Shen X, Steyrer E, Retzek H, Sanders E J, Schneider W J
Department of Medicine, University of Manitoba, Winnipeg, Canada.
Cell Tissue Res. 1993 Jun;272(3):459-71. doi: 10.1007/BF00318552.
During the rapid final stage of growth, chicken oocytes take up massive amounts of plasma components and convert them to yolk. The oocyte expresses a receptor that binds both major yolk lipoprotein precursors, vitellogenin (VTG) and very low density lipoprotein (VLDL). In the present study, in vivo transport tracing methodology, isolation of coated vesicles, ligand- and immuno-blotting, and ultrastructural immunocytochemistry were used for the analysis of receptor-mediated yolk formation. The VTG/VLDL receptor was identified in coated profiles in the oocyte periphery, in isolated coated vesicles, and within vesicular compartments both outside and inside membrane-bounded yolk storage organelles (yolk spheres). VLDL particles colocalized with the receptor, as demonstrated by ultrastructural visualization of VLDL-gold following intravenous administration, as well as by immunocytochemical analysis with antibodies to VLDL. Lipoprotein particles were shown to reach the oocyte surface by passage across the basement membrane, which possibly plays an active and selective role in yolk precursor accessibility to the oocyte surface, and through gaps between the follicular granulosa cells. Following delivery of ligands from the plasma membrane into yolk spheres, proteolytic processing of VTG and VLDL by cathepsin D appears to correlate with segregation of receptors and ligands which enter disparate sub-compartments within the yolk spheres. In small, quiescent oocytes, the VTG/VLDL receptor was localized to the central portion of the cell. At onset of the rapid growth phase, it appears that this pre-existing pool of receptors redistributes to the peripheral region, thereby initiating yolk formation. Such a redistribution mechanism would obliterate the need for de novo synthesis of receptors when the oocyte's energy expenditure is to be utilized for plasma membrane synthesis, establishment and maintenance of intracellular topography and yolk formation, and preparation for ovulation.
在快速生长的最后阶段，鸡的卵母细胞会摄取大量血浆成分并将其转化为卵黄。卵母细胞表达一种受体，该受体可结合两种主要的卵黄脂蛋白前体，即卵黄原蛋白（VTG）和极低密度脂蛋白（VLDL）。在本研究中，采用体内转运追踪方法、包被小泡的分离、配体和免疫印迹以及超微结构免疫细胞化学来分析受体介导的卵黄形成。在卵母细胞周边的包被结构、分离的包被小泡以及膜结合的卵黄储存细胞器（卵黄球）内外的泡状区室中都鉴定出了VTG/VLDL受体。静脉注射后通过VLDL-金的超微结构可视化以及用VLDL抗体进行的免疫细胞化学分析表明，VLDL颗粒与受体共定位。脂蛋白颗粒通过穿过基底膜到达卵母细胞表面，基底膜可能在卵黄前体接近卵母细胞表面的过程中发挥积极的选择性作用，并且还通过卵泡颗粒细胞之间的间隙。在配体从质膜传递到卵黄球后，组织蛋白酶D对VTG和VLDL的蛋白水解加工似乎与进入卵黄球内不同亚区室的受体和配体的分离相关。在小的、静止的卵母细胞中，VTG/VLDL受体定位于细胞的中央部分。在快速生长阶段开始时，似乎这种预先存在的受体池重新分布到周边区域，从而启动卵黄形成。当卵母细胞的能量消耗要用于质膜合成、细胞内拓扑结构的建立和维持以及卵黄形成和排卵准备时，这样的重新分布机制将消除对受体从头合成的需求。
