allowing it to function despite the absence of angiotensin II (Fig. 3, right panel). Angiotensin II stimulates the secretion of aldosterone, which induces SGK1. SGK1 influences both the WNK-SPAK-NCC and SGK1-ENaC signaling cascades. There are multiple SGK1 phosphorylation sites in the C-terminal region of WNK4 located within or close to the calmodulin-binding domain. SGK1-mediated phosphorylation of these sites is thought to disrupt the effect of the C-terminal inhibitory domain and concomitantly increase WNK4 kinase activity. The alteration of SGK1 phosphorylation by the R1185C mutation is another indication that the mutation disrupts the C-terminal inhibitory mechanism in WNK4 (Fig. 3, right panel). Dysregulation of WNK4 abundance Besides WNK1 and WNK4, mutations in two other genes, CUL3 (encoding Cullin 3) and KLHL3 (encoding Kelch Like Family Member 3) have been found in patients with PHAII. These two proteins are part of the ubiquitin E3 ligase complex involved in the ubiquitin-mediated degradation of WNK1 and WNK4. The PHAII-causing mutations in KLHL3 and cullin 3 prevent the interactions of these proteins with each other and with WNK1/4. The mutations in these proteins impair the degradation of WNK1/4. This in turn increases the protein abundance of WNK1/4 and concomitantly enhances the total kinase activity. The increased WNK4 kinase activity leads to the hyperactivation of NCC through WNK4-SPAK and/or the OSR1-NCC cascades, ultimately resulting in the retention of sodium and potassium by the kidney. Elevated WNK4 activity The primary effect of the elevated WNK4 kinase activity is the increase of NCC-mediated sodium reabsorption in the distal convoluted tubule of the kidney. The increase in sodium reabsorption in this segment of the nephron reduces the sodium load in the collecting duct, where sodium reabsorption by the ENaC provides the driving force for potassium secretion through ROMK (Fig. 4). The sodium reabsorption by hyperactive NCC overrides the loss of reabsorption by ENaC, and the net effect is moderate sodium retention. Over time, this potentially contributes to the elevated blood pressure observed in PHAII patients. The reduction of potassium secretion by ROMK contributes to the development of hyperkalemia. The direct effects of the elevated WNK4 activity on other channels and transporters, such as ENaC, ROMK, and Ca2+-activated maxi K+ channels, may also contribute to the pathogenesis of PHAII; however, the primary features of PHAII could be explained by the gain-of-function of NCC. References External links HumanProteinAtlas/WNK4 GeneCard/WNK4 UniProt/WNK4 OMIM/WNK4 EC 2.7.11