Reptile Toll-like receptor 5 unveils adaptive evolution of bacterial flagellin recognition

Carlos G P Voogdt,Marja J L Kik,Lieneke I Bouwman,Jaap A Wagenaar,Jos P M Van Putten

doi:10.1038/srep19046

Carlos G P Voogdt, Marja J L Kik + Show 3 more

Open Access

https://doi.org/10.1038/srep19046

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Abstract

Toll-like receptors (TLR) are ancient innate immune receptors crucial for immune homeostasis and protection against infection. TLRs are present in mammals, birds, amphibians and fish but have not been functionally characterized in reptiles despite the central position of this animal class in vertebrate evolution. Here we report the cloning, characterization, and function of TLR5 of the reptile Anolis carolinensis (Green Anole lizard). The receptor (acTLR5) displays the typical TLR protein architecture with 22 extracellular leucine rich repeats flanked by a N- and C-terminal leucine rich repeat domain, a membrane-spanning region, and an intracellular TIR domain. The receptor is phylogenetically most similar to TLR5 of birds and most distant to fish TLR5. Transcript analysis revealed acTLR5 expression in multiple lizard tissues. Stimulation of acTLR5 with TLR ligands demonstrated unique responsiveness towards bacterial flagellin in both reptile and human cells. Comparison of acTLR5 and human TLR5 using purified flagellins revealed differential sensitivity to Pseudomonas but not Salmonella flagellin, indicating development of species-specific flagellin recognition during the divergent evolution of mammals and reptiles. Our discovery of reptile TLR5 fills the evolutionary gap regarding TLR conservation across vertebrates and provides novel insights in functional evolution of host-microbe interactions.

Highlights

One major gap in our knowledge on vertebrate Toll-like receptors (TLRs) evolution is the complete lack of information about the structure, function, and ligand specificity of TLRs in any species of reptile
To assess whether reptile cells respond to TLR ligands we first stimulated IgH-2 Iguana iguana cells carrying a NF-κ B luciferase reporter plasmid with the canonical mammalian TLR ligands; LTA (TLR2), Pam3CSK4 (TLR2/1), FSL-1 (TLR2/6), LPS (TLR4), FliC (TLR5), CL097 (TLR7), ODN2006 (TLR9) and the avian TLR15 activator Proteinase K
That we had identified acTLR5 as a specific receptor for bacterial flagellin, we examined the conservation of residues involved in flagellin binding by aligning acTLR5 with zebrafish TLR5b of which the crystal structure in complex with flagellin has been determined[29]

Summary

Introduction

One major gap in our knowledge on vertebrate TLR evolution is the complete lack of information about the structure, function, and ligand specificity of TLRs in any species of reptile. Development of the amniotic egg and a water impermeable skin allowed these early reptiles to be the first vertebrates that could permanently colonize terrestrial habitats. This pioneering step must have brought the first reptiles into contact with the prehistorical terrestrial flora, fauna and microbiota that undoubtedly shaped the immune system of reptiles and descendant animals. Differential sensitivity of acTLR5 compared to human TLR5 to Pseudomonas aeruginosa but not Salmonella Enteritidis flagellins indicate host specific adaptation of flagellin recognition

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