Hepatic GPR125: ligands and liver repair

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Project Summary G protein-coupled receptors (GPCRs) are 7 transmembrane domain proteins that aid in the maintenance of homeostasis by responding to cues produced by the internal and external environment. In humans, there are more than 800 known GPCRs, and while more than 20% of all FDA-approved drugs target GPCRs, the vast majority of these receptors are understudied. In fact, many of these uncharacterized ‘orphan’ receptors, termed GPRs, are expressed at extremely high levels in different tissues suggesting that they likely have important, yet unknown, physiological functions. The liver is the largest metabolic tissue in the human body and is responsible for a number of important functions including sensing and detoxifying xenobiotics, metabolizing glucose and other energy sources, and synthesizing and secreting bile acids. Thus, it is well- poised to take advantage of some sensory GPRs to aid in these processes. We recently identified a total of 57 GPRs that are expressed to varying degrees in the murine liver, including 7 with expression profiles that rival other well characterized receptors. One of these high expressing receptors is adhesion receptor GPR125 (ADGRA3). As a family, adhesion receptors have been linked to the maintenance of cell-cell junctions and have been reported to be activated by proteolytically cleavage of their non-covalently attached N-terminal domain. However, GPR125 is an atypical adhesion receptor with a mutated cleavage site and it is unclear if this receptor is activated via the conventional processes. GPR125 has been linked to the development of planar cell polarity in a Zebrafish model suggesting that this receptor may be involved in tissue repair in adult tissues. Using RNAscope and immunofluorescence, we have localized GPR125 to the basolateral membrane of cholangiocytes. Lining the bile ducts, cholangiocytes play a key role in the liver regeneration following injury. Thus, GPR125 is uniquely suited to contribute to this process. We hypothesize that GPR125, on the basolateral membrane of cholangiocytes, is activated by components of the extracellular matrix in order to contribute to liver repair following injury. This will be tested using the PRESTO-TANGO assay to deorphanize the receptor alongside a unique liver-specific knockout mediated by adenoviral delivery of CRISPR guide RNAs. Collectively, our discoveries will unveil new functions for a highly expressing, yet under-characterized, receptor and will set the stage for future physiological analysis in both mouse and man.