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

ANTXR1 as a potential sensor of extracellular mechanical cues

Cell adhesion molecules mediate cell-cell or cell-matrix interactions, some of which are mechanical sensors, such as integrins. Emerging evidence indicates that anthrax toxin receptor 1 (ANTXR1), a newly identified cell adhesion molecule, can also sense extracellular mechanical signals such as hydrostatic pressure and extracellular matrix (ECM) rigidity. ANTXR1 can interact with ECM through connecting intracellular cytoskeleton and ECM molecules (just like integrins) to regulate numerous biological processes, such as cell adhesion, cell migration or ECM homeostasis. Although with high structural similarity to integrins, its functions and downstream signal transduction are independent from those of integrins. In this perspective, based on existing evidence in literature, we analyzed the structural and functional evidence that ANTXR1 can act as a potential sensor for extracellular mechanical cues. To our knowledge, this is the first in-depth overview of ANTXR1 from the perspective of mechanobiology. STATEMENT OF SIGNIFICANCE: An overview of ANTXR1 from the perspective of mechanobiology; An analysis of mechanical sensitivity of ANTXR1 in structure and function; A summary of existing evidence of ANTXR1 as a potential mechanosensor.

Anthrax protective antigen is a calcium-dependent serine protease

Bacillus anthracis secretes a three component exotoxin-complex, which contributes to anthrax pathogenesis. Formation of this complex starts with the binding of protective antigen (PA) to its cellular receptor. In this study, we report that PA is a calcium-dependent serine protease and that the protein potentially uses this proteolytic activity for receptor binding. Additionally our findings shed new light on previous research describing the inhibition of anthrax toxins and exotoxin formation. Importantly, inhibition of the proteolytic activity of protective antigen could be a novel therapeutic strategy in fighting B. anthracis-related infections.

Effects from metal ion in tumor endothelial marker 8 and anthrax protective antigen: BioLayer Interferometry experiment and molecular dynamics simulation study

One of the anthrax receptors, tumor endothelial marker 8 (TEM8), is reported to be a potential anticancer target due to its over-expression during tumor angiogenesis. To extend our BioLayer Interferometry study in PA-TEM8 binding, we present a computational approach to reveal the role of an integral metal ion on receptor structure and binding thermodynamics. We estimated the interaction energy between PA and TEM8 using computer simulation. Consistent with experimental study, computational results indicate the metal ion in TEM8 contributes significantly to the binding affinity, and PA-TEM8 binding is more favorable in the presence of Mg²⁺ than Ca²⁺ . Further, computational analysis suggests that the differences in PA-TEM8 binding affinity are comparable to the closely related integrin proteins. The conformation change, which linked to changes in activity of integrins, was not found in TEM8. In the present of Mg²⁺ , TEM8 remains in a conformation analogous to an integrin open (high-affinity) conformation. © 2017 Wiley Periodicals, Inc.

Binding of filamentous actin to anthrax toxin receptor 1 decreases its association with protective antigen

ANTXR1 is a type I membrane protein that binds the protective antigen (PA) component of anthrax toxin. The cytosolic domain of ANTXR1 has a novel actin-binding region that influences the interaction of the ectodomain with PA. Here, we have investigated features of the cytosolic domain of ANTXR1 that reduce the association of the receptor with PA. We mutated a stretch of conserved acidic amino acids adjacent to the actin-binding region and found that the mutation increased the affinity for monomeric actin in vitro. ANTXR1 bearing this mutation exhibited increased association with the cytoskeleton and bound less PA compared to the wild-type receptor, confirming the inverse correlation between the two interactions. To determine whether binding of actin is sufficient to regulate the ectodomain, we replaced the actin-binding region of ANTXR1 with that from the yeast protein abp140 and with the WH2 domain of WAVE2. Although both of these domains bound monomeric actin in vitro, only the sequence from abp140 reduced binding of PA to a hybrid receptor. The actin binding regions of ANTXR1 and abp140, but not the WH2 domain, colocalized with actin stress fibers, which suggested that filamentous actin regulates ANTXR1. Consistent with this notion, disruption of actin filaments using latrunculin A increased the amount of PA bound to cells. This work provides evidence that cytoskeletal dynamics regulate ANTXR1 function.

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.

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.