Project Summary/Abstract: The intricate tissue architecture established during organ of Corti development requires precise temporal coordination between arrest of cell proliferation and initiation of sensory cell differentiation. Despite the necessity for this complex patterning, the mechanism regulating cell-cycle exit in the developing organ of Corti remains poorly understood. This proposal is aimed at investigating the role of Hippo signaling—a pathway that has recently emerged as an important regulator of growth control—in organ of Corti morphogenesis. Unlike most signaling cascades, Hippo is repressive in nature, and it is activated mechanically through increased cell density, cell- cell contacts, and tissue stiffness. The main experimental objective of the current proposal is to investigate whether genetic manipulation of the transcription co-factor Yap—a downstream target of Hippo—regulates cell proliferation and cell-cycle exit in the developing inner ear. To interrogate this premise, Cre-loxP mediated gene knockout will be implemented for loss of function experiments (Aim 1), and viral expression of a constitutively active form of Yap will be used to assess gain-of-function conditions (Aim 3). Additionally, a novel three-dimensional cochlear organ culture will be used to study the role of mechanical force of Yap protein degradation and cell-cycle exit in the developing organ of Corti. The proposed basic research has a long-term therapeutic potential. Understanding the role of Yap in triggering the arrest of prosensory-cell proliferation during early stages of organ of Corti development is critical in efforts to characterize the molecular signaling network governing its morphogenesis. A more complete understanding of organ of Corti development will aid in relieving the restriction on proliferative response in the surviving populations of inner ear supporting cells, which is the major impediment to hair-cell regeneration in mammals.