Endocytosis endosomes

Functional Cyclodextrin Polyrotaxanes for Drug Delivery Uptake Endocytosis Endosomal escape... 

Fig. 4 Transfection process of TMC-Cys nanocomplexes (NC) including cell binding through electrostatic affinity and disulfide bonding, uptake via endocytosis, endosomal escape, intracellular plasmid DNA release, and nuclear transport of plasmid DNA [39]...

Fig. 4 Top Cationization of the protein human serum albumin carrying multiple positively charged primary amino groups. Images show that such albumin polycations (stained red) reveal efficient cellular uptake by clathrin-mediated endocytosis, endosomal release (yellow arrows), and allow gene delivery and release into cells due to tight interaction of DNA, as exemplified by the isothermal titration calorimetry graph...

In addition to binding to sialic acid residues of the carbohydrate side chains of cellular proteins that the virus exploits as receptors, hemagglutinin has a second function in the infection of host cells. Viruses, bound to the plasma membrane via their membrane receptors, are taken into the cells by endocytosis. Proton pumps in the membrane of endocytic vesicles that now contain the bound viruses cause an accumulation of protons and a consequent lowering of the pH inside the vesicles. The acidic pH (below pH 6) allows hemagglutinin to fulfill its second role, namely, to act as a membrane fusogen by inducing the fusion of the viral envelope membrane with the membrane of the endosome. This expels the viral RNA into the cytoplasm, where it can begin to replicate. 

Biochemical characterization of clathrin-coated vesicles revealed that their major coat components are clathrin and various types of adaptor complexes. Clathrin assembles in triskelions that consist of three heavy chains of approximately 190 kDa and three light chains of 30 40 kDa. Four types of adaptor complexes have been identified to date, AP-1, AP-2, AP-3 and AP-4 (AP for adaptor protein). Whereas AP-1, AP-3 and AP-4 mediate sorting events at the TGN and/or endosomes, AP-2 is involved in endocytosis at the plasma membrane. Each adaptor complex is a hetero-tetrameric protein complex, and the term adaptin was extended to all subunits of these complexes. One complex is composed of two large adaptins (one each of y/a/S/s and [31-4, respectively, 90-130 kDa), one medium adaptin (pi -4, <50 kDa), and one small adaptin (ol-4, <20 kDa). In contrast to AP-1, AP-2 and AP-3, which interact directly with clathrin and are part of the clathrin-coated vesicles, AP-4 seems to be involved in budding of a certain type of non-clathrin-coated vesicles at the TGN. 

NHE5. The distribution of this isoform is distinct, being in neuronal-rich areas of the central nervous system. Low levels have also been found in testis, spleen and skeletal muscle. Like the preceding isoforms, NHE5 is found in the plasma membrane and is internalised by clathrin-associated endocytosis into recycling endosomes. The normal role of NHE5 is unknown but its malfunction is speculated to contribute to the development of neurodegenerative disease. 

There are receptors (TfRs) on the surfaces of many cells for transferrin, it binds to these receptors and is internalized by receptor-mediated endocytosis (compare the fate of LDL Chapter 25). The acid pH inside the lysosome causes the iron to dissociate from the protein. The dissociated iron leaves the endosome via DMTl to enter the cytoplasm. Unlike the protein component of LDL, apoTf is not degraded within the lysosome. Instead, it remains associated with its receptor, returns to the plasma membrane, dissociates from its receptor, reenters the plasma, picks up more iron, and again delivers the iron to needy ceils. 

Lee, R. J. Wang, S. Low, R S., Measurement of endosome pH following folate receptor-mediated endocytosis, Biochim. Biophys. Acta 1312, 237-242 (1996). 

Fig. 6.22. Folate-FRET sensor structure and its application to measure disulfide bond reduction in endosomes. The molecule contains the folate moiety which is recognized by the folate receptor situated at the plasma membrane. This recognition leads to endocytosis and after some time to cleavage of the probe. 

Figure 13.9 represents the TEM image of LDH particles and their cellular internalization. As expected, LDH particles are internalized by endocytosis. Figure 13.9(A) shows the cellular uptake process of LDHs after 3h of treatment, and demonstrates a successive entry of LDH by endocytosis first the LDH particles were located around the cell membrane due to their positive charge ( ), then they migrate to the membrane ruffles which are considered as endocytic bodies ( ), finally the coated intracellular vesicles were formed as early endosomes ( ). Figure 13.9(B)...

Fig. 1 Bioresponsive polyplexes. (a) Systemic circulation of shielded polyplexes in blood stream and attachment to cell surface receptor (b) endocytosis into endosomes, deshielding by cleavage of PEG linkers and activation of membrane-destabilizing component by acidic pH or other means (c) endosomal escape into cytosol (d) siRNA transfer to form a cytosolic RNA-induced silencing complex complex (e) cytosolic migration and intranuclear import of pDNA (/) presentation of pDNA in accessible form to the transcription machinery...

Fig. 2 Deshielding of polyplexes. After endocytosis of polyplexes into endosomes, deshielding by cleavage of PEG hydrazone or acetal linkers...

Figure 4.6 Likely mechanisms by which macromolecules cross cellular barriers in order to reach the bloodstream from (in this case) the lung. Transcytosis entails direct uptake of the macromolecule at one surface via endocytosis, travel of the endosome vesicle across the cell, with subsequent release on the opposite cell face via exocytosis. Paracellular transport entails the passage of the macromolecules through leaky tight junctions found between some cells...

Some attempts have been made to rationally increase the efficiency of endosomal escape. One such avenue entails the incorporation of selected hydrophobic (viral) peptides into the gene delivery systems. Many viruses naturally enter animal cells via receptor-mediated endocytosis. These viruses have evolved efficient means of endosomal escape, usually relying upon membrane-disrupting peptides derived from the viral coat proteins. 

Figure 14.10 Overview of cellular entry of (non-viral) gene delivery systems, with subsequent plasmid relocation to the nucleus. The delivery systems (e.g. lipoplexes and polyplexes) initially enter the cell via endocytosis (the invagination of a small section of plasma membrane to form small membrane-bound vesicles termed endosomes). Endosomes subsequently fuse with golgi-derived vesicles, forming lysosomes. Golgi-derived hydrolytic lysosomal enzymes then degrade the lysosomal contents. A proportion of the plasmid DNA must escape lysosomal destruction via entry into the cytoplasm. Some plasmids subsequently enter the nucleus. Refer to text for further details...

However, not all proteins proceed directly to their eventual destination. Some proteins relocate from one plasma membrane compartment to another by means of trans-cytosis. Transcytosis involves endocytosis of selected proteins in one membrane compartment, followed by subsequent transport through early endosomes to recycling endosomes and finally translocation to a different membrane compartment, for example from the apical to the basolateral surfaces. Sorting at the TGN and endo-some recycling steps appear to have a primary role in the steady state distribution of proteins in different plasma membrane domains [47], However, selective retention of proteins at the plasma membrane by scaffolding proteins or selective removal may also contribute to normal distributions. Finally, microtubule-motor regulatory mechanisms have been discovered that might explain the specific delivery of membrane proteins to discrete plasma membrane domains [48]. 

Under some circumstances, lysosomal hydrolases may fail to be properly packaged in the TGN, so they enter the default pathway to the cell surface, where they are secreted. Although these hydrolases do little harm at the nearly neutral pH of most extracellular fluids, they can also be returned to lysosomes by a pathway known as receptor-mediated endocytosis. In this pathway, M6P receptors are sent to the plasma membrane, where they bind escaped lysosomal hydrolases and bring them back to lysosomes through the early and late endosomes. Receptor-mediated endocytosis is a major component of the endocytic pathways for trafficking of membrane proteins and merit more detailed consideration. 

Hacker, H. et al., CpG-DNA-specific activation of antigen-presenting cells requires stress kinase activity and is preceded by non-specific endocytosis and endosomal maturation, Embo J., 17, 6230, 1998. 

Many of the investigations into endosomal pathways have concentrated on receptor-mediated endocytosis, as in the iron-transferrin-receptor complex, and it is not clear how the systems vary depending on whether or not the pathway is clathrin-dependent or clathrin-independent [54],... 

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