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Brain cells called oligodendrocytes wrap neurons with a sheath of insulating fat called myelin to improve electrical conduction. In people carrying the APOE4 gene variant, which increases Alzheimer's risk, the cells accumulate lipid droplets. Researchers studying why also learned a way to treat them in the lab. Here, an enlarged figure from the paper shows untreated APOE4 oligodendrocytes (left) accumulate lipid droplets (yellow), but are rescued by Gsk3b inhibition (right).
Image Credit: Courtesy of the Tsai Lab/Picower Institute.
Scientific Frontline: Extended "At a Glance" Summary: APOE4 Gene and Myelination Disruption
The Core Concept: The APOE4 gene variant, the primary genetic risk factor for late-onset Alzheimer’s disease, disrupts the cellular process of insulating neurons with a fatty sheath called myelin.
Key Distinction/Mechanism: Unlike the neutral APOE3 variant, APOE4 suppresses Wnt signaling in oligodendrocytes by downregulating beta-catenin and overexpressing GSK3b. This molecular imbalance causes oligodendrocytes to accumulate lipid droplets instead of utilizing fats to properly myelinate neuronal axon branches.
Major Frameworks/Components:
- Wnt Signaling Pathway: A crucial molecular pathway for lipid metabolism and myelination, which becomes suppressed in APOE4 carriers.
- Oligodendrocytes: Central nervous system cells responsible for wrapping neuronal axons in myelin sheaths to optimize electrical transmission.
- GSK3b and Beta-Catenin: Competing proteins regulating Wnt activity, where excessive GSK3b suppresses Wnt, leading to lipid droplet accumulation.
- CRISPR Screening and ATAC-seq: Advanced genomic technologies utilized to map accessible genes and identify therapeutic targets for lipid metabolism defects.
Branch of Science: Neuroscience, Molecular Biology, Molecular Genetics
Future Application: Inhibiting the GSK3b protein with targeted therapeutics (such as the chemical CHIR99021) represents a proof-of-concept strategy to restore Wnt signaling, clear lipid droplets, and promote axon myelination in Alzheimer's patients.
Why It Matters: Understanding how APOE4 undermines brain circuit insulation provides critical new targets for drug development, offering potential pathways to delay or reverse the neurodegenerative progression of late-onset Alzheimer’s disease.
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| An enlarged figure from the paper shows increased axon growth (magenta) in APOE4 organoids with Gsk3b inhibition treatment (right). Image Credit: Courtesy of the Tsai Lab/Picower Institute. |
One of the main ways that the APOE4 gene variant, the biggest genetic risk factor for late-onset Alzheimer’s disease, undermines brain circuit function is by disrupting the process of insulating neurons with a fatty sheath that optimizes their electrical transmission. A new study led by neuroscientists at the Picower Institute for Learning and Memory at MIT provides several lines of evidence to implicate a specific molecular pathway in that disruption and uses that insight to demonstrate an intervention for preventing it in lab experiments.
The study, published October 15 in Cell Stem Cell, 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, they showed, 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.
The research team, led by Picower Professor Li-Huei Tsai and study lead author Leyla Akay, PhD ’25, a former graduate student in Tsai’s lab, 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 absent as well.
“This proof of principle of doing CRISPR screens for genetic modifiers of APOE4-associated lipid accumulation in a central nervous system cell type is very exciting,” says Tsai, who also directs MIT’s Aging Brain Initiative. “I think this type of screen not only provides new insights into cellular pathways contributing to lipid metabolism defects, but really also has the potential to provide new targets for drug development.”
For Want of Wnt
In 2022, Tsai’s lab, led by Akay, Djuna Von Maydell, PhD ’25, Jose Davila-Velderrain, and Joel Blanchard (also a co-author of the new study), showed that APOE4 caused lipid droplet accumulation in oligodendrocytes and disrupted myelination. But, Akay says, that droplet accumulation could happen for many reasons. The new study sought to dig deeper into the underlying molecular mechanisms.
“This CRISPR screen allowed us an unbiased way to test every single possibility,” Akay says. “What kind of lipid enzymes are involved in this process? And what are upstream pathways that could be targeted?”
In parallel with the CRISPR screen, the team also analyzed the chromatin accessibility of the DNA in human oligodendrocyte cell cultures and in postmortem human brain samples, in each case comparing APOE4-carrying samples to ones carrying the neutral APOE3 variant. This analysis of ATAC-seq data allowed them to assess which genes were most accessible for expression versus which ones were locked down from transcription. The chromatin analysis showed that genes promoting Wnt signaling were less accessible in APOE4 carriers.
“There was a striking overlap between ATAC-seq and CRISPR screen results,” Tsai says. “Both pointed to a dysregulation of the Wnt signaling pathway.”
Toward a Therapy
Further analysis, including direct observations in cells at the lab bench, 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. As a final test, they treated mice carrying APOE4 and genetically modeling Alzheimer’s tau pathology with doses of CHIR99021. The treated mice exhibited a reduction in lipid droplets in the hippocampus of the brain, a key memory-making region. Moreover, they also showed increased myelination compared to untreated controls.
In addition to highlighting the suppression of Wnt signaling, the CRISPR screen and chromatin analysis provided key clues about the molecular mechanisms by which the loss of Wnt alters how oligodendrocytes metabolize lipids. Akay and Tsai say they are eager to follow up on those clues. They are also interested in investigating GSK3b’s linkage to tau phosphorylation, a major feature of Alzheimer’s pathology.
Funding: The National Institutes of Health, the Robert and Renee E. Belfer Family Foundation, the Manthripragada McManama Alzheimer's Fund, the CAMA Seed Fund, the J. Crayton Pruitt Foundation, Joseph P. DiSabato and Nancy Sakamoto, and the Freedom Together Foundation provided funding for the study.
Disclosure: Akay and Tsai have applied for a patent on the CRISPR screen results.
Published in journal: Cell Stem Cell
Authors: Leyla Anne Akay, Anna Bright, Daiyun Dong, Ethan D. Smith, Robert C. Hurt, Carles A. Boix, Kate Louderback, Janine Medrano, Dingcheng Sun, Haonan Lin, OisÃn King, Gwyneth M. Welch, Hiba Nawaid, Emre Agbas, Alan Jiang, Adele Bubnys, Ji-Xin Cheng, Joel W. Blanchard, and Li-Huei Tsai
Source/Credit: Massachusetts Institute of Technology | David Orenstein / The Picower Institute for Learning and Memory
Edited by: Scientific Frontline
Reference Number: ns102826_01
