The APOEε4 Variant Interferes in Myelination in the Aging Brain
Of the common variants of the APOE gene, APOEε4 presents the greatest risk of Alzheimer's disease. APOE is involved in lipid metabolism, and APOEε4 is associated with lipid-related contributions to inflammation in microglia, suggesting a primarily inflammatory mechanism for the increased risk of neurodegeneration. Here, researchers find that APOEε4 also interferes in the ability of oligodendrocyte cells to maintain the myelin structures that sheath the axons that connect neurons into neural networks, again via lipid-related mechanisms. This myelin sheathing is required for effective communication between neurons. Myelin maintenance does falter to some degree in normal aging, and the consequent loss of myelin is thought to contribute to dysfunction in the aging brain. It is reasonable to hypothesize that greater myelin loss driven by APOEε4 is a factor in accelerated neurodegeneration.
A new study shows that APOE4 leads to a suppression of Wnt signaling in the oligodendrocyte cells that provide neurons with a fatty insulation, called myelin. When Wnt signaling is deficient, the cells accumulate droplets of lipids rather than deploying fats to properly myelinate the neurons' axon branches. When the researchers restored Wnt signaling in APOE4-carrying oligodendrocytes, myelination rebounded, both in human cell cultures and in mice modeling Alzheimer's.
Researchers discovered Wnt's key role via multiple methods including a screen in which they used CRISPR gene editing to systematically knock out each protein-coding gene in the genome, one by one, in cultures of human APOE4-carrying oligodendrocytes. The screen showed that when genes promoting Wnt activity in oligodendrocytes were lacking, lipid droplets accumulated, and that when genes suppressing Wnt were lacking, lipid droplets were, too. Further analysis revealed how Wnt signaling became depressed in cultured APOE4 oligodendrocytes. Two proteins, beta-catenin and GSK3b, each compete to regulate Wnt-beta-catenin promotes it and GSK3b suppresses it. The team found that in APOE4 oligodendrocytes compared to APOE3 ones, there was reduced beta-catenin expression and excessive GSK3b expression.
The team then tested interventions that manipulate GSK3b activity. In human APOE3 oligodendrocyte cultures, they showed that increasing GSK3b with a chemical called Dif3 increased lipid droplets. In APOE4 oligodendrocytes, they showed that inhibiting GSK3b with a chemical called CHIR99021 reduced droplets. In more complex "organoid" cultures combining APOE4 oligodendrocytes with neurons, they found that inhibiting GSK3b increased myelination and promoted the growth of neural axons. The tests suggested that GSK3b inhibition could be a proof-of-concept therapeutic strategy for restoring Wnt signaling and myelination.
Link: https://picower.mit.edu/news/how-biggest-alzheimers-risk-gene-disrupts-process-insulating-neurons