|TEM micrograph of poliovirus virions.
Scale bar, 50 nm.
|Group:||Group IV ((+)ssRNA)|
|Human enterovirus C
Poliovirus is composed of an RNA genome and a protein capsid. The genome is a single-stranded positive-sense RNA genome that is about 7500 nucleotides long. The viral particle is about 30 nanometres in diameter with icosahedral symmetry. Because of its short genome and its simple composition—only RNA and a non-enveloped icosahedral protein coat that encapsulates it—poliovirus is widely regarded as the simplest significant virus.
Poliovirus was first isolated in 1909 by Karl Landsteiner and Erwin Popper. In 1981, the poliovirus genome was published by two different teams of researchers— by Vincent Racaniello and David Baltimore at MIT and by Naomi Kitamura and Eckard Wimmer at the State University of New York, Stony Brook. Poliovirus is one of the most well-characterized viruses, and has become a useful model system for understanding the biology of RNA viruses.
Replication cycle 
Poliovirus infects human cells by binding to an immunoglobulin-like receptor, CD155, (also known as the poliovirus receptor (PVR)) on the cell surface. Interaction of poliovirus and CD155 facilitates an irreversible conformational change of the viral particle necessary for viral entry. The precise mechanism poliovirus uses to enter the host cell has not been firmly established. Attached to the host cell membrane, entry of the viral nucleic acid was thought to occur one of two ways: via the formation of a pore in the plasma membrane through which the RNA is then “injected” into the host cell cytoplasm, or that the virus is taken up by receptor-mediated endocytosis. Recent experimental evidence supports the latter hypothesis and suggests that poliovirus binds to CD155 and is taken up via endocytosis. Immediately after internalization of the particle, the viral RNA is released. However, any mechanism by which poliovirus enters the cell is very inefficient; as an infection is initiated only about 1% of the time.
Poliovirus is a positive stranded RNA virus. Thus the genome enclosed within the viral particle can be used as messenger RNA and immediately translated by the host cell. On entry the virus hijacks the cell's translation machinery; causing inhibition of cellular protein synthesis in favor of virus–specific protein production. Unlike the host cell's mRNAs the 5' end of poliovirus RNA is extremely long—over 700 nucleotides—and is highly structured. This region of the viral genome is called internal ribosome entry site (IRES) and it directs translation of the viral RNA. Genetic mutations in this region prevent viral protein production.
- 3Dpol, an RNA dependent RNA polymerase whose function is to copy and multiply the viral RNA genome.
- 2Apro and 3Cpro/3CDpro, proteases which cleave the viral polypeptide.
- VPg (3B), a small protein that binds viral RNA and is necessary for synthesis of viral positive and negative strand RNA.
- 2BC, 2B, 2C, 3AB, 3A, 3B proteins which comprise the protein complex needed for virus replication.
- VP0, VP1, VP2, VP3, VP4 proteins of the viral capsid.
The assembly of new virus particles, (i.e. the packaging of progeny genome into a capsid which can survive outside the host cell) is not fully understood. Fully assembled poliovirus leaves the confines of its host cell 4 to 6 hours following initiation of infection in cultured mammalian cells. The mechanism of viral release from the cell is unclear, but each dying cell can release up to 10,000 polio virions.
Origin and serotypes 
Poliovirus is structurally similar to other human enteroviruses (coxsackieviruses, echoviruses, and rhinoviruses), which also use immunoglobulin-like molecules to recognize and enter host cells. Phylogenetic analysis of the RNA and protein sequences of poliovirus (PV) suggests that PV may have evolved from a C-cluster coxsackie A virus ancestor, that arose through a mutation within the capsid. The distinct speciation of poliovirus probably occurred as a result of change in cellular receptor specificity from intercellular adhesion molecule-1 (ICAM-1), used by C-cluster coxsackie A viruses, to CD155; leading to a change in pathogenicity, and allowing the virus to infect nervous tissue.
The mutation rate in the virus is relatively high even for an RNA virus with a synonymous substitution rate of 1.0 x 10−2 substitutions/site/year and non synonymous substitution rate of 3.0 x 10−4 substitutions/site/year. Base distribution within the genome is non random with adenosine being less common than expected at the 5' end and higher at the 3' end. Codon use is non random with codons ending in adenosine being favoured and those ending in cytosine or guanine being avoided. Codon use differs between the three genotypes and appears to be driven by mutation rather than selection.
There are three serotypes of poliovirus, PV1, PV2, and PV3; each with a slightly different capsid protein. Capsid proteins define cellular receptor specificity and virus antigenicity. PV1 is the most common form encountered in nature, however all three forms are extremely infectious. Wild polioviruses can be found in two continents. As of 2012, PV1 is highly localized to regions in Pakistan and Afghanistan in Asia, and Nigeria, Niger and Chad in Africa. Wild poliovirus type 2 has probably been eradicated; it was last detected in October 1999 in Uttar Pradesh, India. Wild PV3 is found in parts of only two countries, Nigeria and Pakistan.
Specific strains of each serotype are used to prepare vaccines against polio. Inactive polio vaccine (IPV) is prepared by formalin inactivation of three wild, virulent reference strains, Mahoney or Brunenders (PV1), MEF-1/Lansing (PV2), and Saukett/Leon (PV3). Oral polio vaccine (OPV) contains live attenuated (weakened) strains of the three serotypes of poliovirus. Passaging the virus strains in monkey kidney epithelial cells introduces mutations in the viral IRES, and hinders (or attenuates) the ability of the virus to infect nervous tissue.
Polioviruses were formerly classified as a distinct species belonging to the genus Enterovirus in the family Picornaviridae. In 2008 the Poliovirus species was eliminated from the genus Enterovirus and the three serotypes were assigned to the species Human enterovirus C, in the genus Enterovirus in the family Picornaviridae. The type species of the genus Enterovirus was changed from Poliovirus to Human enterovirus C.
The primary determinant of infection for any virus is its ability to enter a cell and produce additional infectious particles. The presence of CD155 is thought to define the animals and tissues that can be infected by poliovirus. CD155 is found (outside of laboratories) only on the cells of humans, higher primates, and Old World monkeys. Poliovirus is however strictly a human pathogen, and does not naturally infect any other species (although chimpanzees and Old World monkeys can be experimentally infected).
The CD155 gene appears to have been subject to positive selection. The protein has several domains of which domain D1 contains the polio virus binding site. Within this domain 37 amino acids are responsible for binding the virus.
Poliovirus is an enterovirus. Infection occurs via the fecal-oral route, meaning that one ingests the virus and viral replication occurs in the alimentary tract. Virus is shed in the feces of infected individuals. In 95% of cases only a primary, transient presence of viremia (virus in the bloodstream) occurs, and the poliovirus infection is asymptomatic. In about 5% of cases, the virus spreads and replicates in other sites such as brown fat, reticuloendothelial tissue, and muscle. The sustained viral replication causes secondary viremia and leads to the development of minor symptoms such as fever, headache and sore throat. Paralytic poliomyelitis occurs in less than 1% of poliovirus infections. Paralytic disease occurs when the virus enters the central nervous system (CNS) and replicates in motor neurons within the spinal cord, brain stem, or motor cortex, resulting in the selective destruction of motor neurons leading to temporary or permanent paralysis. In rare cases, paralytic poliomyelitis leads to respiratory arrest and death. In cases of paralytic disease, muscle pain and spasms are frequently observed prior to onset of weakness and paralysis. Paralysis typically persists anywhere from days to weeks prior to recovery.
In many respects the neurological phase of infection is thought to be an accidental diversion of the normal gastrointestinal infection. The mechanisms by which poliovirus enters the CNS are poorly understood. Three non-mutually exclusive hypotheses have been suggested to explain its entry. All theories require primary viremia. The first hypothesis predicts that virions pass directly from the blood into the central nervous system by crossing the blood brain barrier independent of CD155. A second hypothesis suggests that the virions are transported from peripheral tissues that have been bathed in the viremic blood, for example muscle tissue, to the spinal cord through nerve pathways via retrograde axonal transport. A third hypothesis is that the virus is imported into the CNS via infected monocytes or macrophages.
Poliomyelitis is a disease of the central nervous system. However, CD155 is believed to be present on the surface of most or all human cells. Therefore receptor expression does not explain why poliovirus preferentially infects certain tissues. This suggests that tissue tropism is determined after cellular infection. Recent work has suggested that the type I interferon response (specifically that of interferon alpha and beta) is an important factor that defines which types of cells support poliovirus replication. In mice expressing CD155 (through genetic engineering) but lacking the type I interferon receptor, poliovirus not only replicates in an expanded repertoire of tissue types, but these mice are also able to be infected orally with the virus.
Immune system avoidance 
Poliovirus uses two key mechanisms to evade the immune system. First, it is capable of surviving the highly acidic conditions of the gastrointestinal tract, allowing the virus to infect the host and spread throughout the body via the lymphatic system. Second, because it can replicate very quickly, the virus overwhelms the host organs before an immune response can be mounted.
Individuals who are exposed to poliovirus, either through infection or by immunization with polio vaccine, develop immunity. In immune individuals, antibodies against poliovirus are present in the tonsils and gastrointestinal tract (specifically IgA antibodies) and are able to block poliovirus replication; IgG and IgM antibodies against poliovirus can prevent the spread of the virus to motor neurons of the central nervous system. Infection with one serotype of poliovirus does not provide immunity against the other serotypes, however second attacks within the same individual are extremely rare.
PVR transgenic mouse 
Although humans are the only known natural hosts of poliovirus, monkeys can be experimentally infected and they have long been used to study poliovirus. In 1990-91, a small animal model of poliomyelitis was developed by two laboratories. Mice were engineered to express a human receptor to poliovirus (hPVR).
Unlike normal mice, transgenic poliovirus receptor (TgPVR) mice are susceptible to poliovirus injected intravenously or intramuscularly, and when injected directly into the spinal cord or the brain. Upon infection, TgPVR mice show signs of paralysis that resemble those of poliomyelitis in humans and monkeys, and the central nervous systems of paralyzed mice are histocytochemically similar to those of humans and monkeys. This mouse model of human poliovirus infection has proven to be an invaluable tool in understanding poliovirus biology and pathogenicity.
Three distinct types of TgPVR mice have been well studied:
- In TgPVR1 mice the transgene encoding the human PVR was incorporated into mouse chromosome 4. These mice express the highest levels of the transgene and the highest sensitivity to poliovirus. TgPVR1 mice are susceptible to poliovirus through the intraspinal, intracerebral, intramuscular, and intravenous pathways, but not through the oral route.
- TgPVR21 mice have incorporated the human PVR at chromosome 13. These mice are less susceptible to poliovirus infection through the intracerebral route, possibly because they express decreased levels of hPVR. TgPVR21 mice have been shown to be susceptible to poliovirus infection through intranasal inoculation, and may be useful as a mucosal infection model.
- In TgPVR5 mice the human transgene is located on chromosome 12. These mice exhibit the lowest levels of hPVR expression and are the least susceptible to poliovirus infection.
Recently a fourth TgPVR mouse model was developed. These "cPVR" mice carry hPVR cDNA, driven by a β-actin promoter, and have proven susceptible to poliovirus through intracerebral, intramuscular, and intranasal routes. In addition, these mice are capable of developing the bulbar form of polio after intranasal inoculation.
The development of the TgPVR mouse has had a profound effect on oral poliovirus vaccine (OPV) production. Previously, monitoring the safety of OPV had to be performed using monkeys, because only primates are susceptible to the virus. In 1999 the World Health Organization approved the use of the TgPVR mouse as an alternative method of assessing the effectiveness of the vaccine against poliovirus type-3. In 2000 the mouse model was approved for tests of vaccines against type-1 and type-2 poliovirus.
Cloning and synthesis 
In 1981 Racaniello and Baltimore used recombinant DNA technology to generate the first infectious clone of an animal RNA virus, poliovirus. DNA encoding the RNA genome of poliovirus was introduced into cultured mammalian cells and infectious poliovirus was produced. Creation of the infectious clone propelled understanding of poliovirus biology, and has become a standard technology used to study many other viruses.
In 2002 Eckard Wimmer's group at SUNY Stony Brook succeeded in synthesizing poliovirus from its chemical code, producing the world's first synthetic virus. Scientists first converted poliovirus's published RNA sequence, 7741 bases long, into a DNA sequence, as DNA was easier to synthesize. Short fragments of this DNA sequence were obtained by mail-order, and assembled. The complete viral genome was then assembled by a gene synthesis company. This whole painstaking process took two years. Nineteen markers were incorporated into the synthesized DNA, so that it could be distinguished from natural poliovirus. Enzymes were used to convert the DNA back into RNA, its natural state. Other enzymes were then used to translate the RNA into a polypeptide, producing functional viral particle. The newly minted synthetic virus was injected into PVR transgenic mice, to determine if the synthetic version was able to cause disease. The synthetic virus was able to replicate, infect, and cause paralysis or death in mice. However, the synthetic version was between 1,000 and 10,000 times less lethal than the original virus.
- INTERNATIONAL COMMITTEE ON TAXONOMY OF VIRUSES (August 2011). ICTV 2009 MASTER SPECIES LIST VERSION 10.
- Ryan KJ, Ray CG (editors) (2004). Sherris Medical Microbiology (4th ed.). McGraw Hill. ISBN 0-8385-8529-9.
- Hogle J (2002). "Poliovirus cell entry: common structural themes in viral cell entry pathways". Annu Rev Microbiol 56: 677–702. doi:10.1146/annurev.micro.56.012302.160757. PMC 1500891. PMID 12142481.
- Goodsell DS (1998). The machinery of life. New York: Copernicus. ISBN 0-387-98273-6.
- Paul JR (1971). A History of Poliomyelitis. (Yale studies in the history of science and medicine). New Haven, Conn: Yale University Press. ISBN 0-300-01324-8.
- Racaniello and Baltimore; Baltimore, D (1981). "Molecular cloning of poliovirus cDNA and determination of the complete nucleotide sequence of the viral genome". Proceedings of the National Academy Science USA 78 (8): 4887–91. doi:10.1073/pnas.78.8.4887. PMC 320284. PMID 6272282.
- Kitamura N, Semler B, Rothberg P, et al. (1981). "Primary structure, gene organization and polypeptide expression of poliovirus RNA". Nature 291 (5816): 547–53. doi:10.1038/291547a0. PMID 6264310.
- De Jesus NH (2007). "Epidemics to eradication: the modern history of poliomyelitis". Virol. J. 4 (1): 70. doi:10.1186/1743-422X-4-70. PMC 1947962. PMID 17623069.
- Mendelsohn Cl, Wimmer E, Racaniello VR (1989). "Cellular receptor for poliovirus: molecular cloning, nucleotide sequence, and expression of a new member of the immunoglobin superfamily". Cell 56 (5): 855–865. doi:10.1016/0092-8674(89)90690-9. PMID 2538245.
- He Y, Mueller S, Chipman P, et al. (2003). "Complexes of poliovirus serotypes with their common cellular receptor, CD155". J Virol 77 (8): 4827–35. doi:10.1128/JVI.77.8.4827-4835.2003. PMC 152153. PMID 12663789.
- Dunnebacke TH, Levinthal JD, Williams RC (1 October 1969). "Entry and release of poliovirus as observed by electron microscopy of cultured cells". J. Virol. 4 (4): 505–13. PMC 375900. PMID 4309884.
- Kaplan G, Freistadt MS, Racaniello VR (1 October 1990). "Neutralization of poliovirus by cell receptors expressed in insect cells". J. Virol. 64 (10): 4697–702. PMC 247955. PMID 2168959.
- Gomez Yafal A, Kaplan G, Racaniello VR, Hogle, JM (1993). "Characterization of poliovirus conformational alteration mediated by soluble cell receptors". Virology 197 (1): 501–505. doi:10.1006/viro.1993.1621. PMID 8212594.
- ed. by Samuel Baron (1996). Picornaviruses: The Enteroviruses: Polioviruses in: Baron's Medical Microbiology (Baron S et al., eds.) (4th ed.). Univ of Texas Medical Branch. ISBN 0-9631172-1-1.
- Mueller S, Wimmer E, Cello J (2005). "Poliovirus and poliomyelitis: a tale of guts, brains, and an accidental event". Virus Res 111 (2): 175–93. doi:10.1016/j.virusres.2005.04.008. PMID 15885840.
- Brandenburg B, Lee LY, Lakadamyali M, Rust MJ, Zhuang X, Hogle JM, (2007). "Imaging poliovirus entry in live cells". PLOS Biology 5 (7): e183. doi:10.1371/journal.pbio.0050183. PMC 1914398. PMID 17622193.
- Charles Chan and Roberto Neisa. "Poliomyelitis". Brown University.
- Chen CY, Sarnow P (1995). "Initiation of protein synthesis by the eukaryotic translational apparatus on circular RNAs". Science 268 (5209): 415–7. doi:10.1126/science.7536344. PMID 7536344.
- Pelletier J, Sonenberg N (1988). "Internal initiation of translation of eukaryotic mRNA directed by a sequence derived from poliovirus RNA". Nature 334 (6180): 320–325. doi:10.1038/334320a0. PMID 2839775.
- Jang SK, Kräusslich HG, Nicklin MJ, Duke GM, Palmenberg AC, Wimmer E (1 August 1988). "A segment of the 5' nontranslated region of encephalomyocarditis virus RNA directs internal entry of ribosomes during in vitro translation". J. Virol. 62 (8): 2636–43. PMC 253694. PMID 2839690.
- Kew O, Sutter R, de Gourville E, Dowdle W, Pallansch M (2005). "Vaccine-derived polioviruses and the endgame strategy for global polio eradication". Annu Rev Microbiol 59: 587–635. doi:10.1146/annurev.micro.58.030603.123625. PMID 16153180.
- Jiang P, Faase JA, Toyoda H, et al. (2007). "Evidence for emergence of diverse polioviruses from C-cluster coxsackie A viruses and implications for global poliovirus eradication". Proc. Natl. Acad. Sci. U.S.A. 104 (22): 9457–62. doi:10.1073/pnas.0700451104. PMC 1874223. PMID 17517601.
- Jorba J, Campagnoli R, De L, Kew O. (2008). "Calibration of multiple poliovirus molecular clocks covering an extended evolutionary range". J Virol. 82 (9): 4429–4440. doi:10.1128/JVI.02354-07. PMC 2293050. PMID 18287242.
- Rothberg PG, Wimmer E (1981). "Mononucleotide and dinucleotide frequencies, and codon usage in poliovirion RNA". Nucleic Acids Res. 9 (23): 6221–6229. doi:10.1093/nar/9.23.6221. PMC 327599. PMID 6275352.
- Zhang J, Wang M, Liu WQ, Zhou JH, Chen HT, Ma LN, Ding YZ, Gu YX, Liu YS. et al. (2011). "Analysis of codon usage and nucleotide composition bias in polioviruses". Virol J. 8 (1): 146.
- "Transmission of wild poliovirus type 2--apparent global interruption". Wkly. Epidemiol. Rec. 76 (13): 95–7. March 2001. PMID 11315462.
- Carstens, E. B.; Ball, L. A. (July, 2009). "Ratification vote on taxonomic proposals to the International Committee on Taxonomy of Viruses (2008)". Archives of Virology (Springer Wien) 154 (7): 1181–1188. doi:10.1007/s00705-009-0400-2. ISSN 1432-8798. PMID 19495937.
- Mueller S, Wimmer E (2003). "Recruitment of nectin-3 to cell-cell junctions through trans-heterophilic interaction with CD155, a vitronectin and poliovirus receptor that localizes to alpha(v)beta3 integrin-containing membrane microdomains". J Biol Chem 278 (33): 31251–60. doi:10.1074/jbc.M304166200. PMID 12759359.
- Suzuki Y (2006). "Ancient positive selection on CD155 as a possible cause for susceptibility to poliovirus infection in simians". Gene 373: 16–22. doi:10.1016/j.gene.2005.12.016. PMID 16500041.
- Bodian D and Horstmann DH (1969). Polioviruse. Philadelphia, Penn: Lippincott. pp. 430–73.
- Sabin A (1956). "Pathogenesis of poliomyelitis; reappraisal in the light of new data". Science 123 (3209): 1151–7. doi:10.1126/science.123.3209.1151. PMID 13337331.
- Yang W, Terasaki T, Shiroki K, et al. (1997). "Efficient delivery of circulating poliovirus to the central nervous system independently of poliovirus receptor". Virology 229 (2): 421–8. doi:10.1006/viro.1997.8450. PMID 9126254.
- Ohka S. Yang WX, Terada E, Iwasaki K, Nomot A (1998). "Retrograde transport of intact poliovirus through the axon via the first transport system". Virology 250 (1): 67–75. doi:10.1006/viro.1998.9360. PMID 9770421.
- Ren R, Racaniello V (1992). "Poliovirus spreads from muscle to the central nervous system by neural pathways". J Infect Dis 166 (4): 747–52. doi:10.1093/infdis/166.4.747. PMID 1326581.
- Lancaster KZ, Pfeiffer JK (2010). "Limited trafficking of a neurotropic virus through inefficient retrograde axonal transport and the type I interferon response". In Gale, Michael. PLoS Pathog 6 (3): e1000791. doi:10.1371/journal.ppat.1000791. PMC 2832671. PMID 20221252.
- Ida-Hosonuma M, Iwasaki T, Yoshikawa T, et al. (April 2005). "The alpha/beta interferon response controls tissue tropism and pathogenicity of poliovirus". J. Virol. 79 (7): 4460–9. doi:10.1128/JVI.79.7.4460-4469.2005. PMC 1061561. PMID 15767446.
- Ohka S, Igarashi H, Sakai M, Koike S, Nochi T, Kiyono A, Nomoto A (2007). "Esstablishment of a poliovirus oral infection system in human poliovirus receptor-expressing transgenic mice that are deficient in alpha/beta interferon receptor". J. Virol. 81 (15): 7902–12. doi:10.1128/JVI.02675-06. PMC 1951287. PMID 17507470.
- Ren RB, Costantini F, Gorgacz EJ, Lee JJ, Racaniello VR (1990). "Transgenic mice expressing a human poliovirus receptor: a new model for poliomyelitis". Cell 63 (2): 353–62. doi:10.1016/0092-8674(90)90168-E. PMID 2170026.
- Koike S, Taya C, Kurata T, et al. (1991). "Transgenic mice susceptible to poliovirus". Proc. Natl. Acad. Sci. U.S.A. 88 (3): 951–5. doi:10.1073/pnas.88.3.951. PMC 50932. PMID 1846972.
- Horie H, Koike S, Kurata T, et al. (1 February 1994). "Transgenic mice carrying the human poliovirus receptor: new animal models for study of poliovirus neurovirulence". J. Virol. 68 (2): 681–8. PMC 236503. PMID 8289371.
- Ohka S, Nomoto A (2001). "Recent insights into poliovirus pathogenesis". Trends Microbiol. 9 (10): 501–6. doi:10.1016/S0966-842X(01)02200-4. PMID 11597452.
- Koike S, Taya C, Aoki J, et al. (1994). "Characterization of three different transgenic mouse lines that carry human poliovirus receptor gene--influence of the transgene expression on pathogenesis". Arch. Virol. 139 (3–4): 351–63. doi:10.1007/BF01310797. PMID 7832641.
- Nagata N, Iwasaki T, Ami Y, et al. (2004). "A poliomyelitis model through mucosal infection in transgenic mice bearing human poliovirus receptor, TgPVR21". Virology 321 (1): 87–100. doi:10.1016/j.virol.2003.12.008. PMID 15033568.
- Dragunsky E, Nomura T, Karpinski K, et al. (2003). "Transgenic mice as an alternative to monkeys for neurovirulence testing of live oral poliovirus vaccine: validation by a WHO collaborative study". Bull. World Health Organ. 81 (4): 251–60. doi:10.1590/S0042-96862003000400006. PMC 2572431. PMID 12764491.
- Racaniello V, Baltimore D (1981). "Cloned poliovirus complemenatry DNA is infectious in mammalian cells". Science 214 (453): 916–9. doi:10.1126/science.6272391. PMID 6272391.
- Cello J, Paul AV, Wimmer E (2002). "Chemical synthesis of poliovirus cDNA: generation of infectious virus in the absence of natural template". Science 297 (5583): 1016–8. doi:10.1126/science.1072266. PMID 12114528.
- Couzin J (2002). "Virology. Active poliovirus baked from scratch". Science 297 (5579): 174–5. doi:10.1126/science.297.5579.174b. PMID 12114601.
- ICTV MASTER SPECIES LIST 2009 VERSION 4
- ICTVdb virus classification 2006
- Home of Picornaviruses (latest updates of species, serotypes, & proposed changes)
- Poliovirus and Rhinovirus August 2001 Molecule of the Month by David Goodsell
- 3D macromolecular structures of the Poliovirus archived in the EM Data Bank(EMDB)