Hyperactive memory circuits in young mice

In young mice engineered to carry APOE4, the researchers recorded excessive firing in two regions of the hippocampus, the brain's memory hub. Neurons there were also physically smaller than in mice carrying APOE3, the lower-risk version of the same gene. Smaller neurons respond more readily to stimulation.

Mice with the most hippocampal hyperactivity in youth later performed worst on spatial learning and memory tests. Neurons in APOE3 mice became more excitable too, but only in old age. The authors propose that APOE4 accelerates a process that resembles normal aging, which could explain why carriers develop the disease earlier.

The signal comes from inside neurons

Most APOE protein in a healthy brain comes from astrocytes, the support cells that surround neurons. Scientists had long assumed those cells drove APOE4's harmful effects.

The team's mouse experiments pointed elsewhere. Deleting APOE4 from astrocytes left the brain hyperactivity unchanged. Deleting APOE4 from neurons restored normal cell size and firing. The result reframes the field, suggesting that neurons themselves, not their support cells, are the critical site of the variant's early damage.

Nell2 rises as a treatment candidate

To find the molecular messenger, the researchers profiled gene activity in single cells from the hippocampus. One molecule stood out: Nell2.

Nell2 levels were elevated in neurons carrying APOE4. Earlier work had tied higher Nell2 to poorer cognition in Alzheimer's patients, but the protein had not been linked directly to APOE4 before.

The team then used CRISPRi — a gene-silencing tool that lowers gene activity without permanently altering DNA — to dial back Nell2 in hippocampal neurons from adult APOE4 mice. The neurons grew larger and fired less.

"What's exciting about Nell2 is that we were able to reverse the disease manifestations in adult mice by lowering its level," says Yadong Huang, MD, PhD, associate director of the Gladstone Institute of Neurological Disease and a senior author. "That tells us the damage is not irreversible, and that there may be a window for intervention even after disease processes have been triggered."

What still needs testing

The findings rest on mouse models. Whether Nell2 plays the same role in human APOE4 carriers remains unknown. The researchers have not yet shown that lowering Nell2 improves memory in older animals, nor that long-term suppression is safe.

Any human therapy would need clinical trials confirming both efficacy and side effects, and would have to target Nell2 without disrupting its other roles in the brain.

Even so, the study offers one of the first mechanistic pictures of how a single Alzheimer's risk gene reshapes neurons decades before symptoms — and a hint that those changes may not be permanent.