Mature Human Collecting-Duct Organoids Link Lithium Exposure to Reduced Swelling

Maturation Changes the Functional Question an Organoid Can Address

Researchers led by Kumamoto University investigated whether the high-salt environment of the kidney medulla helps collecting ducts mature. Their work connected developmental experiments in mice with human induced pluripotent stem cell (iPSC)-derived collecting-duct organoids. After combining tonicity conditioning with hormones, the human models displayed water-channel localization and vasopressin-receptor responses that enabled tests of lithium exposure and an engineered inherited-disease variant.

The human iPSC backgrounds came from two healthy donors. The disease comparisons were generated experimentally, including knockout of the transcription factor NFAT5 and Q174R mutation of AVPR2, the vasopressin V2 receptor gene, with isogenic controls. The study therefore examined defined perturbations in human renal cells, alongside separate mouse developmental evidence.

Salt and Hormones Contribute Different Maturation Signals

In mouse ureteric-bud cultures, increased tonicity altered epithelial shape, gene expression and selected physiological functions. Salt and sucrose induced maturation-associated changes, whereas urea did not produce the same response. These comparisons linked the effect to tonicity, the osmotic activity of relatively non-permeating solutes, rather than simply to a larger total solute concentration.

NFAT5 mediated a substantial part of the transcriptional response. Mouse knockout experiments supported its role, while reducing medullary salt accumulation with furosemide impaired selected maturation features in neonatal kidneys. Tonicity alone had only a limited effect on Aqp2 expression, making hormonal support important for a water-transport model.

For human organoids, the investigators gradually raised medium osmolality to 900 mOsm/kg and combined this conditioning with arginine vasopressin (AVP) and aldosterone. Forskolin supported tolerance during maturation. The combined condition increased aquaporin 2 (AQP2) at the apical membrane and aquaporin 3 (AQP3) at the basolateral membrane. In single-cell analysis, 97.0% of collecting-duct cells expressed AQP2, compared with 52.3% under tonicity alone and 2.7% under the reported hormone-only condition.

After washing out the conditioning factors, the AVPR2-selective vasopressin analog desmopressin (ddAVP) restored apical AQP2 in combined-protocol organoids. Hormone-only organoids did not show the same response. Comparison with adult human kidney datasets supported a more mature medullary principal-cell profile, although the authors state that additional developmental reference data are needed to define the maturation stage precisely.

Measuring Lithium-Associated Functional Impairment

Lithium chloride exposure at 20 mM reduced AQP2 and AQP3 while maintaining CDH1 expression. Preservation of the epithelial marker supported a selective water-channel phenotype rather than attributing the result solely to generalized cell loss. The matched salt comparator used the same concentration of sodium chloride.

The functional assay changed the surrounding medium from 900 to 450 mOsm/kg, creating a relative hypotonic challenge. Time-lapse microscopy followed the response over 12 minutes. Clearly defined image regions were measured at five time points and normalized to their starting area. Although the paper describes estimated volume changes, the calculation used two-dimensional regional area, so the measurement is best understood as an image-based swelling estimate.

Control organoids reached a reported peak swelling estimate of 25.9% at nine minutes. Lithium-treated organoids showed a smaller peak, 12.0%, at six minutes. Twenty regions per condition were analyzed across four independent experiments.

Genetic Comparisons Define the Model’s Scope

The engineered AVPR2 Q174R model supplied a second test of the water-transport phenotype. After ddAVP exposure, wild-type organoids accumulated AQP2 at the luminal membrane, whereas variant organoids did not. Their swelling response was also smaller, consistently across the reported clones. This comparison involved 20 image regions per condition from three independent experiments.

NFAT5 knockout organoids provided a different limitation: they could not tolerate 900 mOsm/kg, so subsequent knockout comparisons used 700 mOsm/kg. Their reduced AQP2 and altered transport-related expression supported a maturation mechanism under that distinct condition.

The organoids contain collecting ducts without complete nephrons, a natural corticomedullary gradient or a urinary outlet. The authors propose larger-scale cultures for functional variant and drug studies. Applying the maturation conditions to multisegment kidney assembloids will require additional design work because cortical tissue is normally protected from the high tonicity used here.