Binding and endocytosis

Lee, K.-D., Hong, K. and Papahadjopoulos, D. (1992). Recognition of liposomes by cells in vitro binding and endocytosis mediated by specific lipid headgroups and surface charge density, Biochim. Biophys. Acta, 1103, 185-197. 

Lee, K.-D., Nir, S. and Papahadjopoulos, D. (1993). Quantitative analysis of liposome-cell interactions in vitro rate constants of binding and endocytosis with suspension and adherent J774 cells and human monocytes, Biochemistry, 32, 889-899. 

Miller CR, Bondurant B, McLean SD, et al. Liposome-cell interactions in vitro effect of liposome surface charge on the binding and endocytosis of conventional and sterically stabilized liposomes. Biochemistry 1998 37 12875. 

Pardridge, W.M., et al. 1993. Protamine-mediated transport of albumin into brain and other organs of the rat. Binding and endocytosis of protamine-albumin complex by microvascular endothelium. J Clin Invest 92 2224. 

Connolly DT, Townsend RR, Kawaguchi K, Bell WR, Lee YC. Binding and endocytosis of cluster glycosides by rabbit hepato-cytes. Evidence for a short-circuit pathway that does not lead to degradation. J. Biol. Chem. 1982 257 939-945. 

Okamoto, P. M., Herskovits, J. S., and Vallee, R. B. (1997). Role of the basic, proline-rich region of dynamin in Sre homology 3 domain binding and endocytosis. J. Biol. Chem. 272, 11629-11635. 

C T, Garbern J. and Wu J -Y. (1982a) Light and electron microscopic immunocytochemical localization of clathnn in rat cerebellum and kidney / Histochem Cytochem 30, 853-863 C T, Mukai K., and Lee C Y (1982b) Electron microscopic immunocytochemical studies of hCG binding and endocytosis in rat ovary Cell Tissue Res, 224, 647-653... 

Most cells release macromolecules to the exterior by exocytosis. This process is also involved in membrane remodeling, when the components synthesized in the Colgi apparatus are carried in vesicles to the plasma membrane. The signal for exocytosis is often a hormone which, when it binds to a cell-surface receptor, induces a local and transient change in Ca concentration. Ca triggers exocytosis. Figure 41—16 provides a comparison of the mechanisms of exocytosis and endocytosis. 

Figure 8 Ubiquitin and endocytosis. Receptors on the plasma membrane undergo monoubiquitination as a result of ligand (e.g., neurotransmitter). Ubiquitinated receptors bind to proteins called epsins, which in turn interact with adaptor proteins (adaptin) bound to clathrin-coated pits. Ubiquitination also functions to sort the internalized membrane protein into early endosomes, which directs them to degradation by lysosome through the multivesicular body. If ubiquitin from the endocytosed receptors is removed by an UBP, the receptor recycles back to the membrane. Proteasome inhibitors block endocytotic degradation of some proteins such as glutamate receptor subunits indicating a possible role for the proteasome.

Freshly isolated capillaries from different species are a valuable model system of the BBB [51], because they retain the native repertoire of receptors and enzymes. In particular, receptor binding and carrier-mediated uptake or receptor-mediated endocytosis can be conveniently studied. On which side binding or uptake takes place cannot be easily determined, as both the luminal and abluminal side are exposed in such a preparation. 

In the kiss-and-run mode exocytosis and endocytosis are directly coupled to each other, while in the case of classical complete vesicle fusion, exocytosis and slow clathrin-mediated endocytosis are timely and spatially separated. However, it appears that also in the latter case exocytosis and endocytosis occur coordinated, as both are stimulated by an increase of the cytoplasmic calcium concentration. It has been shown that after calcium entry the enzyme phospho-inositol-5 kinase Iy, which is enriched in the synapse, catalyzes the synthesis of phosphatidylinos-itol (4,5)-bisphosphate and that this mechanism is important for synaptic vesicle trafficking (Di Paolo et al. 2004). As many proteins involved in clathrin-mediated endocytosis are recruited to the plasma membrane by binding to phosphatidylinosi-tol (4,5)-bisphosphate (e.g., amphiphysin, dynamin, epsin, AP-180, and AP-2) it is attractive to speculate that elevated levels of calcium mediate the recruitment of en-docytic proteins to the plasma membrane by this mechanism. The increased level of phosphatidylinositol (4,5)-bisphosphate could be in part degraded by synaptojanin that thereby initiates the disassembly of the clathrin coat. Hence, calcium-induced transient increases in the level of phosphatidylinositol (4,5)-bisphosphate appear to play a central role for coupling exocytosis to clathrin-mediated endocytosis. In addition, it has been demonstrated that calcium also leads to the dephosphorylation of endocytic proteins as amphiphysin, dynamin, and synaptojanin, which in vitro is important for efficient coat assembly (Cousin and Robinson 2001). 

Many putative receptors for anti-dsDNA on the membrane of various cell types have been noted. Some workers found a 30-kDa protein involved in the binding and internalization of [3H]DNA via receptor-mediated endocytosis (B12, B13). Other possible receptors include nucleosomes on human leukocytes (R8), Fc receptors on human T cells (A6), DNA on mouse and human mononuclear cells (06), a 94-kDa protein on several cells lines (J2), DNase-resistant target on human fibroblasts and PK 15 cells (K9), membrane determinant precisely resembling DNA in murine renal tubular cells (Zl), Hp8 on human and murine tubular cells (Z2), ribosomal P protein on rat and human glomerular mesangial cells (S30, S31), brush border myosin 110 kDa on rat hepatoma cells, and a diverse set of membrane proteins on a series of human tumor cell lines (R3). 

The plasma membrane surrounds the cell, separating it from the external environment. The plasma membrane is a selectively permeable barrier due to the presence of specific transport proteins. It is also involved in receiving information when ligands bind to receptor proteins on its surface, and in the processes of exocytosis and endocytosis. 

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