A spinal cord injury can change a person’s life in an instant. Damage to the spinal cord can cause long-term problems with movement, sensation and independence. When the injury occurs in the neck, the consequences can be even more serious because the damaged nerves may interfere with the signals that control breathing.
Now, researchers at Gladstone Institutes have reported a promising step toward repairing this damage. In a new study published in Science Translational Medicine, scientists showed that human stem cell-derived spinal neurons could survive after being transplanted into injured rats, connect with existing neural circuits and improve breathing-related motor function.
The research does not yet represent a treatment for people. However, it provides important evidence that damaged spinal cord networks may eventually be repaired by replacing specific types of neurons that have been lost.
Why spinal cord injuries are so difficult to repair
The spinal cord acts as a major communication highway between the brain and the rest of the body. Nerve signals travel through this complex network to control movement, sensation and many automatic functions.
After a serious spinal cord injury, some of these connections are destroyed. Unlike many other tissues, the central nervous system has a very limited ability to rebuild complex neural connections on its own.
This is why even with emergency treatment, rehabilitation and modern medical care, people can be left with permanent disabilities.
Injuries to the cervical, or neck, region are particularly dangerous. The nerves in this area are connected to circuits responsible for controlling the diaphragm—the main muscle involved in breathing.
If these circuits are badly damaged, a person may have difficulty breathing independently and could require mechanical ventilation.
Researchers have therefore been searching for ways to rebuild the lost connections rather than simply helping patients adapt to the damage.
Turning stem cells into specialized neurons
The Gladstone team focused on a particular type of spinal neuron called a V2a interneuron.
Interneurons can be thought of as biological communication links. They connect different neurons within the nervous system and help coordinate complex circuits.
V2a interneurons are involved in controlling movement and are also connected to neural circuits involved in breathing.
The researchers wanted to determine whether replacing these cells after a spinal cord injury could help rebuild damaged communication pathways.
To do this, they developed a method for producing human V2a interneurons from stem cells in the laboratory.
This work builds on almost a decade of research involving stem cell-derived spinal neurons. Scientists initially used mouse embryonic stem cells to create specific populations of spinal neurons. Over time, the technology moved toward human induced pluripotent stem cells, which can be converted into different specialized cell types.
The researchers then refined the process to produce human V2a interneurons suitable for transplantation.
Getting the recipe right was not easy. According to the researchers, it took roughly a year and a half of experimentation to reliably produce the desired neurons.
Another important step was making sure the cells could be frozen, stored and later thawed without losing their usefulness. This could become important if the technology eventually reaches human clinical trials.
Testing the treatment in injured rats
The scientists transplanted the human V2a interneurons into adult rats one week after they suffered cervical spinal cord injuries.
The researchers chose breathing as the first major test because the breathing system is relatively well understood and is directly relevant to people with severe neck-level spinal cord injuries.
The results were encouraging.
Two months after transplantation, the human neurons had survived inside the injured spinal cord. More importantly, they had formed connections with nearby cells belonging to the rats.
The scientists then stimulated the area containing the transplanted cells. This produced increased activity in the diaphragm, suggesting that the new neurons had become functionally connected to the breathing system.
The researchers also activated neurons in the rats' brainstem—the part of the nervous system that normally sends signals toward the spinal cord.
The transplanted human neurons responded to these signals.
This was an important finding because it suggested that the transplanted cells were not simply surviving in the spinal cord. They were actually becoming part of the existing neural network.
The biggest test came under stress
Under normal conditions, the difference between treated and untreated animals was relatively small.
But the researchers wanted to know whether the new neurons could provide additional breathing capacity when the respiratory system was placed under stress.
They exposed the animals to conditions involving low oxygen or high carbon dioxide. These situations force the breathing muscles to work harder.
The results were striking.
Most injured rats that did not receive the cell transplant developed signs of respiratory failure during these challenges.
In contrast, about three-quarters of the rats that received the V2a interneuron transplant successfully handled the breathing challenges.
This suggests that the transplanted cells may have provided additional functional capacity to a damaged breathing circuit.
The significance could be important for people living with spinal cord injuries. Even if a person has enough breathing ability under normal conditions, an infection such as a cold or another respiratory challenge can potentially push a weakened system beyond its limits.
A therapy that strengthens damaged breathing circuits could therefore provide meaningful protection against such challenges.
Scientists are looking for the most effective cells
The researchers also noticed that not every transplant worked equally well.
They identified a particular subset of transplanted V2a interneurons that appeared especially capable of connecting with the host breathing circuit.
Understanding why these cells perform better could help scientists develop more precise and reliable cell therapies in the future.
Instead of simply transplanting large numbers of neurons, researchers may eventually be able to produce specific populations of cells designed to repair particular damaged circuits.
That could make future treatments more effective and predictable.
Could this eventually help people?
There is still a long road between successful experiments in rats and an approved human therapy.
Before testing this approach in people, scientists need to determine whether the treatment remains safe and effective in larger animal models.
They also need to answer another major question: Can the therapy work long after an injury?
In this experiment, the cells were transplanted relatively soon after the injury. But many people with spinal cord injuries have lived with their condition for months or years.
Researchers therefore need to determine whether the damaged spinal cord remains capable of accepting and integrating new neurons long after the original injury.
Safety will also be critical. Scientists must carefully evaluate whether transplanted cells remain stable, connect appropriately and avoid unwanted effects.
Beyond breathing
The researchers don't want to stop with breathing.
Cervical spinal cord injuries can severely affect the arms and hands, and restoring hand function is often one of the most important priorities for people living with these injuries.
The team is therefore investigating whether similar stem cell-derived neurons could eventually help rebuild circuits responsible for arm and hand movement.
If successful, this could expand the technology from repairing one specific breathing circuit to repairing multiple neural networks throughout the spinal cord.
A promising step toward spinal cord repair
The study does not mean that spinal cord injuries can currently be cured. Human trials are still needed, and many scientific and safety challenges remain.
But the research demonstrates something important: human stem cell-derived neurons can survive inside an injured spinal cord, connect with existing neural networks and contribute to the recovery of a critical function.
For decades, spinal cord damage has been considered extremely difficult to repair because the nervous system does not naturally recreate the complex connections that are destroyed by injury.
This research offers a different possibility—rather than simply working around damaged circuits, scientists may eventually be able to rebuild them.
The ultimate goal is ambitious: create specialized human neurons in the laboratory, transplant them into damaged spinal cords and have them integrate with the body's existing nervous system.
For now, the work remains an early proof of concept. But if future studies confirm the findings and overcome the remaining challenges, stem cell-derived spinal neurons could one day become part of a new generation of regenerative treatments for people living with spinal cord injuries.
Reference:
- Lyandysha V. Zholudeva et al.
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