Molecular characterization and functional significance of the Vti family of SNARE proteins in tick salivary glands

Exocytosis involves membrane fusion between secretory vesicles and the plasma membrane. The Soluble N-ethylmaleimide-sensitive factor attachment proteins (SNAPs) and their receptor proteins (SNAREs) interact to fuse vesicles with the membrane and trigger the release of their sialosecretome out of the tick salivary gland cells. In this study, we examined the functional significance of the Vti family of SNARE proteins of blood-feeding Amblyomma maculatum and Amblyomma americanum. Vti1A and Vti1B have been implicated in multiple functional roles in vesicle transport. QRT-PCR studies demonstrated that the highest transcriptional expression of vti1a and vti1b genes occurs in unfed salivary glands, suggesting that elevated secretory vesicle formation occurs prior to feeding but continues at low rates after blood feeding commences. Vti1A and Vti1B localize to the secretory vesicles in unfed tick salivary glands in immunofluorescence microscopy studies. Knockdown of vti1a and vti1b by RNA interference resulted in a significant decrease in the engorged tick weight compared to the control during prolonged blood-feeding on the host. RNA interference of vti1a or vti1b impaired oviposition and none of the ticks produced eggs masses. Surprisingly, the double knockdown did not produce a strong phenotype and ticks fed normally on the host and produced egg masses, suggesting a compensatory mechanism exists within the secretory system which may have been activated in the double knockdown. These results suggest an important functional role of the Vti family of SNARE proteins in tick blood feeding and ultimately oviposition. Understanding the basic functions of the Vti family of SNARE proteins in salivary glands may lead to better ways to prevent tick attachment and transmission of tick-borne diseases.

Physiology of a Tick feeding (left) and the general morphology of salivary gland acini (top-right). Secretory granules (present in salivary gland acini) undergo vesicle fusion events driven in part by the interactions with VTI proteins syntaxins (bottom-right).Figure optionsDownload high-quality image (231 K)Download as PowerPoint slideHighlights
► Functional significance of the VTI family of SNARE proteins, AmVti1a, AmVti1b, and AaVti1b was determined.
► The qRT-PCR analysis revealed the highest expression of the VTI family in unfed salivary glands.
► The VTI family of SNARE proteins is localized differently in unfed and partially fed salivary glands.
► Knockdown of the VTI family of SNARE proteins suggests an important role in tick blood feeding, survival and oviposit.