A bitter taste for the microbiome

Taste is a fine thing - nuanced by the presence of many compounds, their ratios to one another, and past experience. That is true of foods that enter the mouth. Could this also be true of the mix of food waste and bacteria in the colon? Although sampling a spoonful of Firmicutes or Bacteroidetes is admittedly something quite revolting, my thinking is leaning toward "yes." The brain likely has an idea of who is in residence in the colon and which metabolic byproducts are present.

A stir was created in 2007 when it was reported that two taste receptors and gustducin have a role in glucose-mediated responses, suggesting function as a previously undescribed glucose sensor in the gut lumen (Jang, Kokrashvili, et al. 2007 Proc Natl Acad Sci USA 104:15069–15074). One can find many news articles from that time highlighting this finding. It seems that the repertoire of taste receptors expressed in the gut, particularly in the colon, is much more extensive.

The ATLAS gene expression tool at the EBI is a semantically enriched database of meta-analysis based summary statistics over a curated subset of ArrayExpress gene expression data. ATLAS supports queries for condition-specific gene expression patterns as well as broader exploratory searches for biologically interesting genes. Using ATLAS, I found that the following ten taste receptors are expressed either in the small intestine or the colon:

TAS1R1
TAS2R13
TAS2R14
TAS2R16
TAS2R3
TAS2R38
TAS2R4
TAS2R7
TAS2R8
TAS2R9

Every one of these, except for one, are described as responding primarily to bitter tastants. TAS2R38 is sensitive to glucosinolate, a plant derived family of compounds, which do have a bitter-like taste. TAS1R1 is a type of sweet receptor, detecting primarily L-enantiomers of certain amino acids. It is highly likely that querying other gene expression databases will turn up other members of the taste receptor family as expressed in the lower gastrointestinal tract.

It makes sense to me that proteins that are designed to communicate what is in essence the composition of the external environment should be given such roles as sentry or monitor beyond the taste buds on the tongue. Taste receptors are designed to sense specific classes of chemicals and to relay a signal to the brain. Fecal fermentation of proanthocyanins, phytochemicals and other complex molecules is likely to be monitored in some way for the benefit of the human host. Perhaps taste receptors have a role in that process.