Stalled protein processing may underlie Alzheimer’s disease

A recent study published in eLife offers a new perspective on the development of Alzheimer’s disease, suggesting that the condition may be triggered by disruptions in how the brain processes proteins. Researchers found that mutations in a specific gene can cause protein processing to stall, potentially leading to the brain changes associated with Alzheimer’s. This finding could pave the way for the development of new treatments focused on correcting these protein processing issues.

For many years, scientists have been trying to understand what causes Alzheimer’s disease, a devastating condition that robs people of their memory and thinking abilities. One of the main ideas guiding research has been the amyloid cascade hypothesis. This idea proposes that the disease begins with the buildup of a protein called amyloid beta in the brain. It is thought that this buildup starts a chain of events that ultimately damages brain cells and leads to dementia.

While much research has focused on this amyloid beta protein, the precise ways in which it becomes harmful and how to effectively target it with treatments remain unclear. Many clinical trials aimed at reducing amyloid beta or preventing its buildup have had limited success. This has led scientists to reconsider whether amyloid beta is truly the primary cause of Alzheimer’s disease and to explore other potential mechanisms.

In this new study, researchers turned their attention to the process that creates amyloid beta in the first place. This process, called proteolysis, involves an enzyme known as gamma-secretase. Gamma-secretase acts like a pair of molecular scissors, trimming a larger protein, called amyloid precursor protein, into smaller pieces, one of which is amyloid beta.

Previous work by the research team had indicated that mutations in a gene called presenilin-1, which is linked to early-onset Alzheimer’s disease, might disrupt the normal trimming action of gamma-secretase. They discovered that these mutations seemed to prevent gamma-secretase from efficiently cutting amyloid precursor protein. This led to a buildup of incompletely processed protein fragments. Furthermore, in a simple laboratory model using worms, they found that these stalled protein complexes could cause brain cell damage even when amyloid beta was not present.

Free daily newsletter

“Given the problems with the long-standing amyloid hypothesis of the cause of Alzheimer’s disease, we sought an alternative explanation,” said senior author Michael Wolfe, the Mathias P. Mertes Professor of Medicinal Chemistry at the University of Kansas.

“We focused on rare mutations that cause early-onset Alzheimer’s disease. These mutations are found in the genes that encode the enzyme and substrate that produce the amyloid beta-peptide, so there must be something about this process that is triggering neurodegeneration. In a previous report last year, we showed that it’s the stalled process, not the amyloid products that can cause degeneration of neural connections in a simple genetic animal model system.”

Building on this earlier work, the scientists wanted to investigate further how different mutations in the presenilin-1 gene affect the entire protein processing pathway. They focused on six additional mutations known to cause early-onset Alzheimer’s disease, which strikes people at a younger age, typically between their late twenties and fifties. These mutations are of particular interest because they are studied by a large research network called the Dominantly Inherited Alzheimer Network, which focuses on inherited forms of the disease.

To examine the effects of these mutations, the researchers first created different versions of the gamma-secretase enzyme in the laboratory, each carrying one of the six mutations. They then purified these mutant enzymes and mixed them with fragments of amyloid precursor protein. This allowed them to observe how each mutated enzyme processed the precursor protein.

To measure the results of this protein processing, they used a technique called mass spectrometry. This method allowed them to identify and precisely measure the amounts of different protein fragments produced by each mutated enzyme. By analyzing these fragments, the team could determine exactly how each mutation altered the way gamma-secretase trimmed the amyloid precursor protein.

The researchers discovered that all six mutations disrupted the normal protein processing carried out by gamma-secretase. However, the specific nature of these disruptions varied depending on the particular mutation. They found that all the mutations caused problems at multiple steps in the trimming process. This indicated that the mutations were not simply stopping the process entirely but rather causing it to become inefficient and error-prone.

“The enzyme that produces amyloid does so through a series of cleavage reactions on the precursor protein,” Wolfe told PsyPost. “We were surprised that disease-causing mutations can disrupt distinct cleavage steps. It apparently doesn’t matter which of these steps are stalled. Stalling at any stage may be sufficient to trigger loss of neuronal connections.”

In addition to analyzing the protein fragments, the researchers also wanted to understand how these mutations affected the interaction between the gamma-secretase enzyme and the amyloid precursor protein. They hypothesized that the mutations might be causing the enzyme to get ‘stuck’ to the precursor protein, leading to the stalled processing they had observed.

To test this idea, they used a technique involving fluorescently labeled antibodies. They attached fluorescent tags to antibodies that would bind to both the amyloid precursor protein fragment and the gamma-secretase enzyme. By measuring the fluorescent signal, they could determine when the enzyme and the precursor protein were in close proximity, indicating that they were bound together.

The researchers found that for all the mutations they tested, the fluorescent signal was reduced compared to the normal, functional enzyme. This reduction in signal suggested that the mutations were indeed causing the enzyme and the precursor protein to bind together more tightly and for a longer duration. In other words, the enzyme-substrate complexes were more stable in the mutated versions.

This finding supported their idea that the mutations were causing protein processing to stall. The enzyme was binding to the precursor protein but not efficiently completing the trimming process, resulting in a ‘stuck’ or stalled complex.

According to the researchers, these results align with their ‘stalled complex’ hypothesis. They suggest that it is not just the amyloid beta protein itself, but these stalled enzyme-substrate complexes that may be triggering the brain cell damage seen in Alzheimer’s disease. Even if amyloid beta protein production is reduced, the presence of these stuck protein processing complexes could still initiate the disease process.

“Amyloid may be coincidental with the true cause of the disease,” Wolfe said. “Our evidence suggests that the stalled process of trying to make amyloid, rather than amyloid itself, may initiate the degeneration of neuronal connections.”

Future research will need to confirm these findings in more complex systems, such as animal models that more closely mimic human Alzheimer’s disease. It will also be important to investigate whether similar mechanisms are involved in the more common, late-onset form of Alzheimer’s disease, which is not directly caused by these specific genetic mutations.

Looking ahead, the researchers aim to continue exploring this ‘stalled complex’ hypothesis and to identify potential drug candidates that can help restore normal protein processing in Alzheimer’s disease. They hope that this new direction in research will lead to truly effective treatments for this devastating condition.

“We hope to identify a therapeutic strategy that is truly effective, unlike the currently approved drugs for Alzheimer’s disease,” Wolfe said. “We have already begun this search to find drug candidates that rescue stalled enzymes.”

“This field of investigation has been long invested in the amyloid hypothesis. It will take many more studies to turn the direction of research toward this new amyloid-independent hypothesis.”

The study, “Alzheimer-mutant γ-secretase complexes stall amyloid β-peptide production,” was authored by Parnian Arafi, Sujan Devkota, Emily Williams, Masato Maesako, and Michael S. Wolfe.