Timing and Dosage of Notch Inhibition and Wnt Activation Enhanced Hair Cell-Like Cell Generation and Stereocilia Elongation in Otic Organoids

The modulation of Notch and Wnt signaling was evaluated for its ability to promote the differentiation and maturation of hair cell (HC)-like cells in otic organoids derived from mouse embryonic stem cells (mESCs). Using R1/E mESCs, we generated otic organoids and treated them with the Notch pathway inhibitor (γ-secretase inhibitor, DAPT) and the Wnt signaling agonist (CHIR99021) during two-time windows: days (D) D12-D18 (early treatment) or D28-D34 (late treatment). Among the late treatment conditions, lower concentration (5 μM DAPT and 2.5 μM CHIR99021) yielded greater expression of MyoVIIa and Atoh1 compared with higher doses (10 and 5 μM, respectively). Ultrastructural analysis by transmission electron microscopy further demonstrated that late treatment with lower-dose DAPT and CHIR99021 generated HC-like cells harboring longer and more prominent stereocilia-like bundles than those observed in other conditions. In contrast, early treatment produced HCs arranged in a typical otic organoid pattern, resulting in a more morphologically stable but less robustly induced HC-like population. These findings indicate that temporally and dose-controlled modulation of Notch and Wnt signaling efficiently enhances HC-like cell differentiation and stereocilia maturation in mESC-derived otic organoids, providing a useful platform for future studies in inner ear regeneration and cell-based therapies.