A new preprint study from Calico has found that the local brain environment is the primary driver of microglial aging. After being transplanted into old brains, young cells adopted aged characteristics, but their susceptibility to these signals could be turned off [1].
Microglia grow old and irritable
Brain aging holds a special place in the longevity field as a strong limiting factor: even if we can rejuvenate the body by replacing its various parts and organs, the brain is what contains our memories and personality, hence it cannot be simply replaced. This makes rejuvenating the brain a crucial step in achieving meaningful lifespan extension.
Studies have consistently found that the support cells known as glia age faster than neurons. Of these glial cells, the brain’s specialized immune cells (microglia) are among the most profoundly affected by aging [2]. Aged microglia often develop a more aggressive, pro-inflammatory phenotype that is suspected to drive neurodegeneration [3].
One crucial question about microglial aging is about whether these cells age because of a pre-programmed internal clock (a cell-intrinsic process) or are pushed into an aged state by signals from their deteriorating neighborhood (a cell-extrinsic process). This study from Calico, the Alphabet-owned longevity company, was designed to test cell-intrinsic versus environmental effects by replacing microglia with donor myeloid cells across young and old brains.
Young cells for aged brains
The researchers developed a method to replace the native microglia in mice with new myeloid cells derived from the bone marrow of donor mice. They first produced a pool of donor hematopoietic stem cells (HSCs) taken from young female mice. The cells were genetically engineered to produce two extra proteins: an enhanced version of the green fluorescent protein (EGFP), which allows the tracking of cells that express it in vivo, and Cas9, an element of the CRISPR gene editing platform. This created “ready-to-edit” cells: to knock out a gene later in the study, the team simply needed to introduce a small piece of guide RNA to direct the pre-existing Cas9 to its target.
The bone marrow niche of the old recipient mice was then depleted to make room for donor stem cells. However, the researchers also needed to rid the brain of old microglia. This was done by adding a drug that inhibits CSF1R, a protein crucial for microglial survival. With the original microglia gone, the donor-derived myeloid cells circulating in the blood could now enter the brain. There, they settled down and became microglia-like cells.
With their system established, the researchers set out to investigate what happens when young, healthy ‘reconstituted cells’ are placed in an aged brain. Apparently, the environment is the dominant force. Young cells in old brains rapidly began to look and act old, especially in the cerebellum, adopting aged gene expression patterns. The researchers defined a “Cerebellar Accelerated Aging Signature” (CAAS), a molecular fingerprint of 403 genes, and watched the young cells in the old brain acquire this signature.
“What did the old brain tell the young cells? To get old, fast,” said Oliver Hahn, Ph.D., a principal investigator at Calico and the paper’s senior author, in an X thread. “The aged brain environment overrode the intrinsic youth of donor cells! These young ‘reconstituted cells’ acquired the molecular aging signatures we see in old microglia, an effect strongest in the cerebellum.”
To confirm that the brain environment could not only make young cells old but also make old cells young, the researchers performed the reverse transplantation. When transplanted into young brains, cells from old mice showed transcriptional and morphological rejuvenation.
Aging signals blocked
When the researchers compared the gene expression profiles of microglia from young and old brains, one of the strongest molecular patterns they found was a heightened pro-inflammatory interferon response signature. To see if dampening interferon response would rescue microglia aging, the team decided to knock out Stat1, a well-known master regulator of this signaling pathway.
Using their Cas9 “edit-ready” platform, the researchers produced Stat1-deficient young cells and repeated their repopulation protocol. Unlike in the previous experiment, these cells were largely protected from the rapid aging that the researchers had previously observed: they resisted the aging signals from the environment and did not activate the CAAS signature.
The researchers wanted to know which type of cells produced these aging signals. As it turned out, at least for the interferon response, the culprit was natural killer (NK) cells rather than T cells, which initially were the researchers’ primary suspect. Depleting NK cells in aged mice blunted the age-related interferon response in microglia.
“Our findings are clear,” Hahn said. “The local brain environment drives microglia aging, with NK cells acting as an unexpected upstream trigger. Crucially, this is blockable, as Stat1-KO shields young cells from pro-aging cues. This challenges simple ‘rejuvenation-by-replacement’ ideas. This is just the beginning. We’re now using this platform to map out other pro-aging signaling axes. We hope our new, scalable eHSC system will be a powerful resource for the field, enabling future in vivo screens to find new targets for neuroinflammation.”
Literature
[1] Gizowski, C., Popova, G., Shin, H., Mader, M. M., Craft, W., Kong, W., Shibuya, Y., Wranik, B. J., Fu, Y. C., Depp, C., Lin, T. D., Martin-McNulty, B., Yoo, Y., Tai, P.-H., Hingerl, M., Leung, K., Atkins, M., Fong, N., Jogran, D., … Hahn, O. (2025, September 7). Heterochronic myeloid cell replacement reveals the local brain environment as key driver of microglia aging [Preprint]. bioRxiv.
[2] Costa, J., Martins, S., Ferreira, P. A., Cardoso, A. M., Guedes, J. R., Peça, J., & Cardoso, A. L. (2021). The old guard: Age-related changes in microglia and their consequences. Mechanisms of ageing and development, 197, 111512.[3] Adamu, A., Li, S., Gao, F., & Xue, G. (2024). The role of neuroinflammation in neurodegenerative diseases: current understanding and future therapeutic targets. Frontiers in aging neuroscience, 16, 1347987.
[3] Adamu, A., Li, S., Gao, F., & Xue, G. (2024). The role of neuroinflammation in neurodegenerative diseases: current understanding and future therapeutic targets. Frontiers in aging neuroscience, 16, 1347987.
View the article at lifespan.io
