New Clues for Healing Spinal Cord Injuries Found in Single-Cell Studies in Zebrafish

Each year in the U.S. there are about 18,000 new spinal cord injuries, which damage the bundle of nerves and nerve fibers that send signals from the brain to other parts of the body and can affect feeling, movement, strength, and function below the injured site. A severe spinal cord injury can lead to immediate and permanent paralysis, as our spinal cords lack the capacity to regenerate the damaged tissues and heal.

So far, even the most groundbreaking regenerative therapies have yielded only modest improvements after spinal cord injuries. Now, an NIH-supported study reported in Nature Communications offers some new clues that may one day lead to ways to encourage healing of spinal cord injuries in people. The researchers uncovered these clues through detailed single-cell analysis in what might seem an unlikely place: the zebrafish spinal cord.

Why zebrafish? Unlike mammals, zebrafish have a natural ability to spontaneously heal and recover after spinal cord injuries, even when the injuries are severe. Remarkably, after a complete spinal cord injury, a zebrafish can reverse the paralysis and start swimming again within six to eight weeks. Earlier studies in zebrafish after spinal cord injury found that this regenerative response involves many types of cells, including immune cells, progenitor cells, neurons, and supportive glial cells, all of which work together to successfully repair damage.

In the new work, a team at Washington University School of Medicine in St. Louis led by Mayssa Mokalled took advantage of tools that make it possible to study the gene activity underlying this spinal healing response by sequencing RNA transcripts within individual neurons. The goal was to learn in much more detail about the zebrafish’s response to spinal cord injury in major neural cell types. The researchers also wanted to compare what they found in zebrafish to other single-cell studies in mice, which lack this regenerative capacity.

To do it, the researchers sequenced RNA from spinal cells at the time of injury and then again one, three, and six weeks later. Their analyses show that zebrafish neurons demonstrate a dramatic response indicated by changes in gene activity, followed by the growth of new neurons to restore essential connections. Importantly, the researchers also found that some of the existing injured neurons get reprogrammed, showing a regenerative pattern of gene activity after a week that encourages their survival and increased flexibility to allow healing. When those injury-responsive neurons, which the researchers call iNeurons, were disabled, the zebrafish didn’t regain the ability to swim, even when regenerative stem cells remained active.

In people or mice, by comparison, a spinal cord injury sets off a toxic chain of events that kills the existing neurons and prevents repair. Interestingly, the new study suggests that, in the zebrafish, the regeneration is not only due to stem cells sprouting new neurons as long suspected, and may be more related to the processes that protect and save the injured neurons and lend them more flexibility to heal. It’s possible that this kind of protective and regenerative capacity is present in mammalian neurons, even if the regenerative process doesn’t automatically switch on in the way seen in zebrafish, according to the researchers. Indeed, many of the genes that play important roles in the zebrafish healing process are also present in the human genome.

In future work, the researchers plan to conduct similar studies in the many other cell types known to play some role in spinal cord healing in zebrafish, including supportive glia and immune cells. They’re also continuing to explore how the activities they see in the zebrafish spinal cord compare to what happens in mice and humans. With much more study, these kinds of findings in zebrafish may lead to promising new ideas and even treatments that encourage neural protection, flexibility, and recovery in the human nervous system after spinal cord injuries.

Reference:

Muraleedharan Saraswathy V, et al. Single-cell analysis of innate spinal cord regeneration identifies intersecting modes of neuronal repair . Nature Communications. DOI: 10.1038/s41467-024-50628-y (2024).

NIH Support: National Institute of Neurological Disorders and Stroke