Bacteria live in our gut, acting as agents for food digestion, producing essential vitamins and conversing with our immune systems to permit residence in our tissues. This ?communication? occurs as the metabolites produced by the bacteria interact with our cells, changing signaling responses and transcriptional mechanisms. Although current studies have profiled many bacterial species living in our gut and linked them to a wide variety of normal or pathological physiological states, these studies of complex microbiota can not tell us how the individual bacterial products change our cellular function. Without this knowledge, we can't understand which cellular pathways are targeted, or what the effects of therapeutics that mimic or block bacterial responses would be. To address this open, critical question, we have paired two genetic models, C. elegans and E. coli, to determine which bacterial pathways affected fat accumulation in the host. C. elegans consume bacteria, but many of the microorganisms remain intact, reside and proliferate in the gut, similar to mammals. Using a visual screen in C. elegans, we found that multiple E. coli mutants deficient in the methylcitrate cycle, which converts propionate to pyruvate or succinate, stimulate a lipogenic reporter. This gene, the sterol Co-A desaturase (SCD1) fat-7, is conserved to humans, as is its regulation by the sterol response element binding protein (SREBP) family of transcription factors. Short chain fatty acids (SCFAs) such as propionate have been implicated as bacterial products influencing host responses. Strikingly, we found propionate also induces SCD expression in human intestine-derived cell lines. We will use C. elegans determine the mechanisms that link propionate to lipogenic gene expression, testing if this requires SREBPs, along with other transcription factors, then expand these studies to mammalian cell culture. Metabolic disease is often linked to two ?hits?: lipogenesis and immune function. We previously found that a conserved metabolic pathway increasing fat-7 in C. elegans also stimulated innate immune responses. Therefore, we will also determine if propionate changes immune function in C. elegans and in mammalian cells. Use of C. elegans allows a rapid genetic dissection of lipogenic and immune responses, identifying regulatory pathways in the context of a whole animals model. Our next steps, evaluating these high confidence models in human cells, confirms relevance to mammals and, importantly, allows us to examine these interactions in the more complex mammalian context.
Bacteria in the gut may have broad effects on host metabolism and impact the development of diseases such as Type 2 Diabetes. Changes in bacterial metabolites are predicted to have diverse effects on host function, from providing energy to signaling to peripheral tissues. Thus understanding how bacterial metabolites affect host processes is critical to understand the basis of metabolic disease. We have used a simple system, involving C. elegans and it's bacterial food, E. coli, to show that propionate metabolism can induce markers of lipogenesis in this metazoan model. We hypothesize that by using this straightforward model to understand the mechanisms of this induction, we will uncover regulatory linkages that will be relevant to mammalian physiology.
