The biology of the cytolethal distending toxins.

The cytolethal distending toxins (CDTs), produced by a variety of Gram-negative pathogenic bacteria, are the first bacterial genotoxins described, since they cause DNA damage in the target cells. CDT is an A-B(2) toxin, where the CdtA and CdtC subunits are required to mediate the binding on the surface of the target cells, allowing internalization of the active CdtB subunit, which is functionally homologous to the mammalian deoxyribonuclease I. The nature of the surface receptor is still poorly characterized, however binding of CDT requires intact lipid rafts, and its internalization occurs via dynamin-dependent endocytosis. The toxin is retrograde transported through the Golgi complex and the endoplasmic reticulum, and subsequently translocated into the nuclear compartment, where it exerts the toxic activity. Cellular intoxication induces DNA damage and activation of the DNA damage responses, which results in arrest of the target cells in the G1 and/or G2 phases of the cell cycle and activation of DNA repair mechanisms. Cells that fail to repair the damage will senesce or undergo apoptosis. This review will focus on the well-characterized aspects of the CDT biology and discuss the questions that still remain unanswered.

Thioredoxin 80-activated-monocytes (TAMs) inhibit the replication of intracellular pathogens.

BACKGROUND: Thioredoxin 80 (Trx80) is an 80 amino acid natural cleavage product of Trx, produced primarily by monocytes. Trx80 induces differentiation of human monocytes into a novel cell type, named Trx80-activated-monocytes (TAMs). PRINCIPAL FINDINGS: In this investigation we present evidence for a role of TAMs in the control of intracellular bacterial infections. As model pathogens we have chosen Listeria monocytogenes and Brucella abortus which replicate in the cytosol and the endoplasmic reticulum respectively. Our data indicate that TAMs efficiently inhibit intracellular growth of both L. monocytogenes and B. abortus. Further analysis shows that Trx80 activation prevents the escape of GFP-tagged L. monocytogenes into the cytosol, and induces accumulation of the bacteria within the lysosomes. Inhibition of the lysosomal activity by chloroquine treatment resulted in higher replication of bacteria in TAMs compared to that observed in control cells 24 h post-infection, indicating that TAMs kill bacteria by preventing their escape from the endosomal compartments, which progress into a highly degradative phagolysosome. SIGNIFICANCE: Our results show that Trx80 potentiates the bactericidal activities of professional phagocytes, and contributes to the first line of defense against intracellular bacteria.

Cytolethal distending toxins and activation of DNA damage-dependent checkpoint responses.

Cytolethal distending toxins (CDTs) are unique among bacterial protein toxins in their ability to cause DNA damage, due to their functional similarity to the mammalian deoxyribonuclease I (DNase I). The cellular response to CDT intoxication is characterised by activation of DNA damage-induced checkpoint responses, and the final outcome is cell type dependent. Cells of epithelial origin and normal keratinocytes are arrested in the G2 phase of the cell cycle, normal fibroblasts are also arrested in G1, while B cells die of apoptosis. CDTs are encoded by three linked genes (cdtA, cdtB and cdtC), and CdtB is the toxin subunit which possesses the DNase I-like activity. All the three genes have to be present in the bacterium in order to produce an active cytotoxin, however cytotoxic Haemophilus ducreyi CDT, purified from a CdtABC recombinant E. coli strain, contains the CdtB and CdtC subunits, suggesting that they constitute the holotoxin and that CdtC may be required for CdtB internalization. The role of the CdtA subunit is currently unknown, but it might modify and therefore activate CdtC. This review will focus on the cellular responses induced by CDTs in mammalian cells.

The Haemophilus ducreyi cytolethal distending toxin induces DNA double-strand breaks and promotes ATM-dependent activation of RhoA.

Among bacterial protein toxins, the cytolethal distending toxins (CDTs) are unique in their ability to activate the DNA damage checkpoint responses, causing cell cycle arrest or apoptosis in intoxicated cells. We provide direct evidence that natural intoxication of cells with the Haemophilus ducreyi CDT (HdCDT) holotoxin induces DNA double-strand breaks similarly to ionizing radiation. Upon DNA damage, epithelial cells and fibroblasts promote the formation of actin stress fibres via activation of the small GTPase RhoA. This phenomenon is not toxin specific, but is part of the ATM-induced cellular responses to genotoxic stresses, including ionizing radiation. Activation of RhoA is associated with prolonged cell survival, as HdCDT-treated epithelial cells expressing a dominant-negative form of RhoA detach and consequently die faster than cells expressing a functional RhoA. Our data highlight several novel aspects of CDT biology: (i) we show that a member of the CDT family causes DNA double-strand breaks in naturally intoxicated cells, acting as a true genotoxic agent; and (ii) we disclose the existence of a novel signalling pathway for intracellularly triggered activation of the RhoA GTPase via the ATM kinase in response to DNA damage, possibly required to prolong cell survival.