# Targeting the Early Endosome-to-Golgi Transport of Shiga Toxins as a Therapeutic Strategy
## 4 Small-Molecule Inhibitors Targeting Early Endosome-to-Golgi Transport of STx, STx1, and STx2
### Manganese  
A serendipitous discovery in the field was that exposure to 0.5-500 \(\upmu\)M manganese induces rapid degradation of GPP130, the endosomal receptor of STxB/STx1B (Figure 2B) [73; 74; 75]. Initial studies  
Figure 5: Small-molecule inhibitors of STx/STx1 and/or STx2 trafficking. Compound 134 (active concentration 50 \(\upmu\)M) and the _S_-enantiomer of retro2.1 (median effective concentration (EC\({}_{50}\)) 54 nM) were tested only against STx/STx1, but the parent retro-2 compound is active against both STx/STx1 and STx2 at 20 \(\upmu\)M. Tamoxifen is active against STx1 and STx2 at 2.5 to 10 \(\upmu\)M. Tamoxifen derivatives with the tertiary amine base (tested at active concentrations of 10 \(\upmu\)M) are protective against STx2, but were not tested against STx/STx1. Opemifene lacks the tertiary amine base and is not protective.  
revealed that the mechanism related to an increase in intra-Golgi manganese, which reroutes GPP130 to late endosomes/lysosomes from the Golgi apparatus for degradation (Figure 2B) [73; 74]. Subsequently, it was discovered that the increased intra-Golgi manganese levels induce GPP130 oligomerization, which acts as a trafficking signal to reroute the protein to the degradative pathway [76]. This line of work led to the important discovery of a novel quality control mechanism in the Golgi through which aggregated proteins, in general, are targeted to lysosomes for degradation [76; 77]. More relevant to the current discussion, as GPP130 is required for STxB/STx1B transport (Figure 2A), the protective effect of manganese against STx1 was then tested. Mechanistic assays in cell culture revealed that manganese blocks the early endosome-to-Golgi transport of STxB/STx1B; under these conditions, STxB/STx1B is rerouted to late endosomes/lysosomes and degraded (Figure 2B) [19]. This effect is rescued by expression of manganese-insensitive versions of GPP130 [19]. Moreover, in cells, treatment with 125-500 uM manganese provided a protection factor of 3800 against STx1-induced cell death [19]. Importantly, in mice, treatment with 10-50 mg manganese/kg conferred complete protection against STx1-induced lethality and kidney damage without inducing overt manganese toxicity [19]. However, since the early endosome-to-Golgi transport of STx2B is GPP130-independent, manganese does not protect cells against STx2 under the same conditions [10]. Further development assays (safety, non-human primate, and clinical studies) are necessary before it can be determined whether manganese may be a safe therapeutic option to treat human patients infected with STx-producing bacteria (e.g., _Shigella_). Particular concerns are off-target effects because GPP130 is required for the endosome-to-Golgi trafficking of host proteins, such as GP73 and TGN46 [26], as well as the possibility that Mn therapy may induce neurotoxicity, despite the likely short duration necessary to counter STx-induced disease [78]. Moreover, due to the lack of effect against STx2, manganese by itself is unlikely to be effective against STEC.