No abstract

WNK bodies are large punctate membraneless cytosolic signaling foci that sequester WNK serine–threonine kinases and form in renal distal tubular epithelial cells during shifts in total body potassium balance. The assembly of these structures requires KS-WNK1, a truncated isoform of the WNK1 gene that is exclusively expressed in the distal tubule.

We characterized mouse blood pressure and ion transport in the setting of commonly used rodent diets that drive K+ intake to the extremes of deficiency and excess. Male 129S2/Sv mice were fed either K+-deficient, control, high-K+ basic, or high-KCl diets for 10 days. Mice maintained on a K+-deficient diet exhibited no change in blood pressure, whereas K+-loaded mice developed an ~10-mmHg blood pressure increase. Following challenge with NaCl, K+-deficient mice developed a salt-sensitive 8 mmHg increase in blood pressure, whereas blood pressure was unchanged in mice fed high-K+ diets. Notably, 10 days of K+ depletion induced diabetes insipidus and upregulation of phosphorylated NaCl cotransporter, proximal Na+ transporters, and pendrin, likely contributing to the K+-deficient NaCl sensitivity. While the anionic content with high-K+ diets had distinct effects on transporter expression along the nephron, both K+ basic and KCl diets had a similar increase in blood pressure. The blood pressure elevation on high-K+ diets correlated with increased Na+-K+-2Cl− cotransporter and γ-epithelial Na+ channel expression and increased urinary response to furosemide and amiloride. We conclude that the dietary K+ maneuvers used here did not recapitulate the inverse effects of K+ on blood pressure observed in human epidemiological studies. This may be due to the extreme degree of K+ stress, the low-Na+-to-K+ ratio, the duration of treatment, and the development of other coinciding events, such as diabetes insipidus. These factors must be taken into consideration when studying the physiological effects of dietary K+ loading and depletion.

The multiligand receptors megalin and cubilin and their endocytic adaptor protein Dab2 play essential roles in maintaining the integrity of the apical endocytic pathway of proximal tubule (PT) cells, and have complex and poorly understood roles in the development of chronic kidney disease. Here we used RNA-Seq and CRISPR/Cas9 knockout (KO) technology in a well-differentiated cell culture model to identify PT-specific transcriptional changes that are directly consequent to the loss of megalin (Lrp2), cubilin (Cubn), or Dab2 (Dab2) expression. KO of Lrp2 had the greatest transcriptional effect, and nearly all genes whose expression was affected in Cubn KO and Dab2 KO cells were also changed in the Lrp2 KO cells. Pathway analysis and more granular inspection of the altered gene profiles suggest changes in pathways with immunomodulatory functions that might trigger the pathologic changes observed in KO mice and in Donnai-Barrow patients. Additionally, differences in transcription patterns between Lrp2 and Dab2 KO cells suggest the possibility that altered spatial signaling by aberrantly localized receptors contributes to transcriptional changes upon disruption of PT endocytic function. A reduction in transcripts encoding Sglt2, was confirmed in Lrp2 KO mouse kidney lysates by qPCR analysis. Our results highlight the role of megalin as a master regulator and coordinator of ion transport, metabolism, and endocytosis in the PT. Compared with studies in animal models, this approach provides a means to identify PT-specific transcriptional changes that are directly consequent to the loss of these target genes.

Ongoing research seeks to define the relationship between potassium and volume homeostasis by elucidating pathways that couple renal K+ sensing and tubular function during the potassium stress response.