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Bile acid, immune-metabolism, lipid and glucose homeostasis

Periodic Reporting for period 2 - ImmunoBile (Bile acid, immune-metabolism, lipid and glucose homeostasis)

Reporting period: 2018-03-01 to 2019-08-31

The role of chronic inflammation in obesity, metabolic and cardiovascular diseases is increasingly recognized. Bile acids (BA) which are synthesized in the liver and modified by the gut flora, facilitate lipid absorption in the intestine acting as soaps. Recent findings indicate that BA also modulate lipid and glucose homeostasis by activating the nuclear receptor FXR and the GPCR TGR5. Intriguingly, peripheral BA concentrations are elevated in type 2 diabetes (T2D) and FXR mediates the beneficial metabolic response to gastric bypass in mice. Since the immune system plays an important role in the cross-talk with metabolic tissues, such as liver, intestine and adipose tissues, we aimed to determine whether BA modulate immune cell function. Our results identify FXR and TGR5 expression in lymphoid cells, which prompted us to study their role in the regulation of glucose and lipid metabolism through immune cell modulation. Using reporter mice and specific ligands, we are characterizing the immune cells expressing active FXR and TGR5. In a second step, ee will determine their role in metabolism and inflammation by immune cell-specific gene inactivation in models of obesity, T2D and non-alcoholic fatty liver disease (NAFLD). Application of mass cytometry, cell sorting and single cell transcriptomic analysis allow the identification of gene networks regulated by BA and their receptors. As microbiota generate biologically active secondary BA, we also assess the impact of microbiota depletion and subsequent BA acid pool modifications on immune cell populations. Translational studies are performed in humans with altered BA metabolism and pharmacological treatment with anti-diabetic BA sequestrants to assess the impact of alterations in BA metabolism and signaling on immune functions. Our project aims to identify an hitherto unexplored role of BA through modulation of the immune system on T2D, NAFLD and dyslipidemia by applying an integrative approach, possible thanks the unique multidisciplinary expertise in basic and translational biology of my laboratory.
- In an additional screening of bile acid receptor expression in cells of the immune system, platelets which are produced from hematopoietic megakaryocytes, but lack a nucleus, express bile acid receptors such as FXR. Functional studies with pharmacological ligands revealed that FXR activation inhibits platelet activation in response to stimulation of collagen or thrombin receptors, resulting in diminished intracellular calcium mobilization, secretion, fibrinogen binding, aggregation. integrin αIIbβ3 outside-in signaling, spreading and ability to stimulate clot retraction. FXR function was associated with the modulation of cGMP levels and associated downstream inhibitory signaling. Furthermore, platelets from FXR-deficient mice were refractory to treatment with FXR activators. This study provides identifies a role for FXR ligands to modulate platelet activation suggesting that FXR may be a potential target for the prevention of athero-thrombotic disease (Moraes et al., 2016).
- As secondary BA are produced by microbiota which impact on the immune system, we sought to determine whether alterations in microbiota composition and/or function through the administration of “prebiotics” impacts through bile acid changes on vascular function. In a mouse model of metabolically-induced endothelial dysfunction (n-3 PUFA-depleted apolipoprotein E-deficient mice), inulin-type fructan supplementation was found to reverse endothelial dysfunction in mesenteric and carotid arteries of via activation of the nitric oxide (NO) synthase/NO pathway. Prebiotic administration increased NO-producing bacteria, replenishned flora with the beneficial bacterium Akkermansia and decreased abundance in bacterial taxa involved in secondary BA synthesis. Changes in gut and liver gene expression also occur upon inulin-type fructan administration suggesting increased GLP-1 production and BA turnover as drivers of endothelium function preservation, emphasizing the key role of BA in the regulation of endothelial function (Catry et al., 2018).
- As alternative for murine models, the minipig is increasingly used in particular in the field of surgery. We took advantage of this model to functionally assess the link between the increase of BA concentrations and the metabolic benefits of bariatric surgery, in particular Roux en Y gastric bypass (RYGB). We first determined the BA composition in pig, which was found more similar to the human, than to the mouse bile acid pool, yet displaying some interesting differences, in particular due to an enrichment with hyocholic acid species (Spinelli et al., 2016). Systemic total BA concentrations increased after RYGB, due to an increase in conjugated BAs. The ratio of portal:systemic conjugated BAs decreased after RYGB, indicating alterations of liver function in the changes in systemic BA concentrations upon RYGB surgery. Our results show that the increase in systemic BAs after surgery is due to decreased selective hepatic recapture. Thus, alterations in hepatic function contribute to the increase in systemic BAs after RYGB (Chavez-Talavera et al., 2017a).
- To improve the available array of BA receptor agonists and antagonists, a medicinal chemistry program was undertaken to identify TGR5 agonists devoid of deleterious systemic on-target effects, but retaining beneficial effects on the entero-endocrine and entero-immune systems. Therefore, a screen for TGR5 agonists with limited exposure mainly in the intestine was performed by measuring GLP-1 secretion by enteroendocrine L-cells. A potent GLP-1 secretagogue with low effect on gallbladder volume, was identified to improve glucose homeostasis in a preclinical murine model of diet-induced obesity and IR (Lasalle et al., 2017). This topical intestinal TGR5 agonist is now further tested for its potential actions on the immune-inflammatory system as a tool and as a potential therapeutic agent.
- In addition, several translational studies were initiated to determine the role o
In conclusion, in the first part of the Immunibile program, the required animal models and technologies to assess the role of BA receptors in the control of immune-metabolism have been developed. Using these tools, we are now performing the functional studies in the second part of the program.