Insight into the functional role of a tick salivary Selenoprotein M protein

The Gulf-coast ticks (Amblyomma maculatum) transmit disease-causing pathogens to humans and animals. Rickettsia parkeri is notable among the pathogens transmitted by A. maculatum to humans. Heavy infestations of A. maculatum on animal ears cause it to become thickened and curled, a condition commonly called “gotch ear”. The tick’s multifunctional salivary glands are vital to their biological success and likely also play a critical role in transmission of disease; tick saliva contains a broad array of secretory products that facilitate prolonged tick attachment and feeding. Because ticks transmit a broad array of disease-causing agents, disrupting tick blood feeding or inactivating key tick salivary proteins presents a novel strategy for tick-borne disease prevention. Sequencing of A. maculatum salivary gland normalized cDNA revealed a gene sequence homologous to Selenoprotein M. Trace element Selenium exhibits a variety of functions in the form of Selenoproteins, most importantly, as an antioxidant enzyme. Selenoprotein M is expressed in A. maculatum salivary glands. RNA interference (RNAi) was used to assess the role of this molecule for tick feeding success. The salivary Selenoprotein M gene expression was disrupted by injecting adult ticks with 500ng of dsRNA complementing the gene sequence. Silencing was demonstrated by reduced transcript in salivary glands removed from partially fed ticks. Disrupting expression of Selenoprotein M by RNAi reduced the ability of ticks to feed successfully, as demonstrated by low survival rate. Our studies demonstrate a critical role of Selenoprotein M in prolonged tick feeding on the host.