Control of cochlear amplification by cellular and acellular elements of the mammalian cochlea

The hearing organ of mammals, the cochlea, permits them to listen to sounds with remarkable acuity and sensitivity over enormous frequency and dynamic ranges. This ability is due to the novel design of the mammalian cochlea which facilitates electromechanical interaction between sensory hair cells, supporting cells of the sophisticated cellular architecture of the sensory organ of Corti, and the extracellular matrixes of the tectorial and basilar membranes that sandwich the organ of Corti. The basilar membrane, which supports the organ of Corti, separates sounds into constituent frequency components and underlies cochlear tonotopicity. Viscous damping in the fluid filled cochlea is counteracted through the action of sensory-motor outer hair cells, which boost and sharpen cochlear responses that are sensed by the inner hair cells. Resultant inner hair cell excitation and consequent transmitter release generates a flow of signals in the auditory nerve to the brain.
Hair cells die when damaged by exposure to intense sounds, ototoxicity, disease, age and genetic disorders. According to WHO, 5% of the world population suffer from irrecoverable hearing loss. Exciting possibilities for hair cell regeneration and hearing restoration are now becoming available. Understanding of the complex in vivo interaction between the sensory hair cells, supporting cells and tectorial and basilar membranes is essential for the future development of successful treatments for hearing loss, especially those involving recovery of damaged, or replacement of, dead sensory hear cells. Therefore, our prime objective is to study this complex interaction between elements of the cochlear sensory epithelium underlying the unique features of mammalian audition using in vivo measurements supported by ex vivo measurements. This research proposal is motivated by recent discoveries from our laboratory and elsewhere showing that the frequency and level-dependent interaction between different elements of the cochlea are far more complex than is presented in classical models of cochlear function, with the basilar membrane passively separating the frequency constituents of sounds and outer hair cells amplifying and sharpening the basilar membrane responses, which are then seen as sharp, sensitive, neural responses. To achieve our goals we record mechanical, acoustic, electrical and neural cochlear responses and combine them with predictive modelling to validate our ideas and gain further insights into the functional significance of interaction between different structures of the organ of Corti. More specifically, we will determine the timing of outer hair cell excitatory input which is optimised to deliver energy to the movement of cochlear structures at the appropriate time and place. We will investigate the significance of extracellular voltage for controlling outer hair cell motility and electrical and mechanical interaction between hair cells, which permits them to boost cochlear responses at ultrasonic frequencies with very sharp frequency resolution. Accordingly, the electrical properties of the organ of Corti will be characterised. Mechanical properties of the tectorial membrane, their dependence on the stimulus parameters and contribution to the build-up of cochlear amplification and sharpening of cochlear response will be determined. Using optogenetic mice that express channel rhodopsins in supporting cells, we will derive the mechanisms by which supporting cells regulate hair cell operation and regulate the longitudinal flow of energy within the organ of Corti.

The prime objective of this research proposal is to understand the complex electromechanical functional in vivo relationships between cellular and noncellular elements of the organ of Corti in the mammalian cochlea, control of energy production by the outer hair cells and flow of energy in the active healthy and impaired cochleae as an essential basis for restoring hearing function. We will make in vivo microelectrode recordings from single sensory cells in guinea pigs to determine the timing of hair cell excitatory input relative to responses of other cochlear structures. Furthermore, we will investigate the importance of tectorial membrane viscoelastic properties in normal cochlear function. Outcomes from these investigations are essential for understanding the mechanisms of cochlear frequency tuning. We will express channelrhodopsins in Deiters cells in mice and selectively excite them with light to determine the mechanisms by which Deiters' cells regulate outer hair cell sensitivity, operating point and the longitudinal flow of energy in the cochlear partition. We will characterise the ionic conductances of Deiters cells and their contribution to organ of Corti electrical impedance, which is essential for understanding the basis of cochlear amplification at high frequencies. Through predictive modelling, based on in vivo and in vitro experimental measurements, further insight will be gained into the functional significance of interaction between sensory cells, their supporting cell cages, and extracellular matrixes of the basilar and tectorial membranes. This detailed understanding of functional relationship between different elements of the organ of Corti is necessary to fully exploit the exciting, evolving, possibilities for hearing restoration through repair and regeneration.