SAN DIEGO—A team at the University of California, San Diego (UCSD) School of Medicine believe they have found a critical clue to one of the causes of amyotrophic lateral sclerosis (ALS)—a promising discovery about a disease that has no known cure. Through cellular manipulation in the laboratory, the team found that exposing a critical protein, TDP-43, to stressors that impede normal function may create conditions that allow for the protein to form clumps of excess in the surrounding cytoplasm, which may contribute to the motor neuron degeneration found in ALS. The results of their research have been published in the July issue of the journal Neuron.
The TDP-43 protein performs crucial and healthy functions, such as binding to DNA and regulating genetic transcription, or protein production. It can also bind to RNA and ensure its stability by processing messenger RNA (mRNA) in various configurations to produce different proteins, serving as the genetic blueprint for the regulation of cell function. The UCSD researchers found that samples from ALS patients reviewed under the microscope indicated that TDP-43 was collecting into clumps that prevented it from performing its normal function, leading to a protein build-up thought to impact motor neurons.
Senior author Dr. Gene Yeo, a professor at the university’s medical school and faculty member in the Sanford Consortium for Regenerative Medicine, elucidated their scientific process: “The hypothesis we were working with is that stress granules which are RNA binding protein-RNA aggregates that form when cells are under duress lead chronically to the recruitment of TDP-43 into these stress granules,” he explained. “Ultimately, sequestration of TDP-43 in the cytoplasm precipitated or accelerated by stress granules results in loss of nuclear presence and function of TDP-43, leading to neuronal death.”
Cellular stresses come in many forms, such as heat shock (fever), virus infections, oxidative stress, inflammation or neuronal/brain trauma. In most forms, it would be difficult to avoid stressors in daily life, but most individuals will have a resilient stress granule response and resolution upon reduction of the stressor. However, genetically predisposed individuals—such as those with ALS-linked mutations—will have issues clearing these granules.
According to the paper, Yeo and his team generated motor neurons from induced pluripotent stem cells that had been converted from human skin cells. To mimic cellular aspects of ALS, they exposed these laboratory motor neurons to toxins such as puromycin, which stressed the cells and led to TDP-43 clumps. They were then able to identify corresponding compounds that helped in preventing the TDP-43 accumulation, and seemed to aid in extending the lifespan of the associated neurons before neural death occurred. “These compounds could provide a starting point for new ALS therapeutics,” said Yeo.
After testing thousands of candidates, they identified compounds that not only reduced the overall amount of clumping by up to 75 percent, but also added varieties to the makeup of the clumps, affecting the number and size per cell. As noted by Dr. Mark Fang, a first author on the study, these discoveries could be the basis for a long-awaited therapeutic for ALS patients.
“While these findings still need to be tested in model organisms, and there is more work to do before a potential therapy could one day be tested in patients,” he said, “these compounds already expand our toolbox for unraveling the relationship between RNA-protein aggregations and neurological disease.”
The scientists also found that some of their compounds were molecules able to insert into DNA or RNA. Known as planar-moiety containing compounds, they believe that this insertion may block TDP-43 from properly binding with RNA and impacting clump growth, a factor that makes sense to Yeo.
“Earlier work by Aaron Gitler’s lab and our own studies suggests that modulating stress granule proteins can have ameliorating effects on toxicity driven by mutant forms of TDP43 or FUS/TLS or dipeptide repeats from C9ORF72 mutation,” Yeo elaborates. “Our current study approaches this from another angle, which is to identify small molecules that modulate stress granule formation and/or dissipation. We were excited to find compounds that affected stress granule size and mature composition, preventing persistence of TDP-43 in the cytoplasm. Interestingly, some of these planar-moiety containing compounds lead to improved survival in primary neurons expressing ALS-linked mutations in TDP-43.”
While a truly therapeutic intervention will require some additional testing on in-vivo samples, this is a welcome glimmer of hope. More than 20,000 Americans suffer from this disease, which is poorly understood and, so far, invariably fatal.