BI1 is associated with microvascular protection in cardiac ischemia reperfusion injury via repressing Syk-Nox2-Drp1-mitochondrial fission pathways

Mitochondrial fission has been identified as the pathogenesis underlying the development of cardiac microvascular ischemia reperfusion (IR) injury, although the regulatory signaling upstream from fission is far from clear. Bax inhibitor is a novel anti-apoptotic factor, and, however, its role of cardiac microvascular IR injury and mitochondrial homeostasis remains unclear. The cardiac microvascular IR injury was performed in WT mice and BI1 transgenic (BI (TG) ) mice. The alterations of microvascular structure and function were detected via electron microscope, immunohistochemistry and immunofluorescence in vivo. Cardiac microvascular endothelial cells were isolated form WT and BI (TG) mice and underwent hypoxia/reoxygenation injury in vitro. Cellular viability and apoptosis were analyzed via MTT assay and caspase-3 activity. Mitochondrial function, morphology and apoptosis were detected. Signaling pathways were analyzed via inhibitor, siRNA and mutant plasmid. Herein, we demonstrated that Bax inhibitor 1 (BI1) was downregulated following cardiac microvascular IR injury, and its expression correlated negatively with microvascular collapse, endothelial cell apoptosis and mitochondrial damage. However, compared to wild-type mice, BI1 transgenic mice were actually protected from the acute microvascular injury and mitochondrial dysfunction. Functional studies illustrated that reintroduced BI1 directly interacted with and inhibited the Syk pathway, leading to the inactivation of Nox2. Subsequently, less Nox2 was associated with ROS downregulation, inhibiting Drp1 phosphorylated activation. Through repression of the Syk-Nox2-Drp1 signaling axis, BI1 strongly disrupted mitochondrial fission, abolishing mitochondrial apoptosis and thus sustaining endothelial cell viability. In summary, our report illustrates that BI1 functions as a novel microvascular guardian in cardiac IR injury that operates via inhibition of the Syk-Nox2-Drp1-mitochondrial fission signaling axis. Thus, novel therapeutic strategies to regulate the balance between BI1 and mitochondrial fission could provide a survival advantage to microvasculature following IR stress.