How gut microbiota impact diabetes
An obesity-driven T2D epidemic is ongoing with an estimated 600 million patients affected by 2035. Caused by the functional failure of pancreatic beta cells, a reversal or stabilisation of beta-cell function could slow the course of this devastating disease. One recent theory is that high energy/fat intake leads to gut microbiota imbalance, causing alterations in circulating bacteria, microbiota-related metabolites, and endotoxins through leakage. Therefore, direct exposure of beta cells to gram negative bacteria as well as their toxins might trigger beta cell inflammation in T2D. “We at the BETA-BACT project devised a set of experiments to determine whether gut microbiota could play a role in the development of islet-cell inflammation and islet-cell dysfunction in type 2 diabetes,” outlines Dr Daniël van Raalte, project coordinator. Microbial involvement Researchers confirmed that high faecal content of gram-negative bacterium Enterobacter cloacae (E. cloacae) was associated with T2D incidence. Moreover, there were indications of effects on blood sugar control. In clonal cell lines and islet organoids that lack immune cells, incubation with E. cloacae had no effect. By contrast, in primary mouse islets with abundant immune cells, the enterobacterium increased insulin release as well as inflammation while reducing insulin content, demonstrating signs of beta-cell stress. Testing of gene expression products showed reduced islet metabolism and insulin production along with increased inflammatory molecules. When beneficial bacteria replaced E. cloacae, there was no change in islet cell function and no inflammation. Toll-like receptor 4 (TLR4) protein is classically thought to mediate the innate immune system response to bacteria. However, results showed that when TLR4 was inactivated, the islets responded the same as control islets that were inoculated with the bug, suggesting that another molecule is responsible for the E. Cloacae-induced changes. When the experiment was repeated after depletion of macrophages (immune cells) which harbour the TLR5, the inflammatory response was fully prevented. Interestingly, TLR5 is a receptor for flagellin, involved in the movement of bacteria along the intestine by flagella. Still on the research path The BETA-BACT team are running several experiments – blocking TLR5 with pharmacological compounds, modifying E. cloacae to delete its flagellar protein, and characterising the macrophages by RNA sequencing. “With these data, we get more insight into the mechanisms of how microbiota are linked to diabetes development through islet cell inflammation and dysfunction,” states Dr van Raalte. When the underlying pathways and mechanisms are established, compounds that block the processes can be developed for treating or preventing T2D. Another possibility is that diet can be modified to manipulate the levels of different gut microbiota. Marie Skłodowska-Curie funding opens research doors and progress BETA-BACT was funded under the Marie Skłodowska-Curie fellowship programme and Dr van Raalte describes how many research opportunities have developed from the project. Together with grants on more clinical topics, he has built his research group (currently consisting of 13 PhD students, 2 technicians, 3 trial nurses and several master students) and been appointed Associate Professor and Principal Investigator at Amsterdam University. When all the data is complete, a significant knowledge base about how gram negative bacteria may cause beta-cell inflammation will be created. Dr van Raalte sums up: “If the gram-negative bacteria from the gut or the products they secrete are a driver in beta-cell failure, they form a clear target for therapy, as beta-cell failure is a driving force in the development and progression of diabetes.”
Keywords
BETA-BACT, inflammation, type 2 diabetes (T2D), microbiota, E. cloacae, insulin, beta cell
