Since the discovery that variants in the microglial receptor TREM2 triple the risk of Alzheimer’s disease, researchers have tried to figure out why. Studies conflict on what TREM2 does in AD models, particularly when it comes to mopping up amyloid. Now, researchers led by Christian Haass at the German Center for Neurodegenerative Diseases (DZNE), Munich, strengthen the case for a phagocytic role. In cell culture studies, microglia and macrophages lacking the receptor engulfed less amyloid than their wild-type counterparts did, the researchers report in the July 8 EMBO Molecular Medicine. Coating aggregated amyloid with anti-Aβ antibodies enabled all phagocytes to gobble more amyloid. The knockouts lagged behind controls at low antibody concentrations, though dramatically upping the dose did whet the appetites of TREM2 knockout cells up to control levels. 

If the finding holds up it may imply that Alzheimer’s patients whose microglia are impaired would need higher doses of therapeutic anti-Aβ antibodies, the authors suggest. Others agreed. “This study hints that TREM2 can affect the potency and efficacy of immunotherapy targeting Aβ,” commented Jason Ulrich at Washington University in St. Louis. “It will be interesting to see if these results can be recapitulated in vivo.” 

Appetite for Amyloid.

Amyloid (green) inside a macrophage (purple) feasting on an AD mouse brain section. [Courtesy of EMBO.]

Previous work has supported a role for TREM2 in modulating phagocytosis. Cultured microglia lacking the receptor poorly swallow beads, Haass’ group found (see Jul 2014 webinarApr 2015 conference news). Others reported that TREM2 knockouts failed to clean up damaged tissue and myelin debris in healthy mouse brain (see Feb 2015 news; Cantoni et al., 2015). In AD mouse models, evidence suggests plaques build up in the absence of TREM2 (Wang et al., 2015), but how TREM2 controls deposits is unclear. In some studies, TREM2 appears to help microglia live longer, steer them to plaques, or wall off deposits from surrounding tissue, rather than drive phagocytosis (see Feb 2015 conference news; May 2016 news). 

To nail down whether TREM2 does affect phagocytosis, first author Xianyuan Xiang and colleagues applied aggregates of synthetic Aβ42 to five types of cultured cells: a murine microglial line with TREM2 knocked out, bone marrow-derived macrophages from wild-type and TREM2 knockout mice, and primary microglia isolated from neonatal mice of both genotypes. In all cases, TREM2 knockout cells ingested about half as much amyloid as wild-types did.

The authors then stimulated phagocytosis by coating amyloid with either 2D8 or mAb11, antibodies that target the N-terminus of Aβ. The latter binds amyloid fibrils much like gantenerumab, which is in Phase 3 clinical trials (see Bohrmann et al., 2012; Lathuiliere et al., 2016). Antibody treatment boosted amyloid uptake two- to sevenfold in both wild-type and TREM2 knockout cells. Nonetheless, knockout cells remained sluggish compared with their wild-type counterparts, ingesting one-half to two-thirds as much amyloid.

To see whether the phagocytes were also able to clean up amyloid in the brain, the authors added macrophages to tissue sections from six-month-old APPPS1 mice, which sport abundant plaques. Wild-type cells mopped up about one-fourth of the plaques, while TREM2 knockout cells left them untouched. When sections were pre-incubated with mAb11 for one hour, wild-type cells gobbled up three-quarters of the deposits, and TREM2 knockouts cleared about half. Amyloid appeared inside vesicles in macrophages (see image above). Boosting antibody concentration by 10- to 100-fold bumped up the performance of TREM2-negative macrophages to levels close to wild-type. TREM2 knockouts began to clear amyloid at antibody concentrations of 0.1 μg/ml, a level achievable in brain with gantenerumab treatment, the authors noted. However, they did not test microglia, the brain resident macrophages, in these titration experiments.

If TREM2 aids phagocytosis, how do cells lacking the receptor clear amyloid? Microglia and macrophages express other receptors that recognize the Fcγ tail of antibodies and are known to latch onto and engulf antibody-coated targets. In TREM2 knockout cells, expression of these receptors shot up two- to fourfold, and about 20-30 percent more reached the cell surface compared to wild-type cells, the authors found. Moreover, the kinase Syk, which promotes phagocytosis, became twice as active in knockouts. Together, the findings suggest that compensatory mechanisms for activating phagocytosis may take over in TREM2-deficient cells, Haass said. He speculated that this may also explain why knocking out TREM2 has only a minimal effect on plaque density in AD mouse models. In addition, he noted that TREM2’s ability to stimulate plaque clearance was modest in the absence of antibodies.

Commenters found the data intriguing. “The potential crosstalk between TREM2 and Fcγ receptors is an interesting avenue to explore,” Ulrich said. Gary Landreth at the Stark Neurosciences Research Institute, Indiana University School of Medicine, Indianapolis, noted that unlike Fcγ receptors, TREM2 itself may not latch onto amyloid, since other cell-surface receptors, such as scavenger receptors and Toll-like receptors (TLRs), have been found to bind fibrillar Aβ. “It seems reasonable that TREM2 influences the phagocytic machinery well downstream of the receptors themselves,” Landreth speculated. In ongoing work, Haass and colleagues are trying to dissect how TREM2 affects phagocytosis by examining what genes are turned on in knockout versus wild-type cells after antibody stimulation. They are also investigating the effects of harmful TREM2 variants such as R47H.

It is unclear how TREM2’s role in phagocytosis relates to its other functions and how these interact to modulate pathology in AD models. Monica Carson at the University of California, Riverside, noted that the overall effects of TREM2 likely depend on environmental factors, such as the age and disease status of the animal as well as the activation state of its microglia. Because TREM2 is an injury-response gene, its levels in people may vary depending on a person’s life history of hits to the head, infections, and other insults, she noted. “This may help explain how [therapeutic] antibodies can have very different effects on different people,” Carson speculated.—Madolyn Bowman Rogers

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References

Webinar Citations

  1. Mutations Impair TREM2 Maturation, Processing, and Microglial Phagocytosis

News Citations

  1. Microglia—Who Are You Really? New Clues Emerge
  2. TREM2 Buoys Microglial Disaster Relief Efforts in AD and Stroke
  3. United in Confusion: TREM2 Puzzles Researchers in Taos
  4. Barrier Function: TREM2 Helps Microglia to Compact Amyloid Plaques

Therapeutics Citations

  1. Gantenerumab

Research Models Citations

  1. APPPS1

Paper Citations

  1. . TREM2 regulates microglial cell activation in response to demyelination in vivo. Acta Neuropathol. 2015 Mar;129(3):429-47. Epub 2015 Jan 29 PubMed.
  2. . TREM2 lipid sensing sustains the microglial response in an Alzheimer's disease model. Cell. 2015 Mar 12;160(6):1061-71. Epub 2015 Feb 26 PubMed.
  3. . Gantenerumab: A Novel Human Anti-Aβ Antibody Demonstrates Sustained Cerebral Amyloid-β Binding and Elicits Cell-Mediated Removal of Human Amyloid-β. J Alzheimers Dis. 2012;28(1):49-69. PubMed.
  4. . A subcutaneous cellular implant for passive immunization against amyloid-β reduces brain amyloid and tau pathologies. Brain. 2016 May;139(Pt 5):1587-604. Epub 2016 Mar 8 PubMed.

Further Reading

Primary Papers

  1. . TREM2 deficiency reduces the efficacy of immunotherapeutic amyloid clearance. EMBO Mol Med. 2016 Sep 1;8(9):992-1004. PubMed.