Lesson 52: Viral replication and antivirals

Growth of viruses

• Viral replication studied in vitro bc very fast, one virion can produce thousands

• Eclipse period: time after virus has penetrated before progeny of virion are visible

  • Important time where antivirals can interfere

• Eclipse time unrelated to incubation time

Growth curve

1. Inoculation: virus binds to cell

2. Eclipse: virion penetrates cell

3. Burst: host cell releases many viral particles

4. Burst size: number of virions released

Unculturable virus

Does not grow in cell culture, construct infectious clone: insert viral genome into plasmid

Circumvents need for receptors and promotors

Steps of viral replication

1. Attachment

2. Penetration

3. uncoating

4. Transcription of early mRNA

5. Translation of early proteins

6. Replication of viral DNA

7. Transcription of late mRNA

8. Translation of late proteins

9. Assembly of virion

10. Release

1. Viral attachment

Receptors on the viral receptors and coreceptors on the cell membrane envelope or capsid become connected to complementary expressed in susceptible cells

2. Viral penetration

Viral capsid or genome enters host cytoplasm

• Membrane fusion: Enveloped virus

• Endocytosis: Naked virus

• Genetic ejection: Bacteriophage

3. Viral uncoating

Viral capsid opens and frees genome: partial or complete

Cellular proteins called chaperones take genome to nucleus

4-8. Transcription, Translation, Replication

Differ in order depending if virus has DNA or RNA and if its Positive or negative polarity

• DNA viruses that replicate in the nucleus use the cellular DNA-dependent RNA polymerase to produce their mRNAs

• In most single stranded + RNA viruses the nucleic acid can directly bind to ribosomes and start translating either

  partially or fully

• Single stranded negative sense RNA viruses must carry their own polymerase enzyme inside their nucleocapsid, a specific RNA-dependent RNA polymerase

9. Assembly

Structural proteins of viruses form capsomers that self assemble into capsids in which the nucleic acid is packed

One end of the viral genome has a sequence” that binds to a protein that enables the entering to the procapsid

10. release

Exit cell

Some naked viruses accumulate in nucleus or cytoplasm until lysis

Enveloped virus exit the cell by budding or exocytosis

Retrovirus replication

use their own with RNA-dependent DNA polymerase capability) to first produce an RNA-DNA hybrid and then a double-stranded DNA with LTR that can insert into the host genome.

Retrovirus genomes have 2 copies of single stranded + polarity RNA but they replicate through a DNA intermediate inserts into the host genome

Transcription and translation happen from this DNA

LTR

Long terminal repeats

• Allows viral genome to integrate into host genome

• Acts as a strong promoter

Why can antivirals be toxic to cells/organisms

• virus use metabolic pathway of host to replicate so agents that interfere with viral replication are toxic to cell

• Targets specific to virus are safer but also virus specific

Antiviral use

Usually ease symptoms and shorten length of viral infection but are not curative

Antiviral drugs best suited for persistant infections(Ebola/flu)

Why aren’t antivirals used in vet med

Cost and lack. of efficacy and safety clinical trials

Most infections treated symptomatically or palliatively

Antivirals cannot be used in food animals

Antiviral drug types

Targeting viral function

• Proteases

• Integrases

• Neuraminidases

• Nucleic acid polymerases

  • D+NA dependent DNA polymerase

  • RNA dependent RNA polymerase

  • DNA dependent RNA polymerase

Nucleotide Analog

Something similar to nucleotides to incorporate into growing DNA strands

Acts as chain terminators and stop DNA or RNA polymerase from adding further nucleotides

Where would antivirals act

Monoclonal antibody: Receptors

Protease inhibitor: Creation of capsid

Spike maturation: Assembly of viral particle

Polymerase inhibitor: Replication and transcription

Endosomal fusion inhibitor: Entry and exit of viral particle