Brain cell ‘powerhouses’ may fuel dementia pathology

A new study reveals that specific chemical signals, generated within the energy-producing structures of the brain’s support cells, may actively promote the pathology seen in dementia. Researchers found that blocking the production of these signals at their source in mice reduced brain inflammation, protected neurons, and extended lifespan, suggesting a new path for treating neurodegenerative disorders. The findings were published in the journal Nature Metabolism.

Our cells contain tiny structures called mitochondria, often described as cellular power plants because they generate energy from the food we eat. A natural byproduct of this energy production is a group of chemically reactive molecules known as reactive oxygen species, or ROS. At low levels, these molecules are important for normal cell communication, but when produced in excess, they can cause damage to cellular components. This damaging state is sometimes referred to as oxidative stress.

For many years, this type of damage has been linked to neurodegenerative diseases like Alzheimer’s and frontotemporal dementia. This connection led to clinical trials of general antioxidant therapies, which are designed to neutralize ROS throughout the body.

“But most antioxidants tested in clinical studies have failed,” said Adam Orr, an assistant professor of research in neuroscience in the Feil Family Brain and Mind Research Institute at Weill Cornell Medicine, who co-led the research. He suggested that this lack of success might be because these general antioxidants cannot block ROS selectively at their specific source without also disrupting normal cell metabolism.

The research team, co-led by Orr and Anna Orr, the Nan and Stephen Swid Associate Professor of Frontotemporal Dementia Research at Weill Cornell Medicine, decided to investigate with more precision.Adam Orr had previously developed a method for identifying molecules that could suppress ROS production from singular sites within the mitochondria without affecting the organelle’s main job of energy production. This work yielded a set of small molecules the team calls S3QELs, which specifically block ROS from a mitochondrial site known as Complex III.

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The researchers began their investigation using brain cells grown in laboratory dishes. They exposed these cells to stimuli associated with neurodegenerative disease, including inflammatory molecules and the amyloid-beta protein, a hallmark of Alzheimer’s disease. They observed that these stimuli prompted the cells to produce more ROS. Using their specialized S3QEL molecules, the team confirmed that a significant portion of this increase was coming specifically from Complex III within the mitochondria.

A surprising observation came when the team was studying neurons cultured alongside star-shaped support cells called astrocytes. They found that the S3QEL molecules protected neurons from damage, but only when astrocytes were also present in the culture. “This suggested that ROS coming from Complex III caused at least some of the neuronal pathology,” said Daniel Barnett, a graduate student in the Orr laboratory and the study’s lead author. The finding pointed to astrocytes, not neurons themselves, as the key source of this particular damaging signal.

To understand how these signals were being generated, the scientists traced the molecular chain of events inside the astrocytes. They determined that the process begins with the activation of a master switch for gene activity called nuclear factor-κB. This switch then appears to engage a mitochondrial channel called the sodium-calcium exchanger, or NCLX, which in turn triggers the production of ROS at Complex III. This step-by-step pathway shows a specific and regulated mechanism, not a random burst of chemical byproducts.

Having identified the source and the trigger, the team next sought to understand what these ROS signals were actually doing to the cell. They used advanced techniques to map the effects of the signals on the cell’s proteins, metabolism, and gene activity. The analysis showed that the ROS from Complex III did not cause widespread, random damage. Instead, they selectively modified a distinct set of proteins involved in the cell’s immune and metabolic functions.

This highly specific effect was not anticipated. “The precision of these mechanisms had not been previously appreciated, especially not in brain cells,” said Anna Orr. “This suggests a very nuanced process in which specific triggers induce ROS from specific mitochondrial sites to affect specific targets.” The ROS signals also amplified the activity of thousands of genes inside the astrocytes, particularly those related to inflammation. A protein called STAT3 appeared to be a major mediator of these genetic changes.

The final step was to see if blocking this pathway could have a therapeutic effect in a living animal. The researchers used a mouse model of frontotemporal dementia that develops brain pathology similar to that seen in human patients. They administered an S3QEL inhibitor to the mice after the disease process had already begun. The treatment reduced signs of astrocyte activation and blunted the activity of neuroinflammatory genes in the brain. It also reduced a chemical modification on the tau protein, a change that is a common feature of several forms of dementia.

When the treatment was given to the mice for a longer period, it extended their lifespan. The compound was well tolerated and did not produce obvious side effects, which the researchers attribute to its highly specific action. “I’m really excited about the translational potential of this work,” said Anna Orr. “We can now target specific mechanisms and go after the exact sites that are relevant for disease.”

This study presents a different perspective on the role of reactive oxygen species in disease. Rather than viewing them as simply agents of indiscriminate damage, the findings frame them as specific signaling molecules in a complex communication network. The failure of broad antioxidant therapies in the past may be explained if the key is not to eliminate all ROS, but to selectively inhibit their production at the specific sources that become overactive in disease states.

The researchers plan to continue exploring how factors linked to dementia influence this signaling pathway in the brain. They also intend to investigate whether genes that increase or decrease a person’s risk for neurodegenerative disease might affect ROS generation from these specific mitochondrial sites. The team hopes to further develop these inhibitor compounds into a new class of therapeutics for human use. “The study has really changed our thinking about free radicals and opened up many new avenues of investigation,” Adam Orr said.

The study, “Mitochondrial complex III-derived ROS amplify immunometabolic changes in astrocytes and promote dementia pathology,” was authored by Daniel Barnett, Till S. Zimmer, Caroline Booraem, Fernando Palaguachi, Samantha M. Meadows, Haopeng Xiao, Man Ying Wong, Wenjie Luo, Li Gan, Edward T. Chouchani, Anna G. Orr, and Adam L. Orr.