Anthrax toxin receptor 2 is expressed in murine and tumor vasculature and functions in endothelial proliferation and morphogenesis

Tumor endothelial marker 8 amplifies canonical Wnt signaling in blood vessels

Tumor Endothelial Marker 8/Anthrax Toxin Receptor 1 (TEM8/ANTXR1) expression is induced in the vascular compartment of multiple tumors and therefore, is a candidate molecule to target tumor therapies. This cell surface molecule mediates anthrax toxin internalization, however, its physiological function in blood vessels remains largely unknown. We identified the chicken chorioallantoic membrane (CAM) as a model system to study the endogenous function of TEM8 in blood vessels as we found that TEM8 expression was induced transiently between day 10 and 12 of embryonic development, when the vascular tree is undergoing final development and growth. We used the cell-binding component of anthrax toxin, Protective Antigen (PA), to engage endogenous TEM8 receptors and evaluate the effects of PA-TEM8 complexes on vascular development. PA applied at the time of highest TEM8 expression reduced vascular density and disrupted hierarchical branching as revealed by quantitative morphometric analysis of the vascular tree after 48h. PA-dependent reduced branching phenotype was partially mimicked by Wnt3a application and ameliorated by the Wnt antagonist, Dikkopf-1. These results implicate TEM8 expression in endothelial cells in regulating the canonical Wnt signaling pathway at this day of CAM development. Consistent with this model, PA increased beta catenin levels acutely in CAM blood vessels in vivo and in TEM8 transfected primary human endothelial cells in vitro. TEM8 expression in Hek293 cells, which neither express endogenous PA-binding receptors nor Wnt ligands, stabilized beta catenin levels and amplified beta catenin-dependent transcriptional activity induced by Wnt3a. This agonistic function is supported by findings in the CAM, where the increase in TEM8 expression from day 10 to day 12 and PA application correlated with Axin 2 induction, a universal reporter gene for canonical Wnt signaling. We postulate that the developmentally controlled expression of TEM8 modulates endothelial cell response to canonical Wnt signaling to regulate vessel patterning and density.

Tumor endothelium marker-8 based decoys exhibit superiority over capillary morphogenesis protein-2 based decoys as anthrax toxin inhibitors

Anthrax toxin is the major virulence factor produced by Bacillus anthracis. The toxin consists of three protein subunits: protective antigen (PA), lethal factor, and edema factor. Inhibition of PA binding to its receptors, tumor endothelium marker-8 (TEM8) and capillary morphogenesis protein-2 (CMG2) can effectively block anthrax intoxication, which is particularly valuable when the toxin has already been overproduced at the late stage of anthrax infection, thus rendering antibiotics ineffectual. Receptor-like agonists, such as the mammalian cell-expressed von Willebrand factor type A (vWA) domain of CMG2 (sCMG2), have demonstrated potency against the anthrax toxin. However, the soluble vWA domain of TEM8 (sTEM8) was ruled out as an anthrax toxin inhibitor candidate due to its inferior affinity to PA. In the present study, we report that L56A, a PA-binding-affinity-elevated mutant of sTEM8, could inhibit anthrax intoxication as effectively as sCMG2 in Fisher 344 rats. Additionally, pharmacokinetics showed that L56A and sTEM8 exhibit advantages over sCMG2 with better lung-targeting and longer plasma retention time, which may contribute to their enhanced protective ability in vivo. Our results suggest that receptor decoys based on TEM8 are promising anthrax toxin inhibitors and, together with the pharmacokinetic studies in this report, may contribute to the development of novel anthrax drugs.

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.

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.