There are drugs on the market that can slow the progression of neurodegenerative diseases, but there are still no cures. Studies by researchers at Boston Children’s Hospital and Harvard Medical School have now found that proteins involved in the innate immune system may be involved in mechanisms that drive the fatal motor neuron disorder amyotrophic lateral sclerosis (ALS). In their search for pathways that could slow the neuronal dysfunction and potentially treat ALS, the team discovered that a molecule in the brain called gasdermin-E (GSDME), which is linked with inflammation, plays a role in mitochondrial damage and axon loss. Their experiments showed that knocking down GSDME prevented cellular damage in human motor neurons derived from ALS patients, and delayed the progression of ALS in mice.
“The unmet need for therapies for neurodegenerative diseases is huge, and our work opens up a whole new pathology that we could address,” said Judy Lieberman, MD, PhD, a researcher in cellular and molecular medicine at Boston Children’s and a co-principal investigator on the reported study. “We revealed an innate immune molecule playing a role in neurodegeneration, which opens up a new avenue for thinking about neuronal health,” added co-principal researcher Isaac Chiu, PhD, associate professor in immunology at Harvard Medical School. The team suggests that the work is the first to report the significance of GSDME in a classic model of motor neuron disease.
Lieberman, Chiu, and colleagues described their findings in Neuron, in a paper titled “Gasdermin-E mediates mitochondrial damage in axons and neurodegeneration,” in which they concluded, “We identify GSDME as an executioner of neuronal mitochondrial dysfunction that may contribute to neurodegeneration.”
Neurologic diseases are commonly characterized by early mitochondrial dysfunction and axon loss that precedes cell death, the authors noted. However, they acknowledged, “The myriad mechanisms that amplify local mitochondrial collapse and neurite loss are not clear,” so mapping early molecular mechanisms leading to neuronal dysfunction could have implications for a wide range of neurological disorders.
When cells recognize danger, such as an infection, immune molecules are activated to sound an alarm that recruits and activates immune cells to the site of damage, to try to eliminate the cause and orchestrate tissue repair. This immune response can sometimes involve a family of proteins called gasdermins, which trigger cells to die through a highly inflammatory process called pyroptosis. “GSDMs are a family of pore-forming proteins that have been linked to inflammation and cell death,” the team wrote. However, they pointed out, “The function of GSDMs in neuronal cell biology and their action in axonal processes has not been studied.” Gasdermin-E is one of the gasdermin family of proteins that is expressed in the brain, particularly in nerve cells. But to date, its function hasn’t been clear. “GSDME, one member of the GSDM family, is expressed in both the brain and spinal cord,” the team continued. “However, its functional role in the nervous system is largely unknown.”
For their newly reported study, the research team led by Lieberman and Chiu first examined how gasdermin-E affects neurons. They developed models of neurons from mice and from humans, and looked at the effects of gasdermin-E on axons, the parts of neurons that send electrical signals. They discovered that when neurons detect a hazard, gasdermin-E drives damage to the mitochondria—the powerhouses of the cell—and to the axons. As a result, the axons degenerate, but the cells don’t die.
“If you look at a plate of neurons, you see a jungle of axons. But if you look at a plate where gasdermin-E is activated, you see retractions of these cellular processes,” explained Himanish Basu, PhD, a postdoctoral researcher in Chiu’s lab at Harvard University who led the study. This retraction happens in nerves in the muscles of patients with ALS, a progressive disease that is characterized by muscle twitching and weakness, but which eventually progresses to muscle atrophy and paralysis.
To better understand the relationship between gasdermin-E and neurodegeneration, the team created models of ALS motor neurons by transforming induced pluripotent stem cells (iPSCs) derived from ALS patients into neurons. The researchers found that gasdermin-E was present at high levels in these neurons, but that they could protect axons and mitochondria from damage by silencing gasdermin-E.
The team then tested whether the effects they saw in cells could translate to improvements in symptoms related to neurodegeneration, in a mouse model (SOD1G93A) of ALS. They found that silencing gasdermin-E in these animals resulted in delayed the progression of ALS symptoms and effectively protected the motor neurons, which demonstrated longer axons, and there was less overall inflammation. “Genetic knockout (KO) or short hairpin RNA (shRNA)-based knockdown is neuroprotective both in vitro and in vivo,” they stated. “Knockout of GSDME in SOD1G93A ALS mice prolonged survival, ameliorated motor dysfunction, rescued motor neuron loss, and reduced neuroinflammation,” they stated.
These findings suggested that gasdermin-E drives changes to neurons that may contribute to disease progression. “Taken together, this data suggests that GSDME is activated in a classic mouse model of ALS and drives pathology—namely neuronal loss and gliosis—that contributes to decreased motor function and survival,” the scientists commented. Chiu added, “Inflammation is a double-edged sword, and could be very destructive based on context.” As the authors further pointed out, “Given its wide expression in the brain, GSDME may function in broad contexts, including in normal development, pathogen infection, and aging, which may warrant future investigation.
The work represents an important first step towards developing new approaches for treating ALS. “We describe a pathway and molecules that you can target for treating many neurodegenerative diseases,” Lieberman suggested. “Our study is an example of how immunology can help explain neurodegeneration on a mechanistic level, and what drives axon loss and neuronal injury,” added Dylan Neel, an MD/PhD student in Chiu’s lab who co-led this study.
Although some drugs can block the effects of other gasdermins, it is still unclear whether gasdermin-E can be targeted pharmacologically. “Small-molecule inhibitors of GSDMD activation have been identified, although it is unclear whether GSDME is similarly druggable,” the investigators noted. “Future work should determine the feasibility of targeting GSDME expression or function.”