Millions of people carry APOE4, the strongest known genetic risk factor for Alzheimer’s disease. New research suggests the gene variant may begin altering brain activity well before memory problems become noticeable.
Researchers at Gladstone Institutes have now mapped out a molecular sequence that could help explain these early effects. Their findings also point to a possible way to reverse some of the changes.
In a study using mouse models, published in Nature Aging, the researchers found that APOE4 increases production of a protein called Nell2. Higher Nell2 levels caused neurons to become smaller and unusually active. Mice with the greatest brain hyperactivity when they were young later developed the most severe memory problems.
The team then reduced Nell2 production. Even in adult mice carrying APOE4, neurons returned toward their normal size and firing behavior. The result raises the possibility that future drugs targeting Nell2 could help people with APOE4 who face an elevated risk of Alzheimer’s disease.
“To the best of our knowledge, this is the first study that has directly examined what APOE4 does to the function of neurons at different ages,” says Misha Zilberter, PhD, principal staff research scientist at Gladstone and a senior author of the study. “We found fundamental changes in brain circuits occurring in young mice that still had normal learning and memory, and importantly, that those changes predicted the development of cognitive deficits at older ages.”
A Major Genetic Risk Factor for Alzheimer’s
APOE4 is one of three common forms of the APOE gene, but it has a much stronger connection to Alzheimer’s risk than the others. Roughly one in four people carry APOE4, and the variant is estimated to occur in 60 to 75 percent of people with Alzheimer’s.
“This study is a big breakthrough for the field of Alzheimer’s research,” says Yadong Huang, MD, PhD, associate director of the Gladstone Institute of Neurological Disease and a senior author of the study. “It opens the door to a better understanding of how APOE4 alters the function of neurons at a young age to increase risk of cognitive decline, and to the development of therapies that could block the detrimental effects of APOE4 early on.”
APOE4 Makes Memory Circuits Hyperactive Early
Earlier research had already found signs of unusually high brain activity in human APOE4 carriers before middle age. That early hyperactivity has also been associated with later cognitive decline. What remained unclear was how APOE4 produced these cellular changes and why they might contribute to memory problems later in life.
To investigate, the researchers studied recordings of brain activity in young mice and examined individual neurons from their brains. Young mice carrying APOE4 showed excessive neuronal activity in two areas of the hippocampus, a brain region central to memory.
Notably, those same hippocampal regions have also been found to be hyperactive in people carrying APOE4.
“We found that the extent of hyperactivity in young mice predicted how poorly they performed on spatial learning and memory tests later in life,” says Dennis Tabuena, PhD, a scientist co-mentored by Zilberter and Huang, and first author of the new paper.
The scientists also compared these animals with mice carrying APOE3, a version of the APOE gene associated with a lower risk of Alzheimer’s disease in humans.
Neurons in the affected brain regions were smaller in APOE4 mice than in mice with APOE3. Smaller neurons tend to respond more easily to stimulation, making them more likely to fire excessively.
The hippocampal neurons of APOE3 mice eventually became more excitable as well, but that shift did not appear until the animals were old.
“This suggests APOE4 accelerates a process that resembles normal aging, and could explain why people with the gene variant are more likely to develop Alzheimer’s disease earlier in life,” Huang says.
The Effect Comes From APOE4 Inside Neurons
Most APOE4 in a healthy brain is produced by astrocytes, cells that help support neurons. Because of this, researchers had long suspected that astrocytes were largely responsible for the connection between APOE4 and Alzheimer’s risk.
The new results point in a different direction. According to the researchers, the hippocampal hyperactivity associated with APOE4 was entirely driven by APOE4 produced within neurons themselves.
“When we deleted the APOE4 gene from astrocytes, nothing changed,” Zilberter says. “But when we deleted it from neurons, the cells became larger and started functioning normally again.”
Nell2 Emerges as a Possible Treatment Target
The researchers next searched for the molecular process responsible for making APOE4 neurons smaller and more excitable. They examined patterns of gene activity in individual cells across several types of cells in the hippocampus.
That analysis highlighted Nell2. The molecule appeared at unusually high levels in neurons carrying APOE4.
The researchers then used CRISPRi, a method that lowers the activity of a gene without permanently changing DNA, to reduce Nell2 in hippocampal neurons from adult APOE4 mice.
After Nell2 levels fell, the neurons became larger and less excitable. The finding indicates that elevated Nell2 is responsible for the excessive neuronal activity seen in brains carrying APOE4.
Nell2 had not previously been investigated specifically in connection with APOE4. However, earlier research found elevated levels of the protein in the brains of Alzheimer’s patients, with higher amounts associated with poorer cognitive function.
“What’s exciting about Nell2 is that we were able to reverse the disease manifestations in adult mice by lowering its level,” Huang says. “That tells us the damage is not irreversible, and that there may be a window for intervention even after disease processes have been triggered.”
The research was supported by the National Institute on Aging (R01AG061150, R01AG087323, R01AG092390, R01AG085468, R01AG055682, R01AG071697, P01AG073082, F32AG0859612), the National Institute of Neurological Disorders and Stroke (K99NS134734), and the National Center for Research Resources (C06 RR018928).