3DEM-analyses of Golgi apparatus subunits during physiologic and pathologic reorganizations

Abstract

The Golgi apparatus is a dynamic organelle that rapidly reorganizes upon changes of functional states, and environmental conditions. Physiologic reorganizations take place continuously with secretory and endocytic traffic into and across the stacks of Golgi cisternae [1, 2]. We show here 3D-electron microscopic (3DEM) analyses of newly formed and transformed subcompartments of the Golgi apparatus during endocytosis of Wheat germ agglutinin (WGA 33µg/100µl, labelled with peroxidase and visualized electron microscopically by means of oxidation of diaminobenzidine) in HepG2 hepatoma cells within internalization periods of 10 to 60 min. WGA is an N-acetylglucosamine and sialic acid-specific lectin, which is known for many years to be transported to the Golgi apparatus in retrograde direction [3], and also is used for developments of drug delivery systems [4]. Following internalization, WGA is rapidly transported to a cell region close to the Golgi apparatus. Early globular endosomes accumulate at the trans-Golgi side, and an endocytic trans-Golgi network (endocytic TGN) is formed. Portions of the endocytic TGN attach to trans-Golgi cisternae, thus becoming integrated parts of the Golgi apparatus stacks. The trans-Golgi attachment of the endocytic TGN leads to interconnections of the small Golgi stacks, and causes the formation of Golgi apparatus ribbons. Three-dimensional reconstructions of the endocytic TGN following different preparation methods show that this compartment consists of globular and cylindrical pieces, which in dimensions and shapes resemble the earlier globular endosomes, and which are interconnected by cristae and fine bridges.