Effects of Animal Viruses on Host Cells

1
Effects of Animal Viruses on Host Cells

• (What does the virus do to the host cell?)

2
Morphological Changes

• Virus lytic infections often cause very
  distinctive, visible changes in the infected
  cell. These changes are called cytopathic
  effects (CPE) and they include
• Inclusion bodies microscopically these are
  visible sites of viral assembly or cellular
  damage. They are often used as a diagnostic
  tool. Examples include
• Virions in the nucleus (Adenovirus)
• Virions in the cytoplasm (Rhabdovirus- negri
  bodies of rabies virus)

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Negri Bodies of Rabies Virus
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Morphological changes

• Viral protein associated with host microtubules
  (Reovirus)
• Factories of viral replication in the cytoplasm
  (Poxvirus)
• Clumps of ribosomes in capsids (Arenavirus)
• Clumps of chromatin (herpesviruses)
• Morphological alterations
• Nuclear pyknosis (shrinking) (Picornaviruses)
• Proliferation of membranes (Picornaviruses)
• Proliferation of the nuclear membrane
  (Alphaviruses)
• Formation of vacuoles in the cytoplasm
  (Papovaviruses)
• Apoptosis (will discuss this more later)
• Formation of syncytia (Paramyxoviruses and
  Coronaviruses) which are giant, multinucleated
  cells formed by the fusion of plasma membranes

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Syncytia formation
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Morphological changes

• Margination and breakage of chromosomes
  (Herpesviruses)
• Rounding up and detachment of tissue culture
  cells due to apoptosis (Herpes and
  Rhabdoviruses)

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Morphological changes

• CPE is very rarely caused by a harmful protein
  with no other purpose in the infective process.
• CPE is usually a secondary result of changes in
  the host metabolism caused by viral replication.
• Viruses may halt or alter host cell DNA
  synthesis, transcription, and/or protein
  synthesis (translation)

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Changes in the Host Cell Due To Viral Infection

• Eukaryotic viruses are more likely to target host
  cell translation rather than transcription or DNA
  replication because the half-life of mRNAs is so
  long in eukaryotic cells (T1/2600 minutes).
• If the virus wants to use the host translation
  machinery, it will need to compete with host
  mRNAs for a long time.
• Therefore, it needs to shut off or slow down host
  mRNA translation while allowing viral translation
  to continue.
• There are three steps in protein synthesis
  (translation) that are potential targets for
  viral action.

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Inhibition of Eukaryotic Protein Synthesis

• Initiation requires mRNA, 2 ribosomal subunits,
  special met-tRNAmet, initiation factors (eIF
  1-6), ATP, and GTP
• Elongation requires charged tRNAs, elongation
  factors (EF1, and EF1B), and GTP
• Termination requires a termination factor and
  GTP
• Mechanisms to inhibit host cell protein synthesis
  may vary for the same virus in different hosts.
  They include
• Competition
• The virus may produce an overabundance of mRNAs
  an/or produce viral products that actually bind
  to and tie up host cell mRNAS to make them
  inaccessible to ribosomes (rhabdovirus and
  reovirus).

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Inhibition of Eukaryotic Protein Synthesis

• The viral mRNAs may have highly accessible and
  easily recognized initiation sequences that allow
  them to compete more effectively for the
  initiation factor 2 (eIF2). Therefore, there is a
  higher initiation of translation of the viral
  proteins (mengovirus and influenza virus).
• There may be an inhibition of transport of host
  mRNA from the nucleus. Adenovirus allows
  selective transport of viral mRNAs from the
  nucleus, while inhibiting the transport of the
  host mRNAs.
• The virus may degrade the host mRNAs
• Bunyavirus uses primers derived from host cell
  mRNA for its own transcription in the cytoplasm.
  The 5 cap of the host mRNA plus 10-14 bases are
  cleaved by a viral enzyme. This is then used as
  the primer for transcription. This is similar to
  what influenza virus does in the nucleus.

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Inhibition of Eukaryotic Protein Synthesis

• Herpes viruses (remember that they control the
  timing of their gene expression through positive
  and negative feedback loops)
• First host mRNA is degraded. This is mediated by
  a protein found as part of the virion
• Next, at the time that the ? genes get expressed,
  there is a complete shut down of host protein
  synthesis. This is due to a newly synthesized
  viral protein.
• Blockage of initiation complex formation
• Pox virus alters the specificity of the host cell
  ribosomes so that the viral mRNA is
  preferentially recognized
• A phosphoprotein that is part of the virion acts
  on the 40S ribosomal subunit to hinder its
  association with the met-tRNA met, GTP, and eIF2.
  This equally inhibits host and viral translation.

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Inhibition of Eukaryotic Protein Synthesis

• Another viral protein that is not a part of the
  virion, somehow allows specific translation of
  viral messages only.
• Covalent modification of translation related
  components
• Inactivation by cleavage Poliovirus cleaves the
  cap-binding complex (eIF4) so that capped host
  mRNAs are no longer translated. How is viral mRNA
  then translated?
• Inactivation by phosphorylation
• Influenza A virus and adenoviruses inactivate the
  cap binding reaction by removing the required
  phosphate from eIF-4E
• Picornaviruses act indirectly on the cap binding
  complex by phosphorylating 4E-bp1 which then
  competes with eIF-4G for binding to the active
  form of eIF-4E

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Inactivation of the Cap-Binding Complex
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Inhibition of Eukaryotic Protein Synthesis

• A poxvirus structural core protein sequentially
  phosphorylates and inactivates three ribosomal
  proteins
• VSV, adenovirus, mengovirus, and reovirus
  phosphorylate the ? subunit of eIF2. When eIF2 is
  phosphorylated, it forms a complex with GEF (GTP
  exchanging factor or eIF-2B) that cant exchange
  GDP for GTP. The exchange of GDP for GTP
  normally occurs upon formation of the initiation
  complex. This exchange allows ? to participate in
  the formation of a new initiation complex. When
  the exchange does not occur, ? is blocked from
  participating in the formation of a new
  initiation complex and the rate of translation
  initiation is slowed down for both host and viral
  mRNA. Since there is an overabundance of viral
  mRNA, this tips the scale even more in favor of
  viral mRNA translation.

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Inhibition of Eukaryotic Protein Synthesis
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Inhibition of Eukaryotic Protein Synthesis

• The enzyme that phosphorylates eIE2? is called
  protein kinase RNA dependent (PKR). Its
  mechanism of action will be discussed later.
• Increases in intracellular cation concentrations
  one of the environmental conditions that can
  influence the rate or efficiency of an enzymatic
  reaction is the concentration of particular
  cations (Na, K).
• Some viruses alter the membrane permeability to
  cause an increase in intracellular Na.
• This inhibits host cell, but not viral
  translation. (Sindbis virus inhibits the Na pump
  in the plasma membrane).

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Inhibition of Eukaryotic Protein Synthesis

• Some viruses produce inhibitory proteins
• Mengoviruses produce a protein that binds to
  ribosomes and blocks translation after formation
  of the initiation complex, i.e., it inhibits
  elongation.
• In summary, as an example, mengoviruses may use
  three different methods to halt host cell protein
  synthesis
• Competition for initiation complexes
• Inhibition of elongation
• Phosphorylation of eIF-2.
• The viral RNA of this virus is so efficient in
  initiation that it must use the other two
  mechanisms to free viral RNA for packaging into
  the genome!

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Changes in the Host Cell Due To Viral Infection

• Inhibition of host cell transcription
• This may be secondary to the shut off of host
  mRNA translation.
• Factors required for host cell transcription are
  not replaced when translation is halted.
• Some viruses appear to actually inhibit host cell
  mRNA synthesis by acting directly or indirectly
  on RNA polymerase II by unknown mechanisms (polio
  and reovirus)
• The viral RNA may inhibit transcription there
  is evidence that the VSV leader sequence may bind
  to an initiation factor for host transcription
  making it unavailable for host transcription.

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Changes in the Host Cell Due To Viral Infection

• Inhibition of host cell DNA replication
• This may be secondary to decreased translation
• Some viruses have proteins that may specifically
  inhibit host DNA synthesis (reovirus)
• Poxvirus produces a protein that degrades SS DNA
  at the replication fork
• Herpesviruses displace host chromatin from its
  normal association with nuclear matrix proteins