GLP-1 receptor agonists improve glycemia and lipidemia through changes in gut microbiota and the intestinal immune system. 

Background 
Anti-diabetic drugs need to demonstrate benefits in addition to glucose control, in order to target niches of type 2 diabetic patients. 
Aim 
To differentiate the GLP-1 receptor agonists liraglutide and exendin-4 (ex-4) from each other and to identify discriminant therapeutic features, we studied metabolic parameters, gut microbiota and the intestinal immune system in diet-induced obese (DIO) mice.
Method 
DIO mice were treated i.p. with vehicle, ex-4 10µg/kg BID or liraglutide 100µg/kg QD and drug effects were evaluated after 14 days of treatment. The intestinal immune system was studied by cell sorting analyses and the gut microbiota by targeted 16S sequencing.
Results 
Compared with vehicle, ex-4 induced a significant body weight loss (-3.6g) after treatment, while liraglutide showed a more pronounced effect (-6.2g). Ex-4 displayed more effective glycemic control compared to liraglutide after an oral glucose tolerance test, which was due to a stronger impact on gastric emptying (as measured with acetaminophen oral administration). However, following a test meal, both ex-4 and liraglutide significantly reduced blood glucose by 40% and triglycerides levels by 25%. Ex-4 significantly reduced intestinal cholesterol absorption by 40% after a 14C-cholesterol labeled olive oil oral gavage, while liraglutide showed a more pronounced effect (50% reduction). Fecal cholesterol mass excretion was significantly increased by 62% and 75% with ex-4 and liraglutide, respectively.  Gut microbiota taxonomic profiling further discriminated between ex-4 and liraglutide with significant changes in various bacteria taxa, including bifidobacteriaceae, clostridiaceae and lactobaciliceae. 
The intestinal immune system was oriented towards a dramatic reduction of the frequency of Th1-inf+ cells (80%), no change in Th17 cells, and an increased frequency of TReg cells (60%) with liraglutide treatment. All of these effects were abolished by antibiotic treatment, suggesting a role of gut microbiota. It was then demonstrated that germ-free mice colonized with the microbiota from liraglutide-treated animals became more glucose tolerant than germ-free mice colonized with the microbiota from vehicle-treated animals.
Discussion 
Ex-4 and liraglutide differentially alter gut microbiota and the intestinal adaptive immune system. The changes in gut microbiota improve the intestinal adaptive immune system and contribute to the amelioration of the metabolic disorders. The DIO mouse model, combined with in vivo phenotyping by microbiota sequencing and intestinal immune system analyses, represent useful tools to differentiate anti-diabetics and demonstrate their benefits beyond glucose control.

GLP-1 receptor agonists improve glycemia and lipidemia through changes in gut microbiota and intestinal immune system