Endocytosis, molecular transport mechanism

In contrast to the above-discussed molecular transport mechanisms, endocytosis, exocytosis, and transcytosis represent the transport of compounds across plasma membranes along with the bulk of their surrounding aqueous medium through vesicle formation (invagination), translocation of the vesicle, and subsequent membrane fusion, a process that also requires energy. These topics are not covered in this article. 

The transferrin receptor was one of the first to be exploited for receptor-mediated gene delivery. All actively metabolising cells require iron that is internalised by the cell as a transferrin-iron complex by means of receptor mediated endocytosis. To exploit this ubiquitous and efficient transport mechanism for introducing DNA into cells, conjugates of chicken or human transferrin with polycations (polylysine or protamine) were synthesised and used to form complexes with plasmid DNA. The number of transferrin molecules attached to each polylysine molecule varies according to the molecular weight of the polymer, but is generally around 1 transferrin molecule for every 50 lysine residues. ... 

Gentamicin is more toxic to LLC-PKj monolayers when exposed at the apical side, indicating a preferential uptake from the luminal membrane [136]. The uptake mechanism is proposed to be via megalin mediated endocytosis, a protein which is abundantly expressed in the proximal tubule [137]. A pathway delineated in LLC-PKl cells is proposed, whereby internalized aminoglycosides and other small molecular weight cationic compounds are transported from the early and late endosomes, through the Golgi complex. 

The trematode gastrodermis epithelium is probably the principal site of absorption for large-molecular-weight nutrients. These nutrients seem to be critical for growth and long-term survival and for egg production by females (88a). The mechanisms of absorption are uncharacterized. Endocytosis occurs to a very limited extent in F. hepatica, but there is no evidence for this process in schistosomes. There is no information available on the role of the trematode gastrodermis in the transport of inorganic ions or water. 

An additional mechanism for transport of metal complexes is by endo-cytosis/exocytosis (for review see Ballatori 1991). Fluid-phase, adsorptive, and receptor-mediated endocytosis make a major contribution to the transport of metals that are bound to high molecular weight ligands, and in particular to ligands such as ferritin, transferrin, and other proteins that are selectively cleared by receptor-mediated endocytosis. Because these proteins also have some affinity for toxic metals, they may play an important role in their transport across cell membranes (Ballatori 1991). The mechanism by which metallothionein and its associated metals are removed from the circulation is not known, but the kidney appears to be the principal site of removal (Tanaka et al. 1975). When rats are injected intravenously with ° Cd-labeled metallothionein, the radioactivity is rapidly and nearly completely accumulated in the kidney (Tanaka et al. 1975). 

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