Glia as regulators of auditory nerve function

Mammals have an extraordinary ability to analyse the sounds within their environment and this can provide them with important advantages for survival. Humans hearing appears best-tuned to the range of sound frequencies that are used for speech-based communication, and this is also relevant for our appreciation of music. The sensory organ of hearing, the "cochlea", analyses incoming sound waves and sends electrically coded signals to the brain via groups of excitable cells within the auditory nerve. All the necessary information we require to understand the pitch, loudness, site of origin of sounds etc, are carried within the fine detail of the electrical code that the nerve passes to the brain. The nerve cells are able to work continually, at very high rates, and for many years without being replaced, because the environment they inhabit is kept stable by non-sensory cells called "glia". There is evidence from elsewhere in the nervous system that there is continual communication between nerve cells and their attendant glia, and that this can inform glia to be reactive to changes in nerve activity. The details of how this happens in the cochlea are unknown. Failures of this nerve-glia communication are suggested to cause death of nerve cells and conditions such as chronic pain. Glia are also responsible for coating the nerve cells with an insulating layer called "myelin". Myelin acts like the plastic coating on household wires, to improve electrical conductivity and to minimise energy losses as signals are carried over long distances. It is thought that the way in which myelin is laid down during development acts as an important cue for the maturation of nerve function. This has important implications for our understanding of the onset of sensory function in humans, and is particularly relevant to how hearing develops. This project is aimed towards a better understanding of glial function in the cochlea, and how glia preserve essential signalling in the auditory nerve. The data from this study would help explain some of the complexities of normal hearing, and may identify potential targets for therapies aimed at enhancing nerve cell survival in the inner ear. In addition the project would assist in the future design of devices such as cochlear implants. In some people who have lost their hearing the glia ensure the survival of some of the nerve cells in the deafened ear, even for years after deafness first occurs. This survival means the nerves can be electrically stimulated via a cochlear implant, providing the profoundly deaf with some hearing. Glia are clearly important in both the hearing and deaf ear, but it is not currently obvious how they carry out these essential roles. The findings of the study would not be specific to hearing though, as many of the mechanisms to be studied are common throughout the nervous system. A more comprehensive picture of how glial cells function would be of value to other scientists and clinicians studying the nervous system, including the other sense organs and the brain.

This project aims to provide a comprehensive mechanistic description of the interactions between glial cell subtypes and primary afferent neurons in the cochlea. This would provide a better understanding of the roles of glia during the development and mature function of the auditory nerve. We shall study the spatio-temporal patterns of expression and functional properties of ligand-gated and voltage-gated ion channels, connexins, and other signalling molecules, to investigate the importance of K+ buffering and intercellular purinergic signalling. In particular we wish to define the cellular roles of K+ channels, gap junctional intercellular communication, non-junctional hemi-channels, and P2X and P2Y receptors in normal glial function. We have developed cochlear slice preparations to study these mechanisms in developing or functionally mature cells in situ, or alternatively where complex anatomy normally restricts access to glia and neurons simultaneously we can employ simplified culture models. We shall derive novel cultures of purified glia in which we can study the function of wild-type and mutant channels. Heterologous protein expression assays are available for studying membrane proteins in isolation, and for testing antibody and toxin specificity. We propose to use a combination of whole-cell patch recordings in conjunction with ion channel-specific pharmacology, single channel recordings, imaging experiments (dye injection, dye uptake assays, calcium imaging), electron microscopy, and confocal immunofluorescence. The data will feed models of cochlear function, and will be exploitable for the development of ion channel-targeted therapeutics and optimisation of implantable devices such as cochlear implants.

Who will benefit?
1. Industrial partners
2. Clinicians
3. Hearing-impaired patients
4. Charities
5. The wider public

How will they benefit?
There are various aspects within the project that have the potential to be exploited by the pharmaceutical industry, but also by the rapidly-expanding industry producing implantable prostheses for auditory impairments. Of particular relevance is the growing field of therapies designed to influence the activity of the nervous system via an action on ion channels ("electroceuticals"). The project will characterise the mechanisms that underlie the homeostasis of cochlear neuronal signalling. These findings will identify novel ion channel and receptor targets for drug-based therapies. These therapies would be of interest not just to companies interested in sensory impairment (eg retinal neuropathies, chronic pain syndromes), but also those developing therapies for motor impairments brought on by glial-associated neuropathies (eg multiple sclerosis).

The project will provide new insight into the control of excitability of afferent neurons in the ear. This information will enable cochlear implant companies to refine prosthesis design and to optimise stimulus paradigms for improved patient outcomes. The project will provide electrical and image data for these purposes to shed new light on the basic function of the auditory nerve. In particular we would provide new mechanistic description of the biophysical properties of the peripheral extent of spiral ganglion neuron fibres, which are normally stimulated by cochlear implants. Currently the electrodes for these devices operate via quite simple paradigms of wide-field stimuli, and require significant technical refinement to help improve speech appreciation etc.

The data will provide new insight into connexins in the inner ear. A large proportion of genetic hearing loss is caused by mutations in connexins, and there is consequently a broad clinical interest in developing gene therapy or pharmaceutical approaches to correct the inherited errors. The project will provide a mechanistic description of the role of Connexin29 in normal hearing, and provide a first model for the cellular pathogenesis caused by mutations of the Connexin29 gene. This will be beneficial clinically, by enabling doctors and health workers to inform patients better about the nature of their sensory impairments. We are ideally positioned for regular communication with health workers in the adjoining Royal National Throat Nose & Ear hospital. Similarly, the findings of the project will have relevance for deafness charities and their clients, particularly people using cochlear implants. The data generated will be used by charities we work with to produce informative content for leaflets, their website, and an educational roadshow.

The project will generate significant image data in an accessible form for the public understanding of science arena, and artistic ventures such as film-making and installations. The mechanistic nature of the processes to be investigated will readily lend themselves to the production of educational materials for schools.