Published October 2, 2020
New research reveals for the first time that despite the fragility of axons, Schwann cells — which surround axons within nerves like a glove covers a hand — can come to the assistance of injured axons.
Axons are long, finger-like projections of neurons that transmit critical signals throughout the nervous system. But because they are energetically difficult to maintain, they are often among the first casualties of certain neurodegenerative diseases, causing symptoms such as muscle weakness or numbness of limbs.
“When axons are damaged early by neurodegenerative diseases, the neuronal network is disrupted, leading to debilitating symptoms,” says Bogdan K. Beirowski, MD, PhD, assistant professor of biochemistry and a principal investigator at the Hunter James Kelly Research Institute (HJKRI).
Beirowski explains that therapies to specifically protect fragile axons are just beginning to be developed, and they focus exclusively on neurons, thereby neglecting Schwann cells.
“We have discovered a new metabolic function of Schwann cells,” he says. “This is a paradigm shift away from the neuron-focused mechanisms of axon protection.”
“We have discovered that when axons are stressed, they develop an increased appetite for sugars produced by Schwann cells, which can help the axons recover,” he says. “Because axon demise is a hallmark of many degenerative conditions, our study paves the way for exploiting this endogenous mechanism to develop novel therapeutic approaches that are focused on the glia, the supportive, non-neuronal cells of the nervous system to stabilize axons in disease.”
In a review, the findings were described as a major breakthrough.
Beirowski says the discovery is “universally applicable to different neurodegenerative conditions affecting axons in the peripheral nervous system.” In particular, he says the findings should be of special interest to neuroscientists working on amyotrophic lateral sclerosis (ALS or Lou Gehrig’s disease) and peripheral neuropathies.
The researchers discovered that Schwann cells “sense” when axons become sick, and in response provide them with increasing amounts of sugar products as an energy source to promote axon stability.
At the molecular level, the nerve injury causes a dramatic upregulation of glycolysis in Schwann cells, the metabolic process where glucose is broken down to form two molecules of pyruvate, a simpler sugar. That, together with the upregulation of specialized sugar transporters in the cell membrane, promote the release of sugars from Schwann cells that support the sick axons.
“It’s as if the Schwann cells become a kind of energetic support system for injured axons,” Beirowski says.
By pharmacologically and genetically manipulating this metabolic pathway, the researchers were able to protect axons that had been injured in different ways in mice.
The researchers say that while axons and Schwann cells form an inextricable relationship, they are generally studied separately by neurobiologists.
“In this study, we bring axons and Schwann cells together to study their metabolic interactions for the first time in the context of axon injury,” says lead author Elisabetta Babetto, PhD, research assistant professor of pharmacology and toxicology as well as biochemistry in the Jacobs School of Medicine and Biomedical Sciences and an investigator at the HJKRI.
Beirowski adds that the research could lead to further investigations into similar relationships between axons and the glial cell counterparts of the central nervous system, called oligodendrocytes and astrocytes.
“What’s especially intriguing about these findings is that we show for the first time a physiological transcellular mechanism that has a soothing and reparative effect for injured axons,” Babetto says.
The researchers say that in the future it could be possible to develop a treatment that would strengthen axons by manipulating glycolytic metabolism in nerves.
The research may also lead to a preventive treatment that could reduce axon damage in scenarios where axons are especially vulnerable, such as in chemotherapy-induced peripheral neuropathy.
The research could also lead to insights into why neuropathy occurs so often in diabetes.
“Because a nutrient and energy sensor is involved in the molecular mechanism that regulates glycolysis, our findings may also improve our understanding as to how diet, exercise and other environmental factors impact axon integrity in the peripheral nervous system,” Beirowski says.
The paper, “A Glycolytic Shift in Schwann Cells Supports Injured Axons,” was published in Nature Neuroscience.
Keit Men Wong, PhD, who did her doctoral research at the HJKRI and earned her doctorate from the University at Buffalo, is a co-author; she is a postdoctoral researcher at the University Medical Center Gottingen in Germany.
The work was funded by two grants from the Muscular Dystrophy Association awarded to Beirowski and Babetto.