Experimental Colitis Enhances Temporal Variations in CX3CR1 Cell Colonization of the Gut and Brain Following Irradiation

New insights into muscularis macrophages in the gut: from their origin to therapeutic targeting

Muscularis macrophages, as the most abundant immune cells in the intestinal muscularis externa, exhibit tissue protective phenotype in the steady state. Owing to tremendous advances in technology, we now know the fact that muscularis macrophages are a heterogeneous population of cells which could be divided into different functional subsets depending on their anatomic niches. There is emerging evidence showing that these subsets, through molecular interactions with their neighbours, take part in a wide range of physiological and pathophysiological processes in the gut. In this review, we summarize recent progress (particularly over the past 4 years) on distribution, morphology, origin and functions of muscularis macrophages and, where possible, the characteristics of specific subsets in response to the microenvironment they occupy, with particular emphasis on their role in muscular inflammation. Furthermore, we also integrate their role in inflammation-related gastrointestinal disorders, such as post-operative ileus and diabetic gastroparesis, in order to propose future therapeutic strategies.

A leaky gut dysregulates gene networks in the brain associated with immune activation, oxidative stress, and myelination in a mouse model of colitis

The gut and brain are increasingly linked in human disease, with neuropsychiatric conditions classically attributed to the brain showing an involvement of the intestine and inflammatory bowel diseases (IBDs) displaying an ever-expanding list of neurological comorbidities. To identify molecular systems that underpin this gut-brain connection and thus discover therapeutic targets, experimental models of gut dysfunction must be evaluated for brain effects. In the present study, we examine disturbances along the gut-brain axis in a widely used murine model of colitis, the dextran sodium sulfate (DSS) model, using high-throughput transcriptomics and an unbiased network analysis strategy coupled with standard biochemical outcome measures to achieve a comprehensive approach to identify key disease processes in both colon and brain. We examine the reproducibility of colitis induction with this model and its resulting genetic programs during different phases of disease, finding that DSS-induced colitis is largely reproducible with a few site-specific molecular features. We focus on the circulating immune system as the intermediary between the gut and brain, which exhibits an activation of pro-inflammatory innate immunity during colitis. Our unbiased transcriptomics analysis provides supporting evidence for immune activation in the brain during colitis, suggests that myelination may be a process vulnerable to increased intestinal permeability, and identifies a possible role for oxidative stress and brain oxygenation. Overall, we provide a comprehensive evaluation of multiple systems in a prevalent experimental model of intestinal permeability, which will inform future studies using this model and others, assist in the identification of druggable targets in the gut-brain axis, and contribute to our understanding of the concomitance of intestinal and neuropsychiatric dysfunction.

Tongqiao Huoxue Decoction ameliorates traumatic brain injury-induced gastrointestinal dysfunction by regulating CD36/15-LO/NR4A1 signaling, which fails when CD36 and CX3CR1 are deficient

AIMS

Gastrointestinal (GI) dysfunction, as a common peripheral-organ complication after traumatic brain injury (TBI), is primarily characterized by gut inflammation and damage to the intestinal mucosal barrier (IMB). Previous studies have confirmed that TongQiao HuoXue Decoction (TQHXD) has strong anti-inflammatory properties and protects against gut injury. However, few have reported on the therapeutic effects of TQHXD in a TBI-induced GI dysfunction model. We aimed to explore the effects of TQHXD on TBI-induced GI dysfunction and the underlying mechanism thereof.

METHODS

We assessed the protective effects and possible mechanism of TQHXD in treating TBI-induced GI dysfunction via gene engineering, histological staining, immunofluorescence (IF), 16S ribosomal ribonucleic acid (rRNA) sequencing, real-time polymerase chain reaction (RT-PCR), enzyme-linked immunosorbent assay (ELISA), Western blot (WB), and flow cytometry (FCM).

RESULTS

TQHXD administration ameliorated TBI-induced GI dysfunction by modulating the abundance and structure of bacteria; reconstructing the destroyed epithelial and chemical barriers of the IMB; and improving M1/M2 macrophage, T-regulatory cell (Treg)/T helper 1 cell (Th1 ), as well as Th17 /Treg ratios to preserve homeostasis of the intestinal immune barrier. Notably, Cluster of Differentiation 36 (CD36)/15-lipoxygenase (15-LO)/nuclear receptor subfamily 4 group A member 1 (NR4A1) signaling was markedly stimulated in colonic tissue of TQHXD-treated mice. However, insufficiency of both CD36 and (C-X3-C motif) chemokine receptor 1 (CX3CR1) worsened GI dysfunction induced by TBI, which could not be rescued by TQHXD.

CONCLUSION

TQHXD exerted therapeutic effects on TBI-induced GI dysfunction by regulating the intestinal biological, chemical, epithelial, and immune barriers of the IMB, and this effect resulted from the stimulation of CD36/NR4A1/15-LO signaling; however, it could not do so when CX3CR1 and CD36 were deficient. TQHXD might therefore be a potential drug candidate for treating TBI-induced GI dysfunction.

The Gut–Immune–Brain Axis: An Important Route for Neuropsychiatric Morbidity in Inflammatory Bowel Disease

Inflammatory bowel disease (IBD) comprises Crohn’s disease (CD) and ulcerative colitis (UC) and is associated with neuropsychiatric symptoms like anxiety and depression. Both conditions strongly worsen IBD disease burden. In the present review, we summarize the current understanding of the pathogenesis of depression and anxiety in IBD. We present a stepwise cascade along a gut–immune–brain axis initiated by evasion of chronic intestinal inflammation to pass the epithelial and vascular barrier in the gut and cause systemic inflammation. We then summarize different anatomical transmission routes of gut-derived peripheral inflammation into the central nervous system (CNS) and highlight the current knowledge on neuroinflammatory changes in the CNS of preclinical IBD mouse models with a focus on microglia, the brain-resident macrophages. Subsequently, we discuss how neuroinflammation in IBD can alter neuronal circuitry to trigger symptoms like depression and anxiety. Finally, the role of intestinal microbiota in the gut–immune–brain axis in IBD will be reviewed. A more comprehensive understanding of the interaction between the gastrointestinal tract, the immune system and the CNS accounting for the similarities and differences between UC and CD will pave the path for improved prediction and treatment of neuropsychiatric comorbidities in IBD and other inflammatory diseases.