Nedd4 mediates control of an epithelial Na ⁺ channel in salivary duct cells by cytosolic Na ⁺

Abstract

Epithelial Na ؉ channels are expressed widely in absorptive epithelia such as the renal collecting duct and the colon and play a critical role in f luid and electrolyte homeostasis. Recent studies have shown that these channels interact via PY motifs in the C terminals of their ␣, ␤, and ␥ subunits with the WW domains of the ubiquitin-protein ligase Nedd4. Mutation or deletion of these PY motifs (as occurs, for example, in the heritable form of hypertension known as Liddle's syndrome) leads to increased Na ؉ channel activity. Thus, binding of Nedd4 by the PY motifs would appear to be part of a physiological control system for down-regulation of Na ؉ channel activity. The nature of this control system is, however, unknown. In the present paper, we show that Nedd4 mediates the ubiquitin-dependent down-regulation of Na ؉ channel activity in response to increased intracellular Na ؉ . We further show that Nedd4 operates downstream of G o in this feedback pathway. We find, however, that Nedd4 is not involved in the feedback control of Na ؉ channels by intracellular anions. Finally, we show that Nedd4 has no inf luence on Na ؉ channel activity when the Na ؉ and anion feedback systems are inactive. We conclude that Nedd4 normally mediates feedback control of epithelial Na ؉ channels by intracellular Na ؉ , and we suggest that the increased Na ؉ channel activity observed in Liddle's syndrome is attributable to the loss of this regulatory feedback system.

Intracellular Sodium Regulates Proteolytic Activation of the Epithelial Sodium Channel

“…Third, mutation of ENaC PY motifs (which bind to Nedd4-2) did not prevent Na ϩ from inhibiting ENaC cleavage. These findings contrast with previous observations in oocytes and salivary duct cells that implicated a role for Nedd4 and ENaC PY motifs in the inhibition of ENaC by intracellular Na ϩ (13,36). Thus, it seems likely that intracellular Na ϩ regulates ENaC activity by more than one mechanism.…”

β-Adrenoceptor-mediated control of apical membrane conductive properties in fetal distal lung epithelia

“…In both experimental systems, the transport of Na ϩ and the transport of Cl Ϫ thus seem to be linked in some way. Interestingly, changes in [Cl Ϫ ] i have recently been shown to influence epithelial Na ϩ channel activity in some systems (8,22). It is therefore possible that the apparent coupling between Na ϩ and Cl Ϫ transport (16,29) may, at least in part, be due to [Cl Ϫ ] i -dependent control of G Na ϩ.…”

“…It is possible, however, that rapid changes in G Na ϩ might occur in intact cells, since permeabilizing the basolateral membrane could allow the loss of a cytoplasmic component essential to the regulation of G Na ϩ. Indeed, recent work has shown that ENaC can be rapidly controlled via changes in internal [Na ϩ ] and [Cl Ϫ ] (8,22), and it is very unlikely that such ionic control of G Na ϩ (8,22) would be detected in studies of basolaterally permeabilized cells. In this experimental situation, transient changes in internal ionic composition may occur as apical conductances are activated, but the relatively large volume of the external solution implies that such changes would be rapidly compensated for by ions entering/leaving the cytoplasm via the permeabilized membrane (21).…”

NEDD4-2 (neural precursor cell expressed, developmentally down-regulated 4-2) negatively regulates TGF-β (transforming growth factor-β) signalling by inducing ubiquitin-mediated degradation of Smad2 and TGF-β type I receptor

“…NEDD4-2 (neural precursor cell expressed, developmentally down-regulated 4-2) and a structurally related protein, NEDD4, have been shown to interact with the epithelial sodium channel, and to down-regulate its activity via the ubiquitin-proteasome pathway [23][24][25][26][27][28][29][30][31]. In the present study, we identified NEDD4-2 as a novel binding partner of Smad7, and examined the function of NEDD4-2 in TGF-β superfamily signalling.…”