The Long-Lived Nature of Clostridium perfringens Iota Toxin in Mammalian Cells Induces Delayed Apoptosis

Abstract

Mono-ADP ribosylation of actin by bacterial toxins, such as Clostridium perfringens iota or Clostridium botulinum C2 toxins, results in rapid depolymerization of actin filaments and cell rounding. Here we report that treatment of African green monkey kidney (Vero) cells with iota toxin resulted in delayed caspase-dependent death. Unmodified actin did not reappear in toxin-treated cells, and enzyme-active toxin was detectable in the cytosol for at least 24 h. C2 toxin showed comparable, long-lived effects in cells, while a C2 toxin control lacking ADP-ribosyltransferase activity did not induce cell death. To address whether the remarkable stability of the iota and C2 toxins in cytosol was crucial for inducing cell death, we treated cells with C/SpvB, the catalytic domain of Salmonella enterica SpvB. Although C/SpvB also mono-ADP ribosylates actin as do the iota and C2 toxins, cells treated with a cell-permeating C/SpvB fusion toxin became rounded but recovered and remained viable. Moreover, unmodified actin reappeared in these cells, and ADP-ribosyltransferase activity due to C/SpvB was not detectable in the cytosol after 24 h, a result most likely due to degradation of C/SpvB. Repeated application of C/SpvB prevented recovery of cells and reappearance of unmodified actin. In conclusion, a complete but transient ADP ribosylation of actin was not sufficient to trigger apoptosis, implying that longterm stability of actin-ADP-ribosylating toxins, such as iota and C2, in the cytosol is crucial for inducing delayed, caspase-dependent cell death.Various bacterial toxins destroy the actin cytoskeleton of eukaryotic cells by mono-ADP ribosylation of G-actin at arginine-177 (1, 3, 23). ADP-ribosylated actin caps the barbed, fastgrowing ends of actin filaments (F-actin), thus preventing further assembly of unmodified G-actin into filaments (32). Although ADP-ribosylated G-actin does not affect the pointed, slowgrowing ends of F-actin, the critical concentration for actin polymerization does increase and leads to complete depolymerization of actin filaments (33). Therefore, treatment of cells with these toxins disrupts the actin cytoskeleton and causes rounding of adherent cells within hours.Binary ADP-ribosylating toxins that target actin can be divided into family members that include Clostridium botulinum C2 toxin (20), Clostridium perfringens iota toxin (29), CDT from Clostridium difficile (26), Clostridium spiroforme toxin, also known as CST (25), and the vegetative insecticidal proteins from Bacillus cereus (9). The clostridial and bacillus binary toxins are typical exotoxins, produced by extracellular bacteria, that ultimately enter the cytosol of targeted cells without the toxin-producing bacteria. In contrast, SpvB from Salmonella enterica (21, 31) also targets actin but is delivered into the host cell's cytosol by intracellularly located bacteria.All binary actin-ADP-ribosylating toxins are composed of two nonlinked proteins, a binding/translocation component and a separate enzyme component (3). In recent years, t...

A Cell-Permeable Fusion Protein Based on Clostridium botulinum C2 Toxin for Delivery of p53 Tumorsuppressor into Cancer Cells

“…GST-C2IN-p53 accumulated in the cytosol after 5 h, but was no longer detectable after 24 h (data not shown). In consistency, a cytosolic degradation has also been reported for other C2IN-based fusion proteins, such as C2IN-C3 [36], C2IN-SpvB [37] and C2IN-streptavidin [38]. Upon internalization into HeLa and A549 cells GST-C2IN-p53 was primarily detected in the cytoplasm and less prominently in the nucleus as observed by confocal microscopy.…”

Cyclophilin-Facilitated Membrane Translocation as Pharmacological Target to Prevent Intoxication of Mammalian Cells by Binary Clostridial Actin ADP-Ribosylated Toxins

“…This toxin-catalyzed modification turns G-actin into a capping molecule that prevents further polymerization of actin filaments [2][3][4][5] and results in the breakdown of the actin cytoskeleton. Finally, the mode of action of these toxins results in cell rounding [6][7][8][9][10][11] and cell death [12,13] associated with severe enteric diseases in humans and animals [14][15][16][17][18].…”

Toxin Plasmids of Clostridium perfringens

“…Inside the cytoplasm, IA exerts its enzymatic activity, which involves ADP-ribosylating actin at Arg-177 to disassemble the host cell cytoskeleton (126). ITX can persist for at least 24 h inside host cells, which results in a delayed apoptosis (127).…”