Microglial cells may ensure their own survival by cutting a receptor loose from their surface. As described February 16 in the Journal of Experimental Medicine, the soluble portion of triggering receptor expressed on myeloid cells 2, aka sTREM2, activates microglia and shields them from death (Zhong et al., 2017). Researchers led by Guojun Bu at the Mayo Clinic in Jacksonville, Florida, also reported that mutations known to increase risk of Alzheimer’s disease sapped sTREM2’s protective and inflammatory prowess. The findings suggest that the freewheeling fragment of TREM2 counteracts the previously reported anti-inflammatory role of its membrane-bound parent.
While the role of sTREM2 in AD pathogenesis is unclear, Bu said he views its pro-inflammatory function as a double-edged sword. On one hand, sTREM2 may trigger microglia to remove Aβ. On the other, it may push the cells into chowing down on precious synapses, hastening neurodegeneration.
An immune receptor expressed on microglia, TREM2 is poised to steer responses of the brain’s primary immune cells. While the TREM2 field has been marked by complex and sometimes contradictory findings since researchers first linked it to AD risk, the predominant view is that signaling through the receptor exerts an anti-inflammatory, pro-phagocytic effect. Various TREM2 ligands—including phospholipids and even ApoE—promote microglial survival and clearance of debris, including Aβ plaques (Wang et al., 2015; Kawabori et al., 2015; Chang et al., 2015; Atagi et al., 2015; Bailey et al., 2015). Thickening the plot, ADAM proteases can liberate the TREM2 ectodomain from the microglial surface. In cerebrospinal fluid, the concentration of this shed fragment rises in the prodromal phase of AD as well as in other neurodegenerative diseases, and tracks with the elevation of tau and p-tau (see Piccio et al., 2008; Piccio et al., 2016; Helsegrave et al., 2016). What part this soluble fragment plays in immune responses mediated by TREM2 in the healthy or diseased brain remains to be addressed.
To investigate, co-first authors Li Zhong and Xiao-Fen Chen of Xiamen University in China and colleagues started by generating a recombinant version of the TREM2 extracellular domain, fused to an antibody Fc fragment. This allowed them to purify sufficient quantities of sTREM2 for injection into animals or cell cultures. They then incubated primary mouse microglia with 20 nM of the fusion protein, and found that microglia survived just as well as they did when cultured with GM-CSF, an essential support factor for microglia in culture. Strikingly, sTREM2-Fc delivered the same survival benefit to TREM2 knock-out microglia, or to microglia lacking DAP12, the adaptor molecule through which full-length TREM2 signals. These findings indicated that sTREM2 did not signal via its own membrane-bound precursor. This is contrary to the prevailing idea that the soluble fragment functions primarily as a decoy receptor, hoarding available ligands away from its parent.
The researchers next explored how sTREM2 affects microglial activation. They found it ramped up the transcription of IL-1β, IL-6, TNF-α, and IL-10 in a dose-dependent manner. All these cytokines promote inflammatory responses. Conversely, sTREM2 did not budge expression of Arg-1 and Ym-1, two proteins known to dampen inflammation.
Microglia treated with sTREM2-Fc also morphed into an activated form, pulling in the spindly processes they use to continually survey their environment and assuming a rounder shape. Just as it did for cell survival, sTREM2-Fc activated microglia that lacked TREM2 or DAP12, again hinting a separate signal transduction pathway was involved. The researchers observed similar effects on microglial survival and activation when they used an Fc-free version of human sTREM2, or when they used mouse sTREM2-Fc.
Microglia in the brain also responded to sTREM2. Compared to injection of Fc alone, injection of sTREM2-Fc into the hippocampi of normal or TREM2 knock-out mice stimulated the expression of pro-inflammatory cytokines, and coaxed microglia there to round up into their activated state.
Armed with a tool kit of inhibitors, the researchers homed in on the signaling pathways that might be responsible for sTREM2’s effects. They found that sTREM2 promoted microglial survival via the well-known anti-apoptotic Akt-GSK3β-β-catenin pathway, dovetailing with the lab’s recent report that knocking out TREM2 attenuates Akt/GSK3β signaling and microglial survival via this pathway (see Zheng et al., 2017). They also found that sTREM2 switched on pro-inflammatory cytokines via the transcription factor NF-κB.
How do AD risk variants fit into this picture? To find out, the researchers treated microglia with sTREM2-Fc fusion proteins harboring the R47H or R62H mutations in their extracellular domains. These mutations reportedly interfere with TREM2’s ability to bind various ligands. They found that both of these mutations lessened the protection and activation of microglia by sTREM2. Notably, these AD-linked mutations are distinct from two other missense mutations, T66M and Y38C, which are associated with frontotemporal dementia. Researchers led by Christian Haass of the German Center for Neurodegenerative Diseases in Munich had previously reported that these mutants bungled the transport of TREM2 to the cell surface and prevented shedding of sTREM2, while R47H had lesser effects on TREM2 maturation (see Kleinberger et al., 2014).
Bu said his findings support the idea that the R47H and R62H variants elevate AD risk through a loss-of-function mechanism, perhaps thwarting sTREM2’s ability to enlist microglia in the clean-up of Aβ-laden debris. The pro-inflammatory effects of sTREM2 appear to run counter to the reported anti-inflammatory effects of full-length TREM2, supporting the idea that the soluble fragment is unleashed in response to injury, Bu said. He suggested that promoting sTREM2 function in the earliest stages of AD could be beneficial, although too much inflammation could turn harmful once neurodegeneration is initiated.
Monica Carson of the University of California, Riverside, interpreted the data slightly differently. She believes the soluble fragment may be crucial for mounting acute responses to injuries or infections that occur throughout life. “Maybe sTREM2 allows microglia to live longer in the face of various bouts of inflammation,” she said. However, in the face of chronic injury (such as Aβ aggregation), sTREM2 may do more harm than good. “Once chronic inflammation sets in, perhaps it would be better if those microglia didn’t survive,” she said. She suggested that blocking sTREM2 function, rather than promoting it, would have a better chance at slowing neurodegeneration.
“Although it is known that there is an increase in sTREM2 in many disease states, the role that sTREM2 plays is still relatively unknown,” wrote Marco Colonna of Washington University in St. Louis. “The data presented here provide a first step toward this goal, suggesting a protective function of sTREM2 on microglia in steady state.” He added that the study raises many questions about the nature of sTREM2’s role in disease, and what its receptor might be.
“Conceptually, these data support recent proposals that the presence of sTREM2, as detected in human biological samples, indicate microglial activation,” wrote Gary Landreth of Stark Neurosciences Research Institute, Indiana University School of Medicine, Indianapolis, in a comment to Alzforum. “Unanswered is the question of whether sTREM2 has analogous deleterious effects on astrocytes and neurons, and what relationship sTREM2 has with full-length TREM2,” he added.
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