Plus Strand RNA Viruses Flashcards
Picornaviruses
naming, history, and lab use
- The name picornavirusis derived from two words: Pico (= small) RNA-virus. Meaning small RNA viruses.Pico-rna-virus = small-rna-virus
- Picornavirusbelongs to the virus family-Picornaviridae.
- They commonly infect mammals and birds.
- They are one of the first viruses to be propagated in cell culture.
- Also, one of the first viruses to be plaque purified.
- They are easy to purify and are stable, making them the most popular viruses for laboratory studies
Characteristics of picornaviruses
- They belong to the class iv viruses.
- Their genome is plus-stranded RNA (ssRNA+) that functions as mRNA once it is released into the host cell.
- Their gene do not undergo transcription since they are ssRNA+.
- Absence of transcription makes them unique from other viruses (whose gene must undergo transcription before virus protein synthesis can start).
- Therefore, the first virus molecules to be synthesized from infected cells are proteins including those that will replicate virus RNA.
- Once sufficient virus protein is achieved, the role of virus RNA in producing mRNA is switched into templates for the replication of virus genome
Examples of Picornaviruses
- Hepatitis A virus
- Poliovirus
- Coxsackieviruses
- Rhinoviruses
- Foot and mouth disease virus
Hepatitis A virus
picornaviruses
- Infection results in an inflammation of the liver which could develop mild to severe illness.
- Disease is transmitted through ingestion of contaminated food or water or contact with an infected person.
- It is mostly prevalent in developing countries with poor sanitation.
- In most infants and children, infection is mostly asymptomatic (no symptoms) or mild, leading to life long immunity.
- About 75% of infected adult develop jaundice.
- Severe hepatitis occurs rarely and could be fat
Polivirus
picornaviruses
- Caused by one of the three serotypes of the poliovirus (PV 1-3).
- The virus is transmitted through poor hygiene arising from contact with infected persons or ingesting contaminated food or water.
- Widely studied because infection can result in devastating paralyses.
- Most poliovirus infection are relatively harmless infections of the oropharnyx(throat area) and the gut.
- Severe infection is rare in babies due to immunity from anti-polio vaccines acquired from their mothers.
- Severe infection can occur only after other tissues are infected, resulting in viremia(virus in the blood) and spread of infection to the central nervous system (CNS).
- Infection of the CNS is more likely to occur if virus antibodies have disappeared in the body due to absence of the virus in human environment.
- Thus ironically, polio is a disease primarily associated with improved hygiene and sanitation.
Poliovirus
Meningitis
Meningitis: Polio virus can infect the meninges (membranes around the brain and spinal cord) causing severe headache, stiff neck and light sensitivity which hurts the eyes.
Polioivirus
Encephalitis
infection of the brain arising from inflammation of the grey matter of the brain stem
poliovirus
Paralytic poliomyelitis
Paralytic poliomyelitis: Caused by virus replication in motor neuronesof the spinal cord or the brain stem, resulting in paralysis of limbs and/or breathing muscles
Poliovirus epidemiology
there is still a risk of spread in parts of west africa, parts of afganistan, pakistan, yemen and china
Poliovirus eradication effort
polio has been eradicated from many parts of the world due to the development of vaccines against the virus
polio vaccines
developed and used in preventing infection are either inactivated or live attenuated vaccine
Coxsackieviruses
transmission
through faecal contaminaiton of food or water
Coxsackieviruses
(picornaviruses)
origin
symptoms of the virus were first discovered in a US town of Coxackie in 1948
Coxsackieviruses
method of discovery
researchers injected faecal samples of two suspected polio patients into suckling mice
Coxsackieviruses
virus groups
two groups of the virus have been identified from studies of mice (groups A and B)
Coxsackieviruses
disease
Medical conditions include myocarditis (heart disease), hepatitis, spastic paralysis (unusual tightness or pool of muscles), meningitis and rashes
Hand, foot and mouth disease virus
(In humans)
(picornaviruses)
- The disease is commonly caused by the CoxsackievirusA16 in the USA.
- The virus is transmitted through contact with infected person, respiratory droplets containing the virions, faecal-oral contamination and fomite.
- Symptoms include fever, flu, sore throat, mouth sores (blisters), skin rash (palms of the hand and soles of the feet
Rhinoviruses
picornaviruses
•Transmission is by inhaling aerosols of respiratory droplets or contact with fomite (contaminated surfaces)
.•Are the most common agents causing upper respiratory tract infections in human.
- Rhinovirus accounts for about 50% of all cold in adults.
- Most children are likely to have been exposed to the virus before the age of two.
most replicate in the epithelium of the upper respiratory tract, where temperature range between 330C -350C.
•Some human rhinoviruses replicate around 330C -350C, while many are capable of replicating at 370C.
Foot and mouth disease virus
(In livestock)
(picornaviruses)
- The virus can infect goat, cattle, pigs, deer and sheep, causing lesions of the feet and mouth.
- Transmission is through contact with infected body parts (fluid from blisters), fomites or airborne (outbreak in Isle of wright in 1981).
- The virus has a broader host range than other picornaviruses.
- Infection results in economic losses for farmers, due to reduced milk yield, death of young animals.•In 2001, outbreak of the virus in the UK, results in loss of about 8 billion pound
Picornavirus virion capsid
- They are small RNA viruses of relatively simple structure.
- A capsid encloses the RNA, which is roughly spherical, with a diameter between 25-30 nm.
- It has icosahedral symmetry which is made up of 60 copies of each of four virus proteins numbered 1-4 (numbered from the largest to smallest
Picornavirus virion capsid
crayons
•Many picornaviruses(poliovirus and rhinovirus) possess a deep cleft around each of the 12 vertices of the icosahedral-shaped capsid
These clefts are about 2 nm deep and were named canyons.Canyons make up about 7% of the entire virus (the size of an entire virus is about 30nm).Canyons are surrounded by the C-termini of VP1 and VP3 molecules and contain the virus attachment sites
Picornavirus genome
- The genome is made up of 7-8 kb ssRNA.
- The 5’ end of the RNA is covalently linked with a small 2-3 kDprotein called VPg(virus protein, genome-linked. Figure 4.
- The covalent link is through the –OH group of tyrosine residue at position 3 of VPg
- The 3’ end of the RNA is polyadenylated(addition of PolyAto an RNA transcript).
Picornavirus genome %’ UTR
Within the untranslatedregion at the 5’ end, there exist some amount of secondary structure.For instance, the poliovirus RNA has six domains (i-vi), five of which form the internal ribosome entry site (IRES
Picornavirus replication
- Attachment
- Entry
- Translation and post-translational modification
- Transcription/genome replication
- Assembly and exit
- Inhibition of host gene expressio
Picornavirusreplication-Attachment
•The virus attach themselves to specific sites on the surface of host cell membrane known as cell receptors
.•The poliovirus receptor is the glycoprotein CD155.
- The receptor is found on many cell types, though its function is yet to be clearly defined.
- CD155 is found only in humans and some other primate species
.•CD155 is a member of the immunoglobulin superfamily of molecules with three immunoglobulin-like domains; the attachment site for picornavirusesis situated at the outer domain
Picornavirusreplication-Attachment to CD155
- Binding of CD155 to poliovirus particles result in major conformational changes in the structure of the capsid.
- Binding results in the movement of the N-termini of VP1 from the interior to the exterior surface of the capsid while VP4 is lost from the virionsurface (VP4 forms the initial interior part of the virionprotein)
Picornavirus attachement sites
- Poliovirus attachment: possesses canyonson their surfaces whose pockets serves as sites of attachment to host receptors.
- Virus attachment sites are protected from antibody attack as they are too large to penetrate these pockets; hence protecting them from immune attack.
- The other part of the virionsurface can mutate to avoid host immune attack.
- Foot and mouth virus: Note that the attachment sites for foot and mouth virus are on surface protrusionson the capsid and is not on the canyons.
Picoronavirus replication - entry
•Several modes of virus genome entry into host cells have been proposed:
- Transfer of virus RNAfrom the virioninto the cytoplasm at the plasma membrane; leaving the capsid at the cell membrane.
- Endocytosis: common among rhinoviruses and foot and mouth virus.Endocytosis is followed by either release from the capsid after disruption of the endosome membrane,Or release of RNA from the capsid then transport across the endosome membrane
•Once the virus genome is free in the cytoplasm the VPgis removed from the 5’ end by a cell enzyme
Picornavirusreplication-Translation and post-translational modifications
- The picornavirusgenome acts as mRNA, but is a slightly different from normal mRNA at the preliminary stages of translation since the virus mRNA isn’t capped (cap-independent translation).
- The 40S ribosomal subunit doesn’t bind at the 5’ end of the RNA, but at the internal ribosome entry sites (IRES). Fig. 4, slide 1
- Due to the small size of the picornaviruses, its genome encodes a single glycoproteinwhich undergoes a series of cleavages to give rise to all the structural and functional proteins
SEE SLIDES
Class IV virus transcription/ genome replication
(picornavirus?)
- All known class IV viruses of eukaryotes replicate their RNA in the cytoplasmic surfaces of membranous vesicles that develop in the cytoplasm of infected cells.
- For (+) RNA to be replicated, multiple copies of (-) RNA must be transcribed and used as templates for more (+) RNA synthesis.
- The small protein VPgis the primer for both (+) and (-) strand synthesi
Poliovirus transcripton/ genome replication
- Approximately 2500 RNA molecules are synthesized per minute in a poliovirus infected cell.
- The virus controls RNA synthesis in a way that about 50 times more (+) RNA than (-) RNA strands are produced.
- Some of the (+) RNA molecules are used as templates for more (-) RNA synthesis, some function as mRNA and some are destined to be the genomes of progeny virions.
Virus assembly and exit
+ stranded RNA viruses
Figure 9: Virus assembly and exitFive copies each of virus protein (VP0, VP3 and VP1) assemble into a pentamer. Note that 12pentamersform a procapsid.A copy of virus genome enters the procapsidforming a virionProgeny virus leaves the infected cell after lysisto infect new cells
- Each procapsidacquires a copy of the virus genome with VPgstill attached to the 5’ end (figure 9).
- Soon after, 60 copies of VP0 are cleaved into VP4 and VP2
.•Virus infected host cells undergo structural changes (cytopathogenic effects) leading to cell lysis
.•Lysisof the cells releases the virions; approximately 100,000 virionsare produced in each poliovirus-infected cell.
•Shortly after infection with poliovirus, the expression of host cell genes is stopped (host RNA polymerases are inhibited) and manipulated into synthesis of more virus genes
