Proximal Tubule Cells Contribute to the Thin Descending Limb of the Loop of Henle during Mouse Kidney Development

Background: The thin descending limb of the loop of Henle is crucial for urine concentration, as it facilitates passive water reabsorption. Despite its importance, little is known about how this nephron segment forms during kidney development. Methods: We assembled a large single-cell RNA sequencing (scRNA-seq) dataset by integrating multiple datasets of non-mutant developing mouse kidneys to identify developing thin descending limb cells. To test whether those cells originate from proximal tubule cells, we generated a proximal tubule-specific Cre line, Slc34a1eGFPCre , and conducted lineage tracing. Additionally, given that the transcription factor Hnf4a directly binds to the Aqp1 gene, we examined whether the loss of Hnf4a affects Aqp1 expression in thin descending limb cells. Results: From our scRNA-seq dataset, we identified a small cluster of cells distinct from both the proximal tubule and the thick ascending limb of the loop of Henle. Those cells exhibited high expression of thin descending limb marker genes, including Aqp1 and Bst1 . Notably, a subset of proximal tubule cells also expressed thin descending limb marker genes, suggesting that proximal tubule cells may give rise to thin descending limb cells. Using lineage tracing with the Slc34a1eGFPCre line, we found that, at least, a subset of thin descending limb cells are descendants of proximal tubule cells. Furthermore, the loss of Hnf4a, a transcription factor essential for mature proximal tubule cell formation, disrupted proper Aqp1 expression in thin descending limb cells, providing additional evidence of a developmental link between proximal tubule cells and thin descending limb cells. Conclusions: Our findings shed new light on the developmental origin of thin descending limb cells and highlight the importance of Hnf4a in regulating their formation.