Scientists have found a pathway that regulates protein aggregation, a cause of several age-related neurodegenerative diseases.
United by abnormal protein aggregation
Most neurodegenerative diseases are age-related, and many of them are similar in other ways. For instance, amyotrophic lateral sclerosis (ALS), Huntington’s disease, and Alzheimer’s disease have all been linked to abnormal protein aggregation [1], albeit with different proteins involved. This is unsurprising, since one of the key hallmarks of aging is the decline of proteostasis.
While the protein culprits are different between these diseases, this similarity hints at possible common mechanisms that scientists have been looking for. In a new study from the University of Cologne, published in Nature Aging, the researchers might have made a big leap towards understanding these common causes.
One protein causes several others to aggregate
The team worked with models based on C. elegans, a nematode worm and a workhorse of longevity research. Researchers focused on a signaling pathway involving two proteins, EPS-8 and RAC. Previous work had found that as C. elegans ages, EPS-8 accumulates, which upregulates RAC signaling. This hyperactivation shortens the worm’s lifespan [2].
The key question was whether this known aging pathway could also be the trigger for disease-related protein aggregation. To find out, the team used genetically engineered worms that express the toxic human proteins responsible for Huntington’s (polyglutamine, or polyQ) and ALS (mutant FUS and TDP-43).
Using RNA interference (RNAi) to knock down the eps-8 gene and its partner rac genes, the researchers observed a significant decrease in the aggregation of all the disease-related proteins. This effect occurred without simply reducing the total amount of the protein, suggesting that it specifically prevented the clumping process.
This reduction in protein clumps had a direct functional benefit. The worms’ neuronal health was preserved, as evidenced by their improved performance on behavioral tests like avoidance response and the ability to sense and move towards chemicals (chemotaxis).
Digging for the mechanisms
To confirm that high levels of EPS-8 were the direct cause, the team used a mutant worm producing a reinforced form of EPS-8. This mutation increases the protein’s levels from a young age. The worms showed accelerated protein aggregation and earlier onset of neuronal deficits, directly linking EPS-8 accumulation to the disease pathology.
The researchers found that the overactivated EPS-8/RAC pathway drives aggregation through at least two downstream mechanisms. The first one is excessive actin polymerization, which leads to the overproduction and destabilization of the cell’s actin cytoskeleton. While the exact link isn’t definitive, the authors hypothesize that a destabilized actin cytoskeleton could promote protein aggregation by causing actin to form aggregates, which then act as “seeds” for other toxic proteins to clump onto.
The second mechanism is hyperactivation of another signaling protein called JNK. Knocking down the worm’s JNK homolog, kgb-1, also effectively prevented protein aggregation. The authors suggest that the chronic upregulation of the JNK stress pathway during aging may lead to broader cellular changes that hurt the cell’s ability to maintain healthy proteostasis.
Moving further upstream, the team investigated why EPS-8 accumulates with age. They discovered that the culprit is a deubiquitinating enzyme (DUB), USP-4. Proteins in a cell are often tagged with a ubiquitin molecule to mark them for recycling. DUBs remove these tags, which prevents protein degradation.
The study showed that as worms age, levels of usp-4 increase. This leads to more EPS-8 being deubiquitinated and saved from destruction, which causes its accumulation. Importantly, knocking down usp-4 in aging worms reduced protein aggregation and extended their lifespan.
Confirmed in human cells
To confirm that these findings are relevant to humans, the researchers replicated their experiments in human cell models, including motor neurons derived from induced pluripotent stem cells (iPSCs) taken from an ALS patient. Remarkably, the results were similar. Knocking down human EPS-8 or USP-4 significantly reduced the aggregation of mutant HTT, FUS, and TDP-43 proteins. In the ALS patient-derived motor neurons, this intervention also reduced apoptosis and necroptosis, two forms of cell death.
“We are delighted to uncover a molecular mechanism that could shed light on how aging contributes to diseases like ALS and Huntington’s,” said first author Dr. Seda Koyuncu. “For years, we’ve known that age is the major common risk factor for different neurodegenerative diseases. However, how exactly age-related changes contribute to these diseases remains largely unknown. This study may contribute to filling in a part of that puzzle.”
Literature
[1] Hommen, F., Bilican, S., & Vilchez, D. (2022). Protein clearance strategies for disease intervention. Journal of Neural Transmission, 129(2), 141-172.
[2] Koyuncu, S., Loureiro, R., Lee, H. J., Wagle, P., Krueger, M., & Vilchez, D. (2021). Rewiring of the ubiquitinated proteome determines ageing in C. elegans. Nature, 596(7871), 285-290.
View the article at lifespan.io
