Millions of people are carriers of APOE4, the strongest known genetic risk factor for Alzheimer’s disease. New research suggests that the genetic variant may begin to alter brain activity long before memory problems become noticeable.
Gladstone Institutes researchers have 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 aging of nature, Researchers found that APOE4 increases the production of a protein called Nell2. Higher levels of Nell2 caused the neurons to become smaller and unusually active. Mice with the most brain hyperactivity when they were young later developed the most severe memory problems.
The team then reduced production of Nell2. Even in adult mice carrying APOE4, neurons returned to 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 our knowledge, this is the first study that has directly examined what APOE4 does to neuron function at different ages,” says Misha Zilberter, PhD, senior research scientist at Gladstone and lead author of the study. “We found fundamental changes in brain circuits occurring in young mice that still had normal learning and memory and, more importantly, that these 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. About one in four people are carriers of APOE4, and the variant is estimated to occur in 60 to 75 percent of people with Alzheimer’s.
“This study is a major advance for the field of Alzheimer’s research,” says Yadong Huang, MD, PhD, associate director of the Gladstone Institute of Neurological Diseases and senior author of the study. “It opens the door to a better understanding of how APOE4 alters the function of neurons at an early age to increase the risk of cognitive decline, and to the development of therapies that could block the detrimental effects of APOE4 early on.”
APOE4 makes memory circuits overactive early
Previous research had already found signs of unusually high brain activity in human APOE4 carriers before middle age. Such early hyperactivity has also been associated with later cognitive decline. What was not clear 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 in 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 regions of the hippocampus have also been found to be hyperactive in APOE4 carriers.
“We found that the degree of hyperactivity in young mice predicted their poor performance on memory and spatial learning 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 to 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 APOE3 mice. Smaller neurons tend to respond more easily to stimulation, making them more likely to fire excessively.
The hippocampal neurons of the APOE3 mice also eventually became more excitable, but that change didn’t appear until the animals aged.
“This suggests that APOE4 accelerates a process that resembles normal aging and could explain why people with the genetic variant are more likely to develop Alzheimer’s disease earlier in life,” says Huang.
The effect comes from APOE4 within the neurons
Most of 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 driven entirely by APOE4 produced within the neurons themselves.
“When we removed the APOE4 gene from astrocytes, nothing changed,” says Zilberter. “But when we removed it from the neurons, the cells became larger and began to function normally again.”
Nell2 emerges as a potential treatment target
Next, the researchers looked for the molecular process responsible for making APOE4 neurons smaller and more excitable. They examined patterns of gene activity in single cells of various hippocampal cell types.
That analysis highlighted Nell2. The molecule appeared at unusually high levels in neurons carrying APOE4.
The researchers then used CRISPRi, a method that reduces the activity of a gene without permanently changing the DNA, to reduce Nell2 in the hippocampal neurons of adult APOE4 mice.
After Nell2 levels fell, the neurons became larger and less excitable. The finding indicates that the elevated level of Nell2 is responsible for the excessive neuronal activity observed in brains carrying APOE4.
Nell2 had not previously been investigated specifically in relation to APOE4. However, previous research found elevated levels of the protein in the brains of Alzheimer’s patients, and higher amounts were associated with poorer cognitive function.
“The interesting thing about Nell2 is that we were able to reverse the manifestations of the disease in adult mice by reducing its level,” says Huang. “That tells us that the damage is not irreversible and that there may be a window for intervention even after the pathological 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 Cerebrovascular Accidents. (K99NS134734) and the National Research Resources Center (C06 RR018928).