Lo Woo-Kuen, Wen Xiao-Jun, Zhou Cheng-Jing
Department of Anatomy and Neurobiology, Morehouse School of Medicine, 720 Westview Drive, SW, Atlanta, GA 30310, USA.
Exp Eye Res. 2003 Nov;77(5):615-26. doi: 10.1016/s0014-4835(03)00176-3.
This study examines the microtubule configuration and its close association with the Golgi complex and Golgi-derived membranous vesicles in elongating fiber cells of the rat lens. Since fiber cells elongate tremendously during lens differentiation, we hypothesize that a microtubule-based motor system exists in the elongating fiber cells for transporting important membrane proteins and organelles to the target regions for cell growth. The newly synthesized membrane proteins are known to be transported from the trans-Golgi network in the form of vesicles to the target plasma membrane. By thin-section TEM, we observed a large number of vesicles of various sizes and shapes randomly distributed throughout the cytoplasm of elongating fiber cells. Both Golgi complex and vesicles exhibited characteristic normal structural features seen in other cell types and thus represented real vesicular organelles in the fiber cells. A large number of microtubules were regularly arranged into bundles parallel to the long axis of fiber cells as examined in both longitudinal and cross-section views. Many of these microtubules were closely associated or in intimate contact with the Golgi complex and vesicles in elongating fiber cells. The microtubule polarity assay revealed that microtubules exhibited a unidirectional polarity for the entire length of fiber cells as examined in both anterior and posterior cortical fiber segments. Namely, the minus end of microtubules was towards the anterior lens pole while the plus end was headed towards the posterior pole. This suggests that multiple molecular motors such as kinesin and dynein are needed for carrying the vesicles to both lens poles, since conventional kinesin is known to transport vesicular organelles towards the plus end whereas cytoplasmic dynein carries them towards the minus end of microtubules. By immunoblot analysis, we indeed detected the presence of both kinesin (120 kD) and dynein (70 kD) in homogenate prepared from lens cortical fibers. Moreover, immunogold TEM demonstrated that the aquaporin 0 (formally MIP26) antibody was localized on the membranous vesicles as well as plasma membranes of the cortical fiber cells. This study suggests that a microtubule-based motor system exists in the lens and plays an important role in transporting membrane proteins such as aquaporin 0 in the vesicles during fiber cell differentiation and elongation.
本研究检测了大鼠晶状体伸长纤维细胞中的微管结构及其与高尔基体复合体和高尔基体衍生膜泡的紧密联系。由于纤维细胞在晶状体分化过程中会极大地伸长，我们推测在伸长的纤维细胞中存在一个基于微管的马达系统，用于将重要的膜蛋白和细胞器运输到细胞生长的目标区域。已知新合成的膜蛋白以囊泡的形式从反式高尔基体网络运输到目标质膜。通过超薄切片透射电子显微镜，我们观察到大量大小和形状各异的囊泡随机分布在伸长纤维细胞的整个细胞质中。高尔基体复合体和囊泡均呈现出在其他细胞类型中可见的典型正常结构特征，因此代表了纤维细胞中真正的膜泡细胞器。在纵切面和横切面观察中均发现，大量微管沿纤维细胞的长轴规则排列成束。在伸长的纤维细胞中，许多这些微管与高尔基体复合体和囊泡紧密相连或紧密接触。微管极性分析显示，在前皮质纤维段和后皮质纤维段观察时，微管在纤维细胞的整个长度上呈现单向极性。也就是说，微管的负端朝向晶状体前极，而正端朝向晶状体后极。这表明需要多种分子马达如驱动蛋白和动力蛋白将囊泡运输到晶状体的两极，因为已知传统驱动蛋白将膜泡细胞器运输到微管的正端，而细胞质动力蛋白将它们运输到微管的负端。通过免疫印迹分析，我们确实在从晶状体皮质纤维制备的匀浆中检测到了驱动蛋白（120 kD）和动力蛋白（70 kD）的存在。此外，免疫金透射电子显微镜显示水通道蛋白0（原MIP26）抗体定位于皮质纤维细胞的膜泡以及质膜上。本研究表明，晶状体中存在一个基于微管的马达系统，在纤维细胞分化和伸长过程中，该系统在运输囊泡中的水通道蛋白0等膜蛋白方面发挥着重要作用。
