TREM2 Alleviates Neuroinflammation and Improves Neurogenesis in ApoE^-/- Mice by Regulating M1/M2 Microglial...

Triggering receptor expressed on myeloid cells 2 (TREM2) is an innate immune receptor abundantly expressed in microglia in the brain. Our previous study indicated that TREM2 promoted microglia polarization to the M2 phenotype in APP/PS1 transgenic mice and BV2 cells. It is reported that M2 microglia release brain-derived neurotrophic factor (BDNF) to enhance adult neurogenesis in the hippocampus. However, the role of TREM2 in hippocampal neurogenesis and the underlying mechanism are still less known. Apolipoprotein E knockout (ApoE^-/-) mice exhibit cholinergic dysfunction, tau hyperphosphorylation, synaptic loss and dysfunction that may affect brain function and simulate Alzheimer's disease (AD). In this study, overexpression of TREM2 significantly increased the number of minichromosome maintenance 2 (MCM2) and doublecortin (DCX) cells in the subgranular zone (SGZ) of ApoE^-/- mice. Additionally, the protein levels of MCM2 and DCX showed a similar trend. Furthermore, we found that overexpression of TREM2 promoted a phenotypical switch from M1 to M2 in microglia, as the levels of tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β), and CD86 were decreased, whereas the levels of IL-4, Arginase-1(Arg-1), BDNF, and CD206 were increased. Importantly, overexpression of TREM2 activated the phosphatidylinositol 3-kinase/protein kinase B (PI3K/Akt) and extracellular signal-regulated protein kinase 1 and 2 (ERK1/2) signaling pathways. In vitro, overexpression of TREM2 in primary microglia increased the production of anti-inflammatory factors (IL-4, Arg-1) and BDNF, while decreasing the production of pro-inflammatory factors (TNF-α, IL-1β). Furthermore, conditioned medium (CM) from TREM2 overexpressing primary microglia facilitated neural stem cells (NSCs) proliferation and differentiation into neurons. Moreover, the mechanistic study indicated that overexpression of TREM2 modulated microglial M2 polarization and promoted the proliferation and differentiation of NSCs partly via the PI3K/Akt and ERK1/2 signaling pathways. Collectively, these findings revealed that TREM2 may modulate microglial M2 polarization to inhibit neuroinflammation and increase the M2 microglia-derived BDNF to rescue hippocampal neurogenesis in ApoE^-/- mice, and TREM2 can be considered a promising therapeutic factor to promote neurogenesis in AD and other brain diseases.