New functions and signaling mechanisms for the class of adhesion G protein-coupled receptors

Abstract

The class of adhesion G protein-coupled receptors (aGPCRs), with 33 human homologs, is the second largest family of GPCRs. In addition to a seven-transmembrane α-helix-a structural feature of all GPCRs-the class of aGPCRs is characterized by the presence of a large N-terminal extracellular region. In addition, all aGPCRs but one (GPR123) contain a GPCR autoproteolysis-inducing (GAIN) domain that mediates autoproteolytic cleavage at the GPCR autoproteolysis site motif to generate N- and a C-terminal fragments (NTF and CTF, respectively) during protein maturation. Subsequently, the NTF and CTF are associated noncovalently as a heterodimer at the plasma membrane. While the biological function of the GAIN domain-mediated autocleavage is not fully understood, mounting evidence suggests that the NTF and CTF possess distinct biological activities in addition to their function as a receptor unit. We discuss recent advances in understanding the biological functions, signaling mechanisms, and disease associations of the aGPCRs.

Keywords: adhesion G protein-coupled receptor; cancer; development; myelination; signal transduction; structural biology; synaptogenesis.

Figure 1

Structure topology of aGPCRs. Adhesion GPCRs are characterized by a large N-terminus that features several adhesive and functional domains. While the adhesive domains are thought to play roles in cell–cell or cell–matrix interactions, the highly conserved GPCR proteolysis site (GPS), is part of a bigger GPCR autoproteolysis–inducing (GAIN) domain. Most aGPCRs undergo a GAIN domain–mediated autoproteolysis process at the GPS during protein maturation to generate an N-terminal fragment (NTF) and a C-terminal fragment (CTF) that remain in the form of a non-covalently associated heterodimer on the plasma membrane.

Figure 2

Possible GPR126 signaling modalities during heart development. GPR126 possesses a GPS motif and an extended NTF containing a CUB (Complement, Uegf, Bmp1) domain, a PTX (pentraxin) domain, and a hormone-binding domain. Study results in Engel’s lab indicate that GPR126 is expressed in endocardial cells (ECs) during mouse heart development. Since ECs and cardiomyocytes exhibit cellular defects in Gpr126 knockout mice, Engle and colleagues hypothesize that EC function depends on CTF-mediated signaling, while cardiomyocyte function depends on NTF-mediated signaling. (A) Signaling in ECs depends on ligand binding to the NTF, inducing CTF-dependent signaling. (B) Interaction of the NTF with a receptor or cell surface molecule induces CTF-dependent signaling in ECs and an unknown signaling pathway in cardiomyocytes. (C) Cleavage and dissociation of the NTF induces CTF-dependent signaling in ECs. The shed NTF induces signaling in cardiomyocytes.

Figure 3

Working model for GPR116 function in pulmonary surfactant homeostasis. Loss of GPR116 function in mice perturbs the balance of surfactant synthesis and secretion versus catabolism and recycling, resulting in surfactant accumulation in the airspaces. Surfactant accumulation in Gpr116 knockout animals is associated with increased phospholipid (PL) synthesis and secretion, and increased ATP- and thapsigargin-induced calcium flux, in type II epithelial cells. ATP/P2RY2 signaling is known to stimulate surfactant secretion in vitro, suggesting that GPR116 may modulate P2RY2 activity, or other receptor-dependent pathways implicated in PL secretion such as ADRB2 or ADORA2b, to control surfactant homeostasis in vivo. GPR116 expression is also detected in type I alveolar epithelial cells; the specific function of GPR116 in this cell type has yet to be determined.

Figure 4

Working model for GPR56 signaling in skeletal muscle. In response to mechanical overload, GPR56 signals through the Gα_12/13 subunit to activate mTOR and downstream protein synthesis. GPR56 gain of function induces IGF-1 mRNA expression, which is dependent on a functional Gα_12/13 subunit. Together, GPR56 signaling is a novel pathway linking mechanical loading to muscle anabolism.

Figure 5

Myelination defects in BFPP brain. Loss-of-function mutations in GPR56 cause a devastating human brain malformation called bilateral frontoparietal polymicrogyria (BFPP), in which the normal convoluted brain surface is replaced by numerous small gyri. In addition to this cortical defect, BFPP brains also show signs of myelination abnormalities. In contrast to the white matter in the normal brain (left), the BFPP brain (right) presents with signal changes (arrows) on MRI, indicating defective myelination. This figure is adapted from Figure 1a in Ref. .

Figure 6

Multiple alternative transcription start sites allow fine control of temporal and spatial expression of GPR56. Human GPR56 has at least 17 alternative transcription start sites, whereas mouse Gpr56 has only five. Some of the transcription start sites arose as a result of retrotransposon insertions. For example, exon 1m, which causes perisylvian polymicrogyria when one of the upstream noncoding elements is mutated, is placental mammal–specific and shows homology at its 3′ end to a long interspersed nuclear element (LINE), a family of retrotransposons; whereas noncoding exon 1m’s noncoding element is conserved between humans and mice, another noncoding GPR56 exon (exon 2) is present only in primates, derived from a primate-specific Alu retrotransposon. Comprehensive cataloging of RNA splice forms may soon suggest that complex expression patterns of other GPCRs may be regulated by alternative promoters. This figure is adapted from Figure 4A in Ref. .

Figure 7

GPR56 inhibits melanoma progression by removing TG2 in ECM. In melanoma cells, GPR56 internalizes TG2 via endocytosis, leading to its degradation in lysosomes. The loss of TG2 is associated with destabilized fibronectin deposition in ECM and compromised cell adhesion, which might result in inactivation of PKCα and subsequent impairment of VEGF secretion and angiogenesis, and ultimate inhibition of melanoma progression.

Figure 8

Gpr116 knock-down inhibits breast cancer cell bone metastasis in mouse tumor metastasis model. Two shRNA-mediated stable Gpr116 knock-down MDA-MB-231 cells (shGPR116 #1 and #2) or control cells (shNTC) (2 × 10⁵ cells) were injected directly into the left ventricle of 4- to 5-week-old female nu/nu mice (n = 10 mice in each group). Bioluminescent (left) and radiographic (right) imaging of representative mice in each experimental group at the indicated days are shown in the same color scale. White arrows indicate osteolytic lesions (right).

Figure 9

Participants of the 7th International Adhesion GPCR Workshop in the Folkman Auditorium, Boston Children’s Hospital, Harvard Medical School, Boston.