Airway Sentinels

Researchers identify rare sensory neurons that guard upper airway against threats

airway neurons

Newly identified P2RY1 neurons in the mouse larynx. Image: Liberles lab.

With every bite of food or sip of water, danger lurks for the body. Should these or other substances enter the airways, even in small amounts, they could impair breathing or damage the sensitive cells of the lungs. So, to guard the upper airway, the body executes automatic defense programs—reflexes such as coughing and swallowing—in response to an assault.

In a new study in mice, Harvard Medical School scientists have discovered a rare population of sensory neurons, only around 100 in number, that reside in the throat and are responsible for detecting such threats and initiating coordinated airway defense programs.

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The newly identified neurons are finely tuned to sense water and acid in the larynx and are one group among at least 37 different classes of internal sensory neurons identified by the research team.

The findings, published in Cell in April, shed light on the mechanisms of airway defense and how the brain detects and responds to internal sensations, according to the authors.

“There are a number of dedicated pathways that sense things in the airway and evoke a suite of protective reflexes,” said study co-first author Sara Prescott, research fellow in the lab of Stephen Liberles, professor of cell biology in the Blavatnik Institute at HMS. “The more we understand about these processes, the more we can take advantage of that knowledge in the future.”

Most animals rely on five senses to interact with the world—sight, hearing, touch, smell and taste. These phenomena are among the best studied in biology, from the sensory neurons that detect external stimuli to the programs they activate in the brain and body.

Less well studied are internal sensations. To control the body’s myriad functions, including breathing, heartbeat, digestion and more, the brain and body must constantly communicate. This responsibility falls upon the vagus nerve, a long, meandering neural highway that connects the brain to essentially every major organ, relaying sensory information and providing motor control.

Despite the importance of the vagus nerve, most of its sensory pathways have remained poorly understood.

“Scientists have studied taste for a long time, for example, and we know it has essentially five distinct modalities—sweet, salt, sour, bitter and umami,” said Liberles, a Howard Hughes Medical Institute Investigator. “But we have no idea how many modalities there are in the vagus. It’s still vastly understudied in comparison.”

Finely tuned

In their study, Liberles, Prescott, co-first author Benjamin Umans and colleagues focused on one of the most important functions of the vagus nerve—protection of the upper airway.

Food, water and other ingested substances normally travel down the esophagus to the stomach, while air travels through the larynx, the organ that houses vocal cords, and the trachea to reach the lungs. The larynx is responsible for protecting foreign substances from entering the trachea and employs several programs to do so.

In mice, the primary reflexes to expel threats away from the airway are swallowing, expiration—analogous to a human cough—a temporary halt of breathing and closing of the vocal folds. The team found that four stimuli—water, high salt, acid and force—trigger these reflexes in the larynx.

To identify the sensory neurons responsible, the researchers conducted a broad scan of the different cell types within the vagus nerve. Using single-cell RNA sequencing, they looked at gene expression in over 46,000 individual cells, which revealed at least 37 distinct classes of sensory neurons, each with potentially different functions and responsibilities.

Then, using these data, the team devised ways to precisely control sensory neuron activity. They identified unique genetic markers for each of the different classes and applied optogenetics, a technique in which neurons are genetically modified to respond to light.

This allowed them to activate selectively each class of vagal neuron in the larynx and observe the resulting reflex. They found that neurons expressing one gene in particular, P2RY1, stood out because they strongly and immediately evoked a coordinated airway defense program.

When P2RY1 neurons were specifically and selectively eliminated, water and acid no longer evoked airway defense reflexes, while salt and force continued to do so, suggesting that P2RY1 neurons specifically sense and respond to water and acid. Remarkably, only around 100 of these neurons exist in the mouse larynx, the researchers noted.

“It’s really exciting to think that one type of vagus neuron might innervate the larynx and control airway protection, while others control feeding, heart rate or other reflexes,” Liberles said. “Our current model is that different neuron types convey different threats, and our focus is to now understand the mechanisms by which these sensations are occurring.”

The team has found evidence that communication between P2RY1 neurons and epithelial cells, possibly taste buds, are essential for threat detection to occur. They are now working to identify the mechanisms involved, as well as the functions of the other sensory neuron classes.

Of particular interest is the role of the vagus nerve in infectious diseases such as COVID-19. The Liberles lab has ongoing collaborations with the lab of Isaac Chiu, assistant professor of immunology at HMS, to study the potential role of internal sensory neurons in host defense and inflammation.

“In some cases, airway sensory neurons are beneficial in suppressing immune responses, and in other cases they can be harmful,” Liberles said. “If we can find different pathways by which immune cells are communicating with neurons, we could manipulate them in different infectious disease models to see what the outcomes are. That would be something we’re very interested in exploring.”

In addition, these neurons may also play a role in other conditions, such as respiratory dysregulation in premature newborns as well as certain age-related complications of respiration and swallowing, the authors said.

“The vagus nerve is a really critical mediator of a lot of internal physiological processes,” Prescott said. “A better understanding of the molecular mechanisms that the vagus employs gives us an opportunity to do much more targeted therapeutics in the future.”

Additional authors on the study include Erika Williams and Rachael Brust.

The work was supported by the National Institutes of Health (DP1 AT009497, R01 HL132255, F31 HL132645, F30 CA177170) the Howard Hughes Medical Institute, American Diabetes Association and the Life Sciences Research Foundation.