Association of rare missense variants in the second intracellular loop of NaV1.7 sodium channels with familial autism

Autism spectrum disorder (ASD) is a complex neurodevelopmental disorder often accompanied by intellectual disability, language impairment and medical co-morbidities. The heritability of autism is high and multiple genes have been implicated as causal. However, most of these genes have been identified in de novo cases. To further the understanding of familial autism, we performed whole-exome sequencing on five families in which second- and third-degree relatives were affected. By focusing on novel and protein-altering variants, we identified a small set of candidate genes. Among these, a novel private missense C1143F variant in the second intracellular loop of the voltage-gated sodium channel Na_V1.7, encoded by the SCN9A gene, was identified in one family. Through electrophysiological analysis, we show that Na_V1.7^C1143F exhibits partial loss-of-function effects, resulting in slower recovery from inactivation and decreased excitability in cultured cortical neurons. Furthermore, for the same intracellular loop of Na_V1.7, we found an excess of rare variants in a case-control variant-burden study. Functional analysis of one of these variants, M932L/V991L, also demonstrated reduced firing in cortical neurons. However, although this variant is rare in Caucasians, it is frequent in Latino population, suggesting that genetic background can alter its effects on phenotype. Although the involvement of the SCN1A and SCN2A genes encoding Na_V1.1 and Na_V1.2 channels in de novo ASD has previously been demonstrated, our study indicates the involvement of inherited SCN9A variants and partial loss-of-function of Na_V1.7 channels in the etiology of rare familial ASD.

Influence of physical exercise on quality of life in individuals with spinal cord injury

STUDY DESIGN

Retrospective cross-sectional study with anonymous postal data collection.

OBJECTIVE

Regaining the best possible mobility and independence is not only the focus of the rehabilitation process for individuals with spinal cord injury (SCI), but also represents an important criterion for the individual's quality of life (QoL). Therefore, if and to what extent physical exercise (PE) influences the QoL of individuals with SCI was investigated.

SETTING

The period of investigation extended from September 2007 to January 2008. Data were acquired from the BG Trauma Hospital Hamburg database and the German Wheelchair Sport Federation databases.

METHODS

Analysis of 277 questionnaires of individuals with acquired SCI between the age of 16 and 65 years with complete wheelchair dependency in everyday life and lesion level lower C5.

RESULTS

In all, 51.5% of all individuals were reported being actively involved in sports as opposed to 48.5% individuals not participating in sports. Individuals actively involved in sports have higher employment rate than physically inactive individuals with SCI. PE was identified as the main influencing determinant of QoL. This was particularly within the physical and psychological dimensions.

CONCLUSION

In discovering the potential of individuals with SCI for getting involved in PE, the improvement of physical and coordinative skills with interaction between individuals with SCI and external sport groups should be an inherent part of the rehabilitation process. Individuals not having access to PE should be given the opportunity to participate in wheelchair mobility courses. This may improve the adherence to PE of individuals with SCI in post-clinical settings.

Control of neuronal excitability by phosphorylation and dephosphorylation of sodium channels

Currents through voltage-gated sodium channels drive action potential depolarization in neurons and other excitable cells. Smaller currents through these channels are key components of currents that control neuronal firing and signal integration. Changes in sodium current have profound effects on neuronal firing. Sodium channels are controlled by neuromodulators acting through phosphorylation of the channel by serine/threonine and tyrosine protein kinases. That phosphorylation requires specific molecular interaction of kinases and phosphatases with the channel molecule to form localized signalling complexes. Such localization is required for effective neurotransmitter-mediated regulation of sodium channels by protein kinase A. Analogous molecular complexes between sodium channels, kinases and other signalling molecules are expected to be necessary for specific and localized transmitter-mediated modulation of sodium channels by other protein kinases.

Differential interactions of lamotrigine and related drugs with transmembrane segment IVS6 of voltage-gated sodium channels

Voltage-gated sodium channels are blocked by local anesthetic and anticonvulsant drugs. A receptor site for local anesthetics has been defined in transmembrane segment S6 in domain IV (IVS6) of the alpha subunit, but the anticonvulsant lamotrigine and related compounds have more complex structures than local anesthetics and may interact with additional amino acid residues. Apparent K(D) values for inactivated-state block of rat brain type IIA sodium channels expressed in Xenopus oocytes were 31.9 micro M, 17.3 micro M, 3.7 micro M and 10.3 micro M for lamotrigine and compounds 227c89, 4030w92 and 619c89, respectively. Compound 619c89 was the strongest frequency-dependent blocker, which correlated with higher affinity and a five-fold slower recovery from drug block compared to lamotrigine. Examination of lamotrigine block of mutant sodium channel alpha subunits, in which alanine had been substituted for each individual amino acid in IVS6, identified mutations I1760A, F1764A and Y1771A as causing the largest reductions in affinity (six-, seven- and 12-fold, respectively). The ratios of effects of these three mutations differed for compounds 227c89, 4030w92, and 619c89. The amino acid residues interacting with these pore-blocking drugs define a surface of IVS6 that is exposed to the pore and may rotate during gating.

Role of the C-terminal domain in inactivation of brain and cardiac sodium channels

Inactivation is a fundamental characteristic of Na(+) channels, and small changes cause skeletal muscle paralysis and myotonia, epilepsy, and cardiac arrhythmia. Brain Na(v)1.2a channels have faster inactivation than cardiac Na(v)1.5 channels, but minor differences in inactivation gate structure are not responsible. We constructed chimeras in which the C termini beyond the fourth homologous domains of Na(v)1.2a and Na(v)1.5 were exchanged. Replacing the C-terminal domain (CT) of Na(v)1.2a with that of Na(v)1.5 (Na(v)1.2/1.5CT) slowed inactivation at +40 mV approximately 2-fold, making it similar to Na(v)1.5. Conversely, replacing the CT of Na(v)1.5 with that of Na(v)1.2a (Nav1.5/1.2CT) accelerated inactivation, making it similar to Na(v)1.2a. Activation properties were unaffected. The voltage dependence of steady-state inactivation of Na(v)1.5 is 16 mV more negative than that of Na(v)1.2a. The steady-state inactivation curve of Na(v)1.2a was shifted +12 mV in Na(v)1.2/1.5CT, consistent with destabilization of the inactivated state. Conversely, Na(v)1.5/1.2CT was shifted -14 mV relative to Na(v)1.5, consistent with stabilization of the inactivated state. Although these effects of exchanging C termini were consistent with their effects on inactivation kinetics, they magnified the differences in the voltage dependence of inactivation between brain and cardiac channels rather than transferring them. Thus, other parts of these channels determine the basal difference in steady-state inactivation. Deletion of the distal half of either the Na(v)1.2 or Na(v)1.5 CTs accelerated open-state inactivation and negatively shifted steady-state inactivation. Thus, the C terminus has a strong influence on kinetics and voltage dependence of inactivation in brain Na(v)1.2 and cardiac Na(v)1.5 channels and is primarily responsible for their differing rates of channel inactivation.

Differential modulation of sodium channel gating and persistent sodium currents by the beta1, beta2, and beta3 subunits

Brain sodium channels are complexes of a pore-forming alpha subunit with auxiliary beta subunits, which are transmembrane proteins that modulate alpha subunit function. The newly cloned beta3 subunit is shown to be expressed broadly in neurons in the central and peripheral nervous systems, but not in glia and most nonneuronal cells. Beta1, beta2, and beta3 subunits are coexpressed in many neuronal cell types, but are differentially expressed in ventromedial nucleus of the thalamus, brain stem nuclei, cerebellar Purkinje cells, and dorsal root ganglion cells. Coexpression of beta1, beta2, and beta3 subunits with Na(v)1.2a alpha subunits in the tsA-201 subclone of HEK293 cells shifts sodium channel activation and inactivation to more positive membrane potentials. However, beta3 is unique in causing increased persistent sodium currents. Because persistent sodium currents are thought to amplify summation of synaptic inputs, expression of this subunit would increase the excitability of specific groups of neurons to all of their inputs.

Allosteric modulation of Ca2+ channels by G proteins, voltage-dependent facilitation, protein kinase C, and Ca(v)beta subunits

N-type and P/Q-type Ca(2+) channels are inhibited by neurotransmitters acting through G protein-coupled receptors in a membrane-delimited pathway involving Gbetagamma subunits. Inhibition is caused by a shift from an easily activated "willing" (W) state to a more-difficult-to-activate "reluctant" (R) state. This inhibition can be reversed by strong depolarization, resulting in prepulse facilitation, or by protein kinase C (PKC) phosphorylation. Comparison of regulation of N-type Ca(2+) channels containing Cav2.2a alpha(1) subunits and P/Q-type Ca(2+) channels containing Ca(v)2.1 alpha(1) subunits revealed substantial differences. In the absence of G protein modulation, Ca(v)2.1 channels containing Ca(v)beta subunits were tonically in the W state, whereas Ca(v)2.1 channels without beta subunits and Ca(v)2.2a channels with beta subunits were tonically in the R state. Both Ca(v)2.1 and Ca(v)2.2a channels could be shifted back toward the W state by strong depolarization or PKC phosphorylation. Our results show that the R state and its modulation by prepulse facilitation, PKC phosphorylation, and Ca(v)beta subunits are intrinsic properties of the Ca(2+) channel itself in the absence of G protein modulation. A common allosteric model of G protein modulation of Ca(2+)-channel activity incorporating an intrinsic equilibrium between the W and R states of the alpha(1) subunits and modulation of that equilibrium by G proteins, Ca(v)beta subunits, membrane depolarization, and phosphorylation by PKC accommodates our findings. Such regulation will modulate transmission at synapses that use N-type and P/Q-type Ca(2+) channels to initiate neurotransmitter release.

Control of gating mode by a single amino acid residue in transmembrane segment IS3 of the N-type Ca2+ channel

N-type Ca(2+) channels can be inhibited by neurotransmitter-induced release of G protein betagamma subunits. Two isoforms of Ca(v)2.2 alpha1 subunits of N-type calcium channels from rat brain (Ca(v)2.2a and Ca(v)2.2b; initially termed rbB-I and rbB-II) have different functional properties. Unmodulated Ca(v)2.2b channels are in an easily activated "willing" (W) state with fast activation kinetics and no prepulse facilitation. Activating G proteins shifts Ca(v)2.2b channels to a difficult to activate "reluctant" (R) state with slow activation kinetics; they can be returned to the W state by strong depolarization resulting in prepulse facilitation. This contrasts with Ca(v)2.2a channels, which are tonically in the R state and exhibit strong prepulse facilitation. Activating or inhibiting G proteins has no effect. Thus, the R state of Ca(v)2.2a and its reversal by prepulse facilitation are intrinsic to the channel and independent of G protein modulation. Mutating G177 in segment IS3 of Ca(v)2.2b to E as in Ca(v)2.2a converts Ca(v)2.2b tonically to the R state, insensitive to further G protein modulation. The converse substitution in Ca(v)2.2a, E177G, converts it to the W state and restores G protein modulation. We propose that negatively charged E177 in IS3 interacts with a positive charge in the IS4 voltage sensor when the channel is closed and produces the R state of Ca(v)2.2a by a voltage sensor-trapping mechanism. G protein betagamma subunits may produce reluctant channels by a similar molecular mechanism.

Molecular determinants of voltage-dependent gating and binding of pore-blocking drugs in transmembrane segment IIIS6 of the Na(+) channel alpha subunit

Mutations of amino acid residues in the inner two-thirds of the S6 segment in domain III of the rat brain type IIA Na(+) channel (G1460A to I1473A) caused periodic positive and negative shifts in the voltage dependence of activation, consistent with an alpha-helix having one face on which mutations to alanine oppose activation. Mutations in the outer one-third of the IIIS6 segment all favored activation. Mutations in the inner half of IIIS6 had strong effects on the voltage dependence of inactivation from closed states without effect on open-state inactivation. Only three mutations had strong effects on block by local anesthetics and anticonvulsants. Mutations L1465A and I1469A decreased affinity of inactivated Na(+) channels up to 8-fold for the anticonvulsant lamotrigine and its congeners 227c89, 4030w92, and 619c89 as well as for the local anesthetic etidocaine. N1466A decreased affinity of inactivated Na(+) channels for the anticonvulsant 4030w92 and etidocaine by 3- and 8-fold, respectively, but had no effect on affinity of the other tested compounds. Leu-1465, Asn-1466, and Ile-1469 are located on one side of the IIIS6 helix, and mutation of each caused a positive shift in the voltage dependence of activation. Evidently, these amino acid residues face the lumen of the pore, contribute to formation of the high-affinity receptor site for pore-blocking drugs, and are involved in voltage-dependent activation and coupling to closed-state inactivation.

Molecular determinants of diltiazem block in domains IIIS6 and IVS6 of L-type Ca(2+) channels

The benzothiazepine diltiazem blocks ionic current through L-type Ca(2+) channels, as do the dihydropyridines (DHPs) and phenylalkylamines (PAs), but it has unique properties that distinguish it from these other drug classes. Wild-type L-type channels containing alpha(1CII) subunits, wild-type P/Q-type channels containing alpha(1A) subunits, and mutants of both channel types were transiently expressed in tsA-201 cells with beta(1B) and alpha(2)delta subunits. Whole-cell, voltage-clamp recordings showed that diltiazem blocks L-type Ca(2+) channels approximately 5-fold more potently than it does P/Q-type channels. Diltiazem blocked a mutant P/Q-type channel containing nine amino acid changes that made it highly sensitive to DHPs, with the same potency as L-type channels. Thus, amino acids specific to the L-type channel that confer DHP sensitivity in an alpha(1A) background also increase sensitivity to diltiazem. Analysis of single amino acid mutations in domains IIIS6 and IVS6 of alpha(1CII) subunits confirmed the role of these L-type-specific amino acid residues in diltiazem block, and also indicated that Y1152 of alpha(1CII), an amino acid critical to both DHP and PA block, does not play a role in diltiazem block. Furthermore, T1039 and Y1043 in domain IIIS5, which are both critical for DHP block, are not involved in block by diltiazem. Conversely, three amino acid residues (I1150, M1160, and I1460) contribute to diltiazem block but have not been shown to affect DHP or PA block. Thus, binding of diltiazem to L-type Ca(2+) channels requires residues that overlap those that are critical for DHP and PA block as well as residues unique to diltiazem.

Inhibition of cardiac L-type calcium channels by protein kinase C phosphorylation of two sites in the N-terminal domain

We have investigated the mechanism underlying the modulation of the cardiac L-type Ca(2+) current by protein kinase C (PKC). Using the patch-clamp technique, we found that PKC activation by 4-alpha-phorbol 12-myristate 13-acetate (PMA) or rac-1-oleyl-2-acetylglycerol (OAG) caused a substantial reduction in Ba(2+) current through Ca(v)1.2 channels composed of alpha(1)1.2, beta(1b), and alpha(2)delta(1) subunits expressed in tsA-201 cells. In contrast, Ba(2+) current through a cloned brain isoform of the Ca(v)1.2 channel (rbC-II) was unaffected by PKC activation. Two potential sites of PKC phosphorylation are present at positions 27 and 31 in the cardiac form of Ca(v)1.2, but not in the brain form. Deletion of N-terminal residues 2-46 prevented PKC inhibition. Conversion of the threonines at positions 27 and 31 to alanine also abolished the PKC sensitivity of Ca(v)1.2. Mutant Ca(v)1.2 channels in which the threonines were converted singly to alanines were also insensitive to PKC modulation, suggesting that phosphorylation of both residues is required for PKC-dependent modulation. Consistent with this, mutating each of the threonines individually to aspartate in separate mutants restored the PKC sensitivity of Ca(v)1.2, indicating that a change in net charge by phosphorylation of both sites is responsible for inhibition. Our results define the molecular basis for inhibition of cardiac Ca(v)1.2 channels by the PKC pathway.

Ca2+/calmodulin-dependent facilitation and inactivation of P/Q-type Ca2+ channels

Trains of action potentials cause Ca(2+)-dependent facilitation and inactivation of presynaptic P/Q-type Ca(2+) channels that can alter synaptic efficacy. A potential mechanism for these effects involves calmodulin, which associates in a Ca(2+)-dependent manner with the pore-forming alpha(1A) subunit. Here, we report that Ca(2+) and calmodulin dramatically enhance inactivation and facilitation of P/Q-type Ca(2+) channels containing the auxiliary beta(2a) subunit compared with their relatively small effects on channels with beta(1b). Tetanic stimulation causes an initial enhancement followed by a gradual decline in P/Q-type Ca(2+) currents over time. Recovery of Ca(2+) currents from facilitation and inactivation is relatively slow (30 sec to 1 min). These effects are strongly inhibited by high intracellular BAPTA, replacement of extracellular Ca(2+) with Ba(2+), and a calmodulin inhibitor peptide. The Ca(2+)/calmodulin-dependent facilitation and inactivation of P/Q-type Ca(2+) channels observed here are consistent with the behavior of presynaptic Ca(2+) channels in neurons, revealing how dual feedback regulation of P/Q-type channels by Ca(2+) and calmodulin could contribute to activity-dependent synaptic plasticity.

A sodium channel signaling complex: modulation by associated receptor protein tyrosine phosphatase beta

Voltage-gated sodium channels in brain neurons were found to associate with receptor protein tyrosine phosphatase beta (RPTPbeta) and its catalytically inactive, secreted isoform phosphacan, and this interaction was regulated during development. Both the extracellular domain and the intracellular catalytic domain of RPTPbeta interacted with sodium channels. Sodium channels were tyrosine phosphorylated and were modulated by the associated catalytic domains of RPTPbeta. Dephosphorylation slowed sodium channel inactivation, positively shifted its voltage dependence, and increased whole-cell sodium current. Our results define a sodium channel signaling complex containing RPTPbeta, which acts to regulate sodium channel modulation by tyrosine phosphorylation.

Potent blockade of sodium channels and protection of brain tissue from ischemia by BIII 890 CL

We have synthesized a new benzomorphan derivative, 2R-[2alpha,3(S*), 6alpha]-1,2,3,4,5,6-hexahydro-6,11, 11-trimethyl-3-[2-(phenylmethoxy)propyl]-2, 6-methano-3-benzazocin-10-ol hydrochloride (BIII 890 CL), which displaced [(3)H]batrachotoxinin A-20alpha-benzoate from neurotoxin receptor site 2 of the Na(+) channel in rat brain synaptosomes (IC(50) = 49 nM), but exhibited only low affinity for 65 other receptors and ion channels. BIII 890 CL inhibited Na(+) channels in cells transfected with type IIA Na(+) channel alpha subunits and shifted steady-state inactivation curves to more negative potentials. The IC(50) value for the inactivated Na(+) channel was much lower (77 nM) than for Na(+) channels in the resting state (18 microM). Point mutations F1764A and Y1771A in transmembrane segment S6 in domain IV of the alpha subunit reduced the voltage- and frequency-dependent block, findings which suggest that BIII 890 CL binds to the local anesthetic receptor site in the pore. BIII 890 CL inhibited veratridine-induced glutamate release in brain slices, as well as glutamate release and neurotoxicity in cultured cortical neurons. BIII 890 CL (3-30 mg/kg s.c.) reduced lesion size in mice and rats when administered 5 min after permanent focal cerebral ischemia at doses that did not impair motor coordination. In contrast to many other agents, BIII 890 CL was neuroprotective in both cortical and subcortical regions of the rat brain. Our results demonstrate that BIII 890 CL is a potent, selective, and highly use-dependent Na(+) channel blocker that protects brain tissue from the deleterious effects of focal cerebral ischemia in rodents.

Differential interaction of R-mexiletine with the local anesthetic receptor site on brain and heart sodium channel alpha-subunits

Mexiletine is a class I antiarrhythmic drug with neuroprotective effects in models of brain ischemia attributable to inhibition of brain sodium channels. We compared effects of R-mexiletine on wild-type and mutant rat brain (rbIIA) and heart (rh1) sodium channel alpha-subunits transiently expressed in tsA-201 cells. R-mexiletine induced tonic and frequency-dependent block and bound with a 26-fold (brain) or 35-fold (heart) higher affinity to inactivated sodium channels. Affinities of both resting and inactivated channels for R-mexiletine block were approximately 2-fold higher for heart than for brain channels. Mutations in transmembrane segment IVS6 of heart (rhF1762A) and brain (rbF1764A and rbY1771A) channels, which reduce block by other local anesthetics, reduced high-affinity block of inactivated channels and frequency-dependent block of open channels by R-mexiletine and abolished the difference in affinity between brain and heart sodium channels. Unlike previous local anesthetics studied, the strongest effect was observed for mutation rbY1771A. Comparison of mutations of the homologous phenylalanine residue in brain and heart channels showed striking differences in the effects of the mutations. rbF1764A reduced drug block by slowing R-mexiletine binding to inactivated channels, whereas rhF1762A reduced block by increasing the rate of dissociation from inactivated and resting channels. Thus, rbF1764/rhF1762 is a critical determinant of affinity and tissue-specific differences in mexiletine block of brain and heart sodium channels, but its role in drug interaction differs in these two channel isoforms.

The extracellular domain of the beta1 subunit is both necessary and sufficient for beta1-like modulation of sodium channel gating

The type IIA voltage-gated sodium Na(+) channel from rat brain is composed of a large, pore-forming alpha subunit and the auxiliary subunits beta1 and beta2. When expressed in Xenopus oocytes, the beta1 subunit modulates the gating properties of the type IIA alpha subunit, resulting in acceleration of both inactivation and recovery from inactivation and in a negative shift in the voltage dependence of fast inactivation. The beta1 subunit is composed of an extracellular domain with a single immunoglobulin-like fold, a single transmembrane segment, and a small intracellular domain. A series of chimeras with exchanges of domains between the Na(+) channel beta1 and beta2 subunits and between beta1 and the structurally related protein myelin P0 were constructed and analyzed by two-microelectrode voltage clamp in Xenopus oocytes. Only chimeras containing the beta1 extracellular domain were capable of beta1-like modulation of Na(+) channel gating. Neither the transmembrane segment nor the intracellular domain was required for modulation, although mutation of Glu(158) within the transmembrane domain altered the voltage dependence of steady-state inactivation. A truncated beta1 subunit was engineered in which the beta1 extracellular domain was fused to a recognition sequence for attachment of a glycosylphosphatidylinositol membrane anchor. The beta1(ec)-glycosylphosphatidylinositol protein fully reproduced modulation of Na(+) channel inactivation and recovery from inactivation by wild-type beta1. Our findings demonstrate that extracellular domain of the beta1 subunit is both necessary and sufficient for the modulation of Na(+) channel gating.

Functional roles of the extracellular segments of the sodium channel alpha subunit in voltage-dependent gating and modulation by beta1 subunits

Voltage-gated sodium channels consist of a pore-forming alpha subunit associated with beta1 subunits and, for brain sodium channels, beta2 subunits. Although much is known about the structure and function of the alpha subunit, there is little information on the functional role of the 16 extracellular loops. To search for potential functional activities of these extracellular segments, chimeras were studied in which an individual extracellular loop of the rat heart (rH1) alpha subunit was substituted for the corresponding segment of the rat brain type IIA (rIIA) alpha subunit. In comparison with rH1, wild-type rIIA alpha subunits are characterized by more positive voltage-dependent activation and inactivation, a more prominent slow gating mode, and a more substantial shift to the fast gating mode upon coexpression of beta1 subunits in Xenopus oocytes. When alpha subunits were expressed alone, chimeras with substitutions from rH1 in five extracellular loops (IIS5-SS1, IISS2-S6, IIIS1-S2, IIISS2-S6, and IVS3-S4) had negatively shifted activation, and chimeras with substitutions in three of these (IISS2-S6, IIIS1-S2, and IVS3-S4) also had negatively shifted steady-state inactivation. rIIA alpha subunit chimeras with substitutions from rH1 in five extracellular loops (IS5-SS1, ISS2-S6, IISS2-S6, IIIS1-S2, and IVS3-S4) favored the fast gating mode. Like wild-type rIIA alpha subunits, all of the chimeric rIIA alpha subunits except chimera IVSS2-S6 were shifted almost entirely to the fast gating mode when coexpressed with beta1 subunits. In contrast, substitution of extracellular loop IVSS2-S6 substantially reduced the effectiveness of beta1 subunits in shifting rIIA alpha subunits to the fast gating mode. Our results show that multiple extracellular loops influence voltage-dependent activation and inactivation and gating mode of sodium channels, whereas segment IVSS2-S6 plays a dominant role in modulation of gating by beta1 subunits. Evidently, several extracellular loops are important determinants of sodium channel gating and modulation.

Dopaminergic modulation of voltage-gated Na+ current in rat hippocampal neurons requires anchoring of cAMP-dependent protein kinase

Activation of D1-like dopamine (DA) receptors reduces peak Na(+) current in acutely isolated hippocampal neurons via a modulatory mechanism involving phosphorylation of the Na(+) channel alpha subunit by cAMP-dependent protein kinase (PKA). Peak Na(+) current is reduced 20-50% in the presence of the D1 agonist SKF 81297 or the PKA activator Sp-5,6-dichloro-l-beta-d-ribofuranosyl benzimidazole-3',5'-cyclic monophosphorothionate (cBIMPS). Co-immunoprecipitation experiments show that Na(+) channels are associated with PKA and A-kinase-anchoring protein 15 (AKAP-15), and immunocytochemical labeling reveals their co-localization in the cell bodies and proximal dendrites of hippocampal pyramidal neurons. Anchoring of PKA near the channel by an AKAP, which binds the RII alpha regulatory subunit, is necessary for Na(+) channel modulation in acutely dissociated hippocampal pyramidal neurons. Intracellular dialysis with the anchoring inhibitor peptides Ht31 from a human thyroid AKAP and AP2 from AKAP-15 eliminated the modulation of the Na(+) channel by the D1-agonist SKF 81297 and the PKA activator cBIMPS. In contrast, dialysis with the inactive proline-substituted control peptides Ht31-P and AP2-P had little effect on the D1 and PKA modulation. Therefore, we conclude that modulation of the Na(+) channel by activation of D1-like DA receptors requires targeted localization of PKA near the channel to achieve phosphorylation of the alpha subunit and to modify the functional properties of the channel.

Voltage-dependent neuromodulation of Na+ channels by D1-like dopamine receptors in rat hippocampal neurons

Activation of D1-like dopamine (DA) receptors reduces peak Na+ current in acutely isolated hippocampal neurons through phosphorylation of the alpha subunit of the Na+ channel by cAMP-dependent protein kinase (PKA). Here we report that neuromodulation of Na+ currents by DA receptors via PKA is voltage-dependent in the range of -110 to -70 mV and is also sensitive to concurrent activation of protein kinase C (PKC). Depolarization enhanced the ability of D1-like DA receptors to reduce peak Na+ currents via the PKA pathway. Similar voltage-dependent modulation was observed when PKA was activated directly with the membrane-permeant PKA activator DCl-cBIMPS (cBIMPS; 20 microM), indicating that the membrane potential dependence occurs downstream of PKA. PKA activation caused only a small (-2.9 mV) shift in the voltage dependence of steady-state inactivation and had no effect on slow inactivation or on the rates of entry into the fast or slow inactivated states, suggesting that another mechanism is responsible for coupling of membrane potential changes to PKA modulation. Activation of PKC with a low concentration of the membrane-permeant diacylglycerol analog oleylacetyl glycerol also potentiated modulation by SKF 81297 or cBIMPS, and these effects were most striking at hyperpolarized membrane potentials where PKA modulation was not stimulated by membrane depolarization. Thus, activation of D1-like DA receptors causes a strong reduction in Na+ current via the PKA pathway, but it is effective primarily when it is combined with depolarization or activation of PKC. The convergence of these three distinct signaling modalities on the Na+ channel provides an intriguing mechanism for integration of information from multiple signaling pathways in the hippocampus and CNS.

Ca2+/calmodulin binds to and modulates P/Q-type calcium channels

Neurotransmitter release at many central synapses is initiated by an influx of calcium ions through P/Q-type calcium channels, which are densely localized in nerve terminals. Because neurotransmitter release is proportional to the fourth power of calcium concentration, regulation of its entry can profoundly influence neurotransmission. N- and P/Q-type calcium channels are inhibited by G proteins, and recent evidence indicates feedback regulation of P/Q-type channels by calcium. Although calcium-dependent inactivation of L-type channels is well documented, little is known about how calcium modulates P/Q-type channels. Here we report a calcium-dependent interaction between calmodulin and a novel site in the carboxy-terminal domain of the alpha1A subunit of P/Q-type channels. In the presence of low concentrations of intracellular calcium chelators, calcium influx through P/Q-type channels enhances channel inactivation, increases recovery from inactivation and produces a long-lasting facilitation of the calcium current. These effects are prevented by overexpression of a calmodulin-binding inhibitor peptide and by deletion of the calmodulin-binding domain. Our results reveal an unexpected association of Ca2+/calmodulin with P/Q-type calcium channels that may contribute to calcium-dependent synaptic plasticity.

Solution structure of the sodium channel inactivation gate

The sodium channel initiates action potentials by opening in response to membrane depolarization. Fast channel inactivation, which is required for proper physiological function, is mediated by a cytoplasmic loop proposed to occlude the ion pore via a hinged lid mechanism with the triad IFM serving as a hydrophobic "latch". The NMR solution structure of the isolated inactivation gate reveals a stably folded core comprised of an alpha-helix capped by an N-terminal turn, supporting a model in which the tightly folded core containing the latch motif pivots on a more flexible hinge region to occlude the pore during inactivation. The structure, in combination with substituted cysteine mutagenesis experiments, indicates that the IFM triad and adjacent Thr are essential components of the latch and suggests differing roles for the residues of the IFMT motif in fast inactivation.

Interaction of batrachotoxin with the local anesthetic receptor site in transmembrane segment IVS6 of the voltage-gated sodium channel

The voltage-gated sodium channel is the site of action of more than six classes of neurotoxins and drugs that alter its function by interaction with distinct, allosterically coupled receptor sites. Batrachotoxin (BTX) is a steroidal alkaloid that binds to neurotoxin receptor site 2 and causes persistent activation. BTX binding is inhibited allosterically by local anesthetics. We have investigated the interaction of BTX with amino acid residues I1760, F1764, and Y1771, which form part of local anesthetic receptor site in transmembrane segment IVS6 of type IIA sodium channels. Alanine substitution for F1764 (mutant F1764A) reduces tritiated BTX-A-20-alpha-benzoate binding affinity, causing a 60-fold increase in Kd. Alanine substitution for I1760, which is adjacent to F1764 in the predicted IVS6 transmembrane alpha helix, causes only a 4-fold increase in Kd. In contrast, mutant Y1771A shows no change in BTX binding affinity. For wild-type and mutant Y1771A, BTX shifted the voltage for half-maximal activation approximately 40 mV in the hyperpolarizing direction and increased the percentage of noninactivating sodium current to approximately 60%. In contrast, these BTX effects were eliminated completely for the F1764A mutant and were reduced substantially for mutant I1760A. Our data suggest that the BTX receptor site shares overlapping but nonidentical molecular determinants with the local anesthetic receptor site in transmembrane segment IVS6 as well as having unique molecular determinants in transmembrane segment IS6, as demonstrated in previous work. Evidently, BTX conforms to a domain-interface allosteric model of ligand binding and action, as previously proposed for calcium agonist and antagonist drugs acting on L-type calcium channels.

Voltage sensor-trapping: enhanced activation of sodium channels by beta-scorpion toxin bound to the S3-S4 loop in domain II

Polypeptide neurotoxins alter ion channel gating by binding to extracellular receptor sites, even though the voltage sensors are in their S4 transmembrane segments. By analysis of sodium channel chimeras, a beta-scorpion toxin is shown here to negatively shift voltage dependence of activation and enhance closed state inactivation by binding to a receptor site that requires glycine 845 (Gly-845) in the S3-S4 loop at the extracellular end of the S4 segment in domain II of the alpha subunit. Toxin action requires prior depolarization to drive the S4 voltage sensors outward, but these effects are lost in the mutant G845N. The results reveal a voltage sensor-trapping model of toxin action in which the IIS4 voltage sensor is trapped in its outward, activated position by toxin binding.

Primary structure and function of an A kinase anchoring protein associated with calcium channels

Rapid, voltage-dependent potentiation of skeletal muscle L-type calcium channels requires phosphorylation by cAMP-dependent protein kinase (PKA) anchored via an A kinase anchoring protein (AKAP). Here we report the isolation, primary sequence determination, and functional characterization of AKAP15, a lipid-anchored protein of 81 amino acid residues with a single amphipathic helix that binds PKA. AKAP15 colocalizes with L-type calcium channels in transverse tubules and is associated with L-type calcium channels in transfected cells. A peptide fragment of AKAP15 encompassing the RII-binding domain blocks voltage-dependent potentiation. These results indicate that AKAP15 targets PKA to the calcium channel and plays a critical role in voltage-dependent potentiation and regulation of skeletal muscle contraction. The expression of AKAP15 in the brain and heart suggests that it may mediate rapid PKA regulation of L-type calcium channels in neurons and cardiac myocytes.

Molecular determinants of Na+ channel function in the extracellular domain of the beta1 subunit

The rat brain voltage-gated Na+ channel is composed of three glycoprotein subunits: the pore-forming alpha subunit and two auxiliary subunits, beta1 and beta2, which contain immunoglobulin (Ig)-like folds in their extracellular domains. When expressed in Xenopus oocytes, beta1 modulates the gating properties of the channel-forming type IIA alpha subunit, resulting in an acceleration of inactivation. We have used a combination of deletion, alanine-scanning, site-directed, and chimeric mutagenesis strategies to examine the importance of different structural features of the beta1 subunit in the modulation of alphaIIA function, with an emphasis on the extracellular domain. Deletion analysis revealed that the extracellular domain is required for function, but the intracellular domain is not. The mutation of four putative sites of N-linked glycosylation showed that they are not required for beta1 function. Mutations of hydrophobic residues in the core beta sheets of the Ig fold disrupted beta1 function, whereas substitution of amino acid residues in connecting segments had no effect. Mutations of acidic residues in the A/A' strand of the Ig fold reduced the effectiveness of the beta1 subunit in modulating the rate of inactivation but did not significantly affect the association of the mutant beta1 subunit with the alphaIIA subunit or its effect on recovery from inactivation. Our data suggest that the Ig fold of the beta1 extracellular domain serves as a scaffold that presents the charged residues of the A/A' strands for interaction with the pore-forming alpha subunit.

A critical role for the S4-S5 intracellular loop in domain IV of the sodium channel alpha-subunit in fast inactivation

Na+ channel fast inactivation is thought to involve the closure of an intracellular inactivation gate over the channel pore. Previous studies have implicated the intracellular loop connecting domains III and IV and a critical IFM motif within it as the inactivation gate, but amino acid residues at the intracellular mouth of the pore required for gate closure and binding have not been positively identified. The short intracellular loops connecting the S4 and S5 segments in each domain of the Na+ channel alpha-subunit are good candidates for this role in the Na+ channel inactivation process. In this study, we used scanning mutagenesis to examine the role of the IVS4-S5 region in fast inactivation. Mutations F1651A, near the middle of the loop, and L1660A and N1662A, near the COOH-terminal end, substantially disrupted Na+ channel fast inactivation. The mutant F1651A conducted Na+ currents that decayed very slowly, while L1660A and N1662A had large sustained Na+ currents at the end of 30-ms depolarizing pulses. Inactivation of macroscopic Na+ currents was nearly abolished by the N1662A mutation and the combination of the F1651A/L1660A mutations. Single channel analysis revealed frequent reopenings for all three mutants during 40-ms depolarizing pulses, indicating a substantial impairment of the stability of the inactivated state compared with wild type (WT). The F1651A and N1662A mutants also had increased mean open times relative to WT, indicating a slowed rate of entry into the inactivated state. In addition to these effects on inactivation of open Na+ channels, mutants F1651A, L1660A, and N1662A also impaired fast inactivation of closed Na+ channels, as assessed from measurements of the maximum open probability of single channels. The peptide KIFMK mimics the IFM motif of the inactivation gate and provides a test of the effect of mutations on the hydrophobic interaction of this motif with the inactivation gate receptor. KIFMK restores fast inactivation of open channels to the F1651A/L1660A mutant but does not restore fast inactivation of closed F1651A/L1660A channels, suggesting that these residues interact with the IFM motif during inactivation of closed channels. Our results implicate F1651, L1660, and N1662 of the IVS4-S5 loop in inactivation of both closed and open Na+ channels and suggest that the IFM motif of the inactivation gate interacts with F1651 and/or L1660 in the IVS4-S5 loop during inactivation of closed channels.

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

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

Dopaminergic modulation of sodium current in hippocampal neurons via cAMP-dependent phosphorylation of specific sites in the sodium channel alpha subunit

Phosphorylation of brain Na+ channel alpha subunits by cAMP-dependent protein kinase (PKA) decreases peak Na+ current in cultured brain neurons and in mammalian cells and Xenopus oocytes expressing cloned brain Na+ channels. We have studied PKA regulation of Na+ channel function by activation of D1-like dopamine receptors in acutely isolated hippocampal neurons using whole-cell voltage-clamp recording techniques. The D1 agonist SKF 81297 reversibly reduced peak Na+ current in a concentration-dependent manner. No changes in the voltage dependence or kinetics of activation or inactivation were observed. This effect was mediated by PKA, as it was mimicked by application of the PKA activator Sp-5, 6-dichloro-1-beta-D-ribofuranosylbenzimidazole-3', 5'-monophosphorothioate(cBIMPS) and was inhibited by the specific PKA inhibitor peptide PKAI5-24. cBIMPS had similar effects on type IIA brain Na+ channel alpha subunits expressed in tsA-201 cells, but no effect was observed on a mutant Na+ channel alpha subunit in which serine residues in five PKA phosphorylation sites in the intracellular loop connecting domains I and II (LI-II) had been replaced by alanine. A single mutation, S573A, similarly eliminated cBIMPS modulation. Thus, activation of D1-like dopamine receptors results in PKA-dependent phosphorylation of specific sites in LI-II of the Na+ channel alpha subunit, causing a reduction in Na+ current. Such modulation is expected to exert a profound influence on overall neuronal excitability. Dopaminergic input to the hippocampus from the mesocorticolimbic system may exert this influence in vivo.

Type II regulatory subunits are not required for the anchoring-dependent modulation of Ca2+ channel activity by cAMP-dependent protein kinase

Preferential phosphorylation of specific proteins by cAMP-dependent protein kinase (PKA) may be mediated in part by the anchoring of PKA to a family of A-kinase anchor proteins (AKAPs) positioned in close proximity to target proteins. This interaction is thought to depend on binding of the type II regulatory (RII) subunits to AKAPs and is essential for PKA-dependent modulation of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid/kainate receptor, the L-type Ca2+ channel, and the KCa channel. We hypothesized that the targeted disruption of the gene for the ubiquitously expressed RIIalpha subunit would reveal those tissues and signaling events that require anchored PKA. RIIalpha knockout mice appear normal and healthy. In adult skeletal muscle, RIalpha protein levels increased to partially compensate for the loss of RIIalpha. Nonetheless, a reduction in both catalytic (C) subunit protein levels and total kinase activity was observed. Surprisingly, the anchored PKA-dependent potentiation of the L-type Ca2+ channel in RIIalpha knockout skeletal muscle was unchanged compared with wild type although it was more sensitive to inhibitors of PKA-AKAP interactions. The C subunit colocalized with the L-type Ca2+ channel in transverse tubules in wild-type skeletal muscle and retained this localization in knockout muscle. The RIalpha subunit was shown to bind AKAPs, although with a 500-fold lower affinity than the RIIalpha subunit. The potentiation of the L-type Ca2+ channel in RIIalpha knockout mouse skeletal muscle suggests that, despite a lower affinity for AKAP binding, RIalpha is capable of physiologically relevant anchoring interactions.

Increased expression of the cardiac L-type calcium channel in estrogen receptor-deficient mice

Steroid hormones control the expression of many cellular regulators, and a role for estrogen in cardiovascular function and disease has been well documented. To address whether the activity of the L-type Ca2+ channel, a critical element in cardiac excitability and contractility, is altered by estrogen and its nuclear receptor, we examined cardiac myocytes from male mice in which the estrogen receptor gene had been disrupted (ERKO mice). Binding of dihydropyridine Ca2+ channel antagonist isradipine (PN200-110) was increased 45.6% in cardiac membranes from the ERKO mice compared to controls, suggesting that a lack of estrogen receptors in the heart increased the number of Ca2+ channels. Whole-cell patch clamp of acutely dissociated adult cardiac ventricular myocytes indicated that Ca2+ channel current was increased by 49% and action potential duration was increased by 75%. Examination of electrocardiogram parameters in ERKO mice showed a 70% increase in the QT interval without significant changes in PQ or QRS intervals. These results show that the membrane density of the cardiac L-type Ca2+ channel is regulated by the estrogen receptor and suggest that decreased estrogen may lead to an increase in the number of cardiac L-type Ca2+ channels, abnormalities in cardiac excitability, and increased risk of arrhythmia and cardiovascular disease.

Persistent sodium currents through brain sodium channels induced by G protein betagamma subunits

Persistent Na+ currents are thought to be important for integration of neuronal responses. Here, we show that betagamma subunits of G proteins can induce persistent Na+ currents. Coexpression of G beta2gamma3, G beta1gamma3, or G beta5gamma3, but not G beta1gamma1 subunits with rat brain type IIA Na+ channel alpha subunits in tsA-201 cells greatly enhances a component of Na+ current with a normal voltage dependence of activation but with dramatically slowed and incomplete inactivation and with steady-state inactivation shifted +37 mV. Synthetic peptides containing the proposed G betagamma-binding motif, Gln-X-X-Glu-Arg, from either adenylyl cyclase 2 or the Na+ channel alpha subunit C-terminal domain reversed the effect of G beta2gamma3 subunits. These results are consistent with direct binding of G betagamma subunits to the C-terminal domain of the Na+ channel, stabilizing a gating mode responsible for slowed and persistent Na+ current. Modulation of Na+ channel gating by G betagamma subunits is expected to have profound effects on neuronal excitability.

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

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

A patch clamp study of a glutamatergic chloride channel on pharyngeal muscle of the nematode Ascaris suum

Glutamatergic chloride channels on the pharyngeal muscle of Ascaris suum could be activated with glutamate and ivermectin and reversibly blocked with picrotoxin using the patch clamp technique. No activation was observed with GABA, glycine and acetylcholine. Most of the current was carried by the main subconductance state of 21 pS. Two smaller subconductance states occurred rarely. Open time histograms could be best fitted by two time constants of tau(o1) = 0.33 ms and tau(o2) = 9.8 ms present at all glutamate concentrations applied. The results suggest that some properties of the channel investigated here are different from other glutamatergic chloride channels reported from various animals.

Molecular determinants of high affinity phenylalkylamine block of L-type calcium channels in transmembrane segment IIIS6 and the pore region of the alpha1 subunit

Recent studies of the phenylalkylamine binding site in the alpha1C subunit of L-type Ca2+ channels have revealed three amino acid residues in transmembrane segment IVS6 that are critical for high affinity block and are unique to L-type channels. We have extended this analysis of the phenylalkylamine binding site to amino acid residues in transmembrane segment IIIS6 and the pore region. Twenty-two consecutive amino acid residues in segment IIIS6 were mutated to alanine and the conserved Glu residues in the pore region of each homologous domain were mutated to Gln. Mutant channels were expressed in tsA-201 cells along with the beta1b and alpha2delta auxiliary subunits. Assay for block of Ba2+ current by (-)-D888 at -60 mV revealed that mutation of five amino acid residues in segment IIIS6 and the pore region that are conserved between L-type and non-L-type channels (Tyr1152, Phe1164, Val1165, Glu1118, and Glu1419) and one L-type-specific amino acid (Ile1153) decreased affinity for (-)-D888 from 10-20-fold. Combination of the four mutations in segment IIIS6 increased the IC50 for block by (-)-D888 to approximately 9 microM, similar to the affinity of non-L-type Ca2+ channels for this drug. These results indicate that there are important determinants of phenylalkylamine binding in both the S6 segments and the pore regions of domains III and IV, some of which are conserved across the different classes of voltage-gated Ca2+ channels. A model of the phenylalkylamine receptor site at the interface between domains III and IV of the alpha1 subunit is presented.

Molecular analysis of potential hinge residues in the inactivation gate of brain type IIA Na+ channels

During inactivation of Na+ channels, the intracellular loop connecting domains III and IV is thought to fold into the channel protein and occlude the pore through interaction of the hydrophobic motif isoleucine-phenylalanine-methionine (IFM) with a receptor site. We have searched for amino acid residues flanking the IFM motif which may contribute to formation of molecular hinges that allow this motion of the inactivation gate. Site-directed mutagenesis of proline and glycine residues, which often are components of molecular hinges in proteins, revealed that G1484, G1485, P1512, P1514, and P1516 are required for normal fast inactivation. Mutations of these residues slow the time course of macroscopic inactivation. Single channel analysis of mutations G1484A, G1485A, and P1512A showed that the slowing of macroscopic inactivation is produced by increases in open duration and latency to first opening. These mutant channels also show a higher probability of entering a slow gating mode in which their inactivation is further impaired. The effects on gating transitions in the pathway to open Na+ channels indicate conformational coupling of activation to transitions in the inactivation gate. The results are consistent with the hypothesis that these glycine and proline residues contribute to hinge regions which allow movement of the inactivation gate during the inactivation process of Na+ channels.

Molecular analysis of the putative inactivation particle in the inactivation gate of brain type IIA Na+ channels

Fast Na+ channel inactivation is thought to involve binding of phenylalanine 1489 in the hydrophobic cluster IFM in L(III-IV) of the rat brain type IIA Na+ channel. We have analyzed macroscopic and single channel currents from Na+ channels with mutations within and adjacent to hydrophobic clusters in L(III-IV). Substitution of F1489 by a series of amino acids disrupted inactivation to different extents. The degree of disruption was closely correlated with the hydrophilicity of the amino acid at position 1489. These mutations dramatically destabilized the inactivated state and also significantly slowed the entry into the inactivated state, consistent with the idea that F1489 forms a hydrophobic interaction with a putative receptor during the fast inactivation process. Substitution of a phe residue at position 1488 or 1490 in mutants lacking F1489 did not restore normal inactivation, indicating that precise location of F1489 is critical for its function. Mutations of T1491 disrupted inactivation substantially, with large effects on the stability of the inactivated state and smaller effects on the rate of entry into the inactivated state. Mutations of several other hydrophobic residues did not destabilize the inactivated state at depolarized potentials, indicating that the effects of mutations at F1489 and T1491 are specific. The double mutant YY1497/8QQ slowed macroscopic inactivation at all potentials and accelerated recovery from inactivation at negative membrane potentials. Some of these mutations in L(III-IV) also affected the latency to first opening, indicating coupling between L(III-IV) and channel activation. Our results show that the amino acid residues of the IFM hydrophobic cluster and the adjacent T1491 are unique in contributing to the stability of the inactivated state, consistent with the designation of these residues as components of the inactivation particle responsible for fast inactivation of Na+ channels.

High affinity binding of pyrethroids to the alpha subunit of brain sodium channels

Na+ channels are the primary molecular targets of the pyrethroid insecticides. Na+ channels consisting of only a type IIA alpha subunit expressed in Chinese hamster ovary cells responded to pyrethroid treatment in a normal manner: a sustained Na+ current was induced progressively after each depolarizing pulse in a train of stimuli, and this Na+ current decayed slowly on repolarization. These modified Na+ channels could be reactivated at much more negative membrane potentials (V0.5 = -139 mV) than unmodified Na+ channels (V0.5 = -28 mV). These results indicate that pyrethroids can modify the functional properties of the Na+ channel alpha subunit expressed alone by blocking their inactivation, shifting their voltage dependence of activation, and slowing their deactivation. To demonstrate directly the specific interaction of pyrethroids with the alpha subunit of voltage-gated Na+ channels, a radioactive photosensitive derivative, [3H]RU58487, was used in binding and photolabeling studies. In the presence of a low concentration of the nonionic detergent Triton X-100, specific pyrethroid binding to Na+ channels in rat brain membrane preparations could be measured and reached 75% of total binding under optimal conditions. Binding approached equilibrium within 1 hr at 4 degrees, dissociated with a half-time of approximately 10 min, and had K(D) values of approximately 58-300 nM for three representative pyrethroids. Specific pyrethroid binding was enhanced by approximately 40% in the presence of 100 nM alpha-scorpion toxin, but no allosteric enhancement was observed in the presence of toxins acting at other Na+ channel receptor sites. Extensive membrane washing increased specific binding to 89%. Photolabeling with [3H]RU58487 under these optimal binding conditions revealed a radiolabeled band with an apparent molecular mass of 240 kDa corresponding to the Na+ channel alpha subunit. Anti-peptide antibodies recognizing sequences within the alpha subunit were able to specifically immunoprecipitate the covalently modified channel. Together, these results demonstrate that the pyrethroids can modify the properties of cells expressing only the alpha subunit of Na+ channels and can bind specifically to a receptor site on the alpha subunit.

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.

Modulation of the cloned skeletal muscle L-type Ca2+ channel by anchored cAMP-dependent protein kinase

Ca2+ influx through skeletal muscle Ca2+ channels and the force of contraction are increased in response to beta-adrenergic stimulation and high-frequency electrical stimulation. These effects are thought to be mediated by cAMP-dependent phosphorylation of the skeletal muscle Ca2+ channel. Modulation of the cloned skeletal muscle Ca2+ channel by cAMP-dependent phosphorylation and by depolarizing prepulses was reconstituted by transient expression in tsA-201 cells and compared to modulation of the native skeletal muscle Ca2+ channel as expressed in mouse 129CB3 skeletal muscle cells. The heterologously expressed Ca2+ channel consisting of alpha1, alpha2delta, and beta subunits gave currents that were similar in time course, current density, and dihydropyridine sensitivity to the native Ca2+ channel. cAMP-dependent protein kinase (PKA) stimulation by Sp-5,6-DCl-cBIMPS (cBIMPS) increased currents through both native and expressed channels two- to fourfold. Tail currents after depolarizations to potentials between -20 and +80 mV increased in amplitude and decayed more slowly as either the duration or potential of the depolarization was increased. The time- and voltage-dependent slowing of channel deactivation required the activity of PKA, because it was enhanced by cBIMPS and reduced or eliminated by the peptide PKA inhibitor PKI (5-24) amide. This voltage-dependent modulation of the cloned skeletal muscle Ca2+ channel by PKA also required anchoring of PKA by A-Kinase Anchoring Proteins because it was blocked by peptide Ht 31, which disrupts such anchoring. The results show that the skeletal muscle Ca2+ channel expressed in heterologous cells is modulated by PKA at rest and during depolarization and that this modulation requires anchored protein kinase, as it does in native skeletal muscle cells.

Movement of the Na+ channel inactivation gate during inactivation

Phenylalanine 1489 in the inactivation gate of the rat brain IIA sodium channel alpha subunit is required for stable inactivation. It is proposed to move into the intracellular mouth of the pore and occlude it during inactivation, but direct evidence for movement of this residue during inactivation has not been presented. We used the substituted cysteine accessibility method to test the availability of a cysteine residue substituted at position 1489 to modification by methanethiosulfonate reagents applied from the cytoplasmic side. Mutation of Phe-1489 to Cys results in a small (8%) fraction of noninactivating current. Ag+ and methanethiosulfonate reagents irreversibly slowed the inactivation rate and increased the fraction of noninactivating current of F1489C but not wild-type channels. Single channel analysis showed that modification slowed inactivation from both closed and open states and destabilized the inactivated state. Depolarization prevented rapid modification of Cys-1489 by these reagents, and the voltage dependence of their reaction rate correlated closely with steady-state inactivation. Modification was not detectably voltage-dependent at voltages more negative than channel gating. Our results show that, upon inactivation, Phe-1489 in the inactivation gate moves from an exposed and modifiable position outside the membrane electric field to a buried and inaccessible position, perhaps in or near the intracellular mouth of the channel pore.

Distinct effects of mutations in transmembrane segment IVS6 on block of L-type calcium channels by structurally similar phenylalkylamines

The phenylalkylamines (-)-D888, verapamil, and D600, cause voltage- and use-dependent block of L-type Ca2+ channels and differ from each other only in the number of methoxy groups on each of their two terminal phenyl rings. To study the effects of mutations in the phenylalkylamine receptor site on block by these drugs, wild-type and mutant Ca2+ channels were transiently expressed in the tsA-201 clone of human embryonic kidney 293 cells. The combined mutations Y1463A, A1467S, and I1470A (mutant YAI) in transmembrane segment S6 of domain IV of the alpha 1c subunit disrupted block by all three phenylalkylamines. Surprisingly, although this mutation reduced both resting block at -60 mV and depolarized block at +10 mV by (-)-D888, resting and depolarized block by verapamil and D600 were relatively unaffected. In contrast, for all three drugs, use-dependent block during repetitive stimulations was sharply reduced, and the rate of recovery from depolarized block was accelerated for YAI channels. Thus, the effects of the YAI mutation on apparent affinity were specific to (-)-D888, whereas effects on the kinetics of block were observed for all three drugs. Additional experiments with substitution of phenylalanine for Y1463 suggested that (-)-D888 affinity is specifically sensitive to removal of the hydroxyl group of Y1463, whereas effects on the kinetics of block by all three phenylalkylamines require larger molecular changes, perhaps related to residue size and hydrophobicity. Analysis of the data using a state-dependent model of drug block suggests that these kinetic differences are caused by both changes in drug access to the receptor site and affinity for binding to the inactivated state of the channel. The different effects of the YAI mutations on the actions of (-)-D888, verapamil, and D600 indicate that these residues interact differently with these closely related drugs.

Phosphorylation of S1505 in the cardiac Na+ channel inactivation gate is required for modulation by protein kinase C

Inactivation of both brain and cardiac Na+ channels is modulated by activation of protein kinase C (PKC) but in different ways. Previous experiments had shown that phosphorylation of serine 1506 in the highly conserved loop connecting homologous domains III and IV (LIII/IV) of the brain Na+ channel alpha subunit is necessary for all effects of PKC. Here we examine the importance of the analogous serine for the different modulation of the rH1 cardiac Na+ channel. Serine 1505 of rH1 was mutated to alanine to prevent its phosphorylation, and the resulting mutant channel was expressed in 1610 cells. Electrophysiological properties of these mutant channels were indistinguishable from those of wild-type (WT) rH1 channels. Activation of PKC with 1-oleoyl-2-acetyl-sn-glycerol (OAG) reduced WT Na+ current by 49.3 +/- 4.2% (P < 0.01) but S1505A mutant current was reduced by only 8.5 +/- 5.4% (P = 0.29) when the holding potential was -94 mV. PKC activation also caused a -17-mV shift in the voltage dependence of steady-state inactivation of the WT channel which was abolished in the mutant. Thus, phosphorylation of serine 1505 is required for both the negative shift in the inactivation curve and the reduction in Na+ current by PKC. Phosphorylation of S1505/1506 has common and divergent effects in brain and cardiac Na+ channels. In both brain and cardiac Na+ channels, phosphorylation of this site by PKC is required for reduction of peak Na+ current. However, phosphorylation of S1506 in brain Na+ channels slows and destabilizes inactivation of the open channel. Phosphorylation of S1505 in cardiac, but not S1506 in brain, Na+ channels causes a negative shift in the inactivation curve, indicating that it stabilizes inactivation from closed states. Since LIII/IV containing S1505/S1506 is completely conserved, interaction of the phosphorylated serine with other regions of the channel must differ in the two channel types.

Common molecular determinants of local anesthetic, antiarrhythmic, and anticonvulsant block of voltage-gated Na+ channels

Voltage-gated Na+ channels are the molecular targets of local anesthetics, class I antiarrhythmic drugs, and some anticonvulsants. These chemically diverse drugs inhibit Na+ channels with complex voltage- and frequency-dependent properties that reflect preferential drug binding to open and inactivated channel states. The site-directed mutations F1764A and Y1771A in transmembrane segment IVS6 of type IIA Na+ channel alpha subunits dramatically reduce the affinity of inactivated channels for the local anesthetic etidocaine. In this study, we show that these mutations also greatly reduce the sensitivity of Na+ channels to state-dependent block by the class Ib antiarrhythmic drug lidocaine and the anticonvulsant phenytoin and, to a lesser extent, reduce the sensitivity to block by the class Ia and Ic antiarrhythmic drugs quinidine and flecainide. For lidocaine and phenytoin, which bind preferentially to inactivated Na+ channels, the mutation F1764A reduced the affinity for binding to the inactivated state 24.5-fold and 8.3-fold, respectively, while Y1771A had smaller effects. For quinidine and flecainide, which bind preferentially to the open Na+ channels, the mutations F1764A and Y1771A reduced the affinity for binding to the open state 2- to 3-fold. Thus, F1764 and Y1771 are common molecular determinants of state-dependent binding of diverse drugs including lidocaine, phenytoin, flecainide, and quinidine, suggesting that these drugs interact with a common receptor site. However, the different magnitude of the effects of these mutations on binding of the individual drugs indicates that they interact in an overlapping, but nonidentical, manner with a common receptor site. These results further define the contributions of F1764 and Y1771 to a complex drug receptor site in the pore of Na+ channels.

Molecular determinants of high affinity binding of alpha-scorpion toxin and sea anemone toxin in the S3-S4 extracellular loop in domain IV of the Na+ channel alpha subunit

alpha-Scorpion toxins and sea anemone toxins bind to a common extracellular site on the Na+ channel and inhibit fast inactivation. Basic amino acids of the toxins and domains I and IV of the Na+ channel alpha subunit have been previously implicated in toxin binding. To identify acidic residues required for toxin binding, extracellular acidic amino acids in domains I and IV of the type IIa Na+ channel alpha subunit were converted to neutral or basic amino acids using site-directed mutagenesis, and altered channels were transiently expressed in tsA-201 cells and tested for 125I-alpha-scorpion toxin binding. Conversion of Glu1613 at the extracellular end of transmembrane segment IVS3 to Arg or His blocked measurable alpha-scorpion toxin binding, but did not affect the level of expression or saxitoxin binding affinity. Conversion of individual residues in the IVS3-S4 extracellular loop to differently charged residues or to Ala identified seven additional residues whose mutation caused significant effects on binding of alpha-scorpion toxin or sea anemone toxin. Moreover, chimeric Na+ channels in which amino acid residues at the extracellular end of segment IVS3 of the alpha subunit of cardiac Na+ channels were substituted into the type IIa channel sequence had reduced affinity for alpha-scorpion toxin characteristic of cardiac Na+ channels. Electrophysiological analysis showed that E1613R has 62- and 82-fold lower affinities for alpha-scorpion and sea anemone toxins, respectively. Dissociation of alpha-scorpion toxin is substantially accelerated at all potentials compared to wild-type channels. alpha-Scorpion toxin binding to wild type and E1613R had similar voltage dependence, which was slightly more positive and steeper than the voltage dependence of steady-state inactivation. These results indicate that nonidentical amino acids of the IVS3-S4 loop participate in alpha-scorpion toxin and sea anemone toxin binding to overlapping sites and that neighboring amino acid residues in the IVS3 segment contribute to the difference in alpha-scorpion toxin binding affinity between cardiac and neuronal Na+ channels. The results also support the hypothesis that this region of the Na+ channel is important for coupling channel activation to fast inactivation.

Muscarinic modulation of sodium current by activation of protein kinase C in rat hippocampal neurons

Phosphorylation of brain Na+ channels by protein kinase C (PKC) decreases peak Na+ current and slows macroscopic inactivation, but receptor-activated modulation of Na+ currents via the PKC pathway has not been demonstrated. We have examined modulation of Na+ channels by activation of muscarinic receptors in acutely-isolated hippocampal neurons using whole-cell voltage-clamp recording. Application of the muscarinic agonist carbachol reduced peak Na+ current and slowed macroscopic inactivation at all potentials, without changing the voltage-dependent properties of the channel. These effects were mediated by PKC, since they were eliminated when the specific PKC inhibitor (PKCI19-36) was included in the pipette solution and mimicked by the extracellular application of the PKC activator, OAG. Thus, activation of endogenous muscarinic receptors on hippocampal neurons strongly modulates Na+ channel activity by activation of PKC. Cholinergic input from basal forebrain neurons may have this effect in the hippocampus in vivo.

Modulation of Ca2+ channels by G-protein beta gamma subunits

Calcium ions entering cells through voltage-gated Ca2+ channels initiate rapid release of neurotransmitters and secretion of hormones. Ca2+ currents can be inhibited in many cell types by neurotransmitters acting through G proteins via a membrane-delimited pathway independently of soluble intracellular messengers. Inhibition is typically caused by a positive shift in the voltage dependence and a slowing of channel activation and is relieved by strong depolarization resulting in facilitation of Ca2+ currents. This pathway regulates the activity of N-type and P/Q-type Ca2+ channels, which are localized in presynaptic terminals and participate in neurotransmitter release. Synaptic transmission is inhibited by neurotransmitters through this mechanism. G-protein alpha subunits confer specificity in receptor coupling, but it is not known whether the G alpha or G beta gamma subunits are responsible for modulation of Ca2+ channels. Here we report that G beta gamma subunits can modulate Ca2+ channels. Transfection of G beta gamma into cells expressing P/Q-type Ca2+ channels induces modulation like that caused by activation of G protein-coupled receptors, but G alpha subunits do not. Similarly, injection or expression of G beta gamma subunits in sympathetic ganglion neurons induces facilitation and occludes modulation of N-type channels by noradrenaline, but G alpha subunits do not. In both cases, the G gamma subunit is ineffective by itself, but overexpression of exogenous G beta subunits is sufficient to cause channel modulation.