Microglia constantly survey the brain microenvironment and rapidly adopt different phenotypes in response to environmental stimuli. Such dynamic functions require a unique metabolism and bioenergetics. However, little is known about the basic metabolism of microglia and how metabolic changes regulate microglia function. Here, we uncover that microglia activation is accompanied by extensive transcriptional changes in glucose and lipid metabolism‐related genes. Using metabolic flux assays, we found that LPS, a prototype of the pathogen‐associated molecular patterns (PAMPs), significantly enhanced glycolysis but suppressed oxidative phosphorylation (OXPHOS) in primary cultured microglia. By contrast, ATP, a known damage‐associated molecular pattern (DAMPs) that triggers sterile activation of microglia, boosted both glycolysis and OXPHOS. Importantly, both LPS and ATP activated the mechanistic target of rapamycin (mTOR) pathway and enhanced the intracellular reactive oxygen species (ROS). Inhibition of mTOR activity suppressed glycolysis and ROS production in both conditions but exerted different effects on OXPHOS: it attenuated the ATP‐induced elevation of OXPHOS, yet had no impact on the LPS‐induced suppression of OXPHOS. Further, inhibition of mTOR or glycolysis decreased production of LPS‐induced proinflammatory cytokines and ATP‐induced tumor necrosis factor‐α (TNF‐α) and brain derived neurotrophic factor (BDNF) in microglia. Our study reveals a critical role for mTOR in the regulation of metabolic programming of microglia to shape their distinct functions under different states and shed light on the potential application of targeting metabolism to interfere with microglia‐mediated neuroinflammation in multiple disorders.

BackgroundInterleukin-33 (IL-33) is increasingly being recognized as a key immunomodulatory cytokine in many neurological diseases.MethodsIn the present study, wild-type (WT) and IL-33−/− mice received intracerebroventricular (i.c.v.) injection of lipopolysaccharide (LPS) to induce neuroinflammation. Intravital microscopy was employed to examine leukocyte–endothelial interactions in the brain vasculature. The degree of neutrophil infiltration was determined by myeloperoxidase (MPO) staining. Real-time PCR and western blotting were used to detect endothelial activation. Enzyme-linked immunosorbent assay and quantitative PCR were conducted to detect pro-inflammatory cytokine levels in the brain.ResultsIn IL-33−/− mice, neutrophil infiltration in the brain cortex and leukocyte–endothelial cell interactions in the cerebral microvessels were significantly decreased as compared to WT mice after LPS injection. In addition, IL-33−/− mice showed reduced activation of microglia and cerebral endothelial cells. In vitro results indicated that IL-33 directly activated cerebral endothelial cells and promoted pro-inflammatory cytokine production in LPS-stimulated microglia.ConclusionsOur study indicated that IL-33/ST2 signaling plays an important role in the activation of microglia and cerebral endothelial cells and, therefore, is essential in leukocyte recruitment in brain inflammation.Graphical abstractThe role of IL-33/ST2 in LPS induced neuroinflammation

Microglia continuously survey the brain parenchyma and actively shift status following stimulation. These processes demand a unique bioenergetic programme; however, little is known about the metabolic determinants in microglia. By mining large datasets and generating transgenic tools, here we show that hexokinase 2 (HK2), the most active isozyme associated with mitochondrial membrane, is selectively expressed in microglia in the brain. Genetic ablation of HK2 reduced microglial glycolytic flux and energy production, suppressed microglial repopulation, and attenuated microglial surveillance and damage-triggered migration in male mice. HK2 elevation is prominent in immune-challenged or disease-associated microglia. In ischaemic stroke models, however, HK2 deletion p ro mo ted n eu ro in fl am mation and potentiated cerebral damages. The enhanced inflammatory responses after HK2 ablation in microglia are associated with aberrant mitochondrial function and reactive oxygen species accumulation. Our study demonstrates that HK2 gates both glycolytic flux and mitochondrial activity to shape microglial functions, changes of which contribute to metabolic abnormalities and maladaptive inflammation in brain diseases.Metabolism has emerged as a key regulator in both innate and adaptive immunity 1 . Microglia, the primary immune cells and key guardians of brain activity in the brain, adopt different metabolic states in response to exogenous stimuli 2-4 . For example, pro-inflammatory microglia utilize aerobic glycolysis to promote synthesis of inflammatory cytokines, whereas immunomodulatory microglia in mice increase fatty acid metabolism to drive anti-inflammatory responses 2,3 . Moreover, metabolic dysfunction of microglia has been implicated in multiple diseases including Alzheimer's disease (AD) and chronic demyelination disease models [4][5][6] . Microglia deficient in triggering the receptor expressed on

Sphingosine kinases (Sphks) are the rate-limiting enzymes in the conversion of sphingosine to biologically active sphingosine-1-phosphate. The present study aimed to determine the role of Sphk2 and its downstream targets in renal fibroblast activation and interstitial fibrosis. In the kidney interstitium of patients with renal fibrosis, Sphk2-expressing cells (mainly interstitial fibroblasts) were significantly elevated and highly correlated with disease progression in patients. In a murine model of renal interstitial fibrosis, Sphk2 was upregulated in the kidney of wild-type mice in response to disease progression. Importantly, Sphk2-knockout (KO) mice exhibited significantly lower levels of extracellular matrix (ECM) production and a suppressed inflammatory response in the kidney tissues, compared to those in their wild-type counterparts, whereas the expression of TGF-β1 was unaffected. TGF-β1 effectively upregulated Sphk2 expression in the renal interstitial fibroblast line, NRK-49F, independent of canonical Smad signaling activation. Furthermore, siRNA-mediated Sphk2 knockdown or suppression of Sphk2 activity by ABC294640 exposure effectively attenuated AKT and STAT3 activation and ECM production, but had no effects on Smad2 and Smad3 activation. Sphk2 phosphorylated Fyn to activate downstream STAT3 and AKT, thereby promoting ECM synthesis. Therefore, our findings indicate that targeting Sphk2-Fyn-STAT3/AKT signaling pathway may be a novel therapeutic approach for renal fibrosis.

Fingolimod (FTY720), an immunomodulator, is approved as an oral treatment for patients with relapsing forms of multiple sclerosis. Its effects are largely attributed to its mechanism of selectively retaining lymphocytes in the lymph nodes to reduce autoreactive T-cell recruitment in the CNS. In this study, we investigated the therapeutic effect of FTY720 on an animal model of CNS inflammation induced by intracerebral ventricle LPS injection. We found that FTY720 treatment significantly prevented LPS-induced neutrophil recruitment in the CNS by inhibiting leukocyte recruitment in cerebral microvessels. Furthermore, FTY720 also inhibited the expressions of adhesion molecules on the cerebral endothelium, but did not affect the expression levels of pro-inflammatory cytokines (TNF-and IL-6) and chemokines (CXCL1 and CXCL2) in the CNS parenchyma. The inhibition of endothelial activation was accompanied by reduced phosphorylation of signaling molecules, including serine/threonine-specific protein kinase (Akt), STAT6, and nuclear factor-B. This FTY720-attenuated inhibition of leukocyte recruitment and endothelial activation was reversed by blocking the functions of sphingosine kinase 2 or sphingosine-1-phosphate receptor 1. Our study demonstrated, for the first time, that FTY720 directly inhibits the phosphorylation of multiple signaling molecules in endothelial cells, thereby effectively blocking leukocyte recruitment in the CNS. K E Y W O R D S

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Neuropathic pain is a refractory condition that involves de novo protein synthesis in the nociceptive pathway. The mechanistic target of rapamycin (mTOR) is a master regulator of protein translation; however, mechanisms underlying its role in neuropathic pain remain elusive. Using the spared nerve injury-induced neuropathic pain model, we found that mTOR was preferentially activated in large-diameter dorsal root ganglion (DRG) neurons and spinal microglia. However, selective ablation of mTOR in DRG neurons, rather than microglia, alleviated acute neuropathic pain in mice. We showed that injury-induced mTOR activation promoted the transcriptional induction of Npy likely via signal transducer and activator of transcription 3 (STAT3) phosphorylation.NPY further acted primarily on Y2 receptors (Y2R) to enhance neuronal excitability.Peripheral replenishment of NPY reversed pain alleviation upon mTOR removal, whereas Y2R antagonists prevented pain restoration. Our findings reveal an unexpected link between mTOR and NPY/Y2R in promoting nociceptor sensitization and neuropathic pain.

Supplemental Digital Content is Available in the Text.Microglia and macrophages play an essential role in the development and maintenance of lumbar disc herniation–induced sciatica.