Multiple Signaling Pathways Converge on KLHL3 and WNKs

   The With No Lysine [K] (WNK) family of enzymes are characterized by a unique placement of the catalytic lysine, a positioning which allows chloride sensing in cells and may allow phosphorylation of otherwise inaccessible substrates [1-3]. WNKs are mutated in familial hypertensive disorders, such as Pseudohypoaldosteronism type II (PHA II), with intronic deletions of WNK1 and point mutations of WNK4 leading to increased WNK protein levels and hence signaling by these enzymes [4]. Other PHA II mutations occur in KLHL3 and Cullin-3, an adaptor and ubiquitin-ligase respectively, and these mutations diminish their interaction with WNKs and thereby lead to WNK accumulation [5-9]. Defining the signals and regulatory inputs controlling WNK activity, stability and degradation is essential for our understanding of blood pressure control and the mechanisms that go awry in hypertension.

   In attempts to identify modifications of KLHL3 that could impact its interaction with WNKs, two independent studies (Shibata et al, 2014 and Yoshizaki et al, 2015) performed mass spectrometry on over-expressed KLHL3 and identified multiple phosphorylation sites: T10, T295, and S433 were identified in both studies, while phosphorylation at T264, S275 and S376 were identified in one of the studies [10, 11]. The S433 residue lies within one of the Kelch domains of KLHL3, which mediates interaction with WNKs, and this site is mutated in PHA II patients [5, 6]. Prediction software suggested that the S433 residue lies within motifs that could be phosphorylated by PKC, PKA and Akt, and this was shown in vitro and in cell culture under different stimuli [10, 11]. Furthermore, Angiotensin II infusion of mice resulted in increased S433 phosphorylation, leading to increased WNK4 levels in the kidney [10]. These phosphorylation events at S433 of KLHL3 lead to diminished interaction between WNKs and KLHL3, thereby decreasing WNK degradation and turnover.

   The convergence of multiple, diverse signaling pathways on KLHL3 phosphorylation provides further evidence to the significance of maintaining proper WNK levels and signaling. These data also point to the need for further studies to elucidate signaling pathways regulating WNK activity and turnover, and also raise questions regarding the potential cross-talk between WNKs and other pathways in cells.

References:

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