Identifying and Characterizing Hepatic Sensory Receptors

Abstract

Sensory receptors, including olfactory receptors (ORs), taste receptors (TAS), and orphan G‐protein coupled receptors (GPRs) have recently been found in a variety of non‐sensory tissues where they have important physiological functions. As G protein‐coupled receptors (GPCRs), these receptors serve as chemosensors by interpreting and responding to chemical cues in their native environment (i.e. blood, urine) and can be activated by naturally occurring metabolites. We reasoned that the liver, the largest metabolic organ in the body, is primed to take advantage of some of these sensory receptors in order to regulate its internal environment. To identify the complete complement of sensory receptors expressed in murine liver, we performed an unbiased screen of whole liver using TaqMan qPCR array cards. This screen identified a total of 16 ORs, 10 TAS, 3 opsins, and 57 GPRs – with 7 of the GPRs expressed at levels equal to well‐characterized hepatic GPCRs. In order to understand the functions of these sensory receptors, it is imperative to determine where they localize within the liver and their ligand profile. Unfortunately, due to the lack of reliable antibodies for most sensory receptors, localizing these receptors and resolving their binding profiles can be challenging. To begin, we focused initially on the hepatic ORs. Using gene‐specific primers and RT‐PCR, we successfully confirmed the presence of 16 ORs consistently expressed in male and female murine liver. The majority of the identified hepatic ORs are orphan receptors with no known ligands. This is due, in part, to the inability to functionally express ORs in heterologous cell systems. To examine functional expression of Olfr99 and Olfr267, the two highest expressing liver ORs, we cloned the full length ORs from the liver into an expression system containing an N‐terminal Flag tag and expressed them in HEK293T cells. Using live‐cell surface labeling for the extracellular Flag epitope, we were able to identify conditions that promoted functional expression of Olfr99, but not Olfr267, using a cleavable LUCY tag as well as two chaperone proteins, RTP1s and Ric8b. With expression conditions established, we began a large‐scale ligand screen using a cAMP driven Dual‐Luciferase assay. To date, we have screened Olfr99 against a total of 49 compounds and mixtures including small odorant molecules that cover a wide odorant space and physiologically‐relevant molecules including hepatic metabolites and bile acids. Although a strong activator has yet to be found for Olfr99, our results have uncovered the identification of novel sensory receptors in the liver. Efforts are currently underway to determine the localization and physiological function of these receptors to better understand how the liver responds to its immediate environment.Support or Funding InformationNIDDKThis abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.