Hearing Development

A Balancing Act Before the Onset of Hearing

By Sonja J. Pyott, Ph.D.

Our ability to hear relies on the proper connections between the sensory hair cells in the inner ear and the brain. Activity of the sensory hair cells (red) and these connections ( green) before hearing begins is essential for the proper development of hearing. The research conducted by Sonja J. Pyott, Ph.D., and colleagues investigated the mechanisms that regulate this activity.

Our ability to hear relies on the proper connections between the sensory hair cells in the inner ear and the brain. Activity of the sensory hair cells (red) and these connections ( green) before hearing begins is essential for the proper development of hearing. The research conducted by Sonja J. Pyott, Ph.D., and colleagues investigated the mechanisms that regulate this activity.

The development of the auditory system begins in the womb and culminates in a newborn’s ability to hear upon entering the world. While the age at which hearing begins varies across mammals, the sensory structures of the inner ears are active before the onset of hearing. This activity instructs the maturation of the neural connections between the inner ear and brain, an essential component of the proper development of hearing. However, we still know very little about the mechanisms regulating the activity of these sensory structures and their neural connections, specifically during the critical period just before the onset of hearing.

In our paper, “mGluR1 enhances efferent inhibition of inner hair cells in the developing rat cochlea,” soon to be published in an upcoming issue of The Journal of Physiology, we investigate the role of glutamate, a neurotransmitter, in regulating activity of the sensory structures and their connections in the inner ear before the start of hearing.

Neurotransmitters assist in the communication between neurons and are typically classified as either excitatory or inhibitory based on their action. Excitatory action results in stimulation; inhibitory action assists in the calming of the brain. Our research found that although glutamate typically excites activity, it also elicits inhibitory activity. This dual role for glutamate occurs because it activates two distinct classes of glutamate receptors: ionotropic glutamate receptors (iGluRs) and metabotropic glutamate receptors (mGluRs).

Importantly, this dual activation balances excitatory and inhibitory activity of the sensory structures, a balance of which is likely important in the final refinement of the neural connections between the inner ear and brain prior to the onset of hearing.

As part of future research, we will further investigate the role of mGluRs, one the distinct classes of glutamate receptors, in the development of hearing. We will also investigate if mGluRs balance excitatory and inhibitory activity in the adult inner ear, similar to its role prior to the onset of hearing. Insights into these mechanisms may identify new ways to modulate activity and prevent congenital or acquired hearing loss.

Study coauthor Sonja J. Pyott, Ph.D., was a 2007 and 2008 Hearing Health Foundation Emerging Research Grants recipient.

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New Player Identified in Hair Cell Development

By Betty Zou, Sunnybrook Research Institute

Sensory hair cells (red) and supporting cells (green) are intricately organized in the developed cochlea. Supporting cells have high levels of the Kremen1 protein, which is stained with a green fluorescent marker here. [Image courtesy of Dr. Alain Dabdoub]

Sensory hair cells (red) and supporting cells (green) are intricately organized in the developed cochlea. Supporting cells have high levels of the Kremen1 protein, which is stained with a green fluorescent marker here. [Image courtesy of Dr. Alain Dabdoub]

There are roughly 37.2 trillion cells in the human body, each of which can be categorized into one of about 200 different types. What’s remarkable about this immense number and diversity of cells is that they all came from a microscopic cluster that comprises the embryo. Many of these early progenitor cells start out the same, but they receive different programming instructions along the way that enable them to replicate and differentiate to form various tissues and organs.



Signalling pathways are cellular communication systems that govern whether a cell keeps dividing or stops, where it goes and, ultimately, what it becomes. One such pathway is Wnt (pronounced “wint”) signalling, a group of signal transmission networks that play a critical role in embryonic development. Dr. Alain Dabdoub, a scientist in Biological Sciences at Sunnybrook Research Institute, is studying how Wnt signalling affects inner ear development and hearing. A new study by his team has shown for the first time that Kremen1, a poorly understood member of the Wnt network, plays a direct role in the formation of the cochlea, a spiral-shaped auditory sensory organ in the inner ear.

“We know that initially at the very early stages [of development], Wnt signalling pushes cells to proliferate,” says Dabdoub. “Then division stops and cell differentiation occurs. We’re trying to find out what promotes this high level of Wnt and also what decreases it.”

Kremen1 is a protein that sits on the cell surface where it receives and transmits signals to the cellular machinery inside. Previous studies have shown that it blocks Wnt signalling, so Dabdoub and his team decided to investigate whether Kremen1 is involved in cell differentiation in the cochlea.

The researchers found that at an early embryonic stage Kremen1 was present in the precursor cells that give rise to hair cells and supporting cells. Shortly thereafter, Kremen1 was only found in the supporting cells that surround hair cells. When the researchers forced the precursor cells to overproduce Kremen1, fewer of them went on to become hair cells and more became supporting cells. In contrast, knocking down levels of Kremen1 resulted in more hair cells. The results were published in August 2016 in the journal Scientific Reports.

The cochlea contains tens of thousands of hair cells, which have hair bundles on their surface to detect and amplify sound. In mammals, when these cells are damaged or destroyed, they are not replaced and hearing loss results. Supporting cells, on the other hand, remain abundant during an individual’s lifetime and do not appear to be affected by the insults that batter hair cells.

Dabdoub’s research seeks to understand how the cochlea and hair cells form, as well as how these sensory cells can be replenished to restore hearing. “If you think about regeneration, where are the cells that you’re going to regenerate coming from?” he says.

The survival of supporting cells makes them excellent candidates from which to regrow hair cells, but they must first replicate to ensure there are enough to maintain a stable number of supporting cells and form new hair cells. Dabdoub thinks that exploiting the proliferation-enhancing properties of Wnt signalling will help achieve this. His finding that Kremen1 plays an important role in cell fate decisions in the cochlea will be critical to future efforts to regenerate hair cells. “This is a molecule that we should keep an eye on as we work towards regeneration,” he says.

Funding for this study came from the Hearing Health Foundation’s Hearing Restoration Project, Koerner Foundation and Sunnybrook Hearing Regeneration Initiative.

This blog was reposted with the permission of Sunnybrook Research Institute.
 

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