Divalent counterions tether membrane-bound carbohydrates to promote the cohesion of auditory hair bundles

The cell membranes in the hair bundle of an auditory hair cell confront a difficult task as the bundle oscillates in response to sound: for efficient mechanotransduction, all the component stereocilia of the hair bundle must move essentially in unison, shearing at their tips yet maintaining contact without membrane fusion. One mechanism by which this cohesion might occur is counterion-mediated attachment between glycan components of apposed stereociliary membranes. Using capillary electrophoresis, we showed that the stereociliary glycocalyx acts as a negatively charged polymer brush. We found by force-sensing photomicrometry that the stereocilia formed elastic connections with one another to various degrees depending on the surrounding ionic environment and the presence of N-linked sugars. Mg²⁺ was a more potent mediator of attachment than was Ca²⁺. The forces between stereocilia produced chaotic stick-slip behavior. These results indicate that counterion-mediated interactions in the glycocalyx contribute to the stereociliary coherence that is essential for hearing.