Functional interactions between anthrax toxin receptors and the WNT signalling protein LRP6

Abstract

SummaryTo exert its activity, anthrax toxin must be endocytosed and its enzymatic toxic subunits delivered to the cytoplasm. It has been proposed that, in addition to the anthrax toxin receptors (ATRs), lipoproteinreceptor-related protein 6 (LRP6), known for its role in Wnt signalling, is also required for toxin endocytosis. These findings have however been challenged. We show that LRP6 can indeed form a complex with ATRs, and that this interaction plays a role both in Wnt signalling and in anthrax toxin endocytosis. We found that ATRs control the levels of LRP6 in cells, and thus the Wnt signalling capacity. RNAi against ATRs indeed led to a drastic decrease in LRP6 levels and a subsequent drop in Wnt signalling. Conversely, LRP6 plays a role in anthrax toxin endocytosis, but is not essential. We indeed found that toxin binding triggered tyrosine phosphorylation of LRP6, induced its redistribution into detergent-resistant domains, and its subsequent endocytosis. RNAis against LRP6 strongly delayed toxin endocytosis. As the physiological role of ATRs is probably to interact with the extracellular matrix, our findings raise the interesting possibility that, through the ATR-LRP6 interaction, adhesion to the extracellular matrix could locally control Wnt signalling.

Targeted Silencing of Anthrax Toxin Receptors Protects against Anthrax Toxins

“…Indeed, ANTXRs are an obvious target for therapy, and approaches that have been presented previously by others include the use of decoy receptors (7,8) and RNAi (36,37). We show for the first time that efficient and effective silencing of ANTXRs using specific siRNAs can significantly protect mouse and human macrophages from anthrax toxins in vitro.…”

Heterodimeric integrin complexes containing β1-integrin promote internalization and lethality of anthrax toxin

“…Diverse cell lines treated with LF-PA show a wide range of sensitivities to the toxin (9,(20)(21)(22)(23)(24)(25), and cell death occurs only in cells that have impaired MAPK kinase function (21). We hypothesized that such phenotypic differences might enable microarray-based identification of other genes whose differentially elevated or reduced expression correlates with differential toxin lethality.…”

“…Entry of the PA-LF complex is modulated by the GTPase-activating protein, ARAP3 (4) and LDL-related protein 6 (LRP6) (5), as well as by other genes that affect autophagy (6) or clathrin-mediated endocytosis (7,8). Whereas the mechanisms underlying the effects of ARAP3 on anthrax toxin entry are not known, LRP6 has been shown to interact with TEM8 and CMG2 at the cell surface (5) and to accelerate their endocytosis (9).…”

Capillary morphogenesis protein-2 is the major receptor mediating lethality of anthrax toxin in vivo

“…1B). The residual PA binding on TEM8 Ϫ/Ϫ CMG2 Ϫ/Ϫ MEFs may be mediated by the TMdeleted forms of TEM8 and CMG2, which may be transiently associated with the cell surface during secretion, possibly by association with LRP6 (7,8), or by other unidentified VWA proteins that can bind PA but cannot mediate toxin internalization. At 37°C the WT cells and the cells with either the TEM8 or CMG2 mutations bound and proteolytically processed PA to the PA63 heptamer, which was then internalized into endocytic vesicles to form the characteristic SDS/heat-resistant heptamer, resulting in LF delivery to the cytosol and MEK3 cleavage (Fig.…”

“…These 3 proteins are individually nontoxic, but can assemble on the cell surface to form toxic complexes. To intoxicate host mammalian cells, PA binds to its cellular receptors, tumor endothelium marker-8 [TEM8, also named anthrax toxin receptor 1 (ANTXR1)] and capillary morphogenesis protein-2 [CMG2, also named anthrax toxin receptor 2 (ANTXR2)] (5, 6) with the involvement of a coreceptor LRP6 (7,8) and is then proteolytically processed to the active form, cell-surface bound PA63. PA63 spontaneously oligomerizes to form a heptamer that binds and delivers LF and EF into the cytosol.…”