overview virology

Overview of viruses

A virus is a subcellular agent, consisting of a core of nucleic acid (DNA or RNA) surrounded by a protein coat. Viruses use the metabolic machinery of a living host to replicate and produce more viral particles.

The smallest viruses have only a few genes; the largest viruses have up to 200.

A completely assembled and infective virus is called a virion.

Characteristics of viruses:

1. Small; filterable - pass through 0.22 mm. filters

2. Multiply only as obligatory parasites in living cells.

3. Simple organization - Virions = free infectious viral particles. Composition:

a. genome:

DNA or RNA
single stranded or double stranded
most viral nucleic acids are linear molecules.
Segmented (some RNA viruses) or nonsegmented

Single-stranded RNA genome may be either a sense strand (plus strand) which can function as mRNA or an antisense strand (minus strand), which is complementary to the sense strand. The latter has a viral RNA transcriptase that synthesizes a complementary strand to serve as mRNA.

DNA viruses have mutation rates similar to those of eukaryotic cells because, like eukaryotic DNA polymerases, their replicatory enzymes have proofreading functions. 1 error/108 to 1011 nucleotides.

RNA viruses lack a proofreading function in their replicatory enzymes, and some have mutation rates that are as high as 1 error/103 to 104 nucleotides.

b. capsid is a protein coat that encloses the genome and gives the virion its characteristic shape. It is made up of protein molecules which are specified by viral genes. The capsid itself is made up of identical protein subunits called capsomers. The capsid together with the nucleic acid form the nucleocapsid.

capsid: protects nucleic acid

attachment to cell receptors

c. Proteins with enzymatic functions.

Examples: DNA-dependent RNA polymerase or RNA-dependent DNA polymerase (reverse transcriptase), and neuraminidase.

d. membranous envelope composed of lipid and protein - optional

Capsid and envelope protect the genome from nucleases present in the environment.

The lipid envelope mimics the composition of the cytoplasmic membrane of the infected host. The presence of lipids makes enveloped viruses sensitive to damage by lipid solvents. Enveloped viruses - less resistant to drying, pH, temperature, detergents. Enveloped viruses require intact envelope for infectivity.

e. Proteins with structural functions

i. Glycoproteins are found on the surface of viral envelopes and are in the form of projections or spikes

important as antigenic determinants

facilitate attachment of virions to host tissues

penetration of their genomes into the host cell

hemagglutination

fusion of the viral membrane with the cell membrane

hemolysis

Viruses infect animal cells, bacterial cells, plant cells. Within each class the virus is able to infect only cells of a certain species or of a certain type. Host range is determined in part by the specificity of cell attachment which depend on properties of the virion's coat and specific cell receptors.

ii. Membrane or matrix proteins reinforce the envelope, connect the nucleocapsid to the glycoproteins, and function in assembly of the virions.

4. Characteristic mode of replication. Virions lack the machinery for using and transforming energy and for making the proteins specified by the viral genes. The virus uses the machinery of the host cell to make the constituents of viruses in the cell's cytoplasm or nucleus. Progeny virions are then assembled from these constituents.

Virions can also be subdivided on the basis of their morphological type (how the nucleic acid is packaged):

a. icosahedral (20-sided) virions resemble small crystals. These virions have an icosahedral protein shell (capsid) surrounding a condensed core of nucleic acid and proteins.

b. helical virions form long rods. The extended nucleic acid is surrounded by a cylindrical capsid.

c. enveloped virions contain lipids. The nucleocapsid (icosahedral or helical) is surrounded by a membranous envelope. Most enveloped virions are roughly spherical but may be pleomorphic because the envelope is not rigid.

d. complex virion structures may have several coats around the nucleic acid or are like bacteriophages which have a capsid to which additional structures are appended.

Viral detection: by physical methods, immunologic techniques, or by their infectivity.

1. counting particles by electron microscopy

2. hemagglutination

3. plaque method - bacteriophages on lawn of susceptible bacterial cells or animal viruses on a monolayer of cells growing on a solid support.

4. immunologic techniques: immunofluorescence, EIA, RIA, complement fixation, latex agglutination

5. nucleic acid hybridization

6. PCR

Transmission:

Viruses enter the mammalian host via the respiratory tract, GI tract, direct inoculation by arthropods into the bloodstream, or breaks in the skin. Viruses have an affinity (tropism) for certain cells and this seems to be based on the chemical composition of the viral surface and the cytoplasmic membrane of the host cell.

Once the host has been invaded, the infection process can be broken down into 5 stages:

1. Adsorption. The host cell invaded by virus possesses many receptors which are dispersed over its surface. The number of receptors varies from 104 to 105 per cell and are major determinants in susceptibility to infection. Most viral receptors are glycoproteins.

2. Penetration. Three mechanisms for this:

a. endocytosis. Virus-receptor complexes are concentrated in coated pits, which invaginate to form vesicles. As the vesicles move deeper into the cytoplasm, they fuse with lysosomes to produce a lysosomal vesicle. Lysosomal enzymes digest away the viral layers and release the nucleocapsid into the cytoplasm.

b. fusion. Some enveloped viruses gain access to the cytoplasm of the cell by fusing with the cytoplasmic membrane. Fusion produces an opening through which the nucleocapsid passes directly into the cytoplasm.

c. Direct penetration - small, nonenveloped viruses; bacteriophages.

3. Uncoating - separates genome from protective shell.

Most viral nucleocapsids are uncoated in the cytoplasm, although some are uncoated after entering the cell nucleus.

4. Genome expression - two functions are necessary:

a. formation of mRNA so that specific proteins will be translated

b. viral nucleic acid replication

5. Maturation and assembly - after adsorption there is an eclipse period during which no new viral particles can be detected. During this period of growth,

a. early proteins are synthesized

b. viral nucleic acids are synthesized

c. late proteins are made that are structural and used in the construction of the capsid. Capsid assembly may occur in the nucleus (most DNA viruses) or in the cytoplasm (most RNA viruses).

6. Release.

Many nonenveloped viruses are released from the cell only after the cell has disintegrated.

For enveloped viruses the lipid bilayer of the viral envelope is acquired from the host's membrane system (the nuclear or cytoplasmic membrane)

However, the viral envelope also possesses glycoprotein spikes that are encoded by the virus. The viral glycoprotein is synthesized on the ER where sugars are added. The glycosylated protein is transported to the Golgi where it is further modified. Then it is transported to and inserted into the cell membrane. When the viral nucleocapsid makes contact with the membrane, it protrudes and encloses the nucleocapsid. The enveloped nucleocapsid then pinches off or buds from the cell membrane. The continuity of the cell membrane is retained.

The multiplication cycle for animal viruses: 6 - 48 hours.

Effects of viral infection on the host cell - cytopathic effects (CPE)

a. Direct damage. No viral toxins reported. Some viruses shut down host macromolecule synthesis and thus directly damage host cells. The damaged cells lyse: plaques on cultured cells. Or may see other alterations in the cells called cytopathic effects, e.g. rounding of cells, vacuolization, inclusion bodies (intracellular accumulation of viral products - nucleic acids, proteins, capsids).

b. Indirect damage - affect function of tissues or host organs. Example: influenza virus damages the respiratory epithelium and ciliary activity is severely affected. This results in the accumulation of bacteria that normally would be eliminated by ciliary action. AIDS virus affects immune cells bringing about immunological changes that result in susceptibility to other infectious agents.

Immunopathology - results from virus-induced host reactions to cells expressing virus genes. Some viruses modify host gene expression.

c. cell fusion (syncytia formation). Enveloped viruses release specific proteins that become incorporated into the cytoplasmic membrane of the infected cell. These proteins attract uninfected cells to their surface. Eventually the cells fuse, producing a giant multinucleated cell or syncytium. May be a mechanism by which virus spreads from infected to uninfected cells.

d. changes in surface antigens. For enveloped viruses, incorporation of virus-specified glycoproteins into the cytoplasmic membrane. For viruses without envelopes, viral proteins may also be associated with the cell surface.

e. interferon production. Interferons are a family of small proteins secreted by virally infected cells into the extracellular fluids; they have the ability to prevent the infection of healthy cells. Secreted interferon binds to healthy cells and induces production w/in these cells of antiviral proteins. The antiviral proteins produced intracellularly block various stages of viral replication.

Persistence. The intracellular location of some viruses permits them to remain as cellular occupants for prolonged periods of time: persistence. It allows virus to remain undetected and safe from immune forces that exist outside the cell. The ultimate mechanism of escape from immune forces is integration of the viral genome into host DNA.