Mechanism of diphtheria toxin catalytic domain delivery to the eukaryotic cell cytosol and the cellular factors that directly participate in the process

Overview of Bacterial Protein Toxins from Pathogenic Bacteria: Mode of Action and Insights into Evolution

Bacterial protein toxins are secreted by certain bacteria and are responsible for mild to severe diseases in humans and animals. They are among the most potent molecules known, which are active at very low concentrations. Bacterial protein toxins exhibit a wide diversity based on size, structure, and mode of action. Upon recognition of a cell surface receptor (protein, glycoprotein, and glycolipid), they are active either at the cell surface (signal transduction, membrane damage by pore formation, or hydrolysis of membrane compound(s)) or intracellularly. Various bacterial protein toxins have the ability to enter cells, most often using an endocytosis mechanism, and to deliver the effector domain into the cytosol, where it interacts with an intracellular target(s). According to the nature of the intracellular target(s) and type of modification, various cellular effects are induced (cell death, homeostasis modification, cytoskeleton alteration, blockade of exocytosis, etc.). The various modes of action of bacterial protein toxins are illustrated with representative examples. Insights in toxin evolution are discussed.

Development of plug-and-deliverable intracellular protein delivery platforms based on botulinum neurotoxin

Intracellular protein delivery systems have great potential in the fields of therapeutics development and biomedical research. However, targeted delivery, passing through the cell membrane without damaging the cells, and escaping from endosomal entrapment of endocytosed molecular cargos are major challenges of the system. Here, we present a novel intracellular protein delivery system based on modularly engineered botulinum neurotoxin type A (BoNT/A). LHNA domain, consisting of light chain and endosomal escape machinery of BoNT/A, was genetically fused with SpyCatcher (SC) and EGFR targeting affibody (EGFRAfb) to create SC-LHNA-EGFRAfb, a target-specific and protein cargo-switchable BoNT/A-based intracellular protein delivery platform. SC-LHNA-EGFRAfb was purely purified in large quantities, efficiently ligated with multiple ST-fused protein cargos individually, generating a variety of protein cargo-containing intracellular delivery complexes, and successfully delivered ligated protein cargos into the cytosol of target cells via receptor-mediated endocytosis, followed by endosomal escape and subsequent cytosolic delivery. SC-LHNA-EGFRAfb enhanced intracellular delivery efficiency of protein toxin, gelonin, by approximately 100-fold, highlighting the crucial roles of EGFRAfb and LHNA domain as a targeting ligand and an endosomal escape machinery, respectively, in the delivery process. The BoNT-based plug-and-deliverable intracellular protein delivery system has the potential to expand its applications in protein therapeutics and manipulating cellular processes.

Unveiling Sticholysin II and plasmid DNA interaction: Implications for developing non-viral vectors

Non-viral gene delivery systems offer significant potential for gene therapy due to their versatility, safety, and cost advantages over viral vectors. However, their effectiveness can be hindered by the challenge of efficiently releasing the genetic cargo from endosomes to prevent degradation in lysosomes. To overcome this obstacle, functional components can be incorporated into these systems. Sticholysin II (StII) is one of the pore-forming proteins derived from the sea anemone Stichodactyla helianthus, known for its high ability to permeabilize cellular and model membranes. In this study, we aimed to investigate the interaction between StII, and a model plasmid (pDNA) as an initial step towards designing an improved vector with enhanced endosomal escape capability. The electrophoretic mobility shift assay (EMSA) confirmed the formation of complexes between StII and pDNA. Computational predictions identified specific residues involved in the StII-DNA interaction interface, highlighting the importance of electrostatic interactions and hydrogen bonds in mediating the binding. Atomic force microscopy (AFM) of StII-pDNA complexes revealed the presence of nodular fiber and toroid shapes. These complexes were found to have a predominantly micrometer size, as confirmed by dynamic light scattering (DLS) measurements. Despite increase in the overall charge, the complexes formed at the evaluated nitrogen-to-phosphorus (N/P) ratios still maintained a negative charge. Moreover, StII retained its pore-forming capacity regardless of its binding to the complexes. These findings suggest that the potential ability of StII to permeabilize endosomal membranes could be largely maintained when combined with nucleic acid delivery systems. Additionally, the still remaining negative charge of the complexes would enable the association of another positively charged component to compact pDNA. However, to minimize non-specific cytotoxic effects, it is advisable to explore methods to regulate the protein's activity in response to the microenvironment.

Neuropilin-1 Binding Peptide as Fusion to Diphtheria Toxin Induces Apoptosis in Non-small Cell Lung Cancer Cell Line

BACKGROUND

Targeted cancer therapy can be considered as a new strategy to overcome the side effects of current cancer treatments. Neuropilin-1 (NRP-1) is a transmembrane glycoprotein that is expressed in endothelial cells and tumor vessels to stimulate angiogenesis progression. Targeted diphtheria toxin (DT)- based therapeutics are promising tools for cancer treatment. This study aimed to construct a novel NRP-1 binding peptide (as three repeats) (CRGDK) as a fusion to truncated DT (DTA) (DTA-triCRGDK) for targeted delivery of DT into NRP-1 expressing cells.

METHODS

The concept of DTA-triCRGDK was designed, synthesized and cloned into the bacterial host. Expression of DTA-triCRGDK was induced by Isopropyl ß-D-1-thiogalactopyranoside (IPTG) and purification was performed using Ni-NTA chromatography. Biological activity of DTA-triCRGDK was evaluated using MTT, apoptosis, and wound healing assays. In addition, expression levels of apoptotic Bax, Bcl2, and Casp3 genes were determined by Real-time PCR.

RESULTS

Cytotoxicity analysis showed the IC50 values of DTA-triCRGDK for A549 and MRC5 were 0.43 nM and 4.12 nM after 24 h, respectively. Bcl2 expression levels decreased 0.4 and 0.72 fold in A549 and MRC5, respectively. However, Bax and Casp3 expression level increased by 6.75 and 8.19 in A549 and 2.51 and 3.6 in MRC5 cells.

CONCLUSION

Taken together, DTA-triCRGDK is a promising tool for targeted therapy of NRP-1 overexpressing cancer cells.

Discovery of natural bispecific antibodies: Is psoriasis induced by a toxigenic Corynebacterium simulans and maintained by CIDAMPs as autoantigens?

The high abundance of Corynebacterium simulans in psoriasis skin suggests a contribution to the psoriasis aetiology. This hypothesis was tested in an exploratory study, where western blot (WB) analyses with extracts of heat-treated C. simulans and psoriasis serum-derived IgG exhibited a single 16 kDa-WB-band. Proteomic analyses revealed ribosomal proteins as candidate C. s.-antigens. A peptidomic analysis unexpectedly showed that psoriasis serum-derived IgG already contained 31 immunopeptides of Corynebacteria ssp., suggesting the presence of natural bispecific antibodies (BsAbs). Moreover, peptidomic analyses gave 372 DECOY-peptides with similarity to virus- and phage proteins, including Corynebacterium diphtheriae phage, and similarity to diphtheria toxin. Strikingly, a peptidomic analysis for human peptides revealed 64 epitopes of major psoriasis autoantigens such as the spacer region of filaggrin, hornerin repeats and others. Most identified immunopeptides represent potential cationic intrinsically disordered antimicrobial peptides (CIDAMPs), which are generated within the epidermis. These may form complexes with bacterial disordered protein regions, representing chimeric antigens containing discontinuous epitopes. In addition, among 128 low-abundance immunopeptides, 48 are putatively psoriasis-relevant such as epitope peptides of PGE2-, vitamin D3- and IL-10-receptors. Further, 47 immunopeptides originated from tumour antigens, and the endogenous retrovirus HERV-K. I propose that persistent infection with a toxigenic C. simulans initiates psoriasis, which is exacerbated as an autoimmune disease by CIDAMPs as autoantigens. The discovery of natural BsAbs allows the identification of antigen epitopes from microbes, viruses, autoantigens and tumour-antigens, and may help to develop epitope-specific peptide-vaccines and therapeutic approaches with antigen-specific regulatory T cells to improve immune tolerance in an autoimmune disease-specific-manner.

Unconventional structure and mechanisms for membrane interaction and translocation of the NF-κB-targeting toxin AIP56

Bacterial AB toxins are secreted key virulence factors that are internalized by target cells through receptor-mediated endocytosis, translocating their enzymatic domain to the cytosol from endosomes (short-trip) or the endoplasmic reticulum (long-trip). To accomplish this, bacterial AB toxins evolved a multidomain structure organized into either a single polypeptide chain or non-covalently associated polypeptide chains. The prototypical short-trip single-chain toxin is characterized by a receptor-binding domain that confers cellular specificity and a translocation domain responsible for pore formation whereby the catalytic domain translocates to the cytosol in an endosomal acidification-dependent way. In this work, the determination of the three-dimensional structure of AIP56 shows that, instead of a two-domain organization suggested by previous studies, AIP56 has three-domains: a non-LEE encoded effector C (NleC)-like catalytic domain associated with a small middle domain that contains the linker-peptide, followed by the receptor-binding domain. In contrast to prototypical single-chain AB toxins, AIP56 does not comprise a typical structurally complex translocation domain; instead, the elements involved in translocation are scattered across its domains. Thus, the catalytic domain contains a helical hairpin that serves as a molecular switch for triggering the conformational changes necessary for membrane insertion only upon endosomal acidification, whereas the middle and receptor-binding domains are required for pore formation.

AB Toxins as High-Affinity Ligands for Cell Targeting in Cancer Therapy

Conventional targeted therapies for the treatment of cancer have limitations, including the development of acquired resistance. However, novel alternatives have emerged in the form of targeted therapies based on AB toxins. These biotoxins are a diverse group of highly poisonous molecules that show a nanomolar affinity for their target cell receptors, making them an invaluable source of ligands for biomedical applications. Bacterial AB toxins, in particular, are modular proteins that can be genetically engineered to develop high-affinity therapeutic compounds. These toxins consist of two distinct domains: a catalytically active domain and an innocuous domain that acts as a ligand, directing the catalytic domain to the target cells. Interestingly, many tumor cells show receptors on the surface that are recognized by AB toxins, making these high-affinity proteins promising tools for developing new methods for targeting anticancer therapies. Here we describe the structure and mechanisms of action of Diphtheria (Dtx), Anthrax (Atx), Shiga (Stx), and Cholera (Ctx) toxins, and review the potential uses of AB toxins in cancer therapy. We also discuss the main advances in this field, some successful results, and, finally, the possible development of innovative and precise applications in oncology based on engineered recombinant AB toxins.

Prospects of Using Protein Engineering for Selective Drug Delivery into a Specific Compartment of Target Cells

A large number of proteins are successfully used to treat various diseases. These include natural polypeptide hormones, their synthetic analogues, antibodies, antibody mimetics, enzymes, and other drugs based on them. Many of them are demanded in clinical settings and commercially successful, mainly for cancer treatment. The targets for most of the aforementioned drugs are located at the cell surface. Meanwhile, the vast majority of therapeutic targets, which are usually regulatory macromolecules, are located inside the cell. Traditional low molecular weight drugs freely penetrate all cells, causing side effects in non-target cells. In addition, it is often difficult to elaborate a small molecule that can specifically affect protein interactions. Modern technologies make it possible to obtain proteins capable of interacting with almost any target. However, proteins, like other macromolecules, cannot, as a rule, freely penetrate into the desired cellular compartment. Recent studies allow us to design multifunctional proteins that solve these problems. This review considers the scope of application of such artificial constructs for the targeted delivery of both protein-based and traditional low molecular weight drugs, the obstacles met on the way of their transport to the specified intracellular compartment of the target cells after their systemic bloodstream administration, and the means to overcome those difficulties.

In silico prediction of molecular mechanisms of toxicity mediated by the leptospiral PF07598 gene family-encoded virulence-modifying proteins

Mechanisms of leptospirosis pathogenesis remain unclear despite the identification of a number of potential leptospiral virulence factors. We recently demonstrated potential mechanisms by which the virulence-modifying (VM) proteins-defined as containing a Domain of Unknown function (DUF1561), encoded by the PF07598 gene family-found only in group 1 pathogenic Leptospira-might mediate the clinical pathogenesis of leptospirosis. VM proteins belongs to classical AB toxin paradigm though have a unique AB domain architecture, unlike other AB toxins such as diphtheria toxin, pertussis toxin, shiga toxin, or ricin toxin which are typically encoded by two or more genes and self-assembled into a multi-domain holotoxin. Leptospiral VM proteins are secreted R-type lectin domain-containing exotoxins with discrete N-terminal ricin B-like domains involved in host cell surface binding, and a C-terminal DNase/toxin domain. Here we use the artificial intelligence-based AlphaFold algorithm and other computational tools to predict and elaborate on details of the VM protein structure-function relationship. Comparative AlphaFold and CD-spectroscopy defined the consistent secondary structure (Helix and ß-sheet) content, and the stability of the functional domains were further supported by molecular dynamics simulation. VM proteins comprises distinctive lectic family (QxW)₃ motifs, the Mycoplasma CARDS toxin (D3 domain, aromatic patches), C-terminal similarity with mammalian DNase I. In-silico study proposed that Gln412, Gln523, His533, Thr59 are the high binding energy or ligand binding residues plausibly anticipates in the functional activities. Divalent cation (Mg⁺²-Gln412) and phosphate ion (PO₄]^-3-Arg615) interaction further supports the functional activities driven by C-terminal domain. Computation-driven structure-function studies of VM proteins will guide experimentation towards mechanistic understandings of leptospirosis pathogenesis, which underlie development of new therapeutic and preventive measures for this devastating disease.

Analysis of Corynebacterium silvaticum genomes from Portugal reveals a single cluster and a clade suggested to produce diphtheria toxin

Background

Corynebacterium silvaticum is a pathogenic, gram-positive bacterial species that causes caseous lymphadenitis in wild boars, domestic pigs and roe deer in Western Europe. It can affect animal production and cause zoonosis. Genome analysis has suggested that one strain from Portugal and one from Austria could probably produce the diphtheria toxin (DT), which inhibits protein synthesis and can cause death.

Methods

To further investigate the species genetic diversity and probable production of DT by Portuguese strains, eight isolates from this country were sequenced and compared to 38 public ones.

Results

Strains from Portugal are monophyletic, nearly identical, form a unique cluster and have 27 out of 36 known Corynebacterium virulence or niche factors. All of them lack a frameshift in the tox gene and were suggested to produce DT. A phylogenetic analysis shows that the species has diverged into two clades. Clade 1 is composed of strains that were suggested to have the ability to produce DT, represented by the monophyletic strains from Portugal and strain 05-13 from Austria. Clade 2 is composed of strains unable to produce DT due to a frameshifted tox gene. The second clade is represented by strains from Austria, Germany and Switzerland. Ten genome clusters were detected, in which strains from Germany are the most diverse. Strains from Portugal belong to an exclusive cluster. The pangenome has 2,961 proteins and is nearly closed (α = 0.968). Exclusive genes shared by clusters 1 and 2, and Portuguese strains are probably not related to disease manifestation as they share the same host but could play a role in their extra-host environmental adaptation. These results show the potential of the species to cause zoonosis, possibly diphtheria. The identified clusters, exclusively shaded genes, and exclusive STs identified in Portugal could be applied in the identification and epidemiology of the species.

Molecularly imprinted nanoparticles with recognition properties towards diphtheria toxin for ELISA applications

Plastic antibodies can be used for in vitro neutralization of biomacromolecules with different fragments due to their potential in separation, purification, chemical sensor, catalysis and drug production studies. These polymer nanoparticles with binding affinity and selectivity comparable to natural antibodies were prepared using functional monomer synthesis and copolymerization of acrylic monomers via miniemulsion polymerization. As a result, the in vitro cytotoxic effect from diphtheria toxin was reduced by MIPs. In vitro imaging experiments of polymer nanoparticles (plastic antibodies) were performed to examine the interaction of diphtheria toxin with actin filaments, and MIPs inhibited diphtheria toxin damage on actin filaments. The enzyme-linked immunosorbent assay (ELISA) was performed with plastic antibodies labeled with biotin, and it was determined that plastic antibodies could also be used for diagnostic purposes. We report that molecularly imprinted polymers (MIPs), which are biocompatible polymer nanoparticles, can capture and reduce the effect of diphtheria toxic and its fragment A.

Establishment of a pig CRISPR/Cas9 knockout library for functional gene screening in pig cells

BACKGROUND

As an important farm animal, pig functional genomic study can help understand the molecular mechanism related to the key economic traits of pig, such as growth, reproduction, or disease. The genome-scale library based on clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR-associated endonuclease Cas9 (Cas9) system facilitates discovery of key genes involved in a specific function or phenotype, allowing for an effective "phenotype-to-genotype" strategy for functional genomic study.

METHODS AND RESULTS

We designed and constructed a pig genome-scale CRISPR/Cas9 knockout library targeting 16,888 genes with 970,001 unique sgRNAs. The library is a single-vector system including both Cas9 and sgRNA, and packaged into lentivirus for an easy cell delivery for screening. To establish a screening method in pig cells, we used diphtheria toxin (DT)-induced cell death as a model to screen the host genes critical for DT toxicity in pig PK-15 cells. After lentiviral transduction and two sequential screening with DT treatment, the highest-ranking candidates we identified were previously validated genes, HBEGF, DPH1, DPH2, DPH3, DPH5, DNAJC24, and ZBTB17, which are DT receptor and the key factors involved in biosynthesis of diphthamide, the target of DT action. The function and gene essentiality of candidates were further confirmed by gene knockout and DT toxicity assay in PK-15 cells.

CONCLUSIONS

Our CRISPR knockout library targeting pig whole genome establishes a promising platform for pig functional genomic analysis.

Thiol Modifications in the Extracellular Space—Key Proteins in Inflammation and Viral Infection

Posttranslational modifications (PTMs) allow to control molecular and cellular functions in response to specific signals and changes in the microenvironment of cells. They regulate structure, localization, stability, and function of proteins in a spatial and temporal manner. Among them, specific thiol modifications of cysteine (Cys) residues facilitate rapid signal transduction. In fact, Cys is unique because it contains the highly reactive thiol group that can undergo different reversible and irreversible modifications. Upon inflammation and changes in the cellular microenvironment, many extracellular soluble and membrane proteins undergo thiol modifications, particularly dithiol–disulfide exchange, S-glutathionylation, and S-nitrosylation. Among others, these thiol switches are essential for inflammatory signaling, regulation of gene expression, cytokine release, immunoglobulin function and isoform variation, and antigen presentation. Interestingly, also the redox state of bacterial and viral proteins depends on host cell-mediated redox reactions that are critical for invasion and infection. Here, we highlight mechanistic thiol switches in inflammatory pathways and infections including cholera, diphtheria, hepatitis, human immunodeficiency virus (HIV), influenza, and coronavirus disease 2019 (COVID-19).

Bacterial-Based Methods for Cancer Treatment: What We Know and Where We Are

A severe disease, cancer is caused by the exponential and uncontrolled growth of cells, leading to organ dysfunction as well as disorders. This disease has been recognized as one of the significant challenges to health and medicine. Various treatment procedures for cancer are associated with diverse side effects; the most conventional cancer treatments include chemotherapy, surgery, and radiotherapy, among others. Numerous adverse and side effects, low specificity and sensitivity, narrow therapeutic windows, and, recently, the emergence of tumor cells resistant to such treatments have been documented as the shortcomings of conventional treatment strategies. As a group of prokaryotic microorganisms, bacteria have great potential for use in cancer therapy. Currently, utilizing bacteria for cancer treatment has attracted the attention of scientists. The high potential of bacteria to become non-pathogenic by genetic manipulation, their distinguished virulence factors (which can be used as weapons against tumors), their ability to proliferate in tissues, and the contingency to control their population by administrating antibiotics, etc., have made bacteria viable candidates and live micro-medication for cancer therapies. However, the possible cytotoxicity impacts of bacteria, their inability to entirely lyse cancerous cells, as well as the probability of mutations in their genomes are among the significant challenges of bacteria-based methods for cancer treatment. In this article, various available data on bacterial therapeutics, along with their pros and cons, are discussed.

Interactions between the Re-Emerging Pathogen Corynebacterium diphtheriae and Host Cells

Corynebacterium diphtheriae, the etiological agent of diphtheria, is a re-emerging pathogen, responsible for several thousand deaths per year. In addition to diphtheria, systemic infections, often by non-toxigenic strains, are increasingly observed. This indicates that besides the well-studied and highly potent diphtheria toxin, various other virulence factors may influence the progression of the infection. This review focuses on the known components of C. diphtheriae responsible for adhesion, invasion, inflammation, and cell death, as well as on the cellular signaling pathways activated upon infection.

AB5 Enterotoxin-Mediated Pathogenesis: Perspectives Gleaned from Shiga Toxins

Foodborne diseases affect an estimated 600 million people worldwide annually, with the majority of these illnesses caused by Norovirus, Vibrio, Listeria, Campylobacter, Salmonella, and Escherichia coli. To elicit infections in humans, bacterial pathogens express a combination of virulence factors and toxins. AB₅ toxins are an example of such toxins that can cause various clinical manifestations, including dehydration, diarrhea, kidney damage, hemorrhagic colitis, and hemolytic uremic syndrome (HUS). Treatment of most bacterial foodborne illnesses consists of fluid replacement and antibiotics. However, antibiotics are not recommended for infections caused by Shiga toxin-producing E. coli (STEC) because of the increased risk of HUS development, although there are conflicting views and results in this regard. Lack of effective treatment strategies for STEC infections pose a public health threat during outbreaks; therefore, the debate on antibiotic use for STEC infections could be further explored, along with investigations into antibiotic alternatives. The overall goal of this review is to provide a succinct summary on the mechanisms of action and the pathogenesis of AB₅ and related toxins, as expressed by bacterial foodborne pathogens, with a primary focus on Shiga toxins (Stx). The role of Stx in human STEC disease, detection methodologies, and available treatment options are also briefly discussed.

Structure of a bacterial Rhs effector exported by the type VI secretion system

The type VI secretion system (T6SS) is a widespread protein export apparatus found in Gram-negative bacteria. The majority of T6SSs deliver toxic effector proteins into competitor bacteria. Yet, the structure, function, and activation of many of these effectors remains poorly understood. Here, we present the structures of the T6SS effector RhsA from Pseudomonas protegens and its cognate T6SS spike protein, VgrG1, at 3.3 Å resolution. The structures reveal that the rearrangement hotspot (Rhs) repeats of RhsA assemble into a closed anticlockwise β-barrel spiral similar to that found in bacterial insecticidal Tc toxins and in metazoan teneurin proteins. We find that the C-terminal toxin domain of RhsA is autoproteolytically cleaved but remains inside the Rhs ‘cocoon’ where, with the exception of three ordered structural elements, most of the toxin is disordered. The N-terminal ‘plug’ domain is unique to T6SS Rhs proteins and resembles a champagne cork that seals the Rhs cocoon at one end while also mediating interactions with VgrG1. Interestingly, this domain is also autoproteolytically cleaved inside the cocoon but remains associated with it. We propose that mechanical force is required to remove the cleaved part of the plug, resulting in the release of the toxin domain as it is delivered into a susceptible bacterial cell by the T6SS.

Bacteria have developed a variety of strategies to compete for nutrients and limited resources. One system widely used by Gram-negative bacteria is the T6 secretion system which delivers a plethora of effectors into competing bacterial cells. Known functions of effectors are degradation of the cell wall, the depletion of essential metabolites such as NAD⁺ or the cleavage of DNA. RhsA is an effector from the widespread plant-protecting bacteria Pseudomonas protegens. We found that RhsA forms a closed cocoon similar to that found in bacterial Tc toxins and metazoan teneurin proteins. The effector cleaves its polypeptide chain by itself in three pieces, namely the N-terminal domain including a seal, the cocoon and the actual toxic component which potentially cleaves DNA. The toxic component is encapsulated in the large cocoon, so that the effector producing bacterium is protected from the toxin. In order for the toxin to exit the cocoon, we propose that the seal, which closes the cocoon at one end, is removed by mechanical forces during injection of the effector by the T6 secretion system. We further hypothesize about different scenarios for the delivery of the toxin into the cytoplasm of the host cell. Together, our findings expand the knowledge of the mechanism of action of the T6 secretion system and its essential role in interbacterial competition.

The Urokinase Receptor (uPAR) as a "Trojan Horse" in Targeted Cancer Therapy: Challenges and Opportunities

Simple Summary

Discovered more than three decades ago, the urokinase-type plasminogen activator receptor (uPAR) has now firmly established itself as a versatile molecular target holding promise for the treatment of aggressive malignancies. The copious abundance of uPAR in virtually all human cancerous tissues versus their healthy counterparts has fostered a gradual shift in the therapeutic landscape targeting this receptor from function inhibition to cytotoxic approaches to selectively eradicate the uPAR-expressing cells by delivering a targeted cytotoxic insult. Multiple avenues are being explored in a preclinical setting, including the more innovative immune- or stroma targeting therapies. This review discusses the current state of these strategies, their potentialities, and challenges, along with future directions in the field of uPAR targeting.

Abstract

One of the largest challenges to the implementation of precision oncology is identifying and validating selective tumor-driving targets to enhance the therapeutic efficacy while limiting off-target toxicity. In this context, the urokinase-type plasminogen activator receptor (uPAR) has progressively emerged as a promising therapeutic target in the management of aggressive malignancies. By focalizing the plasminogen activation cascade and subsequent extracellular proteolysis on the cell surface of migrating cells, uPAR endows malignant cells with a high proteolytic and migratory potential to dissolve the restraining extracellular matrix (ECM) barriers and metastasize to distant sites. uPAR is also assumed to choreograph multiple other neoplastic stages via a complex molecular interplay with distinct cancer-associated signaling pathways. Accordingly, high uPAR expression is observed in virtually all human cancers and is frequently associated with poor patient prognosis and survival. The promising therapeutic potential unveiled by the pleiotropic nature of this receptor has prompted the development of distinct targeted intervention strategies. The present review will focus on recently emerged cytotoxic approaches emphasizing the novel technologies and related limits hindering their application in the clinical setting. Finally, future research directions and emerging opportunities in the field of uPAR targeting are also discussed.

Clostridial Neurotoxins: Structure, Function and Implications to Other Bacterial Toxins

Gram-positive bacteria are ancient organisms. Many bacteria, including Gram-positive bacteria, produce toxins to manipulate the host, leading to various diseases. While the targets of Gram-positive bacterial toxins are diverse, many of those toxins use a similar mechanism to invade host cells and exert their functions. Clostridial neurotoxins produced by Clostridial tetani and Clostridial botulinum provide a classical example to illustrate the structure-function relationship of bacterial toxins. Here, we critically review the recent progress of the structure-function relationship of clostridial neurotoxins, including the diversity of the clostridial neurotoxins, the mode of actions, and the flexible structures required for the activation of toxins. The mechanism clostridial neurotoxins use for triggering their activity is shared with many other Gram-positive bacterial toxins, especially molten globule-type structures. This review also summarizes the implications of the molten globule-type flexible structures to other Gram-positive bacterial toxins. Understanding these highly dynamic flexible structures in solution and their role in the function of bacterial toxins not only fills in the missing link of the high-resolution structures from X-ray crystallography but also provides vital information for better designing antidotes against those toxins.

Immunotoxins and nanobody-based immunotoxins: review and update

Immunotoxins (ITs) are protein-based drugs that compose of targeting and cytotoxic moieties. After binding the IT to the specific cell-surface antigen, the IT internalises into the target cell and kills it. Targeting and cytotoxic moieties usually include monoclonal antibodies and protein toxins with bacterial or plant origin, respectively. ITs have been successful in haematologic malignancies treatment. However, ITs penetrate poorly into solid tumours because of their large size. Use of camelid antibody fragments known as nanobodies (Nbs) as a targeting moiety may overcome this problem. Nbs are the smallest fragment of antibodies with excellent tumour tissue penetration. The ability to recognise cryptic (immuno-evasive) target antigens, low immunogenicity, and high-affinity are other fundamental characteristics of Nbs that make them suitable candidates in targeted therapy. Here, we reviewed and discussed the structure and function of ITs, Nbs, and nanobody-based ITs. To gain sound insight into the issue at hand, we focussed on nanobody-based ITs.

Targeted Toxins for the Treatment of Prostate Cancer

Prostate cancer is the second most common cancer and the fifth leading cause of cancer deaths worldwide. Despite improvements in diagnosis and treatment, new treatment options are urgently needed for advanced stages of the disease. Targeted toxins are chemical conjugates or fully recombinant proteins consisting of a binding domain directed against a target antigen on the surface of cancer cells and a toxin domain, which is transported into the cell for the induction of apoptosis. In the last decades, targeted toxins against prostate cancer have been developed. Several challenges, however, became apparent that prevented their direct clinical use. They comprise immunogenicity, low target antigen binding, endosomal entrapment, and lysosomal/proteasomal degradation of the targeted toxins. Moreover, their efficacy is impaired by prostate tumors, which are marked by a dense microenvironment, low target antigen expression, and apoptosis resistance. In this review, current findings in the development of targeted toxins against prostate cancer in view of effective targeting, reduction of immunogenicity, improvement of intracellular trafficking, and overcoming apoptosis resistance are discussed. There are promising approaches that should lead to the clinical use of targeted toxins as therapeutic alternatives for advanced prostate cancer in the future.

Amoebae as Targets for Toxins or Effectors Secreted by Mammalian Pathogens

Numerous microorganisms, pathogenic for mammals, come from the environment where they encounter predators such as free-living amoebae (FLA). The selective pressure due to this interaction could have generated virulence traits that are deleterious for amoebae and represents a weapon against mammals. Toxins are one of these powerful tools that are essential for bacteria or fungi to survive. Which amoebae are used as a model to study the effects of toxins? What amoeba functions have been reported to be disrupted by toxins and bacterial secreted factors? Do bacteria and fungi effectors affect eukaryotic cells similarly? Here, we review some studies allowing to answer these questions, highlighting the necessity to extend investigations of microbial pathogenicity, from mammals to the environmental reservoir that are amoebae.

In Vivo Tumor Therapy with Novel Immunotoxin Containing Programmed Cell Death Protein-1 and Diphtheria Toxin

Immunotoxins, as a class of antitumor agents, consist of tumor-selective ligands linked to highly toxic protein molecules. This type of modified antibody has been designed for the therapy of cancers and a few viral infections. In this study, we designed immunotoxin consisting of mouse programmed cell death protein-1 (PD1), which genetically fused to diphtheria toxin (DT) subunit A (DT386). DNA construct was cloned, expressed in a bacterial system, purified, and confirmed by western blotting. The immunotoxin potency in the treatment of tumorous C57BL/6 mice was evaluated. Immunotoxin was injected intratumoral to mice, and through eight injections, 67% of the tumor volume of the test group started shrinking dramatically. On the contrary, the tumor size of the control group, treated with phosphate-buffered saline, continued its growth. The successful targeting of solid tumor cells by PD1-DT immunotoxin demonstrates the potential therapeutic utility of these conjugates.

Production of IgY polyclonal antibody against diphtheria toxin and evaluation of its neutralization effect by Vero cell assay

BACKGROUND

Diphtheria is a bacterial disease which is caused by Corynebacterium diphtheriae. The symptoms are due to the diphtheria toxin produced by the bacteria. Antibiotic therapy and the use of diphtheria antitoxin is a recommended strategy to control diphtheria. Although mammalian antibodies are used to treat patients, IgY antibody has advantages over mammalian ones, including cost-effectiveness and production through non-invasive means. Moreover, in contrast to mammalian antibodies, IgY does not bind to the rheumatoid factor and does not activate the complement system. The objective of this study was to evaluate the in vitro neutralizing effect of IgY against diphtheria toxin.

RESULTS

Anti-DT IgY was produced by immunization of the laying white leghorn chickens. Indirect enzyme-linked immunosorbent assay revealed successful immunization of the animals, and the IgY was purified with a purity of 93% via polyethylene glycol precipitation method. The neutralizing activity of the purified IgY was evaluated by Vero cell viability assay. This assay confirmed that 1.95 μg (8.6 μg/ml of culture medium) of anti-DT IgY would neutralize 10 fold of cytotoxic dose 99% of DT, which was 0.3 ng (1.33 ng/ml of culture medium).

CONCLUSION

This anti-DT IgY may be applicable for diphtheria treatment and quality controls in vaccine production.

Resolving the Molecular Steps in Clostridial Neurotoxin Light Chain Translocation

The clostridial neurotoxins (CNTs), botulinum toxin and tetanus toxin, are the most toxic proteins for humans. Neurotoxicity is based upon the specificity of the CNTs for neural host receptors and substrates. CNTs are organized into three domains, a Light Chain (LC) that is a metalloprotease and a Heavy Chain (HC) that has two domains, an N-terminal LC translocation domain (HCN) and a C-terminal receptor binding domain (HCC). While catalysis and receptor binding functions of the CNTs have been developed, our understanding of LC translocation is limited. This is due to the intrinsic complexity of the translocation process and limited tools to assess the step-by-step events in LC translocation. Recently, we developed a novel, cell-based TT-reporter to measure LC translocation as the translocation of a β-lactamase reporter across a vesicle membrane in neurons. Using this approach, we identified a role for a cis-Loop, located within the HCN, in LC translocation. In this commentary, we describe our current understanding of how CNTs mediate LC translocation and place the role of the cis-Loop in the LC translocation process relative to other independent functions that have been implicated in LC translocation. Understanding the basis for LC translocation will enhance the use of CNTs in vaccine development and as human therapies.

A novel diphtheria toxin-based bivalent human EGF fusion toxin for treatment of head and neck squamous cell carcinoma

Epidermal growth factor receptor (EGFR) is often overexpressed in head and neck squamous cell carcinoma (HNSCC) and represents a top candidate for targeted HNSCC therapy. However, the clinical effectiveness of current Food and Drug Administration (FDA)-approved drugs targeting EGFR is moderate, and the overall survival rate for HNSCC patients remains low. Therefore, more effective treatments are urgently needed. In this study, we generated a novel diphtheria toxin-based bivalent human epidermal growth factor fusion toxin (bi-EGF-IT) to treat EGFR-expressing HNSCC. Bi-EGF-IT was tested for in vitro binding affinity, cytotoxicity, and specificity using 14 human EGFR-expressing HNSCC cell lines and three human EGFR-negative cancer cell lines. Bi-EGF-IT had increased binding affinity for EGFR-expressing HNSCC compared with the monovalent version (mono-EGF-IT), and both versions specifically depleted EGFR-positive HNSCC, but not EGFR-negative cell lines, in vitro. Bi-EGF-IT exhibited a comparable potency to that of the FDA-approved EGFR inhibitor, erlotinib, for inhibiting HNSCC tumor growth in vivo using both subcutaneous and orthotopic HNSCC xenograft mouse models. When tested in an experimental metastasis model, survival was significantly longer in the bi-EGF-IT treatment group than the erlotinib treatment group, with a significantly reduced number of metastases compared with mono-EGF-IT. In addition, in vivo off-target toxicities were significantly reduced in the bi-EGF-IT treatment group compared with the mono-EGF-IT group. These results demonstrate that bi-EGF-IT is more effective and markedly less toxic at inhibiting primary HNSCC tumor growth and metastasis than mono-EGF-IT and erlotinib. Thus, the novel bi-EGF-IT is a promising drug candidate for further development.

Crystal structure of bacterial cytotoxic necrotizing factor CNFY reveals molecular building blocks for intoxication

Cytotoxic necrotizing factors (CNFs) are bacterial single-chain exotoxins that modulate cytokinetic/oncogenic and inflammatory processes through activation of host cell Rho GTPases. To achieve this, they are secreted, bind surface receptors to induce endocytosis and translocate a catalytic unit into the cytosol to intoxicate host cells. A three-dimensional structure that provides insight into the underlying mechanisms is still lacking. Here, we determined the crystal structure of full-length Yersinia pseudotuberculosis CNFY . CNFY consists of five domains (D1-D5), and by integrating structural and functional data, we demonstrate that D1-3 act as export and translocation module for the catalytic unit (D4-5) and for a fused β-lactamase reporter protein. We further found that D4, which possesses structural similarity to ADP-ribosyl transferases, but had no equivalent catalytic activity, changed its position to interact extensively with D5 in the crystal structure of the free D4-5 fragment. This liberates D5 from a semi-blocked conformation in full-length CNFY , leading to higher deamidation activity. Finally, we identify CNF translocation modules in several uncharacterized fusion proteins, which suggests their usability as a broad-specificity protein delivery tool.

Conformational switching, refolding and membrane insertion of the diphtheria toxin translocation domain

Diphtheria toxin is among many bacterial toxins that utilize the endosomal pathway of cellular entry, which is ensured by the bridging of the endosomal membrane by the toxin's translocation (T) domain. Endosomal acidification triggers a series of conformational changes of the T-domain, that take place first in aqueous and subsequently in membranous milieu. These rearrangements ultimately result in establishing membrane-inserted conformation(s) and translocation of the catalytic moiety of the toxin into the cytoplasm. We discuss here the strategy for combining site-selective labeling with various spectroscopic methods to characterize structural and thermodynamic aspects of protonation-dependent conformational switching and membrane insertion of the diphtheria toxin T-domain. Among the discussed methods are FRET, FCS and depth-dependent fluorescence quenching with lipid-attached bromine atoms and spin probes. The membrane-insertion pathway of the T-domain contains multiple intermediates and is governed by staggered pH-dependent transitions involving protonation of histidines and acidic residues. Presented data demonstrate that the lipid bilayer plays an active part in T-domain functioning and that the so-called Open-Channel State does not constitute the translocation pathway, but is likely to be a byproduct of the translocation. The spectroscopic approaches presented here are broadly applicable to many other systems of physiological and biomedical interest for which conformational changes can lead to membrane insertion (e.g., other bacterial toxins, host defense peptides, tumor-targeting pHLIP peptides and members of Bcl-2 family of apoptotic regulators).

Structure of the Diphtheria Toxin at Acidic pH: Implications for the Conformational Switching of the Translocation Domain

Diphtheria toxin, an exotoxin secreted by Corynebacterium that causes disease in humans by inhibiting protein synthesis, enters the cell via receptor-mediated endocytosis. The subsequent endosomal acidification triggers a series of conformational changes, resulting in the refolding and membrane insertion of the translocation (T-)domain and ultimately leading to the translocation of the catalytic domain into the cytoplasm. Here, we use X-ray crystallography along with circular dichroism and fluorescence spectroscopy to gain insight into the mechanism of the early stages of pH-dependent conformational transition. For the first time, we present the high-resolution structure of the diphtheria toxin at a mildly acidic pH (5–6) and compare it to the structure at neutral pH (7). We demonstrate that neither catalytic nor receptor-binding domains change their structure upon this acidification, while the T-domain undergoes a conformational change that results in the unfolding of the TH2–3 helices. Surprisingly, the TH1 helix maintains its conformation in the crystal of the full-length toxin even at pH 5. This contrasts with the evidence from the new and previously published data, obtained by spectroscopic measurements and molecular dynamics computer simulations, which indicate the refolding of TH1 upon the acidification of the isolated T-domain. The overall results imply that the membrane interactions of the T-domain are critical in ensuring the proper conformational changes required for the preparation of the diphtheria toxin for the cellular entry.

Engineering Cell-Permeable Proteins through Insertion of Cell-Penetrating Motifs into Surface Loops

Effective delivery of proteins into the cytosol of mammalian cells would open the door to a wide range of applications. However, despite great efforts from numerous investigators, effective protein delivery in a clinical setting is yet to be accomplished. Herein we report a potentially general approach to engineering cell-permeable proteins by genetically grafting a short cell-penetrating peptide (CPP) to an exposed loop of a protein of interest. The grafted peptide is conformationally constrained, exhibiting enhanced proteolytic stability and cellular entry efficiency. Applying this technique to enhanced green fluorescent protein (EGFP), protein-tyrosine phosphatase 1B (PTP1B), and purine nucleoside phosphorylase (PNP) rendered all three proteins cell-permeable and biologically active in cellular assays. When added into growth medium at 0.5-5 μM concentrations, the engineered PTP1B dose-dependently reduced the phosphotyrosine levels of intracellular proteins, while the modified PNP corrected the metabolic deficiency of PNP-deficient mouse T lymphocytes, providing a potential enzyme replacement therapy for a rare genetic disease.

Bispecific Targeting of EGFR and Urokinase Receptor (uPAR) Using Ligand-Targeted Toxins in Solid Tumors

Ligand-targeted toxins (LTTs) are bioengineered molecules which are composed of a targeting component linked to a toxin that induces cell death once the LTT binds its target. Bispecific targeting allows for the simultaneous targeting of two receptors. In this review, we mostly focus on the epidermal growth factor receptor (EGFR) as a target. We discuss the development and testing of a bispecific LTT targeting EGFR and urokinase-type plasminogen activator receptor (uPAR) as two attractive targets implicated in tumor growth and in the regulation of the tumor microvasculature in solid tumors. In vitro and mouse xenograft studies have shown that EGFR-targeted bispecific angiotoxin (eBAT) is effective against human solid tumors. Canine studies have shown that eBAT is both safe and effective against canine hemangiosarcoma, which is physiologically similar to human angiosarcoma. Finding the appropriate dosing strategy and sequencing of eBAT administration, in combination with other therapeutics, are among important factors for future directions. Together, the data indicate that eBAT targets cancer stem cells, it may have a role in inhibiting human tumor vasculature, and its bispecific conformation may have a role in reducing toxicity in comparative oncologic trials in dogs.

Novel Toxin-Antitoxin Module SlvT-SlvA Regulates Megaplasmid Stability and Incites Solvent Tolerance in Pseudomonas putida S12

Sustainable alternatives for high-value chemicals can be achieved by using renewable feedstocks in bacterial biocatalysis. However, during the bioproduction of such chemicals and biopolymers, aromatic compounds that function as products, substrates, or intermediates in the production process may exert toxicity to microbial host cells and limit the production yield. Therefore, solvent tolerance is a highly preferable trait for microbial hosts in the biobased production of aromatic chemicals and biopolymers. In this study, we revisit the essential role of megaplasmid pTTS12 from solvent-tolerant Pseudomonas putida S12 for molecular adaptation to an organic solvent. In addition to the solvent extrusion pump (SrpABC), we identified a novel toxin-antitoxin module (SlvAT) which contributes to short-term tolerance in moderate solvent concentrations, as well as to the stability of pTTS12. These two gene clusters were successfully expressed in non-solvent-tolerant strains of P. putida and Escherichia coli strains to confer and enhance solvent tolerance.

Pseudomonas putida S12 is highly tolerant of organic solvents in saturating concentrations, rendering this microorganism suitable for the industrial production of various aromatic compounds. Previous studies revealed that P. putida S12 contains the single-copy 583-kbp megaplasmid pTTS12. pTTS12 carries several important operons and gene clusters facilitating P. putida S12 survival and growth in the presence of toxic compounds or other environmental stresses. We wished to revisit and further scrutinize the role of pTTS12 in conferring solvent tolerance. To this end, we cured the megaplasmid from P. putida S12 and conclusively confirmed that the SrpABC efflux pump is the major determinant of solvent tolerance on the megaplasmid pTTS12. In addition, we identified a novel toxin-antitoxin module (proposed gene names slvT and slvA, respectively) encoded on pTTS12 which contributes to the solvent tolerance phenotype and is important for conferring stability to the megaplasmid. Chromosomal introduction of the srp operon in combination with the slvAT gene pair created a solvent tolerance phenotype in non-solvent-tolerant strains, such as P. putida KT2440, Escherichia coli TG1, and E. coli BL21(DE3).

IMPORTANCE Sustainable alternatives for high-value chemicals can be achieved by using renewable feedstocks in bacterial biocatalysis. However, during the bioproduction of such chemicals and biopolymers, aromatic compounds that function as products, substrates, or intermediates in the production process may exert toxicity to microbial host cells and limit the production yield. Therefore, solvent tolerance is a highly preferable trait for microbial hosts in the biobased production of aromatic chemicals and biopolymers. In this study, we revisit the essential role of megaplasmid pTTS12 from solvent-tolerant Pseudomonas putida S12 for molecular adaptation to an organic solvent. In addition to the solvent extrusion pump (SrpABC), we identified a novel toxin-antitoxin module (SlvAT) which contributes to short-term tolerance in moderate solvent concentrations, as well as to the stability of pTTS12. These two gene clusters were successfully expressed in non-solvent-tolerant strains of P. putida and Escherichia coli strains to confer and enhance solvent tolerance.

Bacteriotherapy in gastrointestinal cancer

The most prevalent gastrointestinal (GI) cancers include colorectal cancer, stomach cancer, and liver cancer, known as the most common causes of cancer-related death in both men and women populations in the world. Traditional therapeutic approaches, including surgery, radiotherapy, and chemotherapy have failed in the effective treatment of cancer. Therefore, there is an urgent need for finding new effective anticancer agents. The available evidence and also the promising results of using bacteria as the anticancer agents on numerous cancer cell lines have attracted the attention of scientists for the therapeutic role of bacteria in the field of cancer therapy. Moreover, several studies on the bacteriotherapy agents have used genetic engineering to overcome the challenges and enhance the efficacy with the least drawbacks. Numerous bacterial species that can specifically target and internalize into the tumor cells are used live, attenuated, or genetically as compared to selectively consider the hypoxic condition of tumor, which results in the tumor suppression. The present study is a comprehensive review of the current literature on the use of bacteria and their substances such as bacteriocins and toxins in the treatment of different types of gastrointestinal cancers.

The bacterial instrument as a promising therapy for colon cancer

OBJECTIVE

Colon cancer is a great health concern worldwide, as it is the second leading cause of cancer-related death. Conventional treatment of cancer such as surgery, radiotherapy, and chemotherapy are faced with limitations and side effects. Therefore, strategies for the treatment of cancer need to be modified or new strategies replacing the old one.

AIMS

The aim of this study is to review the role of bacteria or their products (such as peptides, bacteriocins, and toxins) as a therapeutic agent for colon cancer.

RESULTS AND CONCLUSION

Recently, the therapeutic role of bacteria and their products in colon cancer treatment holds promise as emerging novel anti-cancer agents. Unlike the conventional treatments, targeted therapy based on the use of bacteria that are able to directly target tumor cells without affecting normal cells is evolving as an alternative strategy. Moreover, several bacterial species were used in live, attenuated or genetically modified that are able to multiply selectively in tumors and inhibiting their growth.

Revisiting Old Ionophore Lasalocid as a Novel Inhibitor of Multiple Toxins

The ionophore lasalocid is widely used as a veterinary drug against coccidiosis. We found recently that lasalocid protects cells from two unrelated bacterial toxins, the cytotoxic necrotizing factor-1 (CNF1) from Escherichia. coli and diphtheria toxin. We evaluated lasalocid’s capacity to protect cells against other toxins of medical interest comprising toxin B from Clostridium difficile, Shiga-like toxin 1 from enterohemorrhagic E. coli and exotoxin A from Pseudomonas aeruginosa. We further characterized the impact of lasalocid on the endolysosomal and the retrograde pathways and organelle integrity, especially the Golgi apparatus. We found that lasalocid protects cells from all toxins tested and impairs the drop of vesicular pH along the trafficking pathways that are required for toxin sorting and translocation to the cytoplasm. Lasalocid also has an impact on the cellular distribution of GOLPH4 and GOLPH2 Golgi markers. Other intracellular trafficking compartments positive for EEA1 and Rab9A display a modified cellular pattern. In conclusion, lasalocid protects cells from multiple deadly bacterial toxins by corrupting vesicular trafficking and Golgi stack homeostasis.

Cho W, Soleimanpour S. Bacteriotherapy in Breast Cancer

Breast cancer is the second most common cause of cancer-related mortality among women around the world. Conventional treatments in the fight against breast cancer, such as chemotherapy, are being challenged regarding their effectiveness. Thus, strategies for the treatment of breast cancer need to be continuously refined to achieve a better patient outcome. We know that a number of bacteria are pathogenic and some are even associated with tumor development, however, recent studies have demonstrated interesting results suggesting some bacteria may have potential for cancer therapy. Therefore, the therapeutic role of bacteria has aroused attention in medical and pharmaceutical studies. Furthermore, genetic engineering has been used in bacterial therapy and may led to greater efficacy with few side effects. Some genetically modified non-pathogenic bacterial species are more successful due to their selectivity for cancer cells but with low toxicity for normal cells. Some live, attenuated, or genetically modified bacterias are capable to multiply in tumors and inhibit their growth. This article aims to review the role of bacteria and their products including bacterial peptides, bacteriocins, and toxins for the treatment of breast cancer.

The role of the single interchains disulfide bond in tetanus and botulinum neurotoxins and the development of antitetanus and antibotulism drugs

A large number of bacterial toxins consist of active and cell binding protomers linked by an interchain disulfide bridge. The largest family of such disulfide-bridged exotoxins is that of the clostridial neurotoxins that consist of two chains and comprise the tetanus neurotoxins causing tetanus and the botulinum neurotoxins causing botulism. Reduction of the interchain disulfide abolishes toxicity, and we discuss the experiments that revealed the role of this structural element in neuronal intoxication. The redox couple thioredoxin reductase-thioredoxin (TrxR-Trx) was identified as the responsible for reduction of this disulfide occurring on the cytosolic surface of synaptic vesicles. We then discuss the very relevant finding that drugs that inhibit TrxR-Trx also prevent botulism. On this basis, we propose that ebselen and PX-12, two TrxR-Trx specific drugs previously used in clinical trials in humans, satisfy all the requirements for clinical tests aiming at evaluating their capacity to effectively counteract human and animal botulism arising from intestinal toxaemias such as infant botulism.

A novel, safe, fast and efficient treatment for Her2-positive and negative bladder cancer utilizing an EGF-anthrax toxin chimera

Bladder cancer is the sixth most common cancer in the United States, and it exhibits an alarming 70% recurrence rate. Thus, the development of more efficient antibladder cancer approaches is a high priority. Accordingly, this work provides the basis for a transformative anticancer strategy that takes advantage of the unique characteristics of the bladder. Unlike mucin-shielded normal bladder cells, cancer cells are exposed to the bladder lumen and overexpress EGFR. Therefore, we used an EGF-conjugated anthrax toxin that after targeting EGFR was internalized and triggered apoptosis in exposed bladder cancer cells. This unique agent presented advantages over other EGF-based technologies and other toxin-derivatives. In contrast to known agents, this EGF-toxin conjugate promoted its own uptake via receptor microclustering even in the presence of Her2 and induced cell death with a LC₅₀ < 1 nM. Furthermore, our data showed that exposures as short as ≈3 min were enough to commit human (T24), mouse (MB49) and canine (primary) bladder cancer cells to apoptosis. Exposure of tumor-free mice and dogs with the agent resulted in no toxicity. In addition, the EGF-toxin was able to eliminate cells from human patient tumor samples. Importantly, the administration of EGF-toxin to dogs with spontaneous bladder cancer, who had failed or were not eligible for other therapies, resulted in ~30% average tumor reduction after one treatment cycle. Because of its in vitro and in vivo high efficiency, fast action (reducing treatment time from hours to minutes) and safety, we propose that this EGF-anthrax toxin conjugate provides the basis for new, transformative approaches against bladder cancer.

Tc Toxin Complexes: Assembly, Membrane Permeation, and Protein Translocation

Tc toxin complexes are virulence factors of many bacteria, including insect and human pathogens. Tc toxins are composed of three subunits that act together to perforate the host membrane, similar to a syringe, and translocate toxic enzymes into the host cell. The reactions of the toxic enzymes lead to deterioration and ultimately death of the cell. We review recent high-resolution structural and functional data that explain the mechanism of action of this type of bacterial toxin at an unprecedented level of molecular detail. We focus on the steps that are necessary for toxin activation and membrane permeation. This is where the largest conformational transitions appear. Furthermore, we compare the architecture and function of Tc toxins with those of anthrax toxin and vertebrate teneurin.

HSC70 and HSP90 chaperones perform complementary roles in translocation of the cholera toxin A1 subunit from the endoplasmic reticulum to the cytosol

Cholera toxin (CT) travels by vesicle carriers from the cell surface to the endoplasmic reticulum (ER) where the catalytic A1 subunit of CT (CTA1) dissociates from the rest of the toxin, unfolds, and moves through a membrane-spanning translocon pore to reach the cytosol. Heat shock protein 90 (HSP90) binds to the N-terminal region of CTA1 and facilitates its ER-to-cytosol export by refolding the toxin as it emerges at the cytosolic face of the ER membrane. HSP90 also refolds some endogenous cytosolic proteins as part of a foldosome complex containing heat shock cognate 71-kDa protein (HSC70) and the HSC70/HSP90-organizing protein (HOP) linker that anchors HSP90 to HSC70. We accordingly predicted that HSC70 and HOP also function in CTA1 translocation. Inactivation of HSC70 by drug treatment disrupted CTA1 translocation to the cytosol and generated a toxin-resistant phenotype. In contrast, the depletion of HOP did not disrupt CT activity against cultured cells. HSC70 and HSP90 could bind independently to disordered CTA1, even in the absence of HOP. This indicated HSP90 and HSC70 recognize distinct regions of CTA1, which was confirmed by the identification of a YYIYVI-binding motif for HSC70 that spans residues 83-88 of the 192-amino acid CTA1 polypeptide. Refolding of disordered CTA1 occurred in the presence of HSC70 alone, indicating that HSC70 and HSP90 can each independently refold CTA1. Our work suggests a novel translocation mechanism in which sequential interactions with HSP90 and HSC70 drive the N- to C-terminal extraction of CTA1 from the ER.

Lipid-modulation of membrane insertion and refolding of the apoptotic inhibitor Bcl-xL

Bcl-xL is a member of the Bcl-2 family of apoptotic regulators, responsible for inhibiting the permeabilization of the mitochondrial outer membrane, and a promising anti-cancer target. Bcl-xL exists in the following conformations, each believed to play a role in the inhibition of apoptosis: (a) a soluble folded conformation, (b) a membrane-anchored (by its C-terminal α8 helix) form, which retains the same fold as in solution and (c) refolded membrane-inserted conformations, for which no structural data are available. Previous studies established that in the cell Bcl-xL exists in a dynamic equilibrium between soluble and membranous states, however, no direct evidence exists in support of either anchored or inserted conformation of the membranous state in vivo. In this in vitro study, we employed a combination of fluorescence and EPR spectroscopy to characterize structural features of the bilayer-inserted conformation of Bcl-xL and the lipid modulation of its membrane insertion transition. Our results indicate that the core hydrophobic helix α6 inserts into the bilayer without adopting a transmembrane orientation. This insertion disrupts the packing of Bcl-xL and releases the regulatory N-terminal BH4 domain (α1) from the rest of the protein structure. Our data demonstrate that both insertion and refolding of Bcl-xL are modulated by lipid composition, which brings the apparent pK_a of insertion to the threshold of physiological pH. We hypothesize that conformational rearrangements associated with the bilayer insertion of Bcl-xL result in its switching to a so-called non-canonical mode of apoptotic inhibition. Presented results suggest that the alteration in lipid composition before and during apoptosis can serve as an additional factor regulating the permeabilization of the mitochondrial outer membrane.

Clinical Activity and Tolerability of SL-401 (Tagraxofusp): Recombinant Diphtheria Toxin and Interleukin-3 in Hematologic Malignancies

Overcoming the leukemia stem cell resistance to intensive chemotherapy has been an area of extensive research over the last two decades. Advances and greater understanding of the molecular biology of leukemia stem cells are in rapid progress. Targeted therapies are currently being used in clinical practice with reasonable response rates, but a cure is being achieved in only a small percentage of patients, most likely due to tumor mutational heterogeneity. A genetically engineered diphtheria toxin fused with interleukin-3 (SL-401 or tagraxofusp) has shown robust activity in blastic plasmacytoid dendritic cell neoplasm and promising response rates in different myeloid malignancies, including eradication of minimal residual disease. Multiple clinical trials are being conducted using this drug and the preliminary results are encouraging. This article reviews the clinical trials for SL-401, its mechanism of action, clinical activity, and the adverse event profile.

Hydrogen-Deuterium Exchange Epitope Mapping Reveals Distinct Neutralizing Mechanisms for Two Monoclonal Antibodies against Diphtheria Toxin

The diphtheria toxoid (DT) antigen is one of the major components in pediatric and booster combination vaccines and is known to raise a protective humoral immune response upon vaccination. However, a structurally resolved analysis of diphtheria toxin (DTx) epitopes with underlying molecular mechanisms of antibody neutralization has not yet been reported. Using hydrogen-deuterium exchange mass spectrometry (HDX-MS) and Biolayer Interferometry (BLI) assays, we have characterized two neutralizing anti-DTx monoclonal antibodies (mAbs), 2-25 and 2-18, by identifying the specific epitopes on the diphtheria toxin responsible for antibody binding. Our results show that both epitopes are conformational, and mechanistically distinct. Monoclonal antibody 2-25 binds selectively to the B-subunit (translocation and receptor domain) of DTx, blocking the heparin-binding EGF-like growth factor (HBEGF) binding site. In contrast, mAb 2-18 binds to the A-subunit (catalytic domain), partially covering the catalytic loop region that shuttles NAD during catalysis. The results are discussed in the context of antigen neutralization mechanisms and can ultimately help to reveal the underlying factors that contribute to Diptheria vaccine efficacy.

Bacterial infections and cancer

Infections are estimated to contribute to 20% of all human tumours. These are mainly caused by viruses, which explains why a direct bacterial contribution to cancer formation has been largely ignored. While epidemiological data link bacterial infections to particular cancers, tumour formation is generally assumed to be solely caused by the ensuing inflammation responses. Yet, many bacteria directly manipulate their host cell in various phases of their infection cycle. Such manipulations can affect host cell integrity and can contribute to cancer formation. We here describe how bacterial surface moieties, bacterial protein toxins and bacterial effector proteins can induce host cell DNA damage, and thereby can interfere with essential host cell signalling pathways involved in cell proliferation, apoptosis, differentiation and immune signalling.

Scavenger receptor-C acts as a receptor for Bacillus thuringiensis vegetative insecticidal protein Vip3Aa and mediates the internalization of Vip3Aa via endocytosis

The vegetative insecticidal proteins (Vip), secreted by many Bacillus thuringiensis strains during their vegetative growth stage, are genetically distinct from known insecticidal crystal proteins (ICPs) and represent the second-generation insecticidal toxins. Compared with ICPs, the insecticidal mechanisms of Vip toxins are poorly understood. In particular, there has been no report of a definite receptor of Vip toxins to date. In the present study, we identified the scavenger receptor class C like protein (Sf-SR-C) from the Spodoptera frugiperda (Sf9) cells membrane proteins that bind to the biotin labeled Vip3Aa, via the affinity magnetic bead method coupled with HPLC-MS/MS. We then certified Vip3Aa protoxin could interact with Sf-SR-C in vitro and ex vivo. In addition, downregulation of SR-C expression in Sf9 cells and Spodoptera exigua larvae midgut reduced the toxicity of Vip3Aa to them. Coincidently, heterologous expression of Sf-SR-C in transgenic Drosophila midgut significantly enhanced the virulence of Vip3Aa to the Drosophila larvae. Moreover, the complement control protein domain and MAM domain of Sf-SR-C are involved in the interaction with Vip3Aa protoxin. Furthermore, endocytosis of Vip3Aa mediated by Sf-SR-C correlates with its insecticidal activity. Our results confirmed for the first time that Sf-SR-C acts as a receptor for Vip3Aa protoxin and provides an insight into the mode of action of Vip3Aa that will significantly facilitate the study of its insecticidal mechanism and application.

Bacillus thuringiensis Vip3A has potential in control of Lepidopteran pest and has been used in transgenic plants. However, studies of the insecticidal mechanisms of Vip3A are rare, and none of their definite receptors have been reported so far, which seriously restricts the study of its insecticidal mechanism and application. This work identified and confirmed the scavenger receptor class C like protein (Sf-SR-C) acts as the receptor of Vip3Aa protoxin, demonstrated that Sf-SR-C mediates the toxicity of Vip3Aa to Sf9 cells in an internalized manner. These results extend our understanding of SR-C proteins in insects and explain the specificity of Vip3Aa insecticidal activity, which strongly support it as a safe biopesticide. More importantly, it suggests the insecticidal mechanism of Vip3Aa different from the well-known “pore formation” model, “signal transduction” model, as well as newly found “necrosis” model of Cry toxins, which will significantly promote the relevant study of Vip3Aa. Last but not least, because scavenger receptors play a crucial role in innate immunity, our results provide relevant insights into host-pathogen interactions.

The chaperonin TRiC/CCT is essential for the action of bacterial glycosylating protein toxins like Clostridium difficile toxins A and B

Various bacterial protein toxins, including Clostridium difficile toxins A (TcdA) and B (TcdB), attack intracellular target proteins of host cells by glucosylation. After receptor binding and endocytosis, the toxins are translocated into the cytosol, where they modify target proteins (e.g., Rho proteins). Here we report that the activity of translocated glucosylating toxins depends on the chaperonin TRiC/CCT. The chaperonin subunits CCT4/5 directly interact with the toxins and enhance the refolding and restoration of the glucosyltransferase activities of toxins after heat treatment. Knockdown of CCT5 by siRNA and HSF1A, an inhibitor of TRiC/CCT, blocks the cytotoxic effects of TcdA and TcdB. In contrast, HSP90, which is involved in the translocation and uptake of ADP ribosylating toxins, is not involved in uptake of the glucosylating toxins. We show that the actions of numerous glycosylating toxins from various toxin types and different species depend on TRiC/CCT. Our data indicate that the TRiC/CCT chaperonin system is specifically involved in toxin uptake and essential for the action of various glucosylating protein toxins acting intracellularly on target proteins.