Functional maturation of kidney organoid tubules: PIEZO1-mediated Ca²⁺ signaling

Kidney organoids cultured on adherent matrices in the presence of superfusate flow generate vascular networks and exhibit more mature podocyte and tubular compartments compared with static controls (Homan KA, Gupta N, Kroll KT, Kolesky DB, Skylar-Scott M, Miyoshi T, Mau D, Valerius MT, Ferrante T, Bonventre JV, Lewis JA, Morizane R. Nat Methods 16: 255-262, 2019; Takasato M, Er PX, Chiu HS, Maier B, Baillie GJ, Ferguson C, Parton RG, Wolvetang EJ, Roost MS, Chuva de Sousa Lopes SM, Little MH. Nature 526: 564-568, 2015.). However, their physiological function has yet to be systematically investigated. Here, we measured mechano-induced changes in intracellular Ca²⁺ concentration ([Ca²⁺]_i) in tubules isolated from organoids cultured for 21-64 days, microperfused in vitro or affixed to the base of a specimen chamber, and loaded with fura-2 to measure [Ca²⁺]_i. A rapid >2.5-fold increase in [Ca²⁺]_i from a baseline of 195.0 ± 22.1 nM (n = 9; P ≤ 0.001) was observed when microperfused tubules from organoids >40 days in culture were subjected to luminal flow. In contrast, no response was detected in tubules isolated from organoids <30 days in culture. Nonperfused tubules (41 days) subjected to a 10-fold increase in bath flow rate also exhibited a threefold increase in [Ca²⁺]_i from baseline (P < 0.001). Mechanosensitive PIEZO1 channels contribute to the flow-induced [Ca²⁺]_i response in mouse distal tubule (Carrisoza-Gaytan R, Dalghi MG, Apodaca GL, Kleyman TR, Satlin LM. The FASEB J 33: 824.25, 2019.). Immunodetectable apical and basolateral PIEZO1 was identified in tubular structures by 21 days in culture. Basolateral PIEZO1 appeared to be functional as basolateral exposure of nonperfused tubules to the PIEZO1 activator Yoda 1 increased [Ca²⁺]_i (P ≤ 0.001) in segments from organoids cultured for >30 days, with peak [Ca²⁺]_i increasing with advancing days in culture. These results are consistent with a maturational increase in number and/or activity of flow/stretch-sensitive Ca²⁺ channels, including PIEZO1, in tubules of static organoids in culture.