Viruses and Prions

1. Viruses and Prions • Chapter 14

2. 14.1 Structure and Classification of Animal Viruses • Structure • DNA or RNA genome • Double stranded (ds) or single stranded (ss) • Surrounded by a capsid (protein coat) • The nucleic acid and capsid are termed nucleocapsid • Some viruses have an envelope • The envelope is a phospholipid bilayer membrane that was obtained from the cell in which the virus arose

3. 14.1 Structure and Classification of Animal Viruses • Viruses are obligate intracellular parasites • They occur in many shapes, some of which are distinctive Human papillomavirus Rhabdovirus Ebola virus

4. 14.1 Structure and Classification of Animal Viruses • Viral genomes exhibit a range of complexity • Polioviruses: single-stranded RNA virus • Herpesviruses: double-stranded DNA • Retroviruses: diploid single-stranded RNA • Influenza viruses: multiple gene segments of single-stranded RNA • Genome sizes • Hantaviruses have 3 genes that encode 4 polypeptides • Pox viruses have nearly 200 genes • There are thousands of known viruses (and probably tens of thousands of unknown viruses)

5. 14.1 Structure and Classification of Animal Viruses • Virus Classification • Genome structure • Virus particle structure • Presence or absence of an envelope • Nomenclature rule: Viruses are named for the geographic region in which they are discovered

6. 14.1 Structure and Classification of Animal Viruses • Groupings by Transmission Mechanism • Enteric viruses: fecal-oral route • Respiratory viruses: aerosols • Zoonotic agents • Biting • Respiratory route • Sexually-transmitted

7. 14.2 Interactions of Animal Viruses and Their Hosts • Viruses tend to be species- and cell-specific • Infection is a 9-step process • Attachment • Entry • Targeting to site of viral replication • Uncoating • Nucleic acid replication and protein synthesis • Maturation • Release from cells • Shedding from host • Transmission to other hosts

8. 14.2 Interactions of Animal Viruses and Their Hosts • Step 1: Attachment • Mediated by cell-surface molecule(s) and viral spike proteins • HIV gp120 is specific for CD4 • CD4 is principally found on helper T cells • Occurs by noncovalent interactions

9. 14.2 Interactions of Animal Viruses and Their Hosts • Step 2: Entry into the cell • Some viruses fuse with the cell’s plasma membrane • HIV’s gp41 interacts with a cellular chemokine receptor to induce fusion • Other viruses are internalized by endocytosis • In either case, the capsid, containing the nucleic acid and viral enzymes, is dumped into the cytoplasm

11. 14.2 Interactions of Animal Viruses and Their Hosts • Step 3: Targeting to the site of viral replication • Most DNA viruses replicate in the nucleus • Most RNA viruses replicate in the cytoplasm • Some viruses integrate their dsDNA into the host cell’s genome (i.e., chromosomes) • Some viruses copy their RNA into dsDNA, which is then integrated into the host cell’s genome

12. 14.2 Interactions of Animal Viruses and Their Hosts • Step 4: Uncoating • The capsid is composed of protein subunits • The nucleic acid dissociates from the subunits • This causes the capsid to disintegrate, liberating the nucleic acid

13. 14.2 Interactions of Animal Viruses and Their Hosts • Step 5: Nucleic acid replication and protein synthesis • RNA viruses • Some RNA virus genomes act as a mRNA (”plus-strand” viruses) • All others (minus-strand viruses) possess a prepackaged, virus-encoded RNA-dependent RNA polymerase • DNA viruses encode RNA polymerases • Many viruses have polycistronic mRNAs • Viral polypeptides are synthesized by the cell’s translational machinery

14. 14.2 Interactions of Animal Viruses and Their Hosts • Step 6: Maturation • Cleavage of polycistronic polypeptides into subunits • HIV gp160 polypeptide is cleaved into its gp120 and gp41 mature polypeptides • This step is inhibited by the HIV protease inhibitors taken by HIV+ patients • Nucleic acids and capsid proteins spontaneously polymerize into nucleocapsid

15. 14.2 Interactions of Animal Viruses and Their Hosts • Step 7: Release from cells • Some viruses rely upon cell lysis for release into the extracellular environment • Other viruses rely upon budding, whereby they exit from the cell, taking part of its membrane (viral envelope) • Budding occurs at the plasma membrane, ER or Golgi, depending on the viral species • If the rate of budding exceeds the rate of membrane synthesis, then the cell will die

17. 14.2 Interactions of Animal Viruses and Their Hosts • Step 8: Shedding from the host • Viruses must leave the infected host to infect other hosts • Shedding can be a minor event (such as cold viruses) or a catastrophic event (such as hemorrhagic fever viruses) • Step 9: Transmission to other hosts • Transmission routes usually reflect the sites of infection for viruses (e.g., respiratory, GI, STD)

18. 14.2 Interactions of Animal Viruses and Their Hosts • Persistent infections • Latent - periods of inactivation and activation (e.g., herpesviruses); usually limited pathology • Chronic - infectious virus can be detected for years or decades with little discernible pathology, but can eventually lead to disease (e.g., hepatitis B and C viruses) • Slow infections - short period of acute infection (weeks) followed by the apparent disappearance of virus for months or years, with pathology ensuing (e.g., HIV)

19. 14.3 Viruses and Human Tumors • Tumor viruses drive cell proliferation • Several mechanisms account for this phenomenon • Viral oncogenes that stimulate cell proliferation • Viral DNA integrates adjacent to genes that drive cell division • Expression of the viral genes leads to aberrant expression of the cellular gene • Some viruses encode growth factors that stimulate cellular proliferation • Epstein-Barr virus encodes viral interleukin-10 that causes B cell proliferation, leading to Burkitt’s lymphoma

20. 14.4 Viral Genetic Alterations • Segmented viruses contain multiple genetic elements that encode different genes • Influenza viruses are the best characterized of segmented viruses • The gene sequences of these segments within the same species can vary, thus provide genetic diversity • Coinfection of a cell with two or more different strains of a virus, such as influenza A viruses, can lead to the emergence of reassortant viruses that have distinct characteristics • The process is termed reassortment

21. 14.4 Viral Genetic Alterations • Influenza A viruses have 8 gene segments that encode 10 polypeptides • Segment 1 (2,341 nt): PB2 • Segment 2 (2,341 nt): PB1 • Segment 3 (2,233 nt): PA • Segment 4 (1,778 nt): HA (hemagglutinin) - 16 known subtypes • Segment 5 (1,565 nt): NP • Segment 6 (1,413 nt): NA (neuraminidase) - 9 known subtypes • Segment 7 (1,027 nt): M1, M2 • Segment 8 (890 nt): NS1, NS2 The H5N1 influenza virus has subtype 5 HA segment and subtype 1 NA segment

23. 14.5 Methods Used to Study Viruses • Cultivation of host cells • Embryonated chicken eggs • Must be susceptible to the virus • Two principal targets • Chorioallantoic fluid (CAF) • Embryo

24. 14.5 Methods Used to Study Viruses • Cell culture • Cells must be susceptible to virus • Cells are grown attached to flasks in a monolayer • Cells are inoculated with virus • Within days, cytopathic effect (CPE) can be seen

26. 14.7 Other Infectious Agents • Prions • Proteinaceous infectious particle • Cause spongiform encephalopathies • Characteristics • They contain no nucleic acids • They are a normal cellular protein (PrPc) that has misfoldedinto a pathogenic protein • The prion protein “replicates” itself by causing copies of the normal protein to misfold into the prion protein • Diseases • Creutzfeldt-Jakob (New Variant CJ from “mad” cows) • Kuru (religious consumption of brains from deceased) • Chronic wasting disease (elk, deer, moose)