Calcicludine binding to the outer pore of L-type calcium channels is allosterically coupled to dihydropyridine binding

Cav 1.3-selective inhibitors of voltage-gated L-type Ca2+ channels: Fact or (still) fiction?

Voltage-gated L-type Ca2+ -channels (LTCCs) are the target of Ca2+ -channel blockers (CCBs), which are in clinical use for the evidence-based treatment of hypertension and angina. Their cardiovascular effects are largely mediated by the Cav 1.2-subtype. However, based on our current understanding of their physiological and pathophysiological roles, Cav 1.3 LTCCs also appear as attractive drug targets for the therapy of various diseases, including treatment-resistant hypertension, spasticity after spinal cord injury and neuroprotection in Parkinson's disease. Since CCBs inhibit both Cav 1.2 and Cav 1.3, Cav 1.3-selective inhibitors would be valuable tools to validate the therapeutic potential of Cav 1.3 channel inhibition in preclinical models. Despite a number of publications reporting the discovery of Cav 1.3-selective blockers, their selectivity remains controversial. We conclude that at present no pharmacological tools exist that are suitable to confirm or refute a role of Cav 1.3 channels in cellular responses. We also suggest essential criteria for a small molecule to be considered Cav 1.3-selective.

Cation Permeability of Voltage-Gated Hair Cell Ca2+ Channels of the Vertebrate Labyrinth

Some hearing, vestibular, and vision disorders are imputable to voltage-gated Ca²⁺ channels of the sensory cells. These channels convey a large Ca²⁺ influx despite extracellular Na⁺ being 70-fold more concentrated than Ca²⁺; such high selectivity is lost in low Ca²⁺, and Na⁺ can permeate. Since the permeation properties and molecular identity of sensory Ca²⁺ channels are debated, in this paper, we examine the Na⁺ current flowing through the L- and R-type Ca²⁺ channels of labyrinth hair cells. Ion currents and cytosolic free Ca²⁺ concentrations were simultaneously monitored in whole-cell recording synchronous to fast fluorescence imaging. L-type and R-type channels were present with different densities at selected sites. In 10 nM Ca²⁺, the activation and deactivation time constants of the L-type Na⁺ current were accelerated and its maximal amplitude increased by 6-fold compared to physiological Ca²⁺. The deactivation of the R-type Na⁺ current was not accelerated, and its current amplitude increased by 2.3-fold in low Ca²⁺; moreover, it was partially blocked by nifedipine in a voltage- and time-dependent manner. In conclusion, L channel gating is affected by the ion species permeating the channel, and its selectivity filter binds Ca²⁺ more strongly than that of R channel; furthermore, external Ca²⁺ prevents nifedipine from perturbing the R selectivity filter.

Molecular Determinants of the Differential Modulation of Cav1.2 and Cav1.3 by Nifedipine and FPL 64176

Nifedipine and FPL 64176 (FPL), which block and potentiate L-type voltage-gated Ca2+ channels, respectively, modulate Cav1.2 more potently than Cav1.3. To identify potential strategies for developing subtype-selective inhibitors, we investigated the role of divergent amino acid residues in transmembrane domains IIIS5 and the extracellular IIIS5-3P loop region in modulation of these channels by nifedipine and FPL. Insertion of the extracellular IIIS5-3P loop from Cav1.2 into Cav1.3 (Cav1.3+) reduced the IC50 of nifedipine from 289 to 101 nM, and substitution of S1100 with an A residue, as in Cav1.2, accounted for this difference. Substituting M1030 in IIIS5 to V in Cav1.3+ (Cav1.3+V) further reduced the IC50 of nifedipine to 42 nM. FPL increased current amplitude with an EC50 of 854 nM in Cav1.3, 103 nM in Cav1.2, and 99 nM in Cav1.3+V. In contrast to nifedipine block, substitution of M1030 to V in Cav1.3 had no effect on potency of FPL potentiation of current amplitude, but slowed deactivation in the presence and absence of 10 μM FPL. FPL had no effect on deactivation of Cav1.3/dihydropyridine-insensitive (DHPi), a channel with very low sensitivity to nifedipine block (IC50 ∼93 μM), but did shift the voltage-dependence of activation by ∼-10 mV. We conclude that the M/V variation in IIIS5 and the S/A variation in the IIIS5-3P loop of Cav1.2 and Cav1.3 largely determine the difference in nifedipine potency between these two channels, but the difference in FPL potency is determined by divergent amino acids in the IIIS5-3P loop.

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.'

The ionic mechanism of membrane potential oscillations and membrane resonance in striatal LTS interneurons

Striatal low-threshold spiking (LTS) interneurons spontaneously transition to a depolarized, oscillating state similar to that seen after sodium channels are blocked. In the depolarized state, whether spontaneous or induced by sodium channel blockade, the neurons express a 3- to 7-Hz oscillation and membrane impedance resonance in the same frequency range. The membrane potential oscillation and membrane resonance are expressed in the same voltage range (greater than -40 mV). We identified and recorded from LTS interneurons in striatal slices from a mouse that expressed green fluorescent protein under the control of the neuropeptide Y promoter. The membrane potential oscillation depended on voltage-gated calcium channels. Antagonism of L-type calcium currents (Ca_V1) reduced the amplitude of the oscillation, whereas blockade of N-type calcium currents (Ca_V2.2) reduced the frequency. Both calcium sources activate a calcium-activated chloride current (CaCC), the blockade of which abolished the oscillation. The blocking of any of these three channels abolished the membrane resonance. Immunohistochemical staining indicated anoctamin 2 (ANO2), and not ANO1, as the CaCC source. Biophysical modeling showed that Ca_V1, Ca_V2.2, and ANO2 are sufficient to generate a membrane potential oscillation and membrane resonance, similar to that in LTS interneurons. LTS interneurons exhibit a membrane potential oscillation and membrane resonance that are both generated by Ca_V1 and Ca_V2.2 activating ANO2. They can spontaneously enter a state in which the membrane potential oscillation dominates the physiological properties of the neuron.

Fasxiator, a novel factor XIa inhibitor from snake venom, and its site-specific mutagenesis to improve potency and selectivity

BACKGROUND

Bleeding remains a major limitation of standard anticoagulant drugs that target the extrinsic and common coagulation pathways. Recently, intrinsic coagulation factors are increasingly being investigated as alternative targets for developing anticoagulant drugs with lower bleeding risk.

OBJECTIVES

Goals were to (i) identify novel anticoagulants selectively targeting intrinsic coagulation pathway and (ii) characterize and further improve the properties of the identified anticoagulants.

METHODS AND RESULTS

We have isolated and sequenced a specific factor XIa (FXIa) inhibitor, henceforth named Fasxiator, from the venom of the banded krait snake, Bungarus fasciatus. It is a Kunitz-type protease inhibitor that prolonged activated partial thromboplastin time without significant effects on prothrombin time. Fasxiator was recombinantly expressed (rFasxiator), purified, and characterized to be a slow-type inhibitor of FXIa that exerts its anticoagulant activities (doubled activated partial thromboplastin time at ~ 3 μmol L(-1) ) by selectively inhibiting human FXIa in in vitro assays. A series of mutants were subsequently generated to improve the potency and selectivity of recombinant rFasxiator. rFasxiatorN17R,L19E showed the best balance between potency (IC50 ~ 1 nmol L(-1) ) and selectivity (> 100 times). rFasxiatorN17R,L19E is a competitive slow-type inhibitor of FXIa (Ki  = 0.86 nmol L(-1) ), possesses anticoagulant activity that is ~ 10 times stronger in human plasma than in murine plasma, and prolonged the occlusion time of mice carotid artery in FeCl3 -induced thrombosis models.

CONCLUSION

We have isolated an exogenous FXIa specific inhibitor, engineered it to improve its potency by ~ 1000 times and demonstrated its in vitro and in vivo efficacy. These proof-of-principle data supported the further development of Fasxiator as a novel anticoagulant candidate.

Use of venom peptides to probe ion channel structure and function

Venoms of snakes, scorpions, spiders, insects, sea anemones, and cone snails are complex mixtures of mostly peptides and small proteins that have evolved for prey capture and/or defense. These deadly animals have long fascinated scientists and the public. Early studies isolated lethal components in the search for cures and understanding of their mechanisms of action. Ion channels have emerged as targets for many venom peptides, providing researchers highly selective and potent molecular probes that have proved invaluable in unraveling ion channel structure and function. This minireview highlights molecular details of their toxin-receptor interactions and opportunities for development of peptide therapeutics.

A single amino acid change in Ca(v)1.2 channels eliminates the permeation and gating differences between Ca(2+) and Ba(2+)

Glutamate scanning mutagenesis was used to assess the role of the calcicludine binding segment in regulating channel permeation and gating using both Ca(2+) and Ba(2+) as charge carriers. As expected, wild-type Ca(V)1.2 channels had a Ba(2+) conductance ~2x that in Ca(2+) (G(Ba)/G(Ca) = 2) and activation was ~10 mV more positive in Ca(2+) vs. Ba(2+). Of the 11 mutants tested, F1126E was the only one that showed unique permeation and gating properties compared to the wild type. F1126E equalized the Ca(V)1.2 channel conductance (G(Ba)/G(Ca) = 1) and activation voltage dependence between Ca(2+) and Ba(2+). Ba(2+) permeation was reduced because the interactions among multiple Ba(2+) ions and the pore were specifically altered for F1126E, which resulted in Ca(2+)-like ionic conductance and unitary current. However, the high-affinity block of monovalent cation flux was not altered for either Ca(2+) or Ba(2+). The half-activation voltage of F1126E in Ba(2+) was depolarized to match that in Ca(2+), which was unchanged from that in the wild type. As a result, the voltages for half-activation and half-inactivation of F1126E in Ba(2+) and Ca(2+) were similar to those of wild-type in Ca(2+). This effect was specific to F1126E since F1126A did not affect the half-activation voltage in either Ca(2+) or Ba(2+). These results indicate that residues in the outer vestibule of the Ca(V)1.2 channel pore are major determinants of channel gating, selectivity, and permeation.

A genetic screen for dihydropyridine (DHP)-resistant worms reveals new residues required for DHP-blockage of mammalian calcium channels

Dihydropyridines (DHPs) are L-type calcium channel (Ca_v1) blockers prescribed to treat several diseases including hypertension. Ca_v1 channels normally exist in three states: a resting closed state, an open state that is triggered by membrane depolarization, followed by a non-conducting inactivated state that is triggered by the influx of calcium ions, and a rapid change in voltage. DHP binding is thought to alter the conformation of the channel, possibly by engaging a mechanism similar to voltage dependent inactivation, and locking a calcium ion in the pore, thereby blocking channel conductance. As a Ca_v1 channel crystal structure is lacking, the current model of DHP action has largely been achieved by investigating the role of candidate Ca_v1 residues in mediating DHP-sensitivity. To better understand DHP-block and identify additional Ca_v1 residues important for DHP-sensitivity, we screened 440,000 randomly mutated Caenorhabditis elegans genomes for worms resistant to DHP-induced growth defects. We identified 30 missense mutations in the worm Ca_v1 pore-forming (α₁) subunit, including eleven in conserved residues known to be necessary for DHP-binding. The remaining polymorphisms are in eight conserved residues not previously associated with DHP-sensitivity. Intriguingly, all of the worm mutants that we analyzed phenotypically exhibited increased channel activity. We also created orthologous mutations in the rat α_1C subunit and examined the DHP-block of current through the mutant channels in culture. Six of the seven mutant channels examined either decreased the DHP-sensitivity of the channel and/or exhibited significant residual current at DHP concentrations sufficient to block wild-type channels. Our results further support the idea that DHP-block is intimately associated with voltage dependent inactivation and underscores the utility of C. elegans as a screening tool to identify residues important for DHP interaction with mammalian Ca_v1 channels.

L-type calcium channels are important drug targets because they regulate many physiological processes throughout the body. For example, L-type calcium channels regulate cardiac myocytes and vascular smooth muscle contraction. Antagonists are therefore commonly used to lower blood pressure and treat other related ailments. Despite their medical importance, the mechanism by which L-type antagonists inactivate calcium channels is not fully understood, due in large part to the lack of a channel crystal structure. Here, we present the first large-scale genetic screen for L-type calcium channel residues that are important for sensitivity to a new drug analog that we discovered called nemadipine. We performed the screen using nematodes, and then recreated similar mutations in a mammalian channel to investigate how the mutant residues alter interactions with the antagonists using electrophysiological techniques. Together, our analyses revealed eight new L-type calcium channel residues that are important for DHP-sensitivity and highlight the utility of using a simple animal model system for understanding how drugs interact with their targets.