A Novel Human Distal Tubuloid-on-a-Chip Model for Investigating Sodium and Water Transport Mechanisms.

Background: The distal nephron, which includes the thick ascending limb, distal convoluted tubule, connecting tubule, and collecting duct, is essential for the regulation of water and electrolyte balance. Injury or dysfunction of these segments contributes to significant kidney disease and systemic complications. Studying these regions directly in vivo is limited by the complexity of kidney architecture. Recently, three-dimensional cultures of human kidney tubule cells, called tubuloids, and microfluidic platforms that support dynamic flow have provided new opportunities to model kidney function in vitro.

Methods: We developed a human distal tubuloid-on-a-chip model by integrating primary human tubuloid cells with the OrganoPlate® microfluidic culture system. Tubuloids were seeded to form three-dimensional tubular structures adjacent to a collagen type I matrix and exposed to alternating flow. Differentiation into a distal nephron phenotype was evaluated using quantitative polymerase chain reaction, immunohistochemistry, and barrier integrity assays. Functional sodium and water transport were investigated using radiolabeled sodium uptake, transepithelial resistance, dextran diffusion, and dome formation. Pharmacological studies were conducted using trimethoprim, an inhibitor of epithelial sodium channel activity.

Results: Three-dimensional cultures under flow conditions showed increased expression of markers specific to distal nephron segments compared to conventional two-dimensional cultures. Immunostaining confirmed the formation of a highly polarized epithelium with apical and basolateral localization of key transport proteins. The tubules formed a leak-tight barrier, demonstrated by reduced dextran permeability and elevated transepithelial resistance. Radiolabeled assays revealed active sodium transport driven by apical epithelial sodium channels and basolateral sodium-potassium ATPase. Water transport accompanied sodium movement, indicated by dome formation beneath the epithelial layer. Exposure to trimethoprim reduced sodium uptake and dome formation, confirming functional responsiveness of the model.

Conclusions: This distal tubuloid-on-a-chip system reproduces key structural and functional features of the human distal nephron. It enables the study of salt and water transport in primary human tubule cells under controlled microfluidic flow. The model provides a physiologically relevant and scalable platform for investigating kidney physiology, pathology, and pharmacological responses, with potential applications in drug discovery and toxicity testing.