Editing Tetanus toxin

{{Short description|Extremely potent neurotoxin}}
{{cs1 config|name-list-style=vanc|display-authors=6}}
{{Infobox nonhuman protein
|Symbol= tetX
|Organism=''Clostridium tetani'' E88
|TaxID = 212717
|UniProt=P04958
|image=PDB 1a8d EBI.jpg
|caption=Tetanus Toxin Heavy Chain C Fragment ({{PDB|1a8d}})
| DrugBank = 
| EntrezGene = 24255210
| HomoloGene = 
| PDB = 
| RefSeqmRNA = 
| RefSeqProtein = WP_011100836.1
| ECnumber = 
| Chromosome = Genomic
| EntrezChromosome = NC_004565.1
| GenLoc_start = 68047
| GenLoc_end = 73178
}}
{{Infobox enzyme
| Name       = Tentoxilysin
| EC_number  = 3.4.24.68
| CAS_number = 107231-12-9
| GO_code    =
| image      = 
| width      = 
| caption    =  
}}
[[Image:Diagram of structure of tetanospasmin.png|thumb|340px|right|Structure of tetanospasmin]]
[[Image:Mechanism of action of tetanospasmin.gif|thumb|340px|right|Mechanism of action of tetanospasmin]]

'''Tetanus toxin''' ('''TeNT''') is an extremely potent [[neurotoxin]] produced by the vegetative cell of ''[[Clostridium tetani]]''<ref>{{DorlandsDict|eight/000107584|Tetanospasmin}}</ref> in [[Hypoxia (environmental)|anaerobic]] conditions, causing [[tetanus]]. It has no known function for clostridia in the soil environment where they are normally encountered. It is also called '''spasmogenic toxin''', '''tentoxilysin''', '''tetanospasmin''', or '''tetanus neurotoxin'''. The LD<sub>50</sub> of this toxin has been measured to be approximately 2.5&ndash;3&nbsp;ng/kg,<ref>{{Cite web|url=https://www.cdc.gov/pinkbook/hcp/table-of-contents/chapter-21-tetanus.html |title=Chapter 21: Tetanus |website=CDC |access-date=2017-01-18}}</ref><ref>{{Cite web|url=http://www.ehs.ufl.edu/programs/bio/toxins/toxin-table/|title=Toxin Table |website=Environmental Health & Safety » University of Florida |access-date=2017-01-18|archive-date=2017-01-18|archive-url=https://web.archive.org/web/20170118221643/http://www.ehs.ufl.edu/programs/bio/toxins/toxin-table/|url-status=dead}}</ref> making it second only to the related [[botulinum toxin]] (LD<sub>50</sub> 2&nbsp;ng/kg)<ref>{{Cite web|url=https://www.who.int/mediacentre/factsheets/fs270/en/|title=Botulism|website=World Health Organization |access-date=2017-01-18}}</ref> as the deadliest toxin in the world. However, these tests are conducted solely on mice, which may react to the toxin differently from humans and other animals.

''C. tetani'' also produces the [[exotoxin]] [[tetanolysin]], a hemolysin, that causes destruction of tissues.<ref>{{cite book| vauthors = Willey J |title=Prescott's Principles of Microbiology|url=https://archive.org/details/prescottsprincip00will|url-access=limited|year=2009|publisher=McGraw-Hill|location=New York City, NY|isbn=978-0-07-337523-6|pages=[https://archive.org/details/prescottsprincip00will/page/n497 481]}}</ref>

==Distribution==
Tetanus toxin spreads through tissue spaces into the [[lymphatic system|lymphatic]] and [[vascular system]]s. It enters the nervous system at the [[neuromuscular junction]]s and migrates through nerve trunks and into the [[central nervous system]] (CNS) by retrograde axonal transport by using [[dynein]]s.<ref name = Farrar>{{cite journal | vauthors = Farrar JJ, Yen LM, Cook T, Fairweather N, Binh N, Parry J, Parry CM | title = Tetanus | journal = Journal of Neurology, Neurosurgery, and Psychiatry | volume = 69 | issue = 3 | pages = 292–301 | date = September 2000 | pmid = 10945801 | pmc = 1737078 | doi = 10.1136/jnnp.69.3.292 }}</ref><ref>{{cite journal | vauthors = Lalli G, Gschmeissner S, Schiavo G | title = Myosin Va and microtubule-based motors are required for fast axonal retrograde transport of tetanus toxin in motor neurons | journal = Journal of Cell Science | volume = 116 | issue = Pt 22 | pages = 4639–4650 | date = November 2003 | pmid = 14576357 | doi = 10.1242/jcs.00727 | doi-access = free }}</ref>

== Structure ==
The tetanus toxin [[protein]] has a molecular weight of 150&nbsp;[[Atomic mass unit|kDa]]. It is translated from the ''tetX'' gene as one protein which is subsequently cleaved into two parts: a 100&nbsp;kDa heavy or B-chain and a 50&nbsp;kDa light or A-chain. The chains are connected by a [[Disulfide#Occurrence in proteins|disulfide bond]].
* The  B-chain binds to disialo[[ganglioside]]s (GD2 and GD1b) on the neuronal membrane and contains a translocation domain which aids the movement of the protein across that membrane and into the neuron.
* The A-chain, an M27-family zinc [[endopeptidase]], attacks the [[vesicle-associated membrane protein]] (VAMP).

The TetX gene encoding this protein is located on the PE88 plasmid.<ref>{{cite journal | vauthors = Eisel U, Jarausch W, Goretzki K, Henschen A, Engels J, Weller U, Hudel M, Habermann E, Niemann H | title = Tetanus toxin: primary structure, expression in E. coli, and homology with botulinum toxins | journal = The EMBO Journal | volume = 5 | issue = 10 | pages = 2495–2502 | date = October 1986 | pmid = 3536478 | pmc = 1167145 | doi = 10.1002/j.1460-2075.1986.tb04527.x }}</ref><ref name="Alp">{{cite journal | vauthors = Popp D, Narita A, Lee LJ, Ghoshdastider U, Xue B, Srinivasan R, Balasubramanian MK, Tanaka T, Robinson RC | title = Novel actin-like filament structure from Clostridium tetani | journal = The Journal of Biological Chemistry | volume = 287 | issue = 25 | pages = 21121–21129 | date = June 2012 | pmid = 22514279 | pmc = 3375535 | doi = 10.1074/jbc.M112.341016 | doi-access = free }}</ref>

Several structures of the binding domain and the peptidase domain have been solved by [[X-ray crystallography]] and deposited in the [[Protein Data Bank|PDB]].<ref>{{cite web | title = Advanced Search for UniProt ID P04958 | work = Protein Databank in Europe (PDBe) | url = https://www.ebi.ac.uk/pdbe/entry/search/index/index?uniprot:P04958 }}</ref>

==Mechanism of action==

The mechanism of TeNT action can be broken down and discussed in these different steps:

;Transport
:# Specific binding in the [[Peripheral nervous system|periphery neurons]]
:# [[Axoplasmic transport|Retrograde axonal transport]] to the CNS [[interneuron|inhibitory interneurons]]
:# [[Transcytosis]] from the axon into the inhibitory interneurons
;Action
:# Temperature- and pH-mediated translocation of the light chain into the cytosol
:# [[Redox|Reduction]] of the disulfide bridge to [[thiol]]s, severing the link between the light and heavy chain
:# Cleavage of synaptobrevin at -Gln<sup>76</sup>-Phe- bond

The first three steps outline the travel of tetanus toxin from the peripheral nervous system to where it is taken up to the CNS and has its final effect. The last three steps document the changes necessary for the final mechanism of the neurotoxin.

Transport to the CNS inhibitory interneurons begins with the B-chain mediating the neurospecific binding of TeNT to the nerve terminal membrane. It binds to GT1b polysialo[[gangliosides]], similarly to the ''C. botulinum'' neurotoxin. It also binds to another poorly characterized [[Glycophosphatidylinositol|GPI-anchored protein]] receptor more specific to TeNT.<ref name=GPI>{{cite journal | vauthors = Munro P, Kojima H, Dupont JL, Bossu JL, Poulain B, Boquet P | title = High sensitivity of mouse neuronal cells to tetanus toxin requires a GPI-anchored protein | journal = Biochemical and Biophysical Research Communications | volume = 289 | issue = 2 | pages = 623–629 | date = November 2001 | pmid = 11716521 | doi = 10.1006/bbrc.2001.6031 }}</ref><ref name=Gt1b>
{{cite journal | vauthors = Winter A, Ulrich WP, Wetterich F, Weller U, Galla HJ | title = Gangliosides in phospholipid bilayer membranes: interaction with tetanus toxin | journal = Chemistry and Physics of Lipids | volume = 81 | issue = 1 | pages = 21–34 | date = June 1996 | pmid = 9450318 | doi = 10.1016/0009-3084(96)02529-7 }}</ref> Both the ganglioside and the GPI-anchored protein are located in [[lipid microdomain]]s and both are requisite for specific TeNT binding.<ref name="Gt1b" /> Once it is bound, the neurotoxin is then endocytosed into the nerve and begins to travel through the axon to the spinal neurons. The next step, transcytosis from the axon into the CNS inhibitory interneuron, is one of the least understood parts of TeNT action. At least two pathways are involved, one that relies on the recycling of synaptic vesicle 2 (SV2) system and one that does not.<ref name=SV2>{{cite journal | vauthors = Yeh FL, Dong M, Yao J, Tepp WH, Lin G, Johnson EA, Chapman ER | title = SV2 mediates entry of tetanus neurotoxin into central neurons | journal = PLOS Pathogens | volume = 6 | issue = 11 | pages = e1001207 | date = November 2010 | pmid = 21124874 | pmc = 2991259 | doi = 10.1371/journal.ppat.1001207 | doi-access = free }}</ref>

Once the vesicle is in the inhibitory interneuron, its translocation is mediated by pH and temperature, specifically a low or acidic pH in the vesicle and standard physiological temperatures.<ref name="Time and pH">{{cite journal | vauthors = Pirazzini M, Rossetto O, Bertasio C, Bordin F, Shone CC, Binz T, Montecucco C | title = Time course and temperature dependence of the membrane translocation of tetanus and botulinum neurotoxins C and D in neurons | journal = Biochemical and Biophysical Research Communications | volume = 430 | issue = 1 | pages = 38–42 | date = January 2013 | pmid = 23200837 | doi = 10.1016/j.bbrc.2012.11.048 }}</ref><ref name="Translocation pH">{{cite journal | vauthors = Burns JR, Baldwin MR | title = Tetanus neurotoxin utilizes two sequential membrane interactions for channel formation | journal = The Journal of Biological Chemistry | volume = 289 | issue = 32 | pages = 22450–22458 | date = August 2014 | pmid = 24973217 | pmc = 4139251 | doi = 10.1074/jbc.m114.559302 | doi-access = free }}</ref> Once the toxin has been translocated into the cytosol, chemical reduction of the disulfide bond to separate thiols occurs, mainly by the enzyme [[thioredoxin reductase|NADPH-thioredoxin reductase-thioredoxin]]. The light chain is then free to cleave the Gln76-Phe77 bond of synaptobrevin.<ref name=Thioredoxin>{{cite journal | vauthors = Pirazzini M, Bordin F, Rossetto O, Shone CC, Binz T, Montecucco C | title = The thioredoxin reductase-thioredoxin system is involved in the entry of tetanus and botulinum neurotoxins in the cytosol of nerve terminals | journal = FEBS Letters | volume = 587 | issue = 2 | pages = 150–155 | date = January 2013 | pmid = 23178719 | doi = 10.1016/j.febslet.2012.11.007 | doi-access = free | bibcode = 2013FEBSL.587..150P }}</ref> Cleavage of synaptobrevin affects the stability of the SNARE core by restricting it from entering the low-energy conformation, which is the target for NSF binding.<ref name="NSF target">{{cite journal | vauthors = Pellegrini LL, O'Connor V, Lottspeich F, Betz H | title = Clostridial neurotoxins compromise the stability of a low energy SNARE complex mediating NSF activation of synaptic vesicle fusion | journal = The EMBO Journal | volume = 14 | issue = 19 | pages = 4705–4713 | date = October 1995 | pmid = 7588600 | pmc = 394567 | doi = 10.1002/j.1460-2075.1995.tb00152.x }}</ref> Synaptobrevin is an integral [[SNARE (protein)|V-SNARE]] necessary for vesicle fusion to membranes. The final target of TeNT is the cleavage of [[synaptobrevin]] and, even in low doses, has the effect of interfering with [[exocytosis]] of [[neurotransmitter]]s from inhibitory [[interneuron]]s. The blockage of the neurotransmitters [[gamma-aminobutyric acid|γ-aminobutyric acid]] (GABA) and [[glycine]] is the direct cause of the physiological effects that TeNT induces. GABA inhibits motor neurons, so by blocking GABA, tetanus toxin causes violent spastic paralysis.<ref>{{cite book | vauthors = Kumar V, Abbas AK, Fausto N, Aster JC | title = Robbins and Cotran Pathologic Basis of Disease | edition = Professional: Expert Consult - Online Kindle | publisher = Elsevier Health }} </ref>  The action of the A-chain also stops the affected neurons from releasing excitatory transmitters,<ref>{{cite journal | vauthors = Kanda K, Takano K | title = Effect of tetanus toxin on the excitatory and the inhibitory post-synaptic potentials in the cat motoneurone | journal = The Journal of Physiology | volume = 335 | pages = 319–333 | date = February 1983 | pmid = 6308220 | pmc = 1197355 | doi = 10.1113/jphysiol.1983.sp014536 }}</ref> by degrading the protein [[synaptobrevin 2]].<ref>{{cite journal | vauthors = Schiavo G, Benfenati F, Poulain B, Rossetto O, Polverino de Laureto P, DasGupta BR, Montecucco C | title = Tetanus and botulinum-B neurotoxins block neurotransmitter release by proteolytic cleavage of synaptobrevin | journal = Nature | volume = 359 | issue = 6398 | pages = 832–835 | date = October 1992 | pmid = 1331807 | doi = 10.1038/359832a0 | s2cid = 4241066 | bibcode = 1992Natur.359..832S }}</ref> The combined consequence is dangerous overactivity in the [[muscle]]s from the smallest sensory stimuli, as the damping of [[motor reflex]]es is inhibited, leading to generalized contractions of the agonist and antagonist musculature, termed a "tetanic spasm".

==Clinical significance==
The clinical manifestations of tetanus are caused when tetanus toxin blocks inhibitory impulses, by interfering with the release of [[neurotransmitter]]s, including [[glycine]] and [[gamma-aminobutyric acid]]. These inhibitory neurotransmitters inhibit the [[alpha motor neuron]]s. With diminished inhibition, the resting firing rate of the alpha motor neuron increases, producing rigidity, unopposed muscle contraction and spasm. Characteristic features are [[risus sardonicus]] (a rigid smile), [[trismus]] (commonly known as "lock-jaw"), and [[opisthotonus]] (rigid, arched back). [[Seizures]] may occur, and the [[autonomic nervous system]] may also be affected. Tetanospasmin appears to prevent the release of neurotransmitters by selectively cleaving a component of synaptic vesicles called [[synaptobrevin]] II.<ref name=Todar>{{cite book| vauthors = Todar K |date=2005 |chapter-url=http://textbookofbacteriology.net/clostridia_3.html |access-date=24 June 2018 |chapter=Pathogenic Clostridia, including Botulism and Tetanus |title=Todar's Online Textbook of Bacteriology}}</ref> Loss of inhibition also affects preganglionic sympathetic neurons in the lateral [[gray matter]] of the spinal cord and produces sympathetic hyperactivity and high circulating [[catecholamine]] levels. [[Hypertension]] and [[tachycardia]] alternating with [[hypotension]] and [[bradycardia]] may develop.<ref name="isbn0-07-146633-9">{{cite book | vauthors = Loscalzo J, Fauci AS, Braunwald E, Kasper DL, Hauser SL, Longo DL |title=Harrison's principles of internal medicine |publisher=McGraw-Hill Medical |year=2008 |isbn=978-0-07-146633-2 }}</ref><ref name="urlTetanus in Emergency Medicine">{{cite web | vauthors = Yabes Jr JM, McLaughlin R | veditors = Brusch JL |url=http://emedicine.medscape.com/article/786414-overview |title=Tetanus in Emergency Medicine |work=Emedicine |access-date=2011-09-01}}</ref>

Tetanic spasms can occur in a distinctive form called ''[[opisthotonos]]'' and be sufficiently severe to fracture long bones. The shorter nerves are the first to be inhibited, which leads to the characteristic early symptoms in the face and jaw, ''[[risus sardonicus]]'' and [[Tetanus|lockjaw]].

==Immunity and vaccination==
Due to its extreme potency, even a lethal dose of tetanospasmin may be insufficient to provoke an immune response. Naturally acquired tetanus infections thus do not usually provide immunity to subsequent infections. Immunization (which is impermanent and must be repeated periodically) instead uses the less deadly [[toxoid]] derived from the toxin, as in the [[tetanus vaccine]] and some [[combination vaccine]]s (such as [[DTP vaccine|DTP]]).

== References ==
{{Reflist}}

== Further reading ==
{{refbegin}}
* {{cite journal | vauthors = Pellizzari R, Rossetto O, Schiavo G, Montecucco C | title = Tetanus and botulinum neurotoxins: mechanism of action and therapeutic uses | journal = Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences | volume = 354 | issue = 1381 | pages = 259–268 | date = February 1999 | pmid = 10212474 | pmc = 1692495 | doi = 10.1098/rstb.1999.0377 }}
* {{cite journal | vauthors = Rossetto O, Scorzeto M, Megighian A, Montecucco C | title = Tetanus neurotoxin | journal = Toxicon | volume = 66 | issue = | pages = 59–63 | date = May 2013 | pmid = 23419592 | doi = 10.1016/j.toxicon.2012.12.027 | bibcode = 2013Txcn...66...59R }}
* {{cite journal | vauthors = Lalli G, Bohnert S, Deinhardt K, Verastegui C, Schiavo G | title = The journey of tetanus and botulinum neurotoxins in neurons | journal = Trends in Microbiology | volume = 11 | issue = 9 | pages = 431–7 | date = September 2003 | pmid = 13678859 | doi = 10.1016/s0966-842x(03)00210-5 }}
* {{cite journal | vauthors = Montecucco C | title = How do tetanus and botulinum toxins bind to neuronal membranes? | journal = Trends in Biochemical Sciences | date = August 1986 | volume = 11 | issue = 8 | pages = 314–317 | doi = 10.1016/0968-0004(86)90282-3 }}
{{refend}}

== External links ==
* {{MeshName|tetanospasmin}}
*{{MeshName|Tentoxilysin}}
*{{Commons category-inline|Tetanus neurotoxin}}

{{Toxins}}
{{Metalloendopeptidases}}
{{Enzymes}}

[[Category:EC 3.4.24]]
[[Category:Membrane channels]]
[[Category:Neurotoxins]]
[[Category:Bacterial toxins]]
[[Category:Tetanus]]
[[Category:Proteins]]<!-- I'm not completly sure about that.-->