Selective inhibition of mucosal serotonin as treatment for IBD?

Peripheral Serotonin: Cultivating Companionship with Gut Microbiota in Intestinal Homeostasis

Serotonin, also known as 5-hydroxytryptamine (5-HT), is an evolutionarily ancient and phylogenetically conserved monoamine that regulates multifaceted physiological functions in mammals. 5-HT was, at one time, most extensively studied as a neurotransmitter within the central nervous system but is now known to regulate non-neuronal functions including immune responses in an autocrine-paracrine-endocrine manner. Compelling evidence from intervention studies using germ-free mice or antibiotic-associated microbiota perturbation suggests that novel interactions between 5-HT and the gut microbiota are essential in maintaining intestinal homeostasis. Importantly, recent studies reveal that bidirectional host-microbial interactions mediated by the host serotonergic system can promote distinct changes within the gut microbiota. These changes may potentially lead to a state known as 'dysbiosis' which has been strongly associated with various gut pathologies including inflammatory bowel disease (IBD). In this review, we update the current understanding of host-microbiota interaction by focusing on the impact of peripheral 5-HT signaling within this dynamic. We also briefly highlight key environmental risk factors for IBD, such as Western diet, and draw attention to the interaction of synthetic food colorants with 5-HT signaling that may facilitate future research.

Fructus Mume (Wu Mei) Attenuates Acetic Acid-Induced Ulcerative Colitis by Regulating Inflammatory Cytokine, Reactive Oxygen Species, and Neuropeptide Levels in Model Rats

Ulcerative colitis (UC) is a chronic idiopathic inflammatory disorder of the large intestine. Fructus mume (FM), a natural food with nutritive and pharmaceutical value, has demonstrated therapeutic efficacy against UC. In this study, we investigated the protective effects and mechanisms of FM against UC. We induced UC in rats with 4% (v/v) acetic acid (AA), orally administered 0.7 or 0.325 g/kg FM and 0.3 g/kg sulfasalazine (SASP) for 7 days, and explored the responses the drugs elicited in the rats. We assessed the general conditions of the rats by the disease active index. We evaluated colon tissue damage macroscopically and by Hematoxylin & Eosin, Alcian Blue-periodic acid-Schiff, and Masson's staining, and explored the potential mechanisms of FM on inflammation, oxidative stress, and neuropeptides by measuring TNF-α, IL-6, IL-8, IL-10, MMP9, CXCR-1, SOD, GSH-px, MDA, ROS, SIRT3, SP, VIP, ghrelin, and 5-HT. FM treatment significantly attenuated colon damage and submucosal fibrosis compared with the model. It lowered serum proinflammatory TNF-α, IL-8, and colonic MMP9 and CXCR-1, and raised serum anti-inflammatory IL-10 levels. FM upregulated the antioxidant enzymes SOD, GSH-px, and SITR3 protein but inhibited ROS and MDA production. It downregulated colonic SP, VIP, ghrelin, and 5-HT. The beneficial effects of FM might be dose dependent. Around 0.7 g/kg FM and SASP displayed similar efficacy for treating AA-induced colitis in rats. Our results provide empirical evidence that FM protects against AA-induced UC in rats via anti-inflammatory and antioxidant mechanisms, and regulates neuropeptides; thus, FM may be a promising, safe, and efficacious alternative therapy for UC, if its efficacy can be confirmed in human trials.

Prebiotics, Probiotics, Synbiotics, Paraprobiotics and Postbiotic Compounds in IBD

The increasing incidence of inflammatory bowel diseases (IBD) and the increasing severity of the course of these diseases create the need for developing new methods of therapy. The gut microbiome is extensively studied as a factor influencing the development and course of IBD. The composition of intestinal microbiota can be relatively easily modified by diet (i.e., prebiotics, mainly dietary fibers) and bacterial supplementation using beneficial bacteria strains called probiotics. Additionally, the effects of the improved microbiome could be enhanced or gained by using paraprobiotics (non-viable, inactivated bacteria or their components) and/or postbiotics (products of bacterial metabolism or equal synthetic products that beneficially modulate immunological response and inflammation). This study summarizes the recent works on prebiotics, probiotics, synbiotics (products merging pre- and probiotics), paraprobiotics and postbiotics in IBD.

Crosstalk Between Intestinal Serotonergic System and Pattern Recognition Receptors on the Microbiota-Gut-Brain Axis

Disruption of the microbiota-gut-brain axis results in a wide range of pathologies that are affected, from the brain to the intestine. Gut hormones released by enteroendocrine cells to the gastrointestinal (GI) tract are important signaling molecules within this axis. In the search for the language that allows microbiota to communicate with the gut and the brain, serotonin seems to be the most important mediator. In recent years, serotonin has emerged as a key neurotransmitter in the gut-brain axis because it largely contributes to both GI and brain physiology. In addition, intestinal microbiota are crucial in serotonin signaling, which gives more relevance to the role of the serotonin as an important mediator in microbiota-host interactions. Despite the numerous investigations focused on the gut-brain axis and the pathologies associated, little is known regarding how serotonin can mediate in the microbiota-gut-brain axis. In this review, we will mainly discuss serotonergic system modulation by microbiota as a pathway of communication between intestinal microbes and the body on the microbiota-gut-brain axis, and we explore novel therapeutic approaches for GI diseases and mental disorders.

Modulation of Crustacean Innate Immune Response by Amino Acids and Their Metabolites: Inferences From Other Species

Aquaculture production of crustaceans (mainly shrimp and crabs) has expanded globally, but disease outbreaks and pathogenic infections have hampered production in the last two decades. As invertebrates, crustaceans lack an adaptive immune system and mainly defend and protect themselves using their innate immune system. The immune system derives energy and metabolites from nutrients, with amino acids constituting one such source. A growing number of studies have shown that amino acids and their metabolites are involved in the activation, synthesis, proliferation, and differentiation of immune cells, as well as in the activation of immune related signaling pathways, reduction of inflammatory response and regulation of oxidative stress. Key enzymes in amino acid metabolism have also been implicated in the regulation of the immune system. Here, we reviewed the role played by amino acids and their metabolites in immune-modulation in crustaceans. Information is inferred from mammals and fish where none exists for crustaceans. Research themes are identified and the relevant research gaps highlighted for further studies.

Postbiotic-Enabled Targeting of the Host-Microbiota-Pathogen Interface: Hints of Antibiotic Decline?

Mismanagement of bacterial infection therapies has undermined the reliability and efficacy of antibiotic treatments, producing a profound crisis of the antibiotic drug market. It is by now clear that tackling deadly infections demands novel strategies not only based on the mere toxicity of anti-infective compounds. Host-directed therapies have been the first example as novel treatments with alternate success. Nevertheless, recent advances in the human microbiome research have provided evidence that compounds produced by the microbial metabolism, namely postbiotics, can have significant impact on human health. Such compounds target the host-microbe-pathogen interface rescuing biotic and immune unbalances as well as inflammation, thus providing novel therapeutic opportunities. This work discusses critically, through literature review and personal contributions, these novel nonantibiotic treatment strategies for infectious disease management and resistance prevention, which could represent a paradigm change rocking the foundation of current antibiotic therapy tenets.

A colon-specific prodrug of metoclopramide ameliorates colitis in an experimental rat model

Background

We examined whether metoclopramide (MCP), a modulator of dopamine and serotonin receptors, alleviated colitis and had synergistic effects when coadministered with 5-aminosalicylic acid (5-ASA) in an experimental model of colitis.

Methods

MCP azo-linked to 5-ASA (5-[4-chloro-2-{2-(diethylamino)ethylcarbamoyl}– 1-methoxyphenyl]azosalicylic acid, MCP-azo-ASA) was synthesized, where 5-ASA was used as a colon-targeting carrier and an anti-colitic agent, and the ability of MCP-azo-ASA to target the colon in vitro and in vivo was evaluated.

Results

Our results indicate that MCP-azo-ASA was cleaved to MCP and 5-ASA in the cecal contents, but not in the contents of the small intestine. Oral gavage with equimolar concentrations of MCP-azo-ASA and sulfasalazine (SSZ, a colon-specific prodrug of 5-ASA widely used clinically) demonstrated that the two prodrugs delivered comparable amounts of 5-ASA to the cecum. MCP was barely detected in the blood after oral gavage with MCP-azo-ASA. In a rat model of 2,4-dinitrobenzene sulfonic acid hydrate (DNBS)-induced colitis, MCP-azo-ASA alleviated colonic damage in a dose-dependent manner. Moreover, MCP-azo-ASA reduced the concentrations of inflammatory mediators in the inflamed colon. At low equimolar doses, MCP-azo-ASA, but not SSZ, resulted in significant anti-colitic effects, which indicates that MCP has anti-colitic activity. MCP-azo-ASA had anti-colitic effects equal to those of SSZ at high equimolar doses.

Conclusion

Thus, our results indicate that MCP-azo-ASA is a colon-specific prodrug of MCP. Targeted delivery of MCP to the colon ameliorated DNBS-induced colitis in rats, and we did not observe any synergistic effects of MCP after co-delivery with 5-ASA.

Impact of the Gut Microbiota on Intestinal Immunity Mediated by Tryptophan Metabolism

The gut microbiota influences the health of the host, especially with regard to gut immune homeostasis and the intestinal immune response. In addition to serving as a nutrient enhancer, L-tryptophan (Trp) plays crucial roles in the balance between intestinal immune tolerance and gut microbiota maintenance. Recent discoveries have underscored that changes in the microbiota modulate the host immune system by modulating Trp metabolism. Moreover, Trp, endogenous Trp metabolites (kynurenines, serotonin, and melatonin), and bacterial Trp metabolites (indole, indolic acid, skatole, and tryptamine) have profound effects on gut microbial composition, microbial metabolism, the host's immune system, the host-microbiome interface, and host immune system-intestinal microbiota interactions. The aryl hydrocarbon receptor (AhR) mediates the regulation of intestinal immunity by Trp metabolites (as ligands of AhR), which is beneficial for immune homeostasis. Among Trp metabolites, AhR ligands consist of endogenous metabolites, including kynurenine, kynurenic acid, xanthurenic acid, and cinnabarinic acid, and bacterial metabolites, including indole, indole propionic acid, indole acetic acid, skatole, and tryptamine. Additional factors, such as aging, stress, probiotics, and diseases (spondyloarthritis, irritable bowel syndrome, inflammatory bowel disease, colorectal cancer), which are associated with variability in Trp metabolism, can influence Trp-microbiome-immune system interactions in the gut and also play roles in regulating gut immunity. This review clarifies how the gut microbiota regulates Trp metabolism and identifies the underlying molecular mechanisms of these interactions. Increased mechanistic insight into how the microbiota modulates the intestinal immune system through Trp metabolism may allow for the identification of innovative microbiota-based diagnostics, as well as appropriate nutritional supplementation of Trp to prevent or alleviate intestinal inflammation. Moreover, this review provides new insight regarding the influence of the gut microbiota on Trp metabolism. Additional comprehensive analyses of targeted Trp metabolites (including endogenous and bacterial metabolites) are essential for experimental preciseness, as the influence of the gut microbiota cannot be neglected, and may explain contradictory results in the literature.

Telotristat ethyl: a novel agent for the therapy of carcinoid syndrome diarrhea

Carcinoid syndrome (CS), characterized by diarrhea and flushing, is present in 20% of patients with neuroendocrine tumors at diagnosis and becomes more frequent with progression. The diarrhea of CS is caused mainly by tumoral secretion of serotonin. It may not be fully controlled by somatostatin analogs, the currently indicated drugs for symptomatic relief. Telotristat ethyl is a novel inhibitor of tryptophan hydroxylase, the rate-limiting enzyme in serotonin biosynthesis. Administration of the drug decreases diarrhea in patients with CS. Telotristat ethyl was approved in February 2017 (USA) and September 2017 (European Commission) for the treatment of CS diarrhea in adults inadequately controlled by somatostatin analog alone. This drug is expected to greatly improve the health and quality of life of patients with CS diarrhea.

NOD1 downregulates intestinal serotonin transporter and interacts with other pattern recognition receptors

Serotonin (5-HT) is an essential gastrointestinal modulator whose effects regulate the intestinal physiology. 5-HT effects depend on extracellular 5-HT bioavailability, which is controlled by the serotonin transporter (SERT) expressed in both the apical and basolateral membranes of enterocytes. SERT is a critical target for regulating 5-HT levels and consequently, modulating the intestinal physiology. The deregulation of innate immune receptors has been extensively studied in inflammatory bowel diseases (IBD), where an exacerbated defense response to commensal microbiota is observed. Interestingly, many innate immune receptors seem to affect the serotonergic system, demonstrating a new way in which microbiota could modulate the intestinal physiology. Therefore, our aim was to analyze the effects of NOD1 activation on SERT function, as well as NOD1's interaction with other immune receptors such as TLR2 and TLR4. Our results showed that NOD1 activation inhibits SERT activity and expression in Caco-2/TC7 cells through the extracellular signal-regulated kinase (ERK) signaling pathway. A negative feedback between 5-HT and NOD1 expression was also described. The results showed that TLR2 and TLR4 activation seems to regulate NOD1 expression in Caco-2/TC7 cells. To assess the extend of cross-talk between NOD1 and TLRs, NOD1 expression was measured in the intestinal tract (ileum and colon) of wild type mice and mice with individual knockouts of TLR2, and TLR4 as well as double knockout TLR2/TLR4 mice. Hence, we demonstrate that NOD1 acts on the serotonergic system decreasing SERT activity and molecular expression. Additionally, NOD1 expression seems to be modulated by 5-HT and other immune receptors as TLR2 and TLR4. This study could clarify the relation between both the intestinal serotonergic system and innate immune system, and their implications in intestinal inflammation.

Serotonin drives the acquisition of a profibrotic and anti-inflammatory gene profile through the 5-HT7R-PKA signaling axis

Peripheral serotonin (5-hydroxytryptamine, 5-HT) regulates cell growth and differentiation in numerous cell types through engagement of seven types of cell surface receptors (HTR1-7). Deregulated 5-HT/HTR levels contribute to pathology in chronic inflammatory diseases, with macrophages being relevant targets for the physio-pathological effects of 5-HT. In fact, 5-HT skews human macrophage polarization through engagement of 5-HT2BR and 5-HT7R receptors. We now report that 5-HT primes macrophages for reduced pro-inflammatory cytokine production and IFN type I-mediated signaling, and promotes an anti-inflammatory and pro-fibrotic gene signature in human macrophages. The acquisition of the 5-HT-dependent gene profile primarily depends on the 5-HT7R receptor and 5-HT7R-initiated PKA-dependent signaling. In line with the transcriptional results, 5-HT upregulates TGFβ1 production by human macrophages in an HTR7- and PKA-dependent manner, whereas the absence of Htr7 in vivo results in diminished macrophage infiltration and collagen deposition in a mouse model of skin fibrosis. Our results indicate that the anti-inflammatory and pro-fibrotic activity of 5-HT is primarily mediated through the 5-HT7R-PKA axis, and that 5-HT7R contributes to pathology in fibrotic diseases.

Gastrointestinal neuroendocrine peptides/amines in inflammatory bowel disease

Inflammatory bowel disease (IBD) is a chronic recurrent condition whose etiology is unknown, and it includes ulcerative colitis, Crohn’s disease, and microscopic colitis. These three diseases differ in clinical manifestations, courses, and prognoses. IBD reduces the patients’ quality of life and is an economic burden to both the patients and society. Interactions between the gastrointestinal (GI) neuroendocrine peptides/amines (NEPA) and the immune system are believed to play an important role in the pathophysiology of IBD. Moreover, the interaction between GI NEPA and intestinal microbiota appears to play also a pivotal role in the pathophysiology of IBD. This review summarizes the available data on GI NEPA in IBD, and speculates on their possible role in the pathophysiology and the potential use of this information when developing treatments. GI NEPA serotonin, the neuropeptide Y family, and substance P are proinflammatory, while the chromogranin/secretogranin family, vasoactive intestinal peptide, somatostatin, and ghrelin are anti-inflammatory. Several innate and adaptive immune cells express these NEPA and/or have receptors to them. The GI NEPA are affected in patients with IBD and in animal models of human IBD. The GI NEPA are potentially useful for the diagnosis and follow-up of the activity of IBD, and are candidate targets for treatments of this disease.

五羟色胺信号系统与胃肠道疾病的研究进展

五羟色胺(5-hydroxytryptamine, 5-HT)是脑肠轴调控路径中的一个重要组成部分. 5-HT信号系统在多种胃肠道疾病的发病机制中扮演重要的角色, 参与调节胃肠运动、感觉及分泌等功能. 5-HT的合成、释放、与特异性受体结合及再摄取过程中任意一个环节发生异常, 均与胃肠道功能紊乱的发生密切相关, 其不仅在功能性疾病中存在作用, 与器质性疾病也有很大的关系. 临床上5-HT受体激动剂、拮抗剂及选择性再摄取抑制剂已被广泛运用到多种胃肠道疾病的治疗中. 本文阐述了5-HT信号系统与胃肠道疾病病理生理的关系, 从而研究5-HT受体在胃肠道中的作用及其相关药物的临床效用, 为胃肠道疾病的防治提供理论依据.

Anticolitis activity of Chinese herbal formula yupingfeng powder via regulating colonic enterochromaffin cells and serotonin

Objective:

To investigate whether traditional Chinese herbal formula Yupingfeng (YPF) powder has an anti-inflammatory effect on colonic inflammation, and to explore the mechanism involved.

Materials and Methods:

YPF powder was orally administrated to trinitrobenzene sulfonic acid (TNBS)-induced colitis mice at the dose of 3, 6, and 12 g/kg/d for 7 consecutive days. Body weight, stool consistency, histopathological score, and myeloperoxidase (MPO) activity were tested to evaluate the effect of YPF powder on colonic inflammation while colonic enterochromaffin (EC) cell density and serotonin 5-hydroxytryptamine (5-HT) content were investigated to identify the effect of YPF powder on colonic 5-HT availability.

Results:

The results showed that the body weight of colitis mice was markedly decreased by 10, 12, 14, and 17% at 1, 3, 5, and 7 days (P < 0.05), whereas stool consistency score (3.6 vs. 0.4, P < 0.05), histopathological score (3.6 vs. 0.3, P < 0.05), and MPO activity (2.7 vs. 0.1, P < 0.05) in colitis mice were significantly increased compared to that of the normal mice; YPF powder treatment dose-dependently increased the body weight (7–13% increase) and decreased the stool consistency score (0.4–1.4 decrease), histopathological score (0.2–0.7 decrease), and MPO activity (0.1–0.9 decrease) in colitis mice. Colonic EC cell density (70% increase) and 5-HT content (40% increase) were markedly increased in colitis mice (P < 0.05), YPF powder treatment dose-dependently reduced EC cell density (20–50% decrease), and 5-HT content (5–27% decrease) in colitis mice.

Conclusion:

The findings demonstrate that the anti-inflammatory effect of YPF powder on TNBS - induced colitis may be mediated via reducing EC cell hyperplasia and 5-HT content. The important role of YPF powder in regulating colonic EC cell number and 5-HT content may provide an alternative therapy for colonic inflammation.

Comorbidity between depression and inflammatory bowel disease explained by immune-inflammatory, oxidative, and nitrosative stress; tryptophan catabolite; and gut-brain pathways

The nature of depression has recently been reconceptualized, being conceived as the clinical expression of activated immune-inflammatory, oxidative, and nitrosative stress (IO&NS) pathways, including tryptophan catabolite (TRYCAT), autoimmune, and gut-brain pathways. IO&NS pathways are similarly integral to the pathogenesis of inflammatory bowel disease (IBD). The increased depression prevalence in IBD associates with a lower quality of life and increased morbidity in IBD, highlighting the role of depression in modulating the pathophysiology of IBD.This review covers data within such a wider conceptualization that better explains the heightened co-occurrence of IBD and depression. Common IO&NS underpinning between both disorders is evidenced by increased pro-inflammatory cytokine levels, eg, interleukin-1 (IL-1) and tumor necrosis factor-α, IL-6 trans-signalling; Th-1- and Th-17-like responses; neopterin and soluble IL-2 receptor levels; positive acute phase reactants (haptoglobin and C-reactive protein); lowered levels of negative acute phase reactants (albumin, transferrin, zinc) and anti-inflammatory cytokines (IL-10 and transforming growth factor-β); increased O&NS with damage to lipids, proteinsm and DNA; increased production of nitric oxide (NO) and inducible NO synthase; lowered plasma tryptophan but increased TRYCAT levels; autoimmune responses; and increased bacterial translocation. As such, heightened IO&NS processes in depression overlap with the biological underpinnings of IBD, potentially explaining their increased co-occurrence. This supports the perspective that there is a spectrum of IO&NS disorders that includes depression, both as an emergent comorbidity and as a contributor to IO&NS processes. Such a frame of reference has treatment implications for IBD when "comorbid" with depression.

Intestinal barrier homeostasis in inflammatory bowel disease

The single-cell thick intestinal epithelial cell (IEC) lining with its protective layer of mucus is the primary barrier protecting the organism from the harsh environment of the intestinal lumen. Today it is clear that the balancing act necessary to maintain intestinal homeostasis is dependent on the coordinated action of all cell types of the IEC, and that there are no passive bystanders to gut immunity solely acting as absorptive or regenerative cells: Mucin and antimicrobial peptides on the epithelial surface are continually being replenished by goblet and Paneth's cells. Luminal antigens are being sensed by pattern recognition receptors on the enterocytes. The enteroendocrine cells sense the environment and coordinate the intestinal function by releasing neuropeptides acting both on IEC and inflammatory cells. All this while cells are continuously and rapidly being regenerated from a limited number of stem cells close to the intestinal crypt base. This review seeks to describe the cell types and structures of the intestinal epithelial barrier supporting intestinal homeostasis, and how disturbance in these systems might relate to inflammatory bowel disease.