protease inhibitor cocktail synthesis In this review we will

In this review, we will present the traditional and actual strategies to study orphan receptors and identify their ligands. An extensive description of the orphan GPCR field has been published in 2013 by Davenport et al. [22]. Therefore, we will focus on the deorphanizations that were reported since 2013. Because of the importance of the reproducibility of initial pairings, we also discuss prominent ongoing controversies regarding problematic orphan GPCRs pairings.
Current strategies to assign function to orphan GPCRs Each orphan GPCR bares the potential of being the kick-start of a whole new research area with novel therapeutic options. However, the identification of a clearly defined function for the remaining orphans is a daunting task and these understudied receptors must be extensively investigated with innovative tools in preclinical research before reaching the status of validated target. There are many hurdles that preclude research on orphan GPCRs and the field is obviously suffering from a “streetlight effect”, where investigators are biased toward the same well-described receptors, just because there are tools to study them. This phenomenon is not restricted to GPCR and is observed in many other fields [26]. Therefore, the relatively limited literature on some orphans is probably the consequence of a paucity of useful probes and adequate tools [27]. An additional possible explanation for lack of literature expansion on particular orphans is that attempts by other labs to reproduce the receptor-ligand pair failed, thus precluding further research initiatives on the topic. For investigators interested by obscure orphans, the very first step is to carefully design and generate innovative tools to address the questions about the receptor function. Schematically, three types of experimental strategies can be envisaged: a pharmacological approach, a genetic approach or both combined. Strangely, whereas the identification of a ligand is usually followed by studies in transgenic models and the characterization of a protease inhibitor cocktail synthesis followed by the quest for a ligand, strategies build on both approaches simultaneously are not so common, although they would be probably highly efficient. We present in this section the current trends in technologies applied to orphans and discuss their respective advantages and weaknesses.
Current deorphanization landscape In this section, we present a detailed overview of the current state of ongoing investigations on recently deorphanized receptors. We also discuss some of the ligand-receptor pairs that have not yet reached consensus due to divergent data among different labs (see also Table 2).
Conclusions Firstly, the number of deorphanization has decreased and remained low for the past 10 years (Fig. 1). However, there is nothing that suggests that it is because of insurmountable technical hurdles. Recently, significant advances have been made in many aspects of physiology following deorphanizations (See Table 1 and Davenport et al. [22]). Therefore, current and future research on the remaining orphans should be maintained and amplified with new innovative tools in order to maintain or even increase the current rate of deorphanization. Secondly, we noticed a surprisingly high number of irreproducible ligand-receptor pairs along with discrepancies difficult to reconcile (see Table 2). Given the importance of proper definition of function for the remaining orphans, novel deorphanizations should be carefully assessed before claiming a new ligand-receptor pair. The reports that were later debated or unconfirmed have several points in common. Firstly, they are usually based on a single technique to assay the receptor activation. The gold standard when characterizing a new ligand is to use at least one (preferably two) orthogonal assays. These can be defined as assays demonstrating the same cellular effect (such as activation of a given signaling pathway) but in a completely unrelated technical environment. For instance, the detection of arrestin recruitment with a luciferase-based complementation assay should be confirmed with a system with no luciferase or even enzyme involved, such as the redistribution of signal from an arrestin fused with GFP by confocal microscopy. In addition, distinct GPCR signaling pathways should be assayed and the test should be as close as possible to the receptor activation event. From this perspective, reporter gene assays should be avoided because they are remote from receptor activation and are prone to artefacts and indirect effects due to amplification. Secondly, we noticed that in several cases there was already an important effect on non-transfected cells. Rather than normalization, the best option would be to find a system where the putative ligand has no effect on the background. When the elimination of background is technically difficult, for instance in case of ligand-receptor promiscuity, surrogate agonists and antagonists represent invaluable tools as positive and negative controls. Another popular strategy consists in using cellular background more remote from humans and mammals such as yeast [187], to avoid the presence of endogenous receptors.