Approximately 5,000 people in the US develop amyotrophic lateral sclerosis (ALS) each year. According to the Centers for Disease Control and Prevention, they only survive two to five years on average after they are diagnosed. The rapidly progressive neurodegenerative disease causes the death of neurons in the brain and spinal cord, resulting in muscle weakness, respiratory failure and dementia. Despite the devastating nature of the disease, little is known about what first causes the deterioration of motor neurons at the onset of ALS.
Now researchers from the University of California San Diego and their colleagues report that they have identified a key pathway that causes neurodegeneration in the early stages of the disease. The findings could lead to the development of therapies to prevent or slow the progression of ALS at an early stage, before major damage has been done. The study was published on October 31, 2024 in Neuron.
A protein called TDP-43 is usually found in the nucleus of motor neurons, where it regulates the gene expression needed for the cells to function. Studies have shown that when TDP-43 instead accumulates in the cytoplasm, outside the nucleus, it is a telltale sign of ALS. How the protein ends up in the wrong place, leading to neuronal degeneration, has baffled researchers until now.
By the time you see a patient with ALS and you see the TDP-43 protein aggregated in the cytoplasm, it looks like the accident scene, where all the cars have already crashed, but that is not the initiating event.”
Gene Yeo, Ph.D., corresponding author, professor in the Department of Cellular and Molecular Medicine at UC San Diego School of Medicine and director of the Center for RNA Technologies and Therapeutics and the Sanford Stem Cell Institute Innovation Center
Tracing the events that led to the “accident”, Yeo explains that another protein called CHMP7 -; normally found in the cytoplasm -; Instead, it accumulates in the nucleus, causing a cascade of events that ultimately lead to motor neuron degeneration. But what actually causes CHMP7 to accumulate in the nucleus?
Yeo and his team screened for RNA-binding proteins that could influence the assembly of CHMP7 in the nucleus. This yielded 55 proteins, 23 of which had a potential link to ALS pathogenesis. Inhibiting the production of several of these proteins led to an increase in CHMP7 in the nucleus. Further experiments with motor neurons created from ALS patient-derived induced pluripotent stem cells resulted in the surprising discovery that depleting one of these, an RNA splicing-associated protein called SmD1, previously not known to affect CHMP7 levels, led to the largest increase in nuclear accumulation.
A buildup of CHMP7 in the nucleus damages nucleoporins, which Yeo likens to tiny portals in the membrane that separate the nucleus from the cytoplasm and orchestrate the movement of proteins and RNA between the two cellular spaces. Dysfunctional nucleoporins allow TDP-43 to leave the nucleus and accumulate in the cytoplasm. Once there, the protein can no longer oversee the gene expression programs necessary for neurons to function.
However, when the researchers increased SmD1 expression in cells, CHMP7 was restored to its usual location in the cytoplasm, leaving the nucleopores intact, allowing TDP-43 to remain in the nucleus, sparing the motor neurons from degeneration.
“You can actually restore the localization of this CHMP7 protein and therefore all the downstream effects,” says Norah Al-Azzam, first author of the study, then a neuroscience student in the Yeo lab who later received her PhD. in the spring of 2024.
Furthermore, the SmD1 protein is part of SMN, a multiprotein complex. SMN dysfunction has been implicated in another neurodegenerative disorder, spinal muscular atrophy.
“We’re intrigued because there are actually therapies for spinal muscular atrophy,” Yeo said. “One of these, risdiplam, is a small molecule compound that enhances the splicing and expression of SMN2, a gene closely related to the SMN1 gene that becomes dysfunctional in ALS.”
This suggests the possibility that using risdiplam to increase SMN levels could prevent ALS from progressing beyond the earliest stages of the disease.
“It’s not like all the neurons die at once,” says Yeo. “Some neurons die first, then they spread to other neurons. Maybe once you get symptoms, we can treat the patient so that the rest of the neurons don’t crash and hope that you stop the progression of ALS.”
The researchers think the SMN complex could play a crucial role in the development of ALS, but further research is needed. The next steps will be to raise funds to continue research into animal and other genetic models of ALS, and ultimately test the effectiveness of risdiplam or other compounds for short-circuiting ALS.
Other co-authors of the study include: Jenny H. To, Vaishali Gautam, Dylan C. Lam, Chloe B. Nguyen, Jack T. Naritomi, Assael A. Madrigal, Benjamin Lee, Anthony Avina, Orel Mizrahi, Jasmine R. Mueller, Willard Ford, Anthony Q. Vu, Steven M. Blue, Yashwin L. Madakamutil, Uri Manor, Cara R. Schiavon and Elena Rebollo, all at UC San Diego; Wenhao Jin at Sanford Laboratories for Innovative Medicines; Lena A. Street and Marko Jovanovic at Columbia University; Jeffrey D. Rothstein and Alyssa N. Coyne at Johns Hopkins University School of Medicine.
The study was funded in part by the National Institutes of Health (grants R01HG004659, U24 HG009889, R35GM128802, R01AG071869, and R01HG012216), the National Science Foundation (MCB-2224211), and the Chan-Zuckerberg Initiative.
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Magazine reference:
Al-Azzam, N., et al. (2024). Inhibition of RNA splicing causes the nuclear entry of CHMP7, which impacts the function of TDP-43 and leads to the onset of cellular ALS phenotypes. Neuron. doi.org/10.1016/j.neuron.2024.10.007.