Evolutionary Regression and Species-Specific Codon Usage of TLR15.

Carlos G P Voogdt,Jaap A Wagenaar,Mark E Merchant,Jos P M Van Putten

doi:10.3389/fimmu.2018.02626

Abstract

Toll-like receptors (TLRs) form an ancient family of innate immune receptors that detect microbial structures and activate the host immune response. Most subfamilies of TLRs (including TLR3, TLR5, and TLR7) are highly conserved among vertebrate species. In contrast, TLR15, a member of the TLR1 subfamily, appears to be unique to birds and reptiles. We investigated the functional evolution of TLR15. Phylogenetic and synteny analyses revealed putative TLR15 orthologs in bird species, several reptilian species and also in a shark species, pointing to an unprecedented date of origin of TLR15 as well as large scale reciprocal loss of this TLR in most other vertebrates. Cloning and functional analysis of TLR15 of the green anole lizard (Anolis carolinensis), salt water crocodile (Crocodylus porosus), American alligator (Alligator mississippiensis), and chicken (Gallus gallus) showed for all species TLR15 specific protease-induced activation of NF-κB, despite highly variable TLR15 protein expression levels. The variable TLR15 expression was consistent in both human and reptilian cells and could be attributed to species-specific differences in TLR15 codon usage. The species-specific codon bias was not or barely noted for more evolutionarily conserved TLRs (e.g., TLR3). Overall, our results indicate that TLR15 originates before the divergence of chondrichthyes fish and tetrapods and that TLR15 of both avian and reptilian species has a conserved function as protease activated receptor. The species-specific codon usage and large scale loss of TLR15 in most vertebrates suggest evolutionary regression of this ancient TLR.

Highlights

Toll-like receptors (TLRs) are innate immune receptors that have a critical role in the early detection of infection [1]
In order to identify potential TLR15 sequences in vertebrate genomes we investigated the evolutionary relationship among TLR1 subfamily members from a diverse set of species (Supplementary Table 1) using a maximum likelihood based phylogenetic tree (Figure 1)
In the present work we provide evidence that (i) TLR15 is evolutionarily older than expected, (ii) tlr15 genes display species-specific codon usage, (iii) the tlr15 gene underwent evolutionary regression in most vertebrates including certain reptilian lineages, and (iv) that the activation of reptilian TLR15 by external proteases is a conserved feature that functionally distinguishes TLR15 from other TLR1 subfamily members

Summary

Introduction

Toll-like receptors (TLRs) are innate immune receptors that have a critical role in the early detection of infection [1]. The general architecture of TLRs consists of a ligand-binding extracellular domain containing multiple leucine rich repeats (LRR), a single transmembrane domain and an intracellular Toll-interleukin-1 (TIR) signaling domain [2]. Ligand-induced TLR signaling activates immune-related transcription factors (e.g., nuclear factor κB, NF-κB) which induce expression of pro-inflammatory genes. The importance of TLRs in the immune system is underlined by the strong evolutionary conservation of this family of receptors. Extensive gene duplication and gene loss events have resulted in 10 different TLRs in some mammals (including humans) to more than 20 TLRs in teleost fish [4,5,6]. Evolutionary diversification of the TLR ligand-binding domain to detect diverse types of microbial structures has resulted in distinct TLR subfamilies

Methods

Results

Conclusion