Scientists Have Discovered a Hidden Change in the Brain that Begins Around the Age of 50

A study funded by the National Institutes of Health (NIH) has revealed a significant change in the immune environment of the hippocampus, the part of the brain that plays a central role in learning and memory. The findings suggest that this remodeling of the immune system begins as early as middle age and may help explain how aging contributes to the long-lasting inflammation in the brain that is commonly observed in neurodegenerative diseases. “Aging is by far the greatest risk factor for dementia, yet our understanding of how it drives the disease remains incomplete,” said Dr. Richard Hodes, director of the National Institute on Aging (NIA) at the NIH. “This previously hidden change in microglia, which has now been uncovered thanks to innovative technologies and new approaches, could be a key clue to help us complete the puzzle.”

Immune Cells in the Brain Begin to Change in Middle Age

Researchers at the University of California, San Diego, the New York Genome Center, and the University of California, Irvine used state-of-the-art single-cell methods to analyze postmortem hippocampal tissue from 40 neurologically healthy adults aged 20 to 95. Their analysis revealed that microglia—the brain’s primary immune cells—gradually decline between the ages of approximately 50 and 75. At the same time, they appear to be replaced by cells that emit stronger inflammatory signals and exhibit other characteristics similar to those of immune cells derived from peripheral blood. This discovery challenges a long-held assumption about microglia. These cells first develop during embryonic growth, and scientists generally assumed that they remain in the brain and continuously renew themselves throughout a person’s lifetime.

To study the effects of aging on the human brain in extraordinary detail, the team combined standard measurements of gene activity with newer methods that map the 3D structure of the genome and its chemical modifications—the so-called epigenome. “Gene expression tells us what a cell is doing today, but epigenetic signatures preserve information about where a cell came from,” said first author Dr. Nathan Zemke, head of single-cell genomics at the Center for Epigenomics at UC San Diego. “By combining these approaches, we have uncovered a significant change in the identity and lineage of immune cells in the aging human brain that could not have been detected using gene expression data alone.” These combined methods enabled the researchers to identify changes in the identity and origin of immune cells that would have remained hidden had they examined gene activity alone.

Aging Also Affects the Blood-brain Barrier

In addition to the changes in immune cells, the scientists found further clear evidence that the blood-brain barrier (BBB) changes over the course of a lifetime. The blood-brain barrier forms a highly specialized protective barrier between the bloodstream and nervous tissue. It ensures that important nutrients can reach the brain, while many potentially harmful substances, pathogens, and certain inflammatory signals are largely blocked.

This barrier does not consist of a single layer of cells. Among others, endothelial cells of the blood vessels, pericytes, and astrocytes are involved in its function. Together, they very precisely control which substances pass from the blood into the brain tissue. With advancing age, however, this finely tuned system appears to lose some of its stability. The study revealed an age-related decline in certain cells involved in maintaining the blood-brain barrier. As a result, the barrier may become more susceptible to changes with age. A less stable blood-brain barrier, in turn, could make it easier for certain molecules or inflammatory signals to come into contact with brain tissue. This could further intensify the inflammatory environment already present due to altered microglial activity.

It is also interesting to note that the researchers did not merely observe changes in individual cells. In numerous cell types throughout the brain, they found widespread changes in the organization of the genome. The genome is not simply distributed randomly within the cell nucleus but possesses a complex three-dimensional structure. For example, certain regions of DNA are spatially closer together so that genes can be specifically activated or silenced.

With increasing age, this spatial organization of the genome appears to change systematically in many brain cells. At the same time, gene regulation—that is, the extent to which certain genes are expressed—also changed. The researchers found a close correlation between structural changes in the genome and changes in the identity and function of the cells. This is significant because it suggests that brain aging is not merely the accumulation of individual damages. Instead, it may be a coordinated biological process in which the structure of the genome, gene activity, and cellular properties influence one another.

Possible Links to Alzheimer’s Disease

Dr. Bing Ren summarized this relationship as follows: The progressive changes in genome structure were closely associated with changes in gene regulation and cell identity. This could reveal a fundamental characteristic of the human aging process. This connection is particularly interesting in the context of neurodegenerative diseases. If, at the same time, the blood-brain barrier weakens, the immune cell landscape changes, and gene regulation in many brain cells reorganizes, this could create an environment in which the brain responds less effectively to damage and inflammatory processes persist for longer.

However, this does not mean that age-related changes in the blood-brain barrier automatically lead to Alzheimer’s. The study primarily highlights biological changes associated with aging. Whether and to what extent these changes actually contribute to the development of Alzheimer’s, Parkinson’s, or other neurodegenerative diseases must be clarified through further research.

Future research will investigate why resident microglia are lost with advancing age and whether the newly identified transition of immune cells directly contributes to Alzheimer’s disease and other age-related neurological disorders. “Understanding these cellular transitions could open up new avenues for developing interventions that preserve brain function and reduce susceptibility to neurodegenerative diseases,” said Dr. Xiangmin Xu, professor and director of the Center for Neural Circuit Mapping at UC Irvine and corresponding author of the study.

Nearly Half of All Dementia Cases May be Linked to Modifiable Risks

But lifestyle can also play a significant role. A long-term study of the brain conducted by Lund University found that common risk factors such as smoking, high blood pressure, heart disease, and elevated blood lipid levels are closely associated with the damage seen in vascular dementia. The findings suggest that a healthier lifestyle could help delay the onset of dementia by simultaneously reducing several harmful changes in the brain.

The study included nearly 500 participants with an average age of 65 who showed no signs of cognitive impairment. The researchers followed them over a four-year period, measuring changes in the brain’s white matter—the nerve fibers that are often damaged in vascular dementia. The team also monitored the levels of amyloid-β and tau, two proteins associated with Alzheimer’s disease. Their goal was to determine the extent to which both modifiable and non-modifiable risk factors were linked to changes in the brain over time.

Vascular Risk Factors Are Linked to White Matter Damage

The researchers found that most modifiable risk factors—including smoking, cardiovascular disease, elevated blood lipid levels, and high blood pressure—were associated with damage to the brain’s blood vessels and a faster accumulation of so-called white matter changes. “This damage impairs blood vessel function and leads to vascular brain damage—and can ultimately result in vascular dementia,” said Isabelle Glans, a doctoral student at Lund University and a resident in neurology at Skåne University Hospital.

The researchers also identified potential links between certain risk factors and the proteins involved in Alzheimer’s disease. Diabetes was associated with increased accumulation of amyloid-β, while people with a lower BMI showed a faster accumulation of tau. However, these findings need to be further investigated and validated in future studies.

Healthy Lifestyle Habits Can Reduce Various Types of Damage

The results show how important it can be to reduce modifiable risk factors as early as middle age. Regular physical activity—such as brisk walking, cycling, swimming, or strength training—strengthens the cardiovascular system and can help improve blood pressure, blood sugar, and blood lipid levels.

Equally important is a balanced, Mediterranean-style diet rich in vegetables, fruits, legumes, whole grains, nuts, and fish. At the same time, highly processed foods, excessive sugar, too much salt, and unhealthy fats should be minimized as much as possible. Quitting smoking is particularly important, as smoking damages blood vessels and increases the risk of strokes and other vascular diseases. High blood pressure, elevated blood lipid levels, and diabetes should also be monitored regularly and treated consistently as needed. In addition, getting enough sleep and engaging in mental and social activities can support overall brain health.

Overall, it is not so much a single measure that matters, but rather the combination of various healthy lifestyle habits. Since many people with dementia experience both vascular damage and changes typical of Alzheimer’s disease, reducing these risk factors could help limit several harmful processes in the brain simultaneously and potentially delay the onset of dementia symptoms.