Fatty acid-binding protein 4 drives microglia-mediated neuroinflammation through promoting S100A9 expression and lipid droplet accumulation after intracerebral hemorrhage

Abstract Background As primary immune sentinels of the central nervous system (CNS), microglia respond rapidly to acute brain injury and engage in dynamic crosstalk with infiltrating peripheral immune cells. This interplay critically shapes the neuroinflammatory microenvironment—a key determinant of secondary brain injury (SBI) following intracerebral hemorrhage (ICH). Fatty acid-binding protein 4 (FABP4), an adipokine associated with metabolic disorders, is recognized as a pivotal modulator of inflammatory responses; however, its role in ICH-induced SBI remains undefined. Objectives To investigate the pathogenic functions of FABP4 in microglia after ICH, elucidate its molecular mechanisms, and develop targeted therapeutic strategies. Methods Blood and brain tissue samples from ICH patients were analyzed to evaluate the relationships between FABP4 expression and prognosis. Behavioral tests, Nissl staining, and Golgi-Cox staining were used to quantify neuronal damage. Immunofluorescence and flow cytometry were used to assess microglial activation and immune cell infiltration. Transcriptomic, proteomic, co-immunoprecipitation, western blotting, and ChIP‒qPCR analyses were used to examine the FABP4 regulatory network. Brain-targeted nanoparticles were engineered to deliver FABP4-specific siRNA. Results Clinical analyses revealed microglia-specific FABP4 upregulation in ICH patients, correlating with poor neurological outcomes. Microglial Fabp4 knockout in mice attenuated neuronal loss, ameliorated cerebral edema, and enhanced functional recovery after ICH. Mechanistically, FABP4 promoted lipid droplet accumulation and inhibited the ubiquitin–proteasome-mediated degradation of S100A9 in microglia, synergistically amplifying neuroinflammation. Moreover, the activity of FABP4 in microglia facilitated neutrophil transendothelial migration into the brain parenchyma, exacerbating injury via the release of neutrophil extracellular traps (NETs). Finally, pharmacological FABP4 inhibition using brain-targeted nanoparticles conferred significant neuroprotective effects in ICH models. Conclusion This study establishes that FABP4 acts as a novel orchestrator of post-ICH neuroinflammation through dual enzymatic and nonenzymatic pathways. We also demonstrate a targeted nanotherapeutic strategy to suppress FABP4 and improve neurological outcomes.