Probing sodium channel cytoplasmic domain structure. Evidence for the interaction of the rSkM1 amino and carboxyl termini

Perturbation of sodium channel structure by an inherited Long QT Syndrome mutation

The cardiac voltage-gated sodium channel (Na_V1.5) underlies impulse conduction in the heart and its depolarization-induced inactivation is essential in control of the duration of the QT interval of the electrocardiogram (ECG). Perturbation of Nav1.5 inactivation by drugs or inherited mutation can underlie and trigger cardiac arrhythmias. The carboxy terminus plays an important role in channel inactivation, but complete structural information on its predicted structural domain is unknown. Here we measure interactions between the functionally critical distal C-T alpha helix (H6) and the proximal structured EF hand motif using transition metal ion FRET. We measure distances at three loci along H6 relative to an intrinsic tryptophan, demonstrating the proximal-distal interaction in a contiguous carboxy terminus polypeptide. Using these data together with the existing Na_V1.5 carboxy terminus NMR structure, we construct a model of the predicted structured region of the carboxy terminus. An arrhythmia associated H6 mutant which impairs inactivation decreases FRET, indicating destabilization of the distal-proximal intramolecular interaction. These data provide a structural correlate to the pathological phenotype of the mutant channel.

A novel Na+ channel splice form contributes to the regulation of an androgen-dependent social signal

Na(+) channels are often spliced but little is known about the functional consequences of splicing. We have been studying the regulation of Na(+) current inactivation in an electric fish model in which systematic variation in the rate of inactivation of the electric organ Na(+) current shapes the electric organ discharge (EOD), a sexually dimorphic, androgen-sensitive communication signal. Here, we examine the relationship between an Na(+) channel (Na(v)1.4b), which has two splice forms, and the waveform of the EOD. One splice form (Na(v)1.4bL) possesses a novel first exon that encodes a 51 aa N-terminal extension. This is the first report of an Na(+) channel with alternative splicing in the N terminal. This N terminal is present in zebrafish suggesting its general importance in regulating Na(+) currents in teleosts. The extended N terminal significantly speeds fast inactivation, shifts steady-state inactivation, and dramatically enhances recovery from inactivation, essentially fulfilling the functions of a beta subunit. Both splice forms are equally expressed in muscle in electric fish and zebrafish but Na(v)1.4bL is the dominant form in the electric organ implying electric organ-specific transcriptional regulation. Transcript abundance of Na(v)1.4bL in the electric organ is positively correlated with EOD frequency and lowered by androgens. Thus, shaping of the EOD waveform involves the androgenic regulation of a rapidly inactivating splice form of an Na(+) channel. Our results emphasize the role of splicing in the regulation of a vertebrate Na(+) channel and its contribution to a known behavior.

Interdomain cytoplasmic interactions govern the intracellular trafficking, gating, and modulation of the Kv2.1 channel

Voltage-gated potassium (Kv) channels comprise four transmembrane alpha subunits, often associated with cytoplasmic beta subunits that impact channel expression and function. Here, we show that cell surface expression, voltage-dependent activation gating, and phosphorylation-dependent modulation of Kv2.1 are regulated by cytoplasmic N/C interaction within the alpha subunit. Kv2.1 surface expression is greatly reduced by C-terminal truncation. Tailless Kv2.1 channels exhibit altered voltage-dependent gating properties and lack the bulk of the phosphorylation-dependent modulation of channel gating. Remarkably, the soluble C terminus of Kv2.1 associates with tailless channels and rescues their expression, function, and phosphorylation-dependent modulation. Soluble N and C termini of Kv2.1 can also interact directly. We also show that the N/C-terminal interaction in Kv2.1 is governed by a 34 aa motif in the juxtamembrane cytoplasmic C terminus, and a 17 aa motif located in the N terminus at a position equivalent to the beta subunit binding site in other Kv channels. Deletion of either motif disrupts N/C-terminal interaction and surface expression, function, and phosphorylation-dependent modulation of Kv2.1 channels. These findings provide novel insights into intrinsic mechanisms for the regulation of Kv2.1 trafficking, gating, and phosphorylation-dependent modulation through cytoplasmic N/C-terminal interaction, which resembles alpha/beta subunit interaction in other Kv channels.

A genetically targetable fluorescent probe of channel gating with rapid kinetics

We have developed a genetically targetable, optical channel-gating reporter that converts rapid membrane potential changes into changes in fluorescence intensity. We have named this construct SPARC (sodium channel protein-based activity reporting construct). Green fluorescent protein was inserted into an intracellular loop of a reversibly nonconducting form of the rat mu I skeletal muscle voltage-gated sodium channel. Rapid changes of the membrane potential modulate the fluorescence of the inserted green fluorescent protein. This change in fluorescence can faithfully report depolarizing pulses as short as 2 ms. The fluorescence signal does not inactivate during extended depolarizations. Several features of the probe's response properties indicate that it likely reports gating charge movement of a single domain of rat mu I skeletal muscle. This probe provides a new approach for studying rapid channel movements and may possibly act as a fluorescent activity reporter in excitable cells.

A carboxy-terminal alpha-helical segment in the rat skeletal muscle voltage-dependent Na+ channel is responsible for its interaction with the amino-terminus

Cytoplasmic segments of the adult rat skeletal muscle sodium channel alpha-subunit (rSkM1) comprise a major portion (approximately 40%) of the total protein and are involved in channel functions both general, such as inactivation, and isoform-specific, for example, protein kinase A modulation. Far ultraviolet circular dichroism measurements of synthetic peptides and overexpressed fusion proteins containing individual channel cytoplasmic segments suggest that cytoplasmic domains of rSkM1 contain ordered secondary structures even in the absence of adjoining transmembrane segments. Intrinsic fluorescence experiments with a nested set of carboxy-terminal deletion proteins confirm a specific interaction between the channel's amino- and carboxy-termini and identify residues 1716-1737 in the carboxy-terminus as the region that binds to the amino-terminus. Circular dichroism measurements suggest that this same region is organized as an alpha-helix and that electrostatic forces may contribute to this association. The interaction of the amino- and carboxy-termini is not accompanied by secondary structure changes detectable by circular dichroism spectroscopy, but a decrease in intrinsic fluorescence indicates that this association is accompanied by a change in the environment of Trp1617.

Molecular determinants of beta 1 subunit-induced gating modulation in voltage-dependent Na+ channels

Recombinant brain, skeletal muscle, and heart voltage-gated Na+ channel alpha subunits differ in their functional responses to an accessory beta 1 subunit when coexpressed in Xenopus oocytes. We exploited the distinct beta 1 subunit responses observed for the human heart (hH1) and human skeletal muscle (hSkM1) isoforms to identify determinants of this response. Chimeric alpha subunits were constructed by exchanging the S5-S6 interhelical loops of each domain between hH1 and hSkM1 and then examined for effects on inactivation induced by coexpressed beta 1 subunit in oocytes. Substitution of single S5-S6 loops in either domain 1 (D1/S5-S6) or domain 4 (D4/S5-S6) of hSkM1 by the corresponding segments of hH1 produced channels that exhibited an attenuated response to coexpressed beta 1 subunit. Substitutions of both D1/S5-S6 and D4/S5-S6 in hSkM1 by the corresponding loops from hH1 completely abolished the effects of the beta 1 subunit on inactivation. The reciprocal chimera in which both D1/S5-S6 and D4/S5-S6 from hSkM1 were transplanted into hH1 exhibited significant beta 1 responsiveness (accelerated inactivation). The region within D4/S5-S6 that conferred beta 1 responsiveness was determined to reside primarily within an extracellular loop between the putative pore-forming segment SS2 and D4/S6. Gating modulation was also demonstrated using a chimeric beta subunit consisting of the extracellular domains of beta 1 and the transmembrane and C-terminal domains of the rat brain beta 2 subunit. These results suggest that the D1/S5-S6 and D4/S5-S6 loops in the alpha subunit and the extracellular domain of the beta 1 subunit are important determinants of the beta 1 subunit-induced gating modulation in Na+ channels.

Immunocytochemical localization of rH1 sodium channel in adult rat heart atria and ventricle. Presence in terminal intercalated disks

BACKGROUND

Of the five sodium channel subtypes expressed in cardiac tissues, the rat (rH1) and human (hH1) isoforms are thought to be the predominant subtypes on the basis of heterologous expression studies. In this study, subtype-specific antibodies and immunocytochemistry were used to confirm protein expression and to localize rH1 protein in cardiac tissues.

METHODS AND RESULTS

Subtype-specific antibodies immunolabeled adult rat heart tissue in a manner identical to that obtained with subtype-nonselective antibodies. All antibodies specifically bound to the surface and t-tubular systems of atrial and ventricular muscle cells. Cytoplasmic labeling, reflecting nascent sodium channels or cytoplasmic stores of sodium channel protein, was apparent. Most notably, all antibodies also specifically labeled the subset of intercalated disks located at the ends but not the sides of adjacent ventricular muscle cells.

CONCLUSIONS

rH1 is the predominant subtype expressed on rat atrial and ventricular muscle cells. rH1 protein localization in surface and t-tubular membranes is consistent with its proposed role in coordinating membrane depolarization along the length and deep within cardiac muscle cells. rH1 protein localization in terminal intercalated disks suggests that sodium channels may also act as a localized voltage-dependent current amplifier, raising the safety margin for conduction; they also may contribute to anisotropic or saltatory conduction in cardiac tissues. These electrophysiological properties would be particularly important under conditions of altered, channel function resulting from ion channel gene defects (e.g., long QT syndrome), antiarrhythmic drug therapy, ischemia, or other heart diseases by influencing the electrophysiological substrate for ventricular tachyarrhythmias.