Brakes and gas-regulation of ENaC by sodium

The epithelial Na⁺ Channel (ENaC) is rate limiting to Na⁺ entry across many epithelia. Its activity modifies transepithelial Na⁺ transport and by extension salt and water absorption. It has long been known that the channel's conductance increases, albeit non-linearly, as the Na⁺ concentration increases.¹ However, this channel is unique in that it is also inhibited by elevating the concentration of the transported ion leading to inhibition of permeability.² This effect occurs at 2 different time domains stemming from at least 2 distinct mechanisms. The initial acute time course occurs by interaction of the sodium ion with the channel or channel associated proteins and occurs within seconds.³ The prolonged time course spans minutes to days. This has been recognized early on and termed feedback inhibition.⁴

Feedback inhibition is ubiquitously observed for ENaC in many native and heterologous systems and has been reported to involve the ubiquitin ligase protein Nedd4-2⁵, and protein kinase C.⁶ The physiological significance of feedback inhibition has been demonstrated by Palmer and colleagues⁷; however, an exact link to the channel has remained missing.

In the 8(5) issue of Channels Patel et al.⁸ examined the mechanism of feedback inhibition of ENaC by [Na⁺]_i . They subdivide feedback inhibition into one with a 1–2 hour time course and one with an overnight (>8 h) time course. In the early phase, inhibition was not accompanied by detectable effects on subunit trafficking or plasma membrane density but was dependent on the presence of intact channel subunit C-termini. By utilizing a truncated C-terminus β subunit they demonstrated that the sensitivity to increased [Na⁺]_i was rightward shifted or reduced. This demonstrates for the first time that feedback inhibition may affect individual channel activity or open probability (P_o), but interestingly, in a manner that depends on the presence of intact intracellular C-termini. The broader implications are that Po cannot be simply assigned to a single amino acid but is rather the collective activity of numerous extra, intra and transmembrane domains. The second implication is that prolonged increases of [Na⁺]_i may involve downstream modification or signaling with the intracellular C-termini.

The second phase of feedback inhibition was observed at periods >8 h. This longer phase was dependent on internalization of γENaC possibly accompanied by reduced cleavage of this subunit indicating reduced channel activity by reducing membrane protein density and by reducing subunit cleavage- a process that leads to channel activation. Their results indicate the importance of the C-termini in this phase and present a continuum of events whereby early feedback inhibition likely occurs by inactivating membrane resident channels, while prolonged inactivation occurs by reduced endogenous and presumably intracellular cleavage of subunits accompanied by enhanced internalization from the plasma membrane.

The physiological significance of these differences could reside in the recovery from an intracellular Na⁺ load in the kidneys. These data would predict that rapid recovery from inhibition after a 1–2 h increase of tubular [Na⁺] is allowed with ease possibly due to the reversibility of inhibition of channel P_o; a process that requires that no changes of membrane protein density have occurred. Conversely, recovery from inhibition after a prolonged increase of tubular [Na⁺] is not allowed to progress without synthesis and/or trafficking of new channels owing to internalization and reduction of channel density. This would in turn allow the body and kidneys time to eliminate and recover from the excess Na⁺ load before restoring ENaC homeostasis and collecting duct Na⁺ reabsorption.