Shiga toxin is transported into the nucleoli of intestinal epithelial cells via a carrier-dependent process

Analysis of the Sequence Preference of Saporin by Deep Sequencing

Ribosome-inactivating proteins (RIPs) are RNA:adenosine glycosidases that inactivate eukaryotic ribosomes by depurinating the sarcin-ricin loop (SRL) in 28S rRNA. The GAGA sequence at the top of the SRL or at the top of a hairpin loop is assumed to be their target motif. Saporin is a RIP widely used to develop immunotoxins for research and medical applications, but its sequence specificity has not been investigated. Here, we combine the conventional aniline cleavage assay for depurinated nucleic acids with high-throughput sequencing to study sequence-specific depurination of oligonucleotides caused by saporin. Our data reveal the sequence preference of saporin for different substrates and show that the GAGA motif is not efficiently targeted by this protein, neither in RNA nor in DNA. Instead, a preference of saporin for certain hairpin DNAs was observed. The observed sequence-specific activity of saporin may be relevant to antiviral or apoptosis-inducing effects of RIPs. The developed method could also be useful for studying the sequence specificity of depurination by other RIPs or enzymes.

CRISPR Screen Reveals that EHEC's T3SS and Shiga Toxin Rely on Shared Host Factors for Infection

Enterohemorrhagic Escherichia coli (EHEC) has two critical virulence factors—a type III secretion system (T3SS) and Shiga toxins (Stxs)—that are required for the pathogen to colonize the intestine and cause diarrheal disease. Here, we carried out a genome-wide CRISPR/Cas9 (clustered regularly interspaced short palindromic repeats with Cas9) loss-of-function screen to identify host loci that facilitate EHEC infection of intestinal epithelial cells. Many of the guide RNAs identified targeted loci known to be associated with sphingolipid biosynthesis, particularly for production of globotriaosylceramide (Gb3), the Stx receptor. Two loci (TM9SF2 and LAPTM4A) with largely unknown functions were also targeted. Mutations in these loci not only rescued cells from Stx-mediated cell death, but also prevented cytotoxicity associated with the EHEC T3SS. These mutations interfered with early events associated with T3SS and Stx pathogenicity, markedly reducing entry of T3SS effectors into host cells and binding of Stx. The convergence of Stx and T3SS onto overlapping host targets provides guidance for design of new host-directed therapeutic agents to counter EHEC infection.

Enterohemorrhagic Escherichia coli (EHEC) has two critical virulence factors—a type III secretion system (T3SS) and Shiga toxins (Stxs)—that are required for colonizing the intestine and causing diarrheal disease. We screened a genome-wide collection of CRISPR mutants derived from intestinal epithelial cells and identified mutants with enhanced survival following EHEC infection. Many had mutations that disrupted synthesis of a subset of lipids (sphingolipids) that includes the Stx receptor globotriaosylceramide (Gb3) and hence protect against Stx intoxication. Unexpectedly, we found that sphingolipids also mediate early events associated with T3SS pathogenicity. Since antibiotics are contraindicated for the treatment of EHEC, therapeutics targeting sphingolipid biosynthesis are a promising alternative, as they could provide protection against both of the pathogen’s key virulence factors.

Shiga Toxins Activate the NLRP3 Inflammasome Pathway To Promote Both Production of the Proinflammatory Cytokine Interleukin-1β and Apoptotic Cell Death

Shiga toxin (Stx)-mediated immune responses, including the production of the proinflammatory cytokines tumor necrosis-α (TNF-α) and interleukin-1β (IL-1β), may exacerbate vascular damage and accelerate lethality. However, the immune signaling pathway activated in response to Stx is not well understood. Here, we demonstrate that enzymatically active Stx, which leads to ribotoxic stress, triggers NLRP3 inflammasome-dependent caspase-1 activation and IL-1β secretion in differentiated macrophage-like THP-1 (D-THP-1) cells. The treatment of cells with a chemical inhibitor of glycosphingolipid biosynthesis, which suppresses the expression of the Stx receptor globotriaosylceramide and subsequent endocytosis of the toxin, substantially blocked activation of the NLRP3 inflammasome and processing of caspase-1 and IL-1β. Processing and release of both caspase-1 and IL-1β were significantly reduced or abolished in Stx-intoxicated D-THP-1 cells in which the expression of NLRP3 or ASC was stably knocked down. Furthermore, Stx mediated the activation of caspases involved in apoptosis in an NLRP3- or ASC-dependent manner. In Stx-intoxicated cells, the NLRP3 inflammasome triggered the activation of caspase-8/3, leading to the initiation of apoptosis, in addition to caspase-1-dependent pyroptotic cell death. Taken together, these results suggest that Stxs trigger the NLRP3 inflammasome pathway to release proinflammatory IL-1β as well as to promote apoptotic cell death.

Analyzing of expression of novel polypeptide complexes consisting of Shiga toxin B subunit and Adherence Fimbriae of Escherichia coli based on in silico modeling

Enterohemorrhagic (EHEC) and enteroaggregative (EAEC) are two pathotypes of diarrheagenic Escherichia coli. EAEC strains express adhesins called aggregate adherence fimbriae (AAFs) which the bacteria use to adhere to intestinal mucosa. EHEC virulence factor is Shiga toxin which belongs to the AB5 toxin family. B subunit, the nontoxic part of Shiga toxin (StxB), forms a homo pentamer and is responsible for binding to target cells. StxB has recently been proven to have adjuvant activity. In the current study we fused StxB encoding gene to 3' end of genes encoding two variants of AAFs, i.e., AAF/I and AAF/II. The in silico studies on tertiary structure and biochemical characteristics of Shiga toxin A subunit (StxA) revealed more resemblance to AAF/II than AAF/I. The constructs were prepared in a way that StxB could imitate its natural structure (pentamer formation) and its position (C-terminus) in the native toxin complex. The expression of these constructs showed the formation of AAF/II-B as a protein complex but with lower molecular mass than its expected size. In contrast, the AAF/I-B complex was not formed. Overall, the results of in silico studies and expression experiments together revealed that despite AAF/II-B expression, StxB failed to form pentamer. Therefore the observed protein complex has lower molecular mass. Since StxB is bound to AAF/II through disulfide bond, this bond prevents pentamer formation of StxB. However, due to the lack of disulfide bond between AAF/I and StxB, no protein complex is formed, thus StxB maintains its pentamer structure.

The induction of apoptosis by Shiga toxins and ricin

Shiga toxins and ricin are ribosome-inactivating proteins which share the property of inhibiting protein synthesis by catalytic inactivation of eukaryotic ribosomes. There is now abundant evidence that Shiga toxins and ricin induce apoptosis in epithelial, endothelial, lymphoid and myeloid cells in vitro, and in multiple organs in animals when administered these toxins. Many studies suggest that protein synthesis inhibition and apoptosis induction mediated by Shiga toxins and ricin may be dissociated. In some cells, non-enzymatic toxin components (Shiga toxin B-subunits, ricin B-chain) appear capable of inducing apoptosis. The toxins appear capable of activating components of both the extrinsic or death receptor-mediated and intrinsic or mitochondrial-mediated pathways of apoptosis induction. Although the toxins have been shown to be capable of activating several cell stress response pathways, the precise signaling mechanisms by which Shiga toxins and ricin induce apoptosis remain to be fully characterized. This chapter provides an overview of studies describing Shiga toxin- and ricin-induced apoptosis and reviews evidence that signaling through the ribotoxic stress response and the unfolded protein response may be involved in apoptosis induction in some cell types.