Loss-of-function variants in the microglial protein TREM2 increase the risk for late-onset Alzheimer’s disease. Now science suggests a reason: Deletion of TREM2 causes a metabolic meltdown for microglia, whereby the cells cannot meet their own energy demands and undergo apoptosis.
In the August 10 issue of Cell, Marco Colonna and colleagues at Washington University in St. Louis show that microglia lacking TREM2 have global changes in their metabolism that result in lowered ATP levels and signs of stress and death. Microglia in these mice were too moribund to ring around amyloid plaques or prevent amyloid toxicity to the surrounding neurons. Correcting this “metabolic derailment,” as the authors call it, with dietary cyclocreatine raised microglial ATP levels. Lo and behold, the cells once again rallied around amyloid plaques, where they protected neurons from toxicity.
“In this extremely exciting paper, the Colonna lab shows that TREM2-deficient dysfunctional microglia can be therapeutically modulated to regain their original defense mechanisms,” wrote Christian Haass and Gernot Kleinberger of the German Center for Neurodegenerative Diseases in Munich.
The Haass lab’s own recent analyses of microglia from TREM2-knockout mice painted a picture of immune cells stuck in neutral with an apparent deficit in glucose uptake. Held back in a quiescent state, the cells were unable to migrate normally, or respond to amyloid (see May 2017 news). Alas, the depiction in this new study is even worse—the microglia are not just sleeping, they are actually quite ill.
Microglia, the resident immune cells of the brain, are emerging as key defenders against the damage caused by amyloid plaques. Microglia normally form a protective barrier around amyloid deposits, which prevents plaque spread and shields nearby neurons from toxicity. Pathogenic TREM2 mutations or TREM2 knockout impair this protective function (May 2016 news). However, exactly how loss of TREM2 immobilized microglia was unclear.
To get at that question, first authors Tyler Ulland and Wilbur Song isolated microglia from TREM2 knockout mice crossed with 5XFAD mice, a model that develops early and abundant plaque pathology with neuronal death. Electron micrograph images revealed microglia loaded with multilamellar vesicles resembling autophagosomes. In brain tissue, more than 80 percent of microglia stained positive for the autophagosome marker LC3, compared with 20 percent in either the TREM2 knockout or 5XFAD parental strains. The investigators saw a similar increase in LC3 in postmortem human tissue: Microglia in brains from AD patients carrying the R47H or R62H TREM2 risk allele had three to four times more LC3+ microglia than cases without the risk variants.
Signs of stress: Autophagosomes (yellow arrows) proliferate in microglia when TREM2 is knocked out in the 5XFAD AD mouse model. [Courtesy of Cell, Ulland et al., 2017.]
Autophagosomes are telltale signs of metabolic stress, where cells cannot keep up with the energy demands of proliferation. Looking more closely at brain microglia ex vivo, the investigators found a significant deficit in activity in the mammalian target of rapamycin (mTOR) pathway, a crucial mediator of cell metabolism and proliferation. In isolated microglia, they found impaired phosphorylation of mTOR pathway intermediates, indicative of deficient mTOR activation and a low energy state in the cells. Compared to microglia from wild-type or 5XFAD mice, the TREM-deficient cells had lower mitochondrial mass, and changes in gene expression tied to aberrant glycolysis and protein synthesis. More of the microglia contained traces of cleaved caspase3, a marker for apoptosis.
Cells without TREM2 responded haplessly to stress in general. In macrophages derived from the bone marrow of TREM2 knockouts, growth factor deprivation increased autophagy and the same mTOR and metabolic failures as seen in the microglia from TREM2-/-5XFAD mice. Metabolomics analysis showed widespread decreases of metabolic intermediates involved in nucleotide synthesis, glycolysis, and energy production, along with more anabolic metabolites.
The results suggest that TREM2 provides trophic support for macrophages and microglia in stressful conditions. “In AD, microglia undergo prolonged stress and activation of proliferation, and that is sustained by mTOR signaling. If there is no TREM2, all the functions of microglia, and ultimately survival, are compromised,” Colonna explained. This suggests that microglia in TREM2 knockout mice are not simply ignoring plaques. “They are being actively driven into a stressed state that is normally compensated by TREM2-depending survival signals,” he told Alzforum
In the short term, autophagy may facilitate clearing of plague by stimulating microglial phagocytosis, Colonna said. But in the long term, microglia lacking TREM2 die, and that may open the gates to further spreading of plaques.
Can the cells be saved? To try, the researchers turned to cyclocreatine, a compound that raises ATP levels in cells. Treating bone-marrow-derived macrophages in vitro with cyclocreatine raised their metabolic rate, lowered the number of autophagic vesicles, and restored mTOR signaling and cell viability. To treat mice, the investigators added cyclocreatine to the drinking water continuously from 10 weeks of age. At eight months of age, they saw fewer microglia containing autophagosomes, less LC3 staining, and less caspase 3 cleavage in treated mice compared with untreated littermates. Cyclocreatine treatment increased the number of microglia clustered around plaques, and the expression of the microglial activation marker osteopontin.
Importantly, cyclocreatine also improved the cells’ barrier function. In TREM2-/- 5XFAD mice, plaques were diffuse and not surrounded by microglia. After treatment with cyclocreatine, more microglia appeared around plagues, which were denser, resembling those in parental 5XFAD mice. Cyclocreatine treatment did not change the number of plaques or the engulfment of amyloid by microglia, but it did reduce the number of dystrophic neurites around plaques.
The work provides a “proof of principle that if we can rescue the metabolic pathway, somehow we can restore the function of microglia,” Colonna said. “I hope this will inspire pharmacology studies and focus attention on this pathway.”
Cyclocreatine’s relative, creatine, is sold over the counter in the United States and widely used by athletes as a nutritional supplement. Colonna strongly advises against people trying creatine to ward off AD. The activity he sees in mice is no proof of activity in people, and the compound can harm the heart and kidneys with long-term use.—Pat McCaffrey
- Paper Alert: TREM2 Crucial for Microglial Activation
- Barrier Function: TREM2 Helps Microglia to Compact Amyloid Plaques
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- Ulland TK, Song WM, Huang SC, Ulrich JD, Sergushichev A, Beatty WL, Loboda AA, Zhou Y, Cairns NJ, Kambal A, Loginicheva E, Gilfillan S, Cella M, Virgin HW, Unanue ER, Wang Y, Artyomov MN, Holtzman DM, Colonna M. TREM2 Maintains Microglial Metabolic Fitness in Alzheimer's Disease. Cell. 2017 Aug 10;170(4):649-663.e13. PubMed.