Functional Genomics of Tick Thioester-Containing Proteins Reveal the Ancient Origin of the Complement System

Abstract

Ticks are important ectoparasites and vectors of multiple human and animal diseases. The obligatory hemophagy of ticks provides a formidable route for parasite transmission from one host to another. Parasite survival inside the tick relies on the ability of a pathogen to escape or inhibit tick immune defenses, but the molecular interactions between the tick and its pathogens remain poorly understood. Here we report that tick genomes are unique in that they contain all known classes of the α₂-macroglobulin family (α₂M-F) proteins: α₂-macroglobulin pan-protease inhibitors, C3 complement components, and insect thioester-containing and macroglobulin-related proteins. By using RNA interference-mediated gene silencing in the hard tick Ixodes ricinus we demonstrated the central role of a C3-like molecule in the phagocytosis of bacteria and revealed nonredundant functions for α₂M-F proteins. Assessment of α₂M-F functions in a single organism should significantly contribute to the general knowledge on the evolution and function of the complement system. Importantly, understanding the tick immune mechanisms should provide new concepts for efficient transmission blocking of tick-borne diseases.

Evolution of the Complement System

“…All deuterostome species analyzed thus far such as various vertebrates, urochordate sea squirt [13], cephalochordate lancelet and echinoderm sea urchin [2], possess both the C3 and A2M family members, whereas many protostome genomes deciphered thus far such as fly [14], mosquito [15], honeybee [16], parasitoid wasp [17], aphid [18], flour beetle [19], and Caenorhabditis elegans [20] possessed only the A2M family members. Among protostomes, only horseshoe crab [21,22], spider [6], tick [23], clam [24] and squid [25] were reported to possess the C3 family, indicating that the loss of the C3 family has occurred multiple times during the protostome evolution [6]. The loss of the C3 family also occurred at least once in cnidaria, since hydra has only the A2M family gene [26].…”

Evolution of the complement system in protostomes revealed by de novo transcriptome analysis of six species of Arthropoda

“…Although the C3 gene has been identified from all deuterostome species analyzed thus far (Nonaka, 2014), earlier genomic analyses showed its absence from the genomes of Drosophila melanogaster (Adams et al, 2000) and Caenorhabditis elegans (C. elegans Sequencing Consortium, 1998), indicating that this gene has been lost in at least some lineages of protostomes. On the other hand, the C3 gene has been reported from several other protostome species, such as the clam (Prado-Alvarez et al, 2009), the squid (Castillo et al, 2009), horseshoe crabs (Ariki et al, 2008;Zhu et al, 2005), the spider (Sekiguchi et al, 2012) and ticks (Buresova et al, 2011;Urbanová et al, 2015), and FB has also been reported from the clam (Prado-Alvarez et al, 2009) and horseshoe crab (Tagawa et al, 2012;Zhu et al, 2005). However, to date, no comprehensive analysis of the complement genes has been performed in protostome species possessing the C3 gene and it is still an unsolved question as to whether the protostome complement system shares complement components other than C3 and FB with deuterostomes.…”

RNA Interference – A Powerful Functional Analysis Tool for Studying Tick Biology and its Control

“…The function of some components of immunity, such as α2-macroglobulin proteins [132,133], antimicrobial peptides [131], Janus kinase (JAK)-signaling transducer activator of transcription (STAT) pathway [135], dityrosine network [136], and cysteine protease inhibitor in the hemocytes [140] have been analyzed using RNAi. The α2-macroglobulin proteins of I. ricinus, related to vertebrate complement system, were shown to be involved in the phagocytic activity of hemocytes against Gram-negative bacteria [132,133].…”

“…The α2-macroglobulin proteins of I. ricinus, related to vertebrate complement system, were shown to be involved in the phagocytic activity of hemocytes against Gram-negative bacteria [132,133]. In contrast, a cysteine protease Rmcystatin3 identified in R.…”