Molecular Mechanisms Regulating Tick Vitellogenesis

我们的研究主要集中在卵黄蛋白原(VG)的特性和调节该蛋白在软体扁虱中表达的机制上。在脂肪体中合成的VG由血淋巴携带到卵巢，并以卵黄蛋白(VN)的形式并入卵母细胞。先前的研究表明，卵黄生成诱导因子(VIF)和脂肪体刺激因子(FSF)两种因素调节VG的合成。VIF是一种多肽，而FSF似乎是一种类固醇。注射高浓度的蜕皮类固醇会刺激未喂养的雌性VG分枝数增加。对处女和交配雌体的血淋巴、全身和生殖道中蜕皮类固醇滴度的分析表明，蜕皮类固醇调节着蜱类的生殖。蜕皮激素和VG浓度保持在低水平，卵母细胞不成熟，处女雌性没有产卵。另一方面，在交配的雌性血淋巴和全身提取物中分别出现了三个和四个明显的蜕皮类固醇峰。第一个峰值与FSF分泌开始时相吻合。第二个峰大而宽，与VG被卵母细胞吸收相吻合。第三个高峰是一个小高峰，出现在产卵开始时。第四个大的宽峰，只有在全身，似乎是由于生殖道中的蜕皮类固醇。实验正在进行中，以确定蜕皮类固醇(ECR)和视黄醇X(RXR)受体。这些研究表明，这种扁虱体内至少存在三种蜕皮类固醇受体(ECR)亚型。蜕皮激素受体基因在卵黄发生的不同阶段的表达目前正在研究中。卵黄蛋白原基因的全序列尚未完全分析，但一旦完成，将使我们能够确定该基因是否具有蜕皮类固醇反应元件，并更仔细地研究卵黄发生。

Our research has concentrated on characterization of vitellogenin (Vg) and elucidation of mechanisms regulating the expression of this protein in a soft tick. Vg synthesized in the fat body is carried by the hemolymph to the ovary and incorporated into the oocytes as vitellin (Vn). Previous studies have shown two factors, vitellogenesis-inducing factor (VIF) and fat body stimulating factor (FSF), regulate Vg synthesis. VIF is a peptide and FSF appears to be a steroid. Injection of high concentrations of ecdysteroids stimulates an increase in Vg tilers of unfed females. Analysis of ecdysteroid titers in the hemolymph, whole body and reproductive tracts of virgin and mated fed females indicate ecdysteroids regulate reproduction in ticks. Ecdysteroid and Vg concentrations remain at low levels, oocytes do not mature and no oviposition occurs in virgin females. On the other hand, three and four distinct ecdysteroid peaks are seen in mated female hemolymph and whole body extracts, respectively. The first peak coincides with the beginning of FSF secretion. The second, a large broad peak, coincides with absorption of Vg into the oocytes. The third peak, a small peak, occurs at the start of oviposition. The fourth large broad peak, only in the whole body, appears to be due to ecdysteroids in the reproductive tract. Experiments are underway to identify the ecdysteroid (EcR) and retinoid X (RXR) receptors of O. moubata. These studies indicate there are at least three isoforms of ecdysteroid receptors (EcR) in this tick. Expression of the ecdysteroid receptor genes during the different phases of vitellogenesis is under investigation now. The full sequence of the vitellogenin gene has not yet been completely analyzed, but when completed will allow us to determine if this gene has an ecdysteroid responsive element and to investigate vitellogenesis more carefully.