Endothelial HOXB9/SDC4 signaling exacerbates post-ischemic blood-brain and blood-spinal cord barrier disruption by promoting PKCα activation

Ischemic stroke is one of the most deadly chronic diseases, it damages blood-brain barrier (BBB) and blood-spinal cord barrier (BSCB). Syndecan-4 (SDC4) affects BBB integrity and function, but the specific mechanism is unclear. The middle cerebral artery occlusion (MCAO) rat model and the oxygen-glucose deprivation/reoxygenation (OGD/R) cell model were conducted. The influences of SDC4 on the integrity and function of BBB were detected by the smembrane resistance (TEER) assay, immunofluorescence staining and Evans Blue assay. Luxol Fast Blue staining was used to observe the changes of motoneurons structure. Additionally, the effect of SDC4 on protein kinase Cα (PKCα) activation was measured. Finally, the effect and mechanism of Homeobox B9 (HOXB9) on SDC4 was measured with chromatin immunoprecipitation assays and dual-luciferase reporter assay. SDC4 was upregulated in brain of MCAO rat, hCMEC/D3 cells and rBMVEC cells treated with OGD/R. SDC4 overexpression reduced cell viability, TEER and the expression of tight junction proteins. Importantly, SDC4 overexpression promoted PKCα activation. SDC4 interference had the opposite effects. In vivo experiments, SDC4 interference hindered Evans Blue entering brain, protected neurons structure and reduced IL-6 and TNF-α levels. As the transcription factor of SDC4, HOXB9 was upregulated in hCMEC/D3 cells induced by OGD/R. HOXB9 overexpression promoted the expression of SDC4, reduced cell viability, TEER, and tight junction protein expressions. Importantly, HOXB9 interacts with SDC4 promoter region site 2 to affect SDC4 transcription. HOXB9 interference inhibited SDC4 transcription to attenuate PKCα activation, finally alleviate BBB disruption, spinal cord injury and BSCB damage.