underlying arousal.

Autoradiographic localization of [3H]-cyclic GMP binding sites in the rat brain

Both the atriopeptides and nitric oxide act in the nervous system by activating guanylyl cyclases to stimulate the production of cyclic GMP. Thus a key to understanding the roles of these messengers is to understand the functions of cyclic GMP in the nervous system. Three potential targets for cyclic GMP have been identified, phosphodiesterases, protein kinases and ion channels. In this study we describe a method using autoradiography to localize specific [3H]-cGMP binding sites in the brain. The specific binding of [3H]-cGMP to rat brain sections was saturable (Bmax = 1.5 pmol/mg protein) and of high affinity (KD = 164 nM). The pharmacological characteristics were consistent with binding to the cGMP-dependent protein kinase. Highest densities of binding were seen in the medial habenula, basal ganglia, locus ceruleus and nucleus of the solitary tract. The CA1 pyramidal cells of the hippocampus, the neocortex, thalamus and cerebellum were also labelled. This method should prove useful in studies of potential targets for cyclic GMP in the brain.

Expression of the olfactory cyclic nucleotide gated channel (CNG1) in the rat brain

The expression of the olfactory cyclic nucleotide-gated channel (CNG1) was studied in the rat brain. Using RT-PCR, levels of CNG1 mRNA were determined relative to the expression of a constantly expressed gene, alpha-tubulin. RT-PCR showed that CNG1 mRNA was detectable in the pituitary gland, the olfactory bulb, and the cerebellum of adult and 5-day-old rats. A 3.4 kb mRNA was detected in the olfactory bulb by Northern blotting. In situ hybridization analysis showed that CNG1 mRNA expression is present in the olfactory bulb and in the Purkinje cells of the cerebellum. RT-PCR studies on Purkinje cell-enriched cultures obtained from the cerebellum of 16-day-old embryos (E16) confirmed the expression of CNG1 mRNA in these neurones. Our results show that CNG1 is not restricted to the olfactory epithelium but is also present in specific regions of the brain. These results suggest that cyclic nucleotides may act in the regions that possess CNG1 gene expression to affect the electrical activity of certain neurones directly.

Molecular characterization of a type II cyclic GMP-dependent protein kinase expressed in the rat brain

We applied reverse transcription-PCR to examine the gene expression of cyclic GMP (cGMP)-dependent protein kinase in the rat brain. A PCR product with the size predicted from the type II cGMP-dependent protein kinase (cGK II) cDNA was detected in various regions of the brain, with highest expression in the thalamus. The amplified product of this cDNA was subcloned, sequenced, and consequently shown to be cGK II. Northern analysis confirmed that this kinase was highly expressed in the thalamus. In situ hybridization with riboprobes derived from this cDNA indicated that cGK II mRNA was highly expressed in the outer layers of the cortex, the septum, amygdala, and olfactory bulb with highest levels in the thalamus. High amounts of cGK II mRNA were also found in specific brainstem loci, including the medial habenula, the subthalamic nucleus, the locus ceruleus, the pontine nucleus, the inferior olivary nuclei, and the nucleus of the solitary tract. Only low levels of cGK II mRNA were detected in the striatum, cerebellum, and hippocampus. These data suggest that the effects of guanylyl cyclase activators, such as nitric oxide and the atriopeptides, in various regions of the CNS may be mediated through cGK II.

Characterization and localization of [3H] cyclosporin A binding sites in rat brain

The immunosuppressant drug [3H]cyclosporin A binds specifically and with high affinity to rat brain membrane preparations. The highest density of binding sites was observed in the hippocampus, cerebellum, cortex and basal ganglia. A similar distribution pattern was seen using a quantitative autoradiographic analysis. This distribution agrees with the localizations of cyclophilin and calcineurin reported in immunohistochemical and in situ hybridization studies. Thus inhibition of calcineurin activity following cyclosporin A binding to cyclophilin may occur in neurones, as it does in T-cells. These results suggest that the neurological side-effects of cyclosporin A may be mediated through its interaction with these proteins in neurones.

Low-affinity binding sites for 1,4-dihydropyridines in skeletal muscle transverse tubule membranes revealed by changes in the fluorescence of felodipine

The fluorescence changes accompanying the binding of the fluorescent calcium channel antagonist, felodipine, to transverse tubule membranes from rabbit skeletal muscle have been used to characterize low-affinity binding sites for 1,4-dihydropyridine derivatives in these preparations. In competition experiments, felodipine inhibited the high-affinity binding of (+)-[3H]PN200-110 to transverse tubule membranes with an apparent Ki of 5 +/- 2 nM. Binding of felodipine to additional low-affinity sites resulted in a large, saturable (Kd = 6 +/- 2 microM) increase in its fluorescence which could be excited either directly (380 nm) or indirectly via energy transfer from membrane protein (290 nm). The observed fluorescence enhancement was competitively inhibited by other 1,4-dihydropyridines with inhibition constants of 3-21 microM but was unaffected by the structurally unrelated calcium channel antagonists, diltiazem and verapamil, or by Ca2+, Cd2+, and La3+. Both high- and low-affinity binding sites appear to be localized in the transverse tubular system, since the magnitude of the observed fluorescence enhancement was higher in these membranes than in microsomal preparations and was directly proportional to the density of high-affinity sites for (+)-[3H]PN200-110. Furthermore, both high- and low-affinity sites appear to be conformationally coupled since, over the same concentration range that the fluorescence changes were observed, felodipine accelerated the rate of dissociation of [3H]PN200-110 previously bound to its high-affinity sites. Similar behavior has previously been reported for other 1,4-dihydropyridines [Dunn, S. M. J., & Bladen, C. (1991) Biochemistry 30, 5716-5721].(ABSTRACT TRUNCATED AT 250 WORDS)

Kinetics of binding of dihydropyridine calcium channel ligands to skeletal muscle membranes: evidence for low-affinity sites and for the involvement of G proteins

Detailed kinetic studies of the binding of the calcium channel antagonist (+)-[3H]PN200-110 to membrane preparations from rabbit skeletal muscle have demonstrated that, in addition to the high-affinity sites (Kd = 0.30 +/- 0.05 nM) that are readily measured in equilibrium and kinetic experiments, there are also dihydropyridine binding sites with much lower affinities. These sites were detected by the ability of micromolar concentrations of several dihydropyridines to accelerate the rate of dissociation of (+)-[3H]-PN200-110 from its high-affinity sites. The observed increase in rate was dependent on the concentration of competing ligand, and half-maximal effects occurred at approximately 10 microM for the agonist (+/-)-Bay K8644 and for the antagonists nifedipine, (+/-)-nitrendipine, and (+)-PN200-110. The low-affinity sites appear to be stereospecific since (-)-PN200-110 (1-200 microM) did not affect the dissociation rate. The possible involvement of guanine nucleotide binding proteins in dihydropyridine binding has been investigated by studying the effects of guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) and guanosine 5'-O-(2-thiodiphosphate) (GDP beta S) on binding parameters. At a concentration of 10 microM, neither GTP gamma S nor GDP beta S significantly affected the binding of dihydropyridines to their high-affinity sites. GTP gamma S did, however, increase the ability of (+/-)-Bay K8644, but not of (+/-)-nitrendipine, to accelerate the rate of dissociation of tightly bound (+)-[3H]PN200-110. GDP beta S did not affect the dose dependence of either the agonist or the antagonist.(ABSTRACT TRUNCATED AT 250 WORDS)

Hypertension and the Department of Health and Human Services

Hypertension affects from 17% to 25% of all Americans. Because of its fundamental charge to help protect the public health, the Department of Health and Human Services (HHS) is substantially concerned with the condition. Additionally, hypertension represents a significant source of underwriting risk to which HHS is exposed in its role as health insurer of the poor and the elderly. HHS has invested hundreds of millions of dollars in hypertension research, development and testing of treatment regimens, and education of health care providers and consumers. However, much of the etiology of hypertension still eludes us. Sodium's apparent importance as a "causal" agent to the development of hypertension, and in its treatment, has waxed and waned over the past several generations; research to date has not yet finally settled the issue. Is sodium or some other cation the key? While research on this issue continues, HHS is currently faced with deciding whether and how to require inclusion of sodium content in nutrition labelling. In the debate, attention has to be given not only to the current best evidence on sodium; additional issues of consumer choice, costs, and education are also of importance.