Clathrin-coated pits/vesicles endocytosis

In the classic model of synaptic vesicle recycling in nerve terminals, synaptic vesicles fuse completely with the plasma membrane and the integrated vesicle proteins move away from the active zone to adjacent membrane regions (Fig. 9-9A). In these regions, clathrin-mediated synaptic vesicle endocytosis takes place rapidly after neurotransmitter release (within seconds) [64]. The process starts with the formation of a clathrin-coated pit that invaginates toward the interior of the cell and pinches off to form a clathrin-coated vesicle [83]. Coated vesicles are transient organelles that rapidly shed their coats in an ATP/chaperone dependent process. Once uncoated, the recycled vesicle fuses with a local EE for reconstitution as a synaptic vesicle. Subsequently, the recycled synaptic vesicle is filled with neurotransmitter and it returns to the release site ready for use. This may be the normal pathway when neurotransmitter release rates are modest. Clathrin/ EE-based pathways become essential when synaptic proteins have been incorporated into the presynaptic plasma membrane. 

Receptor-mediated endocytosis is the selective uptake of extracellular macromolecules (such as cholesterol) through their binding to specific cell-surface receptors. The receptor-macromolecule complex then accumulates in clathrin-coated pits and is endocytosed via a clathrin-coated vesicle. 

Both endocytosis of material at the plasma membrane and exocytosis from the Golgi apparatus involve the formation of clathrin-coated pits and vesicles. On the cytosolic side of the membrane these structures have an electron-dense coat consisting mainly of the protein clathrin, the polypeptides of which form a three-legged structure known as a triskelion. The clathrin triskelions assemble into a basket-like convex framework that causes the membrane to invaginate at that point and eventually to pinch off and form a vesicle. In endocytosis these clathrin-coated vesicles migrate into the cell where the clathrin coats are lost before delivering their contents to the lysosomes. 

Fig. 3. Receptor-mediated endocytosis involves clathrin-coated pits and vesicles.

Gene delivery systems can distribute plasmids to the desired target cells, after which the plasmid is internalized into the cell by a number of mechanisms, such as adsorptive endocytosis, receptor-mediated endocytosis, micropinocytosis, caveolae-mediated endocytosis and phagocytosis (see Section 1.3.3.2). The intracellular fate of plasmids depends on the means by which they are internalized and translocated to the cytoplasms and then to the nucleus. In coated-pit endocytosis, DNA complexes first bind to the cell surface, then migrate to clathrin-coated pits about 150 ran in diameter and are internalized from the plasma membrane to form coated vesicles. 

The activation of vesicle recycling in the above experiments was associated with a massive accumulation of clathrin-coated pits in the plasma membrane around active zones (Fig. 7B). At early times (10-20 s) after addition of Ca +, early stages of coated pits (i.e. shallow coated pits) were relatively more abundant, whereas at later times (2 min), late stages (i.e. invaginated coated pits with narrow necks) predominated. Synaptic vesicle recycling under conditions of low-frequency stimulation also appears to be predominantly or exclusively mediated by clathrin-mediated endocytosis. In axons maintained at rest, clathrin-coated pits... 

Synaptic vesicles are formed primarily by endocytic budding from the plasma membrane of axon terminals. Endocytosis usually involves clathrin-coated pits and is quite specific, in that several membrane proteins unique to the synaptic vesicles (e.g., neurotransmitter transporters) are specifically incorporated into the endocytosed vesicles. In this way, synaptic-vesicle membrane proteins can be reused and the recycled vesicles refilled with neurotransmitter (see Figure 17-36). 

Fig. 10.16. Lysosomes in receptor-mediated endocytosis via clathrin-coated pits. 1 Endo-cytotic vesicles fuse to form early endosomes. 2 Vesicle contents are sorted, and receptors, clathrin, and lipids are sent back to the plasma membrane. 3 Transport vesicles from the trans-Golgi carry lysosomal hydrolases to the late endosome. 4 Lysosomes containing concentrated hydrolases digest proteins and other components acquired from endocytotic vesicles.

The best-documented way of endocytosis is receptor-mediated uptake of ligands via clathrin-coated vesicles (reviewed in Schmid 1997). Receptors are recruited and concentrated into clathrin-coated pits at the plasma membrane. After coated vesicle formation, the clathrin coat is removed by the concerted action of auxihn and heat shock protein 70 (Ungewickell et al. 1995). The uncoated vesicles fuse with early endosomes in a rab5-regulated manner (Rubino et al. 2000). 

Coated Pit A cell membrane pit that is lined on its cytosolic side by a meshwork of the protein clathrin. Coated pits participate in the mechanism of receptor-mediated Endocytosis, in which surface receptors that have bound specific Extracellular substances are gathered into coated pits, which pinch oflf to become cytoplasmic vesicles. 

Figure 41 -15. Two types of endocytosis. An endocytotic vesicle (V) forms as a result of invagination of a portion of the plasma membrane. Fluid-phase endocytosis (A) is random and nondirected. Receptor-mediated endocytosis (B) is selective and occurs in coated pits (CP) lined with the protein clathrin (the fuzzy material). Targeting is provided by receptors (black symbols) specific for a variety of molecules. This results in the formation of a coated vesicle (CV).

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