A Sodium Wasting Phenotype and Disrupted Collecting Duct Function in Ae1 R607H and L919X Knock-In Mice

The kidney collecting duct, composed of principal and intercalated cells (ICs), fine-tunes urine composition by regulating fluid, electrolyte, and acid-base homeostasis. Type A intercalated cells (A-ICs) facilitate proton secretion into the urine through an apical proton ATPase and mediate bicarbonate reabsorption into the blood via a basolateral kidney anion exchanger 1 (kAE1). Genetic mutations that impact IC function, for example mutations to SLC4A1 (kAE1), can lead to distal renal tubular acidosis (dRTA). dRTA results in metabolic acidosis, failed urinary acidification, and an unexplained urinary sodium-wasting phenotype. In vivo data from transgenic mice and dRTA patients’ kidney biopsies indicate that a loss of kAE1 results in a decreased abundance of ICs. This is evident in the previously published Ae1 R607H knock-in (KI) mouse model (orthologous to human AE1 R589H mutation) that recapitulates classical metabolic acidosis. Our lab obtained a second, unpublished mouse model, the Ae1 L919X KI mice (orthologous to human AE1 R901X mutation). We hypothesize that these mice, with a decreased abundance of A-ICs, are an effective model for studying the persistent urinary sodium loss. The objectives of our work were to 1) characterize the Ae1 L919X KI mice, 2) determine whether the two mouse models accurately reflect the urinary ion loss seen in dRTA patients, and 3) begin investigating the mechanisms causing the urinary sodium loss in these mice. To test our hypothesis, wildtype and dRTA mutant mice were fed a salt-depleted diet, an acid diet, or a salt- depleted acid diet. Following these dietary challenges, analysis of plasma and urine ion concentrations and pH, and qRT-PCR and immunoblots on whole kidney tissues was completed. At baseline, similar to homozygous R607H KI mice, the homozygous L919X KI mice exhibit typical dRTA features including alkaline urine and a reduced number of A-ICs. The salt-depletion and acid challenge alone revealed preliminary evidence of disrupted collecting duct function in the mutant animals compared to WT. Following the salt-depleted acid diet, both dRTA mutant mice exhibited a urinary sodium loss. Additionally, both homozygous mutant mice showed evidence of upregulated gene and protein expression of tight junction proteins claudin-4 and claudin-10b, suggesting an involvement of the paracellular pathway to compensate for the failing transcellular pathways within the collecting duct. Overall, our findings reveal that both mutant mice appropriately model dRTA pathophysiology, including the urinary sodium wasting phenotype seen in human patients, and suggest a compensatory upregulation of the paracellular transport pathway. As kidney disease is a prominent health concern in Canada, this project contributes to the growing body of knowledge on how various nephron segments communicate with each other to regulate ion homeostasis in our body. Our understanding of the relationship between acid-base balance and sodium reabsorption, can not only provide improved treatment options for dRTA patients, but also to those living with other electrolyte imbalance disorders.

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