Transcription factors for promoting sensory hair cell differentiation

Partial or complete deafness is a condition that will affect most people in time. The majority of deafness results from loss or damage of the sensory receptor cells in the inner ear, called hair cells. Once lost or damaged, mammals including humans are not able to regenerate these hair cells. There is much interest in finding ways to stimulate the inner ear to regenerate new hair cells (a process that occurs naturally in other animals such as fish and birds). The key to this is to take lessons from our understanding of the mechanisms that generate hair cells in the developing embryo. These lessons might then be applied to therapy. For example, gene therapy may be used to trigger adult hair cell regeneration directly, or cell therapy may be used in which hair cell formation is triggered in stem cells that could then be transplanted into the inner ear.

Our research aims at understanding mechanisms of hair cell formation in normal development. Already, much research has been aimed at a gene called Atoh1, which produces a 'master regulator' factor that triggers hair cell formation during development; it has even been shown in animal models that Atoh1 gene therapy can induce partial regeneration of hair cells, at least in young animals. It is widely believed that Atoh1's ability to promote the generation of hair cells is dependent on other unknown 'cofactors' within the cells of the developing ear, and that lack of such cofactors restrict its function in the adult ear. Our previous work has led to the identification of promising candidates for these cofactors. We propose to show that these cofactors are required for Atoh1's function, and then to apply this information to stem cell culture systems to determine if they can help Atoh1 to trigger hair cell formation efficiently.

Our research will be conducted in mouse stem cells and in zebrafish as they are convenient, relevant and well characterised models for these kinds of analysis. If successful, our findings will be applicable to human stem cell studies, leading to potential new therapeutic routes. There are also more immediate benefits to establishing protocols for hair cell formation in stem cells. Stem cells provide a source of specialised cell types in the lab that can be used for modelling disease and for testing the effects of drugs. For instance, skin cells could be taken from patients suffering from hearing diseases and lab protocols used to convert them into hair cells in a dish. These could then be studied to determine the nature of the disease and could be used to screen for therapeutically useful drugs. Overall, our research has the potential to lead to discoveries that facilitate gene therapy, stem cell therapy, and the production of lab models for disease and drug screening.

Sensorineural deafness most often results from loss of or damage to hair cells in the inner ear. Understanding the developmental biology of hair cell formation may provide strategies leading to potential therapeutic avenues to tackle deafness through: (1) gene therapy - to reactivate hair cell development in the adult inner ear; (2) stem cell therapy - to promote differentiation of stem cells for transplantation; (3) stem cell models for in vitro investigation - to use stem cells as a source of hair cells for drug screening and disease modelling. The bHLH transcription factor Atoh1 triggers hair cell formation during development and is a key factor in all the strategies outlined above. However, its function appears to be limited by as yet unknown cofactors. Our previous work on Drosophila auditory and proprioceptive sensory cells (which are homologous to vertebrate hair cells) has led to the implication of Gfi1 and SUMO as potential Atoh1 cofactors in vertebrates. Here we propose to garner critical preliminary information on the role of these factors in promoting Atoh1 function in a mouse ESC model and in zebrafish development. We shall then apply this knowledge to the development of stem cell models of hair cell formation. Firstly we shall investigate whether forced expression of Atoh1 and cofactors can promote transcriptional programming of mouse ESCs to induce hair cell differentiation. Using mouse ESCs (rather than human) allows us to take advantage of new genetic tools that we have previously developed for forced expression. We shall then apply the strategy to mouse embryonic fibroblasts. Finally, if forced expression alone is not sufficient, we shall determine whether it can be used to improve current protocols for generating hair cell-like cells from ESCs by directed differentiation using extrinsic factors, which currently have many limitations.

The impact of this research will mainly be technological in the short term, with many direct beneficiaries of the information, tools and protocols that we develop. In the longer term, this should lead to enhanced quality of life for people with hearing defects.

1. Academics in stem cell research, developmental biology, and hearing research. The knowledge and tools we will generate will be of immediate benefit. Also applicable to neurogenesis. A major impact will be on 3Rs, as an ES cell model of hair cell formation will reduce the need for animal experiments.

2. Groups working in technology development: the University of Edinburgh has a strong base for developing new technologies for high-throughput drug discovery platforms (Manfred Auer, Mark Bradley), and we will be in a good position to apply our stem cell models of hair cell formation directly into these platforms. This provides a good opportunity to extend the impact of our research beyond the goals proposed.

3. Biotechnology industry: Biotech companies will benefit from our ESC tools and protocols in high-throughput screens to identify small molecules that can be used to manipulate ES cell differentiation for hair cell formation and neurogenesis. Furthermore, the biotech sector will benefit from methods to generate ES-derived or iPS-derived hair cells that can feed into disease modelling and drug testing programmes.

4. Patient groups: the academic and commercial activities described above will contribute to development of new treatments for hair cell-related diseases including deafness. It may also help deliver ES-derived or iPS-derived cells for regenerative strategies.

5. The wider public: since deafness is so pervasive, much of the wider public may benefit directly from the above. In the long term the impact will be enhanced quality of life for many. More generally, there is considerable interest in news on strategies to combat deafness, and particularly in stem cell/regenerative approaches (witness several recent BBC website news articles (Aug/Sept 2012) that generated high page hit rates). There will also be public interest from the point of view of reducing reliance on animal research. Our research will contribute to ongoing outreach activities particularly those supported by impressive outreach programmes at the Centre for Regenerative Medicine, including presentations as science festivals and science centres, adult education courses, web-based education tools, and public workshops and debates.

6. UK Regenerative Medicine Platform. MRC is among the research councils investing £25M in this initiative to take basic science through to clinical impact. Of course, such initiatives rely entirely on the flow of discoveries grounded in advances in the basic science underlying regeneration, and it is vital that MRC continue to support this in parallel with the advancement of direct clinical impact. Our proposed research may allow us to apply for translational funds through this initiative in the future.