Interaction between sensory and supporting cells in the organ of Corti: basis for sensitivity and frequency selectivity of mammalian cochlea.

Our prime objective is to study interaction between individual cells and other elements of the complex sensory epithelium of the organ of Corti (OC) of the cochlea that determines the exquisite sensitivity and frequency selectivity of mammalian audition. Understanding this interaction is essential for future development of successful treatments for hearing loss, especially those involving recovery of damaged, or replacement of dead, sensory hair cells (HCs). HCs 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.
HCs in the OC are not replaced when they die. Subsequent hearing depends on remaining, usually low frequency, HCs. Why HCs of non-mammalian vertebrates are replaced, but not those in the mammalian cochlea, is not known. We suggest it is a consequence of the mechanism, by which HCs are tuned to acoustic frequencies. HC frequency tuning of non-mammalian vertebrates is due usually to intrinsic electrical and mechanical resonances and each HC is surrounded by a ring of supporting cells (SC)s that provide a source of replacement HCs. HCs in the mammalian cochlea rely on an extrinsic source of mechanical tuning: the basilar membrane (BM), which constitutes a spiralling acellular ribbon with graded stiffness increasing from apex to the base of the cochlea and is intimately attached to the OC. BM vibrations deflect the HC sensory hair bundles. For the three rows of sensory-motor outer HCs (OHCs) extending the length of the OC, the resultant receptor potentials drive motile forces that feedback energy to the BM. The forces boost BM vibrations close to the frequency place of the OHC, which are transmitted to the row of sensory inner hair cells (IHCs). Resultant deflections of IHC hair bundles generate voltages that control transmitter release and flow of afferent signals in the auditory nerve.
To interact with the BM, each OHC is restrained in a complex, flexible, fluid filled cage of specialized, interconnected, SCs comprising pillar cells (PCs) and Deiter's cells (DCs). The cage is proposed to optimize exchange and control of energy between OHCs and other elements of the cochlear partition, including the BM. It has been suggested that their complexity renders mammalian SCs unavailable as sources for HC replacement. Recently, however, we have shown that SCs can be converted into HCs at various postnatal stages, but remain immature, possibly due to lack of interaction with surrounding SCs, which is why it is essential to understand this interaction for restoration of hearing. To this end, we will systematically modify and delete specific proteins in OHCs, PCs, DCs and BM in mice and produce mouse models of age-related and congenital hearing loss. With mice that express channel rhodopsins in OHCs and SCs, we can excite and reversibly change the mechanical properties of the cochlea with light flashes. Through modelling, based on in vivo and in vitro acoustical, mechanical, and electrical measurements, our understanding of the functional significance of interaction between OHCs and their SC cages can be developed and tested, leading to the detailed understanding necessary to fully exploit the exciting regenerative possibilities now becoming available.
SCs, but not HCs, are interconnected by gap junctions that are thought to play a role in fluid homeostasis in the cochlea and/or intercellular signalling. Gap junctions are mediated by special proteins (connexins). The majority of hereditary hearing disorders, including age-related hearing-loss (ARHL) are associated with defects in, or lack of expression of, connexions 26 (Cx26) and Cx30. Cx26 and Cx30, which interconnect DCs and PCs, have been recently implicated in the transmission of forces within the OC. We wish to discover how they might do this and how a specific Cx30 mutation can rescue hearing loss in a mouse strain with severe ARHL.

Our prime objective is to study interaction between outer hair cells (OHCs) and the supporting cells (SCs) that comprise a restraining, complex, flexible, fluid filled cage that is proposed to optimise exchange and control of energy between OHCs and the cochlear partition, including the basilar membrane (BM). Cochlear HCs die and are not replaced when damaged by exposure to intense sounds, disease, age, and genetic disorders. Recently, however, we have shown that SCs can be converted into HCs at various postnatal stages, but remain immature. HC immaturity is likely due to lack of interaction with surrounding SCs, which is why it is essential to understand this interaction for restoration of hearing. To this end, we will systematically modify and delete the motor protein, prestin, in OHCs and cytoskeletal proteins in SCs in mice and produce mouse models of age-related and congenital hearing loss. With mice that express channel rhodopsins in OHCs and SCs, we can excite and reversibly change the mechanical properties of the cochlea with light flashes. Through modelling, based on in vivo and in vitro acoustical, mechanical, and electrical measurements, our understanding of the functional significance of interaction between OHCs and their SC cages can be developed and tested, leading to the detailed understanding necessary to fully exploit the exciting regenerative possibilities now becoming available.
SCs, but not HCs, are interconnected by gap junctions that are attributed with roles in fluid homeostasis and/or intercellular signalling. Connexins 26 (Cx26) and CX30 gap junctions, which interconnect SCs, have also been implicated in the transmission of forces within the OC. Most hereditary hearing disorders, including age-related hearing-loss (ARHL) are associated with defects in CX26 and CX30 expression. We aim to discover how Cx26 and Cx30 mutations alter cochlear mechanical properties and the Cx30 mutation rescues hearing loss in a mouse strain with severe ARHL.

Research community: We will use novel approaches and techniques to investigate issues, central to the study of hearing, that have previously eluded direct investigation. Outcomes of our research will be of immediate relevance to those who investigate and model the workings of the cochlea and explore ways of repairing and replenishing the sensory epithelium of the organ of Corti. We will provide fundamental information about sensory processing in the cochlea that should contribute to the training and knowledge base of neuroscientists, medical practitioners and bioengineers. Techniques and approaches that will be employed in the proposed research programme have potential utility in research fields outside neuroscience. We envisage application for the combination of channel rhodopsin expression in motile cells and AFM in fields including bioengineering, stem cell research, microvascular control, cardiovascular research, and renal physiology. The output of our research should be of immediate use to computational and analytical modellers of cochlear mechanics.
Public Health Sector: According to Action on Hearing Loss (RNID), more than 10 million people in the UK have some form of hearing loss. It has high personal and social costs and it is expensive, costing our economy billions. Hearing research at £1.34 per person affected is massively underfunded compared with cardiovascular and vision research at £49.74 and £14.50 respectively (Action on Hearing Loss, 2013). One outcome of underfunding is lack of basic understanding of the peripheral auditory system that is necessary to produce effective and radical new treatments. A main objective of our proposed research is to contribute towards filling this knowledge gap. A potential outcome of immediate to medium-term application in public health is a validated model of hearing impairment that could be used to investigate the behaviour of different types of cochlear implant and prostheses in terms of their action at the cochlear level. This might inspire novel strategies for signal processing or implant stimulation, which exploit particular features of the impaired cochlea. A better model of the hearing-impaired cochlea would drive the development of hearing loss simulation algorithms and hence enable better and more reliable testing of new signal processing strategies for hearing prostheses.
St. Jude Children's Research Hospital has established an ototoxicity programme to monitor and study cisplatin induced hearing loss in >50% of paediatric cancer patients who are undergoing chemotherapy. Our proposed study will have direct impact on how we treat such patients to protect and restore their hearing.
Commercial & Applied Research Sector: This proposal will build on AFM-based techniques previously developed by Dr. Gavara to measure tissue impedance at acoustic frequencies. As described in Pathways to Impact, novel protocols, which will be written for a commercial system (Bruker) for data acquisition and analysis, have potential application in the wider research fields of material science and bioengineering. Once our protocol is successfully implemented, we will establish a dialogue with Bruker and establish ways to incorporate it into their existing software for instrument control.
Training of Skilled Individuals: Those who work on the project, including research and project students and clinicians, will become highly skilled at working on cross-disciplinary problems of importance to the research, public health and commercial sectors. They will gain key physiological, molecular biological and modelling skills and experience in using and understanding novel equipment including interferometers, state-of-the-art AFM, imaging, and electrophysiological equipment. They will be able to apply these skills to a unique set of technically challenging in vivo, in situ, and in vitro measurements of the cochlea and its components and more widely in the fields of bioscience, mater