A rare but powerful genetic mutation that alters proteins in the brain’s immune cells known as microglia can triple the risk of developing Alzheimer’s disease. A new study by researchers at the Massachusetts Institute of Technology’s Picower Institute for Learning and Memory details how this mutation impairs microglial function and explains how it creates their high risk. has been done.
“This TREM2 R47H/+ mutation is a fairly important risk factor for Alzheimer’s disease,” said study lead author Jay Penny, a former postdoctoral fellow in Picower Professor Lee Huei Tsai’s MIT lab. Penny is currently an incoming assistant professor at the University of Prince Edward Island. “This study adds clear evidence that microglial dysfunction contributes to Alzheimer’s disease risk.”
in magazine research gliaTsai and Penny’s team showed that human microglia with the R47H/+ mutation in the TREM2 protein exhibit several defects associated with Alzheimer’s disease pathology. Mutant microglia are more prone to inflammation, have a worse response to neuron damage, and are less able to clear harmful debris such as amyloid beta, the hallmark protein of Alzheimer’s disease. And when scientists transferred TREM2-mutant human microglia into the brains of mice, the mice had significantly fewer synapses, or connections between neurons, and the circuits that enable brain functions such as memory may be impaired. There is sex.
Penny said this study is not the first to ask how the TREM2 R47H/+ mutation contributes to Alzheimer’s disease, but it may advance scientists’ new understanding. Initial studies suggested that the mutation simply robbed the protein of its function, but new evidence paints a deeper, more nuanced picture. Although microglia exhibit debris clearance and reduced injury responses, they become hyperactive in other ways, including excessive inflammation and synaptic pruning.
“In addition to partial loss of functionality, you may also gain functionality in certain areas,” Penny says.
Microglia cheating
Rather than relying on a mouse model of the TREM2 R47H/+ mutation, Penney, Tsai, and their coauthors focused their work on human microglial cell cultures. To do this, they used a stem cell line derived from skin cells donated by a healthy 75-year-old woman. Then, in some stem cells, we used CRISPR gene editing to insert her R47H/+ mutation and cultured both edited and non-edited stem cells to become microglia. This strategy provided mutated and healthy microglia that served as experimental controls but were otherwise genetically identical.
The researchers then examined how this mutation affected gene expression in each cell line. The scientists measured more than 1,000 differences, but the most notable finding was that the mutated microglia had increased expression of genes associated with inflammation and immune responses. Next, when microglia in culture were exposed to chemicals that simulated infection, the mutant microglia showed a more pronounced response than normal microglia, suggesting that the mutation made them more susceptible to inflammation.
In further experiments with the cells, the researchers exposed them to three types of microglial debris that are normally removed in the brain: myelin, synaptic proteins, and amyloid beta. Mutant microglia were cleared less than healthy microglia.
Another job of microglia is to respond when cells such as neurons are damaged. Penny and Tsai’s team co-cultured microglia and neurons and zapped the neurons with a laser. For 90 minutes after the injury, the team tracked the movement of microglia around them. Compared to normal microglia, microglia with the mutation were found to be less likely to target damaged cells.
Finally, to test how mutant microglia act in the living brain, the scientists introduced mutant microglia or normal control microglia into a memory-focused region of the brain called the hippocampus in mice. Ported. The scientists then stained the area to highlight the different proteins of interest. Although having mutant or normal human microglia did not matter in some measurements, mice transplanted with mutant microglia had significant reductions in proteins associated with synapses.
By combining evidence from gene expression measurements and microglial function experiments, the researchers were able to formulate new ideas about the causes of at least some microglial malfunctions. For example, Penny and Tsai’s team found reduced expression of “purinergic” receptor proteins involved in sensing nerve damage, which likely explains why mutant microglia struggle with their role. it seems to do. They also noticed that mice with this mutation overexpressed the “complement” protein, which is used to tag and remove synapses. Professor Penny said this could explain why the mutant microglia were so eager to wipe out synapses in mice, but it was also possible that increased inflammation caused damage to neurons overall.
Professor Penny said that as the molecular mechanisms underlying microglial dysfunction become clearer, drug developers will gain important insights into how to target the high disease risk associated with the TREM2 R47H/+ mutation. .
“Our findings highlight multiple effects of the TREM2 R47H/+ mutation that likely underlie its association with Alzheimer’s disease risk and suggest new nodes that can be exploited for therapeutic intervention,” the authors wrote. conclude.
In addition to Penny and Tsai, the paper’s other authors are William Larvenius, Anjanette Luhn, Oik Sellit, Vishnu Dileep, Breta Miro, Pinchy Pao, and Hannah Wolf.
The Robert A. Rennie Belfer Family Foundation, Alzheimer’s Treatment Fund, National Institutes of Health, JPB Foundation, Picower Institute for Learning and Memory, and Human Frontier Science Program funded this research.