Molecular Studies on the Regulation of Tick Vitellogenesis and Expression of Hormone Receptors

在过去的几年里，我们集中精力在卵黄蛋白原（Vg）的特性和阐明调控这种蛋白质在软蜱中的表达的机制。Vg在脂肪体中合成，由血淋巴携带到卵巢，并经历蛋白水解成为卵黄蛋白（Vn）。然后将Vn掺入卵母细胞中。先前的研究表明，由联合神经节产生的肽，卵黄发生诱导因子（VIF），刺激后半部分的组织产生第二种因子，脂肪体刺激因子（FSF），刺激脂肪体产生Vg。我们推测第二个因素是类固醇。注射高浓度的蜕皮激素刺激Vg滴度的增加，未进食的女性和蜕皮激素滴度在血淋巴中的分析表明，蜕皮激素在调节卵黄发生的作用。为了阐明这种作用，正在进行实验以鉴定O.穆巴塔通过蜱基因组DNA的PCR产生的克隆的分析揭示了与昆虫EcR和USP受体的DNA结合结构域相似的部分序列。与EcR受体DNA结合结构域相似的部分序列与美洲钝眼蜱（Amblyomma americanum）同源性为100%，与家蚕同源性为83%，与人FXR受体同源性为85%。PCR产物与A. americanum，B. B. mori和人RXR受体。此外，单独的克隆显示94，89和87%的同源性的DNA结合结构域的果蝇melongaster七喜，HR3和HR78核孤儿受体基因。

Over the past several years we have concentrated our efforts on characterization of vitellogenin (Vg) and elucidation of mechanisms regulating the expression of this protein in the soft ticks. Vg is synthesized in the fat body, carried by the hemolymph to the ovary and undergoes proteolysis to become vitellin (Vn). Vn is then incorporated into the oocytes. Previous studies have shown a peptide, vitellogenesis-inducing factor (VIF), produced by the synganglion, stimulates a tissue in the posterior half to produce a second factor, fat body stimulating factor (FSF), which stimulates the fat body to produce Vg. We speculate this second factor to be a steroid. Injection of high concentrations of ecdysteroids stimulates an increase in Vg titers of unfed females and analysis of ecdysteroid titers in the hemolymph indicates a role for ecdysteroids in the regulation of vitellogenesis. To clarify this role, experiments are underway to identify the ecdysteroid (EcR) and retinoid X (RXR) receptors of O. moubata. Analysis of clones produced by PCR of tick genomic DNA revealed partial sequences similar to the DNA binding domains of insect EcR and USP receptors. The partial sequences similar to the DNA binding domains of the EcR receptors showed 100% homology to the ixodid tick, Amblyomma americanum, 83% to Bombyx mori and 85% to the human FXR receptor. The products from PCR with the USP primers showed homologies of 93, 95 and 85% with the DNA binding domains of A. americanum, B. mori and human RXR receptors, respectively. In addition, separate clones showed 94, 89 and 87% homologies to the DNA binding domains of Drospholia melongaster seven-up, HR3 and HR78 nuclear orphan receptor genes.