Hair Cell Shape Key to Mechanotransduction

Deep within the spiral cavity of the inner ear are intricately complex assemblies called hair bundles that are responsible for hearing. These bundles convert vibrational energy into electrical signals that are translated into the sensation of hearing. Once these hair bundles are lost in mammals—whether by loud noise, toxins, disease, or aging—they do not naturally regenerate.

A study published today in Current Biology provides important clues that may help scientists develop new techniques to regenerate hair cells and reverse hearing loss. The researchers discovered that the development of hair bundles occurs in a kind of feedback loop in which form follows function and function drives form.

In mice, the researchers found that stereocilia—roughly 100 of which are assembled into a hair bundle—widened simultaneously with the onset of mechanotransduction. The stereocilia only elongated to their mature lengths after transduction had been established.

It turns out that form and function are mutually reinforcing. “We’ve been looking at these as separate pathways,” says first author Jocelyn Krey of OHSU. “But in the course of this research, we observed the change in form occurs at the same time as the conversion of mechanical to electrical signals. So we’re seeing these happen together, and feeding each other in a way we hadn’t seen before.”

When the researchers examined mice lacking transduction, they discovered that the animals did not develop the classic staircase-shaped form of mature hair bundles.

Researchers say the study suggests that new techniques to reverse hearing loss should focus on the critical importance of early development. “In the future, with the rapid development of gene editing tools like CRISPR, we will be able to turn on genes at will,” says senior author Peter Barr-Gillespie. “I have no doubt we will be there in 5 or 10 years.”

Image: Electron microscopy images reveal the classic stair-stepping form of mature hair bundles in the inner ear of mice. New research provides important clues about how scientists might develop techniques to regenerate hair cells and reverse hearing loss. Image courtesy of Oregon Health & Science University.