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The incidence of inflammatory bowel diseases (IBD) worldwide has resulted in a global public health challenge. Intestinal fibrosis leading to stricture formation and bowel obstruction is a frequent complication in Crohn's disease (CD), and the lack of anti-fibrotic therapies makes elucidation of fibrosis mechanisms a priority. Progress has shown that mesenchymal cells, cytokines, microbial products, and mesenteric adipocytes are jointly implicated in the pathogenesis of intestinal fibrosis. This recent information puts prevention or reversal of intestinal strictures within reach through innovative therapies validated by reliable clinical trial endpoints. Here, we review the role of immune and non-immune components of the pathogenesis of intestinal fibrosis, including new cell clusters, cytokine networks, host-microbiome interactions, creeping fat, and their translation for endpoint development in anti-fibrotic clinical trials.

Obesity has repeatedly been linked to reorganization of the gut microbiome, yet to this point obesity therapeutics have been targeted exclusively toward the human host. Here, we show that gut microbe-targeted inhibition of the trimethylamine N-oxide (TMAO) pathway protects mice against the metabolic disturbances associated with diet-induced obesity (DIO) or leptin deficiency (Lepob/ob). Small molecule inhibition of the gut microbial enzyme choline TMA-lyase (CutC) does not reduce food intake but is instead associated with alterations in the gut microbiome, improvement in glucose tolerance, and enhanced energy expenditure. We also show that gut microbial CutC inhibition is associated with reorganization of host circadian control of both phosphatidylcholine and energy metabolism. This study underscores the relationship between microbe and host metabolism and provides evidence that gut microbe-derived trimethylamine (TMA) is a key regulator of the host circadian clock. This work also demonstrates that gut microbe-targeted enzyme inhibitors have potential as anti-obesity therapeutics.

Essentials Microbe-dependent production of trimethylamine N-oxide (TMAO) contributes to thrombosis risk. The impact of host flavin monooxygenase 3 (FMO3) modulation on platelet function is unknown. Genetic manipulation of FMO3 in mice alters systemic TMAO levels and thrombosis potential. Genetic manipulation of FMO3 is associated with alteration of gut microbial community structure. Summary Background Gut microbes play a critical role in the production of trimethylamine N-oxide (TMAO), an atherogenic metabolite that impacts platelet responsiveness and thrombosis potential. Involving both microbe and host enzymatic machinery, TMAO generation utilizes a metaorganismal pathway, beginning with ingestion of trimethylamine (TMA)-containing dietary nutrients such as choline, phosphatidylcholine and carnitine, which are abundant in a Western diet. Gut microbial TMA lyases use these nutrients as substrates to produce TMA, which upon delivery to the liver via the portal circulation, is converted into TMAO by host hepatic flavin monooxygenases (FMOs). Gut microbial production of TMA is rate limiting in the metaorganismal TMAO pathway because hepatic FMO activity is typically in excess. Objectives FMO3 is the major FMO responsible for host generation of TMAO; however, a role for FMO3 in altering platelet responsiveness and thrombosis potential in vivo has not yet been explored. Methods The impact of FMO3 suppression (antisense oligonucleotide-targeting) and overexpression (as transgene) on plasma TMAO levels, platelet responsiveness and thrombosis potential was examined using a murine FeCl3 -induced carotid artery injury model. Cecal microbial composition was examined using 16S analyses. Results Modulation of FMO3 directly impacts systemic TMAO levels, platelet responsiveness and rate of thrombus formation in vivo. Microbial composition analyses reveal taxa whose proportions are associated with both plasma TMAO levels and in vivo thrombosis potential. Conclusions The present studies demonstrate that host hepatic FMO3, the terminal step in the metaorganismal TMAO pathway, participates in diet-dependent and gut microbiota-dependent changes in both platelet responsiveness and thrombosis potential in vivo.

The human intestine is home to trillions of bacteria, which symbiose with our mammalian cells to promote health or disease in the human metaorganism. To maintain this symbiotic relationship, gut microbes and host cells engage in bidirectional cross-talk that relies heavily on common nutrients used by both cell types. It is important to understand this bidirectional metabolic cross-talk because this metaorganismal communication is involved in the pathogenesis of obesity, insulin resistance, cardiovascular disease, and cancer. A key constraint on the types of microbes that can inhabit our intestine is the quality and quantity of our diet. However, the distinct dietary nutrients that impact microbial community structure are poorly understood. Now, Holmes et al . provide evidence that dietary nitrogen content impacts gut microbial diversity and associated host metabolic responses. To identify dietary factors that influence microbiome-host interactions, the researchers designed a series of well-controlled diets with a dynamic range of macronutrients (carbohydrates, protein, and fat) and overall energy density. These defined diets were then fed to mice to examine their effects on gut microbiome community structure, as well as microbe and host nutrient metabolism using stable isotope methodology. This work also leveraged an innovative bioinformatics platform to predict guilds of microbes that have distinct nutrient acquisition preferences. A key observation was that dietary protein intake constrains the host-microbiome metabolic interplay. In particular, the availability of nitrogen can impact both gut microbial community structure and host metabolism. Diets that restricted nitrogen availability to gut microbes promoted healthy aging in the mice, potentially by shaping community structure. These insights offer a potential explanation for the effect of high protein intake on the metabolic health in the host. This work also provides evidence that diet may have a large impact on host-microbiome interactions in health and disease. Forging ahead, it is imperative that we understand that studying microbe-host interactions in chow-fed mice likely has little direct relevance to the human situation. Humans do not consume diets even remotely similar to mouse chow, which is an extremely high-fiber, high-carbohydrate, and low-fat diet. As we move forward to therapeutically intervene on microbe-host interactions to improve human health, consideration of the dietary constraints will be key. A. J. Holmes et al. , Diet-microbiome interactions in health are controlled by intestinal nitrogen source constraints. Cell Metab. 10.1016/j.cmet.2016.10.021 (2016). [[Abstract]][1] [1]: http://www.cell.com/cell-metabolism/abstract/S1550-4131(16)30553-8