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TGR5 (also known as G protein–coupled bile acid receptor 1, GPBAR1) is a G protein–coupled bile acid receptor that is expressed in many diverse tissues. TGR5 is involved in various metabolic processes, including glucose metabolism and energy expenditure; however, TGR5's function in skeletal muscle is not fully understood. Using both gain- and loss-of-function mouse models, we demonstrate here that Tgr5 activation promotes muscle cell differentiation and muscle hypertrophy. Both young and old transgenic mice with muscle-specific Tgr5 expression exhibited increased muscle strength. Moreover, we found that Tgr5 expression is increased by the unfolded protein response (UPR), which is an adaptive response required for maintenance of endoplasmic reticulum (ER) homeostasis. Both ER stress response element (ERSE)- and unfolded protein response element (UPRE)-like sites are present in the 5′ upstream region of the Tgr5 gene promoter and are essential for Tgr5 expression by Atf6α (activating transcription factor 6α), a well known UPR-activated transcriptional regulator. We observed that in the skeletal muscle of mice, exercise-induced UPR increases Tgr5 expression, an effect that was abrogated in Atf6α KO mice, indicating that Atf6α is essential for this response. These findings indicate that the bile acid receptor Tgr5 contributes to improved muscle function and provide an additional explanation for the beneficial effects of exercise on skeletal muscle activity.

TGR5, a G protein-coupled bile acid receptor, is expressed in various tissues and regulates several physiological processes. In the skeletal muscle, TGR5 activation is known to induce muscle hypertrophy; however, the effects on glucose and lipid metabolism are not well understood, despite the fact that the skeletal muscle plays a major role in energy metabolism. Here, we demonstrate that skeletal muscle-specific TGR5 transgenic (Tg) mice exhibit increased glucose utilization, without altering the expression of major genes related to glucose and lipid metabolism. Metabolite profiling analysis by capillary electrophoresis time-of-flight mass spectrometry showed that glycolytic flux was activated in the skeletal muscle of Tg mice, leading to an increase in glucose utilization. Upon long-term, high-fat diet challenge, blood glucose clearance was improved in Tg mice without an accompanying increase in insulin sensitivity in skeletal muscle and a reduction of body weight. Moreover, Tg mice showed improved age-associated glucose intolerance. These results strongly suggest that TGR5 ameliorated glucose metabolism disorder that is caused by diet-induced obesity and aging by enhancing the glucose metabolic capacity of the skeletal muscle. Our study demonstrates that TGR5 activation in the skeletal muscle is effective in improving glucose metabolism and may be beneficial in developing a novel strategy for the prevention or treatment of hyperglycemia.

Transmembrane protein 100 (TMEM100) comprises 134 amino acid residues and is highly conserved among vertebrates. Tmem100 has been recently reported as a key factor in angiogenesis, pain transmission, and tumor suppression. Although the importance of TMEM100 function is well supported, few studies have elucidated its expression mechanism. In the current study, we found that activating transcription factor 6α, a transcription factor activated by endoplasmic reticulum (ER) stress, enhanced Tmem100 promoter activity. Two ER stress response element-like motifs were identified in the mouse Tmem100 promoter region. However, additional experiments using another type of ER stress inducer demonstrated that calcium signaling was more important than ER stress in the regulation of TMEM100 expression. Intracellular calcium signaling controls biological processes such as cell proliferation and embryonic development. This study suggested that TMEM100 performs various functions in response to alterations in calcium signaling in addition to those in response to ER stress.