S100A9 induces neuroinflammation and aggravates early brain injury after subarachnoid hemorrhage by activating the TLR4/MYD88/NF-κB pathway

BackgroundSubarachnoid hemorrhage (SAH) is a stroke subtype with an extremely high mortality rate, and its severity is closely related to the short-term prognosis of patients with SAH. The S100 calcium-binding protein A9 (S100A9) has been shown to be associated with some neurological diseases, and this study aimed to investigate the relationship between S100A9 and neuroinflammation, as well as its mechanism in SAH.MethodsAn enzyme-linked immunosorbent assay (ELISA) was used to detect the concentration of S100A9 in clinical cerebrospinal fluid samples. Furthermore, an in vivo mouse SAH model was established using intravascular perforation; S100A9 knockout mice were used for the in vivo experiments. S100A9 recombinant protein was administered via lateral ventricular injection 1 h before SAH model induction. SAH grade, neurological function score, and brain water content were measured after a specific time. BV2 and HT22 cells and co-culture models were treated with heme chloride to establish an in vitro model of SAH. Paquinimod was used to explore the potential neuroprotective mechanisms of S100A9 inhibition. Western blotting and immunofluorescence staining were used to explore microglial activation, inflammatory responses, and its related protein pathways.ResultsThe expression of S100A9 protein in the cerebrospinal fluid of patients with SAH increased and was related to the short-term prognosis of patients with SAH; S100A9 was highly expressed in the microglia. S100A9 knockout significantly improved neurological function scores, reduced brain edema, and reduced neuronal apoptosis. S100A9 inhibition with Paquinimod restrained neuronal apoptosis, while administration of recombinant S100A9 aggravated neuroinflammation, activated the TLR4 receptor, promoted NF-κB nuclear transcription, and ultimately aggravated nerve injury.ConclusionS100A9 protein expression increased after SAH, which induced neuroinflammation and promote neuronal apoptosis by activating the TLR4/MYD88/ NF-κB pathway, ultimately aggravating nerve injury after SAH.