WNK1 et WNK4, nouveaux acteurs de l’homéostasie hydrosodée WNK1 and WNK4, new players in salt and water homeostasis

Résumé

L’étude d’une forme rare d’hypertension artérielle de transmission mendélienne, l’hypertension hyperkaliémique familiale (HHF), a récemment permis d’identifier des mutations dans les gènes WNK1 et WNK4, qui codent pour des protéines appartenant à une nouvelle famille de sérine-thréonine kinases (with no lysine [K] kinase). Plusieurs éléments du tableau clinique de l’HHF, caractérisé par une hyperkaliémie, une hyperchlorémie et une grande sensibilité aux diurétiques thiazidiques, sont en faveur d’une anomalie du transport ionique dans le tubule distal rénal. En accord avec cette hypothèse, WNK1 et WNK4 sont fortement exprimés dans cette partie du néphron. On les retrouve également dans de nombreux épithéliums impliqués dans le transport du chlore, tels que celui du côlon. In vitro, WNK4 règle à la fois le transport de Na⁺, K⁺ et Cl^-, et pourrait donc constituer une voie de régulation importante des transports ioniques rénal et extra-rénal.

Summary

Arterial hypertension is a complex trait influenced by a variety of environmental and genetic factors. Several approaches can be used to identify its susceptibility genes : one is to study rare monogenic forms of hypertension, like familial hyperkalemic hypertension (FHH). Also known as pseudohypoaldosteronism type 2 or Gordon syndrome, FHH is characterized by hypertension, hyperkalemia despite normal renal glomerular filtration rate, abnormalities which are particularly sensitive to thiazide diuretics. Mild hyperchloremia, metabolic acidosis, and suppressed plasma renin activity are associated findings. Despite its phenotypic and genetic heterogeneity, mutations in two related genes, WNK1 and WNK4, were recently identified. These genes belong to a newly identified family of serine-threonine (with no lysine [K]) kinases. Both are highly expressed in the kidney and in a variety of epithelia involved in chloride transport. It has thus been postulated that these two kinases could be implicated in a new pathway of ionic transport regulation. Several studies have very recently confirmed this hypothesis in vitro, in Xenopus oocytes or kidney cell lines. They have shown that, in the renal distal tubule, WNK4 inhibits sodium reabsorption and potassium secretion, via inhibition of NCC (thiazide-sensitive Na⁺-Cl^- cotransporter) and K⁺ channel ROMK activity, respectively. Interestingly, FHH mutations have opposite effects : while they lead to loss of NCC inhibition, they increase ROMK inhibition. Moreover, they also increase paracellular permeability to chloride of MDCK cells. WNK4 also inhibits apical and basal chloride transporters present in extra-renal epithelia, such as CFEX and Na⁺-K⁺-2 Cl^-, respectively. It is also interesting to note that the WNK4-mediated negative regulation of NCC activity is in turn inhibited by WNK1. By its role on several transporters, WNK4 appears as a putative key regulator of ionic transport and blood pressure.