Amino acid residues outside of the pore region contribute to N-type calcium channel permeation

It is widely believed that the selectivity of voltage-dependent calcium channels is mainly controlled by amino acid residues contained within four p-loop motifs forming the pore of the channel. An examination of the amino acid sequences of high voltage-activated calcium channels reveals that their domain III S5-H5 regions contain a highly conserved motif with homology to known EF hand calcium binding proteins, hinting that this region may contribute to channel permeation. To test this hypothesis, we used site-directed mutagenesis to replace three conserved negatively charged residues in the N-type calcium channel alpha1B subunit (Glu-1321, Asp-1323, and Glu-1332) with positively charged amino acids (lysine and arginine) and studied their effect on ion selectivity using whole cell and single channel patch clamp recordings. Whereas the wild type channels conducted barium much more effectively than calcium, the mutant displayed nearly equal permeabilities for these two ions. Individual replacement of residue 1332 or a double substitution of residues 1321 and 1323 with lysine and arginine, respectively, were equally effective. Disruption of the putative EF hand motif through replacement of the central glycine residue (1326) with proline resulted in a similar effect, indicating that the responses observed with the triple mutant were not due to changes in the net charge of the channel. Overall, our data indicate that residues outside of the narrow region of the pore have the propensity to contribute to calcium channel permeation. They also raise the possibility that interactions of calcium ions with a putative calcium binding domain at the extracellular side of the channel may underlie the differential permeabilities of the channel for barium and calcium ions.

Voltage-dependent calcium channels--beyond dihydropyridine antagonists

The blockade of L-type calcium channels by dihydropyridines, phenylalkylamines and benzothiazepines has been well described and forms the basis of a multibillion dollar market for the treatment of cardiovascular disease and migraine. More recently, neuron-specific calcium channels have become the subject of intense interest regarding their potential as therapeutic targets for the treatment of chronic and neuropathic pain. A number of recently described agents that selectively target neuronal calcium channels have been described and appear promising for a variety of pain conditions.

Cross-talk between G-protein and protein kinase C modulation of N-type calcium channels is dependent on the G-protein beta subunit isoform

The modulation of N-type calcium current by protein kinases and G-proteins is a factor in the fine tuning of neurotransmitter release. We have previously shown that phosphorylation of threonine 422 in the alpha(1B) calcium channel domain I-II linker region resulted in a dramatic reduction in somatostatin receptor-mediated G-protein inhibition of the channels and that the I-II linker consequently serves as an integration center for cross-talk between protein kinase C (PKC) and G-proteins (Hamid, J., Nelson, D., Spaetgens, R., Dubel, S. J., Snutch, T. P., and Zamponi, G. W. (1999) J. Biol. Chem. 274, 6195-6202). Here we show that opioid receptor-mediated inhibition of N-type channels is affected to a lesser extent compared with that seen with somatostatin receptors, hinting at the possibility that PKC/G-protein cross-talk might be dependent on the G-protein subtype. To address this issue, we have examined the effects of four different types of G-protein beta subunits on both wild type and mutant alpha(1B) calcium channels in which residue 422 has been replaced by glutamate to mimic PKC-dependent phosphorylation and on channels that have been directly phosphorylated by protein kinase C. Our data show that phosphorylation or mutation of residue 422 antagonizes the effect of Gbeta(1) on channel activity, whereas Gbeta(2), Gbeta(3), and Gbeta(4) are not affected. Our data therefore suggest that the observed cross-talk between G-proteins and protein kinase C modulation of N-type channels is a selective feature of the Gbeta(1) subunit.

Cysteine string protein regulates G protein modulation of N-type calcium channels

Cysteine string proteins (CSPs) are secretory vesicle proteins bearing a "J domain" and a palmitoylated cysteine-rich "string" region that are critical for neurotransmitter release. The precise role of CSP in neurotransmission is controversial. Here, we demonstrate a novel interaction between CSP, receptor-coupled trimeric GTP binding proteins (G proteins), and N-type Ca2+ channels. G. subunits interact with the J domain of CSP in an ATP-dependent manner; in contrast, Gbetagamma subunits interact with the C terminus of CSP in both the presence and absence of ATP. The interaction of CSP with both G proteins and N-type Ca2+ channels results in a tonic G protein inhibition of the channels. In view of the crucial importance of N-type Ca2+ channels in presynaptic vesicle release, our data attribute a key role to CSP in the fine tuning of neurotransmission.

Differential modulation of N-type 1B and P/Q-type 1A calcium channels by different G protein subunit isoforms

Using transient calcium phosphate transfection into the human embryonic kidney tsa-201 cell line and subsequent whole-cell patch-clamp protocols, we examined the tonic modulation of cloned N- and P/Q-type calcium channels by five different G protein beta subunits via strong depolarizing voltage prepulses. For N- and P/Q-type channels, the magnitude of inhibition was dependent on the Gbeta subtype co-expressed. Both the absolute and relative magnitudes of Gbeta subunit-induced inhibition of P/Q-type channels differed from those observed with the N-type channel. For each calcium channel subtype, kinetics of both the prepulse-mediated recovery from inhibition and the re-inhibition following the prepulse were examined for each of the Gbeta subunits by varying either the duration between the pre- and the test pulse or the length of the prepulse. For each channel subtype, we observed a differential Gbeta subunit rank order with regard to the rates of re-inhibition and recovery from inhibition. On average, P/Q-type channels exhibited more rapid rates of recovery from inhibition than those observed with N-type channels. Different Gbeta subtypes mediated different degrees of slowing of activation kinetics. The differential modulation of P/Q- and N-type channels by various Gbeta subtypes may provide a mechanism for fine tuning the amount of calcium entering the presynaptic nerve termini.

Fast inactivation of voltage-dependent calcium channels. A hinged-lid mechanism?

We recently described domains II and III as important determinants of fast, voltage-dependent inactivation of R-type calcium channels (Spaetgens, R. L., and Zamponi, G. W. (1999) J. Biol. Chem. 274, 22428-22438). Here we examine in greater detail the structural determinants of inactivation using a series of chimeras comprising various regions of wild type alpha(1C) and alpha(1E) calcium channels. Substitution of the II S6 and/or III S6 segments of alpha(1E) into the alpha(1C) backbone resulted in rapid inactivation rates that closely approximated those of wild type alpha(1E) channels. However, neither individual or combined substitution of the II S6 and III S6 segments could account for the 60 mV more negative half-inactivation potential seen with wild type alpha(1E) channels, indicating that the S6 regions contribute only partially to the voltage dependence of inactivation. Interestingly, the converse replacement of alpha(1E) S6 segments of domains II, III, or II+III with those of alpha(1C) was insufficient to significantly slow inactivation rates. Only when the I-II linker region and the domain II and III S6 regions of alpha(1E) were concomitantly replaced with alpha(1C) sequence could inactivation be abolished. Conversely, introduction of the alpha(1E) domain I-II linker sequence into alpha(1C) conferred alpha(1E)-like inactivation rates, indicating that the domain I-II linker is a key contributor to calcium channel inactivation. Overall, our data are consistent with a mechanism in which inactivation of voltage-dependent calcium channels may occur via docking of the I-II linker region to a site comprising, at least in part, the domain II and III S6 segments.

Block of voltage-dependent calcium channels by aliphatic monoamines

We have recently identified farnesol, an intermediate in the mevalonate pathway, as a potent endogenous modulator and blocker of N-type calcium channels (Roullet, J. B., R. L. Spaetgens, T. Burlingame, and G. W. Zamponi. 1999. J. Biol. Chem. 274:25439-25446). Here, we investigate the action of structurally related compounds on various types of voltage-dependent Ca(2+) channels transiently expressed in human embryonic kidney cells. 1-Dodecanol, despite sharing the 12-carbon backbone and headgroup of farnesol, exhibited a significantly lower blocking affinity for N-type Ca(2+) channels. Among several additional 12-carbon compounds tested, dodecylamine (DDA) mediated the highest affinity inhibition of N-type channels, indicating that the functional headgroup is a critical determinant of blocking affinity. This inhibition was concentration-dependent and relatively non-discriminatory among N-, L-, P/Q-, and R-Ca(2+) channel subtypes. However, whereas L-type channels exhibited predominantly resting channel block, the non-L-type isoforms showed substantial rapid open channel block manifested by a speeding of the apparent time course of current decay and block of the inactivated state. Consistent with these findings, we observed significant frequency-dependence of block and dependence on external Ba(2+) concentration for N-type, but not L-type, channels. We also systematically investigated the drug structural requirements for N-type channel inhibition. Blocking affinity varied with carbon chain length and showed a clear maximum at C12 and C13, with shorter and longer molecules producing progressively weaker peak current block. Overall, our data indicate that aliphatic monoamines may constitute a novel class of potent inhibitors of voltage-dependent Ca(2+) channels, with block being governed by rigid structural requirements and channel-specific state dependencies.

Block of voltage-dependent calcium channel by the green mamba toxin calcicludine

A number of peptide toxins derived from marine snails and various spiders have been shown to potently inhibit voltage-dependent calcium channels. Here, we describe the effect of calcicludine, a 60 amino-acid peptide isolated from the venom of the green mamba (Dendroaspis angusticeps), on transiently expressed high voltage-activated calcium channels. Upon application of calcicludine, L-type (alpha(1)(C)) calcium channels underwent a rapid, irreversible decrease in peak current amplitude with no change in current kinetics, or any apparent voltage-dependence. However, even at saturating toxin concentrations, block was always incomplete with a maximum inhibition of 58%, indicating either partial pore block, or an effect on channel gating. Block nonetheless was of high affinity with an IC(50) value of 88 nm. Three other types of high voltage activated channels tested (alpha(1)(A), alpha(1)(B), and alpha(1)(E)) exhibited a diametrically different response to calcicludine. First, the maximal inhibition observed was around 10%, furthermore, the voltage-dependence of channel activation was shifted slightly towards more negative potentials. Thus, at relatively hyperpolarized test potentials, calcicludine actually upregulated current activity of (N-type) alpha(1)(B) channels by as much as 50%. Finally, the use of several chimeric channels combining the major transmembrane domains of alpha(1)(C) and alpha(1)(E) revealed that calcicludine block of L-type calcium channels involves interactions with multiple structural domains. Overall, calcicludine is a potent and selective inhibitor of neuronal L-type channels with a unique mode of action.

G protein modulation of N-type calcium channels is facilitated by physical interactions between syntaxin 1A and Gbetagamma

The direct modulation of N-type calcium channels by G protein betagamma subunits is considered a key factor in the regulation of neurotransmission. Some of the molecular determinants that govern the binding interaction of N-type channels and Gbetagamma have recently been identified (see, i.e., Zamponi, G. W., Bourinet, E., Nelson, D., Nargeot, J., and Snutch, T. P. (1997) Nature 385, 442-446); however, little is known about cellular mechanisms that modulate this interaction. Here we report that a protein of the presynaptic vesicle release complex, syntaxin 1A, mediates a crucial role in the tonic inhibition of N-type channels by Gbetagamma. When syntaxin 1A was coexpressed with (N-type) alpha(1B) + alpha(2)-delta + beta(1b) channels in tsA-201 cells, the channels underwent a 18 mV negative shift in half-inactivation potential, as well as a pronounced tonic G protein inhibition as assessed by its reversal by strong membrane depolarizations. This tonic inhibition was dramatically attenuated following incubation with botulinum toxin C, indicating that syntaxin 1A expression was indeed responsible for the enhanced G protein modulation. However, when G protein betagamma subunits were concomitantly coexpressed, the toxin became ineffective in removing G protein inhibition, suggesting that syntaxin 1A optimizes, rather than being required for G protein modulation of N-type channels. We also demonstrate that Gbetagamma physically binds to syntaxin 1A, and that syntaxin 1A can simultaneously interact with Gbetagamma and the synprint motif of the N-type channel II-III linker. Taken together, our experiments suggest a mechanism by which syntaxin 1A mediates a colocalization of G protein betagamma subunits and N-type calcium channels, thus resulting in more effective G protein coupling to, and regulation of, the channel. Thus, the interactions between syntaxin, G proteins, and N-type calcium channels are part of the structural specialization of the presynaptic terminal.

Modulation of neuronal voltage-gated calcium channels by farnesol

The modulation of presynaptic voltage-dependent calcium channels by classical second messenger molecules such as protein kinase C and G protein betagamma subunits is well established and considered a key factor for the regulation of neurotransmitter release. However, little is known of other endogenous mechanisms that control the activity of these channels. Here, we demonstrate a unique modulation of N-type calcium channels by farnesol, a dephosphorylated intermediate of the mammalian mevalonate pathway. At micromolar concentrations, farnesol acts as a relatively non-discriminatory rapid open channel blocker of all types of high voltage-activated calcium channels, with a mild specificity for L-type channels. However, at 250 nM, farnesol induces an N-type channel-specific hyperpolarizing shift in channel availability that results in approximately 50% inhibition at a typical neuronal resting potential. Additional experiments demonstrated the presence of farnesol in the brain (rodents and humans) at physiologically relevant concentrations (100-800 pmol/g (wet weight)). Altogether, our results indicate that farnesol is a selective, high affinity inhibitor of N-type Ca(2+) channels and raise the possibility that endogenous farnesol and the mevalonate pathway are implicated in neurotransmitter release through regulation of presynaptic voltage-gated Ca(2+) channels.

Osteoblasts derived from load-bearing bones of the rat express both L- and T-like voltage-operated calcium channels and mRNA for alpha 1C, alpha 1D and alpha 1G subunits

Voltage operated calcium channels (VOCCs) are implicated in osteoblastic mechano- and hormonal transduction. Very little, however, is known about the expression of VOCCs in osteoblasts of load-bearing bones. Here we describe two types of whole-cell calcium current in rat femoral explant-derived osteoblasts. The first is high-voltage activated and sensitive to nifedipine, Bay K8644 and FPL 64176. The second is low-voltage activated and is sensitive to micromolar concentrations of Ni2+. The properties of these two currents are consistent with those of L-type and T-type calcium currents respectively. T-type currents were detected in most cells on the day of passage, the level of expression being significantly lower on subsequent days. L-type currents were also most common on the day of passage but were detected consistently throughout the 4-day period of study. The reverse transcription polymerase chain reaction with non-specific primers directed against all L-type VOCC alpha 1 subunits and then with specific primers directed against sequences from rat brain alpha 1C (L-type), alpha 1D (L-type) and alpha 1G (T-type) VOCC subunits detected transcripts of appropriate size in all four cases. Products from the three sets of specific primer pairs (alpha 1C, alpha 1D, alpha 1G) were sequenced and were identical to their respective rat brain templates.

Multiple structural domains contribute to voltage-dependent inactivation of rat brain alpha(1E) calcium channels

We have investigated the molecular determinants that mediate the differences in voltage-dependent inactivation properties between rapidly inactivating (R-type) alpha(1E) and noninactivating (L-type) alpha(1C) calcium channels. When coexpressed in human embryonic kidney cells with ancillary beta(1b) and alpha(2)-delta subunits, the wild type channels exhibit dramatically different inactivation properties; the half-inactivation potential of alpha(1E) is 45 mV more negative than that observed with alpha(1C), and during a 150-ms test depolarization, alpha(1E) undergoes 65% inactivation compared with only about 15% for alpha(1C). To define the structural determinants that govern these intrinsic differences, we have created a series of chimeric calcium channel alpha(1) subunits that combine the major structural domains of the two wild type channels, and we investigated their voltage-dependent inactivation properties. Each of the four transmembrane domains significantly affected the half-inactivation potential, with domains II and III being most critical. In particular, substitution of alpha(1C) sequence in domains II or III with that of alpha(1E) resulted in 25-mV negative shifts in half-inactivation potential. Similarly, the differences in inactivation rate were predominantly governed by transmembrane domains II and III and to some extent by domain IV. Thus, voltage-dependent inactivation of alpha(1E) channels is a complex process that involves multiple structural domains and possibly a global conformational change in the channel protein.

Splicing of alpha 1A subunit gene generates phenotypic variants of P- and Q-type calcium channels

P-type and Q-type calcium channels mediate neurotransmitter release at many synapses in the mammalian nervous system. The alpha 1A calcium channel has been implicated in the etiologies of conditions such as episodic ataxia, epilepsy and familial migraine, and shares several properties with native P- and Q-type channels. However, the exact relationship between alpha 1A and P- and Q-type channels is unknown. Here we report that alternative splicing of the alpha 1A subunit gene results in channels with distinct kinetic, pharmacological and modulatory properties. Overall, the results indicate that alternative splicing of the alpha 1A gene generates P-type and Q-type channels as well as multiple phenotypic variants.

Identification of an integration center for cross-talk between protein kinase C and G protein modulation of N-type calcium channels

The modulation of presynaptic calcium channel activity by second messengers provides a fine tuning mechanism for neurotransmitter release. In neurons, the activation of certain G protein-coupled receptors reduces N-type channel activity by approximately 60%. In contrast, activation of protein kinase C (PKC) results in an approximately 50% increase in N-type channel activity, and subsequent G protein inhibition is antagonized. Here, we describe the molecular determinants that control the dual effects of PKC-dependent phosphorylation. The double substitution of two adjacent PKC consensus sites in the calcium channel domain I-II linker (Thr422, Ser425) to alanines abolished both PKC-dependent up-regulation and the PKC-G protein cross-talk. The single substitution of Ser425 to glutamic acid abolished PKC up-regulation but had no effect on G protein modulation. Replacement of Thr422 with glutamic acid eliminated PKC-dependent up-regulation and mimicked the effects of PKC phosphorylation on G protein inhibition. Our data suggest that Thr422 mediates the antagonistic effect of PKC on G protein modulation, while phosphorylation of either Thr422 or Ser425 are sufficient to increase N-type channel activity. Thus, Thr422 serves as a molecular switch by which PKC is able to simultaneously trigger the up-regulation of channel activity and antagonize G protein inhibition.

Cytoplasmic determinants of piperidine blocking affinity for N-type calcium channels

Piperidines are a relatively novel class of calcium channel blockers which act at a unique receptor site associated with the calcium channel alpha1 subunit. Calcium channel blocking affinities ranging from subnanomolar to several hundred micromolar have been reported in the literature, suggesting that piperidine block is highly sensitive to the cellular environment experienced by the channel. Here, I have investigated some of the cytoplasmic determinants of haloperidol block of N-type calcium channels expressed in human embryonic kidney cells. In perforated patch clamp recordings, haloperidol blocks N-type calcium channels with an inhibition constant of 120 microM. Upon internal dialysis with chloride containing pipette solution, the blocking affinity increases by 40-fold. This effect could be attributed in part to the presence of internal chloride ions, as replacement of intracellular chloride with methanesulfonate reduced haloperidol blocking affinity by almost one order of magnitude. Tonic inhibition of N-type channels by Gbetagamma subunits further enhanced the blocking effects of haloperidol, suggesting the possibility of direct effects of Gbetagamma binding on the local environment of the piperidine receptor site. Overall, depending on the cytoplasmic environment experienced by the channel, the blocking affinity of N-type calcium channels for haloperidol may vary by more than two orders of magnitude. Thus, absolute blocking affinities at the piperidine receptor site must be interpreted cautiously and in the context of the particular experimental setting.

Molecular and kinetic determinants of local anaesthetic action on sodium channels

(1) Local anaesthetics (LA) rely for their clinical actions on state-dependent inhibition of voltage-dependent sodium channels. (2) Single, batrachoxin-modified sodium channels in planar lipid bilayers allow direct observation of drug-channel interactions. Two modes of inhibition of single-channel current are observed: fast block of the open channels and prolongation of a long-lived closed state, some of whose properties resemble those of the inactivated state of unmodified channels. (3) Analogues of different parts of the LA molecule separately mimic each blocking mode: amines--fast block, and water-soluble aromatics--closed state prolongation. (4) Interaction between a mu-conotoxin derivative and diethylammonium indicate an intrapore site of fast, open-state block. (5) Site-directed mutagenesis studies suggest that hydrophobic residues in transmembrane segment 6 of repeat domain 4 of sodium channels are critical for both LA binding and stabilization of the inactivated state.

Inhibition of neuronal calcium channels by a novel peptide spider toxin, DW13.3

Peptide toxins have proved to be useful agents, both in discriminating between different components of native calcium channel currents and in the molecular isolation and designation of their cloned channel counterparts. Here, we describe the isolation and characterization of the biochemical and physiological properties of a novel 74-amino acid peptide toxin (DW13.3) extracted from the venom of the spider Filistata hibernalis. The subtype specificity of DW13.3 was investigated using calcium channel currents recorded from two separate expression systems and several different cultured mammalian cell preparations. Overall, DW13.3 potently blocked all native calcium channel currents studied, with the exception of T-type currents recorded from GH3 cells. Examination of transiently expressed calcium channels in oocytes showed that DW13.3 had the highest affinity for alpha1A, followed by alpha1B > alpha1C > alpha1E. The affinity of DW13.3 for alpha1B N-type currents varied by 10-fold between expressed channels and native currents. Although block occurred in a similar 1:1 manner for all subtypes, DW13.3 produced a partial block of both alpha1A currents and P-type currents in cerebellar Purkinje cells. Selective occlusion of the P/Q-type channel ligand omega-conotoxin MVIIC (but not omega-agatoxin IVA) from its binding site in Purkinje neurons suggests that DW13.3 binds to a site close to the pore of the channel. The inhibition of different subtypes of calcium channels by DW13.3 reflects a common "macro" binding site present on all calcium channels except T-type.

Modulation of voltage-dependent calcium channels by G proteins

Voltage-gated calcium channels are found in all excitable cells, in which they regulate many important physiological functions, including excitability, gene transcription, muscle contraction, and neurotransmitter and hormone release. The differential modulation of calcium channels by intracellular second messengers constitutes a key mechanism for controlling calcium influx. Recent advances have provided important clues to the underlying molecular mechanisms involved in the inhibition of N-type and P/Q-type calcium channels by a membrane-delimited G-protein-dependent pathway.

Decay of prepulse facilitation of N type calcium channels during G protein inhibition is consistent with binding of a single Gbeta subunit

We have examined the modulation of cloned and stably expressed rat brain N type calcium channels (alpha1B + beta1b + alpha2delta subunits) by exogenously applied purified G protein betagamma subunits. In the absence of Gbetagamma, barium currents through N type channels are unaffected by application of strong depolarizing prepulses. In contrast, inclusion of purified Gbetagamma in the patch pipette results in N type currents that initially facilitated upon application of positive prepulses followed by rapid reinhibition. Examination of the kinetics of Gbetagamma-dependent reinhibition showed that as the duration between the test pulse and the prepulse was increased, the degree of facilitation was attenuated in a monoexponential fashion. The time constant tau for the recovery from facilitation was sensitive to exogenous Gbetagamma, so that the inverse of tau linearly depended on the Gbetagamma concentration. Overall, the data are consistent with a model whereby a single Gbetagamma molecule dissociates from the channel during the prepulse, and that reassociation of Gbetagamma with the channel after the prepulse occurs as a bimolecular reaction.

Crosstalk between G proteins and protein kinase C mediated by the calcium channel alpha1 subunit

The modulation of voltage-dependent Ca2+ channels at presynaptic nerve terminals is an important factor in the control of neurotransmitter release and synaptic efficacy. Some terminals contain multiple Ca2(+)-channel subtypes (N and P/Q), which are differentially regulated by G-protein activation and by protein kinase C (PKC)-dependent phosphorylation. Regulation of channel activity by crosstalk between second messenger pathways has been reported although the molecular mechanisms underlying crosstalk have not been described. Here we show that crosstalk occurs at the level of the presynaptic Ca2(+)-channel complex. The alpha1 subunit domain I-II linker, which connects the first and second transmembrane domains, contributes to the PKC-dependent upregulation of channel activity, while G-protein-dependent inhibition occurs through binding of Gbetagamma to two sites in the I-II linker. Crosstalk results from the PKC-dependent phosphorylation of one of the Gbetagamma binding sites which antagonizes Gbetagamma-induced inhibition. The results provide a mechanism for the highly regulated and dynamic control of neurotransmitter release that depends on the integration of multiple presynaptic signals.

The alpha 1E calcium channel exhibits permeation properties similar to low-voltage-activated calcium channels

The physiological and pharmacological properties of the alpha 1E calcium (Ca) channel subtype do not exactly match any of the established categories described for native neuronal Ca currents. Many of the key diagnostic features used to assign cloned Ca channels to their native counterparts, however, are dependent on a number of factors, including cellular environment, beta subunit coexpression, and modulation by second messengers and G-proteins. Here, by examining the intrinsic pore characteristics of a family of transiently expressed neuronal Ca channels, we demonstrate that the permeation properties of alpha 1E closely resemble those described for a subset of low-threshold Ca channels. The alpha 1A (P-/Q-type), alpha 1B (N-type), and alpha 1C (L-type) high-threshold Ca channels all exhibit larger whole-cell currents with barium (Ba) as the charge carrier as compared with Ca or strontium (Sr). In contrast, macroscopic alpha 1E currents are largest in Sr, followed by Ca and then Ba. The unique permeation properties of alpha 1E are maintained at the single-channel level, are independent of the nature of the expression system, and are not affected by coexpression of alpha 2 and beta subunits. Overall, the permeation characteristics of alpha 1E are distinct from those described for R-type currents and share some similarities with native low-threshold Ca channels.

Nickel block of a family of neuronal calcium channels: subtype- and subunit-dependent action at multiple sites

Nickel ions have been reported to exhibit differential effects on distinct subtypes of voltage-activated calcium channels. To more precisely determine the effects of nickel, we have investigated the action of nickel on four classes of cloned neuronal calcium channels (alpha1A, alpha1B, alpha1C, and alpha1E) transiently expressed in Xenopus oocytes. Nickel caused two major effects: (i) block detected as a reduction of the maximum slope conductance and (ii) a shift in the current-voltage relation towards more depolarized potentials which was paralleled by a decrease in the slope of the activation-curve. Block followed 1:1 kinetics and was most pronounced for alpha1C, followed by alpha1E > alpha1A > alpha1B channels. In contrast, the change in activation-gating was most dramatic with alpha1E, with the remaining channel subtypes significantly less affected. The current-voltage shift was well described by a simple model in which nickel binding to a saturable site resulted in altered gating behavior. The affinity for both the blocking site and the putative gating site were reduced with increasing concentration of external permeant ion. Replacement of barium with calcium reduced both the degree of nickel block and the maximal effect on gating for alpha1A channels, but increased the nickel blocking affinity for alpha1E channels. The coexpression of Ca channel beta subunits was found to differentially influence nickel effects on alpha1A, as coexpression with beta2a or with beta4 resulted in larger current-voltage shifts than those observed in the presence of beta1b, while elimination of the beta subunit almost completely abolished the gating shifts. In contrast, block was similar for the three beta subunits tested, while complete removal of the beta subunit resulted in an increase in blocking affinity. Our data suggest that the effect of nickel on calcium channels is complex, cannot be described by a single site of action, and differs qualitatively and quantitatively among individual subtypes and subunit combinations.

Beta subunit coexpression and the alpha1 subunit domain I-II linker affect piperidine block of neuronal calcium channels

The effects of local anesthetics were examined on a family of transiently expressed neuronal calcium channels. Fomocaine, a local anesthetic containing a morpholine ring, preferentially blocked alpha1E channels (Ki = 100 microM), and had a lower affinity (3- to 15-fold) for alpha1A, alpha1B, and alpha1C channels. Block was incompletely reversible, followed 1:1 kinetics, and did not affect steady-state inactivation properties. Fomocaine block was sensitive to the concentration of permeant ion and enhanced in the presence of external pore blockers, suggesting a site of action in the conducting pathway. Flecainide, which carries a piperidine ring, and the diphenylbutylpiperidine antipsychotic, penfluridol, caused qualitatively similar block, suggesting that morpholine rings are compatible with the piperidine receptor site. In contrast, procaine, which contains an alkyl chain, caused reversible low affinity block of the different calcium channels (Kd values between 2 and 5 mM) and was least effective on alpha1E and did not compete with fomocaine, suggesting that local anesthetics interact with at least two distinct receptor sites. Compared to coexpression with the Ca channel beta1b subunit, block at the piperidine receptor site was significantly weakened with the beta2a subunit suggesting that the nature of the beta subunit contributes to drug binding. Amino acid changes in the cytoplasmic linker between domains I and II resulted in decreased fomocaine and penfluridol blocking affinity. Furthermore, the blocking affinity observed with alpha1B, was conferred on alpha1A by substitution of the domain I-II linker of alpha1B into alpha1A. Taken together, the data suggest that beta subunit binding and the domain I-II linker contribute to the piperidine receptor site on neuronal calcium channels.

Interactions between a pore-blocking peptide and the voltage sensor of the sodium channel: an electrostatic approach to channel geometry

Few experimental data illuminate the relationship between the molecular structures that mediate ion conduction through voltage-dependent ion channels and the structures responsible for sensing transmembrane voltage and controlling gating. To fill this void, we have used a strongly cationic, mutated mu-conotoxin peptide, which only partially blocks current through voltage-dependent sodium channels, to study voltage-dependent activation gating in both bound and unbound channels. When the peptide binds to the ion-conducting pore, it inhibit channel opening, necessitating stronger depolarization for channel activation. We show that this activation shift could result entirely from electrostatic inhibition of the movement of the voltage-sensing S4 charges and estimate the approximate physical distance through which the S4 charges move.

Evidence for a specific site for modulation of calcium channel activation by external calcium ions

We have investigated the effect of permeant ion species on activation of transiently expressed neuronal alpha1A Ca channels. Equimolar replacement of Ba with Ca resulted in a consistent depolarizing shift of the half-activation potential whose magnitude (∼10 mV) was constant over a range of 2 to 100 mM permeant ion, suggesting that the effects of Ca ions were fully developed at concentrations below 2 mM and indicating that Ba and Ca screened surface charges equally. In mixtures of Ba and Ca at constant divalent cation concentration the voltage-shift, as a function of Ca mole fraction, was well described by a model in which Ba and Ca compete for a single site but only Ca ions produce a gating effect. Overall, our data are consistent with Ca ions exerting their effects on activation via a specific regulatory site.