Analyzing the aging brain offers hope for dementia

Living a long, healthy life is the goal for most people, but longevity has its own risks. Few of these risks loom as large as losing memory and the ability to reason—the chief symptoms of dementia. Alzheimer’s disease and other dementias affect 6.2 million Americans, of whom 72% are age 75 or older. And of those aged 65 and older, 11.3% have Alzheimer’s dementia.

While researchers have figured out how to diagnose Alzheimer’s disease (the most common form of dementia) through the presence of tau protein and amyloid plaques, they do not know whether these characteristics are results or causes of the disease. The source of other dementias remains similarly mysterious.

Yale School of Medicine researchers are investigating the diverse processes involved in dementia and are finding new and complex interactions. They hope that research will yield opportunities for treatments that could delay or mitigate the devastating effects of Alzheimer’s disease, perhaps within the next decade.

Jaime Grutzendler, MD, the Dr. Harry M. Zimmerman and Dr. Nicholas and Viola Spinelli Professor of Neurology and Neuroscience and the director of the Center for Experimental Neuroimaging, leads a team at the Grutzendler Lab that studies the mechanisms of dementia. “Many things happen with aging, including vascular disease, myelin disruption, and the accumulation of abnormal proteins inside and outside the cell. There is a complex interplay between different pathological processes that eventually lead to dementia,” said Grutzendler. “After a certain age, everybody has a mixture of these pathologies, rather than one pure type of pathology.”

The Grutzendler Lab focuses on three areas of technical innovation: microscopy; methods for manipulating, labeling, and separating cells in vivo; and therapeutics. “All of these tools allow us to understand and potentially manipulate the process,” Grutzendler said. “[Brain] processes are multifactorial. By developing subtle ways to manipulate the system to initiate certain processes, we can better determine what comes first and what is a consequence.”

The lab uses imaging to observe live models of cellular injury or repair as they occur, much like examining a crime scene video for clues. “A major problem is understanding neurodegeneration in the brain. We can’t know what happens with aging without clearly seeing who the players are, what’s dying, when it died, and how it’s removed,” said Eyiyemisi Damisah, MD, an assistant professor of neurosurgery who works in Grutzendler’s lab. “When I joined the lab, I wanted to see if it was possible to kill a single cell in a live brain and observe what happens.”

To accomplish this task, Grutzendler’s team developed a technique called two-photon chemical apoptotic targeted ablation (2Phatal), which uses focal illumination with a femtosecond-pulsed laser to bleach a specific dye, H33342, that binds to the DNA of a single cell. This photobleaching causes the cell to die without damaging the cells around it. The researchers then observe how the cells around the dead cell compensate, and which cells remove pieces of the dead cell. This process, in essence, is how the brain removes its cellular garbage.

Watching the brain clean itself in real time has revealed an amazing system. Microglia—resident macrophages in the brain—swallow up dead cell bodies, while astrocytes remove the dendritic branches spanning the brain’s hemispheres. Prior to 2Phatal, this process had never been seen in a live brain.

Seeing the system function also identified a crucial component in the brain’s self-cleaning process. One type of cleaning has redundancy built into it, but the other does not. “There’s a division of labor,” said Damisah. “If we remove microglia, the astrocytes rally at a slow pace to remove debris. But when you disable astrocytes, it becomes a terrible problem.” She describes dementia as a cleanup process gone awry, likening it to a city with faulty garbage disposal. As debris accumulates, it becomes impossible for cars to travel and people to leave their homes. Eventually, the volume of trash halts movement entirely, and commerce in the neighborhood grinds to a halt.

As research uncovers new intricacies, Grutzendler notes that many processes are happening over time. “In dementia, we think the myelin wrapping is affected in subtle ways over decades,” he said. His lab developed a microscopy method called spectral confocal reflectance microscopy, or SCoRe, to image these wrappings. “When light is shined into the brain, it bounces off the myelin wraps’ surface, allowing us to visualize them beautifully,” he said. “SCoRe opens up the possibility of studying this cell type, which was difficult to study before, and understand it in the context of aging, dementia, and the garbage collection process.”

Le Zhang, PhD, assistant professor of neurology at Yale School of Medicine, is involved in a different area of dementia research. Zhang works in the field of single-cell RNA sequencing. “This technique is being applied in all kinds of research in animal models of human disease,” she said. “We’re able to do single-nucleus RNA sequencing and can extend this to frozen or postmortem human brain tissue, enabling the study of Alzheimer’s or other brain diseases.”

RNA molecules are fundamental to interpreting the functional elements of the human genome and understanding disease. Thus an RNA analysis of a single cell can reveal a genetic issue for that cell. It is then possible to extrapolate about what may have caused the problem. Zhang is also using this technique to study sex differences in the development of dementia.

Zhang explained that it is possible to gather the RNA information in parallel and study thousands of cells in a single run of analysis. “Eventually, we will have the whole transcriptome and a road map for how all the cells function and all the cellular information regarding the different cell types,” she said.

By comparing the RNA in a healthy brain and that from a diseased brain, scientists can see which cells are being damaged and what is happening within all of the different cells. “Alzheimer’s disease could be the result of damage to many different cells,” Zhang said. “Recently we found that there are changes in the neurons, microglia, astrocytes, and even oligodendrocytes.”

As the secrets of the aging brain are unraveled, researchers remain optimistic about the development of effective targeted therapeutics to treat dementia.