Regenerating hair cells in the mammalian inner ear: defining conditions in the vestibular sensory epithelia.

Balance dysfunction results in dizziness and vertigo. It is often disabling and is a significant contributor to falls in the elderly. Dizziness is the most common cause of visits to the GP amongst the elderly population. It is also prevalent amongst those with acquired deafness. One major factor underlying balance dysfunction is the loss of sensory ?hair? cells from the vestibular (balance) system in the inner ear. This study will assess the potential for regenerating hair cells in the vestibular system. Vestibular tissue taken from humans and from young and aged mice will be maintained in culture. A consortium of ENT surgeons across the UK will provide human vestibular tissue, normally discarded during surgery for acoustic neuromas (benign tumours of the auditory nerve). This presents a unique opportunity for direct experimental comparison of inner ear tissues of animals and humans. When hair cells die the supporting cells that surround each one close the lesions. There is some evidence that after loss of hair cells, supporting cells can sometimes convert into hair cells, to replace those lost. This project will compare the capacity to regenerate functional hair cells in these vestibular tissues after insertion of a gene, Atoh1, that is known to induce conversion of supporting cells into hair cells, when delivered to supporting cells by a non-virulent virus engineered to contain it. The project will also assess possibilities for inducing supporting cell conversion by pharmacological manipulation of a biochemical pathway that normally is active during development of the inner ear to regulate the differential fate of precursor cells as hair cells or supporting cells. There is also evidence that the vestibular system of mice contains a small population of stem cells from which hair cells can be generated. The project will examine whether such a population exists in the human vestibular sensory tissues, and if so whether such cells could provide an alternative source of replacement hair cells. Any successful hair cell replacement strategy, however, depends upon the supporting cells that remain after hair cell loss maintaining an environment that will support the survival of functional replacement hair cells. The project therefore will also identify the characteristics of the supporting cell population during recovery after hair cell loss to determine the conditions under which regeneration occurs. Defining the conditions for hair cell regeneration in the vestibular system will lead to clinical treatments for a wide-spread, disabling condition.

Loss of sensory ?hair? cells from the vestibular sensory epithelia is a major cause of balance dysfunction, dizziness and vertigo. Dizziness is the most common reason for visits to the GP by elderly people and balance dysfunction is a significant risk factor for falls in the elderly. Loss of vestibular hair cells and balance dysfunction are also side-effects of some otherwise useful drugs such as aminoglycoside antibiotics. Recent work has shown possibilities for regenerating hair cells in the mammalian inner ear by inducing the direct non-mitotic conversion of supporting cells into hair cells. There is also evidence that a stem cell population, able to produce cells with hair cell like characteristics, exists in mammalian vestibular organs. In this project explant cultures of human vestibular tissues, supported by parallel experiments with tissues from mice of different ages, will be used to determine conditions necessary for regenerating functional hair cells by induced conversion of supporting cells, and whether a stem cell population, that could provide an alternative source of replacement hair cells, exists in the human vestibular system. A consortium of surgeons from 6 UK centres will provide human vestibular tissue obtained at operations for acoustic neuromas. The characteristics of supporting cells in the repaired epithelium following hair cell loss and with hair cell regeneration will be examined to determine whether they retain specialisations conducive to sustain differentiation and survival of functional replacement hair cells. The distribution of ion transport proteins will be explored. Gap junction-mediated intercellular communication pathways will be examined by dye transfer in slice preparations. The capacity of the repaired epithelium of humans and of mice to produce functional replacement hair cells of both vestibular types following viral-mediated transfection with the gene encoding Atoh1 will be determined. The physiological signatures of regenerated hair cells will be identified by patch clamping in slice preparations. The activity of the Notch-Delta lateral inhibition signalling pathway will also be examined in the repaired epithelium to assess whether inhibition of Notch activation provides an alternative means to induce phenotypic conversion of supporting cells. Published procedures for isolating stem-like cells from inner ear epithelia and nurturing their differentiation will also be applied to the human tissues. In addition to assessing the potential for replacing lost hair cells in the vestibular system, the project will provide a ?test bed? for defining conditions and procedures that might encourage hair cell regeneration in the auditory epithelium.