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Jan 14

C9orf72 toxicity driven by neuronal activity, according to new study

ALS Headlines, Packard Center News
A new Packard study found that found that excitotoxic stress and hyperactivity in neurons increased the production of toxic peptides associated with the C9orf72 repeat expansion.

A six nucleotide repeat in the C9orf72 gene is the most common genetic cause of ALS and frontotemporal dementia (FTD). In a new study in EMBO Molecular Medicine, a team led by Packard scientist Davide Trotti and Aaron Haeusler, in collaboration with Packard science director Piera Pasinelli, all at the Weinberg ALS Center of the Vickie and Jack Farber Institute for Neuroscience at Jefferson, found that excitotoxic stress and hyperactivity in neurons, which are often coupled to an integrated stress response, increased the production of toxic peptides associated with the C9orf72 repeat expansion.

“This work identified drivers of and cell-type specific differences for the production of potentially toxic dipeptides linked to the aberrant C9orf72 repeat expansions in ALS/FTD. Intriguingly, we found that the production of these peptides is coupled to neuronal activity. The more these cells are excited and/or stressed, the more they produce these potentially neurotoxic peptides. Understanding of these basic pathogenic processes of disease led us to uncover novel therapeutic candidates that may be repurposed in the near future to reduce the production of toxic DPRs and begin treating C9orf72 ALS and FTD neurodegeneration,” Trotti says.

Since the C9orf72 repeat expansion was discovered in 2011, scientists have been working to understand how these hundreds to thousands of repeats cause disease. The repeats may interfere with the normal function of the gene by preventing the synthesis of the normal C9orf72 protein specifically and many cellular proteins more broadly. The large number of repeats also confuse the cell’s translation machinery that turns DNA into RNA and then into protein. Thus, instead of producing the ‘normal’ C9orf72 protein, the machinery directs the synthesis of five unique toxic dipeptide repeat proteins (DPRs). Each DPR is localized to a different part of the cell and interacts with a different group of proteins, although all lead to neural degeneration and death. Understanding the factors that drive the production of DPRs could allow researchers to target the process with newer therapeutics.

Using a fluorescent tag to monitor DPR production and decay, Haeusler, Trotti, and colleagues showed that DPRs have a long half-life that varies depending on cellular localization. When they treated cultured motor neurons and C9orf72 patient-derived motor neurons with stress-inducing compounds and induced excitotoxic stress, the production of DPRs increased. Repeatedly depolarizing rat cortical neurons that carried the C9orf72 repeat expansion also increased DPR production. Pretreating primary cortical neurons with glutamate, homocysteine, and NMDA antagonists reduced the stress-induced production of DPRs. This increased synthesis is part of the cell’s integrated stress response protocol, as the authors showed that the stress response protein PERK, a kinase that phosphorylates eif2α, increased in tandem with DPR production. Targeting these stress response proteins also reduced DPR synthesis.

These results, taken with other findings that show cellular stress can shift the types of RNA the cell makes, indicate that specifically targeting stress response proteins may help reduce DPR assembly in C9orf72 ALS/FTD patients.

“Our treatment results using a few select FDA-approved compounds that target components of the integrated stress response show that we can reduce overall DPR levels in vitro, demonstrating the potential to reduce toxicity linked to the C9orf72 mutation in patients. In future efforts, we will further expand our drug discovery efforts to identify more effective compounds and explore combinatorial therapy approaches that target both the cellular stressor and the deleterious response,” Hausler says.

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