Tekt2 regulates hair cell development and function by maintaining normal kinocilium morphology in zebrafish.

Background: Sensory hair cells (HCs) rely on a highly polarized apical apparatus, composed of actin-rich stereocilia and a microtubule-based kinocilium, to detect mechanical stimuli. Although tektins are conserved microtubule-associated proteins enriched in cilia and flagella, their roles in sensory HC morphogenesis and function remain poorly understood. Methods: We investigated the expression and function of tektin 2 (tekt2) in zebrafish HCs using single-cell RNA sequencing, whole-mount in situ hybridization, genetic loss-of-function and rescue analyses, behavioral assays, scanning electron microscopy, and FM-dye uptake assays. Results: Single-cell RNA sequencing and whole-mount in situ hybridization showed that tekt2 is highly expressed in neuromast, macular, and crista HCs. A tekt2-mCherry reporter further revealed enrichment of Tekt2 at the HC apex and within the kinocilium. The tekt2 deficiency caused auditory and vestibular dysfunction and disrupted otolith formation, accompanied by reduced and shortened otic cilia. In neuromasts and cristae, tekt2 deficiency decreased HC number, shortened hair bundles, and produced abnormal kinocilia. These kinocilium defects were associated with disorganized stereocilia and disrupted HC orientation. Mechanotransduction was consistently compromised, as indicated by reduced FM-dye uptake, and could be partially restored by exogenous tekt2. Conclusion: Together, our findings identify Tekt2 as an HC-enriched microtubule-associated factor required for normal kinocilium morphology, apical polarity, hair-bundle organization, and mechanotransduction function in zebrafish HCs.