Human voltage-gated sodium channel mutations that cause inherited neuronal and muscle channelopathies increase resurgent sodium currents

Inherited mutations in voltage-gated sodium channels (VGSCs; or Nav) cause many disorders of excitability, including epilepsy, chronic pain, myotonia, and cardiac arrhythmias. Understanding the functional consequences of the disease-causing mutations is likely to provide invaluable insight into the roles that VGSCs play in normal and abnormal excitability. Here, we sought to test the hypothesis that disease-causing mutations lead to increased resurgent currents, unusual sodium currents that have not previously been implicated in disorders of excitability. We demonstrated that a paroxysmal extreme pain disorder (PEPD) mutation in the human peripheral neuronal sodium channel Nav1.7, a paramyotonia congenita (PMC) mutation in the human skeletal muscle sodium channel Nav1.4, and a long-QT3/SIDS mutation in the human cardiac sodium channel Nav1.5 all substantially increased the amplitude of resurgent sodium currents in an optimized adult rat–derived dorsal root ganglion neuronal expression system. Computer simulations indicated that resurgent currents associated with the Nav1.7 mutation could induce high-frequency action potential firing in nociceptive neurons and that resurgent currents associated with the Nav1.5 mutation could broaden the action potential in cardiac myocytes. These effects are consistent with the pathophysiology associated with the respective channelopathies. Our results indicate that resurgent currents are associated with multiple channelopathies and are likely to be important contributors to neuronal and muscle disorders of excitability.

Voltage-gated sodium channels (VGSCs; or Nav) are crucial to the generation and propagation of the all-or-none action potentials (APs) in excitable cells, such as neurons and muscle. More than 200 different missense mutations in 7 VGSCs have been identified as causing disorders of excitability, or channelopathies, in humans (1, 2). Although these channelopathies are relatively rare, understanding the functional consequences of the disease mutations provides invaluable insight into the roles that VGSCs play in normal and abnormal excitability. Mutant channels have been extensively studied in nonexcitable heterologous expression systems, providing substantial knowledge; however, a major concern is that the functional properties of VGSCs in neurons and muscle cells are not always accurately reproduced in nonexcitable cells. A prime example of this phenomenon is VGSC resurgent currents. Although resurgent VGSC currents have been recorded from neurons, it has not been possible to record resurgent currents in nonexcitable heterologous expression systems (3–5), and it is not known whether VGSC mutations that cause channelopathies alter resurgent currents.

Resurgent sodium currents were first described in cerebellar Purkinje neurons (3) and more recently have been observed in dorsal root ganglion (DRG) neurons (4). Normally, VGSCs open, and subsequently rapidly inactivate, upon membrane depolarization. Inactivated channels cannot reopen until they have been substantially hyperpolarized for many milliseconds; this is the basis for the AP refractory period (6). In contrast, resurgent currents reopen during the repolarization of the membrane potential and therefore are likely to contribute to spontaneous and high-frequency firing (7). These unusual sodium currents are proposed to result from a putative intracellular blocking factor that binds to open VGSCs and prevents classical VGSC inactivation, but unbinds during repolarization at potentials at which channels normally remain inactivated (8). In both cerebellar Purkinje and DRG neurons isolated from Nav1.6 knockout mice, resurgent currents are greatly reduced, which indicates that Nav1.6 is normally the predominant generator of resurgent currents in neurons (3, 4). However, artificial slowing of VGSC inactivation via toxin application can induce resurgent currents in Purkinje neurons isolated from Nav1.6 knockout mice, which indicates that VGSCs other than Nav1.6 may have the capacity to generate resurgent sodium currents (9). In this study, we hypothesized that disease mutations that slow or destabilize sodium channel inactivation lead to increased resurgent currents. We have previously shown that recombinant Nav1.6 channels expressed in DRG neurons can generate resurgent currents in approximately 60% of the transfected neurons (4), which indicates that DRG neurons should be an ideal cell background for testing our hypothesis. Although DRG neurons express multiple VGSCs, it is worth noting that not all isoforms are natively expressed in DRG neurons (10). Adult DRG sensory neurons can express combinations of Nav1.1, Nav1.6, Nav1.7, Nav1.8, and Nav1.9. In addition, Nav1.2, Nav1.3, and Nav1.5 are expressed, at least at low levels, in embryonic rat DRG. However, because the cellular background of DRG neurons, which may include expression of particular kinases and auxiliary subunits, is appropriate to generate resurgent sodium currents in Nav1.6, we proposed that cultured DRG neurons might provide an opportune expression system for investigating the capability of other wild-type and mutant VGSCs to generate resurgent current. Using this expression system, we showed for the first time to our knowledge that disease mutations in 3 different VGSC isoforms, associated with 3 distinct disorders of excitability, increased resurgent sodium currents. Based on our data, we propose that resurgent sodium currents play important and previously unrecognized roles in disorders of excitability.

A Nav1.7 inactivation gate mutation implicated in PEPD increases resurgent sodium currents. We first asked whether a mutation in Nav1.7 that slows inactivation and causes paroxysmal extreme pain disorder (PEPD; ref. 11) generates resurgent sodium currents. Highly expressed in DRG neurons, Nav1.7 channels are essential in nociception, as evidenced by single-point missense mutations causing a spectrum of pain syndromes, including PEPD, and by nonsense mutations resulting in human insensitivity to pain (12). Although Nav1.7 channels have not previously been shown to produce resurgent currents, PEPD mutations destabilize inactivation, shifting voltage dependence and decreasing the rate of inactivation (11, 13). Therefore, we hypothesized that PEPD mutations induce resurgent currents. We studied the Nav1.7 PEPD mutation I1461T, located within the highly conserved D3-D4 inactivation particle (IFMT) critical for VGSC inactivation (14, 15). Modified wild-type Nav1.7 and Nav1.7-I1461T (referred to herein as Nav1.7r and Nav1.7r-I1461T, respectively) channels that generate currents that can be pharmacologically isolated were expressed in adult rat DRG neurons (Figure 1, A and B). In addition to the recombinant channel of interest, neurons were also cotransfected with a second plasmid encoding for both EGFP, to help identify transfected neurons, and a specific Nav1.8 shRNA, to minimize endogenous Nav1.8 currents (see Methods for details). In DRG neurons, the I1461T PEPD mutation impaired inactivation (Figure 1, C and D, and Table 1), but did not alter activation (data not shown), of Nav1.7r channels. The persistent component, measured at the end of a 50-ms pulse to –10 mV, was also significantly larger for Nav1.7r-I1461T channels than for Nav1.7r channels (Nav1.7r, 0.2% ± 0.2 % of peak transient current, n = 13; Nav1.7r-I1461T, 1.0% ± 0.3%, n = 22; P < 0.05). These changes were identical to those observed for wild-type and I1461T Nav1.7 channels expressed in HEK293 cells (13). However, in DRG neurons, both Nav1.7r and Nav1.7r-I1461T channels also generated resurgent sodium currents. Resurgent sodium currents were observed in 5 of 21 neurons transfected with Nav1.7r (Figure 2, A and B), with an average amplitude — expressed as a percentage of the peak transient sodium current elicited with a test pulse to –10 mV — of 1.0% ± 0.5% for these 5 neurons. Conversely, 20 of 30 neurons expressing Nav1.7r-I1461T channels produced resurgent sodium current (Figure 2, C and D). The frequency of resurgent current with Nav1.7r-I1461T channels was significantly increased (P < 0.05, χ² test) compared with Nav1.7r channels. Moreover, the relative amplitude of the resurgent current (2.0% ± 0.1%) was also significantly greater for Nav1.7r-I1461T than for Nav1.7r channels (P < 0.05). Because all PEPD mutations characterized to date result in slower inactivation of Nav1.7 (11, 13), we predict that all PEPD mutations are likely to increase resurgent current generation.

Figure 1

Currents generated by recombinant Nav1.7 channels expressed in DRG neurons. (A) Representative Nav1.7r current traces recorded from a transfected DRG neuron. (B) Representative Nav1.7r-I1461T current traces recorded from a transfected DRG neuron. Currents were elicited with step depolarizations to voltages ranging from –80 to +40 mV from a holding potential of –100 mV. (C) The painful mutation I1461T slowed the rate of inactivation of Nav1.7r. Black trace, Nav1.7r; red trace, Nav1.7r-I1461T. Currents were elicited with a step depolarization to +10 mV. (D) Steady-state inactivation curves for Nav1.7r (black) and Nav1.7r-I1461T (red) channels expressed in DRG neurons. Cultured adult rat DRG neurons were transfected with the recombinant VGSC construct and Nav1.8 shRNA, and recordings were done in the presence of 500 nM TTX.

Figure 2

Resurgent currents are produced by recombinant Nav1.7 channels expressed in DRG neurons. (A and B) Representative current traces recorded from DRG neurons expressing Nav1.7r that did not (A) and that did (B) generate resurgent currents. (C and D) Representative current traces recorded from DRG neurons expressing Nav1.7r-I1461T channels that did not (C) and that did (D) generate resurgent currents. Resurgent currents were larger on average for Nav1.7r-I1461T than for Nav1.7 channels. Currents are magnified ×30 relative to the peak transient current (elicited with a pulse to –10 mV) to emphasize the resurgent current components. (E) Resurgent current voltage protocol. (F) Voltage dependence of resurgent current shown in D.

Table 1

Biophysical properties of wild-type and mutant sodium channels

A PEPD mutation that increases resurgent currents increases AP repetitive firing in simulated neurons. Based on work done in cerebellar Purkinje neurons (3), we expected increased resurgent sodium currents generated by PEPD mutations to result in spontaneous and/or repetitive APs in DRG neurons. To test this, we performed computer simulations of DRG neuron excitability. We used an established model of DRG neuron excitability (16) that had been implemented in the NEURON simulation environment (17) and, with modifications only to the appropriate sodium channel formulation, simulated and evaluated the impact of the I1461T mutation and resurgent currents on AP firing. Mathematical models of Nav1.7 and Nav1.7-I1461T currents with and without a resurgent current blocking factor were developed using a multistate Markov-type model of Nav1.7 (Figure 3). The resurgent current blocking factor was implemented using the strategy developed by Khaliq et al. (18). The forward and reverse transition rate expressions for the blocking factor were adjusted (see Supplemental Table 2; supplemental material available online with this article; doi: 10.1172/JCI40801DS1) in order to match the relative amplitude of the experimentally observed resurgent currents. In our model, the blocking factor–induced resurgent current was 1.0% of the peak transient current in the modeled Nav1.7 conductance and 2.0% of the peak transient current in the modeled Nav1.7-I1461T conductance (Figure 3C). The computer simulations of AP firing in DRG neurons indicated that, whereas the destabilization of inactivation caused by the I1461T mutation was sufficient to decrease the threshold for eliciting an AP, inclusion of the resurgent current blocking factor led to high-frequency trains of APs (Figure 3D). It is important to note that even when the transition rate expressions for the blocking factor were adjusted so that the wild-type resurgent current was doubled (2% of peak), a 70-pA stimulus still only elicited a single AP in the model neuron. These results indicate the resurgent current blocking factor and the I1461T mutations act synergistically to increase neuronal excitability and that resurgent currents probably contribute to the extreme pain sensations associated with PEPD mutations.

Figure 3

Computer simulation of sodium conductances and DRG neuron excitability. (A) Diagram of Markov models for VGSC conductances. An 8-state Markov model was used for simulation of VGSC conductances without resurgent currents. C1–C3, closed (nonconducting) states; O, open (conducting) state; I1–I4, inactivated (also nonconducting) states. A 9-state Markov model incorporated the resurgent current blocking factor. The OB state (boxed) represents channels blocked by this factor. (B) Simulated Nav1.7 (black trace) and Nav1.7-I1461T (red trace) currents elicited by a voltage step from –100 mV to +10 mV. (C) Simulated resurgent currents generated by model Nav1.7 (black trace) and Nav1.7-I1461T (red trace) conductances. Model currents were elicited with the standard resurgent current voltage protocol shown in Figure 2E. (D) In simulated DRG neurons with Nav1.7 channels, 70 pA depolarizing current is required to elicit an AP with or without resurgent current simulation. Conversely, only 40 pA is needed to elicit an AP in a simulated neuron with Nav1.7-I1461T channels, and a train of high-frequency APs is generated when the modeled Nav1.7-I1461T channels generate resurgent currents.

A cardiac long-QT3/sudden infant death syndrome mutation that slows inactivation increases resurgent Nav1.5 current and broadens AP waveform in a modeled myocyte. Disease mutations that impair inactivation have also been identified in several other VGSCs, including Nav1.1 and Nav1.3 mutations associated with epilepsies, Nav1.4 mutations associated with skeletal muscle myotonias, and Nav1.5 mutations associated with cardiac arrhythmias (1, 2). More than 50 different disease mutations that impair inactivation have been characterized (see Supplemental Table 1 for a partial listing). However, these mutations have all been characterized in heterologous expression systems that do not support the generation of resurgent sodium currents. Therefore, we next used the DRG expression system to determine whether the Nav1.5 mutation F1486L (19), associated with long-QT3/sudden infant death syndrome (LQT3/SIDS), generates increased resurgent currents. As was previously shown in HEK293 cells (19), the F1486L mutation, which is also located in the IFMT inactivation particle, slowed the rate of inactivation, increased the fraction of persistent currents, and shifted the voltage dependence of activation in the depolarizing direction (Table 1). However, 9 of 18 DRG neurons expressing wild-type Nav1.5 generated resurgent currents (Figure 4, A and B). In our expression system, the Nav1.5-F1486L LQT3/SIDS mutation significantly increased the relative amplitude of the resurgent currents (Table 1). Of 17 neurons expressing Nav1.5-F1486L channels, 8 generated resurgent currents, with an average relative amplitude of 2.0% ± 0.4% (Figure 4, C and D). As resurgent sodium currents are activated during repolarization, we expected increased resurgent currents in Nav1.5 to broaden the AP, increase the QT interval, and thus contribute to the potentially lethal cardiac arrhythmias associated with LQT3/SIDS mutations. Mathematical modeling of resurgent sodium currents and computer simulations of a cardiac myocyte (20) indicated that this may indeed be the case (Figure 5).

Figure 4

Resurgent currents are produced by Nav1.5 channels. (A and B) Representative current traces recorded from DRG neurons expressing Nav1.5 that did not (A) and that did (B) generate resurgent currents. (C and D) Representative current traces recorded from DRG neurons expressing Nav1.5-F1486L channels that did not (C) and that did (D) generate resurgent currents. Resurgent currents were larger on average for Nav1.5-F1486L than for Nav1.5 channels. Currents were elicited with the standard resurgent current protocol shown in Figure 2E and are magnified ×30 relative to the peak current amplitude.

Figure 5

Computer simulations of Nav1.5 currents and cardiac myocyte APs. (A) Simulated Nav1.5 (black trace) and Nav1.5-F1486L (blue trace) currents elicited by a voltage step from –100 mV to +10 mV. (B) Simulated resurgent currents generated by model Nav1.5 (black trace) and Nav1.5-F1486L (blue trace) conductances. Model currents were elicited with the standard resurgent current voltage protocol shown in Figure 2E. The modeled resurgent current was 0.9% of the peak current for Nav1.5 and 1.7% of the peak current for Nav1.5-F1486L. (C) Simulated APs from a modeled cardiac myocyte. There was little difference between the APs of a model cell with Nav1.5 that did not include resurgent currents (black trace) and those of a cell with Nav1.5 that did include resurgent currents (green trace). Nav1.5-F1486L simulated without resurgent current generation broadened the AP (red trace), and this effect was exacerbated in the simulation of Nav1.5-F1486L with resurgent current generation (blue trace).

A D4/S4 paramyotonia congenita mutation in Nav1.4 that replaces a charged residue and uncouples fast inactivation generates resurgent sodium currents. We next asked whether a mutation that slows inactivation of Nav1.4 and causes paramyotonia congenita (PMC) induces resurgent sodium currents. We studied the R1448P mutation (21), which alters the outermost extracellular charged residue in the sodium channel voltage sensor that couples channel activation and inactivation (22). Interestingly, although this mutation slows the inactivation of Nav1.4 currents by approximately 10-fold (Figure 6A), it causes a hyperpolarizing shift in the voltage dependence of inactivation (Table 1). In a previous study, resurgent sodium currents were not detected in any of 41 DRG neurons transfected with wild-type skeletal muscle sodium channel Nav1.4r (4). Again, we did not detect resurgent currents in any of 11 neurons expressing Nav1.4r (Figure 6B); however, 13 of 20 neurons expressing Nav1.4r-R1448P channels generated resurgent currents (Figure 6C), with an average relative amplitude of 4.8% ± 0.7% of peak. Thus, at least for Nav1.4, slowing the rate of inactivation seemed to be crucial to the production of resurgent sodium currents, and the impact of the mutation on the voltage dependence of inactivation may be less important. It should be noted that although the R1448P mutation does not alter the voltage dependence of activation, it slows deactivation (the rate at which open channels transit to the closed state; refs. 21, 23), and slower deactivation might also contribute to enhanced resurgent currents, especially in combination with slower inactivation. Resurgent currents generated by Nav1.4 are likely to increase repetitive AP firing in skeletal muscle, which is one of the hallmarks of PMC. However, it should be noted that patients with PMC can also experience episodes of muscle weakness or paralysis in addition to myotonia (24). Although resurgent currents could clearly contribute to myotonic activity associated with PMC, incomplete inactivation observed with mutant PMC channels is likely to be an important factor in muscle weakness associated with PMC (2).

Figure 6

A PMC mutation induces resurgent currents in Nav1.4. (A) The paramyotonia R1448P mutation caused a pronounced slowing of the rate of Nav1.4r inactivation. Currents were elicited with a step depolarization to +10 mV. (B) Resurgent currents were not detectable in any of the neurons expressing Nav1.4r channels. (C) Conversely, the majority of neurons expressing Nav1.4r-R1448P channels generated robust resurgent currents. Resurgent currents were elicited with the protocol shown in Figure 2E and are magnified ×20 relative to the peak current amplitude.

A homologous inactivation gate mutation engineered into Nav1.6 channels dramatically increases resurgent currents and destabilizes transition to an inactivated state. Because Nav1.6 appears to be the predominant generator of resurgent currents in cerebellar Purkinje neurons and DRG neurons (3, 4), we next compared the amplitude of resurgent currents generated by Nav1.6 channels with those produced by the Nav1.4, Nav1.5, and Nav1.7 disease mutations. Resurgent currents were detected in 8 of 14 DRG neurons expressing wild-type Nav1.6r channels, with an average relative amplitude of 2.4% ± 0.3% of the peak current (Figure 7A). This relative amplitude is similar to that produced by the Nav1.7r-I1461T PEPD mutant and the Nav1.5-F1486L LQT3/SIDS mutant, but slightly smaller than that produced by the Nav1.4r-R1448P PMC mutant (Figure 7C). This indicates that the resurgent currents produced by the disease mutants are indeed likely to have important impact on cellular excitability. Remarkably, gain-of-function disease mutations, to date, have not been identified in Nav1.6 channels. To determine the potential impact of mutations in Nav1.6 that impair inactivation, we next asked if a mutation in Nav1.6 (I1477T) corresponding to the I1461T mutation in Nav1.7 could increase resurgent sodium currents generated by Nav1.6. The Nav1.6r-I1477T mutation shifted the voltage dependence of inactivation in the depolarizing direction, slowed the rate of inactivation and significantly increased the persistent sodium current (Table 1). Resurgent currents were observed in 7 of 14 neurons expressing Nav1.6r-I1477T channels. Notably, these currents were approximately 8-fold larger than those produced by either Nav1.6r or Nav1.7r-I1461T channels, with an average relative amplitude of 15.3% ± 3.4% (Figure 7C). These data suggest that mutations impairing inactivation of Nav1.6 may be lethal as a result of the proclivity of Nav1.6 to generate resurgent currents.

Figure 7

Nav1.6 channels generate large resurgent currents. (A and B) Representative resurgent currents recorded from a neuron expressing Nav1.6r (A) and Nav1.6r-I1477T (B) channels. Current traces are magnified ×20 and ×5, respectively, relative to the peak current. (C) Comparison of the relative resurgent current amplitude, expressed as a percentage of peak transient current, for wild-type and mutant VGSCs. *P < 0.05 versus the wild-type channel for the given isoform.

Our data show, for the first time to our knowledge, that Nav1.4, Nav1.5, and Nav1.7 channels have the capability to generate resurgent currents, and that the relative resurgent sodium current amplitudes observed with the disease mutations that impair inactivation are of approximately the same magnitude as the resurgent currents generated by Nav1.6 under control conditions in cerebellar Purkinje neurons and DRG neurons (3, 4). An extensive series of studies on cerebellar Purkinje neurons indicates that resurgent currents have a major impact on excitability, contributing to spontaneous firing and accelerating the rate of repetitive firing of APs (3, 7, 8, 18, 25). As a consequence, it is predicted that the resurgent currents associated with the inherited neuronal and muscle channelopathies could markedly affect AP firing in neurons and muscle and contribute to disease pathophysiology. Indeed, our computer simulations of a DRG neuron and cardiac myocyte indicated that resurgent currents could substantially exacerbate the effects of the disease mutations on cellular excitability.

Our data provided further evidence that the sodium channel isoforms differ in their proclivity to produce resurgent sodium currents. We showed that Nav1.6 was more inclined to generate resurgent currents than Nav1.4, Nav1.5, or Nav1.7. Furthermore, the identical mutation produced a greater increase of resurgent currents in Nav1.6 than in Nav1.7 (compare Nav1.6r-I1477T data with that of Nav1.7r-I1461T; Table 1 and Figures 2 and 7). Although studies of Nav1.6 knockout mice strongly indicate that Nav1.6 is the predominant generator of resurgent currents in cerebellar Purkinje and DRG neurons (3, 4), resurgent currents have been detected in Purkinje and other central nervous system neurons from Nav1.6 knockout mice (26). Nav1.2 channels have been shown to produce resurgent currents when expressed in DRG neurons, albeit in only 2 of 25 transfected cells (27). Although it is not known whether Nav1.1 and Nav1.3 channels produce resurgent currents, mutations that slow the rate of inactivation have been identified in these isoforms in patients with epilepsy (28, 29), and it is therefore conceivable that resurgent currents could contribute to the pathophysiology of some inherited epilepsies. In our study, we showed that a mutation in the voltage-sensing segment 4 of the fourth domain, as well as mutations in the IFMT inactivation particle, induced enhanced resurgent currents, which suggests that any mutation that slows the rate of open channel fast inactivation might be able to induce resurgent currents. However, it is important to note that the rate of inactivation cannot be the only determinant of resurgent current production. Nav1.8 currents are slower to inactivate than are Nav1.7r-I1461T channels. Regardless of this, cultured rat DRG neurons treated with tetrodotoxin (TTX) that produced large endogenous Nav1.8 currents, but no recombinant currents, did not produce resurgent currents (n = 11; see Supplemental Figure 1). Thus, other factors must also contribute to the inclination of specific VGSC isoforms to generate resurgent currents.

The data presented here, in conjunction with the findings of our previous study (4), clearly show that DRG neurons have the appropriate cellular environment for production of resurgent currents. Conversely, adult DRG sensory neurons are not the native tissue for expression of Nav1.4 and Nav1.5, and it is not known whether cardiac and skeletal muscle cells have the appropriate cellular environment for the production of resurgent currents. Data from cerebellar Purkinje neurons indicate that the auxiliary sodium channel β4 subunit may be the putative blocking factor crucial for generation of resurgent currents with Nav1.6 channels (30). Previous work has demonstrated that DRG neurons and cardiac and skeletal muscle all express high levels of β4 (31). However, although the β4 subunit may be necessary, it does not appear to be sufficient. Phosphorylation of the sodium channel, the β4 subunit, or possibly an unidentified protein also seems to be required (32), at least in cerebellar Purkinje neurons. This indicates that changes in kinase and phosphatase activity are likely to affect the generation of resurgent currents. Therefore, even if the conditions in normal cardiac and skeletal muscle are not appropriate for generation of resurgent currents, under specific, possibly pathological, conditions, these cells might express the appropriate accessory subunits and kinases. Because our optimized expression system used adult DRG neurons, in which Nav1.4 and Nav1.5 are not normally expressed, it would be of great interest to determine whether Nav1.4 and Nav1.5 wild-type and mutant channels produce resurgent sodium currents in skeletal and cardiac muscle, respectively, under either normal or pathophysiological conditions.

It is important to note that not all of the DRG neurons expressing recombinant channels capable of generating resurgent currents produced detectable resurgent currents. This further indicates that the cellular environment is a crucial determinant of resurgent current production. Regional differences in the properties of sensory afferents that affect resurgent current generation might contribute to the phenotypical association of pain with particular body regions in PEPD. Interestingly, cells that generated detectable resurgent currents generally generated larger peak transient currents (Supplemental Table 4). Although this difference was only significant for 3 of the 7 channel constructs that generated resurgent currents, it might suggest that cellular factors underlying resurgent current generation could also affect peak current expression. However, more than 40 cells with peak transient current amplitudes greater than 20 nA were recorded that lacked detectable resurgent currents. Furthermore, the relative amplitude of resurgent currents is poorly correlated with peak transient current amplitude (Supplemental Figure 3). At this time, we do not know how DRG neurons that do and do not produce resurgent currents differ in terms of β subunit expression and/or kinase activity. Our optimized DRG expression system could be useful in identifying cellular factors that modulate resurgent currents.

Our data clearly show that resurgent sodium currents are likely to play a role in the functional consequences of inherited neuronal and muscle channelopathies. In addition, our data — in conjunction with the previous study indicating that β-pompilidotoxin can artificially induce resurgent currents in cerebellar Purkinje neurons (9) — indicate that any manipulation that slows or destabilizes inactivation has the potential to induce resurgent currents. Many posttranslational modifications have been reported to slow the rate of inactivation or increase the amplitude of persistent sodium currents, including hypoxia (33), phosphorylation (34), altered calcium signaling (35), G protein activation (36), and oxidation (37). We propose that these alterations could also result in abnormal resurgent current generation. The induction of resurgent sodium currents likely contributes to the more extreme electrophysiological changes and disease sequelae that can be associated with both inherited and acquired disorders of neuronal and muscle excitability.

View Supplemental data

We thank E. Thompson and M. Vasko for assistance with the Nav1.8 shRNA. B.W. Jarecki was supported by an Indiana Clinical and Translational Sciences Institute Career Development Award (PHS grant 5TL1RR025759), and A.D. Piekarz was partially supported by NIH training fellowship NS066663. This work was supported by NIH research grant NS053422 (to T.R. Cummins).

Address correspondence to: Theodore R. Cummins, Indiana University School of Medicine, 950 W. Walnut St., Indianapolis, Indiana 46202, USA. Phone: (317) 278-9342; Fax: (317) 278-5849; E-mail: trcummin@iupui.edu.

Conflict of interest: The authors have declared that no conflict of interest exists.

Reference information: J. Clin. Invest.120:369–378 (2010). doi:10.1172/JCI40801.

See the related article at Sodium channels gone wild: resurgent current from neuronal and muscle channelopathies.

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