We here report the existence of a new cluster of adhesion-GPCRs in human immune cells. Analysis of a comprehensive immune cell transcriptome dataset indicated that expression of the closely related receptors, GPR56, GPR97, and GPR114, is associated with single lymphocyte and granulocyte subsets. Applying flow cytometric analysis with newly generated mAb, we show that expression of GPR56 is restricted to cytotoxic NK and T lymphocytes, including CD8+, CD4+, and γδ T cells. Primary infection with human CMV, which generates a vast population of CD8+ T cells with an effector phenotype, induced a strong increase in GPR56 expression in virus-specific CD8+ T cells that remained detectable during latency. In NK-92 cells, ectopic expression of GPR56 inhibited spontaneous and SDF-1-stimulated cell migration. Our data suggest that GPR56 expression is a common trait of human cytotoxic lymphocytes and might affect the migratory properties of these cells.

Natural killer (NK) cells possess potent cytotoxic mechanisms that need to be tightly controlled. Here, we explored the regulation and function of GPR56/ADGRG1, an adhesion G protein-coupled receptor implicated in developmental processes and expressed distinctively in mature NK cells. Expression of GPR56 was triggered by Hobit (a homolog of Blimp-1 in T cells) and declined upon cell activation. Through studying NK cells from polymicrogyria patients with disease-causing mutations in ADGRG1, encoding GPR56, and NK-92 cells ectopically expressing the receptor, we found that GPR56 negatively regulates immediate effector functions, including production of inflammatory cytokines and cytolytic proteins, degranulation, and target cell killing. GPR56 pursues this activity by associating with the tetraspanin CD81. We conclude that GPR56 inhibits natural cytotoxicity of human NK cells.

Loss-of-function mutations in the gene encoding G protein-coupled receptor 56 (GPR56) lead to bilateral frontoparietal polymicrogyria (BFPP), an autosomal recessive disorder affecting brain development. The GPR56 receptor is a member of the adhesion-GPCR family characterized by the chimeric composition of a long ectodomain (ECD), a GPCR proteolysis site (GPS), and a seven-pass transmembrane (7TM) moiety. Interestingly, all identified BFPP-associated missense mutations are located within the extracellular region of GPR56 including the ECD, GPS, and the extracellular loops of 7TM. In the present study, a detailed molecular and functional analysis of the wild-type GPR56 and BFPP-associated point mutants shows that individual GPR56 mutants most likely cause BFPP via different combination of multiple mechanisms. These include reduced surface receptor expression, loss of GPS proteolysis, reduced receptor shedding, inability to interact with a novel protein ligand, and differential distribution of the 7TM moiety in lipid rafts. These results provide novel insights into the cellular functions of GPR56 receptor and reveal molecular mechanisms whereby GPR56 mutations induce BFPP.

Cells respond to diverse types of mechanical stimuli using a wide range of plasma membrane-associated mechanosensitive receptors to convert extracellular mechanical cues into intracellular signaling. G protein-coupled receptors (GPCRs) represent the largest cell surface protein superfamily that function as versatile sensors for a broad spectrum of bio/chemical messages. In recent years, accumulating evidence has shown that GPCRs can also engage in mechano-transduction. According to the GRAFS classification system of GPCRs, adhesion GPCRs (aGPCRs) constitute the second largest GPCR subfamily with a unique modular protein architecture and post-translational modification that are well adapted for mechanosensory functions. Here, we present a critical review of current evidence on mechanosensitive aGPCRs.

The adhesion class G protein-coupled receptors (adhesion-GPCRs) play important roles in diverse biological processes ranging from immunoregulation to tissue polarity, angiogenesis, and brain development. These receptors are uniquely modified by self-catalytic cleavage at a highly conserved GPCR proteolysis site (GPS) dissecting the receptor into an extracellular subunit (α) and a seven-pass transmembrane subunit (β) with cellular adhesion and signaling functions, respectively. Using the myeloid cell-restricted EMR2 receptor as a paradigm, we exam the mechanistic relevance of the subunit interaction and demonstrate a critical role for GPS autoproteolysis in mediating receptor signaling and cell activation. Interestingly, two distinct receptor complexes are identified as a result of GPS proteolysis: one consisting of a noncovalent α-β heterodimer and the other comprising two completely independent receptor subunits which distribute differentially in membrane raft microdomains. Finally, we show that receptor ligation induces subunit translocation and colocalization within lipid rafts, leading to receptor signaling and inflammatory cytokine production by macrophages. Our present data resolve earlier conflicting results and provide a new mechanism of receptor signaling, as well as providing a paradigm for signal transduction within the adhesion-GPCR family.