Multiplication of Human Viruses

MULTIPLICATION OF HUMAN VIRUSES

The objective of the replication cycle is to ensure the multiplication of the virus with the formation of identical viral progeny. Viruses differ in their replication cycle and the time to produce and release new virions. The multiplication cycle of human viruses is generally slow, from 4 to more than 40 hours. Bacterial viruses are generally faster and can take as little as 20 minutes to replicate within the bacterial host. The replication cycle can be divided into six distinct phases (Figure 5.4) that are common to all viruses, although detail within each phase varies greatly between viruses. Understanding the viral multiplication process is crucial for the development of new antiviral drugs.

1) Attachment To The Host Cell

Viral attachment to the cell surface can be divided into three phases: (1) an initial contact mainly dependent on brownian motion, (2) a reversible phase during which electrostatic repulsion is reduced and (3) irreversible changes in virusreceptor–hostreceptor configuration that initiates viral penetration through the cell membrane.

All viruses possess receptors on their surface, usually in the form of glycoproteins embedded in the viral envelope or protruding as spike from the viral capsid. These structures recognize and bind receptors on the host cell and provide the virus with its high specificity although different viruses might share the same receptor. The virus–cell recognition event is similar to any protein–protein interaction in that it occurs through a stereospecific network of hydrogen bonds and lipophilic associations. For example, the haemagglutinin receptor of influenza virus binds the terminal glycoside residues of gangliosides (cell surface glycolipids) of the target cell leading directly to the virus particle adhering to the cell. Similarly, the interaction between the HIV receptor (i.e. gp120) and the Tlymphocyte receptor (i.e. CD4) has been intensively studied.

2) Penetration Of The Viral Particle

Following the irreversible attachment of the virus to the host cell, penetration of the virus through the cell membrane is initiated following two energydependent mechanisms, endocytosis or fusion. A third mechanism has been identified in some bacteriophages that can inject their nucleic acid inside the bacterium (see section 8.1). During endocytosis, the association between virus receptor and host receptor triggers a number of mechanisms that draw the cell membrane to engulf the virus particle forming a cytosolic vacuole. This process is widespread among nonenveloped viruses, but is also used with some enveloped viruses such as influenza (orthomyxoviruses). Certain enveloped viruses (e.g. herpes simplex virus, HIV) can penetrate following fusion of their envelope with the host cell membrane, liberating the viral capsid within the cell cytoplasm.

3) Uncoating Of The Viral Particle

Following penetration of the virus in a vacuole or directly into the cell cytoplasm, the viral nucleic acid then needs to be released from the capsid/coat(s) to initiate viral replication. This is the uncoating process. For viruses that penetrate by endocytoses, the acidification of the cytosolic vacuoles following endosome fusion induces a conformational change in the capsid and the release of viral nucleocapsid (some helper proteins are associated with the viral nucleic acid) into the cytoplasm. For certain viruses, such as reovirus, only a partial uncoating is necessary for the expression of the viral genome. The release of the nucleocapsid from vacuoles can occur in the cytoplasm, close to the nucleus or within the cell nucleus.

4) Replication Of Viral Nucleic Acids And Translation Of The Genome

This stage of viral replication ensures that (1) the host cell synthesis machinery is taken over by the virus, and

the viral genome is replicated. The structure, size and nature of the viral genome are extremely diverse and thus this stage of the viral multiplication cycle reflects this diversity. Three main mechanisms are, however, common to all viruses: the transcription of viral genes into viral mRNA, the translation of the viral genome into proteins, and the replication of the viral genome. Early transcription and translation usually occurring immediately after the release of the nucleocapsid in the cytoplasm is also common, and ensures the production of early proteins such as viral polymerases, and the hijacking of the cell synthesis machinery. In addition, some viruses can encode for genes the products of which regulate the host synthetic processes according to the needs of the virus (e.g. tat gene in HIV).

The replication of the viral genome depends on the type of nucleic acid carried by the virus. The positive strand RNA in viruses such as the poliovirus can be used directly as mRNA following the acquisition of a terminal sequence from the host cell. Negative strand RNA (e.g. in influenza virus) is transcribed into a positive RNA complementary in base sequence to the parent RNA using an RNAdependent RNA polymerase carried by the virus. In ds DNA viruses (e.g. adenoviruses), the nucleic acid passes into the nucleus where it is usually transcribed by a host DNAdependent RNA polymerase. In some viruses (e.g. poxvirus), this enzyme is contained within the virus and released during uncoating, allowing the viral genome to be replicated in the cell cytoplasm. In retroviruses (e.g. HIV), a single stranded proviral DNA is produced from the viral ss RNA by a viral enzyme called reverse transcriptase. This unique enzyme acts both as an RNA and DNA directed DNA polymerase, and has associated RNAase activity. The proviral DNA can be transported to the cell nucleus where it can be integrated within the cell host genome by a viral integrase.

One important difference between the host cell and the virus is in the nature of their mRNA. Host cell mRNA encodes directly for functional proteins, whereas viral mRNA is polycistronic, which means several distinct proteins are encoded within a single piece of mRNA. This implies that the virus needs to use a virus-specific protease to cut at the correct place the polyprotein produced by translation to restore the functionality of viral proteins.

Late protein synthesis during the replication cycle concerns the production of structural components (e.g. capsomeres) of the new virions.

5) Maturation Or Assembly Of Virions

Towards the end of the multiplication process, large amounts of viral materials accumulate within the host. Viral capsid starts to form from individual structural proteins. In certain viruses (e.g. poliovirus) the capsid self-assembles. The replicated viral genome and some viral proteins become packaged within the capsid. Most nonenveloped viruses accumulate within the cytoplasm or nucleus and are only released when the cell lyses. Packaging of viral components can occur within the cytoplasm or in the cell nucleus. For example, with influenza virus, the capsomeres are transported to the cell nucleus where they combined with the viral RNA and assemble into helical capsids. The envelope of enveloped viruses originates from the host. With the influenza virus, viral proteins such as neuraminidase and haemagglutinin migrate to the cell membrane, displacing cell protein. The assembled nucleocapsids pass out from the nucleus to the cytoplasm and as they impinge on the altered cytoplasmic membrane they cause it to bulge and bud off completed enveloped particles from the cell. In the herpesvirus, the envelope originates from the nucleus membrane. The nucleocapsid assembles into the nucleus and it acquires its envelope as it passes through the inner nuclear membrane. The complete virus is then incorporated into a vesicle which migrates to the cell surface.

The maturation of viruses and their assembly is not well understood at present. The presence of chaperone proteins may play an important role in the interaction between the viral nucleic acid and the structural proteins.

6) Release Of Virions Into The Surrounding Environment

At the end of the multiplication process, the mature virions are released from the host cell. This can occur in a number of different ways. For most nonenveloped viruses, the virus progeny accumulates within the host cell cytoplasm and is released following cell lysis. Some viruses (e.g. bacteriophages) produce a lytic enzyme (peptide) or proteases to lyse the host enabling the release of infectious particles, although the host often selfdisintegrates as it cannot maintain normal housekeeping functions during a viral infection. Enveloped viruses are usually released by a budding process over a period of hours. Ultimately the host cell will die following damage to its metabolism and housekeeping functions during viral replication.