Viruses: intimate parasites

1
Viruses intimate parasites

• Are viruses alive?
• Not made of cells, in violation of Cell Theory
• Do not grow (but self assemble)
• Do not metabolize (but steal energy)
• Cannot reproduce w/o a host cell (but other
  organisms may require another species in order to
  reproduce)
• Can evolve over time
• Some can respond to environmental stimuli
• Have a complex, organized structure

2
If not cells, then what?

• Viruses are particles
• Some components are essential
• A genetic material with the blueprint for making
  more
• Could be ds DNA, ss DNA, ss RNA, ds RNA
• Space is limited, so genes are few
• A covering to protect the genetic material
• Capsid, made of one or more proteins
• Capsid nucleic acid nucleocapsid
• Viruses are obligate intracellular parasites

3
Viruses may have other parts

• Envelope piece of organelle membrane or cell
  membrane covering capsid
• Virus is formed by budding, pushes through
  membrane taking a piece.
• Viral envelope usually contains viral proteins.
• Envelope makes virus susceptible to some
  disinfectants
• Spikes (peplomers) extend from envelope
• Used for attachment, escape
• Accessory enzymes
• Reverse transcriptase, RNA RNA enzymes

4
Viral size and shape

• Viruses range from 20 nm to 300 nm
• Ribosomes are about 30 nm
• The smallest known bacteria are about 200 nm
• Viral shapes
• helical, polyhedral, and complex

http//www.glencoe.com/qe/images/b136/q4323/ch18_0
_a.jpg www.blc.arizona.edu/.../
Figures/Icos_Virus.GIF http//www.foresight.org/U
pdates/Update48/Images/T4Schematic.jpg
5
Examples of virus shapes
Ebola
Adenovirus
http//www-cgi.cnn.com/HEALTH/9604/16/nfm/ebola.le
vine/ebola.reston.large.jpg http//www.virology.n
et/Big_Virology/EM/Adeno-FD.jpg
6
Specificity

• A recurring theme in biology
• Enzymes, membrane receptors, antibodies, etc.
• Viruses are limited to certain types of host
  cells
• Species barriers rabies not specific, but most
  are
• Tissue type rabies specific to nerve, salivary
  tissue
• Cell type HIV infection mostly restricted to
  Helper T cells, a kind of lymphocyte.
• Different characteristics of host cells involved
• Attachment to cell surface often a major point
• Every type of organism has a virus that infects
  it?

7
Viruses across kingdoms

• A densovirus newly isolated from the smoky-brown
  cockroachPeriplaneta fuliginosa
• Acanthamoeba castellanii Promotion of In Vitro
  Survival and Transmission of Coxsackie B3 Viruses
  
• The causal organism is the Tulip Breaking Virus
  (TBV). The pathogen is a potyvirus and is divided
  into two strains,
• Genome characterization of Botrytis virus F, a
  flexuous rod-shaped mycovirus

8
Classification

• Microbes problematic, viruses especially so.
• No sexual reproduction, no asexual reproduction,
  just assembly.
• No clear evolutionary relationships
• Classification scheme (from David Baltimore)
• First, by nucleic acid type, e.g. ds DNA, sense
  RNA
• Next, structural characteristics (presence of
  envelope, capsid shape), type of organism
  infected, etc.

9
Life cycle of a virus

• Manner of infection and reproduction depends on
  whether host is prokaryotic or eukaryotic.
• Life cycle here outlined is general
• ADSORPTION following contact,
• molecules on surface of virus bind
• to particular molecules on host cell.
• PENETRATION the nucleic acid
• must get access to the machinery
• of the cell to replicate.

http//www.tthhivclinic.com/Fig_20.gif
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Life cycle continued

• SYNTHESIS/REPLICATION once inside the nucleic
  acid issues orders leading to
• Replication of the nucleic acid
• Transcription (usually) and translation,
  producing the necessary capsid proteins.
• ASSEMBLY a spontaneous process
• Capsid proteins and nucleic acid combine to make
  virion.
• Cheap but highly inefficient process.
• RELEASE successful parasite must spread to
  others
• Virus causes lysis of cell or pushes through cell
  membrane. Virions may acquire an envelope.

11
Bacteriophage lytic vs. lysogenic

• Most bacteriophages multiply then lyse the host
  cell
• This life cycle is called a lytic cycle
• Others are temperate, enter a lysogenic cycle.
• Lysogeny is an effective way to multiply the
  viral DNA
• Viral DNA inserts into the bacterial chromosome
• Now called a prophage
• Bacterial replication also replicates viral DNA
• Prophage may bring new genes for use by bacterium
• Damage to bacterial DNA (e.g. UV) prompts virus
  to begin lytic cycle DNA excises, virus
  multiplies.

12
Measuring numbers of virionsthe Plaque Assay

• Virus and host cells are mixed
• Bacteria cover in a Petri dish as a lawn,
  eukaryotic cells cover bottom of a dish.
• Multiplication of virus leads to release, spread
  to and destruction of nearby cells.
• Visible as holes, plaques, on bacterial lawn
  eukaryotic cells in culture are first stained for
  easier view to see plaques.

http//dept.kent.edu/biology/Courses/30171/imageQB
T.JPG news.bbc.co.uk/.../ _230333_cell_culture_3
00.jpg
13
Growing viruses

• Obligate intracellular parasites require a host
  cell!!
• Whole organism
• Animal models, human volunteers
• Ethically, humans require consent, safety, pay
• Eggs aseptic incubator
• Various cells and membranes support growth of
  viruses
• Shell provides protection from bacterial
  contamination
• Used for large batches of viruses for vaccines
• Egg allergies a problem sometimes

http//www.fao.org/ag/againfo/foto/egg-facts.gif
14
Growing viruses continued

• Organ/tissue/cell culture
• Minced tissue, separated into cells by enzyme
  treatment
• Grown in sterile plastic dish with nutrient
  solution
• Cells prepared this way grow until dish is
  covered, stop.
• Scrape up, use some to inoculate new culture
• Limited number of rounds of replication
• Transformed cells, with cancer properties, grow
  forever.
• Must be subcultured when dish bottom is covered
• Cell culture major reason for advances in virology

15
Viruses and disease

• Most of discomfort, some of damage, is due to
  host defense response to viral infection.
• Cytopathic effects damage caused by viruses
• Cells change shape, change size (round up, swell)
• Cells become sticky (clump, fuse to form
  syncytia)
• Syncytia are giant, multinucleated cells
• Cells change internally (vacuoles, inclusions)
• Location, appearance of inclusions characteristic
  of certain viruses comprised of viral parts or
  cell debris
• Cells die.

16
Other effects of viruses

• Transformation cells become cancer-like
• Lose contact inhibition
• Continue to multiply
• In vivo, produce tumors
• Certain Herpes-type viruses, genital wart virus,
  etc.
• Some viruses cause birth defects teratogenesis
• Damage to critical cells in fetus, failure to
  form parts
• Typically, virus is mild and able to cross
  placenta
• Rubella (German measles virus), CMV

17
NOT viruses

• Viroids naked RNA
• Cause of infectious disease in plants
• Prions infectious protein particles
• Got Stanley Prusiner the Nobel Prize, but still
  controversial.
• Cause of Mad Cow Disease, Scrapie, Elk Wasting
  disease, Creutzfeldt-Jakob disease, Kuru, etc.
• Transmissible spongioform encephalopathies
• Misshapen protein causes normal protein to also
  fold incorrectly, increasing their numbers
• Replication without the presence of nucleic acid

18
Prions
Normal cell protein
Misfolded version prion
www.stanford.edu/.../2004anderson/index.html
19
Attack and defense

• Virus has ways of attacking
• Specific binding to host cell receptor
• Receptors involved in normal cell functions
• Virus may regulate cell division for its own
  replication
• Insertion of viral DNA into chromosome allows
  virus to hide from immune system
• Causes adjacent cells to fuse, allows cell to
  cell spread without leaving cell.

20
Defense by host

• Host has ways of defending
• Cell mediated immunity (T cells) infected cells
  killed.
• Antibodies intercept virions between cells, in
  fluids
• Interferon produces anti-viral state, prevents
  replication
• Natural killer cells act
• Using antibodies as tags
• Quickly before B T cells ready

21
Pathogenic Viruses

• Name of virus
• what family it belongs to
• what disease it causes, organ system affected
• DNA or RNA?
• Route of transmission reservoirs, vectors
• Viral virulence factors (selected viruses)
• Immunizations

22
A molecular biology lesson

• RNA mutates more readily than DNA
• Copying mistakes by the enzymes are not corrected
• Not double stranded, so no mismatches noticed
  that can be fixed.
• Many variants result
• Less chance of lasting immunity
• Harder to create vaccines

23
HIV Human Immunodeficiency Virus

• Host range
• Main types of cells infected T helper cells and
  dendritic cells (including macrophages,
  microglia)
• Have CD4 and CCR5 glycoproteins on surface
• Infection process
• RNA is copied into cDNA by reverse transcriptase
• cDNA inserts into host chromosome
• New RNA made
• Protein precursor made, then processed assembly
  occurs
• Virions bud through cell membrane

24
HIV life cycle

1. Binding to host cell
2. Copying RNA into DNA by Rev Trans
3. Integration into host DNA
4. Transcription
5. Translation
6. Assembly and exit by budding

www.aidsmeds.com/images/HIVLifeCycle1.gif
25
Disease process

• Chronic infection
• T cells continually made, continually destroyed
• Eventually, host loses
• AIDS diagnosis
• Acquired Immuno Deficiency Syndrome
• CD4 cell count below 200/µl
• opportunistic infections
• Examples of opportunistic infections
• Pneumocystis carinii pneumonia (PCP pneumonia)
• Kaposis sarcoma Tuberculosis several others

26
Prevention and Treatment

• Prevention is easy
• Practice monogamous sex, avoid shared needles
• HIV cannot be spread by casual contact, skeeters
• Drug treatment
• Nucleoside analogs such as AZT
• Protease inhibitors prevent processing of viral
  proteins

Nifty animation at http//www.hopkins-aids.edu/hi
v_lifecycle/hivcycle_txt.html