The retromer component sorting nexin-1 is required for efficient retrograde transport of Shiga toxin from early endosome to the trans Golgi network

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Abstract
The mammalian retromer complex is a multi-protein complex that regulates retrograde transport of the cation-independent mannose 6-phosphate receptor (CI-MPR) from early endosomes to the trans Golgi network (TGN). It consists of two subcomplexes: a membrane-bound coat comprising sorting nexin-1 (SNX1) and possibly sorting nexin-2 (SNX2), and a cargo-selective subcomplex, composed of VPS26, VPS29 and VPS35. In addition to the retromer, a variety of other protein complexes has been suggested to regulate endosome-to-TGN transport of not only the CI-MPR but a wide range of other cargo proteins. Here, we have examined the role of SNX1 and SNX2 in endosomal sorting of Shiga and cholera toxins, two toxins that undergo endosome-to-TGN transport en route to their cellular targets located within the cytosol. By using small interfering RNA (siRNA)-mediated silencing combined with single-cell fluorescent-toxin-uptake assays and well-established biochemical assays to analyze toxin delivery to the TGN, we have established that suppression of SNX1 leads to a significant reduction in the efficiency of endosome-to-TGN transport of the Shiga toxin B-subunit. Furthermore, we show that for the B subunit of cholera toxin, retrograde endosome-to-TGN transport is less reliant upon SNX1. Overall, our data establish a role for SNX1 in the endosome-to-TGN transport of Shiga toxin and are indicative for a fundamental difference between endosomal sorting of Shiga and cholera toxins into endosome-to-TGN retrograde transport pathways.