Humans can regenerate their peripheral nerves (PNS), but the regenerative ability does not extend to the central nervous system (CNS). So, what changed? Previously, the focus had been on identifying the cellular and molecular contributors that differentiate this regenerative ability in CNS vs. PNS. But now there seems to be a shift towards recognizing the underlying genetic makeup differences between the two.
Researchers at Washington University School of Medicine in St. Louis have identified some of the critical steps taken by peripheral nerves – those in the arms and legs – as they regenerate. The findings, published the week of Dec. 10 in Proceedings of the National Academy of Sciences, lay out a path that neurons of the spinal cord might be able to follow – potentially leading to improved recovery for people paralyzed by spinal cord injuries.
“We’ve figured out some of the events that are required for injured peripheral nerves to repair themselves, and we can see that these things fail to happen in the central nervous system,” said Valeria Cavalli, Ph.D., an associate professor of neuroscience. “So now we’re trying to see if turning on these networks can help spinal cord neurons regenerate.”
Spinal cord nerves do not regenerate, and thus its damage rarely heals. Often the regeneration is hindered by the formation of scar tissue. However, despite intervention strategies to remove the scar and promote regeneration using chondroitinase ABC enzyme, it only showed limited success. Combined with the challenges associated with the delivery of the enzyme, it prompted the researchers to look more closely at the central nervous system neurons itself. Watch the featured breakthrough in chondroitinase research at King's College London published in June 2018 to know more.
While CNS counterparts in the peripheral can regenerate what makes the CNS nerves differently. So, Cavalli realized that studying peripheral neurons could help us understand why some damaged neurons regenerate and others do not. She turned to a unique kind of sensory cell that spans both nervous systems. Known as dorsal root ganglion neurons, these cells have long tendrils, called axons, with two offshoots. One branch of the axon connects to cells in the body’s periphery and can regenerate if cut; the other side links up with cells in the spinal cord and cannot regrow after injury.
Mouse neurons are shown with the cell bodies in the center and long tendrils radiating outward. Valeria Cavalli, Ph.D., of Washington University School of Medicine in St. Louis, used such neurons to study how cells regrow after injury. Her findings could one day lead to better treatments for spinal cord injury.
Cavalli and first author Young Mi Oh, Ph.D., a staff scientist, and colleagues grew mouse dorsal root ganglion neurons in the lab and then cut them to find out what biological processes occur as the cells regrow their axons. They also cut the sciatic nerve – which runs up the leg and into the spinal cord through the dorsal root ganglia – in mice. The researchers then identified a suite of genes needed to be turned off for the axons to regenerate.
“Other people also have shown that a big swath of genes is turned down during regeneration, but as a field, we’ve just said, ‘Eh’ and ignored them to focus on the genes that are activated,” Cavalli said. “Here, we showed that establishing a regeneration program means some genes have to be turned on, but a lot has to be turned off.”
In particular, a set of genes related to sending and receiving chemical and electrical signals – the primary duty of mature neurons – had to be silenced for the injury to heal, the researchers showed. The study revealed an epigenetic mechanism that silences gene expression by ubiquitin-like containing PHD ring finger 1 (UHRF1)-dependent DNA methylation to promote axon regeneration.
“The injured neuron has to stop functioning as a neuron and focus on repairing itself,” Cavalli said. “This means the neuron has to transition back to an immature state so it can re-engage developmental programs and regrow.”
The idea that cells must become less mature to regenerate is not new, but Cavalli and Oh’s study provides evidence in support of that idea. The researchers identified the fundamental molecular and genetic players involved in regressing to a less mature state and showed that the timing of the regression was crucial to successful recovery.
“If you’re triggering a system that makes the neuron less mature, you have to make sure it’s not forever,” Cavalli said. “It has to remain a neuron, albeit an immature one, so it can re-mature and start functioning again after it repairs itself.” The authors also suggest that the transient increase in the transcriptional regulator REST, controlled by miR-9 and UHRF1, allows neurons to enter a less mature state that favors a regenerative state.
Cavalli and colleagues are working on developing a more detailed understanding of when and for how long specific genes must be shut off, and whether silencing the genes in neurons from the central nervous system will induce them to regrow after injury.
“We haven’t found a cure, but we have a better understanding now of what injured neurons do,” Cavalli said. “From here, we can build new hypotheses and work toward applying them to people.”