A National Institutes of Health (NIH)-funded study indicates that the immune cell landscape of the hippocampus, a brain region critical for learning and memory, rapidly undergoes substantial remodeling beginning in midlife. This previously unrecognized shift suggests a potential mechanism by which aging may contribute to chronic neuroinflammation commonly observed in neurodegenerative disease.
"Aging is the single largest risk factor for dementia, but our understanding of how it drives disease is still incomplete," said Richard Hodes, M.D., director of NIH's National Institute on Aging (NIA). "This previously hidden microglial shift, now uncovered by innovations in technology and thinking, may be an important clue to help us complete the puzzle."
Using advanced single-cell analysis techniques, a collaborative team of researchers from the University of California, San Diego, the New York Genome Center and the University of California, Irvine analyzed postmortem hippocampal tissue from 40 neurologically healthy adults ranging in age from 20 to 95 years. The findings suggest that the brain's primary immune cells, called microglia, progressively decline between approximately 50 and 75 years of age, becoming replaced by cells with elevated inflammatory signatures and other features resembling characteristics of peripheral blood-derived immune cells.
These results call into question the longevity of microglia which emerge during embryonic development, which scientists have long thought renew throughout the human lifespan.
To investigate how aging affects the human brain at an unprecedented resolution, the researchers applied traditional measures of gene expression alongside more advanced techniques to analyze the genome's 3D architecture and array of chemical modifications, called the 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 Nathan Zemke, Ph.D., director of single-cell genomics at the UC San Diego Center for Epigenomics. "By combining these approaches, we uncovered a major shift in the identity and lineage of immune cells in the aging human brain's immune cells that gene expression data alone would not have revealed."
Their findings also indicated that cells known to maintain the protective blood-brain barrier deteriorated with age. And across many brain cell types, aging accompanied a widespread and coordinated disruption of genome architecture.
"The progressive structural disruptions were closely linked to shifts in gene regulation and cell identity, potentially revealing a fundamental feature of aging in the human brain," said Bing Ren, Ph.D., a corresponding author of the study, scientific director and CEO of the New York Genome Center, and professor of genetics and development at Columbia University.
Future studies will investigate the mechanisms driving the loss of resident microglia and determine whether the newly identified immune-cell transition contributes directly to Alzheimer's disease and other age-related neurological disorders.
"Understanding these cellular transitions may provide new opportunities to develop interventions that preserve brain function and reduce vulnerability to neurodegenerative disease," said Xiangmin Xu, Ph.D., professor and director of Center for Neural Circuit Mapping at UC Irvine, and a corresponding author of the study.
NIH supported this research through NIA grants R01AG067153 and R01AG082127 and the NIH Common Fund 4D Nucleome (4DN) program grant 1U01DA052769. This study is part of a collection of 4DN-supported papers published in Science and Science Advances that offer unprecedented insight into how the human genome's 3D organization influences development, aging, and a host of diseases.
