Before the substantial advances of modern medicine, little could be done for treating inner ear disease. Beethoven used his pencil to bypass his ailing middle ear to improve his hearing while at the piano. Van Gogh’s unabated health problems, which likely included a component of Meniere’s disease, may have contributed to his tragic premature death. Had these great men lived today, the combination of modern science, technology and medicine, uncovering important ways to prevent hearing loss, reverse disease (especially in the middle ear) and restore hearing, may have been of use to them. Several options are currently available for treating ears and enhancing hearing, including the following:
• Hearing aids that amplify sound – extremely helpful for persons with conductive hearing loss and welcomed by many who also have a sensorineural component
• Antibiotics, routinely used to treat infections that might otherwise pose risks to the auditory system, and steroids that may help restore hearing lost due to sudden deafness attacks or autoimmune disease
• Surgeries treat many middle ear disorders present from birth and acquired later in life
• Cochlear implants that restore hearing for many who are profoundly or severely deaf.
Though antibiotics and steroids are examples of biological cures that eliminate disease in the middle ear, a biological cure for most types of inner ear disease is still unavailable. The majority of cases of severe and profound deafness, whether hereditary or environmental, involve a loss of hair cells, as well as a disruption of the fragile microenvironment in and around the region where hair cells reside: the organ of Corti. However, technology, science and medicine in the last decade have greatly advanced our understanding of inner ear diseases and are beginning to yield feasible biological strategies for preventing and restoring hearing.
A colossal step forward for many fields of medicine, the recent sequencing of the genome (of humans and other species) has facilitated an increase in our understanding of inner ear development and function at the molecular level. Knowledge of the molecular components that participate in building the ear during its development and facilitate its function throughout life has helped us understand how changes in these building blocks (proteins or genes) lead to inner ear disease.
Prevention Therapy
Steering clear of loud noise and wearing earplugs are still the best prevention against noise-induced hearing loss. In addition, there are several families of molecules that may protect the inner ear against lesions caused by noise and toxic drugs. Most studies on inner ear protection have dealt with protecting hair cells, because these cells are the most common victims of inner ear trauma and their loss is the most common cause of sensorineural hearing loss. Other cell types, such as supporting cells and spiral ganglion neurons, are also important to inner ear function and should also be considered as possible targets for preventing hearing loss.
It just makes sense to direct prevention therapy at reducing hair cell loss. Should this fail, a second tier prevention becomes necessary to enhance survival of the spiral ganglion neurons and other supporting cells to maximize the immediate benefits of a cochlear implant and preserve the nerve for possible future hair cell regeneration therapy.
Several families of molecules can protect the structure and function of the inner ear against trauma. These include, among others, antioxidants and agents called growth factors. Experiments on lab animals show promise for these molecules, and the next challenge is to develop ways to deliver these beneficial molecules to the inner ear. Highly targeted intervention cannot be achieved by taking a pill or getting a shot. Some ways of moving the molecules to the inner ear still under investigation include the use of miniature osmotic pumps and gene transfer. However, animal studies indicate that antioxidants, taken orally, enhance survival of inner ear hair cells. Aspirin is an inexpensive and widely available option that has been shown to protect animals’ inner ears. Another strategy to protect the inner ear is based on blocking the molecules that participate in cell death, which in many cases is an active and well-regulated process. Studies of the pathways and signals that mediate cell death may help us prevent a hair cell from dying.
Cell Replacement Therapies
Should preventive measures fail to save hair cells, the only way to biologically restore hearing is by replacing the lost cells. Replacing lost auditory neurons may also be necessary in some cases; such therapy is not currently available. Several lines of research are aimed at designing clinically feasible ways to repopulate the cochlea with new hair cells, as well as neurons, once original cells are lost. Research avenues that show promise for restoring the population of inner ear hair cells include stem cell therapy and inducing proliferation and manipulating expression of genes that signal hair cell development.
Stem Cell Therapy
Stem cells can become many different types of cells. The laboratory of Harvard University researcher Stefan Heller, Ph.D., recently identified stem cells in the inner ear, and, in another study, found them capable of being integrated in a recipient’s inner ear and developing new hair cells in chick ears. The next major task is to design strategies to integrate stem cells into the inner ears of mature mammals. When this technology is in place, it may be possible to introduce and incorporate a controlled number of new hair cells in the organ of Corti. Based on current knowledge, we expect that these new cells will be able to connect to the brain and contribute to hearing restoration.
In some cases, after hair cells are lost, auditory neurons also die, which makes the use of a cochlear implant impossible. The laboratories of Juichi Ito in Kyoto, Japan, Mats Ulfendahl in Stockholm, Sweden, and others, have been making promising progress in research on generating new neurons from stem cells and incorporating them into the ear. Based on these studies, we believe that new neurons can be generated, placed in their normal site in the cochlea, survive, and connect to hair cells. Establishing connections with the brain may become the next challenge in this set of neural repopulation studies.
Manipulating the Cell Cycle
In the above-mentioned stem cell approaches, the strategy for generating new hair cells involves introducing cells into the cochlea. In contrast, other strategies aim at manipulating the work of genes in cells already present in the ear, in order to make them proliferate or become new hair cells. We expect that some of the newly generated cells would then differentiate into new inner ear hair cells.
Some genes give the order to cells to stop dividing and start becoming specialized cells. In the cochlea, the p27Kip1 gene signals supporting cells to stop dividing. Using mice without the p27Kip1 gene, researchers Ping Chen and Neil Segil in Los Angeles and Hubert Lowenheim in the Rubel lab in Seattle demonstrated that organ of Corti cells continue to reproduce after they normally would have stopped had p27Kip1 been present. As a result, the mice ears contained an excessive number of hair cells. When hair cells were destroyed in these mice ears, new hair cells later developed. For both technical and ethical reasons, we cannot genetically manipulate humans in the way these mice were engineered. However, the principles of the research can be used for developing ways to induce proliferation of hair cells in the inner ear. Such a strategy was already successfully used in another set of experiments in the laboratory of Zheng-Yi Chen in Harvard. Blocking expression of RB1, another gene that regulates proliferation, led to the generation of a large number of new hair cells in developing ears. The major challenges now are to test whether the mature inner ear can respond similarly to disruption of RB1, to determine if disabling the gene might predispose the ear to other pathologies such as cancer, and to verify that the new hair cells can survive and contribute to hearing.
Using Developmental Genes
During embryonic development, hair cells and supporting cells in the cochlea have the same origin. Then, specific genes signal some of the cells to become hair cells. Some 10 years ago, my research, as well as that of others, demonstrated that supporting cells in the chick cochlea can spontaneously become new hair cells, if the original hair cells die. Inspired by what bird ears can do (and mammals cannot), we then formulated a working hypothesis that called for inserting hair cell genes (the developmental genes that kick off the process of hair cell development) into the supporting cells that remain in the deaf ear. We hoped such genetic manipulation would induce a change of identity (type) of the supporting cells and turn them into hair cells.
Technically, it is necessary to physically deliver the hair cell gene into the supporting cells. The most commonly used method for delivering genes into cells is via a modified virus or viral vector. Viral vectors are engineered to become harmless while maintaining their invasive characteristics. The gene needed for the therapy is inserted into the viral genome and it hitches a ride to the target as the virus invades the target cells.
We developed a surgical access that allowed us to deliver the hair cell gene into deafened guinea pig ears using a virus as its “taxi.” This special gene, called Atohl, triggered the growth of new hair cells in the organ of Corti and significantly restored hearing in the deafened guinea pigs. These data prove the principle that developmental genes can be harnessed for hair cell regeneration therapy. Several important aspects of this technology, including safety, reliability and efficiency issues, need to be resolved or improved before clinical applications can be planned.
Hereditary Disease
Identifying and characterizing genes involved in hereditary inner ear disease has enabled us to provide more accurate diagnostic and prognostic information. Now we need to develop strategies for using this knowledge for a cure. We know how to generate new cells but that is not enough. The genetic mutation that caused the loss of the original cells will remain a problem. Using stem cells to replace the cells affected by the mutation may be helpful because the stem cells are taken from a source that ensures absence of the mutation. Still, in many cases the primary effect of the mutation is actually in the supporting cell (or some other cell) and the loss of hair cells is secondary. Thus, any practical therapy for hereditary inner ear disease will require that a normal (wild-type) gene producing the normal protein be inserted into the ear. Experiments in the laboratories of Sally Camper in Michigan and Richard Smith in Iowa have proven that introducing a normal gene can prevent inner ear disease, and are paving the way for the use of this technology for treating hereditary disease in humans.
Scientists are designing new strategies for prevention and treatment of inner ear disease. The rapid increase in understanding of the molecular and genetic basis for cell division, differentiation, death and regeneration, along with the technological advances facilitating genetic manipulation and gene transfer, will soon enable us to perform these novel therapeutic interventions. Combi-nation therapies will also likely be developed. For instance, cochlear implants will be combined with neuronal or hair cell regeneration to enhance the function of the implant, resulting in better hearing. Some of the new technologies will have applications for the vestibular (balance) system as well, because hair cell loss is the major cause of balance problems for which no treatment is now available. All this is on the horizon, awaiting further improvement of the technological tools to ensure that these methods are efficient, reliable and safe.
Yehoash Raphael, Ph.D., attended Tel Aviv University, Israel, and received his Ph.D. from the Department of Embryology & Teratology at Tel Aviv University Medical School. After a postdoctoral training fellowship, Dr. Raphael joined the faculty of the Kresge Hearing Research Institute at the University of Michigan in Ann Arbor in 1990 and is currently the director of the otopathology laboratory. When not in the lab, Dr. Raphael dedicates his time to his family and his main hobbies, music and stereophile audio. Dr. Raphael acknowledges B. and A. Hirschfield, the Center for Hearing Disorders, General Motors, the United Auto Workers Union and the National Institute on Deafness and Other Communication Disorders for the support of his research.
