A glimpse into the world of microRNAs and their putative roles in hard ticks

Abstract

Ticks are important blood feeding ectoparasites that transmit pathogens to wildlife, domestic animals, and humans. Hard ticks can feed for several days to weeks, nevertheless they often go undetected. This phenomenon can be explained by a tick's ability to release analgesics, immunosuppressives, anticoagulants, and vasodilators within their saliva. Several studies have identified extracellular vesicles (EVs) as carriers of some of these effector molecules. Further, EVs, and their contents, enhance pathogen transmission, modulate immune responses, and delay wound healing. EVs are double lipid-membrane vesicles that transport intracellular cargo, including microRNAs (miRNAs) to recipient cells. miRNAs are involved in regulating gene expression post-transcriptionally. Interestingly, tick-derived miRNAs have been shown to enhance pathogen transmission and affect vital biological processes such as oviposition, blood digestion, and molting. miRNAs have been found within tick salivary EVs. This review focuses on current knowledge of miRNA loading into EVs and homologies reported in ticks. We also describe findings in tick miRNA profiles, including miRNAs packed within tick salivary EVs. Although no functional studies have been done to investigate the role of EV-derived miRNAs in tick feeding, we discuss the functional characterization of miRNAs in tick biology and pathogen transmission. Lastly, we propose the possible uses of tick miRNAs to develop management tools for tick control and to prevent pathogen transmission. The identification and functional characterization of conserved and tick-specific salivary miRNAs targeting important molecular and immunological pathways within the host could lead to the discovery of new therapeutics for the treatment of tick-borne and non-tick-borne human diseases.

Keywords: RNA transport proteins; extracellular vesicles; miRNA transport; small RNA; tick feeding.

FIGURE 1

Comparison of the miRNA biogenesis canonical pathway in humans, Drosophila, and ticks. The transcription of a miRNA gene is completed by RNA polymerase II, which leads to the production of a miRNA. The primary miRNA is cleaved into a miRNA precursor by Drosha/DGCR8 and then exported out of the nucleus via Exportin 5/Ran/GTP. Next the precursor is cleaved by Dicer and forms a mature duplex that will be loaded into the miRNA Induced Silencing Complex by an Argonaut protein. The guide strand is then degraded, and the mature miRNA sequence will be translocated to its mRNA target. The exact complementation between the mRNA and miRNA leads to the degradation of the transcript, while the partial match represses translation. Ribonuclease III enzymes represented in dark or light brown have been functionally characterized in the species whereas their representation in white signifies that their role in silencing remains to be tested in members of this group. The ID of the gene encoding each of the ribonucleases has been added to each organism. In the case of ticks, the gene IDs from the Ixodes scapularis genome were used based on the results from Schnettler et al. (2014). Only Dicer 1 and Ago 1 IDs are provided as these proteins are involved in the miRNA silencing pathway. The IDs for Dicer 2 and Ago 2 in I. scapularis have been described in Schnettler et al. (2014). Images were created with BioRender.

FIGURE 2

miRNA translocation into exosomes in ticks. After the cleavage of a precursor miRNA into a mature miRNA a motif in the seed sequence is recognized by RNA binding proteins, such as heterogenous nuclear ribonuclear protein A2B1 (hnRNPa2b1) and Synaptotagmin Binding Cytoplasmic RNA Interacting Protein (SYNCRIP). Bound miRNAs are loaded into exosomes. The exosomes will then be released when the multivesicular bodies fuse with the plasma membrane. The RNA binding proteins depicted in this figure have been shown to participate in miRNA translocation into exosomes in humans. Whether these proteins complete the same function in ticks remains to be determined. Image was created with BioRender.

FIGURE 3

The conserved miRNA-1 and miRNA-375 and their site of expression. The site of expression of miRNA-1 (green arrows) and miRNA-375 (blue arrows) have been investigated in Ixodes scapularis (Isc), Rhipicephalus microplus (Rmi), Hyalomma Anatolicum (Hy), Haemaphysalis longicornis (Hlo), and Ixodes ricinus (Iri).