Some older adults develop the brain changes associated with Alzheimer’s disease but never experience dementia. Researchers at the Netherlands Institute for Neuroscience have found a possible clue: rare immature neurons in a small region of the brain’s memory center appear to activate protective biological programs in people who remain cognitively resilient. The findings do not prove that these cells prevent dementia or that they are newly generated, but they suggest that how immature neurons respond to disease may matter more than how many are present.
The mystery of cognitive resilience
Some people experience worsening memory and thinking problems as Alzheimer’s-related changes develop, while others remain mentally alert despite evidence of disease in their brains. Scientists have long struggled to explain this difference.
This ability to maintain memory and thinking despite brain damage is known as cognitive resilience.
“About 30 percent of older adults who develop Alzheimer’s never experience symptoms,” says Evgenia Salta, senior author of the study. “We really don’t know why. It’s a big mystery and a very important mystery.”
Understanding cognitive resilience could change how researchers approach Alzheimer’s disease. Instead of focusing only on what causes brain cells to deteriorate, scientists could also investigate how some parts of the brain remain healthy and functional.
Can the aging brain be repaired?
One possible explanation is that some brains become better able to repair or defend themselves as Alzheimer’s disease progresses.
“Perhaps we can add new brain cells to networks that are degenerating,” Salta says.
This possibility involves adult neurogenesis, the generation of new nerve cells, or neurons, in the adult brain. Neurons transmit information throughout the nervous system, allowing people to think, learn, remember and respond to their surroundings.
Scientists have demonstrated adult neurogenesis in several animal species. Whether it continues in the adult human brain, however, has been debated for decades.
If the human brain can generate new neurons later in life, those cells could help preserve important connections when aging and disease damage existing networks. But finding immature neurons does not necessarily prove that they formed recently.
To investigate the possibility, Salta and her colleagues analyzed donated human brain tissue obtained through the Netherlands Brain Bank, a research resource that collects and stores brain samples.
The researchers compared three groups: people without detectable brain lesions, people who developed Alzheimer’s disease, and people who had Alzheimer’s-related changes in their brains but remained cognitively resilient.
The team focused on a small region within the brain’s memory center, which is thought to be one of the few areas where new neurons may develop.
“These cells are so rare that we needed to develop new ways to find them,” Salta says. “We actually zoomed in on exactly where we expected them to be.”
The researchers also developed a new method for analyzing the data. This allowed them to identify cells more accurately without relying as heavily on assumptions derived from animal studies.
Immature neurons found in people over 80
The search produced an unexpected result: immature neurons were present in all three groups, including the brains of older adults.
The cells resemble neurons that have not fully matured, suggesting that the aging brain may retain some cellular characteristics associated with development and adaptability.
“Even though the average age was over 80 years, we still found these immature neurons in all groups,” Salta says.
The most notable differences emerged when the researchers compared people who developed dementia with those who remained mentally alert despite Alzheimer’s-related brain changes.
At first, the scientists expected resilient people to have more immature neurons. If the cells helped replace damaged neurons, a larger population might explain why some brains withstand disease better than others.
Instead, the difference in the number of immature neurons between the groups was much smaller than expected.
This finding pointed to another possibility: cognitive resilience may depend less on how many immature neurons the brain contains and more on how those cells respond when the brain is under attack.
Possible protective responses
When the researchers examined the activity of the immature neurons more closely, they found striking differences between the groups.
In cognitively resilient individuals, the cells showed signs of activating biological processes associated with survival and protection from damage.
“In resilient people, these cells appear to activate programs that help them survive and cope with injury,” Salta says. “We also saw a reduction in signals associated with inflammation and cell death.”
Inflammation is part of the body’s normal defense system. When it persists in the brain, however, it can cause damage and interfere with communication between neurons. Cell death gradually destroys the networks involved in memory and thinking.
The findings suggest that immature neurons in resilient brains may be better able to withstand these harmful conditions.
Their possible role may extend beyond protecting themselves or replacing neurons that have died.
“It may be more than just replacing lost neurons,” Salta explains. “These cells may support surrounding tissues and help keep the brain functional and ‘youthful’. They may act as a kind of fertilizer for a garden that is starting to crumble.”
In this interpretation, immature cells could help maintain the environment around existing brain cells rather than serving solely as replacements for dying neurons. That support might allow neural networks to continue functioning as Alzheimer’s-related damage accumulates.
Why the findings remain preliminary
The researchers emphasize that the study does not establish exactly what the immature neurons do. Their conclusions about a possible protective role are based on patterns observed in donated brain tissue, not on direct experiments demonstrating the cells’ function.
“We make inferences about cell function based on data, but this type of study cannot confirm that,” Salta explains.
This distinction is important because research into adult neurogenesis has attracted considerable public attention, sometimes leading to claims that go beyond what scientists have demonstrated.
The presence of immature neurons does not prove that new neurons are continuously generated in older people. It also does not prove that these cells prevent dementia or cause cognitive resilience.
Other biological mechanisms almost certainly contribute to the ability to resist cognitive decline.
“This is one piece of a very large puzzle,” Salta concludes. “There is never just one factor that explains resilience.”
Could resilience research help protect the brain?
The findings raise broader questions about aging. Why do some brains remain relatively stable while others undergo progressive cognitive decline?
“There’s a kind of decision point somewhere along this trajectory,” Salta explains. “Some people remain stable, while others develop dementia. We want to understand what causes the difference.”
As a next step, the researchers plan to investigate how immature neurons communicate and interact with neighboring brain cells. These relationships could help determine whether the cells actively contribute to resilience and how their protective responses work.
The study also reflects a broader shift in Alzheimer’s research. Traditionally, much of the field has focused on processes that damage the brain, including the accumulation of abnormal proteins and the loss of neurons. Researchers are increasingly asking why some brains continue to function despite those changes.
By identifying the mechanisms behind this natural resistance, scientists hope to find new ways to preserve memory and cognitive abilities as people age.
“Cognitive resilience is really interesting,” Salta says. “Understanding what it is that protects these brains could ultimately lead to new therapeutic strategies.”
A cure for Alzheimer’s disease remains a long way off, but the findings suggest that the aging human brain may possess protective capabilities that scientists are only beginning to understand.
Source: www.sciencedaily.com
