In the developing organ of Corti (the mammalian auditory sensory epithelium), all progenitors exit the cell cycle in a single coordinated wave between E12.5 and E14.5, directed by an apex-to-base wave of transcription of the cyclin-dependent-kinase inhibitor gene Cdkn1b (p27Kip1). Gnedeva et al. (PNAS 2020) showed that final organ size is governed by Yap/Tead-mediated progenitor self-renewal: Tead factors directly control cell-cycle genes, Yap protein is degraded in the prosensory domain as progenitors become postmitotic, Yap loss shrinks the sensory epithelia, and constitutively active Yap5SA blocks cell-cycle exit in the vestibular sensory epithelia (the same inducible allele was not expressed in organ of Corti progenitors in that study). The 2020 paper also reported that conditional Yap1 ablation before the onset of cell-cycle exit does not affect the timing of Cdkn1b upregulation, so the wave trigger is Yap-independent; a 2025 follow-up from the same group restated that result as definitive and resolved further parts of the problem, showing that Yap drives Skp2-mediated p27 protein degradation and that Hippo and p27Kip1 synergistically suppress postnatal regenerative proliferation. The surviving open core is developmental-mechanistic: what initiates and spatially patterns the Cdkn1b transcriptional wave, what triggers Yap inactivation in the prosensory domain, how redundant Yap and Taz are in maintaining progenitors, and how Yap/Taz/Tead output is integrated with Wnt/β-catenin signaling to fix progenitor number and organ size. That core remains open.

Background

The organ of Corti is the auditory sensory epithelium of the mammalian cochlea: a precisely arrayed strip of sensory hair cells and non-sensory supporting cells that runs along the cochlear spiral. Its progenitors occupy the prosensory domain, a Sox2-positive strip within the embryonic cochlear duct. In the mouse, every prosensory progenitor withdraws from the cell cycle in one coordinated wave between embryonic days 12.5 and 14.5 (E12.5–E14.5), long before hair cell differentiation begins; this terminal-mitosis wave is directed by a wave of expression of p27Kip1p27^{Kip1}, a cyclin-dependent-kinase inhibitor encoded by the Cdkn1b gene, and it spreads from the apex (tip) of the duct toward the base — the reverse of the later base-to-apex wave of hair cell differentiation. Chen and Segil (1999) showed that p27Kip1 links proliferation to morphogenesis in this system, and Lee, Liu and Segil (2006) showed that the p27Kip1 wave is controlled at the transcriptional level. In the twenty years since, the molecular trigger that initiates and orients this wave has not been identified; because p27Kip1 loss causes organ of Corti dysplasia rather than simple overgrowth, this transcriptional control is unusually central to how the organ is built.

The proliferative state of the progenitors is maintained by the Hippo pathway effectors Yap and Taz. Yap (Yes-associated protein, encoded by Yap1) and its paralog Taz (encoded by Wwtr1) are transcriptional coactivators that, in the nucleus, bind Tead-family transcription factors to activate stemness and cell-cycle genes; the Hippo kinase cascade (Mst1/2 and Lats1/2 kinases) phosphorylates Yap/Taz, causing their cytoplasmic retention and degradation, thereby shutting off Tead-dependent transcription. Gnedeva et al. (2020) established that this module governs organ of Corti size: Tead factors bind putative regulatory elements of many cell-cycle and stemness genes in prosensory progenitors; Yap protein is degraded specifically in the Sox2-positive prosensory domain with a spatiotemporal pattern that coincides with the p27Kip1 wave; conditional Yap loss in the inner ear yields significantly smaller auditory and vestibular epithelia; and a constitutively active Yap allele (Yap5SA, mutated at Lats phosphorylation sites) prevents cell-cycle exit in the tissue where it is induced — robust induction and overproliferation were obtained in the vestibular sensory epithelia, whereas the construct was not induced in organ of Corti progenitors, in which no excess proliferating Sox2-positive cells were found at E14.5. Skp2, the substrate-recognition component of an SCF ubiquitin ligase complex that degrades both p27Kip1 and, in some contexts, Yap, was identified as a direct Tead target in the sensory epithelia.

Two further results frame the unresolved regulatory questions. First, Gnedeva et al. (2020) showed that conditional ablation of Yap1 before the onset of cell-cycle exit does not affect the timing of Cdkn1b upregulation, so the p27Kip1 wave trigger is Yap-independent. Second, canonical Wnt/β-catenin signaling (a mitogenic pathway in which Wnt ligands stabilize β-catenin, which then co-activates Tcf/Lef-dependent transcription) is active in the proliferating prosensory domain and can drive progenitor proliferation, yet Jansson et al. (2019) found that conditional Ctnnb1 (β-catenin) loss from E10.5 does not shorten the organ of Corti, whereas Yap loss drastically does. Progenitors also do not completely lose mitotic capacity in Yap mutants, and Taz itself is expressed in cochlear progenitors and can drive Tead-dependent transcription, so Taz may compensate for Yap loss. What remains unknown is how these pieces fit together: what initiates the Cdkn1b transcriptional wave and its apex-to-base directionality, what upstream mechanism triggers Yap degradation in the prosensory domain, how much of progenitor maintenance is Yap versus Taz, and how Yap/Taz/Tead activity is integrated with Wnt/β-catenin signaling to set final progenitor number — that is, the regulatory architecture that switches the prosensory domain from self-renewal to cell-cycle exit and thereby fixes the size of the organ of Corti.

Problem Statement

Determine the regulatory architecture by which organ of Corti progenitors terminate proliferation in a single apex-to-base wave, and thereby fix final organ size. The central unresolved question is component (1): identify the molecular mechanism, whether a discrete transcriptional regulator or a distributed system of inputs, that initiates the wave of Cdkn1b (p27Kip1p27^{Kip1}) transcription in the cochlear duct and accounts for its apex-to-base directionality and timing; the transcriptional regulator(s), the Cdkn1b cis-regulatory elements they act on, and any upstream inductive signal(s) all belong to this mechanism. That the trigger is Yap-independent is already established and is itself a constraint any answer must respect. Three further open questions about the same regulatory module are: (2) the upstream mechanism that triggers Yap phosphorylation, cytoplasmic retention, and degradation in the embryonic Sox2-positive prosensory domain; (3) the relative contributions of Yap and Taz to maintaining the prosensory progenitor pool, including whether Taz compensates for Yap loss; and (4) how Yap/Taz/Tead-dependent transcription is integrated with canonical Wnt/β-catenin signaling in setting progenitor number, the timing of cell-cycle exit and differentiation, and hence the length and cell count of the mature organ of Corti. These are intertwined yet distinct open questions about Yap and p27Kip1 as upstream regulators of the cell cycle in the inner ear.

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