S100A8 Promotes Inflammation via Toll-Like Receptor 4 After Experimental Traumatic Brain Injury

S100 calcium-binding protein A8 (S100A8) is also known as macrophage-related protein 8, which is involved in various pathological processes in the central nervous system post-traumatic brain injury (TBI), and plays a critical role in inducing inflammatory cytokines. Accumulating evidences have indicated that toll-like receptor 4 (TLR4) is considered to be involved in inflammatory responses post TBI. The present study was designed to analyze the hypothesis that S100A8 is the key molecule that induces inflammation via TLR4 in TBI.

The weight-drop TBI model was used and randomly implemented on mice that were categorized into six groups: Sham, NS, S100A8, S100A8+TAK-242, TBI, and TBI+TAK-242 groups. In the S100A8+TAK-242 and TBI+TAK-242 groups, at half an hour prior to the intracerebroventricular administration of S100A8 or TBI, mice were intraperitoneally treated with TAK-242 that acts as a selective antagonist and inhibitor of TLR4. Furthermore, the protein recombinant of S100A8 was injected into the lateral ventricle of the brain of mice in the S100A8 and S100A8+TAK-242 groups. Sterile normal saline was injected into the lateral ventricle in the NS group. To evaluate the association between S100A8 and TLR4, Western blot, immunofluorescence, enzyme-linked immunosorbent assay (ELISA), and Nissl staining were employed. Simultaneously, the neurological score and brain water content were assessed. In the in vitro analysis, BV-2 microglial cells were stimulated with lipopolysaccharide LPS or S100A8 recombinant protein, with or without TAK-242. The expression of the related proteins was subsequently detected by Western blot or enzyme-linked immunosorbent assay.

The levels of S100A8 protein and pro-inflammatory cytokines were significantly elevated after TBI. There was a reduction in the neurological scores of non-TBI animals with remarkable severe brain edema after the intracerebroventricular administration of S100A8. Furthermore, the TLR4, p-p65, and myeloid differentiation factor 88 (MyD88) levels were elevated after the administration of S100A8 or TBI, which could be restored by TAK-242. Meanwhile, in the in vitro analysis, due to the stimulation of S100A8 or LPS, there was an upregulation of p-p65 and MyD88, which could also be suppressed by TAK-242.

The present study demonstrated that the TLR4-MyD88 pathway was activated by S100A8, which is essential for the development of inflammation in the brain after TBI.