Phylogenetic and expression analysis of amphibian Xenopus Toll-like receptors

Abstract

An anuran amphibian, South African clawed frog (Xenopus laevis), is used to study the immune system, as it possesses a set of acquired immune system represented by T and B lymphocytes and the immunoglobulins. The acquired immune system is impaired throughout the larva and the metamorphosis stage in the amphibians. On the other hand, the role of innate immune system in the tadpole remains unclear. Recently, insect Toll protein homologues, namely, Toll-like receptors (TLRs), have been identified as sensors recognizing microbe-pattern molecules in vertebrates. Whole-genome analysis of Xenopus tropicalis supported the existence of the tlr genes in the frog. In this study, we annotated 20 frog tlr gene nucleotide sequences from the latest genome assembly version 4.1 on the basis of homology and identified cDNAs of the predicted frog TLR proteins. Phylogenetic analysis showed that the repertoire of the frog TLRs consisted of both fish- and mammalian-type TLRs. We showed that the frog TLRs are constitutively expressed in the tadpole as well as in the adult frog. Our results suggest that tadpoles are protected from microbes by the innate system that includes TLRs, despite impaired acquired immune system in tadpoles. This is the first report on the properties of TLRs in the most primitive terrestrial animals like amphibia.

Combinational recognition of bacterial lipoproteins and peptidoglycan by chicken Toll-like receptor 2 subfamily

“…In these mammals, TLR1, TLR6 and TLR10 are mapped closely together on one chromosome and TLR2 is mapped to another chromosome (8). In contrast, fish and xenopus have unique TLR2 subfamilies of TLR1, TLR2 and TLR14 (2,8,16). Even the lamprey appears to have the TLR2 subfamily (17).…”

Characterization of bbtTICAM from amphioxus suggests the emergence of a MyD88-independent pathway in basal chordates

“…Homologs of TI-CAM2 have not been identified in teleosts [15,16,40], Xenopus, [22] or chickens [17,23], except in mammals and sharks [21]. As two TICAM homologs were identified in lamprey [14], Iliev et al [40] assumed that a TI-CAM2 ortholog existed in the common predecessor of fish and tetrapods.…”

“…However, unlike orthologs of mammalian TICAM1 in early vertebrates, homologs of TICAM2 have not been identified in other vertebrates besides mammals and shark [21], such as chicken, Xenopus, zebrafish, and lamprey [14,17,22,23]. TICAM2 specifically acts as a bridge for TICAM1 to be successfully recruited when TLR4 recognizes LPS by coupling with CD14 and MD-2 [5,24,25].…”

“…TICAM2 specifically acts as a bridge for TICAM1 to be successfully recruited when TLR4 recognizes LPS by coupling with CD14 and MD-2 [5,24,25]. Since endotoxin recognition by TLR4 is absent in lamprey [14], jawfish [19], and Xenopus [22], it was previously believed that the endotoxin recognition complex and the downstream signaling pathway in mammals arose after the divergence of fish and tetrapods [19]. Thus, the absence of TICAM2 in bony fish and the rapid evolution from tetrapods to mammals may have been associated with the development of the functional requirement for the formation of the endotoxin recognition complex.…”

Toll-like receptor 3 regulates Mx expression in rare minnow Gobiocypris rarus after viral infection

“…They are evolutionarily conserved components and act as sentinels of the innate immune system, sensing a variety of ligands from lipopolysaccharide to flagellin to dsRNA by leucine-rich-repeat motifs (LRRs) in their extracellular domain (West et al 2006;Xu et al 2000;Ishii et al 2007). TLRs are type I transmembrane pattern recognition receptors (PRRs) that detect invading pathogens by binding conserved, microbially derived molecules (pathogen-associated molecular patterns, PAMPs) that are not found in higher eukaryotes, and that signal and induce expression of multiple host defense genes including proinflammatory cytokines and chemokines (Rudd et al 2005).…”