Abstract
Tick feeding requires the secretion of a huge number of pharmacologically dynamic proteins and other molecules which are vital for the formation of the cement cone, the establishment of the blood pool and to counter against the host immune response. Glycine-rich proteins (GRP) are found in many organisms and can function in a variety of cellular processes and structures. The functional characterization of the GRPs in the tick salivary glands has not been elucidated. GRPs have been found to play a role in the formation of the cement cone; however, new evidence suggests repurposing of GRPs in the tick physiology. In this study, an RNA interference approach was utilized to silence two glycine-rich protein genes expressed in early phase of tick feeding to determine their functional role in tick hematophagy, cement cone structure, and microbial homeostasis within the tick host. Additionally, the transcriptional regulation of GRPs was determined after exposure to biotic and abiotic stresses including cold and hot temperature, injury, and oxidative stress. This caused a significant up-regulation of AamerSigP-34358, Aam-40766, AamerSigP-39259, and Aam-36909. Our results suggest ticks repurpose these proteins and further functional characterization of GRPs may help to design novel molecular strategies to disrupt the homeostasis and the pathogen transmission.
Highlights
The Lone-Star tick Amblyomma americanum is of significant health concern in the United States, given its expanding geographic range and vector-competence for diseases such as by Ehrlichia chaffeensis, Borrelia lonestari, Ehrlichia ewingii, Francisella tularensis, Theileria cervi and heartland virus (Childs and Paddock, 2003; Goddard and Varela-Stokes, 2009)
To determine if the Glycine-rich proteins (GRP) selected from the A. americanum sialotranscriptome (Karim and Ribeiro, 2015) could be grouped, the tripeptide and pentapeptide repeats commonly found in GRPs were used as a way to group the proteins (Table 1)
Multiple sequence alignment of all selected nine GRP amino acid sequences only showed GGX and GXG repeats (Supplementary Figure S2). This pattern does not hold true when looking at GRPs from other organisms or even other ticks of the same species, so this information may not be a usable criterion for identifying GRP classes
Summary
The Lone-Star tick Amblyomma americanum is of significant health concern in the United States, given its expanding geographic range and vector-competence for diseases such as by Ehrlichia chaffeensis, Borrelia lonestari, Ehrlichia ewingii, Francisella tularensis, Theileria cervi and heartland virus (Childs and Paddock, 2003; Goddard and Varela-Stokes, 2009). A. americanum has recently been associated with delayed anaphylaxis to red meat and is the first recorded example of an ectoparasite causing food allergy in the United States (Commins et al, 2011; Crispell et al, 2019). The tick must establish a firm attachment by secreting multiple pharmacologically active proteins and other compounds in its saliva. Some of these compounds solidify once inside the host skin to form a proteinaceous matrix called the cement cone (Bishop et al, 2002; Bullard R. et al, 2016). Other amino acids overrepresented include serine (12–33% in 17 proteins) and alanine (12–17% in 2 proteins) (Zhang et al, 2013)
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