Unraveling the Role of Mitochondrial Dysfunction in the Pathogenesis of Clc-k2-Deficient Bartter Syndrome Mice

Background: Mutations in human ClC-Kb (Clc-k2 in mice) lead to Bartter syndrome (BS) type 3, a congenital renal tubulopathy. Our previous research showed the constitutive deletion of Clc-k2 in mouse kidneys hindered the postnatal development of thick ascending limbs (TALs) through mechanisms not yet fully understood. Our recent extracellular flux analysis revealed that low transport activity in renal tubules resulted in low mitochondrial mass and respiratory capacity. This alarming discovery prompted us to delve deeper into the mitochondrial functions in Clc-k2-deficient TALs and explore the potential of peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC1-α), the master activator of mitochondrial biogenesis, in rescuing Bartter syndrome. Methods: We analyzed the mitochondrial biogenesis and bioenergetics in isolated TALs using mitochondrial DNA copy numbers, seahorse-based mitochondrial assays, and electron microscopy. We also conducted a comparative transcriptome analysis using isolated Clc-k2 KO vs. wild-type TALs. Uromodulin-positive TAL/DCT cells were pulled down and used for cell cycle and proliferation assays. Pseudohypoaldosteronism type 2 (PHA2) WNK4D561A/+ mutant knockin mice and PGC1-α transgenic mice were crossed with Clc-k2 KO mice to rescue the transport activity/mitochondrial function, respectively. Mouse phenotypes then analyzed and compared. Results: Clc-k2-deficient TALs had reduced mitochondrial-to-nuclear DNA ratio and mitochondrial respiratory capacities, indicating downregulated mitochondrial biogenesis and bioenergetics. Transcriptome analysis confirmed suppression of mitochondrial genes in Clc-k2-deficient TALs. These cells were prone to G1-S cell cycle transition and proliferation delay. PHA2 WNK4D561A/+ mutant improved the mitochondrial mass/function, cell cycle, and proliferation of Clc-k2-deficient TAL cells and rescued the activity of NKCC2 and NCC cotransporter of Clc-k2-deficient mice. Overexpression of PGC1-α in Clc-k2 KO mice improved mitochondrial function of TAL, NKCC2/NCC activities, and BS phenotype. Conclusion: Our results support that low transport activity suppresses mitochondrial biogenesis/bioenergetics in renal tubules, thereby aggravating renal salt wasting, cell cycle delay, and TAL hypoplasia in Clc-k2-deficient BS mice. Mitochondrial-targeting therapy could be promising to BS. Funding: Other NIH Support - R01