Electrophysiological properties of mutant Nav1.7 sodium channels in a painful inherited neuropathy

Abstract

Although the physiological basis of erythermalgia, an autosomal dominant painful neuropathy characterized by redness of the skin and intermittent burning sensation of extremities, is not known, two mutations of Na(v)1.7, a sodium channel that produces a tetrodotoxin-sensitive, fast-inactivating current that is preferentially expressed in dorsal root ganglia (DRG) and sympathetic ganglia neurons, have recently been identified in patients with primary erythermalgia. Na(v)1.7 is preferentially expressed in small-diameter DRG neurons, most of which are nociceptors, and is characterized by slow recovery from inactivation and by slow closed-state inactivation that results in relatively large responses to small, subthreshold depolarizations. Here we show that these mutations in Na(v)1.7 produce a hyperpolarizing shift in activation and slow deactivation. We also show that these mutations cause an increase in amplitude of the current produced by Na(v)1.7 in response to slow, small depolarizations. These observations provide the first demonstration of altered sodium channel function associated with an inherited painful neuropathy and suggest that these physiological changes, which confer hyperexcitability on peripheral sensory and sympathetic neurons, contribute to symptom production in hereditary erythermalgia.

Figure 1.

The I848T and L858H mutations of hNa_v1.7 alter activation and deactivation. A, Current traces recorded from representative HEK293 cells expressing either wild-type hNa_v1.7 or mutant channels, I848T or L858H. Cells were held at -100 mV, and currents were elicited with 50 msec test pulses to potentials ranging from -80 to 40 mV. B, Normalized peak current-voltage relationship for wild-type (filled squares; n = 29), I848T (open circles; n = 27), and L858H (open triangles; n = 27) channels. C, Representative tail currents of WT, I848T, and L858H channels. Cells were held at -100 mV and depolarized to -20 mV for 0.5 msec, followed by a repolarization to -50 mV to elicit tail currents. D, Time constants for tail current deactivation at repolarization potentials ranging from -40 to -100 mV for wild-type (filled squares; n = 7), I848T (open circles; n = 7), and L858H (open triangles; n = 7) hNa_v1.7 channels. Time constants were obtained with single exponential fits to the deactivation phase of the currents. Error bars represent SE.

Figure 2.

The I848T and L858H mutations differentially alter inactivation of hNa_v1.7. A, Fast inactivation kinetics as a function of voltage for wild-type (filled squares; n = 8), I848T (open circles; n = 8), and L858H (open triangles; n = 8) hNa_v1.7 channels. Currents elicited as described in Figure 1 A were fit with Hodgkin-Huxley type m ³h model to estimate the inactivation time constants. B, Comparison of steady-state fast inactivation for wild-type (filled squares; n = 20), I848T (open circles; n = 19), and L858H (open triangles; n = 17) hNa_v1.7 channels. Currents were elicited with test pulses to 0 mV after 500 msec inactivating prepulses. C, Comparison of steady-state slow inactivation for wild-type hNa_v1.7 (filled squares; n = 9), I848T (open circles; n = 8), and L858H (open triangles; n = 9) hNa_v1.7 channels. Slow inactivation was induced with 30 sec prepulses, followed by 100 msec pulses to -120 mV to allow recovery from fast inactivation. A test pulse to 0 mV for 20 msec was used to determine the fraction of current available. Error bars represent SE.

Figure 3.

The I848T and L858H mutations enhance ramp currents of hNa_v1.7. Representative ramp currents elicited with 500 msec ramp depolarizations from -100 to 0 mV from HEK293 cells expressing wild-type, I848T, and L858H channels.