Pathways of clathrin-independent endocytosis

Abstract

There are numerous ways that endocytic cargo molecules may be internalized from the surface of eukaryotic cells. In addition to the classical clathrin-dependent mechanism of endocytosis, several pathways that do not use a clathrin coat are emerging. These pathways transport a diverse array of cargoes and are sometimes hijacked by bacteria and viruses to gain access to the host cell. Here, we review our current understanding of various clathrin-independent mechanisms of endocytosis and propose a classification scheme to help organize the data in this complex and evolving field.

Figure 1. Pathways of entry into cells.

Large particles can be taken up by phagocytosis, whereas fluid uptake occurs by macropinocytosis. Both processes appear to be triggered by and are dependent on actin-mediated remodelling of the plasma membrane at a large scale. Compared with the other endocytic pathways, the size of the vesicles formed by phagocytosis and macropinocytosis is much larger. Numerous cargoes (TABLE 1) can be endocytosed by mechanisms that are independent of the coat protein clathrin and the fission GTPase, dynamin. This Review focuses on the clathrin-independent pathways, some of which are also dynamin independent (FIGS 2,3). Most internalized cargoes are delivered to the early endosome via vesicular (clathrin- or caveolin-coated vesicles) or tubular intermediates (known as clathrin- and dynamin-independent carriers (CLICs)) that are derived from the plasma membrane. Some pathways may first traffic to intermediate compartments, such as the caveosome or glycosyl phosphatidylinositol-anchored protein enriched early endosomal compartments (GEEC), en route to the early endosome.

Figure 2. Electron micrographs of early intermediates in clathrin-dependent and independent pathways of endocytosis.

a|Thin-section views of reticulocytes incubated with gold-conjugated transferrin (AuTf) for 5 min at 37°C, which shows AuTf clustering into clathrin-coated pits and subsequently being endocytosed via coated vesicles. b | Rapid-freeze, deep-etch views of clathrin lattices on the inner surface of a normal chick fibroblast. c | Thin-section (left panel) and rapid-freeze, deep-etch images (right panel) of caveolae in endothelial cells. The arrow in the left panel points to the endoplasmic reticulum near deeply invaginated caveolae. d | Thin-section surface view of horseradish peroxidase (HRP)-conjugated cholera toxin in the process of internalization via grape-like caveolae in mouse embryonic fibroblasts. e | Thin-section images of green fluorescent protein (GFP) with a glycosyl phosphatidylinositol (GPI) anchor expressed in Chinese hamster ovary cells and incubated with gold-conjugated antibodies against GFP. The antibodies show putative sites for clathrin- and dynamin-independent endocytosis. These represent surface-connected tubular invaginations where the gold probe is concentrated with respect to the rest of the plasma membrane. f | Thin-section micrographs of internalized HRP-conjugated cholera toxin incubated at 37°C in mouse embryonic fibroblasts, showing early intermediates in clathrin- and caveolin-independent endocytosis. Clathrin- and dynamin-independent carriers (CLICs) are observed after 15 s (top panel) and GPI-anchored protein enriched early endosomal compartments (GEECs) are observed after 5 min (bottom panel). The scale bar in a and b is 100 nm, in c is 200 nm (left panel) and 100 nm (right panel), and in d–f is 200 nm. Part a reproduced with permission from REF. 101 © (1983) Rockefeller University Press. Part b reproduced with permission from REF. 102 © (1989) Rockefeller University Press. Part c reproduced with permission from REF. 103 © (1998) Annual Reviews. Parts d and f reproduced with permission from REF. 31 © (2005) Rockefeller University Press. Part e reproduced with permission from REF. 37 © (2002) Elsevier.

Figure 3. Proposed classification system for endocytic mechanisms.

A cargo protein can be endocytosed by either clathrin-dependent or clathrin-independent (CI) mechanisms. We propose that the CI pathways can be further categorized first by their dependence on the large GTPase dynamin, and then by other mechanistic components of the internalization pathway. This system is based on the current literature, which shows that interfering with or modifying the function of a particular GTPase affects the internalization or trafficking of one set of CI endocytic markers but not another. See TABLE 1 for endocytic markers that have been studied. The aim of this classification system for CI pathways is to categorize the molecular machinery that is associated with these pathways in the most parsimonious fashion consistent with the current literature. It is not meant to imply that a given GTPase is directly involved in regulating a particular internalization pathway because many other important cellular functions may also be regulated by that GTPase. There may be other, as-yet-unidentified CI pathways.