Tissue-Dependent Expression of Bitter Receptor TAS2R38 mRNA

Abstract

TAS2R38 is a human bitter receptor gene with a common but inactive allele; people homozygous for the inactive form cannot perceive low concentrations of certain bitter compounds. The frequency of the inactive and active forms of this receptor is nearly equal in many human populations, and heterozygotes with 1 copy of the active form and 1 copy of the inactive form have the most common diplotype. However, even though they have the same genotype, heterozygotes differ markedly in their perception of bitterness, perhaps in part because of differences in TAS2R38 mRNA expression. Other tissues express this receptor too, including the nasal sinuses, where it contributes to pathogen defense. We, therefore, wondered whether heterozygous people had a similar wide range of TAS2R38 mRNA in sinonasal tissue and whether those with higher TAS2R38 mRNA expression in taste tissue were similarly high expressers in nasal tissue. To that end, we measured gene expression by quantitative PCR in taste and sinonasal tissue and found that expression abundance in one tissue was not related to the other. We confirmed the independence of expression in other tissue pairs expressing TAS2R38 mRNA, such as pancreas and small intestine, using autopsy data from the Genotype-Tissue Expression project (although people with high expression of TAS2R38 mRNA in colon also tended to have higher expression in the small intestine). Thus, taste tissue TAS2R38 mRNA expression among heterozygotes is unlikely to predict expression in other tissues, perhaps reflecting tissue-dependent function, and hence regulation, of this protein.

Figure 1.

Overview of tissue collection, processing, and qPCR analysis.

Figure 2.

Experimental flowchart.

Figure 3.

Correlation between abundance of the TAS2R38 PAV mRNA in nasal and taste tissue (A) and between PTC taste intensity ratings and abundance of the TAS2R38 PAV mRNA in taste tissue (B), or nasal tissue (C) (n = 6). Each dot represents data from 1 of 6 heterozygous subjects that provided samples in which we could detect appropriate cell-specific markers and no residual genomic DNA. The solid lines show linear correlation; the gray shading shows confidence intervals. Scales at the x-axis are the log of the relative abundance of the TAS2R8 PAV mRNA relative to GAPDH.

Figure 4.

Correlation of TAS2R38 mRNA expression between small intestine and pancreas from the GTEx database. Each dot represents data from 1 GTEx donor. (RNAseq data expressed as RPKM). The solid line shows linear correlation; the gray shading shows confidence intervals. Scales at the x-axis are the log of the relative abundance of the TAS2R8 PAV mRNA relative to GAPDH.