Antiplasmodial Activity Is an Ancient and Conserved Feature of Tick Defensins.

Alejandro Cabezas-Cruz,Miray Tonk,James J Valdés,Anne Bouchut,Jamal Khalife,Andreas Vilcinskas,Mohammad Rahnamaeian,Michalis Kotsyfakis,Raymond J Pierce,Christine Pierrot

doi:10.3389/fmicb.2016.01682

Abstract

Ancestral sequence reconstruction has been widely used to test evolution-based hypotheses. The genome of the European tick vector, Ixodes ricinus, encodes for defensin peptides with diverse antimicrobial activities against distantly related pathogens. These pathogens include fungi, Gram-negative, and Gram-positive bacteria, i.e., a wide antimicrobial spectrum. Ticks do not transmit these pathogens, suggesting that these defensins may act against a wide range of microbes encountered by ticks during blood feeding or off-host periods. As demonstrated here, these I. ricinus defensins are also effective against the apicomplexan parasite Plasmodium falciparum. To study the general evolution of antimicrobial activity in tick defensins, the ancestral amino acid sequence of chelicerate defensins, which existed approximately 444 million years ago, was reconstructed using publicly available scorpion and tick defensin sequences (named Scorpions-Ticks Defensins Ancestor, STiDA). The activity of STiDA was tested against P. falciparum and the same Gram-negative and Gram-positive bacteria that were used for the I. ricinus defensins. While some extant tick defensins exhibit a wide antimicrobial spectrum, the ancestral defensin showed moderate activity against one of the tested microbes, P. falciparum. This study suggests that amino acid variability and defensin family expansion increased the antimicrobial spectrum of ancestral tick defensins.

Highlights

Pauling and Zuckerkandl (1963) proposed that sequences of molecules from extant organisms can be used to infer sequences that were present in their common ancestors
Our results show that STiDA is a defensin peptide with a limited antimicrobial spectrum; it is only active against P. falciparum
The BLAST hits with the lowest E-values (3−18 and 5−16, respectively) were produced with defensins from the scorpion Heterometrus laoticus (GenBank accession number: P0C2F4) and the tick Haemaphysalis longicornis (GenBank accession number: ABW08118)

Summary

Introduction

Pauling and Zuckerkandl (1963) proposed that sequences of molecules from extant organisms can be used to infer sequences that were present in their common ancestors. The statistical methods to infer such ancestral sequences have been improved (Yang et al, 1995; Pupko et al, 2000; Nielsen, 2002), prediction has become more accurate (Thornton, 2004; Harms and Thornton, 2010) and chemically synthesized peptides are routine This permits a study of ancestral molecules using widely available methods in scientific research. Using this approach, interesting insights into molecular evolution have been gained, for instance, on lysozymes (Malcolm et al, 1990), ribonucleases (Jermann et al, 1995), thermophilic proteins (Akanuma et al, 2015), and hemoglobin in pikas (Tufts et al, 2015). The functional implications of such sequence diversification have not been experimentally tested

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