Ear to the Groundby ANNA WILLIAMS | photography by TERESA CRAWFORD
Northwestern investigators listen to the science of hearing loss.
Deep inside the ear sits the cochlea, a small snail-shaped structure that holds the hair cells responsible for human hearing. Named for the hair-like tufts that protrude off their tops, these cells are critical to converting sound waves into neural messages. But unlike skin or other cell types, human hair cells do not regenerate on their own. And once the cells are injured — whether due to loud noises, toxins or normal aging — the damage can lead to lasting hearing loss. Today more than 36 million Americans suffer from hearing loss to some degree.
There has been significant interest in artificially regenerating new hair cells as a treatment for hearing loss. However, the research has long been thwarted by a fundamental roadblock: There are two types of hair cells, inner and outer hair cells, which play complementary roles in hearing and are both essential. Yet the genetic factors required to make one cell type versus the other were completely unknown.
Now, a landmark genetic discovery in the laboratory of Jaime García-Añoveros, PhD, professor of Anesthesiology, Neurology and Physiology, may have finally cleared a path toward novel regenerative treatments. The findings, published in Nature in late 2018, demonstrated that a gene called INSM1 is essential for the development of outer hair cells. The death of these cells is the most common cause of deafness.
Above: A mutant ear lacking the INSM1 gene, in which about half of the outer hair cells have converted (trans-differentiated) into inner hair cells.
“Until now, no one had a clue of how to make an outer hair cell as opposed to an inner hair cell.”
“This discovery is a milestone toward hearing restoration,” says Teerawat Wiwatpanit, ’18 PhD, first author of the paper and a recent alumnus of Feinberg’s Driskill Graduate Program in Life Sciences (DGP). “With the mechanisms we identified, we’ve advanced the knowledge for the development of regenerative approaches to reverse hearing loss.”
The discovery is the result of years of dedicated investigation in the García-Añoveros laboratory and strong collaborations in auditory research across Northwestern. It’s also one with deep scientific roots that stem back to an obscure discovery made in worms more than two decades ago.
Jaime García-Añoveros, PhD, professor of Anesthesiology, Neurology and Physiology, is committed to identifying treatments for hearing loss.
An illustration of the stereocilia, hearing cells in the cochlear duct in the cochlea,
In the late 1990s, García-Añoveros and Anne Duggan, PhD, were doctoral candidates studying nerve cells in the worm Caenorhabditis elegans. While working in the laboratory of Nobel Laureate Martin Chalfie, Duggan molecularly identified for the first time a transcription factor in the worm that is required to make two types of mechanosensory cells (cells that respond to mechanical stimuli, such as touch or vibration) different from each other. Curiously, when the gene was removed, cells of the first type appeared to be transformed into cells of the second type.
While the findings, published in Genes & Development, were intriguing, García-Añoveros and Duggan put the project aside as they went on to complete postdoctoral fellowships at Harvard Medical School and later establish a lab at Northwestern in 2002.
Initially, García-Añoveros and Duggan, now a research assistant professor of Anesthesiology, focused their research on pain neurons. After moving on to study other sensory cells in mammals, they decided to investigate whether the mammalian equivalent to the gene Duggan had first identified in C. elegans — called INSM1 in mammals — might play a similar role in mice. Looking at inner and outer hair cells — two similar types of mechanosensory cells — the scientists found that INSM1 was expressed in outer hair cells but not in inner hair cells.
“This was reminiscent of what Anne had been studying in worms in graduate school 20 years ago. We thought, what if it’s doing the same thing?” García-Añoveros says.
When the scientists mutated INSM1 in the mouse cochlea, they discovered that cells born embryonically as outer hair cells transformed into mature inner hair cells, according to García-Añoveros. “This is remarkable because nematodes [roundworms] have mechanosensory cells but nothing like outer hair cells, which are unique to mammals.”
The team demonstrated that in outer hair cells, INSM1 blocks the expression of a key set of genes specific to inner hair cells. In other words: in order to make an outer hair cell and prevent conversion to an inner hair cell, the INSM1 gene is critical.
“The data was very clear. There was little ambiguity, which rarely happens in science,” explains Wiwatpanit, who recently accepted a faculty position at Thailand’s equivalent of the National Institutes of Health. “When we removed the INSM1 gene from the mice, they had a form of hearing loss characteristic of outer hair cell impairment. And then when we looked into the morphology of the ear, it was absolutely gorgeous. You see a complete transformation of many outer hair cells into inner hair cells.”
The scientists had pinpointed a novel mechanism for how two cells born seemingly identical become two very different cell types.
“This has opened up an entirely new area of research. Until now, no one had a clue of how to make an outer hair cell as opposed to an inner hair cell,” says García-Añoveros, also a member of Northwestern’s Knowles Hearing Center and of the Robert H. Lurie Comprehensive Cancer Center of Northwestern University. “This opens the way to try generating outer hair cells in adults and to developing treatments for most cases of deafness.”
Decades of work by García-Añoveros and Anne Duggan, PhD, led to the recent discovery that the INSM1 gene is critical to the development of outer hair cells.
Teerawat Wiwatpanit, ’18 PhD, is first author of the paper and a recent alumnus of Feinberg’s Driskill Graduate Program in Life Sciences (DGP).
García-Añoveros notes that it was the scientific environment at Northwestern that helped make the discovery possible. “Collaboration is critical,” he says. “When we first moved here, we had very little work on hearing. What allowed us to excel was the ability to team up and learn from investigators here. Northwestern has been a powerhouse in hearing for decades.”
For example, the team has worked closely with collaborator Mary Ann Cheatham, PhD, research professor in the Auditory Physiology Laboratory at Northwestern’s School of Communication, who led the testing of mouse hearing.
Throughout the years, the investigators have also provided important insights in other aspects of hearing. In 2008, García-Añoveros, in collaboration with James Bartles, PhD, professor of Cell and Molecular Biology, published a study in Proceedings of the National Academy of Sciences identifying a novel mechanism of hearing loss. In 2015, the research team discovered a pain pathway in the inner ear that warns of dangerously loud noise, findings spotlighted in Nature Reviews Neuroscience and published in Current Biology. Additionally, in early 2018, Wiwatpanit was the first author of a paper published in The Journal of Neuroscience that found the loss of two lysosomal calcium channels leads to early-onset hearing loss. This discovery suggested the importance of lysosomes in the survival of hair cells.
Now, on the heels of the Nature paper, the scientists are forging ahead to deepen understanding of INSM1 to strengthen the potential for translation to treatment.
“A lot more basic research will have to be done, but we’re already providing tools for people who are trying to develop these treatments,” García-Añoveros says. “We found one key player, and now we want to find all the other players. My goal is to, in the next decade, crack in detail how the two cells become different because that will be an essential tool for regeneration attempts.”
For García-Añoveros and Duggan, the Nature study also underscores an important tenet of biomedical research: Basic science will build the path toward discoveries that eventually improve human health.
“If you focus on something that is a fundamental problem, it’s likely to have repercussions. Although we have now moved on to the medically relevant organ, the cochlea, we benefited from the early work in model systems,” García-Añoveros says. “You have to understand the basic process first. To me, what is fascinating is not why we grow deaf, but how is it that we hear?”
“When we first moved here, we had very little work on hearing. What allowed us to excel was the ability to team up and learn from investigators here. Northwestern has been a powerhouse in hearing for decades.”
Improving Outcomes for Deaf Children
Auditory research at Northwestern spans a wide spectrum from basic science discoveries to clinical interventions. For example, in a recent international collaboration co-led by Nancy Young, MD, ’87 GME, professor of Otolaryngology — Head and Neck Surgery and medical director of Audiology and Cochlear Implant Programs at Ann & Robert H. Lurie Children’s Hospital of Chicago, scientists developed a machine learning algorithm that uses brain scans to predict language ability in deaf children after they receive a cochlear implant. The findings were published in Proceedings of the National Academy of Sciences in early 2018.
“Our study is the first to provide clinicians and caregivers with concrete information about how much language improvement can be expected given the child’s brain development immediately before surgery,” Young says. “The ability to forecast children at risk is the critical first step to improving their outcomes.”
A child can actively participate in sports with a cochlear implant.