Sepsis is a common, potentially lethal, but poorly understood clinical syndrome in the world [30]. Although with the continuous progress of medical technology, the death caused by sepsis is still increasing rapidly, which is because the pathogenesis has not been fully elucidated. Therefore, it is the common wish of researchers to elucidate the molecular mechanism of sepsis all over the world. In the present study, we showed that mice injected with LPS (5 mg/kg, i.p.) for twenty-four hours could induce severe inflammatory reactions including an increased IL-1β, IL-18 in serum, activation of NLRP3 inflammasome, Slit2-Robo4 signaling pathway and disruption of tight junction in intestine. The injection of VX765 (10 mg/kg, i.p.), an inhibitor of NLRP3 inflammasome, could reverse these effects induced by LPS. Altogether, our findings revealed that Slit2-Robo4 signaling pathway and tight junction in intestinal probably involved in LPS-induced inflammation in mice, which may account for the molecular mechanism of sepsis.
A large number of papers have shown that barrier dysfunction and microvascular leakage of intestinal endothelial cells are the key factors of organ failure in sepsis and sepsis-related complications [31, 32]. The barrier consists of endothelial cells, adherens junctions, tight junctions, and extracellular components. Normally, the endothelial barrier is semi-permeable, which enables the transport of fluids and solutes from the blood to the tissues [33]. However, in patients with sepsis, abnormal barrier function leads to enhanced protein and solute transport. Breed ER, et, al. and other researchers have reported that the intestinal permeability of young C57BL/6J female mice in cecal ligation and puncture group was higher than that of in the control group, which was related to the changes of claudins 1,2,3,4,5 and 8[34, 35]. Wang et, al. reported that the activation of autophagy and NLRP3 inflammasome by LPS + ATP can result in paracellular permeability increase and morphological disruption of both ZO-1 and occluding [36]. In this study, tight junction protein ZO-1, occludin, and claudin-5 in the intestine of LPS group and Con group were examined by western blot. The results showed that LPS can induce decreased expression of tight junction protein ZO-1, occludin, and claudin-5. These effects can be reversed by VX765. All of these results demonstrated that inflammation induced by LPS is related to the increased permeability of intestine. Frankly speaking, this conclusion is preliminary, which also needs more experiments such as gene knockdown to prove it in the future.
Many studies have shown that endothelial cells specifically express receptor Robo4 which binding to its ligand Slit can inhibit inflammation and endothelial permeability by enhancing adhesion and adjusting cytoskeleton dynamics [37, 38]. Slit2 bind to the receptors Robo1 and Robo4, and then induces a series of intracellular signaling events. Slit2 regulates angiogenesis and protects endothelial integrity during sepsis and HIV infection [27]. In this study, Slit2 and Robo4 were examined by western blot in LPS group and Con group. The results showed that LPS can decrease the expression of Slit2 and Robo4. These effects also can be reversed by VX765. The mechanisms by which Slit2 regulates endothelial permeability are complex. The research results of Vincent PA et, al. showed that Slit2N was the active fragment of Slit2, which could promote and accelerate the localization of VE cadherin and p120-catenin on the surface of endothelial cells [26, 39]. Therefore, Slit2N can reduce the permeability changes of endothelial cells induced by inflammatory mediators such as VEGF, LPS, TNF-α, and IL-1β [40, 41]. It was found that Slit2N reduced the accumulation of neutrophils and protein exudates in the alveolar cavity of LPS treated mice. But this effect is not present in Robo4 knockout mice, which proves the importance of this receptor [42]. In human umbilical vein endothelial cells (HUVECs), LPS downregulated Slit2, Robo4 and VE-cadherin protein expression and increased endothelial cell permeability in vitro during inflammation. Chen et, al. reported that hPMSC-derived Slit2 may inhibit LPS-induced CD11b CD18 expression to decrease cell migration and increase adhesion through modulating the activity and motility of inflammatory macrophages in placenta [43–44]. Vascular endothelial growth factor (VEGF), a kind of glycoprotein produced by endothelial cells, regulates vascular permeability through binding to VEGF receptors and activating the corresponding signal pathway [45]. Recently, some studies have focused on the effects of sepsis [46]. In this study, VEGF was examined, and the results showed that LPS can induce higher expression of VEGF which can be reversed by VX765. These results indicated that the increased permeability of the endothelium is associated with high expression of VEGF, but the underlying mechanisms need to be elucidated.
Certainly, although the experimental results of this paper are very interesting, the study still has some limitations. Firstly, this study did not carry out functional experiments to verify the established sepsis animal model, such as measuring heart rate, blood pressure and statistical mortality. Secondly, in the experimental design, the detection indexes that measured only after LPS injection for 24 hours. The time point was single, and it would be more meaningful to examine the relevant detection indexes of animals after LPS injection at different time points, which can reflect the whole dynamic process of inflammatory factors with time. Third, there was no corresponding inhibitor and/or agonist group or used gene knockout technology to verify the Slit2-Robo4 signaling pathway. These deficiencies are interesting and worth for studying in the future by setting more experimental groups, time points and inflammatory factors and so on. What is more, Slit2 knockout mouse model could also be established, which can be used for further observing tight junction and other indicators by immunofluorescence or transmission electron microscope.
In conclusion, our study indicated that LPS could induce severe inflammatory reactions including an increased IL-1β, IL-18 in serum, activation of NLRP3 inflammasome, Slit2-Robo4 signal pathway and disruption of tight junction in intestine. VX765, an inhibitor of NLRP3 inflammasome, could reverse these effects induced by LPS. Altogether, our findings revealed that Slit2-Robo4 signaling pathway and tight junction in intestinal may be involved in LPS-induced inflammation in mice, which may account for the molecular mechanism of sepsis.