A repertoire of protease inhibitor families in Amblyomma americanum and other tick species: inter-species comparative analyses

Abstract

BackgroundProtease inhibitors (PIs) are important regulators of physiology and represent anti-parasitic druggable and vaccine targets. We conducted bioinformatic analyses of genome and transcriptome data to determine the protease inhibitor (PI) repertoire in Amblyomma americanum and in 25 other ixodid tick species. For A. americanum, we compared the PI repertoires in fed and unfed, male and female A. americanum ticks. We also analyzed PI repertoires of female 48, 96 and 120 h-fed midgut (MG) and salivary gland (SG) tissues.ResultsWe found 1,595 putative non-redundant PI sequences across 26 ixodid tick species. Ticks express PIs from at least 18 different families: I1, I2, I4, I8, I21, I25, I29, I31, I32, I35, I39, I43, I51, I53, I63, I68, I72 and I74 (MEROPS). The largest PI families were I2, I4 and I8 and lowest in I21, I31, I32, I35 and I68. The majority (75%) of tick PIs putatively inhibit serine proteases, with ~11 and 9% putatively regulating cysteine or metalloprotease-mediated pathways, respectively, and ~4% putatively regulating multiple/mixed protease types. In A. americanum, we found 370 PIs in female and 354 in male ticks. In A. americanum we found 231 and 442 in unfed and fed ticks, respectively. In females, we found 206 and 164 PIs in SG and MG, respectively. The majority of highly cross-tick species conserved PIs were in families I1, I2, I8, I21, I25, I29, I39 and I43.ConclusionsTicks appear to express large and diverse repertoires of PIs that primarily target serine protease-mediated pathways. We speculate that PI families with the highest repertoires may contain functionally redundant members while those with the lowest repertoires are functionally non-redundant PIs. We found some highly conserved PIs in the latter category, which we propose as potential candidates for broad-spectrum anti-tick vaccine candidates or druggable targets in tick control.Electronic supplementary materialThe online version of this article (doi:10.1186/s13071-017-2080-1) contains supplementary material, which is available to authorized users.

Time-resolved proteomic profile ofAmblyomma americanumtick saliva during feeding

“…We previously documented at least 18 of the 99 Merops database protease inhibitor (PI) that might be expressed by A. americanum and other tick species (62). Here we show that adult A. americanum ticks secreted at least 155 PIs belonging into eight PI families.…”

“…Of the PI families in this study, serpins are the most studied (42, 62–67), presumably because functional roles of this protein category are relatable to tick feeding physiology. To successfully feed and transmit TBD agents, ticks have to overcome serine protease-mediated host defense pathways that are tightly controlled by inhibitors, including serpins.…”

“…On this basis, such proteins could be targeted for tick vaccine development. We have previously shown that RNAi silencing of A. americanum tick saliva serpin 19, an anti-coagulant (69), which is also conserved in I. scapularis ticks (42, 62), caused significant mortality demonstrating the importance of this protein in tick physiology (32).…”

The Use of Tick Salivary Proteins as Novel Therapeutics

“…Serine protease inhibitors form four groups – Kunitz-domain inhibitors, serpins, trypsin inhibitor-like cysteine-rich domain inhibitors (TIL-domain inhibitors), and Kazal-domain inhibitors – while the cysteine protease inhibitors usually belong to the cystatin family. Tick protease inhibitors and their functions are reviewed elsewhere (Schwarz et al, 2012; Blisnick et al, 2017; Porter et al, 2017; Parizi et al, 2018), and the therapeutic potential of serpins and cystatins was outlined in our previous review (Chmelar et al, 2017). Here we discuss the therapeutic potential of the other two groups, Kunitz- and TIL-domain inhibitors.…”

Peptidase inhibitors in tick physiology

“…This huge redundancy is not found for other functions such as host immune response modulation, where tick inhibitors seem to be more specific. Importantly, among the hundreds of tick inhibitors reported in the literature, only a few have been functionally characterized (Porter et al, 2017). Future studies dealing with these neglected inhibitors, for example thyropins and cytotoxic T-lymphocyte antigen (CTLA) families, can reveal their roles in tick physiology and expand the roll of canonical functions of tick inhibitors.…”