Deletion of ALMS1 (Alstrom syndrome 1) in Thick Ascending Limbs (TALs) increases surface NKCC2 expression, NaCl reabsorption and causes salt sensitive hypertension

Inactivating mutation in the ALMS1 gene cause Alström syndrom (ALMS) in humans, characterized by the early onset of obesity, hypertension, and chronic kidney disease (CKD). in the general population, single-nucleotide polymorphisms (SNPs) in the ALMS1 gene are associated with lower GFR, hypertension and metabolic syndrome. ALMS1 is expressed in a unset of cells ubiquitously expressed where it localizes to endosomes, centrosomes, and the base of cilia. We previously found that ALMS1 is expressed in Thick Ascending Limb (TAL) where it binds and decreases NKCC2 cotransporter activity. Whole body deletion of ALMS1 in salt-sensitive (SS) rats, increases NKCC2 surface expression, renal NaCl reabsorption, blood pressure, and inhibits NKCC2 endocytosis. However, whole body ALMS1 KO rats or mice develop obesity which may complicate the interpretation of our data. Therefore, we hypothesized that ALMS1, specifically in TALs is a part of protein complex that binds apical NKCC2 and mediates its endocytosis to regulate surface expression, NaCl reabsorption and blood pressure. To study role of ALMS1 in TALs, we generated a loxP flanked exon 7 ALMS1 transgenic mouse line, crossed with TAL specific inducible Umod-ERT2 Cre mice (Umod-ALMS1 KO). 3 weeks after inducing Cre expression with tamoxifen, ALMS1 expression in TAL suspensions decreased by 85% (p<0.01) (n=5). In isolated medullary TALs from Umod-ALMS1 KO surface to total NKCC2 ratio was increased by 37±15% compared to floxed ALMS1 (p<0.01) (n=5). Interestingly, phosphorylation of NKCC2 at thr 96 increased by 4-fold in Umod-ALMS1 KO, something we did not observed in whole animal KO. In a separate group of KO mice we measured bumetanide-induced natriuresis (4h), an index of NKCC2-mediated NaCl absorption. We found that bumetanide induced natriuresis was higher in Umod-ALMS1 KO (control: 110±7 vs Umod-ALMS1 KO: 173±26 µmols Na/4h, p<0.01). We finally studied conscious blood pressure (BP) in Umod-ALMS1 KO in normal or high salt diet. Radiotelemetric devices were implanted, and 1 week later, BP was monitored for 7 days in normal salt (0.2% Na) and then mice were fed high salt diet (4% Na) for 2 weeks. At baseline, we found that systolic BP was higher in Umod-ALMS1 KO (Control: 108±1.5 mmHg vs Umod-ALMS1 KO: 115±1 mmHg p<0.025). A high salt diet increased SBP very fast in Umod-ALMS1 KO mice (within 24 h) and after 13 day remained significantly higher than floxed control mice (Control: 112±1.4 mmHg, Umod-ALMS1 KO: 124±2.2 mmHg). In summary, ALMS1 regulates NKCC2 surface expression such that its deletion in TALs increases surface NKCC2, NKCC2-mediated NaCl reabsorption, and causes salt sensitive hypertension. To date, it is unknown whether ALMS1 expression in the kidney and TAL is transcriptionally regulated, and whether SNPs in ALMS1 present in the general population are associated to hypertension. NIH R01, Henry Ford Fund This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.