Slow inactivation of muscle mu1 Na+ channels in permanently transfected mammalian cells

Differential effects of modified batrachotoxins on voltage-gated sodium channel fast and slow inactivation

Voltage-gated sodium channels (NaVs) are targets for a number of acute poisons. Many of these agents act as allosteric modulators of channel activity and serve as powerful chemical tools for understanding channel function. Herein, we detail studies with batrachotoxin (BTX), a potent steroidal amine, and three ester derivatives prepared through de novo synthesis against recombinant NaV subtypes (rNaV1.4 and hNaV1.5). Two of these compounds, BTX-B and BTX-cHx, are functionally equivalent to BTX, hyperpolarizing channel activation and blocking both fast and slow inactivation. BTX-yne-a C20-n-heptynoate ester-is a conspicuous outlier, eliminating fast but not slow inactivation. This property differentiates BTX-yne among other NaV modulators as a unique reagent that separates inactivation processes. These findings are supported by functional studies with bacterial NaVs (BacNaVs) that lack a fast inactivation gate. The availability of BTX-yne should advance future efforts aimed at understanding NaV gating mechanisms and designing allosteric regulators of NaV activity.

The pharmacology of voltage-gated sodium channel activators

Toxins and venom components that target voltage-gated sodium (Na_V) channels have evolved numerous times due to the importance of this class of ion channels in the normal physiological function of peripheral and central neurons as well as cardiac and skeletal muscle. Na_V channel activators in particular have been isolated from the venom of spiders, wasps, snakes, scorpions, cone snails and sea anemone and are also produced by plants, bacteria and algae. These compounds have provided key insight into the molecular structure, function and pathophysiological roles of Na_V channels and are important tools due to their at times exquisite subtype-selectivity. We review the pharmacology of Na_V channel activators with particular emphasis on mammalian isoforms and discuss putative applications for these compounds. This article is part of the Special Issue entitled 'Venom-derived Peptides as Pharmacological Tools.'

Block of human cardiac sodium channels by lacosamide: evidence for slow drug binding along the activation pathway

Lacosamide is an anticonvulsant hypothesized to enhance slow inactivation of neuronal Na(+) channels for its therapeutic action. Cardiac Na(+) channels display less and incomplete slow inactivation, but their sensitivity toward lacosamide remains unknown. We therefore investigated the action of lacosamide in human cardiac Nav1.5 and Nav1.5-CW inactivation-deficient Na(+) channels. Lacosamide showed little effect on hNav1.5 Na(+) currents at 300 µM when cells were held at -140 mV. With 30-second conditioning pulses from -90 to -50 mV; however, hNav1.5 Na(+) channels became sensitive to lacosamide with IC50 (50% inhibitory concentration) around 70-80 µM. Higher IC50 values were found at -110 and -30 mV. The development of lacosamide block at -70 mV was slow in wild-type Na(+) channels (τ; 8.04 ± 0.39 seconds, n = 8). This time constant was significantly accelerated in hNav1.5-CW inactivation-deficient counterparts. The recovery from lacosamide block at -70 mV for 10 seconds was relatively rapid in wild-type Na(+) channels (τ; 639 ± 90 milliseconds, n = 8). This recovery was accelerated further in hNav1.5-CW counterparts. Unexpectedly, lacosamide elicited a time-dependent block of persistent hNav1.5-CW Na(+) currents with an IC50 of 242 ± 19 µM (n = 5). Furthermore, both hNav1.5-CW/F1760K mutant and batrachotoxin-activated hNav1.5 Na(+) channels became completely lacosamide resistant, indicating that the lacosamide receptor overlaps receptors for local anesthetics and batrachotoxin. Our results together suggest that lacosamide targets the intermediate preopen and open states of hNav1.5 Na(+) channels. Lacosamide may thus track closely the conformational changes at the hNav1.5-F1760 region along the activation pathway.

A naturally occurring amino acid substitution in the voltage-dependent sodium channel selectivity filter affects channel gating

The pore of sodium channels contains a selectivity filter made of 4 amino acids, D/E/K/A. In voltage sensitive sodium channel (Nav) channels from jellyfish to human the fourth amino acid is Ala. This Ala, when mutated to Asp, promotes slow inactivation. In some Nav channels of pufferfishes, the Ala is replaced with Gly. We studied the biophysical properties of an Ala-to-Gly substitution (A1529G) in rat Nav1.4 channel expressed in Xenopus oocytes alone or with a β1 subunit. The Ala-to-Gly substitution does not affect monovalent cation selectivity and positively shifts the voltage-dependent inactivation curve, although co-expression with a β1 subunit eliminates the difference between A1529G and WT. There is almost no difference in channel fast inactivation, but the β1 subunit accelerates WT current inactivation significantly more than it does the A1529G channels. The Ala-to-Gly substitution mainly influences the rate of recovery from slow inactivation. Again, the β1 subunit is less effective on speeding recovery of A1529G than the WT. We searched Nav channels in numerous databases and noted at least four other independent Ala-to-Gly substitutions in Nav channels in teleost fishes. Thus, the Ala-to-Gly substitution occurs more frequently than previously realized, possibly under selection for alterations of channel gating.

Sodium Channel Inactivation: Molecular Determinants and Modulation

Voltage-gated sodium channels open (activate) when the membrane is depolarized and close on repolarization (deactivate) but also on continuing depolarization by a process termed inactivation, which leaves the channel refractory, i.e., unable to open again for a period of time. In the “classical” fast inactivation, this time is of the millisecond range, but it can last much longer (up to seconds) in a different slow type of inactivation. These two types of inactivation have different mechanisms located in different parts of the channel molecule: the fast inactivation at the cytoplasmic pore opening which can be closed by a hinged lid, the slow inactivation in other parts involving conformational changes of the pore. Fast inactivation is highly vulnerable and affected by many chemical agents, toxins, and proteolytic enzymes but also by the presence of β-subunits of the channel molecule. Systematic studies of these modulating factors and of the effects of point mutations (experimental and in hereditary diseases) in the channel molecule have yielded a fairly consistent picture of the molecular background of fast inactivation, which for the slow inactivation is still lacking.

Adenoviral-mediated expression of functional Na+ channel beta1 subunits tagged with a yellow fluorescent protein

Voltage-gated sodium (Na(+)) channels typically contain a pore-forming alpha subunit and one or two auxiliary beta subunits. Although initial characterization of known alpha and beta subunits has been facilitated by expression in heterologous cells, to understand fully the differences between individual subunits and the functional consequences of selective subunit expression, there is a need to acutely manipulate expression in cells that endogenously express Na(+) channels. To this end, we have constructed a recombinant adenovirus containing a cDNA for a mouse Na(+) channel beta1 subunit with a yellow fluorescent protein fused to its C-terminus (Ad-beta1-EYFP), and with fluorescence microscopy detected beta1-EYFP expression in primary cerebellar neurons and Chinese hamster ovary (CHO) cells upon transduction with this adenovirus, including expression in the plasma membrane. Consistent with this, patch clamp recordings confirmed that Na(+) currents in CHO cells expressing mouse Na(v)1.4 alpha subunits were appropriately modified by the viral-mediated expression of beta1-EYFP subunits. The results demonstrate that adenoviral-mediated gene delivery can be used effectively to express epitope-tagged Na(+) channel subunits with properties similar to wild-type subunits, and suggest that Ad-beta1-EYFP will be a useful reagent for investigating Na(+) channels in a variety of excitable cell types, including neurons.

The selectivity filter of the voltage-gated sodium channel is involved in channel activation

Amino acids located in the outer vestibule of the voltage-gated Na+ channel determine the permeation properties of the channel. Recently, residues lining the outer pore have also been implicated in channel gating. The domain (D) IV P-loop residue alanine 1529 forms a part of the putative selectivity filter of the adult rat skeletal muscle (mu1) Na+ channel. Here we report that replacement of alanine 1529 by aspartic acid enhances entry to an ultra-slow inactivated state. Ultra-slow inactivation is characterized by recovery time constants on the order of approximately 100 s from prolonged depolarizations and by the fact that entry to this state can be reduced by binding to the pore of a mutant mu-conotoxin GIIIA, suggesting that ultra-slow inactivation may reflect a structural rearrangement of the outer vestibule. The voltage dependence of ultra-slow inactivation in DIV-A1529D is U-shaped, with a local maximum near -60 mV, whereas activation is maximal only above -20 mV. Furthermore, a train of brief depolarizations produces more ultra-slow inactivation than a single maintained depolarization of the same duration. These data suggest that ultra-slow inactivation emanates from "partially activated" closed states and that the P-loop in DIV may undergo a conformational change during channel activation, which is accentuated by DIV-A1529D.

Voltage-gated ion channels and hereditary disease

By the introduction of technological advancement in methods of structural analysis, electronics, and recombinant DNA techniques, research in physiology has become molecular. Additionally, focus of interest has been moving away from classical physiology to become increasingly centered on mechanisms of disease. A wonderful example for this development, as evident by this review, is the field of ion channel research which would not be nearly as advanced had it not been for human diseases to clarify. It is for this reason that structure-function relationships and ion channel electrophysiology cannot be separated from the genetic and clinical description of ion channelopathies. Unique among reviews of this topic is that all known human hereditary diseases of voltage-gated ion channels are described covering various fields of medicine such as neurology (nocturnal frontal lobe epilepsy, benign neonatal convulsions, episodic ataxia, hemiplegic migraine, deafness, stationary night blindness), nephrology (X-linked recessive nephrolithiasis, Bartter), myology (hypokalemic and hyperkalemic periodic paralysis, myotonia congenita, paramyotonia, malignant hyperthermia), cardiology (LQT syndrome), and interesting parallels in mechanisms of disease emphasized. Likewise, all types of voltage-gated ion channels for cations (sodium, calcium, and potassium channels) and anions (chloride channels) are described together with all knowledge about pharmacology, structure, expression, isoforms, and encoding genes.

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

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