WNK1 -related Familial Hyperkalemic Hypertension results from an increased expression of L-WNK1 specifically in the distal nephron

Abstract

Large deletions in the first intron of the With No lysine (K) 1 (WNK1) gene are responsible for Familial Hyperkalemic Hypertension (FHHt), a rare form of human hypertension associated with hyperkalemia and hyperchloremic metabolic acidosis. We generated a mouse model of WNK1-associated FHHt to explore the consequences of this intronic deletion. WNK1 +/FHHt mice display all clinical and biological signs of FHHt. This phenotype results from increased expression of long WNK1 (L-WNK1), the ubiquitous kinase isoform of WNK1, in the distal convoluted tubule, which in turn, stimulates the activity of the Na-Cl cotransporter. We also show that the activity of the epithelial sodium channel is not altered in FHHt mice, suggesting that other mechanisms are responsible for the hyperkalemia and acidosis in this model. Finally, we observe a decreased expression of the renal outer medullary potassium channel in the late distal convoluted tubule of WNK1 +/FHHt mice, which could contribute to the hyperkalemia. In summary, our study provides insights into the in vivo mechanisms underlying the pathogenesis of WNK1-mediated FHHt and further corroborates the importance of WNK1 in ion homeostasis and blood pressure. The human mutations identified at the WNK1 locus do not modify the coding sequence but are large deletions in the 60-kb-long first intron, which result in an overexpression of WNK1 in the leukocytes of patients (3). The WNK1 gene generates two isoforms through alternative promoters. The long isoform, long WNK1 (L-WNK1), is expressed ubiquitously, whereas the shorter isoform, kidney-specific WNK1 (KS-WNK1), which lacks a functional kinase domain, is expressed specifically in the kidney (5). In the kidney, L-WNK1 is expressed at a low level in all nephron segments, whereas KS-WNK1 is expressed only in the distal nephron (6). We previously generated a transgenic mouse model that exhibited an ectopic expression of KS-WNK1 and an increased expression of L-WNK1 in the distal nephron on deletion of the first intron (7). This model, however, did not allow the study of the functional consequences of the deletion of WNK1 first intron, because a reporter gene was inserted under the control of each WNK1 promoter within the transgene.Several in vitro experiments suggest that an increase in L-WNK1 expression in the distal nephron could trigger the development of FHHt. The kinase can, indeed, stimulate the activity of the Na + -Cl − cotransporter (NCC), which has been established as an essential component of the FHHt phenotype, through its interaction with either WNK4 and/or Ste20-related proline-alanine rich kinase (SPAK) (review in ref. 4). WNK4 inhibits NCC, and L-WNK1 relieves the cotransporter from this inhibition. L-WNK1 phosphorylates and thus, activates SPAK, which in turn, stimulates NCC membrane expression by phosphorylation. However, the characterization of L-WNK1 function in the distal nephron has been hampered by the absence of a valid mouse model, because L-WNK1 inactivation results in embryonic death caused by cardiovasc...

Hyperkalemic hypertension–associated cullin 3 promotes WNK signaling by degrading KLHL3

“…Although many mechanistic details about the disease remain unclear, WNK1, WNK4, KLHL3, and CUL3 all appeared to affect the abundance and activity of NCC and to modulate the response to dietary salt and potassium challenge. As noted above, most data suggest that mutations in either WNK1 or WNK4 increase the abundance and activity of NCC by activating the intermediary kinase SPAK (4,22,23). In contrast, KLHL3 is an adaptor protein that binds to WNK kinases and, in association with CUL3, facilitates their ubiquitylation and degradation (5)(6)(7)(8).…”

“…These kinases, in turn, phosphorylate and activate NCC directly, causing both hyperkalemia and hypertension. The described mutations in WNK1 are intronic (and hence noncoding) mutations; these increase expression of the kinase-active form of WNK1 along the distal convoluted tubule (DCT), a nephron segment where its expression levels are typically low (4), thereby activating SPAK and NCC (4). WNK4 mutations are missense mutations in 1 of 2 coiledcoiled motifs.…”

Alternatively spliced proline-rich cassettes link WNK1 to aldosterone action

“…With regards to NCC regulation, L-WNK1 is a stimulator (28,56), while KS-WNK1 (35). Although NEDD4-2 and NCC formed complexes that could be disrupted by SGK1, the exact nature of the interaction remains undefined, as NCC contains no canonical PY motif.…”

Distal Convoluted Tubule

“…Instead, these mutations increase total WNK1 mRNA and protein expression (41). The increased protein expression overrides ubiquitination and degradation by the KLHL3/CUL3 complex, and the net effect of the mutation is to increase total WNK1 kinase activity (55). Thus, in FHHt caused by mutations of WNK1, increased WNK1 protein expression should suppress the inhibitory effect of WNK4 on NCC traffic while stimulating SPAK and OSR1, causing NCC to be overactive ( Figure 6C).…”