After a spinal-cord injury, adult nerve cells do something frustrating: they hunker down and survive, but they refuse to regrow. For decades, scientists have tried to coax those neurons back into action. Now a team at the Icahn School of Medicine at Mount Sinai has found the molecular switch that keeps them stuck, and a hormone pill prescribed for birth control appears to flip it off.
The study, published in Nature in May 2025, identifies a protein called the aryl hydrocarbon receptor, or AhR, as a central brake on nerve-fiber regrowth. When the researchers blocked AhR in injured neurons, the cells shifted from a defensive crouch into active rebuilding, extending new axons in laboratory models. The drug that did the blocking, lynestrenol, has been on pharmacy shelves for decades.
Why adult nerves stop growing, and what AhR has to do with it
Developing neurons are prolific builders. They send out long axons that wire the brain to the spinal cord and the spinal cord to the muscles. But once those circuits mature, the growth machinery largely shuts down. If an injury severs an axon in an adult, the neuron activates a stress-defense program instead of a repair program. It stays alive, but it does not rebuild.
The Mount Sinai team discovered that AhR orchestrates that trade-off. After an axon is cut, AhR triggers what is known as the integrated stress response, or ISR. The ISR dials down the protein-synthesis machinery the cell would need to construct a new axon. In simple terms, the neuron chooses self-preservation over regrowth, and AhR is the protein enforcing that choice.
When the researchers genetically deleted AhR or blocked it with small molecules, injured neurons resumed building proteins and pushed out new axon extensions. The effect was consistent across multiple experimental systems described in the paper, establishing a clear cause-and-effect chain: AhR activates the stress response, the stress response suppresses growth-related protein production, and removing AhR from the equation restores it.
Why lynestrenol matters
AhR is best known outside neuroscience as a sensor for environmental toxins, including dioxins and polycyclic aromatic hydrocarbons. Because it has been studied for decades in toxicology and immunology, a library of compounds that activate or inhibit it already exists. Among the inhibitors the Mount Sinai team flagged is lynestrenol, a synthetic progestin cataloged in the National Library of Medicine’s PubChem database under compound identifier CID 5857.
Lynestrenol has been prescribed in oral contraceptives and hormone-replacement therapies for more than 50 years. That history gives regulators something they rarely have when evaluating a new therapeutic idea: a thick file of human safety data, including side effects, drug interactions, and long-term outcomes. Under the FDA’s 505(b)(2) regulatory pathway, a sponsor can reference that existing safety record when seeking approval for a new indication, potentially shaving years off the timeline compared to developing a brand-new molecule from scratch.
That regulatory shortcut is what makes the finding especially striking. Spinal-cord injury affects roughly 18,000 new patients each year in the United States alone, according to the National Spinal Cord Injury Statistical Center, and current treatments focus on stabilization and rehabilitation rather than nerve regrowth. A repurposed pill with a known safety profile could, in theory, reach clinical testing faster than a novel compound would.
What the study does and does not show
The Nature paper establishes the AhR mechanism and demonstrates that blocking it restarts axon extension in laboratory models. That is a significant advance. But several gaps remain before anyone should expect a prescription for nerve repair.
First, no publicly available data yet show that lynestrenol specifically inhibits neuronal AhR in an injury model at a dose achievable in humans. The paper identifies it as a candidate based on its known biochemistry, but the dose-response curve in damaged spinal-cord tissue has not been reported in a primary dataset accessible to the public.
Second, there are no clinical-trial registry entries listing lynestrenol as an investigational therapy for spinal-cord injury. The Mount Sinai release described the therapeutic direction but did not announce a trial design, timeline, or funding commitment. Human testing of lynestrenol for this indication has not begun, as far as public records show.
Third, most of the experimental data come from controlled conditions that cannot fully replicate the complexity of a real spinal-cord injury, which involves inflammation, scar tissue, immune-cell infiltration, and variable timing of medical care. A molecule that works in a dish or a mouse does not always work in a person.
Independent expert commentary published alongside the Nature paper described the result as a meaningful step in understanding the tension between stress protection and regrowth in injured neurons. But that analysis stopped short of predicting when or whether the work would translate into a human therapy, a caution worth taking seriously.
A pattern worth watching: repurposing shelf drugs for nerve repair
The lynestrenol finding fits into a broader research trend. In 2007, a separate team showed that common nonsteroidal anti-inflammatory drugs, including ibuprofen and indomethacin, could promote axon regeneration in rodent models by inhibiting a different molecular brake called RhoA. That work, indexed in PubMed, demonstrated that cheap, familiar medications could influence growth pathways that adult neurons normally keep switched off.
The two discoveries target different control points. RhoA affects the cytoskeleton, the structural scaffolding a growing axon needs to push forward. AhR affects the translational machinery, the protein-building apparatus the cell needs to supply that scaffolding with raw materials. In principle, blocking both brakes simultaneously could produce a stronger regenerative response than targeting either one alone. But no published experiment has tested dual inhibition in cell culture or animal models, so that idea remains a hypothesis rather than a result.
What the pattern does confirm is that the adult nervous system is not as permanently locked down as scientists once believed. Multiple molecular brakes hold regeneration in check, and at least some of them can be released by drugs that already exist. Each new brake identified is another potential lever for therapy.
What comes next for AhR research
The immediate next steps, based on the trajectory outlined in the Nature paper and the Mount Sinai release, are likely to include testing lynestrenol and other AhR antagonists in animal models of spinal-cord injury, measuring not just axon extension but functional recovery: movement, sensation, bladder control. Those studies would need to establish effective doses, optimal treatment windows (immediately after injury versus weeks or months later), and whether the drug reaches the injury site in sufficient concentration.
If animal results are promising, the existing human safety data for lynestrenol could accelerate the path to a Phase I or Phase II clinical trial. But “accelerate” is relative. Even with a known drug, designing a spinal-cord-injury trial involves complex decisions about patient selection, outcome measures, and control groups. Realistic timelines for early human data, assuming animal studies begin soon, likely stretch into the late 2020s at the earliest.
For patients and families living with spinal-cord injuries today, the AhR discovery is worth following closely. It identifies a clear, druggable target, pairs it with a compound that has a long safety record, and fits into a growing body of evidence that the adult nervous system retains more regenerative potential than previously thought. None of that guarantees a therapy. But it does mean the scientific foundation for one is stronger than it was before this paper was published.
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*This article was researched with the help of AI, with human editors creating the final content.
