Hepatocytes internalize trophic receptors at large endocytic "Hot Spots".

Clathrin-mediated endocytosis in mammalian epithelial cells is believed to require the synergistic action of structural coat proteins and mechanochemical enzymes to deform and sever the plasma membrane (PM) into discreet vesicles. It is generally believed that the formation of clathrin-coated pits in epithelial cells occurs randomly along the apical and basolateral plasma membranes. In this study we visualized the endocytic machinery in living hepatocytes using green fluorescent protein (GFP)-tagged dynamin, a large mechanochemical guanosine triphosphate (GTP)ase implicated in the liberation of nascent vesicles from the plasma membrane and a variety of internal membrane compartments. Confocal microscopy of living cells expressing the epithelial isoform of GFP-tagged dynamin [Dyn2-GFP] revealed a distribution along the ventral PM in discrete vesicle-like puncta or in large (2-10 μm) tubuloreticular plaques. Remarkably, these large structures are dynamic as they form and then disappear, while generating large numbers of motile endocytic vesicles with which dynamin associates. Inhibiting dynamin function by microinjection of purified dynamin antibodies increases the number and size of the tubuloreticular plaques. Importantly, these "hot spots" sequester specific trophic receptors and cognate ligands such as transferrin receptor 1 (TfR1), but not TfR2. These findings suggest that hepatocytes sequester or prerecruit both structural and enzymatic components of the clathrin-based endocytic machinery to functional hot spots, from which large numbers of coated pits form and vesicles are generated. This process may mimic the endocytic organization found at the synapse in neuronal cells.