Treponema pallidum Tp0751 alters the expression of tight junction proteins by promoting bEnd3 cell apoptosis and IL-6 secretion

Abstract

BackgroundNeurosyphilis is a serious complication caused by the invasion of the central nervous system by Treponema pallidum subsp. pallidum (T. pallidum). However, the molecular mechanism by which T. pallidum crosses the blood-brain barrier has not been fully elucidated. ObjectivesThe primary purpose of this experimental design was to explore the effect of the T. pallidum adhesion protein Tp0751 on the blood-brain barrier and cerebrovascular endothelial cells. MethodsBEnd3 cells were used to construct a monolayer blood-brain barrier model in vitro. The integrity of blood-brain barrier model was evaluated by a transendothelial cell resistance meter and transmission electron microscope after the stimulation of recombinant protein TP0751. Hoechst 33258 staining and flow cytometry were used to detect the apoptosis rate. Western blotting assay was used to measure the expression of tight junction proteins and apoptosis-related proteins. The enzyme activity detection kit was responsible for detecting the enzyme activities of Caspase 3, Caspase 8 and Caspase 9. The expression of pro-inflammatory cytokines TNF-α, IL-1β and IL-6 at the transcription and translation levels were detected by qRT-PCR and ELISA, respectively. ResultsThe results showed that, the tight junction structures between cells showed no obvious fragmentation, but the levels of the tight junction proteins zonula occludens-1 and occludin were reduced by the effects of Tp0751 on bEnd3 cells. In addition, further research demonstrated that after incubation with bEnd3 cells, Tp0751 induced cell apoptosis in a concentration- and time-dependent manner via the caspase 8/caspase 3 pathway. These apoptotic processes may have contributed to the changes in tight junction proteins expression. Furthermore, the Tp0751 protein may be involved in the pathogenic process by which T. pallidum crosses the blood-brain barrier by promoting secretion of the proinflammatory factor interleukin-6. ConclusionsOn the whole, this study is the first to reveal and highlight that Tp0751 may affect the expression of tight junction proteins by inducing apoptosis and promoting the secretion of the inflammatory cytokine IL-6, thus playing a role in the progression of neurosyphilis caused by T. pallidum.