UNC and NC State researchers team up to reverse TDP–43 protein aggregation, a hallmark of degenerative diseases.
Scientists have long known that a protein called TDP–43 clumps together in brain cells of people with amyotrophic lateral sclerosis (ALS). Now, UNC School of Medicine and NC State researchers found that a specific chemical modification called acetylation promotes TDP–43 clumping in animals. Using a natural anti–clumping method in mouse models, the scientists reversed protein clumping in muscle cells and prevented the sIBM–related muscle weakness.

The discovery, published in the journal Nature Communications, has important implications for understanding ALS and sIBM, and for the creation of potential treatments down the road.

For many years, no one knew how TDP–43 moved out of its normal workspace in the cell nucleus. In a 2015 study, Cohen and colleagues found one possible factor: a chemical modification known as acetylation.

Cells commonly use acetylation to switch the activity of proteins on or off. In 2015, Cohen and colleagues reported that acetylation at two spots on TDP–43 caused the protein to detach from RNA. The protein then drifted into the cytoplasm and started to aggregate. This research was done in cells grown in lab dishes. But to underscore the potential relevance to human disease, the scientists examined spinal motor neurons from ALS patients and identified aggregates of TDP–43 that had been acetylated in the same way.

For the new study, the researchers examined the effect of acetylated TDP–43 in living animals. In this case, they sought to mimic sIBM in mice, in which TDP–43 clumps in muscle cells.

Michael Bereman, PhD, an author on the paper and assistant professor at NC State University, uses mass spectrometry to study modifications to proteins.

“It’s pretty amazing that a modification that represents 0.1 percent of the protein mass can have such a detrimental effect in essentially initiating the processes culminating in cell death,” said Bereman, a member of the Center for Human Health and the Environment at NC State. “But that just emphasizes how complex disease biology can be. It’s quite possible that other modifications to this protein could act in a similar manner. So it’s imperative we investigate this with new technologies in sample preparation and mass spectrometry.”

Cohen and his team used a special method to inject acetylated TDP–43 proteins directly into mouse muscle cells. In contrast to ordinary TDP–43 proteins, these acetylated proteins quickly aggregated outside the nucleus. The aggregate–burdened cells showed multiple features that are also seen in human sIBM.

The researchers observed cellular markers indicating that the muscle cells were actively trying to get rid of the TDP–43 aggregates. Cohen and his team found that they could boost these cell defense mechanisms and swiftly remove most of the aggregates by adding heat shock factor 1 (HSF1), a naturally occurring protein that is known to work as a master switch for anti–aggregation processes in cells.

Cohen’s team now hopes to identify compounds suitable for use in oral drugs that have the same anti–clumping effect. Bereman hopes to develop targeted protein assays of acetylated and ubiquitinated forms of TDP–43 to aid Cohen’s drug screening efforts as a measure of drug effectiveness.