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1 Neurology, Yale Univ Sch Med, New Haven, Connecticut, United States; Center for Neuroscience and Regeneration Research, Yale Univ Sch Med, West Haven, Connecticut, United States; Rehabilitation Research Center, Veterans Administration Connecticut Healthcare System, West Haven, Connecticut, United States
* To whom correspondence should be addressed. E-mail: sulayman.dib-hajj{at}yale.edu.
Sodium channel Nav1.8 produces a slowly-inactivating, tetrodotoxin-resistant current which is characterized by recovery from inactivation with fast and slow components, and contributes a substantial fraction of the current underlying the depolarizing phase of the action potential of dorsal root ganglion (DRG) neurons. Nav1.8 C-terminus carries a conserved calmodulin-binding isoleucine-glutamine (IQ) motif. We show here that calmodulin co-immunoprecipitates with endogenous Nav1.8 channels from native DRG, suggesting that the two proteins can interact in vivo. Treatment of native DRG neurons with a calmodulin binding peptide (CBP) reduced the current density of Nav1.8 by ~65%, without changing voltage-dependence of activation or steady-state inactivation. To investigate the functional role of CaM binding to the IQ motif in the Nav1.8 C-terminus, the IQ dipeptide was substituted by DE; we show that this impairs the binding of CaM to the IQ motif. Mutant Nav1.8IQ/DE channels produce currents with ~50% amplitude, but with unchanged voltage-dependence of activation and inactivation when expressed in DRG neurons from Nav1.8-null mice. We also show that blocking the interaction of CaM and Nav1.8 using CBP or the IQ/DE substitution causes a build-up of inactivated channels, and in the case of the IQ/DE mutation, stimulation even at a low frequency of 0.1 Hz significantly enhances the frequency-dependent inhibition of the Nav1.8 current. This study presents, for the first time, evidence that calmodulin associates with a sodium channel, Nav1.8, in native neurons, and demonstrates a regulation of Nav1.8 currents which can significantly affect electrogenesis of DRG neurons in which Nav1.8 is normally expressed.
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