Epigenetic and 3D genome reprogramming during the aging of the human hippocampus

Why it matters

Gene activity changes in the aging brain, but the control systems behind those changes are poorly understood. Those controls include DNA methylation, how open the DNA is to being read, and how the genome is folded in three dimensions. This study looks at all of these layers at once in single cells from the human hippocampus, a region central to memory. That makes it one of the most detailed views yet of how the genome's packaging, not just its sequence, changes in the aging brain. Every cell in the brain carries essentially the same DNA sequence, so differences between cell types, and between young and old cells, come largely from how that DNA is marked, opened and folded. Erosion of that organisation is part of the 'epigenetic alterations' hallmark of aging, and seeing it directly in human tissue is rare.

What they did

The team analysed hippocampal tissue from human donors across the adult lifespan. For individual cell nuclei they measured which genes were active, which stretches of DNA were open (chromatin accessibility), where DNA was methylated, and how the chromosomes were folded in 3D. They then looked for programmes of gene regulation that changed with age, steadily or in bursts, in each cell type.

What they found

They found regulatory programmes that changed in a straight line with age and others that changed non-linearly. Between about ages 50 and 75, the original microglia (immune cells that come from the embryonic yolk sac) were depleted and replaced by cells that resemble microglia derived from blood monocytes. Hippocampal astrocytes, including ones that help regulate signalling between neurons, fell substantially with age. In every cell type, the 3D architecture of the genome underwent a global erosion. The authors suggest that these altered regulatory programmes help produce cell-type-specific aging in the human brain.

Limits

The work uses post-mortem tissue from different people at different ages, so it shows changes across a population rather than following one brain over time. The abstract does not show that the 3D-genome erosion causes memory loss or disease. Only one brain region was studied, and the abstract does not give the number of donors.

Key terms

Hippocampus

A brain region essential for forming new memories.

Chromatin accessibility

How open or tightly packed a stretch of DNA is, which affects whether its genes can be read.

3D genome architecture

The way chromosomes fold inside the nucleus, bringing distant genes and switches together.

Microglia

The brain's resident immune cells.

Astrocytes

Star-shaped support cells that help neurons signal and keep the brain's environment stable.

Single-nucleus sequencing

Reading the molecules from one cell nucleus at a time, so each cell type can be studied separately.

Monocyte

A white blood cell that can leave the blood and turn into tissue immune cells.