Adherens junction protein nectin-4 is the epithelial receptor for measles virus

Measles virus is an aerosol-transmitted virus that affects more than 10 million children each year and accounts for approximately 120,000 deaths. Although it was long believed to replicate in the respiratory epithelium before disseminating, it was recently shown to infect initially macrophages and dendritic cells of the airways using signalling lymphocytic activation molecule family member 1 (SLAMF1; also called CD150) as a receptor. These cells then cross the respiratory epithelium and transport the infection to lymphatic organs where measles virus replicates vigorously. How and where the virus crosses back into the airways has remained unknown. On the basis of functional analyses of surface proteins preferentially expressed on virus-permissive human epithelial cell lines, here we identify nectin-4 (ref. 8; also called poliovirus-receptor-like-4 (PVRL4)) as a candidate host exit receptor. This adherens junction protein of the immunoglobulin superfamily interacts with the viral attachment protein with high affinity through its membrane-distal domain. Nectin-4 sustains measles virus entry and non-cytopathic lateral spread in well-differentiated primary human airway epithelial sheets infected basolaterally. It is downregulated in infected epithelial cells, including those of macaque tracheae. Although other viruses use receptors to enter hosts or transit through their epithelial barriers, we suggest that measles virus targets nectin-4 to emerge in the airways. Nectin-4 is a cellular marker of several types of cancer, which has implications for ongoing measles-virus-based clinical trials of oncolysis.     

Claudin-1 is a hepatitis C virus co-receptor required for a late step in entry

Hepatitis C virus (HCV) is a leading cause of cirrhosis and liver cancer worldwide. A better understanding of the viral life cycle, including the mechanisms of entry into host cells, is needed to identify novel therapeutic targets. Although HCV entry requires the CD81 co-receptor, and other host molecules have been implicated, at least one factor critical to this process remains unknown (reviewed in refs 1-3). Using an iterative expression cloning approach we identified claudin-1 (CLDN1), a tight junction component that is highly expressed in the liver, as essential for HCV entry. CLDN1 is required for HCV infection of human hepatoma cell lines and is the first factor to confer susceptibility to HCV when ectopically expressed in non-hepatic cells. Discrete residues within the first extracellular loop (EL1) of CLDN1, but not protein interaction motifs in intracellular domains, are critical for HCV entry. Moreover, antibodies directed against an epitope inserted in the CLDN1 EL1 block HCV infection. The kinetics of this inhibition indicate that CLDN1 acts late in the entry process, after virus binding and interaction with the HCV co-receptor CD81. With CLDN1 we have identified a novel key factor for HCV entry and a new target for antiviral drug development.     

Angiotensin-converting enzyme 2 is a functional receptor for the SARS coronavirus

Spike (S) proteins of coronaviruses, including the coronavirus that causes severe acute respiratory syndrome (SARS), associate with cellular receptors to mediate infection of their target cells. Here we identify a metallopeptidase, angiotensin-converting enzyme 2 (ACE2), isolated from SARS coronavirus (SARS-CoV)-permissive Vero E6 cells, that efficiently binds the S1 domain of the SARS-CoV S protein. We found that a soluble form of ACE2, but not of the related enzyme ACE1, blocked association of the S1 domain with Vero E6 cells. 293T cells transfected with ACE2, but not those transfected with human immunodeficiency virus-1 receptors, formed multinucleated syncytia with cells expressing S protein. Furthermore, SARS-CoV replicated efficiently on ACE2-transfected but not mock-transfected 293T cells. Finally, anti-ACE2 but not anti-ACE1 antibody blocked viral replication on Vero E6 cells. Together our data indicate that ACE2 is a functional receptor for SARS-CoV.     

SLAM (CDw150) is a cellular receptor for measles virus

Measles virus continues to be a major killer of children, claiming roughly one million lives a year. Measles virus infection causes profound immunosuppression, which makes measles patients susceptible to secondary infections accounting for high morbidity and mortality. The Edmonston strain of measles virus, and vaccine strains derived from it, use as a cellular receptor human CD46 (refs 3, 4), which is expressed on all nucleated cells; however, most clinical isolates of measles virus cannot use CD46 as a receptor. Here we show that human SLAM (signalling lymphocyte-activation molecule; also known as CDw150), a recently discovered membrane glycoprotein expressed on some T and B cells, is a cellular receptor for measles virus, including the Edmonston strain. Transfection with a human SLAM complementary DNA enables non-susceptible cell lines to bind measles virus, support measles virus replication and develop cytopathic effects. The distribution of SLAM on various cell lines is consistent with their susceptibility to clinical isolates of measles virus. The identification of SLAM as a receptor for measles virus opens the way to a better understanding of the pathogenesis of measles virus infection, especially the immunosuppression induced by measles virus.     

Intercellular adhesion molecule-1 is an endothelial cell adhesion receptor for Plasmodium falciparum

The primary event in the pathogenesis of severe malaria in Plasmodium falciparum infection is thought to be adherence of trophozoite- and schizont-infected erythrocytes to capillary endothelium, a process called sequestration. Identifying the endothelial molecules used as receptors is an essential step in understanding this disease process. Recent work implicates the membrane glycoprotein CD36 (platelet glycoprotein IV; refs 2-5) and the multi-functional glycoprotein thrombospondin as receptors. Although CD36 has a widespread distribution on microvascular endothelium, it may not be expressed on all capillary beds where sequestration occurs, especially in the brain. The role of thrombospondin in cell adhesion, in vitro or in vivo, is less certain. We have noticed that some parasites bind to human umbilical-vein endothelial cells independently of CD36 or thrombospondin. To screen for alternative receptors, we have developed a novel cell-adhesion assay using transfected COS cells, which confirms that CD36 is a cell-adhesion receptor. In addition, we find that an endothelial-binding line of P. falciparum binds to COS cells transfected with a complementary DNA encoding intercellular adhesion molecule-1. As this molecule is widely distributed on capillaries and is inducible, this finding may be relevant to the pathogenesis of severe malaria.     

The CD4 (T4) antigen is an essential component of the receptor for the AIDS retrovirus

Acquired immune deficiency syndrome (AIDS) is characterized by opportunistic infections and by 'opportunistic neoplasms' (for example, Kaposi's sarcoma). Persistent generalized lymphadenopathy (PGL) is epidemiologically associated with AIDS, especially in male homosexuals. A subset of T lymphocytes positive for the CD4 antigen (also termed T4 antigen), is depleted in AIDS and PGL patients. A retrovirus found in T-cell cultures from these patients is strongly implicated in the aetiology of AIDS because of the high frequency of isolation and the prevalence of specific antibodies in the patients. Here we have detected cell-surface receptors for the AIDS retrovirus (human T-cell leukaemia virus-III (HTLV-III) and lymphadenopathy-associated virus-1 (LAV-1) isolates) by testing the susceptibility of cells to infection with pseudotypes of vesicular stomatitis virus bearing retroviral envelope antigens, and by the formation of multinucleated syncytia on mixing virus-producing cells with receptor-bearing cells. Receptors were present only on cells expressing CD4 antigen; among 155 monoclonal antibodies tested, each of the 14 anti-CD4 antibodies inhibited formation of syncytia and blocked pseudotypes. Productive infection of CD4+ cells with HTLV-III or LAV-1 markedly reduced cell-surface expression of CD4. In contrast, receptors for HTLV-I and HTLV-II were not restricted to CD4+ cells, were not blocked by anti-CD4 antibodies; cells productively infected with HTLV-I and HTLV-II expressed surface CD4. Hence, we conclude that the CD4 antigen is an essential and specific component of the receptor for the causative agent of AIDS.     

Structure of the influenza virus haemagglutinin complexed with its receptor, sialic acid

The three-dimensional structures of influenza virus haemagglutinins complexed with cell receptor analogues show sialic acids bound to a pocket of conserved amino acids surrounded by antibody-binding sites. Sialic acid fills the conserved pocket, demonstrating that it is the influenza virus receptor. The proximity of the antibody-binding sites suggests that antibodies neutralize virus infectivity by preventing virus-to-cell binding. The structures suggest approaches to the design of anti-viral drugs that could block attachment of viruses to cells.     

BAI1 is an engulfment receptor for apoptotic cells upstream of the ELMO/Dock180/Rac module

Engulfment and subsequent degradation of apoptotic cells is an essential step that occurs throughout life in all multicellular organisms. ELMO/Dock180/Rac proteins are a conserved signalling module for promoting the internalization of apoptotic cell corpses; ELMO and Dock180 function together as a guanine nucleotide exchange factor (GEF) for the small GTPase Rac, and thereby regulate the phagocyte actin cytoskeleton during engulfment. However, the receptor(s) upstream of the ELMO/Dock180/Rac module are still unknown. Here we identify brain-specific angiogenesis inhibitor 1 (BAI1) as a receptor upstream of ELMO and as a receptor that can bind phosphatidylserine on apoptotic cells. BAI1 is a seven-transmembrane protein belonging to the adhesion-type G-protein-coupled receptor family, with an extended extracellular region and no known ligands. We show that BAI1 functions as an engulfment receptor in both the recognition and subsequent internalization of apoptotic cells. Through multiple lines of investigation, we identify phosphatidylserine, a key 'eat-me' signal exposed on apoptotic cells, as a ligand for BAI1. The thrombospondin type 1 repeats within the extracellular region of BAI1 mediate direct binding to phosphatidylserine. As with intracellular signalling, BAI1 forms a trimeric complex with ELMO and Dock180, and functional studies suggest that BAI1 cooperates with ELMO/Dock180/Rac to promote maximal engulfment of apoptotic cells. Last, decreased BAI1 expression or interference with BAI1 function inhibits the engulfment of apoptotic targets ex vivo and in vivo. Thus, BAI1 is a phosphatidylserine recognition receptor that can directly recruit a Rac-GEF complex to mediate the uptake of apoptotic cells.     

Structure of the human M2 muscarinic acetylcholine receptor bound to an antagonist

The parasympathetic branch of the autonomic nervous system regulates the activity of multiple organ systems. Muscarinic receptors are G-protein-coupled receptors that mediate the response to acetylcholine released from parasympathetic nerves. Their role in the unconscious regulation of organ and central nervous system function makes them potential therapeutic targets for a broad spectrum of diseases. The M2 muscarinic acetylcholine receptor (M2 receptor) is essential for the physiological control of cardiovascular function through activation of G-protein-coupled inwardly rectifying potassium channels, and is of particular interest because of its extensive pharmacological characterization with both orthosteric and allosteric ligands. Here we report the structure of the antagonist-bound human M2 receptor, the first human acetylcholine receptor to be characterized structurally, to our knowledge. The antagonist 3-quinuclidinyl-benzilate binds in the middle of a long aqueous channel extending approximately two-thirds through the membrane. The orthosteric binding pocket is formed by amino acids that are identical in all five muscarinic receptor subtypes, and shares structural homology with other functionally unrelated acetylcholine binding proteins from different species. A layer of tyrosine residues forms an aromatic cap restricting dissociation of the bound ligand. A binding site for allosteric ligands has been mapped to residues at the entrance to the binding pocket near this aromatic cap. The structure of the M2 receptor provides insights into the challenges of developing subtype-selective ligands for muscarinic receptors and their propensity for allosteric regulation.     

A receptor for the immunosuppressant FK506 is a cis-trans peptidyl-prolyl isomerase

The structurally novel macrolide FK506 (refs 1,2) has recently been demonstrated to have potent immunosuppressive activity at concentrations several hundredfold lower than cyclosporin A (CsA). Cyclosporin A, a cyclic peptide, has found widespread clinical use in the prevention of graft rejection following bone marrow and organ transplantation. The mechanisms of immunosuppression mediated by FK506 and CsA appear to be remarkably similar, suggesting that these unrelated structures act on a common receptor or on similar molecular targets, perhaps the CsA receptor, cyclophilin, which has recently been shown by Fischer et al. and Takahashi et al. to have cis-trans peptidyl-prolyl isomerase activity. We have prepared an FK506 affinity matrix and purified a binding protein for FK506 from bovine thymus and from human spleen. This FK506-binding protein (FKBP) has a relative molecular mass (Mr) of approximately 14,000(14K), a pI of 8.8-8.9, and does not cross-react with antisera against cyclophilin. The first 40 N-terminal residues of the bovine and 16 residues of the human FKBP were determined; the 16-residue fragments are identical to each other and unrelated to any known sequences. This protein catalyses the cis-trans isomerization of the proline amide in a tetrapeptide substrate and FK506 inhibits the action of this new isomerase. The FKBP and cyclophilin appear to be members of an emerging class of novel proteins that regulate T cell activation and other metabolic processes, perhaps by the recognition (and possibly the isomerization) of proline-containing epitopes in target proteins.     

Aminopeptidase N is a major receptor for the entero-pathogenic coronavirus TGEV.

Coronaviruses, like many animal viruses, are characterized by a restricted host range and tissue tropism. Transmissible gastroenteritis virus (TGEV), a major pathogen causing a fatal diarrhoea in newborn pig, replicates selectively in the differentiated enterocytes covering the villi of the small intestine. To investigate the molecular determinants of the infection, we characterized the surface molecule used by the virus for binding and entry into host cells. Here we report that aminopeptidase N, an ectoenzyme abundantly expressed at the apical membrane of the enterocytes, serves as a receptor for TGEV. Monoclonal antibodies were selected for their ability to block infection by TGEV of porcine cell lines. They recognized a brush-border membrane protein of M(r) 150K, which was identified as aminopeptidase N by amino-terminal sequencing. Two lines of evidence supported the view that the peptidase itself acts as a receptor. First, virions bound specifically to aminopeptidase N that was purified to homogeneity. Second, recombinant expression of aminopeptidase N conferred infectivity by TGEV to an otherwise non-permissive cell line.     

Melanin-concentrating hormone is the cognate ligand for the orphan G-protein-coupled receptor SLC-1.

The underlying causes of obesity are poorly understood but probably involve complex interactions between many neurotransmitter and neuropeptide systems involved in the regulation of food intake and energy balance. Three pieces of evidence indicate that the neuropeptide melanin-concentrating hormone (MCH) is an important component of this system. First, MCH stimulates feeding when injected directly into rat brains; second, the messenger RNA for the MCH precursor is upregulated in the hypothalamus of genetically obese mice and in fasted animals; and third, mice lacking MCH eat less and are lean. MCH antagonists might, therefore, provide a treatment for obesity. However, the development of such molecules has been hampered because the identity of the MCH receptor has been unknown until now. Here we show that the 353-amino-acid human orphan G-protein-coupled receptor SLC-1 expressed in HEK293 cells binds MCH with sub-nanomolar affinity, and is stimulated by MCH to mobilize intracellular Ca2+ and reduce forskolin-elevated cyclic AMP levels. We also show that SLC-1 messenger RNA and protein is expressed in the ventromedial and dorsomedial nuclei of the hypothalamus, consistent with a role for SLC-1 in mediating the effects of MCH on feeding.     

Boc is a receptor for sonic hedgehog in the guidance of commissural axons

In the spinal cord, sonic hedgehog (Shh) is secreted by the floor plate to control the generation of distinct classes of ventral neurons along the dorsoventral axis. Genetic and in vitro studies have shown that Shh also later acts as a midline-derived chemoattractant for commissural axons. However, the receptor(s) responsible for Shh attraction remain unknown. Here we show that two Robo-related proteins, Boc and Cdon, bind specifically to Shh and are therefore candidate receptors for the action of Shh as an axon guidance ligand. Boc is expressed by commissural neurons, and targeted disruption of Boc in mouse results in the misguidance of commissural axons towards the floor plate. RNA-interference-mediated knockdown of Boc impairs the ability of rat commissural axons to turn towards an ectopic source of Shh in vitro. Taken together, these data suggest that Boc is essential as a receptor for Shh in commissural axon guidance.     

A macrophage Fc gamma receptor and the mast cell receptor for IgE share an identical subunit

Fc receptors for immunoglobulins are found on many immune cells and trigger essential functions of the immune defence system. With the exception of the high-affinity receptor for immunoglobulin E (Fc epsilon RI), these receptors were thought to consist of single polypeptides. Fc epsilon RI is a tetrameric complex of one alpha-subunit, one beta-subunit and two gamma-subunits. Here we report the cloning of a polypeptide identical to the gamma-chains of Fc epsilon RI, from mouse macrophages that do not express this receptor. Biosynthetic labelling and gene transfer together show that these gamma-chains associate with one of the macrophage receptors (Fc gamma RIIa). The human homologue, Fc gamma RIII (CD16), from natural killer cells is also expected to associate with gamma-chains. It is possible that these gamma-chains and the homologous zeta-chains of the T-cell antigen receptor belong to a new family of related proteins which share a common role in the signal transducing pathway.     

The mouse insulin-like growth factor type-2 receptor is imprinted and closely linked to the Tme locus.

T-associated maternal effect (Tme) is the only known maternal-effect mutation in the mouse. The defect is nuclear-encoded and embryos that inherit a deletion of the Tme locus from their mother die at day 15 of gestation. There are many genomically imprinted regions known in the mouse genome but so far no imprinted genes have been cloned. The Tme locus is absent in two chromosome-17 deletion mutants, Thp and the tLub2, and its position has been localized using these deletions to a 1-cM region. We report here that the genes for insulin-like growth factor type-2 receptor (Igf2r) and mitochondrial superoxide dismutase-2 (Sod-2) are absent from both deletions. Probes for these genes and for plasminogen (Plg) and T-complex peptide 1 (Tcp-1) were used in pulsed-field gel mapping to show that Tme must lie within a region of 800-1,100 kb. We also demonstrate that embryos express Igf2r only from the maternal chromosome, and that Tcp-1, Plg and Sod-2 are expressed from both chromosomes. Therefore Igf2r is imprinted and closely linked or identical to Tme.     

Cloning of cDNA for the glutamate-binding subunit of an NMDA receptor complex.

The amino acids L-glutamic and L-aspartic acids form the most widespread excitatory transmitter network in mammalian brain. The excitation produced by L-glutamic acid is important in the early development of the nervous system, synaptic plasticity and memory formation, seizures and neuronal degeneration. The receptors activated by L-glutamic acid are a target for therapeutic intervention in neurodegenerative diseases, brain ischaemia and epilepsy. There are two types of receptors for the excitatory amino acids, those that lead to the opening of cation-selective channels and those that activate phospholipase C (ref. 11). The receptors activating ion channels are NMDA (N-methyl-D-aspartate) and kainate/AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate)-sensitive receptors. The complementary DNAs for the kainate/AMPA receptor and for the metabotropic receptor have been cloned. We report here on the isolation and characterization of a protein complex of four major proteins that represents an intact complex of the NMDA receptor ion channel and on the cloning of the cDNA for one of the subunits of this receptor complex, the glutamate-binding protein.