The EGF-TM7 family of the rat

EGF-TM7 receptors are adhesion class heptahelical molecules predominantly expressed by cells of the immune system. Based on an analysis of the recently unraveled genome, the EGF-TM7 family of the rat is described here. Like the mouse, the rat has three EGF-TM7 receptors—CD97, EMR1 and EMR4. The highest conservation between the orthologues lies within the membrane-spanning part and emphasizes the functional importance of this region.The nucleotide sequences reported here have been submitted to the GenBank database with the following accession numbers: AY686632 and AY686633.     

The epidermal growth factor-seven transmembrane (EGF-TM7) receptor CD97 is required for neutrophil migration and host defense

The epidermal growth factor-seven transmembrane (EGF-TM7) family is a group of seven-span transmembrane receptors predominantly expressed by cells of the immune system. Family members CD97, EGF module-containing mucin-like receptor (EMR) 1, EMR2, EMR3, EMR4, and EGF-TM7-latrophilin-related protein are characterized by an extended extracellular region with a variable number of N-terminal EGF-like domains. EGF-TM7 receptors bind cellular ligands as demonstrated by the interaction of CD97 with decay accelerating factor (CD55) and dermatan sulfate. Investigating the effect of newly generated mAb on the migration of neutrophilic granulocytes, we here report for the first time in vivo data on the function of CD97. In dextran sulfate sodium-induced experimental colitis, we show that homing of adoptively transferred neutrophils to the colon was significantly delayed when cells were preincubated with CD97 mAb. The consequences of this defect in neutrophil migration for host defense are demonstrated in a murine model of Streptococcus pneumoniae-induced pneumonia. Mice treated with CD97 mAb to EGF domain 1 (1B2) and EGF domain 3 (1C5) displayed a reduced granulocytic inflammatory infiltrate at 20 h after inoculation. This was associated with a significantly enhanced outgrowth of bacteria in the lungs at 44 h and a strongly diminished survival. Together, these findings indicate an essential role for CD97 in the migration of neutrophils.     

The EGF-TM7 family: a postgenomic view

With the human and mouse genome projects now completed, the receptor repertoire of mammalian cells has finally been elucidated. The EGF-TM7 receptors are a family of class B seven-span transmembrane (TM7) receptors predominantly expressed by cells of the immune system. Within the large TM7 superfamily, the molecular structure and ligand-binding properties of EGF-TM7 receptors are unique. Derived from the processing of a single polypeptide, they are expressed at the cell surface as heterodimers consisting of a large extracellular region associated with a TM7 moiety. Through a variable number of N-terminal epidermal growth factor (EGF)-like domains, EGF-TM7 receptors interact with cellular ligands such as CD55 and chondroitin sulfate. Recent in vivo studies demonstrate a role of the EGF-TM7 receptor CD97 in leukocyte migration. The different number of EGF-TM7 genes in man compared with mice, the chimeric nature of EMR2 and the inactivation of human EMR4 point toward a still-evolving receptor family. Here we discuss the currently available information on this intriguing receptor family.     

Human EMR2, a novel EGF-TM7 molecule on chromosome 19p13.1, is closely related to CD97

The epidermal growth factor (EGF)-TM7 proteins [EMR1, (EGF-like molecule containing mucin-like hormone receptor 1) F4/80, and CD97] constitute a recently defined class B GPCR subfamily and are predominantly expressed on leukocytes. These molecules possess N-terminal EGF-like domains coupled to a seven-span transmembrane (7TM) moiety via a mucin-like spacer domain. Genomic mapping analysis has suggested a possible EGF-TM7 gene family on the human chromosome 19p13 region. In this study, a new member of the EGF-TM7 family, EMR2, which shares strikingly similar molecular characteristics with CD97, is described. In addition to mapping closely to CD97 on human chromosome 19p13.1, EMR2 contains a total of five tandem EGF-like domains and expresses similar protein isoforms consisting of various numbers of EGF-like domains as a result of alternative RNA splicing. Furthermore, EMR2 and CD97 exhibit highly homologous EGF-like domains and share identical gene organization, indicating that both genes are the products of a recent gene duplication event. The homologous EGF-like domains enable the identification of both EMR2 and CD97 by monoclonal antibodies (mAbs) raised against the first EGF-like domain of CD97, whereas mAbs directed against the extracellular spacer domain of CD97 are able to differentiate these two proteins. Both EMR2 and CD97 are highly expressed in immune tissues; however, unlike CD97, which is ubiquitously expressed in most cell types, EMR2 expression is restricted to monocytes/Mφ and granulocytes. EMR2 fails to interact with CD55, the cellular ligand for CD97, suggesting the possibility of a different cellular ligand(s). EMR2 may therefore have a unique function in cells of monocyte/Mφ and granulocyte lineages.     

Human epidermal growth factor (EGF) module-containing mucin-like hormone receptor 3 is a new member of the EGF-TM7 family that recognizes a ligand on human macrophages and activated neutrophils

The epidermal growth factor (EGF)-TM7 subgroup of G-protein-coupled receptors is composed predominantly of leukocyte-restricted glycoproteins defined by their unique hybrid structure, in which extracellular EGF-like domains are coupled to a seven-span transmembrane moiety via a mucin-like stalk. The EGF-TM7 group comprises mouse F4/80, human EGF module-containing mucin-like hormone receptor (EMR) 1, human EMR2, and human and mouse CD97, the genes for which map to human chromosome 19p13 and the syntenic regions of the mouse genome. In this study we describe the cloning and characterization of EMR3, a novel human EGF-TM7 molecule, and show the existence of its cellular ligand. The EMR3 gene maps closely to the existing members of the EGF-TM7 family on human chromosome 19p13.1 and, in common with other EGF-TM7 genes, is capable of generating different protein isoforms through alternative splicing. Two alternative splice forms have been isolated: one encoding a 652-amino acid cell surface protein consisting of two EGF-like domains, a mucin stalk, and a putative G-protein-coupled receptor domain and the other encoding a truncated soluble form containing only two EGF-like domains. As with other members of the EGF-TM7 family, EMR3 mRNA displays a predominantly leukocyte-restricted expression pattern, with highest levels in neutrophils, monocytes, and macrophages. Through the use of soluble EMR3 multivalent probes we have shown the presence of a ligand at the surface of monocyte-derived macrophages and activated human neutrophils. These interactions suggest a potential role for EMR3 in myeloid-myeloid interactions during immune and inflammatory responses.     

Site-specific N-glycosylation regulates the GPS auto-proteolysis of CD97

Auto-proteolysis at the G protein-coupled receptor (GPCR) proteolytic site (GPS) is a hallmark of adhesion-GPCRs. Although defects in GPS auto-proteolysis have been linked to genetic disorders, information on its regulation remains elusive. Here, we investigated the GPS proteolysis of CD97, a human leukocyte-restricted and tumor-associated adhesion-GPCR. We found that CD97 is incompletely processed, unlike its close homolog, epidermal growth factor-like module-containing mucin-like hormone receptor 2. A unique pattern of N-glycosylation within the GPS motif of related adhesion-GPCRs was identified. The use of N-glycosylation inhibitors and mutants confirm site-specific N-glycosylation is an important determinant of GPS proteolysis in CD97. Our results suggest that N-glycosylation may regulate the processing of adhesion-GPCRs leading to the production of either cleaved or uncleaved molecules.     

The role of receptor oligomerization in modulating the expression and function of leukocyte adhesion-G protein-coupled receptors

The human leukocyte adhesion-G protein-coupled receptors (GPCRs), the epidermal growth factor (EGF)-TM7 proteins, are shown here to function as homo- and hetero-oligomers. Using cell surface cross-linking, co-immunoprecipitation, and fluorescence resonance energy transfer analysis of EMR2, an EGF-TM7 receptor predominantly expressed in myeloid cells, we demonstrate that it forms dimers in a reaction mediated exclusively by the TM7 moiety. We have also identified a naturally occurring but structurally unstable EMR2 splice variant that acts as a dominant negative modulator by dimerizing with the wild type receptor and down-regulating its expression. Additionally, heterodimerization between closely related EGF-TM7 members is shown to result in the modulation of expression and ligand binding properties of the receptors. These findings suggest that receptor homo- and hetero-oligomerization play a regulatory role in modulating the expression and function of leukocyte adhesion-GPCRs.     

A short 5' splice site RNA oligo can participate in both steps of splicing in mammalian extracts.

A short 5' splice site RNA oligonucleotide (5'SS RNA oligo) undergoes both steps of splicing when a second RNA containing the 3' splice site region (3'SS RNA) is added in trans. This trans-splicing reaction displays the same 5' and 3' splice site sequence requirements as cis-splicing of full-length pre-mRNA. The analysis of RNA-snRNP complexes formed on each of the two splice site RNAs is consistent with the formation of partial complexes, which then associate to form the complete spliceosome. Specifically, U2 snRNP bound to the 3'SS RNA associates with U4/U5/U6 snRNP bound to the 5'SS RNA oligo. Thus, as expected, trans-splicing depends on the integrity of U2, U4, and U6 snRNAs. However, unlike cis-splicing, trans-splicing is enhanced when the 5' end of U1 snRNA is blocked or removed or when the U1 snRNP is depleted. Thus, the early regulatory requirement for U1 snRNP, which is essential in cis-splicing, is bypassed in this trans-splicing system. This simplified trans-splicing reaction offers a unique model system in which to study the mechanistic details of pre-mRNA splicing.     

GPS autoproteolysis is required for CD97 to up-regulate the expression of N-cadherin that promotes homotypic cell-cell aggregation

Most adhesion-class G protein-coupled receptors (adhesion-GPCRs) undergo a novel self-catalytic cleavage at the GPCR proteolysis site (GPS) to form a hetero-dimeric complex containing the extracellular and seven-span transmembrane subunits. However, little is known about the role of GPS auto-proteolysis in the function of adhesion-GPCRs. Here we show that GPS cleavage is essential for the homotypic cell aggregation promoted by CD97 receptor, a leukocyte-restricted adhesion-GPCR often aberrantly expressed in carcinomas. We find that CD97 does not mediate cell aggregation directly. Instead, expression of the wild type – but not the GPS cleavage-deficient CD97 up-regulates the expression of N-cadherin, leading to Ca⁺⁺-dependent cell–cell aggregation. Our results provide a clear evidence for the role of GPS proteolytic modification in the cellular function of adhesion-GPCRs.     

Structural and functional characterization of a novel T cell receptor co-regulatory protein complex, CD97-CD55

CD97, the archetypal member of the EGF-TM7 protein family, is constitutively expressed on granulocytes and monocytes and rapidly up-regulated on T and B cells following activation. The key isoform of CD97 expressed on leukocytes binds the complement regulatory protein CD55 (also termed decay-accelerating factor). CD97 has been shown recently to mediate co-stimulation of T cells via CD55. Here, we demonstrate that blocking the interaction between CD55 on monocytes and CD97 on T cells leads to inhibition of proliferation and interferon-gamma secretion. This implies that bidirectional interactions between CD97 and CD55 are involved in T cell regulation. Structural studies presented here reveal the molecular basis for this activity. We have solved the structure of EMR2, a very close homolog of CD97, using x-ray crystallography. NMR-based chemical shift mapping of the EMR2-CD55 interaction has allowed us to generate a model for the CD97-CD55 complex. The structure of the complex reveals that the T cell and complement regulatory activities of CD55 occur on opposite faces of the molecule. This suggests that CD55 might simultaneously regulate both the innate and adaptive immune responses, and we have shown that CD55 can still regulate complement when bound to CD97.     

The human and mouse repertoire of the adhesion family of G-protein-coupled receptors

The adhesion G-protein-coupled receptors (GPCRs) (also termed LN-7TM or EGF-7TM receptors) are membrane-bound proteins with long N-termini containing multiple domains. Here, 2 new human adhesion-GPCRs, termed GPR133 and GPR144, have been found by searches done in the human genome databases. Both GPR133 and GPR144 have a GPS domain in their N-termini, while GPR144 also has a pentraxin domain. The phylogenetic analyses of the 2 new human receptors show that they group together without close relationship to the other adhesion-GPCRs. In addition to the human genes, mouse orthologues to those 2 and 15 other mouse orthologues to human were identified (GPR110, GPR111, GPR112, GPR113, GPR114, GPR115, GPR116, GPR123, GPR124, GPR125, GPR126, GPR128, LEC1, LEC2, and LEC3). Currently the total number of human adhesion-GPCRs is 33. The mouse and human sequences show a clear one-to-one relationship, with the exception of EMR2 and EMR3, which do not seem to have orthologues in mouse. EST expression charts for the entire repertoire of adhesion-GPCRs in human and mouse were established. Over 1600 ESTs were found for these receptors, showing widespread distribution in both central and peripheral tissues. The expression patterns are highly variable between different receptors, indicating that they participate in a number of physiological processes.     

Molecular analysis of the epidermal growth factor-like short consensus repeat domain-mediated protein-protein interactions: dissection of the CD97-CD55 complex

Epidermal growth factor-like (EGF) and short consensus repeat (SCR) domains are commonly found in cell surface and soluble proteins that mediate specific protein-protein recognition events. Unlike the immunoglobulin (Ig) superfamily, very little is known about the general properties of intermolecular interactions encoded by these common modules, and in particular, how specificity of binding is achieved. We have dissected the binding of CD97 (a member of the EGF-TM7 family) to the complement regulator CD55, two cell surface modular proteins that contain EGF and SCR domains, respectively. We demonstrate that the interaction is mediated solely by these domains and is characterized by a low affinity (86 microm) and rapid off-rate (at least 0.6 s(-1)). The interaction is Ca(2+) -dependent but is unaffected by glycosylation of the EGF domains. Using biotinylated multimerized peptides in cell binding assays and surface plasmon resonance, we show that a CD97-related EGF-TM7 molecule (termed EMR2), differing by only three amino acids within the EGF domains, binds CD55 with a K(D) at least an order of magnitude weaker than that of CD97. These results suggest that low affinity cell-cell interactions may be a general feature of highly expressed cell surface proteins and that specificity of SCR-EGF binding can be finely tuned by a small number of amino acid changes on the EGF module surface.     

U small nuclear ribonucleoprotein requirements for nematode cis- and trans-splicing in vitro.

In nematodes, a fraction of mRNAs acquires a common 22-nucleotide 5'-terminal spliced leader sequence via a trans-splicing reaction. The same premessenger RNAs which receive the spliced leader are also processed by conventional cis-splicing. Whole cell extracts prepared from synchronous embryos of the parasitic nematode Ascaris lumbricoides catalyze both cis- and trans-splicing. We have used this cell-free system and oligodeoxynucleotide directed RNase H digestion to assess the U small nuclear RNA requirements for nematode cis- and trans-splicing. These experiments indicated that both cis- and trans-splicing require intact U2 and U4/U6 small nuclear ribonucleoproteins (snRNPs). However, whereas cis-splicing displays the expected requirement for an intact U1 snRNP, trans-splicing is unaffected when approximately 90% of U1 snRNP is degraded. These results suggest that 5' splice site identification differs in nematode cis- and trans-splicing.     

Functional analyses of positions across the 5' splice site of the trypanosomatid spliced leader RNA. Implications for base-pair interaction with U5 and U6 snRNAs

In this study, we have used a genetic compensatory approach to examine the functional significance of the previously proposed interaction of spliced leader (SL) RNA with U5 small nuclear RNA (snRNA) (Dungan, J. D., Watkins, K. P., and Agabian, N. (1996) EMBO J. 15, 4016-4029; Xu, Y.-X., Ben Shlomo, H., and Michaeli, S. (1997) Proc. Natl. Acad. Sci. U. S. A. 94, 8473-8478) and the interaction of the SL RNA intron with U6 snRNA analogous to cis-splicing. Mutations were introduced at positions -4, -1, +1, +4, +5, and +7/+8 relative to the SL RNA 5' splice site that were proposed to interact with U5 and U6 snRNAs. All mutants exhibited altered splicing phenotypes compared with the parental strain, showing the importance of these intron and exon positions for trans-splicing. Surprisingly, mutation at invariant +1 position did not abolish splicing completely, unlike cis-splicing, but position +2 had the most severe effect on trans-splicing. Compensatory mutations were introduced in U5 and U6 snRNAs to examine whether the defects resulted from failure to interact with these snRNAs by base pairing. Suppression was observed only for positions +5 and +7/+8 with U5 compensatory mutations and for position +5 with a U6 compensatory mutation, supporting the existence of a base pair interaction of U5 and U6 with the SL RNA intron region. The failure to suppress the other SL RNA mutants by the U5 compensatory mutations suggests that another factor(s) interacts with these key SL RNA positions.     

Extending the B7 (CD80) gene family.

B7-1 and B7-2 are members of the immunoglobulin superfamily (IgSF) and important regulators of T cell-mediated immune responses. Despite sharing only limited sequence identity, B7-1 and B7-2 bind common receptors, CD28 and CTLA-4, on T cells and have similar functional properties. We have found that the extracellular V (ariable)-like domains of B7-1 and B7-2 share significant sequence similarities with 3 major histocompatibility complex (MHC)-encoded members of the IgSF: butyrophilin, myelin/oligodendrocyte glycoprotein, and the chicken MHC molecule, B-G. This raises the question whether there is an evolutionary link between the MHC, which encodes molecules regulating the antigen specificity of T lymphocyte responses, and B7 molecules, which co-stimulate these responses in antigen-nonspecific fashion.     

Effects of the allosteric modulator SCH-202676 on adenosine and P2Y receptors

The G protein-coupled receptor allosteric modulator SCH-202676 (N-(2,3-diphenyl-1,2,4-thiadiazol-5-(2H)-ylidene)methanamine), which affects a wide range of structurally unrelated G protein-coupled receptors, has highly divergent effects on purine receptors. SCH-202676 inhibited radioligand binding to human adenosine A(1), A(2A), and A(3) receptors (IC(50) = 0.5-0.8 microM) and affected dissociation kinetics, but at the human P2Y(1) nucleotide receptor it had no effect. SCH-202676 (10 microM) selectively accelerated agonist dissociation at adenosine A(3) receptors and either slowed (adenosine A(1) receptors) or accelerated (adenosine A(2A) receptors) antagonist dissociation. Thus, SCH-202676 differentially modulated A(1), A(2A), and A(3) receptors as well as agonist- and antagonist-occupied receptors.     

Expression of macrophage-selective markers in human and rodent adipocytes

CD14, CD68 and/or mouse F4/80 or human epidermal growth factor module-containing mucin-like receptor 1 (EMR1) are widely used as macrophage-specific markers. Since macrophages infiltrate several tissues during inflammatory processes, CD14, CD68 and EMR1-F4/80 have been employed to discriminate between tissue-containing macrophages, like adipose tissue (AT), and other cells. Using real-time PCR experiments, we show that isolated adipocytes from humans and mice AT express high levels of CD14 and CD68 mRNA, whereas EMR1-F4/80 is mainly present in the macrophage-containing stroma-vascular fraction. Furthermore, fibroblasts-like cells (adipoblasts), preadipocytes and adipocytes from the murine cell lines, 3T3-F442A and BFC-1, express CD14 and CD68 mRNA and protein as determined by fluorescence-activated cell sorter, but not F4/80 which, as expected, is strongly expressed in the macrophage cell line RAW264.7. These results reinforce the view that EMR1-F4/80 is the best macrophage marker to date and show that CD14 and CD68 are not macrophage-specific proteins.