Identifying Compounds That May Stimulate Nerve Regeneration in the Inner Ear

April, 2012 — In 2007, Donna Whitlon, PhD, Department of Otolaryngology at Northwestern University in Chicago was the recipient of an AHRF Birtman Grant. The Birtman Grant was for $75,000 covering a year of research, and was used to hire a postdoctoral student, Dr. Mihaela Lie to help her investigate the effects of chemical compounds on the growth of spiral ganglion nerve fibers.
Spiral ganglion neurons (SGNs) make contact with the sensory cells of the inner ear- the hair cells, and convey their signals to the brain where they are interpreted as sound. When hair cells die, due to noise trauma or chemical toxicity, the SGNs connected to them retract and the connections are lost, leading to hearing loss. Encouraging SGNs to regenerate and make new contacts with cochlear implants or with future regenerated hair cells is thought to be an important step in restoring hearing loss.

Dr. Whitlon and Dr. Lie investigated several enzymes to determine their effect on the regrowth of SGNs. She found that inhibiting an enzyme called Rho kinase led to accelerated growth of SGN nerve fibers. Whitlon used a novel technique she developed to culture SGNs taken from mice. Whitlon then applied the Rho kinase inhibitor to the cells. “This was the first time in our field that we have seen that the direct inhibition of an enzyme increased nerve fiber growth of auditory neurons,” she said.

Mouse spiral ganglion neurons. Blue=nuclei of all cell types. Red=neurons.

Since then, Whitlon has used her cell culture technique to prescreen other compounds for effects on cochlear neuron survival and regeneration. “In fields like cancer research, there are vast chemical “libraries” made up of drugs and other pharmacologic agents that are tested for their effects on tumor and other cells,” says Whitlon. “I wanted to take advantage of all those available libraries and use them for hearing drug discovery.”

Using the spiral ganglion cell culture technique that she developed, Whitlon took advantage of a technique that lets her screen hundreds of chemicals to determine their effects on SGN growth. Different groups of SGN cell cultures are exposed to different chemical compounds. The cultures are stained, automatically imaged and the nerve fibers are measured with a specialized computer program. Automated imaging and measurements let Whitlon measure nerve fiber growth under hundreds of chemical conditions and zero in on potential compounds that have promise. In effect, it lets her perform hundreds of experiments at once, saving vast amounts of labor and time and animals.
“The AHRF grant helped me investigate an idea that hadn’t been looked at before in the auditory system,” Whitlon says. “Since then, I have been able to leverage the results of my AHRF grant to secure larger federal funding and take the idea further.”

Whitlon hopes that her rapid screening of chemical compounds will help identify a pharmacological intervention that can be used to regrow SGNs in the inner ear and serve as a fundamental building block of hearing restoration.