Abstract: SA-PO0475
Juxtaglomerular Effects of SPAK Deficiency Enhance the Glomerular Filtration Rate
Session Information
- Fluid, Electrolyte, and Acid-Base Disorders: Basic Research
November 08, 2025 | Location: Exhibit Hall, Convention Center
Abstract Time: 10:00 AM - 12:00 PM
Category: Fluid, Electrolytes, and Acid-Base Disorders
- 1101 Fluid, Electrolyte, and Acid-Base Disorders: Basic
Authors
- Mutig, Kerim, I.M. Sechenov First Moscow State Medical University (Sechenov University), Moscow, Russian Federation
- Yilmaz, Duygu Elif, Charité-Universitätsmedizin Berlin, Berlin, Germany
- Saritas, Turgay, University Hospital RWTH Aachen, Aachen, Germany
- Demirci, Hasan, Charité-Universitätsmedizin Berlin, Berlin, Germany
- Ferdaus, Mohammed Zubaerul, Vanderbilt University School of Medicine, Nashville, Tennessee, United States
- McCormick, James A., Oregon Health and Science University, Portland, Oregon, United States
Background
Na+-K+-2Cl- cotransporter (NKCC2) of the thick ascending limb (TAL) and macula densa (MD) cells is critical to regulation of the glomerular filtration rate (GFR) via the tubulo-glomerular feedback (TGF). NKCC2 function relies on activating phosphorylation provided by two homologous STE20-related kinases, SPAK and OSR1, whereas a truncated, catalytically inactive, kidney-specific SPAK variant (KS-SPAK) limits this process via competitive binding to NKCC2. KS-SPAK is produced via alternative splicing or proteolytic cleavage by the aspartyl aminopeptidase (DNPEP). We hypothesized that SPAK isoforms participate in regulation of GFR via distinct effects on NKCC2 activity in TAL vs. MD cells.
Methods
The hypothesis was investigated in SPAK-deficient (SPAK-/-) vs. wild-type (WT) mice and in cultured TAL vs. MD cell using physiological, immunohistochemical, and biochemical methods.
Results
Compared to WT, SPAK-/- mice exhibited strongly enhanced baseline NKCC2 phosphorylation in TAL due to deletion of the dominant-negative KS-SPAK and disinhibition of OSR1. In contrast, MD cells of SPAK-/- mice showed no changes in NKCC2 phosphorylation suggesting the absence of KS-SPAK in this cell type. Indeed, analysis of SPAK isoforms in cultured MD vs. TAL cells revealed high expression of the full-length SPAK in MD cells, whereas TAL cells predominantly exhibited KS-SPAK. Accordingly, DNPEP expression was markedly stronger in TAL compared to MD cells. Juxtaglomerular expression of cyclooxygenase 2 and nitric oxide synthase 1 were increased in SPAK-/- mice likely reflecting a reduced NaCl delivery to MD and TGF inhibition. Baseline GFR was higher in SPAK-/- compared to WT mice as determined by transcutaneous measurement using FITS-Sinistrin.
Conclusion
Thus, deletion of KS-SPAK along TAL prior to MD may reduce the NaCl load at MD, whereas deletion of FL-SPAK in MD cells may interfere with their NaCl sensitivity; both effects would synergistically augment GFR. Stronger DNPEP expression in TAL vs. MD cells further explains their distinct SPAK isoform profiles and functional effects of SPAK deletion. Since pharmacological SPAK inhibition is emerging as a therapeutic option for management of hypertension, improved understanding of renal SPAK effects is mandatory.