When a mouthful of water goes down the “wrong pipe”—heading toward the lungs instead of the gut—it triggers uncontrollable coughing. That’s because the upper airway senses the water and quickly signals to the brain. The same coughing reflex is also set off in people with acid reflux, when acid from the stomach reaches the throat.

Newly reported research by David Julius, PhD, winner of the 2021 Nobel Prize in Physiology or Medicine, and professor and chair of physiology at the University of California, San Francisco (UCSF), working with lab colleague Laura F. Seeholzer, PhD, has identified a rare cell type in the throat that senses when fluid is aspirated, or when acid is regurgitated. The researchers’ studies detailed exactly how these laryngeal and tracheal neuroendocrine (NE) cells can sense water or acid in the upper airway and pass the information along by releasing chemical messengers that activate nerves leading to the brain.

The findings, the researchers suggested, could contribute to a better understanding of how such coughing reflexes diminish with aging and disease, and help scientists develop interventions to prevent pneumonia or treat certain types of chronic cough. “This study gives us a lot of insight into not only how our bodies are protecting our airways in this profoundly surprising way, but also more broadly how internal organs can act as gatekeepers to the outside world,” said Julius.

The investigators reported on their findings in Science, in a paper titled, “Neuroendocrine cells initiate protective upper airway reflexes,” in which they concluded, “… our detailed characterization of the molecular mechanisms underlying NE cell activation may reveal additional therapeutic strategies for treating disorders of cough or other aspects of airway protection and hyperreactivity.”

The respiratory tract must guard against the inhalation of noxious agents, such as foods and liquids, or corrosive acid from gastric reflux,” the authors wrote. “Airway protection is mediated by reflexes, such as cough, swallowing, or apneas.” However, they pointed out, dysfunction of these reflexes promotes aspiration of foreign entities into the lung, and this is a major cause of morbidity and mortality.

NE cells are found throughout the respiratory and digestive systems and have a double role. Like endocrine cells they produce and release hormones, and like neurons they also send and receive electric messages. And while airway neuroendocrine cells have been proposed to serve as specialized sensory epithelial cells that modulate respiratory behavior by communicating with nearby nerve endings, the team continued, “…  their functional properties and physiological roles in the healthy lung, trachea, and larynx remain largely unknown.”

For the reported murine study, Seeholzer, a UCSF postdoctoral fellow in the Julius lab who led the new work, developed methods to isolate the tiny and very rare NE cells from the lungs, larynx, and upper and lower trachea and characterize the molecular details of the different groups of cells.

Seezholzer noted, “We knew something about what these cells were doing in the lungs, but not the upper airway. The only reason we even knew they existed in the larynx is because some people get very rare neuroendocrine tumors in the larynx.”

She studied what genes were active in each cell, as well as what stimuli caused the cells to release chemical signals. The results showed that, unlike NE cells from the lungs, those found in the trachea and larynx did not respond to pressure changes. Using techniques including electrophysiology, calcium imaging, and histological evaluation, the investigators found that, in contrast to the lung, “… the tracheal and laryngeal NE cells were largely unresponsive to mechanical stimuli.”

However, the airway NE cells did release signals in response to water and acid. “Water ‘going down the wrong pipe’ and acid reflux are two commonly experienced examples of noxious stimuli that elicit protective airway reflexes,” the researchers wrote. Their calcium imaging studies in mouse tissues showed that “ … water and acid robustly activated tracheal and laryngeal NE cells.”

Seeholzer also demonstrated, in isolated tissues from mice, how the NE cells could pass these messages directly to the sensory neurons that lead to the brain. The collective results showed that “ … ATP release from laryngeal or tracheal NC cells activates airway-innervating nerve fibers.” Julius stated, “We really wanted to know the details of what makes these cells tick. No one had ever showed before that these cells talk to sensory neurons, and we wanted to follow the whole message trail.”

Seeholzer, in addition, showed that activating these NE cells in live mice made the animals swallow and cough, which are important reflexes for protecting our lungs. On the flip side, mice completely lacking NE cells did not respond to water in their airways. “We used two orthologous diphtheria toxin–based methods to genetically ablate NE cells and the putatively acid-sensitive type III laryngeal taste cells …,” the study authors wrote. “With both genetic strategies, we found that water and acid elicited fewer swallows after NE cell ablation compared with that observed in littermate control mice.”

Seeholzer said the new findings suggest that the larynx and trachea NE cells are analogous to taste buds or ear hairs; they are not nerve cells themselves, but they are connected to nerves that send sensory information to the brain. “If you’ve ever had the experience of aspirating water or acid reflux, you know that both of these things are incredibly painful; you immediately cough and gag and try to clean out your airway,” Seeholzer said. “Now we better understand how the body triggers that.” The authors further stated, “Our work uncovers the broad molecular and biophysical diversity of NE cells across the airways and reveals mechanisms by which these specialized excitable cells serve as sentinels for activating protective responses.”

The cough reflex often becomes less sensitive as we age, making older adults, and individuals with certain diseases more prone to aspirating fluid into their lungs and developing pneumonia. “Dysphagia and aspiration into the lungs account for morbidity and mortality across a range of age groups and in clinical settings, especially in the elderly and/or those afflicted with disorders such as stroke or neurodegenerative diseases that compromise motor function,” the investigators pointed out.

The new understanding of how NE cells relate to the cough reflex could eventually lead to ways of increasing sensitivity and preventing this aspiration. Targeting the cells could also treat the chronic cough associated with acid reflux in some people. “Our work now raises the possibility that functional interactions between NE cells and purinoceptive sensory afferents contribute to disease symptoms,” they noted.

“More work is needed to better understand how the NE cells are potentially changing with disease, smoking, or aging,” said Seeholzer, who is collaborating with UCSF clinicians to undertake this next phase of the research.

Previous articleGene Network Interactions Shed Insight into Breast Cancer Onset and Development
Next articleSCG Cell Therapy and A*STAR Collaborate on Scalable GMP Manufacturing of Cellular Immunotherapies