Pharmacological characterization of recombinant N-type calcium channel (Cav2.2) mediated calcium mobilization using FLIPR

Pronociceptive role of spinal Cav2.3 (R-type) calcium channels in a mouse model of postoperative pain

BACKGROUND

More than 80% of patients may experience acute pain after a surgical procedure, and this is often refractory to pharmacological intervention. The identification of new targets to treat postoperative pain is necessary. There is an association of polymorphisms in the Cav2.3 gene with postoperative pain and opioid consumption. Our study aimed to identify Cav2.3 as a potential target to treat postoperative pain and to reduce opioid-related side effects.

EXPERIMENTAL APPROACH

A plantar incision model was established in adult male and female C57BL/6 mice. Cav2.3 expression was detected by qPCR and suppressed by siRNA treatment. The antinociceptive efficacy and safety of a Cav2.3 blocker-alone or together with morphine-was also assessed after surgery.

KEY RESULTS

Paw incision in female and male mice caused acute nociception and increased Cav2.3 mRNA expression in the spinal cord but not in the incised tissue. Intrathecal treatment with siRNA against Cav2.3, but not with a scrambled siRNA, prevented the development of surgery-induced nociception in both male and female mice, with female mice experiencing long-lasting effects. High doses of i.t. SNX-482, a Cav2.3 channel blocker, or morphine injected alone, reversed postoperative nociception but also induced side effects. A combination of lower doses of morphine and SNX-482 mediated a long-lasting reversal of postsurgical pain in female and male mice.

CONCLUSION

Our results demonstrate that Cav2.3 has a pronociceptive role in the induction of postoperative pain, indicating that it is a potential target for the development of therapeutic approaches for the treatment of postoperative pain.

Inhibition of N-type calcium ion channels by tricyclic antidepressants - experimental and theoretical justification for their use for neuropathic pain

A number of tricyclic antidepressants (TCAs) are commonly prescribed off-label for the treatment of neuropathic pain. The blockade of neuronal calcium ion channels is often invoked to partially explain the analgesic activity of TCAs, but there has been very limited experimental or theoretical evidence reported to support this assertion. The N-type calcium ion channel (CaV2.2) is a well-established target for the treatment of neuropathic pain and in this study a series of eleven TCAs and two closely related drugs were shown to be moderately effective inhibitors of this channel when endogenously expressed in the SH-SY5Y neuroblastoma cell line. A homology model of the channel, which matches closely a recently reported Cryo-EM structure, was used to investigate via docking and molecular dynamics experiments the possible mode of inhibition of CaV2.2 channels by TCAs. Two closely related binding modes, that occur in the channel cavity that exists between the selectivity filter and the internal gate, were identified. The TCAs are predicted to position themselves such that their ammonium side chains interfere with the selectivity filter, with some, such as amitriptyline, also appearing to hinder the channel's ability to open. This study provides the most comprehensive evidence to date that supports the notion that the blockade of neuronal calcium ion channels by TCAs is at least partially responsible for their analgesic effect.

Structure of human Cav2.2 channel blocked by the painkiller ziconotide

The neuronal-type (N-type) voltage-gated calcium (Ca_v) channels, which are designated Ca_v2.2, have an important role in the release of neurotransmitters^1-3. Ziconotide is a Ca_v2.2-specific peptide pore blocker that has been clinically used for treating intractable pain^4-6. Here we present cryo-electron microscopy structures of human Ca_v2.2 (comprising the core α1 and the ancillary α2δ-1 and β3 subunits) in the presence or absence of ziconotide. Ziconotide is thoroughly coordinated by helices P1 and P2, which support the selectivity filter, and the extracellular loops (ECLs) in repeats II, III and IV of α1. To accommodate ziconotide, the ECL of repeat III and α2δ-1 have to tilt upward concertedly. Three of the voltage-sensing domains (VSDs) are in a depolarized state, whereas the VSD of repeat II exhibits a down conformation that is stabilized by Ca_v2-unique intracellular segments and a phosphatidylinositol 4,5-bisphosphate molecule. Our studies reveal the molecular basis for Ca_v2.2-specific pore blocking by ziconotide and establish the framework for investigating electromechanical coupling in Ca_v channels.

Discovery of a Potent, Selective T-type Calcium Channel Blocker as a Drug Candidate for the Treatment of Generalized Epilepsies

We report here the discovery and pharmacological characterization of N-(1-benzyl-1H-pyrazol-3-yl)-2-phenylacetamide derivatives as potent, selective, brain-penetrating T-type calcium channel blockers. Optimization focused mainly on solubility, brain penetration, and the search for an aminopyrazole metabolite that would be negative in an Ames test. This resulted in the preparation and complete characterization of compound 66b (ACT-709478), which has been selected as a clinical candidate.

Optimization of a High-Throughput Assay for Calcium Channel Modulators on IonWorks Barracuda

Voltage-gated calcium channels represent important drug targets. The implementation of higher throughput electrophysiology assays is necessary to characterize the interaction of test compounds with several conformational states of the channel, but has presented significant challenges. We report on the development of a high-throughput, automated electrophysiology assay for Cav2.2 on the IonWorks Barracuda™ platform. The assay provides an assessment of the potency of the test compound on the resting/closed and inactivated states of the channel in the same assay run. Inclusion of the heavy metal chelator 2,3-bis(sulfanyl)propane-1-sulfonate in the assay solutions improved the data quality by reversing a loss of current seen in wells directly above the ground electrodes. We hypothesize that the loss of current is caused by block of Cav2.2 currents by silver ions originating from the electrodes.

The Physiology, Pathology, and Pharmacology of Voltage-Gated Calcium Channels and Their Future Therapeutic Potential

Voltage-gated calcium channels are required for many key functions in the body. In this review, the different subtypes of voltage-gated calcium channels are described and their physiologic roles and pharmacology are outlined. We describe the current uses of drugs interacting with the different calcium channel subtypes and subunits, as well as specific areas in which there is strong potential for future drug development. Current therapeutic agents include drugs targeting L-type Ca(V)1.2 calcium channels, particularly 1,4-dihydropyridines, which are widely used in the treatment of hypertension. T-type (Ca(V)3) channels are a target of ethosuximide, widely used in absence epilepsy. The auxiliary subunit α2δ-1 is the therapeutic target of the gabapentinoid drugs, which are of value in certain epilepsies and chronic neuropathic pain. The limited use of intrathecal ziconotide, a peptide blocker of N-type (Ca(V)2.2) calcium channels, as a treatment of intractable pain, gives an indication that these channels represent excellent drug targets for various pain conditions. We describe how selectivity for different subtypes of calcium channels (e.g., Ca(V)1.2 and Ca(V)1.3 L-type channels) may be achieved in the future by exploiting differences between channel isoforms in terms of sequence and biophysical properties, variation in splicing in different target tissues, and differences in the properties of the target tissues themselves in terms of membrane potential or firing frequency. Thus, use-dependent blockers of the different isoforms could selectively block calcium channels in particular pathologies, such as nociceptive neurons in pain states or in epileptic brain circuits. Of important future potential are selective Ca(V)1.3 blockers for neuropsychiatric diseases, neuroprotection in Parkinson's disease, and resistant hypertension. In addition, selective or nonselective T-type channel blockers are considered potential therapeutic targets in epilepsy, pain, obesity, sleep, and anxiety. Use-dependent N-type calcium channel blockers are likely to be of therapeutic use in chronic pain conditions. Thus, more selective calcium channel blockers hold promise for therapeutic intervention.

Expression and pharmacology of endogenous Cav channels in SH-SY5Y human neuroblastoma cells

SH-SY5Y human neuroblastoma cells provide a useful in vitro model to study the mechanisms underlying neurotransmission and nociception. These cells are derived from human sympathetic neuronal tissue and thus, express a number of the Ca_v channel subtypes essential for regulation of important physiological functions, such as heart contraction and nociception, including the clinically validated pain target Ca_v2.2. We have detected mRNA transcripts for a range of endogenous expressed subtypes Ca_v1.3, Ca_v2.2 (including two Ca_v1.3, and three Ca_v2.2 splice variant isoforms) and Ca_v3.1 in SH-SY5Y cells; as well as Ca_v auxiliary subunits α₂δ_1–3, β₁, β₃, β₄, γ₁, γ_4–5, and γ₇. Both high- and low-voltage activated Ca_v channels generated calcium signals in SH-SY5Y cells. Pharmacological characterisation using ω-conotoxins CVID and MVIIA revealed significantly (∼ 10-fold) higher affinity at human versus rat Ca_v2.2, while GVIA, which interacts with Ca_v2.2 through a distinct pharmacophore had similar affinity for both species. CVID, GVIA and MVIIA affinity was higher for SH-SY5Y membranes vs whole cells in the binding assays and functional assays, suggesting auxiliary subunits expressed endogenously in native systems can strongly influence Ca_v2.2 channels pharmacology. These results may have implications for strategies used to identify therapeutic leads at Ca_v2.2 channels.

Venom peptides as a rich source of cav2.2 channel blockers

Ca_v2.2 is a calcium channel subtype localized at nerve terminals, including nociceptive fibers, where it initiates neurotransmitter release. Ca_v2.2 is an important contributor to synaptic transmission in ascending pain pathways, and is up-regulated in the spinal cord in chronic pain states along with the auxiliary α2δ1 subunit. It is therefore not surprising that toxins that inhibit Ca_v2.2 are analgesic. Venomous animals, such as cone snails, spiders, snakes, assassin bugs, centipedes and scorpions are rich sources of remarkably potent and selective Ca_v2.2 inhibitors. However, side effects in humans currently limit their clinical use. Here we review Ca_v2.2 inhibitors from venoms and their potential as drug leads.

An automated electrophysiological assay for differentiating Ca(v)2.2 inhibitors based on state dependence and kinetics

Ca(V)2.2 (N-type) calcium channels are key regulators of neurotransmission. Evidence from knockout animals and localization studies suggest that Ca(V)2.2 channels play a critical role in nociceptive transmission. Additionally, ziconotide, a selective peptide inhibitor of Ca(V)2.2 channels, is clinically used to treat refractory pain. However, the use of ziconotide is limited by its low therapeutic index, which is believed, at least in part, to be a consequence of ziconotide inhibiting Ca(V)2.2 channels regardless of the channel state. Subsequent efforts have focused on the discovery of state-dependent inhibitors that preferentially bind to the inactivated state of Ca(V)2.2 channels in order to achieve an improved safety profile relative to ziconotide. Much less attention has been paid to understanding the binding kinetics of these state-dependent inhibitors. Here, we describe a novel electrophysiology-based assay on an automated patch platform designed to differentiate Ca(V)2.2 inhibitors based on their combined state dependence and kinetics. More specifically, this assay assesses inactivated state block, closed state block, and monitors the kinetics of recovery from block when channels move between states. Additionally, a use-dependent assay is described that uses a train of depolarizing pulses to drive channels to a similar level of inactivation for comparison. This use-dependent protocol also provides information on the kinetics of block development. Data are provided to show how these assays can be utilized to screen for kinetic diversity within and across chemical classes.

Development and optimization of FLIPR high throughput calcium assays for ion channels and GPCRs

Ca(2+) permeable ion channels and GPCRs linked to Ca(2+) release are important drug targets, with modulation of Ca(2+) signaling increasingly recognized as a valid therapeutic strategy in a range of diseases. The FLIPR is a high throughput imaging plate reader that has contributed substantially to drug discovery efforts and pharmacological characterization of receptors and ion channels coupled to Ca(2+). Now in its fourth generation, the FLIPR(TETRA) is an industry standard for high throughput Ca(2+) assays. With an increasing number of excitation LED banks and emission filter sets available; FLIPR Ca(2+) assays are becoming more versatile. This chapter describes general methods for establishing robust FLIPR Ca(2+) assays, incorporating practical aspects as well as suggestions for assay optimization, to guide the reader in the development and optimization of high throughput FLIPR assays for ion channels and GPCRs.

An integrated multiassay approach to the discovery of small-molecule N-type voltage-gated calcium channel antagonists

Abstract The N-type voltage-gated calcium channel (Cav2.2) has been intensively explored as a target for novel, small-molecule analgesic drugs because of its distribution in the pain pathway and its role in nociceptive processing. For example, Cav2.2 is localized at presynaptic terminals of pain fibers in the dorsal horn, and it serves as a downstream effector of μ-opioid receptors. Most importantly, antagonism of the channel by the highly specific and potent Cav2.2 blocker ω-conotoxin MVIIA (ziconotide) produces clinical efficacy in the treatment of severe, intractable pain. To identify novel small-molecule Cav2.2 inhibitors, we developed new tools and screening methods critical to enhance the efficiency and probability of success. First, we established and characterized a new cell line stably expressing the three subunits of the Cav2.2, including an α-subunit splice variant that is uniquely expressed by dorsal root ganglion neurons. Second, using this cell line, we validated and employed a fluorescence-based calcium flux assay. Third, we developed a new "medium-throughput" electrophysiology assay using QPatch-HT to provide faster turnaround on high-content electrophysiology data that are critical for studying ion channel targets. Lastly, we used a therapeutically relevant, ex vivo spinal cord calcitonin gene-related peptide-release assay to confirm activities in the other assays. Using this approach we have identified compounds exhibiting single-digit nM IC₅₀ values and with a positive correlation across assay methods. This integrated approach provides a more comprehensive evaluation of small-molecule N-type inhibitors that may lead to improved therapeutic pharmacology.

Acid-evoked Ca2+ signalling in rat sensory neurones: effects of anoxia and aglycaemia

Ischaemia excites sensory neurones (generating pain) and promotes calcitonin gene-related peptide release from nerve endings. Acidosis is thought to play a key role in mediating excitation via the activation of proton-sensitive cation channels. In this study, we investigated the effects of acidosis upon Ca²⁺ signalling in sensory neurones from rat dorsal root ganglia. Both hypercapnic (pH_o 6.8) and metabolic–hypercapnic (pH_o 6.2) acidosis caused a biphasic increase in cytosolic calcium concentration ([Ca²⁺]_i). This comprised a brief Ca²⁺ transient (half-time approximately 30 s) caused by Ca²⁺ influx followed by a sustained rise in [Ca²⁺]_i due to Ca²⁺ release from caffeine and cyclopiazonic acid-sensitive internal stores. Acid-evoked Ca²⁺ influx was unaffected by voltage-gated Ca²⁺-channel inhibition with nickel and acid sensing ion channel (ASIC) inhibition with amiloride but was blocked by inhibition of transient receptor potential vanilloid receptors (TRPV1) with (E)-3-(4-t-butylphenyl)-N-(2,3-dihydrobenzo[b][1,4] dioxin-6-yl)acrylamide (AMG 9810; 1 μM) and N-(4-tertiarybutylphenyl)-4-(3-cholorphyridin-2-yl) tetrahydropryazine-1(2H)-carbox-amide (BCTC; 1 μM). Combining acidosis with anoxia and aglycaemia increased the amplitude of both phases of Ca²⁺ elevation and prolonged the Ca²⁺ transient. The Ca²⁺ transient evoked by combined acidosis, aglycaemia and anoxia was also substantially blocked by AMG 9810 and BCTC and, to a lesser extent, by amiloride. In summary, the principle mechanisms mediating increase in [Ca²⁺]_i in response to acidosis are a brief Ca²⁺ influx through TRPV1 followed by sustained Ca²⁺ release from internal stores. These effects are potentiated by anoxia and aglycaemia, conditions also prevalent in ischaemia. The effects of anoxia and aglycaemia are suggested to be largely due to the inhibition of Ca²⁺-clearance mechanisms and possible increase in the role of ASICs.

TRPM8, a versatile channel in human sperm

Background

The transient receptor potential channel (TRP) family includes more than 30 proteins; they participate in various Ca²⁺ dependent processes. TRPs are functionally diverse involving thermal, chemical and mechanical transducers which modulate the concentration of intracellular Ca²⁺ ([Ca²⁺]i). Ca²⁺ triggers and/or regulates principal sperm functions during fertilization such as motility, capacitation and the acrosome reaction. Nevertheless, the presence of the TRPM subfamily in sperm has not been explored.

Principal Findings

Here we document with RT-PCR, western blot and immunocitochemistry analysis the presence of TRPM8 in human sperm. We also examined the participation of this channel in sperm function using specific agonists (menthol and temperature) and antagonists (BCTC and capsazepine). Computer-aided sperm analysis revealed that menthol did not significantly alter human sperm motility. In contrast, menthol induced the acrosome reaction in human sperm. This induction was inhibited about 70% by capsazepine (20 µM) and 80% by BCTC (1.6 µM). Activation of TRPM8 either by temperature or menthol induced [Ca²⁺]i increases in human sperm measured by fluorescence in populations or individual sperm cells, effect that was also inhibited by capsazepine (20 µM) and BCTC (1.6 µM). However, the progesterone and ZP3-induced acrosome reaction was not inhibited by capsazepine or BCTC, suggesting that TRPM8 activation triggers this process by a different signaling pathway.

Conclusions

This is the first report dealing with the presence of a thermo sensitive channel (TRPM8) in human sperm. This channel could be involved in cell signaling events such as thermotaxis or chemotaxis.

Antidepressants inhibit P2X4 receptor function: a possible involvement in neuropathic pain relief

BACKGROUND

Neuropathic pain is characterized by pain hypersensitivity to innocuous stimuli (tactile allodynia) that is nearly always resistant to known treatments such as non-steroidal anti-inflammatory drugs or even opioids. It has been reported that some antidepressants are effective for treating neuropathic pain. However, the underlying molecular mechanisms are not well understood. We have recently demonstrated that blocking P2X4 receptors in the spinal cord reverses tactile allodynia after peripheral nerve injury in rats, implying that P2X4 receptors are a key molecule in neuropathic pain. We investigated a possible role of antidepressants as inhibitors of P2X4 receptors and analysed their analgesic mechanism using an animal model of neuropathic pain.

RESULTS

Antidepressants strongly inhibited ATP-mediated Ca2+ responses in P2X4 receptor-expressing 1321N1 cells, which are known to have no endogenous ATP receptors. Paroxetine exhibited the most powerful inhibition of calcium influx via rat and human P2X4 receptors, with IC50 values of 2.45 microM and 1.87 microM, respectively. Intrathecal administration of paroxetine produced a striking antiallodynic effect in an animal model of neuropathic pain. Co-administration of WAY100635, ketanserin or ondansetron with paroxetine induced no significant change in the antiallodynic effect of paroxetine. Furthermore, the antiallodynic effect of paroxetine was observed even in rats that had received intrathecal pretreatment with 5,7-dihydroxytryptamine, which dramatically depletes spinal 5-hydroxytryptamine.

CONCLUSION

These results suggest that paroxetine acts as a potent analgesic in the spinal cord via a mechanism independent of its inhibitory effect on serotonin transporters. Powerful inhibition on P2X4 receptors may underlie the analgesic effect of paroxetine, and it is possible that some antidepressants clinically used in patients with neuropathic pain show antiallodynic effects, at least in part via their inhibitory effects on P2X4 receptors.