Anthrax toxin triggers the activation of src-like kinases to mediate its own uptake

ER-Golgi-localized proteins TMED2 and TMED10 control the formation of plasma membrane lipid nanodomains

To promote infections, pathogens exploit host cell machineries such as structural elements of the plasma membrane. Studying these interactions and identifying molecular players are ideal for gaining insights into the fundamental biology of the host cell. Here, we used the anthrax toxin to screen a library of 1,500 regulatory, cell-surface, and membrane trafficking genes for their involvement in the intoxication process. We found that endoplasmic reticulum (ER)-Golgi-localized proteins TMED2 and TMED10 are required for toxin oligomerization at the plasma membrane of human cells, an essential step dependent on localization to cholesterol-rich lipid nanodomains. Biochemical, morphological, and mechanistic analyses showed that TMED2 and TMED10 are essential components of a supercomplex that operates the exchange of both cholesterol and ceramides at ER-Golgi membrane contact sites. Overall, this study of anthrax intoxication led to the discovery that lipid compositional remodeling at ER-Golgi interfaces fully controls the formation of functional membrane nanodomains at the cell surface.

The Capillary Morphogenesis Gene 2 Triggers the Intracellular Hallmarks of Collagen VI-Related Muscular Dystrophy

Collagen VI-related disorders (COL6-RD) represent a severe form of congenital disease for which there is no treatment. Dominant-negative pathogenic variants in the genes encoding α chains of collagen VI are the main cause of COL6-RD. Here we report that patient-derived fibroblasts carrying a common single nucleotide variant mutation are unable to build the extracellular collagen VI network. This correlates with the intracellular accumulation of endosomes and lysosomes triggered by the increased phosphorylation of the collagen VI receptor CMG2. Notably, using a CRISPR-Cas9 gene-editing tool to silence the dominant-negative mutation in patients’ cells, we rescued the normal extracellular collagen VI network, CMG2 phosphorylation levels, and the accumulation of endosomes and lysosomes. Our findings reveal an unanticipated role of CMG2 in regulating endosomal and lysosomal homeostasis and suggest that mutated collagen VI dysregulates the intracellular environment in fibroblasts in collagen VI-related muscular dystrophy.

Protein Lipidation by Palmitate Controls Macrophage Function

Macrophages are present in all tissues within our body, where they promote tissue homeostasis by responding to microenvironmental triggers, not only through clearance of pathogens and apoptotic cells but also via trophic, regulatory, and repair functions. To accomplish these divergent functions, tremendous dynamic fine-tuning of their physiology is needed. Emerging evidence indicates that S-palmitoylation, a reversible post-translational modification that involves the linkage of the saturated fatty acid palmitate to protein cysteine residues, directs many aspects of macrophage physiology in health and disease. By controlling protein activity, stability, trafficking, and protein–protein interactions, studies identified a key role of S-palmitoylation in endocytosis, inflammatory signaling, chemotaxis, and lysosomal function. Here, we provide an in-depth overview of the impact of S-palmitoylation on these cellular processes in macrophages in health and disease. Findings discussed in this review highlight the therapeutic potential of modulators of S-palmitoylation in immunopathologies, ranging from infectious and chronic inflammatory disorders to metabolic conditions.

Infantile systemic hyalinosis: Variable grades of severity

BACKGROUND

Infantile systemic hyalinosis (ISH) is an autosomal recessively inherited disorder. The classical natural history of the disease is characterised by hypotonia, multiple contractures, skin lesions, osteopenia, joint pain, bone fractures, persistent diarrhoea and growth deficiency.

MATERIALS AND METHODS

Two children manifested the severe type of ISH underwent genotypic confirmation. In order to identify which other family members have inherited the disease. We included siblings and cousins in this study. The baseline tool to study other family subjects was based on the phenotypic characterisations of each child.

RESULTS

. Two children with the severe type of ISH showed craniosynostosis (brachycephaly and scaphocephaly) associated with multiple contractures, progressive joint osteolysis ending up with multiple joint dislocations. The full exome sequencing was carried out, revealing a previously reported heterozygous nonsense mutation с.1294С>Т and a novel heterozygous non-synonymous substitution c. 58T>A in ANTRX2 gene. Three children (sibling and cousins) manifested variable clinical manifestations relevant to ISH. Specifically, asymptoamtic skin and skeletal abnormalities of hypoplastic clavicles and 'shepherd's crook' deformity and coxa vara.

CONCLUSION

It is mandatory to perform extensive family pedigree search to detect asymptomatic clinical features in siblings and cousins in families with first degree related marriages. Interestingly, in the mild and the moderate types of ISH, we observed undescribed combination of asymptomatic skin and skeletal abnormalities. This is a comparative study between the severe and the mild/moderate types in a group of children from consanguineous families. Our current study extends the phenotypic characterisations of ISH.

Genetic deletion of CMG2 exacerbates systemic-to-pulmonary shunt-induced pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) secondary to congenital heart disease (CHD-PAH) with systemic-to-pulmonary shunt (SPS) is characterized by proliferative vascular remodeling. Capillary morphogenesis gene-2 (CMG2) plays a key role in cell proliferation and apoptosis. This study aimed to determine the role of CMG2 in the pathogenesis of SPS-induced PAH. CMG2 levels were significantly downregulated in pulmonary arterioles from patients with Eisenmenger syndrome and rats with SPS-induced PAH. CMG2 was highly expressed in several cells including human pulmonary arterial smooth muscle cells (HPASMCs). CMG2^-/- rats exhibited more severe PAH and pulmonary vascular remodeling than wild-type rats when exposed to SPS for 8 weeks. Overexpression of CMG2 significantly inhibited proliferation and promoted apoptosis of HPASMCs, while knockdown of CMG2 promoted cell proliferation and inhibited cell apoptosis. Next-generation sequencing and subsequent validation results suggested that PI3K-AKT was the most prominent signaling pathway regulated by differentially expressed genes (DEGs) in CMG2^-/- rat lungs. Our work identified a novel role for CMG2 in SPS-induced PAH based on the findings that CMG2 deficiency exacerbates SPS-induced vascular remodeling in the development of PAH, indicating that CMG2 might act as a potential target for the treatment of CHD-PAH.

Antagonistic regulation controls clathrin-mediated endocytosis: AP2 adaptor facilitation vs restraint from clathrin light chains

Orchestration of a complex network of protein interactions drives clathrin-mediated endocytosis (CME). A central role for the AP2 adaptor complex beyond cargo recognition and clathrin recruitment has emerged in recent years. It is now apparent that AP2 serves as a pivotal hub for protein interactions to mediate clathrin coated pit maturation, and couples lattice formation to membrane deformation. As a key driver for clathrin assembly, AP2 complements the attenuating role of clathrin light chain subunits, which enable dynamic lattice rearrangement needed for budding. This review summarises recent insights into AP2 function with respect to CME dynamics and biophysics, and its relationship to the role of clathrin light chains in clathrin assembly.

Ligand Binding to the Collagen VI Receptor Triggers a Talin-to-RhoA Switch that Regulates Receptor Endocytosis

Capillary morphogenesis gene 2 (CMG2/ANTXR2) is a cell surface receptor for both collagen VI and anthrax toxin. Biallelic loss-of-function mutations in CMG2 lead to a severe condition, hyaline fibromatosis syndrome (HFS). We have here dissected a network of dynamic interactions between CMG2 and various actin interactors and regulators, describing a different behavior from other extracellular matrix receptors. CMG2 binds talin, and thereby the actin cytoskeleton, only in its ligand-free state. Extracellular ligand binding leads to src-dependent talin release and recruitment of the actin cytoskeleton regulator RhoA and its effectors. These sequential interactions of CMG2 are necessary for the control of oriented cell division during fish development. Finally, we demonstrate that effective switching between talin and RhoA binding is required for the intracellular degradation of collagen VI in human fibroblasts, which explains why HFS mutations in the cytoskeleton-binding domain lead to dysregulation of extracellular matrix homeostasis.

A Canstatin-Derived Peptide Provides Insight into the Role of Capillary Morphogenesis Gene 2 in Angiogenic Regulation and Matrix Uptake

Capillary Morphogenesis Gene 2 protein (CMG2) is a transmembrane, integrin-like receptor and the primary receptor for the anthrax toxin. CMG2 also plays a role in angiogenic processes. However, the molecular mechanism that mediates the observed CMG2-related angiogenic effects is not fully elucidated. Previous studies have reported that CMG2 binds type IV collagen (Col-IV), a vital component of the vascular basement membrane, as well as other ECM proteins. Here, we further characterize the interaction between CMG2 and individual peptides from Col-IV and explore the effects of this interaction on angiogenesis. Using a peptide array, we observed that CMG2 preferentially binds peptide fragments of the NC1 (noncollagenous domain 1) domains of Col-IV. These domains are also known as the fragments arresten (from the α1 chain) and canstatin (from the α2 chain) and have documented antiangiogenic properties. A second peptide array was probed to map a putative peptide-binding epitope onto the Col-IV structure. A top hit from the initial array, a canstatin-derived peptide, binds to the CMG2 ligand-binding von Willebrand factor A (vWA) domain with a submicromolar affinity (peptide S16, K_d = 400 ± 200 nM). This peptide competes with anthrax protective antigen (PA) for CMG2 binding and does not bind CMG2 in the presence of EDTA. Together these data suggest that, like PA, S16 interacts with CMG2 at the metal-ion dependent adhesion site (MIDAS) of its vWA domain. CMG2 specifically mediates endocytic uptake of S16; both CMG2-/- endothelial cells and WT cells treated with PA show markedly reduced S16 uptake. Furthermore, S16 dramatically reduces directional endothelial cell migration with no impact on cell proliferation. These data demonstrate that this canstatin-derived peptide acts via CMG2 to elicit a marked effect on a critical process required for angiogenesis.

Converging physiological roles of the anthrax toxin receptors

The anthrax toxin receptors-capillary morphogenesis gene 2 (CMG2) and tumor endothelial marker 8 (TEM8)-were identified almost 20 years ago, although few studies have moved beyond their roles as receptors for the anthrax toxins to address their physiological functions. In the last few years, insight into their endogenous roles has come from two rare diseases: hyaline fibromatosis syndrome, caused by mutations in CMG2, and growth retardation, alopecia, pseudo-anodontia, and optic atrophy (GAPO) syndrome, caused by loss-of-function mutations in TEM8. Although CMG2 and TEM8 are highly homologous at the protein level, the difference in disease symptoms points to variations in the physiological roles of the two anthrax receptors. Here, we focus on the similarities between these receptors in their ability to regulate extracellular matrix homeostasis, angiogenesis, cell migration, and skin elasticity. In this way, we shed light on how mutations in these two related proteins cause such seemingly different diseases and we highlight the existing knowledge gaps that could form the focus of future studies.

Bacillus anthracis Protective Antigen Shows High Specificity for a UV Induced Mouse Model of Cutaneous Squamous Cell Carcinoma

Squamous cell carcinoma (SCC) accounts for the majority of non-melanoma skin cancer related deaths, particularly in immunosuppressed persons. Identification of biomarkers that could be used to identify or treat SCC would be of significant benefit. The anthrax toxin receptors, Tumor Endothelial Marker 8 (TEM8) and Capillary Morphogenesis Gene 2 (CMG2), are endothelial receptors involved in extracellular matrix homeostasis and angiogenesis that are selectively upregulated on numerous tumors. One method of targeting these receptors is Protective Antigen (PA), a protein produced by B. anthracis that mediates binding and translocation of anthrax toxins into cells. PA targeted toxins have been demonstrated to selectively inhibit tumor growth and angiogenesis, but tumor selectivity of PA is currently unknown. In this work fluorescently labeled PA was shown to maintain receptor dependent binding and internalization in vitro. Utilizing a human papillomavirus transgenic mouse model that develops cutaneous SCC in response to ultraviolet irradiation we identified tumor uptake of PA in vivo. The intravenously administered PA resulted in tumor specific localization, with exclusive tumor detection 24 h post injection. Ex vivo analysis identified significantly higher fluorescence in the tumor compared to adjacent healthy tissue and major clearance organs, demonstrating low non-specific uptake and rapid clearance. While both TEM8 and CMG2 were observed to be overexpressed in SCC tumor sections compared to control skin, the intravenously administered PA was primarily co-localized with TEM8. These results suggest that PA could be systemically administered for rapid identification of cutaneous SCC, with potential for further therapeutic development.

TEM8 functions as a receptor for uPA and mediates uPA-stimulated EGFR phosphorylation

Background

TEM8 is a cell membrane protein predominantly expressed in tumor endothelium, which serves as a receptor for the protective antigen (PA) of anthrax toxin. However, the physiological ligands for TEM8 remain unknown.

Results

Here we identified uPA as an interacting partner of TEM8. Binding of uPA stimulated the phosphorylation of TEM8 and augmented phosphorylation of EGFR and ERK1/2. Finally, TEM8-Fc, a recombinant fusion protein comprising the extracellular domain of human TEM8 linked to the Fc portion of human IgG1, efficiently abrogated the interaction between uPA and TEM8, blocked uPA-induced migration of HepG2 cells in vitro and inhibited the growth and metastasis of human MCF-7 xenografts in vivo. uPA, TEM8 and EGFR overexpression and ERK1/2 phosphorylation were found co-located on frozen cancer tissue sections.

Conclusions

Taken together, our data provide evidence that TEM8 is a novel receptor for uPA, which may play a significant role in the regulation of tumor growth and metastasis.

Electronic supplementary material

The online version of this article (10.1186/s12964-018-0272-8) contains supplementary material, which is available to authorized users.

Protein Palmitoylation and Its Role in Bacterial and Viral Infections

S-palmitoylation is a reversible, enzymatic posttranslational modification of proteins in which palmitoyl chain is attached to a cysteine residue via a thioester linkage. S-palmitoylation determines the functioning of proteins by affecting their association with membranes, compartmentalization in membrane domains, trafficking, and stability. In this review, we focus on S-palmitoylation of proteins, which are crucial for the interactions of pathogenic bacteria and viruses with the host. We discuss the role of palmitoylated proteins in the invasion of host cells by bacteria and viruses, and those involved in the host responses to the infection. We highlight recent data on protein S-palmitoylation in pathogens and their hosts obtained owing to the development of methods based on click chemistry and acyl-biotin exchange allowing proteomic analysis of protein lipidation. The role of the palmitoyl moiety present in bacterial lipopolysaccharide and lipoproteins, contributing to infectivity and affecting recognition of bacteria by innate immune receptors, is also discussed.

Lysenin Toxin Membrane Insertion Is pH-Dependent but Independent of Neighboring Lysenins

Pore-forming toxins form a family of proteins that act as virulence factors of pathogenic bacteria, but similar proteins are found in all kingdoms of life, including the vertebrate immune system. They are secreted as soluble monomers that oligomerize on target membranes in the so-called prepore state; after activation, they insert into the membrane and adopt the pore state. Lysenin is a pore-forming toxin from the earthworm Eisenida foetida, of which both the soluble and membrane-inserted structures are solved. However, the activation and membrane-insertion mechanisms have remained elusive. Here, we used high-speed atomic force microscopy to directly visualize the membrane-insertion mechanism. Changing the environmental pH from pH 7.5 to below pH 6.0 favored membrane insertion. We detected a short α-helix in the soluble structure that comprised three glutamic acids (Glu92, Glu94, and Glu97) that we hypothesized may represent a pH-sensor (as in similar toxins, e.g., Listeriolysin). Mutant lysenin still can form pores, but mutating these glutamic acids to glutamines rendered the toxin pH-insensitive. On the other hand, toxins in the pore state did not favor insertion of neighboring prepores; indeed, pore insertion breaks the hexagonal ordered domains of prepores and separates from neighboring molecules in the membrane. pH-dependent activation of toxins may represent a common feature of pore-forming toxins. High-speed atomic force microscopy with single-molecule resolution at high temporal resolution and the possibility of exchanging buffers during the experiments presents itself as a unique tool for the study of toxin-state conversion.

Bacterial Toxins for Cancer Therapy

Several pathogenic bacteria secrete toxins to inhibit the immune system of the infected organism. Frequently, they catalyze a covalent modification of specific proteins. Thereby, they block production and/or secretion of antibodies or cytokines. Moreover, they disable migration of macrophages and disturb the barrier function of epithelia. In most cases, these toxins are extremely effective enzymes with high specificity towards their cellular substrates, which are often central signaling molecules. Moreover, they encompass the capacity to enter mammalian cells and to modify their substrates in the cytosol. A few molecules, at least of some toxins, are sufficient to change the cellular morphology and function of a cell or even kill a cell. Since many of those toxins are well studied concerning molecular mechanisms, cellular receptors, uptake routes, and structures, they are now widely used to analyze or to influence specific signaling pathways of mammalian cells. Here, we review the development of immunotoxins and targeted toxins for the treatment of a disease that is still hard to treat: cancer.

CMG2/ANTXR2 regulates extracellular collagen VI which accumulates in hyaline fibromatosis syndrome

Loss-of-function mutations in capillary morphogenesis gene 2 (CMG2/ANTXR2), a transmembrane surface protein, cause hyaline fibromatosis syndrome (HFS), a severe genetic disorder that is characterized by large subcutaneous nodules, gingival hypertrophy and severe painful joint contracture. Here we show that CMG2 is an important regulator of collagen VI homoeostasis. CMG2 loss of function promotes accumulation of collagen VI in patients, leading in particular to nodule formation. Similarly, collagen VI accumulates massively in uteri of Antxr2-/- mice, which do not display changes in collagen gene expression, and leads to progressive fibrosis and sterility. Crossing Antxr2-/- with Col6a1-/- mice leads to restoration of uterine structure and reversion of female infertility. We also demonstrate that CMG2 may act as a signalling receptor for collagen VI and mediates its intracellular degradation.

Diagnosis implications of the whole genome sequencing in a large Lebanese family with hyaline fibromatosis syndrome

BACKGROUND

Hyaline fibromatosis syndrome (HFS) is a recently introduced alternative term for two disorders that were previously known as juvenile hyaline fibromatosis (JHF) and infantile systemic hyalinosis (ISH). These two variants are secondary to mutations in the anthrax toxin receptor 2 gene (ANTXR2) located on chromosome 4q21. The main clinical features of both entities include papular and/or nodular skin lesions, gingival hyperplasia, joint contractures and osteolytic bone lesions that appear in the first few years of life, and the syndrome typically progresses with the appearance of new lesions.

METHODS

We describe five Lebanese patients from one family, aged between 28 and 58 years, and presenting with nodular and papular skin lesions, gingival hyperplasia, joint contractures and bone lesions. Because of the particular clinical features and the absence of a clinical diagnosis, Whole Genome Sequencing (WGS) was carried out on DNA samples from the proband and his parents.

RESULTS

A mutation in ANTXR2 (p. Gly116Val) that yielded a diagnosis of HFS was noted.

CONCLUSIONS

The main goal of this paper is to add to the knowledge related to the clinical and radiographic aspects of HFS in adulthood and to show the importance of Next-Generation Sequencing (NGS) techniques in resolving such puzzling cases.

The Ins and Outs of Anthrax Toxin

Anthrax is a severe, although rather rare, infectious disease that is caused by the Gram-positive, spore-forming bacterium Bacillus anthracis. The infectious form is the spore and the major virulence factors of the bacterium are its poly-γ-D-glutamic acid capsule and the tripartite anthrax toxin. The discovery of the anthrax toxin receptors in the early 2000s has allowed in-depth studies on the mechanisms of anthrax toxin cellular entry and translocation from the endocytic compartment to the cytoplasm. The toxin generally hijacks the endocytic pathway of CMG2 and TEM8, the two anthrax toxin receptors, in order to reach the endosomes. From there, the pore-forming subunit of the toxin inserts into endosomal membranes and enables translocation of the two catalytic subunits. Insertion of the pore-forming unit preferentially occurs in intraluminal vesicles rather than the limiting membrane of the endosome, leading to the translocation of the enzymatic subunits in the lumen of these vesicles. This has important consequences that will be discussed. Ultimately, the toxins reach the cytosol where they act on their respective targets. Target modification has severe consequences on cell behavior, in particular on cells of the immune system, allowing the spread of the bacterium, in severe cases leading to host death. Here we will review the literature on anthrax disease with a focus on the structure of the toxin, how it enters cells and its immunological effects.

Anthrax Pathogenesis

Anthrax is caused by the spore-forming, gram-positive bacterium Bacillus anthracis. The bacterium's major virulence factors are (a) the anthrax toxins and (b) an antiphagocytic polyglutamic capsule. These are encoded by two large plasmids, the former by pXO1 and the latter by pXO2. The expression of both is controlled by the bicarbonate-responsive transcriptional regulator, AtxA. The anthrax toxins are three polypeptides-protective antigen (PA), lethal factor (LF), and edema factor (EF)-that come together in binary combinations to form lethal toxin and edema toxin. PA binds to cellular receptors to translocate LF (a protease) and EF (an adenylate cyclase) into cells. The toxins alter cell signaling pathways in the host to interfere with innate immune responses in early stages of infection and to induce vascular collapse at late stages. This review focuses on the role of anthrax toxins in pathogenesis. Other virulence determinants, as well as vaccines and therapeutics, are briefly discussed.

Hyaline fibromatosis syndrome with mutation c.1074delT of the CMG2 gene: a case report

INTRODUCTION

Juvenile hyaline fibromatosis and infantile systemic hyalinosis are variants of the same autosomal recessive syndrome; hyaline fibromatosis syndrome, characterized by papulonodular skin lesions, gingival hypertrophy, flexion contractures of joints, osteolytic bone lesions and stunted growth. Infantile systemic hyalinosis is distinguished from juvenile hyaline fibromatosis by its more severe phenotype, which includes hyaline deposits in multiple organs, recurrent infections and death within the first two years of life.Hyaline fibromatosis syndrome is due to mutations of the gene-encoding capillary morphogenesis protein 2 (CMG2). Cases have been reported in different countries but to the best of our knowledge, this is the first reported Moroccan patient with hyaline fibromatosis syndrome and carrying the CMG2 mutation.

CASE PRESENTATION

We report the case of an eight-year-old Moroccan male patient with typical features of hyaline fibromatosis syndrome: multiple recurring subcutaneous tumors, gingival hypertrophy, joint contractures and other anomalies carrying a homozygous mutation in the CMG2 gene. The identification of the mutation in our patient allowed us to do a presymptomatic diagnosis in our patient's sister, a two-day-old newborn, who is carrying the familial mutation in the heterozygous state. Early recognition of this condition is important for genetic counseling and early treatment.

CONCLUSIONS

Hyaline fibromatosis syndrome might be underdiagnosed. Molecular diagnosis will help clinicians and geneticists, firstly to conduct genetic counseling, prenatal diagnosis and early treatment, and secondly to gain better understanding of the disease and genotype-phenotype correlations.

Endocytic trafficking of membrane-bound cargo: a flotillin point of view

The ubiquitous and highly conserved flotillin proteins, flotillin-1 and flotillin-2, have been shown to be involved in various cellular processes such as cell adhesion, signal transduction through receptor tyrosine kinases as well as in cellular trafficking pathways. Due to the fact that flotillins are acylated and form hetero-oligomers, they constitutively associate with cholesterol-enriched lipid microdomains. In recent years, such microdomains have been appreciated as platforms that participate in endocytosis and other cellular trafficking steps. This review summarizes the current findings on the role of flotillins in membrane-bound cargo endocytosis and endosomal trafficking events. We will discuss the proposed function of flotillins in endocytosis in the light of recent findings that point towards a role for flotillins in a step that precedes the actual endocytic uptake of cargo molecules. Recent findings have also revealed that flotillins may be important for endosomal sorting and recycling of specific cargo molecules. In addition to these aspects, the cellular trafficking pathway of flotillins themselves as potential cargo in the context of growth factor signaling will be discussed.

Designing inhibitors of anthrax toxin

INTRODUCTION

Present-day rational drug design approaches are based on exploiting unique features of the target biomolecules, small- or macromolecule drug candidates and physical forces that govern their interactions. The 2013 Nobel Prize in chemistry awarded 'for the development of multiscale models for complex chemical systems' once again demonstrated the importance of the tailored drug discovery that reduces the role of the trial-and-error approach to a minimum. The intentional dissemination of Bacillus anthracis spores in 2001 via the so-called anthrax letters has led to increased efforts, politically and scientifically, to develop medical countermeasures that will protect people from the threat of anthrax bioterrorism.

AREAS COVERED

This article provides an overview of the recent rational drug design approaches for discovering inhibitors of anthrax toxin. The review also directs the readers to the vast literature on the recognized advances and future possibilities in the field.

EXPERT OPINION

Existing options to combat anthrax toxin lethality are limited. With the only anthrax toxin inhibiting therapy (protective antigen-targeting with a monoclonal antibody, raxibacumab) approved to treat inhalational anthrax, the situation, in our view, is still insecure. Further, the FDA's animal rule for drug approval, which clears compounds without validated efficacy studies on humans, creates a high level of uncertainty, especially when a well-characterized animal model does not exist. Better identification and validation of anthrax toxin therapeutic targets at the molecular level as well as elucidation of the parameters determining the corresponding therapeutic windows are still necessary for more effective therapeutic options.

Arrestins in host-pathogen interactions

In the context of host-pathogen interaction, host cell receptors and signaling pathways are essential for both invading pathogens, which exploit them for their own profit, and the defending organism, which activates early mechanism of defense, known as innate immunity, to block the aggression. Because of their central role as scaffolding proteins downstream of activated receptors, β-arrestins are involved in multiple signaling pathways activated in host cells by pathogens. Some of these pathways participate in the innate immunity and the inflammatory response. Other β-arrestin-dependent pathways are actually hijacked by microbes and toxins to penetrate into host cells and spread in the organism.

ANTXR1, a stem cell-enriched functional biomarker, connects collagen signaling to cancer stem-like cells and metastasis in breast cancer

Cancer stem-like cells are thought to contribute to tumor recurrence. The anthrax toxin receptor 1 (ANTXR1) has been identified as a functional biomarker of normal stem cells and breast cancer stem-like cells. Primary stem cell-enriched basal cells (CD49f(+)/EpCAM(-)/Lin(-)) expressed higher levels of ANTXR1 compared with mature luminal cells. CD49f(+)/EpCAM(-), CD44(+)/EpCAM(-), CD44(+)/CD24(-), or ALDEFLUOR-positive subpopulations of breast cancer cells were enriched for ANTXR1 expression. CD44(+)/CD24(-)/ANTXR1(+) cells displayed enhanced self-renewal as measured by mammosphere assay compared with CD44(+)/CD24(-)/ANTXR1(-) cells. Activation of ANTXR1 by its natural ligand C5A, a fragment of collagen VI α3, increased stem cell self-renewal in mammosphere assays and Wnt signaling including the expression of the Wnt receptor-lipoprotein receptor-related protein 6 (LRP6), phosphorylation of GSK3α/β, and elevated expression of Wnt target genes. RNAi-mediated silencing of ANTXR1 enhanced the expression of luminal-enriched genes but diminished Wnt signaling including reduced LRP6 and ZEB1 expression, self-renewal, invasion, tumorigenicity, and metastasis. ANTXR1 silencing also reduced the expression of HSPA1A, which is overexpressed in metastatic breast cancer stem cells. Analysis of public databases revealed ANTXR1 amplification in medullary breast carcinoma and overexpression in estrogen receptor-negative breast cancers with the worst outcome. Furthermore, ANTXR1 is among the 10% most overexpressed genes in breast cancer and is coexpressed with collagen VI. Thus, ANTXR1:C5A interactions bridge a network of collagen cleavage and remodeling in the tumor microenvironment, linking it to a stemness signaling network that drives metastatic progression.

Identification of novel host-targeted compounds that protect from anthrax lethal toxin-induced cell death

Studying how pathogens subvert the host to cause disease has contributed to the understanding of fundamental cell biology. Bacillus anthracis, the causative agent of anthrax, produces the virulence factor lethal toxin to disarm host immunity and cause pathology. We conducted a phenotypic small molecule screen to identify inhibitors of lethal toxin-induced macrophage cell death and used an ordered series of secondary assays to characterize the hits and determine their effects on cellular function. We identified a structurally diverse set of small molecules that act at various points along the lethal toxin pathway, including inhibitors of endocytosis, natural product inhibitors of organelle acidification (e.g., the botulinum neurotoxin inhibitor, toosendanin), and a novel proteasome inhibitor, 4MNB (4-methoxy-2-[2-(5-methoxy-2-nitrosophenyl)ethyl]-1-nitrosobenzene). Many of the compounds, including three drugs approved for use in humans, also protected against the related Clostridium difficile toxin TcdB, further demonstrating their value as novel tools for perturbation and study of toxin biology and host cellular processes and highlighting potential new strategies for intervening on toxin-mediated diseases.

Small molecule inhibitors of Bacillus anthracis protective antigen proteolytic activation and oligomerization

Protective antigen (PA), lethal factor, and edema factor, the protein toxins of Bacillus anthracis , are among its most important virulence factors and play a key role in infection. We performed a virtual ligand screen of a library of 10000 members to identify compounds predicted to bind to PA and prevent its oligomerization. Four of these compounds slowed PA association in a FRET-based oligomerization assay, and two of those protected cells from intoxication at concentrations of 1-10 μM. Exploration of the protective mechanism by Western blot showed decreased SDS-resistant PA oligomer on cells and, surprisingly, decreased amounts of activated PA. In vitro assays showed that one of the inhibitors blocked furin-mediated cleavage of PA, apparently through its binding to the PA substrate. Thus, we have identified inhibitors that can independently block both PA's cleavage by furin and its subsequent oligomerization. Lead optimization on these two backbones may yield compounds with high activity and specificity for the anthrax toxins.

Identification of new dominant-negative mutants of anthrax protective antigen using directed evolution

The anthrax toxin is composed of three proteins: protective antigen (PA), lethal factor (LF), and edema toxin (EF). The PA moiety carries EF and LF into the cytosol of mammalian cells via a mechanism that depends on the oligomerization of PA and transmembrane pore formation by the PA oligomer. Certain mutants of PA, termed dominant-negative (DN) mutants, can co-oligomerize with wild-type PA and disrupt the translocation ability of the pore. Here, we constructed a PA mutant library by introducing random mutations into domain II of PA and screened three new DN mutants of PA: V377E, T380S, and I432C. All the mutants inhibited the anthrax toxin action against sensitive cells. V377E had the strongest inhibitory effect and was further confirmed to be able to protect mice against a challenge with anthrax lethal toxin. Furthermore, we functionally characterized these mutants. The result showed that these mutations did not impair proteolytic activation or oligomer formation of PA, but impeded the prepore-pore conversion of the oligomer. These DN mutants of PA identified in our study may provide valuable information for elucidating the structure-function relationship of PA and for designing therapeutics for anthrax treatment.

Delayed toxicity associated with soluble anthrax toxin receptor decoy-Ig fusion protein treatment

Soluble receptor decoy inhibitors, including receptor-immunogloubulin (Ig) fusion proteins, have shown promise as candidate anthrax toxin therapeutics. These agents act by binding to the receptor-interaction site on the protective antigen (PA) toxin subunit, thereby blocking toxin binding to cell surface receptors. Here we have made the surprising observation that co-administration of receptor decoy-Ig fusion proteins significantly delayed, but did not protect, rats challenged with anthrax lethal toxin. The delayed toxicity was associated with the in vivo assembly of a long-lived complex comprised of anthrax lethal toxin and the receptor decoy-Ig inhibitor. Intoxication in this system presumably results from the slow dissociation of the toxin complex from the inhibitor following their prolonged circulation. We conclude that while receptor decoy-Ig proteins represent promising candidates for the early treatment of B. anthracis infection, they may not be suitable for therapeutic use at later stages when fatal levels of toxin have already accumulated in the bloodstream.

Human genetic variation altering anthrax toxin sensitivity

The outcome of exposure to infectious microbes or their toxins is influenced by both microbial and host genes. Some host genes encode defense mechanisms, whereas others assist pathogen functions. Genomic analyses have associated host gene mutations with altered infectious disease susceptibility, but evidence for causality is limited. Here we demonstrate that human genetic variation affecting capillary morphogenesis gene 2 (CMG2), which encodes a host membrane protein exploited by anthrax toxin as a principal receptor, dramatically alters toxin sensitivity. Lymphoblastoid cells derived from a HapMap Project cohort of 234 persons of African, European, or Asian ancestry differed in sensitivity mediated by the protective antigen (PA) moiety of anthrax toxin by more than four orders of magnitude, with 99% of the cohort showing a 250-fold range of sensitivity. We find that relative sensitivity is an inherited trait that correlates strongly with CMG2 mRNA abundance in cells of each ethnic/geographical group and in the combined population pool (P = 4 × 10(-11)). The extent of CMG2 expression in transfected murine macrophages and human lymphoblastoid cells affected anthrax toxin binding, internalization, and sensitivity. A CMG2 single-nucleotide polymorphism (SNP) occurring frequently in African and European populations independently altered toxin uptake, but was not statistically associated with altered sensitivity in HapMap cell populations. Our results reveal extensive human diversity in cell lethality dependent on PA-mediated toxin binding and uptake, and identify individual differences in CMG2 expression level as a determinant of this diversity. Testing of genomically characterized human cell populations may offer a broadly useful strategy for elucidating effects of genetic variation on infectious disease susceptibility.

Cytoskeleton as an emerging target of anthrax toxins

Bacillus anthracis, the agent of anthrax, has gained virulence through its exotoxins produced by vegetative bacilli and is composed of three components forming lethal toxin (LT) and edema toxin (ET). So far, little is known about the effects of these toxins on the eukaryotic cytoskeleton. Here, we provide an overview on the general effects of toxin upon the cytoskeleton architecture. Thus, we shall discuss how anthrax toxins interact with their receptors and may disrupt the interface between extracellular matrix and the cytoskeleton. We then analyze what toxin molecular effects on cytoskeleton have been described, before discussing how the cytoskeleton may help the pathogen to corrupt general cell processes such as phagocytosis or vascular integrity.

The dark sides of capillary morphogenesis gene 2

Capillary morphogenesis gene 2 (CMG2) is a type I membrane protein involved in the homeostasis of the extracellular matrix. While it shares interesting similarities with integrins, its exact molecular role is unknown. The interest and knowledge about CMG2 largely stems from the fact that it is involved in two diseases, one infectious and one genetic. CMG2 is the main receptor of the anthrax toxin, and knocking out this gene in mice renders them insensitive to infection with Bacillus anthracis spores. On the other hand, mutations in CMG2 lead to a rare but severe autosomal recessive disorder in humans called Hyaline Fibromatosis Syndrome (HFS). We will here review what is known about the structure of CMG2 and its ability to mediate anthrax toxin entry into cell. We will then describe the limited knowledge available concerning the physiological role of CMG2. Finally, we will describe HFS and the consequences of HFS-associated mutations in CMG2 at the molecular and cellular level.

Streptolysin O clearance through sequestration into blebs that bud passively from the plasma membrane

Cells survive exposure to bacterial pore-forming toxins, such as streptolysin O (SLO), through mechanisms that remain unclear. Previous studies have suggested that these toxins are cleared by endocytosis. However, the experiments reported here failed to reveal any evidence for endocytosis of SLO, nor did they reveal any signs of damage to endosomal membranes predicted from such endocytosis. Instead, we illustrate that SLO induces a characteristic form of plasma membrane blebbing that allows cells to shed SLO by the process known as ectocytosis. Specifically, 'deep-etch' electron microscopy of cells exposed to SLO illustrates that the toxin is rapidly sequestered into domains in the plasmalemma greatly enriched in SLO pores, and these domains bleb outwards and bud from the cell surface into the medium. Such ectocytosis is even observed in cells that have been chemically fixed before exposure to SLO, suggesting that it is caused by a direct physical action of the toxin on the cell membrane, rather than by an active cellular reaction. We conclude, therefore, that ectocytosis is an important means for SLO clearance and hypothesize that this is a primary method by which cells defend themselves generally against pore-forming toxins.

Hyaline fibromatosis syndrome inducing mutations in the ectodomain of anthrax toxin receptor 2 can be rescued by proteasome inhibitors

Hyaline Fibromatosis Syndrome (HFS) is a human genetic disease caused by mutations in the anthrax toxin receptor 2 (or cmg2) gene, which encodes a membrane protein thought to be involved in the homeostasis of the extracellular matrix. Little is known about the structure and function of the protein or the genotype–phenotype relationship of the disease. Through the analysis of four patients, we identify three novel mutants and determine their effects at the cellular level. Altogether, we show that missense mutations that map to the extracellular von Willebrand domain or the here characterized Ig-like domain of CMG2 lead to folding defects and thereby to retention of the mutated protein in the endoplasmic reticulum (ER). Mutations in the Ig-like domain prevent proper disulphide bond formation and are more efficiently targeted to ER-associated degradation. Finally, we show that mutant CMG2 can be rescued in fibroblasts of some patients by treatment with proteasome inhibitors and that CMG2 is then properly transported to the plasma membrane and signalling competent, identifying the ER folding and degradation pathway components as promising drug targets for HFS.

Clostridium botulinum C2 toxin is internalized by clathrin- and Rho-dependent mechanisms

Clostridium botulinum C2 toxin is an ADP-ribosyltransferase, causing depolymerization of the actin cytoskeleton in eukaryotic cells. The C2 toxin is a binary toxin consisting of the enzymatic subunit C2I and the binding subunit C2II. Proteolytical activation of the binding subunit triggers the formation of heptameric structures (C2IIa), which bind to cellular receptors. C2I is able to bind to C2IIa oligomers, and it has been suggested that the whole complex is internalized by a raft-dependent mechanism. Here we analysed by which mechanism C2 toxin is endocytosed. In HeLa cells expressing a dominant-negative dynamin mutant, cytotoxicity and C2 toxin uptake were blocked. Furthermore, siRNA-mediated knockdown of flotillins or inhibition of Arf6 function, proteins suggested to be involved in dynamin-independent endocytosis, did not affect C2 toxicity. Knockdown of caveolin did not inhibit endocytosis of C2 toxin, whereas inhibition of clathrin function reduced the uptake of C2 toxin and delayed the cytotoxic effect. Finally, we found evidence for a Rho-mediated uptake of C2 toxin. In conclusion, C2 toxin is endocytosed by dynamin-dependent mechanisms and we provide evidence for involvement of clathrin and Rho.

Targeting the anthrax receptors, TEM-8 and CMG-2, for anti-angiogenic therapy

The anthrax toxin receptors tumor endothelial marker-8 (TEM-8) and capillary morphogenesis gene-2 (CMG-2) are responsible for allowing entry of anthrax toxin into host cells. These receptors were first discovered due to their enhanced expression on endothelial cells undergoing blood vessel growth or angiogenesis in model systems. Inhibition of angiogenesis is an important strategy for current anti-cancer therapies and treatment of retinal diseases. Functional roles for TEM-8 and CMG-2 in angiogenesis have recently emerged. TEM-8 appears to regulate endothelial cell migration and tubule formation whereas a role for CMG-2 in endothelial proliferation has been documented. TEM-8 and CMG-2 bind differentially to extracellular matrix proteins including collagen I, collagen IV and laminin and these properties may be responsible for their apparent roles in regulating endothelial cell behavior during angiogenesis. TEM-8-binding moieties have also been suggested to be useful in selectively targeting anti-angiogenic and anti-tumorigenic therapies to tumor endothelium. Additionally, studies of modified forms of lethal toxin (LeTx) have demonstrated that targeted inhibition of MAPKs within tumor vessels may represent an efficacious anti-angiogenic strategy.

Heterodimeric integrin complexes containing beta1-integrin promote internalization and lethality of anthrax toxin

To kill macrophages, the lethal factor component of Bacillus anthracis toxin binds to a carrier protein (PA), which then interacts with the CMG2 receptor protein on the cell surface and is endocytosed into the cytoplasm. CMG2, as well as TEM8, a second PA receptor not present on macrophages, contain a von Willebrand A domain that is crucial for toxin binding. Here we report that integrin beta1, another cell surface von Willebrand A domain protein, can mediate and potentiate anthrax toxin endocytosis. By using microarray-based analysis to globally correlate gene expression profiles with toxin sensitivity, we associated toxin effects with the integrin-activating proteins osteopontin and CD44. Further study showed that PA binds to alpha4beta1- and alpha5beta1-integrin complexes, leading to their conjoint endocytosis, and also interacts-weakly relative to CMG2 but comparably to TEM8--with purified alpha5beta1 complex in vitro. Monoclonal antibody directed against beta1-integrin or its alpha integrin partners reduced PA/integrin endocytosis and anthrax toxin lethality, and hyaluronic acid--which interferes with CD44-mediated integrin activation--had similar effects. Remarkably, whereas deficiency of CMG2 protected macrophages from rapid killing by large toxin doses (>50 ng/mL), by 24 h the toxin-treated cells were dead. Such late killing of CMG2-deficient cells by high dose toxin as well as the late death observed during exposure of CMG2-producing macrophages to low-dose toxin (<1 ng/mL), was dependent on integrin function. Effects of inactivating both CMG2 and integrin were synergistic. Collectively, our findings argue strongly that beta1-integrin can both potentiate CMG2-mediated endocytosis and serve independently as a low-affinity PA receptor.

Hijacking the endocytic machinery by microbial pathogens

Understanding the mechanisms that microbes exploit to invade host cells and cause disease is crucial if we are to eliminate their threat. Although pathogens use a variety of microbial factors to trigger entry into non-phagocytic cells, their targeting of the host cell process of endocytosis has emerged as a common theme. To accomplish this, microbes often rewire the normal course of particle internalization, frequently usurping theoretical maximal sizes to permit entry and reconfiguring molecular components that were once thought to be required for vesicle formation. Here, we discuss recent advances in our understanding of how toxins, viruses, bacteria, and fungi manipulate the host cell endocytic machinery to generate diseases. Additionally, we will reveal the advantages of using these organisms to expand our general knowledge of endocytic mechanisms in eukaryotic cells.

AB toxins: a paradigm switch from deadly to desirable

To ensure their survival, a number of bacterial and plant species have evolved a common strategy to capture energy from other biological systems. Being imperfect pathogens, organisms synthesizing multi-subunit AB toxins are responsible for the mortality of millions of people and animals annually. Vaccination against these organisms and their toxins has proved rather ineffective in providing long-term protection from disease. In response to the debilitating effects of AB toxins on epithelial cells of the digestive mucosa, mechanisms underlying toxin immunomodulation of immune responses have become the focus of increasing experimentation. The results of these studies reveal that AB toxins may have a beneficial application as adjuvants for the enhancement of immune protection against infection and autoimmunity. Here, we examine similarities and differences in the structure and function of bacterial and plant AB toxins that underlie their toxicity and their exceptional properties as immunomodulators for stimulating immune responses against infectious disease and for immune suppression of organ-specific autoimmunity.

Endocytosis of the anthrax toxin is mediated by clathrin, actin and unconventional adaptors

The anthrax toxin is a tripartite toxin, where the two enzymatic subunits require the third subunit, the protective antigen (PA), to interact with cells and be escorted to their cytoplasmic targets. PA binds to cells via one of two receptors, TEM8 and CMG2. Interestingly, the toxin times and triggers its own endocytosis, in particular through the heptamerization of PA. Here we show that PA triggers the ubiquitination of its receptors in a β-arrestin-dependent manner and that this step is required for clathrin-mediated endocytosis. In addition, we find that endocytosis is dependent on the heterotetrameric adaptor AP-1 but not the more conventional AP-2. Finally, we show that endocytosis of PA is strongly dependent on actin. Unexpectedly, actin was also found to be essential for efficient heptamerization of PA, but only when bound to one of its 2 receptors, TEM8, due to the active organization of TEM8 into actin-dependent domains. Endocytic pathways are highly modular systems. Here we identify some of the key players that allow efficient heptamerization of PA and subsequent ubiquitin-dependent, clathrin-mediated endocytosis of the anthrax toxin.

Bacillus anthracis is the bacterium responsible for the anthrax disease. Its virulence is mainly due to 2 factors, the anthrax toxin and the anti-phagocytic capsule. This toxin is composed of three independent polypeptide chains. Two of these have enzymatic activity and are responsible for the effects of the toxin. The third has no activity but is absolutely required to bring the 2 enzymatic subunits into the cell where they act. If one blocks entry into the cells, one blocks the effects of these toxins, which is why it is important to understand how the toxin enters into the cell at the molecular level. Here we identified various molecules that are involved in efficiently bringing the toxin into the cell. First, we found that the actin cytoskeleton plays an important role in organizing one of the two anthrax toxin receptors at the cell surface. Second, we found a cytosolic protein, β-arrestin, that is required to modify the intracellular part of the toxin receptor, to allow uptake. Finally, we directly show, for the first time, that anthrax toxin uptake is mediated by the so-called clathrin-dependent pathway, a very modular entry pathway, but that the toxin utilizes this pathway in an unconventional way.