Retrovirus replication

We have discussed in a general way the nature of animal viruses in the first part of this chapter. Now we discuss in some detail the structure and molecular biology of a number of important animal viruses. Viruses will be discussed which illustrate different ways of replicating, and both RNA and DNA viruses will be covered. One group of animal viruses, those called the retroviruses, have both an RNA and a DNA phase of replication. Retroviruses are especially interesting not only because of their unusual mode of replication, but because retroviruses cause such important diseases as certain cancers and acquired immunodeficiency syndrome (AIDS). 

The human immunodeficiency vims (HIV) is the causative agent of the acquired immunodeficiency syndrome (AIDS). HIV is a retrovirus, whose replication includes the transcription of the single-stranded RNA genome into double stranded DNA (reverse transcription) and the covalent insertion of the DNA... 

Viral Proteases. Figure 1 Role of virally encoded proteases in the replication cycle of a retrovirus (HIV, part a) and of a (+)-strand RNA virus (HCV, part b). The numbers correspond to the following steps in the infectious cycle ... 

In contrast to retroviruses, proteolysis is an early event in the replication cycle of (+)-strand RNA viruses and both protease and polymerase inhibitors can be expected to halt the propagation of infectious viral particles from already infected cells. 

Bai J, Rossi J, Akkina R (2001) Multivalent anti-CCR ribozymes for stem cell-based HIV type 1 gene therapy. AIDS Res Hum Retroviruses 17 385-399 Bai J, Sui J, Zhu RY, TaUarico AS, Gennari F, Zhang D, Marasco WA (2003) Inhibition of Tat-mediated transactivation and HIV-1 replication by human anti-hCychnTl intrabodies. J Biol Chem 278 1433-1442... 

Human immunodeficiency virus (HIV) is a retrovirus, i.e. its RNA is converted in human cells by the en me reverse transcriptase to DNA which is incorporated into the human genome and is responsible for producing new HIV particles. Zidovudine (azidothymidine, AZT Fig. 5.22F) is a stmctural analogue of thymidine (Fig. 5.22A) and is used to treat AIDS patients. Zidovudine is converted in both infected and uninfected cells to the mono-, di- and eventually triphosphate derivatives. Zidovudine triphosphate, the aetive form, is a potent inhibitor of HIV replication, being mistaken for thymidine by reverse transeriptase. Premature ehain termination of viral DNA ensues. However, AZT is relatively toxic because, as pointed out above, it is converted to the triphosphate by eellular enzymes and is thus also aetivated in uninfected cells. 

The antiviral activity spectrum of the ddN analogues should, in principle, extend to all retroviruses as well as hepadnaviruses [i.e., hepatitis B virus (HBV)], since HBV, like retroviruses, replicates through an RNA template-driven RT process. Indeed, various ddN analogues (particularly, the L-enantiomeric forms 3TC, FTC, and L-DDC) have been shown to inhibit HBV replication [36-38]. Consequently, 3TC is, at present, pursued as a potential drug candidate for the treatment of both HIV and HBV infections. 

Okamoto M, Ono M, Baba M. Potent inhibition of HIV type 1 replication by an anti-inflammatory alkaloid, cepharanthine, in chronically infected monocytic cells. Res Hum Retroviruses 1998 14 1239-1245. 

Enzymes in viruses We have stated that virus particles do not carry out metabolic processes. Outside of a host cell, a virus particle is metabolically inert. However, some viruses do contain enzymes which play roles in the infectious process. For instance, many viruses contain their own nucleic acid polymerases which transcribe the viral nucleic acid into messenger RNA once the infection process has begun. The retroviruses are RNA viruses which replicate inside the cell as DNA intermediates. These viruses possess an enzyme, an RNA-dependent DNA popo called reverse transcriptase, which transcribes the information in the incoming RNA into a DNA intermediate. It should be noted that reverse transcriptase is unique to the retroviruses and is not found in any other viruses or in cells. 

If the virus has double-stranded RNA (ds RNA), this RNA serves as a template in a manner analogous to DNA. There are three classes of viruses with ss RNA and they differ in the mechanism by which mRNA is synthesized. In the simplest case, the incoming viral RNA is the plus sense and hence serves directly as mRNA, and copies of this viral RNA are also copied to make further mRNA molecules. In another class, the viral RNA has a minus (-) sense. In such viruses a copy is made (plus sense) and this copy becomes the mRNA. In the case of the retroviruses (causal agents of certain kinds of cancers and AIDS), a new phenomenon called reverse transcription is seen, in which virion ss RNA is copied to a double-stranded DNA (through a ss DNA intermediate) and the ds DNA then serves as the template for mRNA synthesis (thus ss RNA ss DNA ds DNA). Retrovirus replication is of unusual interest and complexity. 

Li+ is currently administered topically for the relief of HSV and, in addition, it has been demonstrated that the recurrence of HSV infection is inhibited in Li+-treated patients, indicating another potential prophylactic effect of Li+ [245]. Ointment containing 8% lithium succinate has been shown to reduce the severity and the incidence of recurrent genital HSV infection in man [246]. It has also been proposed that Li+ might be efficacious in treating HIV-infected patients, although any benefits have not yet been demonstrated [247]. While HTV is a RNA virus and as such might not be predicted to be affected by Li+, it is a retrovirus and utilizes a DNA intermediate for its replication and it uses a DNA polymerase. 

The antiretrovirus properties of NO were shown in mice infected with Friend leukemia virus, a murine retrovirus. NO produced by NO-generating compounds or activated macrophages inhibits viral replication in fibroblast cultures, and is involved in defens against this murine retrovirus in vivo [134]. It was also reported that NO donors can inhibit HIV-1 replication in human monocytes through induction ofiNOS [135],... 

The life cycle of many viruses, including retroviruses, depends on viral proteases that cleave viral glycoproteins into individual polypeptides, and these enzymes are necessary for viral replication. NO can inactivate coxsackievirus [136]. Since cysteine proteases are critical for the virulence and replication of many viruses, nitrosation of viral cysteine proteases may be a mechanism of antiviral host defense. NO mediates nitrosation of cysteine and aspartyl proteases of HIV-1, and it was suggested that this... 

Reverse transcriptase. This enzyme is involved in the replication of retroviruses in vivo. It synthesizes a complementary DNA (cDNA) strand using RNA instead of DNA as its template. It is widely used to create a strand of cDNA from mRNA extracted from cells or tissue for cloning or for PCR analysis. 

Following the process of virus attachment, penetration, and uncoating described before, the retrovirus undergoes replication. 

The life cycle of HIV is typical of an enveloped retrovirus. As with other viruses, HIV requires a cellular receptor to infect a cell. The receptor for HIV is known as the CD4 antigen. The life cycle for HIV is outlined in Fig. 3. Virus replication is regulated by the products of six genes. 

Reverse transcription, which produces DNA copies of an RNA, is more commonly associated with life cycles of retroviruses, which replicate and express their genome through a DNA intermediate (an integrated provirus). Reverse transcription also occurs to a limited extent in human cells, where it plays a role in amplifying certain highly repetitive sequences in the DNA (Chapter 7). 

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