A link between planar polarity and staircase-like bundle architecture in hair cells

Sensory perception in the inner ear relies on the hair bundle, the highly polarized brush of movement detectors crowning hair cells. We previously showed that, in the mouse cochlea, the edge of the forming bundle is defined by the 'bare zone', a microvilli-free sub-region of apical membrane specified by the mInsc-LGN-Gαi protein complex. We now report that LGN and Gαi also occupy the very tip of stereocilia that directly abut the bare zone. We demonstrate that LGN and Gαi are both essential to promote the elongation and differential identity of stereocilia across rows. Interestingly, we also uncover that total LGN-Gαi protein amounts are actively balanced between the bare zone and stereocilia tips, suggesting that early planar asymmetry of protein enrichment at the bare zone confers adjacent stereocilia their tallest identity. We propose that LGN and Gαi participate in a long-inferred signal originating outside of the bundle to model its staircase-like architecture, a property essential for direction-sensitivity to mechanical deflection and hearing.