Post-translational proteolytic processing of the calcium-independent receptor of alpha-latrotoxin (CIRL), a natural chimera of the cell adhesion protein and the G protein-coupled receptor. Role of the G protein-coupled receptor proteolysis site (GPS) motif

Novel Isoforms of Adhesion G Protein-Coupled Receptor B1 (ADGRB1/BAI1) Generated from an Alternative Promoter in Intron 17

Brain-specific angiogenesis inhibitor 1 (BAI1) belongs to the adhesion G-protein-coupled receptors, which exhibit large multi-domain extracellular N termini that mediate cell-cell and cell-matrix interactions. To explore the existence of BAI1 isoforms, we queried genomic datasets for markers of active chromatin and new transcript variants in the ADGRB1 (adhesion G-protein-coupled receptor B1) gene. Two major types of mRNAs were identified in human/mouse brain, those with a start codon in exon 2 encoding a full-length protein of a predicted size of 173.5/173.3 kDa and shorter transcripts starting from alternative exons at the intron 17/exon 18 boundary with new or exon 19 start codons, predicting two shorter isoforms of 76.9/76.4 and 70.8/70.5 kDa, respectively. Immunoblots on wild-type and Adgrb1 exon 2-deleted mice, reverse transcription PCR, and promoter-luciferase reporter assay confirmed that the shorter isoforms originate from an alternative promoter in intron 17. The shorter BAI1 isoforms lack most of the N terminus and are very close in structure to the truncated BAI1 isoform generated through GPS processing from the full-length receptor. The cleaved BAI1 isoform has a 19 amino acid extracellular stalk that may serve as a receptor agonist, while the alternative transcripts generate BAI1 isoforms with extracellular N termini of 5 or 60 amino acids. Further studies are warranted to compare the functions of these isoforms and examine the distinct roles they play in different tissues and cell types.

Biased signalling is structurally encoded as an autoproteolysis event in adhesion G protein-coupled receptor Latrophilin-3/ADGRL3

Adhesion G protein-coupled receptors (aGPCRs) possess a unique topology, including the presence of a GPCR proteolysis site (GPS), which, upon autoproteolysis, generates two functionally distinct fragments that remain non-covalently associated at the plasma membrane. A proposed activation mechanism for aGPCRs involves the exposure of a tethered agonist, which depends on cleavage at the GPS. However, this hypothesis has been challenged by the observation that non-cleavable aGPCRs exhibit constitutive activity, thus making the function of GPS cleavage widely enigmatic. In this study, we sought to elucidate the function of GPS-mediated cleavage through the study of G protein coupling with Latrophilin-3/ADGRL3, a prototypical aGPCR involved in synapse formation and function. Using BRET-based G protein biosensors, we reveal that an autoproteolysis-deficient mutant of ADGRL3 retains constitutive activity. Surprisingly, we uncover that cleavage deficiency leads to a signalling bias directed at potentiating the activity of select G proteins such as Gi2 and G12/13. These data unveil the underpinnings of biased signalling for aGPCRs defined by GPS autoproteolysis.

GPR114/ADGRG5 is activated by its tethered peptide agonist because it is a cleaved adhesion GPCR

Family B2 or adhesion G protein-coupled receptors (AGPCRs) are distinguished by variable extracellular regions that contain a modular protease, termed the GPCR autoproteolysis-inducing domain that self-cleaves the receptor into an N-terminal fragment (NTF) and a C-terminal fragment (CTF), or seven transmembrane domain (7TM). The NTF and CTF remain bound after cleavage through noncovalent interactions. NTF binding to a ligand(s) presented by nearby cells, or the extracellular matrix anchors the NTF, such that cell movement generates force to induce NTF/CTF dissociation and expose the AGPCR tethered peptide agonist. The released tethered agonist (TA) binds rapidly to the 7TM orthosteric site to activate signaling. The orphan AGPCR, GPR114 was reported to be uncleaved, yet paradoxically capable of activation by its TA. GPR114 has an identical cleavage site and TA to efficiently cleave GPR56. Here, we used immunoblotting and biochemical assays to demonstrate that GPR114 is a cleaved receptor, and the self-cleavage is required for GPR114 TA-activation of Gs and no other classes of G proteins. Mutagenesis studies defined features of the GPR114 and GPR56 GAINA subdomains that influenced self-cleavage efficiency. Thrombin treatment of protease-activated receptor 1 leader/AGPCR fusion proteins demonstrated that acute decryption of the GPR114/56 TAs activated signaling. GPR114 was found to be expressed in an eosinophilic-like cancer cell line (EoL-1 cells) and endogenous GPR114 was efficiently self-cleaved. Application of GPR114 TA peptidomimetics to EoL-1 cells stimulated cAMP production. Our findings may aid future delineation of GPR114 function in eosinophil cAMP signaling related to migration, chemotaxis, or degranulation.

7TM domain structures of adhesion GPCRs: what's new and what's missing?

Adhesion-type G protein-coupled receptors (aGPCRs) have long resisted approaches to resolve the structural details of their heptahelical transmembrane (7TM) domains. Single-particle cryogenic electron microscopy (cryo-EM) has recently produced aGPCR 7TM domain structures for ADGRD1, ADGRG1, ADGRG2, ADGRG3, ADGRG4, ADGRG5, ADGRF1, and ADGRL3. We review the unique properties, including the position and conformation of their activating tethered agonist (TA) and signaling motifs within the 7TM bundle, that the novel structures have helped to identify. We also discuss questions that the kaleidoscope of novel aGPCR 7TM domain structures have left unanswered. These concern the relative positions, orientations, and interactions of the 7TM and GPCR autoproteolysis-inducing (GAIN) domains with one another. Clarifying their interplay remains an important goal of future structural studies on aGPCRs.

PTK7 is a positive allosteric modulator of GPR133 signaling in glioblastoma

The adhesion G-protein-coupled receptor GPR133 (ADGRD1) supports growth of the brain malignancy glioblastoma. How the extracellular interactome of GPR133 in glioblastoma modulates signaling remains unknown. Here, we use affinity proteomics to identify the transmembrane protein PTK7 as an extracellular binding partner of GPR133 in glioblastoma. PTK7 binds the autoproteolytically generated N-terminal fragment of GPR133 and its expression in trans increases GPR133 signaling. This effect requires the intramolecular cleavage of GPR133 and PTK7's anchoring in the plasma membrane. PTK7's allosteric action on GPR133 signaling is additive with but topographically distinct from orthosteric activation by soluble peptide mimicking the endogenous tethered Stachel agonist. GPR133 and PTK7 are expressed in adjacent cells in glioblastoma, where their knockdown phenocopies each other. We propose that this ligand-receptor interaction is relevant to the pathogenesis of glioblastoma and possibly other physiological processes in healthy tissues.

Adhesion G protein-coupled receptors-Structure and functions

Adhesion G protein-coupled receptors (aGPCRs) are an ancient class of receptors that represent some of the largest transmembrane-integrated proteins in humans. First recognized as surface markers on immune cells, it took more than a decade to appreciate their 7-transmembrane structure, which is reminiscent of GPCRs. Roughly 30 years went by before the first functional proof of an interaction with a G protein was published. Besides classic features of GPCRs (extracellular N terminus, 7-transmembrane region, intracellular C terminus), aGPCRs display a distinct N-terminal structure, which harbors the highly conserved GPCR autoproteolysis-inducing (GAIN) domain with the GPCR proteolysis site (GPS) in addition to several functional domains. Several human diseases have been associated with variants of aGPCRs and subsequent animal models have been established to investigate these phenotypes. Much progress has been made in recent years to decipher the structure and functions of these receptors. This chapter gives an overview of our current understanding with respect to the molecular structural patterns governing aGPCR activation and the contribution of these giant molecules to the development of pathologies.

A guide to adhesion GPCR research

Adhesion G protein-coupled receptors (aGPCRs) are a class of structurally and functionally highly intriguing cell surface receptors with essential functions in health and disease. Thus, they display a vastly unexploited pharmacological potential. Our current understanding of the physiological functions and signaling mechanisms of aGPCRs form the basis for elucidating further molecular aspects. Combining these with novel tools and methodologies from different fields tailored for studying these unusual receptors yields a powerful potential for pushing aGPCR research from singular approaches toward building up an in-depth knowledge that will facilitate its translation to applied science. In this review, we summarize the state-of-the-art knowledge on aGPCRs in respect to structure-function relations, physiology, and clinical aspects, as well as the latest advances in the field. We highlight the upcoming most pressing topics in aGPCR research and identify strategies to tackle them. Furthermore, we discuss approaches how to promote, stimulate, and translate research on aGPCRs 'from bench to bedside' in the future.

GPR125 (ADGRA3) is an autocleavable adhesion GPCR that traffics with Dlg1 to the basolateral membrane and regulates epithelial apico-basal polarity

The adhesion family of G protein–coupled receptors (GPCRs) is defined by an N-terminal large extracellular region that contains various adhesion-related domains and a highly-conserved GPCR-autoproteolysis-inducing (GAIN) domain, the latter of which is located immediately before a canonical seven-transmembrane domain. These receptors are expressed widely and involved in various functions including development, angiogenesis, synapse formation, and tumorigenesis. GPR125 (ADGRA3), an orphan adhesion GPCR, has been shown to modulate planar cell polarity in gastrulating zebrafish, but its biochemical properties and role in mammalian cells have remained largely unknown. Here, we show that human GPR125 likely undergoes cis-autoproteolysis when expressed in canine kidney epithelial MDCK cells and human embryonic kidney HEK293 cells. The cleavage appears to occur at an atypical GPCR proteolysis site within the GAIN domain during an early stage of receptor biosynthesis. The products, i.e., the N-terminal and C-terminal fragments, seem to remain associated after self-proteolysis, as observed in other adhesion GPCRs. Furthermore, in polarized MDCK cells, GPR125 is exclusively recruited to the basolateral domain of the plasma membrane. The recruitment likely requires the C-terminal PDZ-domain–binding motif of GPR125 and its interaction with the cell polarity protein Dlg1. Knockdown of GPR125 as well as that of Dlg1 results in formation of aberrant cysts with multiple lumens in Matrigel 3D culture of MDCK cells. Consistent with the multilumen phenotype, mitotic spindles are incorrectly oriented during cystogenesis in GPR125-KO MDCK cells. Thus, the basolateral protein GPR125, an autocleavable adhesion GPCR, appears to play a crucial role in apicobasal polarization in epithelial cells.

Adhesion G protein-coupled receptor gluing action guides tissue development and disease

Phylogenetic analysis of human G protein-coupled receptors (GPCRs) divides these transmembrane signaling proteins into five groups: glutamate, rhodopsin, adhesion, frizzled, and secretin families, commonly abbreviated as the GRAFS classification system. The adhesion GPCR (aGPCR) sub-family comprises 33 different receptors in humans. Majority of the aGPCRs are orphan receptors with unknown ligands, structures, and tissue expression profiles. They have a long N-terminal extracellular domain (ECD) with several adhesion sites similar to integrin receptors. Many aGPCRs undergo autoproteolysis at the GPCR proteolysis site (GPS), enclosed within the larger GPCR autoproteolysis inducing (GAIN) domain. Recent breakthroughs in aGPCR research have created new paradigms for understanding their roles in organogenesis. They play crucial roles in multiple aspects of organ development through cell signaling, intercellular adhesion, and cell-matrix associations. They are involved in essential physiological processes like regulation of cell polarity, mitotic spindle orientation, cell adhesion, and migration. Multiple aGPCRs have been associated with the development of the brain, musculoskeletal system, kidneys, cardiovascular system, hormone secretion, and regulation of immune functions. Since aGPCRs have crucial roles in tissue patterning and organogenesis, mutations in these receptors are often associated with diseases with loss of tissue integrity. Thus, aGPCRs include a group of enigmatic receptors with untapped potential for elucidating novel signaling pathways leading to drug discovery. We summarized the current knowledge on how aGPCRs play critical roles in organ development and discussed how aGPCR mutations/genetic variants cause diseases.

Thwarting of Lphn3 Functions in Cell Motility and Signaling by Cancer-Related GAIN Domain Somatic Mutations

Cancer progression relies on cellular transition states accompanied by changes in the functionality of adhesion molecules. The gene for adhesion G protein-coupled receptor latrophilin-3 (aGPCR Lphn3 or ADGRL3) is targeted by tumor-specific somatic mutations predominantly affecting the conserved GAIN domain where most aGPCRs are cleaved. However, it is unclear how these GAIN domain-altering mutations impact Lphn3 function. Here, we studied Lphn3 cancer-related mutations as a proxy for revealing unknown GAIN domain functions. We found that while intra-GAIN cleavage efficiency was unaltered, most mutations produced a ligand-specific impairment of Lphn3 intercellular adhesion profile paralleled by an increase in cell-matrix actin-dependent contact structures for cells expressing the select S810L mutation. Aberrant remodeling of the intermediate filament vimentin, which was found to coincide with Lphn3-induced modification of nuclear morphology, had less impact on the nuclei of S810L expressing cells. Notoriously, receptor signaling through G13 protein was deficient for all variants bearing non-homologous amino acid substitutions, including the S810L variant. Analysis of cell migration paradigms revealed a non-cell-autonomous impairment in collective cell migration indistinctly of Lphn3 or its cancer-related variants expression, while cell-autonomous motility was potentiated in the presence of Lphn3, but this effect was abolished in S810L GAIN mutant-expressing cells. These data identify the GAIN domain as an important regulator of Lphn3-dependent cell motility, thus furthering our understanding of cellular and molecular events linking Lphn3 genetic somatic mutations to cancer-relevant pathogenesis mechanisms.

Structural basis for the tethered peptide activation of adhesion GPCRs

Adhesion G-protein-coupled receptors (aGPCRs) are important for organogenesis, neurodevelopment, reproduction and other processes^1-6. Many aGPCRs are activated by a conserved internal (tethered) agonist sequence known as the Stachel sequence^7-12. Here, we report the cryogenic electron microscopy (cryo-EM) structures of two aGPCRs in complex with G_s: GPR133 and GPR114. The structures indicate that the Stachel sequences of both receptors assume an α-helical-bulge-β-sheet structure and insert into a binding site formed by the transmembrane domain (TMD). A hydrophobic interaction motif (HIM) within the Stachel sequence mediates most of the intramolecular interactions with the TMD. Combined with the cryo-EM structures, biochemical characterization of the HIM motif provides insight into the cross-reactivity and selectivity of the Stachel sequences. Two interconnected mechanisms, the sensing of Stachel sequences by the conserved 'toggle switch' W^6.53 and the constitution of a hydrogen-bond network formed by Q^7.49/Y^7.49 and the P^6.47/V^6.47φφG^6.50 motif (φ indicates a hydrophobic residue), are important in Stachel sequence-mediated receptor activation and G_s coupling. Notably, this network stabilizes kink formation in TM helices 6 and 7 (TM6 and TM7, respectively). A common G_s-binding interface is observed between the two aGPCRs, and GPR114 has an extended TM7 that forms unique interactions with G_s. Our structures reveal the detailed mechanisms of aGPCR activation by Stachel sequences and their G_s coupling.

Tethered peptide activation mechanism of the adhesion GPCRs ADGRG2 and ADGRG4

Adhesion G protein-coupled receptors (aGPCRs) constitute an evolutionarily ancient family of receptors that often undergo autoproteolysis to produce α and β subunits^1-3. A tethered agonism mediated by the 'Stachel sequence' of the β subunit has been proposed to have central roles in aGPCR activation^4-6. Here we present three cryo-electron microscopy structures of aGPCRs coupled to the G_s heterotrimer. Two of these aGPCRs are activated by tethered Stachel sequences-the ADGRG2-β-G_s complex and the ADGRG4-β-G_s complex (in which β indicates the β subunit of the aGPCR)-and the other is the full-length ADGRG2 in complex with the exogenous ADGRG2 Stachel-sequence-derived peptide agonist IP15 (ADGRG2(FL)-IP15-G_s). The Stachel sequences of both ADGRG2-β and ADGRG4-β assume a U shape and insert deeply into the seven-transmembrane bundles. Constituting the FXφφφXφ motif (in which φ represents a hydrophobic residue), five residues of ADGRG2-β or ADGRG4-β extend like fingers to mediate binding to the seven-transmembrane domain and activation of the receptor. The structure of the ADGRG2(FL)-IP15-G_s complex reveals the structural basis for the improved binding affinity of IP15 compared with VPM-p15 and indicates that rational design of peptidic agonists could be achieved by exploiting aGPCR-β structures. By converting the 'finger residues' to acidic residues, we develop a method to generate peptidic antagonists towards several aGPCRs. Collectively, our study provides structural and biochemical insights into the tethered activation mechanism of aGPCRs.

Therapeutic Application of Brain-Specific Angiogenesis Inhibitor 1 for Cancer Therapy

Brain-specific angiogenesis inhibitor 1 (BAI1/ADGRB1) is an adhesion G protein-coupled receptor that has been found to play key roles in phagocytosis, inflammation, synaptogenesis, the inhibition of angiogenesis, and myoblast fusion. As the name suggests, it is primarily expressed in the brain, with a high expression in the normal adult and developing brain. Additionally, its expression is reduced in brain cancers, such as glioblastoma (GBM) and peripheral cancers, suggesting that BAI1 is a tumor suppressor gene. Several investigators have demonstrated that the restoration of BAI1 expression in cancer cells results in reduced tumor growth and angiogenesis. Its expression has also been shown to be inversely correlated with tumor progression, neovascularization, and peri-tumoral brain edema. One method of restoring BAI1 expression is by using oncolytic virus (OV) therapy, a strategy which has been tested in various tumor models. Oncolytic herpes simplex viruses engineered to express the secreted fragment of BAI1, called Vasculostatin (Vstat120), have shown potent anti-tumor and anti-angiogenic effects in multiple tumor models. Combining Vstat120-expressing oHSVs with other chemotherapeutic agents has also shown to increase the overall anti-tumor efficacy in both in vitro and in vivo models. In the current review, we describe the structure and function of BAI1 and summarize its application in the context of cancer treatment.

Functional genomics of GPR126 in airway smooth muscle and bronchial epithelial cells

GPR126 is an adhesion G protein-coupled receptor which lies on chromosome 6q24. Genetic variants in this region are reproducibly associated with lung function and COPD in genome wide association studies (GWAS). The aims of this study were to define the role of GPR126 in the human lung and in pulmonary disease and identify possible casual variants. Online tools (GTEx and LDlink) identified SNPs which may have effects on GPR126 function/ expression, including missense variant Ser123Gly and an intronic variant that shows eQTL effects on GPR126 expression. GPR126 signaling via cAMP-mediated pathways was identified in human structural airway cells when activated with the tethered agonist, stachel. RNA-seq was used to identify downstream genes/ pathways affected by stachel-mediated GPR126 activation in human airway smooth muscle cells. We identified ~350 differentially expressed genes at 4 and 24 hours post stimulation with ~20% overlap. We identified that genes regulated by GPR126 activation include IL33, CTGF, and SERPINE1, which already have known roles in lung biology. Pathways altered by GPR126 included those involved in cell cycle progression and cell proliferation. Here, we suggest a role for GPR126 in airway remodeling.

Functional impact of intramolecular cleavage and dissociation of adhesion G protein-coupled receptor GPR133 (ADGRD1) on canonical signaling

GPR133 (ADGRD1), an adhesion G protein–coupled receptor (GPCR) whose canonical signaling activates G_αS-mediated generation of cytosolic cAMP, has been shown to be necessary for the growth of glioblastoma (GBM), a brain malignancy. The extracellular N terminus of GPR133 is thought to be autoproteolytically cleaved into N-terminal and C- terminal fragments (NTF and CTF, respectively). However, the role of this cleavage in receptor activation remains unclear. Here, we used subcellular fractionation and immunoprecipitation approaches to show that the WT GPR133 receptor is cleaved shortly after protein synthesis and generates significantly more canonical signaling than an uncleavable point mutant GPR133 (H543R) in patient-derived GBM cultures and HEK293T cells. After cleavage, the resulting NTF and CTF remain noncovalently bound to each other until the receptor is trafficked to the plasma membrane, where we demonstrated NTF–CTF dissociation occurs. Using a fusion of the CTF of GPR133 and the N terminus of thrombin-activated human protease-activated receptor 1 as a controllable proxy system to test the effect of intramolecular cleavage and dissociation, we also showed that thrombin-induced cleavage and shedding of the human protease-activated receptor 1 NTF increased intracellular cAMP levels. These results support a model wherein dissociation of the NTF from the CTF at the plasma membrane promotes GPR133 activation and downstream signaling. These findings add depth to our understanding of the molecular life cycle and mechanism of action of GPR133 and provide critical insights that will inform therapeutic targeting of GPR133 in GBM.

Structures of the glucocorticoid-bound adhesion receptor GPR97-Go complex

Adhesion G-protein-coupled receptors (GPCRs) are a major family of GPCRs, but limited knowledge of their ligand regulation or structure is available^1-3. Here we report that glucocorticoid stress hormones activate adhesion G-protein-coupled receptor G3 (ADGRG3; also known as GPR97)^4-6, a prototypical adhesion GPCR. The cryo-electron microscopy structures of GPR97-G_o complexes bound to the anti-inflammatory drug beclomethasone or the steroid hormone cortisol revealed that glucocorticoids bind to a pocket within the transmembrane domain. The steroidal core of glucocorticoids is packed against the 'toggle switch' residue W^6.53, which senses the binding of a ligand and induces activation of the receptor. Active GPR97 uses a quaternary core and HLY motif to fasten the seven-transmembrane bundle and to mediate G protein coupling. The cytoplasmic side of GPR97 has an open cavity, where all three intracellular loops interact with the G_o protein, contributing to the high basal activity of GRP97. Palmitoylation at the cytosolic tail of the G_o protein was found to be essential for efficient engagement with GPR97 but is not observed in other solved GPCR complex structures. Our work provides a structural basis for ligand binding to the seven-transmembrane domain of an adhesion GPCR and subsequent G protein coupling.

Structures of the glucocorticoid-bound adhesion receptor GPR97-Go complex

Adhesion G-protein-coupled receptors (GPCRs) are a major family of GPCRs, but limited knowledge of their ligand regulation or structure is available^1-3. Here we report that glucocorticoid stress hormones activate adhesion G-protein-coupled receptor G3 (ADGRG3; also known as GPR97)^4-6, a prototypical adhesion GPCR. The cryo-electron microscopy structures of GPR97-G_o complexes bound to the anti-inflammatory drug beclomethasone or the steroid hormone cortisol revealed that glucocorticoids bind to a pocket within the transmembrane domain. The steroidal core of glucocorticoids is packed against the 'toggle switch' residue W^6.53, which senses the binding of a ligand and induces activation of the receptor. Active GPR97 uses a quaternary core and HLY motif to fasten the seven-transmembrane bundle and to mediate G protein coupling. The cytoplasmic side of GPR97 has an open cavity, where all three intracellular loops interact with the G_o protein, contributing to the high basal activity of GRP97. Palmitoylation at the cytosolic tail of the G_o protein was found to be essential for efficient engagement with GPR97 but is not observed in other solved GPCR complex structures. Our work provides a structural basis for ligand binding to the seven-transmembrane domain of an adhesion GPCR and subsequent G protein coupling.

Optimization of a peptide ligand for the adhesion GPCR ADGRG2 provides a potent tool to explore receptor biology

The adhesion GPCR ADGRG2, also known as GPR64, is a critical regulator of male fertility that maintains ion/pH homeostasis and CFTR coupling. The molecular basis of ADGRG2 function is poorly understood, in part because no endogenous ligands for ADGRG2 have been reported, thus limiting the tools available to interrogate ADGRG2 activity. It has been shown that ADGRG2 can be activated by a peptide, termed p15, derived from its own N-terminal region known as the Stachel sequence. However, the low affinity of p15 limits its utility for ADGRG2 characterization. In the current study, we used alanine scanning mutagenesis to examine the critical residues responsible for p15-induced ADGRG2 activity. We next designed systematic strategies to optimize the peptide agonist of ADGRG2, using natural and unnatural amino acid substitutions. We obtained an optimized ADGRG2 Stachel peptide T1V/F3Phe(4-Me) (VPM-p15) that activated ADGRG2 with significantly improved (>2 orders of magnitude) affinity. We then characterized the residues in ADGRG2 that were important for ADGRG2 activation in response to VPM-p15 engagement, finding that the toggle switch W^6.53 and residues of the ECL2 region of ADGRG2 are key determinants for VPM-p15 interactions and VPM-p15-induced Gs or arrestin signaling. Our study not only provides a useful tool to investigate the function of ADGRG2 but also offers new insights to guide further optimization of Stachel peptides to activate adhesion GPCR members.

Mechanisms of adhesion G protein-coupled receptor activation

Adhesion G protein-coupled receptors (AGPCRs) are a thirty-three-member subfamily of Class B GPCRs that control a wide array of physiological processes and are implicated in disease. AGPCRs uniquely contain large, self-proteolyzing extracellular regions that range from hundreds to thousands of residues in length. AGPCR autoproteolysis occurs within the extracellular GPCR autoproteolysis-inducing (GAIN) domain that is proximal to the N terminus of the G protein-coupling seven-transmembrane-spanning bundle. GAIN domain-mediated self-cleavage is constitutive and produces two-fragment holoreceptors that remain bound at the cell surface. It has been of recent interest to understand how AGPCRs are activated in relation to their two-fragment topologies. Dissociation of the AGPCR fragments stimulates G protein signaling through the action of the tethered-peptide agonist stalk that is occluded within the GAIN domain in the holoreceptor form. AGPCRs can also signal independently of fragment dissociation, and a few receptors possess GAIN domains incapable of self-proteolysis. This has resulted in complex theories as to how these receptors are activated in vivo, complicating pharmacological advances. Currently, there is no existing structure of an activated AGPCR to support any of the theories. Further confounding AGPCR research is that many of the receptors remain orphans and lack identified activating ligands. In this review, we provide a detailed layout of the current theorized modes of AGPCR activation with discussion of potential parallels to mechanisms used by other GPCR classes. We provide a classification means for the ligands that have been identified and discuss how these ligands may activate AGPCRs in physiological contexts.

Adhesion GPCRs as a paradigm for understanding polycystin-1 G protein regulation

Polycystin-1, whose mutation is the most frequent cause of autosomal dominant polycystic kidney disease, is an extremely large and multi-faceted membrane protein whose primary or proximal cyst-preventing function remains undetermined. Accumulating evidence supports the idea that modulation of cellular signaling by heterotrimeric G proteins is a critical function of polycystin-1. The presence of a cis-autocatalyzed, G protein-coupled receptor (GPCR) proteolytic cleavage site, or GPS, in its extracellular N-terminal domain immediately preceding the first transmembrane domain is one of the notable conserved features of the polycystin-1-like protein family, and also of the family of cell adhesion GPCRs. Adhesion GPCRs are one of five families within the GPCR superfamily and are distinguished by a large N-terminal extracellular region consisting of multiple adhesion modules with a GPS-containing GAIN domain and bimodal functions in cell adhesion and signal transduction. Recent advances from studies of adhesion GPCRs provide a new paradigm for unraveling the mechanisms by which polycystin-1-associated G protein signaling contributes to the pathogenesis of polycystic kidney disease. This review highlights the structural and functional features shared by polycystin-1 and the adhesion GPCRs and discusses the implications of such similarities for our further understanding of the functions of this complicated protein.

C-terminal phosphorylation of latrophilin-1/ADGRL1 affects the interaction between its fragments

Latrophilin-1 is an adhesion G protein-coupled receptor that mediates the effect of α-latrotoxin, causing massive release of neurotransmitters from nerve terminals and endocrine cells. Autoproteolysis cleaves latrophilin-1 into two parts: the extracellular N-terminal fragment (NTF) and the heptahelical C-terminal fragment (CTF). NTF and CTF can exist as independent proteins in the plasma membrane, but α-latrotoxin binding to NTF induces their association and G protein-mediated signaling. We demonstrate here that CTF in synapses is phosphorylated on multiple sites. Phosphorylated CTF has a high affinity for NTF and copurifies with it on affinity columns and sucrose density gradients. Dephosphorylated CTF has a lower affinity for NTF and can behave as a separate protein. α-Latrotoxin (and possibly other ligands of latrophilin-1) binds both to the NTF-CTF complex and receptor-like protein tyrosine phosphatase σ, bringing them together. This leads to CTF dephosphorylation and facilitates CTF release from the complex. We propose that ligand-dependent phosphorylation-dephosphorylation of latrophilin-1 could affect the interaction between its fragments and functions as a G protein-coupled receptor.

Catching Latrophilin With Lasso: A Universal Mechanism for Axonal Attraction and Synapse Formation

Latrophilin-1 (LPHN1) was isolated as the main high-affinity receptor for α-latrotoxin from black widow spider venom, a powerful presynaptic secretagogue. As an adhesion G-protein-coupled receptor, LPHN1 is cleaved into two fragments, which can behave independently on the cell surface, but re-associate upon binding the toxin. This triggers intracellular signaling that involves the Gαq/phospholipase C/inositol 1,4,5-trisphosphate cascade and an increase in cytosolic Ca²⁺, leading to vesicular exocytosis. Using affinity chromatography on LPHN1, we isolated its endogenous ligand, teneurin-2/Lasso. Both LPHN1 and Ten2/Lasso are expressed early in development and are enriched in neurons. LPHN1 primarily resides in axons, growth cones and presynaptic terminals, while Lasso largely localizes on dendrites. LPHN1 and Ten2/Lasso form a trans-synaptic receptor pair that has both structural and signaling functions. However, Lasso is proteolytically cleaved at multiple sites and its extracellular domain is partially released into the intercellular space, especially during neuronal development, suggesting that soluble Lasso has additional functions. We discovered that the soluble fragment of Lasso can diffuse away and bind to LPHN1 on axonal growth cones, triggering its redistribution on the cell surface and intracellular signaling which leads to local exocytosis. This causes axons to turn in the direction of spatio-temporal Lasso gradients, while LPHN1 knockout blocks this effect. These results suggest that the LPHN1-Ten2/Lasso pair can participate in long- and short-distance axonal guidance and synapse formation.

Characterization of G-protein coupled receptors from the blackback land crab Gecarcinus lateralis Y organ transcriptome over the molt cycle

BACKGROUND

G-protein coupled receptors (GPCRs) are ancient, ubiquitous, constitute the largest family of transducing cell surface proteins, and are integral to cell communication via an array of ligands/neuropeptides. Molt inhibiting hormone (MIH) is a key neuropeptide that controls growth and reproduction in crustaceans by regulating the molt cycle. It inhibits ecdysone biosynthesis by a pair of endocrine glands (Y-organs; YOs) through binding a yet uncharacterized GPCR, which triggers a signalling cascade, leading to inhibition of the ecdysis sequence. When MIH release stops, ecdysone is synthesized and released to the hemolymph. A peak in ecdysone titer is followed by a molting event. A transcriptome of the blackback land crab Gecarcinus lateralis YOs across molt was utilized in this study to curate the list of GPCRs and their expression in order to better assess which GPCRs are involved in the molt process.

RESULTS

Ninety-nine G. lateralis putative GPCRs were obtained by screening the YO transcriptome against the Pfam database. Phylogenetic analysis classified 49 as class A (Rhodopsin-like receptor), 35 as class B (Secretin receptor), and 9 as class C (metabotropic glutamate). Further phylogenetic analysis of class A GPCRs identified neuropeptide GPCRs, including those for Allatostatin A, Allatostatin B, Bursicon, CCHamide, FMRFamide, Proctolin, Corazonin, Relaxin, and the biogenic amine Serotonin. Three GPCRs clustered with recently identified putative CHH receptors (CHHRs), and differential expression over the molt cycle suggests that they are associated with ecdysteroidogenesis regulation. Two putative Corazonin receptors showed much higher expression in the YOs compared with all other GPCRs, suggesting an important role in molt regulation.

CONCLUSIONS

Molting requires an orchestrated regulation of YO ecdysteroid synthesis by multiple neuropeptides. In this study, we curated a comprehensive list of GPCRs expressed in the YO and followed their expression across the molt cycle. Three putative CHH receptors were identified and could include an MIH receptor whose activation negatively regulates molting. Orthologs of receptors that were found to be involved in molt regulation in insects were also identified, including LGR3 and Corazonin receptor, the latter of which was expressed at much higher level than all other receptors, suggesting a key role in YO regulation.

Cancer Cell Mechanics: Adhesion G Protein-coupled Receptors in Action?

In mammals, numerous organ systems are equipped with adhesion G protein-coupled receptors (aGPCRs) to shape cellular processes including migration, adhesion, polarity and guidance. All of these cell biological aspects are closely associated with tumor cell biology. Consistently, aberrant expression or malfunction of aGPCRs has been associated with dysplasia and tumorigenesis. Mounting evidence indicates that cancer cells comprise viscoelastic properties that are different from that of their non-tumorigenic counterparts, a feature that is believed to contribute to the increased motility and invasiveness of metastatic cancer cells. This is particularly interesting in light of the recent identification of the mechanosensitive facility of aGPCRs. aGPCRs are signified by large extracellular domains (ECDs) with adhesive properties, which promote the engagement with insoluble ligands. This configuration may enable reliable force transmission to the ECDs and may constitute a molecular switch, vital for mechano-dependent aGPCR signaling. The investigation of aGPCR function in mechanosensation is still in its infancy and has been largely restricted to physiological contexts. It remains to be elucidated if and how aGPCR function affects the mechanoregulation of tumor cells, how this may shape the mechanical signature and ultimately determines the pathological features of a cancer cell. This article aims to view known aGPCR functions from a biomechanical perspective and to delineate how this might impinge on the mechanobiology of cancer cells.

Adhesion G Protein-Coupled Receptors as Drug Targets

The adhesion G protein-coupled receptors (aGPCRs) are an evolutionarily ancient family of receptors that play key roles in many different physiological processes. These receptors are notable for their exceptionally long ectodomains, which span several hundred to several thousand amino acids and contain various adhesion-related domains, as well as a GPCR autoproteolysis-inducing (GAIN) domain. The GAIN domain is conserved throughout almost the entire family and undergoes autoproteolysis to cleave the receptors into two noncovalently-associated protomers. Recent studies have revealed that the signaling activity of aGPCRs is largely determined by changes in the interactions among these protomers. We review recent advances in understanding aGPCR activation mechanisms and discuss the physiological roles and pharmacological properties of aGPCRs, with an eye toward the potential utility of these receptors as drug targets.

A disease-associated mutation in the adhesion GPCR BAI2 (ADGRB2) increases receptor signaling activity

Mutations in G protein-coupled receptors (GPCRs) that increase constitutive signaling activity can cause human disease. A de novo C-terminal mutation (R1465W) in the adhesion GPCR BAI2 (also known as ADGRB2) was identified in a patient suffering from progressive spastic paraparesis and other neurological symptoms. In vitro studies revealed that this mutation strongly increases the constitutive signaling activity of an N-terminally cleaved form of BAI2, which represents the activated form of the receptor. Further studies dissecting the mechanism(s) underling this effect revealed that wild-type BAI2 primarily couples to Gα_z , with the R1465W mutation conferring increased coupling to Gα_i . The R1465W mutation also increases the total and surface expression of BAI2. The mutation has no effect on receptor binding to β-arrestins, but does perturb binding to the endocytic protein endophilin A1, identified here as a novel interacting partner for BAI2. These studies provide new insights into the signaling capabilities of the adhesion GPCR BAI2/ADGRB2 and shed light on how an apparent gain-of-function mutation to the receptor's C-terminus may lead to human disease.

Biochemical features of the adhesion G protein-coupled receptor CD97 related to its auto-proteolysis and HeLa cell attachment activities

CD97 belongs to the adhesion GPCR family characterized by a long ECD linked to the 7TM via a GPCR proteolytic site (GPS) and plays important roles in modulating cell migration and invasion. CD97 (EGF1-5) is a splicing variant of CD97 that recognizes a specific ligand chondroitin sulfate on cell membranes and the extracellular matrix. The aim of this study was to elucidate the extracellular molecular basis of the CD97 EGF1-5 isoform in protein expression, auto-proteolysis and cell adhesion, including epidermal growth factor (EGF)-like domain, GPCR autoproteolysis-inducing (GAIN) domain, as well as GPS mutagenesis and N-glycosylation. Both wild-type (WT) CD97-ECD and its truncated, GPS mutated, PNGase F-deglycosylated, and N-glycosylation site mutated forms were expressed and purified. The auto-proteolysis of the proteins was analyzed with Western blotting and SDS-PAGE. Small angle X-ray scattering (SAXS) and molecular modeling were used to determine a structural profile of the properly expressed receptor. Potential N-glycosylation sites were identified using MS and were modulated with PNGase F digestion and glyco-site mutations. A flow cytometry-based HeLa cell attachment assay was used for all aforementioned CD97 variants to elucidate the molecular basis of CD97-HeLa interactions. A unique concentration-dependent GPS auto-proteolysis was observed in CD97 EGF1-5 isoform with the highest concentration (4 mg/mL) per sample was self-cleaved much faster than the lower concentration (0.1 mg/mL), supporting an intermolecular mechanism of auto-proteolysis that is distinct to the reported intramolecular mechanism for other CD97 isoforms. N-glycosylation affected the auto-proteolysis of CD97 EGF1-5 isoform in a similar way as the other previously reported CD97 isoforms. SAXS data for WT and deglycosylated CD97ECD revealed a spatula-like shape with GAIN and EGF domains constituting the body and handle, respectively. Structural modeling indicated a potential interaction between the GAIN and EGF5 domains accounting for the absence of expression of the GAIN domain itself, although EGF5-GAIN was expressed similarly in the wild-type protein. For HeLa cell adhesion, the GAIN-truncated forms showed dramatically reduced binding affinity. The PNGase F-deglycosylated and GPS mutated forms also exhibited reduced HeLa attachment compared with WT CD97. However, neither N-glycosylation mutagenesis nor auto-proteolysis inhibition caused by N-glycosylation mutagenesis affected CD97-HeLa cell interactions. A comparison of the HeLa binding affinities of PNGase F-digested, GPS-mutated and N-glycosylation-mutated CD97 samples revealed diverse findings, suggesting that the functions of CD97 ECD were complex, and various technologies for function validation should be utilized to avoid single-approach bias when investigating N-glycosylation and auto-proteolysis of CD97. A unique mechanism of concentration-dependent auto-proteolysis of the CD97 EGF1-5 isoform was characterized, suggesting an intermolecular mechanism that is distinct from that of other previously reported CD97 isoforms. The EGF5 and GAIN domains are likely associated with each other as CD97 expression and SAXS data revealed a potential interaction between the two domains. Finally, the GAIN and EGF domains are also important for CD97-HeLa adhesion, whereas N-glycosylation of the CD97 GAIN domain and GPS auto-proteolysis are not required for HeLa cell attachment.

GPR56/ADGRG1 Activation Promotes Melanoma Cell Migration via NTF Dissociation and CTF-Mediated Gα12/13/RhoA Signaling

GPR56/ADGRG1 is a versatile adhesion G protein-coupled receptor with diverse biological functions. GPR56 expression is variably detected in human melanoma cell lines and correlates inversely with the metastatic potential of melanoma lesions. GPR56 associates with the tetraspanins CD9 and CD81 on the melanoma cell surface. GPR56 activation by immobilized CG4 monoclonal antibody facilitates N-terminal fragment dissociation in a CD9/CD81-dependent manner specifically inducing IL-6 production, which promotes cell migration and invasion. Interestingly, expression of GPR56-C-terminal fragment alone recapitulates the antibody-induced receptor function, implicating a major role for the C-terminal fragment in GPR56 activation and signaling. Analysis of site-directed mutant receptors attests the importance of the conserved N-terminal residues of the C-terminal fragment for its self-activation. Finally, we show that the GPR56-induced signaling in melanoma cells is mediated by the Gα_12/13/RhoA pathway. In summary, the expression and activation of GPR56 may modulate melanoma progression in part by inducing IL-6 production after N-terminal fragment dissociation and C-terminal fragment self-activation.

Heparin interacts with the adhesion GPCR GPR56, reduces receptor shedding, and promotes cell adhesion and motility

GPR56 is an adhesion-class G-protein-coupled receptor responsible for bilateral frontoparietal polymicrogyria (BFPP), a severe disorder of cortical formation. Additionally, GPR56 is involved in biological processes as diverse as hematopoietic stem cell generation and maintenance, myoblast fusion, muscle hypertrophy, immunoregulation and tumorigenesis. Collagen III and tissue transglutaminase 2 (TG2) have been revealed as the matricellular ligands of GPR56 involved in BFPP and melanoma development, respectively. In this study, we identify heparin as a glycosaminoglycan interacting partner of GPR56. Analyses of truncated and mutant GPR56 proteins reveal two basic-residue-rich clusters, R(26)GHREDFRFC(35) and L(190)KHPQKASRRP(200), as the major heparin-interacting motifs that overlap partially with the collagen III- and TG2-binding sites. Interestingly, the GPR56-heparin interaction is modulated by collagen III but not TG2, even though both ligands are also heparin-binding proteins. Finally, we show that the interaction with heparin reduces GPR56 receptor shedding, and enhances cell adhesion and motility. These results provide novel insights into the interaction of GPR56 with its multiple endogenous ligands and have functional implications in diseases such as BFPP and cancer.

Behavioral and transcriptomic profiling of mice null for Lphn3, a gene implicated in ADHD and addiction

Background

The Latrophilin 3 (LPHN3) gene (recently renamed Adhesion G protein‐coupled receptor L3 (ADGRL3)) has been linked to susceptibility to attention deficit/hyperactivity disorder (ADHD) and vulnerability to addiction. However, its role and function are not well understood as there are no known functional variants.

Methods

To characterize the function of this little known gene, we phenotyped Lphn3 null mice. We assessed motivation for food reward and working memory via instrumental responding tasks, motor coordination via rotarod, and depressive‐like behavior via forced swim. We also measured neurite outgrowth of primary hippocampal and cortical neuron cultures. Standard blood chemistries and blood counts were performed. Finally, we also evaluated the transcriptome in several brain regions.

Results

Behaviorally, loss of Lphn3 increases both reward motivation and activity levels. Lphn3 null mice display significantly greater instrumental responding for food than wild‐type mice, particularly under high response ratios, and swim incessantly during a forced swim assay. However, loss of Lphn3 does not interfere with working memory or motor coordination. Primary hippocampal and cortical neuron cultures demonstrate that null neurons display comparatively enhanced neurite outgrowth after 2 and 3 days in vitro. Standard blood chemistry panels reveal that nulls have low serum calcium levels. Finally, analysis of the transcriptome from prefrontal cortical, striatal, and hippocampal tissue at different developmental time points shows that loss of Lphn3 results in genotype‐dependent differential gene expression (DGE), particularly for cell adhesion molecules and calcium signaling proteins. Much of the DGE is attenuated with age, and is consistent with the idea that ADHD is associated with delayed cortical maturation.

Conclusions

Transcriptome changes likely affect neuron structure and function, leading to behavioral anomalies consistent with both ADHD and addiction phenotypes. The data should further motivate analyses of Lphn3 function in the developmental timing of altered gene expression and calcium signaling, and their effects on neuronal structure/function during development.

In silico prediction of the G-protein coupled receptors expressed during the metamorphic molt of Sagmariasus verreauxi (Crustacea: Decapoda) by mining transcriptomic data: RNA-seq to repertoire

Against a backdrop of food insecurity, the farming of decapod crustaceans is a rapidly expanding and globally significant source of food protein. Sagmariasus verreauxi spiny lobster, the subject of this study, are decapods of underdeveloped aquaculture potential. Crustacean neuropeptide G-protein coupled receptors (GPCRs) mediate endocrine pathways that are integral to animal fecundity, growth and survival. The potential use of novel biotechnologies to enhance GPCR-mediated physiology may assist in improving the health and productivity of farmed decapod populations. This study catalogues the GPCRs expressed in the early developmental stages, as well as adult tissues, with a view to illuminating key neuropeptide receptors. De novo assembled contiguous sequences generated from transcriptomic reads of metamorphic and post metamorphic S. verreauxi were filtered for seven transmembrane domains, and used as a reference for iterative re-mapping. Subsequent putative GPCR open reading frames (ORFs) were BLAST annotated, categorised, and compared to published orthologues based on phylogenetic analysis. A total of 85 GPCRs were digitally predicted, that represented each of the four arthropod subfamilies. They generally displayed low-level and non-differential metamorphic expression with few exceptions that we examined using RT-PCR and qPCR. Two putative CHH-like neuropeptide receptors were annotated. Three dimensional structural modelling suggests that these receptors exhibit a conserved extracellular ligand binding pocket, providing support to the notion that these receptors co-evolved with their ligands across Decapoda. This perhaps narrows the search for means to increase productivity of farmed decapod populations.

Introduction: History of the Adhesion GPCR Field

Development of the aGPCR scientific field based on PubMed-listed research articles and selected key findings Since the discovery of adhesion G-protein-coupled receptors (aGPCRs) 20 years ago, reverse genetics approaches have dominated the elucidation of their function and work mechanisms. Seminal findings in this field comprise the description of aGPCRs as seven-transmembrane (7TM) molecules with an extended extracellular region, the identification of matricellular ligands that bind to distinct protein folds at the N-terminus, the clarification of an autoproteolytic cleavage event at a juxtamembranous GPCR proteolysis site (GPS), the elucidation of the crystal structure of the GPCR autoproteolysis-inducing (GAIN) domain that embeds the GPS and connects the receptor fragments, the demonstration that a short N-terminal sequence of the seven-transmembrane (7TM) region can serve as a tethered agonist, and, recently, the notification that aGPCRs can serve as mechanosensors. We here discuss how these discoveries have moved forward aGPCR research and, finally, linked the field to the GPCR field. We argue that crucial questions remain to be addressed before we can fully appreciate the biological nature of these fascinating receptors.

Control of Adhesion GPCR Function Through Proteolytic Processing

Proteolytic processing events in adhesion GPCRs. aGPCRs can undergo multiple autoproteolytic (red asterisks) and proteolytic processing events by exogenous proteases (yellow asterisks) that may be involved in signaling events of the receptors. Proteolytic processing is an unusual property of adhesion family G protein-coupled receptors (aGPCRs) that was observed upon their cloning and biochemical characterization.Ever since, much effort has been dedicated to delineate the mechanisms and requirements for cleavage events in the control of aGPCR function. Most notably, all aGPCRs possess a juxtamembrane protein fold, the GPCR autoproteolysis-inducing (GAIN) domain, which operates as an autoprotease for many aGPCR homologs investigated thus far. Analysis of its autoproteolytic reaction, the consequences for receptor fate and function, and the allocation of physiological effects to this peculiar feature of aGPCRs has occupied the experimental agenda of the aGPCR field and shaped our current understanding of the signaling properties and cell biological effects of aGPCRs. Interestingly, individual aGPCRs may undergo additional proteolytic steps, one of them resulting in shedding of the entire ectodomain that is secreted and can function independently. Here, we summarize the current state of knowledge on GAIN domain-mediated and GAIN domain-independent aGPCR cleavage events and their significance for the pharmacological and cellular actions of aGPCRs. Further, we compare and contrast the proteolytic profile of aGPCRs with known signaling routes that are governed through proteolysis of surface molecules such as the Notch and ephrin pathways.

Tethered Agonism: A Common Activation Mechanism of Adhesion GPCRs

Adhesion GPCRs harbor a tethered agonist sequence (reproduced from [24]) As the past years have seen a magnificent increase in knowledge on adhesion GPCR (aGPCR) signal transduction, the time had come to fill the gap on how these receptors can be activated. Based on experimental observations that deletion of the ectodomain can induce signaling, the idea arose that aGPCRs, just like other atypical GPCRs, may harbor a tethered agonist sequence. In this chapter, we describe the recent findings and characteristics of this agonist, called the Stachel sequence, and discuss potential mechanisms that cause liberation of this encrypted sequence. Further, we provide perspectives for application of Stachel-derived synthetic peptides in future studies of aGPCR function.

An ancient autoproteolytic domain found in GAIN, ZU5 and Nucleoporin98

A large family of G protein-coupled receptors (GPCRs) involved in cell adhesion has a characteristic autoproteolysis motif of HLT/S known as the GPCR proteolysis site (GPS). GPS is also shared by polycystic kidney disease proteins and it precedes the first transmembrane segment in both families. Recent structural studies have elucidated the GPS to be part of a larger domain named GPCR autoproteolysis inducing (GAIN) domain. Here we demonstrate the remote homology relationships of GAIN domain to ZU5 domain and Nucleoporin98 (Nup98) C-terminal domain by structural and sequence analysis. Sequence homology searches were performed to extend ZU5-like domains to bacteria and archaea, as well as new eukaryotic families. We found that the consecutive ZU5-UPA-death domain domain organization is commonly used in human cytoplasmic proteins with ZU5 domains, including CARD8 (caspase recruitment domain-containing protein 8) and NLRP1 (NACHT, LRR and PYD domain-containing protein 1) from the FIIND (Function to Find) family. Another divergent family of extracellular ZU5-like domains was identified in cartilage intermediate layer proteins and FAM171 proteins. Current diverse families of GAIN domain subdomain B, ZU5 and Nup98 C-terminal domain likely evolved from an ancient autoproteolytic domain with an HFS motif. The autoproteolytic site was kept intact in Nup98, p53-induced protein with a death domain and UNC5C-like, deteriorated in many ZU5 domains and changed in GAIN and FIIND. Deletion of the strand after the cleavage site was observed in zonula occluden-1 and some Nup98 homologs. These findings link several autoproteolytic domains, extend our understanding of GAIN domain origination in adhesion GPCRs and provide insights into the evolution of an ancient autoproteolytic domain.

Novel functional complexity of polycystin-1 by GPS cleavage in vivo: role in polycystic kidney disease

Polycystin-1 (Pc1) cleavage at the G protein-coupled receptor (GPCR) proteolytic site (GPS) is required for normal kidney morphology in humans and mice. We found a complex pattern of endogenous Pc1 forms by GPS cleavage. GPS cleavage generates not only the heterodimeric cleaved full-length Pc1 (Pc1(cFL)) in which the N-terminal fragment (NTF) remains noncovalently associated with the C-terminal fragment (CTF) but also a novel (Pc1) form (Pc1(deN)) in which NTF becomes detached from CTF. Uncleaved Pc1 (Pc1(U)) resides primarily in the endoplasmic reticulum (ER), whereas both Pc1(cFL) and Pc1(deN) traffic through the secretory pathway in vivo. GPS cleavage is not a prerequisite, however, for Pc1 trafficking in vivo. Importantly, Pc1(deN) is predominantly found at the plasma membrane of renal epithelial cells. By functional genetic complementation with five Pkd1 mouse models, we discovered that CTF plays a crucial role in Pc1(deN) trafficking. Our studies support GPS cleavage as a critical regulatory mechanism of Pc1 biogenesis and trafficking for proper kidney development and homeostasis.

The BAI subfamily of adhesion GPCRs: synaptic regulation and beyond

The brain-specific angiogenesis inhibitors 1-3 (BAI1-3) comprise a subfamily of adhesion G-protein-coupled receptors (GPCRs). These receptors are highly expressed in the brain and were first studied for their ability to inhibit angiogenesis and tumor formation. Subsequently, BAI1 was found to play roles in apoptotic cell phagocytosis and myoblast fusion. Until recently, however, little was known about the physiological importance of the BAI subfamily in the context of normal brain function. Recent work has provided evidence for key roles of BAI1-3 in the regulation of synaptogenesis and dendritic spine formation. In this review, we summarize the current understanding of the BAI subfamily with regard to downstream signaling pathways, physiological actions, and potential importance as novel drug targets in the treatment of psychiatric and neurological diseases.

Adhesion G-protein-coupled receptors: elusive hybrids come of age

Adhesion G-protein-coupled receptors (GPCRs) are the most recently identified and least understood subfamily of GPCRs. Adhesion GPCRs are characterized by unusually long ectodomains with adhesion-related repeats that facilitate cell- cell and cell-cell matrix contact, as well as a proteolytic cleavage site-containing domain that is a structural hallmark of the family. Their unusual chimeric structure of adhesion-related ectodomain with a seven-pass transmembrane domain and cytoplasmic signaling makes these proteins highly versatile in mediating cellular signaling in response to extracellular adhesion or cell motility events. The ligand binding and cytoplasmic signaling modes for members of this family are beginning to be elucidated, and recent studies have demonstrated critical roles for Adhesion GPCRs in planar polarity and other important cell-cell and cell-matrix interactions during development and morphogenesis, as well as heritable diseases and cancer.

CD97 is a multifunctional leukocyte receptor with distinct roles in human cancers (Review)

G-protein coupled receptors (GPCRs) represent the most diverse and biologically ubiquitous protein receptors. The epidermal growth factor seven-span transmembrane (EGF-TM7) family consists of adhesion GPCRs with a diverse functional repertoire. CD97 is the most broadly expressed member with roles in cell adhesion, migration and regulation of intercellular junctions. CD97 is also expressed in a variety of human malignancies including those of the thyroid, stomach, colon and brain. CD97 confers an invasive phenotype and has been shown to correlate with tumor grade, lymph node invasion, metastatic spread and overall prognosis. More recently, CD97 was found to signal through Gα12/13, resulting in increased RHO-GTP levels. Proven roles in tumor invasion and signaling make CD97 an exciting novel therapeutic target. In this review, we will discuss the structure and function of this receptor, with a specific focus on its mechanistic significance in neoplastic diseases.

Matching structure with function: the GAIN domain of adhesion-GPCR and PKD1-like proteins

Elucidation of structural information can greatly facilitate the understanding of molecular function. A recent example is the description of the G-protein-coupled receptor (GPCR) autoproteolysis-inducing (GAIN) domain, an evolutionarily ancient fold present in Adhesion-GPCRs (aGPCRs) and polycystic kidney disease 1 (PKD1)-like proteins. In the past, the peculiar autoproteolytic capacity of both membrane protein families at the conserved GPCR proteolysis site (GPS) had not been described in detail. The physiological performance of aGPCRs and PKD1-like proteins is thought to be regulated through the GPS, but it is debated how. A recent report provides pivotal details by discovery and analysis of the GAIN domain structure that incorporates the GPS motif. Complementary studies have commenced to analyze physiological requirements of the GAIN domain for aGPCR function, indicating that it serves as the linchpin for multiple receptor signals. Structural analysis and functional assays now allow for the dissection of the biological duties conferred through the GAIN domain.

Dances with black widow spiders: dysregulation of glutamate signalling enters centre stage in ADHD

Attention-deficit/hyperactivity disorder (ADHD) is a common neurodevelopmental disorder with impairments across the lifespan. The persistence of ADHD is associated with considerable liability to neuropsychiatric co-morbidity such as depression, anxiety and substance use disorder. The substantial heritability of ADHD is well documented and recent genome-wide analyses for risk genes revealed synaptic adhesion molecules (e.g. latrophilin-3, LPHN3; fibronectin leucine-rich repeat transmembrane protein-3, FLRT3), glutamate receptors (e.g. metabotropic glutamate receptor-5, GRM5) and mediators of intracellular signalling pathways (e.g. nitric oxide synthase-1, NOS1). These genes encode principal components of the molecular machinery that connects pre- and postsynaptic neurons, facilitates glutamatergic transmission, controls synaptic plasticity and empowers intersecting neural circuits to process and refine information. Thus, identification of genetic variation affecting molecules essential for the formation, specification and function of excitatory synapses is refocusing research efforts on ADHD pathogenesis to include the long-neglected glutamate system.

Comparative genomic analysis and evolution of family-B G protein-coupled receptors from six model insect species

Family-B G protein-coupled receptors (GPCR-Bs) play vital roles in many biological processes, including growth, development and reproduction. However, the evolution and function of GPCR-Bs have been poorly understood in insects. We have identified 87 GPCR-Bs from six model insect species, 20 from Tribolium castaneum, 9 from Apis mellifera, 11 from Bombyx mori, 9 from Acyrthosiphon pisum, 14 from Anopheles gambiae and 24 from Drosophila melanogaster. 22 of them were reported in this study for the first time. Phylogenetic analysis revealed that there are three kinds of evolutionary patterns that occurred among GPCR-Bs during insect evolution: one-to-one orthologous relationships, species-specific expansion and episodic duplication or loss in certain insect lineages. A striking finding was the discovery of a parathyroid hormone receptor like gene (pthrl) in invertebrates, which was independently duplicated in vertebrates and invertebrates, whereas this gene was lost at least twice during insect evolution. These results indicate that PTHRL is possibly divergent in the functions between mammals and insects. The information of family-B GPCRs in nondrosophiline insects has been established, and will promote the further study on the function of these GPCRs and deorphanization of them. On the other hand, this study provides us with multiple function of GPCR-Bs in differential organisms, which will be also the potential attacking targets for new pesticides and drugs.

G Protein-Coupled Receptors in cancer: biochemical interactions and drug design

G Protein-Coupled Receptors (GPCRs) share the same topology made of seven-transmembrane segments and represent the largest family of membrane receptors. Initially associated with signal transduction in differentiated cells, GPCRs and heterotrimeric G proteins were shown to behave as proto-oncogenes whose overexpression or activating mutations confer transforming properties. The first part of this review focuses on the link between biochemical interactions of a GPCR with other receptors, such as dimerization or multiprotein complexes, and their oncogenic properties. Alteration of these interactions or deregulation of transduction cascades can promote uncontrolled cell proliferation or cell transformation that leads to tumorigenicity and malignancy. The second part concerns the design of drugs specifically targeting these complex interactions and their promise in cancer therapy.

A proprotein convertase/MMP-14 proteolytic cascade releases a novel 40 kDa vasculostatin from tumor suppressor BAI1

Brain-specific angiogenesis inhibitor 1 (BAI1), an orphan G protein-coupled receptor-type seven transmembrane protein, was recently found mutated or silenced in multiple human cancers and can interfere with tumor growth when overexpressed. Yet, little is known about its regulation and the molecular mechanisms through which this novel tumor suppressor exerts its anti-cancer effects. Here, we demonstrate that the N terminus of BAI1 is cleaved extracellularly to generate a truncated receptor and a 40-kDa fragment (Vasculostatin-40) that inhibits angiogenesis. We demonstrate that this novel proteolytic processing event depends on a two-step cascade of protease activation: proprotein convertases, primarily furin, activate latent matrix metalloproteinase-14, which then directly cleaves BAI1 to release the bioactive fragment. These findings significantly augment our knowledge of BAI1 by showing a novel post-translational mechanism regulating BAI1 activity through cancer-associated proteases, have important implications for BAI1 function and regulation, and present novel opportunities for therapy of cancer and other vascular diseases.

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

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.