Large clostridial cytotoxins

Exploring the Toxin-Mediated Mechanisms in Clostridioides difficile Infection

Clostridioides difficile infection (CDI) is the leading cause of nosocomial antibiotic-associated diarrhea, and colitis, with increasing incidence and healthcare costs. Its pathogenesis is primarily driven by toxins produced by the bacterium C. difficile, Toxin A (TcdA) and Toxin B (TcdB). Certain strains produce an additional toxin, the C. difficile transferase (CDT), which further enhances the virulence and pathogenicity of C. difficile. These toxins disrupt colonic epithelial barrier integrity, and induce inflammation and cellular damage, leading to CDI symptoms. Significant progress has been made in the past decade in elucidating the molecular mechanisms of TcdA, TcdB, and CDT, which provide insights into the management of CDI and the future development of novel treatment strategies based on anti-toxin therapies. While antibiotics are common treatments, high recurrence rates necessitate alternative therapies. Bezlotoxumab, targeting TcdB, is the only available anti-toxin, yet limitations persist, prompting ongoing research. This review highlights the current knowledge of the structure and mechanism of action of C. difficile toxins and their role in disease. By comprehensively describing the toxin-mediated mechanisms, this review provides insights for the future development of novel treatment strategies and the management of CDI.

Trauma-toxicology: concepts, causes, complications

Trauma and toxic substances are connected in several aspects. On the one hand, toxic substances can be the reason for traumatic injuries in the context of accidental or violent and criminal circumstances. Examples for the first scenario is the release of toxic gases, chemicals, and particles during house fires, and for the second scenario, the use of chemical or biological weapons in the context of terroristic activities. Toxic substances can cause or enhance severe, life-threatening trauma, as described in this review for various chemical warfare, by inducing a tissue trauma accompanied by break down of important barriers in the body, such as the blood-air or the blood-gut barriers. This in turn initiates a "vicious circle" as the contribution of inflammatory responses to the traumatic damage enhances the macro- and micro-barrier breakdown and often results in fatal outcome. The development of sophisticated methods for detection and identification of toxic substances as well as the special treatment of the intoxicated trauma patient is summarized in this review. Moreover, some highly toxic substances, such as the protein toxins from the pathogenic bacterium Clostridioides (C.) difficile, cause severe post-traumatic complications which significantly worsens the outcome of hospitalized patients, in particular in multiply injured trauma patients. Therefore, novel pharmacological options for the treatment of such patients are necessarily needed and one promising strategy might be the neutralization of the toxins that cause the disease. This review summarizes recent findings on the molecular and cellular mechanisms of toxic chemicals and bacterial toxins that contribute to barrier breakdown in the human body as wells pharmacological options for treatment, in particular in the context of intoxicated trauma patients. "trauma-toxicology" comprises concepts regrading basic research, development of novel pharmacological/therapeutic options and clinical aspects in the complex interplay and "vicious circle" of severe tissue trauma, barrier breakdown, pathogen and toxin exposure, tissue damage, and subsequent clinical complications.

Multimodal vaccination targeting the receptor binding domains of Clostridioides difficile toxins A and B with an attenuated Salmonella Typhimurium vector (YS1646) protects mice from lethal challenge

Developing a vaccine against Clostridioides difficile is a key strategy to protect the elderly. Two candidate vaccines using a traditional approach of intramuscular (IM) delivery of recombinant antigens targeting C. difficile toxins A (TcdA) and B (TcdB) failed to meet their primary endpoints in large phase 3 trials. To elicit a mucosal response against C. difficile, we repurposed an attenuated strain of Salmonella Typhimurium (YS1646) to deliver the receptor binding domains (rbd) of TcdA and TcdB to the gut-associated lymphoid tissues, to elicit a mucosal response against C. difficile. In this study, YS1646 candidates with either rbdA or rbdB expression cassettes integrated into the bacterial chromosome at the attTn7 site were generated and used in a short-course multimodal vaccination strategy that combined oral delivery of the YS1646 candidate(s) on days 0, 2, and 4 and IM delivery of recombinant antigen(s) on day 0. Five weeks after vaccination, mice had high serum IgG titers and increased intestinal antigen-specific IgA titers. Multimodal vaccination increased the IgG avidity compared to the IM-only control. In the mesenteric lymph nodes, we observed increased IL-5 secretion and increased IgA+ plasma cells. Oral vaccination skewed the IgG response toward IgG2c dominance (vs IgG1 dominance in the IM-only group). Both oral alone and multimodal vaccination against TcdA protected mice from lethal C. difficile challenge (100% survival vs 30% in controls). Given the established safety profile of YS1646, we hope to move this vaccine candidate forward into a phase I clinical trial.IMPORTANCEClostridioides difficile remains a major public health threat, and new approaches are needed to develop an effective vaccine. To date, the industry has focused on intramuscular vaccination targeting the C. difficile toxins. Multiple disappointing results in phase III trials have largely confirmed that this may not be the best strategy. As C. difficile is a pathogen that remains in the intestine, we believe that targeting mucosal immune responses in the gut will be a more successful strategy. We have repurposed a highly attenuated Salmonella Typhimurium (YS1646), originally pursued as a cancer therapeutic, as a vaccine vector. Using a multimodal vaccination strategy (both recombinant protein delivered intramuscularly and YS1646 expressing antigen delivered orally), we elicited both systemic and local immune responses. Oral vaccination alone completely protected mice from lethal challenge. Given the established safety profile of YS1646, we hope to move these vaccine candidates forward into a phase I clinical trial.

An Updated View on the Cellular Uptake and Mode-of-Action of Clostridioides difficile Toxins

Research on the human gut pathogen Clostridioides (C.) difficile and its toxins continues to attract much attention as a consequence of the threat to human health posed by hypervirulent strains. Toxin A (TcdA) and Toxin B (TcdB) are the two major virulence determinants of C. difficile. Both are single-chain proteins with a similar multidomain architecture. Certain hypervirulent C. difficile strains also produce a third toxin, namely binary toxin CDT (C. difficile transferase). C. difficile toxins are the causative agents of C. difficile-associated diseases (CDADs), such as antibiotics-associated diarrhea and pseudomembranous colitis. For that reason, considerable efforts have been expended to unravel their molecular mode-of-action and the cellular mechanisms responsible for their uptake. Many of these studies have been conducted in European laboratories. Here, we provide an update on our previous review (Papatheodorou et al. Adv Exp Med Biol, 2018) on important advances in C. difficile toxins research.

The TcdE holin drives toxin secretion and virulence in Clostridioides difficile

Clostridioides difficile is the leading cause of healthcare associated infections. The Pathogenicity Locus (PaLoc) toxins TcdA and TcdB promote host disease. These toxins lack canonical N-terminal signal sequences for translocation across the bacterial membrane, suggesting alternate mechanisms of release, which have included targeted secretion and passive release from cell lysis. While the holin TcdE has been implicated in TcdA and TcdB release, its role in vivo remains unknown. Here, we show profound reductions in toxin secretion in ΔtcdE mutants in the highly virulent strains UK1 (epidemic ribotype 027, Clade 3) and VPI10463 (ribotype 087, Clade 1). Notably, tcdE deletion in either strain rescued highly susceptible gnotobiotic mice from lethal infection by reducing acute extracellular toxin to undetectable levels, limiting mucosal damage, and enabling long-term survival, in spite of continued toxin gene expression in ΔtcdE mutants. Our findings confirm TcdE's critical functions in vivo for toxin secretion and C. difficile virulence.

Biofilm Formation of Clostridioides difficile, Toxin Production and Alternatives to Conventional Antibiotics in the Treatment of CDI

Clostridioides difficile is considered a nosocomial pathogen that flares up in patients exposed to antibiotic treatment. However, four out of ten patients diagnosed with C. difficile infection (CDI) acquired the infection from non-hospitalized individuals, many of whom have not been treated with antibiotics. Treatment of recurrent CDI (rCDI) with antibiotics, especially vancomycin (VAN) and metronidazole (MNZ), increases the risk of experiencing a relapse by as much as 70%. Fidaxomicin, on the other hand, proved more effective than VAN and MNZ by preventing the initial transcription of RNA toxin genes. Alternative forms of treatment include quorum quenching (QQ) that blocks toxin synthesis, binding of small anion molecules such as tolevamer to toxins, monoclonal antibodies, such as bezlotoxumab and actoxumab, bacteriophage therapy, probiotics, and fecal microbial transplants (FMTs). This review summarizes factors that affect the colonization of C. difficile and the pathogenicity of toxins TcdA and TcdB. The different approaches experimented with in the destruction of C. difficile and treatment of CDI are evaluated.

S-layer proteins as immune players: tales from pathogenic and non-pathogenic bacteria

In bacteria, as in other microorganisms, surface compounds interact with different pattern recognition receptors expressed by host cells, which usually triggers a variety of cellular responses that result in immunomodulation. The S-layer is a two-dimensional macromolecular crystalline structure formed by (glyco)-protein subunits that covers the surface of many species of Bacteria and almost all Archaea. In Bacteria, the presence of S-layer has been described in both pathogenic and non-pathogenic strains. As surface components, special attention deserves the role that S-layer proteins (SLPs) play in the interaction of bacterial cells with humoral and cellular components of the immune system. In this sense, some differences can be predicted between pathogenic and non-pathogenic bacteria. In the first group, the S-layer constitutes an important virulence factor, which in turn makes it a potential therapeutic target. For the other group, the growing interest to understand the mechanisms of action of commensal microbiota and probiotic strains has prompted the studies of the role of the S-layer in the interaction between the host immune cells and bacteria bearing this surface structure. In this review, we aim to summarize the main latest reports and the perspectives of bacterial SLPs as immune players, focusing on those from pathogenic and commensal/probiotic most studied species.

Detection and molecular characterization of Clostridium perfringens, Paeniclostridium sordellii and Clostridium septicum from lambs and goat kids with hemorrhagic abomasitis in Turkey

BACKGROUND

The pathogenic Clostridia cause neurotoxic, histotoxic and enterotoxic infections in humans and animals. Several Clostridium species have been associated with abomasitis in ruminants. The present study aimed to investigate the frequency, and the presence of virulence genes, of Clostridium perfringens, Paeniclostridium sordellii and Clostridium septicum in lambs and goat kids with hemorrhagic abomasitis.

RESULTS

A total of 38 abomasum samples, collected from lambs and goat kids of 1 week to 1 month of age in different farms located in eastern Turkey between 2021 and 2022, were evaluated by histopathology, culture and PCR. At necropsy, the abomasum of the animals was excessively filled with caseinized content and gas, and the abomasum mucosa was hemorrhagic in varying degrees. In histopathological evaluation, acute necrotizing hemorrhagic inflammation was noted in abomasum samples. The examination of swab samples by culture and PCR revealed that C. perfringens type A was the most frequently detected species (86.84%) either alone or in combination with other Clostridium species. P. sordellii, C. perfringens type F and C. septicum were also harboured in the samples, albeit at low rates. Beta2 toxin gene (cpb2) was found in three of C. perfringens type A positive samples.

CONCLUSION

It was suggested that vaccination of pregnant animals with toxoid vaccines would be beneficial in terms of protecting newborn animals against Clostridial infections. This study investigated the presence of clostridial toxin genes in abomasal samples for the first time in Turkey.

Neutralizing epitopes on Clostridioides difficile toxin A revealed by the structures of two camelid VHH antibodies

Toxin A (TcdA) and toxin B (TcdB) are two key virulence factors secreted by Clostridioides difficile, which is listed as an urgent threat by the CDC. These two large homologous exotoxins are mainly responsible for diseases associated with C. difficile infection (CDI) with symptoms ranging from diarrhea to life threatening pseudomembranous colitis. Single-domain camelid antibodies (VHHs) AH3 and AA6 are two potent antitoxins against TcdA, which when combined with two TcdB-targeting VHHs showed effective protection against both primary and recurrent CDI in animal models. Here, we report the co-crystal structures of AH3 and AA6 when they form complexes with the glucosyltransferase domain (GTD) and a fragment of the delivery and receptor-binding domain (DRBD) of TcdA, respectively. Based on these structures, we find that AH3 binding enhances the overall stability of the GTD and interferes with its unfolding at acidic pH, and AA6 may inhibit the pH-dependent conformational changes in the DRBD that is necessary for pore formation of TcdA. These studies reveal two functionally critical epitopes on TcdA and shed new insights into neutralizing mechanisms and potential development of epitope-focused vaccines against TcdA.

LDLR, LRP1, and Megalin redundantly participate in the uptake of Clostridium novyi alpha-toxin

Clostridium novyi alpha-toxin (Tcnα) is a potent exotoxin that induces severe symptoms including gas gangrene, myositis, necrotic hepatitis, and sepsis. Tcnα binds to sulfated glycosaminoglycans (sGAG) for cell-surface attachment and utilizes low-density lipoprotein receptor (LDLR) for rapid entry. However, it was also shown that Tcnα may use alternative entry receptors other than LDLR. Here, we define that LRP1 and Megalin can also facilitate the cellular entry of Tcnα by employing reconstitutive LDLR family proteins. LDLR, LRP1, and Megalin recognize Tcnα via their ligand-binding domains (also known as LDL receptor type A repeats). Notably, LDLR and LRP1 have contrasting expression levels in many different cells, thus the dominant entry receptor for Tcnα could be cell-type dependent. These findings together increase our knowledge of the Tcnα actions and further help to understand the pathogenesis of C. novyi infection-associated diseases.

Clostridium novyi alpha-toxin (Tcnα) also uses LRP1 and Megalin as cellular entry receptors besides LDLR, and this might be a response to cell-type dependent receptor availability for the exotoxin.

Activation of the Extracytoplasmic Function σ Factor σV in Clostridioides difficile Requires Regulated Intramembrane Proteolysis of the Anti-σ Factor RsiV

Clostridioides (Clostridium) difficile is one of the leading causes of nosocomial diarrhea. Lysozyme is a common host defense against many pathogenic bacteria. C. difficile exhibits high levels of lysozyme resistance, which is due in part to the extracytoplasmic functioning (ECF) σ factor, σ^V. It has been previously demonstrated that genes regulated by σ^V are responsible for peptidoglycan modifications that provide C. difficile with high lysozyme resistance. σ^V is not unique to C. difficile however, and its role in lysozyme resistance and its mechanism of activation has been well characterized in Bacillus subtilis where the anti-σ, RsiV, sequesters σ^V until lysozyme directly binds to RsiV, activating σ^V. However, it remains unclear if the mechanism of σ^V activation is similar in C. difficile. Here, we investigated how activation of σ^V is controlled in C. difficile by lysozyme. We found that C. difficile RsiV was degraded in the presence of lysozyme. We also found that disruption of a predicted signal peptidase cleavage site blocked RsiV degradation and σ^V activation, indicating that the site-1 protease is likely a signal peptidase. We also identified a conserved site-2 protease, RasP, that was required for site-2 cleavage of RsiV and σ^V activation in response to lysozyme. Combined with previous work showing RsiV directly binds lysozyme, these data suggested that RsiV directly binds lysozyme in C. difficile, which leads to RsiV destruction via cleavage at site-1 by signal peptidase and then at site-2 by RasP, ultimately resulting in σ^V activation and increased resistance to lysozyme. IMPORTANCE Clostridioides difficile is a major cause of hospital-acquired diarrhea and represents an urgent concern due to the prevalence of antibiotic resistance and the rate of recurrent infections. We previously showed that σ^V and the regulon under its control were involved in lysozyme resistance. We have also shown in B. subtilis that the anti-σ RsiV acts as a direct sensor for lysozyme. which results in the destruction of RsiV and activation of σ^V. Here, we described the proteases required for degradation of RsiV in C. difficile in response to lysozyme. Our data indicated that the mechanism is highly conserved between B. subtilis and C. difficile.

Human α-Defensin-6 Neutralizes Clostridioides difficile Toxins TcdA and TcdB by Direct Binding

Rising incidences and mortalities have drawn attention to Clostridioides difficile infections (CDIs) in recent years. The main virulence factors of this bacterium are the exotoxins TcdA and TcdB, which glucosylate Rho-GTPases and thereby inhibit Rho/actin-mediated processes in cells. This results in cell rounding, gut barrier disruption and characteristic clinical symptoms. So far, treatment of CDIs is limited and mainly restricted to some antibiotics, often leading to a vicious circle of antibiotic-induced disease recurrence. Here, we demonstrate the protective effect of the human antimicrobial peptide α-defensin-6 against TcdA, TcdB and the combination of both toxins in vitro and in vivo and unravel the underlying molecular mechanism. The defensin prevented toxin-mediated glucosylation of Rho-GTPases in cells and protected human cells, model epithelial barriers as well as zebrafish embryos from toxic effects. In vitro analyses revealed direct binding to TcdB in an SPR approach and the rapid formation of TcdB/α-defensin-6 complexes, as analyzed with fluorescent TcdB by time-lapse microscopy. In conclusion, the results imply that α-defensin-6 rapidly sequesters the toxin into complexes, which prevents its cytotoxic activity. These findings extend the understanding of how human peptides neutralize bacterial protein toxins and might be a starting point for the development of novel therapeutic options against CDIs.

Inhibition of Clostridioides difficile Toxins TcdA and TcdB by Ambroxol

Clostridioides (C.) difficile produces the exotoxins TcdA and TcdB, which are the predominant virulence factors causing C. difficile associated disease (CDAD). TcdA and TcdB bind to target cells and are internalized via receptor-mediated endocytosis. Translocation of the toxins’ enzyme subunits from early endosomes into the cytosol depends on acidification of endosomal vesicles, which is a prerequisite for the formation of transmembrane channels. The enzyme subunits of the toxins translocate into the cytosol via these channels where they are released after auto-proteolytic cleavage. Once in the cytosol, both toxins target small GTPases of the Rho/Ras-family and inactivate them by mono-glucosylation. This in turn interferes with actin-dependent processes and ultimately leads to the breakdown of the intestinal epithelial barrier and inflammation. So far, therapeutic approaches to treat CDAD are insufficient, since conventional antibiotic therapy does not target the bacterial protein toxins, which are the causative agents for the clinical symptoms. Thus, directly targeting the exotoxins represents a promising approach for the treatment of CDAD. Lately, it was shown that ambroxol (Ax) prevents acidification of intracellular organelles. Therefore, we investigated the effect of Ax on the cytotoxic activities of TcdA and TcdB. Ax significantly reduced toxin-induced morphological changes as well as the glucosylation of Rac1 upon intoxication with TcdA and TcdB. Most surprisingly, Ax, independent of its effects on endosomal acidification, decreased the toxins’ intracellular enzyme activity, which is mediated by a catalytic glucosyltransferase domain. Considering its undoubted safety profile, Ax might be taken into account as therapeutic option in the context of CDAD.

Clostridioides difficile: Current overview and future perspectives

The most common world-wide cause of antibiotic-associated infectious diarrhea and colitis is the toxin producing bacterium, Clostridioides difficile (C. difficile). Here we review the background and characteristics of the bacterium and the toxins produced together with the epidemiology and the complex pathogenesis that leads to a broad clinical spectrum of disease. The review describes the difficulties faced in obtaining a quick and accurate diagnosis despite the range of sensitive and specific diagnostic tools available. We also discuss the problem of disease recurrence and the importance of disease prevention. The high rates of infection recurrence mean that treatment strategies are constantly under review and we outline the diverse treatment options that are currently in use and explore the emerging treatment options of pulsed antibiotic use, microbial replacement therapies and the use of monoclonal antibodies. We summarize the future direction of treatment strategies which include the development of novel antibiotics, the administration of oral polyclonal antibody formulations, the use of vaccines, the administration of competitive non-toxigenic spores and the neutralization of antibiotics at the microbiota level. Future successful treatments will likely involve a combination of therapies to provide the most effective and robust approach to C. difficile management.

A Bioluminescent Sensor for Rapid Detection of PPEP-1, a Clostridioides difficile Biomarker

Current assays for Clostridioides difficile in nonhospital settings are outsourced and time-intensive, resulting in both delayed diagnosis and quarantining of infected individuals. We designed a more rapid point-of-care assay featuring a “turn-on” bioluminescent readout of a C. difficile-specific protease, PPEP-1. NanoLuc, a bright and stable luciferase, was “caged” with a PPEP-1-responsive peptide tail that inhibited luminescence. Upon proteolytic cleavage, the peptide was released and NanoLuc activity was restored, providing a visible readout. The bioluminescent sensor detected PPEP-1 concentrations as low as 10 nM. Sensor uncaging was achieved within minutes, and signal was captured using a digital camera. Importantly, the sensor was also functional at ambient temperature and compatible with fecal material, suggesting that it can be readily deployed in a variety of settings.

Structural basis for selective modification of Rho and Ras GTPases by Clostridioides difficile toxin B

Toxin B (TcdB) is a primary cause of Clostridioides difficile infection (CDI). This toxin acts by glucosylating small GTPases in the Rho/Ras families, but the structural basis for TcdB recognition and selectivity of specific GTPase substrates remain unsolved. Here, we report the cocrystal structures of the glucosyltransferase domain (GTD) of two distinct TcdB variants in complex with human Cdc42 and R-Ras, respectively. These structures reveal a common structural mechanism by which TcdB recognizes Rho and R-Ras. Furthermore, we find selective clustering of adaptive residue changes in GTDs that determine their substrate preferences, which helps partition all known TcdB variants into two groups that display distinct specificities toward Rho or R-Ras. Mutations that selectively disrupt GTPases binding reduce the glucosyltransferase activity of the GTD and the toxicity of TcdB holotoxin. These findings establish the structural basis for TcdB recognition of small GTPases and reveal strategies for therapeutic interventions for CDI.

Low-density lipoprotein receptor-related protein 1 is a CROPs-associated receptor for Clostridioides difficile toxin B

As the leading cause of worldwide hospital-acquired infection, Clostridioides difficile (C. difficile) infection has caused heavy economic and hospitalized burden, while its pathogenesis is not fully understood. Toxin B (TcdB) is one of the major virulent factors of C. difficile. Recently, CSPG4 and FZD2 were reported to be the receptors that mediate TcdB cellular entry. However, genetic ablation of genes encoding these receptors failed to completely block TcdB entry, implicating the existence of alternative receptor(s) for this toxin. Here, by employing the CRISPR-Cas9 screen in CSPG4-deficient HeLa cells, we identified LDL receptor-related protein-1 (LRP1) as a novel receptor for TcdB. Knockout of LRP1 in both CSPG4-deficient HeLa cells and colonic epithelium Caco2 cells conferred cells with increased TcdB resistance, while LRP1 overexpression sensitized cells to TcdB at a low concentration. Co-immunoprecipitation assay showed that LRP1 interacts with full-length TcdB. Moreover, CROPs domain, which is dispensable for TcdB's interaction with CSPG4 and FZD2, is sufficient for binding to LRP1. As such, our study provided evidence for a novel mechanism of TcdB entry and suggested potential therapeutic targets for treating C. diff.

Structural basis for CSPG4 as a receptor for TcdB and a therapeutic target in Clostridioides difficile infection

C. difficile is a major cause of antibiotic-associated gastrointestinal infections. Two C. difficile exotoxins (TcdA and TcdB) are major virulence factors associated with these infections, and chondroitin sulfate proteoglycan 4 (CSPG4) is a potential receptor for TcdB, but its pathophysiological relevance and the molecular details that govern recognition remain unknown. Here, we determine the cryo-EM structure of a TcdB–CSPG4 complex, revealing a unique binding site spatially composed of multiple discontinuous regions across TcdB. Mutations that selectively disrupt CSPG4 binding reduce TcdB toxicity in mice, while CSPG4-knockout mice show reduced damage to colonic tissues during C. difficile infections. We further show that bezlotoxumab, the only FDA approved anti-TcdB antibody, blocks CSPG4 binding via an allosteric mechanism, but it displays low neutralizing potency on many TcdB variants from epidemic hypervirulent strains due to sequence variations in its epitopes. In contrast, a CSPG4-mimicking decoy neutralizes major TcdB variants, suggesting a strategy to develop broad-spectrum therapeutics against TcdB.

Chondroitin sulfate proteoglycan 4 (CSPG4) is a potential receptor for C. difficile toxin B (TcdB) during C. difficile infections (CDIs). Here, the cryo-EM structure of a TcdB–CSPG4 complex and CDI mouse models offer insights into CSPG4 role in CDIs and suggest a therapeutic strategy targeting TcdB.

Pharmacological Modulators of Small GTPases of Rho Family in Neurodegenerative Diseases

Classical Rho GTPases, including RhoA, Rac1, and Cdc42, are members of the Ras small GTPase superfamily and play essential roles in a variety of cellular functions. Rho GTPase signaling can be turned on and off by specific GEFs and GAPs, respectively. These features empower Rho GTPases and their upstream and downstream modulators as targets for scientific research and therapeutic intervention. Specifically, significant therapeutic potential exists for targeting Rho GTPases in neurodegenerative diseases due to their widespread cellular activity and alterations in neural tissues. This study will explore the roles of Rho GTPases in neurodegenerative diseases with focus on the applications of pharmacological modulators in recent discoveries. There have been exciting developments of small molecules, nonsteroidal anti-inflammatory drugs (NSAIDs), and natural products and toxins for each classical Rho GTPase category. A brief overview of each category followed by examples in their applications will be provided. The literature on their roles in various diseases [e.g., Alzheimer's disease (AD), Parkinson's disease (PD), Amyotrophic lateral sclerosis (ALS), Frontotemporal dementia (FTD), and Multiple sclerosis (MS)] highlights the unique and broad implications targeting Rho GTPases for potential therapeutic intervention. Clearly, there is increasing knowledge of therapeutic promise from the discovery of pharmacological modulators of Rho GTPases for managing and treating these conditions. The progress is also accompanied by the recognition of complex Rho GTPase modulation where targeting its signaling can improve some aspects of pathogenesis while exacerbating others in the same disease model. Future directions should emphasize the importance of elucidating how different Rho GTPases work in concert and how they produce such widespread yet different cellular responses during neurodegenerative disease progression.

Clostridioides difficile toxins enhanced the in vitro production of CXC chemokine ligand 2 and tumor necrosis factor-α via Toll-like receptors in macrophages

Introduction. Clostridioides difficile infection (CDI) causes toxin-mediated enteropathy, such as antibiotic-associated diarrhoea and pseudomembranous colitis. Rho-glucosylating toxin A (TcdA) and toxin B (TcdB) have been clearly implicated in pathogenesis, whereas the virulence of binary toxin (CDT) is still debated.Hypothesis statement. We hypothesized that CDT is involved in the host immune response and plays a pivotal role in establishing virulence by modulating pro-inflammatory cytokine production; this is achieved through the integral Toll-like receptor (TLR) signalling pathways.Aim. The aim of the present study was to determine whether and how CDT impacts macrophages compared to TcdA or TcdB by examining the induction of CXC chemokine ligand 2 (CXCL2) and tumour necrosis factor-α (TNF-α), both of which are crucial in mediating local and systematic inflammatory responses.Methodology. RAW264.7 cells or transfected human embryonic kidney (HEK) 293 T cells were incubated with TcdA, TcdB, or CDT. In some experiments, a neutralizing antibody against TLR2 or TLR4, or myeloid differentiation 88 inhibitory peptide were added. The amount of CXCL2 and TNF-α secreted was then measured.Results. In RAW264.7 macrophages, CXCL2 and TNF-α were produced via the Toll-like receptor 2 (TLR2) or Toll-like receptor 4 (TLR4) pathway in a TcdA, TcdB, or CDT dose-dependent manner. Interleukin-8 secretion was induced in TLR4/MD2/CD14-transfected, but not in TLR2-transfected, HEK 293 T cells following TcdB or CDT exposure.Conclusion. Our results showed that C. difficile toxins, including CDT, enhanced macrophage-mediated CXCL2 and TNF-α production via TLR2 and TLR4, indicating that CDT affects host immune responses.

Avian intestinal ultrastructure changes provide insight into the pathogenesis of enteric diseases and probiotic mode of action

Epithelial damage and loss of barrier integrity occur following intestinal infections in humans and animals. Gut health was evaluated by electron microscopy in an avian model that exposed birds to subclinical necrotic enteritis (NE) and fed them a diet supplemented with the probiotic Bacillus amyloliquefaciens strain H57 (H57). Scanning electron microscopy of ileal mucosa revealed significant villus damage, including focal erosions of epithelial cells and villous atrophy, while transmission electron microscopy demonstrated severe enterocyte damage and loss of cellular integrity in NE-exposed birds. In particular, mitochondria were morphologically altered, appearing irregular in shape or swollen, and containing electron-lucent regions of matrix and damaged cristae. Apical junctional complexes between adjacent enterocytes were significantly shorter, and the adherens junction was saccular, suggesting loss of epithelial integrity in NE birds. Segmented filamentous bacteria attached to villi, which play an important role in intestinal immunity, were more numerous in birds exposed to NE. The results suggest that mitochondrial damage may be an important initiator of NE pathogenesis, while H57 maintains epithelium and improves the integrity of intestinal mucosa. Potential actions of H57 are discussed that further define the mechanisms responsible for probiotic bacteria’s role in maintaining gut health.

Bacterial intracellularly active toxins: Membrane localisation of the active domain

Numerous bacterial toxins exert their activity by inactivating or modulating a specific intracellular host target. For this purpose, these toxins have developed efficient strategies to overcome the different host cell defences including specific binding to cell surface, internalisation, passage through the endosome or plasma membrane, exploiting intracellular trafficking and addressing to intracellular targets. Several intracellularly active toxins deliver an active domain into the cytosol that interacts with a target localised to the inner face of the plasma membrane. Thus, the large clostridial glucosylating toxins (LCGTs) target Rho/Ras-GTPases, certain virulence factors of Gram negative bacteria, Rho-GTPases, while Pasteurella multocida toxin (PMT) targets trimeric G-proteins. Others such as botulinum neurotoxins and tetanus neurotoxin have their substrate on synaptic vesicle membrane. LCGTs, PMT, and certain virulence factors from Vibrio sp. show a particular structure constituted of a four-helix bundle membrane (4HBM) protruding from the catalytic site that specifically binds to the membrane phospholipids and then trap the catalytic domain at the proximity of the membrane anchored substrate. Structural and functional analysis indicate that the 4HBM tip of the Clostridium sordellii lethal toxin (TcsL) from the LCGT family contain two loops forming a cavity that mediates the binding to phospholipids and more specifically to phosphatidylserine.

A Drosophila Model for Clostridium difficile Toxin CDT Reveals Interactions with Multiple Effector Pathways

Clostridium difficile infections (CDIs) cause severe and occasionally life-threatening diarrhea. Hyper-virulent strains produce CDT, a toxin that ADP-ribosylates actin monomers and inhibits actin polymerization. We created transgenic Drosophila lines expressing the catalytic subunit CDTa to investigate its interaction with host signaling pathways in vivo. When expressed in the midgut, CDTa reduces body weight and fecal output and compromises survival, suggesting severe impairment of digestive functions. At the cellular level, CDTa induces F-actin network collapse, elimination of the intestinal brush border, and disruption of intercellular junctions. We confirm toxin-dependent re-distribution of Rab11 to enterocytes' apical surface and observe suppression of CDTa phenotypes by a Dominant-Negative form of Rab11 or RNAi of the dedicated Rab11GEF Crag (DENND4). We also report that Calmodulin (Cam) is required to mediate CDTa activity. In parallel, chemical inhibition of the Cam/Calcineurin pathway by Cyclosporin A or FK506 also reduces CDTa phenotypes, potentially opening new avenues for treating CDIs.

Human Serum Albumin Is an Essential Component of the Host Defense Mechanism Against Clostridium difficile Intoxication

Background

The pathogenic effects of Clostridium difficile are primarily attributable to the production of the large protein toxins (C difficile toxins [Tcd]) A (TcdA) and B (TcdB). These toxins monoglucosylate Rho GTPases in the cytosol of host cells, causing destruction of the actin cytoskeleton with cytotoxic effects. Low human serum albumin (HSA) levels indicate a higher risk of acquiring and developing a severe C difficile infection (CDI) and are associated with recurrent and fatal disease.

Methods

We used a combined approach based on docking simulation and biochemical analyses that were performed in vitro on purified proteins and in human epithelial colorectal adenocarcinoma cells (Caco-2), and in vivo on stem cell-derived human intestinal organoids and zebrafish embryos.

Results

Our results show that HSA specifically binds via its domain II to TcdA and TcdB and thereby induces their autoproteolytic cleavage at physiological concentrations. This process impairs toxin internalization into the host cells and reduces the toxin-dependent glucosylation of Rho proteins.

Conclusions

Our data provide evidence for a specific HSA-dependent self-defense mechanism against C difficile toxins and provide an explanation for the clinical correlation between CDI severity and hypoalbuminemia.

The Antibiotic Bacitracin Protects Human Intestinal Epithelial Cells and Stem Cell-Derived Intestinal Organoids from Clostridium difficile Toxin TcdB

Bacitracin is an established antibiotic for local application and inhibits the cell wall synthesis of Gram-positive bacteria. Recently, we discovered a completely different mode of action of bacitracin and reported that this drug protects human cells from intoxication by a variety of medically relevant bacterial protein toxins including CDT, the binary actin ADP-ribosylating toxin of Clostridium (C.) difficile. Bacitracin prevents the transport of CDT into the cytosol of target cells, most likely by inhibiting the transport function of the binding subunit of this toxin. Here, we tested the effect of bacitracin towards TcdB, a major virulence factor of C. difficile contributing to severe C. difficile-associated diseases (CDAD) including pseudomembranous colitis. Bacitracin protected stem cell-derived human intestinal organoids as well as human gut epithelial cells from intoxication with TcdB. Moreover, it prevented the TcdB-induced disruption of epithelia formed by gut epithelium cells in vitro and maintained the barrier function as detected by measuring transepithelial electrical resistance (TEER). In the presence of bacitracin, TcdB was not able reach its substrate Rac1 in the cytosol of human epithelial cells, most likely because its pH-dependent transport across cell membranes into the cytosol is decreased by bacitracin. In conclusion, in addition to its direct antibiotic activity against C. difficile and its inhibitory effect towards the toxin CDT, bacitracin neutralizes the exotoxin TcdB of this important pathogenic bacterium.

Structure of the full-length Clostridium difficile toxin B

Clostridium difficile is an opportunistic pathogen that establishes in the colon when the gut microbiota are disrupted by antibiotics or disease. C. difficile infection (CDI) is largely caused by two virulence factors, TcdA and TcdB. Here, we report a 3.87-Å-resolution crystal structure of TcdB holotoxin that captures a unique conformation of TcdB at endosomal pH. Complementary biophysical studies suggest that the C-terminal combined repetitive oligopeptides (CROPs) domain of TcdB is dynamic and can sample open and closed conformations that may facilitate modulation of TcdB activity in response to environmental and cellular cues during intoxication. Furthermore, we report three crystal structures of TcdB-antibody complexes that reveal how antibodies could specifically inhibit the activities of individual TcdB domains. Our studies provide novel insight into the structure and function of TcdB holotoxin and identify intrinsic vulnerabilities that could be exploited to develop new therapeutics and vaccines for the treatment of CDI.

Quantitative Phosphoproteome Analysis of Clostridioides difficile Toxin B Treated Human Epithelial Cells

The large clostridial glucosylating toxin B (TcdB) is a major virulence factor of the nosocomial pathogen Clostridioides difficile. TcdB inhibits small GTPases by glucosylation leading to impaired downstream signaling. TcdB also possesses a glucosyltransferase independent effect described as pyknosis. To elucidate the impact of TcdB and its glucosylation-inactive mutant TcdB_NXN on the kinome of human cells, SILAC labeled HEp-2 cells were treated with 2 nM TcdB for 8 h. Phosphopeptides were enriched using SCX chromatography, IMAC and TiO₂ followed shotgun mass spectrometry analysis. Overall 4,197 phosphopeptides were identified; more than 1,200 phosphosites responded to treatment with TcdB or TcdB_NXN. The data suggested that predominantly stress-activated MAPK-dependent signaling pathways were triggered by toxin B treatment.

The Conserved Cys-2232 in Clostridioides difficile Toxin B Modulates Receptor Binding

Clostridioides difficile toxins TcdA and TcdB are large clostridial glucosyltransferases which are the main pathogenicity factors in C. difficile-associated diseases. Four highly conserved cysteines are present in all large clostridial glucosyltransferases. In this study we focused on the conserved cysteine 2232 within the combined repetitive oligopeptide domain of TcdB from reference strain VPI10463 (clade I). Cysteine 2232 is not present in TcdB from hypervirulent strain R20291 (clade II), where a tyrosine is found instead. Replacement of cysteine 2232 by tyrosine in TcdB_{V PI10463} reduced binding to the soluble fragments of the two known TcdB receptors, frizzled-2 (FZD2) and poliovirus receptor-like protein-3/nectin-3 (PVRL3). In line with this, TcdB_R20291 showed weak binding to PVRL3 in pull-down assays which was increased when tyrosine 2232 was exchanged for cysteine. Surprisingly, we did not observe binding of TcdB_R20291 to FZD2, indicating that this receptor is less important for this toxinotype. Competition assay with the receptor binding fragments (aa 1101–1836) of TcdB_{V PI10463} and TcdB_R20291, as well as antibodies newly developed by antibody phage display, revealed different characteristics of the yet poorly described delivery domain of TcdB harboring the second receptor binding region. In summary, we found that conserved Cys-2232 in TcdB indirectly contributes to toxin–receptor interaction.

The Binary Toxin CDT of Clostridium difficile as a Tool for Intracellular Delivery of Bacterial Glucosyltransferase Domains

Binary toxins are produced by several pathogenic bacteria. Examples are the C2 toxin from Clostridium botulinum, the iota toxin from Clostridium perfringens, and the CDT from Clostridium difficile. All these binary toxins have ADP-ribosyltransferases (ADPRT) as their enzymatically active component that modify monomeric actin in their target cells. The binary C2 toxin was intensively described as a tool for intracellular delivery of allogenic ADPRTs. Here, we firstly describe the binary toxin CDT from C. difficile as an effective tool for heterologous intracellular delivery. Even 60 kDa glucosyltransferase domains of large clostridial glucosyltransferases can be delivered into cells. The glucosyltransferase domains of five tested large clostridial glucosyltransferases were successfully introduced into cells as chimeric fusions to the CDTa adapter domain (CDTaN). Cell uptake was demonstrated by the analysis of cell morphology, cytoskeleton staining, and intracellular substrate glucosylation. The fusion toxins were functional only when the adapter domain of CDTa was N-terminally located, according to its native orientation. Thus, like other binary toxins, the CDTaN/b system can be used for standardized delivery systems not only for bacterial ADPRTs but also for a variety of bacterial glucosyltransferase domains.

Cysteine Protease-Mediated Autocleavage of Clostridium difficile Toxins Regulates Their Proinflammatory Activity

BACKGROUND & AIMS

Clostridium difficile toxin A (TcdA) and C difficile toxin toxin B (TcdB), the major virulence factors of the bacterium, cause intestinal tissue damage and inflammation. Although the 2 toxins are homologous and share a similar domain structure, TcdA is generally more inflammatory whereas TcdB is more cytotoxic. The functional domain of the toxins that govern the proinflammatory activities of the 2 toxins is unknown.

METHODS

Here, we investigated toxin domain functions that regulate the proinflammatory activity of C difficile toxins. By using a mouse ilea loop model, human tissues, and immune cells, we examined the inflammatory responses to a series of chimeric toxins or toxin mutants deficient in specific domain functions.

RESULTS

Blocking autoprocessing of TcdB by mutagenesis or chemical inhibition, while reducing cytotoxicity of the toxin, significantly enhanced its proinflammatory activities in the animal model. Furthermore, a noncleavable mutant TcdB was significantly more potent than the wild-type toxin in the induction of proinflammatory cytokines in human colonic tissues and immune cells.

CONCLUSIONS

In this study, we identified a novel mechanism of regulating the biological activities of C difficile toxins in that cysteine protease-mediated autoprocessing regulates toxins' proinflammatory activities. Our findings provide new insight into the pathogenesis of C difficile infection and the design of therapeutics against the disease.

Quantification of small GTPase glucosylation by clostridial glucosylating toxins using multiplexed MRM analysis

Large clostridial toxins mono-O-glucosylate small GTPases of the Rho and Ras subfamily. As a result of glucosylation, the GTPases are inhibited and thereby corresponding downstream signaling pathways are disturbed. Current methods for quantifying the extent of glucosylation include sequential [¹⁴ C]glucosylation, sequential [³² P]ADP-ribosylation, and Western Blot detection of nonglucosylated GTPases, with neither method allowing the quantification of the extent of glucosylation of an individual GTPase. Here, we describe a novel MS-based multiplexed MRM assay to specifically quantify the glucosylation degree of small GTPases. This targeted proteomics approach achieves a high selectivity and reproducibility, which allows determination of the in vivo substrate pattern of glucosylating toxins. As proof of principle, GTPase glucosylation was analyzed in CaCo-2 cells treated with TcdA, and glucosylation kinetics were determined for RhoA/B, RhoC, RhoG, Ral, Rap1, Rap2, (H/K/N)Ras, and R-Ras2.

Glucosyltransferase Activity of Clostridium difficile Toxin B Triggers Autophagy-mediated Cell Growth Arrest

Autophagy is a bulk cell-degradation process that occurs through the lysosomal machinery, and many reports have shown that it participates in microbial pathogenicity. However, the role of autophagy in Clostridium difficile infection (CDI), the leading cause of antibiotics-associated diarrhea, pseudomembranous colitis and even death in severe cases, is not clear. Here we report that the major virulent factor toxin B (TcdB) of Clostridium difficile elicits a strong autophagy response in host cells through its glucosyltransferase activity. Using a variety of autophagy-deficient cell lines, i.e. HeLa/ATG7^−/−, MEF/atg7^−/−, MEF/tsc2^−/−, we demonstrate that toxin-triggered autophagy inhibits host cell proliferation, which contributes to TcdB-caused cytopathic biological effects. We further show that both the PI3K complex and mTOR pathway play important roles in this autophagy induction process and consequent cytopathic event. Although the glucosyltransferase activity of TcdB is responsible for inducing both cell rounding and autophagy, there is no evidence suggesting the causal relationship between these two events. Taken together, our data demonstrate for the first time that the glucosyltransferase enzymatic activity of a pathogenic bacteria is responsible for host autophagy induction and the following cell growth arrest, providing a new paradigm for the role of autophagy in host defense mechanisms upon pathogenic infection.

Nonantimicrobial drug targets for Clostridium difficile infections

Clostridium difficile infection (CDI) is a major public health problem worldwide. Treatment has become complicated due to the emergence of strains with increased toxigenicity and sporulation rate, together with rampant antibiotics use that disrupts colonization resistance of the colonic microbiota. As a result, there is a critical need for nonantibiotic treatments. Therapies based on inhibiting the toxins, bacterial structures responsible for colonization, virulence and restoration of the gut microbiota are the most important nonantibiotic targets to combat CDI. This report outlines these targets and how they could become the focus of future therapeutic agents. Inhibiting colonization and virulence factors during CDI will disrupt pathogen persistence and decrease exposure to the inflammatory toxins, allowing the immune system to clear the infection.

Glucosyltransferase-dependent and -independent effects of TcdB on the proteome of HEp-2 cells

Toxin B (TcdB) of the nosocomial pathogen C. difficile has been reported to exhibit a glucosyltransferase-dependent and -independent effect on treated HEp-2 cells at toxin concentration above 0.3 nM. In order to investigate and further characterize both effects epithelial cells were treated with wild type TcdB and glucosyltransferase-deficient TcdB_NXN and their proteomes were analyzed by LC-MS. Triplex SILAC labeling was used for quantification. Identification of 5212 and quantification of 4712 protein groups was achieved. Out of these 257 were affected by TcdB treatment, 92 by TcdB_NXN treatment and 49 by both. TcdB mainly led to changes in proteins that are related to "GTPase mediated signaling" and the "cytoskeleton" while "chromatin" and "cell cycle" related proteins were altered by both, TcdB and TcdB_NXN . The obtained dataset of HEp-2 cell proteome helps us to better understand glucosyltransferase-dependent and -independent mechanisms of TcdB and TcdB_NXN , particularly those involved in pyknotic cell death. All proteomics data have been deposited in the ProteomeXchange with the dataset identifier PXD006658 (https://proteomecentral.proteomexchange.org/dataset/PXD006658).

Overexpression of the Endosomal Anion/Proton Exchanger ClC-5 Increases Cell Susceptibility toward Clostridium difficile Toxins TcdA and TcdB

Virulent C. difficile toxins TcdA and TcdB invade host intestinal epithelia by endocytosis and use the acidic environment of intracellular vesicles for further processing and activation. We investigated the role of ClC-5, a chloride/proton exchanger expressed in the endosomes of gastrointestinal epithelial cells, in the activation and processing of C. difficile toxins. Enhanced intoxication by TcdA and TcdB was observed in cells expressing ClC-5 but not ClC-4, another chloride/proton exchanger with similar function but different localization. In accordance with the established physiological function of ClC-5, its expression lowered the endosomal pH in HEK293T cells by approximately 0.6 units and enhanced approximately 5-fold the internalization of TcdA. In colon HT29 cells, 34% of internalized TcdA localized to ClC-5-containing vesicles defined by colocalization with Rab5, Rab4a, and Rab7 as early and early-to-late of endosomes but not as Rab11-containing recycling endosomes. Impairing the cellular uptake of TcdA by deleting the toxin CROPs domain did not abolish the effects of ClC-5. In addition, the transport-incompetent mutant ClC-5 E268Q similarly enhanced both endosomal acidification and intoxication by TcdA but facilitated the internalization of the toxin to a lower extent. These data suggest that ClC-5 enhances the cytotoxic action of C. difficile toxins by accelerating the acidification and maturation of vesicles of the early and early-to-late endosomal system. The dispensable role of electrogenic ion transport suggests that the voltage-dependent nonlinear capacitances of mammalian CLC transporters serve important physiological functions. Our data shed light on the intersection between the endocytotic cascade of host epithelial cells and the internalization pathway of the large virulence C. difficile toxins. Identifying ClC-5 as a potential specific host ion transporter hijacked by toxins produced by pathogenic bacteria widens the horizon of possibilities for novel therapies of life-threatening gastrointestinal infections.

Evidence for dual receptor-binding sites in Clostridium difficile toxin A

TcdA (308 kDa) and TcdB (270 kDa) disrupt the integrity of the intestinal epithelial barrier and provide an environment favorable for Clostridium difficile colonization. Recent evidence suggests that entry of TcdA into cells is mediated by at least two domains. Here, we report the characterization of a second receptor-binding domain (RBD2) for TcdA. While both the isolated combined repetitive oligopeptides (CROPs) and RBD2 fragments are rapidly internalized into cells under physiologic conditions, only the CROPs domain appreciably accumulates at the cell surface. Once internalized, CROPs and RBD2 are trafficked to late endosomal compartments. An internal deletion of RBD2 from TcdA holotoxin ablated toxicity in HT29 cells. These data are consistent with the recently proposed dual receptor model of cellular entry.

CodY-Dependent Regulation of Sporulation in Clostridium difficile

Clostridium difficile must form a spore to survive outside the gastrointestinal tract. The factors that trigger sporulation in C. difficile remain poorly understood. Previous studies have suggested that a link exists between nutritional status and sporulation initiation in C. difficile In this study, we investigated the impact of the global nutritional regulator CodY on sporulation in C. difficile strains from the historical 012 ribotype and the current epidemic 027 ribotype. Sporulation frequencies were increased in both backgrounds, demonstrating that CodY represses sporulation in C. difficile The 027 codY mutant exhibited a greater increase in spore formation than the 012 codY mutant. To determine the role of CodY in the observed sporulation phenotypes, we examined several factors that are known to influence sporulation in C. difficile Using transcriptional reporter fusions and quantitative reverse transcription-PCR (qRT-PCR) analysis, we found that two loci associated with the initiation of sporulation, opp and sinR, are regulated by CodY. The data demonstrate that CodY is a repressor of sporulation in C. difficile and that the impact of CodY on sporulation and expression of specific genes is significantly influenced by the strain background. These results suggest that the variability of CodY-dependent regulation is an important contributor to virulence and sporulation in current epidemic isolates. This report provides further evidence that nutritional state, virulence, and sporulation are linked in C. difficile

IMPORTANCE

This study sought to examine the relationship between nutrition and sporulation in C. difficile by examining the global nutritional regulator CodY. CodY is a known virulence and nutritional regulator of C. difficile, but its role in sporulation was unknown. Here, we demonstrate that CodY is a negative regulator of sporulation in two different ribotypes of C. difficile We also demonstrate that CodY regulates known effectors of sporulation, Opp and SinR. These results support the idea that nutrient limitation is a trigger for sporulation in C. difficile and that the response to nutrient limitation is coordinated by CodY. Additionally, we demonstrate that CodY has an altered role in sporulation regulation for some strains.

Clostridium difficile Toxins A and B: Insights into Pathogenic Properties and Extraintestinal Effects

Clostridium difficile infection (CDI) has significant clinical impact especially on the elderly and/or immunocompromised patients. The pathogenicity of Clostridium difficile is mainly mediated by two exotoxins: toxin A (TcdA) and toxin B (TcdB). These toxins primarily disrupt the cytoskeletal structure and the tight junctions of target cells causing cell rounding and ultimately cell death. Detectable C. difficile toxemia is strongly associated with fulminant disease. However, besides the well-known intestinal damage, recent animal and in vitro studies have suggested a more far-reaching role for these toxins activity including cardiac, renal, and neurologic impairment. The creation of C. difficile strains with mutations in the genes encoding toxin A and B indicate that toxin B plays a major role in overall CDI pathogenesis. Novel insights, such as the role of a regulator protein (TcdE) on toxin production and binding interactions between albumin and C. difficile toxins, have recently been discovered and will be described. Our review focuses on the toxin-mediated pathogenic processes of CDI with an emphasis on recent studies.

Screening Glycolipids Against Proteins in Vitro Using Picodiscs and Catch-and-Release Electrospray Ionization-Mass Spectrometry

This work describes the application of the catch-and-release electrospray ionization-mass spectrometry (CaR-ESI-MS) assay, implemented using picodiscs (complexes comprised of saposin A and lipids, PDs), to screen mixtures of glycolipids (GLs) against water-soluble proteins to detect specific interactions. To demonstrate the reliability of the method, seven gangliosides (GM1, GM2, GM3, GD1a, GD1b, GD2, and GT1b) were incorporated, either individually or as a mixture, into PDs and screened against two lectins: the B subunit homopentamer of cholera toxin (CTB5) and a subfragment of toxin A from Clostridium difficile (TcdA-A2). The CaR-ESI-MS results revealed that CTB5 binds to six of the gangliosides (GM1, GM2, GM3, GD1a, GD1b, and GT1b), while TcdA-A2 binds to five of them (GM1, GM2, GM3, GD1a, and GT1b). These findings are consistent with the measured binding specificities of these proteins for ganglioside oligosaccharides. Screening mixtures of lipids extracted from porcine brain and a human epithelial cell line against CTB5 revealed binding to multiple GM1 isoforms as well as to fucosyl-GM1, which is a known ligand. Finally, a comparison of the present results with data obtained with the CaR-ESI-MS assay implemented using nanodiscs (NDs) revealed that the PDs exhibited similar or superior performance to NDs for protein-GL binding measurements.

Pathogenic effects of glucosyltransferase from Clostridium difficile toxins

The glucosyltransferase domain ofClostridium difficiletoxins modifies guanine nucleotide-binding proteins of Rho family. It is the major virulent domain of the holotoxins. Various pathogenic effects ofC. difficiletoxins in response to Rho glucosylation have been investigated including cytoskeleton damage, cell death and inflammation. The most recent studies have revealed some significant characteristics of the holotoxins that are independent of glucosylating activity. These findings arouse discussion about the role of glucosyltransferase activity in toxin pathogenesis and open up new insights for toxin mechanism study. In this review, we summarize the pathogenic effects of glucosyltransferase domain of the toxins in the past years.

The Tip of the Four N-Terminal α-Helices of Clostridium sordellii Lethal Toxin Contains the Interaction Site with Membrane Phosphatidylserine Facilitating Small GTPases Glucosylation

Clostridium sordellii lethal toxin (TcsL) is a powerful virulence factor responsible for severe toxic shock in man and animals. TcsL belongs to the large clostridial glucosylating toxin (LCGT) family which inactivates small GTPases by glucosylation with uridine-diphosphate (UDP)-glucose as a cofactor. Notably, TcsL modifies Rac and Ras GTPases, leading to drastic alteration of the actin cytoskeleton and cell viability. TcsL enters cells via receptor-mediated endocytosis and delivers the N-terminal glucosylating domain (TcsL-cat) into the cytosol. TcsL-cat was found to preferentially bind to phosphatidylserine (PS)-containing membranes and to increase the glucosylation of Rac anchored to the lipid membrane. We have previously reported that the N-terminal four helical bundle structure (1–93 domain) recognizes a broad range of lipids, but that TcsL-cat specifically binds to PS and phosphatidic acid. Here, we show using mutagenesis that the PS binding site is localized on the tip of the four-helix bundle which is rich in positively-charged amino acids. Residues Y14, V15, F17, and R18 on loop 1, between helices 1 and 2, in coordination with R68 from loop 3, between helices 3 and 4, form a pocket which accommodates L-serine. The functional PS-binding site is required for TcsL-cat binding to the plasma membrane and subsequent cytotoxicity. TcsL-cat binding to PS facilitates a high enzymatic activity towards membrane-anchored Ras by about three orders of magnitude as compared to Ras in solution. The PS-binding site is conserved in LCGTs, which likely retain a common mechanism of binding to the membrane for their full activity towards membrane-bound GTPases.

A novel murine model of Clostridium sordellii myonecrosis: Insights into the pathogenesis of disease

Clostridium sordellii infections have been reported in women following natural childbirth and spontaneous or medically-induced abortion, injection drug users and patients with trauma. Death is rapid and mortality ranges from 70 to 100%. Clinical features include an extreme leukemoid reaction, the absence of fever, and only minimal pain or erythema at the infected site. In the current study, we developed a murine model of C. sordellii soft tissue infection to elucidate the pathogenic mechanisms. Mice received 0.5, 1.0 or 2.0 × 10(6) CFU C. sordellii (ATCC 9714 type strain) in the right thigh muscle. All doses caused fatal infection characterized by intense swelling of the infected limb but no erythema or visible perfusion deficits. Survival rates and time to death were inoculum dose-dependent. Mice developed a granulocytic leukocytosis with left shift, the onset of which directly correlated with disease severity. Histopathology of infected tissue showed widespread edema, moderate muscle damage and minimal neutrophil infiltration. Circulating levels of granulocyte colony-stimulating factor (G-CSF), soluble tumor necrosis factor receptor I (sTNF-RI) and interlukin-6 (IL-6) were significantly increased in infected animals, while TNF-α, and IL-1β levels were only mildly elevated, suggesting these host factors likely mediate the leukocytosis and innate immune dysfunction characteristic of this infection. Thus, this model mimics many of the salient features of this infection in humans and has allowed us to identify novel targets for intervention.

Receptors and Binding Structures for Clostridium difficile Toxins A and B

Two characteristics of toxins A and B from C. difficile (TcdA, TcdB) are important for the understanding of the pathogenic effect of these homologous toxins. First, these toxins are huge single-chain but multidomain proteins that display their action intracellularly within the cytosol of host cells. And second, albeit various cell types highly differ in their sensitivity toward these toxins, no toxin-resistant cell type has been described yet. Investigation of receptor-mediated uptake of these toxins is very ambitious. It demands discrimination between cell surface binding, interaction with more than one functional receptor responsible for uptake as well as other functional receptors that recognize bacterial pathogens and are not necessarily related with endocytosis. The current understanding of a complex uptake process is that TcdB interacts with at least two facultative receptors that mediate entry into host cells by redundant endocytotic pathways. Although both homologous toxins do obviously not share the same receptors, this principle of redundant binding domains found for TcdB does also account for TcdA.

Time-resolved amino acid uptake of Clostridium difficile 630Δerm and concomitant fermentation product and toxin formation

Background

Clostridium difficile is one of the major nosocomial threats causing severe gastrointestinal infections. Compared to the well documented clinical symptoms, little is known about the processes in the bacterial cell like the regulation and activity of metabolic pathways. In this study, we present time-resolved and global data of extracellular substrates and products. In a second part, we focus on the correlation of fermentation products and substrate uptake with toxin production.

Results

Formation of different fermentation products during growth in a comparison between the two different media in a global approach was studied using non-targeted gas chromatography–mass spectrometry (GC-MS) based analysis. During cultivation in a casamino acids medium and minimal medium, the clinical isolate C. difficile 630Δerm showed major differences in amino acid utilization: In casamino acids medium, C. difficile preferred proline, leucine and cysteine as carbon and energy sources while glutamate and lysine were not or hardly used. In contrast, proline and leucine were consumed at a significantly later stage in minimal medium. Due to the more complex substrate mixture more fermentation products were detectable in the casamino acids medium, accompanied by major changes in the ratios between oxidative and reductive Stickland products. Different glucose consumption dynamics were observed in presence of either casamino acids or the minimal set of amino acids, accompanied by major changes in butanoate formation. This was associated with a variation in both the toxin yield and a change in the ratio of toxin A to toxin B.

Conclusions

Since in all media compositions, more than one substrate was available as a suitable carbon source, availability of different carbon sources and their metabolic fate appears to be the key factor for toxin formation.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-015-0614-2) contains supplementary material, which is available to authorized users.