Domain IV voltage-sensor movement is both sufficient and rate limiting for fast inactivation in sodium channels

Abstract

Voltage-gated sodium channels are critical for the generation and propagation of electrical signals in most excitable cells. Activation of Na+ channels initiates an action potential, and fast inactivation facilitates repolarization of the membrane by the outward K+ current. Fast inactivation is also the main determinant of the refractory period between successive electrical impulses. Although the voltage sensor of domain IV (DIV) has been implicated in fast inactivation, it remains unclear whether the activation of DIV alone is sufficient for fast inactivation to occur. Here, we functionally neutralize each specific voltage sensor by mutating several critical arginines in the S4 segment to glutamines. We assess the individual role of each voltage-sensing domain in the voltage dependence and kinetics of fast inactivation upon its specific inhibition. We show that movement of the DIV voltage sensor is the rate-limiting step for both development and recovery from fast inactivation. Our data suggest that activation of the DIV voltage sensor alone is sufficient for fast inactivation to occur, and that activation of DIV before channel opening is the molecular mechanism for closed-state inactivation. We propose a kinetic model of sodium channel gating that can account for our major findings over a wide voltage range by postulating that DIV movement is both necessary and sufficient for fast inactivation.

Pharmacology of the Na v 1.1 domain IV voltage sensor reveals coupling between inactivation gating processes

“…Notably, the concept that slow inactivation is tied to a VSDIV transition subsequent to activation supports previous models proposing that Na v channel voltage sensor immobilization and slow inactivation are coupled (1,(15)(16)(17)(18)36). Taken together, our results provide support for a VSDIV-centric model of Na v 1.1 inactivation in which both fast and slow inactivation processes are coupled to sequential movements of this voltage sensor (Fig.…”

“…6A). The first step initiates fast inactivation and corresponds to the transition of VSDIV from the resting state to the activated state, a step that accounts for the majority of gating charge movement (15,(32)(33)(34)(35). The second, more weakly voltage-dependent, transition moves the activated VSDIV to an immobilized state that is coupled to selectivity filter collapse and slow inactivation, as has been proposed for other voltage-gated channels (14,17,36).…”

“…Discussion Na v channel inactivation is a complex process consisting of fast and slow components that may involve a contribution from one or more voltage sensor domains (1,13,15,16,19). Understanding these processes and elucidating strategies for their pharmacologic modulation may aid drug discovery efforts aimed at treating Na v 1.1-associated disorders, such as epilepsy, migraine, and mechanical pain (2, 3, 5-8, 10, 11).…”

“…Mammalian Na v channels are large proteins composed of four homologous domains that form a pseudo-fourfold symmetric channel comprising a central pore surrounded by four voltage sensors, one from each domain (1). Accumulating evidence suggests that activation gate opening is associated with movement of voltage sensors in domains I-III (VSDI-III), whereas fast inactivation is initiated by subsequent movement of the voltage sensor in domain IV (VSDIV) (1,15,16); however, the role of VSDIV in slow inactivation and the possibility of functional coupling between fast and slow inactivation remain matters of debate (17)(18)(19).…”

Mapping of voltage sensor positions in resting and inactivated mammalian sodium channels by LRET

“…Site-directed fluorimetry shows that the VSDs in DI, DII, and DIII of the rat skeletal muscle voltage-gated sodium channel (Nav1.4) are activated by depolarization faster than in DIV (2). From this observation, it has been hypothesized that DI-III VSDs control the pore opening of the mammalian Nav, whereas the DIV VSD governs its fast inactivation (2)(3)(4)(5).…”