Hennings, J. C. | bioRxiv Science

bioRxiv2024

Urinary Sodium Wasting and Disrupted Collecting Duct Function in Mice with dRTA-Causing SLC4A1 Mutations

Distal renal tubular acidosis (dRTA) results in metabolic acidosis due to impaired urinary acidification and can also result in an unexplained urinary sodium-wasting phenotype. Here, we report the generation and characterization of a novel dRTA mutant mouse line, Ae1 L919X knockin (KI). Homozygous L919X KI mice exhibit typical dRTA features including a reduced ability to acidify urine in response to an acid load. This renal acidification defect was associated with a reduced number of Ae1-positive type A intercalated cells. To assess whether these mice exhibit urinary sodium-wasting as seen in some dRTA patients, homozygous KI L919X and the previously described R607H KI mice were fed a salt-depleted acid diet. In line with human patients, both mouse strains exhibited urinary sodium loss. Additionally, we identified increased expression of tight junction proteins claudin-4 and -10b, suggesting a compensatory paracellular pathway in the loop of Henle. Consistent with data from human patients, L919X KI mice displayed a milder phenotype than R607H KI mice. Our findings reveal that both mouse strains are appropriate models for dRTA with a urinary salt-wasting phenotype and a compensatory up-regulation of the paracellular pathway in the ascending limb of the loop of Henle.

physiology↗

bioRxiv2024

Renal proximal tubules are sensitive to metabolic acidosis

Patients suffering from distal renal tubular acidosis (dRTA) are sometimes diagnosed with proximal tubule dysfunction with leaks of phosphate, uric acid, amino acids, and low-molecular-weight proteins, also known as Fanconi-like syndrome. The underlying molecular basis is largely elusive. We previously reported on Atp6v0a4 knockout (KO) mice, which exhibit severe metabolic acidosis in combination with proximal tubule dysfunction as evidenced by phosphaturia and proteinuria. Here, we show that Rab7, a key regulator of endo-lysosomal trafficking and lysosomal biogenesis, is strongly diminished in proximal tubules of Atp6v0a4 KO mice, while the number of abnormal Ist1-labelled Lamp1-positive vesicles is increased. This is accompanied by the accumulation of autophagosomes, autolysosomes and autophagic substrates. Importantly, correction of metabolic acidosis with bicarbonate therapy resolves proximal tubule dysfunction and trafficking defects in Atp6v0a4 KO mice. Acid-challenged wildtype mice also show trafficking defects with Rab7-downregulation and an increase in Ist1-labeled Lamp1-positive vesicles and develop proximal tubule damage in the long-term. Similar acidosis-induced alterations also occur in human kidney organoids. Altogether, our data provide insights, why patients suffering from severe dRTA may develop a Fanconi-like syndrome, which may contribute to the progression of chronic kidney failure. Translational StatementPatients with renal acidosis caused by impaired proton secretion in the collecting duct (distal renal tubular acidosis - dRTA) sometimes show unexplained symptoms of proximal tubule dysfunction such as proteinuria and phosphaturia. Here, we show that proximal tubules are particularly sensitive to acidosis as evidenced by impaired trafficking, lysosomal damage and accumulation of autophagic substrates. We also show that early treatment of dRTA by alkali supplementation can prevent proximal tubule dysfunction. Because metabolic acidosis represents a well-known risk factor for the progression of chronic kidney disease (CKD), our findings highlight the potential clinical importance of early alkali supplementation to delay disease progression.

molecular biology↗