Microglia can help or hinder in Alzheimer’s disease. A protective variant of the microglial gene phospholipase C-γ2 (PLCG2) may shed light on what makes the difference. In the September 11 Molecular Neurodegeneration, researchers led by Mikko Hiltunen at the University of Eastern Finland, Kuopio, and Christian Haass at Ludwig-Maximilians University in Munich report that mice carrying the protective P522R PLCG2 variant accumulate more activated microglia as they age. These microglia boast a gene expression profile that overlaps with the previously described DAM, or disease-activated microglia. The mobilized cells appear sensitized to defend the brain, because they respond better to inflammatory stimuli than do microglia from wild-type mice. P522R PLCG2 cells also survive longer and gobble up particles more readily.
- P522R PLCG2 variant reduces a person’s risk of AD.
- In P522R PLCG2 knock-in mice, more microglia activate with age than in wild-types.
- The variant acts downstream of TREM2, boosting phagocytosis.
Notably, PLCG2 acts downstream of TREM2, a major AD risk gene. “That AD mutations from different genes fall into the same pathway tells us this pathway is causally involved, and that activating it could be therapeutic,” Haass told Alzforum. In future work, he will test this idea by crossing the knock-in mice with models of amyloidosis.
The P522R variant was found in especially long-lived people who had a low risk of AD, dementia with Lewy bodies, and frontotemporal dementia (Aug 2017 conference news; Sims et al., 2017; May 2019 news).
Pathway to Watch. Plcg2 acts downstream of the TREM2 receptor to snip lipids, activating numerous signaling pathways and modulating gene transcription. [Courtesy of Takalo et al., Molecular Neurodegeneration.]
Recently, researchers led by Joseph Lewcock at Denali Therapeutics in South San Francisco used microglial cultures generated from iPS cells to show that TREM2 activation switched on PLCG2’s phospholipase activity, which in turn regulated lipid processing. The P522R variant greased phospholipase activity and lipid processing, while knocking out PLCG2 caused lipid debris to build up inside microglia (Jun 2020 news).
To examine what the variant does in vivo, Haass and colleagues generated a P522R PLCG2 knock-in mouse. At one year of age, these mice had the same number of microglia, with the same morphology, as did wild-types. However, TSPO PET imaging showed that the knock-ins had more activated microglia in all brain regions. Microglia in the knock-ins also expressed more of the purinergic receptor, P2RY12, which mediates phagocytosis.
Gene expression analysis showed that, like DAMs, P522R PLCG2 knock-in microglia expressed more APOE, TYROBP, CCL3, CST7, and CLEC7A. However, unlike DAMs, P522R PLCG2 cells did not upregulate TREM2, ITGAX, or CD68 (Jun 2017 news). This indicates a partial overlap between the phenotypes. Because the P522R variant is protective, Haass believes this expression profile represents a signature of therapeutic microglia. Studying this signature could help guide anti-inflammatory treatment strategies, he said.
First author Mari Takalo, working in Kuopio, isolated peripheral macrophages from the knock-in mice. Compared to wild-type macrophages, the cells were less prone to apoptosis when Takalo withdrew growth factors. After an inflammatory stimulus, the knock-in cells released more of the proinflammatory cytokines TNFα, IL-6, and IL-1β, but not NO, which has apoptotic effects. The knock-ins were better phagocytes, swallowing about 50 percent more test particles than did wild-types. In a mouse microglial cell line, P522R PLCG2 had an even greater effect, doubling phagocytosis. The data are exactly the opposite of what Denali reported in PLCG2 knockout microglia.
“These findings are entirely consistent with the results from our recent manuscript,” Lewcock wrote to Alzforum (full comment below). “This study is the first to examine how the P522R variant impacts microglial activity in vivo, and provides additional data to substantiate the idea that this variant acts as a mild hypermorph.”
Marco Colonna and Yingyue Zhou at Washington University in St. Louis agreed that the results confirm that P522R acts as a gain-of-function mutation, but noted that the disease relevance is still unclear. “How the mutation affects microglial functions in a neurodegenerative condition, such as AD, remains an important question to be answered,” they wrote (full comment below).
Hiltunen said this is the next goal. In their knock-in/amyloidosis crosses, the researchers will examine whether the variant mitigates pathology as animals age. So far, the data suggest that P522R hits the sweet spot of boosting microglia activity just enough to be helpful. That is no foregone conclusion, however, because too much activation of the TREM2 pathway, as happens with progranulin mutations, can turn microglia into dangerous, proinflammatory monsters that trigger neurodegeneration (Apr 2019 news).
Given this tricky balance, could boosting PLCG2 function ever be a therapeutic approach? Lewcock said finding direct activators for this enzyme would be challenging. In addition, activating PLCG2 would have broader effects than activating TREM2 alone, because PLCG2 also mediates pro-inflammatory signaling through Toll-like receptors.
Haass agrees that the best approach is to target TREM2, rather than PLCG2. “TREM2 is very selective, and that’s a big advantage,” he said.
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- Antibodies Against Microglial Receptors TREM2 and CD33 Head to Trials
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