{{Infobox protein family | Symbol = CDT | Name = Cytolethal distending toxin | image = PDB protein 1sr4 (HdCDT).jpg | width = | caption = Crystal structure of the fully assembled ''Haemophilus ducreyi'' cytolethal distending toxin | Pfam = PF03498 | Pfam_clan = CL0066 | InterPro = IPR003558 | SMART = | PROSITE = | MEROPS = | SCOP = | TCDB = | OPM family = | OPM protein = | CAZy = | CDD = }}
'''Cytolethal distending toxins''' (abbreviated '''CDTs''') are a class of heterotrimeric toxins produced by certain gram-negative bacteria that display DNase activity.<ref name="one">{{cite journal | vauthors = Jinadasa RN, Bloom SE, Weiss RS, Duhamel GE | title = Cytolethal distending toxin: a conserved bacterial genotoxin that blocks cell cycle progression, leading to apoptosis of a broad range of mammalian cell lineages | journal = Microbiology | volume = 157 | issue = Pt 7 | pages = 1851–75 | date = July 2011 | pmid = 21565933 | pmc = 3167888 | doi = 10.1099/mic.0.049536-0 | doi-access = free | url = http://mic.sgmjournals.org/content/157/7/1851.short }} </ref><ref name="nine">{{Cite journal |author=Cherilyn A. Elwell |author2=Lawrence A. Dreyfus | year = 2000 | title = DNase I homologous residues in CdtB are critical for cytolethal distending toxin-mediated cell cycle arrest. | journal = Molecular Microbiology | volume = 37 | issue = 4 | pages = 952–963 | doi = 10.1046/j.1365-2958.2000.02070.x | pmid = 10972814 | s2cid = 10087538 | doi-access = free }} </ref> These toxins trigger G2/M cell cycle arrest in specific mammalian cell lines, leading to the enlarged or distended cells for which these toxins are named.<ref name="four">{{Citation | last = Dreyfus | first = Lawrence A. | contribution = Cytolethal Distending Toxins | year = 2003 | title = Bacterial Protein Toxins | editor= D. Burns| pages = 257–270 | place = Washington, DC | publisher = ASM Press | contribution-url = https://onlinelibrary.wiley.com/doi/abs/10.1128/9781555817893.ch18 |display-editors=etal}} </ref> Affected cells die by apoptosis.<ref name= "one" />
Each toxin consists of three distinct subunits named alphabetically in the order that their coding genes appear in the ''cdt operon''. Cytolethal distending toxins are classified as AB toxins, with an active ("A") subunit that directly damages DNA and a binding ("B") subunit that helps the toxin attach to the target cells. CdtB is the active subunit and a homolog to mammalian DNase I, whereas CdtA and CdtC make up the binding subunit.<ref name="two">{{cite journal | vauthors = Guerra L, Cortes-Bratti X, Guidi R, Frisan T | title = The biology of the cytolethal distending toxins | journal = Toxins | volume = 3 | issue = 12 | pages = 172–90 | date = March 2011 | pmid = 22069704 | doi = 10.3390/toxins3030172 | pmc = 3202825 | doi-access = free }} </ref>
Cytolethal distending toxins are produced by gram-negative pathogenic bacteria from the phylum ''Pseudomonadota''. Many of these bacteria, including ''Shigella dysenteriae'', ''Haemophilus ducreyi'', and ''Escherichia coli'', infect humans. Bacteria that produce CDTs often persistently colonize their host.<ref name= "one" />
==History==
The first recorded observation of a cytolethal-distending toxin was in 1987 in a pathogenic strain in ''E. coli'' isolated from a young patient.<ref name= "four" /> Later that year, scientists W.M. Johnson and H. Lior published the journal article "Production of Shiga toxin and a cytolethal distending toxin (CLDT) by serogroups of ''Shigella spp.''" in ''Microbiology Letters''.<ref name= "one" /> The discovery of other bacteria producing CDT toxins continues to this day.
In 1994 Scott and Kaper cloned and sequenced a ''cdt operon'' from another ''E. coli'' strain, publishing in ''Infection and Immunity''.<ref name="one" /><ref name="ten"> {{cite journal | vauthors = Scott DA, Kaper JB | title = Cloning and sequencing of the genes encoding Escherichia coli cytolethal distending toxin | journal = Infection and Immunity | volume = 62 | issue = 1 | pages = 244–51 | date = January 1994 | doi = 10.1128/iai.62.1.244-251.1994 | pmid = 8262635 | url= | pmc=186093}}</ref> The three genes discovered were denoted ''cdtA'', ''cdtB'', and ''cdtC''.<ref name="ten" />
In 1997, the first paper of many to show G2/M cell cycle arrest caused by a cytolethal distending toxin was published in ''Molecular Microbiology''.<ref name= "one" /> The study focused on another ''E. coli'' strain. This paper was followed by a 1999 publication in ''Infectious Immunity'', which demonstrated that ''Haemophilus ducreyi'' CDT causes cell death via apoptosis. This finding was also confirmed for other cytolethal distending toxins in subsequent studies.
The discovery of the homology of ''cdtB'' to mammalian DNase I and the current AB model for the toxin were published in early 2000. <ref name="nine"/><ref name="five"> {{cite journal | vauthors = Lara-Tejero M, Galán JE | title = CdtA, CdtB, and CdtC form a tripartite complex that is required for cytolethal distending toxin activity | journal = Infection and Immunity | volume = 69 | issue = 7 | pages = 4358–65 | date = July 2001 | pmid = 11401974 | pmc = 98507 | doi = 10.1128/IAI.69.7.4358-4365.2001 }} </ref> Further research and the publication of crystal structures for the CDT toxins from two different species continues to support this model.<ref name="one" />
==Sources==
All known cytolethal distending toxins are produced by gram-negative bacteria in the ''Gammaproteobacteria'' and ''Campylobacterota''. In several cases, the bacteria producing CDT are human pathogens. Medically important CDT producers include:<ref name= "one" />
*''Haemophilus ducreyi'' (chancroids) *''Aggregatibacter actinomycetemcomitans'' (periodontitis) *''Escherichia coli'' (various diseases) *''Shigella dysenteriae'' (dysentery) *''Salmonella enterica'' serotype Typhi (typhoid fever) *''Campylobacter upsaliensis'' (enterocolitis) *''Campylobacter jejuni'' (enterocolitis)
CDT-producing bacteria are often associated with mucosal linings, such as those in the stomach and intestines, and with persistent infections. The toxins are either secreted freely or associated with the membrane of the producing bacteria.<ref name= "one" />
==Nomenclature==
Individual cytolethal distending toxins are named for the bacterial species that they are isolated from. As of 2011, most scientists have adopted the practice of placing the first letter of both the genus and species in front of the toxin name to reflect its source (i.e., the CDT from ''Haemaphilus ducreyi'' is referred to as HdCDT).<ref name= "one" /><ref name="Spinal Tap">{{Cite journal |author=Ximena Cortes-Bratti |author2=Teresa Frisan |author3=Monica Thelestam | year = 2001 | title = The Cytolethal Distending Toxins Induce DNA Damage and Cell Cycle Arrest. | journal = Toxicon | volume = 39 | issue = 11 | pages = 1729–1736 | doi = 10.1016/S0041-0101(01)00159-3 | pmid = 11595635 |bibcode=2001Txcn...39.1729C }} </ref> If several subspecies produce different toxins, as in the case of ''E. coli'', Roman numerals may be added after the second letter.<ref name="Spinal Tap" /> Both complete toxins and individual subunits are labeled using this convention.
In response to the continued discovery of additional cytolethal distending toxins, a 2011 review has proposed that the toxin names be expanded to include the first three letters of the species (i.e., HducCDT for ''Haemaphilus ducreyi'' CDT).<ref name="one" />
==Cellular effects== CDT toxins are genotoxins capable of directly damaging DNA in target cells. They are the only AB-type toxins discovered that display DNase activity, allowing them to introduce breaks into the target cell's DNA.<ref name= "one" /><ref name= "two" />
In many cell lines including human fibroblasts, epithelial cells, endothelial cells, and keratinocytes, CDTs cause G2/M cell cycle arrest, cytoplasmic distension, and eventual cell death via apoptosis.<ref name="one" /><ref name="four" /><ref name="seven"> {{Cite journal |author=Bruce J. Shenker |author2=Mensur Dlakic |author3=Lisa P. Walker |author4=Dave Besack |author5=Eileen Jaffe |author6=Ed LaBelle |author7=Kathleen Boesze-Battaglia | year = 2007 | title = A Novel Mode of Action for a Microbial-Derived Immunotoxin: The Cytolethal Distending Toxin Subunit B Exhibits Phosphatidylinositol 3,4,5-Triphosphate Phosphatase Activity | journal = The Journal of Immunology | volume = 178 | issue = 8 | pages = 5099–5108 | doi = 10.4049/jimmunol.178.8.5099 | pmc = 4472023 | pmid=17404292}}</ref> Most publications attribute the G2/M cycle arrest to the buildup of irreversible DNA damage from the toxin's DNase activity as the trigger for the G2/M cell cycle arrest, but other research suggests that this model is incomplete.<ref name= "seven" /> The cytoplasmic distension is a direct result of the G2/M cell cycle arrest. The cell enlarges in preparation for mitosis, but cannot divide to restore its normal size. Aside from classical apoptosis, signs of cellular senescence has also been observed in normal and cancer cell lines (fibroblasts, HeLa and U2-OS) after CDT intoxication<ref>{{cite journal | vauthors = Blazkova H, Krejcikova K, Moudry P, Frisan T, Hodny Z, Bartek J | title = Bacterial intoxication evokes cellular senescence with persistent DNA damage and cytokine signalling | journal = Journal of Cellular and Molecular Medicine | volume = 14 | issue = 1–2 | pages = 357–67 | date = January 2010 | pmid = 19650831 | doi = 10.1111/j.1582-4934.2009.00862.x | pmc = 3837606 }}</ref>
In lymphocytes, cell death occurs quickly and is not preceded by significant cytoplasmic distension.<ref name= "seven" /> The ability of these toxins to affect lymphocytes differently may be advantageous to the bacteria that utilize these toxins, but the mechanism behind this phenomenon is not yet well understood.
==Toxin structure==
The active, assembled toxin is a tripartite structure with three distinct subunits- CdtA, CdtB, and CdtC. In terms of function, it is an AB toxin. In this context, the CdtB subunit is actually the catalytically active "A" subunit, and the CdtA and CdtC together form the binding "B" subunit, which helps the toxin bind and enter target cells.<ref name="five" /> Some literature refers to the toxin structure as AB<sub>2</sub> to reflect the presence of both CdtA and CdtC.
Different from all other CDTs, ''Salmonella enterica'' serovar Typhi CDT (SeCDT) has no CdtA and CdtC homologues. However, encoded closely to the active subunit ''cdtb'', the Pertussis-like toxin A and B (pltA/pltB) have been shown to be essential for cellular intoxication.<ref name="Spanò 30–38">{{cite journal | vauthors = Spanò S, Ugalde JE, Galán JE | title = Delivery of a Salmonella Typhi exotoxin from a host intracellular compartment | journal = Cell Host & Microbe | volume = 3 | issue = 1 | pages = 30–8 | date = January 2008 | pmid = 18191792 | doi = 10.1016/j.chom.2007.11.001 | doi-access = free }}</ref> PltA and PltB have a different structure from CdtA and CdtC, thus promoting CdtB activity in a different way. Both PltA and PltB have been found to bind directly to CdtB ''in vitro''.<ref name="Spanò 30–38"/> In addition, different from all other CDTs, ''Salmonella'' genotoxin is produced only upon bacterial internalization in infected cells, thus the SeCDT traffic may differ remarkably from the canonical ones.
==CdtB==
CdtB is considered the active subunit of the CDT holotoxin. Microinjection of CdtB into susceptible cells without CdtA or CdtC results in the G2/M cell cycle arrest and cytoplasmic distension characteristic of CDT toxins.<ref name="nine" /> The structure of CdtB is well-conserved between different bacteria. The CdtB subunit is the most sequentially conserved between species.<ref name="two" /> The molecular weight of CdtB ranges from 28 kDa to 29 kDa, depending on the species.<ref name= "one" />
As the active subunit, CdtB is termed the "A" subunit according to the AB toxin model.<ref name="one" /> This confusing nomenclature is due to the naming of the toxin's subunits before their individual functions were understood.
===Activity===
CdtB exhibits at least two enzymatic activities- DNase activity capable of introducing double-strand breaks in DNA, and a phosphatase activity that resembles phosphatidylinositol 3,4,5-triphosphatase.<ref name= "nine" /><ref name= "seven" /> Both activities can be demonstrated ''in vitro'' in the absence of the other two subunits.<ref name= "eight"> {{cite journal | vauthors = Eshraghi A, Maldonado-Arocho FJ, Gargi A, Cardwell MM, Prouty MG, Blanke SR, Bradley KA | title = Cytolethal distending toxin family members are differentially affected by alterations in host glycans and membrane cholesterol | journal = The Journal of Biological Chemistry | volume = 285 | issue = 24 | pages = 18199–207 | date = June 2010 | pmid = 20385557 | pmc = 2881744 | doi = 10.1074/jbc.m110.112912 | doi-access = free }}</ref> The relative importance of each activity ''in vivo'' is unclear.<ref name="eight" /> Mutations that reduce either activity also reduce the toxin's ability to induce G2/M phase arrest in at least some of the susceptible cell lines.<ref name= "nine" /><ref name= "seven" />
===Similarities to mammalian DNase I===
CdtB is functionally homologous to mammalian DNase I and contains a conserved pentapeptide sequence found in all DNase I enzymes to date.<ref name="nine" /> In addition, several residues critical to DNase I's ability to break the phosphodiester bonds in the DNA backbone are found in the CdtB structure. A 2002 paper studying the effect of point mutations on five of these residues found that four of the five mutations tested abolished both CdtB's ability to degrade DNA in cell-free extracts and to cause G2/M arrest upon microinjection. The fifth mutation moderately reduced CdtB's activity.<ref name="nine" />
==CdtA and CdtC==
CdtA and CdtC make up the B subunit of the CDT holotoxin responsible for targeting the CdtB against susceptible cells.<ref name= "five" /> Neither subunit appears highly conserved, with sequence identities between different species often lower than 30%.<ref name= "two" /> The molecular weight of CdtA ranges from 23 kDa to 30 kDa, whereas CdtC ranges from 19 kDa to 21 kDa depending on the species.<ref name= "one" />
===Activity===
CdtA and CdtC are both believed to bind to the surface of target cells. The exact mechanism of this binding is unclear, and may not be conserved between CDT toxins from different species.<ref name="one" /><ref name= "eight" /> Proposed targets of CdtA and CdtC binding have included cholesterol, N-linked glycans, and glycosphingolipids.<ref name="eight" /> Current research has produced conflicting results on the actual importance of these proposed targets.<ref name="one" /><ref name="eight" /> Both CdtA and CdtC contain lectin domains,<ref name= "six"> {{cite journal | vauthors = Nesić D, Hsu Y, Stebbins CE | title = Assembly and function of a bacterial genotoxin | journal = Nature | volume = 429 | issue = 6990 | pages = 429–33 | date = May 2004 | pmid = 15164065 | doi = 10.1038/nature02532 | bibcode = 2004Natur.429..429N | s2cid = 4373500 }}</ref> suggesting that the toxin may bind via carbohydrates on the target cell's surface, whereas other research has suggested that the targets are surface proteins.<ref name= "one" />
==Notes== {{Reflist}}
Category:Toxins