Viscoelastic coupling of stereocilia coordinates whole bundle motion in mammalian auditory inner hair cells

Hair cells are primary sensory cells used by all vertebrates to detect gravity, acceleration, sound, and fluid flow, and are named for the hair-like bundle of mechanically sensitive specialized microvilli, called stereocilia, on their apical surface. Hair bundles consist of an array of stereocilia connected with various extracellular linkages that form a staircased pattern. Stereocilia in columns are connected by filamentous tip-links that align in the direction of mechanosensitivity. Hair cells are activated when the tip link transfers the force of bundle displacement to mechanically gated ion channels near the tops of the shorter stereocilia. Hair cell bundles vary morphologically in number of stereocilia, number of stereocilia rows, stereocilia heights, and stereocilia thickness depending on the species and end organ in which they are found. For example, bullfrog, turtle, and bird hair cells have many rows of stereocilia that are tightly coupled. Mammalian auditory hair cells have only three rows of stereocilia and these stereocilia are not tightly coupled, but rather allow semi-independent stereocilia movement. What is the added value of a non-cohesive bundle with fewer stereocilia rows like that found in the mammalian auditory system? In order to answer that overarching question, we must first determine how non-cohesiveness affects mechanoelectrical transduction, and characterize the underlying mechanical properties of that non-cohesiveness. The first experimental part of my thesis addresses the question of how semi-independent movement of stereocilia in the mammalian hair cell bundle affects mechanoelectrical transduction (MET). Using stiff probes smaller than the width of the inner hair cell (IHC) bundle, I evoked semi-independent stereocilia displacements while recording MET current responses using whole-cell patch-clamp electrophysiology. Small probes directly displaced stereocilia they contacted and recruited adjacent stereocilia with larger displacements. The recruitment of stereocilia resulted in less uniform and less synchronous movement. Step displacements using smaller probes resulted in smaller and slower current activation responses, shallower activation curves, and slower and less complete adaptation. These results showed that the mechanical properties of less cohesive bundles greatly affect the force transfer to MET channels as indicated by the changes in the electrical response of the cell. The second part of my thesis work investigates the mechanical properties of the connections between stereocilia in mammalian auditory inner hair cell bundles. I recorded high speed bright field videos (≥ 25K fps) of bundle displacement using small probes while simultaneously recording the MET response. By using small probes, the motion of any stereocilia not directly contacted by the probe was due to force transfer through stereocilia connections. Non-contacted stereocilia moved less with increasing distance from the stimulus probe, indicating that the connections between stereocilia have an elastic property. Additionally, stereocilia not directly contacting the probe did not maintain their position during the step displacement, but instead moved backwards to a smaller steady state displacement. This backwards motion indicated that the connections between stereocilia have viscoelastic properties. I also measured the displacements of the stereocilia of the second row, which contain MET channels at their tips. The second row stereocilia moved similarly to, although less than, their first row partners, indicating that the bundle lacks coherence between rows as well as between columns, and that this connection is also viscoelastic. Because electrophysiological data were collected simultaneously with high speed videos of the stimulation, I was able to compare the total MET current with the peak displacements of all the stereocilia. In a coherent bundle, all stereocilia move to the same displacement, and so all the MET channels will contribute equally to the total current. If the stereocilia move differently, as is the case with the small probe stimulation of a non-coherent bundle, the MET channels in different parts of the bundle will contribute different amounts of current to the total. By modeling the total current as the sum of the currents carried by the different columns of stereocilia, and fitting the data to that model, I estimated an underlying current-displacement (IX) relationship for the mammalian auditory hair cell bundle. This estimated IX relationship showed higher sensitivity than the IX relationship measured using the large probe. Therefore, the large probe is not able to force perfect bundle coherence, and a different stimulus is needed to achieve the bundle's maximum possible sensitivity. Finally, a mathematical model of viscoelastic coupling between stereocilia captures the motion trajectory properties present in my experimental data. This model represented the bundle as having viscoelastic connections between each stereocilium in a row, and viscoelastic connections anchoring each stereocilium to the cuticular plate. Alone, the model qualitatively matched the experimentally measured motion properties of the stereocilia. By fitting the model to the experimental data, we were able to determine that the strengths of the elastic and damping properties of the links between stereocilia are stronger than the elastic and damping properties anchoring stereocilia to the cuticular plate, but this relationship is much weaker than that seen in non-mammalian bundles with many rows. This model, and specifically the ratio of the linking vs anchoring forces, quantifies the extent of the inherent lack of coherence in the mammalian bundle as compared to the non-mammalian bundle. Overall, the data presented in this thesis characterizes the functional consequences of non-cohesive connectivity between stereocilia in mammalian auditory inner hair cell bundles in terms of mechanoelectrical transduction, relates the displacements of individual stereocilia to the evoked current response of the cell, and describes the viscoelastic nature of the connections between stereocilia that shape bundle motion.

Advisor: Ricci Anthony 
Dunn Alexander Robert 
Goodman Miriam Beth 
Grillet Nicolas