Transferrin transcytosis

Widera A, Kim KJ, Crandall ED, Shen WC (2003) Transcytosis of GCSF-transferrin across rat alveolar epithelial cell monolayers. Pharm Res 20(8) 1231— 1238... 

The process of transcytosis is illustrated in Figure 2.3 for the transferrin receptor (TfR) [37]. The receptor is heavily expressed at the BBB compared to other vascular beds [38]. Transferrin or a monoclonal antibody to the extracellular domain of the receptor protein will bind from the luminal side of the BBB. This triggers cellular uptake by the mechanism of receptor-mediated endocytosis, i.e. the invagination and budding off of parts of the cell membrane as a result of the formation of small vesicles (endosomes). The transceUular passage of ligand (transcytosis) is completed by exocytosis at the abluminal membrane, and the whole process is completed within minutes in vivo. 

SECs are normally able to internalize only small particles (up to 0.23 pm). In conditions of impaired KC function, however, they have also been found to phagocytose larger particles [23]. They are also responsible for the receptor-mediated transcytosis of several compounds, such as insulin [24] and transferrin [25]. 

Concentration of transferrin in CSF does not correlate with serum levels. This suggests consideration of the presence of a speciflc transport system for transferrin in the blood-CSF barrier. This transport system may be similar to that, for instance, for immunoglobulins (transcytosis). Only as an epiphenomenon is there a signiflcant decrease of serum concentrations of transferrin, as in cases of ulcerous meningitis and malignant meningeal inflltration. In this case other methods are required to determine the levels with any degree of precision (A24, Zl). 

Polypeptides are substrates for receptor-mediated transcytosis. Cerebral insulin reaches the brain from the circulation via receptor-mediated transcytosis through the BBB on the brain endothelial insulin receptor. This receptor is upregulated in development and downregulated in streptozotocin-induced diabetes mellitus. Similarly a BBB transferrin receptor mediates the transcytosis of transferrin across the BBB and this explains how the brain is able to extract iron from the circulation. Other RMT pathways consituting portals of entry to the brain for circulating peptides include receptors for insulin-like growth factors, cationic proteins, lectins, acetyl-low density lipoprotein and leptin. 

Liposomes can be targeted to the brain by exploiting receptor-mediated transcytosis systems. For example, a bi-functional PEG-linker has been used to couple anti-transferrin (0X26) receptor antibodies to one end of the PEG strands and liposomes at the other end of the PEG strands (Figure 13.6). Classically, immunoliposomes are prepared by attaching the MAb to the surface of the liposomes (see Section 5.3.1.3). However, this can lead to steric hindrance by the PEG strands with respect to antibody binding to the appropriate receptor. The use of the bifunctional PEG linker overcomes this problem. 

Peptide nucleic acids (PNA) are novel antisense oligonucleotides (see Section 1.6.2) which contain a polypeptide backbone. Receptor-mediated transcytosis can be exploited to promote their delivery to the CNS. For example, the attachment of PNAs to the anti-transferrin (0X26) receptor antibodies has been shown to increase the brain uptake of the PNAs, without loss of the ability of the PNAs to hybridize to target mRNA. However, antisense agents will not exert pharmacologic effects in vivo following delivery to... 

Raub TJ, Newton CR. 1991. Recycling kinetics and transcytosis of transferrin in primary cultures of bovine brain... 

Descamps L, Dehouck M-P, Torpier G, Cecchelli R. 1996. Receptor-mediated transcytosis of transferrin through blood-brain barrier endothelial cells. Am. J. Physiol. Heart Circ. Physiol. 39 1149-58... 

Re cep tor-mediated endocylosis or transcytosis (e.g. transferrin, Insulin, lipoproteins)... 

Another method of delivery of insulin is to conjugate the protein with transferrin. Oral administration of the insulin-transferrin complex and insulin in streptozotocin-induced diabetic mice lowered the blood glucose levels by 28 and 5%, respectively. The blood glucose level was further decreased to 40% when the mice were pretreated with brefeldin A, a fungal metabolite, before the administration of the insulin-transferrin complex. The potentiation by brefeldin A indicated that insulin absorption could be accomplished through a transferrin receptor-mediated transcytosis in the intestinal wall. 

PMN binding to and migration across endothelium initiates a sequence of events that resembles that following histamine treatment [14,30], namely, an increase in the permeability of the EC monolayer. For anionic plasma macromolecules, the plasmalemmal vesicles and transendothelial channels are suitable candidates for exit from the vessel lumen. Indeed, receptor-mediated transcytosis of insulin and transferrin has been identified for brain capillaries [31-33]. The insulin carriers have not yet been identified, but it is speculated that they could be coated vesicles and/or plasmalemmal vesicles. Transferrin [34] as well as ceruloplasmin [35,36] binding has been localized to coated pits and vesicles in the endothelium of bone marrow and liver capillary endothelium, respectively. 

R. Soda and M. Tavassoli. Transendothelial transport (transcytosis) of iron-transferrin complex in bone and marrow. J. Ultrastruct. Res. 88 18-25 (1984). 

To solve this problem, in vivo studies have been performed in rats using the AySl-40 conjugated with a monoclonal antibody specific to the rat transferrin receptor. The transferrin receptor allows receptor-mediated transcytosis through the rats BBB. Conjugation of the peptide with this antibody increased its brain uptake and decreased its plasma clearance [166]. To our... 

Certain proteins, such as insulin, transferrin, and insulin-like growth factors, cross the blood-brain barrier by receptor-mediated transcytosis. Once the protein binds to its membrane receptor, the membrane containing the receptor-protein complex is endocytosed into the endothelial cell to form a vesicle. It is released on the other side of the endothelial cell. Absorptive-mediated transcytosis also can occur. It differs from receptor-mediated transcytosis in that the protein binds nonspecifically to the membrane and not to a distinct receptor. 

Several strategies for increasing the permeability of the brain capillaries to proteins have been developed. The permeability of the BBB can be transiently increased by intra-arterial injection of the solutions with high osmolarity, which disrupts inter-endothelial tight junctions [11]. Certain protein modifications, such as cationization by hexamethyldiamine [12] and anionization by succinylation [13], produce enhanced uptake in the brain. Modification of drugs [14] and proteins [15] by linkage to an anti-transferrin receptor antibody also appears to enhance transport into the brain. This approach depends on receptor-mediated transcytosis of transferrin-receptor complexes by brain endothelial cells substantial uptake also occurs in the liver. 

Wiley DT, Webster P, Gale A, Davis M (2013) Transcytosis and brain uptake of transferrin-containing nanoparticles by tuning avidity to transferrin receptor. Proc Natl Acad Sci USA 110 8662-8667... 

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