Abstract
Vector ticks possess a unique system that enables them to digest large amounts of host blood and to transmit various animal and human pathogens, suggesting the existence of evolutionally acquired proteolytic mechanisms. We report here the molecular and reverse genetic characterization of a multifunctional cysteine protease, longipain, from the babesial parasite vector tick Haemaphysalis longicornis. Longipain shares structural similarity with papain-family cysteine proteases obtained from invertebrates and vertebrates. Endogenous longipain was mainly expressed in the midgut epithelium and was specifically localized at lysosomal vacuoles and possibly released into the lumen. Its expression was up-regulated by host blood feeding. Enzymatic functional assays using in vitro and in vivo substrates revealed that longipain hydrolysis occurs over a broad range of pH and temperature. Haemoparasiticidal assays showed that longipain dose-dependently killed tick-borne Babesia parasites, and its babesiacidal effect occurred via specific adherence to the parasite membranes. Disruption of endogenous longipain by RNA interference revealed that longipain is involved in the digestion of the host blood meal. In addition, the knockdown ticks contained an increased number of parasites, suggesting that longipain exerts a killing effect against the midgut-stage Babesia parasites in ticks. Our results suggest that longipain is essential for tick survival, and may have a role in controlling the transmission of tick-transmittable Babesia parasites.
Highlights
The ixodid ticks are obligate hematophagous organisms that belong to the phylum Arthropda, and are classified with spiders and scorpions in the class Arachnida [1]
Longipain is localized in midgut epithelium and its expression is induced by blood feeding
A series of experiments unveil that longipain-knockdown ticks when fed on Babesia-infected dog, exhibited a significantly increased numbers of parasites compared with controls
Summary
The ixodid ticks are obligate hematophagous organisms that belong to the phylum Arthropda, and are classified with spiders and scorpions in the class Arachnida [1]. Ticks can increase more than 50 times in body weight compared with their original weight due to the acquired host blood meal, which mainly consists of red blood cells. Blood digestion in ticks is a slow intracellular process that takes place via phagocytosis by desquamated epithelial cells in the midgut [4,5]. The tick midgut is considered to contain evolutionally acquired molecules involved in host blood digestion [6,7]. The existence of secreted proteolytic enzymes in blood-sucking ticks suggests that they are required for various functions necessary for survival via successful bloodfeeding behavior, which includes continuous feeding for days, or even weeks [8,9], the precise mechanism responsible for the host blood digestion is unknown
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