Alzheimer’s risk gene undermines insulation of brain ‘wiring’ MIT News > Massachusetts

It is well known that carrying one copy of the APOE4 gene variant increases the risk of Alzheimer’s disease threefold and two copies tenfold, but the basic reasons for this and what can be done to help patients are still largely unknown . A study published by an MIT-based team on Nov. 16 Nature provides some new answers as part of a broader line of research that has revealed the effects of APOE4 cell type by cell type in the brain.

The new study combines evidence from postmortem human brains, laboratory-based human brain cell cultures and Alzheimer’s model mice to show that when humans have one or two copies of APOE4, rather than the more common and risk-neutral APOE3 version, cells called oligodendrocytes will mismanage cholesterol by failing to transport the fat molecule to wrap around the long, vine-like axon “wiring” that neurons project to make circuit connections in the brain. A deficiency in this fatty insulation, called myelin, can be a major contributor to the pathology and symptoms of Alzheimer’s disease because communication between neurons is impaired without proper myelination.

Recent studies by the research group led by MIT Professor Li-Huei Tsai, director of the Picower Institute for Learning and Memory and the Aging Brain Initiative at MIT, have found clear ways in which APOE4 interferes with the handling of fat molecules, or lipids in key types of brain cells, including neurons, astrocytes and microglia. In both the new and previous studies, the team has identified compounds that are appearing in the lab to address these distinct issues, leading to potential pharmaceutical-based treatment strategies.

The new study extends that work not only by discovering how APOE4 disrupts myelination, but also by providing the first systematic analysis across key brain cell types using single-nuclear RNA sequencing (snRNAseq) to compare how gene expression varies in people with APOE4 compared to APOE3.

“This publication very clearly shows, using the snRNAseq of postmortem human brains in a genotype-specific manner, that APOE4 affects different brain cell types very clearly,” said Tsai, a member of the Department of Brain and Cognitive Sciences at MIT. “We see that the convergence of lipid metabolism is disrupted, but if you take a closer look at what kind of lipid pathways are disrupted in different types of brain cells, they are all different.

“I think that lipid dysregulation could be this very basic biology that underlies a lot of the pathology that we see,” she says.

The paper’s lead authors are Joel Blanchard, an assistant professor at the Icahn School of Medicine at Mt. Sinai, who began his postdoctoral work in Tsai’s MIT lab; Djuna Von Maydell and Leyla Akay, who are graduate students in Tsai’s lab; and Jose Davila Velderrain, research group leader at the Human Technopole and former postdoc in the lab of co-author Manolis Kellis, professor of computer science at MIT.

Many methods of studying myelination

Postmortem human brain samples came from the Religious Orders Study and the Rush Memory and Aging Project. The team’s snRNAseq results, a data set made freely available by von Maydell, include more than 160,000 single cells of 11 different types from the prefrontal cortex of 32 people — 12 with two APOE3 copies, 12 with one APOE3 and APOE4 copy and eight with two APOE4 copies. The APOE3/3 and APOE3/4 samples were balanced for Alzheimer’s diagnosis, gender and age. All APOE4/4 carriers had Alzheimer’s and five out of eight were female.

Some results reflected known Alzheimer’s pathology, but other patterns were novel. In particular, one showed that APOE4-carrying oligodendrocytes had greater expression of cholesterol synthesis genes and cholesterol transport disorders. The more APOE4 copies people had, the greater the impact. This was particularly interesting given the results of an earlier 2019 analysis by Tsai and Kellis’ labs, which linked Alzheimer’s disease to decreased expression of myelination genes among oligodendrocytes.

Using various techniques to view the tissue directly, the team found that in APOE4 brains, abnormal levels of cholesterol accumulated in cell bodies, particularly in oligodendrocytes, but were relatively low around neural axons.

To understand why, the team used patient-derived induced pluripotent stem cells to create laboratory cell cultures of oligodendrocytes engineered to differ only by whether they had APOE4 or APOE3. Again, APOE4 cells showed major lipid disruption. The affected oligodendrocytes, in particular, stored extra cholesterol in their bodies, showed evidence that the extra internal fats put stress on organelles called the endoplasmic reticulum that play a role in cholesterol transport, and actually transported less cholesterol to their membranes. Later, when cultured along with neurons, the APOE4 oligodendrocytes could not myelinate the neurons as well as APO3 cells, regardless of whether the neurons carried APOE4 or APOE3.

The team also observed that there was less myelination in postmortem brains in APOE4 carriers than in APOE3 carriers. For example, the sheaths around axons that pass through the corpus callosum (the structure that connects the cerebral hemispheres) were significantly thinner in APOE4 brains. The same was true for mice engineered to harbor human APOE4 versus those engineered to have APOE3.

A productive intervention

Looking for a possible intervention, the team focused on drugs that affect cholesterol levels, including statins (which suppress synthesis) and cyclodextrin, which aids in cholesterol transport. The statins didn’t help, but applying cyclodextrin to APOE4 oligodendrocytes cultured in a dish reduced the accumulation of cholesterol in the cells and improved myelination in co-cultures with neurons. In addition, it also had these effects in APOE4 mice.

Finally, the team treated some APOE4 mice with cyclodextrin, left others untreated, and gave them all two different memory tests. The cyclodextrin-treated mice performed significantly better on both tests, suggesting a link between improved myelination and improved cognition.

Tsai said there is a clear picture emerging where intervention to correct specific lipid dysregulations by cell type could potentially help counteract APOE4’s contributions to Alzheimer’s pathology.

“It’s encouraging that we found a way to salvage oligodendrocyte function and myelination in laboratory and mouse models,” says Tsai. “But in addition to oligodendrocytes, we may also need to find clinically effective ways to nurture microglia, astrocytes and vessels to truly combat the disease.”

Besides lead authors Tsai and Kellis, the other authors of the publication are Hansruedi Mathys, Shawn Davidson, Audrey Effenberger, Chih-Yu Chen, Kristan Maner-Smith, Ihab Jahhar, Eric Orlund, Michael Bula, Emre Agbas, Ayesha Ng, Xueqiao Jiang, Martin Kahn, Cristina Blanco-Duque, Nicolas Lavoie, Liwang Liu, Ricardo Reyes, Yuan-Talin, Tak Ko, Lea R’Bibo, William Ralvenius, David Bennet, and Hugh Cam.

The Robert A. and Renee E. Belfer Foundation, the JPB Foundation, the Carol and Gene Ludwig Family Foundation, the Cure Alzheimer’s Fund, and the National Institutes of Health funded the study.

Source