The adhesion GPCR GPR126 has distinct, domain-dependent functions in Schwann cell development mediated by interaction with laminin-211

Interactions between ADGRF1 (GPR110) and extracellular matrix proteins govern its effects on tumorigenesis in HER2-positive breast cancer

BACKGROUND AND PURPOSE

We and others have previously shown that ADGRF1, an adhesion G protein-coupled receptor, is overexpressed and associated with poor survival in many cancers, including human epidermal growth factor receptor-2 (HER2) breast cancer (BC). Also, we have reported the tumour-promoting function of ADGRF1 using preclinical models of HER2+ BC. In this study, we investigated the effect of ADGRF1 overexpression in an orthotopic in vivo model as well as downstream signalling of ADGRF1 in HER2+ BC.

EXPERIMENTAL APPROACH

We utilized a doxycycline (Dox)-induced ADGRF1 overexpression system in HER2+ BC cell lines and performed various in vitro and in vivo studies. Following ADGRF1 overexpression in the presence/absence of Matrigel, laminin-111 or collagen-IV, we performed the mammosphere assay to assess the tumorigenicity of breast epithelial cells, as well as cAMP/IP1 assays and RNA-sequencing, to understand the receptor function and pharmacology. We conducted cross-linking-aided immunoprecipitation and mass spectrometry to confirm the physical interaction between ADGRF1 and the extracellular matrix proteins present in Matrigel.

KEY RESULTS

We found that ADGRF1 switched from a tumour-promoting to tumour-suppressive function upon interaction with laminin-111. Interaction of ADGRF1 with laminin-111 resulted in inhibition of Gαs coupling and STAT3 phosphorylation, induction of senescence, increase in HER2 expression, and improvement of sensitivity to anti-HER2 drugs in HER2+ BC.

CONCLUSIONS

ADGRF1 switches from a tumour-promoting to tumour-suppressive function upon interaction with laminin-111, leading to improvements in sensitivity to anti-HER2 drugs. Leveraging ADGRF1 interactions with laminin-111 may allow the design of novel therapies against ADGRF1 in HER2+ BC.

Development of an allosteric adhesion GPCR nanobody with therapeutic potential

Allosteric modulation of receptor responses to endogenous agonists has therapeutic value, maintaining ligand profiles, reducing side effects and restoring mutant responses. Adhesion G-protein-coupled receptors (aGPCRs), with large N termini, are ideal for allosteric modulator development. We designed a nanobody strategy targeting ADGRG2 N-terminal fragments and got a specific nanobody Nb23-bi, which promoted dehydroepiandrosterone (DHEA)-induced ADGRG2 activation and reversed mutant-induced dysfunctions. By combining structural characterization, crosslinking mass spectrometry, mutational analysis and molecular dynamics simulations, we clarified the allosteric mechanism of how the Nb23-bi modulates conformational changes in the DHEA-binding pocket. Animal studies showed that Nb23-bi promoted the response of DHEA in alleviating testicular inflammation and reversing mutant defects. In summary, we developed an allosteric nanobody of ADGRG2 and gained insights into its functions in reversing disease-associated dysfunctions. Our study may serve as a template for developing allosteric modulators of other aGPCRs for biological and therapeutic purposes.

Complex G-protein signaling of the adhesion GPCR, ADGRA3

ADGRA3 (GPR125) is an orphan adhesion G protein-coupled receptor (aGPCR) involved in planar cell polarity, primarily through recruitment of the signaling components disheveled (DVL) during vertebrate gastrulation and discs large homolog 1, implicated in cancer. Limited knowledge exists of the canonical G protein-coupled receptor pathways downstream of ADGRA3. Here, we employed a series of human cell line-based signaling assays to gain insight into the G protein-mediated signaling of ADGRA3. We designed ADGRA3 constructs based on transcript variant analysis in publicly available human liver and brain RNA-seq datasets. Cleavage in the GPCR autoproteolysis site (GPS) is an aGPCR hallmark; thus, we generated a truncated ADGRA3 (C-terminal fragment, CTF) corresponding to a potential cleavage at the GPS. We found low-level activation of Gi and Gs by ADGRA3 and slightly more by its CTF. As the N terminus of the CTF constitutes a class-defined tethered agonist (so-called stachel peptide), we removed the initial three amino acids of the CTF. This resulted in abrogated G protein-mediated signaling, as observed for other aGPCRs. Due to the central role of ADGRA3 in planar cell polarity signaling through DVL recruitment, we investigated the G-protein signaling in the absence of DVL1-3 and found it sustained. No transcriptional activation was observed in an assay of downstream β-catenin activity. Collectively, this establishes classical G protein-mediated signaling for ADGRA3.

Mechanosensitive adhesion G protein-coupled receptors: Insights from health and disease

Ontogeny cannot be separated from mechanical forces. Cells are continuously subjected to different types of mechanical stimuli that convert into intracellular signals through mechanotransduction. As a member of the G protein-coupled receptor superfamily, adhesion G protein-coupled receptors (aGPCRs) have attracted extensive attention due to their unique extracellular domain and adhesion properties. In the past few decades, increasing evidence has indicated that sensing mechanical stimuli may be one of the main physiological activities of aGPCRs. Here, we review the general structure and activation mechanisms of these receptors and highlight the lesion manifestations relevant to each mechanosensitive aGPCR.

A force-sensitive adhesion GPCR is required for equilibrioception

Equilibrioception (sensing of balance) is essential for mammals to perceive and navigate the three-dimensional world. A rapid mechanoelectrical transduction (MET) response in vestibular hair cells is crucial for detecting position and motion. Here, we identify the G protein-coupled receptor (GPCR) LPHN2/ADGRL2, expressed on the apical membrane of utricular hair cells, as essential for maintaining normal balance. Loss of LPHN2 specifically in hair cells impaired both balance behavior and the MET response in mice. Functional analyses using hair-cell-specific Lphn2-knockout mice and an LPHN2-specific inhibitor suggest that LPHN2 regulates tip-link-independent MET currents at the apical surface of utricular hair cells. Mechanistic studies in a heterologous system show that LPHN2 converts force stimuli into increased open probability of transmembrane channel-like protein 1 (TMC1). LPHN2-mediated force sensation triggers glutamate release and calcium signaling in utricular hair cells. Importantly, reintroducing LPHN2 into the hair cells of Lphn2-deficient mice restores vestibular function and MET response. Our data reveal that a mechanosensitive GPCR is required for equilibrioception.

Identification of a novel prognostic marker ADGRG6 in pancreatic adenocarcinoma: multi-omics analysis and experimental validation

Background

Pancreatic adenocarcinoma (PAAD) ranks among the most lethal malignancies worldwide. Current treatment options have limited efficacy, underscoring the need for new therapeutic targets.

Methods

This study employed a multi-omics analytical framework to delve into the expression profiles and prognostic implications of ADGRG6 within the pan-cancer dataset of The Cancer Genome Atlas (TCGA) database, highlighting the prognostic value and potential carcinogenic role of ADGRG6 in PAAD, which was further validated using data from multiple PAAD cohorts in the Gene Expression Omnibus (GEO) database. To assess the role of ADGRG6 in the tumor microenvironment of PAAD, we evaluated immune infiltration using CIBERSORT, ssGSEA, xCell and Tracking Tumor Immunophenotype (TIP), and utilized single-cell sequencing data to explore cell-cell interactions. Further cellular and animal experiments, such as colony formation assay, transwell assay, western blot, real-time PCR, and tumor xenograft experiments, were used to investigate the effect of ADGRG6 on the proliferation, metastatic potential and immune marker expression of PAAD and the underlying mechanisms.

Results

ADGRG6 emerged as a potential prognostic biomarker and a therapeutic target for PAAD, which was further corroborated by data extracted from multiple PAAD cohorts archived in the GEO database. Single-cell sequencing and immune infiltration analyses predicted the positive correlation of ADGRG6 with the infiltration of immune cells and with the interaction between malignant cells and fibroblasts/macrophages within the PAAD microenvironment. In vitro cell assays demonstrated that ADGRG6 promoted the proliferation, metastatic potential and immune marker expression of PAAD cells by increasing protein level of mutated p53 (mutp53), which activated a spectrum of gain-of-functions to promote cancer progression via the EGFR, AMPK and NF-κB signaling cascades. Furthermore, subcutaneous xenograft experiments in mice demonstrated that ADGRG6 knockdown substantially suppressed the growth of engrafted PAAD tumors.

Conclusions

ADGRG6 may serve as a novel prognostic marker and a therapeutic targets for PAAD, playing a crucial role in the proliferation, metastasis, and immune marker regulation of PAAD through elevating protein level of mutated p53.

Spatiotemporal profiling of adhesion G protein-coupled receptors in developing mouse and human pancreas reveals a role for GPR56 in islet development

INTRODUCTION

G protein-coupled receptors (GPCRs) are cell-surface proteins that are targeted therapeutically for a range of disorders, including diabetes. Adhesion GPCRs (aGPCRs) are the second largest class of the GPCR superfamily and some members of this family have been implicated in appropriate organ development. However, the role of aGPCRs in endocrine pancreas specification is not yet known.

METHODS

Here, we systematically characterised expression of mRNAs encoding aGPCRs and their ligands in developing mouse and human pancreas using our own and publicly available single-cell RNA sequencing and spatial transcriptomics data, and we conducted qPCR analysis of aGPCR expression in human pancreas at different gestational stages. We then investigated the role of GPR56 (ADGRG1), the most abundant aGPCR in pancreatic endocrine progenitors, in islet development using Gpr56 null mice and their wildtype littermates.

RESULTS

We demonstrated that aGPCRs are dynamically expressed during mouse and human pancreas development, with specific aGPCR mRNAs expressed in distinct endocrine, endothelial, mesenchymal, acinar, ductal, and immune cell clusters. aGPCR ligand mRNAs were mostly expressed by non-endocrine cells, and the most highly expressed receptor-ligand interacting mRNA pairs were those encoding GPR56 and COL3A1. Deletion of Gpr56 in neonatal mice was associated with an altered α-/β-/δ-cell ratio and reduced β-cell proliferation.

CONCLUSION

Our data show that aGPCRs are expressed at key stages of human and mouse pancreas endocrine lineage decisions, and analysis of pancreases from Gpr56 knockout mice implicate this aGPCR in the development of a full complement of β-cells.

The N terminus-only function of adhesion GPCRs: emerging concepts

Adhesion G-protein-coupled receptors (aGPCRs) play key roles in health and disease. They are unique in that they not only activate G-protein pathways but also have distinct functions that rely solely on their N termini, making them complex drug targets. To date there have been only descriptive observations about these enigmatic N terminus-only functions. Emerging evidence from several aGPCRs now indicates that these are a defining characteristic of these receptors that allows them to operate bidirectionally across environments. Recent advances in characterizing aGPCR splice variants and receptor structure have revealed the G protein-independent mechanisms that underlie their N terminus-only functions. This review consolidates current findings, explores how the N termini integrate functions, and identifies common principles across aGPCRs. We consider the therapeutic implications and discuss how specifically targeting N terminus functions provides a novel perspective on the pharmacological potential of aGPCRs.

Schwann cells have a limited window of time in which to initiate myelination signaling during early migration in vivo

The temporal control of mitotic exit of individual Schwann cells (SCs) is essential for radial sorting and peripheral myelination. However, it remains unknown when, during their multiple rounds of division, SCs initiate myelin signaling in vivo. By manipulating SC division during development, we report that when SCs skip their division during migration, but not during radial sorting, they fail to myelinate peripheral axons. This coincides with a sharp decrease in Laminin expression within the posterior lateral line nerve. Interestingly, elevating cAMP levels or forcing Laminin 2 expression within individual SCs restore their ability to myelinate, despite missing mitosis during migration. Our results demonstrate a limited time window during which migrating SCs initiate Laminin expression to gradually activate the Laminin/Gpr126/cAMP signaling required for radial sorting and myelination at later stages in vivo.

Ligand-Independent Spontaneous Activation of Purinergic P2Y6 Receptor Under Cell Culture Soft Substrate

G protein-coupled receptors (GPCRs) exist in the conformational equilibrium between inactive state and active state, where the proportion of active state in the absence of a ligand determines the basal activity of GPCRs. Although many GPCRs have different basal activity, it is still unclear whether physiological stresses such as substrate stiffness affect the basal activity of GPCRs. In this study, we identified that purinergic P2Y6 receptor (P2Y6R) induced spontaneous Ca2+ oscillation without a nucleotide ligand when cells were cultured in a silicon chamber. This P2Y6R-dependent Ca2+ oscillation was absent in cells cultured in glass dishes. Coating substrates, including collagen, laminin, and fibronectin, did not affect the P2Y6R spontaneous activity. Mutation of the extracellular Arg-Gly-Asp (RGD) motif of P2Y6R inhibited spontaneous activity. Additionally, extracellular Ca2+ was required for P2Y6R-dependent spontaneous Ca2+ oscillation. The GPCR screening assay identified cells expressing 10 GPCRs, including purinergic P2Y1R, P2Y2R, and P2Y6R, that exhibited spontaneous Ca2+ oscillation under cell culture soft substrate. Our results suggest that stiffness of the cell adhesion surface modulates spontaneous activities of several GPCRs, including P2Y6R, through a ligand-independent mechanism.

N-terminal fragment shedding contributes to signaling of the full-length adhesion receptor ADGRL3

Most adhesion G protein-coupled receptors (GPCRs) undergo autoproteolytic cleavage during receptor biosynthesis, resulting in noncovalently bound N-terminal fragments (NTFs) and C-terminal fragments (CTFs) that remain associated during receptor trafficking to the plasma membrane. While substantial evidence supports increased G protein signaling when just the CTF is expressed, there is an ongoing debate about whether NTF removal is required to initiate signaling in the context of the WT receptor. Here, we use adhesion GPCR latrophilin-3 (ADGRL3) as a model receptor to investigate tethered agonist (TA)-mediated activation. First, we show that extending the N terminus of the TA in ADGRL3 CTF disrupts G protein signaling. This suggests that if the TA is not fully exposed, it is unlikely to interact with the orthosteric pocket in an optimal manner for G protein activation. Second, we show that when full-length ADGRL3 is expressed in heterologous cells, approximately 5% of the receptor population spontaneously sheds its NTF. We hypothesized that the signaling activity observed for full-length ADGRL3 is largely because of this shedding, which exposes the native TA. To test this hypothesis, we used a full-length cleavage-deficient ADGRL3 mutant. Compared with WT receptor, this mutant lost ∼80% of its signaling through Gα13 and showed a much lower level of spontaneous NTF shedding, approximately 20% of that observed for WT receptor. This loss of spontaneous NTF shedding likely explains its diminished signaling activity. These findings suggest that TA-mediated signal transduction by full-length ADGRL3 requires removal of its NTF.

Intron retention of an adhesion GPCR generates 1TM isoforms required for 7TM-GPCR function

Adhesion G protein-coupled receptors (aGPCRs) are expressed in all organs and are involved in various mechanobiological processes. They are heavily alternatively spliced, forecasting an extraordinary molecular structural diversity. Here, we uncovered the existence of unconventional single-transmembrane (1TM)-containing ADGRL/Cirl proteins devoid of the conventional GPCR layout (i.e., the 7TM signaling unit) in Drosophila. These 1TM proteins are made as a result of intron retention and provide an N-terminal fragment that acts as an interactor to allow Gαo-dependent signaling through conventional 7TM-containing Cirl isoforms encoded by the same gene. This molecular mechanism determines sensory precision of neurons in response to mechanical stimulation in vivo. This action mode of aGPCR provides a promising entry point for experimental and therapeutic approaches to intervene in aGPCR signaling and implicates alternative splicing as a physiological strategy to express a given aGPCR together with its molecular interactor.

Generic residue numbering of the GAIN domain of adhesion GPCRs

The GPCR autoproteolysis inducing (GAIN) domain is an ancient protein fold ubiquitous in adhesion G protein-coupled receptors (aGPCR). It contains a tethered agonist necessary and sufficient for receptor activation. The GAIN domain is a hotspot for pathological mutations. However, the low primary sequence conservation of GAIN domains has thus far hindered the knowledge transfer across different GAIN domains in human receptors as well as species orthologs. Here, we present a scheme for generic residue numbering of GAIN domains, based on structural alignments of over 14,000 modeled GAIN domain structures. This scheme is implemented in the GPCR database (GPCRdb) and elucidates the domain topology across different aGPCRs and their homologs in a large panel of species. We identify conservation hotspots and statistically cancer-enriched positions in human aGPCRs and show the transferability of positional and structural information between GAIN domain homologs. The GAIN-GRN scheme provides a robust strategy to allocate structural homologies at the primary and secondary levels also to GAIN domains of polycystic kidney disease 1/PKD1-like proteins, which now renders positions in both GAIN domain types comparable to one another. In summary, our work enables researchers to generate hypothesis and rationalize experiments related to GAIN domain function and pathology.

Modulating vertebrate physiology by genomic fine-tuning of GPCR functions

G protein-coupled receptors (GPCRs) play a crucial role as membrane receptors, facilitating the communication of eukaryotic species with their environment and regulating cellular and organ interactions. Consequently, GPCRs hold immense potential in contributing to adaptation to ecological niches and responding to environmental shifts. Comparative analyses of vertebrate genomes reveal patterns of GPCR gene loss, expansion, and signatures of selection. Integrating these genomic data with insights from functional analyses of gene variants enables the interpretation of genotype-phenotype correlations. This review underscores the involvement of GPCRs in adaptive processes, presenting numerous examples of how alterations in GPCR functionality influence vertebrate physiology or, conversely, how environmental changes impact GPCR functions. The findings demonstrate that modifications in GPCR function contribute to adapting to aquatic, arid, and nocturnal habitats, influencing camouflage strategies, and specializing in particular dietary preferences. Furthermore, the adaptability of GPCR functions provides an effective mechanism in facilitating past, recent, or ongoing adaptations in animal domestication and human evolution and should be considered in therapeutic strategies and drug development.

Peripheral myelin: From development to maintenance

Peripheral myelin is synthesized by glial cells called Schwann cells (SCs). SC development and differentiation must be tightly regulated to avoid any pathological consequence affecting peripheral nerve function. Neuropathic symptoms can arise from developmental issues in SCs, as well as in adult life through processes affecting mature SCs. In this review we focus on SC differentiation from the immature towards the myelinating and non-myelinating SC stages, defining molecular mechanisms outlining radial sorting, a multi-stepped event essential for immature SC differentiation and myelination. We also describe mechanisms regulating myelin sheath maintenance and SC homeostasis during aging. Finally, we will conclude with some remaining questions in the field of SC biology.

Targeting adhesion G protein-coupled receptors. Current status and future perspectives

G protein-coupled receptors (GPCRs) orchestrate many physiological functions and are a crucial target in drug discovery. Adhesion GPCRs (aGPCRs), the second largest family within this superfamily, are promising yet underexplored targets for treating various diseases, including obesity, psychiatric disorders, and cancer. However, the receptors' unique and complex structure and miscellaneous interactions complicate comprehensive pharmacological studies. Despite recent progress in determining structures and elucidation of the activation mechanism, the function of many receptors remains to be determined. This review consolidates current knowledge on aGPCR ligands, focusing on small molecule orthosteric ligands and allosteric modulators identified for the ADGRGs subfamily (subfamily VIII), (GPR56/ADGRG1, GPR64/ADGRG2, GPR97/ADGRG3, GPR114/ADGRG5, GPR126/ADGRG6, and GPR128/ADGRG7). We discuss challenges in hit identification, target validation, and drug discovery, highlighting molecular compositions and recent structural breakthroughs. ADGRG ligands can offer new insights into aGPCR modulation and have significant potential for novel therapeutic interventions targeting various diseases.

Genetic Variants in the Adhesive G Protein-Coupled Receptor ADGRG6 are Associated with Increased Susceptibility to COPD in the Elderly Han Chinese Population of Southern China

Background

Mutations in ADGRG6 are associated with a variety of cancers and multiple types of diseases. However, the impact of genetic variations in ADGRG6 on chronic obstructive pulmonary disease (COPD) susceptibility has not yet been evaluated.

Methods

Considering the high prevalence of COPD among the elderly population in China, this study specifically targets the elderly Han population in Southern China as the study subject. Following the acquisition of participants' whole-genome DNA, genotyping was conducted using the Agena MassARRAY platform. The online tool 'SNPStats', which utilizes logistic regression, was employed to analyze and assess the correlation. Multi-factor dimensionality reduction was utilized to clarify the impact of "SNP-SNP" interactions on COPD risk. The False-Positive Report Probability (FPRP) was applied to determine whether significant results are noteworthy findings.

Results

The mutant allele "C" of rs11155242 was a protective genetic factor against COPD susceptibility (OR = 0.57, 95% CI = 0.36 to 0.91, p = 0.017). The heterozygous mutant genotype "CA" of rs11155242 was found to be significantly associated with reduced COPD risk (CA Vs AA: OR = 0.53, 95% CI = 0.32 to 0.90, p = 0.018). ADGRG6-rs11155242 was found to be strongly associated with a reduced risk of COPD in males, non-smokers, and subjects with a BMI below 24 kg/m2 (OR < 1, p < 0.05). The FPRP analysis indicated that the positive results identified in this study are noteworthy new findings.

Conclusion

The mutant allele "C" and mutant genotype "CA" of rs11155242 act as protective genetic factors against COPD susceptibility. This study will provide a new research direction for the personalized prevention and treatment of COPD in the elderly Han population in southern China, and lay a potential scientific basis.

Schwann Cell Development and Myelination

Glial cells in the peripheral nervous system (PNS), which arise from the neural crest, include axon-associated Schwann cells (SCs) in nerves, synapse-associated SCs at the neuromuscular junction, enteric glia, perikaryon-associated satellite cells in ganglia, and boundary cap cells at the border between the central nervous system (CNS) and the PNS. Here, we focus on axon-associated SCs. These SCs progress through a series of formative stages, which culminate in the generation of myelinating SCs that wrap large-caliber axons and of nonmyelinating (Remak) SCs that enclose multiple, small-caliber axons. In this work, we describe SC development, extrinsic signals from the axon and extracellular matrix (ECM) and the intracellular signaling pathways they activate that regulate SC development, and the morphogenesis and organization of myelinating SCs and the myelin sheath. We review the impact of SCs on the biology and integrity of axons and their emerging role in regulating peripheral nerve architecture. Finally, we explain how transcription and epigenetic factors control and fine-tune SC development and myelination.

Mechanical force induced activation of adhesion G protein-coupled receptor

Among the various families of G protein-couple receptors (GPCR), the adhesion family of GPCRs is specialized by its expansive extracellular region, which facilitates the recruitment of various ligands. Previous hypothesis proposed that aGPCRs are activated by mechanical force, wherein a Stachel peptide is liberated from the GPCR autoproteolysis-inducing (GAIN) domain and subsequently binds to the transmembrane domain (7TM) upon activation. In this review, we summarize recent advancements in structural studies of aGPCRs, unveiling a conserved structural change of the Stachel peptide from the GAIN domain-embedded β-strand conformation to the 7TM-loaded α-helical conformation. Notably, using single-molecule studies, we directly observed the unfolding of GAIN domain and the release of Stachel peptide under physiological level of force, precisely supporting the mechanosensing mechanism for aGPCRs. We observed that the current complex structures of aGPCR adhesion domains with their respective ligands share a common pattern with the C-termini of each binding partner extending in opposite directions, suggesting a similar shearing stretch geometry for these aGPCRs to transmit the mechanical force generated in the circulating environment to the GAIN domain for its unfolding. Outstanding questions, including the relative orientations and interactions between 7TM and its preceding GAIN and adhesion domains of different aGPCRs, may require further structural and mechanical studies at the full-length receptor scale or cell-based level. Our analysis extends the current view of aGPCR structural organization and activation and offers valuable insights for the development of mechanosensor based on aGPCRs or discovery of mechanotherapy against aGPCRs.

Polymerizing laminins in development, health, and disease

Polymerizing laminins are multi-domain basement membrane (BM) glycoproteins that self-assemble into cell-anchored planar lattices to establish the initial BM scaffold. Nidogens, collagen-IV and proteoglycans then bind to the scaffold at different domain loci to create a mature BM. The LN domains of adjacent laminins bind to each other to form a polymer node, while the LG domains attach to cytoskeletal-anchoring integrins and dystroglycan, as well as to sulfatides and heparan sulfates. The polymer node, the repeating unit of the polymer scaffold, is organized into a near-symmetrical triskelion. The structure, recently solved by cryo-electron microscopy in combination with AlphaFold2 modeling and biochemical studies, reveals how the LN surface residues interact with each other and how mutations cause failures of self-assembly in an emerging group of diseases, the LN-lamininopathies, that include LAMA2-related dystrophy and Pierson syndrome.

GPR126 is a specifier of blood-brain barrier formation in the mouse central nervous system

The blood-brain barrier (BBB) acquires unique properties to regulate neuronal function during development. The formation of the BBB, which occurs in tandem with angiogenesis, is directed by the Wnt/β-catenin signaling pathway. Yet the exact molecular interplay remains elusive. Our study reveals the G protein-coupled receptor GPR126 as a critical target of canonical Wnt signaling, essential for the development of the BBB's distinctive vascular characteristics and its functional integrity. Endothelial cell-specific deletion of the Gpr126 gene in mice induced aberrant vascular morphogenesis, resulting in disrupted BBB organization. Simultaneously, heightened transcytosis in vitro compromised barrier integrity, resulting in enhanced vascular permeability. Mechanistically, GPR126 enhanced endothelial cell migration, pivotal for angiogenesis, acting through an interaction between LRP1 and β1 integrin, thereby balancing the levels of β1 integrin activation and recycling. Overall, we identified GPR126 as a specifier of an organotypic vascular structure, which sustained angiogenesis and guaranteed the acquisition of the BBB properties during development.

ERNEST COST action overview on the (patho)physiology of GPCRs and orphan GPCRs in the nervous system

G protein-coupled receptors (GPCRs) are a large family of cell surface receptors that play a critical role in nervous system function by transmitting signals between cells and their environment. They are involved in many, if not all, nervous system processes, and their dysfunction has been linked to various neurological disorders representing important drug targets. This overview emphasises the GPCRs of the nervous system, which are the research focus of the members of ERNEST COST action (CA18133) working group 'Biological roles of signal transduction'. First, the (patho)physiological role of the nervous system GPCRs in the modulation of synapse function is discussed. We then debate the (patho)physiology and pharmacology of opioid, acetylcholine, chemokine, melatonin and adhesion GPCRs in the nervous system. Finally, we address the orphan GPCRs, their implication in the nervous system function and disease, and the challenges that need to be addressed to deorphanize them.

Adhesion G Protein-Coupled Receptor Gpr126 (Adgrg6) Expression Profiling in Diseased Mouse, Rat, and Human Kidneys

Uncovering the function of understudied G protein-coupled receptors (GPCRs) provides a wealth of untapped therapeutic potential. The poorly understood adhesion GPCR Gpr126 (Adgrg6) is widely expressed in developing kidneys. In adulthood, Gpr126 expression is enriched in parietal epithelial cells (PECs) and epithelial cells of the collecting duct and urothelium. Whether Gpr126 plays a role in kidney disease remains unclear. Here, we characterized Gpr126 expression in diseased kidneys in mice, rats, and humans. RT-PCR data show that Gpr126 expression is altered in kidney disease. A quantitative RNAscope® analysis utilizing cell type-specific markers revealed that Gpr126 expression upon tubular damage is mainly increased in cell types expressing Gpr126 under healthy conditions as well as in cells of the distal and proximal tubules. Upon glomerular damage, an increase was mainly detected in PECs. Notably, Gpr126 expression was upregulated in an ischemia/reperfusion model within hours, while upregulation in a glomerular damage model was only detected after weeks. An analysis of kidney microarray data from patients with lupus nephritis, IgA nephropathy, focal segmental glomerulosclerosis (FSGS), hypertension, and diabetes as well as single-cell RNA-seq data from kidneys of patients with acute kidney injury and chronic kidney disease indicates that GPR126 expression is also altered in human kidney disease. In patients with FSGS, an RNAscope® analysis showed that GPR126 mRNA is upregulated in PECs belonging to FSGS lesions and proximal tubules. Collectively, we provide detailed insights into Gpr126 expression in kidney disease, indicating that GPR126 is a potential therapeutic target.

The far extracellular CUB domain of the adhesion GPCR ADGRG6/GPR126 is a key regulator of receptor signaling

Adhesion G Protein-coupled receptors (aGPCRs) transduce extracellular adhesion signals into cytoplasmic signaling pathways. ADGRG6/GPR126 is an aGPCR critical for axon myelination, heart development and ear development; and is associated with developmental diseases and cancers. ADGRG6 has a large, alternatively-spliced, five-domain extracellular region (ECR) that samples different conformations and regulates receptor signaling. However, the molecular details of how the ECR regulates signaling are unclear. Herein, we studied the conformational dynamics of the conserved CUB domain which is located at the distal N-terminus of the ECR and is deleted in an alternatively-spliced isoform ( Δ CUB). We showed that the Δ CUB isoform has decreased signaling. Molecular dynamics simulations suggest that the CUB domain is involved in interdomain contacts to maintain a compact ECR conformation. A cancer-associated CUB domain mutant, C94Y, drastically perturbs the ECR conformation and results in elevated signaling, whereas another CUB mutant, Y96A, located near a conserved Ca 2+ -binding site, decreases signaling. Our results suggest an ECR-mediated mechanism for ADGRG6 regulation in which the CUB domain instructs conformational changes within the ECR to regulate receptor signaling.

The adhesion GPCR and PCP component flamingo (FMI-1) alters body size and regulates the composition of the extracellular matrix

The extracellular matrix (ECM) is a network of macromolecules that presents a vital scaffold for cells and enables multiple ways of cellular communication. Thus, it is essential for many physiological processes such as development, tissue morphogenesis, homeostasis, the shape and partially the size of the body and its organs. To ensure these, the composition of the ECM is tissue-specific and highly dynamic. ECM homeostasis is therefore tightly controlled by several mechanisms. Here, we show that FMI-1, the homolog of the Adhesion GPCR Flamingo/CELSR/ADGRC in the nematode Caenorhabditis elegans, modulates the composition of the ECM by controlling the production both of ECM molecules such as collagens and also of ECM modifying enzymes. Thereby, FMI-1 affects the morphology and functionality of the nematode´s cuticle, which is mainly composed of ECM, and also modulates the body size. Mechanistic analyses highlight the fact that FMI-1 exerts its function from neurons non-cell autonomously (trans) solely via its extracellular N terminus. Our data support a model, by which the activity of the receptor, which has a well-described role in the planar cell polarity (PCP) pathway, involves the PCP molecule VANG-1, but seems to be independent of the DBL-1/BMP pathway.

Perspectives on the Novel Multifunctional Nerve Guidance Conduits: From Specific Regenerative Procedures to Motor Function Rebuilding

Peripheral nerve injury potentially destroys the quality of life by inducing functional movement disorders and sensory capacity loss, which results in severe disability and substantial psychological, social, and financial burdens. Autologous nerve grafting has been commonly used as treatment in the clinic; however, its rare donor availability limits its application. A series of artificial nerve guidance conduits (NGCs) with advanced architectures are also proposed to promote injured peripheral nerve regeneration, which is a complicated process from axon sprouting to targeted muscle reinnervation. Therefore, exploring the interactions between sophisticated NGC complexes and versatile cells during each process including axon sprouting, Schwann cell dedifferentiation, nerve myelination, and muscle reinnervation is necessary. This review highlights the contribution of functional NGCs and the influence of microscale biomaterial architecture on biological processes of nerve repair. Progressive NGCs with chemical molecule induction, heterogenous topographical morphology, electroactive, anisotropic assembly microstructure, and self-powered electroactive and magnetic-sensitive NGCs are also collected, and they are expected to be pioneering features in future multifunctional and effective NGCs.

The dark sides of the GPCR tree - research progress on understudied GPCRs

A large portion of the human GPCRome is still in the dark and understudied, consisting even of entire subfamilies of GPCRs such as odorant receptors, class A and C orphans, adhesion GPCRs, Frizzleds and taste receptors. However, it is undeniable that these GPCRs bring an untapped therapeutic potential that should be explored further. Open questions on these GPCRs span diverse topics such as deorphanisation, the development of tool compounds and tools for studying these GPCRs, as well as understanding basic signalling mechanisms. This review gives an overview of the current state of knowledge for each of the diverse subfamilies of understudied receptors regarding their physiological relevance, molecular mechanisms, endogenous ligands and pharmacological tools. Furthermore, it identifies some of the largest knowledge gaps that should be addressed in the foreseeable future and lists some general strategies that might be helpful in this process.

The adhesion GPCR GPR116/ADGRF5 has a dual function in pancreatic islets regulating somatostatin release and islet development

Glucose homeostasis is maintained by hormones secreted from different cell types of the pancreatic islets and controlled by manifold input including signals mediated through G protein-coupled receptors (GPCRs). RNA-seq analyses revealed expression of numerous GPCRs in mouse and human pancreatic islets, among them Gpr116/Adgrf5. GPR116 is an adhesion GPCR mainly found in lung and required for surfactant secretion. Here, we demonstrate that GPR116 is involved in the somatostatin release from pancreatic delta cells using a whole-body as well as a cell-specific knock-out mouse model. Interestingly, the whole-body GPR116 deficiency causes further changes such as decreased beta-cell mass, lower number of small islets, and reduced pancreatic insulin content. Glucose homeostasis in global GPR116-deficient mice is maintained by counter-acting mechanisms modulating insulin degradation. Our data highlight an important function of GPR116 in controlling glucose homeostasis.

Structure, ligands, and roles of GPR126/ADGRG6 in the development and diseases

Adhesion G protein-coupled receptors (aGPCRs) are the second largest diverse group within the GPCR superfamily, which play critical roles in many physiological and pathological processes through cell-cell and cell-extracellular matrix interactions. The adhesion GPCR Adgrg6, also known as GPR126, is one of the better-characterized aGPCRs. GPR126 was previously found to have critical developmental roles in Schwann cell maturation and its mediated myelination in the peripheral nervous system in both zebrafish and mammals. Current studies have extended our understanding of GPR126-mediated roles during development and in human diseases. In this review, we highlighted these recent advances in GPR126 in expression profile, molecular structure, ligand-receptor interactions, and associated physiological and pathological functions in development and diseases.

Mechanistic Insights into Peptide Binding and Deactivation of an Adhesion G Protein-Coupled Receptor

Adhesion G protein-coupled receptors (ADGRGs) play critical roles in the reproductive, neurological, cardiovascular, and endocrine systems. In particular, ADGRG2 plays a significant role in Ewing sarcoma cell proliferation, parathyroid cell function, and male fertility. In 2022, a cryo-EM structure was reported for the active ADGRG2 bound by an optimized peptide agonist IP15 and the Gs protein. The IP15 peptide agonist was also modified to antagonists 4PH-E and 4PH-D with mutations of the 4PH residue to Glu and Asp, respectively. However, experimental structures of inactive antagonist-bound ADGRs remain to be resolved, and the activation mechanism of ADGRs such as ADGRG2 is poorly understood. Here, we applied Gaussian accelerated molecular dynamics (GaMD) simulations to probe conformational dynamics of the agonist- and antagonist-bound ADGRG2. By performing GaMD simulations, we were able to identify important low-energy conformations of ADGRG2 in the active, intermediate, and inactive states, as well as explore the binding conformations of each peptide. Moreover, our simulations revealed critical peptide-receptor residue interactions during the deactivation of ADGRG2. In conclusion, through GaMD simulations, we uncovered mechanistic insights into peptide (agonist and antagonist) binding and deactivation of the ADGRG2. These findings will potentially facilitate rational design of new peptide modulators of ADGRG2 and other ADGRs.

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.

Intramolecular activity regulation of adhesion GPCRs in light of recent structural and evolutionary information

The class B2 of GPCRs known as adhesion G protein-coupled receptors (aGPCRs) has come under increasing academic and nonacademic research focus over the past decade due to their physiological importance as mechano-sensors in cell-cell and cell-matrix contexts. A major advance in understanding signal transduction of aGPCRs was achieved by the identification of the so-called Stachel sequence, which acts as an intramolecular agonist at the interface between the N terminus (Nt) and the seven-transmembrane helix domain (7TMD). Distinct extracellular signals received by the Nt are integrated at the Stachel into structural changes of the 7TMD towards an active state conformation. Until recently, little information was available on how the activation process of aGPCRs is realized at the molecular level. In the past three years several structures of the 7TMD plus the Stachel in complex with G proteins have been determined, which provide new insights into the architecture and molecular function of this receptor class. Herein, we review this structural information to extract common and distinct aGPCR features with particular focus on the Stachel binding site within the 7TMD. Our analysis extends the current view of aGPCR activation and exposes similarities and differences not only between diverse aGPCR members, but also compared to other GPCR classes.

Unveiling Mechanical Activation: GAIN Domain Unfolding and Dissociation in Adhesion GPCRs

Adhesion G protein-coupled receptors (aGPCRs) have extracellular regions (ECRs) containing GPCR autoproteolysis-inducing (GAIN) domains. The GAIN domain enables the ECR to self-cleave into N- and C-terminal fragments. However, the impact of force on the GAIN domain's conformation, critical for mechanosensitive aGPCR activation, remains unclear. Our study investigated the mechanical stability of GAIN domains in three aGPCRs (B, G, and L subfamilies) at a loading rate of 1 pN/s. We discovered that forces of a few piconewtons can destabilize the GAIN domains. In autocleaved aGPCRs ADGRG1/GPR56 and ADGRL1/LPHN1, these forces cause the GAIN domain detachment from the membrane-proximal Stachel sequence, preceded by partial unfolding. In noncleavable aGPCR ADGRB3/BAI3 and cleavage-deficient mutant ADGRG1/GPR56-T383G, complex mechanical unfolding of the GAIN domain occurs. Additionally, GAIN domain detachment happens during cell migration. Our findings support the mechanical activation hypothesis of aGPCRs, emphasizing the sensitivity of the GAIN domain structure and detachment to physiological force ranges.

The COPD GWAS gene ADGRG6 instructs function and injury response in human iPSC-derived type II alveolar epithelial cells

Emphysema and chronic obstructive pulmonary disease (COPD) most commonly result from the effects of environmental exposures in genetically susceptible individuals. Genome-wide association studies have implicated ADGRG6 in COPD and reduced lung function, and a limited number of studies have examined the role of ADGRG6 in cells representative of the airway. However, the ADGRG6 locus is also associated with DLCO/VA, an indicator of gas exchange efficiency and alveolar function. Here, we sought to evaluate the mechanistic contributions of ADGRG6 to homeostatic function and disease in type 2 alveolar epithelial cells. We applied an inducible CRISPR interference (CRISPRi) human induced pluripotent stem cell (iPSC) platform to explore ADGRG6 function in iPSC-derived AT2s (iAT2s). We demonstrate that ADGRG6 exerts pleiotropic effects on iAT2s including regulation of focal adhesions, cytoskeleton, tight junctions, and proliferation. Moreover, we find that ADGRG6 knockdown in cigarette smoke-exposed iAT2s alters cellular responses to injury, downregulating apical complexes in favor of proliferation. Our work functionally characterizes the COPD GWAS gene ADGRG6 in human alveolar epithelium.

Functional partnerships between GPI-anchored proteins and adhesion GPCRs

Specific extracellular interaction between glycophosphatidylinositol (GPI)-anchored proteins and adhesion G protein-coupled receptors (aGPCRs) plays an important role in unique biological functions. GPI-anchored proteins are derived from a novel post-translational modification of single-span membrane molecules, while aGPCRs are bona fide seven-span transmembrane proteins with a long extracellular domain. Although various members of the two structurally-distinct protein families are known to be involved in a wide range of biological processes, many remain as orphans. Interestingly, accumulating evidence has pointed to a complex interaction and functional synergy between these two protein families. I discuss herein current understanding of specific functional partnerships between GPI-anchored proteins and aGPCRs.

A screen of pharmacologically active compounds to identify modulators of the Adgrg6/Gpr126 signalling pathway in zebrafish embryos

Adhesion G protein-coupled receptors (GPCRs) are an underrepresented class of GPCRs in drug discovery. We previously developed an in vivo drug screening pipeline to identify compounds with agonist activity for Adgrg6 (Gpr126), an adhesion GPCR required for myelination of the peripheral nervous system in vertebrates. The screening assay tests for rescue of an ear defect found in adgrg6tb233c-/- hypomorphic homozygous mutant zebrafish, using the expression of versican b (vcanb) mRNA as an easily identifiable phenotype. In the current study, we used the same assay to screen a commercially available library of 1280 diverse bioactive compounds (Sigma LOPAC). Comparison with published hits from two partially overlapping compound collections (Spectrum, Tocris) confirms that the screening assay is robust and reproducible. Using a modified counter screen for myelin basic protein (mbp) gene expression, we have identified 17 LOPAC compounds that can rescue both inner ear and myelination defects in adgrg6tb233c-/- hypomorphic mutants, three of which (ebastine, S-methylisothiourea hemisulfate, and thapsigargin) are new hits. A further 25 LOPAC hit compounds were effective at rescuing the otic vcanb expression but not mbp. Together, these and previously identified hits provide a wealth of starting material for the development of novel and specific pharmacological modulators of Adgrg6 receptor activity.

Re-routing GPR56 signalling using Gα12/13 G protein chimeras

Adhesion G protein-coupled receptors (aGPCRs) constitute the second largest subclass of the GPCR superfamily. Although canonical GPCRs are explored pharmacologically as drug targets, no clinically approved drugs target the aGPCR family so far. The aGPCR GPR56/ADGRG1 stands out as an especially promising target, given its direct link to the monogenetic disease bilateral frontoparietal polymicrogyria and implications in cancers. Key to understanding GPCR pharmacology has been mapping out intracellular signalling activity. Detection of GPCR signalling in the Gαs /Gαi /Gαq G protein pathways is feasible with second messenger detection systems. However, in the case of Gα12/13 -coupled receptors, like GPR56, signalling detection is more challenging due to the lack of direct second messenger generation. To overcome this challenge, we engineered a Gαq chimera to translate Gα12/13 signalling. We show the ability of the chimeric GαΔ6q12myr and GαΔ6q13myr to translate basal Gα12/13 signalling of GPR56 to a Gαq readout in transcription factor luciferase reporter systems and show that the established peptide ligands (P7 and P19) function to enhance this signal. We further demonstrate the ability to directly influence the generation of second messengers in inositol-3-phosphate assays. In the future, these chimeric G proteins could facilitate basic functional studies, drug screenings and deorphanization of other aGPCRs.

Downstream signalling of the disease-associated mutations on GPR56/ADGRG1

GPR56/ADGRG1 is an adhesion G protein-coupled receptor (GPCR) and mutations on this receptor cause cortical malformation due to the over-migration of neural progenitor cells on brain surface. At pial surface, GPR56 interacts with collagen III, induces Rho-dependent activation through Gα12/13 and inhibits the neuronal migration. In human glioma cells, GPR56 inhibits cell migration through Gαq/11 -dependent Rho pathway. GPR56-tetraspanin complex is known to couple Gαq/11 . GPR56 is an aGPCR that couples with various G proteins and signals through different downstream pathways. In this study, bilateral frontoparietal polymicrogyria (BFPP) mutants disrupting GPR56 function but remaining to be expressed on plasma membrane were used to study receptor signalling through Gα12 , Gα13 and Gα11 with BRET biosensors. GPR56 showed coupling with all three G proteins and activated heterotrimeric G protein signalling upon stimulation with Stachel peptide. However, BFPP mutants showed different signalling defects for each G protein indicative of distinct activation and signalling properties of GPR56 for Gα12 , Gα13 or Gα11 . β-arrestin recruitment was also investigated following the activation of GPR56 with Stachel peptide using BRET biosensors. N-terminally truncated GPR56 showed enhanced β-arrestin recruitment; however, neither wild-type receptor nor BFPP mutants gave any measurable recruitment upon Stachel stimulation, pointing different activation mechanisms for β-arrestin involvement.

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.

Structural clarity is brought to adhesion G protein-coupled receptor tethered agonism

An elusive problem in the adhesion G protein-coupled receptor (AGPCR) field is full understanding of the activation mechanisms of the 33-member receptor class. With the recent solution of active-state structures of nearly one quarter of AGPCRs, clarity has been brought to how AGPCRs are activated in response to endogenous full agonists. AGPCRs are self-activated via a tethered peptide agonist (TA) that transitions from a concealed or encrypted location to a decrypted state that binds to a typical GPCR orthosteric binding pocket. Here, we summarize the key milestones that led to the discovery of the AGPCR TA activation mechanism and discuss how extracellular shear forces may initiate TA decryption in physiological contexts. We compare the new active-state AGPCR structures and note that the orthosteric site-engaged TAs adopt a remarkably similar partial α-helical hook-like conformation, despite divergence of overall receptor similarity. Further, we contrast the TA-bound AGPCR structures to a partially active AGPCR structure to highlight the transitions AGPCRs may undergo during activation. Finally, we provide commentary on the validity of alternative AGPCR activation mechanisms.

Adhesion GPCR Gpr126 (Adgrg6) Expression Profiling in Zebrafish, Mouse, and Human Kidney

Adhesion G protein-coupled receptors (aGPCRs) comprise the second-largest class of GPCRs, the most common target for approved pharmacological therapies. aGPCRs play an important role in development and disease and have recently been associated with the kidney. Several aGPCRs are expressed in the kidney and some aGPCRs are either required for kidney development or their expression level is altered in diseased kidneys. Yet, general aGPCR function and their physiological role in the kidney are poorly understood. Here, we characterize in detail Gpr126 (Adgrg6) expression based on RNAscope® technology in zebrafish, mice, and humans during kidney development in adults. Gpr126 expression is enriched in the epithelial linage during nephrogenesis and persists in the adult kidney in parietal epithelial cells, collecting ducts, and urothelium. Single-cell RNAseq analysis shows that gpr126 expression is detected in zebrafish in a distinct ionocyte sub-population. It is co-detected selectively with slc9a3.2, slc4a4a, and trpv6, known to be involved in apical acid secretion, buffering blood or intracellular pH, and to maintain high cytoplasmic Ca2+ concentration, respectively. Furthermore, gpr126-expressing cells were enriched in the expression of potassium transporter kcnj1a.1 and gcm2, which regulate the expression of a calcium sensor receptor. Notably, the expression patterns of Trpv6, Kcnj1a.1, and Gpr126 in mouse kidneys are highly similar. Collectively, our approach permits a detailed insight into the spatio-temporal expression of Gpr126 and provides a basis to elucidate a possible role of Gpr126 in kidney physiology.

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.

Single hormone or synthetic agonist induces Gs/Gi coupling selectivity of EP receptors via distinct binding modes and propagating paths

To accomplish concerted physiological reactions, nature has diversified functions of a single hormone at at least two primary levels: 1) Different receptors recognize the same hormone, and 2) different cellular effectors couple to the same hormone-receptor pair [R.P. Xiao, Sci STKE 2001, re15 (2001); L. Hein, J. D. Altman, B.K. Kobilka, Nature 402, 181-184 (1999); Y. Daaka, L. M. Luttrell, R. J. Lefkowitz, Nature 390, 88-91 (1997)]. Not only these questions lie in the heart of hormone actions and receptor signaling but also dissecting mechanisms underlying these questions could offer therapeutic routes for refractory diseases, such as kidney injury (KI) or X-linked nephrogenic diabetes insipidus (NDI). Here, we identified that Gs-biased signaling, but not Gi activation downstream of EP4, showed beneficial effects for both KI and NDI treatments. Notably, by solving Cryo-electron microscope (cryo-EM) structures of EP3-Gi, EP4-Gs, and EP4-Gi in complex with endogenous prostaglandin E2 (PGE2)or two synthetic agonists and comparing with PGE2-EP2-Gs structures, we found that unique primary sequences of prostaglandin E2 receptor (EP) receptors and distinct conformational states of the EP4 ligand pocket govern the Gs/Gi transducer coupling selectivity through different structural propagation paths, especially via TM6 and TM7, to generate selective cytoplasmic structural features. In particular, the orientation of the PGE2 ω-chain and two distinct pockets encompassing agonist L902688 of EP4 were differentiated by their Gs/Gi coupling ability. Further, we identified common and distinct features of cytoplasmic side of EP receptors for Gs/Gi coupling and provide a structural basis for selective and biased agonist design of EP4 with therapeutic potential.

Noncoding SNP at rs1663689 represses ADGRG6 via interchromosomal interaction and reduces lung cancer progression

A previous genome-wide association study (GWAS) revealed an association of the noncoding SNP rs1663689 with susceptibility to lung cancer in the Chinese population. However, the underlying mechanism is unknown. In this study, using allele-specific 4C-seq in heterozygous lung cancer cells combined with epigenetic information from CRISPR/Cas9-edited cell lines, we show that the rs1663689 C/C variant represses the expression of ADGRG6, a gene located on a separate chromosome, through an interchromosomal interaction of the rs1663689 bearing region with the ADGRG6 promoter. This reduces downstream cAMP-PKA signaling and subsequently tumor growth both in vitro and in xenograft models. Using patient-derived organoids, we show that rs1663689 T/T-but not C/C-bearing lung tumors are sensitive to the PKA inhibitor H89, potentially informing therapeutic strategies. Our study identifies a genetic variant-mediated interchromosomal interaction underlying ADGRG6 regulation and suggests that targeting the cAMP-PKA signaling pathway may be beneficial in lung cancer patients bearing the homozygous risk genotype at rs1663689.

Collagen VI Is a Gi-Biased Ligand of the Adhesion GPCR GPR126/ADGRG6

GPR126/ADGRG6, a member of the adhesion G-protein-coupled receptor family, balances cell differentiation and proliferation through fine-tuning of intracellular cAMP levels, which is achieved through coupling to Gs and Gi proteins. While GPR126-mediated cAMP increase has been proven to be essential for differentiation of Schwann cells, adipocytes and osteoblasts, Gi-signaling of the receptor was found to propagate breast cancer cell proliferation. Extracellular ligands or mechanical forces can modulate GPR126 activity but require an intact encrypted agonist sequence, coined the Stachel. Even though coupling to Gi can be seen for constitutively active truncated receptor versions of GPR126 as well as with a peptide agonist derived from the Stachel sequence, all known N-terminal modulators have so far only been shown to modulate Gs coupling. Here, we identified collagen VI as the first extracellular matrix ligand of GPR126 that induces Gi signaling at the receptor, which shows that N-terminal binding partners can mediate selective G protein signaling cascades that are masked by fully active truncated receptor variants.

Drosophila glia take shape to sculpt the nervous system

The importance of glial cells has become increasingly apparent over the past 20 years, yet compared to neurons we still know relatively little about these essential cells. Most critical glial cell functions are conserved in Drosophila glia, often using the same key molecular players as their vertebrate counterparts. The relative simplicity of the Drosophila nervous system, combined with a vast array of powerful genetic tools, allows us to further dissect the molecular composition and functional roles of glia in ways that would be much more cumbersome or not possible in higher vertebrate systems. Importantly, Drosophila genetics allow for in vivo manipulation, and their transparent body wall enables in vivo imaging of glia in intact animals throughout early development. Here we discuss recent advances in Drosophila glial development detailing how these cells take on their mature morphologies and interact with neurons to perform their important functional roles in the nervous system.

Optimized genetic code expansion technology for time-dependent induction of adhesion GPCR-ligand engagement

The introduction of an engineered aminoacyl-tRNA synthetase/tRNA pair enables site-specific incorporation of unnatural amino acids (uAAs) with functionalized side chains into proteins of interest. Genetic Code Expansion (GCE) via amber codon suppression confers functionalities to proteins but can also be used to temporally control the incorporation of genetically encoded elements into proteins. Here, we report an optimized GCE system (GCEXpress) for efficient and fast uAA incorporation. We demonstrate that GCEXpress can be used to efficiently alter the subcellular localization of proteins within living cells. We show that click labeling can resolve co-labeling problems of intercellular adhesive protein complexes. We apply this strategy to study the adhesion G protein-coupled receptor (aGPCR) ADGRE5/CD97 and its ligand CD55/DAF that play central roles in immune functions and oncological processes. Furthermore, we use GCEXpress to analyze the time course of ADGRE5-CD55 ligation and replenishment of mature receptor-ligand complexes. Supported by fluorescence recovery after photobleaching (FRAP) experiments our results show that ADGRE5 and CD55 form stable intercellular contacts that may support transmission of mechanical forces onto ADGRE5 in a ligand-dependent manner. We conclude that GCE in combination with biophysical measurements can be a useful approach to analyze the adhesive, mechanical and signaling properties of aGPCRs and their ligand interactions. This article is protected by copyright. All rights reserved.

Molecular sensing of mechano- and ligand-dependent adhesion GPCR dissociation

Adhesion G-protein-coupled receptors (aGPCRs) bear notable similarity to Notch proteins1, a class of surface receptors poised for mechano-proteolytic activation2-4, including an evolutionarily conserved mechanism of cleavage5-8. However, so far there is no unifying explanation for why aGPCRs are autoproteolytically processed. Here we introduce a genetically encoded sensor system to detect the dissociation events of aGPCR heterodimers into their constituent N-terminal and C-terminal fragments (NTFs and CTFs, respectively). An NTF release sensor (NRS) of the neural latrophilin-type aGPCR Cirl (ADGRL)9-11, from Drosophila melanogaster, is stimulated by mechanical force. Cirl-NRS activation indicates that receptor dissociation occurs in neurons and cortex glial cells. The release of NTFs from cortex glial cells requires trans-interaction between Cirl and its ligand, the Toll-like receptor Tollo (Toll-8)12, on neural progenitor cells, whereas expressing Cirl and Tollo in cis suppresses dissociation of the aGPCR. This interaction is necessary to control the size of the neuroblast pool in the central nervous system. We conclude that receptor autoproteolysis enables non-cell-autonomous activities of aGPCRs, and that the dissociation of aGPCRs is controlled by their ligand expression profile and by mechanical force. The NRS system will be helpful in elucidating the physiological roles and signal modulators of aGPCRs, which constitute a large untapped reservoir of drug targets for cardiovascular, immune, neuropsychiatric and neoplastic diseases13.

Development of a Sensitive Digital Droplet PCR Screening Assay for the Detection of GPR126 Non-Coding Mutations in Bladder Cancer Urine Liquid Biopsies

Recent whole-genome sequencing studies identified two novel recurrent mutations in the enhancer region of GPR126 in urothelial bladder cancer (UBC) tumor samples. This mutational hotspot is the second most common after the TERT promoter in UBC. The aim of the study was to develop a digital droplet PCR screening assay for the simultaneous detection of GPR126 mutations in a single tube. Its performance combined with TERT promoter mutation analysis was evaluated in urine of healthy volunteers (n = 50) and patients with cystitis (n = 22) and UBC (n = 70). The developed assay was validated using DNA constructs carrying the studied variants. None of the mutations were detected in control and cystitis group samples. GPR126 mutations were observed in the urine of 25/70 UBC patients (area under the ROC curve (AUC) of 0.679; mutant allele fraction (MAF) of 21.61 [8.30-44.52] %); TERT mutations-in 40/70 (AUC of 0.786; MAF = 28.29 [19.03-38.08] %); ≥1 mutation-in 47/70 (AUC of 0.836)). The simultaneous presence of GPR126 and TERT mutations was observed in 18/70 cases, with no difference in MAFs for the paired samples (31.96 [14.78-47.49] % vs. 27.13 [17.00-37.62] %, p = 0.349, respectively). The combined analysis of these common non-coding mutations in urine allows the sensitive and non-invasive detection of UBC.

Isoform- and ligand-specific modulation of the adhesion GPCR ADGRL3/Latrophilin3 by a synthetic binder

Adhesion G protein-coupled receptors (aGPCRs) are cell-surface proteins with large extracellular regions that bind to multiple ligands to regulate key biological functions including neurodevelopment and organogenesis. Modulating a single function of a specific aGPCR isoform while affecting no other function and no other receptor is not trivial. Here, we engineered an antibody, termed LK30, that binds to the extracellular region of the aGPCR ADGRL3, and specifically acts as an agonist for ADGRL3 but not for its isoform, ADGRL1. The LK30/ADGRL3 complex structure revealed that the LK30 binding site on ADGRL3 overlaps with the binding site for an ADGRL3 ligand - teneurin. In cellular-adhesion assays, LK30 specifically broke the trans-cellular interaction of ADGRL3 with teneurin, but not with another ADGRL3 ligand - FLRT3. Our work provides proof of concept for the modulation of isoform- and ligand-specific aGPCR functions using unique tools, and thus establishes a foundation for the development of fine-tuned aGPCR-targeted therapeutics.