Poliovirus, the causative agent of poliomyelitis, is a human enterovirus and member of the family
of Picornaviridae. 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 Stony Brook University. 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
It infects human cells by binding to an immunoglobulin-like receptor, CD155,) 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 highly structured. This region of the viral genome is called
internal ribosome entry site and it directs translation of the viral RNA. Genetic mutations
in this region prevent viral protein production. Poliovirus mRNA is translated as one long
polypeptide. This polypeptide is then auto-cleaved by internal proteases into approximately 10
individual viral proteins, including:
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, 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, which is further cleaved into VP2 and VP4, VP1 and VP3 proteins of the viral capsid.
The assembly of new virus particles, 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.
Drake demonstrated that poliovirus is able to undergo multiplicity reactivation. That
is, when polioviruses were irradiated with UV light and allowed to undergo multiple infections
of host cells, viable progeny could be formed even at UV doses that inactivated the virus
in single infections. Origin and serotypes
Poliovirus is structurally similar to other human enteroviruses, which also use immunoglobulin-like
molecules to recognize and enter host cells. Phylogenetic analysis of the RNA and protein
sequences of poliovirus 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, 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 substitutionsyear and non synonymous substitution rate of 3.0
x 10−4 substitutionsyear. 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. As of November 2014, wild
PV3 has not been seen since its 2012 detection in parts of Nigeria and Pakistan.
Specific strains of each serotype are used to prepare vaccines against polio. Inactive
polio vaccine is prepared by formalin inactivation of three wild, virulent reference strains,
Mahoney or Brunenders, MEF-1/Lansing, and Saukett/Leon. Oral polio vaccine contains
live attenuated strains of the three serotypes of poliovirus. Passaging the virus strains
in monkey kidney epithelial cells introduces mutations in the viral IRES, and hinders 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
Enterovirus C. Pathogenesis
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 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. 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 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 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 is an important
factor that defines which types of cells support poliovirus replication. In mice expressing
CD155 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 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.
Unlike normal mice, transgenic poliovirus receptor 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
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.
References
External links
ICTV Master Species List 2009 Version 4 ICTVdb virus classification 2006
Home of Picornaviruses Goodsell, David. "Poliovirus and Rhinovirus".
August 2001 Molecule of the Month. 3D macromolecular structures of the Poliovirus
archived in the EM Data Bank(EMDB) "Human poliovirus 1". NCBI Taxonomy Browser.
12080. "Human poliovirus 3". NCBI Taxonomy Browser.
12086.