Anatomical and histological profiling of orphan G-protein-coupled receptor expression in gastrointestinal tract of C57BL/6J mice

Abstract

G-protein-coupled receptors (GPCRs) constitute the largest family of transmembrane receptors and regulate a variety of physiological and disease processes. Although the roles of many non-odorant GPCRs have been identified in vivo, several GPCRs remain orphans (oGPCRs). The gastrointestinal (GI) tract is the largest endocrine organ and is a promising target for drug discovery. Given their close link to physiological function, the anatomical and histological expression profiles of benchmark GI-related GPCRs, such as the cholecystokinin-1 receptor and GPR120, and 106 oGPCRs were investigated in the mucosal and muscle-myenteric nerve layers in the GI tract of C57BL/6J mice by quantitative real-time polymerase chain reaction. The mRNA expression patterns of these benchmark molecules were consistent with previous in situ hybridization and immunohistochemical studies, validating the experimental protocols in this study. Of 96 oGPCRs with significant mRNA expression in the GI tract, several oGPCRs showed unique expression patterns. GPR85, GPR37, GPR37L1, brain-specific angiogenesis inhibitor (BAI) 1, BAI2, BAI3, and GPRC5B mRNAs were preferentially expressed in the muscle-myenteric nerve layer, similar to GPCRs that are expressed in both the central and enteric nerve systems and that play multiple regulatory roles throughout the gut-brain axis. In contrast, GPR112, trace amine-associated receptor (TAAR) 1, TAAR2, and GPRC5A mRNAs were preferentially expressed in the mucosal layer, suggesting their potential roles in the regulation of secretion, immunity, and epithelial homeostasis. These anatomical and histological mRNA expression profiles of oGPCRs provide useful clues about the physiological roles of oGPCRs in the GI tract.

Gene Expression Profiling Reveals Functional Specialization along the Intestinal Tract of a Carnivorous Teleostean Fish (Dicentrarchus labrax)

“…Another orphan receptor, GPR112 , was highly DE in the AI-MI of European sea bass. This GPCR was also highly expressed in the intestinal mucosal layer of zebrafish (Harty et al, 2015) and rodents (Ito et al, 2009; Badiali et al, 2012). NPBWR1 and NPBWR2 were also differentially upregulated in the AI-MI of European sea bass.…”

Trace Amines and Their Receptors

“…TAAR2 mRNA transcripts have also been reported in nonsensory cells in other mammalian species, in duodenal mucosal cells of the gastrointestinal system in mice (Ito et al, 2009), and in the rat heart and testis . Whether a similar pattern of expression is observed in human tissues requires further study.…”

“…Beyond its role in the mammalian olfactory system, TAAR9 mRNA has been found in the full range of human leukocytes (D'Andrea et al, 2003;Babusyte et al, 2013), in addition to the pituitary gland and skeletal muscle (Vanti et al, 2003) (Table 4). TAAR9 mRNA has also been reported to be present in the mouse gastrointestinal tract, where it is preferentially localized to duodenal mucosal cells (Ito et al, 2009), as well as the spleen (Regard et al, 2008) and spinal cord (Gozal et al, 2014). A loss-of-function mutation in the TAAR9 gene with unknown clinical relevance has been reported to be present in up to 20% of the human population (Vanti et al, 2003;Müller et al, 2010).…”

“…The signal transduction events occurring at TAAR6 have also not been investigated but it is expected that like other olfactory TAARs, TAAR6 will be coupled to G olf and subsequent cAMP accumulation. Beyond the olfactory system, TAAR6 mRNA has been found in mouse duodenal mucosal cells (Ito et al, 2009), in the rat spinal cord (Gozal et al, 2014) but not in the human spinal cord (Duan et al, 2004), and in the rat testis (Table 4). TAAR6 transcripts have been reported in several human brain regions, including the amygdala and hippocampus (Borowsky et al, 2001;Duan et al, 2004), basal ganglia, and frontal cortex and substantia nigra (Duan et al, 2004), and these levels may exceed those of the most thoroughly investigated member of the TAAR family, TAAR1 (Duan et al, 2004).…”

“…In the periphery, TAAR1 is prominently located throughout the gastrointestinal system in cells involved in hormone secretion in response to the presence of nutrients (Table 4). This includes the D cells of the stomach Revel et al, 2013;Adriaenssens et al, 2015;Raab et al, 2015), pancreatic b cells (Regard et al, 2007;Revel et al, 2013;Raab et al, 2015), and intestinal enterochromaffin mucosal cells (Kidd et al, 2008;Ito et al, 2009;Revel et al, 2013;Raab et al, 2015) of both rodents and humans. Multiple groups have also reported the presence of TAAR1 mRNA and protein in various populations of mouse and human leukocytes (D'Andrea et al, 2003;Nelson et al, 2007;Wasik et al, 2012;Babusyte et al, 2013), and the presence of TAAR1 protein in breast tissue was suggested very recently (Vattai et al, 2017), although in this latter instance no details on the cell type(s) in which staining was observed were provided.…”