Inhibition of S100A9 alleviates neurogenic pulmonary edema after subarachnoid hemorrhage

Neurogenic pulmonary edema (NPE) frequently arises as a complication subsequent to subarachnoid hemorrhage (SAH). Heterodimers of S100A8 and S100A9 are commonly formed, thereby initiating an inflammatory reaction through receptor binding on the cell surface. Paquinimod serves as a specific inhibitor of S100A9. The objective of this investigation is to assess the impact of Paquinimod administration and S100A9 knockout on NPE following SAH.In this study, SAH models of C57BL/6J wild-type (WT) and S100A9 knockout mice were established through intravascular perforation. These models were then divided into several groups, including the WT-sham group, S100A9-KO-sham group, WT-SAH group, WT-SAH + Paquinimod group, and S100A9-KO-SAH group. After 24 h of SAH induction, pulmonary edema was assessed using the lung wet-dry weight method and Hematoxylin and eosin (HE) staining. Additionally, the expression levels of various proteins, such as interleukin-1β (IL-1β), tumor necrosis factor α (TNF-α), occludin, claudin-3, Bax, Bcl-2, TLR4, MYD88, and pNF-κB, in lung tissue were analyzed using western blot and immunofluorescence staining. Lung tissue apoptosis was detected by TUNEL staining.Firstly, our findings indicate that the knockout of S100A9 has a protective effect on early brain injury following subarachnoid hemorrhage (SAH). Additionally, the reduction of brain injury after SAH can also alleviate neurogenic pulmonary edema (NPE). Immunofluorescence staining and western blot analysis revealed that compared to SAH mice with wild-type S100A9 expression (WT-SAH), the lungs of S100A9 knockout SAH mice (S100A9-KO-SAH) and mice treated with Paquinimod exhibited decreased levels of inflammatory molecules (IL-1β and TNF-α) and increased levels of tight junction proteins. Furthermore, the knockout of S100A9 resulted in upregulated expression of the apoptotic-associated protein Bax and down-regulated expression of Bcl-2. Furthermore, a decrease in TLR4, MYD88, and phosphorylated pNF-κB was noted in S100A9-KO-SAH and Paquinimod treated mice, indicating the potential involvement of the TLR4/MYD88/NF-κB signaling pathway in the inhibition of the protective effect of S100A9 on NPE following SAH.The knockout of S100A9 not only ameliorated initial cerebral injury following subarachnoid hemorrhage (SAH), but also mitigated SAH-associated neurogenic pulmonary edema (NPE). Additionally, Paquinimod was found to diminish NPE. These findings imply a correlation between the central nervous system and peripheral organs, highlighting the potential of safeguarding the brain to mitigate harm to peripheral organs.