Understanding the mode of action of diphtheria toxin: a perspective on progress during the 20th century

TRPV1 nociceptors are required to optimize antigen-specific primary antibody responses to novel antigens

BACKGROUND

Key to the advancement of the field of bioelectronic medicine is the identification of novel pathways of neural regulation of immune function. Sensory neurons (termed nociceptors) recognize harmful stimuli and initiate a protective response by eliciting pain and defensive behavior. Nociceptors also interact with immune cells to regulate host defense and inflammatory responses. However, it is still unclear whether nociceptors participate in regulating primary IgG antibody responses to novel antigens.

METHODS

To understand the role of transient receptor potential vanilloid 1 (TRPV1)-expressing neurons in IgG responses, we generated TRPV1-Cre/Rosa-ChannelRhodopsin2 mice for precise optogenetic activation of TRPV1 + neurons and TRPV1-Cre/Lox-diphtheria toxin A mice for targeted ablation of TRPV1-expressing neurons. Antigen-specific antibody responses were longitudinally monitored for 28 days.

RESULTS

Here we show that TRPV1 expressing neurons are required to develop an antigen-specific immune response. We demonstrate that selective optogenetic stimulation of TRPV1+ nociceptors during immunization significantly enhances primary IgG antibody responses to novel antigens. Further, mice rendered deficient in TRPV1- expressing nociceptors fail to develop primary IgG antibody responses to keyhole limpet hemocyanin or haptenated antigen.

CONCLUSION

This functional and genetic evidence indicates a critical role for nociceptor TRPV1 in antigen-specific primary antibody responses to novel antigens. These results also support consideration of potential therapeutic manipulation of nociceptor pathways using bioelectronic devices to enhance immune responses to foreign antigens.

Single-cell division tracing and transcriptomics reveal cell types and differentiation paths in the regenerating lung

Understanding the molecular and cellular processes involved in lung epithelial regeneration may fuel the development of therapeutic approaches for lung diseases. We combine mouse models allowing diphtheria toxin-mediated damage of specific epithelial cell types and parallel GFP-labeling of functionally dividing cells with single-cell transcriptomics to characterize the regeneration of the distal lung. We uncover cell types, including Krt13+ basal and Krt15+ club cells, detect an intermediate cell state between basal and goblet cells, reveal goblet cells as actively dividing progenitor cells, and provide evidence that adventitial fibroblasts act as supporting cells in epithelial regeneration. We also show that diphtheria toxin-expressing cells can persist in the lung, express specific inflammatory factors, and transcriptionally resemble a previously undescribed population in the lungs of COVID-19 patients. Our study provides a comprehensive single-cell atlas of the distal lung that characterizes early transcriptional and cellular responses to concise epithelial injury, encompassing proliferation, differentiation, and cell-to-cell interactions.

Diphthamide - a conserved modification of eEF2 with clinical relevance

Diphthamide, a complex modification on eukaryotic translation elongation factor 2 (eEF2), assures reading-frame fidelity during translation. Diphthamide and enzymes for its synthesis are conserved in eukaryotes and archaea. Originally identified as target for diphtheria toxin (DT) in humans, its clinical relevance now proves to be broader than the link to pathogenic bacteria. Diphthamide synthesis enzymes (DPH1 and DPH3) are associated with cancer, and DPH gene mutations can cause diphthamide deficiency syndrome (DDS). Finally, new analyses provide evidence that diphthamide may restrict propagation of viruses including SARS-CoV-2 and HIV-1, and that DPH enzymes are targeted by viruses for degradation to overcome this restriction. This review describes how diphthamide is synthesized and functions in translation, and covers its clinical relevance in human development, cancer, and infectious diseases.

Engineering Proteins for Cell Entry

Proteins are essential for life, as they participate in all vital processes in the body. In the past decade, delivery of active proteins to specific cells and organs has attracted increasing interest. However, most proteins cannot enter the cytoplasm due to the cell membrane acting as a natural barrier. To overcome this challenge, various proteins have been engineered to acquire cell-penetrating capacity by mimicking or modifying natural shuttling proteins. In this review, we provide an overview of the different types of engineered cell-penetrating proteins such as cell-penetrating peptides, supercharged proteins, receptor-binding proteins, and bacterial toxins. We also discuss some strategies for improving endosomal escape such as pore formation, the proton sponge effect, and hijacking intracellular trafficking pathways. Finally, we introduce some novel methods and technologies for designing and detecting engineered cell-penetrating proteins.

Diphtheria toxin-derived, anti-PD-1 immunotoxin, a potent and practical tool to selectively deplete PD-1+ cells

Programmed death-1 (PD-1), an immune checkpoint receptor, is expressed on activated lymphocytes, macrophages, and some types of tumor cells. While PD-1+ cells have been implicated in outcomes of cancer immunity, autoimmunity, and chronic infections, the exact roles of these cells in various physiological and pathological processes remain elusive. Molecules that target and deplete PD-1+ cells would be instrumental in defining the roles unambiguously. Previously, an immunotoxin has been generated for the depletion of PD-1+ cells though its usage is impeded by its low production yield. Thus, a more practical molecular tool is desired to deplete PD-1+ cells and to examine functions of these cells. We designed and generated a novel anti-PD1 diphtheria immunotoxin, termed PD-1 DIT, targeting PD-1+ cells. PD-1 DIT is comprised of two single chain variable fragments (scFv) derived from an anti-PD-1 antibody, coupled with the catalytic and translocation domains of the diphtheria toxin. PD-1 DIT was produced using a yeast expression system that has been engineered to efficiently produce protein toxins. The yield of PD-1 DIT reached 1-2 mg/L culture, which is 10 times higher than the previously reported immunotoxin. Flow cytometry and confocal microscopy analyses confirmed that PD-1 DIT specifically binds to and enters PD-1+ cells. The binding avidities between PD-1 DIT and two PD-1+ cell lines are approximately 25 nM. Moreover, PD-1 DIT demonstrated potent cytotoxicity toward PD-1+ cells, with a half maximal effective concentration (EC50 ) value of 1 nM. In vivo experiments further showed that PD-1 DIT effectively depleted PD-1+ cells and enabled mice inoculated with PD-1+ tumor cells to survive throughout the study. Our findings using PD-1 DIT revealed the critical role of pancreatic PD-1+ T cells in the development of type-1 diabetes (T1D). Additionally, we observed that PD-1 DIT treatment ameliorated relapsing-remitting experimental autoimmune encephalomyelitis (RR-EAE), a mouse model of relapsing-remitting multiple sclerosis (RR-MS). Lastly, we did not observe significant hepatotoxicity in mice treated with PD-1 DIT, which had been reported for other immunotoxins derived from the diphtheria toxin. With its remarkable selective and potent cytotoxicity toward PD-1+ cells, coupled with its high production yield, PD-1 DIT emerges as a powerful biotechnological tool for elucidating the physiological roles of PD-1+ cells. Furthermore, the potential of PD-1 DIT to be developed into a novel therapeutic agent becomes evident.

Gaining New Insights into Fundamental Biological Pathways by Bacterial Toxin-Based Genetic Screens

Genetic screen technology has been applied to study the mechanism of action of bacterial toxins-a special class of virulence factors that contribute to the pathogenesis caused by bacterial infections. These screens aim to identify host factors that directly or indirectly facilitate toxin intoxication. Additionally, specific properties of certain toxins, such as membrane interaction, retrograde trafficking, and carbohydrate binding, provide robust probes to comprehensively investigate the lipid biosynthesis, membrane vesicle transport, and glycosylation pathways, respectively. This review specifically focuses on recent representative toxin-based genetic screens that have identified new players involved in and provided new insights into fundamental biological pathways, such as glycosphingolipid biosynthesis, protein glycosylation, and membrane vesicle trafficking pathways. Functionally characterizing these newly identified factors not only expands our current understanding of toxin biology but also enables a deeper comprehension of fundamental biological questions. Consequently, it stimulates the development of new therapeutic approaches targeting both bacterial infectious diseases and genetic disorders with defects in these factors and pathways.

Poly(β-amino ester)s-based nanovehicles: Structural regulation and gene delivery

The first poly(β-amino) esters (PβAEs) were synthesized more than 40 years ago. Since 2000, PβAEs have been found to have excellent biocompatibility and the capability of ferrying gene molecules. Moreover, the synthesis process of PβAEs is simple, the monomers are readily available, and the polymer structure can be tailored to meet different gene delivery needs by adjusting the monomer type, monomer ratio, reaction time, etc. Therefore, PβAEs are a promising class of non-viral gene vector materials. This review paper presents a comprehensive overview of the synthesis and correlated properties of PβAEs and summarizes the progress of each type of PβAE for gene delivery. The review focuses in particular on the rational design of PβAE structures, thoroughly discusses the correlations between intrinsic structure and effect, and then finishes with the applications and perspectives of PβAEs.

A Novel Immunotoxin Targeting Epithelial Cell Adhesion Molecule Using Single Domain Antibody Fused to Diphtheria Toxin

Epithelial Cell Adhesion Molecule (EpCAM) is overexpressed in a variety of cancers such as colon, stomach, pancreas, and prostate adenocarcinomas. Inhibition of EpCAM is considered as a potential target for cancer therapy. In current study, anti-EpCAM immunotoxin (α-EpCAM IT) was developed using genetic fusion of α-EpCAM single domain antibody (nanobody) (α-EpCAM Nb) to truncated form of diphtheria toxin. The expression of recombinant α-EpCAM IT was induced by Isopropyl β-d-1-thiogalactopyranoside (IPTG) and confirmed by SDS-PAGE and western blot. Recombinant α-EpCAM IT was purified from the inclusion bodies and refolded using urea gradient procedure. The cytotoxicity and apoptosis activity of α-EpCAM IT on EpCAM over-expressing (MCF7), low-expressing (HEK293), and no-expressing (HUVEC) cells were evaluated by 3-4,5-Dimethylthiazol-2-yl (MTT) assay and annexin V-FITC-PI assay as well. In addition, anti-tumor activity of α-EpCAM IT was evaluated on nude mice bearing MCF7 tumor cells. Results showed success expression and purification of α-EpCAM IT. The α-EpCAM IT showed time and dose-dependent anti-proliferative activity on MCF-7 cells. However, α-EpCAM IT did not show any anti-proliferative activity on HEK293 and HUVEC cells as well. In addition, the annexin V-FITC-PI assay results showed that α-EpCAM IT significantly increased apoptotic rate in MCF-7 cells with no effect on HEK293 and HUVEC as well. Moreover, α-EpCAM IT significantly reduced tumor size in vivo study. The achieved results indicate the potential of designing α-EpCAM IT as a novel therapeutic for cancer therapy.

Structural Insights into Pseudomonas aeruginosa Exotoxin A-Elongation Factor 2 Interactions: A Molecular Dynamics Study

Exotoxin A (ETA) is an extracellular secreted toxin and a single-chain polypeptide with A and B fragments that is produced by Pseudomonas aeruginosa. It catalyzes the ADP-ribosylation of a post-translationally modified histidine (diphthamide) on eukaryotic elongation factor 2 (eEF2), which results in the inactivation of the latter and the inhibition of protein biosynthesis. Studies show that the imidazole ring of diphthamide plays an important role in the ADP-ribosylation catalyzed by the toxin. In this work, we employ different in silico molecular dynamics (MD) simulation approaches to understand the role of diphthamide versus unmodified histidine in eEF2 on the interaction with ETA. Crystal structures of the eEF2-ETA complexes with three different ligands NAD⁺, ADP-ribose, and βTAD were selected and compared in the diphthamide and histidine containing systems. The study shows that NAD⁺ bound to ETA remains very stable in comparison with other ligands, enabling the transfer of ADP-ribose to the N3 atom of the diphthamide imidazole ring in eEF2 during ribosylation. We also show that unmodified histidine in eEF2 has a negative impact on ETA binding and is not a suitable target for the attachment of ADP-ribose. Analyzing of radius of gyration and COM distances for NAD⁺, βTAD, and ADP-ribose complexes revealed that unmodified His affects the structure and destabilizes the complex with all different ligands throughout the MD simulations.

Tissue-resident glial cells associate with tumoral vasculature and promote cancer progression

Cancer cells are embedded within the tissue and interact dynamically with its components during cancer progression. Understanding the contribution of cellular components within the tumor microenvironment is crucial for the success of therapeutic applications. Here, we reveal the presence of perivascular GFAP+/Plp1+ cells within the tumor microenvironment. Using in vivo inducible Cre/loxP mediated systems, we demonstrated that these cells derive from tissue-resident Schwann cells. Genetic ablation of endogenous Schwann cells slowed down tumor growth and angiogenesis. Schwann cell-specific depletion also induced a boost in the immune surveillance by increasing tumor-infiltrating anti-tumor lymphocytes, while reducing immune-suppressor cells. In humans, a retrospective in silico analysis of tumor biopsies revealed that increased expression of Schwann cell-related genes within melanoma was associated with improved survival. Collectively, our study suggests that Schwann cells regulate tumor progression, indicating that manipulation of Schwann cells may provide a valuable tool to improve cancer patients' outcomes.

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.

The Diphtheria Toxin Translocation Domain Impairs Receptor Selectivity in Cancer Cell-Targeted Protein Nanoparticles

Protein-based materials intended as nanostructured drugs or drug carriers are progressively gaining interest in nanomedicine, since their structure, assembly and cellular interactivity can be tailored by recruiting functional domains. The main bottleneck in the development of deliverable protein materials is the lysosomal degradation that follows endosome maturation. This is especially disappointing in the case of receptor-targeted protein constructs, which, while being highly promising and in demand in precision medicines, enter cells via endosomal/lysosomal routes. In the search for suitable protein agents that might promote endosome escape, we have explored the translocation domain (TD) of the diphtheria toxin as a functional domain in CXCR4-targeted oligomeric nanoparticles designed for cancer therapies. The pharmacological interest of such protein materials could be largely enhanced by improving their proteolytic stability. The incorporation of TD into the building blocks enhances the amount of the material detected inside of exposed CXCR4⁺ cells up to around 25-fold, in absence of cytotoxicity. This rise cannot be accounted for by endosomal escape, since the lysosomal degradation of the new construct decreases only moderately. On the other hand, a significant loss in the specificity of the CXCR4-dependent cellular penetration indicates the unexpected role of the toxin segment as a cell-penetrating peptide in a dose-dependent and receptor-independent fashion. These data reveal that the diphtheria toxin TD displayed on receptor-targeted oligomeric nanoparticles partially abolishes the exquisite receptor specificity of the parental material and it induces nonspecific internalization in mammalian cells.

New Corynebacterium Species with the Potential to Produce Diphtheria Toxin

Only three Corynebacterium species are known to produce a lethal exotoxin called diphtheria toxin. These are C. diphtheriae, C. ulcerans and C. pseudotuberculosis. The diphtheria toxin gene (tox) is carried in a family of closely related corynebacteriophages and therefore the toxin can be produced only through lysogenisation, in which the corynephage encoding tox is stably inserted into the chromosome. However, 'nontoxigenic tox gene-bearing' (NTTB) strains, which are genotypically tox-positive but do not express the protein, have been described. The emergence of NTTB strains was first observed during the 1990s diphtheria epidemic in Eastern Europe and nowadays such isolates have been detected in many countries in the world. Recently, novel species of Corynebacterium genus have been described which might have the potential of producing the diphtheria toxin due to the possession of the diphtheria toxin gene but it has not produced toxin in laboratory tests. The circulation of NTTB strains could be related to the increased risk for diphtheria disease arising from the risk of re-emerging toxin expression. The article presents the mechanism of diphtheria toxin expression and action, recently described novel species of NTTB corynebacteria as well as the taxonomic changes within the C. diphtheriae group.

Nano-Based Approved Pharmaceuticals for Cancer Treatment: Present and Future Challenges

Cancer is one of the main causes of death worldwide. To date, and despite the advances in conventional treatment options, therapy in cancer is still far from optimal due to the non-specific systemic biodistribution of antitumor agents. The inadequate drug concentrations at the tumor site led to an increased incidence of multiple drug resistance and the appearance of many severe undesirable side effects. Nanotechnology, through the development of nanoscale-based pharmaceuticals, has emerged to provide new and innovative drugs to overcome these limitations. In this review, we provide an overview of the approved nanomedicine for cancer treatment and the rationale behind their designs and applications. We also highlight the new approaches that are currently under investigation and the perspectives and challenges for nanopharmaceuticals, focusing on the tumor microenvironment and tumor disseminate cells as the most attractive and effective strategies for cancer treatments.

A diphtheria toxin-based nanoparticle achieves specific cytotoxic effect on CXCR4+ lymphoma cells without toxicity in immunocompromised and immunocompetent mice

High rates of relapsed and refractory diffuse large B-cell lymphoma (DLBCL) patients and life-threatening side effects associated with immunochemotherapy make an urgent need to develop new therapies for DLBCL patients. Immunotoxins seem very potent anticancer therapies but their use is limited because of their high toxicity. Accordingly, the self-assembling polypeptidic nanoparticle, T22-DITOX-H6, incorporating the diphtheria toxin and targeted to CXCR4 receptor, which is overexpressed in DLBCL cells, could offer a new strategy to selectively eliminate CXCR4⁺ DLBCL cells without adverse effects. In these terms, our work demonstrated that T22-DITOX-H6 showed high specific cytotoxicity towards CXCR4⁺ DLBCL cells at the low nanomolar range, which was dependent on caspase-3 cleavage, PARP activation and an increase of cells in early/late apoptosis. Repeated nanoparticle administration induced antineoplastic effect, in vivo and ex vivo, in a disseminated immunocompromised mouse model generated by intravenous injection of human luminescent CXCR4⁺ DLBCL cells. Moreover, T22-DITOX-H6 inhibited tumor growth in a subcutaneous immunocompetent mouse model bearing mouse CXCR4⁺ lymphoma cells in the absence of alterations in the hemogram, liver or kidney injury markers or on-target or off-target organ histology. Thus, T22-DITOX-H6 demonstrates a selective cytotoxicity towards CXCR4⁺ DLBCL cells without the induction of toxicity in non-lymphoma infiltrated organs nor hematologic toxicity.

Epitope Profiling of Diphtheria Toxoid Provides Enhanced Monitoring for Consistency Testing during Manufacturing Process Changes

In the vaccine industry, multiple physicochemical, immunological, in vitro and in vivo analytical methods are applied throughout the manufacturing process to characterize and monitor the quality of vaccines. Presented here is the Single Epitope Antigenicity Test (SEAT), an innovative, quantitative epitope profiling method which provides an extended immunochemical analysis for diphtheria toxoid (DTxd) to be used for consistency testing during manufacturing process changes. The method uses BioLayer Interferometry (BLI) and a panel of monoclonal antibodies (mAbs) to independently assess nine individual antigenic sites of DTxd. The panel includes mAbs which are functional, bind distinct sites on DTxd and are able to distinguish intact DTxd from that which has been exposed to heat treatment. The SEAT method was qualified for precision, accuracy, and linearity, and was used to define a preliminary comparability range for DTxd made using the current manufacturing process. DTxd lots manufactured using alternate processes were assessed in the context of this range to determine the impact on DTxd antigenicity. Epitope profiling by SEAT provides quantitative information on the integrity of multiple important antigenic regions of DTxd, and therefore represents a valuable tool in a comprehensive analytical test package which can be used to support manufacturing process changes for vaccines.

Structures of distant diphtheria toxin homologs reveal functional determinants of an evolutionarily conserved toxin scaffold

Diphtheria toxin (DT) is the archetype for bacterial exotoxins implicated in human diseases and has played a central role in defining the field of toxinology since its discovery in 1888. Despite being one of the most extensively characterized bacterial toxins, the origins and evolutionary adaptation of DT to human hosts remain unknown. Here, we determined the first high-resolution structures of DT homologs outside of the Corynebacterium genus. DT homologs from Streptomyces albireticuli (17% identity to DT) and Seinonella peptonophila (20% identity to DT), despite showing no toxicity toward human cells, display significant structural similarities to DT sharing both the overall Y-shaped architecture of DT as well as the individual folds of each domain. Through a systematic investigation of individual domains, we show that the functional determinants of host range extend beyond an inability to bind cellular receptors; major differences in pH-induced pore-formation and cytosolic release further dictate the delivery of toxic catalytic moieties into cells, thus providing multiple mechanisms for a conserved structural fold to adapt to different hosts. Our work provides structural insights into the expanding DT family of toxins, and highlights key transitions required for host adaptation.

Toxic proteins are responsible for the disease symptoms accompanying bacterial infections. Sugiman-Marangos et al. report crystal structures of two diphtheria toxin homologs, therefore providing structural insights into this family of toxins and the evolutionary adaptation of these toxins to human hosts.

A Structural Approach to Anti-Virulence: A Discovery Pipeline

The anti-virulence strategy is designed to prevent bacterial virulence factors produced by pathogenic bacteria from initiating and sustaining an infection. One family of bacterial virulence factors is the mono-ADP-ribosyltransferase toxins, which are produced by pathogens as tools to compromise the target host cell. These toxins are bacterial enzymes that exploit host cellular NAD+ as the donor substrate to modify an essential macromolecule acceptor target in the host cell. This biochemical reaction modifies the target macromolecule (often protein or DNA) and functions in a binary fashion to turn the target activity on or off by blocking or impairing a critical process or pathway in the host. A structural biology approach to the anti-virulence method to neutralize the cytotoxic effect of these factors requires the search and design of small molecules that bind tightly to the enzyme active site and prevent catalytic function essentially disarming the pathogen. This method requires a high-resolution structure to serve as the model for small molecule inhibitor development, which illuminates the path to drug development. This alternative strategy to antibiotic therapy represents a paradigm shift that may circumvent multi-drug resistance in the offending microbe through anti-virulence therapy. In this report, the rationale for the anti-virulence structural approach will be discussed along with recent efforts to apply this method to treat honey bee diseases using natural products.

The Corynebacterium diphtheriae HbpA hemoglobin-binding protein contains a domain that is critical for hemoprotein-binding, cellular localization and function

The acquisition of hemin-iron from hemoglobin-haptoglobin (Hb-Hp) by Corynebacterium diphtheriae requires the iron-regulated surface proteins HtaA, ChtA, ChtC, and the recently identified Hb-Hp binding protein HbpA. We previously showed that a purified form of HbpA (HbpA-S), lacking the C-terminal region, was able to bind Hb-Hp. In this study, we show that the C-terminal region of HbpA significantly enhances binding to Hb-Hp. A purified form of HbpA that includes the C-terminal domain (HbpA-FL) exhibits much stronger binding to Hb-Hp than HbpA-S. Size exclusion chromatography (SEC) showed that HbpA-FL as well as HtaA-FL, ChtA-FL, and ChtC-FL exist as high molecular weight complexes, while HbpA-S is present as a monomer, indicating that the C-terminal region is required for formation of large aggregates. Growth studies showed that expression of HbpA-FL in the ΔhbpA mutant restored wild-type levels of growth in low-iron medium that contained Hb-Hp as the sole iron source, while HbpA-S failed to complement the ΔhbpA mutant. Protein localization studies in C. diphtheriae showed that HbpA-FL is present in both in the supernatant and in the membrane fractions, and that the C-terminal region is required for membrane anchoring. Purified HbpA-FL was able to enhance growth of the ΔhbpA mutant when added to culture medium that contained Hb-Hp as a sole iron source, suggesting that secreted HbpA is involved in the use of hemin-iron from Hb-Hp. These studies extend our understanding of this novel Hb-Hp binding protein in this important human pathogen. IMPORTANCE Hemoproteins, such as Hb, are an abundant source of iron in humans and are proposed to be required by numerous pathogens to cause disease. In this report, we expand on our previous studies in further defining the role of HbpA in hemin-iron acquisition in C. diphtheriae. HbpA is unique to C. diphtheriae, and appears to function unlike any previously described bacterial iron-regulated Hb- or Hb-Hp-binding protein. HbpA is both secreted and present in the membrane, and exists as a large aggregate that enhances its ability to bind Hb-Hp and promote hemin-iron uptake. Current studies with HbpA will increase our understanding of iron transport systems in C. diphtheriae.

Deletion of TLR2+ erythro-myeloid progenitors leads to embryonic lethality in mice

Early embryonic hematopoiesis in mammals is defined by three successive waves of hematopoietic progenitors which exhibit a distinct hematopoietic potential and provide continuous support for the development of the embryo and adult organism. Although the functional importance of each of these waves has been analyzed, their spatio-temporal overlap and the lack of wave-specific markers hinders the accurate separation and assessment of their functional roles during early embryogenesis. We have recently shown that TLR2, in combination with c-kit, represents the earliest signature of emerging precursors of the second hematopoietic wave, erythro-myeloid precursors (EMPs). Since the onset of Tlr2 expression distinguishes EMPs from primitive progenitors which coexist in the yolk sac from E7.5, we generated a novel transgenic "knock in" mouse model, Tlr2^Dtr , suitable for inducible targeted depletion of TLR2⁺ EMPs. In this model, the red fluorescent protein and diphtheria toxin receptor sequences are linked via a P2A sequence and inserted into the Tlr2 locus before its stop codon. We show that a timely controlled deletion of TLR2⁺ EMPs in Tlr2^Dtr embryos results in a marked decrease in both erythroid as well as myeloid lineages and, consequently, in embryonic lethality peaking before E13.5. These findings validate the importance of EMPs in embryonic development.

Mycoplasma pneumoniae CARDS toxin exploits host cell endosomal acidic pH and vacuolar ATPase proton pump to execute its biological activities

Mycoplasma pneumoniae is the leading cause of bacterial community-acquired pneumonia among hospitalized children in the United States. It is also responsible for a spectrum of other respiratory tract disorders and extrapulmonary manifestations in children and adults. The main virulence factor of M. pneumoniae is a 591 amino acid multifunctional protein called Community Acquired Respiratory Distress Syndrome (CARDS) toxin. The amino terminal region of CARDS toxin (N-CARDS) retains ADP-ribosylating activity and the carboxy region (C-CARDS) contains the receptor binding and vacuolating activities. After internalization, CARDS toxin is transported in a retrograde manner from endosome through the Golgi complex into the endoplasmic reticulum. However, the mechanisms and criteria by which internalized CARDS toxin is transported and activated to execute its cytotoxic effects remain unknown. In this study, we used full-length CARDS toxin and its mutant and truncated derivatives to analyze how pharmacological drugs that alter pH of intracellular vesicles and electrical potential across vesicular membranes affect translocation of CARDS toxin in mammalian cells. Our results indicate that an acidic environment is essential for CARDS toxin retrograde transport to endoplasmic reticulum. Moreover, retrograde transport facilitates toxin clipping and is required to induce vacuole formation. Additionally, toxin-mediated cell vacuolation is strictly dependent on the function of vacuolar type-ATPase.

Large Clostridial Toxins: Mechanisms and Roles in Disease

Large clostridial toxins (LCTs) are a family of bacterial exotoxins that infiltrate and destroy target cells. Members of the LCT family include Clostridioides difficile toxins TcdA and TcdB, Paeniclostridium sordellii toxins TcsL and TcsH, Clostridium novyi toxin TcnA, and Clostridium perfringens toxin TpeL. Since the 19th century, LCT-secreting bacteria have been isolated from the blood, organs, and wounds of diseased individuals, and LCTs have been implicated as the primary virulence factors in a variety of infections, including C. difficile infection and some cases of wound-associated gas gangrene. Clostridia express and secrete LCTs in response to various physiological signals. LCTs invade host cells by binding specific cell surface receptors, ultimately leading to internalization into acidified vesicles. Acidic pH promotes conformational changes within LCTs, which culminates in translocation of the N-terminal glycosyltransferase and cysteine protease domain across the endosomal membrane and into the cytosol, leading first to cytopathic effects and later to cytotoxic effects. The focus of this review is on the role of LCTs in infection and disease, the mechanism of LCT intoxication, with emphasis on recent structural work and toxin subtyping analysis, and the genomic discovery and characterization of LCT homologues. We provide a comprehensive review of these topics and offer our perspective on emerging questions and future research directions for this enigmatic family of toxins.

A protein A based Staphylococcus aureus vaccine with improved safety

Exposure to Staphylococcus aureus does not lead to immunity as evidenced by the persistent colonization of one third of the human population. S. aureus immune escape is mediated by factors that preempt complement activation, destroy phagocytes, and modify B and T cell responses. One such factor, Staphylococcal protein A (SpA) encompasses five Immunoglobulin binding domains (IgBDs) that associate with the Fcγ domain to block phagocytosis. IgBDs also associate with Fab encoded by V_H3 clan related genes. SpA binding to V_H3-IgM that serves as a B cell receptor results in B cell expansion and secretion of antibodies with no specificity for S. aureus. SpA crosslinking of V_H3-IgG and V_H3-IgE bound to cognate receptors of mast cells and basophils promotes histamine release and anaphylaxis. Earlier work developed a prototype variant SpA_KKAA with four amino acid substitutions in each IgBD. When tested in animal models, SpA_KKAA elicited neutralizing antibodies and protection against infection. We show here that SpA_KKAA retains crosslinking activity for V_H3-IgG and V_H3-IgE. We use a rational approach to design and test 67 new SpA variants for loss of V_H3 binding and anaphylactic activities. We identify two detoxified candidates that elicit SpA-neutralizing antibodies and protect animals from S. aureus colonization and bloodstream infection. The new detoxified SpA candidates bear three instead of four amino acid substitutions thus increasing the development of SpA-specific antibodies. We propose that detoxified SpA variants unable to crosslink V_H3-idiotypic immunoglobulin may be suitably developed as clinical-grade vaccines for safety and efficacy testing in humans.

Chemical engineering of bacterial effectors for regulating cell signaling and responses

Bacteria have evolved a variety of effector proteins to facilitate their survival and proliferation within the host environment. Continuous competition at the host-pathogen interface has empowered these effectors with unique mechanism and high specificity toward their host targets. The rich repertoire of bacterial effectors has thus provided us an attractive toolkit for investigating various cellular processes, such as signal transductions. With recent advances in protein chemistry and engineering, we now have the capability for on-demand control of protein activity with high precision. Herein, we review the development of chemically engineered bacterial effectors to control kinase-mediated signal transductions, inhibit protein translation, and direct genetic editing within host cells. We also highlight future opportunities for harnessing diverse prokaryotic effectors as powerful tools for mechanistic investigation and therapeutic intervention of eukaryotic systems.

What the Wild Things Do: Mechanisms of Plant Host Manipulation by Bacterial Type III-Secreted Effector Proteins

Phytopathogenic bacteria possess an arsenal of effector proteins that enable them to subvert host recognition and manipulate the host to promote pathogen fitness. The type III secretion system (T3SS) delivers type III-secreted effector proteins (T3SEs) from bacterial pathogens such as Pseudomonas syringae, Ralstonia solanacearum, and various Xanthomonas species. These T3SEs interact with and modify a range of intracellular host targets to alter their activity and thereby attenuate host immune signaling. Pathogens have evolved T3SEs with diverse biochemical activities, which can be difficult to predict in the absence of structural data. Interestingly, several T3SEs are activated following injection into the host cell. Here, we review T3SEs with documented enzymatic activities, as well as T3SEs that facilitate virulence-promoting processes either indirectly or through non-enzymatic mechanisms. We discuss the mechanisms by which T3SEs are activated in the cell, as well as how T3SEs modify host targets to promote virulence or trigger immunity. These mechanisms may suggest common enzymatic activities and convergent targets that could be manipulated to protect crop plants from infection.

An adult Chinese patient with developmental delay with short stature, dysmorphic features, and sparse hair (Loucks-Innes syndrome)

Variants of the diphthamide biosynthesis I (DPH1, OMIM*603527) are associated with developmental delay, short stature, and sparse hair syndrome (DEDSSH/DPH1 syndrome) (OMIM# 616901). Another name is Loucks-Innes syndrome. DPH1 syndrome is an ultrarare and severe neurodevelopmental disorder. Less than 20 patients were reported from different ethnicities. Here, we described the first Chinese adult with genetically confirmed DPH1 syndrome. We summarized previously reported patients in the literature and found that developmental delay, unusual skull shape, sparse hair, and facial dysmorphism were consistently present in all DPH1 syndrome patients. Dysplastic toenails and dental abnormalities are age-dependent characteristics of DPH1 syndrome. Our patient was the first reported patient with documented growth hormone deficiency. Dental and endocrine checkup should be considered in the routine follow-up of DPH1 syndrome patients.

Characterisation of diphtheria monoclonal antibodies as a first step towards the development of an in vitro vaccine potency immunoassay

Immunoassays are used for routine potency assessment of several vaccines, in some cases having been specifically developed as alternatives to in vivo potency tests. These methods require at least one well characterised monoclonal antibody (mAb) that is specific for the target antigen. In this paper we report the results of the comprehensive characterisation of a panel of mAbs against diphtheria with a view to select antibodies that can be used for development of an in vitro potency immunoassay for diphtheria vaccines. We have assessed binding of the antibodies to native antigen (toxin), detoxified antigen (toxoid), adsorbed antigen and heat-altered antigen. Antibody function was determined by a cell-based toxin neutralisation test and diphtheria toxin-domain recognition was determined by Western blotting. In addition, antibody affinity was measured, and epitope competition analysis was performed to identify pairs of antibodies that could be deployed in a sandwich immunoassay format. Not all characterisation tests provided evidence of "superiority" of one mAb over another, but together the results from all characterisation studies allowed for selection of an antibody pair to be taken forward to assay development.

The N-terminus of Paenibacillus larvae C3larvinA modulates catalytic efficiency

C3larvinA was recently described as a mono-ADP-ribosyltransferase (mART) toxin from the enterobacterial repetitive intergenic consensus (ERIC) III genotype of the agricultural pathogen, Paenibacillus larvae. It was shown to be the full-length, functional version of the previously described C3larvin_trunc toxin, due to a 33-residue extension of the N-terminus of the protein. In the present study, a series of deletions and substitutions were made to the N-terminus of C3larvinA to assess the contribution of the α₁-helix to toxin structure and function. Catalytic characterization of these variants identified Asp²³ and Ala³¹ residues as supportive to enzymatic function. A third residue, Lys³⁶, was also found to contribute to the catalytic activity of the enzyme. Analysis of the C3larvinA homology model revealed that these three residues were participating in a series of interactions to properly orient both the Q-X-E and S-T-S motifs. Ala³¹ and Lys³⁶ were found to associate with a structural network of residues previously identified in silico, whereas Asp²³ forms novel interactions not previously described. At last, the membrane translocation activity into host target cells of each variant was assessed, highlighting a possible relationship between protein dipole and target cell entry.

Cell Death Signaling Pathway Induced by Cholix Toxin, a Cytotoxin and eEF2 ADP-Ribosyltransferase Produced by Vibrio cholerae

Pathogenic microorganisms produce various virulence factors, e.g., enzymes, cytotoxins, effectors, which trigger development of pathologies in infectious diseases. Cholera toxin (CT) produced by O1 and O139 serotypes of Vibrio cholerae (V. cholerae) is a major cytotoxin causing severe diarrhea. Cholix cytotoxin (Cholix) was identified as a novel eukaryotic elongation factor 2 (eEF2) adenosine-diphosphate (ADP)-ribosyltransferase produced mainly in non-O1/non-O139 V. cholerae. The function and role of Cholix in infectious disease caused by V. cholerae remain unknown. The crystal structure of Cholix is similar to Pseudomonas exotoxin A (PEA) which is composed of an N-terminal receptor-recognition domain and a C-terminal ADP-ribosyltransferase domain. The endocytosed Cholix catalyzes ADP-ribosylation of eEF2 in host cells and inhibits protein synthesis, resulting in cell death. In a mouse model, Cholix caused lethality with severe liver damage. In this review, we describe the mechanism underlying Cholix-induced cytotoxicity. Cholix-induced apoptosis was regulated by mitogen-activated protein kinase (MAPK) and protein kinase C (PKC) signaling pathways, which dramatically enhanced tumor necrosis factor-α (TNF-α) production in human liver, as well as the amount of epithelial-like HepG2 cancer cells. In contrast, Cholix induced apoptosis in hepatocytes through a mitochondrial-dependent pathway, which was not stimulated by TNF-α. These findings suggest that sensitivity to Cholix depends on the target cell. A substantial amount of information on PEA is provided in order to compare/contrast this well-characterized mono-ADP-ribosyltransferase (mART) with Cholix.

Treating brain diseases using systemic parenterally-administered protein therapeutics: Dysfunction of the brain barriers and potential strategies

The parenteral administration of protein therapeutics is increasingly gaining importance for the treatment of human diseases. However, the presence of practically impermeable blood-brain barriers greatly restricts access of such pharmaceutics to the brain. Treating brain disorders with proteins thus remains a great challenge, and the slow clinical translation of these therapeutics may be largely ascribed to the lack of appropriate brain delivery system. Exploring new approaches to deliver proteins to the brain by circumventing physiological barriers is thus of great interest. Moreover, parallel advances in the molecular neurosciences are important for better characterizing blood-brain interfaces, particularly under different pathological conditions (e.g., stroke, multiple sclerosis, Parkinson's disease, and Alzheimer's disease). This review presents the current state of knowledge of the structure and the function of the main physiological barriers of the brain, the mechanisms of transport across these interfaces, as well as alterations to these concomitant with brain disorders. Further, the different strategies to promote protein delivery into the brain are presented, including the use of molecular Trojan horses, the formulation of nanosystems conjugated/loaded with proteins, protein-engineering technologies, the conjugation of proteins to polymers, and the modulation of intercellular junctions. Additionally, therapeutic approaches for brain diseases that do not involve targeting to the brain are presented (i.e., sink and scavenging mechanisms).

Pre-clinical and Clinical Implications of "Inside-Out" vs. "Outside-In" Paradigms in Multiple Sclerosis Etiopathogenesis

Multiple Sclerosis (MS) is an immune-mediated neurological disorder, characterized by central nervous system (CNS) inflammation, oligodendrocyte loss, demyelination, and axonal degeneration. Although autoimmunity, inflammatory demyelination and neurodegeneration underlie MS, the initiating event has yet to be clarified. Effective disease modifying therapies need to both regulate the immune system and promote restoration of neuronal function, including remyelination. The challenge in developing an effective long-lived therapy for MS requires that three disease-associated targets be addressed: (1) self-tolerance must be re-established to specifically inhibit the underlying myelin-directed autoimmune pathogenic mechanisms; (2) neurons must be protected from inflammatory injury and degeneration; (3) myelin repair must be engendered by stimulating oligodendrocyte progenitors to remyelinate CNS neuronal axons. The combined use of chronic and relapsing remitting experimental autoimmune encephalomyelitis (C-EAE, R-EAE) (“outside-in”) as well as progressive diphtheria toxin A chain (DTA) and cuprizone autoimmune encephalitis (CAE) (“inside-out”) mouse models allow for the investigation and specific targeting of all three of these MS-associated disease parameters. The “outside-in” EAE models initiated by myelin-specific autoreactive CD4⁺ T cells allow for the evaluation of both myelin-specific tolerance in the absence or presence of neuroprotective and/or remyelinating agents. The “inside-out” mouse models of secondary inflammatory demyelination are triggered by toxin-induced oligodendrocyte loss or subtle myelin damage, which allows evaluation of novel therapeutics that could promote remyelination and neuroprotection in the CNS. Overall, utilizing these complementary pre-clinical MS models will open new avenues for developing therapeutic interventions, tackling MS from the “outside-in” and/or “inside-out”.

Identification of the transcription factor Miz1 as an essential regulator of diphthamide biosynthesis using a CRISPR-mediated genome-wide screen

Diphthamide is a unique post-translationally modified histidine residue (His⁷¹⁵ in all mammals) found only in eukaryotic elongation factor-2 (eEF-2). The biosynthesis of diphthamide represents one of the most complex modifications, executed by protein factors conserved from yeast to humans. Diphthamide is not only essential for normal physiology (such as ensuring fidelity of mRNA translation), but is also exploited by bacterial ADP-ribosylating toxins (e.g., diphtheria toxin) as their molecular target in pathogenesis. Taking advantage of the observation that cells defective in diphthamide biosynthesis are resistant to ADP-ribosylating toxins, in the past four decades, seven essential genes (Dph1 to Dph7) have been identified for diphthamide biosynthesis. These technically unsaturated screens raise the question as to whether additional genes are required for diphthamide biosynthesis. In this study, we performed two independent, saturating, genome-wide CRISPR knockout screens in human cells. These screens identified all previously known Dph genes, as well as further identifying the BTB/POZ domain-containing transcription factor Miz1. We found that Miz1 is absolutely required for diphthamide biosynthesis via its role in the transcriptional regulation of Dph1 expression. Mechanistically, Miz1 binds to the Dph1 proximal promoter via an evolutionarily conserved consensus binding site to activate Dph1 transcription. Therefore, this work demonstrates that Dph1-7, along with the newly identified Miz1 transcription factor, are likely to represent the essential protein factors required for diphthamide modification on eEF2.

Diphthamide is a unique post-translationally modified histidine residue (His⁶⁹⁹ in yeast, His⁷¹⁵ in all mammals) found only in eukaryotic elongation factor-2 (eEF-2). Mice that are deficient in diphthamide biosynthesis are embryonic lethal, attesting to the importance of diphthamide in normal physiology. It has taken four decades to identify the seven non-redundant genes in diphthamide biosynthesis, but whether additional factors are required and how the pathway is regulated remained elusive. To address these issues, we performed two saturating, independent, and unbiased genome-wide CRISPR knockout screens. The screens concluded independently that Dph1-Dph7 and additionally transcription factor Miz1 are the key factors required for diphthamide biosynthesis. Mechanistically, Miz1 binds to the Dph1 proximal promoter via an evolutionarily conserved consensus binding site to activate Dph1 transcription. While diphthamide biosynthesis machinery (Dph1-Dph7) exists across eukaryotes, Miz1 orthologues do not exist in lower species such as yeast, C. elegans, and Drosophila, indicating that the regulation of diphthamide modification by Miz1 emerged much later in evolution. The work opens a new avenue for understanding the role that diphthamide modification plays in normal physiology and bacterial toxin pathogenesis.

DNA methyltransferase inhibition overcomes diphthamide pathway deficiencies underlying CD123-targeted treatment resistance

The interleukin-3 receptor α subunit, CD123, is expressed in many hematologic malignancies including acute myeloid leukemia (AML) and blastic plasmacytoid dendritic cell neoplasm (BPDCN). Tagraxofusp (SL-401) is a CD123-targeted therapy consisting of interleukin-3 fused to a truncated diphtheria toxin payload. Factors influencing response to tagraxofusp other than CD123 expression are largely unknown. We interrogated tagraxofusp resistance in patients and experimental models and found that it was not associated with CD123 loss. Rather, resistant AML and BPDCN cells frequently acquired deficiencies in the diphthamide synthesis pathway, impairing tagraxofusp's ability to ADP-ribosylate cellular targets. Expression of DPH1, encoding a diphthamide pathway enzyme, was reduced by DNA CpG methylation in resistant cells. Treatment with the DNA methyltransferase inhibitor azacitidine restored DPH1 expression and tagraxofusp sensitivity. We also developed a drug-dependent ADP-ribosylation assay in primary cells that correlated with tagraxofusp activity and may represent an additional novel biomarker. As predicted by these results and our observation that resistance also increased mitochondrial apoptotic priming, we found that the combination of tagraxofusp and azacitidine was effective in patient-derived xenografts treated in vivo. These data have important implications for clinical use of tagraxofusp and led to a phase 1 study combining tagraxofusp and azacitidine in myeloid malignancies.

Genetic manipulation of ureteric bud tip progenitors in the mammalian kidney through an Adamts18 enhancer driven tet-on inducible system

The ureteric epithelial progenitor (UEP) population within the embryonic kidney generates the arborized epithelial network of the kidney's collecting system and plays a critical role in the expansion and induction of the surrounding nephron progenitor pool. Adamts18 shows UEP- restricted expression in the kidney and progenitor tip-restricted expression in several other organs undergoing branching epithelial growth. Adamts18 is encoded by 23 exons. Genetic removal of genomic sequence spanning exons 1 to 3 led to a specific loss of Adamts18 expression in UEPs, suggesting this region may encode a UEP-specific enhancer. Intron 2 (3 ​kb) was shown to have enhancer activity driving expression of the doxycycline inducible tet-on transcriptional regulator (rtTA) in an Adamts18en-rtTA transgenic mouse strain. Crossing Adamts18en-rtTA mice to a doxycycline dependent GFP reporter mouse enabled the live imaging of embryonic kidney explants. This facilitated the analysis of ureteric epithelial branching events at the cellular level. Ablation of UEPs at the initiation of ureteric bud outgrowth through the doxycycline-mediated induction of Diphtheria Toxin A (DTA) generated a range of phenotypes from complete kidneys agenesis, to duplex kidneys with double ureters. The latter outcome points to the potential of regulative processes to restore UEPs. In contrast, overexpression of YAP prior to ureteric bud outgrowth led to a complete failure of kidney development. Elevating YAP levels at later stages retarded branching growth. A similar phenotype was observed with the overexpression of MYC within the branch-tip localized UEP population. These experiments showcase the utility of the Adamts18en-rtTA transgenic model to the investigation of cellular and molecular events specific to branch tip progenitors within the mammalian kidney complementing existing CRE-dependent genetic tools. Further, the illustrative examples point to areas where new insight may be gained into the regulation of UEP programs.

The C. difficile toxin B membrane translocation machinery is an evolutionarily conserved protein delivery apparatus

Large Clostridial Toxins (LCTs) are a family of six homologous protein toxins that are implicated in severe disease. LCTs infiltrate host cells using a translocation domain (LCT-T) that contains both cell-surface receptor binding sites and a membrane translocation apparatus. Despite much effort, LCT translocation remains poorly understood. Here we report the identification of 1104 LCT-T homologs, with 769 proteins from bacteria outside of clostridia. Sequences are widely distributed in pathogenic and host-associated species, in a variety of contexts and architectures. Consistent with these homologs being functional toxins, we show that a distant LCT-T homolog from Serratia marcescens acts as a pH-dependent translocase to deliver its effector into host cells. Based on evolutionary footprinting of LCT-T homologs, we further define an evolutionarily conserved translocase region that we show is an autonomous translocase capable of delivering heterologous cargo into host cells. Our work uncovers a broad class of translocating toxins and provides insights into LCT translocation.

Large Clostridial toxins infiltrate host cells using a translocation domain (LCT-T). Here, using a genomics-driven approach and functional assays, the authors uncover the presence of distant LCT-T homologs in bacteria outside clostridia and provide evidence for a toxic effector function in the gammaproteobacterium Serratia marcescens.

Fumagillin, a Mycotoxin of Aspergillus fumigatus: Biosynthesis, Biological Activities, Detection, and Applications

Fumagillin is a mycotoxin produced, above all, by the saprophytic filamentous fungus Aspergillus fumigatus. This mold is an opportunistic pathogen that can cause invasive aspergillosis, a disease that has high mortality rates linked to it. Its ability to adapt to environmental stresses through the production of secondary metabolites, including several mycotoxins (gliotoxin, fumagillin, pseurotin A, etc.) also seem to play an important role in causing these infections. Since the discovery of the A. fumigatus fumagillin in 1949, many studies have focused on this toxin and in this review we gather all the information currently available. First of all, the structural characteristics of this mycotoxin and the different methods developed for its determination are given in detail. Then, the biosynthetic gene cluster and the metabolic pathway involved in its production and regulation are explained. The activity of fumagillin on its target, the methionine aminopeptidase type 2 (MetAP2) enzyme, and the effects of blocking this enzyme in the host are also described. Finally, the applications that this toxin and its derivatives have in different fields, such as the treatment of cancer and its microsporicidal activity in the treatment of honeybee hive infections with Nosema spp., are reviewed. Therefore, this work offers a complete review of all the information currently related to the fumagillin mycotoxin secreted by A. fumigatus, important because of its role in the fungal infection process but also because it has many other applications, notably in beekeeping, the treatment of infectious diseases, and in oncology.

Targeted Diphtheria Toxin-Based Therapy: A Review Article

Cancer remains one of the leading causes of death worldwide. Conventional therapeutic strategies usually offer limited specificity, resulting in severe side effects and toxicity to normal tissues. Targeted cancer therapy, on the other hand, can improve the therapeutic potential of anti-cancer agents and decrease unwanted side effects. Targeted applications of cytolethal bacterial toxins have been found to be especially useful for the specific eradication of cancer cells. Targeting is either mediated by peptides or by protein-targeting moieties, such as antibodies, antibody fragments, cell-penetrating peptides (CPPs), growth factors, or cytokines. Together with a toxin domain, these molecules are more commonly referred to as immunotoxins. Targeting can also be achieved through gene delivery and cell-specific expression of a toxin. Of the available cytolethal toxins, diphtheria toxin (DT) is one of the most frequently used for these strategies. Of the many DT-based therapeutic strategies investigated to date, two immunotoxins, Ontak^TM and Tagraxofusp^TM, have gained FDA approval for clinical application. Despite some success with immunotoxins, suicide-gene therapy strategies, whereby controlled tumor-specific expression of DT is used for the eradication of malignant cells, are gaining prominence. The first part of this review focuses on DT-based immunotoxins, and it then discusses recent developments in tumor-specific expression of DT.

FPR1 is the plague receptor on host immune cells

The plague agent, Yersinia pestis, employs a type III secretion system (T3SS) to selectively destroy human immune cells, thereby enabling its replication in the bloodstream and transmission to new hosts via fleabite. The host factors responsible for the selective destruction of immune cells by plague bacteria were not known. Here we show that LcrV, the needle cap protein of the Y. pestis T3SS, binds N-formylpeptide receptor (FPR1) on human immune cells to promote the translocation of bacterial effectors. Plague infection in mice is characterized by high mortality, however N-formylpeptide receptor deficient animals exhibit increased survival and plague-protective antibody responses. We identified FPR1 p.R190W as a candidate human resistance allele that protects neutrophils from Y. pestis T3SS. These findings reveal the plague receptor on immune cells and show that FPR1 mutations provide for plague survival, which appears to have shaped human immune responses towards other infectious diseases and malignant neoplasms.

Peptides derived from a short stretch of diphtheria toxin bind to heparin-binding epidermal growth factor-like growth factor

Membrane-anchored heparin-binding EGF-like growth factor (HB-EGF) is the receptor for diphtheria toxin (DT). Mutated or truncated, non-toxic DT has been used earlier for HB-EGF-targeted drug delivery and to modulate HB-EGF signaling. In the present work, we have synthesized a peptide corresponding to a 26 amino acid long stretch of the receptor-binding domain of DT. This region of DT makes multiple contacts with HB-EGF and has residues critical for binding to HB-EGF. We show that this peptide and two of its mutants bind to HB-EGF. We have also created recombinant proteins by fusing Maltose-binding Protein (MBP) with these peptides. These recombinant MBP-tagged peptides bind to HB-EGF with affinities in the range of 10^-7 to 10^-8 M. We have observed that these MBP-tagged peptides can modulate molecular signaling of HB-EGF. Therefore, this 26 amino acid long stretch of DT can be considered as an independent functional segment for binding to HB-EGF. Peptides corresponding to this region may be used for HB-EGF targeted cellular delivery of molecular cargo or to modulate HB-EGF signaling.

The asymmetric function of Dph1-Dph2 heterodimer in diphthamide biosynthesis

Diphthamide, the target of diphtheria toxin, is a post-translationally modified histidine residue found in archaeal and eukaryotic translation elongation factor 2 (EF2). In the first step of diphthamide biosynthesis, a [4Fe-4S] cluster-containing radical SAM enzyme, Dph1-Dph2 heterodimer in eukaryotes or Dph2 homodimer in archaea, cleaves S-adenosylmethionine and transfers the 3-amino-3-carboxypropyl group to EF2. It was demonstrated previously that for the archaeal Dph2 homodimer, only one [4Fe-4S] cluster is necessary for the in vitro activity. Here, we demonstrate that for the eukaryotic Dph1-Dph2 heterodimer, the [4Fe-4S] cluster-binding cysteine residues in each subunit are required for diphthamide biosynthesis to occur in vivo. Furthermore, our in vitro reconstitution experiments with Dph1-Dph2 mutants suggested that the Dph1 cluster serves a catalytic role, while the Dph2 cluster facilitates the reduction of the Dph1 cluster by the physiological reducing system Dph3/Cbr1/NADH. Our results reveal the asymmetric functional roles of the Dph1-Dph2 heterodimer and may help to understand how the Fe-S clusters in radical SAM enzymes are reduced in biology.

Site-specific analysis of the Asp- and Glu-ADP-ribosylated proteome by quantitative mass spectrometry

ADP-ribosylation is a protein post-translational modification that is critically involved in a wide array of biological processes connected to cell stress responses. Enzymes known as poly-ADP-ribose polymerases (PARPs) catalyze the addition of the ADP-ribose units to amino acids with various side chain chemistries. In particular, the PARP family member PARP1 is responsible for the modification of a large number of proteins and is involved in initiation of the DNA damage response, although the mechanisms through which PARP1 functions are still incompletely understood. The analysis of protein ADP-ribosylation is challenging because PARylation is a low-abundance, labile and heterogeneous protein modification. Recently, we developed an integrative proteomic platform for the site-specific analysis of protein ADP-ribosylation on Asp and Glu residues. Herein, we describe the method, and demonstrate its utility in quantitative characterization of the human Asp- and Glu-ADP-ribosylated proteome.

Targeting ADP-ribosylation as an antimicrobial strategy

ADP-ribosylation (ADPr) is an ancient reversible modification of cellular macromolecules controlling major biological processes as diverse as DNA damage repair, transcriptional regulation, intracellular transport, immune and stress responses, cell survival and proliferation. Furthermore, enzymatic reactions of ADPr are central in the pathogenesis of many human diseases, including infectious conditions. By providing a review of ADPr signalling in bacterial systems, we highlight the relevance of this chemical modification in the pathogenesis of human diseases depending on host-pathogen interactions. The post-antibiotic era has raised the need to find alternative approaches to antibiotic administration, as major pathogens becoming resistant to antibiotics. An in-depth understanding of ADPr reactions provides the rationale for designing novel antimicrobial strategies for treatment of infectious diseases. In addition, the understanding of mechanisms of ADPr by bacterial virulence factors offers important hints to improve our knowledge on cellular processes regulated by eukaryotic homologous enzymes, which are often involved in the pathogenesis of human diseases.

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.

Disulfide bond of Mycoplasma pneumoniae community-acquired respiratory distress syndrome toxin is essential to maintain the ADP-ribosylating and vacuolating activities

Mycoplasma pneumoniae is the leading cause of bacterial community-acquired pneumonia among hospitalised children in United States and worldwide. Community-acquired respiratory distress syndrome (CARDS) toxin is a key virulence determinant of M. pneumoniae. The N-terminus of CARDS toxin exhibits ADP-ribosyltransferase (ADPRT) activity, and the C-terminus possesses binding and vacuolating activities. Thiol-trapping experiments of wild-type (WT) and cysteine-to-serine-mutated CARDS toxins with alkylating agents identified disulfide bond formation at the amino terminal cysteine residues C230 and C247. Compared with WT and other mutant toxins, C247S was unstable and unusable for comparative studies. Although there were no significant variations in binding, entry, and retrograde trafficking patterns of WT and mutated toxins, C230S did not elicit vacuole formation in intoxicated cells. In addition, the ADPRT domain of C230S was more sensitive to all tested proteases when compared with WT toxin. Despite its in vitro ADPRT activity, the reduction of C230S CARDS toxin-mediated ADPRT activity-associated IL-1β production in U937 cells and the recovery of vacuolating activity in the protease-released carboxy region of C230S indicated that the disulfide bond was essential not only to maintain the conformational stability of CARDS toxin but also to properly execute its cytopathic effects.

Soritesidine, a Novel Proteinous Toxin from the Okinawan Marine Sponge Spongosorites sp

A novel protein, soritesidine (SOR) with potent toxicity was isolated from the marine sponge Spongosorites sp. SOR exhibited wide range of toxicities over various organisms and cells including brine shrimp (Artemia salina) larvae, sea hare (Aplysia kurodai) eggs, mice, and cultured mammalian cells. Toxicities of SOR were extraordinary potent. It killed mice at 5 ng/mouse after intracerebroventricular (i.c.v.) injection, and brine shrimp and at 0.34 µg/mL. Cytotoxicity for cultured mammalian cancer cell lines against HeLa and L1210 cells were determined to be 0.062 and 12.11 ng/mL, respectively. The SOR-containing fraction cleaved plasmid DNA in a metal ion dependent manner showing genotoxicity of SOR. Purified SOR exhibited molecular weight of 108.7 kDa in MALDI-TOF MS data and isoelectric point of approximately 4.5. N-terminal amino acid sequence up to the 25th residue was determined by Edman degradation. Internal amino acid sequences for fifteen peptides isolated from the enzyme digest of SOR were also determined. None of those amino acid sequences showed similarity to existing proteins, suggesting that SOR is a new proteinous toxin.

DPH1 syndrome: two novel variants and structural and functional analyses of seven missense variants identified in syndromic patients

DPH1 variants have been associated with an ultra-rare and severe neurodevelopmental disorder, mainly characterized by variable developmental delay, short stature, dysmorphic features, and sparse hair. We have identified four new patients (from two different families) carrying novel variants in DPH1, enriching the clinical delineation of the DPH1 syndrome. Using a diphtheria toxin ADP-ribosylation assay, we have analyzed the activity of seven identified variants and demonstrated compromised function for five of them [p.(Leu234Pro); p.(Ala411Argfs*91); p.(Leu164Pro); p.(Leu125Pro); and p.(Tyr112Cys)]. We have built a homology model of the human DPH1-DPH2 heterodimer and have performed molecular dynamics simulations to study the effect of these variants on the catalytic sites as well as on the interactions between subunits of the heterodimer. The results show correlation between loss of activity, reduced size of the opening to the catalytic site, and changes in the size of the catalytic site with clinical severity. This is the first report of functional tests of DPH1 variants associated with the DPH1 syndrome. We demonstrate that the in vitro assay for DPH1 protein activity, together with structural modeling, are useful tools for assessing the effect of the variants on DPH1 function and may be used for predicting patient outcomes and prognoses.

The DTA Mouse Model for Oligodendrocyte Ablation and CNS Demyelination

Genetic mouse models facilitate investigation of mechanisms underpinning human diseases and aid the development of novel therapeutic treatments. To better understand the demyelination and remyelination processes in adult-onset demyelinating diseases like multiple sclerosis (MS), we have developed the DTA mouse model system that allows for the widespread ablation of the mature oligodendrocytes, resulting in demyelination throughout the central nervous system (CNS). Induction of oligodendrocyte death in young adult DTA mice causes extensive CNS demyelination that leads to a severe neurological disease, followed by a full recovery that is associated with extensive replenishment of oligodendrocytes and remyelination. Thus, the DTA mouse enables investigation of the mechanisms that promote remyelination in MS and other adult-onset demyelinating diseases. Approximately 30 weeks later, the recovered DTA mice develop a fatal secondary demyelinating disease that is mediated by autoimmune T cells. Therefore, the DTA mouse model is also ideal for elucidating the role of oligodendrocyte death in eliciting autoimmunity in MS. In this chapter we describe the methods we used to generate the DTA mouse model and to analyze both the primary and secondary demyelinating diseases in DTA mice.

Macrophages restrict the nephrogenic field and promote endothelial connections during kidney development

The origins and functions of kidney macrophages in the adult have been explored, but their roles during development remain largely unknown. Here we characterise macrophage arrival, localisation, heterogeneity, and functions during kidney organogenesis. Using genetic approaches to ablate macrophages, we identify a role for macrophages in nephron progenitor cell clearance as mouse kidney development begins. Throughout renal organogenesis, most kidney macrophages are perivascular and express F4/80 and CD206. These macrophages are enriched for mRNAs linked to developmental processes, such as blood vessel morphogenesis. Using antibody-mediated macrophage-depletion, we show macrophages support vascular anastomoses in cultured kidney explants. We also characterise a subpopulation of galectin-3⁺ (Gal3⁺) myeloid cells within the developing kidney. Our findings may stimulate research into macrophage-based therapies for renal developmental abnormalities and have implications for the generation of bioengineered kidney tissues.

The kidneys clean our blood by filtering out waste products while ensuring that useful components, like nutrients, remain in the bloodstream. Blood enters the kidneys through a network of intricately arranged blood vessels, which associate closely with the ‘cleaning tubes’ that carry out filtration. Human kidneys start developing during the early phases of embryonic development. During this process, the newly forming blood vessels and cleaning tubes must grow in the right places for the adult kidney to work properly.

Macrophages are cells of the immune system that clear away foreign, diseased, or damaged cells. They are also thought to encourage growth of the developing kidney, but how exactly they do this has remained unknown. Munro et al. therefore wanted to find out when macrophages first appeared in the embryonic kidney and how they might help control their development.

Experiments using mice revealed that the first macrophages arrived in the kidney early during its development, alongside newly forming blood vessels. Further investigation using genetically modified mice that did not have macrophages revealed that these immune cells were needed at this stage to clear away misplaced kidney cells and help ‘set the scene’ for future development.

At later stages, macrophages in the kidney interacted closely with growing blood vessels. As well as producing molecules linked with blood vessel formation, the macrophages wrapped around the vessels themselves, sometimes even eating cells lining the vessels and the blood cells carried within them. These observations suggested that macrophages actively shaped the network of blood vessels developing within the kidneys. Experiments removing macrophages from kidney tissue confirmed this: in normal kidneys, the blood vessels grew into a continuous network, but in kidneys lacking macrophages, far fewer connections formed between the vessels.

This work sheds new light on how the complex structures in the adult kidney first arise and could be useful in future research. For example, adding macrophages to simplified, laboratory-grown ‘mini-kidneys’ could make them better models to study kidney growth, while patients suffering from kidney diseases might benefit from new drugs targeting macrophages.

Purification and characterization of native human elongation factor 2

Human elongation factor 2 is the translocase that is responsible for the movement of tRNA from the A- to P- and P- to E-site on the ribosome during the elongation phase of translation. Being a vital factor of protein biosynthesis, its function is highly controlled and regulated. It has been implicated in numerous diseases and pathologies, and as such it is important to have a source for isolated pure and active protein for biomedical and biochemical studies. Here we report development of a purification protocol for native human elongation factor 2 from HEK-293S cells. The resulting protein is active, pure, has an intact diphtamide and is obtainable in yields suitable for functional and structural studies.