Determining the Mechanical Properties of Hair-Bundle Components From Physiological Measurements in Outer Hair Cells

Determining the physical properties of cells from experimental data is a challenge. Here we show how to estimate the mechanical properties of the mechanosensory organelle of outer hair cells, the hair bundle, from experimental data. Hair bundles are required for hearing and comprise rod-like projections called stereocilia, which pivot on the hair-cell surface and are connected by gating-springs that regulate ion channels in the stereocilia. Sound-induced forces deflect the stereocilia, modulating gating-spring extensions and generating oscillating channel currents, which sum to equal the receptor current. We hypothesize that measurement uncertainties and morphological assumptions cause uncertainties in the estimated values of a bundle's mechanical properties. We test our hypothesis by building a computational model of the outer-hair-cell hair bundle, whose morphology (stereociliary heights, widths, and pivot positions) is assigned and mechanical properties are deduced from published experimental data. We show that, contrary to previous assumptions, channel currents are different and that these differences decrease the receptor current. Additionally, we show that the estimated values of mechanical properties are highly sensitive to changes in bundle morphology. It is important to constrain hair bundle morphology using experimental measurements to accurately deduce a bundle's mechanical properties.