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Birds never go deaf, but why do humans?

By studying how birds regrow damaged inner ear cells to fix hearing loss, scientists hope to learn how to cure deafness in humans.

For decades now, scientists studying hearing loss have been trying to figure out how to make humans more like birds.

When a bird loses its hearing due to loud noises or trauma, the damaged sensory hair cells in the inner ear essential for hearing simply grow back. Any hearing loss is quickly restored. Not so for humans. As I explained in an article in the new issue of Stanford Medicine magazine:

If these hair cells are destroyed in humans, they lie around like flattened wheat after a rainstorm, permanently powerless to help us hear. The cells die for various reasons, including injuries, aging and loud noises. And no new cells replace them. But in birds, amphibians and reptiles these cells grow back. So birds, unlike humans, never go permanently deaf.

My article tells the story of how Stefan Heller, PhD, a professor of otolaryngology who has been studying these inner ear cells for more than a decade, is following a new line of research to understand the process by which individual cells, rather than entire groups of cells, regenerate into functioning hair cells. As Heller said:

The goal is to follow the trail of these regenerating cells in birds. Then we have to figure out why this trail is not working in mammals. What are the missing parts? Maybe we will discover one critical trigger and we can cleverly fix this trigger in mammals.

In other words, the devil is in the details. The good news is there is now new technology available, which also originated at Stanford, to help uncover the mysterious steps of each of these essential cells. 

"Technology has really turned the tables for us," Heller told me. As my story explains, about six years ago, Heller’s lab adopted a technique, called single cell transcriptome analysis, invented in part in the lab of Stephen Quake, PhD, a professor of bioengineering and of applied physics. The process uses high-tech tools to decode the genetic instructions that enable cells to differentiate into other cells. And it's use has become increasingly popular among scientists.

But even these tools have their limitations. Heller explained further:

We are analyzing all the genes expressed in individual cells during regeneration. The cells are mixed up together and when we isolate the cells, we lose critical information, such as their precise location in the organ. Currently there are no informatics tools that allow us to reconstruct that so we’re creating our own.

Since my story came out, though, there have been some important developments: Heller published a new study in Cell Reports that introduces one such informatics tool. The "CellTrails" software, developed in Heller’s lab primarily by Daniel Ellwanger, PhD, a postdoctoral scholar in bioinformatics, provides a new method of making sense out of the single cell transcriptomic data. The software's predications were validated by postdoctoral fellow Mirko Scheibinger, PhD.

“Single cell transcriptome analysis and reconstruction of spatial and temporal relationships among cells is an exploding new technology," Heller told me. "A lot of labs are faced with the challenge of analyzing the data from single cells. This [Cell Reports] study is a rather extensive study that goes beyond the inner ear field because it provides a new way to analyze single cell transcriptomic data.” Access to the software is posted on his lab's website, he said, because "I want the world to use it."

For Heller, his quest remains to find solutions, cures, remedies, for hearing loss: "In this field, there is no magical cure. It might not be a perfect fix. It might not fully cure deafness. But it may turn a hard-of-hearing person into a better-hearing person. That is the first step we have to take." 

Photo by Atlantios

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