Anthrax toxins cooperatively inhibit endocytic recycling by the Rab11/Sec15 exocyst

Abstract

Bacillus anthracis is the causative agent of anthrax in humans and other mammals1, 2. In lethal systemic anthrax, proliferating bacilli secrete large quantities of the toxins lethal factor (LF) and edema factor (EF), leading to widespread vascular leakage and shock. While host targets of LF (MAPKKs) and EF (cAMP-dependent processes)3 have been implicated in the initial phase of anthrax1, 2, less is understood about toxin action during the final stage of infection. Here, we use Drosophila to identify the Rab11/Sec15 exocyst, which acts at the last step of endocytic recycling, as a novel target of both EF and LF. EF reduces levels of apically localized Rab11, and indirectly blocks vesicle formation by its binding partner and effector Sec15 (Sec15-GFP), while LF acts more directly to reduce Sec15-GFP vesicles. Convergent effects of EF and LF on Rab11/Sec15 inhibit expression of and signaling by the Notch ligand Delta and reduce DE-cadherin levels at adherens junctions (AJ). In human endothelial cells, the two toxins act in a conserved fashion to block formation of Sec15 vesicles, inhibit Notch signaling, and reduce cadherin expression at AJ. This coordinated disruption of the Rab11/Sec-15 exocyst by anthrax toxins may contribute to toxin-dependent barrier disruption and vascular dysfunction during B. anthracis infection.

Identification of host factors required for Yersinia pestis macrophage intracellular survival and their impact on vacuole maturation, acidification and trafficking.

“…Anthrax toxins were shown to disrupt recycling in Drosophila wing discs, and overexpression of Rab11 restored wildtype morphology [248,313]. This theme has been observed for Tetanus and Shiga toxin B through their disruption and association with markers for recycling endosomes [240,309,310].…”

“…Recently, studies using Anthrax toxin demonstrate disruption of host recycling endosomes was directly attributed to the toxins impact on Rab11b and Sec15 exocyst assembly [248]. However, through overexpression of Rab11b they were able to restore wildtype Drosophila wing structure and normal recycling [248].…”

“…Recently, studies using Anthrax toxin demonstrate disruption of host recycling endosomes was directly attributed to the toxins impact on Rab11b and Sec15 exocyst assembly [248]. However, through overexpression of Rab11b they were able to restore wildtype Drosophila wing structure and normal recycling [248]. Within this context, we were intrigued that during our screen Arf6 RNAi, a well-studied Rab4 effector and a negative regulator of rapid recycling [249][250][251][252][253], had a slight hypervirulent phenotype at 10 h post-infection This observation could be suggestive, that inhibition of rapid recycling forces cellular compensation by the slow recycling arm and Rab11b through an increase in slow recycling endosomes.…”

Opening moves: early events in anthrax pathogenesis and their consequences for dissemination

“…One possibility is that the bacteria are able to increase the permeability of the endothelium [252,254,256]. This would possibly allow the bacilli to more easily access the lymphatic capillaries and disseminate to the draining lymph node.…”

“…LF and EF reduces both Notch receptor and surface protein levels of the Notch ligand Delta when endogenously expressed by a GAL4 promoter in Drosophila melanogaster [256]. Notch and Delta are members of the Notch signaling pathway, which have crucial roles in embryogenesis and cell to cell communication.…”