Transcytosis toxins

The apical clathrin-independent pathway is selectively stimulated by reagents that raise intracellular cAMP, such as mastoparan, fluoride, or cholera toxin. Apical endocytosis is also stimulated by brefeldin A (BFA) (106) or by PMA. For an excellent review on transcytosis, see Tuma and Hubbard (138). 

All botulin neurotoxins act in a similar way. They only differ in the amino-acid sequence of some protein parts (Prabakaran et al., 2001). Botulism symptoms are provoked both by oral ingestion and parenteral injection. Botulin toxin is not inactivated by enzymes present in the gastrointestinal tracts. Foodborne BoNT penetrates the intestinal barrier, presumably due to transcytosis. It is then transported to neuromuscular junctions within the bloodstream and blocks the secretion of the neurotransmitter acetylcholine. This results in muscle limpness and palsy caused by selective hydrolysis of soluble A-ethylmalemide-sensitive factor activating (SNARE) proteins which participate in fusion of synaptic vesicles with presynaptic plasma membrane. SNARE proteins include vesicle-associated membrane protein (VAMP), synaptobrevin, syntaxin, and synaptosomal associated protein of 25 kDa (SNAP-25). Their degradation is responsible for neuromuscular palsy due to blocks in acetylcholine transmission from synaptic terminals. In humans, palsy caused by BoNT/A lasts four to six months. 

Lencer Wl, Moe S, Rufo PA, et al. (1995) Transcytosis of cholera toxin subunits across model human intestinal epithelia. In Proc. Natl. Acad. Sci. USA 92 10094-10098. 

The B subunit of the E. coli heat-labile toxin binds to the brush border of intestinal epithelial cells in a highly specific, lectinlike manner. Uptake of this toxin and transcytosis to the basolateral side of the enterocytes was observed both in vivo (63) and in vitro (64). 

Staphylococcus aureus produces a set of proteins [e.g., staphylococcal enterotoxin A (SEA), SEB, toxic shock syndrome toxin 1 (TSST-1)], which act both as superantigens and toxins. Hamad and coworkers (66)found dose-dependent, facilitated transcytosis of SEB and TSST-1, but not SEA, in Caco-2 cells. They extended their studies in mice in vivo by showing that ingested SEB appears in the blood more efficiently than does SEA. 

Another potential explanation for the imique epidemiology of human botulism was provided in a study of botulinum toxin binding and transcytosis across polarized monolayers of two hiunan colon carcinoma cell lines (T-84 and Caco-2). Substantial binding of iodinated BoNT/A and BoNT/B to hiunan colon carcinoma cells was observed, while minimal binding of type Cl neuro-toxin was detected (Maksymowych and Simpson, 1998). Both type A and B neurotoxins were also efficiently taken up, transcytosed, and released, by the polarized human carcinoma cells, whereas minimal transcytosis of type Cl neurotoxin was observed. The patterns of neurotoxin transcytosis (A and B, but not Cl) observed in these human gut epithelial cell lines correlate with human susceptibility to foodbome botulism (Maksymowych and... 

Simpson, 1998). The authors speculated that since human tissues are fully sensitive to the neuromuscular blocking properties of Cl neurotoxin (Coffield et al., 1997 Eleopra et al., 2004), the relative absence of human foodbome type C botulism could be due to the inability of this toxin to penetrate from the gut to the general circulation. Human susceptibility to type Cl and D neurotoxins remains unclear however, clarification of this issue will be important in interpreting data derived both from in vitro studies on toxin transcytosis and from animal models for oral intoxication. 

Transwell experiments were also performed to investigate BoNT / A transcytosis across a human pulmonary adenocarcinoma cell line (Calu-3), the MDCK cell line, and a primary rat alveolar epithelial cell line (Park and Simpson, 2003). Efficient BoNT/A transcytosis in both directions across polarized Calu-3 monolayers was observed, while toxin transcytosis occurred at a much lower rate across MDCK cells. These findings were in agreement with previous work demonstrating that the efficiency of BoNT/A transcytosis across MEXZK mono-layers was much lower than that observed across gut epithelial cells (Maksymowych and Simpson, 1998). BoNT/ A transcytosis was also observed across primary rat alveolar cells, although at a slightly slower rate than that seen for the human adenocarcinoma cells (Park... 

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