The role of the ENaC-regulatory complex in aldosterone-mediated sodium transport

Abstract

The mineralocorticoid aldosterone is indispensable for the control of blood pressure and fluid volume in mammals. It acts in large part to increase the abundance and activity of the epithelial Na(+) channel (ENaC), which mediates apical Na(+) entry in the distal parts of the kidney tubules. Aldosterone acts through the mineralocorticoid receptor to alter the transcription of specific genes, including SGK1 and GILZ1. Recent evidence suggests that these key aldosterone-regulated factors function within a unique multi-protein ENaC-regulatory-complex that governs the net cell surface expression and activity of the channel. Another aldosterone-induced protein, CNK3 (connector enhancer of kinase suppressor of Ras 3), also stimulates ENaC and has all of the features of a scaffolding protein. With these observations in mind, we discuss the possibility that CNK3 coordinates the dynamic assembly of the ENaC-regulatory-complex, and promotes context-appropriate aldosterone signal transduction in the regulation of epithelial Na(+) transport.

Figure 1. Annotated mouse SGK1 (A), GILZ1 (B) and CNK3 (C) protein schematics showing key motifs and interaction domains

A. SGK1 is an important aldosterone-regulated protein kinase that stimulates renal ENaC function. The ER-associated degradation signal (or ‘degron’) lies within the N-terminus of SGK1, upstream of its kinase domain. The Ser residue within the hydrophobic motif that is critical for mTORC2-dependent kinase activation is depicted. SGK1 also possesses a standard Class I PDZ domain-interaction motif at its C-terminus. B. GILZ1 is a small aldosterone-induced chaperone that plays a central role in protein trafficking and signaling. The TSC-22 signature box defines region of homology with the TGFβ-stimulated clone-22 protein and other family members. The leucine zipper (LZ) motif mediates GILZ dimerization. Also shown is the sequence of the Class I PDZ domain-interaction motif at the C-terminus. C. CNK3 is a recently identified MR-target shown to be critical for ENaC activity. As with other members of the CNK family, CNK3 contains a sterile-α-motif (SAM), a conserved-region-in-CNK (CRIC) domain, a classic PDZ domain, and a downstream region commonly referred to as the domain-of-unknown-function or DUF.

Figure 2. Schematic depiction of hypothetical mechanism for ERC regulation of ENaC

In the absence of aldosterone, ENaC activity is limited by Raf-1 and Nedd4-2, the two major negative regulatory components governing ENaC, within the ERC. In the absence of GILZ1 and SGK1, this regulatory complex is thus inhibitory. Aldosterone-induced SGK1 and GILZ1 act together to stimulate ENaC by interfering with the inhibitory components of this complex. Aldosterone simultaneously also markedly upregulates the expression of CNK3, a PDZ domain-containing scaffold protein that assists in the assembly and functioning of an ENaC-stimulatory multi-protein complex, that includes GILZ1 and SGK1. The net result is increased cell surface expression and activity of ENaC. Red and green arrows represent Na⁺ movement through the channel, while black arrows represent aldosterone action. Dashed red arrow shows ENaC in the “inhibited” state (red ‘X’, top panel), while solid green arrow in the bottom panel shows the activated or “disinhibited” state of ENaC.