Dietary electrolyte-driven responses in the renal WNK kinase pathway in vivo.

WNK1 and WNK4 are unusual serine/threonine kinases with atypical positioning of the catalytic active-site lysine (WNK: With-No-K[lysine]). Mutations in these WNK kinase genes can cause familial hyperkalemic hypertension (FHHt), an autosomal dominant, hypertensive, hyperkalemic disorder, implicating this novel WNK pathway in normal regulation of BP and electrolyte balance. Full-length (WNK1-L) and short (WNK1-S) kinase-deficient WNK1 isoforms previously have been identified. Importantly, WNK1-S is overwhelmingly predominant in kidney. Recent Xenopus oocyte studies implicate WNK4 in inhibition of both thiazide-sensitive co-transporter-mediated Na+ reabsorption and K+ secretion via renal outer medullary K+ channel and now suggest that WNK4 is inhibited by WNK1-L, itself inhibited by WNK1-S. This study examined WNK pathway gene expression in mouse kidney and its regulation in vivo. Expression of WNK1-S and WNK4 is strongest in distal tubule, dropping sharply in collecting duct and with WNK4 also expressed in thick ascending limb and the macula densa. These nephron segments that express WNK1-S and WNK4 mRNA have major influence on long-term NaCl reabsorption, BP, K+, and acid-base balance, processes that all are disrupted in FHHt. In vivo, this novel WNK pathway responds with significant upregulation of WNK1-S and WNK4 with high K+ intake and reduction in WNK1-S on chronic lowering of K+ or Na+ intake. A two-compartment distal nephron model explains these in vivo findings and the pathophysiology of FHHt well, with WNK and classic aldosterone pathways responding to drivers from K+ balance, extracellular volume, and aldosterone and cross-talk through distal Na+ delivery regulating electrolyte balance and BP.

WNK1, a gene within a novel blood pressure control pathway, tissue-specifically generates radically different isoforms with and without a kinase domain.

WNK1 is a member of a novel serine/threonine kinase family, With-No-K, (lysine). Intronic deletions in the encoding gene cause Gordon syndrome, an autosomal dominant, hypertensive, hyperkalemic disorder particularly responsive to thiazide diuretics, a first-line treatment in essential hypertension. To elucidate the novel WNK1 BP control pathway active in distal nephron, WNK1 expression in mouse was studied. It was found that WNK1 is highly expressed in testis > heart, lung, kidney, placenta > skeletal muscle, brain, and widely at low levels. Several WNK1 transcript classes are demonstrated, showing tissue-, developmental-, and nephron-segment-specific expression. Importantly, in kidney, the most prominent transcripts are smaller than elsewhere, having the first four exons replaced by an alternative 5'-exon, deleting the kinase domain, and showing strong distal nephron expression, whereas larger transcripts show low-level widespread distribution. Alternative splicing of exons 11 and 12 is prominent-for example, transcripts containing exon 11 are abundant in neural tissues, testis, and secondary renal transcripts but are predominantly absent in placenta. The transcriptional diversity generated by these events would produce proteins greatly differing in both structure and function. These findings help further define and clarify the role of WNK1 and the thiazide-responsive pathway relevant to essential hypertension in which it participates.

Sgk1 gene expression in kidney and its regulation by aldosterone: spatio-temporal heterogeneity and quantitative analysis.

The serine-threonine kinase sgk1 was recently identified as a gene rapidly induced by mineralocorticoids, resulting in increased sodium transport in vitro. To carefully localize and quantify the renal sgk1 expression response to aldosterone, in situ hybridization was performed on kidneys of mice having aldosterone excess over a range of doses and durations. In control and adrenalectomized animals, the glomeruli and inner medullary collecting ducts were the major sites of sgk1 expression, which was maintained independent of aldosterone. Sgk1 upregulation induced by aldosterone excess exhibited spatio-temporal heterogeneity. Both acute (3-h) and chronic (6-d) aldosterone excess stimulated sgk1 expression in the distal nephron, i.e., from the distal convoluted tubules through to the outer medullary collecting ducts. Treatments for 6 d with low sodium diet (0.03% [I]) and aldosterone infusions (50 microg/kg per d [II], 150 microg/kg per d [III], and 750 microg/kg per d [IV]) generated elevation of circulating aldosterone. Across these treatments (I through IV), the circulating level correlated with the progressive induction of sgk1 expression, with highly stimulated tubules first appearing in cortex (I) and continuing downward (II) until there was a strong stimulation throughout outer medulla (III and IV). Interestingly, chronic but not acute aldosterone excess caused a slight increase of sgk1 expression in glomerulus (30 to 50%; P < 0.01) and a dramatic downregulation in the initial portion of inner medulla, which could result from diminished interstitial osmolarity. Relative quantification (versus control) of sgk1 upregulation in individual tubules revealed: (1) a 1.8-fold increase of sgk1 mRNA at 3 h (150 microg/kg injection) and (2) a dose-dependence of chronic upregulation reaching a ceiling of eightfold elevation.