Divergent Neuroinflammatory Regulation of Microglial TREM Expression and Involvement of NF-κB

Abstract

The triggering receptor expressed on myeloid cells (TREM) family of proteins are cell surface receptors with important roles in regulation of myeloid cell inflammatory activity. In the central nervous system, TREM2 is implicated in further roles in microglial homeostasis, neuroinflammation and neurodegeneration. Different TREM receptors appear to have contrasting roles in controlling myeloid immune activity therefore the relative and co-ordinated regulation of their expression is important to understand but is currently poorly understood. We sought to determine how microglial TREM expression is affected under neuroinflammatory conditions in vitro and in vivo. Our data show that microglial Trem1 and Trem2 gene expression are regulated in an opposing manner by lipopolysaccharide (LPS) in vitro in both adult murine and human microglia. LPS caused a significant induction of Trem1 and a contrasting suppression of Trem2 expression. We also observed similar divergent Trem1 and Trem2 responses in vivo in response to acute brain inflammation and acute cerebral ischaemia. Our data show that inhibition of NF-κB activation prevents the LPS-induced alterations in both Trem1 and Trem2 expression in vitro indicating NF-κB as a common signaling intermediate controlling these divergent responses. Distinct patterns of microglial Trem1 induction and Trem2 suppression to different Toll-like receptor (TLR) ligands were also evident, notably with Trem1 induction restricted to those ligands activating TLRs signaling via TRIF. Our data show co-ordinated but divergent regulation of microglial TREM receptor expression with a central role for NF-κB. Neuroinflammatory conditions that alter the balance in TREM expression could therefore be an important influence on microglial inflammatory and homeostatic activity with implications for neuroinflammatory and neurodegenerative disease.

Trem2promotes anti-inflammatory responses in microglia and is suppressed under pro-inflammatory conditions

“…In agreement with previous findings (Owens et al 2017), LPS largely suppressed Trem2 expression with a decrease in expression beginning as early as 3 hr after LPS application and reaching more than an 85% decrease by 24 hr. This finding is consistent with previous studies showing that LPS/IFNγ stimulation down-regulated Trem2 expression via TLR4 receptors in cell lines and primary cultures of microglia/macrophages and also in mouse brains (Owens et al 2017;Schmid et al 2002;Turnbull et al 2006;Zheng et al 2016). Moreover, Owens and colleagues (2017) showed that treatment with pro-inflammatory mediators engaging other Toll-like receptors (TLRs) also suppressed Trem2 expression in the BV-2 cell line, albeit with slightly different potency.…”

“…To begin to investigate how Trem2 expression is regulated, microglia were stimulated with LPS to induce classical pro-inflammatory responses. We first found that, in the control cells, LPS treatment strongly down-regulated Trem2 expression ( Figure 4a), which was consistent with the previous findings in primary mixed glial cell cultures and peritoneal macrophage cell cultures (Schmid et al 2002) and primary microglia (Owens et al 2017). The time course analysis showed that the Trem2 expression started to drop to around 90% of the levels measured in non-stimulated cells from as early as 3 hr after LPS addition, and then continued decreasing and reached levels as low as 10% of non-stimulated levels by 24 hr.…”

Microglial homeostasis requires balanced CSF-1/CSF-2 receptor signaling

“…The LXR/RXR pathway has been reported to increase cholesterol efflux and repress TLR4-induced genes (Hiebl et al, 2018), but also to promote inflammasome activation in microglia (Jang et al, 2016). However, inhibition of classical pro-inflammatory pathways, such as the acute phase response and NFκB signaling, as well as of TREM1 signaling, that sustains inflammation (Owens et al, 2017), was also predicted ( Fig. 5E and Supplemental Table 5), suggesting that Csf1r +/microglia are not pro-inflammatory.…”

“…Consistent with the downregulation of IL-10 receptor signaling in Csf1r +/microglia, its downstream signaling mediator, Stat3 and several IL-10 transcriptional targets (Ddit4, Nfil3, Tsc22d3) (Ip et al, 2017, Lang et al, 2002, Berrebi et al, 2003, Hoppstadter et al, 2015 were also downregulated ( Fig 5B, F). Other potentially relevant downregulated genes encode transcripts associated with Alzheimer's disease (Sorl1) (Nicolas et al, 2016), leukodystrophy (Abcd1 and its downstream mediator of pathology, Ch25h) (Gong et al, 2017, Jang et al, 2016 and the CSF-2 target gene Pkch, encoding protein kinase Cη, a regulator of lipid metabolism and suppressor of NO production by macrophages (Torisu et al, 2016, Ozawa et al, 2016 ( Fig 5B). Pathway analysis revealed that the transcriptomic changes associated with Csf1r heterozygosity are consistent with activation of the RhoGDI signaling and LXR/RXR pathways ( Fig.…”

Microglial Homeostasis Requires Balanced CSF-1/CSF-2 Receptor Signaling

“…The LXR/RXR pathway has been reported to increase cholesterol efflux and repress Toll-like receptor 4 (TLR4)-induced genes (Hiebl et al, 2018) but also promote inflammasome activation in microglia (Jang et al, 2016). However, inhibition of classical pro-inflammatory pathways, such as the acute phase response and nuclear factor kB (NF-kB) signaling, as well as of TREM1 signaling, which sustains inflammation (Owens et al, 2017), was also predicted ( Figure 5E; Table S5), suggesting that Csf1r +/À microglia are not pro-inflammatory. Analysis of the biological processes affected by Csf1r heterozygosity predicted active neurodegeneration, increased cellular protrusions and microtubule dynamics, as well as elevated paired-pulse facilitation of synapses ( Figure 5F; Table S6).…”