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Background: Sweet taste receptors (STRs) are formed by two proteins, T1R2 and T1R3, and are expressed in many tissues outside of the tongue, such as the pancreas and the intestine. There they function as nutrient sensors regulating endocrine secretion. We found STRs are involved in skeletal muscle function by improving its oxidative capacity. We reasoned that STR expression in different fiber types might account for these observations. Thus, we set to assess the expression of STRs in quadriceps, soleus, and tibialis anterior muscles using skeletal muscle-specific mRNA analysis.

Methods: The RiboTag method allows for analysis of STR expression specifically in muscle fibers. This technology utilizes ribosomal tagging to analyze cell specific gene expression. For this experiment, we crossed Myogenin-Cre mice with RiboTag-fl/fl mice which generate mice that express a specific marker on ribosomes in myocytes. This marker allows for immunoprecipitation (IP) of mRNA selectively from myocytes. Reverse transcription and qPCR analysis of the resulting cDNA followed. Specific gene markers were chosen to assess their expression in the elution, which includes mRNA only from muscle fiber compared to the input, which is mRNA from total muscle tissue. Based on this, myocyte-specific genes should be robustly amplified, while genes specific to other cell types such as immune or satellite cells will be de-enriched.

Results: Compared to whole-muscle input control, all the muscle-specific marker genes Myh7, Myh2, Myh4, and Myh1 were enriched in the elution from RiboTag/Myogenin mRNA IP. T1R2, T1R3, and Gnat3 (involved in STR signaling) were also enriched in all muscle elutions. As expected, genes for other cell types such as immune cells, satellite cells, and macrophages were significantly de-enriched in the elution, confirming the specificity of the experiment. Using principal component analysis (PCA) across all muscles, we found that STR genes cluster with specific type I and IIa fiber markers suggesting that their expression may be linked to oxidative fibers.

Conclusions: STRs are expressed in myofibers of all muscles but their expression is more closely associated with oxidative fibers. To confirm these associations future investigation will focus on RiboTag x T1R2-Cre mice to assess the profile of T1R2-expressing fibers.