Aquaporin regulates cell rounding through vacuole formation during endothelial-to-hematopoietic transition

Endothelial-to-hematopoietic transition (EHT) is crucial for hematopoietic stem cell (HSC) generation. During EHT, the morphology of hemogenic endothelial cells (HECs) changes from flat and adherent to spherical HSCs, which detach from the dorsal aorta. HECs attain a rounded shape in a mitosis-independent manner before cell adhesion termination, suggesting an atypical cell-rounding mechanism. However, the direct mechanisms underlying this change in cell morphology during EHT remain unclear. Here, we show that large vacuoles were transiently formed in HECs and that aquaporin-1 (AQP1) was localized in the vacuole and plasma membranes. Overexpression of AQP1 in non-HECs induced ectopic vacuole expansion, cell rounding, and subsequent cell detachment from the endothelium into the bloodstream, mimicking EHT. Loss of redundant AQP functions by CRISPR/Cas9 gene editing in HECs impeded the morphological EHT. Our findings provide the first evidence indicating that morphological segregation of HSCs from endothelial cells is regulated by water influx into vacuoles. These findings provide important insights for further exploration of the mechanisms underlying cell/tissue morphogenesis through water-adoptive cellular responses. SUMMARY STATEMENT Hemogenic endothelial cells transiently form large vacuoles during endothelial-to-hematopoietic transition. Aquaporin water channels regulate cell rounding and detachment of emerging hematopoietic stem cells through vacuole formation. ### Competing Interest Statement M.S. is an employee of JST. M.S.K. is an employee of Daiichisankyo RD Novare Co., Ltd. The remaining authors declare no competing interests.