Transcript Morphology_and_physiology_of_viruses

Chair of Microbiology, Virology, and Immunology
MORPHOLOGY
AND PHYSIOLOGY OF
VIRUSES
Welcome to Vira
Kingdom!
Virus:
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a noncellular small obligate intracellular
parasites (genetic element)
contains either a RNA or DNA genome
surrounded by a protective, virus-coded protein
coat
has no its own biosynthetic machinery for energy
generation and protein synthesis
For propagation virus depends on specialized
host cells
infects a cell for its own replication, it has an
extracellular state.
Virus:
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A virus particle containing nucleic acid
surrounded by protein and other macromolecular
components is called virion.
Viruses have a heavy dependence on host-cell
structural and metabolic components.
Viruses can confer important new properties on
their host cell.
Viruses may be non productive, latent,
oncogenic, chronic or lytic.
The main purpose of a virus is to
deliver its genome into the host cell to
allow its expression (transcription and
translation) by the host cell.
Some viruses
infected human
body
Viruses
How Viruses are classified
Main criteria presently used are:
 Acid type: RNA or DNA; single-stranded or double-stranded; strategy of replication.
 Size and morphology, including type of symmetry, number of capsomeres, and
presence of membranes.
 Presence of specific enzymes, particularly RNA and DNA polymerases, and
neuraminidase
 Susceptibility to physical and chemical agents, especially ether.
 Immunologic properties.
 Natural methods of transmission.
 Host, tissue, and cell tropisms.
 Pathology; inclusion body formation.
 Symptomatology.
Classification of Viruses
By genome structure (DNA or RNA, single or
double stranded, particle structure, presence or
absence of envelope, chemical and
immunological properties - most widely used)
 By symptomatology - oldest
 By routes of transmission
 Plant, animal or bacterial
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Classification by Symptomatology
 Dermatotropic
- lesions of skin and mucous
membranes
 cold
sores, shingles, warts
 Pneumotropic
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flue, parainfluenza, respiratory syncytial viral pneumonia,
 Neurotropic
- CNS
 encephalitis,
 Viscerotropic
others?
- organs
 hepatitis, infectious parotitis
 Generalized
Classification According to Routes of Transmission
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Respiratory transmission
 Influenza A virus
Faecal-oral transmission
 Enterovirus
Blood-borne transmission
 Hepatitis B virus
Sexual Transmission
 HIV
Animal or insect vectors
 Rabies virus, Western equine encephalitis,
yellow fever, West Nile fever, dengue fever
Classification according to Biologic,
Chemical, and Physical Properties
DNA-Containing
Viruses
RNA-Containing
Hepadnaviridae
Viruses
Picornaviridae Paramyxoviruses
Parvoviridae
Caliciviridae
Orthomyxoviruses
Papovaviridae
Togaviruses
Bunyaviridae
Adenoviridae
Flaviviridae
Arenaviridae
Herpesviridae
Coronaviridae Reoviridae
Poxviridae
Rhabdoviridae Birnaviridae
Iridovoridae
Filoviridae
Retroviridae
Classification of viruses
Viral morphological
types
Structure of viruses
There are two major structures of viruses called the
naked nucleocapsid virus (left) and the enveloped
virus (right)
CAPSID FUNCTIONS
PACKAGING OR CONDENSATION OF
NUCLEIC ACID
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PROTECTION OF NUCLEIC ACID
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TRANSPORT NUCLEIC ACID FROM CELL TO CELL
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PROVIDES SPECIFICITY FOR ATTACHMENT
Virus Specific Enzymes
 Some viruses have enzymes for
 Penetration of the host cell
 ex. Haemagglutinin, Neuraminidase, bacteriophages have
Lysozyme for penetration of bacterial cell walls
 Replication of viral nucleic acid
 ex. Retroviruses carry Reverse transcriptase
Viral structure
Definitions:
Virion - physical particle of the virus
Core - nucleic acid and tightly associated proteins within the virion
Capsid - protein shell around NA or core
Capsomere - protein subunit making up the capsid
Nucleocapsid - core and capsid
Envelope - lipid membrane found on some viruses,
often derived by budding from infected cells.
Peplomer - ("spike”) - morphological unit projecting
from the envelope or surface of a naked virion
VIRUS
STRUCTURE
Capsid symmetry
Helical
Complex
Icosahedral
Helical Capsid
In the replication of viruses with
helical symmetry, identical
protein subunits (protomers) selfassemble into a helical array
surrounding the nucleic acid,
which follows a similar spiral
path. Such nucleocapsids form
rigid, highly elongated rods or
flexible filaments
This category includes many of the best known
human pathogens, e.g. influenza virus, mumps &
measles viruses, & Rabies virus
Tobacco mosaic diseases virus
ICOSAHEDRAL CAPSID
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polyhedron with 20 equilateral faces and 12 vertices
 capsomers
 ring
or knob-shaped units made of 5 or 6 protomers
 pentamers (pentons) – 5 subunit capsomers
 hexamers (hexons) – 6 subunit capsomers
VIRUSES WITH CAPSIDS OF COMPLEX SYMMETRY
 many viruses do not fit into helical or icosahedral symmetry
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Examples: poxviruses and large bacteriophages
Vaccinia virus
T4 phage
Binal symetry: head icosahedron, tail helical.
Tail fibers and sheath used for binding and
pins for injecting genome
200x400x250 nm, enveloped virus DNA
With double membrane envelope.
Examples of medically important DNA viruses.
Examples of medically important RNA viruses.
Chemical composition of viruses
 Nucleic acids (1-40 %)
 Proteins
 Lipids
 Glycolipids
 Glycoproteins
Viral proteins (70-90 %):
 structural (capside, envelope, matrix, core, associated with nucleic
acid)
 non-structural
 Structural proteins are in virion in its extracellular state.
Functions:
 protection of nucleic acid,
 interaction with the membrane of susceptible cell
 provide viral penetration into the cell,
 have RNA- and DNA-polymerase activity etc.
 Non-structural proteins are absent in virion in its extracellular state, but
they are formed during viral reproduction
Functions:
 provide regulation of viral genome expression,
 are viral precursor proteins and can inhibit cell biosynthesis.
Lipids (15-35 %) are in enveloped viruses in their envelope
Functions:
 Stabilization of viral shell,
 Protection of inner virion shells and nucleic acid,
 Deproteinization of virions
Carbohydrates molecules are in glycoproteins and
glycolipids (3,5-9 %).
They protect these molecules from cell proteases action
Viral reproduction
Feature: disjunctive way of reproduction
Synthesis of viral genome and viral proteins is
interrupted in the space and in the time:
 Nucleic acids are synthesized in the cell nucleus and proteins – in the
cytoplasm
Note:
 Each virus requires different strategy depending on its nucleic acid
 DNA viruses often enter the nucleus
 RNA viruses typically replicate in cytoplasm
 Must consider:
 What serves as template for replication and how mRNA is transcribed
adsorption
penetration
uncoating
synthesis
Transcription
of viral genes
DNA replication
maturation
Assembly
Replication of viruses
budding
Translation
Proteins
Attachment of viruses
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Chemical attraction
 Animal
viruses do not have tails or tail fibers
Have glycoprotein spikes or other attachment molecules that
mediate attachment
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Penetration
Penetration of animal viruses occur
by direct
penetration (a),
fusion (b) between the viral
envelope and the the host cell
membrane or endocytosis (c) clathrin-coated pits
Uncoating and synthesis
of viruses rely on the
host’s metabolic
systems.
Types of Viral Genomes and Their Replication
Two events critical to viral infection:
 The production of virus structural proteins and enzymes
 Replication of the viral genome (dsDNA, ssDNA, dsRNA, ssRNA)
dsDNA Viruses
Contain dsDNA genome
Most dsDNA viruses replicate their genomes in the nucleus of
the cell
Use host’s DNA and RNA synthesizing machinery
Adapted from D. R. Harper. Molecular Virology, Second Edition. BIOS Scientific Publishers, 1999.
ssDNA Viruses
Contain ssDNA genomes
Adapted from D. R. Harper. Molecular Virology, Second Edition. BIOS Scientific Publishers, 1999.
ss/dsDNA Viruses (Using an RNA intermediate)
Virus carries it’s
own reverse
transcriptase
 dsDNA enters
the nucleus,
forms an episome
 Virus does not
encode an
integrase gene
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dsRNA viruses
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Contain dsRNA segmented genomes
Viral polymerase
Adapted from D. R. Harper. Molecular Virology, Second Edition. BIOS Scientific Publishers, 1999.
+ssRNA Viruses
Contain +ssRNA
nonsegmented genomes
 The RNA in the virus particle
functions as mRNA
 Viral mRNA is recognized by
cellular translational machinery
 Contain a viral RNAdependent RNA polymerase in
order to replicate viral genomes
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Adapted from D. R. Harper. Molecular Virology, Second Edition. BIOS Scientific Publishers, 1999.
-ssRNA viruses
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Contain -ssRNA segmented or nonsegmented genomes
Contain a viral RNA-dependent RNA polymerase gene
Adapted from D. R. Harper. Molecular Virology, Second Edition. BIOS Scientific Publishers, 1999.
Viruses with ssRNA Genomes That Use a
dsDNA Intermediate to Replicate
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Unique biology
Viral genome is reverse transcribed and integrated as a cDNA
into the host’s chromosome
Assembly
All of the components of the virus assembled into a particle
 Occurs when an appropriate concentration of virus proteins
and genomic nucleic acids are reached and localized at
specific sites within the infected cell
 Some particles self-assemble
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Viral release from cell
Cell lysis, “burst”
Simple viruses
Budding
Enveloped viruses
> 4000 (poxviruses) - >100 000 (polyoviruse)
Types of viral infection
Productive
Аbortive
Virogeny
Viruses caused the virogeny are temperately ones
Measuring the Size of Viruses
Filtration Through Membranes of
Graded Porosity:
 Sedimentation
Ultracentrifuge
in
the
Direct Observation in the Electron
Microscope:
 Ionizing Radiation:
Comparative Measurements:
Staphylococcus has a diameter of about 1000 nm;
Bacteriophages vary in size (10-100 nm).
Representative protein molecules range in diameter from
serum albumin (5 nm) and globulin (7 nm) to certain
hemocyanins (23 nm).
Cultivation of viruses
Laboratory
animals
Chick embryos
Tissues
cultures
Bacteria (for
bacteriophages)
The early developing bird embryo contains a protective case,
providing an ideal environment for viral propagation.
POCKS on Chorio-Allantoic
Membrane of Chick Embryo:
Vaccinia virus (left) HSV-1 (right)
1949 - Enders, Weller, & Robinson - grow polio in non-neural cells
Cells can grow as monolayers or in suspension cultures: glass (in vitro) and
plastic
Types of cell culture
Primary
Diploid
Continuous
Tissue Culture
Primary cell lines are derived directly from tissue after treatment with
trypsin. Die after a few generations - Limited to 5-20 cell divisions
Fibroblasts of human embryo, Rhesus monkey kidney, Chiken fibroblasts
Continuous cell lines - immortal cells derived from tumors or mutagenesis of
primary cells.
HeLa (Henrietta Lacks), HEp-2 (Hu. Epithelial), BHK (Baby Hamster Kidney),
Detroit-6.
Disadvantages:
 May not resemble the original cell of origin
 Less differentiated - lost morphological and biochemical features
 Can be tumorigenic
Diploid cell lines - homogeneous population of a single type (fibroblastliked cell). Typically derived from tumors. Can divide up to 100 times.
Remain diploid
WI-38, MRC-5, MRC-9, IMR-90
Morphologic and Structural Effects
 Cytocidal effect, or necrosis
of cells
Normal cells
 Hemadsorption
 Formation of syncytia, or polykaryocytes
Giant multynucleated skin cells
(simple herpes virus)
Infected cells
Morphologic and Structural Effects
 Inclusion Body Formation granules in cytoplasm and/or nucleus
of infected cells
Large cells with typical nuclear
“owl’s eye” inclusions
 Cell rounding
 Morphologic transformation by an
oncogenic virus
Normal cells
Infected cells
Cell culture obtaining
Inoculation of laboratory animal
Intracerebrally
Intraperitoneally
Intramuscularly
Intravenously
Intranasally
Bacteriophage
Bacterial virus
 Multiplication is similar to animal viruses except
for the penetration (inject DNA), release (lyses)
and prophage (lysogeny) stages
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T-even bacteriophage penetrate the host cell by specifically binding and
injecting their DNA into the host cell
After viral multiplication inside the host cell, viral enzymes will weaken the
host cell membrane, rupture the cell (lyses), and release numerous virions
A weakened bacterial cell, crowed with viruses.
Prions (Proteinaceous infectious particles)
Protein particle with no nucleic acid, no envelope, no
capsid
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The nature of these agents is still controversial
 Presently thought to be unusual proteins lacking nucleic acid and more
resistant to proteases than normal proteins
 They resist inactivation by UV, ionizing radiation, formaldehyde and
heat
 Have amino acid sequence identical to normal brain protein, PrPc, (gene
on chromosome 20) but folded differently (designated PrPs)
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Current theory is that prion protein enters brain tissue and binds to normal
pre-prion protein and converts it into a prion protein form
They increase in number during long incubation periods
 Much smaller that the smallest virus
 In 1997 Dr. Stanley Prusiner received a Nobel Prize for work on these
agents
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Hypothesis of how a protein can be infectious
Prions
 Spongiform encephalopathies
 slow neurological diseases, fatal
 spongelike appearance of brain tissue
as neurons degenerate
 main signs are dementia and
unsteadiness
 Animal diseases
 BSE - bovine spongiform
encephalopathy
 i.e. mad cow disease
 chronic wasting disease (in mules,
deer, elk)
 mink encepahlopathy
 scrapie in sheep
Human Prion Diseases
Kuru
 Creutzfeldt-Jakob disease (CJD)
 Gerstmann-Straussler-Scheinker disease (GSS)
 Fatal familial insomnia (FFI)
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Kuru (means shivering or trembling)
Disease limited to the Fore tribe of New Guinea who practiced cannibalism
Congestion of blood vessels.
Degeneration of cerebral cortex
in long term cases
Observe vacuoles or holes of Kuru
infected brain
Creutzfeldt-Jakob Disease
The human variant of mad cow disease
 Transmission between humans is unusual
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 injection,
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transplantation (e.g., corneas)
Symptoms
 progressive dementia, muscle wasting, unsteadiness
Possible genetic link because the disease runs in families
Viroids
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Much smaller than viruses
Single stranded RNA, no capsid
 300-400
nucleotides
 1/10 the size of the smallest infectious viral RNA
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Cause plant diseases
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potato spindle tuber (PSTV)
chrysanthemum stunt
cucumber pale fruit
Hopstunt
Tomatoes pathogens
Potato Spindle Tuber Viroid (PSTV)