The dimerized pentraxin-like domain of the adhesion G protein-coupled receptor 112 (ADGRG4) suggests function in sensing mechanical forces

Adhesion G protein-coupled receptors (aGPCRs) feature large extracellular regions with modular domains that often resemble protein classes of various function. The pentraxin (PTX) domain, which is predicted by sequence homology within the extracellular region of four different aGPCR members, is well known to form pentamers and other oligomers. Oligomerization of GPCRs is frequently reported and mainly driven by interactions of the seven-transmembrane region and N or C termini. While the functional importance of dimers is well-established for some class C GPCRs, relatively little is known about aGPCR multimerization. Here, we showcase the example of ADGRG4, an orphan aGPCR that possesses a PTX-like domain at its very N-terminal tip, followed by an extremely long stalk containing serine-threonine repeats. Using X-ray crystallography and biophysical methods, we determined the structure of this unusual PTX-like domain and provide experimental evidence for a homodimer equilibrium of this domain which is Ca2+-independent and driven by intermolecular contacts that differ vastly from the known soluble PTXs. The formation of this dimer seems to be conserved in mammalian ADGRG4 indicating functional relevance. Our data alongside of theoretical considerations lead to the hypothesis that ADGRG4 acts as an in vivo sensor for shear forces in enterochromaffin and Paneth cells of the small intestine.

The expression profile and tumorigenic mechanisms of CD97 (ADGRE5) in glioblastoma render it a targetable vulnerability

Glioblastoma (GBM) is the most common and aggressive primary brain malignancy. Adhesion G protein-coupled receptors (aGPCRs) have attracted interest for their potential as treatment targets. Here, we show that CD97 (ADGRE5) is the most promising aGPCR target in GBM, by virtue of its de novo expression compared to healthy brain tissue. CD97 knockdown or knockout significantly reduces the tumor initiation capacity of patient-derived GBM cultures (PDGCs) in vitro and in vivo. We find that CD97 promotes glycolytic metabolism via the mitogen-activated protein kinase (MAPK) pathway, which depends on phosphorylation of its C terminus and recruitment of β-arrestin. We also demonstrate that THY1/CD90 is a likely CD97 ligand in GBM. Lastly, we show that an anti-CD97 antibody-drug conjugate selectively kills tumor cells in vitro. Our studies identify CD97 as a regulator of tumor metabolism, elucidate mechanisms of receptor activation and signaling, and provide strong scientific rationale for developing biologics to target it therapeutically in GBM.

The adhesion G protein-coupled receptor GPR56/ADGRG1 in cytotoxic lymphocytes

GPR56/ADGRG1 is an adhesion G protein-coupled receptor connected to brain development, haematopoiesis, male fertility, and tumorigenesis. Nevertheless, expression of GPR56 is not restricted to developmental processes. Studies over the last years have demonstrated a marked presence of GPR56 in human cytotoxic NK and T cells. Expression of GPR56 in these cells is driven by the transcription factor HOBIT, corresponds with the production of cytolytic mediators and the presence of CX3 CR1 and CD57, indicates a state of terminal differentiation and cellular exhaustion, and disappears upon cellular activation. Functional studies indicate that GPR56 regulates cell migration and effector functions and thereby acts as an inhibitory immune checkpoint. We here discuss the current state of knowledge regarding GPR56 in cytotoxic lymphocytes.

Modulation of GPR133 (ADGRD1) Signaling by its Intracellular Interaction Partner Extended Synaptotagmin 1 (ESYT1)

GPR133 (ADGRD1) is an adhesion G protein-coupled receptor that signals through Gαs and is required for growth of glioblastoma (GBM), an aggressive brain malignancy. The regulation of GPR133 signaling is incompletely understood. Here, we use proximity biotinylation proteomics to identify ESYT1, a Ca2+-dependent mediator of endoplasmic reticulum-plasma membrane bridge formation, as an intracellular interactor of GPR133. ESYT1 knockdown or knockout increases GPR133 signaling, while its overexpression has the opposite effect, without altering GPR133 levels in the plasma membrane. The GPR133-ESYT1 interaction requires the Ca2+-sensing C2C domain of ESYT1. Thapsigargin-mediated increases in cytosolic Ca2+ relieve signaling-suppressive effects of ESYT1 by promoting ESYT1-GPR133 dissociation. ESYT1 knockdown or knockout in GBM impairs tumor growth in vitro, suggesting functions of ESYT1 beyond the interaction with GPR133. Our findings suggest a novel mechanism for modulation of GPR133 signaling by increased cytosolic Ca2+, which reduces the signaling-suppressive interaction between GPR133 and ESYT1 to raise cAMP levels.

Loss of Adgra3 causes obstructive azoospermia with high penetrance in male mice

The adhesion receptor ADGRA3 (GPR125) is a known spermatogonial stem cell marker, but its impact on male reproduction and fertility has not been examined. Using a mouse model lacking Adgra3 (Adgra3-/- ), we show that 55% of the male mice are infertile from puberty despite having normal spermatogenesis and epididymal sperm count. Instead, male mice lacking Adgra3 exhibited decreased estrogen receptor alpha expression and transient dilation of the epididymis. Combined with an increased estradiol production, this indicates a post-pubertal hormonal imbalance and fluid retention. Dye injection revealed a blockage between the ejaculatory duct and the urethra, which is rare in mice suffering from infertility, thereby mimicking the etiologies of obstructive azoospermia found in human male infertility. To summarize, male reproductive tract development is dependent on ADGRA3 function that in concert with estrogen signaling may influence fluid handling during sperm maturation and storage.

GPR56 C-terminal fragment mediates signal received by N-terminal fragment of another adhesion GPCR Latrophilin1 in neurons

Adhesion GPCRs (aGPCRs) are a subfamily of GPCRs that are involved in cell adhesion, cell proliferation, and cell migration in various tissues. G protein-coupled receptor proteolytic site (GPS) of aGPCR is required to cleave the extracellular domain autocatalytically, generating two fragments; a N-terminal fragment (NTF) and a C-terminal fragment (CTF) containing seven transmembrane structure. NTF can interact with CTF non-covalently after cleavage, however the physiological significance of the cleavage of aGPCR at GPS, and also the interaction between NTF and CTF have not been fully clarified yet. In this study, we first investigated the expression profiles of two aGPCRs, GPR56/ADGRG1, and LPHN1/ADGRL1 in mouse brain, and found that the NTF and CTF of GPR56 independently expressed in different brain region at different developmental stages. Immunoprecipitation of GPR56CTF co-immunoprecipitated LPHN1NTF from mouse brain and HEK293T cells expressing both fragments. Stimulation with LPHN1 ligand, α-Latrotoxin N4C (αLTXN4C), to cells expressing LPHN1NTF and GPR56CTF increased intracellular Ca²⁺ concentration ([Ca²⁺ ]i). We also demonstrated that GPR56KO mouse neurons attenuated their Ca²⁺ response to αLTXN4C. These results suggest the possibility of functional and chimeric complex containing LPHN1NTF and GPR56CTF in neuronal signal transduction.

Activation of the adhesion G protein-coupled receptor GPR133 by antibodies targeting its N-terminus

We recently demonstrated that GPR133 (ADGRD1), an adhesion G protein-coupled receptor involved in raising cytosolic cAMP levels, is necessary for growth of glioblastoma (GBM) and is de novo expressed in GBM relative to normal brain tissue. Our previous work suggested that dissociation of autoproteolytically generated N-terminal and C-terminal fragments of GPR133 at the plasma membrane correlates with receptor activation and signaling. To promote the goal of developing biologics that modulate GPR133 function, we investigated the effects of antibodies against the N-terminus of GPR133 on receptor signaling. Here, we show that treatment of HEK293T cells overexpressing GPR133 with these antibodies increased cAMP levels in a concentration-dependent manner. Analysis of culture medium following antibody treatment further indicated the presence of complexes of these antibodies with the autoproteolytically cleaved N-terminal fragments of GPR133. In addition, cells expressing a cleavage-deficient mutant of GPR133 (H543R) did not respond to antibody stimulation, suggesting that the effect is cleavage dependent. Finally, we demonstrate the antibody-mediated stimulation of WT GPR133, but not the cleavage-deficient H543R mutant, was reproducible in patient-derived GBM cells. These findings provide a paradigm for modulation of GPR133 function with biologics and support the hypothesis that the intramolecular cleavage in the N-terminus modulates receptor activation and signaling.

Stimulation of Vibratory Urticaria-Associated Adhesion-GPCR, EMR2/ADGRE2, Triggers the NLRP3 Inflammasome Activation Signal in Human Monocytes

EMR2/ADGRE2 is an adhesion G protein-coupled receptor differentially expressed by human myeloid cells. It modulates diverse cellular functions of innate immune cells and a missense EMR2 variant is directly responsible for vibratory urticaria. Recently, EMR2 was found to activate NLRP3 inflammasome in monocytes via interaction with FHR1, a regulatory protein of complement Factor H. However, the functional involvement of EMR2 activation and its signaling mechanisms in eliciting NLRP3 inflammasome activation remain elusive. In this study, we show that EMR2-mediated signaling plays a critical role in triggering the activation (2^nd) signal for the NLRP3 inflammasome in both THP-1 monocytic cell line and primary monocytes. Stimulation of EMR2 by its agonistic 2A1 monoclonal antibody elicits a Gα₁₆-dependent PLC-β activation pathway, inducing the activity of downstream Akt, MAPK, NF-κB, and Ca²⁺ mobilization, eventually leading to K⁺ efflux. These results identify EMR2 and its associated signaling intermediates as potential intervention targets of NLRP3 inflammasome activation in inflammatory disorders.

Cell type-specific evaluation of ADGRG1/GPR56 function in developmental central nervous system myelination

Myelination of axons in the central nervous system (CNS) is a concerted effort between many cell types, resulting in significant cross-talk and communication among cells. Adhesion G protein-coupled receptor ADGRG1 (GPR56) is expressed in all major glial cells and regulates a wide variety of physiological processes by mediating cell-cell and cell-matrix communications. Previous literature has demonstrated the requirement of ADGRG1 in oligodendrocyte precursor cells (OPCs) during developmental myelination. However, it is unknown if ADGRG1 is responsible for myelin formation in a cell-type-specific manner. To that end, here we profiled myelin status in response to deletion of Adgrg1 specifically in OPCs, microglia, astrocytes, and neurons. Interestingly, we find that knocking out Adgrg1 in OPCs significantly decreases OPC proliferation and reduced number of myelinated axons. However, deleting Adgrg1 in microglia, astrocytes, and neurons does not impact developmental myelination. These data support an autonomous functional role for Adgrg1 in OPCs related to myelination.

The expanding functional roles and signaling mechanisms of adhesion G protein-coupled receptors

The adhesion class of G protein-coupled receptors (GPCRs) is the second largest family of GPCRs (33 members in humans). Adhesion GPCRs (aGPCRs) are defined by a large extracellular N-terminal region that is linked to a C-terminal seven transmembrane (7TM) domain via a GPCR-autoproteolysis inducing (GAIN) domain containing a GPCR proteolytic site (GPS). Most aGPCRs undergo autoproteolysis at the GPS motif, but the cleaved fragments stay closely associated, with the N-terminal fragment (NTF) bound to the 7TM of the C-terminal fragment (CTF). The NTFs of most aGPCRs contain domains known to be involved in cell-cell adhesion, while the CTFs are involved in classical G protein signaling, as well as other intracellular signaling. In this workshop report, we review the most recent findings on the biology, signaling mechanisms, and physiological functions of aGPCRs.

Adhesion GPCRs in Kidney Development and Disease

Chronic kidney disease (CKD) represents the fastest growing pathology worldwide with a prevalence of >10% in many countries. In addition, kidney cancer represents 5% of all new diagnosed cancers. As currently no effective therapies exist to restore kidney function after CKD- as well as cancer-induced renal damage, it is important to elucidate new regulators of kidney development and disease as new therapeutic targets. G protein-coupled receptors (GPCRs) represent the most successful class of pharmaceutical targets. In recent years adhesion GPCRs (aGPCRs), the second largest GPCR family, gained significant attention as they are present on almost all mammalian cells, are associated to a plethora of diseases and regulate important cellular processes. aGPCRs regulate for example cell polarity, mitotic spindle orientation, cell migration, and cell aggregation; all processes that play important roles in kidney development and/or disease. Moreover, polycystin-1, a major regulator of kidney development and disease, contains a GAIN domain, which is otherwise only found in aGPCRs. In this review, we assess the potential of aGPCRs as therapeutic targets for kidney disease. For this purpose we have summarized the available literature and analyzed data from the databases The Human Protein Atlas, EURExpress, Nephroseq, FireBrowse, cBioPortal for Cancer Genomics and the National Cancer Institute Genomic Data Commons data portal (NCIGDC). Our data indicate that most aGPCRs are expressed in different spatio-temporal patterns during kidney development and that altered aGPCR expression is associated with a variety of kidney diseases including CKD, diabetic nephropathy, lupus nephritis as well as renal cell carcinoma. We conclude that aGPCRs present a promising new class of therapeutic targets and/or might be useful as diagnostic markers in kidney disease.

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

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.