G protein‐coupled receptors (GPCRs) are the largest gene family in the genome and are involved in almost every aspect of physiology ranging from hormone receptors to eyesight. They also happen to represent the largest class of “druggable” molecules; in fact, 20‐30% of all FDA‐approved drugs on the market target GPCRs. Despite this, the vast majority of GPCRs remain underappreciated with incomplete expression patterns and characterization. These receptors, termed GPRs, have been linked to metabolic pathways and are known to respond to naturally produced metabolites. We reasoned that the liver, the largest metabolic organ in the body, is primed to take advantage of some of these GPRs in order to regulate its internal environment. To identify these receptors, we performed a large‐scale screen for over 300 GPCRs on whole murine liver using TaqMan qPCR array cards. This screen identified a total of 57 GPRs – with 7 of the them deemed to be “high expressing” with ΔCt values comparable to the more well‐characterized hepatic GPCRs (i.e. glucagon receptor). We confirmed expression of each of these GPRs (GPRc5c, GPR125, GPR146, GPR137, GPR116, GPR108, GPR17) via gene‐specific PCR from multiple murine liver samples.In order to characterize these understudied receptors within the liver, we must first understand where they are localized and what activates them. Given the dearth of reliable antibodies, we began by examining expression using RNAscope, a powerful in situ technique that specifically localizes mRNA sequences. GPR108, which has previously been linked to regulation of Toll‐Like Receptor signaling, was found within hepatocytes. GPR146, a known C‐peptide receptor which has been shown to impact blood cholesterol levels, was highly expressed in hepatocytes, and weakly localized to cholangioctyes and vessels. GPR109a, a highly studied receptor known to respond to short chain fatty acids and a potential therapeutic target for liver disease, was found to be primarily expressed in cholangiocytes, which notably play a key role in modification of bile. Finally, GPR125, an atypical adhesion GPCR, exhibited weak expression in hepatocytes but rather strong staining in select cholangiocytes and the liver vasculature. This was confirmed using both immunohistochemistry and immunofluorescence microscopy where we noted basolateral staining along the small cholangiocytes and hepatocytes. Given its localization and function as an adhesion receptor, GPR125 is well‐positioned within the liver to respond to, and to regulate, extracellular matrix proteins and their composition in both health and disease. Efforts are currently underway to deorphanize these receptors using the PRESTO‐Tango assay and to better understand their roles in the pathogenesis of metabolic diseases in the liver.
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