title: Evolution of lipopolysaccharide (LPS) recognition and signaling: fish TLR4 does not recognize LPS and negatively regulates NF-kappaB activation.
abstract: It has long been established that lower vertebrates, most notably fish and amphibians, are resistant to the toxic effect of LPS. Furthermore, the lack of a TLR4 ortholog in some fish species and the lack of the essential costimulatory molecules for LPS activation via TLR4 (i.e., myeloid differentiation protein 2 (MD-2) and CD14) in all the fish genomes and expressed sequence tag databases available led us to hypothesize that the mechanism of LPS recognition in fish may be different from that of mammals. To shed light on the role of fish TLRs in LPS recognition, a dual-luciferase reporter assay to study NF-kappaB activation in whole zebrafish embryos was developed and three different bony fish models were studied: 1) the gilthead seabream (Sparus aurata, Perciformes), an immunological-tractable teleost model in which the presence of a TLR4 ortholog is unknown; 2) the spotted green pufferfish (Tetraodon nigroviridis, Tetraodontiformes), which lacks a TLR4 ortholog; and 3) the zebrafish (Danio rerio, Cypriniformes), which possesses two TLR4 orthologs. Our results show that LPS signaled via a TLR4- and MyD88-independent manner in fish, and, surprisingly, that the zebrafish TLR4 orthologs negatively regulated the MyD88-dependent signaling pathway. We think that the identification of TLR4 as a negative regulator of TLR signaling in the zebrafish, together with the absence of this receptor in most fish species, explains the resistance of fish to endotoxic shock and supports the idea that the TLR4 receptor complex for LPS recognition arose after the divergence of fish and tetrapods.
authors.affiliation: Department of Cell Biology and Histology, Faculty of Biology, University of Murcia, Murcia, Spain.
authors.forename: María P
authors.initials: MP
authors.lastname: Sepulcre
authors.forename: Francisca
authors.initials: F
authors.lastname: Alcaraz-Pérez
authors.forename: Azucena
authors.initials: A
authors.lastname: López-Muñoz
authors.forename: Francisco J
authors.initials: FJ
authors.lastname: Roca
authors.forename: José
authors.initials: J
authors.lastname: Meseguer
authors.forename: María L
authors.initials: ML
authors.lastname: Cayuela
authors.forename: Victoriano
authors.initials: V
authors.lastname: Mulero
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D005075:Biological Evolution; D015153:Blotting, Western; D005399:Fishes / Q000276:immunology*; D015870:Gene Expression; D006801:Humans; D007962:Leukocytes / Q000276:immunology* / Q000378:metabolism; D008070:Lipopolysaccharides / Q000276:immunology*; D008969:Molecular Sequence Data; D053594:Myeloid Differentiation Factor 88 / Q000276:immunology; D016328:NF-kappa B / Q000276:immunology* / Q000378:metabolism; D020133:Reverse Transcriptase Polymerase Chain Reaction; D017386:Sequence Homology, Amino Acid; D015398:Signal Transduction / Q000276:immunology*; D051197:Toll-Like Receptor 4 / Q000235:genetics / Q000276:immunology* / Q000378:metabolism; D014162:Transfection; D015027:Zebrafish
chemical_list: D008070:Lipopolysaccharides; D053594:Myeloid Differentiation Factor 88; D016328:NF-kappa B; D051197:Toll-Like Receptor 4