Brooks Jacqueline M, Wessel Gary M
Department of Molecular Biology, Cell Biology, and Biochemistry, Brown University, Providence, RI 02912, USA.
Dev Biol. 2003 Sep 15;261(2):353-70. doi: 10.1016/s0012-1606(03)00301-4.
The major yolk protein of sea urchins is an iron-binding, transferrin-like molecule that is made in the adult gut. Its final destination though is the developing oocytes that are embedded in somatic accessory cells and encompassed by two epithelial layers of the ovary. In this study, we address the dynamics of yolk transport, endocytosis, and packaging during the vitellogenic phase of oogenesis in the sea urchin by use of fluorescently labeled major yolk protein (MYP). Incorporation of MYP into the accessory cells of the ovary and its packaging into yolk platelets of developing oocytes is visualized in isolated oocytes, ovary explants, and in whole animals. When MYP is introduced into the coelom of adult females, it is first accumulated by the somatic cells of the ovarian capsule and is then transported to the oocytes and packaged into yolk platelets. This phenomenon is specific for MYP and accurately reflects the endogenous MYP packaging. We find that oocytes cultured in isolation are endocytically active and capable of selectively packaging MYP into yolk platelets. Furthermore, oocytes that packaged exogenous MYP are capable of in vitro maturation, fertilization, and early development, enabling an in vivo documentation of MYP utilization and yolk platelet dynamics. These results demonstrate that the endocytic uptake of yolk proteins in sea urchins does not require a signal from their surrounding epithelial cells and can occur autonomous of the ovary. In addition, these results demonstrate that the entire population of yolk platelets is competent to receive new yolk protein input, suggesting that they are all made simultaneously during oogenesis.
海胆的主要卵黄蛋白是一种结合铁的、类似转铁蛋白的分子，由成年海胆的肠道产生。然而，它的最终目的地是发育中的卵母细胞，这些卵母细胞嵌入在体细胞辅助细胞中，并被卵巢的两层上皮细胞所包围。在本研究中，我们通过使用荧光标记的主要卵黄蛋白（MYP）来研究海胆卵子发生的卵黄生成阶段中卵黄运输、内吞作用和包装的动态过程。在分离的卵母细胞、卵巢外植体和完整动物中，可以观察到MYP被整合到卵巢的辅助细胞中，并被包装到发育中卵母细胞的卵黄小体中。当将MYP引入成年雌性海胆的体腔时，它首先被卵巢囊的体细胞积累，然后被运输到卵母细胞并包装到卵黄小体中。这种现象对MYP具有特异性，并准确反映了内源性MYP的包装过程。我们发现，分离培养的卵母细胞具有活跃的内吞作用，能够选择性地将MYP包装到卵黄小体中。此外，包装了外源性MYP的卵母细胞能够进行体外成熟、受精和早期发育，从而能够在体内记录MYP的利用情况和卵黄小体的动态过程。这些结果表明，海胆中卵黄蛋白的内吞摄取不需要来自周围上皮细胞的信号，并且可以在卵巢外自主发生。此外，这些结果表明，整个卵黄小体群体都有能力接受新的卵黄蛋白输入，这表明它们在卵子发生过程中是同时形成的。
