Gut microbe-derived metabolite trimethylamine N-oxide activates PERK to drive fibrogenic mesenchymal differentiation

Trimethylamine N-oxide: a meta-organismal axis linking the gut and fibrosis

BACKGROUND

Tissue fibrosis is a common pathway to failure in many organ systems and is the cellular and molecular driver of myriad chronic diseases that are incompletely understood and lack effective treatment. Recent studies suggest that gut microbe-dependent metabolites might be involved in the initiation and progression of fibrosis in multiple organ systems.

MAIN BODY OF THE MANUSCRIPT

In a meta-organismal pathway that begins in the gut, gut microbiota convert dietary precursors such as choline, phosphatidylcholine, and L-carnitine into trimethylamine (TMA), which is absorbed and subsequently converted to trimethylamine N-oxide (TMAO) via the host enzyme flavin-containing monooxygenase 3 (FMO3) in the liver. Chronic exposure to elevated TMAO appears to be associated with vascular injury and enhanced fibrosis propensity in diverse conditions, including chronic kidney disease, heart failure, metabolic dysfunction-associated steatotic liver disease, and systemic sclerosis.

CONCLUSION

Despite the high prevalence of fibrosis, little is known to date about the role of gut dysbiosis and of microbe-dependent metabolites in its pathogenesis. This review summarizes recent important advances in the understanding of the complex metabolism and functional role of TMAO in pathologic fibrosis and highlights unanswered questions.

Intestinal Fibrogenesis in Inflammatory Bowel Diseases: Exploring the Potential Role of Gut Microbiota Metabolites as Modulators

Fibrosis, sustained by the transformation of intestinal epithelial cells into fibroblasts (epithelial-to-mesenchymal transition, EMT), has been extensively studied in recent decades, with the molecular basis well-documented in various diseases, including inflammatory bowel diseases (IBDs). However, the factors influencing these pathways remain unclear. In recent years, the role of the gut microbiota in health and disease has garnered significant attention. Evidence suggests that an imbalanced or dysregulated microbiota, along with environmental and genetic factors, may contribute to the development of IBDs. Notably, microbes produce various metabolites that interact with host receptors and associated signaling pathways, influencing physiological and pathological changes. This review aims to present recent evidence highlighting the emerging role of the most studied metabolites as potential modulators of molecular pathways implicated in intestinal fibrosis and EMT in IBDs. These studies provide a deeper understanding of intestinal inflammation and fibrosis, elucidating the molecular basis of the microbiota role in IBDs, paving the way for future treatments.

The significance of dynamic monitoring plasma TMAO level in pulmonary arterial hypertension - a cohort study

BACKGROUND

Gut microbiota assumes an essential role in the development and progression of pulmonary arterial hypertension (PAH). Trimethylamine N-oxide (TMAO), a gut microbiota-dependent metabolite, is correlated with the prognosis of patients with PAH. However, the correlation between changes in TMAO (ΔTMAO) and the prognosis of PAH remains elusive.

OBJECTIVES

To investigate the association between ΔTMAO and prognosis of PAH, and explore whether dynamic assessment of TMAO level was superior to measurement at a single time point in predicting prognosis.

DESIGN

Single-center cohort study.

METHODS

Consecutive patients diagnosed with PAH and had at least two TMAO measurements taken from May 2019 to June 2020 were eligible. The outcome events of this study were defined as adverse clinical events.

RESULTS

A total of 117 patients with PAH who had two TMAO measurements and follow-up were included in this study. Patients with ΔTMAO ⩾1.082 μmol/L had over four times increased risk of adverse clinical events than their counterparts after adjusting for confounders [hazard ratio (HR) 4.050, 95% confidence interval (CI): 1.468-11.174; p = 0.007]. Patients with constant high TMAO levels at both time points had the highest risk of adverse clinical events compared with patients with constant low TMAO levels (HR 3.717, 95% CI: 1.627-8.492; p = 0.002). ΔTMAO was also associated with changes in parameters reflecting PAH severity (p < 0.05).

CONCLUSION

Changes in TMAO were independently correlated with prognosis in patients with PAH, irrespective of baseline level of TMAO. ΔTMAO also correlated with alteration in disease severity. Repeated assessment of TMAO level contributes to better identification of patients with increased risk of adverse clinical events.

Pathogenesis of vasculopathy in systemic sclerosis and its contribution to fibrosis

PURPOSE OF REVIEW

In patients with systemic sclerosis (SSc), vascular manifestations precede skin and organ fibrosis. There is increasing evidence demonstrating a pathogenic link between early vascular injury and subsequent development of tissue fibrosis.

RECENT FINDINGS

Our knowledge of cellular and molecular mechanisms underlying a unique relationship between SSc-related vasculopathy and fibrosis has changed over the last few years. There is increasing evidence showing viral infection as a potential trigger elucidating vascular injury. Due to defective vascular repair machinery, this initial event results in endothelial cell activation and apoptosis as well as the recruitment of inflammatory/immune cells, leading to endothelial-to-mesenchymal transition. This sequential process induces destructive vasculopathy in capillaries, fibroproliferative vascular lesions in arteries, and excessive fibrosis in the surrounding tissue. A variety of molecular mechanisms and pathways involved in vascular remodeling linked to subsequent excessive fibrosis have been identified and serve as attractive therapeutic targets for SSc.

SUMMARY

Endothelial injury may play a central role in connecting three features that characterize SSc pathogenesis: vasculopathy, chronic inflammation, and fibrosis. Our understanding of the processes responsible for myofibroblast differentiation triggered by vascular injury will provide the rationale for novel targeted therapies for SSc.

Further insight into systemic sclerosis from the vasculopathy perspective

Systemic sclerosis (SSc) is an autoimmune disease characterized by immune dysfunction, vascular system dysfunction, and tissue fibrosis. Vascular injury, vascular remodeling, and endothelial dysfunction are the hallmark pathological changes of the disease. In the early stages of SSc development, endothelial cell injury and apoptosis can lead to vascular and perivascular inflammation, oxidative stress, and tissue hypoxia, which can cause clinical manifestations in various organs from the skin to the parenchymal organs. Early diagnosis and rational treatment can improve patient survival and quality of life. Ancillary examinations such as nailfold capillaroscopy as well as optical coherence tomography can help early detect vascular injury in SSc patients. Studies targeting the mechanisms of vascular lesions will provide new perspectives for treatment of SSc.

Bacterial Metabolites: A Link between Gut Microbiota and Dermatological Diseases

Dysbiosis has been identified in many dermatological conditions (e.g., psoriasis, atopic dermatitis, systemic lupus erythematosus). One of the ways by which the microbiota affect homeostasis is through microbiota-derived molecules (metabolites). There are three main groups of metabolites: short-chain fatty acids (SCFAs), tryptophan metabolites, and amine derivatives including trimethylamine N-oxide (TMAO). Each group has its own uptake and specific receptors through which these metabolites can exert their systemic function. This review provides up-to-date knowledge about the impact that these groups of gut microbiota metabolites may have in dermatological conditions. Special attention is paid to the effect of microbial metabolites on the immune system, including changes in the profile of the immune cells and cytokine disbalance, which are characteristic of several dermatological diseases, especially psoriasis and atopic dermatitis. Targeting the production of microbiota metabolites may serve as a novel therapeutic approach in several immune-mediated dermatological diseases.

Aorta- and liver-generated TMAO enhances trained immunity for increased inflammation via ER stress/mitochondrial ROS/glycolysis pathways

We determined whether gut microbiota-produced trimethylamine (TMA) is oxidized into trimethylamine N-oxide (TMAO) in nonliver tissues and whether TMAO promotes inflammation via trained immunity (TI). We found that endoplasmic reticulum (ER) stress genes were coupregulated with MitoCarta genes in chronic kidney diseases (CKD); TMAO upregulated 190 genes in human aortic endothelial cells (HAECs); TMAO synthesis enzyme flavin-containing monooxygenase 3 (FMO3) was expressed in human and mouse aortas; TMAO transdifferentiated HAECs into innate immune cells; TMAO phosphorylated 12 kinases in cytosol via its receptor PERK and CREB, and integrated with PERK pathways; and PERK inhibitors suppressed TMAO-induced ICAM-1. TMAO upregulated 3 mitochondrial genes, downregulated inflammation inhibitor DARS2, and induced mitoROS, and mitoTEMPO inhibited TMAO-induced ICAM-1. β-Glucan priming, followed by TMAO restimulation, upregulated TNF-α by inducing metabolic reprogramming, and glycolysis inhibitor suppressed TMAO-induced ICAM-1. Our results have provided potentially novel insights regarding TMAO roles in inducing EC activation and innate immune transdifferentiation and inducing metabolic reprogramming and TI for enhanced vascular inflammation, and they have provided new therapeutic targets for treating cardiovascular diseases (CVD), CKD-promoted CVD, inflammation, transplantation, aging, and cancer.

The Gut Microbial Metabolite Trimethylamine N-Oxide is Linked to Specific Complications of Systemic Sclerosis

Background

Systemic sclerosis (SSc) is a rare immune-mediated connective tissue disease characterized by fibrosis of the skin and internal organs, whose pathogenesis is not fully understood. Recent studies have revealed dysbiosis in patients with systemic sclerosis and have indicated the possible role of the microbiota and its metabolites in the pathogenesis of the disease. Trimethylamine N-oxide (TMAO) is a compound produced by dysbiotic microbiota observed at higher concentrations in several autoimmune diseases.

Objective

To determine concentrations of the bacteria-derived metabolite TMAO in patients with systemic sclerosis and to assess possible correlation between TMAO and a specific manifestation of the disease.

Patients and Methods

The study included 63 patients with SSc and 47 matched control subjects. The concentration of TMAO was measured with high-performance liquid chromatography.

Results

Plasma TMAO level was significantly increased in patients with SSc (283.0 [188.5-367.5] ng/mL versus 205.5 [101.0-318.0] ng/mL; p < 0.01). An increased concentration of TMAO was observed in patients with concomitant interstitial lung disease (ILD) (302.0 ng/mL [212.0-385.5] ng/mL versus 204.0 [135.5-292.0] ng/mL; p < 0.01) and esophageal dysmotility (289.75 [213.75-387.5] ng/mL versus 209.5 ng/mL [141.5-315.0] ng/mL; p < 0.05) compared to patients without these complications. Furthermore, TMAO concentration exhibited significant correlation with markers of heart involvement (left ventricle ejection fraction, NT-proBNP), marker of ILD severity and Scleroderma Clinical Trials Consortium Damage Index.

Conclusion

The concentration of TMAO, gut microbiota-associated metabolite, is increased in systemic sclerosis, particularly in patients with advanced organ involvement. This is the first study evaluating plasma TMAO in systemic sclerosis. Bacterial metabolites may be a link between dysbiosis and organ involvement in the course of the disease. Modulation of gut bacterial-derived metabolites may represent a new therapeutic approach in the management of systemic sclerosis.