Transcytosis

FIG. 2 Mechanisms of drug transfer in the cellular layers that line different compartments in the body. These mechanisms regulate drug absorption, distribution, and elimination. The figure illustrates these mechanisms in the intestinal wall. (1) Passive transcellular diffusion across the lipid bilayers, (2) paracellular passive diffusion, (3) efflux by P-glycoprotein, (4) metabolism during drug absorption, (5) active transport, and (6) transcytosis [251]. 

Release of a Protein from Its Polymeric Carrier during Transcytosis... 

Polymers, being macromolecules of considerable size and weight, have many limitations when used either as drugs or as drug carriers (1). One of the most serious limitations is the existence of epithelial or endothelial barriers (2). However, macromolecules can be transported by a vesicular process known as transcytosis (3,4). In transcytosis, a polymer can be shuttled across an epithelial cell by first... 

As shown in Table I, free HRP is poorly transported across MDCK cells but, when conjugated to a PLL carrier, HRP transport is increased considerably. The existence of a proteolytic compartment involved in the transcytotic digestion of HRP-S-PLL conjugate was further confirmed by the finding that when PLL was replaced by PDL, the transport of HRP was completely abolished (Table I) (8). In addition, when protease inhibitors such as leupeptin were added to the basal medium, the transcytosis of HRP was also significantly decreased (Table I). We have previously reported that the partial degradation of HRP-S-PLL was not inhibited by lysosomotropic amines (<8), indicating that this proteolytic process does not occur in lysosomes. 

D). Due to lack of proteolytic enzymes in this pathway, HRP will not be released rather, it will adhere to the basal membrane for possible basal-to-apical transcytosis... 

Table I. Basal-to-Apical Transcytosis of HRP-S-PDL in Cultured MDCK Cell Monolayers8...

Figure 1 General pathways through which molecules can actively or passively cross a monolayer of cells. (A) Endocytosis of solutes and fusion of the membrane vesicle with the opposite plasma membrane in an active process called transcytosis. (B) Similar to A, but the solute associates with the membrane via specific (e.g., receptor) or nonspecific (e.g., charge) interactions. (C) Passive diffusion between the cells through the paracellular space. (C, C") Passive diffusion (C ) through the cell membranes and cytoplasm or (C") via partitioning into and lateral diffusion within the cell membrane. (D) Active or carrier-mediated transport of an otherwise poorly membrane permeable solute into and/or out of a cellular barrier.

In addition to the aforementioned effects on paracellular drug transport, Ca2+ also plays an important role in the transcytosis of macromolecules. The entry of the plant lectins abrin, modeccin, and viscumin into Vero cells was inhibited in a Ca2+-free medium a well as in a Ca2+-containing medium containing verapamil and Co2+, both inhibitors of Ca2+ [217], Ca2+ is therefore required at a stage after the binding of the above lectins, perhaps in the fusion and exocytosis of membrane vesicles. 

Recent work in our laboratory (Kompella, Mathias, and Lee, unpublished observation) has revealed that activation of the cAMP-regulated Cl channels in the conjunctiva also enhances the transcytosis of horseradish peroxidase. 8-Bromo-cAMP (a membrane-permeable analog of cAMP) and terbutaline (a p2-adrenergic agonist known to increase intracellular levels of cAMP in other epithelial tissues [238]), at 0.5 mM, were found to enhance the transport of 100 pg/ mL HRP from the mucosal side to the serosal side of the pigmented rabbit conjunctiva by a factor of 4 (Fig. 11). 

Large, non-lipid-soluble molecules may cross the capillary wall by transcytosis. This mechanism involves the transport of vesicles from one side of the capillary wall to the other. Many hormones, including the catecholamines and those derived from proteins, exit the capillaries and enter their target tissues by way of transcytosis. 

Several of the postulated roles for nematode-secreted AChEs assume that they gain access to the intestinal mucosa. Several possibilities exist for transport of parasite AChE across the epithelial cell barrier, such as (i) utilization of existing pathways for receptor-mediated transcytosis (ii) a paracellular route facilitated by parasite-secreted proteases as observed for a bacterial elastase (Azghani et al., 1993) and (iii) increased paracellular permeability resulting from inflammatory events in the mucosa. We consider the latter suggestion most likely, as this has been duplicated by ex vivo perfusion with rat mast cell protease II (Scudamore et al., 1995). Moreover, cholinergic stimulation attenuates epithelial barrier properties to macromolecules in rat ileal crypts (Phillips et al., 1987). 

Wakama, J., Wakamiya, T., Tsuji, A., Adsorptive-mediated transcytosis of a basic peptide, 001-C8 in Caco-2 cells, Pharm. Res. 1998, 15, 1305-1309. 

Hydrophilic peptides and proteins are frequently large molecules they may enter the brain by carrier-mediated transport, receptor-mediated transcytosis, or by adsorptive-mediated transcytosis. Small peptides, such as di- and tripeptides are transported by the specific transporters, PepTl and PepT2, but neither of them is present at the BBB. Nevertheless, there is saturable brain uptake of the tripeptide glutathione and of several opioid peptides, suggesting that specific transporters, as... 

Passage through the epithelium and endothelial cellular barriers likely represents the greatest challenge to absorption. Although the molecular details remain unclear, this absorption process appears to occur via one of two possible means transcytosis or paracellular transport (Figure 4.6). 

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...

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]. 

Ligand-receptor complexes that do not dissociate in the EEs have different fates. In some cells, they may be returned to the same plasma membrane compartment from which they originated, whereas in polarized cells such as endothelial cells or astrocytes they can be moved to a different plasma membrane domain, resulting in transcytosis. In other cases, the complexes go to LEs and lysosomes for degradation. In neurons, these vesicles may serve as signaling organelles that are transported from the EEs back to the cell body where they influence gene expression [71]. 

Cremaschi D, Ghirardelli R, Porta C (1998) Relationship between polypeptide transcytosis and lymphoid tissue in the rabbit nasal mucosa. Biochim Biophys Acta 1369 287-294. 

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