Virology week6

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Virology - Introduction

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• Viruses cause disease in animals of economic and/or welfare importance

• Diagnose viral disease (clinical/lab tests)• Advise clients control (risk to other animals)

• Animal viruses may pose risk to human health (zoonosis)• Can act as important models for human disease•VIRUSES CAN BE USEFUL

VACCINE DEVELOPMENTGENE THERAPYTOOLS TO INVESTIGATE HOST CELLS

Why are viruses important ?

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What are What are Viruses?Viruses?

•A virus is a non-cellular particle made up of genetic material and protein that can invade living cells.

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Size of VirusesSize of Viruses

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Virus Classification

• International Committee on Taxonomy of Viruses

chemical characteristics, genome type, replication strategy, diseases, vectors, geographical distribution, host speciesnucleotide sequence

order family subfamily genus species/strain/type

-virales -viridae -virinae -virus -virus

mononegavirales

paramyxoviridae

paramyxovirinae morbillivirus canine distemper virus

herpesviridae alphaherpesvirinae varicellovirus

equid herpesvirus 1

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Basic virus structure

Capsid protein

NucleocapsidNaked capsid virus

DNA

RNA

or =+

NucleocapsidLipid

membrane, glycoproteins

Enveloped virus

+

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1. genome

± 3. envelopeenvelope glycoproteins

2. capsid

Virion (virus particle) structure

nucleocapsid

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DNA Viruses

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RNA Viruses

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DNA or RNA [cell genetic material is DNA] ss or ds ss -/+/mixed sense [mRNA= +sense] linear or circular segmented/non-segmented size 2-300 kb(p) [cell genome 3x106kbp]

1- Viral Nucleic Acids

Genetic ‘heritage’ Codes for virus proteins Controls virus protein production - promoters, transcriptional enhancers, splice signals Contains elements necessary for replication and genome packaging

Viral Proteins

Structural Components of capsid (protective coat) and other components of the virion

Non-structural Required for viral replication and interaction with host

Capsid is protein coat that protects the nucleic acid:

physical, chemical, enzymatic attack

Nucleocapsid comprises the capsid and enclosed nucleic acid

facilitates entry into cell and delivery of nucleic acid

exposed to immune system

2- Nucleocapsid

genome

capsid

nucleocapsid

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Viruses come in a variety of

shapes and sizes dictated

by their protein and nucleic acid

composition

- but there are common elements in their architecture due to SYMMETRY

ICOSAHEDRALHELICAL

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Capsid symmetry

Icosahedral Helical

Naked capsid

Enveloped

Lipid

Glycoprotein

Matrix

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Icosahedral (or cubic)

20 faces each face an equilateral triangleaxes of 2-, 3- and 5-fold rotational symmetryCapsomer structure enclosing maximum volume

Foot and mouth disease virus (picornavirus)

Some icosahedral animal viruses are enveloped

adenovirus

herpesvirus

Icosahedral naked capsid viruses

AdenovirusElectron

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Foot and mouth disease virus

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Icosahedral enveloped viruses

Herpes simplex virusElectron micrograph

Herpes simplex virusNucleocapsid cryoEM model

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Helical

Rabies virus (rhabdovirus)

Simple viruses with small genomes use this architecture to provide protection for the genome without the need to encode multiple capsid proteins.

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Helical

All animal viruses with helical symmetry are ENVELOPED

paramyxovirus07/04/23 MASDIANA PADAGA

Helical enveloped viruses

Influneza A virusElectron

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ParamyxovirusElectron

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3 – Virus Envelope Envelopes are LIPID BILAYERS acquired from cellular membranes e.g. endoplasmic reticulum, nuclear membrane, plasma membrane

viral proteins are associated with/inserted into membrane – surface proteins often glycosylated

Adsorption and entry of virus into cells (and exit) -access to target cells-binding to receptors-fusion of envelope with cellular membranes to release genomeInteraction with immune system components- binding of antibody - Targets of immune system

Complex Virus StructuresMost animal viruses fall into three structural classes,

helical capsid (enveloped)icosahedral capsid (nonenveloped)or icosahedral capsid (enveloped)

However, more complex structures do exist e.g. pox viruses

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5 BASIC TYPES OF VIRAL STRUCTURE

HELICAL ENVELOPED HELICAL

ENVELOPED ICOSAHEDRAL

COMPLEX

ICOSAHEDRAL

Adapted from Schaechter et al., Mechanisms of Microbial Disease

nucleocapsid icosahedral nucleocapsid

nucleocapsid

helical nucleocapsid

lipid bilayer

lipid bilayer

glycoprotein spikes= peplomers

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Stability of Viruses

•Non enveloped viruses more ‘hardy’ than enveloped viruses (e.g. foot and mouth disease hardier than influenza virus)

•Different viruses have differential ability to survive

• sensitive to temperature, pH, dessication, lipid solvents, detergents

Most inactivated at >55-60oCDetergents used to disrupt viral envelopes Rotavirus survives pH of stomach

•Clinical sample collection / Diagnostics

Properties of enveloped viruses

• Envelope is sensitive to– Drying– Heat– Detergents– Acid

• Consequences– Must stay wet during

transmission– Transmission in large

droplets and secretions– Cannot survive in the

gastrointestinal tract– Do not need to kill

cells in order to spread– May require both a

humoral and a cellular immune response

Adapted from Murray, P.R. Rosenthal K.S., Pfaller, M.A. (2005) Medical Microbiology, 5th edition, Elsevier Mosby, Philadelphia,

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Properties of naked capsid viruses

• Capsid is resistant to– Drying– Heat– Detergents– Acids– Proteases

• Consequences– Can survive in the

gastrointestinal tract– Retain infectivity on drying– Survive well on

environmental surfaces– Spread easily via fomites– Must kill host cells for

release of mature virus particles

– Humoral antibody response may be sufficient to neutralize infection

Adapted from Murray, P.R. Rosenthal K.S., Pfaller, M.A. (2005) Medical Microbiology, 5th edition, Elsevier Mosby, Philadelphia, PA , Box 6-407/04/23 26MASDIANA PADAGA

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BASIC STEPS IN VIRAL LIFE CYCLE

• ADSORPTION• PENETRATION (injection) of viral DNA or

RNA• UNCOATING AND ECLIPSE• SYNTHESIS OF VIRAL NUCLEIC ACID AND

PROTEIN (REPLICATION)• ASSEMBLY (maturation) of the new viruses• RELEASE of the new viruses into the

environment (cell lyses)07/04/23 MASDIANA PADAGA

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ADSORPTION

• TEMPERATURE INDEPENDENT

• REQUIRES VIRAL ATTACHMENT PROTEIN

• CELLULAR RECEPTORS

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PENETRATION enveloped virus

•FUSION WITH PLASMA MEMBRANE •ENTRY VIA ENDOSOMES

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PENETRATION - ENVELOPED VIRUSES

from Schaechter et al, Mechanisms of Microbial Disease, 3rd ed, 199807/04/23 MASDIANA PADAGA

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PENETRATIONNON-ENVELOPED VIRUSES

entry directly across plasma membrane:

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UNCOATING

• NEED TO MAKE GENOME AVAILABLE

• ONCE UNCOATING OCCURS, ENTER ECLIPSE PHASE

• ECLIPSE PHASE LASTS UNTIL FIRST NEW VIRUS PARTICLE FORMED

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SYNTHESIS OF VIRAL NUCLEIC ACID AND

PROTEIN

• MANY STRATEGIES

• NUCLEIC ACID MAY BE MADE IN NUCLEUS OR CYTOPLASM

• PROTEIN SYNTHESIS IS ALWAYS IN THE CYTOPLASM

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ASSEMBLY AND MATURATION

• NUCLEUS• CYTOPLASM• AT MEMBRANE

RELEASE• LYSIS• BUDDING

THROUGH PLASMA MEMBRANE

• NOT EVERY RELEASED VIRION IS INFECTIOUS

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epithelial cells - adenovirus

uninfected early infection

late infectionslides from CDC

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Bacteriophages

• Bacteriophages ( phages ) are obligate intracellular parasites that multiply inside bacteria by making use of some or all of the host biosynthetic machinery . They are viruses that infect bacteria.

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Composition of Bacteriophage

• Nucleic acid: either DNA or RNA but not both– ds DNA, ss RNA, ss DNA – unusual or modified bases– encode 3-5 gene products to over 100 gene

products

• Protein: function in infection and protect the nucleic acid

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Tail

Tail Fibers

Base Plate

Head

Contractile Sheath

Capsid

DNA

Structure of Bacteriophage

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PrionsPrions•Prions are “infectious proteins”• They are normal body proteins that get converted into an alternate configuration by contact with other prion proteins• They have no DNA or RNA•The main protein involved in human and mammalian prion diseases is called “PrP”07/04/23 MASDIANA PADAGA

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Prion Prion DiseasesDiseases

•Prions form Prions form insoluble insoluble deposits in the braindeposits in the brain•Causes neurons to Causes neurons to rapidly degeneration.rapidly degeneration.•Mad cow disease Mad cow disease (bovine spongiform (bovine spongiform encephalitis: BSE) is an encephalitis: BSE) is an exampleexample•People in New Guinea People in New Guinea used to suffer from used to suffer from kurukuru, which they got , which they got from eating the brains from eating the brains of their enemiesof their enemies07/04/23 MASDIANA PADAGA