Molecular localization of regions in the L-type calcium channel critical for dihydropyridine action

L-type calcium channel regulation of dopamine activity in the ventral tegmental area to nucleus accumbens pathway: Implications for substance use, mood disorders and co-morbidities

L-type calcium channels (LTCCs), including the Cav1.2 and Cav1.3 LTCC subtypes, are important regulators of calcium entry into neurons, which mediates neurotransmitter release and synaptic plasticity. Cav1.2 and Cav1.3 are encoded by the CACNA1C and CACNA1D genes, respectively. These genes are implicated in substance use disorders and depression in humans, as demonstrated by genetic-wide association studies (GWAS). Pre-clinical models have also revealed a critical role of LTCCs on drug and mood related behavior, including the co-morbidity of substance use and mood disorders. Moreover, LTCCs have been shown to regulate the neuronal firing of dopamine (DA) neurons as well as drug and stress-induced plasticity within the ventral tegmental area (VTA) to nucleus accumbens (NAc) pathway. Thus, LTCCs are interesting targets for the treatment of neuropsychiatric diseases. In this review, we provide a brief introduction to voltage-gated calcium channels, specifically focusing on the LTCCs. We place particular emphasis on the ability of LTCCs to regulate DA neuronal activity and downstream signaling in the VTA to NAc pathway, and how such processes mediate substance use and mood disorder-related behavioral responses. We also discuss the bi-directional control of VTA LTCCs on drug and mood-related behaviors in pre-clinical models, with implications for co-morbid psychiatric diagnosis. We conclude with a section on the clinical implications of LTCC blockers, many which are already FDA approved as cardiac medications. Thus, pre-clinical and clinical work should examine the potential of LTCC blockers to be repurposed for neuropsychiatric illness. This article is part of the Special Issue on 'L-type calcium channel mechanisms in neuropsychiatric disorders'.

The Outer Pore and Selectivity Filter of TRPA1

TRPA1 (transient-receptor-potential-related ion channel with ankyrin domains) is a direct receptor or indirect effector for a wide variety of nociceptive signals, and thus is a compelling target for development of analgesic pharmaceuticals such as channel blockers. Recently, the structure of TRPA1 was reported, providing insights into channel assembly and pore architecture. Here we report whole-cell and single-channel current recordings of wild-type human TRPA1 as well as TRPA1 bearing point mutations of key charged residues in the outer pore. These measurements demonstrate that the glutamate at position 920 plays an important role in collecting cations into the mouth of the pore, by changing the effective surface potential by ~16 mV, while acidic residues further out have little effect on permeation. Electrophysiology experiments also confirm that the aspartate residue at position 915 represents a constriction site of the TRPA1 pore and is critical in controlling ion permeation.

Selective pressure modulation of synaptic voltage-dependent calcium channels-involvement in HPNS mechanism

Exposure to hyperbaric pressure (HP) exceeding 100 msw (1.1 MPa) is known to cause a constellation of motor and cognitive impairments named high-pressure neurological syndrome (HPNS), considered to be the result of synaptic transmission alteration. Long periods of repetitive HP exposure could be an occupational risk for professional deep-sea divers. Previous studies have indicated the modulation of presynaptic Ca(2+) currents based on synaptic activity modified by HP. We have recently demonstrated that currents in genetically identified cellular voltage-dependent Ca(2+) channels (VDCCs), CaV 1.2 and CaV 3.2 are selectively affected by HP. This work further elucidates the HPNS mechanism by examining HP effect on Ca(2+) currents in neuronal VDCCs, CaV 2.2 and CaV 2.1, which are prevalent in presynaptic terminals, expressed in Xenopus oocytes. HP augmented the CaV 2.2 current amplitude, much less so in a channel variation containing an additional modulatory subunit, and had almost no effect on the CaV 2.1 currents. HP differentially affected the channels' kinetics. It is, therefore, suggested that HPNS signs and symptoms arise, at least in part, from pressure modulation of various VDCCs.

Meta-Analysis of Public Microarray Datasets Reveals Voltage-Gated Calcium Gene Signatures in Clinical Cancer Patients

Voltage-gated calcium channels (VGCCs) are well documented to play roles in cell proliferation, migration, and apoptosis; however, whether VGCCs regulate the onset and progression of cancer is still under investigation. The VGCC family consists of five members, which are L-type, N-type, T-type, R-type and P/Q type. To date, no holistic approach has been used to screen VGCC family genes in different types of cancer. We analyzed the transcript expression of VGCCs in clinical cancer tissue samples by accessing ONCOMINE (www.oncomine.org), a web-based microarray database, to perform a systematic analysis. Every member of the VGCCs was examined across 21 different types of cancer by comparing mRNA expression in cancer to that in normal tissue. A previous study showed that altered expression of mRNA in cancer tissue may play an oncogenic role and promote tumor development; therefore, in the present findings, we focus only on the overexpression of VGCCs in different types of cancer. This bioinformatics analysis revealed that different subtypes of VGCCs (CACNA1C, CACNA1D, CACNA1B, CACNA1G, and CACNA1I) are implicated in the development and progression of diverse types of cancer and show dramatic up-regulation in breast cancer. CACNA1F only showed high expression in testis cancer, whereas CACNA1A, CACNA1C, and CACNA1D were highly expressed in most types of cancer. The current analysis revealed that specific VGCCs likely play essential roles in specific types of cancer. Collectively, we identified several VGCC targets and classified them according to different cancer subtypes for prospective studies on the underlying carcinogenic mechanisms. The present findings suggest that VGCCs are possible targets for prospective investigation in cancer treatment.

Selective modulation of cellular voltage-dependent calcium channels by hyperbaric pressure-a suggested HPNS partial mechanism

Professional deep sea divers experience motor and cognitive impairment, known as High Pressure Neurological Syndrome (HPNS), when exposed to pressures of 100 msw (1.1 MPa) and above, considered to be the result of synaptic transmission alteration. Previous studies have indicated modulation of presynaptic Ca²⁺ currents at high pressure. We directly measured for the first time pressure effects on the currents of voltage dependent Ca²⁺ channels (VDCCs) expressed in Xenopus oocytes. Pressure selectivity augmented the current in Ca_V1.2 and depressed it in Ca_V3.2 channels. Pressure application also affected the channels' kinetics, such as Ʈ_Rise, Ʈ_Decay. Pressure modulation of VDCCs seems to play an important role in generation of HPNS signs and symptoms.

Na(+)/Ca(2+) selectivity in the bacterial voltage-gated sodium channel NavAb

The recent publication of a number of high resolution bacterial voltage-gated sodium channel structures has opened the door for the mechanisms employed by these channels to distinguish between ions to be elucidated. The way these channels select between Na⁺ and K⁺ has been investigated in computational studies, but the selectivity for Na⁺ over Ca²⁺ has not yet been studied in this way. Here we use molecular dynamics simulations to calculate the energetics of Na⁺ and Ca²⁺ transport through the channel. Single ion profiles show that Ca²⁺ experiences a large barrier midway through the selectivity filter that is not seen by Na⁺. This barrier is caused by the need for Ca²⁺ to partly dehydrate to pass through this region and the lack of compensating interactions with the protein. Multi-ion profiles show that ions can pass each other in the channel, which is why the presence of Ca²⁺ does not block Na⁺ conduction despite binding more strongly in the pore.

Differential sensitivities of CaV1.2 IIS5-S6 mutants to 1,4-dihydropyridine analogs

1,4-Dihydropyridines (DHPs), L-type calcium channel (Ca(V)1) blockers, are known to interact with Ca(V)1.2 subunits through their binding site located at IIIS5-S6 and IVS6 regions. We recently identified two domain II residues (S666 and A752) critical for nifedipine blockade (Kwok et al., 2008). In this study, we examined the blockade effects of two DHP analogues, nemadipine and nicardipine, on wildtype, M1161A (in IIIS6), S666V (in IIS5) and A752T (in IIS6) mutants of the rat alpha(1C) subunit transiently expressed with beta(2a) and alpha(2)delta in cultured tsA201 cells. We found that the IC(50) ratio of the mutants to the wildtype channel was similar in S666V and M1161A mutants for both drugs, but in A752T it was lower for nemadipine than nicardipine (P<0.05). At saturating drug concentrations, not all the current was completely blocked in the mutants. The residual current recorded in 100 microM nemadipine was approximately 10% of the total current for the A752T channel, which was significantly higher than that in 100 microM nicardipine (approximately 2%). In wildtype, S666V and M1161A, there was no significant difference in residual current between nemadipine and nicardipine, although it was greater in S666V (approximately 15%) and M1161A approximately 30%) as compared to the wildtype channel (<5%). Taken together, our findings suggest that the domain II residues alter the DHP effect in a structure-specific manner and may be involved in a pathway downstream of DHP binding.

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.

A novel molecular inactivation determinant of voltage-gated CaV1.2 L-type Ca2+ channel

The inactivation of voltage-gated L-type Ca(2+) channels (Ca(V)1) regulates Ca(2+) entry and controls intracellular Ca(2+) levels that are essential for cellular activity. The molecular entities implicated in L-channel (Ca(V)1.2) inactivation are not fully identified. Here we show for the first time the functional impact of one of the two highly conserved clusters of six negatively charged glutamates and aspartate (802-807; poly ED motif) at the II-III loop of the alpha 1 subunits of rabbit of Ca(v)1.2, alpha(1)1.2 and alpha(1)1.2 DeltaN60-Delta1733) on voltage-dependent inactivation. Mutation of the poly ED motif to alanine or glutamine/asparagine greatly enhanced voltage-dependent inactivation, shifting the voltage dependence to negative potentials by >50 mV and conferring a neuronal like inactivation kinetics onto Ca(V)1.2. The large shift in the midpoint of inactivation of the steady-state inactivation kinetics was observed also in Ca(2+) or Ba(2+) and was not altered by the beta2A subunit. Missing from the fast inactivating neuronal P/Q (Ca(V)2.1)-, N (Ca(V)2.2)- or R (Ca(V)2.3)-type channels and modulating Ca(V)1.2 inactivation kinetics, the poly ED motif is likely to be a specific L-type Ca(2+) channels inactivating domain. Our results fit a model in which the poly ED either by itself or as part of a larger inactivating motif acts as Ca(V)1.2 specific built-in "stopper." In this model, Ca(V)1 accomplishes a large Ca(2+) influx during depolarization, possibly by the poly ED hindering occlusion at the pore. Furthermore, the selective designed poly ED perhaps clarifies major inactivation differences between L- and non-L-type calcium channels.

Phosphoinositide 3-kinase regulates excitation-contraction coupling in neonatal cardiomyocytes

The phosphoinositide 3-kinase (PI3K) inhibitor LY-294002 decreased steady-state contraction in neonatal rat ventricular myocytes (NRVM). To determine whether the effect on steady-state contraction could be due to decreased intracellular Ca2+content, Ca2+content was assessed with fluorescent plate reader analysis by using the caffeine-releasable Ca2+stores as an index of sarcoplasmic reticulum (SR) Ca2+content. Caffeine-releasable Ca2+content was diminished in a dose-dependent manner with LY-294002, suggesting that the decrease in steady-state contraction was due to diminished intracellular Ca2+content. Activation of the L-type Ca2+channel by BAY K 8644 was attenuated by LY-294002, suggesting the effect of LY-294002 is to reduce Ca2+influx at this channel. To investigate whether additional proteins involved in excitation-contraction (EC) coupling are likewise regulated by PI3K activity, the effects of compounds acting at sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA2a), the ryanodine receptor, and the Na/Ca exchanger (NCX) were compared with LY-294002. Inhibition of SERCA2a by thapsigargin increased basal Ca2+levels in contrast to LY-294002, indicating that SERCA2a activity is sustained in the presence of LY-294002. Ryanodine decreased SR Ca2+content. The additive effect with coadministration of LY-294002 could be attributed to a decrease in Ca2+influx at the L-type Ca2+channel. The NCX inhibitor Ni2+was used to investigate whether the decrease in intracellular Ca2+content with LY-294002 could be due to inhibition of the NCX reverse-mode activity. The minimal effect of LY-294002 with Ni2+suggests that the primary effect of LY-294002 on EC coupling occurs through inhibition of PI3K-mediated L-type Ca2+channel activity.

Critical residues of the Caenorhabditis elegans unc-2 voltage-gated calcium channel that affect behavioral and physiological properties

The Caenorhabditis elegans unc-2 gene encodes a voltage-gated calcium channel alpha1 subunit structurally related to mammalian dihydropyridine-insensitive high-threshold channels. In the present paper we describe the characterization of seven alleles of unc-2. Using an unc-2 promoter-tagged green fluorescent protein construct, we show that unc-2 is primarily expressed in motor neurons, several subsets of sensory neurons, and the HSN and VC neurons that control egg laying. Examination of behavioral phenotypes, including defecation, thrashing, and sensitivities to aldicarb and nicotine suggests that UNC-2 acts presynaptically to mediate both cholinergic and GABAergic neurotransmission. Sequence analysis of the unc-2 alleles shows that e55, ra605, ra606, ra609, and ra610 all are predicted to prematurely terminate and greatly reduce or eliminate unc-2 function. In contrast, the ra612 and ra614 alleles are missense mutations resulting in the substitution of highly conserved residues in the C terminus and the domain IVS4-IVS5 linker, respectively. Heterologous expression of a rat brain P/Q-type channel containing the ra612 mutation shows that the glycine to arginine substitution affects a variety of channel characteristics, including the voltage dependence of activation, steady-state inactivation, as well as channel kinetics. Overall, our findings suggest that UNC-2 plays a pivotal role in mediating a number of physiological processes in the nematode and also defines a number of critical residues important for calcium channel function in vivo.

Courtship and other behaviors affected by a heat-sensitive, molecularly novel mutation in the cacophony calcium-channel gene of Drosophila

The cacophony (cac) locus of Drosophila melanogaster, which encodes a calcium-channel subunit, has been mutated to cause courtship-song defects or abnormal responses to visual stimuli. However, the most recently isolated cac mutant was identified as an enhancer of a comatose mutation's effects on general locomotion. We analyzed the cac(TS2) mutation in terms of its intragenic molecular change and its effects on behaviors more complex than the fly's elementary ability to move. The molecular etiology of this mutation is a nucleotide substitution that causes a proline-to-serine change in a region of the polypeptide near its EF hand. Given that this motif is involved in channel inactivation, it was intriguing that cac(TS2) males generate song pulses containing larger-than-normal numbers of cycles--provided that such males are exposed to an elevated temperature. Similar treatments caused only mild visual-response abnormalities and generic locomotor sluggishness. These results are discussed in the context of calcium-channel functions that subserve certain behaviors and of defects exhibited by the original cacophony mutant. Despite its different kind of amino-acid substitution, compared with that of cac(TS2), cac(S) males sing abnormally in a manner that mimics the new mutant's heat-sensitive song anomaly.

Activation of cardiac ryanodine receptors by the calcium channel agonist FPL-64176

We investigated the possibility that the Ca(2+) channel agonist FPL-64176 (FPL) might also activate the cardiac sarcoplasmic reticulum (SR) Ca(2+) release channel ryanodine receptor (RyR). The effects of FPL were tested on single channel activity of purified and crude vesicular RyR (RyR2) isolated from human and dog hearts using the planar lipid bilayer technique. FPL (100-200 microM) increased single channel open probability (P(o)) when added to the cytoplasmic side of the channel (P(o) = 0.070 +/- 0.021 in control RyR2; 0.378 +/- 0.086 in 150 microM FPL, n = 9, P < 0.01) by prolonging open times and decreasing closed times without changing current magnitude. FPL had no effect on P(o) when added to the trans (luminal) side of the bilayer (P(o) = 0.079 +/- 0.036 in control and 0.103 +/- 0.066 in FPL, n = 4, no significant difference). The bell-shaped [Ca(2+)] dependence of [(3)H]ryanodine binding and of P(o) was altered by FPL, suggesting that the mechanism by which FPL increases channel activity is by an increase in Ca(2+)-induced activation at low [Ca(2+)] (without a change in threshold) and suppression of Ca(2+)-induced inactivation at high [Ca(2+)]. However, the fact that inactivation was restored at elevated [Ca(2+)] suggests a competitive interaction between Ca(2+) and FPL on inactivation. FPL had no effect on RyR skeletal channels (RyR1), where P(o) was 0.039 +/- 0.005 in control versus 0.030 +/- 0.006 in 150 microM FPL (no significant difference). These results suggest that, in addition to its ability to activate the L-type Ca(2+) channels, FPL activates cardiac RyR2 primarily by reducing the Ca(2+) sensitivity of inactivation.

The role of region IVS5 of the human cardiac calcium channel in establishing inactivated channel conformation: use-dependent block by benzothiazepines

The role of inactivated channel conformation and use dependence for diltiazem, a specific benzothiazepine calcium channel inhibitor, was studied in chimeric constructs and point mutants created in the IVS5 transmembrane segment of the L-type cardiac calcium channel. All mutations, chimeric or point mutations, were restricted to IVS5, while the YAI-containing segment in IVS6, i.e. the primary interaction site with benzothiazepines, remained intact. Slowed inactivation rate and incomplete steady state inactivation, a behavior of some mutants, were accompanied by a reduced or by a complete loss of use-dependent block by diltiazem. Single channel properties of mutants that lost use dependence toward diltiazem were characterized by drastically elongated mean open times and distinctly slower time constants of open time distribution. Mutation of individual residues of the IVMLF segment in IVS5 did not mimic the complete loss of use dependence as observed for the replacement of the whole stretch. These results establish evidence that amino acids that govern inactivation and the drug-binding site and other amino acids that are located distal from the putative drug-binding site contribute significantly to the function of the benzothiazepine receptor region. The data are consistent with a complex "pocket" conformation that is responsive to a specific class of L-type calcium channel inhibitors. The data allow for a concept that multiple sites within regions of the alpha(1) subunit contribute to auto-regulation of the L-type Ca(2+) channel.

The interaction between the I-II loop and the III-IV loop of Cav2.1 contributes to voltage-dependent inactivation in a beta -dependent manner

We have investigated the molecular mechanisms whereby the I-II loop controls voltage-dependent inactivation in P/Q calcium channels. We demonstrate that the I-II loop is localized in a central position to control calcium channel activity through the interaction with several cytoplasmic sequences; including the III-IV loop. Several experiments reveal the crucial role of the interaction between the I-II loop and the III-IV loop in channel inactivation. First, point mutations of two amino acid residues of the I-II loop of Ca(v)2.1 (Arg-387 or Glu-388) facilitate voltage-dependent inactivation. Second, overexpression of the III-IV loop, or injection of a peptide derived from this loop, produces a similar inactivation behavior than the mutated channels. Third, the III-IV peptide has no effect on channels mutated in the I-II loop. Thus, both point mutations and overexpression of the III-IV loop appear to act similarly on inactivation, by competing off the native interaction between the I-II and the III-IV loops of Ca(v)2.1. As they are known to affect inactivation, we also analyzed the effects of beta subunits on these interactions. In experiments in which the beta(4) subunit is co-expressed, the III-IV peptide is no longer able to regulate channel inactivation. We conclude that (i) the contribution of the I-II loop to inactivation is partly mediated by an interaction with the III-IV loop and (ii) the beta subunits partially control inactivation by modifying this interaction. These data provide novel insights into the mechanisms whereby the beta subunit, the I-II loop, and the III-IV loop altogether can contribute to regulate inactivation in high voltage-activated calcium channels.

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.

Ca2+ transport properties and determinants of anomalous mole fraction effects of single voltage-gated Ca2+ channels in hair cells from bullfrog saccule

We studied the permeation properties of two distinct single voltage-gated Ca2+ channels in bullfrog saccular hair cells to assess the roles of the channels as physiological Ca2+ transporters and multi-ion pores. By varying the permeant ions (Ba2+, Ca2+) and concentrations (2-70 mM), we estimated the affinity constant (K(D)) of the two channels as follows (mM): L-type channel, K(D,Ba) = 7.4 +/- 1.0, K(D,Ca) = 7.1 +/- 2.2 (n = 7); non-L-type channel, K(D,Ba) = 5.3 +/- 3.2, K(D,Ca) = 2.0 +/- 1.0 (n = 8). Using ionic concentrations close to physiological conditions (2 mM Ca2+ and 1.0 mM Mg2+), the conductance of the L-type channel was approximately 2 pS. We determined the mechanisms by which ions traverse the pore of these single Ca2+ channels, using mixtures of Ba2+ and Ca2+ at total concentrations above (70 mM) or close to (5 mM) the K(D) of the channels. We found evidence for an anomalous mole fraction effect (AMFE) only when the total divalent ion concentration was 5 mM, consistent with a multi-ion pore. We show that AMFE arises from the boundaries between the pore and bulk solution in the atria of the channel, which is derived from the presence of depletion zones that become apparent at low divalent cation concentrations. The present findings provide an explanation as to why previous whole-cell Ca2+ currents that were recorded in quasi-physiological Ca2+ concentrations (approximately 2-5 mM) showed clear AMFE, whereas single Ca2+ channel currents that were recorded routinely at high Ca2+ concentrations (20-110 mM) did not.

Independent modulation of L-type Ca2+ channel in guinea pig ventricular cells by nitrendipine and isoproterenol

Dihydropyridine (DHP) Ca2+ channel blockers decrease L-type Ca2+ channel current (I(CaL)) by enhancing steady-state inactivation, whereas beta-adrenergic stimulation increases I(CaL) with small changes in the kinetics. We studied the effects of DHP Ca2+ channel blockers on cardiac I(CaL) augmented by beta-adrenergic stimulation. We recorded I(CaL) as Ba2+ currents (I(Ba)) from guinea pig ventricular myocytes using the whole-cell patch clamp technique. and compared the effects of nitrendipine (NIT) in the absence and presence of isoproterenol (1 microM, ISO) or forskolin (10 microM, FSK). Maximal I(Ba) elicited from a holding potential of -80 mV were diminished to 69.4+/-13.5% (mean and SE, n=5) of control by NIT (100 nM) and the diminished I(Ba) were increased to 180.3+/-23.2% of control by ISO in the presence of NIT, which was similar to the enhancement seen in the absence of NIT. NIT shifted the V(1/2) of the I(Ba) inactivation curve from -34.6+/-1.9 mV (n=5) to -48.7+/-1.2 mV, enhancing I(Ba) decay with shortening T(1/2) at -10 mV from 164.6+/-24.2 ms (n=7) to 105.4+/-15.2 ms. ISO elicited a small additional shift in the V(1/2) of I(Ba) inactivation in the same direction. ISO and FSK each slowed I(Ba) decay in the absence of NIT, but not in its presence. Thus, beta-adrenergic agonists increase and DHP Ca2+ channel blockers decrease the amplitude of cardiac I(CaL) independently and the kinetics of I(CaL) is determined mainly by the latter when these drugs coexist.

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.

Effect of APGW-amide on [Ca2+]i in rat pheochromocytoma PC12 cells

In order to determine whether Ala-Pro-Gly-Try-NH2 (APGW-amide) could affect mammalian excitable cells, we investigated the effect of APGW-amide in PC12 cells. APGW-amide caused a rapid [Ca2+]i elevation, which was completely prevented by elimination of extracellular Ca2+ with EGTA and inhibited by two L-type Ca2+ channel blockers. [Ca2+]i elevation was also blocked by a specific PKC inhibitor and prolonged pretreatment of cells with PMA. These results indicate that APGW-amide elevates [Ca2+]i in PC12 cells, possibly by Ca2+ influx via L-type Ca2+ channel activated by PKC.

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.

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

The role of the inactivated channel conformation in the molecular mechanism of Ca(2+) channel block by the 1,4-dihydropyridine (DHP) (+)-isradipine was analyzed in L-type channel constructs (alpha(1Lc); Berjukow, S., Gapp, F., Aczel, S., Sinnegger, M. J., Mitterdorfer, J., Glossmann, H., and Hering, S. (1999) J. Biol. Chem. 274, 6154-6160) and a DHP-sensitive class A Ca(2+) channel mutant (alpha(1A-DHP); Sinnegger, M. J., Wang, Z., Grabner, M., Hering, S., Striessnig, J., Glossmann, H., and Mitterdorfer, J. (1997) J. Biol. Chem. 272, 27686-27693) carrying the high affinity determinants of the DHP receptor site but inactivating at different rates. Ca(2+) channel inactivation was modulated by coexpressing the alpha(1A-DHP)- or alpha(1Lc)-subunits in Xenopus oocytes with either the beta(2a)- or the beta(1a)-subunit and amino acid substitutions in L-type segment IVS6 (I1497A, I1498A, and V1504A). Contrary to a modulated receptor mechanism assuming high affinity DHP binding to the inactivated state we observed no clear correlation between steady state inactivation and Ca(2+) channel block by (+)-isradipine: (i) a 3-fold larger fraction of alpha(1A-DHP)/beta(1a) channels in steady state inactivation at -80 mV (compared with alpha(1A-DHP)/beta(2a)) did not enhance the block by (+)-isradipine; (ii) different steady state inactivation of alpha(1Lc) mutants at -30 mV did not correlate with voltage-dependent channel block; and (iii) the midpoint-voltages of the inactivation curves of slowly inactivating L-type constructs and more rapidly inactivating alpha(1Lc)/beta(1a) channels were shifted to a comparable extent to more hyperpolarized voltages. A kinetic analysis of (+)-isradipine interaction with different L-type channel constructs revealed a drug-induced inactivated state. Entry and recovery from drug-induced inactivation are modulated by intrinsic inactivation determinants, suggesting a synergism between intrinsic inactivation and DHP block.

The molecular biology of invertebrate voltage-gated Ca(2+) channels

The importance of voltage-gated Ca(2+) channels in cellular function is illustrated by the many distinct types of Ca(2+) currents found in vertebrate tissues, a variety that is generated in part by numerous genes encoding Ca(2+) channel subunits. The degree to which this genetic diversity is shared by invertebrates has only recently become apparent. Cloning of Ca(2+) channel subunits from various invertebrate species, combined with the wealth of information from the Caenorhabditis elegans genome, has clarified the organization and evolution of metazoan Ca(2+) channel genes. Functional studies have employed novel structural information gained from invertebrate Ca(2+) channels to complement ongoing research on mammalian Ca(2+) currents, while demonstrating that the strict correspondence between pharmacological and molecular classes of vertebrate Ca(2+) channels does not fully extend to invertebrate tissues. Molecular structures can now be combined with physiological data to develop a more cogent system of categorizing invertebrate channel subtypes. In this review, we examine recent progress in the characterization of invertebrate Ca(2+) channel genes and its relevance to the diversity of invertebrate Ca(2+) currents.

Calcium channel blockers activate the interleukin-6 gene via the transcription factors NF-IL6 and NF-kappaB in primary human vascular smooth muscle cells

BACKGROUND

Calcium channel blockers (CCB) of all subclasses: the dihydropyridines, benzothiazepines, and phenylalkylamines, at nanomolar concentrations, have been shown to up-regulate interleukin-6 (IL-6) mRNA. We investigated the underlying molecular mechanism responsible for IL-6 induction in response to the CCB amlodipine, diltiazem, and verapamil in primary human vascular smooth muscle cells (VSMC).

METHODS AND RESULTS

All 3 CCB directly activated transcription of the human IL-6 gene in primary human VSMC in a time- and dose-dependent manner, as demonstrated by luciferase reporter gene assays using a 651-bp fragment of the human IL-6 gene promoter. Deletion analysis of the IL-6 promoter revealed that CCB inducible promoter activity was localized to a 160-bp fragment directly upstream of the transcriptional start site of the IL-6 gene. Known transcription factor consensus sequences within this fragment include a NF-IL6 and a NF-kappaB site. Site-directed mutagenesis suggested that both transcription factors had positive regulatory activity and cooperatively transmitted induction of the IL-6 gene by CCB. The data are confirmed by electrophoretic mobility shift analyses using nuclear extracts from CCB-stimulated and control primary VSMC. CCB of all subclasses increased DNA binding of NF-IL6 and NF-kappaB as early as 30 minutes after stimulation with the drugs. This effect was independent of intracellular calcium concentrations because calcium-free medium did not increase NF-IL6 or NF-kappaB activity.

CONCLUSIONS

The results demonstrate that CCB of all 3 subclasses are capable of activating NF-IL6 and NF-kappaB. CCB may thus directly regulate cellular functions by affecting the activity of transcription factors independent of changes of intracellular calcium concentrations, an observation that is of interest considering the biological effects induced by CCB.

N-type calcium channel inactivation probed by gating-current analysis

N-type calcium channels inactivate most rapidly in response to moderate, not extreme depolarization. This behavior reflects an inactivation rate that bears a U-shaped dependence on voltage. Despite this apparent similarity to calcium-dependent inactivation, N-type channel inactivation is insensitive to the identity of divalent charge carrier and, in some reports, to the level of internal buffering of divalent cations. Hence, the inactivation of N-type channels fits poorly with the "classic" profile for either voltage-dependent or calcium-dependent inactivation. To investigate this unusual inactivation behavior, we expressed recombinant N-type calcium channels in mammalian HEK 293 cells, permitting in-depth correlation of ionic current inactivation with potential alterations of gating current properties. Such correlative measurements have been particularly useful in distinguishing among various inactivation mechanisms in other voltage-gated channels. Our main results are the following: 1) The degree of gating charge immobilization was unchanged by the block of ionic current and precisely matched by the extent of ionic current inactivation. These results argue for a purely voltage-dependent mechanism of inactivation. 2) The inactivation rate was fastest at a voltage where only approximately (1)/(3) of the total gating charge had moved. This unusual experimental finding implies that inactivation occurs most rapidly from intermediate closed conformations along the activation pathway, as we demonstrate with novel analytic arguments applied to coupled-inactivation schemes. These results provide strong, complementary support for a "preferential closed-state" inactivation mechanism, recently proposed on the basis of ionic current measurements of recombinant N-type channels (Patil et al., . Neuron. 20:1027-1038).

Regulation of Ca-sensitive inactivation of a 1-type Ca2+ channel by specific domains of beta subunits

Ca2+ channel auxiliary beta subunits have been shown to modulate voltage-dependent inactivation of various types of Ca2+ channels. The beta1 and beta2 subunits, that are differentially expressed with the L-type alpha1 Ca2+ channel subunit in heart, muscle and brain, can specifically modulate the Ca2+-dependent inactivation kinetics. Their expression in Xenopus oocytes with the alpha1C subunit leads, in both cases, to biphasic Ca2+ current decays, the second phase being markedly slowed by expression of the beta2 subunit. Using a series of beta subunit deletion mutants and chimeric constructs of beta1 and beta2 subunits, we show that the inhibitory site located on the amino-terminal region of the beta2a subunit is the major element of this regulation. These results thus suggest that different splice variants of the beta2 subunit can modulate, in a specific way, the Ca2+ entry through L-type Ca2+ channels in different brain or heart regions.

Functional consequences of mutations in the human alpha1A calcium channel subunit linked to familial hemiplegic migraine

Mutations in alpha1A, the pore-forming subunit of P/Q-type calcium channels, are linked to several human diseases, including familial hemiplegic migraine (FHM). We introduced the four missense mutations linked to FHM into human alpha1A-2 subunits and investigated their functional consequences after expression in human embryonic kidney 293 cells. By combining single-channel and whole-cell patch-clamp recordings, we show that all four mutations affect both the biophysical properties and the density of functional channels. Mutation R192Q in the S4 segment of domain I increased the density of functional P/Q-type channels and their open probability. Mutation T666M in the pore loop of domain II decreased both the density of functional channels and their unitary conductance (from 20 to 11 pS). Mutations V714A and I1815L in the S6 segments of domains II and IV shifted the voltage range of activation toward more negative voltages, increased both the open probability and the rate of recovery from inactivation, and decreased the density of functional channels. Mutation V714A decreased the single-channel conductance to 16 pS. Strikingly, the reduction in single-channel conductance induced by mutations T666M and V714A was not observed in some patches or periods of activity, suggesting that the abnormal channel may switch on and off, perhaps depending on some unknown factor. Our data show that the FHM mutations can lead to both gain- and loss-of-function of human P/Q-type calcium channels.

A region in IVS5 of the human cardiac L-type calcium channel is required for the use-dependent block by phenylalkylamines and benzothiazepines

Mutations in motif IVS5 and IVS6 of the human cardiac calcium channel were made using homologous residues from the rat brain sodium channel 2a. [3H]PN200-110 and allosteric binding assays revealed that the dihydropyridine and benzothiazepine receptor sites maintained normal coupling in the chimeric mutant channels. Whole cell voltage clamp recording from Xenopus oocytes showed a dramatically slowed inactivation and a complete loss of use-dependent block for mutations in the cytoplasmic connecting link to IVS5 (HHT-5371) and in IVS5 transmembrane segment (HHT-5411) with both diltiazem and verapamil. However, the use-dependent block by isradipine was retained by these two mutants. For mutants HHT-5411 and HHT-5371, the residual current appeared associated with a loss of voltage dependence in the rate of inactivation indicating a destabilization of the inactivated state. Furthermore, both HHT-5371 and -5411 recovered from inactivation significantly faster after drug block than that of the wild type channel. Our data demonstrate that accelerated recovery of HHT-5371 and HHT-5411 decreased accumulation of these channels in inactivation during pulse trains and suggest a close link between inactivation gating of the channel and use-dependent block by phenylalkylamines and benzothiazepines and provide evidence of a role for the transmembrane and cytoplasmic regions of IVS5 in the use-dependent block by diltiazem and verapamil.

Photochemical localization of the semotiadil binding region within the cardiac Ca2+ channel alpha1 subunit. Comparison with the skeletal muscle counterpart

We have previously identified the binding region of a new Ca2+ antagonist semotiadil in the skeletal muscle Ca2+ channel. To the same semotiadil derivatives, the cardiac counterpart showed distinct and different binding characteristics: semotiadil and its photoaffinity analog D51-4700 inhibited [3H]PN200-110 binding to cardiac membrane preparations with IC50 values of 13-20 microM, which are 10 times higher than those in skeletal muscle. Hill slopes of the binding inhibition were 0.94-1.0 for the cardiac channels compared to 0.63-0.67 for the skeletal muscle channels. A possible explanation for the difference is that the semotiadil binding site is differently conferred in cardiac and skeletal muscle Ca2+ channels. To reveal this within the primary structure, photoaffinity labeling of cardiac membranes was employed. [3H]D51-4700 was photo-incorporated in several polypeptides but only the alpha1 subunit of the Ca2+ channel was photolabeled in a specific manner. Antibody mapping of the [3H]D51-4700-labeled alpha1 subunit with several anti-peptide antibodies revealed that the labeled site was located solely in a peptide fragment between Cys1461 and Lys1529. This region encompasses the labeled site of skeletal muscle, but contains several non-identical amino acid residues, which may participate in expressing different binding characteristics between the two muscle type Ca2+ channels.

Molecular cloning and functional expression of a skeletal muscle dihydropyridine receptor from Rana catesbeiana

In skeletal muscle the dihydropyridine receptor is the voltage sensor for excitation-contraction coupling and an L-type Ca2+ channel. We cloned a dihydropyridine receptor (named Fgalpha1S) from frog skeletal muscle, where excitation-contraction coupling has been studied most extensively. Fgalpha1S contains 5600 base pairs coding for 1688 amino acids. It is highly homologous with, and of the same length as, the C-truncated form predominant in rabbit muscle. The primary sequence has every feature needed to be an L-type Ca2+ channel and a skeletal-type voltage sensor. Currents expressed in tsA201 cells had rapid activation (5-10 ms half-time) and Ca2+-dependent inactivation. Although functional expression of the full Fgalpha1S was difficult, the chimera consisting of Fgalpha1S domain I in the rabbit cardiac Ca channel had high expression and a rapidly activating current. The slow native activation is therefore not determined solely by the alpha1 subunit sequence. Its Ca2+-dependent inactivation strengthens the notion that in rabbit skeletal muscle this capability is inhibited by a C-terminal stretch (Adams, B., and Tanabe, T. (1997) J. Gen. Physiol. 110, 379-389). This molecule constitutes a new tool for studies of excitation-contraction coupling, gating, modulation, and gene expression.

Multiple modulation pathways of calcium channel activity by a beta subunit. Direct evidence of beta subunit participation in membrane trafficking of the alpha1C subunit

In order to study the precise mechanisms of alpha1 subunit modulation by an auxiliary beta subunit of voltage-dependent calcium channels, a recombinant beta3 subunit fusion protein was produced and introduced into oocytes that express the human alpha1C subunit. Injection of the beta3 subunit protein rapidly modulated the current kinetics and voltage dependence of activation, whereas massive augmentation of peak current amplitude occurred over a longer time scale. Consistent with the latter, a severalfold increase in the amount of the alpha1C subunit in the plasma membrane was detected by quantitative confocal laser-scanning microscopy after beta3 subunit injection. Pretreatment of oocytes with bafilomycin A1, a vacuolar type H+-ATPase inhibitor, abolished the increase of the alpha1C subunit in the plasma membrane, attenuated current increase, but did not affect the modulation of current kinetics and voltage dependence by the beta3 subunit. These results provide clear evidence that the beta subunit modifies the calcium channel complex in a binary fashion; one is an allosteric modulation of the alpha1 subunit function and the other is a chaperoning of the alpha1 subunit to the plasma membrane.

Differential interactions of the C terminus and the cytoplasmic I-II loop of neuronal Ca2+ channels with G-protein alpha and beta gamma subunits. I. Molecular determination

Interactions of G-protein alpha (Galpha) and beta gamma subunits (Gbeta gamma) with N- (alpha1B) and P/Q-type (alpha1A) Ca2+ channels were investigated using the Xenopus oocyte expression system. Gi3alpha was found to inhibit both N- and P/Q-type channels by receptor agonists, whereas Gbeta1 gamma2 was responsible for prepulse facilitation of N-type channels. L-type channels (alpha1C) were not regulated by Galpha or Gbeta gamma. For N-type, prepulse facilitation mediated via Gbeta gamma was impaired when the cytoplasmic I-II loop (loop 1) was deleted or replaced with the alpha1C loop 1. Galpha-mediated inhibitions were also impaired by substitution of the alpha1C loop 1, but only when the C terminus was deleted. For P/Q-type, by contrast, deletion of the C terminus alone diminished Galpha-mediated inhibition. Moreover, a chimera of L-type with the alpha1B loop 1 gained Gbeta gamma-dependent facilitation, whereas an L-type chimera with the N- or P/Q-type C terminus gained Galpha-mediated inhibition. These findings provide evidence that loop 1 of N-type channels is a regulatory site for Gbeta gamma and the C termini of P/Q- and N-types for Galpha.

Physiological features of visceral smooth muscle cells, with special reference to receptors and ion channels

Visceral smooth muscle cells (VSMC) play an essential role, through changes in their contraction-relaxation cycle, in the maintenance of homeostasis in biological systems. The features of these cells differ markedly by tissue and by species; moreover, there are often regional differences within a given tissue. The biophysical features used to investigate ion channels in VSMC have progressed from the original extracellular recording methods (large electrode, single or double sucrose gap methods), to the intracellular (microelectrode) recording method, and then to methods for recording from membrane fractions (patch-clamp, including cell-attached patch-clamp, methods). Remarkable advances are now being made thanks to the application of these more modern biophysical procedures and to the development of techniques in molecular biology. Even so, we still have much to learn about the physiological features of these channels and about their contribution to the activity of both cell and tissue. In this review, we take a detailed look at ion channels in VSMC and at receptor-operated ion channels in particular; we look at their interaction with the contraction-relaxation cycle in individual VSMC and especially at the way in which their activity is related to Ca2+ movements and Ca2+ homeostasis in the cell. In sections II and III, we discuss research findings mainly derived from the use of the microelectrode, although we also introduce work done using the patch-clamp procedure. These sections cover work on the electrical activity of VSMC membranes (sect. II) and on neuromuscular transmission (sect. III). In sections IV and V, we discuss work done, using the patch-clamp procedure, on individual ion channels (Na+, Ca2+, K+, and Cl-; sect. IV) and on various types of receptor-operated ion channels (with or without coupled GTP-binding proteins and voltage dependent and independent; sect. V). In sect. VI, we look at work done on the role of Ca2+ in VSMC using the patch-clamp procedure, biochemical procedures, measurements of Ca2+ transients, and Ca2+ sensitivity of contractile proteins of VSMC. We discuss the way in which Ca2+ mobilization occurs after membrane activation (Ca2+ influx and efflux through the surface membrane, Ca2+ release from and uptake into the sarcoplasmic reticulum, and dynamic changes in Ca2+ within the cytosol). In this article, we make only limited reference to vascular smooth muscle research, since we reviewed the features of ion channels in vascular tissues only recently.

Ca2+ channel sensitivity towards the blocker isradipine is affected by alternative splicing of the human alpha1C subunit gene

L-type Ca2+ channels are important targets for drugs, such as dihydropyridines (DHPs), in the treatment of cardiovascular diseases. Channel expression is regulated by alternative splicing. It has been suggested that in the cardiovascular system tissue-specific expression of different L-type Ca2+ channel splice variants may underlie the observed differences in sensitivities to channel block by DHPs. We investigated the sensitivity of Ca2+ channel splice variants derived from the human alpha1C gene to the DHP isradipine. Among seven alpha1C channels we observed up to 10-fold differences in IC50 values for isradipine, as well as changes in the voltage dependence of DHP action.

Structural basis of drug binding to L Ca2+ channels

At least five different types of voltage-gated Ca2+ channels exist in electrically excitable mammalian cells. Only one type, the family of L-type Ca2+ channels (L channels), contains high-affinity binding domains within their alpha 1-subunits for different chemical classes of drugs (Ca2+ channel antagonists; exemplified by isradipine, verapamil and diltiazem). Their stereoselective, high-affinity binding induces block of channel-mediated Ca2+ inward currents in heart and smooth muscle, resulting in antihypertensive, cardiodepressive and antiarrhythmic effects. Amino acids involved in drug binding have recently been identified using photoaffinity labelling, chimeric alpha 1-subunits and site-directed mutagenesis. Insertion of the drug-binding amino acids enabled the transfer of drug-sensitivity into Ca2+ channels that are insensitive to Ca2+ channel antagonists ('gain-of-function' approach). In this review, Jörg Striessing and colleagues summarize the present knowledge about the molecular architecture of L channel drug-binding domains and the implications for Ca2+ channel pharmacology and drug development.

Photochemical identification of transmembrane segment IVS6 as the binding region of semotiadil, a new modulator for the L-type voltage-dependent Ca2+ channel

To identify the binding domain of a new Ca2+ antagonist semotiadil on L-type Ca2+ channels from skeletal muscle, photolabeling was carried out by using an azidophenyl derivative of [3H]semotiadil. Photoincorporation was observed in several polypeptides of membrane triad preparations; the only specific photoincorporation was in the alpha1 subunit of the Ca2+ channel. After solubilization and purification, the photolabeled alpha1 subunit was subjected to proteolytic and CNBr cleavage followed by antibody mapping. Specific labeling was associated solely with the region of transmembrane segment S6 in repeat IV. Quantitative immunoprecipitation was found in the tryptic and the Lys-C/Glu-C fragments of 6.6 and 6.1 kDa, respectively. Further CNBr cleavage of the Lys-C digests produced two smaller fragments of 3.4 and 1.8 kDa that were included in the tryptic and Lys-C/Glu-C fragments. The smallest labeled fragments were: Tyr1350-Met1366 and Leu1367-Met1381 containing IVS6, a possible pore-forming region. The data suggest that semotiadil binds to a region that is overlapped with but not identical to those for phenylalkylamines, dihydropyridines and benzothiazepines. The present study also provides evidence that region IV represents an important component of a binding pocket for Ca2+ antagonists.

Functional characterization of ion permeation pathway in the N-type Ca2+ channel

Multiple types of high-voltage-activated Ca2+ channels, including L-, N-, P-, Q- and R-types have been distinguished from each other mainly employing pharmacological agents that selectively block particular types of Ca2+ channels. Except for the dihydropyridine-sensitive L-type Ca2+ channels, electrophysiological characterization has yet to be conducted thoroughly enough to biophysically distinguish the remaining Ca2+ channel types. In particular, the ion permeation properties of N-type Ca2+ channels have not been clarified, although the kinetic properties of both the L- and N-type Ca2+ channels are relatively well described. To establish ion conducting properties of the N-type Ca2+ channel, we examined a homogeneous population of recombinant N-type Ca2+ channels expressed in baby hamster kidney cells, using a conventional whole cell patch-clamp technique. The recombinant N-type Ca2+ channel, composed of the alpha1B, alpha2a, and beta1a subunits, displayed high-voltage-activated Ba2+ currents elicited by a test pulse more positive than -30 mV, and were strongly blocked by the N-type channel blocker omega-conotoxin-GVIA. In the presence of 110 mM Ba2+, the unitary current showed a slope conductance of 18.2 pS, characteristic of N-type channels. Ca2+ and Sr2+ resulted in smaller ion fluxes than Ba2+, with the ratio 1.0:0. 72:0.75 of maximum conductance in current-voltage relationships of Ba2+, Ca2+, and Sr2+ currents, respectively. In mixtures of Ba2+ and Ca2+, where the Ca2+ concentration was steadily increased in place of Ba2+, with the total concentration of Ba2+ and Ca2+ held constant at 3 mM, the current amplitude went through a clear minimum when 20% of the external Ba2+ was replaced by Ca+2. This anomalous mole fraction effect suggests an ion-binding site where two or more permeant ions can sit simultaneously. By using an external solution containing 110 mM Na+ without polyvalent cations, inward Na+ currents were evoked by test potentials more positive than -50 mV. These currents were activated and inactivated in a kinetic manner similar to that of Ba2+ currents. Application of inorganic Ca2+ antagonists blocked Ba2+ currents through N-type channels in a concentration-dependent manner. The rank order of inhibition was La3+ >/= Cd2+ >> Zn2+ > Ni2+ >/= Co2+. When a short strong depolarization was applied before test pulses of moderate depolarizing potentials, relief from channel blockade by La3+ and Cd2+ and subsequent channel reblocking was observed. The measured rate (2 x 10(8) M-1 s-1) of reblocking approached the diffusion-controlled limit. These results suggest that N-type Ca2+ channels share general features of a high affinity ion-binding site with the L-type Ca2+ channel, and that this site is easily accessible from the outside of the channel pore.

Molecular pharmacology of UK-118, 434-05, a permanently charged amlodipine analog

We studied the effects of UK-118, 434-05, a permanently charged form of amlodipine, on recombinant smooth muscle and cardiac L-type calcium channels to determine the distinct modulatory properties of the ionized form of amlodipine. We found that the short distance between the permanent charge group and the active dihydropyridine (DHP) ring of UK-118, 434-05 reduces the potency of this compound as an inhibitor of smooth muscle (alpha(1c-b)) L-type channels, and is similar to the effects of other charged DHP derivatives on cardiac (alpha(1c-a)) L-type channels. However, we found surprisingly that the tonic block of cardiac (alpha(1c-a)) L-type channels was more pronounced than the tonic block of smooth muscle (alpha(1c-b)) L-type channels. This result contrasts with the previously reported subunit-specificity of neutral DHP compounds, and suggests that interactions between the amlodipine charge group and site(s) on the L-type channel alpha1 subunit distinguish the action of charged from neutral DHPs and may contribute to amlodipine's unique pharmacological profile.

Construction of a high-affinity receptor site for dihydropyridine agonists and antagonists by single amino acid substitutions in a non-L-type Ca2+ channel

The activity of L-type Ca2+ channels is increased by dihydropyridine (DHP) agonists and inhibited by DHP antagonists, which are widely used in the therapy of cardiovascular disease. These drugs bind to the pore-forming alpha1 subunits of L-type Ca2+ channels. To define the minimal requirements for DHP binding and action, we constructed a high-affinity DHP receptor site by substituting a total of nine amino acid residues from DHP-sensitive L-type alpha1 subunits into the S5 and S6 transmembrane segments of domain III and the S6 transmembrane segment of domain IV of the DHP-insensitive P/Q-type alpha1A subunit. The resulting chimeric alpha1A/DHPS subunit bound DHP antagonists with high affinity in radioligand binding assays and was inhibited by DHP antagonists with high affinity in voltage clamp experiments. Substitution of these nine amino acid residues yielded 86% of the binding energy of the L-type alpha1C subunit and 92% of the binding energy of the L-type alpha1S subunit for the high-affinity DHP antagonist PN200-110. The activity of chimeric Ca2+ channels containing alpha1A/DHPS was increased 3.5 +/- 0.7-fold by the DHP agonist (-)Bay K8644. The effect of this agonist was stereoselective as in L-type Ca2+ channels since (+) Bay K8644 inhibited the activity of alpha1A/DHPS. The results show conclusively that DHP agonists and antagonists bind to a single receptor site at which they have opposite effects on Ca2+ channel activity. This site contains essential components from both domains III and IV, consistent with a domain interface model for binding and allosteric modulation of Ca2+ channel activity by DHPs.

Nine L-type amino acid residues confer full 1,4-dihydropyridine sensitivity to the neuronal calcium channel alpha1A subunit. Role of L-type Met1188

Pharmacological modulation by 1,4-dihydropyridines is a central feature of L-type calcium channels. Recently, eight L-type amino acid residues in transmembrane segments IIIS5, IIIS6, and IVS6 of the calcium channel alpha1 subunit were identified to substantially contribute to 1,4-dihydropyridine sensitivity. To determine whether these eight L-type residues (Thr1066, Gln1070, Ile1180, Ile1183, Tyr1490, Met1491, Ile1497, and Ile1498; alpha1C-a numbering) are sufficient to form a high affinity 1,4-dihydropyridine binding site in a non-L-type calcium channel, we transferred them to the 1, 4-dihydropyridine-insensitive alpha1A subunit using site-directed mutagenesis. 1,4-Dihydropyridine agonist and antagonist modulation of barium inward currents mediated by the mutant alpha1A subunits, coexpressed with alpha2delta and beta1a subunits in Xenopus laevis oocytes, was investigated with the two-microelectrode voltage clamp technique. The resulting mutant alpha1A-DHPi displayed low sensitivity for 1,4-dihydropyridines. Analysis of the 1,4-dihydropyridine binding region of an ancestral L-type alpha1 subunit previously cloned from Musca domestica body wall muscle led to the identification of Met1188 (alpha1C-a numbering) as an additional critical constituent of the L-type 1,4-dihydropyridine binding domain. The introduction of this residue into alpha1A-DHPi restored full sensitivity for 1,4-dihydropyridines. It also transferred functional properties considered hallmarks of 1, 4-dihydropyridine agonist and antagonist effects (i.e. stereoselectivity, voltage dependence of drug modulation, and agonist-induced shift in the voltage-dependence of activation). Our gain-of-function mutants provide an excellent model for future studies of the structure-activity relationship of 1, 4-dihydropyridines to obtain critical structural information for the development of drugs for neuronal, non-L-type calcium channels.

Molecular studies on the voltage dependence of dihydropyridine action on L-type Ca2+ channels. Critical involvement of tyrosine residues in motif IIIS6 and IVS6

The interaction site(s) of dihydropyridine (DHP) antagonists and agonists have been identified by site-directed mutagenesis and localized on motifs IIIS5, IIIS6, and IVS6 of L-type voltage-gated calcium channels. In this study, we investigated the voltage-dependent action of DHPs with mutants of the IIIS6 and IVS6 segments of a cardiac calcium channel. Tyrosine residues in both motifs (Tyr1178 and Tyr1489) strongly contributed to the action of DHP agonists and antagonists. When these two sites were mutated, the communication between the voltage sensor and the DHP interaction site(s) was substantially impaired. In contrast, mutants of a nearby Ile (Ile1182) had much less influence on DHP agonist and antagonist interaction, and the voltage dependence of DHP antagonists was very similar to that of the wild type. The effect of a mutating of Ile1182, on agonist or antagonist action, however, depended strongly on the type of amino acid change. When Ile1182 was substituted with alanine, small changes were noted for DHP agonist and antagonist action. Changing this site into phenylalanine, however, significantly decreased the action of the DHP antagonist. These data show that Ile1182 can preferentially interact with DHP antagonists, but has a lesser contribution in agonist interaction. Thus, even though the agonist and antagonist interaction sites for DHPs with L-type calcium channels may overlap, some amino acids in this site may exhibit a preference for either DHP enantiomers.

Transfer of the high affinity dihydropyridine sensitivity from L-type To non-L-type calcium channel

To elucidate the mechanism underlying the interaction between the L-type Ca2+ channel and the dihydropyridines (DHPs), contribution of the repeat III was studied by constructing chimeras between the DHP-sensitive alpha1C and DHP-insensitive alpha1E subunits. The chimeras were transiently expressed in human embryonic kidney 293 cells and the whole-cell Ba2+ current (IBa) was recorded. Mutating Thr1061 to Tyr in IIIS5 of the alpha1C sequence completely abolished the inhibition and stimulation of IBa by the antagonist (+)-isradipine and agonist (-)-Bay K 8644, whereas mutating Gln1065 to Met in IIIS5 decreased the affinity for isradipine 100-fold without affecting the stimulating effect of Bay K 8644. The conserved amino acid residue Tyr1174 in IIIS6 of the alpha1C subunit was necessary for the high affinity DHP block. The DHP-dependent block and stimulation of IBa were transferred to the alpha1E channel by the mutation of two amino acid residues in IIIS5 (Y1295T, M1299Q), three residues in IIIS6 (F1406I, F1409I, V1414M) and three residues in IVS6 (I1706Y, F1707M, L1714I). The mutated alpha1E channel was stimulated 2.8-fold by 1 microM Bay K 8644 and blocked by isradipine with an IC50 value of 60 nM. These results show that mutation of Thr1061 in the alpha1C sequence results in a DHP-insensitive L-type channel and that transfer of the high affinity DHP sensitivity requires mutation of eight amino acid residues in the alpha1E sequence.

Inhibitory effects of a new neuroprotective diltiazem analogue, T-477, on cloned brain Ca2+ channels expressed in Xenopus oocytes

A new neuroprotective agent T-477 ((R)-(+)-2-(4-chlorophenyl)-2,3-dihydro-4-diethylaminoacetyl-4H-1, 4-benzothiazine) and diltiazem are similar in chemical structures but they show different biological properties. To investigate the properties that differentiate T-477 from diltiazem, we examined the effects of the compounds on a cardiac L-type and brain non-L-type Ca2+ channels expressed in Xenopus oocytes. Cardiac L-type currents were inhibited by Ca2+ channel antagonists with an order of potency; PN200-110 isradipine > > diltiazem > T-477. Brain BI (class A)-, BII (class E)- and BIII (class B)-type Ca2+ channel currents were inhibited by T-477 with an IC50 of 45, 74 and 59 microM, respectively, whereas diltiazem barely inhibited the brain non-L-type channels and PN200-110 had no effect. T-477 caused a marked use- and frequency-dependent block of BI Ca2+ channel currents, as demonstrated by a cumulative increase of the block during a train of depolarizing pulses, which seemed to be due to a slow repriming of the drug-bound channels from inactivation. These results suggest that T-477 exerts neuroprotection of brain neurons from ischemic neuronal damage through its inhibitory action on brain Ca2+ channels that differentiates T-477 from cardiac L-type channel blockers such as diltiazem and PN200-110.

Analysis of the dihydropyridine receptor site of L-type calcium channels by alanine-scanning mutagenesis

The dihydropyridine Ca2+ antagonist drugs used in the therapy of cardiovacular disorders inhibit L-type Ca2+ channels by binding to a single high affinity site. Photoaffinity labeling and analysis of mutant Ca2+ channels implicate the IIIS6 and IVS6 segments in high affinity binding. The amino acid residues that are required for high affinity binding of dihydropyridine Ca2+ channel antagonists were probed by alanine-scanning mutagenesis of the alpha1C subunit, transient expression in mammalian cells, and analysis by measurements of ligand binding and block of Ba2+ currents through expressed Ca2+ channels. Eleven amino acid residues in transmembrane segments IIIS6 and IVS6 were identified whose mutation reduced the affinity for the Ca2+ antagonist PN200-110 by 2-25-fold. Both amino acid residues conserved among Ca2+ channels and those specific to L-type Ca2+ channels were found to be required for high affinity dihydropyridine binding. In addition, mutation F1462A increased the affinity for the dihydropyridine Ca2+ antagonist PN200-110 by 416-fold with no effect on the affinity for the Ca2+ agonist Bay K8644. The residues in transmembrane segments IIIS6 and IVS6 that are required for high affinity binding are primarily aligned on single faces of these two alpha helices, supporting a "domain interface model" of dihydropyridine binding and action in which the IIIS6 and IVS6 interact to form a high affinity dihydropyridine receptor site on L-type Ca2+ channels.

Voltage-dependent binding and calcium channel current inhibition by an anti-alpha 1D subunit antibody in rat dorsal root ganglion neurones and guinea-pig myocytes

1. The presence of calcium channel alpha 1D subunit mRNA in cultured rat dorsal root ganglion (DRG) neurones and guinea-pig cardiac myocytes was demonstrated using the reverse transcriptase-polymerase chain reaction. 2. An antipeptide antibody targeted at a region of the voltage-dependent calcium channel alpha 1D subunit C-terminal to the pore-forming SS1-SS2 loop in domain IV (amino acids 1417-1434) only bound to this exofacial epitope if the DRG neurones and cardiac myocytes were depolarized with 30 mM K+. 3. Incubation of cells under depolarizing conditions for 2-4 h with the antibody resulted in a maximal inhibition of inward current density of 49% (P < 0.005) for DRGs and 30% (P < 0.05) for cardiac myocytes when compared with controls. 4. S-(-)-Bay K 8644 (1 microM) enhanced calcium channel currents in DRGs by 75 +/- 19% (n = 5) in neurones incubated under depolarizing conditions with antibody that had been preabsorbed with its immunizing peptide (100 micrograms ml-1). This was significantly (P < 0.05) larger than the enhancement by S-(-)-Bay K 8644 that was seen with cells incubated under identical conditions but with antibody alone, which was 15 +/- 4% (n = 5). 5. These results demonstrate the presence of calcium channel alpha 1D subunits in rat DRG neurones and guinea-pig cardiac myocytes. They also show that amino acids 1417-1434 of the alpha 1D subunit are only exposed to the extracellular face of the membrane following depolarization and that the binding of an antibody to these amino acids attenuates calcium channel current and reduces the ability of S-(-)-Bay K 8644 to enhance this current, indicating that it is an L-type current that is attenuated.

Molecular structures involved in L-type calcium channel inactivation. Role of the carboxyl-terminal region encoded by exons 40-42 in alpha1C subunit in the kinetics and Ca2+ dependence of inactivation

The pore-forming alpha1C subunit is the principal component of the voltage-sensitive L-type Ca2+ channel. It has a long cytoplasmic carboxyl-terminal tail playing a critical role in channel gating. The expression of alpha1C subunits is characterized by alternative splicing, which generates its multiple isoforms. cDNA cloning points to a diversity of human hippocampus alpha1C transcripts in the region of exons 40-43 that encode a part of the 662-amino acid carboxyl terminus. We compared electrophysiological properties of the well defined 2138-amino acid alpha1C,77 channel isoform with two splice variants, alpha1C,72 and alpha1C,86. They contain alterations in the carboxyl terminus due to alternative splicing of exons 40-42. The 2157-amino acid alpha1C,72 isoform contains an insertion of 19 amino acids at position 1575. The 2139-amino acid alpha1C,86 has 80 amino acids replaced in positions 1572-1651 of alpha1C,77 by a non-identical sequence of 81 amino acids. When expressed in Xenopus oocytes, all three splice variants retained high sensitivity toward dihydropyridine blockers but showed large differences in gating properties. Unlike alpha1C,77 and alpha1C,72, Ba2+ currents (IBa) through alpha1C,86 inactivated 8-10 times faster at +20 mV, and its inactivation rate was strongly voltage-dependent. Compared to alpha1C,77, the inactivation curves of IBa through alpha1C,86 and alpha1C,72 channels were shifted toward more negative voltages by 11 and 6 mV, respectively. Unlike alpha1C,77 and alpha1C,72, the alpha1C,86 channel lacks a Ca2+-dependent component of inactivation. Thus the segment 1572-1651 of the cytoplasmic tail of alpha1C is critical for the kinetics as well as for the Ca2+ and voltage dependence of L-type Ca2+ channel gating.