"This study is exciting because we found a new gene mutation that's linked to deafness, and more importantly we have a therapeutic target that can actually mitigate this condition," said lead author Rong Grace Zhai, PhD, Jack Miller Professor for the Study of Neurological Diseases of Neurology at UChicago. Although the study focused on individuals with a rare combination of mutations to the CPD gene, there could be broader implications if single mutations are linked to age-related hearing loss, she added.

The connection between CPD and hearing loss

Researchers began investigating CPD after identifying an unusual combination of mutations in three unrelated Turkish families affected by sensorineural hearing loss (SNHL), a congenital and hereditary condition that causes permanent deafness.

SNHL is typically diagnosed in early childhood and has long been considered irreversible. Hearing aids and cochlear implants can help improve perception of sound, but no direct medical treatment exists to repair the underlying damage.

When the scientists expanded their search through genetic databases, they discovered that individuals with other CPD mutations also showed signs of early-onset hearing loss, strengthening the link between this gene and auditory function.

How CPD protects sensory cells

To understand how CPD influences hearing, the team conducted experiments using mice. The CPD gene normally produces an enzyme responsible for generating the amino acid arginine, which then helps create nitric oxide, a key neurotransmitter involved in nerve signaling. In the inner ear, mutations in CPD disrupted this process, triggering oxidative stress and the death of delicate sensory hair cells that detect sound vibrations.

"It turns out that CPD maintains the level of arginine in the hair cells to allow a quick signaling cascade by generating nitric oxide," Zhai explained. "And that's why, although it's expressed ubiquitously in other cells throughout the nervous system, these hair cells in particular are more sensitive or vulnerable to the loss of CPD."

Fruit fly experiments reveal possible treatments

The researchers also used fruit flies as a model to explore how CPD mutations affect hearing. Flies with the defective gene exhibited behaviors consistent with inner ear dysfunction, such as impaired hearing and balance issues.

To test potential treatments, scientists tried two approaches. One was to provide arginine supplements to replace what was lost due to the gene defect. The other was to use sildenafil (Viagra), a drug known to stimulate one of the signaling pathways disrupted by reduced nitric oxide. Both treatments improved cell survival in patient-derived cells and reduced hearing-loss symptoms in the fruit flies.

"What makes this really impactful is that not only do we understand the underlying cellular and molecular mechanism for this kind of deafness, but we also found a promising therapeutic avenue for these patients. It is a good example of our efforts to repurpose FDA approved drugs for treating rare diseases," Zhai said.

The study also demonstrates the value of fruit fly models for studying neurological diseases, including age-related conditions, Zhai noted. "They give us the capability to not only understand disease pathology, but also to identify therapeutic approaches," she said.

Expanding the research to broader populations

The researchers plan to continue studying how nitric oxide signaling functions in the inner ear's sensory system. They also aim to investigate how common CPD mutations are in larger populations and whether they might contribute to other forms of hearing loss.

"How many people carry variants in this gene and is there a susceptibility to deafness or age-dependent hearing loss?" she said. "In other words, is this a risk factor for other types of sensory neuropathy?"

The study included collaborators from multiple institutions, including the University of Miami, Ege University, Ankara University, Yüzüncü Yıl University, Memorial Şişli Hospital, the University of Iowa, and the University of Northampton (UK).

"We were surprised to find that these specialized cells cannot do both jobs at once," says Douglas Brownfield, Ph.D., senior author of the study, which was published in Nature Communications. "Some commit to rebuilding, while others focus on defense. That division of labor is essential. And by uncovering the switch that controls it, we can start thinking about how to restore balance when it breaks down in disease."

Understanding How Lung Cells Repair and Protect

The study focuses on alveolar type 2 (AT2) cells, which are unique because they both safeguard the lungs and act as reserve stem cells. AT2 cells produce proteins that keep the tiny air sacs open for breathing, while also regenerating alveolar type 1 (AT1) cells -- the thin, flat cells that line the lung surface and enable oxygen exchange.

Scientists have long known that AT2 cells often struggle to regenerate properly in diseases such as pulmonary fibrosis, chronic obstructive pulmonary disease (COPD), and severe viral infections like COVID-19. What had remained unclear was how and why these cells lose their regenerative capacity.

Mapping the Life Cycle of Lung Cells

Using single-cell sequencing, advanced imaging, and preclinical models of lung injury, the Mayo Clinic team tracked the "life history" of AT2 cells. They discovered that new AT2 cells remain flexible for about one to two weeks after birth before they permanently adopt their specialized identity.

That critical transition is governed by a molecular circuit involving three key regulators -- PRC2, C/EBPα, and DLK1. One of these, C/EBPα, acts as a clamp that keeps the cells from behaving like stem cells. To regenerate after injury, adult AT2 cells must release this clamp.

Why Infections Slow Lung Recovery

The same molecular switch also determines whether AT2 cells repair damaged tissue or fight infection. This dual role helps explain why infections can slow down or block recovery in chronic lung diseases.

"When we think about lung repair, it's not just about turning things on -- it's about removing the clamps that normally keep these cells from acting like stem cells," says Dr. Brownfield. "We discovered one of those clamps and how it times the ability of these cells to repair."

Preventing Organ Failure

The findings open new possibilities for regenerative medicine. Drugs that fine-tune C/EBPα activity, for example, could help AT2 cells rebuild lung tissue more effectively or reduce scarring in conditions like pulmonary fibrosis.

"This research brings us closer to being able to boost the lung's natural repair mechanisms, offering hope for preventing or reversing conditions where currently we can only slow progression," says Dr. Brownfield.

The study may also help doctors identify early signs of disease by detecting when AT2 cells are trapped in one state and unable to regenerate. Such insights could lead to new biomarkers that detect lung disease in its earliest, most treatable stages.

Linking Discovery to Mayo Clinic's Regenerative Initiatives

This work aligns with Mayo Clinic's Precure initiative, which focuses on identifying diseases early -- when treatments can have the greatest impact -- and preventing progression before organ failure occurs.

It also advances the Genesis initiative, which aims to prevent organ failure and restore function through regenerative medicine. Building on these findings, the research team is now testing ways to release the repressive clamp in human AT2 cells, grow them in the lab, and explore their potential for future cell-based therapies.