Purpose: Neuroinflammation plays an important part in the pathophysiology of sepsis-associated encephalopathy (SAE). Gut microbiota and gut brain axis are considered as important mediators in the development of neurological diseases. The aim of this study was to investigate the role of intestinal microbiota in sepsis-related brain injury and to explore the underlying mechanisms. Methods: Mouse model of SAE was established using cecal ligation and puncture (CLP). Based on the mouse mortality and the associated time of death, light SAE (LSAE) and severe SAE (SSAE) were classified. Fecal microbiota transplantation (FMT) was performed to verify the role of intestinal microbiota. Feces of mice in the two groups which collected before operation were sequenced for 16S and targeted short chain fatty acids. Results: Intestinal microbiota from SSAE and LSAE mice displayed diverse functions. Interestingly, LSAE mice produced more butyric acid compared with SSAE mice. In the in vivo experiments, sodium butyrate (NaB) reduced the high oxidative stress levels in mice hippocampus and conferred a marked survival superiority to sepsis mice. In addition, NaB prevented the increase in intracellular reactive oxygen species (ROS) generation and inducible nitric-oxide synthase expression in LPS-stimulated primary microglia. The GPR109A/Nrf2/HO-1 signaling pathway was found to be involved in the activation of antioxidant response of primary microglia induced by sodium butyrate. Conclusion: Our findings indicate a crucial role of gut microbiota in the susceptibility to SAE. Butyrate, a metabolite of intestinal microbiota, may have a neuroprotective effect in the process of sepsis by GPR109A/Nrf2/HO-1 pathway.

Axon development plays a pivotal role in the formation of synapse, nodes of Ranvier, and myelin sheath. Interleukin-1β (IL-1β) produced by microglia may cause myelination disturbances through suppression of oligodendrocyte progenitor cell maturation in the septic neonatal rats. Here, we explored if a microglia-derived IL-1β would disturb axon development in the corpus callosum (CC) following lipopolysaccharide (LPS) administration, and if so, whether it is associated with disorder of synapse formation in the cerebral cortex and node of Ranvier. Sprague-Dawley rats (1-day old) in the septic model group were intraperitoneally administrated with lipopolysaccharide (1 mg/kg) and then sacrificed for detection of IL-1β, interleukin-1 receptor (IL-1R1), neurofilament-68, neurofilament-160, and neurofilament-200, proteolipid, synaptophysin, and postsynaptic density 95 (PSD95) expression by western blotting and immunofluorescence. Electron microscopy was conducted to observe alterations of axonal myelin sheath and synapses in the cortex, and proteolipid expression was assessed using in situ hybridization. The effect of IL-1β on neurofilament and synaptophysin expression in primary neuron cultures was determined by western blotting and immunofluorescence. P38-MAPK signaling pathway was investigated to determine whether it was involved in the inhibition of IL-1β on neurofilament and synaptophysin expression. In 1-day old septic rats, IL-1β expression was increased in microglia coupled with upregulated expression of IL-1R1 on the axons. The expression of neurofilament-68, neurofilament-160, and neurofilament-200 (NFL, NFM, NFH) and proteolipid (PLP) was markedly reduced in the CC at 7, 14, and 28 days after LPS administration. Simultaneously, cortical synapses and mature oligodendrocytes were significantly reduced. By electron microscopy, some axons showed smaller diameter and thinner myelin sheath with damaged ultrastructure of node of Ranvier compared with the control rats. In the cerebral cortex of LPS-injected rats, some axo-dendritic synapses appeared abnormal looking as manifested by the presence of swollen and clumping of synaptic vesicles near the presynaptic membrane. In primary cultured neurons incubated with IL-1β, expression of NFL, NFM, and synaptophysin was significantly downregulated. Furthermore, p38-MAPK signaling pathway was implicated in disorder of axon development and synaptic deficit caused by IL-1β treatment. The present results suggest that microglia-derived IL-1β might suppress axon development through activation of p38-MAPK signaling pathway that would contribute to formation disorder of cortical synapses and node of Ranvier following LPS exposure.

The bidirectional interactions between the gut microbiota and sepsis-associated encephalopathy (SAE) are not well characterized. We found that the gut microbiota was more severely disturbed in SAE-susceptible (SES) mice than in SAE-resistant (SER) mice after sepsis modeling.

Cognitive impairment is one of common complications of acute respiratory distress syndrome (ARDS). Increasing evidence suggests that interleukin-1 beta (IL-1β) plays a role in inducing neuronal apoptosis in cognitive dysfunction. The lung protective ventilatory strategies, which serve to reduce pulmonary morbidity for ARDS patients, almost always lead to hypercapnia. Some studies have reported that hypercapnia contributes to the risk of cognitive impairment and IL-1β secretion outside the central nervous system (CNS). However, the underlying mechanism of hypercapnia aggravating cognitive impairment under hypoxia has remained uncertain. This study was aimed to explore whether hypercapnia would partake in increasing IL-1β secretion via activating the NLRP3 (NLR family, pyrin domain-containing 3) inflammasome in the hypoxic CNS and in aggravating cognitive impairment. The Sprague-Dawley (SD) rats that underwent hypercapnia/hypoxemia were used for assessment of NLRP3, caspase-1, IL-1β, Bcl-2, Bax, and caspase-3 expression by Western blotting or double immunofluorescence, and the model was also used for Morris water maze test. In addition, Z-YVAD-FMK, a caspase-1 inhibitor, was used to treat BV-2 microglia to determine whether activation of NLRP3 inflammasome was required for the enhancing effect of hypercapnia on expressing IL-1β by Western blotting or double immunofluorescence. The interaction effects were analyzed by factorial ANOVA. Simple effects analyses were performed when an interaction was observed. There were interaction effects on cognitive impairment, apoptosis of hippocampal neurons, activation of NLRP3 inflammasome, and upregulation of IL-1β between hypercapnia treatment and hypoxia treatment. Hypercapnia + hypoxia treatment caused more serious damage to the learning and memory of rats than those subjected to hypoxia treatment alone. Expression levels of Bcl-2 were reduced, while that of Bax and caspase-3 were increased by hypercapnia in hypoxic hippocampus. Hypercapnia markedly increased the expression of NLRP3, caspase-1, and IL-1β in hypoxia-activated microglia both in vivo and in vitro. Pharmacological inhibition of NLRP3 inflammasome activation and release of IL-1β might ameliorate apoptosis of neurons. The present results suggest that hypercapnia-induced IL-1β overproduction via activating the NLRP3 inflammasome by hypoxia-activated microglia may augment neuroinflammation, increase neuronal cell death, and contribute to the pathogenesis of cognitive impairments.