Marsh Brad J, Volkmann Niels, McIntosh J Richard, Howell Kathryn E
Boulder Laboratory for 3D Electron Microscopy of Cells, Department of Molecular, Cellular, and Developmental Biology, University of Colorado, Boulder, CO 80309, USA.
Proc Natl Acad Sci U S A. 2004 Apr 13;101(15):5565-70. doi: 10.1073/pnas.0401242101. Epub 2004 Apr 2.
Direct continuity between the membranes of cisternae in the Golgi complex in mammalian cells rarely has been observed; when seen, its documentation has been equivocal. Here we have used dual-axis electron microscope tomography to examine the architecture of the Golgi in three dimensions at approximately 6-nm resolution in rapidly frozen, freeze-substituted murine cells that make and secrete insulin in response to glucose challenge. Our data show three types of direct connections between Golgi cisternae that are normally distinct from one another. These connections all "bypass" interceding cisternae. We propose that when pancreatic beta cells are stimulated to synthesize and secrete insulin rapidly in vivo, such connections provide a continuous lumen that facilitates the rapid transit of large amounts of newly made protein for secretion. The heterotypic fusion of cisternae, even transiently, raises important questions about the molecular mechanisms that (i) facilitate the fusion/fission of cisternal membranes and control the directionality and specificity of such events, and (ii) retain Golgi processing enzymes at specific places within individual cisternae when two cisternae at different levels in the Golgi have fused, maintaining the sequential processing hierarchy that is a hallmark of Golgi organization.
在哺乳动物细胞中，高尔基体中潴泡膜之间的直接连续性很少被观察到；即便观察到了，相关记录也模棱两可。在此，我们利用双轴电子显微镜断层扫描技术，以大约6纳米的分辨率，对快速冷冻、冷冻置换的小鼠细胞中的高尔基体结构进行三维观察，这些小鼠细胞在葡萄糖刺激下合成并分泌胰岛素。我们的数据显示，高尔基体潴泡之间存在三种通常彼此不同的直接连接类型。这些连接全都“绕过”中间的潴泡。我们提出，当胰腺β细胞在体内被刺激快速合成并分泌胰岛素时，此类连接提供了一个连续的管腔，便于大量新合成的蛋白质快速转运以进行分泌。潴泡的异型融合，即便只是短暂的，也引发了关于分子机制的重要问题，这些机制包括：（i）促进潴泡膜的融合/裂变并控制此类事件的方向性和特异性；（ii）当高尔基体中不同水平的两个潴泡融合时，将高尔基体加工酶保留在各个潴泡内的特定位置，维持作为高尔基体组织标志的顺序加工层次结构。
