A recent paper published in the journal Nature has revealed a new mechanism by which the genetic risk factor, APOE4, may contribute to Alzheimer’s disease pathology.
The E4 variant of the APOE gene is the predominant genetic risk factor for Alzheimer’s disease. Those who contain one copy of the E4 variant are three times more likely to develop Alzheimer’s, while those who contain two copies of the E4 variant are nearly ten times more likely to develop the disease. The E4 variant has been associated with Alzheimer’s disease for years. However, it is still not understood exactly why or how this genetic variant contributes to the debilitating biological and cognitive symptoms of Alzheimer’s.
Now, studies conducted by a team at the Massachusetts Institute of Technology suggest that the E4 variant of the APOE gene disrupts how fat molecules are processed in the brain. It appears that the disruption of these fat molecules could be the fundamental reason why those that contain the E4 variant are more likely to develop Alzheimer’s symptoms including brain cell death, memory issues, and cognitive decline.
To investigate the role of the E4 variant in Alzheimer’s patients’ brains, the team at MIT began by examining the genetic makeup of thirty-two patients who had donated their brain tissue samples to Alzheimer’s research before passing away.
The first subgroup of donors contained twelve individuals with two copies of the low-risk, E3 variant (E3/E3). The second subgroup contained twelve donors, each with one copy of the E3 variant as well as one copy of the E4 variant (E3/E4). The final subgroup consisted of eight donors with two copies of the high-risk, E4 variant (E4/E4). Only half of the donors in the E3/E3 and E3/E4 subgroups had been diagnosed with Alzheimer’s disease in their lifetime, while all the E4/E4 donors had been afflicted with Alzheimer’s.
One of the primary questions the researchers were interested in investigating was whether the E3 and E4 variants were associated with abnormal cell activity. To determine this, the team at MIT took brain tissue samples from each subgroup and analyzed the tissue with an RNA-sequencing tool. This would allow the researchers to isolate specific types of cells within the brain and figure out which genes seemed to be expressed more or less across different brain cells.
After running their analysis, the researchers found that the E4 variant significantly affected the expression of genes related to intracellular activities. Interestingly, this included increased expression of genes associated with inflammation and other immune-related pathways. Many studies have recently pointed to inflammation as a core feature of Alzheimer’s disease. These studies suggest that increased inflammation in Alzheimer’s patients may be linked to the development of both amyloid plaques and tau tangles and may exacerbate the cognitive symptoms of Alzheimer’s. Researchers also found that genes related to the transmission of information in brain cells were expressed less in patients with the E4 variant.
Curiously, among all the cell RNA tested, differential gene expression was not only found in neurons—the primary brain cells that are responsible for information exchange—but was also found in other cells called oligodendrocytes.
Oligodendrocytes are supportive cells in the brain whose main function is to insulate neurons. Neurons send information to each other through electrical signals in the brain. Much like a wire is insulated to allow for electricity to travel efficiently, neurons are often wrapped with fats called myelin for insulation and to allow information in the brain to travel more quickly. Oligodendrocytes have the essential function of wrapping myelin around neurons.
When researchers investigated the effects of the E4 variant on myelin, they found that neurons seemed to be under-myelinated in patients carrying the high-risk variant. To delve deeper into how oligodendrocytes themselves were being affected, researchers examined the genomic data of isolated oligodendrocytes. They soon found that the E4 variant was associated with increased expression of cholesterol-related genes in oligodendrocytes.
Interestingly, there seemed to be an even greater association between cholesterol gene expression, the E4 variant, and other biomarkers of Alzheimer’s disease like amyloid plaques and tau tangles. Those who had both the E4 variant and some Alzheimer’s disease pathologies displayed the highest levels of cholesterol-related gene expression. This suggested that both the E4 variant and typical Alzheimer’s pathology like amyloid plaques or tau tangles had some effect on the regulation of cholesterol and lipid myelination.
After examining the expression of cholesterol-related genes in the oligodendrocytes of E4 samples, researchers additionally found that only genes related to the formation of cholesterol were expressed more. In contrast, genes associated with the transport of cholesterol through oligodendrocytes were expressed significantly less.
But how did these genetic profiles affect the function of oligodendrocytes in those with the E4 variant? To investigate how a decrease in cholesterol transport gene expression could affect the function of oligodendrocytes, the team at MIT began by examining where cholesterol was located within the oligodendrocyte cell bodies. By staining the cholesterol molecules within the brain samples of patients, the researchers were able to determine exactly where the cholesterol molecules were localized within the cell.
To their surprise, they found that patients with the E4 variant displayed an accumulation of cholesterol around the cell nucleus, where DNA is stored. This was unlike samples taken from E3 patients. In E3 patients, cholesterol was dispersed throughout the cell. These results suggested that those who contain the E4 variant seem to have abnormally functioning oligodendrocytes that produce more cholesterol and are unable to transport the cholesterol throughout the cell body, leading to a significant accumulation of cholesterol molecules.
These unusual results prompted the research team at MIT to explore the relationship between cholesterol, oligodendrocytes, and Alzheimer’s disease more thoroughly. To delve deeper into the biological mechanisms that cause cholesterol to accumulate in the oligodendrocytes of E4 patient cells, the MIT lab set out to use stem cell technology to grow their own oligodendrocytes with either the E3 or E4 APOE variants. In the next installment of this series, we will discuss their fascinating discoveries using this stem cell model and a new route for potential Alzheimer’s treatments.