TLR4 (toll-like receptor 4) activation suppresses autophagy through inhibition of FOXO3 and impairs phagocytic capacity of microglia

Abstract

Macroautophagy/autophagy is a lysosome-dependent catabolic process for the turnover of proteins and organelles in eukaryotes. Autophagy plays an important role in immunity and inflammation, as well as metabolism and cell survival. Diverse immune and inflammatory signals induce autophagy in macrophages through pattern recognition receptors, such as toll-like receptors (TLRs). However, the physiological role of autophagy and its signaling mechanisms in microglia remain poorly understood. Microglia are phagocytic immune cells that are resident in the central nervous system and share many characteristics with macrophages. Here, we show that autophagic flux and expression of autophagy-related (Atg) genes in microglia are significantly suppressed upon TLR4 activation by lipopolysaccharide (LPS), in contrast to their stimulation by LPS in macrophages. Metabolomics analysis of the levels of phosphatidylinositol (PtdIns) and its 3-phosphorylated form, PtdIns3P, in combination with bioinformatics prediction, revealed an LPS-induced reduction in the synthesis of PtdIns and PtdIns3P in microglia but not macrophages. Interestingly, inhibition of PI3K, but not MTOR or MAPK1/3, restored autophagic flux with concomitant dephosphorylation and nuclear translocation of FOXO3. A constitutively active form of FOXO3 also induced autophagy, suggesting FOXO3 as a downstream target of the PI3K pathway for autophagy inhibition. LPS treatment impaired phagocytic capacity of microglia, including MAP1LC3B/LC3-associated phagocytosis (LAP) and amyloid β (Aβ) clearance. PI3K inhibition restored LAP and degradation capacity of microglia against Aβ. These findings suggest a unique mechanism for the regulation of microglial autophagy and point to the PI3K-FOXO3 pathway as a potential therapeutic target to regulate microglial function in brain disorders.

Lipopolysaccharide induces neuroinflammation in microglia by activating the MTOR pathway and downregulating Vps34 to inhibit autophagosome formation

“…Although it has been reported that LPS upregulates autophagy via the receptor TLR4 in RAW264.7 cells [46], Lee et al and He et al observed that autophagy was inhibited in LPSinduced BV2 microglia, which is consistent with our results [33,47]. The study of Lee et al and He et al also examined the effect of LPS on autophagy in RAW264.7 cells and revealed a differential LC3 response to LPS between microglial BV2 cells, in which autophagy was downregulated, and RAW264.7 cells, in which autophagy was upregulated [47]. The researchers speculated that the mechanism of autophagy inhibition may be dependent on cell type or even the state of cells of the same type.…”

“…Here, we found that the expression of Vps34 was significantly inhibited in LPS-stimulated microglia. These results are consistent with a recent metabolomics analysis in microglia which showed that LPS induced a reduction in the synthesis of PI(3)P [47]. Therefore, it appears that LPS impairs autophagic flux in microglia by reducing the formation of autophagosomes.…”

Ibrutinib alleviated LPS-induced neuroinflammation and synaptic defects in a mouse model of depression

“…LPS is widely used to induce neuroin ammation [2,3,49,54,62]. Numerous studies showed that LPS treatment could induce central in ammatory response associated with proin ammatory cytokines production, glial cell activation, as well as ROS and RNS generation [2,5,7,40,44,47,54,[63][64][65][66][67][68][69]. Furthermore, activated NF-κB also promotes neurotoxic cytokines and ROS production of glial cells [2,3,5].…”

PON1 deficiency promotes TREM2 pathway-mediated microglial phagocytosis and inhibits LPS-induced pro-inflammatory cytokines release

“…LPS triggers in ammatory responses in humans and other mammals. Many papers reported that LPS promoted phagocytosis (48)(49)(50), while some studies showed an inhibitory effect of LPS on phagocytosis (51)(52)(53). We demonstrated that LPS signi cantly enhanced the production of TNF-α in microglia and inhibited PON1 KO-induced phagocytosis.…”