Expression of the largest CD97 and EMR2 isoforms on leukocytes facilitates a specific interaction with chondroitin sulfate on B cells

CD97 negatively regulates the innate immune response against RNA viruses by promoting RNF125-mediated RIG-I degradation

The G protein-coupled receptor ADGRE5 (CD97) binds to various metabolites that play crucial regulatory roles in metabolism. However, its function in the antiviral innate immune response remains to be determined. In this study, we report that CD97 inhibits virus-induced type-I interferon (IFN-I) release and enhances RNA virus replication in cells and mice. CD97 was identified as a new negative regulator of the innate immune receptor RIG-I, and RIG-1 degradation led to the suppression of the IFN-I signaling pathway. Furthermore, overexpression of CD97 promoted the ubiquitination of RIG-I, resulting in its degradation, but did not impact its mRNA expression. Mechanistically, CD97 upregulates RNF125 expression to induce RNF125-mediated RIG-I degradation via K48-linked ubiquitination at Lys181 after RNA virus infection. Most importantly, CD97-deficient mice are more resistant than wild-type mice to RNA virus infection. We also found that sanguinarine-mediated inhibition of CD97 effectively blocks VSV and SARS-CoV-2 replication. These findings elucidate a previously unknown mechanism through which CD97 negatively regulates RIG-I in the antiviral innate immune response and provide a molecular basis for the development of new therapeutic strategies and the design of targeted antiviral agents.

The Posttraumatic Increase of the Adhesion GPCR EMR2/ADGRE2 on Circulating Neutrophils Is Not Related to Injury Severity

Trauma triggers a rapid innate immune response to aid the clearance of damaged/necrotic cells and their released damage-associated molecular pattern (DAMP). Here, we monitored the expression of EMR2/ADGRE2, involved in the functional regulation of innate immune cells, on circulating neutrophils in very severely and moderately/severely injured patients up to 240 h after trauma. Notably, neutrophilic EMR2 showed a uniform, injury severity- and type of injury-independent posttraumatic course in all patients. The percentage of EMR2+ neutrophils and their EMR2 level increased and peaked 48 h after trauma. Afterwards, they declined and normalized in some, but not all, patients. Circulating EMR2+ compared to EMR2- neutrophils express less CD62L and more CD11c, a sign of activation. Neutrophilic EMR2 regulation was verified in vitro. Remarkably, it increased, depending on extracellular calcium, in controls as well. Cytokines, enhanced in patients immediately after trauma, and sera of patients did not further affect this neutrophilic EMR2 increase, whereas apoptosis induction disrupted it. Likely the damaged/necrotic cells/DAMPs, unavoidable during neutrophil culture, stimulate the neutrophilic EMR2 increase. In summary, the rapidly increased absolute number of neutrophils, especially present in very severely injured patients, together with upregulated neutrophilic EMR2, may expand our in vivo capacity to react to and finally clear damaged/necrotic cells/DAMPs after trauma.

Single-Cell RNA-Sequencing Reveals Peripheral T Helper Cells Promoting the Development of IgG4-Related Disease by Enhancing B Cell Activation and Differentiation

Abnormal B cell differentiation plays a critical role in IgG4-related disease (IgG4-RD), but the underlying mechanism remains largely unknown. We investigated the cell landscape from three IgG4-RD retroperitoneal tissues and three control tissues using single-cell RNA-sequencing. Critical cell type or markers were further validated in the peripheral blood from the patients with IgG4-RD and healthy controls via flow cytometry as well as in the IgG4-RD and control tissue via immunofluorescence staining. The increases in B cells, plasma cells, and CD4+ T cells were found in IgG4-RD retroperitoneal tissue. Importantly, among CD4+ T cells, an increase in CD4+CXCR5-PD1hi peripheral T helper (Tph) cells with a high expression of IL-21 and TIGIT was discovered in IgG4-RD tissue, which was further validated in peripheral blood of the patients with IgG4-RD. The Tph cell and TIGIT+ Tph cell proportion were remarkably higher in active IgG4-RD patients and correlated with disease activity. Moreover, TIGIT+CD4+ cells were able to promote B cell differentiation via IL-21. Our study revealed that Tph cells are increased in IgG4-RD and probably play critical roles in B cell differentiation through TIGIT-IL-21 axis. Peripheral Tph cell and TIGIT+Tph cell are potential markers for IgG4-RD disease activity.

Dual role of the adhesion G-protein coupled receptor ADRGE5/CD97 in glioblastoma invasion and proliferation

CD97, an adhesion G-protein coupled receptor highly expressed in glioblastoma (GBM), consists of two noncovalently bound domains: the N-terminal fragment (NTF) and C-terminal fragment. The C-terminal fragment contains a GPCR domain that couples to Gα12/13, while the NTF interacts with extracellular matrix components and other receptors. We investigated the effects of changing CD97 levels and its function on primary patient-derived GBM stem cells (pdGSCs) in vitro and in vivo. We created two functional mutants: a constitutively active ΔNTF and the noncleavable dominant-negative H436A mutant. The CD97 knockdown in pdGSCs decreased, while overexpression of CD97 increased tumor size. Unlike other constructs, the ΔNTF mutant promoted tumor cell proliferation, but the tumors were comparable in size to those with CD97 overexpression. As expected, the GBM tumors overexpressing CD97 were very invasive, but surprisingly, the knockdown did not inhibit invasiveness and even induced it in noninvasive U87 tumors. Importantly, our results indicate that NTF was present in the tumor core cells but absent in the pdGSCs invading the brain. Furthermore, the expression of noncleavable H436A mutant led to large tumors that invade by sending massive protrusions, but the invasion of individual tumor cells was substantially reduced. These data suggest that NTF association with CD97 GPCR domain inhibits individual cell dissemination but not overall tumor invasion. However, NTF dissociation facilitates pdGSCs brain infiltration and may promote tumor proliferation. Thus, the interplay between two functional domains regulates CD97 activity resulting in either enhanced cell adhesion or stimulation of tumor cell invasion and proliferation.

Role of Adhesion G Protein-Coupled Receptors in Immune Dysfunction and Disorder

Disorders of the immune system, including immunodeficiency, immuno-malignancy, and (auto)inflammatory, autoimmune, and allergic diseases, have a great impact on a host’s health. Cellular communication mediated through cell surface receptors, among different cell types and between cell and microenvironment, plays a critical role in immune responses. Selective members of the adhesion G protein-coupled receptor (aGPCR) family are expressed differentially in diverse immune cell types and have been implicated recently in unique immune dysfunctions and disorders in part due to their dual cell adhesion and signaling roles. Here, we discuss the molecular and functional characteristics of distinctive immune aGPCRs and their physiopathological roles in the immune system.

Adhesion G protein-coupled receptors: structure, signaling, physiology, and pathophysiology

Adhesion G protein-coupled receptors (AGPCRs) are a family of 33 receptors in humans exhibiting a conserved general structure but diverse expression patterns and physiological functions. The large NH2 termini characteristic of AGPCRs confer unique properties to each receptor and possess a variety of distinct domains that can bind to a diverse array of extracellular proteins and components of the extracellular matrix. The traditional view of AGPCRs, as implied by their name, is that their core function is the mediation of adhesion. In recent years, though, many surprising advances have been made regarding AGPCR signaling mechanisms, activation by mechanosensory forces, and stimulation by small-molecule ligands such as steroid hormones and bioactive lipids. Thus, a new view of AGPCRs has begun to emerge in which these receptors are seen as massive signaling platforms that are crucial for the integration of adhesive, mechanosensory, and chemical stimuli. This review article describes the recent advances that have led to this new understanding of AGPCR function and also discusses new insights into the physiological actions of these receptors as well as their roles in human disease.

Pathophysiological Impact of the Adhesion G-Protein Coupled Receptor Family

G-Protein Coupled Receptors (GPCRs) represent one of the most targeted drug classes in the human genome, accounting for greater than 40% of all FDA-approved drugs. However, the second-largest family of GPCRs, known as Adhesion GPCRs (aGPCR), have yet to serve as a clinical target despite increasing evidence of their physiological and pathological functions, which suggest an opportunity toward the development of novel therapeutics. To date, the pathophysiological function of aGPCRs is associated with a plethora of diseases including cancer, CNS disorders, immunity and inflammation, and others. To highlight their potential as pharmacologic targets, we will review three distinct aGPCR members (ADGRG1, ADGRE5 and ADGRF5), highlighting their molecular mechanisms of action and contributions to the development of pathophysiology.

To Detach, Migrate, Adhere, and Metastasize: CD97/ADGRE5 in Cancer

Tumorigenesis is a multistep process, during which cells acquire a series of mutations that lead to unrestrained cell growth and proliferation, inhibition of cell differentiation, and evasion of cell death. Growing tumors stimulate angiogenesis, providing them with nutrients and oxygen. Ultimately, tumor cells invade the surrounding tissue and metastasize; a process responsible for about 90% of cancer-related deaths. Adhesion G protein-coupled receptors (aGPCRs) modulate the cellular processes closely related to tumor cell biology, such as adhesion and detachment, migration, polarity, and guidance. Soon after first being described, individual human aGPCRs were found to be involved in tumorigenesis. Twenty-five years ago, CD97/ADGRE5 was discovered to be induced in one of the most severe tumors, dedifferentiated anaplastic thyroid carcinoma. After decades of research, the time has come to review our knowledge of the presence and function of CD97 in cancer. In summary, CD97 is obviously induced or altered in many tumor entities; this has been shown consistently in nearly one hundred published studies. However, its high expression at circulating and tumor-infiltrating immune cells renders the systemic targeting of CD97 in tumors difficult.

Chemo- and mechanosensing by dendritic cells facilitate antigen surveillance in the spleen

Spleen dendritic cells (DC) are critical for initiation of adaptive immune responses against blood-borne invaders. Key to DC function is their positioning at sites of pathogen entry, and their abilities to selectively capture foreign antigens and promptly engage T cells. Focusing on conventional DC2 (cDC2), we discuss the contribution of chemoattractant receptors (EBI2 or GPR183, S1PR1, and CCR7) and integrins to cDC2 positioning and function. We give particular attention to a newly identified role in cDC2 for adhesion G-protein coupled receptor E5 (Adgre5 or CD97) and its ligand CD55, detailing how this mechanosensing system contributes to splenic cDC2 positioning and homeostasis. Additional roles of CD97 in the immune system are reviewed. The ability of cDC2 to be activated by circulating missing self-CD47 cells and to integrate multiple red blood cell (RBC)-derived inputs is discussed. Finally, we describe the process of activated cDC2 migration to engage and prime helper T cells. Throughout the review, we consider the insights into cDC function in the spleen that have emerged from imaging studies.

Ligands and Beyond: Mechanosensitive Adhesion GPCRs

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

Genetic analysis in a familial case with high bone mineral density suggests additive effects at two loci

Osteoporosis is the most common bone disease, characterized by a low bone mineral density (BMD) and increased risk of fracture. At the other end of the BMD spectrum, some individuals present strong, fracture‐resistant, bones. Both osteoporosis and high BMD are heritable and their genetic architecture encompasses polygenic inheritance of common variants and some cases of monogenic highly penetrant variants in causal genes. We have investigated the genetics of high BMD in a family segregating this trait in an apparently Mendelian dominant pattern. We searched for rare causal variants by whole‐exome sequencing in three affected and three nonaffected family members. Using this approach, we have identified 38 rare coding variants present in the proband and absent in the three individuals with normal BMD. Although we have found four variants shared by the three affected members of the family, we have not been able to relate any of these to the high‐BMD phenotype. In contrast, we have identified missense variants in two genes, VAV3 and ADGRE5, each shared by two of out of three affected members, whose loss of function fits with the phenotype of the family. In particular, the proband, a woman displaying the highest BMD (sum Z‐score = 7), carries both variants, whereas the other two affected members carry one each. VAV3 encodes a guanine‐nucleotide‐exchange factor with an important role in osteoclast activation and function. Although no previous cases of VAV3 mutations have been reported in humans, Vav3 knockout (KO) mice display dense bones, similarly to the high‐BMD phenotype present in our family. The ADGRE5 gene encodes an adhesion G protein‐coupled receptor expressed in osteoclasts whose KO mouse displays increased trabecular bone volume. Combined, these mouse and human data highlight VAV3 and ADGRE5 as novel putative high‐BMD genes with additive effects, and potential therapeutic targets for osteoporosis. © 2022 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

CD97 stabilises the immunological synapse between dendritic cells and T cells and is targeted for degradation by the Salmonella effector SteD

The Salmonella enterica effector SteD depletes mature MHC class II (mMHCII) molecules from the surface of infected antigen-presenting cells through ubiquitination of the cytoplasmic tail of the mMHCII β chain. This requires the Nedd4 family HECT E3 ubiquitin ligase Wwp2 and a tumor-suppressing transmembrane protein adaptor Tmem127. Here, through a proteomic screen of dendritic cells, we found that SteD targets the plasma membrane protein CD97 for degradation by a similar mechanism. SteD enhanced ubiquitination of CD97 on K555 and mutation of this residue eliminated the effect of SteD on CD97 surface levels. We showed that CD97 localises to and stabilises the immunological synapse between dendritic cells and T cells. Removal of CD97 by SteD inhibited dendritic cell-T cell interactions and reduced T cell activation, independently of its effect on MHCII. Therefore, SteD suppresses T cell immunity by two distinct processes.

Structural basis for CD97 recognition of the decay-accelerating factor CD55 suggests mechanosensitive activation of adhesion GPCRs

The adhesion G protein–coupled receptor CD97 and its ligand complement decay-accelerating factor CD55 are important binding partners in the human immune system. Dysfunction in this binding has been linked to immune disorders such as multiple sclerosis and rheumatoid arthritis, as well as various cancers. Previous literatures have indicated that the CD97 includes 3 to 5 epidermal growth factor (EGF) domains at its N terminus and these EGF domains can bind to the N-terminal short consensus repeat (SCR) domains of CD55. However, the details of this interaction remain elusive, especially why the CD55 binds with the highest affinity to the shortest isoform of CD97 (EGF_1,2,5). Herein, we designed a chimeric expression construct with the EGF_1,2,5 domains of CD97 and the SCR_1–4 domains of CD55 connected by a flexible linker and determined the complex structure by crystallography. Our data reveal that the two proteins adopt an overall antiparallel binding mode involving the SCR_1–3 domains of CD55 and all three EGF domains of CD97. Mutagenesis data confirmed the importance of EGF₅ in the interaction and explained the binding specificity between CD55 and CD97. The architecture of CD55–CD97 binding mode together with kinetics suggests a force-resisting shearing stretch geometry when forces applied to the C termini of both proteins in the circulating environment. The potential of the CD55–CD97 complex to withstand tensile force may provide a basis for the mechanosensing mechanism for activation of adhesion G protein–coupled receptors.

Adhesion G protein-coupled receptors in glioblastoma

Background

Members of the adhesion family of G protein-coupled receptors (GPCRs) have received attention for their roles in health and disease, including cancer. Over the past decade, several members of the family have been implicated in the pathogenesis of glioblastoma.

Methods

Here, we discuss the basic biology of adhesion GPCRs and review in detail specific members of the receptor family with known functions in glioblastoma. Finally, we discuss the potential use of adhesion GPCRs as novel treatment targets in neuro-oncology.

CD36 and CD97 in Pancreatic Cancer versus Other Malignancies

Starting from the recent identification of CD36 and CD97 as a novel marker combination of fibroblast quiescence in lung during fibrosis, we aimed to survey the literature in search for facts about the separate (or concomitant) expression of clusters of differentiation CD36 and CD97 in either tumor- or pancreatic-cancer-associated cells. Here, we provide an account of the current knowledge on the diversity of the cellular functions of CD36 and CD97 and explore their potential (common) contributions to key cellular events in oncogenesis or metastasis development. Emphasis is placed on quiescence as an underexplored mechanism and/or potential target in therapy. Furthermore, we discuss intricate signaling mechanisms and networks involving CD36 and CD97 that may regulate different subpopulations of tumor-associated cells, such as cancer-associated fibroblasts, adipocyte-associated fibroblasts, tumor-associated macrophages, or neutrophils, during aggressive pancreatic cancer. The coexistence of quiescence and activated states in cancer-associated cell subtypes during pancreatic cancer should be better documented, in different histological forms. Remodeling of the local microenvironment may also change the balance between growth and dormant state. Taking advantage of the reported data in different other tissue types, we explore the possibility to induce quiescence (similar to that observed in normal cells), as a therapeutic option to delay the currently observed clinical outcome.

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.

The repertoire of Adhesion G protein-coupled receptors in adipocytes and their functional relevance

BACKGROUND

G protein-coupled receptors (GPCR) are well-characterized regulators of a plethora of physiological functions among them the modulation of adipogenesis and adipocyte function. The class of Adhesion GPCR (aGPCR) and their role in adipose tissue, however, is poorly studied. With respect to the demand for novel targets in obesity treatment, we present a comprehensive study on the expression and function of this enigmatic GPCR class during adipogenesis and in mature adipocytes.

METHODS

The expression of all aGPCR representatives was determined by reanalyzing RNA-Seq data and by performing qPCR in different mouse and human adipose tissues under low- and high-fat conditions. The impact of aGPCR expression on adipocyte differentiation and lipid accumulation was studied by siRNA-mediated knockdown of all expressed members of this receptor class. The biological characteristics and function of mature adipocytes lacking selected aGPCR were analyzed by mass spectrometry and biochemical methods (lipolysis, glucose uptake, adiponectin secretion).

RESULTS

More than ten aGPCR are significantly expressed in visceral and subcutaneous adipose tissues and several aGPCR are differentially regulated under high-caloric conditions in human and mouse. Receptor knockdown of six receptors resulted in an impaired adipogenesis indicating their expression is essential for proper adipogenesis. The altered lipid composition was studied in more detail for two representatives, ADGRG2/GPR64 and ADGRG6/GPR126. While GPR126 is mainly involved in adipocyte differentiation, GPR64 has an additional role in mature adipocytes by regulating metabolic processes.

CONCLUSIONS

Adhesion GPCR are significantly involved in qualitative and quantitative adipocyte lipid accumulation and can control lipolysis. Factors driving adipocyte formation and function are governed by signaling pathways induced by aGPCR yielding these receptors potential targets for treating obesity.

G Protein-Coupling of Adhesion GPCRs ADGRE2/EMR2 and ADGRE5/CD97, and Activation of G Protein Signalling by an Anti-EMR2 Antibody

The experimental evidence that Adhesion G Protein-Coupled Receptors (aGPCRs) functionally couple to heterotrimeric G proteins has been emerging in incremental steps, but attributing biological significance to their G protein signalling function still presents a major challenge. Here, utilising activated truncated forms of the receptors, we show that ADGRE2/EMR2 and ADGRE5/CD97 are G protein-coupled in a variety of recombinant systems. In a yeast-based assay, where heterologous GPCRs are coupled to chimeric G proteins, EMR2 showed broad G protein-coupling, whereas CD97 coupled more specifically to G_α12, G_α13, G_α14 and G_αz chimeras. Both receptors induced pertussis-toxin (PTX) insensitive inhibition of cyclic AMP (cAMP) levels in mammalian cells, suggesting coupling to G_αz. EMR2 was shown to signal via G_α16, and via a G_α16/G_αz chimera, to stimulate IP₁ accumulation. Finally, using an NFAT reporter assay, we identified a polyclonal antibody that activates EMR2 G protein signalling in vitro. Our results highlight the potential for the development of soluble agonists to understand further the biological effects and therapeutic opportunities for ADGRE receptor-mediated G protein signalling.

Adhesion G protein-coupled receptors: opportunities for drug discovery

Adhesion G protein-coupled receptors (aGPCRs) - one of the five main families in the GPCR superfamily - have several atypical characteristics, including large, multi-domain N termini and a highly conserved region that can be autoproteolytically cleaved. Although GPCRs overall have well-established pharmacological tractability, currently no therapies that target any of the 33 members of the aGPCR family are either approved or in clinical trials. However, human genetics and preclinical research have strengthened the links between aGPCRs and disease in recent years. This, together with a greater understanding of their functional complexity, has led to growing interest in aGPCRs as drug targets. A framework for prioritizing aGPCR targets and supporting approaches to develop aGPCR modulators could therefore be valuable in harnessing the untapped therapeutic potential of this family. With this in mind, here we discuss the unique opportunities and challenges for drug discovery in modulating aGPCR functions, including target identification, target validation, assay development and safety considerations, using ADGRG1 as an illustrative example.

CD97 is a critical regulator of acute myeloid leukemia stem cell function

Despite significant efforts to improve therapies for acute myeloid leukemia (AML), clinical outcomes remain poor. Understanding the mechanisms that regulate the development and maintenance of leukemic stem cells (LSCs) is important to reveal new therapeutic opportunities. We have identified CD97, a member of the adhesion class of G protein-coupled receptors (GPCRs), as a frequently up-regulated antigen on AML blasts that is a critical regulator of blast function. High levels of CD97 correlate with poor prognosis, and silencing of CD97 reduces disease aggressiveness in vivo. These phenotypes are due to CD97's ability to promote proliferation, survival, and the maintenance of the undifferentiated state in leukemic blasts. Collectively, our data credential CD97 as a promising therapeutic target on LSCs in AML.

Identification of ADGRE5 as discriminating MYC target between Burkitt lymphoma and diffuse large B-cell lymphoma

Background

MYC is a heterogeneously expressed transcription factor that plays a multifunctional role in many biological processes such as cell proliferation and differentiation. It is also associated with many types of cancer including the malignant lymphomas. There are two types of aggressive B-cell lymphoma, namely Burkitt lymphoma (BL) and a subgroup of diffuse large cell lymphoma (DLBCL), which both carry MYC translocations and overexpress MYC but both differ significantly in their clinical outcome. In DLBCL, MYC translocations are associated with an aggressive behavior and poor outcome, whereas MYC-positive BL show a superior outcome.

Methods

To shed light on this phenomenon, we investigated the different modes of actions of MYC in aggressive B-cell lymphoma cell lines subdivided into three groups: (i) MYC-positive BL, (ii) DLBCL with MYC translocation (DLBCLpos) and (iii) DLBCL without MYC translocation (DLBCLneg) for control. In order to identify genome-wide MYC-DNA binding sites a chromatin immunoprecipitation followed by high-throughput sequencing (ChIP-Seq) was performed. In addition, ChIP-Seq for H3K4me3 was used for determination of genomic regions accessible for transcriptional activity. These data were supplemented with gene expression data derived from RNA-Seq.

Results

Bioinformatics integration of all data sets revealed different MYC-binding patterns and transcriptional profiles in MYC-positive BL and DLBCL cell lines indicating different functional roles of MYC for gene regulation in aggressive B-cell lymphomas. Based on this multi-omics analysis we identified ADGRE5 (alias CD97) - a member of the EGF-TM7 subfamily of adhesion G protein-coupled receptors - as a MYC target gene, which is specifically expressed in BL but not in DLBCL regardless of MYC translocation.

Conclusion

Our study describes a diverse genome-wide MYC-DNA binding pattern in BL and DLBCL cell lines with and without MYC translocations. Furthermore, we identified ADREG5 as a MYC target gene able to discriminate between BL and DLBCL irrespectively of the presence of MYC breaks in DLBCL. Since ADGRE5 plays an important role in tumor cell formation, metastasis and invasion, it might also be instrumental to better understand the different pathobiology of BL and DLBCL and help to explain discrepant clinical characteristics of BL and DLBCL.

Electronic supplementary material

The online version of this article (10.1186/s12885-019-5537-0) contains supplementary material, which is available to authorized users.

Contradictory Role of CD97 in Basal and Tumor Necrosis Factor-Induced Osteoclastogenesis In Vivo

OBJECTIVE

CD97, a member of the 7-transmembrane epidermal growth factor family of adhesion G protein-coupled receptors, is expressed on various cell types. This study was undertaken to elucidate the functions of CD97 in bone and inflammation in an experimental mouse model, by examining the effect of CD97 on osteoclastogenesis in vitro, characterizing the skeletal phenotype of CD97-deficient (CD97-knockout [KO]) mice, and assessing the responses to tumor necrosis factor (TNF) treatment.

METHODS

Femoral tissue and bone marrow (BM)-derived cells from CD97-KO and wild-type (WT) mice were assessed using histomorphometric analyses, in vitro cultures, and reverse transcription-polymerase chain reaction. Serum cytokine and chemokine levels in the presence or absence of TNF challenge were analyzed by multiplex assay.

RESULTS

In cultures of mouse BM-derived macrophages in vitro, RANKL induced the expression of CD97. In vivo, the trabecular bone volume of the femurs of female CD97-KO mice was increased, and this was associated with a decrease in the number of osteoclasts. Compared to WT mice, CD97-KO mice had a reduced potential to form osteoclast-like cells in vitro. Furthermore, TNF treatment augmented the formation of osteoclasts in the calvaria of CD97-KO mice in vivo, by increasing the production of RANKL and other cytokines and chemokines and by reducing the production of osteoprotegerin by calvarial cells.

CONCLUSION

These findings demonstrate that CD97 is a positive regulator of osteoclast-like cell differentiation, a mechanism that influences bone homeostasis. However, the presence of CD97 may be essential to suppress the initial osteoclastogenesis that occurs in response to acute and local inflammatory stimuli.

G Protein-Coupled Receptors in Macrophages

As the largest receptor gene family in the human genome, with >800 members, the signal-transducing G protein-coupled receptors (GPCRs) play critical roles in nearly all conceivable physiological processes, ranging from the sensing of photons and odorants to metabolic homeostasis and migration of leukocytes. Unfortunately, an exhaustive review of the several hundred GPCRs expressed by myeloid cells/macrophages (P.J. Groot-Kormelink, L .Fawcett, P.D. Wright, M. Gosling, and T.C. Kent, BMC Immunol 12:57, 2012, doi:10.1186/1471-2172-13-57) is beyond the scope of this chapter; however, we will endeavor to cover the GPCRs that contribute to the major facets of macrophage biology, i.e., those whose expression is restricted to macrophages and the GPCRs involved in macrophage differentiation/polarization, microbial elimination, inflammation and resolution, and macrophage-mediated pathology. The chemokine receptors, a major group of myeloid GPCRs, will not be extensively covered as they are comprehensively reviewed elsewhere.

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.

Association of the EGF-TM7 receptor CD97 expression with FLT3-ITD in acute myeloid leukemia

Internal tandem duplications within the juxtamembrane region of the FMS-like tyrosine kinase receptor FLT3 (FLT3-ITD) represents one of the most common mutations in patients with acute myeloid leukemia (AML) which results in constitutive aberrant activation, increased proliferation of leukemic progenitors and is associated with an aggressive clinical phenotype. The expression of CD97, an EGF-TM7 receptor, has been linked to invasive behavior in thyroid and colorectal cancer. Here, we have investigated the association of CD97 with FLT3-ITD and its functional consequences in AML.Higher CD97 expression levels have been detected in 208 out of 385 primary AML samples. This was accompanied by a significantly increased bone marrow blast count as well as by mutations in the FLT3 gene. FLT3-ITD expressing cell lines as MV4-11 and MOLM-13 revealed significantly higher CD97 levels than FLT3 wildtype EOL-1, OCI-AML3 and HL-60 cells which were clearly decreased by the tyrosine kinase inhibitors PKC412 and SU5614. CD97 knock down by short hairpin RNA in MV4-11 cells resulted in inhibited trans-well migration towards fetal calf serum (FCS) and lysophosphatidic acid (LPA) being at least in part Rho-A dependent. Moreover, knock down of CD97 led to an altered mechanical phenotype, reduced adhesion to a stromal layer and lower wildtype FLT3 expression.Our results, thus, constitute the first evidence for the functional relevance of CD97 expression in FLT3-ITD AML cells rendering it a potential new theragnostic target.

Characteristics of B cell‑associated gene expression in patients with coronary artery disease

The current study aimed to identify differentially expressed B cell‑associated genes in peripheral blood mononuclear cells and observe the changes in B cell activation at different stages of coronary artery disease. Groups of patients with acute myocardial infarction (AMI) and stable angina (SA), as well as healthy volunteers, were recruited into the study (n=20 per group). Whole human genome microarray analysis was performed to examine the expression of B cell‑associated genes among these three groups. The mRNA expression levels of 60 genes associated with B cell activity and regulation were measured using reverse transcription‑quantitative polymerase chain reaction. The mRNA expression of the B cell antigen receptor (BCR)‑associated genes, CD45, NFAM, SYK and LYN, were significantly upregulated in patients with AMI; however, FCRL3, CD79B, CD19, CD81, FYN, BLK, CD22 and CD5 mRNA expression levels were significantly downregulated, compared with patients in the SA and control group. The mRNA levels of the T‑independent B cell‑associated genes, CD16, CD32, LILRA1 and TLR9, were significantly increased in AMI patients compared with SA and control patients. The mRNA expression of genes associated with T‑dependent B cells were also measured: EMR2 and CD97 were statistically upregulated, whereas SLAMF1, LY9, CD28, CD43, CD72, ICOSL, PD1, CD40 and CD20 mRNAs were significantly downregulated in AMI group patients compared with the two other groups. Additionally the gene expression levels of B cell regulatory genes were measured. In patients with AMI, CR1, LILRB2, LILRB3 and VAV1 mRNA expression levels were statistically increased, whereas, CS1 and IL4I1 mRNAs were significantly reduced compared with the SA and control groups. There was no statistically significant difference in B cell‑associated gene expression levels between patients with SA and the control group. The present study identified the downregulation of genes associated with BCRs, B2 cells and B cell regulators in patients with AMI, indicating a weakened T cell‑B cell interaction and reduced B2 cell activation during AMI. Thus, improving B2 cell‑mediated humoral immunity may be a potential target for medical intervention in patients with AMI.

CD97/ADGRE5 Inhibits LPS Induced NF-κB Activation through PPAR-γ Upregulation in Macrophages

CD97/ADGRE5 protein is predominantly expressed on leukocytes and belongs to the EGF-TM7 receptors family. It mediates granulocytes accumulation in the inflammatory tissues and is involved in firm adhesion of PMNC on activated endothelial cells. There have not been any studies exploring the role of CD97 in LPS induced NF-κB activation in macrophages. Therefore, we first measured the CD97 expression in LPS treated human primary macrophages and subsequently analyzed the levels of inflammatory factor TNF-α and transcription factor NF-κB in these macrophages that have been manipulated with either CD97 knockdown or overexpression. We found that a reported anti-inflammatory transcription factor, PPAR-γ, was involved in the CD97 mediated NF-κB suppression. Furthermore, by immunofluorescence staining, we established that CD97 overexpression not only inhibited LPS induced p65 expression in the nucleus but also promoted the PPAR-γ expression. Moreover, using CD97 knockout THP-1 cells, we further demonstrated that CD97 promoted PPAR-γ expression and decreased LPS induced NF-κB activation. In conclusion, CD97 plays a negative role in LPS induced NF-κB activation and TNF-α secretion, partly through PPAR-γ upregulation.

Classification, Nomenclature, and Structural Aspects of Adhesion GPCRs

Representation of the nine distinct aGPCR subfamilies and their unique N-terminal domain architecture. The illustration also shows the extracellular structural feature shared by all aGPCRs (except ADGRA1), known as the GPCR autoproteolysis-inducing (GAIN) domain, that mediates autoproteolysis and subsequent attachment of the cleaved NTF and CTF fragments The adhesion family of G protein-coupled receptors (aGPCRs) is unique among all GPCR families with long N-termini and multiple domains that are implicated in cell-cell and cell-matrix interactions. Initially, aGPCRs in the human genome were phylogenetically classified into nine distinct subfamilies based on their 7TM sequence similarity. This phylogenetic grouping of genes into subfamilies was found to be in congruence in closely related mammals and other vertebrates as well. Over the years, aGPCR repertoires have been mapped in many species including model organisms, and, currently, there is a growing interest in exploring the pharmacological aspects of aGPCRs. Nonetheless, the aGPCR nomenclature has been highly diverse because experts in the field have used different names for different family members based on their characteristics (e.g., epidermal growth factor-seven-span transmembrane (EGF-TM7)), but without harmonization with regard to nomenclature efforts. In order to facilitate naming of orthologs and other genetic variants in different species in the future, the Adhesion-GPCR Consortium, together with the International Union of Basic and Clinical Pharmacology Committee on Receptor Nomenclature and Drug Classification, proposed a unified nomenclature for aGPCRs. Here, we review the classification and the most recent/current nomenclature of aGPCRs and as well discuss the structural topology of the extracellular domain (ECD)/N-terminal fragment (NTF) that is comparable with this 7TM subfamily classification. Of note, we systematically describe the structural domains in the ECD of aGPCR subfamilies and highlight their role in aGPCR-protein interactions.

Adhesion GPCRs as Modulators of Immune Cell Function

Immune cells express several adhesion G protein-coupled receptors (aGPCRs), including the ADGRE subfamily members EMR1 (F4/80, ADGRE1), EMR2 (ADGRE2), EMR3 (ADGRE3), EMR4 (FIRE, ADGRE4), and CD97 (ADGRE5), the ADGRB subfamily member BAI1 (ADGRB1), and the ADGRG subfamily members GPR56 (ADGRG1), GPR97 (Pb99, ADGRG3), and GPR114 (ADGRG5). Expression of these molecules in hematopoietic stem and progenitor cells, monocytes/macrophages (Mφs), dendritic cells, granulocytes, and lymphocytes depends on lineage diversification and maturation, making them suitable markers for individual leukocyte subsets (e.g., F4/80 on mouse Mφs). Recent studies revealed intriguing activities of aGPCRs in tolerance induction (EMR1), granulopoiesis (CD97), engulfment of apoptotic cells and bacteria (BAI1), hematopoietic stem cell formation (GPR56), and control of cytotoxicity (GPR56). Here, we review these findings and discuss their biological and translational implications.

International Union of Basic and Clinical Pharmacology. XCIV. Adhesion G protein-coupled receptors

The Adhesion family forms a large branch of the pharmacologically important superfamily of G protein-coupled receptors (GPCRs). As Adhesion GPCRs increasingly receive attention from a wide spectrum of biomedical fields, the Adhesion GPCR Consortium, together with the International Union of Basic and Clinical Pharmacology Committee on Receptor Nomenclature and Drug Classification, proposes a unified nomenclature for Adhesion GPCRs. The new names have ADGR as common dominator followed by a letter and a number to denote each subfamily and subtype, respectively. The new names, with old and alternative names within parentheses, are: ADGRA1 (GPR123), ADGRA2 (GPR124), ADGRA3 (GPR125), ADGRB1 (BAI1), ADGRB2 (BAI2), ADGRB3 (BAI3), ADGRC1 (CELSR1), ADGRC2 (CELSR2), ADGRC3 (CELSR3), ADGRD1 (GPR133), ADGRD2 (GPR144), ADGRE1 (EMR1, F4/80), ADGRE2 (EMR2), ADGRE3 (EMR3), ADGRE4 (EMR4), ADGRE5 (CD97), ADGRF1 (GPR110), ADGRF2 (GPR111), ADGRF3 (GPR113), ADGRF4 (GPR115), ADGRF5 (GPR116, Ig-Hepta), ADGRG1 (GPR56), ADGRG2 (GPR64, HE6), ADGRG3 (GPR97), ADGRG4 (GPR112), ADGRG5 (GPR114), ADGRG6 (GPR126), ADGRG7 (GPR128), ADGRL1 (latrophilin-1, CIRL-1, CL1), ADGRL2 (latrophilin-2, CIRL-2, CL2), ADGRL3 (latrophilin-3, CIRL-3, CL3), ADGRL4 (ELTD1, ETL), and ADGRV1 (VLGR1, GPR98). This review covers all major biologic aspects of Adhesion GPCRs, including evolutionary origins, interaction partners, signaling, expression, physiologic functions, and therapeutic potential.

Epidermal growth factor-like module containing mucin-like hormone receptor 2 expression in gliomas

Epidermal growth factor (EGF) module-containing mucin-like receptor 2 (EMR2) is a member of the seven span transmembrane adhesion G-protein coupled receptor subclass. This protein is expressed in a subset of glioblastoma (GBM) cells and associated with an invasive phenotype. The expression pattern and functional significance of EMR2 in low grade or anaplastic astrocytomas is unknown and our goal was to expand and further define EMR2's role in gliomas with an aggressive invasive phenotype. Using the TCGA survival data we describe EMR2 expression patterns across histologic grades of gliomas and demonstrate an association between increased EMR2 expression and poor survival (p < 0.05). This data supports prior functional data depicting that EMR2-positive neoplasms possess a greater capacity for infiltrative and metastatic spread. Genomic analysis suggests that EMR2 overexpression is associated with the mesenchymal GBM subtype (p < 0.0001). We also demonstrate that immunohistorchemistry is a feasible method for screening GBM patients for EMR2 expression. Protein and mRNA analysis demonstrated variable expression of all isoforms of EMR2 in all glioma grades, however GBM displayed the most diverse isoforms expression pattern as well as the highest expression of the EGF1-5 isoform of EMR2. Finally, a correlation of an increased EMR2 expression after bevacizumab treatment in glioma cells lines is identified. This observation should serve as the impetus for future studies to determine if this up-regulation of EMR2 plays a role in the observation of the diffuse and increasingly invasive recurrence patterns witnessed in a subset of GBM patients after bevacizumab treatment.

Skeletal muscle expression of the adhesion-GPCR CD97: CD97 deletion induces an abnormal structure of the sarcoplasmatic reticulum but does not impair skeletal muscle function

CD97 is a widely expressed adhesion class G-protein-coupled receptor (aGPCR). Here, we investigated the presence of CD97 in normal and malignant human skeletal muscle as well as the ultrastructural and functional consequences of CD97 deficiency in mice. In normal human skeletal muscle, CD97 was expressed at the peripheral sarcolemma of all myofibers, as revealed by immunostaining of tissue sections and surface labeling of single myocytes using flow cytometry. In muscle cross-sections, an intracellular polygonal, honeycomb-like CD97-staining pattern, typical for molecules located in the T-tubule or sarcoplasmatic reticulum (SR), was additionally found. CD97 co-localized with SR Ca²⁺-ATPase (SERCA), a constituent of the longitudinal SR, but not with the receptors for dihydropyridine (DHPR) or ryanodine (RYR), located in the T-tubule and terminal SR, respectively. Intracellular expression of CD97 was higher in slow-twitch compared to most fast-twitch myofibers. In rhabdomyosarcomas, CD97 was strongly upregulated and in part more N-glycosylated compared to normal skeletal muscle. All tumors were strongly CD97-positive, independent of the underlying histological subtype, suggesting high sensitivity of CD97 for this tumor. Ultrastructural analysis of murine skeletal myofibers confirmed the location of CD97 in the SR. CD97 knock-out mice had a dilated SR, resulting in a partial increase in triad diameter yet not affecting the T-tubule, sarcomeric, and mitochondrial structure. Despite these obvious ultrastructural changes, intracellular Ca²⁺ release from single myofibers, force generation and fatigability of isolated soleus muscles, and wheel-running capacity of mice were not affected by the lack of CD97. We conclude that CD97 is located in the SR and at the peripheral sarcolemma of human and murine skeletal muscle, where its absence affects the structure of the SR without impairing skeletal muscle function.

The role of epidermal growth factor-like module containing mucin-like hormone receptor 2 in human cancers

G-protein coupled receptors (GPCRs) are among the most diverse and ubiquitous proteins in all of biology. The epidermal growth factor-seven span transmembrane (EGF-TM7) subfamily of adhesion GPCRs is a small subset whose members are mainly expressed on the surface of leukocytes. The EGF domains on the N-terminus add significant size to these receptors and they are considered to be among the largest members of the TM7 family. Although not all of their ligands or downstream targets have been identified, there is evidence implicating the EGF-TM7 family diverse processes such as cell adhesion, migration, inflammation, and autoimmune disease. Recent studies have identified expression of EGF-TM7 family members on human neoplasms including those of the thyroid, stomach, colon, and brain. Their presence on these tissues is not surprising given the ubiquity of GPCRs, but because their functional significance and pathways are not completely understood, they are of tremendous clinical and scientific interest. Current evidence suggests that expression of certain EGF-TM7 receptors is correlated with tumor grade, confers a more invasive phenotype, and increases the likelihood of metastatic disease. In this review, we will discuss the structure, function, and regulation of these receptors. We also describe the expression of these receptors in human cancers and explore their potential mechanistic significance.

Semicircular canal morphogenesis in the zebrafish inner ear requires the function of gpr126 (lauscher), an adhesion class G protein-coupled receptor gene

Morphogenesis of the semicircular canal ducts in the vertebrate inner ear is a dramatic example of epithelial remodelling in the embryo, and failure of normal canal development results in vestibular dysfunction. In zebrafish and Xenopus, semicircular canal ducts develop when projections of epithelium, driven by extracellular matrix production, push into the otic vesicle and fuse to form pillars. We show that in the zebrafish, extracellular matrix gene expression is high during projection outgrowth and then rapidly downregulated after fusion. Enzymatic disruption of hyaluronan in the projections leads to their collapse and a failure to form pillars: as a result, the ears swell. We have cloned a zebrafish mutant, lauscher (lau), identified by its swollen ear phenotype. The primary defect in the ear is abnormal projection outgrowth and a failure of fusion to form the semicircular canal pillars. Otic expression of extracellular matrix components is highly disrupted: several genes fail to become downregulated and remain expressed at abnormally high levels into late larval stages. The lau mutations disrupt gpr126, an adhesion class G protein-coupled receptor gene. Expression of gpr126 is similar to that of sox10, an ear and neural crest marker, and is partially dependent on sox10 activity. Fusion of canal projections and downregulation of otic versican expression in a hypomorphic lau allele can be restored by cAMP agonists. We propose that Gpr126 acts through a cAMP-mediated pathway to control the outgrowth and adhesion of canal projections in the zebrafish ear via the regulation of extracellular matrix gene expression.

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.

Immunohistochemical expression and prognostic value of CD97 and its ligand CD55 in primary gallbladder carcinoma

Background. CD97 as a member of the EGF-TM7 family with adhesive properties plays an important role in tumor aggressiveness by binding its cellular ligand CD55, which is a complement regulatory protein expressed by cells to protect them from bystander complement attack. Previous studies have shown that CD97 and CD55 both play important roles in tumor dedifferentiation, migration, invasiveness, and metastasis. The aim of this study was to investigate CD97 and CD55 expression in primary gallbladder carcinoma (GBC) and their prognostic significance. Methods. Immunohistochemistry was used to investigate the expression of CD97 and CD55 proteins in 138 patients with GBC. Results. CD97 and CD55 were absent or only weakly expressed in the normal epithelium of the gallbladder but in 69.6% (96/138) and 65.2% (90/138) of GBC, respectively, remarkably at the invasive front of the tumors. In addition, CD97 and CD55 expressions were both significantly associated with high histologic grade (both P = 0.009), advanced pathologic T stage (P = 0.01 and 0.009, resp.) and clinical stage (both P = 0.009), and positive venous/lymphatic invasion (both P = 0.009). Multivariate analyses showed that CD97 (hazard ratio, 3.236; P = 0.02) and CD55 (hazard ratio, 3.209; P = 0.02) expressions and clinical stage (hazard ratio, 3.918; P = 0.01) were independent risk factor for overall survival. Conclusion. Our results provide convincing evidence for the first time that the expressions of CD97 and CD55 are both upregulated in human GBC. The expression levels of CD97 and CD55 in GBC were associated with the severity of the tumor. Furthermore, CD97 and CD55 expressions were independent poor prognostic factors for overall survival in patients with GBC.

Orphan G protein-coupled receptors (GPCRs): biological functions and potential drug targets

The superfamily of G protein-coupled receptors (GPCRs) includes at least 800 seven-transmembrane receptors that participate in diverse physiological and pathological functions. GPCRs are the most successful targets of modern medicine, and approximately 36% of marketed pharmaceuticals target human GPCRs. However, the endogenous ligands of more than 140 GPCRs remain unidentified, leaving the natural functions of those GPCRs in doubt. These are the so-called orphan GPCRs, a great source of drug targets. This review focuses on the signaling transduction pathways of the adhesion GPCR family, the LGR subfamily, and the PSGR subfamily, and their potential functions in immunology, development, and cancers. In this review, we present the current approaches and difficulties of orphan GPCR deorphanization and characterization.

LPA receptor heterodimerizes with CD97 to amplify LPA-initiated RHO-dependent signaling and invasion in prostate cancer cells

CD97, an adhesion-linked G-protein-coupled receptor (GPCR), is induced in multiple epithelial cancer lineages. We address here the signaling properties and the functional significance of CD97 expression in prostate cancer. Our findings show that CD97 signals through Gα12/13 to increase RHO-GTP levels. CD97 functioned to mediate invasion in prostate cancer cells, at least in part, by associating with lysophosphatidic acid receptor 1 (LPAR1), leading to enhanced LPA-dependent RHO and extracellular signal-regulated kinase activation. Consistent with its role in invasion, depletion of CD97 in PC3 cells resulted in decreased bone metastasis without affecting subcutaneous tumor growth. Furthermore, CD97 heterodimerized and functionally synergized with LPAR1, a GPCR implicated in cancer progression. We also found that CD97 and LPAR expression were significantly correlated in clinical prostate cancer specimens. Taken together, these findings support the investigation of CD97 as a potential therapeutic cancer target.

Epidermal growth factor module-containing mucin-like receptor 2 is a newly identified adhesion G protein-coupled receptor associated with poor overall survival and an invasive phenotype in glioblastoma

Epidermal growth factor (EGF) module-containing mucin-like receptor 2 (EMR2) is a member of the seven span transmembrane (TM7) adhesion G-protein coupled receptor subclass. It is abundantly expressed in immune cells of myeloid origin and appears to mediate cellular adhesion, migration, and signaling. Based on an analysis showing earlier mortality among glioblastoma patients whose tumors highly express EMR2, we studied its expression patterns in glioblastoma and potential to mediate cellular proliferation and invasion. We performed univariate analysis of overall survival in GBM patients expressing low, moderate, and high amounts of EMR2 based on publicly available microarray data in the Cancer Genome Atlas (TCGA). Using RT-PCR and western blotting, we studied mRNA and protein expression patterns of EMR2, respectively. We then employed siRNA knockdown of EMR2 in two human glioblastoma cell lines expressing high levels of EMR2 to assess functional effects on cellular proliferation and invasion, in vitro. Kaplan-Meier analysis of TCGA survival data for GBM demonstrated that EMR2 levels are inversely correlated with overall time until mortality (log rank, P < 0.01). EMR2 mRNA and protein is variably expressed in primary glioblastoma samples and human glioblastoma cell lines. When comparing cells transfected with siRNA targeting EMR2 to those transfected with a negative, scramble control, there was no difference in proliferation over 72 h in the SF767 and G55 glioblastoma cell lines. However, EMR2 knockout cells demonstrated an almost 3-fold reduction in migration compared to their negative controls (P < 0.05) in the SF767 cell line, and an almost 2-fold reduction (P < 0.05) in migration in the G55 cell line. We provide evidence that EMR2 is expressed in glioblastoma and has significant functional consequences on cellular invasion, but not proliferation. In light of its ability to promote adhesion and migration in immune cells, our data suggest that EMR2 may mediate similar phenomena in glioblastoma. The invasive phenotype conferred by EMR2 correlates with clinical data demonstrating poor survival in glioblastoma patients who express high levels of EMR2 in their tumor.

GPR56 is essential for testis development and male fertility in mice

Testis development is critical for male fertility and continuation of the mammalian species. Essential structural components of testes are seminiferous tubules, which are lined by Sertoli cells and provide nutrients and physical protection for the maturation of sperm. Seminiferous tubule formation is initiated in embryos as testis cords and relies on their remodeling for maturation during development. Recently, three-dimensional image analyses showed that testis cords in different parts of embryonic gonads undergo distinct remodeling processes. How this asymmetric remodeling is regulated has not been investigated. We report here that the absence of an adhesion G protein-coupled receptor, GPR56, leads to partial disruption of seminiferous tubules and reduced fertility in male mice. The defects appear to originate asymmetrically in embryonic gonads, but subsequent to the initial establishment of testis cords, suggesting that GPR56 might act to establish a spatial and/or temporal cue for asymmetric cord remodeling during male gonad development.

GPR56 plays varying roles in endogenous cancer progression

GPR56, a non-classical adhesion receptor, was previously reported to suppress tumor growth and metastasis in xenograft models using human melanoma cell lines. To understand whether GPR56 plays similar roles in the development of endogenous tumors, we analyzed cancer progression in Gpr56 (-/-) mice using a variety of transgenic cancer models. Our results showed that GPR56 suppressed prostate cancer progression in the TRAMP model on a mixed genetic background, similar to its roles in progression of melanoma xenografts. However, its roles in other cancer types appeared to be complex. It had marginal effects on tumor onset of mammary tumors in the MMTV-PyMT model, but had no effects on subsequent tumor progression in either the MMTV-PyMT mice or the melanoma model, Ink4a/Arf (-/-) tyr-Hras. These results indicate diverse roles of GPR56 in cancer progression and provide the first genetic evidence for the involvement of an adhesion GPCR in endogenous cancer development.

Immunity and adhesion-GPCRs

Adhesion-GPCRs are unusual, owing to their unique structure, comprising a large and complex extracellular domain composed of various common protein modules. Adhesion-GPCR family members are expressed ubiquitously; however the expression of each receptor is highly regulated and often restricted to specific cell types. The EGF-TM7 adhesion-GPCR subfamily members are predominantly expressed by leukocytes and involved in coordinating both the innate and acquired immune responses. Here we highlight some immunological insights in relation to EGF-TM7 proteins and other members of the adhesion-GPCR family.

Therapeutic antibody targeting of CD97 in experimental arthritis: the role of antigen expression, shedding, and internalization on the pharmacokinetics of anti-CD97 monoclonal antibody 1B2

CD97 is a member of the EGF-TM7 family of adhesion class receptors, with a proposed role in inflammatory cell recruitment. Neutralization of murine CD97 with the anti-mCD97 mAb 1B2 was efficacious in prevention of murine collagen-induced arthritis, a model with features resembling rheumatoid arthritis. Here, the therapeutic potential of neutralizing CD97 in arthritis was studied with emphasis on the 1B2 pharmacokinetics. Mice with established arthritis were treated with anti-mCD97 or anti-TNF-alpha serum. Ab pharmacokinetics and biodistribution were studied in diseased and nondiseased mice using labeled 1B2. The impact of CD97 expression on Ab pharmacokinetics was studied using CD97 knockout mice. Treatment with 1B2 showed an efficacy comparable to anti-TNF-alpha treatment. Pharmacokinetic analysis of 1B2 in wild-type and CD97 knockout mice indicated a dose-dependent Ab clearance, due to specific interaction with CD97. Biodistribution studies showed accumulation of 1B2 in spleen and lung. In vitro studies using murine splenocytes revealed that CD97 when bound to Ab was internalized. Moreover, soluble CD97 was detected in the supernatant, suggesting Ag shedding. Finally, in arthritic mice, higher levels of soluble CD97 were found and 1B2 treatment led to specific targeting of inflamed paws, resulting in a higher clearance rate of 1B2 in arthritic mice than in wild-type mice. In conclusion, our data support a therapeutic value of CD97 neutralization in experimental arthritis. The pharmacokinetic profile of the 1B2 Ab illustrates the complexity of Ab elimination from an organism and stresses the importance of understanding Ag-Ab interactions when developing therapeutic mAbs.