group v (baltimore) viral classification

BALTIMORE CLASS V

Kavhumbura Gamuchirai MNgara Tanyaradzwa R

PRESENTATION OUTLINE

• Background

• Genome features

• Genome replication

• Examples of class v

• Rhabdovirus as a specific example

• Lifecycle of rhabdovirus

• Replication and pathogenicity

• References

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Classification and Taxonomy• the grouping of viruses into an

ordered system that indicates relationships

Baltimore Classification• Based on genome composition and

structure• allows you to: • 1) deduce the basic steps that must take

place to produce mRNA• 2) simplifies comprehension of the life

cycle of virus

Baltimore Classification

Group V• carry their genetic material in the form

of negative-sense single stranded RNA

• of the order Mononegavirales

• Families: Filoviridae, Paramyxoviridae, Bornaviridae and Rhabdoviridae

• infecting vertebrates, arthropods, amoebae, and plants

• Associated with emerging diseases like Ebola, Marburg

Group V

• RNA is not infectious, viruses in this group encode their own polymerase (RNA dependent RNA polymerase [RDRP]

• first a leader RNA is synthesized, which is followed by sequential transcription of the genes in the 3’ to 5’ order to yield individual mRNAs by a stop-start mechanism guided by the conserved gene-start and gene-end signals.

• The genome encodes for 5 genes in the following order.

• 3’ –N-P-M-G-L- 5’ N-

• Nucleocapsid protein

• P- Phosphoprotein- cofactor of the viral polymerase

• M- Inner virion protein/ helps in budding of the virion.

• G- Glycoprotein that assists in making virion spikes

• L- Large protein that represents RNA dependent RNA polymerase and helps in transcription and replication

Genome features• Linear non-segmented negative sense

RNA genome

• Organization of genome- 3'-Leader-Virion core- Surface proteins-Polymerase-Trailer 5'.

• Helical nucleocapsid contains the RNA dependent RNA polymerase.

• The leader RNA is neither capped nor polyadenylated and is not functional as mRNA.

Genome Features

• Replication occurs when the polymerase complex ignores the transcription stop signals at the 3’ end of each gene and a full-length positive-sense anti-genome is synthesized.

• Transcription at the gene-start site is not perfect, which leads to a gradient of mRNA abundance that decreases according to the distance from the 3’ end of the genome.

Gradient of mRNA abundance from 3’ end towards 5’ end

Replicative Cycle

• As obligate intracellular parasites, Virus must enter and replicate in living cells in order to “reproduce” themselves.

• This “growth cycle” involves specific attachment of virus, penetration and uncoating, nucleic acid transcription, protein synthesis, maturation and assembly of the virions and their subsequent release from the cell by budding or lysis

Viral Life Cycle

• Adsorption

• Penetration

• Uncoating and eclipse

• Synthesis of viral nucliec acid and protein

• Assembly (maturation)

• Release

Adsorption

• TEMPERATURE INDEPENDENT

• REQUIRES VIRAL ATTACHMENT PROTEIN

• CELLULAR RECEPTORS

Penetration

ENVELOPED VIRUSES•Fusion with plasma membrane

•Entry via endosomes

NON-ENVELOPED VIRUSES•entry directly across plasma membrane

Uncoating• Need to make genome available

• Once uncoating occurs, enter eclipse phase

• Eclipse phase lasts until first new virus particle formed

Synthesis of nucliec acid and proteins

• Many strategies• Nucleic acid may be made in the nucleus

or cytoplasm• Protein synthesis is always in the

cytoplasm

Assembly and Maturation• Nucleus• Cytoplasm • At membrane

Release• Lysis

• Budding through plasma membrane

Examples

Nonsegmented genomes:-•Rhadboviruses (rabies)

•Paramoxyviruses

Segmented genomes:-•Orthomyxoviruses (influenza)

•Bunyaviruses

•Arena viruses

Rabies virus (Rhabdoviridae)• Rabies is a zoonotic disease• Responsible for 55 000 deaths in Africa

and Asia (yearly)• caused by the bite of a rabid dog/ animal• Present in the saliva of the infected animal

Rabies virus ( Rhabdoviridae)

Bullet shaped structure

Virion Properties• Contain linear, single stranded and

negative sense RNA genome.• Virions are 45-100nm in diameter, 100-

430nm long.• virion has a cylindrical nucleocapsid

surrounded by an envelope with large glycoprotein spikes.

• encode for their own RNA polymerase (RNA dependent RNA polymerase).

Replication• Replication takes place in cytoplasm• Virus entry is by receptor mediated

endocytosis.• Acetylcholine is the receptor• pH dependent fusion of virus envelope

with endosomal membrane. • As a result of fusion, the nucleocapsid is

released into the cytoplasm .• The first step of replication involves mRNA

transcription from genomic RNA using RDRP.

Replication cntd• For successful replication a large amount

of nucleoprotein (N) and phosphoprotein (P) should be expressed.

• Switching of transcription to positive sense antigenome occurs after a threshold amount of N and P, which are then further used as a template for synthesis of negative stranded genomic RNA.

Replication C’ntd• There is a single promoter site at the 3’

end of the viral genome where the polymerase attaches to the genomic RNA template and moves along the viral RNA.

• While moving it hits with start – stop signals at both the ends of the viral genes.

• Due to this only a small fraction undergoes continuous

• transcription process and hence this phenomenon is also known as attenuated transcription

Replication C’ntd• more mRNA is produced towards the

genes that are located at the 3’ end• *hence producing a gradient of mRNA

in the order of N>P>M>G>L.• As a result of the mRNA gradient, large

amount of structural protein such as nucleocapsid protein is produced as compared to L protein

Rabies Virus replication cycle

PATHOGENICITY OF THE VIRUS

• Rabies virus transmitted through the bite of an infected mammal.

• The virus may enter the peripheral nervous system directly, or may replicate in muscle tissue after entering the host

*remaining at or near the site of introduction for most of the incubation period.

PATHOGENESIS C’ntd• Virus may enter the peripheral

nervous system via the neuromuscular junctions, and moves rapidly centripetally to the central nervous system for replication

- symptoms may develop shortly thereafter.

• The virus then begins to pass centrifugally to many tissues and organs, such as the salivary glands.

Pathogenesis C’ntd

• In the nervous system the virus is formed by budding in various membranes and glands.

• Salivary glands help the virus to bud on plasma membrane and release it in very high concentrations through the saliva

• Death usually occurs due to respiratory arrest.

Progression of rabies virus through neurons

in body:

Orthomyxoviridae (segmented)

• contains viruses of single stranded segmented RNA genome (6-8 segments)

• the most important members of this family which includes influenza virus A, B, and C.

• millions of people have been killed.

Virion Properties

• Orthomyxovirus virions are pleomorphic in shape and around 80–120 nm in diameter

• Contain RNA segments ranging from 8-6

• Genome is single stranded negative sense RNA of 10 – 14Kb in size

• contain surface glycoproteins over the lipid envelope: hemagglutinin protein (H) and neuraminidase protein (N).

Schematic representation of an influenza virus:

Bird Flu

• also known as avian influenza (flu infection in birds).

• The virus caused havoc in the human population when a bird infecting virus mutated to infect humans.

• The first case was reported in Hong Kong in 1997 and that was known as H5N1 (avian influenza A virus)

Viral Replication

• Influenza virus enters the cell after binding to sialic acid receptor present on the cell membrane.

• Different cells of the body contain different sialic acid receptor types which largely determine the host range of these viruses. The types of sialic acid receptor in the epithelium determines the tropism of human as well as avian influenza viruses.

Viral Replication

• Virus enters the cell by receptor mediated endocytosis and uncoats under the low pH condition of endosome.

• RNA synthesis of influenza virus takes place in the nucleus of the cell. Nucleoprotein of influenza virus contains nuclear localization signals (NLS) that help in transportation of ribonucleoprotein complex into the nucleus.

Viral Replication

• Anti sense RNA serves as a template for the synthesis of the sense strand

• undergo splicing phenomena to produce two proteins from one gene such as influenza virus A uses gene segment 7 to produce M1 and M2 protein.

• Viral protein synthesis occurs in the cytoplasm

• maturation takes place in endoplasmic reticulum and Golgi apparatus.

Viral replication

• Progeny virus is released by budding through the plasma membrane.

Schematic representation of an influenza

virus replication cycle:

Pathogenesis

• enters the host via aerosol it replicates in the epithelial cells of upper and lower respiratory tract.

• cause damages in the respiratory epithelium and alveoli

• The alveolar spaces are filled with fibrinous exudates and inflammatory cells.

• Vascular congestions and proliferation of fibroblast also occurs

Pathogenesis

• Varied degree of hepatic damage: as the virus also replicates in the intestine of birds.

• necrosis of different visceral organs, and often succumb to death following few days of infection