Over 20 million Americans have significant hearing deficits that result from loss of sensory hair cells, the acoustic receptors in the ear. Those losses are usually permanent in humans, but comparable losses in other species are followed by dramatic recovery of function. Past work in our lab and others has shown that sharks, bony fish, amphibians, and birds can regenerate damaged auditory and vestibular detector cells in a matter of weeks, leading to dramatic functional recovery from the kinds of deafness and balance disorders that are permanent when they occur in humans.
A decade ago, we discovered that sensory hair cells also can be regenerated in the balance organs from the ears of mammals, including those from humans, but in mammalian tissues those processes occur at very low rates. Of course, we would have been happy to find robust regenerative responses in human tissues and in those from other mammals, but the discovery of even the low rates of regeneration observed showed for the first time that the machinery for biological self-repair existed and could operate in the mature human ear. Our work has extended from those first discoveries in mammalian tissue and we hope that by contributing answers about regenerative processes that work may lead to the development of effective treatments for conditions of neuronal and sensory cell loss.
A primary aim of our recent work was the discovery of pharmacological treatments that might increase the level of regenerative cell production in mammals. Working with pure sensory epithelia from young rodents we have identified a number of the intracellular and extracellular signals that can trigger the cell replacement events that are the first stages of the regenerative processes in non-mammalian ears. The powerful actions of the triggers for inner ear cell production are observed after a sequential two-drug treatment. When balance epithelia from young rodents are exposed to forskolin for just 15 minutes and then cultured with the neuregulin growth factor, rhGGF2 for 72 hours the majority of the supporting cells are induced to enter S-phase, replicate their DNA, and begin the cycle of cell division. Under control conditions only a small fraction of the cells in the hair cell epithelia from newborn rodents will enter S-phase and divide, but other investigations in our lab showed that those cells actually have high capacities for proliferation when they are stimulated by the addition of rhGGF2 and other appropriate mitogenic compounds. That high degree of response to mitogenic stimulation progressively declines during the first two weeks of postnatal life, so that the epithelia eventually become mitotically quiescent and remain so even when treated with potent mitogens. The progressive postnatal decline in mitotic responsiveness appears to explain why mammalian ears are uniquely vulnerability of the to permanent structural and functional deficits that arise through cell loss.
The discovery of the postnatal response decline in mammalian sensory epithelia also provides a potential opportunity for gene expression profiling that is aimed at the discovery of potential targets for drugs that could reverse the mitotic response declines that leave our ears at risk of permanent damage. We have entered into two collaborations that have and are utilizing gene chip (oligonucleotide microarray) technology to simultaneously measure and compare the expression of thousands of genes in the sensory epithelia from embryonic and neonatal rodent ears as those ears pass through the developmental stages when they lose most of the capacity for effective cell replacement. By careful analysis of the large amounts of data that can be rapidly gained in these experiments we have been able to identify promising targets and genes that have expression patterns suggesting their importance for the normal development and functions of the inner ear.
In addition to the projects outlined above, our work is exploring the potential for stem cells to contribute to our ability to repair damaged hearing and balance organs. We have also discovered growth factor treatments that can protect hair cells from drugs that normally cause their death. We have generated cells that secrete antibodies specific to an antigen in all vertebrate hair cells. We have identified certain fluorescent vital dyes that can readily permeate hair cells and other sensory cells and will remain in those cells for many weeks as bright and useful labels after a single injection.
Cell and organ culture, immunocytochemistry, a range of methods in molecular biology, confocal and time-lapse microscopy, laser microbeam cell surgery, scanning and transmission electron microscopy, intracellular injections, and microsurgical methods are used in our in projects involving species from sharks through humans. It is our hope that the results of our research will contribute to the quality of life of many people who have sensorineural hearing losses and balance disorders that are currently considered irreversible. In addition, studies of the relatively simple and defined neurosensory structures of the ear may lead to information that could be useful in attempts to regenerate other elements of the nervous system.