A study by MIT researchers has provided new insights into the mechanisms by which a gene variant that is know to represent a risk factor for Alzheimer’s disease may impact on cellular lipid metabolism and stress response. Their studies in human brain cells and in yeast cells indicated how the APOE4 gene disrupts lipid homeostasis, and also showed that treating the cells with choline, a widely available dietary supplement, could reverse many of the effects of this disruption.
Headed by Li-Huei Tsai, PhD, director of MIT’s Picower Institute for Learning and Memory, the researchers hope that their findings may lead to clinical studies of choline specifically in people who carry the APOE4 gene, to see if the supplement could help to reduce the incidence of Alzheimer’s disease. Previous trials looking at choline’s effects on cognition have shown mixed results, but those trials were not targeted specifically to people with the APOE4 gene. The team is also now studying a mouse model of Alzheimer’s that is engineered to express the human APOE4 gene, with a view to investigating whether choline can help to reverse some of the symptoms of Alzheimer’s in these animals.
“What we would really like to see is whether in the human population, in those APOE4 carriers, if they take choline supplements to a sufficient amount, whether that would delay or give them some protection against developing dementia or Alzheimer’s disease,” said Tsai, the director of MIT’s Picower Institute for Learning and Memory.
Tsai and the late Susan Lindquist, former director of MIT’s Whitehead Institute for Biomedical Research, are the senior authors of the reported in vitro study, which is published in Science Translational Medicine, and titled, “APOE4 disrupts intracellular lipid homeostasis in human iPSC-derived glia.” The paper’s three lead authors are former Whitehead and MIT postdocs Grzegorz Sienski, PhD, and Priyanka Narayan, PhD, and current MIT postdoc Julia Maeve Bonner, PhD.
The human gene for apolipoprotein E (APOE) comes in three variants. APOE4 is linked to a higher risk for Alzheimer’s, and is carried by about 14% of the general population, but by almost half of all Alzheimer’s patients. In contrast, APOE2 is considered protective, and APOE3, the most common variant, is neutral.
APOE is known to be involved in lipid metabolism, and while the E4 allele is an established genetic risk factor for late-onset neurodegenerative disorders—and other diseases, including cardiovascular disease, the authors pointed out—its mechanism of action isn’t well understood. “APOE is a lipid transport protein, and the dysregulation of lipids has recently emerged as a key feature of several neurodegenerative diseases including AD. However, it is unclear how APOE4 perturbs the intracellular lipid state.”
To try to learn more about the effects of APOE4 on lipid metabolism, the researchers created human induced pluripotent stem cells that carry either the APOE3 or APOE4 gene, against an otherwise identical genetic background. They then stimulated these cells to differentiate into astrocytes, the brain cells that produce the most APOE.
They found that the APOE4 astrocytes showed dramatic changes in how they processed lipids, when compared with APOE3 cells. In APOE4 astrocytes, there was a significant build up of neutral lipids and cholesterol. These astrocytes also accumulated droplets containing triglycerides, and these triglycerides had many more unsaturated fatty acid chains than normal. These changes all act to disrupt the normal lipid balance inside the cells. The team also noted APOE4-dependent lipid disruptions in microglia, another key brain cell type.
“When lipid homeostasis is compromised, then a lot of very essential processes are affected, such as intracellular trafficking, vesicular trafficking, and endocytosis,” Tsai said. “A lot of the cells’ essential functions are compromised.” Bonner added, “This balance is really important for cells to be able to perform normal functions like generate membranes and so on, but also to be able to absorb stress. We think that one of the things that’s happening is that these cells are less able to absorb stress because they’re already in this heightened lipid dysregulation state.”
The researchers found that yeast cells engineered to express the human version of APOE4 showed many of the same defects. Using these cells, they performed a systematic genetic screen to determine the molecular basis of the defects seen in APOE4 cells. The screen results showed that turning on a pathway that normally produces phospholipids, an essential component of cell membranes, could reverse some of the damage seen in APOE4 cells. This suggested that APOE4 somehow increases the requirement for phospholipid synthesis. “We combined lipidomics and unbiased genome-wide screens in yeast with functional and genetic characterization to demonstrate that human APOE4 induced altered lipid homeostasis,” they wrote. “These changes resulted in increased unsaturation of fatty acids and accumulation of intracellular lipid droplets in yeast and in APOE4-expressing human iPSC-derived astrocytes.”
Further studies showed that growing APOE4 yeast cells on a very nutrient-rich growth medium helped them to survive better than APOE4 yeast cells grown on the typical growth medium. The team’s experiments identified choline as the nutrient that helped the APOE4 cells to survive. Choline is a building block that cells use to make phospholipids. Encouragingly, treating the human APOE4 astrocyte cells with choline to promote phospholipid synthesis reversed much of the damage that had been observed in the APOE4 cells, including the accumulation of cholesterol and lipid droplets. “These results confirmed that choline supplementation ameliorated the APOE4-induced lipid defects in human iPSC-derived astrocytes,” the scientists stated. “Our study suggests that manipulating lipid metabolism could be a therapeutic approach to help alleviate the consequences of carrying the APOE4 allele.”
Choline is naturally found in foods such as eggs, meat, fish, and some beans and nuts. The minimum recommended intake of choline is 550 milligrams per day for men and 425 milligrams per day for women, but most people don’t consume that much, Tsai said. The new study offers preliminary evidence that people who carry the APOE4 gene may benefit from taking choline supplements, although this suggestion will need to be confirmed in clinical trials. “Our work provides a framework for understanding APOE4 function in disease risk and provides a rationale for genotype-specific dietary supplementation to diminish the detrimental consequences of the APOE4 genetic polymorphism,” the scientists concluded. “Our study suggests that manipulating lipid metabolism could be a therapeutic approach to help alleviate the consequences of carrying the APOE4 allele.”
“What our results suggest is that if you are an APOE2 or APOE3 carrier, even you are somewhat choline deficient you can cope with it,” Tsai commented. “But if you are an APOE4 carrier, then if you don’t take enough choline, then that will have a more dire consequences. The APOE4 carriers are more susceptible to choline deficiency.”