Molecular determinants of voltage-dependent inactivation in calcium channels

Structural elements in domain IV that influence biophysical and pharmacological properties of human alpha1A-containing high-voltage-activated calcium channels

We have cloned two splice variants of the human homolog of the alpha1A subunit of voltage-gated Ca2+ channels. The sequences of human alpha1A-1 and alpha1A-2 code for proteins of 2510 and 2662 amino acids, respectively. Human alpha1A-2alpha2bdeltabeta1b Ca2+ channels expressed in HEK293 cells activate rapidly (tau+10mV = 2.2 ms), deactivate rapidly (tau-90mV = 148 micros), inactivate slowly (tau+10mV = 690 ms), and have peak currents at a potential of +10 mV with 15 mM Ba2+ as charge carrier. In HEK293 cells transient expression of Ca2+ channels containing alpha1A/B(f), an alpha1A subunit containing a 112 amino acid segment of alpha1B-1 sequence in the IVS3-IVSS1 region, resulted in Ba2+ currents that were 30-fold larger compared to wild-type (wt) alpha1A-2-containing Ca2+ channels, and had inactivation kinetics similar to those of alpha1B-1-containing Ca2+ channels. Cells transiently transfected with alpha1A/B(f)alpha2bdeltabeta1b expressed higher levels of the alpha1, alpha2bdelta, and beta1b subunit polypeptides as detected by immunoblot analysis. By mutation analysis we identified two locations in domain IV within the extracellular loops S3-S4 (N1655P1656) and S5-SS1 (E1740) that influence the biophysical properties of alpha1A. alpha1AE1740R resulted in a threefold increase in current magnitude, a -10 mV shift in steady-state inactivation, and an altered Ba2+ current inactivation, but did not affect ion selectivity. The deletion mutant alpha1ADeltaNP shifted steady-state inactivation by -20 mV and increased the fast component of current inactivation twofold. The potency and rate of block by omega-Aga IVA was increased with alpha1ADeltaNP. These results demonstrate that the IVS3-S4 and IVS5-SS1 linkers play an essential role in determining multiple biophysical and pharmacological properties of alpha1A-containing Ca2+ channels.

Voltage and calcium use the same molecular determinants to inactivate calcium channels

During sustained depolarization, voltage-gated Ca2+ channels progressively undergo a transition to a nonconducting, inactivated state, preventing Ca2+ overload of the cell. This transition can be triggered either by the membrane potential (voltage-dependent inactivation) or by the consecutive entry of Ca2+ (Ca2+-dependent inactivation), depending on the type of Ca2+ channel. These two types of inactivation are suspected to arise from distinct underlying mechanisms, relying on specific molecular sequences of the different pore-forming Ca2+ channel subunits. Here we report that the voltage-dependent inactivation (of the alpha1A Ca2+ channel) and the Ca2+-dependent inactivation (of the alpha1C Ca2+ channel) are similarly influenced by Ca2+ channel beta subunits. The same molecular determinants of the beta subunit, and therefore the same subunit interactions, influence both types of inactivation. These results strongly suggest that the voltage and the Ca2+-dependent transitions leading to channel inactivation use homologous structures of the different alpha1 subunits and occur through the same molecular process. A model of inactivation taking into account these new data is presented.

Enhanced slow inactivation by V445M: a sodium channel mutation associated with myotonia

Over 20 different missense mutations in the alpha subunit of the adult skeletal muscle Na channel have been identified in families with either myotonia (muscle stiffness) or periodic paralysis, or both. The V445M mutation was recently found in a family with myotonia but no weakness. This mutation in transmembrane segment IS6 is novel because no other disease-associated mutations are in domain I. Na currents were recorded from V445M and wild-type channels transiently expressed in human embryonic kidney cells. In common with other myotonic mutants studied to date, fast gating behavior was altered by V445M in a manner predicted to increase excitability: an impairment of fast inactivation increased the persistent Na current at 10 ms and activation had a hyperpolarized shift (4 mV). In contrast, slow inactivation was enhanced by V445M due to both a slower recovery (10 mV left shift in beta(V)) and an accelerated entry rate (1.6-fold). Our results provide additional evidence that IS6 is crucial for slow inactivation and show that enhanced slow inactivation cannot prevent myotonia, whereas previous studies have shown that disrupted slow inactivation predisposes to episodic paralysis.

Effect of phosphorylation on L-type calcium current in ventricular myocytes dialysed with proteolytic enzymes

1. L-Type Ca2+ channels play important roles in cardiac excitation and conduction. The present study used the whole-cell patch-clamp technique to investigate properties of Ca2+ channels in guinea-pig isolated ventricular myocytes. The effects of internal application of the proteolytic enzymes trypsin and carboxypeptidase (CBP) on the whole-cell L-type Ca2+ current (ICa) were determined. When the effects of the enzymes on ICa had reached steady state, the effects of isoprenaline (ISP) or 2,3-butane-dione monoxime (BDM), which increase and decrease channel phosphorylation, respectively, were examined. The effects of these agents were compared with those observed in the absence of enzyme pretreatment. 2. The amplitude and inactivation characteristics of ICa during depolarizing voltage-clamp commands to +10 mV (0.1 Hz) were determined at 37 degrees C. 3. Trypsin and CBP (both at concentrations of 1 mg/mL in the pipette solution) increased the amplitude of ICa 4.2- and 2.8-fold, respectively, and each enzyme increased the time constant of the slowly inactivating current by 50%. 4. Trypsin decreased the potential at which ICa was half maximally activated from (mean +/- SD) -1.4 +/- 2.2 mV (n = 9) to -11.3 +/- 2.5 mV (n = 7). Although CBP increased ICa amplitude, it did not shift the half-maximal activation voltage. Maximum conductance was increased 5.3-fold by trypsin and 2.2-fold by CBP. 5. Isoprenaline (1 mumol/L) had no effects in myocytes dialysed with trypsin, but significantly increased the current in myocytes dialysed with CBP by 8%. 6. At 12 mmol/L, BDM had no effect on current amplitude in the presence of trypsin, but decreased the time constant of slow inactivation to control values. After dialysis with CBP, BDM significantly decreased the maximum current by 11% and also decreased the rate of slow inactivation towards control values. 7. These data suggest that trypsin and CBP may have digested a part of the calcium channel that normally restricts current flow, but to different extents. The enzymes interacted with BDM and ISP in a fashion suggesting that two sites may influence the amplitude of the current and at least two other sites may influence the time course of the slowly inactivating current.

Structure and functional characterization of a novel human low-voltage activated calcium channel

We have isolated and characterized overlapping cDNAs encoding a novel, voltage-gated Ca2+ channel alpha1 subunit, alpha1H, from a human medullary thyroid carcinoma cell line. The alpha1H subunit is structurally similar to previously described alpha1 subunits. Northern blot analysis indicates that alpha1H mRNA is expressed throughout the brain, primarily in the amygdala, caudate nucleus, and putamen, as well as in several nonneuronal tissues, with relatively high levels in the liver, kidney, and heart. Ba2+ currents recorded from human embryonic kidney 293 cells transiently expressing alpha1H activated at relatively hyperpolarized potentials (-50 mV), rapidly inactivated (tau = 17 ms), and slowly deactivated. Similar results were observed in Xenopus oocytes expressing alpha1H. Single-channel measurements in human embryonic kidney 293 cells revealed a single-channel conductance of approximately 9 pS. These channels are blocked by Ni2+ (IC50 = 6.6 microM) and the T-type channel antagonists mibefradil (approximately 50% block at 1 microM) and amiloride (IC50 = 167 microM). Thus, alpha1H-containing channels exhibit biophysical and pharmacological properties characteristic of low voltage-activated, or T-type, Ca2+ channels.

Mutations in the EF-hand motif impair the inactivation of barium currents of the cardiac alpha1C channel

Calcium-dependent inactivation has been described as a negative feedback mechanism for regulating voltage-dependent calcium influx in cardiac cells. Most recent evidence points to the C-terminus of the alpha1C subunit, with its EF-hand binding motif, as being critical in this process. The EF-hand binding motif is mostly conserved between the C-termini of six of the seven alpha1 subunit Ca2+ channel genes. The role of E1537 in the C-terminus of the alpha1C calcium channel inactivation was investigated here after expression in Xenopus laevis oocytes. Whole-cell currents were measured in the presence of 10 mM Ba2+ or 10 mM Ca2+ after intracellular injection of 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid. Against all expectations, our results showed a significant reduction in the rate of voltage-dependent inactivation as measured in Ba2+ solutions for all E1537 mutants, whereas calcium-dependent inactivation appeared unscathed. Replacing the negatively charged glutamate residue by neutral glutamine, glycine, serine, or alanine significantly reduced the rate of Ba2+-dependent inactivation by 1.5-fold (glutamine) to 3.5-fold (alanine). The overall rate of macroscopic inactivation measured in Ca2+ solutions was also reduced, although a careful examination of the distribution of the fast and slow time constants suggests that only the slow time constant was significantly reduced in the mutant channels. The fast time constant, the hallmark of Ca2+-dependent inactivation, remained remarkably constant among wild-type and mutant channels. Moreover, inactivation of E1537A channels, in both Ca2+ and Ba2+ solutions, appeared to decrease with membrane depolarization, whereas inactivation of wild-type channels became faster with positive voltages. All together, our results showed that E1537 mutations impaired voltage-dependent inactivation and suggest that the proximal part of the C-terminus may play a role in voltage-dependent inactivation in L-type alpha1C channels.

Molecular basis of drug interaction with L-type Ca2+ channels

Different types of voltage-gated Ca2+ channels exist in the plasma membrane of electrically excitable cells. By controlling depolarization-induced Ca2+ entry into cells they serve important physiological functions, such as excitation-contraction coupling, neurotransmitter and hormone secretion, and neuronal plasticity. Their function is fine-tuned by a variety of modulators, such as enzymes and G-proteins. Block of so-called L-type Ca2+ channels by drugs is exploited as a therapeutic principle to treat cardiovascular disorders, such as hypertension. More recently, block of so-called non-L-type Ca2+ channels was found to exert therapeutic effects in the treatment of severe pain and ischemic stroke. As the subunits of different Ca2+ channel types have been cloned, the modulatory sites for enzymes, G-proteins, and drugs can now be determined using molecular engineering and heterologous expression. Here we summarize recent work that has allowed us to determine the sites of action of L-type Ca2+ channel modulators. Together with previous biochemical, electrophysiological, and drug binding data these results provide exciting insight into the molecular pharmacology of this voltage-gated Ca2+ channel family.

Structural features determining differential receptor regulation of neuronal Ca channels

Dihydropyridine-insensitive Ca channels are subject to direct receptor G-protein-mediated inhibition to differing extents. alpha1B channels are much more strongly modulated than alpha1E channels. To understand the structural basis for this difference, we have constructed and expressed various alpha1B and alpha1E chimeric Ca channels and examined their regulation by kappa-opioid receptors. Replacement of the first membrane-spanning domain of alpha1E with the corresponding region of alpha1B resulted in a chimeric Ca channel that was modulated by kappa-opioid receptors to a significantly greater extent than alpha1E. Transfer of the N terminus and I/II loop from alpha1B in addition to domain I resulted in a chimeric channel that was modulated to the same extent as alpha1B. Other regions of the molecule do not appear to contribute significantly to the degree of inhibition obtained, although the C terminus may contribute to facilitation.

Facilitation of rabbit alpha1B calcium channels: involvement of endogenous Gbetagamma subunits

1. The alpha1B (N-type) calcium channel shows strong G protein modulation in the presence of G protein activators or Gbetagamma subunits. Using transient expression in COS-7 cells of alpha1B together with the accessory subunits alpha2-delta and beta2a, we have examined the role of endogenous Gbetagamma subunits in the tonic modulation of alpha1B, and compared this with modulation by exogenously expressed Gbetagamma subunits. 2. Prepulse facilitation of control alpha1B/alpha2-delta/beta2a currents was always observed. This suggests the existence of tonic modulation of alpha1B subunits. To determine whether endogenous Gbetagamma is involved in the facilitation observed in control conditions, the betaARK1 Gbetagamma-binding domain (amino acids 495-689) was overexpressed, in order to bind free Gbetagamma subunits. The extent of control prepulse-induced facilitation was significantly reduced, both in terms of current amplitude and the rate of current activation. In agreement with this, GDPbetaS also reduced the control facilitation. 3. Co-expression of the Gbeta1gamma2 subunit, together with the alpha1B/alpha2-delta/beta2a calcium channel combination, resulted in a marked degree of depolarizing prepulse-reversible inhibition of the whole-cell ICa or IBa. Both slowing of current activation and inhibition of the maximum current amplitude were observed, accompanied by a depolarizing shift in the mid-point of the voltage dependence of activation. Activation of endogenous Gbetagamma subunits by dialysis with GTPgammaS produced a smaller degree of prepulse-reversible inhibition. 4. The rate of reinhibition of alpha1B currents by activated G protein, following a depolarizing prepulse, was much faster with Gbeta1gamma2 than for the decay of facilitation in control cells. Furthermore, betaARK1 (495-689) co-expression markedly slowed the control rate of reinhibition, suggesting that the kinetics of reinhibition depend on the concentration of free endogenous or exogenously expressed Gbetagamma in the cells. In contrast, the rate of loss of inhibition during a depolarizing prepulse did not vary significantly between the different conditions examined. 5. These findings indicate that, in this system, the voltage-dependent facilitation of alpha1B that is observed under control conditions occurs as a result of endogenous free Gbetagamma binding to alpha1B.

Role of domain I of neuronal Ca2+ channel alpha1 subunits in G protein modulation

1. We studied the G protein inhibition of heteromultimeric neuronal Ca2+ channels by constructing a series of chimeric channels between the strongly modulated alpha1B subunit and the alpha1E(rbEII) subunit, which showed no modulation. 2. In parallel studies, alpha1 subunit constructs were co-expressed together with the accessory Ca2+ channel alpha2-delta and beta2a subunits in mammalian (COS-7) cells and Xenopus oocytes. G protein inhibition of expressed Ca2+ channel currents was induced by co-transfection of Gbeta1 and Ggamma2 subunits in COS-7 cells or activation of co-expressed dopamine (D2) receptors by quinpirole (100 nM) in oocytes. 3. The data indicate that transfer of the alpha1B region containing the N-terminal, domain I and the I-II loop (i.e. the alpha1B1-483 sequence), conferred G protein modulation on alpha1E(rbEII), both in terms of a slowing of activation kinetics and a reduction in current amplitude. 4. In contrast, the data are not consistent with the I-II loop and/or the C-terminal forming a unique site for G protein modulation.

Inactivation of voltage-gated cardiac K+ channels

Inactivation is the process by which an open channel enters a stable nonconducting conformation after a depolarizing change in membrane potential. Inactivation is a widespread property of many different types of voltage-gated ion channels. Recent advances in the molecular biology of K+ channels have elucidated two mechanistically distinct types of inactivation, N-type and C-type. N-type inactivation involves occlusion of the intracellular mouth of the pore through binding of a short segment of residues at the extreme N-terminal. In contrast to this "tethered ball" mechanism of N-type inactivation, C-type inactivation involves movement of conserved core domain residues that result in closure of the external mouth of the pore. Although C-type inactivation can show rapid kinetics that approach those observed for N-type inactivation, it is often thought of as a slowly developing and slowly recovering process. Current models of C-type inactivation also suggest that this process involves a relatively localized change in conformation of residues near the external mouth of the permeation pathway. The rate of C-type inactivation and recovery can be strongly influenced by other factors, such as N-type inactivation, drug binding, and changes in [K+]o. These interactions make C-type inactivation an important biophysical process in determining such physiologically important properties as refractoriness and drug binding. C-type inactivation is currently viewed as arising from small-scale rearrangements at the external mouth of the pore. This review will examine the multiplicity of interactions of C-type inactivation with N-terminal-mediated inactivation and drug binding that suggest that our current view of C-type inactivation is incomplete. This review will suggest that C-type inactivation must involve larger-scale movements of transmembrane-spanning domains and that such movements contribute to the diversity of kinetic properties observed for C-type inactivation.

Subunit interaction sites in voltage-dependent Ca2+ channels: role in channel function

Voltage-dependent Ca2+ channels are heteromeric complexes found in the plasma membrane of virtually all cell types and show a high level of electrophysiological and pharmacological diversity. Associated with the pore-forming alpha 1 subunit are the membrane anchored, largely extracellular alpha2-delta, the cytoplasmic beta and sometimes a transmembrane gamma subunit; these subunits dramatically influence the properties and surface expression of these channels. Effects vary depending on subunit isoforms, suggesting that functional diversity of native channels reflects heterogeneity of combinations. Interaction sites between subunits have been identified and advances have been made in our understanding of the molecular basis of functional effects of the auxiliary subunits, their capacity to be regulated by G proteins, and their interaction with related cellular systems.

Familial hemiplegic migraine mutations change alpha1A Ca2+ channel kinetics

Missense mutations in the pore-forming human alpha1A subunit of neuronal P/Q-type Ca2+ channels are associated with familial hemiplegic migraine (FHM). The pathophysiological consequences of these mutations are unknown. We have introduced the four single mutations reported for the human alpha1A subunit into the conserved rabbit alpha1A (R192Q, T666M, V714A, and I1819L) and investigated possible changes in channel function after functional expression of mutant subunits in Xenopus laevis oocytes. Changes in channel gating were observed for mutants T666M, V714A, and I1819L but not for R192Q. Ba2+ current (IBa) inactivation was slightly faster in mutants T666M and V714A than in wild type. The time course of recovery from channel inactivation was slower than in wild type in T666M and accelerated in V714A and I1819L. As a consequence, accumulation of channel inactivation during a train of 1-Hz pulses was more pronounced for mutant T666M and less pronounced for V714A and I1819A. Our data demonstrate that three of the four FHM mutations, located at the putative channel pore, alter inactivation gating and provide a pathophysiological basis for the postulated neuronal instability in patients with FHM.

Role of the S4 in cooperativity of voltage-dependent potassium channel activation

Charged residues in the S4 transmembrane segment of voltage-gated cation channels play a key role in opening channels in response to changes in voltage across the cell membrane. However, the molecular mechanism of channel activation is not well understood. To learn more about the role of the S4 in channel gating, we constructed chimeras in which S4 segments from several divergent potassium channels, Shab, Shal, Shaw, and Kv3.2, were inserted into a Shaker potassium channel background. These S4 donor channels have distinctly different voltage-dependent gating properties and S4 amino acid sequences. None of the S4 chimeras have the gating behavior of their respective S4 donor channels. The conductance-voltage relations of all S4 chimeras are shifted to more positive voltages and the slopes are decreased. There is no consistent correlation between the nominal charge content of the S4 and the slope of the conductance-voltage relation, suggesting that the mutations introduced by the S4 chimeras may alter cooperative interactions in the gating process. We compared the gating behavior of the Shaw S4 chimera with its parent channels, Shaker and Shaw, in detail. The Shaw S4 substitution alters activation gating profoundly without introducing obvious changes in other channel functions. Analysis of the voltage-dependent gating kinetics suggests that the dominant effect of the Shaw S4 substitution is to alter a single cooperative transition late in the activation pathway, making it rate limiting. This interpretation is supported further by studies of channels assembled from tandem heterodimer constructs with both Shaker and Shaw S4 subunits. Activation gating in the heterodimer channels can be predicted from the properties of the homotetrameric channels only if it is assumed that the mutations alter a cooperative transition in the activation pathway rather than independent transitions.

Slow inactivation of cardiac L-type Ca2+ channel induced by cold acclimation of guinea pig

Whole cell L-type Ca2+ current was recorded in ventricular myocytes dissociated from guinea pigs that were bred at ambient temperatures ranging between daily averages of 4 and 29 degrees C. The dynamic voltage range of inactivation, as measured using 400-ms conditioning pulses and a holding potential of -40 mV, extended from -50 to -20 mV in myocytes prepared in summer. In winter, the inactivation curve was shifted to more negative potentials than in summer. Double-pulse experiments revealed that the negative shift was due to slow-inactivation kinetics. The negative shift of inactivation could be induced in myocytes prepared from animals that had been kept at 5 degrees C for > 3 wk in the summer. The negative shift in Ca2+ current inactivation could be abolished by adding guanosine 5'-O-(2-thiodiphosphate) (5 mM) to the pipette solution, but not by adding staurosporine (2 microM) or 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (100 microM) to the bath. The cold acclimation may introduce the slow inactivation of the cardiac L-type Ca2+ channel through an unknown pertussis toxin-insensitive G protein.

Hair cell-specific splicing of mRNA for the alpha1D subunit of voltage-gated Ca2+ channels in the chicken's cochlea

The L-type voltage-gated Ca2+ channels that control tonic release of neurotransmitter from hair cells exhibit unusual electrophysiological properties: a low activation threshold, rapid activation and deactivation, and a lack of Ca2+-dependent inactivation. We have inquired whether these characteristics result from cell-specific splicing of the mRNA for the L-type alpha1D subunit that predominates in hair cells of the chicken's cochlea. The alpha1D subunit in hair cells contains three uncommon exons: one encoding a 26-aa insert in the cytoplasmic loop between repeats I and II, an alternative exon for transmembrane segment IIIS2, and a heretofore undescribed exon specifying a 10-aa insert in the cytoplasmic loop between segments IVS2 and IVS3. We propose that the alternative splicing of the alpha1D mRNA contributes to the unusual behavior of the hair cell's voltage-gated Ca2+ channels.

Molecular mechanism of use-dependent calcium channel block by phenylalkylamines: role of inactivation

The role of channel inactivation in the molecular mechanism of calcium (Ca2+) channel block by phenylalkylamines (PAA) was analyzed by designing mutant Ca2+ channels that carry the high affinity determinants of the PAA receptor site [Hockerman, G. H., Johnson, B. D., Scheuer, T., and Catterall, W. A. (1995) J. Biol. Chem. 270, 22119-22122] but inactivate at different rates. Use-dependent block by PAAs was studied after expressing the mutant Ca2+ channels in Xenopus oocytes. Substitution of single putative pore-orientated amino acids in segment IIIS6 by alanine (F-1499-A, F-1500-A, F-1510-A, I-1514-A, and F-1515-A) gradually slowed channel inactivation and simultaneously reduced inhibition of barium currents (I(Ba)) by (-)D600 upon depolarization by 100 ms steps at 0.1 Hz. This apparent reduction in drug sensitivity was only evident if test pulses were applied at a low frequency of 0.1 Hz and almost disappeared at the frequency of 1 Hz. (-)D600 slowed I(Ba) recovery after maintained membrane depolarization (1-3 sec) to a comparable extent in all channel constructs. A drug-induced delay in the onset of I(Ba) recovery from inactivation suggests that PAAs promote the transition to a deep inactivated channel conformation. These findings indicate that apparent PAA sensitivity of Ca2+ channels is not only defined by drug interaction with its receptor site but also crucially dependent on intrinsic gating properties of the channel molecule. A molecular model for PAA-Ca2+ channel interaction that accounts for the relationship between drug induced inactivation and channel block by PAA is proposed.

A novel muscle sodium channel mutation causes painful congenital myotonia

Mutations in the skeletal muscle voltage-gated sodium channel alpha-subunit gene (SCN4A) have been associated with a spectrum of inherited nondystrophic myotonias and periodic paralyses. Most disease-associated SCN4A alleles occur in portions of the gene that encode the third and fourth repeat domains with the conspicuous absence of mutations in domain 1. Here we describe a family segregating an unusual autosomal dominant congenital myotonia associated with debilitating pain especially severe in the intercostal muscles. A novel SCN4A mutation causing the replacement of Val445 in the sixth transmembrane segment of domain 1 with methionine was discovered in all affected individuals and is the likely genetic basis for the syndrome. Myotonia was resistant to treatment; however, the most severely affected family member responded dramatically to the sodium channel blocking agent flecainide.

Mutations in the alpha1 subunit of an L-type voltage-activated Ca2+ channel cause myotonia in Caenorhabditis elegans

The control of excitable cell action potentials is central to animal behavior. We show that the egl-19 gene plays a pivotal role in regulating muscle excitation and contraction in the nematode Caenorhabditis elegans and encodes the alphal subunit of a homologue of vertebrate L-type voltage-activated Ca2+ channels. Semi-dominant, gain-of-function mutations in egl-19 cause myotonia: mutant muscle action potentials are prolonged and the relaxation delayed. Partial loss-of-function mutations cause slow muscle depolarization and feeble contraction. The most severe loss-of-function mutants lack muscle contraction and die as embryos. We localized two myotonic mutations in the sixth membrane-spanning domain of the first repeat (IS6) region, which has been shown to be responsible for voltage-dependent inactivation. A third myotonic mutation implicates IIIS4, a region involved in sensing plasma-membrane voltage change, in the inactivation process.

Dissection of functional domains of the voltage-dependent Ca2+ channel alpha2delta subunit

Coexpression of the cloned voltage-dependent Ca2+ channel alpha2delta subunit with the pore-forming alpha1 subunit results in a significant increase in macroscopic current amplitude. To gain insight into the mechanism underlying this interaction, we have examined the regulatory effect of either the alpha2delta complex or the delta subunit on the Ca2+ channel alpha1 subunit. Transient transfection of tsA201 cells with the cardiac L-type alpha1C subunit alone resulted in the expression of inward voltage-activated currents as well as measurable [3H]-PN200-110 binding to membranes from transfected cells. Coexpression of the alpha2delta subunit significantly increased the macroscopic current amplitude, altered the voltage dependence and the kinetics of the current, and enhanced [3H]-PN200-110 binding. Except for the increase in amplitude, coexpression of the delta subunit reproduced entirely the effects of the full-length alpha2delta subunit on the biophysical properties of the alpha1C currents. However, no effect on specific [3H]-PN200-110 binding was observed on delta subunit coexpression. Likewise, profound effects on current kinetics of the neuronal alpha1A subunit were observed on coexpression of the alpha2delta complex in Xenopus oocytes. Furthermore, by using a chimeric strategy, we localized the region involved in this regulation to the transmembrane domain of the delta subunit. These data strongly suggest that the molecular determinants involved in alpha2delta regulation are conserved across L-type and non-L type Ca2+ channels. Taken together, our results indicate that the region of the alpha2delta subunit involved in the modulation of the gating properties of the high voltage-activated calcium channels is localized in the delta domain of the protein. In contrast, the level of membrane expression of functional channels relies on the presence of the alpha2 domain of the alpha2delta complex.

Alternatively spliced IS6 segments of the alpha 1C gene determine the tissue-specific dihydropyridine sensitivity of cardiac and vascular smooth muscle L-type Ca2+ channels

Dihydropyridines (DHPs) block the vascular smooth muscle L-type Ca2+ channel at lower concentrations than the cardiac Ca2+ channel, although their alpha 1 subunit, which binds the DHPs, is derived from the same gene. This alpha 1C gene gives rise to several splice variants, among which the alpha 1C-b variant is affected by lower concentrations of nisoldipine than the alpha 1C-a variant. Functional expression of chimeras of alpha 1C-a and alpha 1C-b subunits demonstrated that the transmembrane segment IS6 is responsible for the different dihydropyridine sensitivity. Northern blot analysis showed that transcripts coding for the IS6 segment of the alpha 1C-a subunit were expressed in heart but not in aorta, whereas the IS6 segment of the alpha 1C-b subunit was expressed predominantly in vascular smooth muscle. In situ hybridization of rat heart sections confirmed this expression pattern of IS6 alpha 1C-a and IS6 alpha 1C-b in ventricular and smooth muscle myocytes, respectively. These results suggest that the different dihydropyridine sensitivities of cardiac and vascular L-type Ca2+ channels are caused at least partially by the tissue-specific expression of alternatively spliced IS6 segments of the alpha 1C gene.

Structural regions of the cardiac Ca channel alpha subunit involved in Ca-dependent inactivation

We investigated the molecular basis for Ca-dependent inactivation of the cardiac L-type Ca channel. Transfection of HEK293 cells with the wild-type alpha or its 3' deletion mutant (alpha) produced channels that exhibited prominent Ca-dependent inactivation. To identify structural regions of alpha involved in this process, we analyzed chimeric alpha subunits in which one of the major intracellular domains of alpha was replaced by the corresponding region from the skeletal muscle alpha subunit (which lacks Ca-dependent inactivation). Replacing the NH terminus or the III-IV loop of alpha with its counterpart from alpha had no appreciable effect on Ca channel inactivation. In contrast, replacing the I-II loop of alpha with the corresponding region from alpha dramatically slowed the inactivation of Ba currents while preserving Ca-dependent inactivation. A similar but less pronounced result was obtained with a II-III loop chimera. These results suggest that the I-II and II-III loops of alpha may participate in the mechanism of Ca-dependent inactivation. Replacing the final 80% of the COOH terminus of alpha with the corresponding region from alpha completely eliminated Ca-dependent inactivation without affecting inactivation of Ba currents. Significantly, Ca-dependent inactivation was restored to this chimera by deleting a nonconserved, 211-amino acid segment from the end of the COOH terminus. These results suggest that the distal COOH terminus of alpha can block Ca-dependent inactivation, possibly by interacting with other proteins or other regions of the Ca channel. Our findings suggest that structural determinants of Ca-dependent inactivation are distributed among several major cytoplasmic domains of alpha.

Electrophysiological properties of the human N-type Ca2+ channel: I. Channel gating in Ca2+, Ba2+ and Sr2+ containing solutions

We have characterized the properties of the human N-type Ca2+ channel produced by the stable co-expression of the alpha(1B-1), alpha(2b)delta and beta(1b) subunits. The channel displayed the expected pharmacology with respect to the toxins omega-CTx-GVIA and omega-CTx-MVIIC, which depressed currents in a voltage-independent fashion. We characterized a variety of biophysical properties of the channel under conditions in which either Ca2+, Ba2+ or Sr2+ was the sole extracellular divalent ion. In all three ions, current-voltage relationships revealed that the channel was clearly high-voltage activated. Current activation was significantly slower in Ca2+ than either Sr2+ or Ba2+. Construction of conductance-voltage relationships from tail current measurements indicated that the channel was more high-voltage activated in Ca2+ than in either Sr2+ or Ba2+. The rank order of current amplitude at +4 mV was Ba2+ > Sr2+ > or = Ca2+. Elevation of the extracellular concentration of Ba2+ increased maximal current amplitude and shifted the current-voltage relationship to the right. In all three ions channel inactivation was complex consisting of three distinct exponentials. Recovery from inactivation was slow taking several seconds to reach completion. Steady-state inactivation curves revealed that channel inactivation became detectable at holding potentials of between -101 and -91 mV depending on the permeating species. The rank order of mid-points of steady state inactivation was (most negative) Sr2+ > Ca2+ > Ba2+ (most positive). Deactivation of the N-type Ca2+ channel was voltage-dependent and very fast in all three ions. The deactivation rate in Ba2+ was significantly slower than that in both Ca2+ and Sr2+, however the voltage-dependence of deactivation rate was indistinguishable in all three ions.

Preferential interaction of omega-conotoxins with inactivated N-type Ca2+ channels

The selective block of N-type Ca2+ channels by omega-conotoxins has been a hallmark of these channels, critical in delineating their biological roles and molecular characteristics. Here we report that the omega-conotoxin-channel interaction depends strongly on channel gating. N-type channels (alpha1B, alpha2, and beta1) expressed in Xenopus oocytes were blocked with a variety of omega-conotoxins, including omega-CTx-GVIA, omega-CTx-MVIIA, and SNX-331, a derivative of omega-CTx-MVIIC. Changes in holding potential (HP) markedly altered the severity of toxin block and the kinetics of its onset and removal. Notably, strong hyperpolarization renders omega-conotoxin block completely reversible. These effects could be accounted for by a modulated receptor model, in which toxin dissociation from the inactivated state is approximately 60-fold slower than from the resting state. Because omega-conotoxins act exclusively outside cells, our results suggest that voltage-dependent inactivation of Ca2+ channels must be associated with an externally detectable conformational change.

Ion-dependent inactivation of barium current through L-type calcium channels

It is widely believed that Ba2+ currents carried through L-type Ca2+ channels inactivate by a voltage-dependent mechanism similar to that described for other voltage-dependent channels. Studying ionic and gating currents of rabbit cardiac Ca2+ channels expressed in different subunit combinations in tsA201 cells, we found a phase of Ba2+ current decay with characteristics of ion-dependent inactivation. Upon a long duration (20 s) depolarizing pulse, IBa decayed as the sum of two exponentials. The slow phase (tau approximately 6 s, 21 degrees C) was parallel to a reduction of gating charge mobile at positive voltages, which was determined in the same cells. The fast phase of current decay (tau approximately 600 ms), involving about 50% of total decay, was not accompanied by decrease of gating currents. Its amplitude depended on voltage with a characteristic U-shape, reflecting reduction of inactivation at positive voltages. When Na+ was used as the charge carrier, decay of ionic current followed a single exponential, of rate similar to that of the slow decay of Ba2+ current. The reduction of Ba2+ current during a depolarizing pulse was not due to changes in the concentration gradients driving ion movement, because Ba2+ entry during the pulse did not change the reversal potential for Ba2+. A simple model of Ca(2+) -dependent inactivation (Shirokov, R., R. Levis, N. Shirokova, and F., Ríos. 1993. J. Gen. Physiol. 102:1005-1030) robustly accounts for fast Ba2+ current decay assuming the affinity of the inactivation site on the alpha 1 subunit to be 100 times lower for Ba2+ than Ca2+.

A mutation in segment I-S6 alters slow inactivation of sodium channels

Slow inactivation occurs in voltage-gated Na+ channels when the membrane is depolarized for several seconds, whereas fast inactivation takes place rapidly within a few milliseconds. Unlike fast inactivation, the molecular entity that governs the slow inactivation of Na+ channels has not been as well defined. Some regions of Na+ channels, such as mu1-W402C and mu1-T698M, have been reported to affect slow inactivation. A mutation in segment I-S6 of mu1 Na+ channels, N434A, shifts the voltage dependence of activation and fast inactivation toward the depolarizing direction. The mutant Na+ current at +50 mV is diminished by 60-80% during repetitive stimulation at 5 Hz, resulting in a profound use-dependent phenomenon. This mutant phenotype is due to the enhancement of slow inactivation, which develops faster than that of wild-type channels (tau = 0.46 +/- 0.01 s versus 2.11 +/- 0.10 s at +30 mV, n = 9). An oxidant, chloramine-T, abolishes fast inactivation and yet greatly accelerates slow inactivation in both mutant and wild-type channels (tau = 0.21 +/- 0.02 s and 0.67 +/- 0.05 s, respectively, n = 6). These findings together demonstrate that N434 of mu1 Na+ channels is also critical for slow inactivation. We propose that this slow form of Na+ channel inactivation is analogous to the "C-type" inactivation in Shaker K+ channels.

The intracellular loop between domains I and II of the B-type calcium channel confers aspects of G-protein sensitivity to the E-type calcium channel

Neuronal voltage-dependent calcium channels undergo inhibitory modulation by G-protein activation, generally involving both kinetic slowing and steady-state inhibition. We have shown previously that the beta-subunit of neuronal calcium channels plays an important role in this process, because when it is absent, greater receptor-mediated inhibition is observed (). We therefore hypothesized that the calcium channel beta-subunits normally may occlude G-protein-mediated inhibition. Calcium channel beta-subunits bind to the cytoplasmic loop between transmembrane domains I and II of the alpha1-subunits (). We have examined the hypothesis that this loop is involved in G-protein-mediated inhibition by making chimeras containing the I-II loop of alpha1B or alpha1A inserted into alpha1E (alpha1EBE and alpha1EAE, respectively). This strategy was adopted because alpha1B (the molecular counterpart of N-type channels) and, to a lesser extent, alpha1A (P/Q-type) are G-protein-modulated, whereas this has not been observed to any great extent for alpha1E. Although alpha1B, coexpressed with alpha2-delta and beta1b transiently expressed in COS-7 cells, showed both kinetic slowing and steady-state inhibition when recorded with GTPgammaS in the patch pipette, both of which were reversed with a depolarizing prepulse, the chimera alpha1EBE (and, to a smaller extent, alpha1EAE) showed only kinetic slowing in the presence of GTPgammaS, and this also was reversed by a depolarizing prepulse. These results indicate that the I-II loop may be the molecular substrate of kinetic slowing but that the steady-state inhibition shown by alpha1B may involve a separate site on this calcium channel.

Molecular determinants of inactivation and G protein modulation in the intracellular loop connecting domains I and II of the calcium channel alpha1A subunit

Synaptic transmission is regulated by G protein-coupled receptors whose activation releases G protein betagamma subunits that modulate presynaptic Ca2+ channels. The sequence motif QXXER has been proposed to be involved in the interaction between G protein betagamma subunits and target proteins including adenylyl cyclase 2. This motif is present in the intracellular loop connecting domains I and II (L I-II) of Ca2+ channel alpha1A subunits, which are modulated by G proteins, but not in alpha1C subunits, which are not modulated. Peptides containing the QXXER motif from adenylate cyclase 2 or from alpha1A block G protein modulation but a mutant peptide containing the sequence AXXAA does not, suggesting that the QXXER-containing peptide from alpha1A can competitively inhibit Gbetagamma modulation. Conversion of the R in the QQIER sequence of alpha1A to E as in alpha1C slows channel inactivation and shifts the voltage dependence of steady-state inactivation to more positive membrane potentials. Conversion of the final E in the QQLEE sequence of alpha1C to R has opposite effects on voltage-dependent inactivation, although the changes are not as large as those for alpha1A. Mutation of the QQIER sequence in alpha1A to QQIEE enhanced G protein modulation, and mutation to QQLEE as in alpha1C greatly reduced G protein modulation and increased the rate of reversal of G protein effects. These results indicate that the QXXER motif in L I-II is an important determinant of both voltage-dependent inactivation and G protein modulation, and that the amino acid in the third position of this motif has an unexpectedly large influence on modulation by Gbetagamma. Overlap of this motif with the consensus sequence for binding of Ca2+ channel beta subunits suggests that this region of L I-II is important for three different modulatory influences on Ca2+ channel activity.

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.

Activity-dependent changes in voltage-dependent calcium currents and transmitter release

Voltage-dependent Ca2+ channels are important in the regulation of neuronal structure and function, and as a result, they have received considerable attention. Recent studies have begun to characterize the diversity of their properties and the relationship of this diversity to their various cellular functions. In particular, Ca2+ channels play a prominent role in depolarization-secretion coupling, where the release of neurotransmitter is very sensitive to changes in voltage-dependent Ca2+ currents. An important feature of Ca2+ channels is their regulation by electrical activity. Depolarization can selectively modulate the properties of Ca2+ channel types, thus shaping the response of the neuron to future electrical activity. In this article, we examine the diversity of Ca2+ channels found in vertebrate and invertebrate neurons, and their short- and long-term regulation by membrane potential and Ca2+ influx. Additionally, we consider the extent to which this activity-dependent regulation of Ca2+ currents contributes to the development and plasticity of transmitter releasing properties. In the studies of long-term regulation, we focus on crustacean motoneurons where activity levels, Ca2+ channel properties, and transmitter releasing properties can be followed in identified neurons.

Multiple structural elements in voltage-dependent Ca2+ channels support their inhibition by G proteins

Molecular determinants of Ca2+ channel responsiveness to inhibition by receptor-coupled G proteins were investigated in Xenopus oocytes. The inhibitory response of alpha1B (N-type) channels was much larger than alpha1A (P/Q-type) channels, while alpha1C (L-type) channels were unresponsive. Differences in both degree and speed of inhibition were accounted for by variations in inhibitor off-rate. We tested proposals that inhibitory G protein and Ca2+ channel beta subunits compete specifically at the I-II loop. G protein-mediated inhibition remained unaltered in alpha1B subunits containing a point mutation in the I-II loop segment critical for Ca2+ channel beta subunit binding, and in chimeras where the I-II loop of alpha1B was replaced with counterparts from alpha1A or alpha1c. Full interconversion between modulatory behaviors of alpha1B and alpha1A was achieved only by swapping both motif I and the C-terminus in combination. Thus, essential structural elements for G protein modulation reside in multiple Ca2+ channel domains.

Identification of a second region of the beta-subunit involved in regulation of calcium channel inactivation

Previous studies have shown that NH2 termini of the type 1 and 2 beta-subunits modulate the rate at which the neuronal alpha 1E calcium channel inactivates in response to voltage and that they do so independently of their common effect to stimulate activation by voltage (R. Olcese, N. Qin, T. Schneider, A. Neely, X. Wei, E. Stefani, and L. Birnbaumer, Neuron 13: 1433-1438, 1994). By constructing NH2-terminal deletions of several splice variants of beta-subunits, we have now found differences in the way they affect the rate of alpha 1E inactivation that lead us to identify a second domain that also regulates the rate of voltage-induced inactivation of the Ca2+ channel. This second domain, named segment 3, lies between two regions of high-sequence identity between all known beta-subunits and exists in two lengths (long and short), each encoded in a separate exon. Beta-Subunits with the longer 45- to 53-amino acid version cause the channel to inactivate more slowly than subunits with the shorter 7-amino acid version. As is the case for the NH2 terminus, the segment 3 does not affect the regulation of channel activation by the beta-subunit. In addition, the effect of the NH2-terminal segment prevails over that of the internal segment. This raises the possibility that phosphorylation, other types of posttranslational modification, or interaction with other auxiliary calcium channel subunits may be necessary to unmask the regulatory effect of the internal segment.

Transfer of high sensitivity for benzothiazepines from L-type to class A (BI) calcium channels

To investigate the molecular basis of the calcium channel block by diltiazem, we transferred amino acids of the highly sensitive and stereoselective L-type (alpha1S or alpha1C) to a weakly sensitive, nonstereoselective class A (alpha1A) calcium channel. Transfer of three amino acids of transmembrane segment IVS6 of L-type alpha1 into the alpha1A subunit (I1804Y, S1808A, and M1811I) was sufficient to support a use-dependent block by diltiazem and by the phenylalkylamine (-)-gallopamil after expression in Xenopus oocytes. An additional mutation F1805M increased the sensitivity for (-)-gallopamil but not for diltiazem. Our data suggest that the receptor domains for diltiazem and gallopamil have common but not identical molecular determinants in transmembrane segment IVS6. These mutations also identified single amino acid residues in segment IVS6 that are important for class A channel inactivation.

Effect of capsaicin and analogues on potassium and calcium currents and vanilloid receptors in Xenopus embryo spinal neurones

1. The potassium current in embryo spinal neurones of Xenopus consists of at least two kinetically distinct components with overlapping voltage-dependencies of activation. We investigated whether capsaicin might specifically block these components in acutely dissociated neurones from stage 37/38 embryos by use of standard patch clamp techniques. 2. Capsaicin caused a time-dependent block of both the slow and fast components of the potassium current. The concentration-dependence was described by the Hill equation with a KD of 21 microM and a coefficient of 1.5 (n = 9-11 at each concentration). Differences between the observed and fitted values were not significant at the 5% level (chi(2) = 2.80, 6 degrees of freedom). 3. Capsaicin did not affect the time course or voltage-sensitivity of activation, but the steady-state block was voltage-dependent. The block could be relieved by hyperpolarization, and the rate of the removal of block was voltage- and time-dependent. The time constant for the blocking reaction was also voltage-dependent for voltage steps below +30 mV, but above this level it was voltage-independent. These results suggest that capsaicin blocks potassium channels by an open channel mechanism. 4. Other derivatives of vanillin, such as capsazepine, resiniferatoxin, and piperine also blocked potassium channels. Capsazepine and resiniferatoxin caused a greater block than similar concentrations of capsaicin, and in the case of capsazepine, the block was also clearly time-dependent. 5. Capsaicin and capsazepine also blocked calcium currents in a time-dependent manner. Fitting the Hill equation to the averaged data gave a KD of 43.5 microM, and a coefficient of 1.35 (n = 11 at each concentration). The fitted values were not significantly different from the observed means at the 5% level (chi(2) = 12.1, 6 degrees of freedom). 6. Six out of 29 Rohon-Beard sensory neurones responded to capsaicin with an inward current that appeared to be similar to the capsaicin activation of mammalian C sensory neurones. This response saturated at 10 microM capsaicin.

Molecular properties of sodium and calcium channels

Voltage-gated sodium and calcium channels are responsible for inward movement of sodium and calcium during electrical signals in cell membranes. Their principal subunits are members of a gene family and can function as voltage-gated ion channels by themselves. They are expressed in association with one or more auxiliary subunits which increase functional expression and modify the functional properties of the principal subunits. Structural elements which are required for voltage-dependent activation, selective ion conductance, and inactivation have been identified, and their mechanisms of action are being explored through mutagenesis, expression in heterologous cells, and functional analysis. These experiments reveal that these two channels are built on a common structural theme with variations appropriate for functional specialization of each channel type.

Transfer of L-type calcium channel IVS6 segment increases phenylalkylamine sensitivity of alpha1A

Conditioned ("use-dependent") inhibition by phenylalkylamines (PAAs) is a characteristic property of L- type calcium (Ca2+) channels. To determine the structural elements of the PAA binding domain we transferred sequence stretches of the pore-forming regions of repeat III and/or IV from the skeletal muscle alpha1 subunit (alpha1S) to the class A alpha1 subunit (alpha1A and expressed these chimeras together with beta1a and alpha2/delta subunits in Xenopus oocytes. The corresponding barium currents (IBa) were tested for PAA sensitivity during trains of depolarizing test pulses (conditioned block). IBa of oocytes expressing the alpha1A subunit were only weakly inhibited by PAAs (less than 10% conditioned block of IBa during a 100-ms pulse train of 0.1 Hz). Transfer of the transmembrane segment IVS6 from alpha1S to alpha1A produced an enhancement of PAA sensitivity of the resulting alpha1A/alpha1S chimera comparable to L-type alpha1 subunits (about 35% conditioned block Of IBa during a 100-ms pulse train of 0.1 Hz). Our results demonstrate that substitution of 11 amino acids within the segment RVS6 of alpha1A with the corresponding residues of alpha1S is sufficient to transfer L-type PAA sensitivity into the low sensitive class A Ca2+ channel.

Reopening of single L-type Ca2+ channels in mouse cerebellar granule cells: dependence on voltage and ion concentration

1. We recorded the activity of single L-type Ca2+ channels from cell-attached patches on mouse cerebellar granule cells. The experiments investigated the mechanism of channel reopening at negative membrane potentials following a strong depolarization. 2. L-type channels that reopened following a strong depolarization showed a wide distribution of single-channel conductances, which ranged from 16 to 28 pS in the presence of 90 mM Ba2+. 3. The distribution of the latencies before reopening was fitted as the sum of two exponential components with time constants tau f approximately 1 and tau s approximately 12 ms at -70 mV. Hyperpolarization reduced the time constant of the slower component approximately e-fold per 43 mV, but had no effect on the faster component. 4. Raising the concentration of external Ba2+ reduced the time constant of the slower component of the reopening latency without altering the fast component. The time constant of the slow component was approximately 27 ms in 10 mM Ba2+ and decreased to 12 ms in 90 mM Ba2+ at -70 mV. The relation between the time constant and external Ba2+ saturated with an apparent KD of approximately 20 mM. 5. The distribution of reopening times was best fitted as the sum of two exponential components with time constants tau f approximately 0.5 ms and tau s approximately 4.5 ms at -70 mV. The conditional latencies before reopening into either the short or long open state were indistinguishable. 6. The results are consistent with the idea that a positively charged blocker occludes the pore during depolarization and channels reopen as the blocker dissociates following repolarization to negative potentials.

Multiple domains contribute to the distinct inactivation properties of human heart and skeletal muscle Na+ channels

Voltage-gated Na+ channels are essential for the normal electrical excitability of neuronal and striated muscle membranes. Distinct isoforms of the Na+ channel alpha-subunit have been identified by molecular cloning, and their functional attributes have been defined by heterologous expression coupled with electrophysiological recording. Two closely related Na+ channel alpha-subunit isoforms, hH1 (human heart) and hSkM1 (human skeletal muscle), exhibit differences in their inactivation properties and in their response to the coexpressed beta 1-subunit. To localize regions that contribute to inactivation and to beta 1-subunit response, we have exploited these functional differences by studying chimeric channels composed of segments from both hH1 and hSkM1. Chimeras in which one or more of the cytoplasmic interdomain regions (ID1-2, ID2-3, and ID3-4) were exchanged between hH1 and hSkM1 exhibit inactivation properties identical with the background channel isoform, suggesting that these regions are not sufficient to cause gating differences. In contrast, inactivation properties of chimeras composed of approximately equal halves of the two channel isoforms were intermediate between hH1 and hSkM1. Furthermore, the response to the coexpressed beta 1-subunit was dependent on structures located in the carboxy-terminal half of the alpha-subunit, although domains D3, D4, and the carboxy terminal are not singularly responsible for this effect. These data indicate that inactivation differences between hH1 and hSkM1 are determined by multiple alpha-subunit domains.

Transfer of 1,4-dihydropyridine sensitivity from L-type to class A (BI) calcium channels

L-type Ca2+ channels are characterized by their unique sensitivity to organic Ca2+ channel modulators like the 1,4-dihydropyridines (DHPs). To identify molecular motifs mediating DHP sensitivity, we transferred this sensitivity from L-type Ca2+ channels to the DHP-insensitive class A brain Ca2+ channel, BI-2. Expression of chimeras revealed minimum sequence stretches conferring DHP sensitivity including segments IIIS5, IIIS6, and the connecting linker, as well as the IVS5-IVS6 linker plus segment IVS6. DHP agonist and antagonist effects are determined by different regions within the repeat IV motif. Sequence regions responsible for DHP sensitivity comprise only 9.4% of the overall primary structure of a DHP-sensitive alpha 1A/alpha 1S construct. This chimera fully exhibits the DHP sensitivity of channels formed by L-type alpha 1 subunits. In addition, it displays the electrophysiological properties of alpha 1A, as well as its sensitivity toward the peptide toxins omega-agatoxin IVA and omega-conotoxin MVIIC.

Structural and functional diversity of voltage-activated calcium channels

Data gathered from the expression of cDNAs that encode the subunits of voltage-dependent Ca2+ channels have demonstrated important structural and functional similarities among these channels. Despite these convergences, there are also significant differences in the nature and functional importance of subunit-subunit and protein-Ca2+ channel interactions. There is evidence demonstrating that the functional differences between Ca2+ channel subtypes is due to several factors, including the expression of distinct alpha 1 subunit proteins, the selective association of structural subunits and modulatory proteins, and differences in posttranslational processing and cell regulation. We summarize several avenues of research that should provide significant clues about the structural features involved in the biophysical and functional diversity of voltage-dependent Ca2+ channels.

C-type inactivation controls recovery in a fast inactivating cardiac K+ channel (Kv1.4) expressed in Xenopus oocytes

1. A fast inactivating transient K+ current (FK1) cloned from ferret ventricle and expressed in Xenopus oocytes was studied using the two-electrode voltage clamp technique. Removal of the NH2-terminal domain of FK1 (FK1 delta 2-146) removed fast inactivation consistent with previous findings in Kv1.4 channels. The NH2-terminal deletion mutation revealed a slow inactivation process, which matches the criteria for C-type inactivation described for Shaker B channels. 2. Inactivation of FK1 delta 2-146 at depolarized potentials was well described by a single exponential process with a voltage-insensitive time constant. In the range -90 to +20 mV, steady-state C-type inactivation was well described by a Boltzmann relationship that compares closely with inactivation measured in the presence of the NH2-terminus. These results suggest that C-type inactivation is coupled to activation. 3. The coupling of C-type inactivation to activation was assessed by mutation of the fourth positively charged residue (arginine 454) in the S4 voltage sensor to glutamine (R454Q). This mutation produced a hyperpolarizing shift in the inactivation relationship of both FK1 and FK1 delta 2-146 without altering the rate of inactivation of either clone. 4. The rates of recovery from inactivation are nearly identical in FK1 and FK1 delta 2-146. 5. To assess the mechanisms underlying recovery from inactivation the effects of elevated [K+]o and selective mutations in the extracellular pore and the S4 voltage sensor were compared in FK1 and FK1 delta 2-146. The similarity in recovery rates in response to these perturbations suggests that recovery from C-type inactivation governs the overall rate of recovery of inactivated channels for both FK1 and FK1 delta 2-146. 6. Analysis of the rate of recovery of FK1 channels for inactivating pulses of different durations (70-2000 ms) indicates that recovery rate is insensitive to the duration of the inactivating pulse.

The temperature dependence of high-threshold calcium channel currents recorded from adult rat dorsal raphe neurones

The temperature dependence of HVA calcium channel currents, using barium as the charge carrier, was studied in acutely isolated adult rat dorsal raphe neurones. Current amplitude was found to be highly sensitive with a Q10 of between 1.7 and 2.5. The most sensitive component of current is that that is activated from hyperpolarized holding potentials, and inactivates during a 200 msec test pulse. The least sensitive is the more sustained current elicited from depolarized potentials. Increases in temperature were also found to cause an irreversible shift in the current-voltage relationship in the hyperpolarizing direction. By far the most temperature-sensitive property was the activation time constant with an extraordinary Q10 of between 10 and 12. This was not significantly affected by holding potential, though the time constant itself is dependent on the test potential. Increases in temperature to 25 degrees C or above revealed a fast inactivating component, not seen at lower temperatures. These findings suggest that there are at least three components of HVA current in dorsal raphe neurones. In addition, the remarkably high Q10 for activation kinetics suggests that the processes underlying calcium channel current activation are multifaceted and complex. The following paper puts forward a new hypothesis which attempts to explain the way in which neurotransmitters modulate the activation kinetics of HVA calcium channel currents.

Exclusion of defects in the skeletal muscle specific regions of the DHPR alpha 1 subunit as frequent causes of malignant hyperthermia

The molecular defect predisposing to the majority of malignant hyperthermia (MH) cases is unknown, although various point mutations in the ryanodine receptor gene (RYR1) have been associated with susceptibility in a small proportion of cases. We report here that one of these, the Arg163Cys substitution, does not cosegregate with MH susceptibility. Comparison of cDNA sequences encoding the skeletal muscle specific components of the dihydropyridine receptor alpha 1 subunit between MH susceptible (MHS) and MH non-susceptible (MHN) patients was made in subjects without the reported MH linked RYR1 mutations. There were no differences within the sequence encoding the II-III loop or the IS3/IS3-IS4 segment, excluding defects in these functional segments of the alpha 1 subunit as frequent causes of MH.

Molecular biology of calcium channels

Pharmacological and electrophysiological studies have established that there are multiple types of voltage-gated Ca2+ channels. Molecular biology has uncovered an even greater number of channel molecules. Thus, the molecular diversity of Ca2+ channels has its basis in the expression of many alpha 1 and beta genes, and also in the splice variants produced from these genes. This ability to mix and match subunits provides the cell with yet another mechanism to control the influx of calcium. Future studies will describe new subunits, the subunit composition of each type of channel, and the cloning of new Ca2+ channel types.

Involvement of the carboxyl-terminal region of the alpha 1 subunit in voltage-dependent inactivation of cardiac calcium channels

Intracellular application of proteases increases cardiac calcium current to a level similar to beta-adrenergic stimulation. Using transiently transfected HEK 293 cells, we studied the molecular mechanism underlying calcium channel stimulation by proteolytic treatment. Perfusion of HEK cells, coexpressing the human cardiac (hHT) alpha 1, alpha 2, and beta 3 subunits, with 1 mg/ml of trypsin or carboxypeptidase A, increased the peak amplitude of the calcium channel current 3-4-fold without affecting the voltage dependence. Similar results were obtained in HEK cells cotransfected with hHT alpha 1 and alpha 2 or with alpha 1 alone, suggesting that modification of the alpha 1 subunit itself is responsible for the current enhancement by proteolysis. To further characterize the modification of the alpha 1 subunit by trypsin, we expressed a deletion mutant in which part of the carboxyl-terminal tail up to amino acid 1673 was removed. The expressed calcium channel currents no longer responded to intracellular application of the proteases; however, a 3-fold higher current density as well as faster inactivation compared with the wild type was observed. The results provide evidence that a specific region of the carboxyl-terminal tail of the cardiac alpha 1 subunit is an important regulatory segment that may serve as a critical component of the gating machinery that influences both inactivation properties as well as channel availability.

Chimeric L-type Ca2+ channels expressed in Xenopus laevis oocytes reveal role of repeats III and IV in activation gating

1. Chimeric alpha 1 subunits consisting of repeat I and II from the rabbit cardiac (alpha 1C-a) and repeat III and IV from the carp skeletal muscle Ca2+ channel (alpha 1S) were constructed and expressed in Xenopus laevis oocytes without co-expressing other channel subunits. Ba2+-current kinetics of five chimeric channel constructs were studied in Xenopus oocytes using the two-microelectrode technique. 2. Exchange of repeats III and IV of alpha 1C-a with sequences of alpha 1S results in a significantly slower and biexponential activation (apparent activation time constants tau 1act = 19.8 +/- 1.8 ms and tau 2act = 214 +/- 28.7 ms, n = 7) of expressed Ca2+ channel currents; no current inactivation was observable during an 800 ms test pulse to 0 mV. 3. Activation of a chimera consisting of repeats I, II and IV from the alpha 1C-a subunit and repeat III from alpha 1S was fast and monoexponential (tau 1act = 6.33 +/- 1.7 ms, n = 5) and the current inactivated during a 350 ms test pulse to 0 mV (tau inact = 175 +/- 22 ms, n = 5). The current kinetics of this construct did not significantly differ from kinetics of a construct consisting of repeats I to IV from alpha 1C-a (tau 1act = 6.6 +/- 2.1 ms; tau inact = 198 +/- 14 ms; n = 9).(ABSTRACT TRUNCATED AT 250 WORDS)