Modulation of Nav1.7 and Nav1.8 Peripheral Nerve Sodium Channels by Protein Kinase A and Protein Kinase C

Abstract

Voltage-gated Na⁺channels (VGSC) are transmembrane proteins that are essential for the initiation and propagation of action potentials in neuronal excitability. Because neurons express a mixture of Na⁺channel isoforms and protein kinase C (PKC) isozymes, the nature of which channel is being regulated by which PKC isozyme is not known. We showed that DRG VGSC Na_v1.7 (TTX-sensitive) and Na_v1.8 (TTX-resistant), expressed in Xenopus oocytes were differentially regulated by protein kinase A (PKA) and PKC isozymes using the two-electrode voltage-clamp method. PKA activation resulted in a dose-dependent potentiation of Na_v1.8 currents and an attenuation of Na_v1.7 currents. PKA-induced increases (Na_v1.8) and decreases (Na_v1.7) in peak currents were not associated with shifts in voltage-dependent activation or inactivation. The PKA-mediated increase in Na_v1.8 current amplitude was prevented by chloroquine, suggesting that cell trafficking may contribute to the changes in Na_v1.8 current amplitudes. A dose-dependent decrease in Na_v1.7 and Na_v1.8 currents was observed with the PKC activators phorbol 12-myristate, 13-acetate (PMA) and phorbol 12,13-dibutyrate. PMA induced shifts in the steady-state activation of Na_v1.7 and Na_v1.8 channels by 6.5 and 14 mV, respectively, in the depolarizing direction. The role of individual PKC isozymes in the regulation of Na_v1.7 and Na_v1.8 was determined using PKC-isozyme-specific peptide activators and inhibitors. The decrease in the Na_v1.8 peak current induced by PMA was prevented by a specific ϵPKC isozyme peptide antagonist, whereas the PMA effect on Na_v1.7 was prevented by ϵPKC and βIIPKC peptide inhibitors. The data showed that Na_v1.7 and Na_v1.8 were differentially modulated by PKA and PKC. This is the first report demonstrating a functional role for ϵPKC and βIIPKC in the regulation of Na_v1.7 and Na_v1.8 Na⁺channels. Identification of the particular PKC isozymes(s) that mediate the regulation of Na⁺channels is essential for understanding the molecular mechanism involved in neuronal ion channel regulation in normal and pathological conditions.