Tick subolesin is an ortholog of the akirins described in insects and vertebrates

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Abstract

The tick protective antigen, subolesin, is a regulatory protein involved in the control of multiple cellular pathways. Subolesin is evolutionary conserved in invertebrates and vertebrates with sequence homology to akirins, a recently renamed group of proteins that were proposed to function as transcription factors in Drosophila and mice. The objective of this research was to provide evidence of the sequence and functional homology between tick subolesin and akirins. The phylogenetic analysis of subolesin and akirins showed that they are evolutionary conserved. The effect of subolesin and akirin2 knockdown was compared in adult ticks and mice, respectively. The results demonstrated that tick subolesin is an ortholog of insect and vertebrate akirins and suggested that these proteins function in the regulation of NF-κB-dependent and independent expression of signal transduction and innate immune response genes. These results suggest that these proteins have an important role in host–pathogen interactions.

Introduction

Akirins constitute a recently renamed group of evolutionary conserved proteins in insects and vertebrates that were proposed to function as transcription factors required for NF-κB-dependent gene expression in Drosophila and mice [1]. Goto et al. [1] also provided evidence of the role of akirin in Drosophila melanogaster innate immune response. However, they failed to recognize that akirins may be homologous to tick subolesin [2]. Ticks are vectors to a large number of pathogens and may constitute a good system to study the role of these molecules in innate immune response in acari.

Tick subolesin was discovered as a tick protective antigen in Ixodes scapularis [3]. Subolesin was shown by both RNA interference (RNAi) gene knockdown and immunization trials using the recombinant protein to protect hosts against tick infestations, reduce tick survival and reproduction, and cause degeneration of gut, salivary gland, reproductive tissues and embryos [2], [4], [5], [6], [7], [8]. The targeting of tick subolesin by RNAi or immunization was also shown to decrease vector capacity of ticks for Anaplasma marginale and A. phagocytophilum, respectively [9]. In addition, subolesin was shown to be differentially expressed in Anaplasma-infected ticks and cultured tick cells [10], [11] and to function in the control of gene expression in ticks through the interaction with other regulatory proteins [12]. These studies demonstrated a role of subolesin in the control of multiple cellular pathways by exerting a regulatory function on global gene expression in ticks. Subolesin was also shown to be conserved in different tick species as well as in other invertebrate and vertebrate organisms [2], [3]. These results suggested that subolesin and akirins may be structurally and functionally homologous but direct comparison of their role in ticks and vertebrates has not been conducted.

The objective of this research was to provide evidence of the sequence and functional homology between tick subolesin and akirins. Herein, the phylogenetic analysis of subolesin and akirins was conducted and the effect of akirin knockdown in adult mice was compared with the results of subolesin knockdown in adult ticks using RNAi and RT-PCR. The results demonstrated that tick subolesin is an ortholog of the akirins described in insects and vertebrates.

Section snippets

Phylogenetic analysis of akirin and subolesin sequences

The evolutionary history of akirins and tick subolesin was inferred using the minimum evolution (ME) method [13]. Stability or accuracy of inferred topology(ies) were assessed via bootstrap analysis of 1000 replications [14]. The evolutionary distances were computed using the Maximum Composite Likelihood method [15]. The ME tree was searched using the Close-Neighbor-Interchange algorithm [16] at a search level of 1. The Neighbor-joining algorithm [17] was used to generate the initial tree. All

Subolesin and akirin sequences are homologous and evolutionary conserved

The phylogenetic analysis of akirin and subolesin sequences showed a high degree of conservation between tick subolesin and insect and vertebrate akirins (Fig. 1). In vertebrates, two closely related akirin homologues exist, akirin1 and akirin2 [1]. In insects, only one akirin has been identified which agrees with the finding of a single subolesin gene in ticks [1 and unpublished results]. These results expanded previous reports [1], [2] and supported that subolesin and akirins are structurally

Conclusions

These results demonstrated that tick subolesin is an ortholog of insect and vertebrate akirins and suggested that these proteins may affect the expression of signal transduction and innate immune response genes as well as positive and negative transcriptional regulators. In ticks, subolesin may function in the regulation of NF-κB-dependent and independent gene expression through interaction with intermediate regulatory proteins such as GI, GII and other as yet unidentified proteins [12]. These

Acknowledgements

We thank María Martín (Ebronatura SL, Spain) for technical assistance. This research was supported by the Ministerio de Ciencia e Innovación, Spain (project BFU2008-01244/BMC), the Consejería de Educación y Ciencia, JCCM, Spain (project PAI06-0046-5285) and the Oklahoma Agricultural Experiment Station (project 1669). R.C. Galindo was funded by MEC, Spain. V. Naranjo was founded by the European Social Fund and the Junta de Comunidades de Castilla-La Mancha (Program FSE 2007-2013), Spain.

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    Evidence of the role of tick subolesin in gene expression

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