Scientists reveal how aging rewires the brain’s molecular landscape

New research unlocks the cellular secrets of aging, with groundbreaking single-cell data mapping how neurons, glial cells and the immune system remodel the aging brain.

Study: Brain-wide cell type-specific transcriptional signatures of healthy aging in mice. Image credit: Monkey Business Images / Shutterstock

In a recent study published in the journal Naturescientists from the Allen Institute for Brain Science in the United States investigated how different types of cells in the mouse brain change at the genetic level with age. By analyzing more than 1.2 million single-cell transcriptomes from young and old mice, the researchers identified key gene expression changes associated with aging. These changes highlight specific molecular mechanisms, such as immune activation and reduced structural integrity, in various cell types. These findings could help reveal the areas of the brain and cells most affected by aging.

Background

Aging is a natural process characterized by cellular and molecular changes that affect overall function. In the brain, aging manifests as altered cellular activity, inflammation, and reduced neurogenesis, among other changes. Previous studies have identified general markers of aging in tissues and some brain-related changes. However, given the complexity of the brain and its numerous cell types and functions, it remains unclear how specific cell types contribute to aging. Emerging evidence has shown that certain regions, such as the third ventricle of the hypothalamus, serve as focal points for aging-related changes. Recent advances in single-cell transcriptomics have provided unprecedented insights into cellular diversity and allowed researchers to detect changes at high resolution.

While these studies have revealed age-related shifts in neurons and glial cells, comprehensive mapping across the entire brain is lacking. This mapping has now revealed distinct, cell-type-specific patterns of aging, including immune activation and neuronal decline. Furthermore, specific changes in smaller, neglected cell populations and their contribution to brain health and aging remain unexplored. Understanding these dynamics is crucial to uncovering the mechanisms leading to age-related cognitive and functional decline and their potential connections to neurodegenerative diseases.

About the study

The present study used single-cell ribonucleic acid sequencing (scRNA-seq) to examine the brains of young (2 months) and aged (18 months) mice. The researchers targeted 16 key brain regions, including the forebrain, midbrain and hindbrain. These regions were selected for their involvement in aging and age-related disorders. Using the 10x Genomics platform, the researchers generated a dataset of approximately 1.2 million high-quality single-cell transcriptomes from neurons and non-neuronal cells. In particular, this represents one of the most comprehensive single-cell datasets for aging research to date. Additional cell sorting strategies ensured comprehensive sampling across cell types, and the study included fluorescence-activated cell sorting (FACS) to unbiasedly sample neurons and other cells.

The Allen Brain Cell Atlas, an open resource developed by the Allen Institute that allows researchers to explore numerous whole-brain datasets, was used to annotate the data. The findings identified 847 clusters of cells representing 172 subtypes in 25 cell categories. Additionally, gene expression changes were modeled using computational methods to detect differentially expressed genes associated with aging. Spatial transcriptome was also used to gain additional validation and visualize gene expression in brain regions of interest.

Numerous other analyzes were used to categorize differentially expressed genes by cell class and subclass, while distinguishing age-related changes in neurons, glial cells, and other cell types. This included the identification of specific proinflammatory microglial clusters and age-depleted populations of neural stem cells. Particular attention was paid to sparsely distributed populations such as ependymal cells and tanycytes, specialized glial cells located in the hypothalamus and involved in the regulation of physiological processes such as energy balance.

In addition, gene ontology or GO enrichment analyzes were performed to identify biological processes affected by aging, such as immune signaling and maintenance of neuronal structure. These analyzes revealed significant losses in neurogenic potential and structural maintenance, especially in tanycytes and neurons near the hypothalamic third ventricle. Key gene expression patterns were identified using in situ hybridization to complement the transcriptional findings.

Results

The study found that aging leads to significant changes in gene expression in various brain cell types and identified 2,449 differentially expressed genes with unique and shared signatures across cell types. Neurons, glial and vascular cells showed distinct patterns of gene expression, with many differentially expressed genes linked to immune activation, structural integrity and cellular senescence.

Specifically, neurons showed decreased expression of synaptic signaling and structural genes such as Ccnd2, while microglia showed increases in inflammatory markers such as Ildr2 and Ccl4. Glial cells, such as astrocytes and oligodendrocytes, showed reduced expression of support-related genes. In contrast, expression of immune-related genes was higher in microglia, macrophages and other immune cell types.

In addition, region-specific changes were observed to be prominent near the hypothalamic third ventricle, where tanycytes and ependymal cells showed marked age-related shifts. These shifts included increased interferon response signaling and decreased markers for structural maintenance. Similarly, oligodendrocytes in aged brains showed altered gene expression patterns, suggesting reduced myelin integrity.

Vascular cells, particularly endothelial cells, also showed aging-related gene expression changes associated with genes involved in major histocompatibility complex (MHC) antigen presentation, with evidence of reduced vascular function. In addition, microglial cells in aged brains formed new clusters associated with pro-inflammatory and aging conditions. Spatial analyzes confirmed increased immunoreactivity localized to subcortical regions, particularly in the midbrain and hindbrain.

conclusions

The results provided a detailed single-cell transcriptome map of brain aging and revealed cell-type and region-specific molecular changes associated with aging. These findings highlight the hypothalamus as a hub for aging-related changes, with important implications for understanding neurodegenerative diseases. Key findings highlighted the roles of immune activation, neuronal decline and glial dysfunction in aging. These insights lay the foundation for investigating how aging affects brain function and its intersection with neurodegenerative diseases.