Molecular mechanism of calcium channel block by isradipine. Role of a drug-induced inactivated channel conformation

Quinpirole inhibits levodopa-induced dyskinesias at structural and behavioral levels: Efficacy negated by co-administration of isradipine

Dopamine depletion associated with parkinsonism induces plastic changes in striatal medium spiny neurons (MSN) that are maladaptive and associated with the emergence of the negative side-effect of standard treatment: the abnormal involuntary movements termed levodopa-induced dyskinesia (LID). Prevention of MSN dendritic spine loss is hypothesized to diminish liability for LID in Parkinson's disease. Blockade of striatal CaV1.3 calcium channels can prevent spine loss and significantly diminish LID in parkinsonian rats. While pharmacological antagonism with FDA approved CaV1 L-type channel antagonist dihydropyridine (DHP) drugs (e.g, isradipine) are potentially antidyskinetic, pharmacologic limitations of current drugs may result in suboptimal efficacy. To provide optimal CaV1.3 antagonism, we investigated the ability of a dual pharmacological approach to more potently antagonize these channels. Specifically, quinpirole, a D2/D3-type dopamine receptor (D2/3R) agonist, has been demonstrated to significantly reduce calcium current activity at CaV1.3 channels in MSNs of rats by a mechanism distinct from DHPs. We hypothesized that dual inhibition of striatal CaV1.3 channels using the DHP drug isradipine combined with the D2/D3 dopamine receptor agonist quinpirole prior to, and in conjunction with, levodopa would be more effective at preventing structural modifications of dendritic spines and providing more stable LID prevention. For these proof-of-principle studies, rats with unilateral nigrostriatal lesions received daily administration of vehicle, isradipine, quinpirole, or isradipine + quinpirole prior to, and concurrent with, levodopa. Development of LID and morphological analysis of dendritic spines were assessed. Contrary to our hypothesis, quinpirole monotherapy was the most effective at reducing dyskinesia severity and preventing abnormal mushroom spine formation on MSNs, a structural phenomenon previously associated with LID. Notably, the antidyskinetic efficacy of quinpirole monotherapy was lost in the presence of isradipine co-treatment. These findings suggest that D2/D3 dopamine receptor agonists when given in combination with levodopa and initiated in early-stage Parkinson's disease may provide long-term protection against LID. The negative interaction of isradipine with quinpirole suggests a potential cautionary note for co-administration of these drugs in a clinical setting.

Pathophysiology of Cav1.3 L-type calcium channels in the heart

Ca²⁺ plays a crucial role in excitation-contraction coupling in cardiac myocytes. Dysfunctional Ca²⁺ regulation alters the force of contraction and causes cardiac arrhythmias. Ca²⁺ entry into cardiomyocytes is mediated mainly through L-type Ca²⁺ channels, leading to the subsequent Ca²⁺ release from the sarcoplasmic reticulum. L-type Ca²⁺ channels are composed of the conventional Ca_v1.2, ubiquitously expressed in all heart chambers, and the developmentally regulated Ca_v1.3, exclusively expressed in the atria, sinoatrial node, and atrioventricular node in the adult heart. As such, Ca_v1.3 is implicated in the pathogenesis of sinoatrial and atrioventricular node dysfunction as well as atrial fibrillation. More recently, Ca_v1.3 de novo expression was suggested in heart failure. Here, we review the functional role, expression levels, and regulation of Ca_v1.3 in the heart, including in the context of cardiac diseases. We believe that the elucidation of the functional and molecular pathways regulating Ca_v1.3 in the heart will assist in developing novel targeted therapeutic interventions for the aforementioned arrhythmias.

The Na+/K+ pump dominates control of glycolysis in hippocampal dentate granule cells

Cellular ATP that is consumed to perform energetically expensive tasks must be replenished by new ATP through the activation of metabolism. Neuronal stimulation, an energetically demanding process, transiently activates aerobic glycolysis, but the precise mechanism underlying this glycolysis activation has not been determined. We previously showed that neuronal glycolysis is correlated with Ca2+ influx, but is not activated by feedforward Ca2+ signaling (Díaz-García et al., 2021a). Since ATP-powered Na+ and Ca2+ pumping activities are increased following stimulation to restore ion gradients and are estimated to consume most neuronal ATP, we aimed to determine if they are coupled to neuronal glycolysis activation. By using two-photon imaging of fluorescent biosensors and dyes in dentate granule cell somas of acute mouse hippocampal slices, we observed that production of cytoplasmic NADH, a byproduct of glycolysis, is strongly coupled to changes in intracellular Na+, while intracellular Ca2+ could only increase NADH production if both forward Na+/Ca2+ exchange and Na+/K+ pump activity were intact. Additionally, antidromic stimulation-induced intracellular [Na+] increases were reduced >50% by blocking Ca2+ entry. These results indicate that neuronal glycolysis activation is predominantly a response to an increase in activity of the Na+/K+ pump, which is strongly potentiated by Na+ influx through the Na+/Ca2+ exchanger during extrusion of Ca2+ following stimulation.

Plant-Derived Nanoscale-Encapsulated Antioxidants for Oral and Topical Uses: A Brief Review

Several plant-based nanoscale-encapsulated antioxidant compounds (rutin, myricetin, β-carotene, fisetin, lycopene, quercetin, genkwanin, lutein, resveratrol, eucalyptol, kaempferol, glabridin, pinene, and whole-plant bio-active compounds) are briefly introduced in this paper, along with their characteristics. Antioxidants’ bioavailability has become one of the main research topics in bio-nanomedicine. Two low patient compliance drug delivery pathways (namely, the oral and topical delivery routes), are described in detail in this paper, for nanoscale colloidal systems and gel formulations. Both routes and/or formulations seek to improve bioavailability and maximize the drug agents’ efficiency. Some well-known compounds have been robustly studied, but many remain elusive. The objective of this review is to discuss recent studies and advantages of nanoscale formulations of plant-derived antioxidant compounds.

P2Y1 purinergic receptor identified as a diabetes target in a small-molecule screen to reverse circadian β-cell failure

The mammalian circadian clock drives daily oscillations in physiology and behavior through an autoregulatory transcription feedback loop present in central and peripheral cells. Ablation of the core clock within the endocrine pancreas of adult animals impairs the transcription and splicing of genes involved in hormone exocytosis and causes hypoinsulinemic diabetes. Here, we developed a genetically sensitized small-molecule screen to identify druggable proteins and mechanistic pathways involved in circadian β-cell failure. Our approach was to generate β-cells expressing a nanoluciferase reporter within the proinsulin polypeptide to screen 2640 pharmacologically active compounds and identify insulinotropic molecules that bypass the secretory defect in CRISPR-Cas9-targeted clock mutant β-cells. We validated hit compounds in primary mouse islets and identified known modulators of ligand-gated ion channels and G-protein-coupled receptors, including the antihelmintic ivermectin. Single-cell electrophysiology in circadian mutant mouse and human cadaveric islets revealed ivermectin as a glucose-dependent secretagogue. Genetic, genomic, and pharmacological analyses established the P2Y1 receptor as a clock-controlled mediator of the insulinotropic activity of ivermectin. These findings identify the P2Y1 purinergic receptor as a diabetes target based upon a genetically sensitized phenotypic screen.

Probing Synaptic Signaling with Optogenetic Stimulation and Genetically Encoded Calcium Reporters

Optogenetics provides a powerful approach for investigating neuronal electrophysiology at the scale required for drug discovery applications. Probing synaptic function with high throughput using optogenetics requires robust tools that enable both precise stimulation of and facile readout of synaptic activity. Here we describe two functional assays to achieve this end: (1) a pre-synaptic calcium assay that utilizes the channelrhodopsin, CheRiff, patterned optogenetic stimulus, and the pre-synaptically targeted calcium reporter jRGECO1a to monitor pre-synaptic changes in calcium influx and (2) a synaptic transmission assay in which CheRiff and cytosolic jRGECO1a are expressed in non-overlapping sets of neurons, enabling pre-synaptic stimulation and post-synaptic readout of activity. This chapter describes the methodology and practical considerations for implementation of these two assays.

T1143 essential for CaV1.2 inhibition by diltiazem

Careful analysis of previously published reports and some new insights into the structure activity studies revealed an important role of Threonine 1143 in drug binding. Substituting T1143 by alanine and other residues significantly reduced channel inhibition by qDil and Dil. Mutation T1143A did not affect channel activation or inactivation while almost completely diminishing channel block by Dil or qDil. These findings support the view that T1143 serves as drug binding determinant. Other mutations in this position than T1143A (T1143L/Y/S/N/C/V/E) diminished channel inhibition by qDil but additionally affected channel activation and inactivation and may therefore affect channel block allosterically. Collectively, our data suggest that T1143 is an essential diltiazem binding determinant.

Ca2+ signaling driving pacemaker activity in submucosal interstitial cells of Cajal in the murine colon

Interstitial cells of Cajal (ICC) generate pacemaker activity responsible for phasic contractions in colonic segmentation and peristalsis. ICC along the submucosal border (ICC-SM) contribute to mixing and more complex patterns of colonic motility. We show the complex patterns of Ca²⁺ signaling in ICC-SM and the relationship between ICC-SM Ca²⁺ transients and activation of smooth muscle cells (SMCs) using optogenetic tools. ICC-SM displayed rhythmic firing of Ca²⁺transients ~ 15 cpm and paced adjacent SMCs. The majority of spontaneous activity occurred in regular Ca²⁺ transients clusters (CTCs) that propagated through the network. CTCs were organized and dependent upon Ca²⁺ entry through voltage-dependent Ca²⁺ conductances, L- and T-type Ca²⁺ channels. Removal of Ca²⁺ from the external solution abolished CTCs. Ca²⁺ release mechanisms reduced the duration and amplitude of Ca²⁺ transients but did not block CTCs. These data reveal how colonic pacemaker ICC-SM exhibit complex Ca^2+-firing patterns and drive smooth muscle activity and overall colonic contractions.

Modest CaV1.342-selective inhibition by compound 8 is β-subunit dependent

Two voltage-gated calcium channel subtypes—Ca_V1.2 and Ca_V1.3—underlie the major L-type Ca²⁺ currents in the mammalian central nervous system. Owing to their high sequence homology, the two channel subtypes share similar pharmacological properties, and at high doses classic calcium channel blockers, such as dihydropyridines, phenylalkylamines and benzothiazepines, do not discriminate between the two channel subtypes. Recent progress in treating Parkinson’s disease (PD) was marked by the discovery of synthetic compound 8, which was reported to be a highly selective inhibitor of the Ca_V1.3 L-type calcium channels (LTCC). However, despite a previously reported IC₅₀ of ~24 μM, in our hands inhibition of the full-length Ca_V1.3₄₂ by compound 8 at 50 μM reaches a maximum of 45%. Moreover, we find that the selectivity of compound 8 towards Ca_V1.3 relative to Ca_V1.2_B15 channels is greatly influenced by the β-subunit type and its splice isoform variants.

Compound 8-1-(3-chlorophenethyl)-3-cyclopentylpyrimidine-2,4,6(1H,3H,5H)-trione was previously reported to be a selective inhibitor of the Ca_V1.3 calcium channel. Now, Huang et al. demonstrate that selectivity towards Ca_V1.3 relative to Ca_V1.2 is dependent on the type of β-subunit and Ca_V1.3 splice variant assayed.

Neuronal voltage-gated calcium channels: structure, function, and dysfunction

Voltage-gated calcium channels are the primary mediators of depolarization-induced calcium entry into neurons. There is great diversity of calcium channel subtypes due to multiple genes that encode calcium channel α1 subunits, coassembly with a variety of ancillary calcium channel subunits, and alternative splicing. This allows these channels to fulfill highly specialized roles in specific neuronal subtypes and at particular subcellular loci. While calcium channels are of critical importance to brain function, their inappropriate expression or dysfunction gives rise to a variety of neurological disorders, including, pain, epilepsy, migraine, and ataxia. This Review discusses salient aspects of voltage-gated calcium channel function, physiology, and pathophysiology.

The delayed rectifier potassium conductance in the sarcolemma and the transverse tubular system membranes of mammalian skeletal muscle fibers

A two-microelectrode voltage clamp and optical measurements of membrane potential changes at the transverse tubular system (TTS) were used to characterize delayed rectifier K currents (IK(V)) in murine muscle fibers stained with the potentiometric dye di-8-ANEPPS. In intact fibers, IK(V) displays the canonical hallmarks of K(V) channels: voltage-dependent delayed activation and decay in time. The voltage dependence of the peak conductance (gK(V)) was only accounted for by double Boltzmann fits, suggesting at least two channel contributions to IK(V). Osmotically treated fibers showed significant disconnection of the TTS and displayed smaller IK(V), but with similar voltage dependence and time decays to intact fibers. This suggests that inactivation may be responsible for most of the decay in IK(V) records. A two-channel model that faithfully simulates IK(V) records in osmotically treated fibers comprises a low threshold and steeply voltage-dependent channel (channel A), which contributes ∼31% of gK(V), and a more abundant high threshold channel (channel B), with shallower voltage dependence. Significant expression of the IK(V)1.4 and IK(V)3.4 channels was demonstrated by immunoblotting. Rectangular depolarizing pulses elicited step-like di-8-ANEPPS transients in intact fibers rendered electrically passive. In contrast, activation of IK(V) resulted in time- and voltage-dependent attenuations in optical transients that coincided in time with the peaks of IK(V) records. Normalized peak attenuations showed the same voltage dependence as peak IK(V) plots. A radial cable model including channels A and B and K diffusion in the TTS was used to simulate IK(V) and average TTS voltage changes. Model predictions and experimental data were compared to determine what fraction of gK(V) in the TTS accounted simultaneously for the electrical and optical data. Best predictions suggest that K(V) channels are approximately equally distributed in the sarcolemma and TTS membranes; under these conditions, >70% of IK(V) arises from the TTS.

Two mechanistically distinct effects of dihydropyridine nifedipine on CaV1.2 L-type Ca²⁺ channels revealed by Timothy syndrome mutation

Dihydropyridine Ca(2+) channel antagonists (DHPs) block Ca(V)1.2 L-type Ca(2+) channels (LTCCs) by stabilizing their voltage-dependent inactivation (VDI); however, it is still not clear how DHPs allosterically interact with the kinetically distinct (fast and slow) VDI. Thus, we analyzed the effect of a prototypical DHP, nifedipine on LTCCs with or without the Timothy syndrome mutation that resides in the I-II linker (L(I)-(II)) of Ca(V)1.2 subunits and impairs VDI. Whole-cell Ba(2+) currents mediated by rabbit Ca(V)1.2 with or without the Timothy mutation (G436R) (analogous to the human G406R mutation) were analyzed in the presence and absence of nifedipine. In the absence of nifedipine, the mutation significantly impaired fast closed- and open-state VDI (CSI and OSI) at -40 and 0 mV, respectively, but did not affect channels' kinetics at -100 mV. Nifedipine equipotently blocked these channels at -80 mV. In wild-type LTCCs, nifedipine promoted fast CSI and OSI at -40 and 0 mV and promoted or stabilized slow CSI at -40 and -100 mV, respectively. In LTCCs with the mutation, nifedipine resumed the impaired fast CSI and OSI at -40 and 0 mV, respectively, and had the same effect on slow CSI as in wild-type LTCCs. Therefore, nifedipine has two mechanistically distinct effects on LTCCs: the promotion of fast CSI/OSI caused by L(I-II) at potentials positive to the sub-threshold potential and the promotion or stabilization of slow CSI caused by different mechanisms at potentials negative to the sub-threshold potential.

Nanodomain control of exocytosis is responsible for the signaling capability of a retinal ribbon synapse

Primary sensory circuits encode both weak and intense stimuli reliably, requiring that their synapses signal over a wide dynamic range. In the retinal circuitry subserving night vision, processes intrinsic to the rod bipolar (RB) cell presynaptic active zone (AZ) permit the RB synapse to encode signals generated by the absorption of single photons as well as by more intense stimuli. In a study using an in vitro slice preparation of the mouse retina, we provide evidence that the location of Ca channels with low open probability within nanometers of the release sites is a critical determinant of the physiological behavior of the RB synapse. This gives rise to apparent one-to-one coupling between Ca channel opening and vesicle release, allowing presynaptic potential to be encoded linearly over a wide dynamic range. Further, it permits a transition from univesicular to multivesicular release (MVR) when two Ca channels/AZ open at potentials above the threshold for exocytosis. MVR permits small presynaptic voltage changes to elicit postsynaptic responses larger than quantal synaptic noise.

Structural model for dihydropyridine binding to L-type calcium channels

1,4-Dihydropyridines (DHPs) constitute a major class of ligands for L-type Ca(2+) channels (LTCC). The DHPs have a boat-like, six-membered ring with an NH group at the stern, an aromatic moiety at the bow, and substituents at the port and starboard sides. Various DHPs exhibit antagonistic or agonistic activities, which were previously explained as stabilization or destabilization, respectively, of the closed activation gate by the portside substituents. Here we report a novel structural model in which agonist and antagonist activities are determined by different parts of the DHP molecule and have different mechanisms. In our model, which is based on Monte Carlo minimizations of DHP-LTCC complexes, the DHP moieties at the stern, bow, and starboard form H-bonds with side chains of the key DHP-sensing residues Tyr_IIIS6, Tyr_IVS6, and Gln_IIIS5, respectively. We propose that these H-bonds, which are common for agonists and antagonists, stabilize the LTCC conformation with the open activation gate. This explains why both agonists and antagonists increase probability of the long lasting channel openings and why even partial disruption of the contacts eliminates the agonistic action. In our model, the portside approaches the selectivity filter. Hydrophobic portside of antagonists may induce long lasting channel closings by destabilizing Ca(2+) binding to the selectivity filter glutamates. Agonists have either hydrophilic substituents or a hydrogen atom at their portside, and thus lack this destabilizing effect. The predicted orientation of the DHP core allows accommodation of long substituents in the domain interface or in the inner pore. Our model may be useful for developing novel clinically relevant LTCC blockers.

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.

Effects of phenylalkylamines and benzothiazepines on Cav1.3-mediated Ca2+ currents in neonatal mouse inner hair cells

Calcium currents (I(Ca)) in inner hair cells (IHCs) are carried by the Ca(v)1.3 subtype of L-type calcium channels. They play an important role in synaptic transmission of sound-evoked mechanical stimuli. L-type calcium channels are targets of the organic blocker classes dihydropyridines, phenylalkylamines and benzothiazepines. Previously a low sensitivity of the Ca(v)1.3 subtype towards dihydropyridines has been demonstrated. Therefore, this study evaluates the effect of two phenylalkylamines (verapamil and gallopamil) and the benzothiazepine diltiazem on I(Ca) through Ca(v)1.3 channels in mouse IHCs. Whole-cell I(Ca) was measured using the patch-clamp technique in mouse IHCs aged postnatal day 3-7 with 5 mM calcium as a charge carrier. The phenylalkylamines verapamil and gallopamil and the benzothiazepine diltiazem inhibited I(Ca) in IHCs in a concentration-dependent manner. This block was largely reversible. Dose-response curves revealed IC(50) values of 199+/-19 microM for verapamil, 466+/-151 microM for gallopamil and 326+/-67 microM for diltiazem. The inhibition of peak I(Ca) by phenylalkylamines and benzothiazepines was voltage-independent. Verapamil (300 microM) enhanced current inactivation from -20 to +20 mV while diltiazem (300 microM) did so only at very depolarised potentials (+20 mV). In conclusion, the concentrations of phenylalkylamines and benzothiazepine necessary to inhibit 50% of I(Ca) in IHCs were one order larger compared to concentrations which inhibited I(Ca) through Ca(v)1.2 channels in native cells or expression systems. However, inhibitory concentrations were in the same range as those required for block of I(Ca) in turtle hair cells.

Modulation of the voltage sensor of L-type Ca2+ channels by intracellular Ca2+

Both intracellular calcium and transmembrane voltage cause inactivation, or spontaneous closure, of L-type (CaV1.2) calcium channels. Here we show that long-lasting elevations of intracellular calcium to the concentrations that are expected to be near an open channel (>/=100 microM) completely and reversibly blocked calcium current through L-type channels. Although charge movements associated with the opening (ON) motion of the channel's voltage sensor were not altered by high calcium, the closing (OFF) transition was impeded. In two-pulse experiments, the blockade of calcium current and the reduction of gating charge movements available for the second pulse developed in parallel during calcium load. The effect depended steeply on voltage and occurred only after a third of the total gating charge had moved. Based on that, we conclude that the calcium binding site is located either in the channel's central cavity behind the voltage-dependent gate, or it is formed de novo during depolarization through voltage-dependent rearrangements just preceding the opening of the gate. The reduction of the OFF charge was due to the negative shift in the voltage dependence of charge movement, as previously observed for voltage-dependent inactivation. Elevation of intracellular calcium concentration from approximately 0.1 to 100-300 microM sped up the conversion of the gating charge into the negatively distributed mode 10-100-fold. Since the "IQ-AA" mutant with disabled calcium/calmodulin regulation of inactivation was affected by intracellular calcium similarly to the wild-type, calcium/calmodulin binding to the "IQ" motif apparently is not involved in the observed changes of voltage-dependent gating. Although calcium influx through the wild-type open channels does not cause a detectable negative shift in the voltage dependence of their charge movement, the shift was readily observable in the Delta1733 carboxyl terminus deletion mutant, which produces fewer nonconducting channels. We propose that the opening movement of the voltage sensor exposes a novel calcium binding site that mediates inactivation.

L-Type Calcium Channels: The Low Down

L-type calcium channels couple membrane depolarization in neurons to numerous processes including gene expression, synaptic efficacy, and cell survival. To establish the contribution of L-type calcium channels to various signaling cascades, investigators have relied on their unique pharmacological sensitivity to dihydropyridines. The traditional view of dihydropyridine-sensitive L-type calcium channels is that they are high-voltage–activating and have slow activation kinetics. These properties limit the involvement of L-type calcium channels to neuronal functions triggered by strong and sustained depolarizations. This review highlights literature, both long-standing and recent, that points to significant functional diversity among L-type calcium channels expressed in neurons and other excitable cells. Past literature contains several reports of low-voltage–activated neuronal L-type calcium channels that parallel the unique properties of recently cloned CaV1.3 L-type channels. The fast kinetics and low activation thresholds of CaV1.3 channels stand in stark contrast to criteria currently used to describe L-type calcium channels. A more accurate view of neuronal L-type calcium channels encompasses a broad range of activation thresholds and recognizes their potential contribution to signaling cascades triggered by subthreshold depolarizations.

Two components of voltage-dependent inactivation in Ca(v)1.2 channels revealed by its gating currents

Voltage-dependent inactivation (VDI) was studied through its effects on the voltage sensor in Ca(v)1.2 channels expressed in tsA 201 cells. Two kinetically distinct phases of VDI in onset and recovery suggest the presence of dual VDI processes. Upon increasing duration of conditioning depolarizations, the half-distribution potential (V(1/2)) of intramembranous mobile charge was negatively shifted as a sum of two exponential terms, with time constants 0.5 s and 4 s, and relative amplitudes near 50% each. This kinetics behavior was consistent with that of increment of maximal charge related to inactivation (Qn). Recovery from inactivation was also accompanied by a reduction of Qn that varied with recovery time as a sum of two exponentials. The amplitudes of corresponding exponential terms were strongly correlated in onset and recovery, indicating that channels recover rapidly from fast VDI and slowly from slow VDI. Similar to charge "immobilization," the charge moved in the repolarization (OFF) transient became slower during onset of fast VDI. Slow VDI had, instead, hallmarks of interconversion of charge. Confirming the mechanistic duality, fast VDI virtually disappeared when Li(+) carried the current. A nine-state model with parallel fast and slow inactivation pathways from the open state reproduces most of the observations.

Molecular modeling of interactions of dihydropyridines and phenylalkylamines with the inner pore of the L-type Ca2+ channel

Domains IIIS5, IIIS6, and IVS6 transmembrane segments of L-type Ca(2+) channels participate in dihydropyridine (DHP) and phenylalkylamine (PAA) binding. The inner pore structure of the Ca(v)1.2 channel was reconstructed from coordinates of the transmembrane alpha-helices of the KcsA channel. S6s were aligned with M2 by comparative analysis of the pore-facing M2 side chains and those required for drug binding. Two neighboring tilted S6 helices of domains III and IV below the selectivity filter formed an interdomain crevice. Docking of DHPs inside this crevice located the DHP ring between Phe-1159 of IIIS6 and Ala-1467 of IVS6, parallel to the pore axis, whereas the 4-aryl ring participated in aromatic and polar interactions with the side chains of Tyr-1152 and Tyr-1463. Nonpolar interactions of the port side ester group with hydrophobic side chains of Ile-1156, Ile-1163, and Ile-1471 on the bottom of the binding cavity, formed by the crossover of IIIS6 and IVS6, could stabilize the channel's closed/inactivated state. Similar arrangements were found for DHP agonist drugs, except for the absence of hydrophobic interactions with the helical crossing. In this arrangement, DHPs do not physically block the pore. Locating the central amine group of desmethoxyverapamil near the selectivity filter domain III glutamic acid allows one aromatic ring through its CH(2)CH(2) linker to interact with the side chain of Tyr-1463 inside the DHP binding site, whereas the opposite aromatic ring is in contact with the side chain of Ile-1470 of IVS6, blocking the pore.

beta-Subunits: fine tuning of Ca(2+) channel block

Ca(2+) channel blockers such as 1,4-dihydropyridines, phenylalkylamines, diltiazem and mibefradil exert their anti-arrhythmic and anti-hypertensive action by restricting Ca(2+) entry into myocardial cells and smooth muscle cells. Binding sites for these drugs are present on the pore-forming alpha(1)-subunits of voltage-dependent Ca(2+) (Ca(v)) channels. However, striking new data show that auxillary beta-subunits also influence drug sensitivity significantly. These findings are summarized and the underlying molecular mechanisms and their pharmacological relevance are discussed.

Synthesis and evaluation of a new class of nifedipine analogs with T-type calcium channel blocking activity

We have synthesized a novel series of 18 dialkyl 1,4-dihydro-4-(2'alkoxy-6'-pentadecylphenyl)-2,6-dimethyl-3,5 pyridine dicarboxylates from anacardic acid, a natural compound found in cashew nut shells, and investigated their blocking action on L- and T-type calcium channels transiently expressed in tSA-201 cells. The IC(50) values for L-type calcium channel block obtained with the series ranged from 1 to approximately 40 microM, with higher affinities being favored by increasing the size of the alkoxy group on the 4-phenyl ring and ester substituent in the 3,5 positions. A detailed analysis of the strongest L-type channel blocker of the series (PPK-12) revealed that block was poorly reversible and mediated an apparent speeding of the time course of inactivation. Moreover, in the presence of PPK-12, the midpoint of the steady state inactivation curve was shifted by 20 mV toward more hyperpolarized potentials, resulting in an increase in blocking efficacy at more depolarized holding potentials. Surprisingly, PPK-12 blocked T- and L-type calcium channels with similar affinities. One of the weakest L-type channel inhibitors (PPK-5) exhibited a T-type channel affinity that was similar to that seen with PPK-12, resulting in a 40-fold selectivity of PPK-5 for T- over L-type channels. Thus, dialkyl 1,4-dihydro-4-(2'alkoxy-6'-pentadecylphenyl)-2,6-dimethyl-3,5 pyridine dicarboxylates may serve as excellent candidates for the development of T-type calcium-channel specific blockers.

Molecular determinants of voltage-dependent slow inactivation of the Ca2+ channel

Ba(2+) current through the L-type Ca(2+) channel inactivates essentially by voltage-dependent mechanisms with fast and slow kinetics. Here we found that slow inactivation is mediated by an annular determinant composed of hydrophobic amino acids located near the cytoplasmic ends of transmembrane segments S6 of each repeat of the alpha(1C) subunit. We have determined the molecular requirements that completely obstruct slow inactivation. Critical interventions include simultaneous substitution of A752T in IIS6, V1165T in IIIS6, and I1475T in IVS6, each preventing in additive manner a considerable fraction of Ba(2+) current from inactivation. In addition, it requires the S405I mutation in segment IS6. The fractional inhibition of slow inactivation in tested mutants caused an acceleration of fast inactivation, suggesting that fast and slow inactivation mechanisms are linked. The channel lacking slow inactivation showed approximately 45% of the sustained Ba(2+) or Ca(2+) current with no indication of decay. The remaining fraction of the current was inactivated with a single-exponential decay (pi(f) approximately 10 ms), completely recovered from inactivation within 100 ms and did not exhibit Ca(2+)-dependent inactivation properties. No voltage-dependent characteristics were significantly changed, consistent with the C-type inactivation model suggesting constriction of the pore as the main mechanism possibly targeted by Ca(2+) sensors of inactivation.

Inactivation determinants in segment IIIS6 of Ca(v)3.1

1. Low threshold, T-type, Ca(2+) channels of the Ca(v)3 family display the fastest inactivation kinetics among all voltage-gated Ca(2+) channels. The molecular inactivation determinants of this channel family are largely unknown. Here we investigate whether segment IIIS6 plays a role in Ca(v)3.1 inactivation as observed previously in high voltage-activated Ca(2+) channels. 2. Amino acids that are identical in IIIS6 segments of all Ca(2+) channel subtypes were mutated to alanine (F1505A, F1506A, N1509A, F1511A, V1512A, F1519A, FV1511/1512AA). Additionally M1510 was mutated to isoleucine and alanine. 3. The kinetic properties of the mutants were analysed with the two-microelectrode voltage-clamp technique after expression in Xenopus oocytes. The time constant for the barium current (I(Ba)) inactivation, tau(inact), of wild-type channels at -20 mV was 9.5 +/- 0.4 ms; the corresponding time constants of the mutants ranged from 9.2 +/- 0.4 ms in V1512A to 45.7 +/- 5.2 ms (4.8-fold slowing) in M1510I. Recovery at -80 mV was most significantly slowed by V1512A and accelerated by F1511A. 4. We conclude that amino acids M1510, F1511 and V1512 corresponding to previously identified inactivation determinants in IIIS6 of Ca(v)2.1 (Hering et al. 1998) have a significant role in Ca(v)3.1 inactivation. These data suggest common elements in the molecular architecture of the inactivation mechanism in high and low threshold Ca(2+) channels.

Neuronal Ca(V)1.3alpha(1) L-type channels activate at relatively hyperpolarized membrane potentials and are incompletely inhibited by dihydropyridines

L-type calcium channels regulate a diverse array of cellular functions within excitable cells. Of the four molecularly defined subclasses of L-type Ca channels, two are expressed ubiquitously in the mammalian nervous system (Ca(V)1.2alpha(1) and Ca(V)1.3alpha(1)). Despite diversity at the molecular level, neuronal L-type channels are generally assumed to be functionally and pharmacologically similar, i.e., high-voltage activated and highly sensitive to dihydropyridines. We now show that Ca(V)1.3alpha(1) L-type channels activate at membrane potentials approximately 25 mV more hyperpolarized, compared with Ca(V)1.2alpha(1). This unusually negative activation threshold for Ca(V)1.3alpha(1) channels is independent of the specific auxiliary subunits coexpressed, of alternative splicing in domains I-II, IVS3-IVS4, and the C terminus, and of the expression system. The use of high concentrations of extracellular divalent cations has possibly obscured the unique voltage-dependent properties of Ca(V)1.3alpha(1) in certain previous studies. We also demonstrate that Ca(V)1.3alpha(1) channels are pharmacologically distinct from Ca(V)1.2alpha(1). The IC(50) for nimodipine block of Ca(V)1.3alpha(1) L-type calcium channel currents is 2.7 +/- 0.3 microm, a value 20-fold higher than the concentration required to block Ca(V)1.2alpha(1). The relatively low sensitivity of the Ca(V)1.3alpha(1) subunit to inhibition by dihydropyridine is unaffected by alternative splicing in the IVS3-IVS4 linker. Our results suggest that functional and pharmacological criteria used commonly to distinguish among different Ca currents greatly underestimate the biological importance of L-type channels in cells expressing Ca(v)1.3alpha(1).

Voltage-dependent acceleration of Ca(v)1.2 channel current decay by (+)- and (-)-isradipine

Inhibition of Ca(v)1.2 by antagonist 1,4 dihydropyridines (DHPs) is associated with a drug-induced acceleration of the calcium (Ca(2+)) channel current decay. This feature is contradictorily interpreted as open channel block or as drug-induced inactivation. To elucidate the underlying molecular mechanism we investigated the effects of (+)- and (-)-isradipine on Ca(v)1.2 inactivation gating at different membrane potentials. alpha(1)1.2 Constructs were expressed together with alpha(2)-delta- and beta(1a)- subunits in Xenopus oocytes and drug-induced changes in barium current (I(Ba)) kinetics analysed with the two microelectrode voltage clamp technique. To study isradipine effects on I(Ba) decay without contamination by intrinsic inactivation we expressed a mutant (V1504A) lacking fast voltage-dependent inactivation. At a subthreshold potential of -30 mV a 200-times higher concentration of (-)-isradipine was required to induce a comparable amount of inactivation as by (+)-isradipine. At +20 mV the two enantiomers were equally efficient in accelerating the I(Ba) decay. Faster recovery from (-)- than from (+)-isradipine-induced inactivation at -80 mV in a Ca(v)1.2 construct (tau((-)-isr.(Cav1.2))=0.74 s<tau((+)-isr.(Cav1.2))=2.85 s) and even more rapid recovery of V1504A (tau((-)-isr.(V1504A))=0.39 s<tau((+)-isr.(V1504A))=1.98 s) indicated that drug-induced determinants and determinants of intrinsic inactivation (V1504) stabilize the DHP-induced channel conformation in an additive manner. In the voltage range between -25 and 20 mV where the channels inactivate predominantly from the open state the (+)- and (-)-isradipine-induced acceleration of the I(Ba) decay in V1504A displayed similar voltage-dependence as intrinsic fast inactivation of Ca(v)1.2. Our data suggest that the isradipine-induced acceleration of the Ca(v)1.2 current decay reflects enhanced fast voltage-dependent inactivation and not open channel block.

alpha 1D (Cav1.3) subunits can form l-type Ca2+ channels activating at negative voltages

In cochlea inner hair cells (IHCs), L-type Ca(2+) channels (LTCCs) formed by alpha1D subunits (D-LTCCs) possess biophysical and pharmacological properties distinct from those of alpha1C containing C-LTCCs. We investigated to which extent these differences are determined by alpha1D itself by analyzing the biophysical and pharmacological properties of cloned human alpha1D splice variants in tsA-201 cells. Variant alpha1D(8A,) containing exon 8A sequence in repeat I, yielded alpha1D protein and L-type currents, whereas no intact protein and currents were observed after expression with exon 8B. In whole cell patch-clamp recordings (charge carrier 15-20 mm Ba(2+)), alpha1D(8A) - mediated currents activated at more negative voltages (activation threshold, -45.7 versus -31.5 mV, p < 0.05) and more rapidly (tau(act) for maximal inward currents 0.8 versus 2.3 ms; p < 0.05) than currents mediated by rabbit alpha1C. Inactivation during depolarizing pulses was slower than for alpha1C (current inactivation after 5-s depolarizations by 90 versus 99%, p < 0.05) but faster than for LTCCs in IHCs. The sensitivity for the dihydropyridine (DHP) L-type channel blocker isradipine was 8.5-fold lower than for alpha1C. Radioligand binding experiments revealed that this was not due to a lower affinity for the DHP binding pocket, suggesting that differences in the voltage-dependence of DHP block account for decreased sensitivity of D-LTCCs. Our experiments show that alpha1D(8A) subunits can form slowly inactivating LTCCs activating at more negative voltages than alpha1C. These properties should allow D-LTCCs to control physiological processes, such as diastolic depolarization in sinoatrial node cells, neurotransmitter release in IHCs and neuronal excitability.

Amino Acids in Segment IVS6 and β-Subunit Interaction Support Distinct Conformational Changes during Cav2.1 Inactivation

Ca(v)2.1 mediates voltage-gated Ca2+ entry into neurons and the release of neurotransmitters at synapses of the central nervous system. An inactivation process that is modulated by the auxiliary beta-subunits regulates Ca2+ entry through Ca(v)2.1. However, the molecular mechanism of this alpha1-beta-subunit interaction remains unknown. Herein we report the identification of new determinants within segment IVS6 of the alpha(1)2.1-subunit that markedly influence channel inactivation. Systematic substitution of residues within IVS6 with amino acids of different size, charge, and polarity resulted in mutant channels with rates of fast inactivation (k(inact)) ranging from a 1.5-fold slowing in V1818I (k(inact) = 0.98 +/- 0.09 s(-1) compared with wild type alpha(1)2.1/alpha2-delta/beta1a k(inact) = 1.35 +/- 0.25 s(-1) to a 75-fold acceleration in mutant M1811Q (k(inact) = 102 +/- 3 s(-1). Coexpression of mutant alpha(1)2.1-subunits with beta(2a) resulted in two different phenotypes of current inactivation: 1) a pronounced reduction in the rate of channel inactivation or 2) an attenuation of a slow component in I(Ba) inactivation. Simulations revealed that these two distinct inactivation phenotypes arise from a beta2a-subunit-induced destabilization of the fast-inactivated state. The IVS6- and beta2a-subunit-mediated effects on Ca(v)2.1 inactivation are likely to occur via independent mechanisms.

Molecular determinants of inactivation in voltage-gated Ca2+ channels

Evolution has created a large family of different classes of voltage-gated Ca2+ channels and a variety of additional splice variants with different inactivation properties. Inactivation controls the amount of Ca2+ entry during an action potential and is, therefore, believed to play an important role in tissue-specific Ca2+ signalling. Furthermore, mutations in a neuronal Ca2+ channel (Ca(v)2.1) that are associated with the aetiology of neurological disorders such as familial hemiplegic migraine and ataxia cause significant changes in the process of channel inactivation. Ca2+ channels of a given subtype may inactivate by three different conformational changes: a fast and a slow voltage-dependent inactivation process and in some channel types by an additional Ca2+-dependent inactivation mechanism. Inactivation kinetics of Ca2+ channels are determined by the intrinsic properties of their pore-forming alpha1-subunits and by interactions with other channel subunits. This review focuses on structural determinants of Ca2+ channel inactivation in different parts of Ca2+ channel alpha1-subunits, including pore-forming transmembrane segments and loops, intracellular domain linkers and the carboxyl terminus. Inactivation is also affected by the interaction of the alpha1-subunits with auxiliary beta-subunits and intracellular regulator proteins. The evidence shows that pore-forming S6 segments and conformational changes in extra- (pore loop) and intracellular linkers connected to pore-forming segments may play a principal role in the modulation of Ca2+ channel inactivation. Structural concepts of Ca2+ channel inactivation are discussed.

Molecular determinants of inactivation in voltage-gated Ca2+ channels

Evolution has created a large family of different classes of voltage-gated Ca2+ channels and a variety of additional splice variants with different inactivation properties. Inactivation controls the amount of Ca2+ entry during an action potential and is, therefore, believed to play an important role in tissue-specific Ca2+ signalling. Furthermore, mutations in a neuronal Ca2+ channel (Ca(v)2.1) that are associated with the aetiology of neurological disorders such as familial hemiplegic migraine and ataxia cause significant changes in the process of channel inactivation. Ca2+ channels of a given subtype may inactivate by three different conformational changes: a fast and a slow voltage-dependent inactivation process and in some channel types by an additional Ca2+-dependent inactivation mechanism. Inactivation kinetics of Ca2+ channels are determined by the intrinsic properties of their pore-forming alpha1-subunits and by interactions with other channel subunits. This review focuses on structural determinants of Ca2+ channel inactivation in different parts of Ca2+ channel alpha1-subunits, including pore-forming transmembrane segments and loops, intracellular domain linkers and the carboxyl terminus. Inactivation is also affected by the interaction of the alpha1-subunits with auxiliary beta-subunits and intracellular regulator proteins. The evidence shows that pore-forming S6 segments and conformational changes in extra- (pore loop) and intracellular linkers connected to pore-forming segments may play a principal role in the modulation of Ca2+ channel inactivation. Structural concepts of Ca2+ channel inactivation are discussed.