Neurons in the brain and spinal cord don't grow back after injury, unlike those in the rest of the body. Cut your finger, and you'll probably be back to using it in days or weeks; slice through your spinal cord, and you likely will never walk again.
Now, working in mice, researchers have identified some of the key steps taken by peripheral nerves - those in the arms and legs - as they regenerate. The findings, published 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.
Neurons in the central nervous system - the brain and spinal cord - and the peripheral nervous system are very similar except in their ability to regenerate. The authors 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.
The team 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," the senior author said. "Here, we showed that establishing a regeneration program means some genes have to be turned on but a lot have 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 injured neuron has to stop functioning as a neuron and focus on repairing itself," the senior author 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 in order to regenerate is not new, but the study provides evidence in support of that idea. The researchers identified the key 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.
The authors show that DNA methylation, which generally leads to gene silencing, is required for robust axon regeneration after peripheral nerve lesion. Ubiquitin-like containing PHD ring finger 1 (UHRF1), a critical epigenetic regulator involved in DNA methylation, increases upon axon injury and is required for robust axon regeneration. The increased level of UHRF1 results from a decrease in miR-9. The level of another target of miR-9, the transcriptional regulator RE1 silencing transcription factor (REST), transiently increases after injury and is required for axon regeneration. Mechanistically, UHRF1 interacts with DNA methyltransferases (DNMTs) and H3K9me3 at the promoter region to repress the expression of the tumor suppressor gene phosphatase and tensin homolog (PTEN) and REST.
"If you're triggering a system that makes the neuron less mature, you have to make sure it's not forever," the senior author 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 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," the senior author said. "From here, we can build new hypotheses and work toward applying them to people."
Regrowing damaged nerves hinges on shutting down key genes
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