Adhesion G protein-coupled receptors: signaling, pharmacology, and mechanisms of activation

Abstract

The adhesion G protein-coupled receptors (GPCRs) are a distinct family of more than 30 receptors in vertebrate genomes. These receptors have been shown to play pivotal roles in a diverse range of biological functions and are characterized by extremely large N termini featuring various adhesion domains capable of mediating cell-cell and cell-matrix interactions. The adhesion GPCR N termini also contain GPCR proteolytic site motifs that undergo autocatalytic cleavage during receptor processing to create mature GPCRs existing as noncovalently attached complexes between the N terminus and transmembrane regions. There is mounting evidence that adhesion GPCRs can couple to G proteins to activate a variety of different downstream signaling pathways. Furthermore, recent studies have demonstrated that adhesion GPCR N termini can bind to multiple ligands, which may differentially activate receptor signaling and/or mediate cell adhesion. In addition, studies on several distinct adhesion GPCRs have revealed that truncations of the N termini result in constitutively active receptors, suggesting a model of receptor activation in which removal of the N terminus may be a key event in stimulating receptor signaling. Because mutations to certain adhesion GPCRs cause human disease and because many members of this receptor family exhibit highly discrete distribution patterns in different tissues, the adhesion GPCRs represent a class of potentially important drug targets that have not yet been exploited. For this reason, understanding the mechanisms of activation for these receptors and elucidating their downstream signaling pathways can provide insights with the potential to lead to novel therapeutic agents.

Fig. 1.

Differential ligand binding to adhesion GPCRs can result in distinct physiological responses. An unliganded adhesion GPCR is shown in the lower portion of the figure, with its large N-terminal region cleaved at the GPS motif but remaining associated with the receptor's seven-transmembrane region. Ligands for adhesion GPCRs are often large secreted glycoproteins and/or components of the extracellular matrix. Some ligands (illustrated here by “Ligand A”) can interact with adhesion GPCRs to facilitate cell adhesion without stimulating downstream receptor signaling. Conversely, other ligands (illustrated here by “Ligand B”) induce either removal of the receptor's N terminus or large-scale N-terminal conformational changes to promote receptor coupling to intracellular G proteins and activation of G protein-mediated signaling pathways.