Expression and purification of functional human anthrax toxin receptor (ATR/TEM8) binding domain from Escherichia coli

Expression of Tumour Endothelial Marker 8 in Canine Mammary Gland Tumour Cells

The aim of this study was to investigate the expression of tumour endothelial marker 8 (TEM8) in canine mammary gland tumours (MGTs) by immunohistochemistry and to evaluate the association between tumour cell TEM8 expression and tumour histological features, histological grades and expression of luminal and basal/myoepithelial cell markers. TEM8 expression was detected in >60 % of neoplastic epithelial cells in all simple adenomas (n = 25), simple carcinomas (n = 43) and invasive micropapillary carcinomas (n = 5) studied. Six of the 18 solid carcinomas studied showed TEM8 expression in >60% of carcinoma cells present in solid structures and in 12 of the 18 solid carcinomas, <30% of the luminal structure-forming carcinoma cells showed TEM8 expression. TEM8 expression in the neoplastic cells was not associated with histological malignancy in canine MGTs. TEM8⁺ tumour cells frequently showed the luminal-like phenotype cytokeratin (CK)19⁺/p63^-/α-smooth muscle actin (SMA)^-, while most TEM8^- tumour cells exhibited the basal-like phenotype CK19^-/p63⁺/αSMA^-. These findings indicate that TEM8 may be involved in maintaining the characteristics of luminal cells in canine MGTs and that TEM8 would be useful in identifying the type of neoplastic epithelial cell in MGTs.

Development of receptor-based inhibitory RNA aptamers for anthrax toxin neutralization

Anthrax toxin excreted by Bacillus anthracis is the key causative agent of infectious anthrax disease. In the present study, we targeted the binding of PA to the ATR/TEM8 Von Willebrand factor type A (VWA) domain, which we cloned into Escherichia coli and purified to homogeneity under denaturing conditions. To develop an anthrax toxin inhibitor, we selected and identified short single strand RNA aptamers (approximately 30mer) consisting of different sequences of nucleic acids with a high binding affinity in the 100 nanomolar range against the recombinant ATR/TEM8 VWA domain using systematic evolution of ligands by exponential enrichment (SELEX). Five candidate aptamers were further characterized by several techniques including secondary structural analysis. The inhibitor efficiency (IC50) of one of the aptamers toward anthrax toxin was approximately 5μM in macrophage RAW 264.7 cells, as determined from cytotoxicity analysis by MTT assay. We believe that the candidate aptamers should be useful for blocking the binding of PA to its receptor in order to neutralize anthrax toxin.

Differential dependence on N-glycosylation of anthrax toxin receptors CMG2 and TEM8

ANTXR 1 and 2, also known as TEM8 and CMG2, are two type I membrane proteins, which have been extensively studied for their role as anthrax toxin receptors, but with a still elusive physiological function. Here we have analyzed the importance of N-glycosylation on folding, trafficking and ligand binding of these closely related proteins. We find that TEM8 has a stringent dependence on N-glycosylation. The presence of at least one glycan on each of its two extracellular domains, the vWA and Ig-like domains, is indeed necessary for efficient trafficking to the cell surface. In the absence of any N-linked glycans, TEM8 fails to fold correctly and is recognized by the ER quality control machinery. Expression of N-glycosylation mutants reveals that CMG2 is less vulnerable to sugar loss. The absence of N-linked glycans in one of the extracellular domains indeed has little impact on folding, trafficking or receptor function of the wild type protein expressed in tissue culture cells. N-glycans do, however, seem required in primary fibroblasts from human patients. Here, the presence of N-linked sugars increases the tolerance to mutations in cmg2 causing the rare genetic disease Hyaline Fibromatosis Syndrome. It thus appears that CMG2 glycosylation provides a buffer towards genetic variation by promoting folding of the protein in the ER lumen.

Potential biological targets ofBacillus anthracisin anti-infective approaches against the threat of bioterrorism

The terrorist attacks of 2001 involving anthrax underscore the imperative that safe and effective medical countermeasures should be readily available. Vaccination appears to be the most effective form of mass protection against a biological attack, but the current vaccines have drawbacks that justify the enormous amount of effort currently being put into developing more effective vaccines and other treatment modalities. After providing a comprehensive overview of the organism Bacillus anthracis as a biological weapon and its pathogenicity, this review briefly summarizes the current knowledge vital to the management of anthrax disease. This knowledge has been acquired since 2001 as a result of the progress on anthrax research and focuses on the possible development of improved human anti-infective strategies targeting B. anthracis spore components, as well as strategies based on host-pathogen interactions.

Imaging tumor endothelial marker 8 using an 18F-labeled peptide

PURPOSE

Tumor endothelial marker 8 (TEM8) has been reported to be upregulated in both tumor cells and tumor-associated endothelial cells in several cancer types. TEM8 antagonists and TEM8-targeted delivery of toxins have been developed as effective cancer therapeutics. The ability to image TEM8 expression would be of use in evaluating TEM8-targeted cancer therapy.

METHODS

A 13-meric peptide, KYNDRLPLYISNP (QQM), identified from the small loop in domain IV of protective antigen of anthrax toxin was evaluated for TEM8 binding and labeled with 18F for small-animal PET imaging in both UM-SCC1 head-and-neck cancer and MDA-MB-435 melanoma models.

RESULTS

A modified ELISA showed that QQM peptide bound specifically to the extracellular vWA domain of TEM8 with an IC50 value of 304 nM. Coupling 4-nitrophenyl 2-(18)F-fluoropropionate with QQM gave almost quantitative yield and a high specific activity (79.2±7.4 TBq/mmol, n=5) of 18F-FP-QQM at the end of synthesis. 18F-FP-QQM showed predominantly renal clearance and had significantly higher accumulation in TEM8 high-expressing UM-SCC1 tumors (2.96±0.84 %ID/g at 1 h after injection) than TEM8 low-expressing MDA-MB-435 tumors (1.38±0.56 %ID/g at 1 h after injection).

CONCLUSION

QQM peptide bound specifically to the extracellular domain of TEM8. 18F-FP-QQM peptide tracer would be a promising lead compound for measuring TEM8 expression. Further efforts to improve the affinity and specificity of the tracer and to increase its metabolic stability are warranted.

Crystallization and preliminary X-ray analysis of the vWA domain of human anthrax toxin receptor 1

The Gram-positive spore-forming bacterium Bacillus anthracis causes anthrax by secreting anthrax toxin, which consists of protective antigen (PA), lethal factor and oedema factor. Binding of PA to receptors triggers the multi-step process of anthrax toxin entry into target cells. Two distinct cellular receptors, ANTXR1 (also known as tumour endothelial marker 8; TEM8) and ANTXR2 (also known as capillary morphogenesis protein 2; CMG2), for anthrax toxin have been identified. Although the crystal structure of the extracellular von Willebrand factor A (vWA) domain of CMG2 has been reported, the difference between the vWA domains of TEM8 and CMG2 remains unclear because there are no structural data for the TEM8 vWA domain. In this report, the TEM8 vWA domain was expressed, purified and crystallized. X-ray diffraction data were collected to 1.8 Å resolution from a single crystal, which belonged to space group P1 with unit-cell parameters a=65.9, b=66.1, c=74.4 Å, α=63.7, β=88.2, γ=59.9°.

Anthrax toxin receptor 1/tumor endothelial marker 8: mutation of conserved inserted domain residues overrides cytosolic control of protective antigen binding

Anthrax toxin receptor 1 (ANTXR1)/tumor endothelial marker 8 (TEM8) is one of two known proteinaceous cell surface anthrax toxin receptors. A metal ion dependent adhesion site (MIDAS) present in the integrin-like inserted (I) domain of ANTXR1 mediates the binding of the anthrax toxin subunit, protective antigen (PA). Here we provide evidence that single point mutations in the I domain can override regulation of ANTXR1 ligand-binding activity mediated by intracellular signals. A previously reported MIDAS mutant of ANTXR1 (T118A) was found to retain normal metal ion binding and secondary structure but failed to bind PA, consistent with a locked inactive state. Conversely, mutation of a conserved I domain phenylalanine residue to a tryptophan (F205W) increased the proportion of cell-surface ANTXR1 that bound PA, consistent with a locked active state. Interestingly, the K(D) and total amount of PA bound by the isolated ANTXR1 I domain were not affected by the F205W mutation, indicating that ANTXR1 is preferentially found in the active state in the absence of inside-out signaling. Circular dichroism (CD) spectroscopy and (1)H-(15)N heteronuclear single-quantum coherence (HSQC) nuclear magnetic resonance (NMR) revealed that structural changes between T118A, F205W, and WT I domains were minor despite a greater than 10(3)-fold difference in their abilities to bind toxin. Regulation of toxin binding has important implications for the design of toxin inhibitors and for the targeting of ANTXR1 for antitumor therapies.

Production of recombinant anthrax toxin receptor (ATR/CMG2) fused with human Fc in planta

Mass vaccination against anthrax with existing vaccines is costly and unsafe due to potential side effects. For post-infection treatment, passive immunotherapy measures are currently available, most based on anthrax protective antigen (PA)-specific therapeutic antibodies. Efficient against wild-type strains, these treatment(s) might fail to protect against infections caused by genetically engineered Bacillus anthracis strains. A recent discovery revealed that the von Willebrand factor A (VWA) domain of human capillary morphogenesis protein 2 (CMG2) is an exceptionally effective anthrax toxin receptor (ATR) proficient in helping to resolve this issue. Here we describe in planta production of chimeric recombinant protein (immunoadhesin) comprised of functional ATR domain fused with the human immunoglobulin Fc fragment (pATR-Fc). The fusion design allowed us to obtain pATR-Fc in plant green tissues in a soluble form making it fairly easy to purify by Protein-A chromatography. Standardized pATR-Fc preparations (purity>90%) were shown to efficiently bind anthrax PA as demonstrated by ELISA and Western blot analysis. Recombinant pATR-Fc was also shown to protect J774A1 macrophage cells against the anthrax toxin. This study confirmed that plant-derived pATR-Fc antibody-like protein is a prospective candidate for anthrax immunotherapy.