Lecture 5. Poliovirus Replication Flashcards
What does picornavirus literally mean?
Small, RNA virus (30nm)
What Baltimore class do picornaviruses come under?
+ve sense ssRNA (Class IV)
What are the characteristic of picornaviruses?
Non enveloped icosahedral particle, 60 copies of VP1 – 4 (Subunit)
RNA typically 7-8 kb, single open reading frame
Genome has covalently attached protein at 5’ end (VPg)
Cytoplasmic replication
Replication typically cytopathic
RNA-dependent RNA polymerase
What are the four genera of picornaviruses that infect humans?
Enterovirus
Hepatovirus
Kobuvirus
Parechovirus
What are examples of enteroviruses?
Poliovirus
Coxsackie virus
Echovirus
Human Rhinovirus
What is an example of a hepatovirus?
Hepatitis A virus
How many human enteroviruses and human rhinoviruses are there estimated to be?
~100 human enteroviruses
~200 human rhinoviruses
Why can rhinoviruses not transit to the gut?
They are acid liable
How are parechoviruses and enteroviruses different?
Phylogenetically distinct from Enteroviruses, but not clinically separable
What are the six stages of poliovirus replication?
Attachment
Penetration
Uncoating
Biosynthesis
Assembly
Release
What is the receptor on the cell membrane that poliovirus binds to?
PVR/CD155
What species is poliovirus restricted to?
Humans and primates
Virus replicates in mouse cells but cannot infect them because they don’t express the PVR receptor
How was PVR initially isolated?
cDNA library transfer screen in murine cells
What is PVR/CD155?
A transmembrane anchor (can induce immune response) that is part of the Ig superfamily
What effect does CD155 have on mice cells?
Renders mouse cells permissive
How does poliovirus attach to CD155?
Receptor binds in a ‘canyon’ - binds next to a 5-fold axis/vertex (not on vertex)
Where do human rhinoviruses receptors attach to the virus?
At the peak of the vertex
What is an example of a picornavirus that binds to a different site?
Foot and mouth disease virus can bind to either integrin αvβ₃ binding site or heparin sulphate receptor binding site
What two groups can receptors be when serving the entry-role during infection?
Hook or unzipper
What are examples of viruses use hook receptors?
FMDV, minor group rhinovirus
What are hook receptors?
Receptor functions to concentrate virions on the surface of the cell; genome uncoating achieved by other means (e.g fusion, low pH in endosome)
What are examples of viruses use unzipper receptors?
Poliovirus, major group rhinovirus
What are unizpper receptors?
Receptor binding triggers and leads to disassembly of virion-capsid (thorugh conformational change)
What is the structure of a canyon and pocket binding site?
‘Mountain’ or vertex formed by V1 pentamer
‘Canyon’ is at the lowest point in the site and contains pocket factor
The pocket factor is the host lipid sphingosine that is important in maintaining the stability of the capsid (capsule dissociates without it)
How does the receptor induce conformational change in poliovirus?
Receptor binding displaces pocket factor
Increases flexibility of VP1; allows VP4 to interact with membrane and creates a pore in the cell membrane allowing RNA to enter cell (uncoating)
Probably needs interaction with multiple CD155 molecules
What is the virus interaction with CD155?
Attachment
Conformational change - irreversible commitment
Entry at cell surface - Delivery of genome (usually in a pit)
Because Baltimore class IV viruses (including picornaviruses) have a +ssRNA genome, what can they do?
+ssRNA genome can act as mRNA
So the genome can be translated straightway by the ribosome to the viral proteins, one of these proteins in fact is also RNA replicase
Why are RNA replicases required in viruses?
Host cells do not contian RNA replicases
What is the role of RNA replicase in genome replication?
The enzyme is used to replicate the viral genome and produce many copies of the +ssRNA that can be used as the viral genome
This along with the viral proteins can then assemble to produce many new viruses
Because poliovirus only has one open reading frame, how many proteins does it encode for (pre-cleavage)?
1 polyprotein (made up of three proteins, P1, P2 and P3)
What does the P1 fragment of the polypeptide code for?
Capsid
What is the P1 fragment cleaved into?
VP1-4
(VP2 and 4 are created by cleavage of VP0 after particle assembly)
What do the P2 and P3 fragments of the polypeptide code for?
Proteases and RNA synthesis
What is the P2 fragment cleaved into?
2A, 2B and 2C
2A releases P1 through co-translational cis cleavage
What is the P3 fragment cleaved into?
3A, 3B (Vpg), 3Cpro, 3Dpol
P3 initially cleaves into 3AB and 3CDpro, cis cleavage to release 3Cpro
What are the roles of 2A and 3C proteases?
Processing the polyprotein and cleaving cellular key proteins
What are the roles of 2B and 3A?
Rearranging cellular membrane vesicles
What is the role of 3D?
RNA dependent RNA polymerase
What are 3B and 2C?
Accessory replication proteins
3B (Vpg) and 2C (helicase)
What is IRES?
Internal ribosome entry site: a highly structured piece of RNA at the 5’ end of PV genome/mRNA
What does the IRES element direct?
Polyprotein translation, proteins synthesis during infection (host proteins disappear, viral proteins appear)
What happens when the IRES element has been replaced in poliovirus-infected cells?
No translation of any protein
What happens when the IRES element is present in poliovirus-infected cells?
Inhibits protein translation that is dependent on 5’ cap. Other protein translation can still take place
What happens when there is no 5’ cap or IRES?
No translation
What does IRES inhibit?
Host protein synthesis
What virus genus has type I IRES?
Enterovirus
What virus genus has type II IRES?
Cardiovirus
What virus genus has type III IRES?
HAV
What virus genus has type IV IRES?
Teschovirus
What is the role of IRES elements?
IRES can directly bind to eIF4G which allows translation to occur without the presence of eIF4E and the capped 5’ end
What cleaves eIF4E off eIF4G and what does this cause?
2A, inhibition of host cell translation
What enzyme do RNA viruses use to replicate?
Replication of RNA viruses is via RNA-dependent RNA polymerase (RdRp) (replicase)
What can RNA-dependent RNA polymerase be used to produce?
Used both for producing the mRNA and for producing the RNA genomes
Remember, viruses need to produce mRNA for the protein synthesis, and they also need to produce their genomes whether this is DNA/RNA
What is a difference between the proteins produced by picornaviruses and other viruses?
Picornaviruses only produce one protein that is cleaved later
In other viruses, the proteins are produced individually from individual mRNA
How many capsomers make up a virus?
60 capsomers of vp1, vp2 and vp3 = 180 subunits
What are many enterovirus diseases due to?
Secondary spread
How do all enteroviruses enter the body?
Faecal-oral route
Enter the mouth and pass into the intestine
From intestine is taken into the blood and spreads from there
What are the only known reservoir for polimyelitis?
Humans
What are the three serotypes of poliovirus?
PV-1,PV-2,PV-3
What is the primary transmission route of poliovirus?
Transmission primarily faecal-oral (possibly by respiratory droplets where hygiene levels are high)
What happens in the initial replication of poliovirus?
Initial replication in the oropharyngeal and intestinal mucosa
Follicle-associated epithelium (M cells and Peyer’s patches)
Virus shed in faeces (major transmission route: faecal-oral)
Where does poliovirus drain into?
Virus drains into cervical and mesenteric lymph nodes and then to blood
Transient viraemia allows replication at extraneural sites (reticuloendothelial tissue, brown fat, muscle)
How do most natural human infections of poliovirus end (~99%)?
After the virus ends up in the blood, that is the final stage
Minor disease with non-specific symptoms (e.g malaise, sore throat, fever)
What happens in 1-2% of poliovirus cases?
Virus enters the central nervous system (CNS)
Replication in motor neurons in spinal chord, brain stem or motor cortex leads to paralysis
What is the range of clinical illnesses caused by Poliovirus infections?
Range from clinically inapparent illness (~90% of infections) to paralytic polio
What is abortive poliomyelitis and how common is it?
(~8% of cases)
Fever, headache, sore throat, no neurological consequences
What is nonparalytic poliomyelitis and how common is it?
(1-2%)
Severe headache, neck stiffness (“aseptic meningitis”)
Full recovery after 2-10 days
What is spinal poliomyelitis and how common is it?
(<1%)
Weakness and flaccid asymmetric lower limb paralysis
Can involve respiratory muscles
Recover from paralysis (often incomplete) can occur
~10% fatality rate
What is bulbar poliomyelitis and how common is it?
(<0.1%)
Cranial nerve paralysis (mostly CN 9, 10)
Vasomotor and respiratory centres involved
May be fatal due to respiratory muscle paralysis
~50% fatality rate
