Friction and Adhesion in the Hair Bundle's Glycocalyx

Abstract

To enable us to hear high-frequency sounds, the mechanosensitive hair bundles of our inner ears must oscillate and amplify at frequencies up to 20 kHz, overcoming both viscous drag and friction within the hair bundles. How can such movements be accomplished for a lifetime without harming cellular structures and causing hearing loss?The hair bundle is a cluster of linked, finger-like projections, the stereocilia, emerging from the hair cell's apical surface. Each stereocilium is composed of an actin core surrounded by membrane endowed with a glycocalyx, a layer of glycoproteins and glycolipids. The stereocilia stand apart from one another at their basal insertions, but lean toward one another at their tips, where they are connected along the axis of mechanosensitivity by fragile tip links. For efficient sensory transduction to occur, mechanotransduction channels must open in unison when the bundle is deflected, suggesting that the stereocilia exhibit some form of low-friction sliding adhesion. One possible mechanism by which such sliding adhesion might be implemented is through charged sugars such as sialic acid. If stereocilia are coated with negatively charged sugars, they will repel each other at certain distance scales owing to electrostatic forces. In the presence of multivalent cations, they may instead exhibit adhesion.Using a flexible glass fiber and photomicrometer to make quantitative force measurements, we investigated the friction and adhesion between individual stereocilia. The charge density of the stereociliary glycocalyx was measured by pairing capillary electrophoresis of individual stereocilia with electron microscopy. Using chemical labeling techniques and fluorophore-conjugated lectins, we identified specific sugars in the glycocalyx. Together, these experiments provide a functional understanding of the hair bundle's glycocalyx and speak to the question of how the hair bundle maintains coherence while simultaneously minimizing friction.