UvA-DARE (Digital Academic Repository) CD97 and EMR2: receptors on the move Kwakkenbos, M.J. Publication date 2004 Link to publication Citation for published version (APA): Kwakkenbos, M. J. (2004). CD97 and EMR2: receptors on the move. General rights It is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). Disclaimer/Complaints regulations If you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: https://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. UvA-DARE is a service provided by the library of the University of Amsterdam (https://dare.uva.nl) Download date:08 Oct 2021 Chapter 2 The EGF-TM7 family: a postgenomic view Immunogenetics, 55: 655-666, 2004 Mark J. Kwakkenbos1, Else N. Kop1,2, Martin Stacey3, Mourad Matmati1, Siamon Gordon3, Hsi-Hsien Lin3 and Jörg Hamann1 laboratory for Experimental Immunology 2Division of Clinical Immunology and Rheumatology, Academic Medical Center, University of Amsterdam, Amsterdam, The Netherlands 3Sir William Dunn School of Pathology, University of Oxford, Oxford, U.K. Chapter 2 ABSTRACT 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 to 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. INTRODUCTION Seven-span transmembrane (TM7) receptors represent the largest gene family in animal genomes.15 TM7 receptors respond to a diverse array of sensory and chemical stimuli such as light, taste, odor, pheromones, calcium ions, neurotransmitters, hormones and chemokines.6 Upon ligation, TM7 receptors regulate a variety of physiological processes via heterotrimeric G proteins which engage subsequent messenger molecules. Although G protein-coupled receptor (GPCR) is an abundantly used synonym for TM7 molecules there is growing evidence that some TM7 receptors signal through alternative, G protein-independent, mechanisms.6,7 TM7 receptors are divided into several classes, which have most likely arisen independently through convergent evolution.8 Best established are the rhodopsin superfamily (class A), the secretin receptor superfamily (class B) and the metabotropic glutamate receptor superfamily (class C). Class B was originally defined as a family of insect and mammalian peptide hormone receptors.9 Subsequent identification of receptors with homologous TM7 moieties but different extracellular regions have led to the classification of class B TM7 receptors into three distinct families: Bl, peptide hormone receptors, B2, TM7 receptors with a long N-terminal extracellular region (LNB-TM7 family) and B3, methuselah-like proteins.1011 The human LNB-TM7 family comprises approximately 30 members.12 They all possess large extracellular regions containing a number of well-defined protein domains.1113 The structural diversity within the LNB-TM7 family is illustrated by molecules such as Cadherin EGF LAG seven-pass G-type receptor 1 (Celsrl), a mosaic protein containing over 15 repeats belonging to three different types of protein module.1415 Another example is the very large G protein-coupled receptor 1 (VLGR1), with ~6300 amino acids the largest cell surface protein yet identified.16 This review will focus on a subfamily of the LNB-TM7 receptors that solely contain extracellular epidermal growth factor (EGF)-like domains, referred to as EGF-TM7 receptors.1718 With the human genome now unravelled,2,3 the complete family comprises six members: CD97,219,20 EGF-like module-containing mucin-like receptor proteinl 14 The EGF-TM7 family CD97 EMR1 EMR2 EMR3 EMR4 ETL Figure 1. Schematic structure of human (black) and mouse (gray) EGF-TM7 family members. EGF-TM7 receptors are expressed as noncovalently associated heterodimers consisting of an extracellular a subunit and a TM7/cytoplasmic p subunit. Due to alternative RNA splicing, they possess a variable number of N-terminal EGF domains, represented as triangles. Depicted here are the largest isoforms. Calcium-binding EGF-like domains are shown in gray. Small circles indicate the position of the processing site (GPS motif). Human EMR4 is shown with a dotted line because the human gene is inactivated by a one-nucleotide deletion in exon 8. See text for details. (EMR1),21 EMR2,22 EMR3,23 EMR424 and EGF-TM7-latrophilin-related protein (ETL).25 Four of these molecules have mouse orthologues: CD97,26,27 EMR1 (also known as F4/80),2829 EMR4 (also called F4/80-like-receptor, FIRE)30'31 and ETL.3233 Interestingly, surveys of the mouse genome4 have failed to identify EMR2 and EMR3 (Stacey and Lin, unpublished observation). More recently EGF-TM7 genes have also been identified in various other vertebrate species including rat, pig, cow and zebrafish (see below). Schematic structure and key characteristics of human and mouse EGF-TM7 receptors are depicted in Figure 1 and Table 1. MOLECULAR STRUCTURE The extracellular region possesses tandemly arranged EGF domains and a conserved cleavage motif EGF-TM7 receptors possess distinct parts including a cleavable signal peptide, an N- terminal EGF domain region, a stalk containing a conserved cleavage site, a class B TM7 region and a short and poorly conserved cytoplasmic tail (Figures 1 and 2).Comprising several hundred amino acids, the extracellular regions of EGF-TM7 receptors represent one of the largest found in TM7 molecules. Located at the N-terminus of every EGF- TM7 receptor are several tandemly arranged EGF-like domains (Figures 1 and 2). EGF domains belong to the most broadly expressed protein modules in animals.2 4 Through the interaction with other protein modules, they are involved in various physiological processes including blood coagulation, fibrinolysis, neural development and cell adhesion.34 Within tandemly linked EGF domains, tertiary structures emerge through contacts between succeeding domains, which are stabilized by calcium ligation of the C-terminal module. Consequently, the EGF domains in EGF-TM7 receptors, except for the most proximal ones, possess a calcium-binding site. The calcium-binding EGF 15 Chapter 2 Table 1: Characteristics of human and mouse EGF-TM7 receptors. See text for details. Name EGF domains3 Expression pattern" Ligands Human CD97 3-5: Activated lymphocytes, monocytes, CD55 (DAF), chondroitin sulfate (EGF1.2.5), macrophages, dendritic cells, granulocytes, (EGF1,2,3,5), smooth muscle cells, malignant cells (EGF1,2,3,4,5) EMR1 1-6: Monocytes, dendritic cells Unknown (EGF4), (EGF4.5), (EGF4,5,6), (EGF1,2,3,4,5,6) 2-5: Monocytes, macrophages, dendritic cells, Chondroitin sulfate (EGF1.2), granulocytes (EGF1,2,5), (EGF1,2,3,5), (EGF1,2,3,4,5) EMR3 Monocytes, macrophages, granulocytes Expressed on monocyte-derived macrophages and activated neutrophils EMR4 2 Monocytes, dendritic cells Unknown ETL 2 Smooth muscle cells Unknown Mouse CD97 3-4: Lymphocytes, monocytes, macrophages, CD55 (DAF), chondroitin sulfate (EGF1,2,4), granulocytes (EGF1,2,3,4), (EGF1,2,X,3,4) EMR1 0-7: Monocytes, macrophages, dendritic cells, Unknown (F4/80) (EGF4,5,6,7), eosinophils (EGF1,2,3,6,7), (EGF1,4,5,6,7), (EGF1,2,3,4,5,6,7) EMR4 2 Monocytes, macrophages, dendritic cells Expressed on B lymphoma cell line (FIRE) A20 ETL 3 Smooth muscle cells, hematopoietic stem Unknown cells aThe EGF composition of isoforms from alternatively spliced receptors is depicted. For EMR2, also alternative splicing that could give rise to truncated, soluble receptors has been observed.22,66 bData derived by detection of transcripts are given in italics. 16 The EGF-TM7 family domains of the EGF-TM7 family belong to the fibrillin-like class I type and are arranged in a near-linear order.35 A characteristic of CD97, EMR1, EMR2 and ETL is the existence of alternative RNA splicing. This results in the expression of various isoforms with differing numbers and arrangements of EGF domains. In contrast, EMR3 and EMR4 exist with two EGF domains. Closer examination of the EGF domain regions has led to some remarkable findings. Firstly, the EGF domains of CD97 and EMR2 are nearly identical,22 differing by only six amino acids. Secondly, unusual isoforms exist of mouse CD97 and mouse and rat ETL. In the largest isoform of mouse CD97, the second and third EGF domains are interrupted by a sequence of 45 amino acids that does not show homology to any known protein module.2627 In rodent ETL, two calcium-binding EGF domains with an identical amino acid sequence are found.2532 A stalk region of 160-260 amino acids separates the EGF domain region from the GPCR-proteolytic site (GPS) (Figure 2). Low amino acid sequence conservation suggests that this region serves as a spacer between the ligand binding EGF domains and the TM7 part.