Shiga toxin regulates its entry in a Syk-dependent manner

Passage through the Golgi is necessary for Shiga toxin B subunit to reach the endoplasmic reticulum

Both Shiga holotoxin and the isolated B subunit, navigate a retrograde pathway from the plasma membrane to the endoplasmic reticulum (ER) of mammalian cells to deliver catalytic A subunits into the cytosol. This route passes through early/recycling endosomes and then through the Golgi. Although passage through the endosomes takes only 30 min, passage through the Golgi is much slower, taking hours. This suggests that Golgi passage is a key step in retrograde traffic. However, there is no empirical data demonstrating that Golgi passage is required for the toxins to enter the ER. In fact, an alternate pathway bypassing the Golgi is utilized by SV40 virus. Here we find that blocking Shiga toxin B access to the entire Golgi with AlF(4)(-) treatment, temperature block or subcellular surgery prevented Shiga toxin B from reaching the ER. This suggests that there is no direct endosome to ER route available for retrograde traffic. Curiously, when Shiga toxin B was trapped in endosomes, it entered the cytosol directly from the endosomal compartment. Our results suggest that trafficking through the Golgi apparatus is required for Shiga toxin B to reach the ER and that diversion into the Golgi may prevent toxin escape from endosomes into the cytosol.

Clathrin-independent endocytosis: from nonexisting to an extreme degree of complexity

Today it is generally accepted that there are several endocytic mechanisms, both the clathrin-dependent one and mechanisms which operate without clathrin and with different requirements when it comes to dynamin, small GTP-binding proteins of the Rho family and specific lipids. It should be noted that clathrin-independent endocytosis can occur even when the cholesterol level in the membrane has been reduced to so low levels that caveolae are gone and clathrin-coated membrane areas are flat. Although new investigators in the field take it for granted that there is a multitude of entry mechanisms, it has taken a long time for this to become accepted. However, more work needs to be done, because one can still ask the question: How many endocytic mechanisms does a cell have, what are their function, and how are they regulated? This article describes some of the history of endocytosis research and attempts to give an overview of the complexity of the mechanisms and their regulation.

The Mitogen-activated protein kinase p38 links Shiga Toxin-dependent signaling and trafficking

Shiga toxin (Stx) binds to the cell, and it is transported via endosomes and the Golgi apparatus to the endoplasmic reticulum and cytosol, where it exerts its toxic effect. We have recently shown that Stx activates the tyrosine kinase Syk, which in turn induces clathrin phosphorylation and up-regulates Stx uptake. Here, we show that toxin-induced signaling can also regulate another step in intracellular Stx transport. We demonstrate that transport of Stx to the Golgi apparatus is dependent on the mitogen-activated protein kinase p38. Treatment of cells with chemical inhibitors or small interfering RNA targeting p38 inhibited Stx transport to the Golgi and reduced Stx toxicity. This p38 dependence is specific to Stx, because transport of the related toxin ricin was not affected by p38 inhibition. Stx rapidly activated p38, and recruited it to early endosomes in a Ca(2+)-dependent manner. Furthermore, agonist-induced oscillations in cytosolic Ca(2+) levels were inhibited upon Stx stimulation, possibly reflecting Stx-dependent local alterations in cytosolic Ca(2+) levels. Intracellular transport of Stx is Ca(2+) dependent, and we provide evidence that Stx activates a signaling cascade involving cross talk between Ca(2+) and p38, to regulate its trafficking to the Golgi apparatus.

Polyunsaturated fatty acids regulate Shiga toxin transport

Shiga toxin (Stx) is internalized by receptor-mediated endocytosis and transported retrogradely to the endoplasmic reticulum from where the enzymatically active part of the toxin is translocated to the cytosol. In this study, we have investigated the effect of polyunsaturated fatty acids (PUFA) on intoxication and retrograde transport of Stx. In HEp-2 cells, PUFA treatment inhibited Stx intoxication by a factor of 10. Moreover, both Stx internalization and endosome-to-Golgi transport were reduced by PUFA and these reductions can together explain the reduced toxicity. Also cholera toxin internalization was reduced by PUFA treatment. Finally, ricin and Pseudomonas exotoxin 1 cytotoxicity were not reduced by PUFA, demonstrating that PUFA do not cause a general block in retrograde transport to the endoplasmic reticulum. In conclusion, these results clearly demonstrate the importance of PUFA for Stx and cholera toxin trafficking.

Receptor affinity, stability and binding mode of Shiga toxins are determinants of toxicity

The closely related Shiga toxins, Shiga toxin 1 (Stx1) and Shiga toxin 2 (Stx2), can bind to Gb3 receptors. However, Stx2-producing enterohemorrhagic Escherichia coli (EHEC) strains are more commonly associated with serious human disease (viz., hemolytic-uremic syndrome) than Stx1-producing strains. To clarify the relationship between properties and toxicity of these toxins, we constructed and analyzed a hybrid holotoxin composed of Stx2A and Stx1B, designated as Stx2A1B, and a B subunit chimeric holotoxin composed of Stx2A and Stx2B (III V), designated as Stx2A2B (III V). The affinity of Stx2A1B to Gb3 was lower than that of Stx1, higher than that of Stx2 and identical to that of Stx2A2B (III V). On the other hand, the 50% lethal dose (LD(50)) for mice of Stx2A1B was lower than that of Stx1, but higher than that of Stx2. These results suggested that pathogenicity in mice was inversely related to the receptor affinity of the holotoxins. However, LD(50) of Stx2A1B was not identical to that of Stx2A2B (III V). Gel filtration analysis indicated that Stx2A2B (III V) was relatively less stable than Stx2A1B. Moreover, cross-linking experiments demonstrated that the modes of cell surface binding of Stx2A2B (III V) and Stx2A1B were different. These results indicated that the receptor affinity, stability and binding mode of Shiga toxins might be important determinants for toxicity in mice.

Protein Kinase Cδ Is Activated by Shiga Toxin and Regulates Its Transport

Protein kinase C (PKC) isozymes regulate different vesicular trafficking steps in the recycling or degradative pathways. However, a possible role of these kinases in the retrograde pathway from endosomes to the Golgi complex has previously not been investigated. We report here the involvement of a specific PKC isozyme, PKCdelta, in the intracellular transport of the glycolipid-binding Shiga toxin (Stx), which utilizes the retrograde pathway to intoxicate cells. Upon binding to cells, Stx was shown to specifically activate PKCdelta and not PKCalpha. The involvement of PKCdelta and PKCalpha in the retrograde transport of Stx was then monitored biochemically and by immunofluorescence after inhibition or depletion of the isozymes. PKCdelta, but not PKCalpha, was shown to selectively regulate the endosome-to-Golgi transport of StxB. Upon inhibition or knockdown of PKCdelta, StxB molecules colocalized less with giantin and more with EEA1, indicating that the molecules were accumulated in endosomes, unable to reach the Golgi complex. The inhibition of Golgi transport of Stx was reflected by a strong reduction in the toxic effect, demonstrating that transport of Stx to the cytosol is dependent on PKCdelta activity. These results are in agreement with our previous data, which show that Stx is able to stimulate its own transport.

Structural requirements for furin-induced cleavage and activation of Shiga toxin

Shiga toxin has a protease-sensitive site in the disulfide loop region of the A-chain. Cleavage of this site by furin is essential for rapid intoxication of cells by Shiga toxin. We have here investigated whether in addition to the Arg-X-X-Arg sequence, there are other structural requirements in the disulfide loop region for furin cleavage. A toxin mutant (Shiga-2D toxin) still containing the consensus motif for cleavage by furin, but lacking ten amino acids in the disulfide loop, was generated. Trypsin was able to cleave Shiga-2D toxin in vitro, demonstrating that the protease-sensitive region is intact. However, Shiga-2D toxin was not efficiently cleaved by furin either in vitro or in vivo. Furthermore, unless it was precleaved with trypsin, Shiga-2D toxin was much less toxic than wild type Shiga toxin in LoVo cells expressing functional furin. In contrast, LoVo/neo cells lacking functional furin were unable to activate both wild type Shiga toxin and Shiga-2D toxin. In conclusion, an extended loop structure is required for furin-induced cleavage of Shiga toxin.

Shiga toxin-producing Escherichia coli: an overview

The objective of this review is to highlight the importance of cattle in human disease due to Shiga toxin-producing Escherichia coli (STEC) and to discuss features of STEC that are important in human disease. Healthy dairy and beef cattle are a major reservoir of a diverse group of STEC that infects humans through contamination of food and water, as well as through direct contact. Infection of humans by STEC may result in combinations of watery diarrhea, bloody diarrhea, and hemolytic uremic syndrome. Systems of serotyping, subtyping, and virulence typing of STEC are used to aid in epidemiology, diagnosis, and pathogenesis studies. Severe disease and outbreaks of disease are most commonly due to serotype O157:H7, which, like most other highly pathogenic STEC, colonize the large intestine by means of a characteristic attaching and effacing lesion. This lesion is induced by a bacterial type III secretion system that injects effector proteins into the intestinal epithelial cell, resulting in profound changes in the architecture and metabolism of the host cell and intimate adherence of the bacteria. Severe disease in the form of bloody diarrhea and the hemolytic uremic syndrome is attributable to Shiga toxin (Stx), which exists as 2 major types, Stx1 and Stx2. The stx genes are encoded on temperate bacteriophages in the chromosome of the bacteria, and production and release of the toxin are highly dependent on induction of the phages. Regulation of the genes involved in induction of the attaching and effacing lesion, and production of Stx is complex. In addition to these genes that are clearly implicated in virulence, there are several putative virulence factors. A major public health goal is to prevent STEC-induced disease in humans. Studies aimed at understanding factors that affect carriage and shedding of STEC by cattle and factors that contribute to development of disease in humans are considered to be important in achieving this objective.

Functionally different pools of Shiga toxin receptor, globotriaosyl ceramide, in HeLa cells

Many studies have investigated the intracellular trafficking of Shiga toxin, but very little is known about the underlying dynamics of its cellular receptor, the glycosphingolipid globotriaosyl ceramide. In this study, we show that globotriaosyl ceramide is required not only for Shiga toxin binding to cells, but also for its intracellular trafficking. Shiga toxin induces globotriaosyl ceramide recruitment to detergent-resistant membranes, and subsequent internalization of the lipid. The globotriaosyl ceramide pool at the plasma membrane is then replenished from internal stores. Whereas endocytosis is not affected in the recovery condition, retrograde transport of Shiga toxin to the Golgi apparatus and the endoplasmic reticulum is strongly inhibited. This effect is specific, as cholera toxin trafficking on GM(1) and protein biosynthesis are not impaired. The differential behavior of both toxins is also paralleled by the selective loss of Shiga toxin association with detergent-resistant membranes in the recovery condition, and comparison of the molecular species composition of plasma membrane globotriaosyl ceramide indicates subtle changes in favor of unsaturated fatty acids. In conclusion, this study demonstrates the dynamic behavior of globotriaosyl ceramide at the plasma membrane and suggests that globotriaosyl ceramide-specific determinants, possibly its molecular species composition, are selectively required for efficient retrograde sorting on endosomes, but not for endocytosis.

About lipids and toxins

Many mono or multicellular organisms secrete soluble proteins, referred to as protein toxins, which alter the behavior of foreign, or target cells, possibly leading to their death. These toxins affect either the cell membrane by forming pores or modifying lipids, or some intracellular target. To reach this target, they must cross one of the cellular membranes, generally that of an intracellular organelle. As described in this minireview, lipids play crucial roles in the intoxication process of most if not all toxins, by allowing/promoting binding, endocytosis, trafficking and/or translocation into the cytoplasm.

Shiga toxin facilitates its retrograde transport by modifying microtubule dynamics

The bacterial exotoxin Shiga toxin is endocytosed by mammalian host cells and transported retrogradely through the secretory pathway before entering the cytosol. Shiga toxin also increases the levels of microfilaments and microtubules (MTs) upon binding to the cell surface. The purpose for this alteration in cytoskeletal dynamics is unknown. We have investigated whether Shiga toxin-induced changes in MT levels facilitate its intracellular transport. We have tested the effects of the Shiga toxin B subunit (STB) on MT-dependent and -independent transport steps. STB increases the rate of MT-dependent Golgi stack repositioning after nocodazole treatment. It also enhances the MT-dependent accumulation of transferrin in a perinuclear recycling compartment. By contrast, the rate of MT-independent transferrin recycling is not significantly different when STB is present. We found that STB normally requires MTs and dynein for its retrograde transport to the juxtanuclear Golgi complex and that STB increases MT assembly. Furthermore, we find that MT polymerization is limiting for STB transport in cells. These results show that STB-induced changes in cytoskeletal dynamics influence intracellular transport. We conclude that the increased rate of MT assembly upon Shiga toxin binding facilitates the retrograde transport of the toxin through the secretory pathway.