Scientists discover a new connection between gut microbiome and inflammation through ‘happy accident’
“This is a perfect example of how you have to follow what the science tells you, regardless of your plans,” says Dr. Stephen Girardin, Professor in the Department of Laboratory Medicine and Pathobiology at the University of Toronto.
Working with PhD candidate, and now Postdoctoral Fellow, Dr. Nathaniel Winsor, they discovered an issue in mice they were using to study the interaction of the gut microbiome and colon cancer. Facing a dilemma – scrap two years of experiments or start again and dig deeper into the issue – they chose to follow their curiosity which could now change our understanding of the role of parasites in gut inflammation. Their study, a culmination of 10 years of investigation, has recently been published in Cell Host & Microbe.
The Girardin lab studies immune regulation of mucosal barriers, specifically in the gut. They investigate how innate sensors in the gut’s epithelial cells detect changes in the microbiome and trigger inflammation as a defense mechanism e.g. when a certain bacteria or virus enters the gut and what the effect of this inflammatory response can have on cancer development. The team was interested in a particular sensor, a protein called NLRP6, which is abundant in these epithelial cells, but its function was not clearly understood.
“What seemed to be a constant in literature was that NLRP6 was responding to changes and imbalances in the microbiome and that it seemed to be somehow involved in colorectal cancer. We wanted to test those two ideas, so we created a mouse model where the cancer would be driven by changes in the microbiome. We discovered that mice which lacked NLRP6 developed more cancer, so we were excited to discover a link between the microbiota, inflammation and colorectal cancer,” says Winsor.
Their premise was that NLRP6 was sensing changes in the microbiota but when they investigated further, they could find no differences in microbiota in the two groups of mice – so what was it responding to? A colleague from Immunology looked at the data and realized that their mice were full of single-celled organisms called Protozoans which they identified as a parasite called Tritrichomonas.
“This was bad news. All the mice in our facility were infected with this parasite and we had no idea,” says Girardin, “This introduced an extra variable into our experiments. We had to make a painful decision – to drop the project or start again and repeat the experiment, which had already taken us two years, with mice who didn’t have the parasite.”
Repeating the experiment with parasite-free mice showed no difference in cancer levels with or without NLRP6 so they realized the protozoan were key. “This completely shifted the project. Now we were less focused on the cancer and more interested in the interaction between the protozoan, the microbiome and NLRP6. As scientists we work in a clean, controlled environment where we’re normally more interested in bacteria or viruses. No one had ever thought about parasites as we rarely encounter them – we’d uncovered an interesting and unusual niche in our field.”
Parasites induce what’s known as a goblet cell response in our intestine – they trigger the gut to produce mucus to create a protective barrier. Girardin and Winsor created an experimental model and assay to measure the goblet cell responses, discovering that there was a connection. Mice with NLRP6 present produced more mucus, those with no NLRP6 produced less which resulted in more inflammation and cancer risk. To investigate how NLRP6 was triggered and why the parasite induced this reaction, they conducted experiments in U of T’s Germ-Free Animal Facility and in collaboration with an expert in ex vivo imaging modalities, Dr. George Birchenough, University of Gothenburg, Sweden.
They discovered that NLRP6 wasn’t responding to the parasite at all, but a change in the microbiome induced by the parasite. “We’ve discovered that the mechanism of detection is quite unexpected. Here we show that we don't respond to the parasite directly, we respond to the changes that the parasites trigger which could be a new paradigm for our field,” explains Girardin.
The infection of their mice with parasites could have shut down the whole project but instead has opened a new avenue of research into how parasites interact with our gut microbiome and have an impact on our overall health. “This accidental discovery has opened our eyes to a previously overlooked group of organisms and a collection of new immunostimulatory molecules and their potential impact on our health,” adds Winsor.
Graphical abstract description
The graphical abstract shows how NLRP6 is present in goblet cells and regulates the mucus barrier in response to microbial ligands known as sphingolipids, that increase as a consequence of changes in the microbiome (increases in Bacteroides and Prevotella bacteria) during protozoan (Tritrichomonas) infection.
This study was conducted in collaboration with the University of Toronto colleagues Dr. Rafael Montenegro-Burke (Department of Molecular Genetics), Dr. John Parkinson (Department of Biochemistry), Dr. Dana Philpott (Department of Immunology), The Centre for the Analysis of Genome Evolution and Function (CAGEF) and the Germ-Free Animal Facility.
Thank you to the funding bodies: Canadian Institute of Health Research (CIHR) and Crohn’s and Colitis Canada.
Read the paper: Cross-kingdom-mediated detection of intestinal protozoa through NLRP6 Winsor, Nathaniel J. et al. Cell Host & Microbe, Volume 33, Issue 3, 388 - 407.e9
This story showcases the following pillars of the LMP strategic plan: Dynamic Collaboration (pillar 2), Impactful Research (pillar 3) and Disruptive Innovation (pillar 4).
