High fat diet causes depletion of intestinal eosinophils associated with intestinal permeability

Diet-Induced Gut Dysbiosis and Leaky Gut Syndrome

Chronic gut inflammation promotes the development of metabolic diseases such as obesity. There is growing evidence which suggests that dysbiosis in gut microbiota and metabolites disrupt the integrity of the intestinal barrier and significantly impact the level of inflammation in various tissues, including the liver and adipose tissues. Moreover, dietary sources are connected to the development of leaky gut syndrome through their interaction with the gut microbiota. This review examines the effects of these factors on intestinal microorganisms and the communication pathways between the gut-liver and gut-brain axis. The consumption of diets rich in fats and carbohydrates has been found to weaken the adherence of tight junction proteins in the gastrointestinal tract. Consequently, this allows endotoxins, such as lipopolysaccharides produced by detrimental bacteria, to permeate through portal veins, leading to metabolic endotoxemia and alterations in the gut microbiome composition with reduced production of metabolites, such as short-chain fatty acids. However, the precise correlation between gut microbiota and alternative sweeteners remains uncertain, necessitating further investigation. This study highlights the significance of exploring the impact of diet on gut microbiota and the underlying mechanisms in the gut-liver and gut-brain axis. Nevertheless, limited research on the gut-liver axis poses challenges in comprehending the intricate connections between diet and the gut-brain axis. This underscores the need for comprehensive studies to elucidate the intricate gut-brain mechanisms underlying intestinal health and microbiota.

Whey protein supplementation reduced the liver damage scores of rats fed with a high fat-high fructose diet

Different functional foods with bioactive nutrients are being explored for the management of NAFLD. Whey proteins are rich in bioactive peptides and are suggested to show antioxidant and anti-inflammatory effects. We aim to test the hypothesis that the whey protein supplementation following a high fat-high fructose (HFHF) diet would protect against liver damage, inflammation, endotoxemia and steatosis in male Wistar rats. 36 rats were randomized into four groups for 8 weeks as the HFHF diet group, HFHF diet and whey protein isolate (WPI-200mg/kg/day) group (HFHF+WPI), control (C) group, and C+WPI (200mg/kg/day) group. Rats fed with a HFHF diet had higher final body weight compared to C and C+WPI groups (p = 0.002). Thus, WPI showed no significant effects for the body weight of rats with a HFHF diet. On the other hand, the HFHF+WPI group had significantly lower abdominal circumference when compared with the HFHF group (p<0,001). Higher serum CRP levels were observed in the groups with a HFHF diet (p<0,001) and WPI supplementation showed no effects on CRP levels. Whey protein supplementation resulted with lower total liver damage score in HFHF+WPI group compared with the HFHF diet group (p<0,001). Conversely, higher liver damage scores were observed with the C+WPI group compared to C group (p<0,001). HFHF diet resulted with higher expression of TLR-4 in the liver meanwhile WPI supplementation showed no effects on liver TLR-4 expression. We observed higher colon Occludin expression in HFHF+WPI and C+WPI groups compared with HFHF and C groups (p<0,001). Our results showed that, whey protein supplementation might help improve liver damage associated with a high fat-high fructose diet and increase the expression of Occludin in the small intestine and colon.

The role of extracellular vesicles in non-alcoholic steatohepatitis: Emerging mechanisms, potential therapeutics and biomarkers

Non-alcoholic steatohepatitis (NASH), an emerging global healthcare problem, has become the leading cause of liver transplantation in recent decades. No effective therapies in the clinic have been proven due to the incomplete understanding of the pathogenesis of NASH, and further studies are expected to continue to delve into the mechanisms of NASH. Extracellular vesicles (EVs), which are small lipid membrane vesicles carrying proteins, microRNAs and other molecules, have been identified to play a vital role in cell-to-cell communication and are involved in the development and progression of various diseases. In recent years, there has been increasing interest in the role of EVs in NASH. Many studies have revealed that EVs mediate important pathological processes in NASH, and the role of EVs in NASH is distinct and variable depending on their origin cells and target cells. This review outlines the emerging mechanisms of EVs in the development of NASH and the preclinical evidence related to stem cell-derived EVs as a potential therapeutic strategy for NASH. Moreover, possible strategies involving EVs as clinical diagnostic, staging and prognostic biomarkers for NASH are summarized.

Restoring SRSF3 in Kupffer cells attenuates obesity-related insulin resistance

BACKGROUND AND AIMS

In obesity, depletion of KCs expressing CRIg (complement receptor of the Ig superfamily) leads to microbial DNA accumulation, which subsequently triggers tissue inflammation and insulin resistance. However, the mechanism underlying obesity-mediated changes in KC complement immune functions is largely unknown.

APPROACH AND RESULTS

Using KC-specific deactivated Cas9 transgenic mice treated with guide RNA, we assessed the effects of restoring CRIg or the serine/arginine-rich splicing factor 3 (SRSF3) abundance on KC functions and metabolic phenotypes in obese mice. The impacts of weight loss on KC responses were evaluated in a diet switch mouse model. The role of SRSF3 in regulating KC functions was also evaluated using KC-specific SRSF3 knockout mice. Here, we report that overexpression of CRIg in KCs of obese mice protects against bacterial DNA accumulation in metabolic tissues. Mechanistically, SRSF3 regulates CRIg expression, which is essential for maintaining the CRIg+ KC population. During obesity, SRSF3 expression decreases, but it is restored with weight loss through a diet switch, normalizing CRIg+ KCs. KC SRSF3 is also repressed in obese human livers. Lack of SRSF3 in KCs in lean and obese mice decreases their CRIg+ population, impairing metabolic parameters. During the diet switch, the benefits of weight loss are compromised due to SRSF3 deficiency. Conversely, SRSF3 overexpression in obese mice preserves CRIg+ KCs and improves metabolic responses.

CONCLUSIONS

Restoring SRSF3 abundance in KCs offers a strategy against obesity-associated tissue inflammation and insulin resistance by preventing bacterial DNA accumulation.

Nutrient-derived signals regulate eosinophil adaptation to the small intestine

Eosinophils are well recognized as effector cells of type 2 immunity, yet they also accumulate in many tissues under homeostatic conditions. However, the processes that govern homeostatic eosinophil accumulation and tissue-specific adaptation, and their functional significance, remain poorly defined. Here, we investigated how eosinophils adapt to the small intestine (SI) microenvironment and the local signals that regulate this process. We observed that eosinophils gradually migrate along the crypt-villus axis, giving rise to a villus-resident subpopulation with a distinct transcriptional signature. Retinoic acid signaling was specifically required for maintenance of this subpopulation, while IL-5 was largely dispensable outside of its canonical role in eosinophil production. Surprisingly, we found that a high-protein diet suppressed the accumulation of villus-resident eosinophils. Purified amino acids were sufficient for this effect, which was a consequence of accelerated eosinophil turnover within the tissue microenvironment and was not due to altered development in the bone marrow. Our study provides insight into the process of eosinophil adaptation to the SI, highlighting its reliance on nutrient-derived signals.

S-amlodipine induces liver inflammation and dysfunction through the alteration of intestinal microbiome in a rat model

S-amlodipine, a commonly prescribed antihypertensive agent, is widely used in clinical settings to treat hypertension. However, the potential adverse effects of long-term S-amlodipine treatment on the liver remain uncertain, given the cautionary recommendations from clinicians regarding its administration in individuals with impaired liver function. To address this, we conducted a study using an eight-week-old male rat model and administered a daily dose of 0.6 ~ 5 mg/kg of S-amlodipine for 7 weeks. Our findings demonstrated that 1.2 ~ 5 mg/kg of S-amlodipine treatment induced liver inflammation and associated dysfunction in rats, further in vitro experiments revealed that the observed liver inflammation and dysfunction were not attributable to direct effects of S-amlodipine on the liver. Metagenome sequencing analysis revealed that S-amlodipine treatment led to alterations in the gut microbiome of rats, with the bloom of E. coli (4.5 ~ 6.6-fold increase) and a decrease in A. muciniphila (1,613.4 ~ 2,000-fold decrease) and B. uniformis (20.6 ~ 202.7-fold decrease), subsequently causing an increase in the gut bacterial lipopolysaccharide (LPS) content (1.4 ~ 1.5-fold increase in feces). S-amlodipine treatment also induced damage to the intestinal barrier and increased intestinal permeability, as confirmed by elevated levels of fecal albumin; furthermore, the flux of gut bacterial LPS into the bloodstream through the portal vein resulted in an increase in serum LPS content (3.3 ~ 4-fold increase). LPS induces liver inflammation and subsequent dysfunction in rats by activating the TLR4 pathway. This study is the first to show that S-amlodipine induces liver inflammation and dysfunction by perturbing the rat gut microbiome. These results indicate the adverse effects of S-amlodipine on the liver and provide a rich understanding of the safety of long-term S-amlodipine administration.

Ketogenic Diet High in Saturated Fat Promotes Colonic Claudin Expression without Changes in Intestinal Permeability to Iohexol in Healthy Mice

Ketogenic diets (KDs) have been studied in preclinical models of intestinal diseases. However, little is known of how the fat source of these diets influences the intestinal barrier. Herein, we studied the impact of four-week feeding with KD high either in saturated fatty acids (SFA-KD) or polyunsaturated linoleic acid (LA-KD) on paracellular permeability of the intestine to iohexol in healthy male C57BL/6J mice. We investigated jejunal and colonic tight junction protein expression, histological changes, and inflammatory markers (Il1b, Il6, Tnf, and Lcn2), as well as the activity and expression of intestinal alkaline phosphatase (IAP) in feces and jejunal tissue, respectively, and plasma lipopolysaccharide. KDs did not change intestinal permeability to iohexol after two or twenty-six days of feeding regardless of fat quality. SFA-KD, but not LA-KD, upregulated the colonic expression of tight junction proteins claudin-1 and -4, as well as the activity of IAP. Both KDs resulted in increased epithelial vacuolation in jejunum, and this was pronounced in SFA-KD. Jejunal Il1β expression was lower and colonic Il6 expression higher in LA-KD compared to SFA-KD. In colon, Tnf mRNA was increased in LA-KD when compared to controls. Overall, the results suggest that KDs do not influence intestinal permeability to iohexol but elicit changes in colonic tight junction proteins and inflammatory markers in both jejunum and colon. Future research will show whether these changes become of importance upon proinflammatory insults.

Gut microbiota induces weight gain and inflammation in the gut and adipose tissue independent of manipulations in diet, genetics, and immune development

Obesity and the metabolic syndrome are complex disorders resulting from multiple factors including genetics, diet, activity, inflammation, and gut microbes. Animal studies have identified roles for each of these, however the contribution(s) specifically attributed to the gut microbiota remain unclear, as studies have used combinations of genetically altered mice, high fat diet, and/or colonization of germ-free mice, which have an underdeveloped immune system. We investigated the role(s) of the gut microbiota driving obesity and inflammation independent of manipulations in diet and genetics in mice with fully developed immune systems. We demonstrate that the human obese gut microbiota alone was sufficient to drive weight gain, systemic, adipose tissue, and intestinal inflammation, but did not promote intestinal barrier leak. The obese microbiota induced gene expression promoting caloric uptake/harvest but was less effective at inducing genes associated with mucosal immune responses. Thus, the obese gut microbiota is sufficient to induce weight gain and inflammation.

The intestinal immune system and gut barrier function in obesity and ageing

Obesity and ageing predispose to numerous, yet overlapping chronic diseases. For example, metabolic abnormalities, including insulin resistance (IR) and type 2 diabetes (T2D) are important causes of morbidity and mortality. Low-grade chronic inflammation of tissues, such as the liver, visceral adipose tissue and neurological tissues, is considered a significant contributor to these chronic diseases. Thus, it is becoming increasingly important to understand what drives this inflammation in affected tissues. Recent evidence, especially in the context of obesity, suggests that the intestine plays an important role as the gatekeeper of inflammatory stimuli that ultimately fuels low-grade chronic tissue inflammation. In addition to metabolic diseases, abnormalities in the intestinal mucosal barrier have been linked to a range of other chronic inflammatory conditions, such as neurodegeneration and ageing. The flow of inflammatory stimuli from the gut is in part controlled by local immunological inputs impacting the intestinal barrier. Here, we will review the impact of obesity and ageing on the intestinal immune system and its downstream consequences on gut barrier function, which is strongly implicated in the pathogenesis of obesity and age-related diseases. In particular, we will discuss the effects of age-related intestinal dysfunction on neurodegenerative diseases.

Modulatory Effects of Co-Fermented Pu-erh Tea with Aqueous Corn Silk Extract on Gut Microbes and Fecal Metabolites in Mice Fed High-Fat Diet

Pu-erh tea is recognized for its weight loss effects, but its potential association with gut microbiota and metabolites remains unclear. This research explored the alterations in gut flora and metabolite composition upon treatment with a co-fermented Pu-erh tea with an aqueous corn silk extract (CPC) in obese mice by employing integrated 16S ribosomal RNA gene sequencing and untargeted metabolomics processes. For 8 weeks, mice were fed control, high-fat, and high-fat diets which included a 46 mg/mL CPC extract. The CPC extract the alleviated high-fat diet (HFD), it stimulated systemic chronic inflammation, and it reduced the body weight, daily energy consumption, and adipose tissue weight of the mice. It also modified the gut microbiota composition and modulated the Lactobacillus, Bifidobacterium, Allobaculum, Turicibacter, and Rikenella genera. Fecal metabolomics analysis revealed that the CPC extract influenced the caffeine, cysteine, methionine, tryptophan, biotin metabolism pathways, primary bile acid, and steroid biosynthesis. This research revealed that the CPC extract could inhibit HFD-stimulated abnormal weight gain and adipose tissue accumulation in mice, and modulate mice gut microbiota composition and multiple metabolic pathways.

Are gut dysbiosis, barrier disruption, and endotoxemia related to adipose tissue dysfunction in metabolic disorders? Overview of the mechanisms involved

Recently, compelling evidence points to dysbiosis and disruption of the epithelial intestinal barrier as major players in the pathophysiology of metabolic disorders, such as obesity. Upon the intestinal barrier disruption, components from bacterial metabolism and bacteria itself can reach peripheral tissues through circulation. This has been associated with the low-grade inflammation that characterizes obesity and other metabolic diseases. While circulating bacterial DNA has been postulated as a common feature of obesity and even type 2 diabetes, almost no focus has been given to the existence and effects of bacteria in peripheral tissues, namely the adipose tissue. As a symbiont population, it is expected that gut microbiota modulate the immunometabolism of the host, thus influencing energy balance mechanisms and inflammation. Gut inflammatory signals cause direct deleterious inflammatory responses in adipose tissue and may also affect key gut neuroendocrine mechanisms governing nutrient sensing and energy balance, like incretins and ghrelin, which play a role in the gut-brain-adipose tissue axis. Thus, it is of major importance to disclose how gut microbiota and derived signals modulate neuroendocrine and inflammatory pathways, which contribute to the dysfunction of adipose tissue and to the metabolic sequelae of obesity and related disorders. This review summarizes the current knowledge regarding these topics and identifies new perspectives in this field of research, highlighting new pathways toward the reduction of the inflammatory burden of metabolic diseases.

Gut lumen-leaked microbial DNA causes myocardial inflammation and impairs cardiac contractility in ageing mouse heart

Emerging evidence indicates the critical roles of microbiota in mediating host cardiac functions in ageing, however, the mechanisms underlying the communications between microbiota and cardiac cells during the ageing process have not been fully elucidated. Bacterial DNA was enriched in the cardiomyocytes of both ageing humans and mice. Antibiotic treatment remarkably reduced bacterial DNA abundance in ageing mice. Gut microbial DNA containing extracellular vesicles (mEVs) were readily leaked into the bloodstream and infiltrated into cardiomyocytes in ageing mice, causing cardiac microbial DNA enrichment. Vsig4⁺ macrophages efficiently block the spread of gut mEVs whereas Vsig4⁺ cell population was greatly decreased in ageing mice. Gut mEV treatment resulted in cardiac inflammation and a reduction in cardiac contractility in young Vsig4^-/- mice. Microbial DNA depletion attenuated the pathogenic effects of gut mEVs. cGAS/STING signaling is critical for the effects of microbial DNA. Restoring Vsig4⁺ macrophage population in ageing WT mice reduced cardiac microbial DNA abundance and inflammation and improved heart contractility.

Emerging functions of tissue-resident eosinophils

Eosinophils are typically considered tissue-damaging effector cells in type 2 immune-related diseases. However, they are also increasingly recognized as important modulators of various homeostatic processes, suggesting they retain the ability to adapt their function to different tissue contexts. In this review, we discuss recent progress in our understanding of eosinophil activities within tissues, with particular emphasis on the gastrointestinal tract, where a large population of these cells resides under non-inflammatory conditions. We further examine evidence of their transcriptional and functional heterogeneity and highlight environmental signals emerging as key regulators of their activities, beyond classical type 2 cytokines.

Capsazepine-Induced Altered Colonic Mucosal Health Limits Isomalto-oligosaccharide Action in High-Fat Diet-Fed C57BL/6J Mice

The present study sought to understand the effects of a combination of altered colonic mucosal health (intrarectal capsazepine administration) and high-fat diet (HFD) administration in mice. Furthermore, we also studied whether this combination prevents protective actions of dietary prebiotic, isomaltooligosaccharides. We studied the alterations in intestinal permeability, histological and transcriptional changes, short-chain fatty acid (SCFA) concentrations, and gut microbial abundance. Capsazepine (CPZ) was administered rectally twice a day along with HFD feeding. Following confirmation of CPZ action (loss of TRPA1 and TRPV1-associated nocifensive behavior), the intrarectal dose of CPZ was reduced to once in 2 days up to 8 weeks. Simultaneous intrarectal administration of CPZ exacerbated the HFD (8 weeks feeding)-induced damage to mucosal lining, intestinal permeability, tight junction protein expression, SCFA levels, and gut bacterial abundances. This higher degree of mucosal damage and pathological alteration in colonic mucosa prevented the previously reported protective actions of isomaltooligosaccharides as a prebiotic in HFD-fed mice. Overall, we present evidence that colonic precondition (gut permeability and mucosal lining) is an important factor in determination of HFD-induced changes in the colon, and success of diet-associated interventions (dietary fibers, pre/probiotics, etc.) is dependent on it.

Diet-induced obesity alters intestinal monocyte-derived and tissue-resident macrophages and increases intestinal permeability in female mice independent of tumor necrosis factor

Macrophages are essential for homeostatic maintenance of the anti-inflammatory and tolerogenic intestinal environment, yet monocyte-derived macrophages can promote local inflammation. Proinflammatory macrophage accumulation within the intestines may contribute to the development of systemic chronic inflammation and immunometabolic dysfunction in obesity. Using a model of high-fat diet-induced obesity in C57BL/6J female mice, we assessed intestinal paracellular permeability by in vivo and ex vivo assays and quantitated intestinal macrophages in ileum and colon tissues by multicolor flow cytometry after short (6 wk), intermediate (12 wk), and prolonged (18 wk) diet allocation. We characterized monocyte-derived CD4^-TIM4^- and CD4⁺TIM4^- macrophages, as well as tissue-resident CD4⁺TIM4⁺ macrophages. Diet-induced obesity had tissue- and time-dependent effects on intestinal permeability, as well as monocyte and macrophage numbers, surface marker phenotype, and intracellular production of the cytokines IL-10 and tumor necrosis factor (TNF). We found that obese mice had increased paracellular permeability, in particular within the ileum, but this did not elicit recruitment of monocytes nor a local proinflammatory response by monocyte-derived or tissue-resident macrophages in either the ileum or colon. Proliferation of monocyte-derived and tissue-resident macrophages was also unchanged. Wild-type and TNF^-/- littermate mice had similar intestinal permeability and macrophage population characteristics in response to diet-induced obesity. These data are unique from reported effects of diet-induced obesity on macrophages in metabolic tissues, as well as outcomes of acute inflammation within the intestines. These experiments also collectively indicate that TNF does not mediate effects of diet-induced obesity on paracellular permeability or intestinal monocyte-derived and tissue-resident intestinal macrophages in young female mice.NEW & NOTEWORTHY We found that diet-induced obesity in female mice has tissue- and time-dependent effects on intestinal paracellular permeability as well as monocyte-derived and tissue-resident macrophage numbers, surface marker phenotype, and intracellular production of the cytokines IL-10 and TNF. These changes were not mediated by TNF.

Lipopolysaccharide-induced hypothalamic inflammation in cancer cachexia-anorexia is amplified by tumour-derived prostaglandin E2

BACKGROUND

Cachexia-anorexia syndrome is a complex metabolic condition characterized by skeletal muscle wasting, reduced food intake and prominent involvement of systemic and central inflammation. Here, the gut barrier function was investigated in pancreatic cancer-induced cachexia mouse models by relating intestinal permeability to the degree of cachexia. We further investigated the involvement of the gut-brain axis and the crosstalk between tumour, gut and hypothalamus in vitro.

METHODS

Two distinct mouse models of pancreatic cancer cachexia (KPC and 4662) were used. Intestinal inflammation and permeability were assessed through fluorescein isothiocyanate dextran (FITC-dextran) and lipopolysaccharide (LPS), and hypothalamic and systemic inflammation through mRNA expression and plasma cytokines, respectively. To simulate the tumour-gut-brain crosstalk, hypothalamic (HypoE-N46) cells were incubated with cachexia-inducing tumour secretomes and LPS. A synthetic mimic of C26 secretome was produced based on its secreted inflammatory mediators. Each component of the mimic was systematically omitted to narrow down the key mediator(s) with an amplifying inflammation. To substantiate its contribution, cyclooxygenase-2 (COX-2) inhibitor was used.

RESULTS

In vivo experiments showed FITC-dextran was enhanced in the KPC group (362.3 vs. sham 111.4 ng/mL, P < 0.001). LPS was increased to 140.9 ng/mL in the KPC group, compared with sham and 4662 groups (115.8 and 115.8 ng/mL, P < 0.05). Hypothalamic inflammatory gene expression of Ccl2 was up-regulated in the KPC group (6.3 vs. sham 1, P < 0.0001, 4662 1.3, P < 0.001), which significantly correlated with LPS concentration (r = 0.4948, P = 0.0226). These data suggest that intestinal permeability is positively related to the cachexic degree. Prostaglandin E2 (PGE2) was confirmed to be present in the plasma and PGE2 concentration (log10) in the KPC group was much higher than in 4662 group (1.85 and 0.56 ng/mL, P < 0.001), indicating a role for PGE2 in pancreatic cancer-induced cachexia. Parallel to in vivo findings, in vitro experiments revealed that the cachexia-inducing tumour secretomes (C26, LLC, KPC and 4662) amplified LPS-induced hypothalamic IL-6 secretion (419%, 321%, 294%, 160%). COX-2 inhibitor to the tumour cells reduced PGE2 content (from 10⁵ to 10²  pg/mL) in the secretomes and eliminated the amplified hypothalamic IL-6 production. Moreover, results could be reproduced by addition of PGE2 alone, indicating that the increased hypothalamic inflammation is directly related to the PGE2 from tumour.

CONCLUSIONS

PGE2 secreted by the tumour may play a role in amplifying the effects of bacteria-derived LPS on the inflammatory hypothalamic response. The cachexia-inducing potential of tumour mice models parallels the loss of intestinal barrier function. Tumour-derived PGE2 might play a key role in cancer-related cachexia-anorexia syndrome via tumour-gut-brain crosstalk.

Modulation of gut microbiota and fecal metabolites by corn silk among high-fat diet-induced hypercholesterolemia mice

Corn silk (CS) is known to reduce cholesterol levels, but its underlying mechanisms remain elusive concerning the gut microbiota and metabolites. The aim of our work was to explore how altered gut microbiota composition and metabolite profile are influenced by CS intervention in mice using integrated 16S ribosomal RNA (rRNA) sequencing and an untargeted metabolomics methodology. The C57BL/6J mice were fed a normal control diet, a high-fat diet (HFD), and HFD supplemented with the aqueous extract of CS (80 mg/mL) for 8 weeks. HFD-induced chronic inflammation damage is alleviated by CS extract intervention and also resulted in a reduction in body weight, daily energy intake as well as serum and hepatic total cholesterol (TC) levels. In addition, CS extract altered gut microbial composition and regulated specific genera viz. Allobaculum, Turicibacter, Romboutsia, Streptococcus, Sporobacter, Christensenella, ClostridiumXVIII, and Rikenella. Using Spearman’s correlation analysis, we determined that Turicibacter and Rikenella were negatively correlated with hypercholesterolemia-related parameters. Fecal metabolomics analysis revealed that CS extract influences multiple metabolic pathways like histidine metabolism-related metabolites (urocanic acid, methylimidazole acetaldehyde, and methiodimethylimidazoleacetic acid), sphingolipid metabolism-related metabolites (sphinganine, 3-dehydrosphinganine, sphingosine), and some bile acids biosynthesis-related metabolites including chenodeoxycholic acid (CDCA), lithocholic acid (LCA), ursodeoxycholic acid (UDCA), and glycoursodeoxycholic acid (GUDCA). As a whole, the present study indicates that the modifications in the gut microbiota and subsequent host bile acid metabolism may be a potential mechanism for the antihypercholesterolemic effects of CS extract.

Gut immune cells—A novel therapeutical target for cardiovascular disease?

Despite scientific and clinical advances during the last 50 years cardiovascular disease continues to be the main cause of death worldwide. Especially patients with diabetes display a massive increased cardiovascular risk compared to patients without diabetes. Over the last two decades we have learned that cardiometabolic and cardiovascular diseases are driven by inflammation. Despite the fact that the gastrointestinal tract is one of the largest leukocyte reservoirs of our bodies, the relevance of gut immune cells for cardiovascular disease is largely unknown. First experimental evidence suggests an important relevance of immune cells in the intestinal tract for the development of metabolic and cardiovascular disease in mice. Mice specifically lacking gut immune cells are protected against obesity, diabetes, hypertension and atherosclerosis. Importantly antibody mediated inhibition of leukocyte homing into the gut showed similar protective metabolic and cardiovascular effects. Targeting gut immune cells might open novel therapeutic approaches for the treatment of cardiometabolic and cardiovascular diseases.

Evidence for Constitutive Microbiota-Dependent Short-Term Control of Food Intake in Mice: Is There a Link with Inflammation, Oxidative Stress, Endotoxemia, and GLP-1?

Aims: Although prebiotics, probiotics, and fecal transplantation can alter the sensation of hunger and/or feeding behavior, the role of the constitutive gut microbiota in the short-term regulation of food intake during normal physiology is still unclear. Results: An antibiotic-induced microbiota depletion study was designed to compare feeding behavior in conventional and microbiota-depleted mice. Tissues were sampled to characterize the time profile of microbiota-derived signals in mice during consumption of either standard or high-fat food for 1 h. Pharmacological and genetic tools were used to evaluate the contribution of postprandial endotoxemia and inflammatory responses in the short-term regulation of food intake. We observed constitutive microbial and macronutrient-dependent control of food intake at the time scale of a meal; that is, within 1 h of food introduction. Specifically, microbiota depletion increased food intake, and the microbiota-derived anorectic effect became significant during the consumption of high-fat but not standard food. This anorectic effect correlated with a specific postprandial microbial metabolic signature, and did not require postprandial endotoxemia or an NOD-, LRR-, and Pyrin domain-containing protein 3-inflammasome-mediated inflammatory response. Innovation and Conclusion: These findings show that the gut microbiota controls host appetite at the time scale of a meal under normal physiology. Interestingly, a microbiota-derived anorectic effect develops specifically with a high-fat meal, indicating that gut microbiota activity is involved in the satietogenic properties of foods. Antioxid. Redox Signal. 37, 349-369.

Protective Effects of High-Fat Diet against Murine Colitis in Association with Leptin Signaling and Gut Microbiome

Inflammatory bowel disease (IBD) is characterized by chronic intestinal-tract inflammation with dysregulated immune responses, which are partly attributable to dysbiosis. Given that diet plays a critical role in IBD pathogenesis and progression, we elucidated the effects of a high-fat diet (HFD) feeding on IBD development in relation to immune dysfunction and the gut microbiota. Five-week-old male C57BL/6J mice were fed either a normal diet (ND) or HFD for 14 weeks. The animals were further divided into ND, ND+ dextran sulfate sodium (DSS), HFD, and HFD+DSS treatment groups. The HFD+DSS mice exhibited lower body weight loss, lower disease activity index, longer colon length, and increased tight-junction protein expression and goblet-cell proportions compared with the ND+DSS mice. The T helper (h)1 and Th17 cell populations and pro-inflammatory cytokines involved in colitis pathogenesis were significantly more reduced in the HFD+DSS mice than in the ND+DSS mice. The HFD+DSS mice showed significantly increased serum leptin concentrations, colonic leptin receptor expression, enhanced anti-apoptotic AKT expression, and reduced pro-apoptotic MAPK and Bax expression compared with the ND+DSS mice, suggesting the involvement of the leptin-mediated pathway in intestinal epithelial cell apoptosis. The alterations in the gut-microbiota composition in the HFD+DSS group were the opposite of those in the ND+DSS group and rather similar to those of the ND group, indicating that the protective effects of HFD feeding against DSS-induced colitis are associated with changes in gut-microbiota composition. Overall, HFD feeding ameliorates DSS-induced colitis and colonic mucosal damage by reinforcing colonic barrier function and regulating immune responses in association with changes in gut-microbiota composition.

Secondary Bile Acids and Tumorigenesis in Colorectal Cancer

Colorectal cancer (CRC) is one of the most common and deadly cancers in the world and is a typical inflammatory tumor. In recent years, the incidence of CRC has been increasing year by year. There is evidence that the intake of high-fat diet and overweight are associated with the incidence of CRC, among which bile acids play a key role in the pathogenesis of the disease. Studies on the relationship between bile acid metabolism and the occurrence of CRC have gradually become a hot topic, improving the understanding of metabolic factors in the etiology of colorectal cancer. Meanwhile, intestinal flora also plays an important role in the occurrence and development of CRC In this review, the classification of bile acids and their role in promoting the occurrence of CRC are discussed, and we highlights how a high-fat diet affects bile acid metabolism and destroys the integrity of the intestinal barrier and the effects of gut bacteria.

Eosinophils in the Gastrointestinal Tract: Key Contributors to Neuro-Immune Crosstalk and Potential Implications in Disorders of Brain-Gut Interaction

Eosinophils are innate immune granulocytes actively involved in defensive responses and in local and systemic inflammatory processes. Beyond these effector roles, eosinophils are fundamental to maintaining homeostasis in the tissues they reside. Gastrointestinal eosinophils modulate barrier function and mucosal immunity and promote tissue development through their direct communication with almost every cellular component. This is possible thanks to the variety of receptors they express and the bioactive molecules they store and release, including cytotoxic proteins, cytokines, growth factors, and neuropeptides and neurotrophines. A growing body of evidence points to the eosinophil as a key neuro-immune player in the regulation of gastrointestinal function, with potential implications in pathophysiological processes. Eosinophil–neuron interactions are facilitated by chemotaxis and adhesion molecules, and the mediators released may have excitatory or inhibitory effects on each cell type, with physiological consequences dependent on the type of innervation involved. Of special interest are the disorders of the brain–gut interaction (DBGIs), mainly functional dyspepsia (FD) and irritable bowel syndrome (IBS), in which mucosal eosinophilia and eosinophil activation have been identified. In this review, we summarize the main roles of gastrointestinal eosinophils in supporting gut homeostasis and the evidence available on eosinophil–neuron interactions to bring new insights that support the fundamental role of this neuro-immune crosstalk in maintaining gut health and contributing to the pathophysiology of DBGIs.

Microbial DNA enrichment promotes liver steatosis and fibrosis in the course of non-alcoholic steatohepatitis

AIM

Low-grade inflammation is the hallmark of non-alcoholic fatty liver diseases (NAFLD) and non-alcoholic steatohepatitis (NASH). The leakage of microbiota-derived products can contribute to liver inflammation during NAFLD/NASH development. Here, we assessed the roles of gut microbial DNA-containing extracellular vesicles (mEVs) in regulating liver cellular abnormalities in the course of NAFLD/NASH.

METHODS

We performed studies with Vsig4^-/- , C3^-/- , cGAS^-/- , and their wild-type littermate mice. Vsig4+ macrophage population and bacterial DNA abundance were examined in both mouse and human liver by either flow cytometric or immunohistochemistry analysis. Gut mEVs were adoptively transferred into Vsig4^-/- , C3^-/- , cGAS^-/- , or littermate WT mice, and hepatocyte inflammation and HSC fibrogenic activation were measured in these mice.

RESULTS

Non-alcoholic fatty liver diseases and non-alcoholic steatohepatitis development was concomitant with a diminished liver Vsig4+ macrophage population and a marked bacterial DNA enrichment in both hepatocytes and HSCs. In the absence of Vsig4+ macrophages, gut mEVs translocation led to microbial DNA accumulation in hepatocytes and HSCs, resulting elevated hepatocyte inflammation and HSC fibrogenic activation. In contrast, in lean WT mice, Vsig4+ macrophages remove gut mEVs from bloodstream through a C3-dependent opsonization mechanism and prevent the infiltration of gut mEVs into hepatic cells. Additionally, Vsig4^-/- mice more quickly developed significant liver steatosis and fibrosis than WT mice after Western diet feeding. In vitro treatment with NASH mEVs triggered hepatocyte inflammation and HSC fibrogenic activation. Microbial DNAs are key cargo for the effects of gut mEVs by activating cGAS/STING.

CONCLUSION

Accumulation of microbial DNAs fuels the development of NAFLD/NASH-associated liver abnormalities.

Targeting colonic macrophages improves glycemic control in high-fat diet-induced obesity

The obesity epidemic continues to worsen worldwide. However, the mechanisms initiating glucose dysregulation in obesity remain poorly understood. We assessed the role that colonic macrophage subpopulations play in glucose homeostasis in mice fed a high-fat diet (HFD). Concurrent with glucose intolerance, pro-inflammatory/monocyte-derived colonic macrophages increased in mice fed a HFD. A link between macrophage numbers and glycemia was established by pharmacological dose-dependent ablation of macrophages. In particular, colon-specific macrophage depletion by intrarectal clodronate liposomes improved glucose tolerance, insulin sensitivity, and insulin secretion capacity. Colonic macrophage activation upon HFD was characterized by an interferon response and a change in mitochondrial metabolism, which converged in mTOR as a common regulator. Colon-specific mTOR inhibition reduced pro-inflammatory macrophages and ameliorated insulin secretion capacity, similar to colon-specific macrophage depletion, but did not affect insulin sensitivity. Thus, pharmacological targeting of colonic macrophages could become a potential therapy in obesity to improve glycemic control.

Expansion of pro-inflammatory macrophages in the colon occurs early after high-fat diet initiation, prior to macrophage accumulation in the adipose tissue, in a microbiome-dependent fashion. Macrophage depletion systemically and/or exclusively in the colon improves glucose metabolism.

The aryl hydrocarbon receptor contributes to tissue adaptation of intestinal eosinophils in mice

Eosinophils are potent sources of inflammatory and toxic mediators, yet they reside in large numbers in the healthy intestine without causing tissue damage. We show here that intestinal eosinophils were specifically adapted to their environment and underwent substantial transcriptomic changes. Intestinal eosinophils upregulated genes relating to the immune response, cell-cell communication, extracellular matrix remodeling, and the aryl hydrocarbon receptor (AHR), a ligand-activated transcription factor with broad functions in intestinal homeostasis. Eosinophils from AHR-deficient mice failed to fully express the intestinal gene expression program, including extracellular matrix organization and cell junction pathways. AHR-deficient eosinophils were functionally impaired in the adhesion to and degradation of extracellular matrix, were more prone to degranulation, and had an extended life span. Lack of AHR in eosinophils had wider effects on the intestinal immune system, affecting the T cell compartment in nave and helminth-infected mice. Our study demonstrates that the response to environmental triggers via AHR partially shapes tissue adaptation of eosinophils in the small intestine.

Microbial DNA Enrichment Promotes Adrenomedullary Inflammation, Catecholamine Secretion, and Hypertension in Obese Mice

Background

Obesity is an established risk factor for hypertension. Although obesity‐induced gut barrier breach leads to the leakage of various microbiota‐derived products into host circulation and distal organs, the roles of microbiota in mediating the development of obesity‐associated adrenomedullary disorders and hypertension have not been elucidated. We seek to explore the impacts of microbial DNA enrichment on inducing obesity‐related adrenomedullary abnormalities and hypertension.

Methods and Results

Obesity was accompanied by remarkable bacterial DNA accumulation and elevated inflammation in the adrenal glands. Gut microbial DNA containing extracellular vesicles (mEVs) were readily leaked into the bloodstream and infiltrated into the adrenal glands in obese mice, causing microbial DNA enrichment. In lean wild‐type mice, adrenal macrophages expressed CRIg (complement receptor of the immunoglobulin superfamily) that efficiently blocks the infiltration of gut mEVs. In contrast, the adrenal CRIg+ cell population was greatly decreased in obese mice. In lean CRIg^−/− or C3^−/− (complement component 3) mice intravenously injected with gut mEVs, adrenal microbial DNA accumulation elevated adrenal inflammation and norepinephrine secretion, concomitant with hypertension. In addition, microbial DNA promoted inflammatory responses and norepinephrine production in rat pheochromocytoma PC12 cells treated with gut mEVs. Depletion of microbial DNA cargo markedly blunted the effects of gut mEVs. We also validated that activation of cGAS (cyclic GMP‐AMP synthase)/STING (cyclic GMP–AMP receptor stimulator of interferon genes) signaling is required for the ability of microbial DNA to trigger adrenomedullary dysfunctions in both in vivo and in vitro experiments. Restoring CRIg+ cells in obese mice decreased microbial DNA abundance, inflammation, and hypertension.

Conclusions

The leakage of gut mEVs leads to adrenal enrichment of microbial DNA that are pathogenic to induce obesity‐associated adrenomedullary abnormalities and hypertension. Recovering the CRIg+ macrophage population attenuates obesity‐induced adrenomedullary disorders.

Gut Barrier Dysfunction and Type 2 Immunity: Implications for Compulsive Behavior

To date, much of the focus of gut-brain axis research has been on gut microbiota regulation of anxiety and stress-related behaviors. Much less attention has been directed to potential connections between gut microbiota and compulsive behavior. Here, we discuss a potential link between gut barrier dysfunction and compulsive behavior that is mediated through "type 2" rather than "type 1" inflammation. We examine connections between compulsive behavior and type 2 inflammation in Tourette syndrome, obsessive-compulsive disorder, autism, addiction, and post-traumatic stress disorder. Next, we discuss potential connections between gut barrier dysfunction, type 2 inflammation, and compulsive behavior. We posit a potential mechanism whereby gut barrier dysfunction-associated type 2 inflammation may drive compulsive behavior through histamine regulation of dopamine neurotransmission. Finally, we discuss the possibility of exploiting the greater accessibility of the gut relative to the brain in identifying targets to treat compulsive behavior disorders.

Accumulation of microbial DNAs promotes to islet inflammation and β cell abnormalities in obesity in mice

Various microbial products leaked from gut lumen exacerbate tissue inflammation and metabolic disorders in obesity. Vsig4+ macrophages are key players preventing infiltration of bacteria and their products into host tissues. However, roles of islet Vsig4+ macrophages in the communication between microbiota and β cells in pathogenesis of obesity-associated islet abnormalities are unknown. Here, we find that bacterial DNAs are enriched in β cells of individuals with obesity. Intestinal microbial DNA-containing extracellular vesicles (mEVs) readily pass through obese gut barrier and deliver microbial DNAs into β cells, resulting in elevated inflammation and impaired insulin secretion by triggering cGAS/STING activation. Vsig4+ macrophages prevent mEV infiltration into β cells through a C3-dependent opsonization, whereas loss of Vsig4 leads to microbial DNA enrichment in β cells after mEV treatment. Removal of microbial DNAs blunts mEV effects. Loss of Vsig4+ macrophages leads to microbial DNA accumulation in β cells and subsequently obesity-associated islet abnormalities.

Intestinal Barrier Function and Immune Homeostasis Are Missing Links in Obesity and Type 2 Diabetes Development

Noncommunicable diseases, such as type 2 diabetes (T2D), place a burden on healthcare systems worldwide. The rising prevalence of obesity, a major risk factor for T2D, is mainly attributed to the adoption of Westernized diets and lifestyle, which cause metabolic dysfunction and insulin resistance. Moreover, diet may also induce changes in the microbiota composition, thereby affecting intestinal immunity. The critical role of intestinal immunity and intestinal barrier function in the development of T2D is increasingly acknowledged, however, limited studies have investigated the link between intestinal function and metabolic disease. In this review, studies reporting specific roles of the intestinal immune system and intestinal epithelial cells (IECs) in metabolic disease are highlighted. Innate chemokine signaling, eosinophils, immunoglobulin A (IgA), T helper (Th) 17 cells and their cytokines were associated with obesity and/or dysregulated glucose homeostasis. Intestinal epithelial cells (IECs) emerged as critical modulators of obesity and glucose homeostasis through their effect on lipopolysaccharide (LPS) signaling and decontamination. Furthermore, IECs create a link between microbial metabolites and whole-body metabolic function. Future in depth studies of the intestinal immune system and IECs may provide new opportunities and targets to develop treatments and prevention strategies for obesity and T2D.

Inflammation in obesity, diabetes, and related disorders

Obesity leads to chronic, systemic inflammation and can lead to insulin resistance (IR), β-cell dysfunction, and ultimately type 2 diabetes (T2D). This chronic inflammatory state contributes to long-term complications of diabetes, including non-alcoholic fatty liver disease (NAFLD), retinopathy, cardiovascular disease, and nephropathy, and may underlie the association of type 2 diabetes with other conditions such as Alzheimer's disease, polycystic ovarian syndrome, gout, and rheumatoid arthritis. Here, we review the current understanding of the mechanisms underlying inflammation in obesity, T2D, and related disorders. We discuss how chronic tissue inflammation results in IR, impaired insulin secretion, glucose intolerance, and T2D and review the effect of inflammation on diabetic complications and on the relationship between T2D and other pathologies. In this context, we discuss current therapeutic options for the treatment of metabolic disease, advances in the clinic and the potential of immune-modulatory approaches.

The relation between the gut microbiome and osteoarthritis: A systematic review of literature

Background

Along with mechanical and genetic factors, emerging evidence suggests that the presence of low-grade inflammation has a role in the pathogenesis of osteoarthritis (OA) and seems to be related to the microbiome composition of the gut.

Purpose

To provide evidence whether there is clinical or preclinical evidence of gut-joint axis in the pathogenesis and symptoms of OA.

Methods

An extensive review of the current literature was performed using three different databases. Human, as well as animal studies, were included. The risk of bias was identified using ROBINS and SYRCLE tools, while the quality of evidence was assessed using GRADE and CAMADARES criteria.

Results

A total of nineteen articles were included. Multiple animal studies demonstrated that both obesity, and high-fat and high-sugar diets resulted in a gut dysbiosis status characterized by increased Firmicutes/Bacteroidetes (F/B) phyla ratio and increased permeability. These changes were associated with increased lipopolysaccharide serum levels, which consequently resulted in synovitis and OA severity. The administration of pre-and probiotics partially reversed this bacterial composition. In addition, in human studies, a decreased amount of gut Bacteroidetes, subsequent increased F/B ratio, have also been observed in OA patients.

Conclusions

Our review confirms preliminary yet sound evidence supporting a gut-joint axis in OA in primarily preclinical models, by showing an association between diet, gut dysbiosis and OA radiological severity and self-reported symptoms. Clinical studies are needed to confirm these findings, and to investigate whether interventions targeting the composition of the microbiome will have a beneficial clinical effect.

A High-Fat Western Diet Attenuates Intestinal Changes in Mice with DSS-Induced Low-Grade Inflammation

BACKGROUND

A Western diet (WD) is associated with increased inflammation in the large intestine, which is often ascribed to the high dietary fat content. Intestinal inflammation in rodents can be induced by oral administration of dextran sodium sulfate (DSS). However, most studies investigating effects of WD and DSS have not used appropriate low-fat diets (LFDs) as control.

OBJECTIVES

To compare the effects of a WD with those of an LFD on colon health in a DSS-induced low-grade colonic inflammation mouse model.

METHODS

Six-week-old male C57BL/6JRj mice were fed an LFD (fat = 10.3% energy, n = 24) or a WD (fat = 41.2% energy, n = 24) for 15 wk [Experiment 1 (Exp.1)]. Half the mice on each diet (n = 12) then received 1% DSS in water for 6 d with the remainder (n = 12 in each diet) administered water. Disease activity, proinflammatory genes, inflammatory biomarkers, and fecal microbiota (16S rRNA) were assessed (Exp.1). Follow-up experiments (Exp.2 and Exp.3) were performed to investigate whether fat source (milk or lard; Exp.2) affected outcomes and whether a shift from LFD to WD 1 d prior to 1% DSS exposure caused an immediate effect on DSS-induced inflammation (Exp.3).

RESULTS

In Exp.1, 1% DSS treatment significantly increased disease score in the LFD group compared with the WD group (2.7 compared with 0.8; P < 0.001). Higher concentrations of fecal lipocalin (11-fold; P < 0.001), proinflammatory gene expression (≤82-fold), and Proteobacteria were observed in LFD-fed mice compared with the WD group. The 2 fat sources in WDs (Exp.2) revealed the same low inflammation in WD+DSS mice compared with LFD+DSS mice. Finally, the switch from LFD to WD just before DSS exposure resulted in reduced colonic inflammation (Exp.3).

CONCLUSIONS

Herein, WDs (with milk or lard) protected mice against DSS-induced colonic inflammation compared with LFD-fed mice. Whether fat intake induces protective mechanisms against DSS-mediated inflammation or inhibits establishment of the DSS-induced colitis model is unclear.

Gut permeability and osteoarthritis, towards a mechanistic understanding of the pathogenesis: a systematic review

Osteoarthritis (OA) is the most common condition affecting human joints. Along with mechanical and genetic factors, low-grade inflammation is increasingly supported as a causal factor in the development of OA. Gut microbiota and intestinal permeability, via the disruption of tight junction competency, are proposed to explain a gut-joint axis through the interaction with the host immune system. Since previous studies and methods have underestimated the role of the gut-joint axis in OA and have only focussed on the characterisation of microbiota phenotypes, this systematic review aims to appraise the current evidence concerning the influence of gut permeability in the pathogenesis of OA. We propose that the tight junction disruption may be due to an increase in zonulin activity as already demonstrated for many other chronic inflammatory disorders. After years of unreliable quantification, one study optimised the methodology, showing a positive validated correlation between plasma lipopolysaccharide (LPS), obesity, joint inflammation, and OA severity. Chemokines show a prominent role in pain development. Our systematic review confirms preliminary evidence supporting a gut-joint axis in OA pathogenesis and progression. Being modifiable by several factors, the gut microbiota is a promising target for treatment. We propose a pathogenetic model in which dysbiosis is correlated to the bipartite graph of tight junctions and bacterially-produced products, aiming to direct future studies in the search of other bacterial products and tight junction disassembly regulators.KEY MESSAGESPrevious studies and methods have underestimated the impact of the gut-joint axis in osteoarthritis and have focussed on the characterisation of microbiota phenotypes rather than clear molecular mediators of disease.Gut dysbiosis is related to higher levels of bacterial toxins that elicit cartilage and synovium inflammatory pathways.Future research may benefit from focussing on both tight junctions and bacterially-produced products.

Dietary suppression of MHC class II expression in intestinal epithelial cells enhances intestinal tumorigenesis

Little is known about how interactions of diet, intestinal stem cells (ISCs), and immune cells affect early-stage intestinal tumorigenesis. We show that a high-fat diet (HFD) reduces the expression of the major histocompatibility complex class II (MHC class II) genes in intestinal epithelial cells, including ISCs. This decline in epithelial MHC class II expression in a HFD correlates with reduced intestinal microbiome diversity. Microbial community transfer experiments suggest that epithelial MHC class II expression is regulated by intestinal flora. Mechanistically, pattern recognition receptor (PRR) and interferon-gamma (IFNγ) signaling regulates epithelial MHC class II expression. MHC class II-negative (MHC-II-) ISCs exhibit greater tumor-initiating capacity than their MHC class II-positive (MHC-II+) counterparts upon loss of the tumor suppressor Apc coupled with a HFD, suggesting a role for epithelial MHC class II-mediated immune surveillance in suppressing tumorigenesis. ISC-specific genetic ablation of MHC class II increases tumor burden cell autonomously. Thus, HFD perturbs a microbiome-stem cell-immune cell interaction that contributes to tumor initiation in the intestine.

Role of Inflammatory Cytokines, Growth Factors and Adipokines in Adipogenesis and Insulin Resistance

Obesity, manifested by increased adiposity, represents a main cause of morbidity in the developed countries, causing increased risk of insulin resistance and type 2 diabetes mellitus. Recruitment of macrophages and activation of innate immunity represent the initial insult, which can be further exacerbated through secretion of chemokines and adipocytokines from activated macrophages and other cells within the adipose tissue. These events can impact adipogenesis, causing dysfunction of the adipose tissue and increased risk of insulin resistance. Various factors mediate adiposity and related insulin resistance including inflammatory and non-inflammatory factors such as pro and anti-inflammatory cytokines, adipokines and growth factors. In this review we will discuss the role of these factors in adipogenesis and development of insulin resistance and type 2 diabetes mellitus in the context of obesity. Understanding the molecular mechanisms that mediate adipogenesis and insulin resistance could help the development of novel therapeutic strategies for individuals at higher risk of insulin resistance and type 2 diabetes mellitus.

Chronic tissue inflammation and metabolic disease

In this review, Lee and Olefsky discuss the characteristics of chronic inflammation in the major metabolic tissues and how obesity triggers these events, including a focus on the role of adipose tissue hypoxia and macrophage-derived exosomes.

Obesity is the most common cause of insulin resistance, and the current obesity epidemic is driving a parallel rise in the incidence of T2DM. It is now widely recognized that chronic, subacute tissue inflammation is a major etiologic component of the pathogenesis of insulin resistance and metabolic dysfunction in obesity. Here, we summarize recent advances in our understanding of immunometabolism. We discuss the characteristics of chronic inflammation in the major metabolic tissues and how obesity triggers these events, including a focus on the role of adipose tissue hypoxia and macrophage-derived exosomes. Last, we also review current and potential new therapeutic strategies based on immunomodulation.

Molecular and Pathophysiological Links between Metabolic Disorders and Inflammatory Bowel Diseases

Despite considerable epidemiological evidence indicating comorbidity between metabolic disorders, such as obesity, type 2 diabetes, and non-alcoholic fatty liver disease, and inflammatory bowel diseases (IBD), such as Crohn’s disease and ulcerative colitis, as well as common pathophysiological features shared by these two categories of diseases, the relationship between their pathogenesis at molecular levels are not well described. Intestinal barrier dysfunction is a characteristic pathological feature of IBD, which also plays causal roles in the pathogenesis of chronic inflammatory metabolic disorders. Increased intestinal permeability is associated with a pro-inflammatory response of the intestinal immune system, possibly leading to the development of both diseases. In addition, dysregulated interactions between the gut microbiota and the host immunity have been found to contribute to immune-mediated disorders including the two diseases. In connection with disrupted gut microbial composition, alterations in gut microbiota-derived metabolites have also been shown to be closely related to the pathogeneses of both diseases. Focusing on these prominent pathophysiological features observed in both metabolic disorders and IBD, this review highlights and summarizes the molecular risk factors that may link between the pathogeneses of the two diseases, which is aimed at providing a comprehensive understanding of molecular mechanisms underlying their comorbidity.

The Interplay between Nutrition, Innate Immunity, and the Commensal Microbiota in Adaptive Intestinal Morphogenesis

The gastrointestinal tract is a functionally and anatomically segmented organ that is colonized by microbial communities from birth. While the genetics of mouse gut development is increasingly understood, how nutritional factors and the commensal gut microbiota act in concert to shape tissue organization and morphology of this rapidly renewing organ remains enigmatic. Here, we provide an overview of embryonic mouse gut development, with a focus on the intestinal vasculature and the enteric nervous system. We review how nutrition and the gut microbiota affect the adaptation of cellular and morphologic properties of the intestine, and how these processes are interconnected with innate immunity. Furthermore, we discuss how nutritional and microbial factors impact the renewal and differentiation of the epithelial lineage, influence the adaptation of capillary networks organized in villus structures, and shape the enteric nervous system and the intestinal smooth muscle layers. Intriguingly, the anatomy of the gut shows remarkable flexibility to nutritional and microbial challenges in the adult organism.

Small Intestinal Tuft Cell Activity Associates With Energy Metabolism in Diet-Induced Obesity

Little is known about the involvement of type 2 immune response-promoting intestinal tuft cells in metabolic regulation. We here examined the temporal changes in small intestinal tuft cell number and activity in response to high-fat diet-induced obesity in mice and investigated the relation to whole-body energy metabolism and the immune phenotype of the small intestine and epididymal white adipose tissue. Intake of high fat diet resulted in a reduction in overall numbers of small intestinal epithelial and tuft cells and reduced expression of the intestinal type 2 tuft cell markers Il25 and Tslp. Amongst >1,700 diet-regulated transcripts in tuft cells, we observed an early association between body mass expansion and increased expression of the gene encoding the serine protease inhibitor neuroserpin. By contrast, tuft cell expression of genes encoding gamma aminobutyric acid (GABA)-receptors was coupled to Tslp and Il25 and reduced body mass gain. Combined, our results point to a possible role for small intestinal tuft cells in energy metabolism via coupled regulation of tuft cell type 2 markers and GABA signaling receptors, while being independent of type 2 immune cell involvement. These results pave the way for further studies into interventions that elicit anti-obesogenic circuits via small intestinal tuft cells.

Alzheimer's disease and gut microbiota: does trimethylamine N-oxide (TMAO) play a role?

Alzheimer's disease (AD) is a neurodegenerative disease that affects memory and cognitive function. Clinical evidence has put into question our current understanding of AD development, propelling researchers to look into further avenues. Gut microbiota has emerged as a potential player in AD pathophysiology. Lifestyle factors, such as diet, can have negative effects on the gut microbiota and thus host health. A Western-type diet has been highlighted as a risk factor for both gut microbiota alteration as well as AD development. The gut-derived trimethylamine N-oxide (TMAO) has been previously implied in the development of cardiovascular diseases with recent evidence suggesting a plausible role of TMAO in AD development. Therefore, the main goal of the present review is to provide the reader with potential mechanisms of action through which consumption of a Western-type diet could increase AD risk, by acting through microbiota-produced TMAO. Although a link between TMAO and AD is far from definitive, this review will serve as a call for research into this new area of research.

Gut-Liver Axis in Nonalcoholic Fatty Liver Disease: the Impact of the Metagenome, End Products, and the Epithelial and Vascular Barriers

Nonalcoholic fatty liver disease (NAFLD) is a systemic, dynamic, heterogeneous, and multiaxis entity, the pathogenesis of which is still uncertain. The gut-liver axis is regulated and stabilized by a complex network encompassing a metabolic, immune, and neuroendocrine cross-talk between the gut, the microbiota, and the liver. Changes in the gut-liver axis affect the metabolism of lipids and carbohydrates in the hepatocytes, and they impact the balance of inflammatory mediators and cause metabolic deregulation, promoting NAFLD and its progression to nonalcoholic steatohepatitis. Moreover, the microbiota and its metabolites can play direct and indirect roles in gut barrier function and fibrosis development. In this review, we will highlight findings from the recent literature focusing on the gut-liver axis and its relation to NAFLD. Finally, we will discuss the impact of technical issues, design bias, and other limitations on current knowledge of the gut microbiota in the context of NAFLD.

Maintenance of Barrier Tissue Integrity by Unconventional Lymphocytes

Mucosal surfaces, as a first barrier with the environment are especially susceptible to damage from both pathogens and physical trauma. Thus, these sites require tightly regulated repair programs to maintain barrier function in the face of such insults. Barrier sites are also enriched for unconventional lymphocytes, which lack rearranged antigen receptors or express only a limited range of such receptors, such as ILCs (Innate Lymphoid Cells), γδ T Cells and MAIT (Mucosal-Associated Invariant T Cells). Recent studies have uncovered critical roles for unconventional lymphocytes in regulating mucosal barrier function, and, in particular, have highlighted their important involvement in barrier repair. The production of growth factors such as amphiregulin by ILC2, and fibroblast growth factors by γδ T cells have been shown to promote tissue repair at multiple barrier sites. Additionally, MAIT cells have been shown to exhibit pro-repair phenotypes and demonstrate microbiota-dependent promotion of murine skin healing. In this review we will discuss how immune responses at mucosal sites are controlled by unconventional lymphocytes and the ways in which these cells promote tissue repair to maintain barrier integrity in the skin, gut and lungs.

Obesity and intestinal stem cell susceptibility to carcinogenesis

Background

Obesity is a top public health problem associated with an increase in colorectal cancer incidence. Stem cells are the chief cells in tissue homeostasis that self-renew and differentiate into other cells to regenerate the organ. It is speculated that an increase in stem cell pool makes cells susceptible to carcinogenesis. In this review, we looked at the recent investigations linking obesity/high-fat diet-induced obesity to intestinal carcinogenesis with regard to intestinal stem cells and their niche.

Findings

High-fat diet-induced obesity may rise intestinal carcinogenesis by increased Intestinal stem cells (ISC)/progenitor’s population, stemness, and niche independence through activation of PPAR-δ with fatty acids, hormonal alterations related to obesity, and low-grade inflammation. However, these effects may possibly relate to the interaction between fats and carbohydrates, and not a fatty acid per se. Nonetheless, literature studies are inconsistency in their results, probably due to the differences in the diet components and limitations of genetic models used.

Conclusion

High-fat diet-induced obesity affects carcinogenesis by changing ISC proliferation and function. However, a well-matched diet and the reliable colorectal cancer models that mimic human carcinogenesis is necessary to clearly elucidate the influence of high-fat diet-induced obesity on ISC behavior.

Deletion of intestinal epithelial AMP-activated protein kinase alters distal colon permeability but not glucose homeostasis

Objective

The intestinal epithelial barrier (IEB) restricts the passage of microbes and potentially harmful substances from the lumen through the paracellular space, and rupture of its integrity is associated with a variety of gastrointestinal disorders and extra-digestive diseases. Increased IEB permeability has been linked to disruption of metabolic homeostasis leading to obesity and type 2 diabetes. Interestingly, recent studies have uncovered compelling evidence that the AMP-activated protein kinase (AMPK) signaling pathway plays an important role in maintaining epithelial cell barrier function. However, our understanding of the function of intestinal AMPK in regulating IEB and glucose homeostasis remains sparse.

Methods

We generated mice lacking the two α1 and α2 AMPK catalytic subunits specifically in intestinal epithelial cells (IEC AMPK KO) and determined the physiological consequences of intestinal-specific deletion of AMPK in response to high-fat diet (HFD)-induced obesity. We combined histological, functional, and integrative analyses to ascertain the effects of gut AMPK loss on intestinal permeability in vivo and ex vivo and on the development of obesity and metabolic dysfunction. We also determined the impact of intestinal AMPK deletion in an inducible mouse model (i-IEC AMPK KO) by measuring IEB function, glucose homeostasis, and the composition of gut microbiota via fecal 16S rRNA sequencing.

Results

While there were no differences in in vivo intestinal permeability in WT and IEC AMPK KO mice, ex vivo transcellular and paracellular permeability measured in Ussing chambers was significantly increased in the distal colon of IEC AMPK KO mice. This was associated with a reduction in pSer425 GIV phosphorylation, a marker of leaky gut barrier. However, the expression of tight junction proteins in intestinal epithelial cells and pro-inflammatory cytokines in the lamina propria were not different between genotypes. Although the HFD-fed AMPK KO mice displayed suppression of the stress polarity signaling pathway and a concomitant increase in colon permeability, loss of intestinal AMPK did not exacerbate body weight gain or adiposity. Deletion of AMPK was also not sufficient to alter glucose homeostasis or the acute glucose-lowering action of metformin in control diet (CD)- or HFD-fed mice. CD-fed i-IEC AMPK KO mice also presented higher permeability in the distal colon under homeostatic conditions but, surprisingly, this was not detected upon HFD feeding. Alteration in epithelial barrier function in the i-IEC AMPK KO mice was associated with a shift in the gut microbiota composition with higher levels of Clostridiales and Desulfovibrionales.

Conclusions

Altogether, our results revealed a significant role of intestinal AMPK in maintaining IEB integrity in the distal colon but not in regulating glucose homeostasis. Our data also highlight the complex interaction between gut microbiota and host AMPK.

•

Deletion of intestinal AMPKα1 and α2 suppresses the stress-polarity signaling (SPS) pathway.

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Loss of the SPS pathway is associated with increased paracellular permeability in the distal colon.

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Intestinal AMPK is dispensable for the acute glucose-lowering action of metformin.

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Loss of intestinal AMPK alters the gut microbiota composition.

CRIg+ Macrophages Prevent Gut Microbial DNA-Containing Extracellular Vesicle-Induced Tissue Inflammation and Insulin Resistance

BACKGROUND & AIMS

Liver CRIg⁺ (complement receptor of the immunoglobulin superfamily) macrophages play a critical role in filtering bacteria and their products from circulation. Translocation of microbiota-derived products from an impaired gut barrier contributes to the development of obesity-associated tissue inflammation and insulin resistance. However, the critical role of CRIg⁺ macrophages in clearing microbiota-derived products from the bloodstream in the context of obesity is largely unknown.

METHODS

We performed studies with CRIg^-/-, C3^-/-, cGAS^-/-, and their wild-type littermate mice. The CRIg⁺ macrophage population and bacterial DNA abundance were examined in both mouse and human liver by either flow cytometric or immunohistochemistry analysis. Gut microbial DNA-containing extracellular vesicles (mEVs) were adoptively transferred into CRIg^-/-, C3^-/-, or wild-type mice, and tissue inflammation and insulin sensitivity were measured in these mice. After coculture with gut mEVs, cellular insulin responses and cGAS/STING-mediated inflammatory responses were evaluated.

RESULTS

Gut mEVs can reach metabolic tissues in obesity. Liver CRIg⁺ macrophages efficiently clear mEVs from the bloodstream through a C3-dependent opsonization mechanism, whereas obesity elicits a marked reduction in the CRIg⁺ macrophage population. Depletion of CRIg⁺ cells results in the spread of mEVs into distant metabolic tissues, subsequently exacerbating tissue inflammation and metabolic disorders. Additionally, in vitro treatment of obese mEVs directly triggers inflammation and insulin resistance of insulin target cells. Depletion of microbial DNA blunts the pathogenic effects of intestinal EVs. Furthermore, the cGAS/STING pathway is crucial for microbial DNA-mediated inflammatory responses.

CONCLUSIONS

Deficiency of CRIg⁺ macrophages and leakage of intestinal EVs containing microbial DNA contribute to the development of obesity-associated tissue inflammation and metabolic diseases.

Shades of white: new insights into tissue-resident leukocyte heterogeneity

Populations of white blood cells (leukocytes) have been found in tissues and organs across the body, in states of both health and disease. The role leukocytes play within these tissues is often highly contested. For many leukocytes, there are studies outlining pro-inflammatory destructive functions, while other studies provide clear evidence of anti-inflammatory homeostatic activities of leukocytes within the same tissue. We discuss how this functional dissonance can be explained by leukocyte heterogeneity. Although cell morphology and surface receptor profiles are excellent methods to segregate cell types, the true degree of leukocyte heterogeneity that exists can only be appreciated by studying the variable and dynamic gene expression profile. Unbiased single-cell RNA sequencing profiling of tissue-resident leukocytes is transforming the way we understand leukocytes across health and disease. Recent investigations into adipose tissue-resident leukocytes have revealed unprecedented levels of heterogeneity among populations of macrophages. We use this example to pose emerging questions regarding tissue-resident leukocytes and review what is currently known (and unknown) about the diversity of tissue-resident leukocytes within different organs.

Adipokine-Modulated Immunological Homeostasis Shapes the Pathophysiology of Inflammatory Bowel Disease

Inflammatory colon diseases, which are a global health concern, include a variety of gastrointestinal tract disorders, such as inflammatory bowel disease and colon cancer. The pathogenesis of these colon disorders involves immune alterations with the pronounced infiltration of innate and adaptive immune cells into the intestines and the augmented expression of mucosal pro-inflammatory cytokines stimulated by commensal microbiota. Epidemiological studies during the past half century have shown that the proportion of obese people in a population is associated with the incidence and pathogenesis of gastrointestinal tract disorders. The advancement of understanding of the immunological basis of colon disease has shown that adipocyte-derived biologically active substances (adipokines) modulate the role of innate and adaptive immune cells in the progress of intestinal inflammation. The biomedical significance in immunological homeostasis of adipokines, including adiponectin, leptin, apelin and resistin, is clear. In this review, we highlight the existing literature on the effect and contribution of adipokines to the regulation of immunological homeostasis in inflammatory colon diseases and discuss their crucial roles in disease etiology and pathogenesis, as well as the implications of these results for new therapies in these disorders.

Supplementation of Bacillus sp. DU-106 reduces hypercholesterolemia and ameliorates gut dysbiosis in high-fat diet rats

Gut microbiota modulation by a probiotic is a novel therapy for hypercholesterolemia mitigation. This study initially investigated the potential hypocholesterolemic effect of Bacillus sp. DU-106 in hypercholesterolemic rats and explored its potential relation with gut microbiota. Sprague-Dawley rats received a high-fat diet, or a high-fat diet supplemented with 7.5 × 10⁹ and 1.5 × 10¹⁰ CFU/kg bw/day Bacillus sp. DU-106 (low-dose and high-dose groups). At the end of 9 weeks, Bacillus sp. DU-106 treatment significantly decreased the body weight, liver index, and total cholesterol. 16S rRNA sequencing showed that Bacillus sp. DU-106 intervention significantly increased bacterial richness and particularly increased the genus abundance of Turicibacter, Acinetobacter, Brevundimonas, and Bacillus and significantly decreased the abundance of Ralstonia. Metabolomic data further indicated that the supplementation of Bacillus sp. DU-106 remarkably changed the gut metabolic profiles of hypercholesterolemic rats and, in particular, elevated the metabolites of indole-3-acetate, methylsuccinic acid, creatine, glutamic acid, threonine, lysine, ascorbic acid, and pyridoxamine. Spearman's correlation analysis showed the close relation between the different genera and metabolites. In conclusion, Bacillus sp. DU-106 supplement ameliorated high-fat diet-induced hypercholesterolemia and showed potential probiotic benefits for the intestine. KEY POINTS: • A novel potential probiotic Bacillus sp. DU-106 ameliorated hypercholesterolemia in rats. • Bacillus sp. DU-106 supplement regulated gut microbiome structure and richness. • Bacillus sp. DU-106 supplement changed metabolic profiles in high-fat diet rats. • Significant correlations were observed between differential genera and metabolites.

Use of Nanovesicles from Orange Juice to Reverse Diet-Induced Gut Modifications in Diet-Induced Obese Mice

We have determined whether orange juice-derived nanovesicles (ONVs) could be used for the treatment of obesity-associated intestinal complications. ONVs were characterized by lipidomic, metabolomic, electron microscopy. In vitro, intestinal barriers (IBs = Caco-2+HT-29-MTX) were treated with ONVs and co-cultured with adipocytes to monitor IB fat release. In vivo, obesity was induced with a high-fat, high-sucrose diet (HFHSD mice) for 12 weeks. Then, half of HFHSD mice were gavaged with ONVs. One-month ONV treatment did not modify HFHSD-induced insulin resistance but reversed diet-induced gut modifications. In the jejunum, ONVs increased villi size, reduced triglyceride content, and modulated mRNA levels of genes involved in immune response (tumor necrosis factor [TNF]-α and interleukin [IL]-1β), barrier permeability (CLDN1, OCLN, ZO1), fat absorption, and chylomicron release. ONVs targeted microsomal triglyceride transfer protein (MTP) and angiopoietin-like protein-4 (ANGPTL4), two therapeutic targets to reduce plasma lipids and inflammation in gastrointestinal diseases. Interestingly, ONV treatment did not aggravate liver steatosis, as MTP mRNA was increased in the liver. Therefore, ONVs protected both intestine and the liver from fat overload associated with the HFHSD. As ONVs concentrated amino acids and bioactive lipids versus orange juice, which are deficient in obese patients, the use of ONVs as a dietary supplement could bring physiological relevant compounds in the jejunum to accelerate the restoration of intestinal functions during weight loss in obese patients.

Adipose Tissue Inflammation Is Directly Linked to Obesity-Induced Insulin Resistance, while Gut Dysbiosis and Mitochondrial Dysfunction Are Not Required

Obesity is associated with adipose tissue hypertrophy, systemic inflammation, mitochondrial dysfunction, and intestinal dysbiosis. Rodent models of high-fat diet (HFD)-feeding or genetic deletion of multifunctional proteins involved in immunity and metabolism are often used to probe the etiology of obesity; however, these models make it difficult to divorce the effects of obesity, diet composition, or immunity on endocrine regulation of blood glucose. We, therefore, investigated the importance of adipose inflammation, mitochondrial dysfunction, and gut dysbiosis for obesity-induced insulin resistance using a spontaneously obese mouse model. We examined metabolic changes in skeletal muscle, adipose tissue, liver, the intestinal microbiome, and whole-body glucose control in spontaneously hyperphagic C57Bl/6J mice compared to lean littermates. A separate subset of lean and obese mice was subject to 8 weeks of obesogenic HFD feeding, or to pair feeding of a standard rodent diet. Hyperphagia, obesity, adipose inflammation, and insulin resistance were present in obese mice despite consuming a standard rodent diet, and these effects were blunted with caloric restriction. However, hyperphagic obese mice had normal mitochondrial respiratory function in all tissues tested and no discernable intestinal dysbiosis relative to lean littermates. In contrast, feeding mice an obesogenic HFD altered the composition of the gut microbiome, impaired skeletal muscle mitochondrial bioenergetics, and promoted poor glucose control. These data show that adipose inflammation and redox stress occurred in all models of obesity, but gut dysbiosis and mitochondrial respiratory dysfunction are not always required for obesity-induced insulin resistance. Rather, changes in the intestinal microbiome and mitochondrial bioenergetics may reflect physiological consequences of HFD feeding.