Physiological and molecular basis of stereociliary bundle growth and maintenance by the Eps8-like family genes and their interacting partners.

Sound is detected by extremely sensitive sensory cells named hair cells that are located in the inner ear. Their name derives from the hair-like elements (stereocilia) that project from their apical surface. In order for the inner ear to analyse the information carried by sound waves (e.g. frequency, intensity and timing) it has to employ a combination of intricate and interrelated mechanisms. Sound enters the ear canal and produces minute vibrations of the hair cell stereocilia. This initiates the conversion of sound into an electrical signal generated by the movement of charged ions through the opening of mechanically gated channels present in the stereocilia; a process known as mechano-electrical transduction. It is this electrical signal that is sent to the brain via specialized nerve fibres, allowing us to perceive different forms of sound such as speech and music and warnings of danger.

It is well established that hair cell stereocilia perform one of most important tasks in sound perception, which is paralleled by their complex structure and the fact that their formation and function require the interplay of several hundred molecules. The length of each stereocilium is scaled precisely to form bundles of 2-3 row of stereocilia (hair bundle) with a staircase-like architecture, similar to the pipes on a church organ. What it is remarkable is that the height of stereocilia within a row is similar not only within a single hair bundle but also between bundles on adjacent hair cells, indicating that stereociliar length is very precisely controlled and tightly coordinated in these sensory cells. Mutations in the molecules that control this mechanism lead to different degrees of hearing loss including profound deafness.

Previous work from our group has shown that Eps8 is an essential molecule present in the stereocilia of mouse auditory hair cells. We have shown that the mechanically sensitive stereociliary bundles of mice lacking Eps8 do not fully grow, causing them to be deaf. More recently, colleagues have also shown that a mutation in the human EPS8 causes profound deafness in people. Despite the essential role of Eps8 in sound detection, we still do not understand the mechanisms used by Esp8 to regulate stereocilia growth, which is crucial for normal hearing. This knowledge is essential to develop suitable diagnostic protocols and therapies.

We will address this important aspect of human biology by performing a series of experiments designed to identify the mechanisms used by Eps8 to regulate the formation and function of the stereociliary bundle. This information will be used to develop a strategy to repair Eps8-induced deafness by the in vivo delivery into the ear of normal molecules with the aim to restore hearing function.

The proposed project is very challenging because it requires the combination of several complex techniques, from the molecular and cellular to genetic, which are difficult to find all within the same research institution. Therefore, to achieve our important goal of restoring hearing, we have created a unique combination of expertise from PIs at the University of Sheffield and the MRC Harwell Institute (Oxford).

Mechanotransduction of sound into an electrical signal depends upon mechanosensitive channels in the stereociliary bundles that project from the apical surface of the auditory sensory hair cells. Individual stereocilia within each bundle contain a core of tightly packed actin filaments, whose length and width are tightly regulated during development to create the characteristic staircase-like structure of each bundle. The additional complication from a biological perspective is that the height of stereocilia within each row is similar, not only within a single hair bundle but also between the bundles of adjacent hair cells, but changes along the length of the cochlea, indicating a sophisticated level of control over their growth and maintenance. Currently, we still have a very limited understanding on how stereociliary bundles are established at the molecular level.

The overall aims of this project are to identify the Eps8-related interactome in hair cells, how it operates and how this knowledge can be used to repair the underlying deafness phenotype. The proposed objectives are: 1) to investigate the molecular and cellular mechanisms leading to hearing loss in mice deficient for known interacting partners of Eps8-like proteins; 2) to identify the Eps8 family interactome in mammalian cochlear hair cells; 3) to restore hearing function in deaf mice using gene replacement therapy. We also have preliminary data that suggests the involvement of Eps8 in the ototoxic side effects of aminoglycosides, the most commonly used antibiotics to treat serious bacterial infections. Therefore, the final objective is: 4) to investigate the cellular and molecular mechanisms linking Eps8 to aminoglycoside-induced ototoxicity. Addressing these complex aims will require the complementary expertise in functional and genetic approaches used by the hearing groups at the University of Sheffield and MRC Harwell Institute.

Academic Impact:
The proposed work will provide an in depth understanding of the molecular and cellular mechanisms underlying Eps8-dependent stereociliary bundle growth in the mammalian cochlea. Therefore, this project will be of great interest not only to the auditory scientists but also to a large proportion of PIs interested in cellular morphogenesis, intracellular signalling and protein-protein interaction, mechanobiology and computational biology.
We will continue to disseminate our results in high-impact peer-reviewed publications and conference presentations. We also expect that the results produced will lead to invitations to give talks and seminars at international institutions, which will be undertaken by the PI, Co-PIs and PDRAs. In addition we are proposing to organize a small international symposium at the University of Sheffield that will bring together PIs from key groups interested in gene-related deafness and gene therapy.

Societal and economic Impact:
Gene-related forms of deafness are a common sensory disorder in humans. People affected by hearing loss struggle to communicate with the public and even their own families, which ultimately lead to their social isolation and cognitive impairment. The proposed project will directly provide new insights into the causes of deafness, which will lead to the development of a gene-based therapy in order to treat the disease. We will also inform the general public via several routes: every year the University of Sheffield organizes together with Action on Hearing Loss an open meeting to discuss in lay terms our scientific work on hearing loss. We also present our findings in lay terms to the public using the several activities organized at Sheffield, such as Discovery night and Science Week, as well as our outreach activities at the MRC Harwell Institute. MRC Harwell carries out regular and diverse outreach and communication activities to schools and the wider public, including the MRC Festival held every two years where we open our doors to the public, engage them in our translational research and the impact on therapeutics and health care.

Post-doctoral scientists:
The proposed work combines a wide-range of genetic, physiological and molecular biological techniques using in vitro and in vivo models, providing excellent training for young PDRAs. Postdocs will receive training not only via the wide range of expertise present in the hearing group at Sheffield, but also via the mouse-genetic training capabilities at the MRC Harwell. The two hearing research groups (Sheffield and MRC Harwell) work very closely with joint projects and regular lab meetings, and we expect this to lead to a considerable interactions and exchange of expertise for the PDRAs employed under the grant.

Undergraduates:
This research will also have a great impact on undergraduates, with the aim to recruit the next generation of scientists. The number of hearing research PIs in the UK has been steadily decreasing in recent years, resulting in the UK falling behind compared to other European countries. The University of Sheffield runs a 3rd year module in Sensory Neuroscience, which normally attracts high-level students. This module is fully research-driven, and as such we share the latest findings from our projects to the students with the additional aim to attract some of them into the hearing field. Since its initiation 2 years ago, we have already attracted two first-class students. Moreover, at the MRC Harwell Institute we continue to attract first-class undergraduates to undertake research into the genetic causes of sensorineural hearing loss. The proposed work will underpin the continued interest and recruitment of undergraduates to our graduate studies programme with the aim of continuing to maintain capacity and UK leadership in this critical area of neuroscience research.