Tmc Reliance Is Biased by the Hair Cell Subtype and Position Within the Ear.

Hair cells are heterogenous, enabling varied roles in sensory systems. An emerging hypothesis is that the transmembrane channel-like (Tmc) proteins of the hair cell's mechanotransduction apparatus vary within and between organs to permit encoding of different mechanical stimuli. Five anatomical variables that may coincide with different Tmc use by a hair cell within the ear are the containing organ, cell morphology, cell position within an organ, axis of best sensitivity for the cell, and the hair bundle's orientation within this axis. Here, we test this hypothesis in the organs of the zebrafish ear using a suite of genetic mutations. Transgenesis and quantitative measurements demonstrate two morphologically distinct hair cell types in the central thickness of a vestibular organ, the lateral crista: short and tall. In contrast to what has been observed, we find that tall hair cells that lack Tmc1 generally have substantial reductions in mechanosensitivity. In short hair cells that lack Tmc2 isoforms, mechanotransduction is largely abated. However, hair cell Tmc dependencies are not absolute, and an exceptional class of short hair cell that depends on Tmc1 is present, termed a short hair cell erratic. To further test anatomical variables that may influence Tmc use, we map Tmc1 function in the saccule of mutant larvae that depend just on this Tmc protein to hear. We demonstrate that hair cells that use Tmc1 are found in the posterior region of the saccule, within a single axis of best sensitivity, and hair bundles with opposite orientations retain function. Overall, we determine that Tmc reliance in the ear is dependent on the organ, subtype of hair cell, position within the ear, and axis of best sensitivity.