Polio Virus

                          A Brief History of Polio

From Ancient Egypt to the 20th Century

   Polio has probably caused paralysis and death for most of human
   history. The oldest clearly identifiable reference to paralytic
   poliomyelitis is an Egyptian stele (stone engraving) over 3,000 years
   old. Cases of poliomyelitis tended to be rare in ancient times,
   though, as sanitation was generally poor. With improvements in waste
   disposal and the widespread use of indoor plumbing in the 20th
   century, epidemics of polio began to occur with regularity in the
   developed world, primarily in cities during the summer. Because sewage
   was dumped away from the drinking water supply (a development which
   helps combat a number of other diseases, including cholera), babies
   were much less likely to be infected with polio and gain protective
   immunity. As the children got older and began playing with others,
   swimming in public pools, and going to school, they were more likely
   to be exposed to the virus, which was then more likely to cause
   paralytic poliomyelitis.

Vaccination and Eradication

   By the time of the Great Depression, paralytic poliomyelitis was
   perhaps the most feared disease known. Polio struck fast, there was no
   cure, and it crippled its victims for life. Hobbling on crutches,
   rolling in wheelchairs, or lying immobile in giant iron lungs, the
   legions of sufferers accumulated from year to year. Even the exact
   mechanism of polio's transmission was a hotly debated subject for many
   years, so many areas were placed under strict quarantine when cases of
   the disease began to manifest themselves. Only the fear surrounding
   AIDS can rival the feelings people had about polio in the first half
   of this century.

   In the early 1960s, the work bore fruit, first with the Salk vaccine,
   and soon after with the Sabin virus strains. Salk used chemical and heat '
   treatment to kill poliovirus, then injected this inactivated virus into
   patients. The proteins of the destroyed virus "taught" the patients'
   immune systems to recognize polio, and they were then protected from
   subsequent infection. Sabin's approach was to grow the virus in the
   laboratory under a variety of conditions, allowing it to accumulate
   mutations.
   Ultimately, this resulted in an attenuated virus which could be given
   to a patient orally. The weaker virus replicates normally in the
   intestine, but cannot grow well enough to invade the central nervous
   system. Once again, the immune system "learns" to recognize polio, and
   this confers protection.

   Once the Sabin and Salk vaccines were proven effective, the disease
   was rapidly eradicated throughout most of the industrialized world.
   The economic effect has been enormous; it has been calculated that the
   polio vaccine pays for the costs of its development approximately
   every three weeks. The benefit to the United States alone for this
   single breakthrough runs into the trillions of dollars. The social
   impact has been incalculable. The crutches, wheelchairs, and iron
   lungs of polio victims have at last been banished from children's and
   parents' nightmares, at least in the developed world. Recently, the
   World Health Organization embarked on a campaign for the worldwide
   eradication of polio.
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                             Polio Pathogenesis

   Poliovirus enters a susceptible host in contaminated drinking water or
   through contact with contaminated surfaces, such as unwashed hands.
   After passing through the stomach, the virus reaches the intestine,
   where it establishes itself in the cells of the intestinal lining (the
   "gut mucosa"). There, it infects cells and replicates. In most cases,
   this results in a transient, self-limiting diarrhea, or it may be
   completely asymptomatic. Unfortunately, the virus is not always so
   benign.

   In approximately 1% of infections, the virus spreads from the
   intestine into the bloodstream and nervous system, eventually reaching
   motor neurons and causing paralysis or, in extreme cases, death. The
   exact mechanism of the virus's spread through the body is still not
   known, though many models have been proposed. Certain clusters of
   cells in the intestine, known as Peyer's Patches, appear to support
   the initial infection. Since the Peyer's Patches are closely
   associated with the body's immune system, it has been suggested that
   the virus migrates from there into the bloodstream. Once in the blood,
   it gains access to the nervous system, where the destruction of motor
   neurons (those which control muscle movement) results in paralysis. In
   an interesting twist on this idea, some researchers have found that
   the virus may be transported directly through nerves rather than
   blood. When a genetically engineered mouse which is susceptible to
   polio is injected with the virus in one limb, the virus migrates to
   the spinal cord and replicates, and the first limb paralyzed is the
   one which was injected. If the nerves connecting the injected limb to
   the body are severed before injection, the virus fails to spread to
   the spinal cord, even though blood still circulates between the limb
   and the body. This suggests that the virus actually travels along the
   nerves to reach the spinal cord. It remains to be determined whether
   or not this mechanism is at work in a normal infection.

   Widespread vaccination against polio has effectively eliminated the
   natural occurrence of the disease in the Western hemisphere. There are
   two types of vaccines: a killed virus, which is injected (the Salk
   vaccine, or IPV), and a live attenuated strain of the virus, which is
   administered orally (the Sabin vaccine, or OPV).

   This convenience of the Sabin Vaccine and effectiveness comes at a price,
   though. The live virus mutates readily:

   In every case studied, vaccination with OPV results in the
   release of a wide variety of live mutant viruses in the feces. Many of
   the mutations enable the virus to regain virulence. Interestingly,
   paralysis from the vaccine occurs only in about one case out of one
   million, despite the fact that the virus seems to mutate the same way
   in all patients. While the reason for this is not known, many
   scientists have suggested that some other pathogen, present in some
   patients at the time of vaccination, suppresses the immune system
   enough to allow the mutant viruses to enter the nervous system. If we
   could find the pathogen responsible for this, we could screen patients
   for it before vaccinating with OPV. A more straightforward solution,
   recently adopted by the Centers for Disease Control and Prevention ,
   is to use the Inactivated Polio Vaccine (IPV), which consists of
   killed viruses and is therefore unable to cause paralytic polio.
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                             Polio Epidemiology

Route of Transmission

   Poliovirus is spread by the so-called "fecal-oral" route, which,
   despite its unsavory name, is a common route of infection. The virus
   can be isolated from human feces and sewage. In areas where raw sewage
   enters an aquifer without treatment, polio can be found in rivers,
   lakes, and streams. When a susceptible person drinks water from one of
   these sources (possibly from the kitchen tap when local water supplies
   are not treated properly), the virus enters his digestive tract. After
   surviving the harsh, acidic conditions of the stomach, the virus
   enters the intestine, where it infects and replicates in the cells of
   the gut mucosa. The progeny virions are then carried through the
   intestine and released into the sewage system to start the cycle over
   again. In addition to untreated drinking water, the virus appears to
   spread through contact with infected patients, especially among
   children.

Paralytic Poliomyelitis

   Polio infection is frequently accompanied only by minor symptoms. In
   some cases, though, the virus enters the central nervous system after
   replicating in the gut and bloodstream, and this can result in
   paralysis of one or more limbs, or death. Even though many infections
   are asymptomatic, the efficiency with which polio is transmitted under
   the right conditions can lead to epidemics of infantile paralysis,
   such as those seen in American cities in the first half of the 20th
   Century. If a large enough portion of the population of a city is
   exposed to polio, even the small percentage of infections which lead
   to paralysis will produce large numbers of casualties, making it a
   major concern for public health authorities. Epidemics of paralytic
   poliomyelitis still occur in some lesser-developed countries today,
   but the World Health Organization (WHO) has embarked on a campaign to
   eradicate polio by the year 2000.

Effects of Development

   Despite its long history, polio has had its most noticeable effect on
   humanity within the past century. Epidemics of poliomyelitis have been
   characterized as a "disease of development," meaning that, ironically,
   major outbreaks seem to accompany an improvement in sanitation and
   living conditions. In highly unsanitary circumstances, virtually all
   children are exposed to the virus during infancy, when infection with
   polio is most likely to be asymptomatic, and these babies then acquire
   lifelong immunity to the disease. When older children or adults are
   infected, they are more likely to be paralyzed or killed by the virus.
   As a society improves its sanitation (a transition which helps
   eliminate a number of other diseases), individuals are likely to be
   exposed to polio later in life, if at all, so the paralytic disease
   starts to occur in sporadic epidemics. Most infectious diseases affect
   the poor more than the rich, but because the wealthy are usually the
   first to benefit from technological advancements, polio did not show
   such snobbish restriction.

Treatment and Control

   As part of the "War on Polio," researchers Albert Sabin and Jonas
   Salk, taking different approaches, developed effective vaccines
   against polio, and a widespread immunization campaign rapidly brought
   the disease under control in developed countries. The Salk vaccine is
   an injection of chemically killed virus, which "teaches" the immune
   system to recognize the virus and eliminate it. This vaccine confers
   lasting, but not always life-long, immunity. Sabin's vaccine is given
   orally, and contains attenuated, live viruses of each of the three
   polio serotypes. The attenuated virus replicates in the patient's
   intestinal tract and induces immunity, but is not virulent enough to
   cause paralysis (except in rare cases). This live vaccine, in addition
   to confering life-long immunity, can also be transmitted to a second
   host via the route normally followed by the virus. It is unclear
   whether or not this "second-hand" vaccination has a significant impact
   on the efficiency of vaccination programs. The chief advantage of the

                         The Poliovirus Life Cycle

I. The Poliovirus Receptor (PVR)

   Polio's first interaction with a susceptible host cell consists of
   binding to a specific cell surface protein, the poliovirus receptor
   (PVR). This protein, whose natural function is not known, is a member
   of a family of proteins called the immunoglobulin (Ig - pronounced as
   two letters, not as a one-syllable word) superfamily, the defining
   feature of which is a "loop" in the protein structure called the Ig
   domain. Different members of the family have different numbers of
   loops, ranging from one to several. Ig superfamily proteins are
   frequently involved in communication between cells and the reception
   of external signals. PVR has three Ig loops (which are outside the
   cell), numbered 1-3 starting with the domain farthest from the cell
   surface. The protein extends through the cell membrane, with a short
   stretch of amino acids inside the cell as well, as represented
   schematically in the figure above.

   Polio appears to bind to its receptor on domain 1. This initial
   binding is followed by conformational changes in the virus's capsid
   which have been hypothesized to prepare it for uncoating. The receptor
   is taken into the cell by the normal process of endocytosis, which is
   most likely involved in PVR's natural function.

   The poliovirus receptor is expressed in many human tissue types,
   apparently including some tissues, such as kidney, which are not
   normal sites of poliovirus replication in the host. Why doesn't polio
   replicate in these cells, if its receptor is available to let it in?
   There are two theories: either the virus's replication is blocked in
   which have been hypothesized to prepare it for uncoating.
   Several experiments have indicated that the receptor is not
   the only determinant of a tissue's susceptibility to poliovirus
   infection (a phenomenon called "tissue tropism").

II. Uncoating

   After binding to its receptor, poliovirus must get its RNA genome into
   the cell's cytoplasm, where translation and replication will occur. In
   this respect, the viral capsid is something of a paradox, since it
   must be stable to harsh conditions in the environment (including the
   low pH of the host's stomach), but must be able to release its RNA
   easily and quickly when stimulated with the proper signal. The signal
   is presumably receptor binding. At physiological temperatures, the
   virus can undergo a major structural change, called alteration, after
   After binding to its receptor, poliovirus must get its RNA genome into
   binding to the receptor. The altered particle is easy to distinguish
   from the native virion, but it is unclear how - or even if - this
   altered stage leads to productive uncoating of the virus genome. For
   every 200 or so virus particles that encounter a cell, only one will
   successfully enter and replicate, so research in this area is often
   confounded by the rarity of successful entry.

   There are two major models for poliovirus entry. In one, the virion,
   after binding its receptor, initiates entry directly from the cell
   surface, and the RNA is injected into the cytoplasm. In the other
   model, the virus particle must be taken into the cell by a process
   called receptor-mediated endocytosis - a mechanism routinely employed
   by cells to take in food and signal proteins. According to this model,
   the virus then uncoats inside an endosome that forms in the cell, and
   the genome is released into the cytoplasm.

   While the three-dimensional structure of the virus is known at high
   resolution, the events of entry are still obscure, as the preceding
   paragraphs indicate. Studying this phenomenon is important not only
   from the standpoint of understanding polio's pathogenesis, but also
   because similar mechanisms are undoubtedly employed by related
   viruses, such as rhinoviruses (which cause the common cold), hepatitis
   A virus, and foot-and-mouth disease virus (an important agricultural
   pathogen). Understanding how the virus enters the cell can lead to new
   therapies that target this vulnerable stage of its life cycle.

III. Protein Synthesis

   The poliovirus genome is a single RNA molecule 7,441 bases in length.
   This RNA is "message sense," meaning that it can be translated
   directly into proteins by the cell's ribosomes. Ordinarily, the cell
   attaches a methyl cap to the 5-prime end of a messenger RNA, and the
   ribosome, after binding to this cap, scans along the RNA until it
   reaches a start codon. This is known as the cap-dependent translation
   mechanism. Poliovirus RNA does not have a 5-prime cap, but a viral
   protein encoded by the 3B gene is attached where a cap would normally
   be found. In addition, the virus has several start codons upstream of
   the start site that is actually used. Instead of binding at the
   protein encoded by the 3B gene is attached where a cap would normally
   be found. In addition, the virus has several start codons upstream of
   the start site that is actually used. Instead of binding at the
   5-prime end and scanning, ribosomes are directed to a "ribosome
   landing pad" sequence in the RNA, and translation proceeds from the
   first start codon after the landing pad. This is called the
   cap-independent mechanism, and is also used by some cellular RNAs.

   Once translation starts, the ribosome continues along the full length
   of the polio genome, producing a long polyprotein rather than
   translating the virus from a series of individual genes. This large
   protein is then cleaved into subsections and finally into the separate
   genes involved in replication and packaging, including the virus
   capsid proteins.

IV. RNA Replication

   RNA viruses have a unique difficulty when it comes to replication, as
   the cell does not have the necessary machinery to replicate an RNA
   molecule (the cell replicates DNA, which is transcribed to produce
   RNA, and RNA is translated to produce proteins). This means that the
   protein is then cleaved into subsections and finally into the separate
   virus must carry its own RNA replication proteins or have a mechanism
   for producing them once inside the cell. For polio, the replication
   functions are carried out by the 3D gene product, which is an
   RNA-directed RNA polymerase. This means that it reads an RNA template
   and produces a new RNA molecule of the opposite polarity.

   First, 3D reads the positive-sense viral RNA genome and produces a
   full-length negative-sense RNA. These negative-sense (or "antisense")
   templates are then read by the polymerase to create new positive-sense
   RNAs, which can then be translated by the host cell machinery to make
   more viral proteins (including more 3D), or packaged into virus
   capsids to be released from the cell. The VPg (3B) protein is attached
   to the 5-prime end of the new genomes after they are replicated but
   before packaging.

V. Protein Processing

   The product of translation is the long viral polyprotein. In a series
   functions are carried out by the 3D gene product, which is an
   of cleavages carried out by virally-encoded proteases, this protein is
   broken down into the component parts that operate as separate genes.
   The two proteases involved in this process are encoded by the 2A and
   3C genes. Since the proteases are contained within the polyprotein
   initially, one of their most important functions is to cleave
   themselves out of the larger structure, freeing them to do the rest of
   their work.

   The proteases first cleave the polyprotein into three subsections,
   named P1, P2, and P3. P1 encodes the virus capsid proteins, which are
   named VP1, VP2, VP3, and VP4 (the occur in the genome in the order
   VP4-VP2-VP3-VP1). The P2 segment carries the 2A, 2B, and 2C gene
   products, and P3 contains 3A, 3B, 3C, and 3D. These individual gene
   products are separated by subsequent protein cleavages.

   In addition to its role in cutting up the polyprotein, the 2A gene
   product is involved in shutting off most of the host cell's own
   protein synthesis. The protease does this by cleaving a component of
   the cell's translational machinery which is required for cap-dependent
   translation. As described above, the virus is translated by a
   The two proteases involved in this process are encoded by the 2A and
   cap-independent mechanism, so it is not affected by this. Shutting
   down the host's mRNA translation serves a dual function for the virus:
   first, it frees up more ribosomes to translate the viral genomes being
   replicated, and second, it insures that the cell will die and break
   down, releasing the progeny virions after they have been assembled.

VI. Packaging and Release

   After the virus has translated its RNA to produce the necessary
   proteins and replicated its genome, it needs to package the newly
   synthesized RNA molecules into the capsids, which will be released as
   infectious virions for the next round of infection. The capsid
   proteins assemble into an immature capsid, a structure which contains
   all of the necessary proteins, but which has not finished cutting them
   into their final form. The viral RNA, with its attached 5-prime VPg
   protein, enters the incomplete capsid and is secured inside when the
   replicated, and second, it insures that the cell will die and break
   viral proteases make the final cleavages. The processes which guide
   the RNA to the capsid are still poorly understood. Once the genomes
   have been packaged into mature virions, the virus particles await the
   cell's lysis (bursting), when they will be released to infect
   neighboring cells.