User:Miacocca/sandbox

Topic: Ebola Virus Disease

Genetics
Ebolaviruses contain negative sense, single-stranded RNA genomes. Ebolavirus genomes are approximately 19 kilobase pairs long and contain seven genes in the order 3'-UTR-NP-VP35-VP40-GP/sGP-VP30-VP24-L-5'-UTR. The genomes of the five different ebolaviruses (BDBV, EBOV, RESTV, SUDV and TAFV) differ slightly in sequence and the number and location of gene overlaps. As all filoviruses, ebolavirions are filamentous particles that may appear in the shape of a shepherd's crook, of a "U" or of a "6," and they may be coiled, toroid or branched. In general, ebolavirions are 80 nanometers (nm) in width and may be as long as 14,000 nm.

The genes NP, VP35, VP30, and L code for proteins that make up the nucleocapsid. This is the inner most part of the virus, where the -ssRNA genome is found wrapped around the nucleoprotein (NP) and the minor nucleoprotein (VP30) in a helical fashion. Bound to the -ssRNA genome is the RNA polymerase cofactor (VP35) and the RNA polymerase (L). The nucleocapsid is then surrounded by the matrix proteins encoded by VP40 and VP24. Lastly, the surface glycoprotein encoded by GP makes up the outside of the virus. Through an RNA editing event, the GP open reading frame also codes for sGP, which is a soluble glycoprotein that is secreted from the virus. Therefore, the ebolavirus genome consists of 7 genes that produce 8 protein products, being 7 structural proteins and 1 non-structural protein (sGP).

Transcription

Ebolaviruses must produce their own RNA polymerases called RNA-dependent RNA polymerases, in order for transcription to take place. This is because the eukaryotic hosts of the virus (such as bats or humans) have DNA genomes, and thus only carry the appropriate DNA-dependent RNA polymerases. During transcription, the RNA-dependent RNA polymerase creates 8 monocistronic (independent) +ssmRNA’s. These +ssmRNA’s resemble host mRNA species, and can now be recognized and translated into proteins by the host's own ribosomal machinery.

Replication

Replication of the viral genome is also done by the same RNA-dependent RNA polymerase. The RNA-dependent RNA polymerase first copies the –ssRNA genome into a full-length positive-strand ssRNA species called the antigenome. The full-length antigenome is then transcribed back into -ssRNA virus progeny.

Life Cycle

Ebolavirus life cycle begins with a virion attaching to specific cell-surface receptors, followed by fusion of the virion envelope with cellular membranes and the concomitant release of the virus nucleocapsid into the cytosol. Ebolavirus' structural glycoprotein (GP) is responsible for the virus' ability to bind to and infect targeted cells. The viral RNA-dependent RNA polymerase, partially uncoats the nucleocapsid and transcribes the genes into positive-strand mRNAs, which are then translated into structural and nonstructural proteins. The most abundant protein produced is the nucleoprotein, whose concentration in the host cell determines when the RNA-dependent RNA polymerase switches from gene transcription to genome replication. Newly synthesized structural proteins and genomes self-assemble and accumulate near the inside of the cell membrane. Virions bud off from the cell, gaining their envelopes from the cellular membrane from which they bud from. The mature progeny particles then infect other cells to repeat the cycle.

Genome Mutation

The ebolavirus is dynamic, as its replication process is a steady source of mutation. An RNA-dependent RNA polymerase is highly error prone as it does not have an error-correction system. The ebolavirus genome may be most stable in its reservoir host (suspected to be bats) as these are the primary carriers. However, as the viral genome crosses over into another species (like humans) there is a strong pressure on the virus to adapt to the new environment that now surrounds it. It has been shown that some of the most substantial genome changes occur during these transitions. The ebolavirus genomes may also mutate as they transition from human to human, especially so in a large outbreak where transmission rates are high. This characteristic is a clear challenge in the development of a vaccine.

Genome Study

With modern technological advancements allowing genomes to be sequenced, researchers can gain important insight on the ebola virus disease in the midst of an outbreak. In particular, comparing mutational changes in the sequenced genomes can give evidence of where the virus originated, its transmission hotspots, its virulence, and how many times it has crossed from the reservoir host to humans. In microarray analysis, researchers are able to study the ebolavirus' gene expression patterns. These patterns are useful when studying cell tissue of infected patients to see what areas of the body the virus has located to.

The role of each viral gene is determined largely through the research use of model organisms. The availability of transgenic and gene knockout mice make them unique compared to all other models in their ability to directly test the role of each viral gene in replication, infection, and immune response. This model system has also been used to tease out genes important for vaccine development.