Drosophila scavenger receptor CI is a pattern recognition receptor for bacteria.
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One hallmark of innate immunity apparently conserved from primitive life forms through to humans is the ability of the host to recognize pathogen-associated molecular patterns (PAMPs). Since macrophage pattern recognition receptors are not well defined in Drosophila, we set out to identify such receptors. Our findings reveal that Drosophila macrophages express multiple pattern recognition receptors and that the Drosophila scavenger receptor, dSR-CI, is one such receptor capable of recognizing both gram-negative and gram-positive bacteria, but not yeast. Our data indicate that scavenger receptor bacterial recognition is conserved from insects to humans and may represent one of the most primitive forms of microbial recognition. (+info)
Macrophages in Drosophila embryos and L2 cells exhibit scavenger receptor-mediated endocytosis.
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Mammalian macrophage scavenger receptors exhibit unusually broad binding specificity and are implicated in atherosclerosis and host defense. Scavenger receptor-like endocytosis was observed in Drosophila melanogaster embryos and in primary embryonic cell cultures. This receptor activity was expressed primarily by macrophages. The Drosophila Schneider L2, but not the Kc, cell line also exhibited a scavenger receptor-mediated endocytic pathway similar to its mammalian counterpart. L2 receptors mediated high-affinity internalization and subsequent temperature- and chloroquine-sensitive degradation of 125I-labeled acetylated low density lipoprotein and displayed characteristic ligand specificity. These findings suggest that scavenger receptors mediate important, well-conserved functions and raise the possibility that they may be pattern recognition receptors that arose early in the evolution of host defense mechanisms. They also establish additional systems for the investigation of endocytosis in Drosophila and scavenger receptor function in disease, host defense, and development. (+info)
Elevated polymorphism and divergence in the class C scavenger receptors of Drosophila melanogaster and D. simulans.
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Scavenger receptor proteins are involved in the cellular internalization of a broad variety of foreign material, including pathogenic bacteria during phagocytosis. I find here that nonsynonymous divergence in three class C scavenger receptors (Sr-C's) between Drosophila melanogaster and D. simulans and between each of these species and D. yakuba is approximately four times the typical genome average. These genes also exhibit unusually high levels of segregating nonsynonymous polymorphism in D. melanogaster and D. simulans populations. A fourth Sr-C is comparatively conserved. McDonald-Kreitman tests reveal a significant excess of replacement fixations between D. melanogaster and D. simulans in the Sr-C's, but tests of polymorphic site frequency spectra do not support models of directional selection. It is possible that the molecular functions of SR-C proteins are sufficiently robust to allow exceptionally high amino acid substitution rates without compromising organismal fitness. Alternatively, SR-Cs may evolve under diversifying selection, perhaps as a result of pressure from pathogens. Interestingly, Sr-CIII and Sr-CIV are polymorphic for premature stop codons. Sr-CIV is also polymorphic for an in-frame 101-codon deletion and for the absence of one intron. (+info)
Two categories of mammalian galactose-binding receptors distinguished by glycan array profiling.
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Profiling of the four known galactose-binding receptors in the C-type lectin family has been undertaken in parallel on a glycan array. The results are generally consistent with those of previous assays using various different formats, but they provide a direct comparison of the properties of the four receptors, revealing that they fall into two distinct groups. The major subunit of the rat asialoglycoprotein receptor and the rat Kupffer cell receptor show similar broad preferences for GalNAc-terminated glycans, while the rat macrophage galactose lectin and the human scavenger receptor C-type lectin (SRCL) bind more restricted sets of glycans. Both of these receptors bind to Lewis x-type structures, but the macrophage galactose lectin also interacts strongly with biantennary galactose- and GalNAc-terminated glycans. Although the similar glycan-binding profiles for the asialoglycoprotein receptor and the Kupffer cell receptor might suggest that these receptors are functionally redundant, analysis of fibroblasts transfected with full-length Kupffer cell receptor reveals that they fail to endocytose glycosylated ligand. (+info)
Scavenger receptor C-type lectin binds to the leukocyte cell surface glycan Lewis(x) by a novel mechanism.
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The scavenger receptor C-type lectin (SRCL) is unique in the family of class A scavenger receptors, because in addition to binding sites for oxidized lipoproteins it also contains a C-type carbohydrate-recognition domain (CRD) that interacts with specific glycans. Both human and mouse SRCL are highly specific for the Lewis(x) trisaccharide, which is commonly found on the surfaces of leukocytes and some tumor cells. Structural analysis of the CRD of mouse SRCL in complex with Lewis(x) and mutagenesis show the basis for this specificity. The interaction between mouse SRCL and Lewis(x) is analogous to the way that selectins and DC-SIGN bind to related fucosylated glycans, but the mechanism of the interaction is novel, because it is based on a primary galactose-binding site similar to the binding site in the asialoglycoprotein receptor. Crystals of the human receptor lacking bound calcium ions reveal an alternative conformation in which a glycan ligand would be released during receptor-mediated endocytosis. (+info)
Expression cloning of dSR-CI, a class C macrophage-specific scavenger receptor from Drosophila melanogaster.
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Mammalian class A macrophage-specific scavenger receptors (SR-A) exhibit unusually broad binding specificity for a wide variety of polyanionic ligands. The properties of these receptors suggest that they may be involved in atherosclerosis and host defense. We have previously observed a similar receptor activity in Drosophila melanogaster embryonic macrophages and in the Drosophila macrophage-like Schneider L2 cell line. Expression cloning was used to isolate from L2 cells a cDNA that encodes a third class (class C) of scavenger receptor, Drosophila SR-CI (dSR-CI). dSR-CI expression was restricted to macrophages/hemocytes during embryonic development. When expressed in mammalian cells, dSR-CI exhibited high affinity and saturable binding of 125I-labeled acetylated low density lipoprotein and mediated its chloroquine-dependent, presumably lysosomal, degradation. Although the broad polyanionic ligand-binding specificity of dSR-CI was similar to that of SR-A, their predicted protein sequences are not similar. dSR-CI is a 609-residue type I integral membrane protein containing several well-known sequence motifs, including two complement control protein (CCP) domains and somatomedin B, MAM, and mucin-like domains. Macrophage scavenger receptors apparently mediate important, well-conserved functions and may be pattern-recognition receptors that arose early in the evolution of host-defense mechanisms. Genetic and physiologic analysis of dSR-CI function in Drosophila should provide further insights into the roles played by scavenger receptors in host defense and development. (+info)