A special focus on rogue proteins may hold future promise in stopping the progression of nerve cell destruction in people who have amyotrophic lateral sclerosis (ALS); or frontotemporal dementia. ALS, a rare but devastating disorder that’s also known as Lou Gehrig’s disease; attacks the body’s nerve cells, resulting in progressive muscle weakness as the neurons degenerate over time. There is no cure. People with ALS eventually lose their strength and the ability to move their arms, legs, and body.
About a third of those with ALS also develop frontotemporal dementia (FTD); destruction of neurons in the brain that causes profound personality changes and disability. The two diseases are similar in both pathology and genetics. FTD tends to affect people earlier than Alzheimer’s disease; the most common type of dementia. RNA’s main function is to translate the molecular instructions from DNA to build proteins. For RNA to translated into protein, it has to correctly processed, and it has to last long enough to undergo translation. Normally, TDP-43 helps regulate both the processing and stability of RNA.
Energy and protein production
Barmada’s team demonstrated that when there is too much TDP-43, RNA is destabilized. The RNAs that most affected were involved in energy and protein production; two pathways that nerve cells need to survive. They found an identical pattern in cells from ALS patients, suggesting that TDP-43 may responsible. So, how does the buildup of TDP-43 cause nerve cells to die? Suspecting it involves TDP-43’s ability to bind RNA; Barmada and colleagues focused on manipulating this function of the protein.
Because protein structure is crucial for function, they wondered if they could alter TDP-43’s function; by changing its structure. By introducing specific mutations; they interrupted an interaction between two parts of the protein needed for RNA binding, creating versions of TDP-43 unable to bind RNA. Unexpectedly, they found that when TDP-43 cannot bind RNA it is quickly degraded; leading them to believe that these versions of TDP-43 would not so lethal for nerve cells.
Versions of TDP-43
To determine whether these engineered versions of TDP-43 are toxic to nerve cells; Barmada’s team used a method called automated microscopy. Here, thousands of nerve cells in culture are over time using a microscope that is entirely controlled by programs. Additional programs analyze the data; determine when each cell dies, and compare different conditions using methods from human clinical trials.
To further support their conclusions, the investigators teamed with Asim Beg, Ph.D., from U-M’s Department of Pharmacology, to create a worm model of TDP-43 with the same structural change using CRISPR/Cas9 genome engineering. Worms expressing these versions of TDP-43 were identical to worms with no TDP-43 at all, suggesting that the structure by the mutations is essential for TDP-43 function and toxicity.
Such an approach would likely take the form of a drug that would interact with TDP-43; interfere with its structure and cause TDP-43 to degraded. Even so, completely knocking TDP-43 out is not feasible, as the protein is essential for life. “But in theory, the dose can be adjustable, something that gives you a dial on TDP-43,” Barmada says. That leads him to the next question: “What’s the magic level?”