Author: Jean Paul Chadarevian

Irvine, Calif., Feb. 14, 2022 — While the word “mutation” may conjure up alarming notions, a mutation in brain immune cells serves a positive role in protecting people against Alzheimer’s disease. Now University of California, Irvine biologists have discovered the mechanisms behind this crucial process. Their paper appears in the journal Alzheimer’s and Dementia.

The investigation centered on a variant of the PLCG2 gene, which makes the instructions for producing an enzyme important to brain immune cells called microglia. “Recently the mutation, which is known as P522R, was shown to lower the risk of developing late-onset Alzheimer’s,” said Hayk Davtyan, Ph.D., senior researcher in the laboratory of Mathew Blurton-Jones, professor of neurobiology & behavior, where the study was conducted. The project was led by assistant project scientist Christel Claes, Ph.D., the paper’s first author.

The scientists used CRISPR gene-editing technology to generate the protective mutation in human stem cells and then implanted microglia derived from those stem cells into humanized rodent models of Alzheimer’s disease.

“Our research showed for the first time that the P522R variant increased expression levels of several microglial genes that are reduced in people with Alzheimer’s. This provides some of the first evidence to explain how this protective mutation might reduce Alzheimer’s risk,” Davtyan said.

The variant also increased the number of T-cells, or white blood immune system cells, in the brain suggesting that it may increase the activation of other important aspects of immune function.

The results will help in designing further studies to understand exactly how microglia and T-cells interact to slow Alzheimer’s progression.

“Beyond that, the next step could be to identify drugs that can safely increase the activity of the PLCG2 enzyme and further promote protective microglial functions,” he said.

First author Christel Claes wonders whether a TREM2 stimulating antibody, like the one currently in a Phase 2 clinical study from Alector (AL002), could exert similar protection in AD patients as the P522R variant.

“It is well known that the PLCG2 P522R mutation increases TREM2 downstream signaling, an AD risk variant, thus it will be very interesting to study the effect of TREM2 stimulating antibodies on microglia-T cell crosstalk. Studies like ours pave the way to find new strategies to treat or prevent this disease that is taking such a toll on humanity, this is what drives us as neuroscientists.” said Davtyan.

The project was supported by a BrightFocus Postdoctoral Fellowship, the Cure Alzheimer’s Fund, National Institutes of Health and National Institute on Aging.

About the University of California, Irvine: Founded in 1965, UCI is the youngest member of the prestigious Association of American Universities and is ranked among the nation’s top 10 public universities by U.S. News & World Report. The campus has produced five Nobel laureates and is known for its academic achievement, premier research, innovation and anteater mascot. Led by Chancellor Howard Gillman, UCI has more than 36,000 students and offers 224 degree programs. It’s located in one of the world’s safest and most economically vibrant communities and is Orange County’s largest employer, contributing $7 billion annually to the local economy and $8 billion statewide. For more on UCI, visit www.uci.edu.

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Congratulations to Morgan Coburn on a successful  Ph.D. Dissertation Defense “Using Chimeric Models to Study the Interactions between Human Microglia and Alzheimer’s Disease Pathology in vivo.“

Congratulations to Amanda McQuade on a successful  Ph.D. Dissertation Defense Development and application of a human microglia model to examine the influence of genetic risk factors on microglial function in Alzheimer’s disease:

Alzheimer’s disease (AD) is a progressive neurodegenerative disease for which there is no cure. Worldwide, AD is estimated to affect 50 million people with 10 million new cases each year. Developing therapeutics for Alzheimer’s disease has been particularly difficult given the complex etiology of this disease. Alzheimer’s disease is characterized by an accumulation of parenchymal beta-amyloid protein plaques, tau neurofibrillary tangles, and neuroinflammation. For sporadic AD, which accounts for around 95 % of AD cases, the direct trigger of neurodegeneration remains unclear. Understanding the mechanistic pathophysiology of this disease will allow for the development of more effective targeted therapeutics and biomarker studies to help patients.

In the last decade, genome-wide association studies (GWAS) have renewed interest in neuroinflammation as a potential disease-modifying mechanism. These large-scale genetic studies have uncovered a striking enrichment of immune-specific genes as risk-factors for Alzheimer’s disease. However, the function of many of these GWAS loci are not well understood. Additionally, many of these genes have poor homology between human cells and traditional murine disease-models. Thus, there is a critical need to develop a model of human microglia in order to understand how these immune risk factors influence human microglial function.

The focus of this dissertation is to develop and characterize a model of human microglia that can be readily studied without the need for isolation of human brain tissue surgically or post-mortem. Building on previous research in the lab, we have developed a model of human microglia differentiated from induced pluripotent stem cells (iPS-Microglia). This model follows developmental ontogeny with a primary differentiation into CD43+ hematopoietic progenitor cells before transition into a microglial differentiation medium containing neuron and astrocyte
derived cytokines to educate our microglia in a homeostatic brain-like environment. The result is a highly pure population of iPS-Microglia which perform key microglial functions and cluster alongside human microglial transcriptomes (Chapter 1).

Because this microglial model is fully defined beginning from iPS cells, it is possible to combine this approach with modern molecular biological manipulation such as CRISPR gene editing. This dissertation focuses on studies surrounding the AD-risk loci Triggering Receptor Expressed on Myeloid Cells II (TREM2). Predicted loss of function mutations in TREM2 increase Alzheimer’s disease risk up to 2-3 fold making it the highest microglial-specific risk factor for AD. By performing CRISPR-mediated knockout of TREM2 in human iPSCs and differentiating into
microglia, we find that TREM2-knockout locks microglia in a homeostatic state. Specifically, microglia lacking TREM2 are deficient in SYK-mediated phagocytosis, CXCR4-mediated migration, and are more sensitive to MCSF-mediated cell death.

In vivo, this translates to an inability to perform chemotaxis towards and compaction of beta-amyloid plaques leading to a build-up of pathology in the brain. We also characterize a distinct lack of transcriptional activation in TREM2 knockout cells suggesting that induction of the Disease Associated Microglia (DAM) profile may be an essential immune response in shielding the brain from dementia (Chapter 2).

Because TREM2-knockout cells are locked in a homeostatic state, they express high levels of homeostatic microglia markers P2RY12 and P2RY13. These purinergic receptors are critical for microglial communication with neurons and clearance of dead cells. We show that purinergic signaling has a profound effect on microglial motility and process extension and that hyper-expression and response of purinergic signaling in TREM2-knockout microglia may render these cells unable to sense gradients and activate against disease pathology. Furthermore, we highlight that partial inhibition of purinergic receptors will rescue migratory deficits characterized in TREM2-knockout microglia suggesting a therapeutic mechanism (Chapter 3).

Taken as a whole, the development of the iPS-Microglia model has yielded critical insight into the interaction of microglial function and the development of Alzheimer’s disease. This model has been readily adopted by many academic labs as well as pharmaceutical companies with the aim of studying human microglial function or other disease risk loci. By further studying AD risk genes in this fashion, we may be able to converge on several mechanisms by which microglial functions are able to attenuate or drive neurodegeneration. Focusing therapeutic efforts on these pathways could lead to the development of immunotherapies for Alzheimer’s disease.