SMALL MOLECULE-DRIVEN ARGINASE-1+ MICROGLIAL PHENOTYPE SUPPRESSES CHRONIC STRESS-INDUCED MICROGLIAL ENGULFMENT AND NEUROINFLAMMATION VIA A PPARγ-DEPENDENT MECHANISM

Background Growing evidence suggests that neuroinflammation is a key contributor to the pathology of depression, a severe psychiatric disorder with rising global prevalence. Although modulating microglial phenotypes has emerged as a promising therapeutic strategy, effective treatments remain scarce. Previous studies have shown that the small molecule luteolin (LUT) possesses anti-inflammatory properties and offers benefits in models of chronic stress-induced depression. Aims & Objectives In this study, we investigated the molecular mechanisms by which LUT regulates the functional phenotypes of microglia in mice with depressive-like behaviors. Method Mice were exposed to chronic restraint stress (CRS) for 7 weeks, and were administered LUT (10, 30, 40 mg·kg−1·d−1, i.g.) in the last 4 weeks. Behavioral tests were conducted to assess depressive-like and anxiety-like behaviors. Primary hippocampal microglia were cultured and treated with LUT or a PPARγ antagonist (GW9662) to investigate microglial phenotype modulation. Western blotting, real-time PCR, immunohistochemistry, and phagocytosis assays were performed. Molecular dynamics simulation and microscale thermophoresis analyzed the interaction between LUT and the target. Results The results showed that LUT administration significantly ameliorated depressive-like behaviors and decreased hippocampal inflammation. LUT administration induced pro-inflammatory microglia to undergo anti-inflammatory arginase (Arg)-1+ phenotypic polarization, which was associated with its antidepressant effects. Furthermore, we showed that LUT concentration-dependently increased the expression of PPARγ in LPS+ATP-treated microglia and the hippocampus of CRS-exposed mice, promoting the subsequent inhibition of the NLRP3 inflammasome. Molecular dynamics simulation and microscale thermophoresis analysis confirmed a direct interaction between LUT and peroxisome proliferator-activated receptor gamma (PPARγ). By using the PPARγ antagonist GW9662, we demonstrated that LUT-driven protection, both in vivo and in vitro, resulted from targeting PPARγ. First, LUT-induced Arg-1+ microglia were no longer detected when PPARγ was blocked. Next, LUT-mediated inhibition of the NLRP3 inflammasome and downregulation of pro-inflammatory cytokine production were reversed by the inhibition of PPARγ. Finally, the protective effects of LUT, which attenuated the microglial engulfment of synapses and prevented apparent synapse loss in the hippocampus of CRS-exposed mice, were eliminated by blocking PPARγ. Discussion & Conclusions This study showed that LUT ameliorates CRS-induced depressive-like behaviors by promoting the Arg-1+ microglial phenotype through a PPARγ-dependent mechanism, thereby alleviating microglial pro-inflammatory responses and reversing microglial phagocytosis-mediated synapse loss. These findings highlight the therapeutic potential of LUT as a modulator of microglial function through PPARγ activation, offering new insights into treatment strategies for neuroinflammation related depression.