Scientists know that carrying the R47H variant of the TREM2 gene triples a person’s chances of developing Alzheimer’s disease, but they don’t yet know how the mutation affects the function of this microglial receptor in vivo. In the January 10 Journal of Experimental Medicine, Marco Colonna and colleagues at Washington University in St. Louis report that activated microglia do not surround plaques in mice expressing the mutated version of the human protein the way they do in AD mice with the common TREM2 variant. Moreover, they found that, normally, soluble TREM2 released from cell membranes latches onto Aβ plaques and nearby neurons, but the R47H variant significantly loses this binding. Overall, their model suggests that R47H impairs TREM2 function and supports the idea that normal TREM2 plays a protective role in disease.
- Human TREM2 expressed in AD mouse models.
- The R47H AD risk variant activates microglia only weakly.
- Soluble TREM2 binds neurons, plaques. Soluble R47H: Not so much.
“This is a very important paper,” wrote Christian Haass, Ludwig-Maximilians University, Munich, to Alzforum (see comment below). “It provides clear evidence that the most relevant AD-associated variant of TREM2, R47H, causes a partial loss of function.”
Dominik Feuerbach of Novartis in Basel, Switzerland, agreed (see comment below). “This pivotal study adds supporting evidence in favor of TREM2-enhancing therapies,” he wrote. Both membrane-bound and shed versions of the healthy protein appear to benefit mice compared to the R47H mutant, Feuerbach added.
TREM2 has been proposed to activate microglia, especially those around plaques, and soluble TREM2 cleaved from the membrane reportedly enhances microglial survival and stokes inflammation (Apr 2017 conference news; Zhong et al., 2017). Previous studies from Colonna and others suggested R47H TREM2 binds ligands more weakly in vitro than does the wild-type receptor, while AD patients who carry the R47H variant have fewer plaque-associated microglia than other AD patients (Wang et al., 2015; Yuan et al., 2016). Together, these results suggest a loss of function for R47H, but scientists have yet to confirm this in an in vivo model.
Unexpected Binding. In mouse cortex, neurons (red) and plaques (blue) bind wild-type sTREM2 (green), but not R47H sTREM2 (right). Soluble TREM2 is absent in TREM2 knockouts (left). [Image ©Song et al., 2018.]
To do that, first authors Wilbur Song and Satoru Joshita knocked out TREM2 in 5XFAD mice and introduced instead the human version of the gene. They engineered mice to express either the common variant (CV) of TREM2 or the R47H version on a bacterial artificial chromosome (BAC) that included regulatory sequences for TREM2. The researchers then compared the brains of these BAC lines at 8.5 months.
As in people, the full-length TREM2 protein appeared exclusively in microglia, with control and R47H versions expressed at similar levels. However, because no known reagent labels both human and mouse TREM2, the scientists were unable to compare BAC gene expression with that of endogenous TREM2 in wild-type mice.
Nevertheless, staining hippocampal and cortical slices of the BAC mice revealed that slices from the CV variant had more activated microglia near plaques than did those from the R47H variant. Microglia from CV mice also expressed higher levels of transcriptional markers of activation, including Spp1 and Gpnmb. However, in both TREM2 strains, soluble Aβ levels and plaque area in the cortex and hippocampus equaled those of 5XFAD-TREM2 knockouts. This suggests TREM2 has no effect on Aβ plaques before 8.5 months, at least in these animals, which begin to aggressively deposit Aβ by 1.5 months of age.
To the researchers’ surprise, the shed, soluble, i.e. extracellular domain of human TREM2 turned up on cell bodies of seemingly healthy neurons and in Aβ plaques in the cortex and hippocampus. The R47H mutation seemed to impede this binding, because the authors detected five times more bound sTREM2 in CV than in R47H mice, despite equal expression.
“That was really an unexpected finding,” Colonna told Alzforum. “We anticipated some soluble TREM2, but we didn’t expect it to bind so closely to cells and to plaques.” Colonna said it is unclear what the soluble protein binds to, or how that interaction affects neurons and plaques. He plans to find out.
What might this have to do with ectodomain shedding? Previously, Haass reported that ADAM proteases cleave R47H TREM2 less efficiently than normal in HEK293 and microglial cell lines (Kleinberger et al., 2014; Jul 2014 webinar). Now, Song and colleagues report that ADAM17 was equally effective at shedding both in macrophage-like cultures. Haass thinks overexpression of TREM2 in these cells might affect shedding; Colonna thinks both wild-type and R47H might be shed equally but the former binds better to plaques and neurons.
Regardless, Haass considers this binding crucial. “This paper shows nicely that we should not overlook the function of shed sTREM2,” he wrote. “Now we need to learn what sTREM2 is doing on the surface of neurons and within plaques. And maybe this will finally refocus AD researchers on the function of soluble APP, which is shed by the same proteases as TREM2, namely ADAM 10 and 17.” Jochen Walter at the University of Bonn, Germany, agreed. “It will be interesting to assess whether this binding of soluble TREM2 has a functional effect,” Walter wrote to Alzforum.
Taken together, the data suggest a partial loss of function of R47H. This agrees with a paper from Haass’ group on the T66M TREM2 mutation associated with frontotemporal dementia, which also reduced microglial clustering and activation in vivo (May 2017 news on Kleinberger et al., 2017). It will be important to observe these mice at later time points to see if TREM2 has the same positive impact, said Colonna, who made his mouse models available through his lab.
“Overall, the hTREM2 mice produced by the Colonna lab will be valuable as animal models to understand the biology of AD-associated variants, test potential TREM2-targeted therapies, and investigate the regulation and possible function, of sTREM2,” wrote Jason Ulrich, Washington University School of Medicine in St. Louis, to Alzforum (see full comment below). “It will also be interesting to compare the effect of variants in human TREM2 to the numerous TREM2 variant knock-in mice that are being generated,” Ulrich added.—Gwyneth Dickey Zakaib
- New Evidence Confirms TREM2 Binds Aβ, Drives Protective Response
- Paper Alert: TREM2 Crucial for Microglial Activation
Research Models Citations
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