Tonicity drives collecting duct maturation in the mammalian kidney

Organ maturation is a fundamental biological process and a major challenge for organoid-based regenerative medicine. During kidney maturation, osmolality increases in the renal medulla for urine concentration, yet whether this extreme environment conversely contributes to kidney maturation remains unclear. Here we show that high tonicity, i.e., salt, drives morphological, transcriptional, and functional maturation of medullary collecting ducts (CDs) primarily via nuclear factor of activated T cells 5 (NFAT5). This occurs both in vitro and in vivo. Combining high tonicity with maturation-promoting hormones, we establish a robust maturation protocol for human medullary CD organoids and a functional assay to measure water permeability in CD organoids, enabling modeling of three disease states: NFAT5 deletion, and lithium-induced and congenital nephrogenic diabetes insipidus. Collectively, high tonicity drives mammalian kidney maturation, offering a promising platform to maximize the potential of organoids for future therapies.