## 2 Cellular Pathways Exploited by Toxins
### Endosome to Golgi Transport  
Toxins have turned out to be extremely useful to study endosome to Golgi transport. Several toxins need to be translocated to the Golgi on their way to the cytosol, and a useful early tool indicating the importance of this transport step for intoxication was the drug Brefeldin A (BFA), which, in many cell types, disrupts most of the Golgi apparatus and induces transport of Golgi cisternae back to the ER [85; 86; 87]. However, not all cells have a BFA-sensitive Golgi apparatus [88; 89; 90], and the drug BFA selectively protects against intoxication in cells with a BFA-sensitive Golgi apparatus [90]. Thus, one can study the effects of different molecules or drugs on cell intoxication to learn about interference with this transport step. Importantly, chemical modifications of genetically modified ricin or Shiga B-chain with sulfate in the trans Golgi or by glycosylation of ricin in the ER [13; 14] have facilitated our studies of this pathway. Moreover, electron and fluorescence microscopy have contributed to a large amount of the knowledge we have today about this transport. A combination of these methods is useful to validate the results, as molecules may be observed in a structure without necessarily moving through that structure. On the other hand, molecules can be transferred by short-lived efficient carriers, which are difficult to observe. Also, if the treatment of the cells changes the level of sulfation or glycosylation, then there is no easy way to \"compensate\" for this change, which could be due to reduced transport of cellular molecules through the cell and therefore less competition for sulfation or glycosylation. Alternatively, it could be due to lower activity of the enzymes involved. Studies performed many years ago with ricin and Shiga toxin (or the Shiga B chain) have revealed that, on their way from endosomes to the Golgi apparatus, toxin molecules did not have to be transported via late endosomes [91; 92]. It is interesting to see in the literature, that Shiga B chain is now mentioned as a well-established tool to study transport to the Golgi apparatus. An important difference between ricin and Shiga toxin is that the endosomal protein GPP130 was identified as a Shiga toxin-binding protein that helps in bringing this toxin to the Golgi apparatus [93; 94]. It should be noted that GPP130 does not interact with Shiga-like toxin 2 (Stc2), which is important for disease caused by toxin-secreting _E. coli_ species, and the mechanism behind transport of this toxin to the Golgi is not known. Over the years, different kinases, sorting nexins, Rab-proteins, Golgins, and SNARE-complexes have been shown to be involved in endosome to Golgi transport (for review, see [95; 96]). For instance, in the case of Shiga toxin, Rab6A' was found to be required for endosome to Golgi transport [97; 98]. For ricin, both Rab6A and Rab6A' seem to be involved in this transport step, as the best inhibition of transport was obtained by knocking down both isoforms [99]. Importantly, these studies have also illustrated the ability of cells to compensate. When knockdown of Rab6A was between 40% and 75%, an inhibition of transport was observed, but if knockdown was better than 75%, then there was an upregulation of Rab6A' and no inhibition of transport. Early experiments with PI-3 kinase inhibitors suggested an involvement of this enzyme for ricin transport to the Golgi [100], and this was later confirmed in studies with hVps34 mutants and siRNA [101]. At about the same time, different laboratories showed the involvement of sorting nexins for transport of both ricin and Shiga toxin to the Golgi apparatus [101; 102; 103], and the retromer and clathrin were also reported to be involved in Shiga toxin transport [104]. Shiga toxin also induces a dissociation of cPLA2\(\alpha\) from a complex with Annexin A1, and the active free form of the enzyme can then facilitate Golgi transport [41].