Novel approaches leading towards peptide GPCR de-orphanisation

Abstract

The discovery of novel ligands for orphan GPCRs has profoundly affected our understanding of human biology, opening new opportunities for research, and ultimately for therapeutic development. Accordingly, much effort has been directed towards the remaining orphan receptors, yet the rate of GPCR de-orphanisation has slowed in recent years. Here, we briefly review contemporary methodologies of de-orphanisation and then highlight our recent integrated computational and experimental approach for discovery of novel peptide ligands for orphan GPCRs. We identified putative endogenous peptide ligands and found peptide receptor sequence and structural characteristics present in selected orphan receptors. With comprehensive pharmacological screening using three complementary assays, we discovered novel pairings of 17 peptides with five different orphan GPCRs and revealed potential additional ligands for nine peptide GPCRs. These promising findings lay the foundation for future studies on these peptides and receptors to characterise their roles in human physiology and disease.

Figure 1

Knowledge state for class A orphan GPCRs. There are 84 class A orphans receptors (excluding tentative pseudogenes), as classified by IUPHAR Committee on Receptor Nomenclature and Drug Classification (NC‐IUPHAR). These receptors generally have low sequence similarity to non‐orphans, making it more challenging to garner reliable data on their evolutionary history or 3D structure than for other GPCRs. Nonetheless, 34 orphan receptors have proposed endogenous ligands (yellow boxes), whereas the majority do not (black boxes). Gene expression data reveals abundant and ubiquitous tissue expression for many orphan receptors (Lachmann et al., 2018; green ring, darker shading denotes higher abundance). Aggregated disease associations for orphan receptors from OpenTargets (Carvalho‐Silva et al., 2019) highlight the clinical relevance and therapeutic potential across disease areas (purple ring, darker shading denotes stronger association). Inner ring: orphan GPCR publication/knowledge scores (black) and tool compounds listed on the ChEMBL database (blue; Nguyen et al., 2017)

Figure 2

Discovery of novel peptides for orphan GPCRs. Putative peptide orphan receptors were selected based on molecular sequence characteristics (top left). An endogenous peptide library was designed from evolutionary tracing and putative cleavage sites found within potential precursor proteins (bottom left); 218 peptides were screened against 21 orphan GPCRs in three independent functional assays covering multiple signalling pathways (middle). Five orphan GPCRs (GPR1, GPR15, GPR55, GPR68, and BB₃) were paired with 17 peptides and validated in at least two orthogonal assays (examples on the right). These novel peptide–receptor interactions represent unexplored aspects of human physiology with considerable implications for drug discovery efforts