Membrane-bound carbohydrates promote the cohesion of 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 should move essentially in unison, shearing at their tips yet maintaining contact without membrane fusion. One mechanism by which this cohesion might occur is through counterion-mediated attachments between the 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 stereocilia form elastic connections with one another to varying degrees dependent on the surrounding ionic environment and the presence of N linked sugars. Mg2+ is a more potent mediator of attachment than is Ca2+. The forces between stereocilia can produce chaotic stick-slip behavior. These results indicate that counterion-mediated interactions in the glycocalyx contribute to the stereociliary coherence essential for hearing.
The top left illustration shows a stereocilium immobilized at the tip of a fiber being rubbed back-and-forth (pink double arrow) against a second stereocilium affixed to the coverslip bottom of the experimental chamber. The distalmost 2 µm of the stereociliary tips, the usual sites of contact between the stereocilia during these experiments, are marked with green brackets. Note the pencil-like taper (yellow arrowhead) that distinguishes the stereociliary base. The schematic lateral view below illustrates the contact between two stereocilia. Lowering the base of the fiber produces a vertical load force perpendicular to the bottom of the chamber (pink arrow). The traces of the middle panel show representative single cycles of movement by a fiber's tip in endolymph. The motion of the free tip (black) is almost indistinguishable from that of the fiber's base (gray). Contact between the stereocilia with a vertical load force of 55 pN produces an irregular trajectory (red) punctuated by stick-slip events. After two cycles with a vertical load force of 83 pN, the stereocilia adhere firmly to one another and the range of motion diminishes (blue). In the right panel, the foregoing data are presented as displacement-force relations. The free tip traces a horizontal line (black) indicative of minimal viscous dissipation. Although the tip moves nearly as far just after contacting the stationary stereocilium (red), the stick-slip events produce a substantial dissipation quantified by the area within the trajectory. When the tip becomes tightly bound (blue), dissipation diminishes; the greater slope of the trajectory reflects the increased stiffness of attachments between stereocilia.