Negative-Strand RNA Viruses of Animals
Negative-strand RNA viruses are single-stranded viruses that can infect several types of animals.
Explain the mechanism of genome replication in negative-strand RNA viruses
- Negative-strand RNA viruses can infect animals, but in several cases they can go from animals into humans, such as the SARS virus of the Ebola Zaire virus.
- The viron RNA is negative sense (complementary to mRNA and cannot encode proteins ), which means it must be replciated over to mRNA before protein production can begin. This is carried out by an RNA-dependent RNA-polymerase.
- Negative-strand viruses can be found in many niches of Earth and are responsible for many common and very deleterious diseases of animals.
- SARS: The SARS coronavirus, sometimes shortened to SARS-CoV, is the virus that causes severe acute respiratory syndrome (SARS).
- RNA-dependent RNA-polymerase: (RdRP, or RNA replicase) An enzyme that catalyzes the replication of RNA from an RNA template. This is in contrast to a typical DNA-dependent RNA polymerase, which catalyzes the transcription of RNA from a DNA template.
The study of animal viruses is important from a veterinary viewpoint, but many animal viruses are also important from a human medical perspective. The emergence of the SARS virus or Ebola Zaire virus in the human population, coming from an animal source, highlights the importance of animals in bearing infectious agents. In addition, research into animal viruses has made an important contribution to our understanding of viruses in general, including their replication, molecular biology, evolution, and interaction with the host. Animal RNA viruses can be classified according to the sense or polarity of their RNA into negative-sense, positive-sense, or ambisense RNA viruses.
The RNA found in a negative-sense virus is not infectious by itself, as it needs to be transcribed into positive-sense RNA. The complementary plus-sense mRNA must be made before proteins can be translated from the viral genome. This RNA negative-strand to positive-strand copying is carried out by an RNA-dependent RNA-polymerase. Each virion that has one negative-strand copy can be transcribed to several positive-sense RNAs. There are several different types of negative-strand RNA viruses that infect animals; two families will be discussed here in further detail.
Rhabdoviruses are a diverse family of single-stranded, negative-sense RNA viruses that can successfully utilize a myriad of ecological niches, ranging from plants and insects, to fish and mammals. This virus family includes pathogens —the rabies virus, vesicular stomatitis virus, potato yellow dwarf virus, etc.—that are of tremendous public health, veterinary, and agricultural significance. Due to the relative simplicity of their genomes and morphology, in recent years rhabdoviruses have become powerful model systems for studying molecular virology.
Paramyxoviruses are a diverse family of non-segmented negative-strand RNA viruses that include many highly pathogenic viruses affecting humans, animals, and birds. In recent years the advent of reverse genetics has led to a greater understanding of their genomics, molecular biology, and viral pathogenesis. Paramyxoviruses cause a range of diseases in animal species: canine distemper virus (dogs), phocine distemper virus (seals), cetacean morbillivirus (dolphins and porpoises), Newcastle disease virus (birds), and rinderpest virus (cattle). Some paramyxoviruses, such as the henipaviruses, are zoonotic pathogens, occurring naturally in an animal host, but also able to infect humans.
Attachment and Entry to the Host Cell
For influenza viral propagation to begin, there first must be viron attachment and entry into a host cell.
Explain the role of hemagglutinin in the attachment and entry processes of influenza virus
- A glycoprotein on the surface of a virus recognizes a receptor on a host, beginning the attachment process.
- After attachment, the influenza virus is brought into the host cell through an endosome. The low pH of the endosome breaks down the viral capsid and releases the viral contents into the cell.
- A well-described example of this is the influenza virus that relies on hemagglutinin, the glycoprotein that allows the influenza virus to attach to target cells, in this case cells in the human respiratory pathway.
- endosome: An endocytic vacuole through which molecules are internalized during endocytosis pass, en route to lysosomes.
- glycoprotein: A protein with covalently bonded carbohydrates.
- sialic: Of or pertaining to sialic acid or its derivatives.
One of the best understood examples of virus entry into the host cell is the influenza viral infection. The glycoprotein responsible for attachment on the surface of an influenza viral particle is hemagglutinin (HA). HA is an antigenic glycoprotein. It is responsible for binding the virus to the cell that is being infected. HA proteins bind to cells with sialic acid on the membranes, such as cells in the upper respiratory tract or erythrocytes.
HA has two functions. First, it allows the recognition of target vertebrate cells, accomplished through the binding of these cells’ sialic acid-containing receptors. Second, once bound, it facilitates the entry of the viral genome into the target cells by causing the fusion of the host endosomal membrane with the viral membrane. HA binds to the monosaccharide sialic acid that is present on the surface of its target cells, which causes the viral particles to stick to the cell’s surface. The cell membrane then engulfs the virus and the portion of the membrane that encloses it pinches off to form a new membrane-bound compartment within the cell called an endosome, which contains the engulfed virus. The cell then attempts to begin digesting the contents of the endosome by acidifying its interior and transforming it into a lysosome.
However, as soon as the pH within the endosome drops to about 6.0, the original folded structure of the HA molecule becomes unstable, causing it to partially unfold and release a very hydrophobic portion of its peptide chain that was previously hidden within the protein. This so-called “fusion peptide” acts like a molecular grappling hook by inserting itself into the endosomal membrane and locking on. Then, when the rest of the HA molecule refolds into a new structure (which is more stable at the lower pH), it “retracts the grappling hook” and pulls the endosomal membrane right up next to the virus particle’s own membrane, causing the two to fuse together. Once this has happened, the contents of the virus, including its RNA genome, are free to pour out into the cell’s cytoplasm.
Replicative Cycle of Influenza A
Influenza A follows the typical life cycle of most influenza virus: infection and replication are a multi-step process.
Contrast the roles of hemagglutinin and neuraminidase throughout the major stages of the replicative cycle of influenza A virus
- A component of the viral coat, hemagglutinin, binds to the surface of target cells. After binding, the virus fuses and is imported by endocytosis.
- Once the vRNA is released into the cytoplasm, it is transported into the nucleus where it is transcribed. The resulting vRNAs are then transported into the cytoplasm and new viral particles are made.
- The endosome ‘s acidic environment breaks down the viron coat, releasing the vRNA. The acidic environment also promotes the release of the vRNA to proteins that are bound to it.
- Once new viral particles are made in the host cell and bud off, the host cell dies.
- neuraminidase: An antigenic enzyme, found on the surfaces of viruses, that catalyzes the hydrolysis of terminal acylneuraminic residues from oligosaccharides, glycoproteins, and glycolipids.
- hemagglutinin: An antigenic glycoprotein that causes agglutination of red blood cells.
- sialic: Of or pertaining to sialic acid or its derivatives.
Influenza A follows the typical life cycle of most influenza viruses. The infection and replication is a multi-step process:
- Binding to and entering the cell
- Delivering the genome to a site where it can produce new copies of viral proteins and RNA
- Assembling these components into new viral particles
- Exiting the host cell
Influenza viruses bind through hemagglutinin onto sialic acid sugars on the surfaces of epithelial cells, typically in the nose, throat, and lungs of mammals, and the intestines of birds (Step 1 in infection figure ). After the hemagglutinin is cleaved by a protease, the cell imports the virus by endocytosis.
The intracellular details are still being worked out. It is known that virions converge to the microtubule organizing center, interact with acidic endosomes, and finally enter the target endosomes for genome release. Once inside the cell, the acidic conditions in the endosome cause two events to happen:
- The hemagglutinin protein fuses the viral envelope with the vacuole’s membrane.
- The M2 ion channel allows protons to move through the viral envelope and acidify the core of the virus, which causes the core to dissemble and release the viral RNA and core proteins.
The viral RNA (vRNA) molecules, accessory proteins, and RNA-dependent RNA polymerase are then released into the cytoplasm (Step 2 in figure). These core proteins and vRNA form a complex that is transported into the cell nucleus, where the RNA-dependent RNA polymerase begins transcribing complementary positive-sense vRNA (Steps 3a and b in figure).
The vRNA either enters into the cytoplasm and translated (Step 4) or remains in the nucleus. Newly synthesized viral proteins are either secreted through the Golgi apparatus onto the cell surface (in the case of neuraminidase and hemagglutinin, Step 5b) or transported back into the nucleus to bind vRNA and form new viral genome particles (Step 5a).
Other viral proteins have multiple actions in the host cell—including degrading cellular mRNA and using the released nucleotides for vRNA synthesis, and also inhibiting translation of host-cell mRNAs.
Negative-sense vRNAs that form the genomes of future viruses, RNA-dependent RNA polymerase, and other viral proteins are assembled into a virion. Hemagglutinin and neuraminidase molecules cluster into a bulge in the cell membrane. The vRNA and viral core proteins leave the nucleus and enter this membrane protrusion (Step 6). The mature virus buds off from the cell in a sphere of the host phospholipid membrane, acquiring hemagglutinin and neuraminidase with this membrane coat (Step 7). As before, the viruses adhere to the cell through hemagglutinin; the mature viruses detach once their neuraminidase has cleaved sialic acid residues from the host cell. Drugs that inhibit neuraminidase, such as oseltamivir, therefore prevent the release of new infectious viruses and halt viral replication. After the release of new influenza viruses, the host cell dies.