News that Boston scientists achieved what was once deemed impossible—inducing regrowth of the vital but perishable sensory hair cells in human inner ear tissue—suggests that the door has partially opened to reversing hearing loss in millions of people.
Humans are born with roughly 15 000 hair cells in each ear that are exquisitely sensitive and arranged linearly by the pitch they respond to, somewhat like piano keys, enabling them to pass along complex information about a sound’s frequency, timing, and intensity. But because they are built to sway and vibrate, they are also easily harmed, and, in adult mammals, once dead they do not regenerate.
Exposure to loud noise is a major cause of hair cell death, affecting the hearing of some 40 million American adults, according to the Centers for Disease Control and Prevention. Additional threats include advancing age and certain antibiotics and chemotherapy drugs. Given an aging, cancer-prone population that, in some cases, has spent a lifetime listening to music full-blast with headphones or at earsplitting rock concerts, the number of people with this type of hearing loss seems destined to climb.
Over 4 decades, researchers have been attempting to explain why hair cells don’t regenerate in mammals, and exploring how one might use this understanding to restore them. The team of Boston researchers has taken a leap forward in this effort.
The breakthrough came when Harvard University stem cell biologist Albert Edge, PhD, and his colleagues built on their previous work showing that certain support cells in the organ of Corti, the sound processing center that is located inside the cochlea and houses hair cells, express a marker protein called LGR5 (leucine-rich repeat-containing G protein-coupled receptor 5). This is the same marker expressed in the stem cells of the intestinal epithelium—stem cells that allow the gut lining to completely renew itself every week. Could the LGR5-positive support cells in the inner ear be coaxed to perform some similar feat?
The cells could and did, the team found. Stimulated biochemically, they made new support cells, which then could differentiate into hair cells, but only in disappointingly small numbers. To make progress in restoring hearing, hair cells must be churned out by the thousands.
For their latest study published in February in Cell Reports, the Boston team devised a cocktail of small molecules that would induce development of large colonies of supportive LGR5-positive stem cells in mouse cochlear tissue, along with a second cocktail that could spur them to differentiate into hair cells.
The cocktails included various growth and inhibitory factors that modulate 2 signaling pathways, Wnt and Notch, previously shown to be involved in stem cell proliferation and hair cell differentiation.
Exposure to the 2 molecular cocktails was pivotal in generating copious amounts of new hair cells. The team reported a 2000-fold or more increase in LGR5-positive support cells, resulting in more than 11 500 fully functional hair cells—a huge increase over the 200 previously derived. The process was tested in situ in both mouse and human tissue, although the work did not address the functionality of the new hair cells in vivo.
Previous studies that tested earlier iterations of the drug cocktails in vivo demonstrated improvements in hearing in deaf mice. But the rodents were affected adversely when the drugs were administered systemically.
“So we changed methods, introducing the drugs directly into the middle ear. It’s done readily, effectively, and had no side effects,” Edge said, noting that it points the way to performing it as an office procedure in humans.
Members of the team report that they have further tweaked the cocktails since publication and, using the middle-ear method of instilling the drugs, observed greater hearing improvement in mice. “We are continually refining our therapeutic combination and have seen promising results,” said Will McLean, PhD, formerly of the Harvard-Massachusetts Institute of Technology (MIT), who was a lead author on the most recent publication.
Researchers have already taken steps to bring the technology to the clinic. A Connecticut company called Frequency Therapeutics, founded by Edge’s cosenior authors on the study, Jeffrey Karp, PhD, of Brigham and Women’s Hospital, and Robert Langer, ScD, of the Massachusetts Institute of Technology, has been licensed to use the technology in the United States and raised $32 million in research money. The company aims to begin human testing “within 18 months,” according to Karp.
Edge, meanwhile, has cast his lot with the Dutch company Audion Therapeutics. Partnering with Eli Lilly, they too are anticipating human trials as early as 3 to 5 years from now, according to Edge.
Although the ability to generate quantities of new hair cells is a promising new avenue for hearing loss treatment, auditory specialists note many challenges must be overcome beyond reconstituting hair cells before the hearing-impaired can once again decipher their companions’ words in loud restaurants and lower the television volume at home.
“I don’t want to have a closed mind, but there are hurdles and risks here. First you have to regenerate two different types of hair cells, inner and outer, [and] then the extracellular matrix cells that are active in the tectorial membrane [that transmits sound vibrations to the hair cells],” said Richard A. Chole, MD, PhD, a practicing otolaryngologist and professor at the Washington University School of Medicine, who was not involved in the work.
“You also have to regenerate the endocochlear potentials in the space that surrounds them, and the dendrites that go from the spiral ganglion cells to the hair cells. People with noise-induced or age-related hearing loss most commonly are deficient also in dendrites and ganglia, and importantly, in synapses between dendrites and hair cells.”
Chole notes that although the work is impressive and a first of its kind, he believes the approach will move into the clinic only after many more studies that build on one another.
“These inner structures are so complex. I’m sure 25 years from now a lot of this will be done, but there’s a lot of steps along the way,” he added.
McLean, however, who is now working for Frequency Therapeutics, was undeterred. He said many of the hearing-related neurons remain viable “for years” after hair cells die, adding that when new hair cells are generated, they release molecules known as neutrophins that signal dormant neurons to form new synapses facilitating reconnection with the auditory nerve.
“The job is done for you,” noted Karp.
Karp said a longer-term goal is to apply the method to other parts of human anatomy. Calling the use of small molecules to initiate tissue regeneration a “platform” that can work wherever suitable progenitor cells exist—for example, the kidneys and the skin, whose cells both also harbor the LGR5 biomarker—he said, “The key is activating the stem cells in the first place. If we can do that, the microenvironment has signals to control the rest.”
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Lyon J. Hearing Restoration: A Step Closer?. JAMA. 2017;318(4):319-320. doi:10.1001/jama.2017.5820