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The pathologic effects of virus diseases result from interplay of several factors:
In most instances the symptoms and signs of acute virus diseases can be directly related to the destruction of cells by the infecting virus.
Infection is associated with the host range of the virus, susceptibility of the host, portal of entry, target cells, etc.
Replication of most animal viruses involved a dual process in which the viral protein and nucleic acid are synthesized separately and then assembled into the infectious particles. This is usually followed by release of mature virions into the extracellular environment.
The cycle of development of animal viruses may be described in 5 separate stages as follows:
To infect a cell, the virion must attach to the cell surface, penetrate the cell, and become sufficiently uncoated to make its genome accessible to viral or host machinery for transcription or translation.
Attachment constitutes specific binding of a virion protein (the antireceptor) to a constituent of the cell surface (the receptor).21
The classic example of an antireceptor is the hemagglutinin of influenza (orthomyxovirus) virus. The antireceptors are distributed throughout the surfaces or viruses infecting human and animal cells.
The cellular receptor identified so far are largely glycoproteins. Since both cells and virus particles are negative charged at pH 7, therefore, positive ions are require as counter-ions to reduce electrostatic repulsion, as a rule, this requirement is met most efficiently by magnesium ions. Attachment is largely temperature and energy-independent.
The susceptibility of a cell is limited by the availability of appropriate receptors and not all cells in an otherwise susceptible organism express receptors, e.g. human kidney cells lack receptors for poliovirus when they reside in the organ, but receptors appear when these cells are propagated in cell culture.
Attachment of viruses to cells in many instances leads to irreversible changes in the virion. In some instances, when penetration does not ensue, the virus can detach and reabsorb to a different cell. Orthomyxoviruses and paramyxoviruses carry a neuraminidase on their surface, these virus can elute from their receptor by cleaving neuraminic acid from polysaccharide chains of receptors. The number of virus particles or infectious units absorbed per cell is referred to as the multiplicity of infection (moi). Animal cells are generally capable of absorbing very large amounts of virus, some host cells contain 100,000 receptors for Sindbis virus.
Penetration is an energy-dependent step most efficient at 37oC. It occurs almost instantaneously after attachment and involves one of three mechanisms: endocytosis, fusion, and translocation.
following attachment to receptors, virions
move down into coated pits-coated with clathrin, fold inward to produce coated vesicles that enter the cytoplasm and fuse with a lysosome to form a phagolysosome. With enveloped viruses, the envelop of endocytosed virion fused with the lysosomal membrane, releasing the viral nucleocapsid into the cytoplasm.
The F (fusion) glycoprotein of paramyxoviruses, enables the envelope of these viruses to fuse directly with the plasma membrane. This may allow the nucleocapsid to be released directly into the cytoplasm.
Some non-enveloped icosahedral viruses appear to be capable of passing through the plasma membrane directly. With the exception of some picorna and 22 reoriruses, direct penetration is uncommon with animal viruses.
In order that at least the early viral genes may become available for transcription, it is necessary that the virion be at least partially uncoated.
Absorption, penetration and uncoating result in loss of infectivity, which is referred to as "eclipse." The only residue infectivity is that due to the viral nucleic acid itself, which is no more than a small fraction of that of the virus particle. The first three stages of infection are usually inefficient processes. The inefficiencies account in large part for the fact that the ratio of infections to total animal virus particles is almost always far less than 1.
The synthesis of viral proteins by the host protein-synthesizing machinery is the key event in viral replication. Irrespective of the size, composition and organization of its genome, the virus must present to the eukaryotic cell protein-synthesizing machinery a messenger RNA that the cell can recognize as such and translate. In this regard, the cell imposes two constraints on viruses:
The viral RNA of (+) sense ssRNA viruses (Picornaviridae, Flavidiridae, Togaviridae, Calicivividae, Coronavividae) binds directly to ribosomes and is translated in full or in part without the need for any prior transcriptional steps. With all other classes of viral genomes, mRNA must be transcribed.
DNA viruses that replicate in the nucleus, the cellular DNA-dependent RNA polymerase II performs the function of mRNA.
All other viruses require unique and specific transcriptases which are virus coded and are integral component of the virion. Cytoplasmic dsDNA viruses carry a DNA-dependent RNA polymerase, whereas dsRNA viruses have dsRNA-dependent RNA polymerase, and (-) sense ssRNA viruses carry a RNA-dependent RNA polymerase.
4.5.3.1 DNA viruses
For all DNA viruses, mRNA must be transcribe by a DNA-dependent RNA polymersae. Not all genes can expressed simultaneously throughout the replication cycle, particular parts of the genome are transcribed in sequence, with the early genes first, and the late genes later in the cycle.
- ds DNA; Cellular transcriptase:
papoviruses, adenoviruses, and herpesviruses, the viral DNA is transcribed within the nucleus by a cellular DNA-dependent RNA polymerase. The transcriptional program consists of at least two cycles, early and late mRNAs of transcription for papovaviruses, and at least three cycles for herpesviruses and adenoviruses. In each instance the structural (virion) polypeptides are made from mRNA generated from the last cycle of transcription. Polycistronic transcripts undergo cleavage and splicing to produce monocistronic mRNA.
- ds DNA; virion transcriptase:
the poxviruses and African swine fever virus, which replicate in the cytoplasm, carry their own transcriptase. The monocistronic mRNA are transcribed directly from the viral DNA. There are at least three cycles of transcription. The transcripts are translated directly into protein, some of which need to undergo post-translational cleavage to yield functional molecules.
- ssDNA; cellular transcriptase:
The negative sense ssDNA of the parvoviruses requires the synthesis of a complementary strand to form dsDNA in the nucleus, and is than transcribe to produce mRNAs.
- ds/ss DNA;
cellular transcriptase, virion DNA poly- merase: the ss DNA portion of the genome of hepadnaviruses is first repaired by a virion-associated DNA polymerase, and the DNA then converted into a supercoiled dsDNA. Transcription of mRNA by cellular RNA polymerase II later.
4.5.3.2 RNA viruses:
Primary RNA transcripts from eukaryotic DNA are subject to a series of postranscriptional alteration in the nucleus, known as processing, prior to export to the cytoplasm as mRNA. A cap, consisting of 7 methylguanosine (m7Gppp) is added to the 5' terminus. The function of this poly (A) tail is uncertain.
Transcription is more complicated for the RNA viruses than for the DNA Viruses, since they are the only form of life that utilize RNA as the repository of genetic information.
There are three main strategies:
- (1) the virion RNA of most viruses with (+) sense RNA is itself infectious, because if functions as mRNA,
- (2) viruses with (-) sense ssRNA, or with dsRNA, carry a virion-associated RNA dependent RNA polymerase which transcribe mRNA from the viral RNA, and
- (3) the (+) sense virion RNA of retroviruses is transcribed into DNA, which serve as a template for transcription of viral mRNAs by a cellular transcriptase:
- ssRNA; (+) sense:
- (1) picornaviruses and flavi-viruses - the genome acting as a single polycistronic mRNA, is translated into a single polyprotein which is subsequently cleaved to give the individual viral polypeptides.
- (2) Togaviruses of the genus Alphavirus, only 2/3 of the viral RNA (the 5' end) is translated; the resulting poly-protein is cleaved into four nonstructural proteins two of which form the RNA polymerase. This enzyme then copies a full-length (-) sense strand, from which 2 species of (+) sense strand are copied: full-lenght virion 3 or 4 R destined for encapsidation, and a 1/3 length RNA is translated into a polyprotein from which 3 or 4 structural proteins are produced by cleavage.
- 3. Coronaviruses have a unique strategy in part of the virion RNA act as mRNA and is translated to produce an RNA poly-merase, which then synthesizes a genome-length (-) sense strand. From this, a "nested set" of overlapping subgenomic RNAs is transcribed, of which only the unique (nonoverlapping) sequence in each is translated.
- ssRNa (-) sense;
virion transcriptase. Primary transcription from the (-) sense ssRNA viruses occurs in the cytoplasm, when the virion RNA is still within the helical nucleocapsid, in association with the nucleoprotein as well as the transcriptase.
- 1. Paramyxoviruses and rhabdoviruses have similartranscription strategies-suggestion of a common ancestry. The (-) sense virion RNA is copied in 2 distinct ways: the replication mode and the transcription mode copying in the replication mode produces a full length (+) sense strand which is used as a template for the synthesis of new virion RNA. In the transcription made, 5 sub-genomic (+) sense RNAs are produced; each is capped and polyadenylated and served as a monocistronic mRNA.
- 2. Orthomyxoviruses, bunyaviruses, and arena viruse have segmented genomes, and each segment is transcribed to yield an mRNA which is translated into one or more protein. In orthomyxoviruses, each segment can be regarded as single gene for they encode single proteins. A virion-associated endonuclease enters the nucleus and remove a short segment from the capped 5' terminus of cell mRNA; this is transported back to the cytoplasm, where it binds to the virion RNA and serves as a primer to initiate transcription.
Each viral RNA segment of the genomes of the bun-yaviruses and arenaviruses codes for more than one protein. Furthermore, the S segment of arenaviruses is ambisense, the replication strategy of ambisense RNA is mixed, with features of both (+) sense and (-) sense ssRNA viruses.
- ds RNA; virion transcriptase:
The two families of viruses with dsRNA (Birnaviridae and Reovirridae) have segmented genomes and each segment is separately transcribed in the cytoplasm by a virion associated RNA dependent RNA polymerase. With reoviruses, each of the 10, 11 or 12 dsRNA segments corresponds to a single gene. Monocistronic mRNAs are transcribed from each segment with the partly uncoated subviral particle; these RNAs complex with a protein before each is copied to produce a dsRNA, which serve as the template for further mRNA transcription.
- ssRNA; (+) sense;
virion reverse transcriptase. Retrovirus genomes are monopartite but diploid (two identical positive-strands of RNA). The sole known function of the genomic RNA is to serve as template for the synthesis of virus DNA. Since eukaryotic cells lack enzyme for this function, the virion also contains addition to the genome, an RNA-dependent DNA polymerase (reverse transcriptase) as well as a mixture of host transfer RNAs one of which serve as a primer.
The key steps in the reproductive cycle are:
- 1. binding of the tRNA-reverse transcriptase complex to the genomic RNA.
- 2. synthesis of a DNA copy complementary to the RNA across the two ends of the RNA molecule in such a fashion as to produce a circular single-stranded DNA molecule hydrogenbonded to the linear genomic RNA.
- 3. digestion of genomic RNA by a nuclease which attack RNA only in DNA-RNA hybrids (ribonuclease H, also packaged in the virion).
- 4. synthesis of the complementary copy of the viral DNA.
- 5. The circular double-stranded DNA is then trans- locate into the nucleus where it integrates intothe host genome (a provirus).
- 6. Virus gene expression may not follow. When it occurs the integrated viral DNA is transcribed by the host cell transcriptase.
Capped, polyadenylated, and processed monocistronic viral mRNAs bind to ribosomes and are translated into protein in the same fashion as cell mRNAs.
4.5.4.1 Early proteins
The proteins translated from the early transcripts of DNA viruses, including enzymes and other proteins required for the replication of viral nucleic acid, as well as proteins that suppress host cell RNA and protein synthesis.
4.5.4.2 Late protein
The viral proteins are translated from late mRNA, most of which is transcribed from progeny viral nucleic acid molecules. Most of the late proteins are viral structural proteins and they are often made in considerable excess.
4.5.4.3 Posttranslation clevage of polyproteins.
Posttranslational clevage occurs in picornaviruses and several other RNA families. The polycistronic viral RNA is translated directly into a single polyprotein which carries protease activity. This virus-coded protease cleaves the polyprotein at defined recognition sites into small proteins.
4.5.4.4 Migration of proteins.
Newly synthesized viral proteins migrate to various sites in the cell where they are needed, e.g. back to the nucleus in the case of viruses that replicate there. Migration is dependent on the structural features of particular proteins. In the case of glycoproteins, the polypeptide is translated on membrane-bond ribosome, i.e. on rough endophasmic reticulum.
4.5.5.1 . ICOSAHEDRAL VIRUSES.
Structural proteins of nonenveloped icosahedral viruses associate spontaneously to from capsomers, which self-assemble to form procapsids, into which viral nucleic acid is packaged. Completion of the virion often involves proteolytic cleavage of one or more species of capsid protein.
In the case of picornaviruses 60 copies of each of the virion proteins designated VPO, VP1, and VP3 assemble in the cytoplasm into a procapsid viral RNA then wrap around the procapsid, and in the process VPO is cleaved to yield VP2 and VP4. The cleavage probably causes rearrangement of the capsid into a thermodynamically stable structure in which the RNA is shielded form access by nucleuses. Poxviruses and reoviruses also assemble in the cytoplasm.
Adenoviruses, papovaviruses, and parvoviruses assemble in the nucleus. All viruses which assemble and acquire infectivity inside the cell depend on the disintegration of the infected cells for their egress.
4.5.5.2 Enveloped viruses.
Enveloped viruses, exemplified by all (-) strand RNA viruses and others such as togaviruses, and retroviruses. Some of the virus proteins become inserted into both the inner and outersurface of the plasma membrane or other cytoplasmic membranes of the infected cells. Those projecting from the outer surface are glycosylated (glycoprotein). The membrane proteins aggregated into patches displacing host membrane proteins. Viral nucleocapsids bind to special virus-specified proteins lining the cytoplasmic side of these patches or cytoplasmic domains of the infected cells. Those projecting form the outer surface are glycosylated (glycoprotein). The membrane protein aggregated into patches displacing host membrane proteins. Viral nucleocapsids bind to special virus-specified proteins lining the cytoplasmic side of these patches or cytoplasmic domains of viral glycoprotein (togavirus) and become wrapped up by the patch. In the process, the nascent virion is "extruded" or "buds" into the exracellular environment. In some instances (e.g. orthomyxoviruses and paramyxoviruses) cleavage and rearrangement of one species of surface protein occurs during or after extrusion and imparts to the newly formed virion the capability of infecting cells.
viruses assembly and maturation by extrustion from the cell surface provides a more efficient mechanism of egress in as much as it does not depend on disintegration of the infected cells. Indeed, viruses that mature and egress in this fashion very considerably in their effects on host cell metabolism and integrity. They range from high cytolytic (togaviruses, paramyxoviruses rhabdoviruses) to virtually noncytolytic (retroviruses). However, by virtue of the insertion of the viral glycoproteins into the cell surface, these viruses impart on the cell a new antigenic specificity, and the infected cell can and does become a target for the immune mechanism of the host.
The herpesvirus nucleocapsid is assembled in the nucleus. The envelopment and maturation occur at the inner lamella of the nuclear membrane. The enveloped virus accumulates in the space between the inner and outer larmellae of the nuclear membrane and in the cisternae of the cytoplasmic reticulum and vesicles carrying the virus to the cell surface. Thus the enveloped virus is uniquely shielded from contact with the cytoplasm. Herpesviruses are cytolytic and invariably destroy the cells in which they multiply. Like other enveloped viruses, herpesviruses import to the infected cell new antigenic specificities.
4.5.6.1 Architecture and "Budding" of enveloped viruses.
A number of animal viruses obtain lipid-containing membrands during maturation by a process of "budding" through the host cells surface.
During infection, RNA and a closely associated viral nucleoprotein (N) form the nucleocapsid of the virus termed RNP. The structure of the nucleosapid varies with virus type. It is a compact, spherical particle in the alpha viruses, a filamentous helical nucleocapsid in paramyxo-and rhabdoviruses, and multisegmented helical nucleocapsid in the orthomyxoviruses. The one or two viral glycoproteins (hemagglutinin and neuraminidase in the case of influenza virus) are anchored in the cellular membrane by a transmembrane hydrophobic peptide. The M (matrix) protein forms a shell around glycoprotein and the underlying M proteins drive the budding process.
4.5.6.2 Viral membranes
The membrane of enveloped viruses is active in 3 stages of the virus:
- 1. a membrane glycoprotein of the virus attaches the virus to target cell by binding to a cellularreceptor.
- 2. a viral membrane fusion activity initiates infection by fusing the viral membrane to a cellular membrane to effect transfer of the nucleocapsid into the cytoplasm of the target cell.
- 3. virus "budding" provides a nonlytic method for virus release from an infected cell.
It was conclusively demonstrated that the influenza virus hemagglutinin (HA) is the only protein necessary for membrane fusion.
The immune systems response to infection by enveloped viruses produces antibodies directly against the viral membrane glycoproteins. These antigens are also the targets of vaccine induced antisera.
