The Domain II S4-S5 Linker in Nav1.9: A Missense Mutation Enhances Activation, Impairs Fast Inactivation, and Produces Human Painful Neuropathy
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Painful small fiber neuropathy is a challenging medical condition with no effective treatment. Non-genetic causes can be identified in one half of the subjects. Gain-of-function variants of sodium channels Nav1.7 and Nav1.8 have recently been associated with painful small fiber neuropathy. More recently, mutations of sodium channel Nav1.9 have been linked to human pain disorders, with two gain-of-function mutations found in patients with painful small fiber neuropathy. Here we report a novel Nav1.9 mutation, a glycine 699 substitution by arginine (G699R) in the domain II S4-S5 linker, identified in a patient with painful small fiber neuropathy. In this study, we assayed the mutant channels by voltage-clamp in superior cervical ganglion neurons, which do not produce endogenous Nav1.8 or Nav1.9 currents, and provide a novel platform where Nav1.9 is expressed at relatively high levels. Voltage-clamp analysis showed that the mutation hyperpolarizes (−10.1 mV) channel activation, depolarizes (+6.3 mV) steady-state fast inactivation, slows deactivation, and enhances ramp responses compared with wild-type Nav1.9 channels. Current-clamp analysis showed that the G699R mutant channels render dorsal root ganglion neurons hyperexcitable, via depolarized resting membrane potential, reduced current threshold and increased evoked firing. These observations show that the domain II S4-S5 linker plays an important role in the gating of Nav1.9 and demonstrates that a mutation in this linker is linked to a common pain disorder.
KeywordsSodium channel Nav1.9 DRG Pain
We thank Lawrence J. Macala, Fadia Dib-Hajj, and Palak Shah for technical assistance. We thank Dr. Mark Estacion and Dr. Jianying Huang for valuable comments. This work was supported in part by Grants from the Rehabilitation Research Service and Medical Research Service, Department of Veterans Affairs (SGW and SDH), and Grant #602273 from the European Union Seventh Framework Programme FP7/2007-2013. The Center for Neuroscience & Regeneration Research is a Collaboration of the Paralyzed Veterans of America with Yale University.
Conflict of interest
The authors report no conflicts of interest.
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