Retroviruses Flashcards
Retrovirus structure and genome
- enveloped with lipid membrane
- glycoprotein gp120 gp41
- capsid and nucelocapsid
- two RNA genome copies
- essential enzymes packaged into genome
- produces DNA of the viral genome in cytoplasm using reverse transcriptase
- viral DNA integrates into genome of host cell
HIV-1 clinical features: acute infection
- initial symptoms 2-4 weeks after exposure, last 1-2 weeks
-acute infection followed by clinical latency (lasts 3-20 years) - gradual decrease in CD4+ T cells
- opportunistic infections and uncommon cancers
Signs and symptoms of advanced disease:
CD4 levels below 500cells/mm3 = AIDS
opportunistic infections, fungal, TB, herpes reactivation, lymphoma epstein barr…etc
What do CD+ T cells do?
help coordinate immune response by stimulating macrophages B cells and CD8+ T cells to fight infection
HIV infects and depletes _____ ____
CD4+ cells (compromises the immune system, allows opportunistic infections & cancer)
Pathogenesis of HIV
- depletes CD4+ gradually (but can lead to aids as fast as 1-2 years)
- prevents body from mounting immune response
- pyroptosis kills none infected CD4+ T cells releasing inflammation (without hurting hosts)
HIV-1 genome organization:
- Gag (structural)
- Pol (enzymatic activities)
- Env (envelope, structural)
- Accessory factors (things that modulate cell environment to promote viral replication)
What are HIV-1 accessory proteins
mostly involved in immune responses
What are viral determinants of HIV-1 entry?
gp120: receptor budding glycoprotein
gp41: fusion machinery
— BOTH ARE class 1 fusion proteins: requires proteolytic cleavage by fusion in the cell
—- assembles as a trimer on the virion
What are cellular determinants of HIV-1 entry?
- receptor: CD4 (helper T cell and myeloid macrophages and dendritic cells)
- co-receptor: CCR5 on macrophages or CXCR4 on CD4+ T cells
(chemokine receptors)
How can you become HIV-1 resistant
polymorphism deletes 32 bp in the coding region of CCR5 and premature stop codon
heterozygote: gets infected but slow progression
homozygote: highly resistant to HIV
Overview of HIV-1 entry ( steps)
- Env glycoprotein gp120+gp41 cleaved by furin — entry
- Receptor binding (by gp120) to CD4 and co-receptor to trigger fusion
- Fusion (by gp41) at plasma membrane
HIV-1 +ve ssRNA is not translated upon cell entry. So then what happens??
+ve sense ssRNA copied by viral enzyme Reverse Transcriptase (RT) to dsDNA
Reverse Transcriptase has three enzymatic activities
- RNA dependant DNA polymerase
- Rnase H
- DNA dependant DNA polymerase
How does HIV-1 become a chronic infection?
integrate their DNA into host cell by viral enzyme integrate, then called a “Provirus”
NOT REVERSIBLE
every time DNA is replicates, virus is also replicated, and transcribed by host machinery
How does integrate work?
- integrase binds to both ends of viral dsDNA (pre-integration complex PIC) through 3 steps
1. 3’ processing
2. strand transfer
3. Gap repair
Step 1 of integrase working:
3’ processing
- integrase catalyzes removal of two nucleotides (dinucleotide GT) at 3’ end of each DNA strand
- leaves a reactive 3’ OH group that can attack host DNA
called Cleaved Donor Complex
Step 2 of integrase working:
Strand transfer
- cleaved donor complex binds to host
- breaks host DNA
- covalently binds viral dsDNA
Step 3 of integrase working:
Gap repair
- cellular enzymes remove protruding viral DNA ends (5’ AC) and repair the gaps
consequences of integration
- HIV genes expressed using cellular transcription machinery
- integration is for life
- 8% of human DNA is from retroviruses (human endogenous retroviruses HERVs)
What happens to viral transcripts that happen every time the cell replicates?
1- exported from nucleus and packages into viral particles
2. Exported from nucleus to make viral protein
3. Splicing generates gene products (accessory proteins)
Maturation of HIV particles
Gag and Pol not cleaved UNTIL its in the viral particle
HIV protease (part of pol) cleaves them once they get into viral particle and makes a MATURE infectious virus
HIV gene and protein expression
1- cellular machinery transcribes HIV mRNA
2- Tat protein needed for efficient transcription
3- HIV accessory proteins made by splicing
4- Rev protein for nuclear export
5- Ribosomal framshifting to produce Gag and Pol poly protein
HIV Assembly and Maturation
HIV RNA packages into new virions with uncleared poly proteins
Cleavge mediated by viral protease leading to maturation
