Download Viruses and Prions (Chapter 13) Lecture Materials for Amy Warenda

Viruses and Prions
(Chapter 13)
Virus: Latin for “poison”
-discovered as “contagious fluid”
-obligate intracellular pathogen
-contains few enzymes of its own
-must get most enzymes and all biomolecule
building blocks and energy from host cell
Lecture Materials
for
Amy Warenda Czura, Ph.D.
Characteristics of a virus:
1. contains a single type of nucleic acid: either
DNA or RNA but not both
2. has a protein coat (capsid) surrounding the
nucleic acid, some also have a lipid
envelope around the capsid
3. multiply inside living cells by using the
synthesizing machinery of the host cell
4. cause the synthesis of specialized viral
structures that can transfer the viral nucleic
acid to other cells
5. have a specific host range
Suffolk County Community College
Eastern Campus
Primary Source for figures and content:
Tortora, G.J. Microbiology An Introduction 8th, 9th, 10th ed. San Francisco: Pearson
Benjamin Cummings, 2004, 2007, 2010.
Host range = the spectrum of host cells types
the virus can infect
-viruses are usually specific to a single species
(or even strain) of host
-a virus has molecules on its surface that
specifically adhere to some molecule on
the host cell surface, each virus is
specialized to attach to and infect one type
of cell
-in multicellular hosts viruses usually infect
only certain specific cell types in that
species
e.g. HIV: human T helper cells
-host range is determined by the virus
requirements for attachment and entry into
the cell and the availability of of host
factors necessary for viral replication
Amy Warenda Czura, Ph.D.
Viral Size
-must be smaller than the cells they infect:
20-1,000nm in length
-smaller than bacteria (E. coli 1000 x 3000nm)
-too small to be seen by light microscopy
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SCCC BIO244 Chapter 13 Lecture Notes
Viral Structure
Virion = infectious viral particle:
completely assembled with a protein coat
surrounding the nucleic acid
1. Nucleic Acid:
- either RNA or DNA, but not both
- single or double stranded
- linear or circular
- if RNA, it can be plus/sense strand (has
codons) or minus/antisense (need to make
complement sense strand for translation)
-2,000 to 250,000 nucleotides
(E. coli ! 4 million, human ! 3 billion)
2. Capsid = protein coat
-composed of subunits called capsomeres
-some capsids have protein-carbohydrate
pointed projections called pentons
-if pentons are present they are
used for attachment to the
host cell
3. Envelope (not all viruses)
-some viruses have an envelope around the
capsid consisting of lipids, proteins and
carbohydrates (cell membrane like)
-with envelope = enveloped virus
-if a virus does not have an envelope it is
called a non-enveloped virus
-the envelope may be coded for by the virus or
taken from the host cell plasma membrane
-some envelopes have
carbohydrate-protein
complexes called
spikes which are
used for attachment
to the host cell
interesting note:
-Coronavirus (cold) and influenza virus (flu)
have high mutation rate in spike genes
-by changing the spikes, they can evade the
host immune system
-you get infected by colds and the flu over and
over since each one with slightly different
spikes looks completely new to your
immune system
Morphology
The capsid architecture can be distinct and
sometimes identifies a particular virus
1. Helical
-cylindrical capsid
-made up of a helical
structure of capsomeres
with the nucleic acid
wound up inside
e.g. Rabies virus, Ebola virus
http://images.encarta.msn.com/xrefmedia/sharemed/targets/images/pho/
t045/T045376A.jpg
Amy Warenda Czura, Ph.D.
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SCCC BIO244 Chapter 13 Lecture Notes
2. Polyhedral
-most are icosahedrons:
20 equilateral triangle
faces & 12 corners
-may have pentons
3. Enveloped Viruses
-appear spherical due to
the lipid envelope, but
contain a shaped capsid:
Enveloped helical
e.g. influenza virus
Enveloped polyhedral
e.g. Herpes Simplex Virus
-may have spikes
e.g. Adenovirus,
Polio virus
Influenza virus
Poliovirus
http://www.zephyr.dti.ne.jp/~john8tam/main/Library/inf
luenza_site/influenza_virus.jpg
Herpes simplex virus
http://www.bact.wisc.edu/themicrobialworld/hsv1struc.jpg
4. Complex Viruses
-unique shape
e.g. bacteriophage: capsid & accessory
structures
e.g. pox virus: no clear capsid, just several
protein layers around the nucleic acid
Amy Warenda Czura, Ph.D.
Taxonomy
-a viral “species” is a group of viruses sharing
the same genetic information and the same
ecological niche (host range)
-species names are not used; usually viruses
are just given a Genus name (ends in
‘virus’) and a common name
Viruses are grouped into families (names end
in ‘-viridae’) based on:
1) Nucleic acid type
2) Strategy for replication
3) Morphology
Viruses are more commonly identified by their
common name – learning the family
groups has little relevance to disease ID.
e.g.
Family: Herpesviridae
Genus: Simplexvirus
Herpes Simplex Virus 2 (HSV2)
(Also known as HHV-2: human
herpesvirus 2)
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SCCC BIO244 Chapter 13 Lecture Notes
Chart of families – Table 13.2
Note all the various possibilities:
-Single stranded DNA, non-enveloped
-Double stranded DNA, non-enveloped
-Double stranded DNA, enveloped
-Single stranded RNA, plus strand,
non-enveloped
-Single stranded RNA, plus strand,
enveloped
-Single stranded RNA, minus strand,
enveloped
-Single stranded RNA, minus strand,
non-enveloped
-Multiple strand RNA, minus strand,
enveloped
-Double strand RNA, non-enveloped
-Double strand RNA, enveloped
Viruses are more commonly identified by their
common name – learning the family
groups has little relevance to disease ID.
Cultivation of viruses for study:
-viruses must be grown in living cells, usually
their specific host
-viruses cannot be grown in culture media
alone (obligate intracellular parasites)
enveloped
nonenveloped
Three ways to grow animal viruses in lab:
1. Animal Models
-infect live animals with the virus
positive aspects:
-provides the potential to study the
complete infection and disease process
caused by the virus in a living animal and
the immune response to the infection which
allows for study of drug treatments and
preventative vaccines
negative aspects:
-only if the virus will infect an animal:
many human viruses only infect humans
thus no animal model exists
enveloped
Double
strand
RNA
enveloped
Amy Warenda Czura, Ph.D.
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SCCC BIO244 Chapter 13 Lecture Notes
e.g. There are no exact models for HIV/AIDS:
it infects and causes disease only in
humans
HIV and AIDS can be studied only in
existing human patients
All drug treatments and vaccines also have
to be tested using humans
negative aspects:
-no ability to study infection or disease
processes or immune response or drug
therapies or vaccines: wrong cells & not
a live functioning animal
3. Cell Culture
-grow the virus in living cells in a Petri dish
-must culture the cells before growing the
virus in them
A. Primary cell lines :
-direct tissue sample grown in culture
positive aspects:
-use the cell type the virus normally
infects: can study normal infection
process
negative aspects:
-primary cells die off quickly: new cells
must be harvested from donors
-cannot study disease process or
immune response: no live animal
2. Embryonated Eggs
-fertilized egg, usually chicken, with a
growing embryo
-used as a "container" to grow virus for
vaccine production
or molecular
studies
positive aspects:
-most viruses
will grow
abundantly in
some part of
the egg
B. Continuous cell lines:
-immortalized, transformed cancer cells
that grow and divide forever
Viral Identification
-viruses are so small they can only be seen by
electron microscopy
-some are so distinct they can be recognized
on sight
-others are identified by:
-symptoms of the disease
-cytopathic effects
-serological methods
(ELISA, Western blot)
-sequencing of DNA or RNA
(RFLP/DNA fingerprinting, PCR)
no contact inhibition or monolayer, no death
positive aspects:
-cells can be grown infinitely in lab: can
grown large quantities of both cells
and virus forever
negative aspects:
-cancer cells are not normal: infection
process being observed may not be
normal
-cannot study disease process or
immune reactions
Amy Warenda Czura, Ph.D.
Viral Multiplication
-replication must occur in a host cell
-the viral genome codes for viral structural
components and a few viral enzymes
needed for processing the viral nucleic acid
-everything else is supplied by the host:
ribosomes, tRNA, nucleotides, amino
acids, energy, etc.
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SCCC BIO244 Chapter 13 Lecture Notes
Bacteriophages
-viruses that infect a specific bacteria
-serves as a well studied example of a virus
life cycle (easy to grow in lab in bacteria)
The Lytic Cycle
1. Attachment
-the phage contacts a bacterium and uses the
tail fibers to attach to proteins on the
bacterial cell wall
Two possible types of infections cycle:
1. Lytic cycle
-ends with the lysis and death of the host
bacterial cell
2. Lysogenic cycle
-host cell remains alive, but carries the
virus in its genome
(all viruses have an attachment site on their
surface which binds to a receptor site on
their host cell
in this case the attachment site = tail fibers
receptor site = specific cell wall proteins)
2. Penetration/Entry
-the phage injects its DNA into the bacterium:
-the phage tail releases lysozyme to break
down the bacterial cell wall
-the sheath contracts to drive the tail core
through the weakened cell wall and
plasma membrane
-the DNA is injected into the bacterium
through the tail core
4. Maturation
-the bacteriophage DNA and capsid
spontaneously assemble into complete
virions
3. Biosynthesis
-synthesis of the viral nucleic acid and protein
occur:
-the virus degrades the host DNA and/or
disrupts host protein synthesis
-the virus then directs viral nucleic acid
replication and transcription and
translation of viral genes
-this results in a pool of viral genomes and
capsid parts
Amy Warenda Czura, Ph.D.
5. Release
-virions leave the bacteria:
-lysozyme encoded by viral genes
causes the cell wall to break down
-the bacteria lyses releasing the virions
Cycle then repeats with new phages
(Phage therapy: using bacteriophage to treat
bacterial infections - experimental)
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SCCC BIO244 Chapter 13 Lecture Notes
The Lysogenic Cycle
-the lysogenic phage infects the cell, but
remains inactive in a stage called lysogeny
1. the phage attaches to the host cell and
injects its DNA
2. the phage genome circularizes
-at this point, the phage could begin a normal
lytic cycle or it can begin the lysogenic
cycle / lysogeny
3. the phage DNA integrates into the
bacterial chromosome
-the phage DNA is now called a prophage
-it synthesizes viral repressor proteins to keep
the rest of its genome inactive (suppress
virion production)
4. replication of the bacterial chromosome
replicates the prophage as well and thus all
progeny cells will also be infected with the
lysogenic virus
5. occasionally, the prophage will recombine
back out of the bacterial chromosome and
initiate a lytic cycle
Multiplication Of Animal Viruses
Three Results of Lysogeny:
1. The host cell is “immune” to reinfection by
the same phage (but not different types)
2. Phage conversion: the host cell may
exhibit new properties due to the
integration of prophage DNA
e.g. cholera toxin (diarrhea) of Vibrio
cholerae is due to a phage gene
3. Makes specialized transduction possible:
-viral genome can
move one of the
host’s genes to a
new bacterium
when it goes
lytic from first
cell ! lysogenic
in second cell
e.g. gene for
galactose
metabolism
2. Penetration:
1
1. Attachment: virus
attachment sites bind
host receptor proteins
on plasma membrane
A. Non-enveloped viruses are endocytosed into a vesicle
B. Enveloped viruses enter by fusion
2A
2B
3. Uncoating: enzymes
degrade capsid releasing
nucleic acid
3
A. Non-enveloped:
viral enzymes for
escape from vesicle
B. Enveloped: host
cytoplasm enzymes
4A
4. Biosynthesis
4B
A. DNA viruses: DNA
replicated in nucleus, protein
made in cytoplasm, virion
assembly in nucleus
B. RNA viruses: RNA and
protein made in cytoplasm,
virions assemble in cytoplasm
C. Retroviruses: reverse
transcribe dsRNA genome to
dsDNA, DNA integrated into
host genome as provirus,
provirus remains latent or is
expressed to create virions in the
cytoplasm
5. Maturation and Release:
capsule and nucleic acid
assemble into virion
4C
5A
5B
A. Enveloped: buds out of
cell taking membrane, cell
survives
B. Non-enveloped: ruptures
out of cell membrane, cell
dies.
Amy Warenda Czura, Ph.D.
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SCCC BIO244 Chapter 13 Lecture Notes
Multiplication of Animal Viruses
1. Attachment
-virus binds to proteins or
glycoproteins on the
host cell plasma membrane
by its attachment sites:
-pentons or spikes or capsid
proteins: no tail fibers
2. Penetration / Entry
-non-enveloped viruses are endocytosed by
the host cell into a vesicle (endosome)
3. Uncoating
uncoating = separation of the
viral nucleic acid from its
protein coat
-Non-enveloped: viral or host enzymes
digest the capsid, viral enzymes allow the
genetic material to escape the vesicle
-Enveloped: done by host enzymes in the host
cytoplasm (proteases)
4. Biosynthesis
A. Biosynthesis of DNA viruses
-viral DNA is replicated in the
host nucleus
-viral proteins are made in the cytoplasm
-viral proteins migrate to the
nucleus to join the DNA and
assemble into virions
-the virions are then transported
through the host endoplasmic reticulum for
release
-enveloped viruses can enter by
fusion: the envelope and cell
membrane fuse and the
capsid is released into the
cytoplasm
B. Biosynthesis of RNA viruses
-both the viral RNA and proteins
are synthesized in the
cytoplasm
-virions assemble in the cytoplasm
C. Biosynthesis of Retroviruses
-retroviruses have a ds RNA genome and
make the enzyme reverse transcriptase
-they synthesize a ds DNA copy of their RNA
using reverse transcriptase and incorporate
the DNA into the host cell genome as a
provirus
5. Maturation and Release
-the viral capsid assembles spontaneously
around the viral nucleic acid
A. if a virus is enveloped,
viral envelope
proteins will be
deposited in the host
cell membrane and
the virion will bud
out of the host cell
taking an envelope
of membrane with it
-the host cell can survive if not too many
virions are released at once
B. if the virus is not enveloped it usually
ruptures out of the
membrane causing
lysis of the host cell
-the provirus can then remain latent in the
genome (protected from host immune
system), or the genes can be expressed to
create virions (e.g. HIV: can remain latent
20+ years before it starts replicating)
Amy Warenda Czura, Ph.D.
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SCCC BIO244 Chapter 13 Lecture Notes
Latency vs. Active Infection
for integrating proviruses:
-some viral infections spend long periods as
latent infections and activate a replication
cycle only on rare occasions
e.g. Herpes
-many integrating viruses activate a persistent
/ chronic replication cycle and will
continue to produce virions until either the
host immune system neutralizes it or until
the virus kills the host
e.g. HIV
A viral cause of cancer in humans was hard to
recognize because:
1. most virions infect cells without inducing
cancer (cold, flu, mumps, measles, etc.)
2. cancer often develops long after the viral
infection (hard to link cause and effect)
3. most cancers are not contagious (prostate,
colon, breast, brain tumor, etc.)
Cell Transformation
normal cells ! tumor cells
-viruses that integrate into the host genome
(retroviruses and some DNA viruses)
change the genetic material of the host
which has the potential to cause cancer
-segments of the DNA where cancer causing
alterations occur are called oncogenes
-oncogene = some kind of cell regulatory or
growth control gene
-an oncogenic virus / oncovirus is capable of
inducing tumors in animals
Viruses and Cancer
-infectious cancer was first observed in
chickens and mice (early 1900s)
-Chicken leukemia was passed to healthy
chickens by cell-free filtrate from sick
chickens
-Mouse mammary gland tumors were
transmitted from mother to offspring in milk
-when the virus integrates, an oncogene is
expressed resulting in a transformed cell
-transformed cells grow uncontrolled: they do
not respect contact inhibition and lead to
tumor formation
-oncogenic viruses are either DNA viruses or
retroviruses that integrate into a
chromosome. They carry a whole
oncogene (some kind of regulatory or
growth gene derived from the animal host)
or carry just a promoter that turns on an
oncogene in the host cell
e.g. Human Papilloma Virus
Cervical cancer
Hepatitis B Virus
Liver cancer
Prions (Proteinaceous infectious particle)
-infectious protein PrPSc
-it is a protein molecule that is misfolded and
can cause misfolding of normal proteins
-results in spongiform encephalopathies
e.g. Mad Cow disease, Sheep scrapie
Creutzfeldt-Jacob Disease, Kuru
Gerstmann-Straussler-Scheinker
Fatal Familial Insomnia
-prion protein in the
brain converts
normal proteins
PrPC into prion
proteins PrPSc
-prion proteins cause
plaques and holes
in neural tissue
resulting in
progressive loss
of function and
eventual death
(Oncolytic viruses: viruses that will naturally
infect and lyse tumor cells. Currently
being studied as possible cancer treatment)
Amy Warenda Czura, Ph.D.
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SCCC BIO244 Chapter 13 Lecture Notes