https://www.alzforum.org/news/research-news/droplets-unsaturated-fats-burden-human-apoe4-astrocytes

Droplets of Unsaturated Fats Burden Human ApoE4 Astrocytes

12 Mar 2021

The E4 allele of the apolipoprotein E, a strong risk factor for many neurodegenerative diseases, most prominently late-onset Alzheimer’s disease, upsets the balance of lipids within cells. Exactly how ApoE4 does this remains unclear. In the March 3 Science Translational Medicine, researchers led by Li-Huei Tsai at MIT found that in human astrocytes and microglia, ApoE4 increased the amount of unsaturated lipids, which then accumulated in lipid droplets. Adding the phospholipid precursor choline to the cells restored their normal lipid metabolism, hinting at potential therapeutic approaches for ApoE4 carriers.

“This study emphasizes the importance of balanced lipid metabolism in prevention and treatment of AD,” Hilkka Soininen from the University of Eastern Finland, Kuopio, wrote to Alzforum. “Since the discovery of ApoE4 as a risk gene for AD, I have been waiting for innovations to reverse its effect and hope to see that in the future.”

This is not the first time scientists have spotted lipid droplets in glia. In fact, Alois Alzheimer himself described lipid buildup in those cells. More recently, one prior study uncovered similar blobs in mouse microglia that were dubbed lipid droplet-associated microglia. LAMs accumulated in the mouse hippocampus with age, increasing neuroinflammation (Aug 2019 news). 

Although ApoE levels remained unchanged in LAMs, other researchers reported that human astrocytes expressing ApoE4 generate more cholesterol than their ApoE3 counterparts and don’t break it down properly, causing lipids to gather inside the cells, possibly increasing susceptibility to AD. Similarly lipid-laden astrocytes were found in AD brain tissue (Aug 2019 news). 

However, the question remains: Exactly how does ApoE4 cause lipids to pile up? Tsai and colleagues focused on astrocytes because they are the main source of APOE in the brain (Boyles et al., 1985). Co-first author Grzegorz Sienski analyzed the lipid composition in ApoE3- or ApoE4-expressing isogenic human astrocytes that he had derived from induced pluripotent stem cells. Using liquid chromatography-mass spectrometry, he discovered that ApoE4 astrocytes contained more unsaturated triacylglycerides. By adding a lipophilic dye to the astrocytes, Sienski saw that ApoE4 cells accumulated lipid droplets.

How would ApoE4 alter the lipidome in the human brain? Sienski examined gene-expression data from postmortem samples of 838 people with different genotypes, ages, and causes of death who had enrolled in the NIH-funded Genotype-Tissue Expression project. He compared the transcriptomic profiles of ApoE4 carriers with those of noncarriers across a set of 609 lipid metabolism genes pulled from the Kyoto Encyclopedia of Genes and Genomes. Genes involved in the biochemistry of neutral lipids and cholesterol were upregulated in ApoE4 carriers, whereas genes involved in the breakdown of fatty acids and neutral lipids were downregulated, suggesting a disturbance in lipid metabolism.

To further explore how ApoE4 causes lipid defects, Sienski and colleagues collaborated with researchers then at the lab of the late Susan Lindquist at MIT. Co-first authors Priyanka Narayan and Julia Bonner created yeast expressing human ApoE3 or ApoE4. Similar to the human astrocytes, ApoE4 yeast had more lipid droplets and grew poorly compared to ApoE3 yeast. The authors used loss-of-function screens to look for genes that might mediate ApoE4’s effects in yeast. Their hits included OPI1, MGA2, and UBX2, all of which encode proteins that regulate lipid metabolism. Focusing on MGA2 and OPI1, they found that neither knockout altered the amount of ApoE4 protein itself, reaffirming that the growth defect seen in these yeasts stemmed from abnormal lipid metabolism.

OPI1 regulates phospholipid synthesis, hence the authors wondered whether precursors of this pathway could reverse ApoE4-related deficiencies. They tried choline and ethanolamine. Lo and behold, when the yeast grew in a medium with choline, both the growth defect and lipid imbalance disappeared. Similarly, ApoE4 astrocytes cultured in a medium supplemented with cytidine 5'-diphosphocholine (CDP-choline) contained fewer triacylglycerides, fewer unsaturated fatty acids, and fewer lipid droplets than untreated cells.

“Choline treatment ameliorated all of the lipid metabolic disruption that we observed,” Tsai told Alzforum. “It brought the yeast and astrocytes back to a homeostatic state.”

Now, one big question is whether it may be possible to mitigate deleterious effects of ApoE4 in people by adding more choline-rich foods, such as nuts, eggs, and fish, to their diet. Tsai said she is studying ApoE4 knock-in mice that are given either a normal diet or a choline-rich one.

Choline is a component of Souvenaid, a medical food designed by Nutricia of Danone Research to enhance the formation and function of synapses. In the LipiDiDiet clinical trial, people with AD who consumed this yogurt-like drink once a day for two years had slightly less shrinkage of the hippocampus than did controls. The trial failed to meet its primary outcome, but follow-up data suggested that Souvenaid might help people who continue consuming the drink for three years (Apr 2020 conference news). 

The authors also noticed that ApoE4 alters lipid droplets in microglia, and are exploring this finding in animal models. “Microglia show tremendous disruption of lipid metabolism,” said Tsai. How this relates to the lipid droplets in LAMs remains to be seen.—Helen Santoro