Quantitative microscopy reveals 3D organization and kinetics of endocytosis in rat hepatocytes

In order to demonstrate the power of quantitative microscopy, the endocytic apparatus of rat hepatocytes was reexamined using in situ liver and short term cultured hepatocyte couplets that were allowed to internalize endocytic markers for various time intervals. Correlative confocal light and electron microscopy demonstrate a tubulovesicular reticulum representing the endocytic apparatus. Volume and membrane area account for 2% of cell volume and 30% plasma membrane surface. Colocalization analysis demonstrated that pathway-specific ligands and fluid-phase markers enter EEA1-positive vesicles, the early endosomal compartment, immediately after internalization. These vesicles are translocated rapidly from basolateral to perinuclear and apical locations. Ligands are sorted within 5 min to their respective pathways. Sequential colocalization of an asialoglycoprotein-pulse with rab7 and lamp3 demonstrates that early endosomes change into or fuse with late endosomes and lysosomes. Alternatively, markers are sequestered into the common endosome consisting of rab11-positive, long tubules that originate from early endosomes and show an affinity for the transcytotic marker pIgA and its receptor. This compartment mediates transcytosis by delivering the receptor-ligand complex to the subapical compartment, a set of apical, rab11-positive vesicles, which are connected to the tubular reticulum. We conclude that vesicular traffic between preexisting compartments, maturation or fusion of endocytic organelles, and transport in tubules act in concert and together mediate transport between compartments of a tubulovesicular endocytic apparatus. In addition, we show that quantitative microscopy using high resolution data sets can detect and characterize kinetics of various parameters thus adding a dynamic component to 3D information.