Metformin ameliorates BSCB disruption by inhibiting neutrophil infiltration and MMP‐9 expression but not direct TJ proteins expression regulation

Blood‐spinal cord barrier (BSCB) disruption is a major process for the secondary injury of spinal cord injury (SCI) and is considered to be a therapeutic target for SCI. Previously, we demonstrated that metformin could improve functional recovery after SCI; however, the effect of metformin on BSCB is still unknown. In this study, we found that metformin could prevent the loss of tight junction (TJ) proteins at day 3 after SCI in vivo, but in vitro there was no significant difference of these proteins between control and metformin treatment in endothelial cells. This indicated that metformin‐induced BSCB protection might not be mediated by up‐regulating TJ proteins directly, but by inhibiting TJ proteins degradation. Thus, we investigated the role of metformin on MMP‐9 and neutrophils infiltration. Neutrophils infiltration is the major source of the enhanced MMP‐9 in SCI. Our results showed that metformin decreased MMP‐9 production and blocked neutrophils infiltration at day 1 after injury, which might be related to ICAM‐1 down‐regulation. Also, our in vitro study showed that metformin inhibited TNF‐α‐induced MMP‐9 up‐regulation in neutrophils, which might be mediated via an AMPK‐dependent pathway. Together, it illustrated that metformin prevented the breakdown of BSCB by inhibiting neutrophils infiltration and MMP‐9 production, but not by up‐regulating TJ proteins expression. Our study may help to better understand the working mechanism of metformin on SCI.