Retrovirus

Retroviruses ( Retroviridae ) (short for reverse transcriptase oncoviruses ) are a large family of enveloped viruses with single ( )- legged RNA genome, the genetic information (ss ( ) RNA ) is present, accordingly, in the form of ribonucleic acid. Unlike the "normal" RNA viruses, the RNA retroviruses but must first be rewritten once by reverse transcription into a DNA molecule, before it can be active as such incorporated in the genome of the host cell, and there.

Retroviruses can be roughly divided into simple and complex retroviruses. Furthermore, we distinguish between infectious exogenous retroviruses ( partially abbreviated XRV ) and endogenous retroviruses ( ERV ), which are inherited vertically above the germ line and are part of the host cell genome in this way.

Retroviruses infect primarily animal cells and are ubiquitous in vertebrates: They infect mammals, birds, amphibians, reptiles and fish, but are usually very host specific. They include the excitation of some widespread infectious diseases, pandemic or epidemic occur both in humans and in animals. As agents of disease in humans include HIV and HTLV- 1 is known.

• 4.1 entry into the host cell
• 4.2 Reverse Transcription
• 4.3 Overcoming the nuclear envelope
• 4.4 Integration into the host genome
• 4.5 expression and particle formation

Taxonomy

Historically, retroviruses were classified A, B, C or D retroviruses make their initial appearance in electron microscopic type. Later, a classification which also took into account biochemical properties and cell tropism followed. The classification differed Onkornaviren, spumaviruses and lentiviruses. The current, currently binding taxonomy by the International Committee on Taxonomy of Viruses ( ICTV ) divides the retroviruses, mainly due to their genetic relationships into two subfamilies and seven genera as follows:

Family: retroviruses ( Retroviridae )

• Alpha retroviruses
• Beta retroviruses
• Gamma retroviruses
• Delta retroviruses
• Epsilonretroviren
• Lentiviruses

• Foamy viruses ( spumaviruses )

When people are so far five retroviruses known: Human T- lymphotropic virus 1 (HTLV -1) ( a delta retrovirus ), Human T- lymphotropic virus 2 (HTLV -2) ( a delta retrovirus ), HIV -I (human immunodeficiency virus type I, a lentivirus ), HIV - II (human immunodeficiency virus type II, a lentivirus ) and Xenotropic murine leukemia virus -related virus ( XMRV a gamma retrovirus ). The human retroviruses are those of other primates so closely related that both groups are often collectively referred to primate retroviruses. It is assumed that the corresponding human retroviruses are caused by transmission of simian retroviruses to humans. In HTLV -1 and HTLV- 2, this transmission has probably occurred thousands of years ago, for HIV -I and HIV - II probably in the 20th century.

The division into simple and complex retroviruses is based on the genomic organization and the translation of accessory proteins. Costs include former alpha, gamma, epsilon and most beta retroviruses, the latter the delta retroviruses and lentiviruses and foamy viruses.

History of Retrovirology

Since the 19th century diseases were in domestic animals such as bovine leukosis or Jaagsiekte in sheep known, its cause remained unclear. In 1904, for the first caused by retroviruses disease Infectious anemia equine, shown by two French veterinarians Vallée and Carré, that they could be transferred to a filtrate to other horses. Oncogenes ( cancer -causing ) retroviruses have been studied since 1908, when the Danish pathologist Vilhelm Ellermann and Oluf Bang showed that was transferred to other chickens by cell-free filtrates chicken leukemia and the first described the contagious cancer. This virus is now known as avian leukosis virus ( ALV), first leukosis was not considered a true leukemia and leukemia were not considered as true tumors. These early studies were largely unnoticed within the scientific community until much later it was recognized their importance in the context of retroviruses. Peyton Rous made ​​in 1911 found that with filtered extracts of Hühnersarkomen healthy chickens could be infected, which then also developed tumors. The virus was later named after him Rous sarcoma virus (RSV ) and Rous was awarded in 1966, 54 years after its first description, the Nobel Prize. Only in 1961 it was found that Rous sarcoma virus ribonucleic acid containing (RNA), they were therefore called ( until 1974 ) and RNA tumor viruses.

The discovery that viruses can induce tumors in 1936 was also confirmed in mammals: John J. Bittner described the mouse mammary tumor virus (MMTV ). 1951 Murine leukemia virus ( MLV) was isolated and first described the vertical transmission from parents to offspring. 1964 suggested by Howard M. Temin provirus hypothesis, since it was observed that cells which have been " transformed" by RSV ( given -tumor properties ), in the absence of virus retained the transformed characteristics. For this reason, Temin postulated on the basis of temperate bacteriophages pension, of which you already knew that they can integrate into the genome of its host that the RNA tumor viruses do likewise. As early as 1960 it was proposed by André Lwoff that DNA tumor viruses ( polyomaviruses ) can integrate into the genome of their host. In 1968 it was shown that this assumption is correct. That RNA tumor viruses can be inherited through the germ line, was still regarded as bizarre.

Endogenous retroviruses were discovered in the late 1960s. The assumption that all viral genomes are inherited by their hosts according to Mendelian rules, was a completely new idea, and the provirus hypothesis of Temin has still not been generally accepted, partly even considered impossible. The reverse transcriptase, is transcribed by the RNA into DNA, was detected in 1970, and renamed the family of RNA tumor viruses as a result of 1974 in retroviruses. Temin's provirus hypothesis proved by the discovery of reverse transcriptase final as applicable.

At the beginning of the 1970s, the first viral proteins have been described and elucidated in outline the course of the following years, the replication cycle of retroviruses gradually. In 1978, the LTR regions ( long terminal repeats ) were detected in the genome of retroviruses, two years later was jumping scheme for the complex process of reverse transcription suggested. The technique of DNA sequencing, which emerged in the early 1980s, led in 1981 to the first publication of the complete genomic sequence of a retrovirus, the Moloney murine leukemia virus.

1980, the first description of human T -cell leukemia virus type 1 (HTLV -1), the first retrovirus that infects humans, after many years trying, unsuccessfully, in all possible tumor tissues of humans by retroviruses. Shortly afterwards, Luc Montagnier and Françoise Barré -Sinoussi discovered (Nobel Prize for Medicine 2008) HIV -1, HIV -2 followed in 1986. Least since 1988 it became clear that HIV is the cause of the immune deficiency disease AIDS, the Retrovirology developed from a rather exotic basic research to the most intense researched area in virology with great importance for the health sciences.

Construction

Virus particles

Infectious retrovirus particles have a diameter of about 100 nm, and they possess a capsid that is surrounded by a viral envelope, which was cut off from the cytoplasmic membrane of the host cell and is interspersed with viral glycoproteins, as well as a "core" within the capsid of other proteins and a ribonucleic acid complex.

Genome

The single-stranded RNA genome of retroviruses is linear and 7-12 kilobase pairs (kb ) in size. Retroviruses are single -stranded RNA viruses that are applied diploid, i.e., each retrovirus has two copies of its genome. They are translated and synthesized by the host's own enzymes and transcription require a specific cellular tRNA. The proviral genome of a single retrovirus generally contains three genes, and two long terminal repeats ( LTRs ) which are located at the beginning and at the end and containing information to control the expression of the viral genes. The three genes are gag ( group specific antigen), pol, and env. gag encodes the matrix, capsid and nucleocapsid proteins. pol encodes the viral enzymes protease, reverse transcriptase ( RNase H) and integrase. At the beta and delta retroviruses, the protease has its own reading frame (pro) and is at the alpha retroviruses, the information for the protease in the gag gene. env codes for the proteins of the envelope. Of regulatory sequences are in the 5'-region a designated by ψ (psi) sequence which is a signal for packaging of the RNA into the viral particle, a primer binding site (PBS), to which the respective tRNA can attach, and a promoter. In the 3 'region there are one or more Polypurintrakte that are essential for reverse transcription.

Complex retroviruses such as the lentiviruses belonging to the HIV virus, which belongs to the delta retroviruses HTLV or foamy viruses contain other regulatory genes, which are called accessory genes. In HIV- 1, these are tat, rev, vif, nef, vpu and vpr, rex in HTLV -1, rof, tax, tof and the foamy viruses tas and bet.

Life cycle

The life cycle of a retrovirus consists of several steps: infection of the cell, reverse transcription, overcoming the nuclear envelope, integration into the host genome, expression of the viral proteins and the RNA genome and the formation of new virus particles.

Entry into the host cell

After the glycoprotein of the viral envelope to its ( n ) cellular receptor (s) has bound to the viral membrane fuses with the membrane of the cell and releases the capsid into the interior of the cell, the cytoplasm. What happens to the capsid in the cytoplasm has not yet been clarified in detail, it probably breaks down into its individual components and are contained in the interior of the proteins, such as reverse transcriptase, and RNA genomes released into the cytoplasm of the host cell.

Reverse transcription

Main article: Reverse transcriptase

Retroviruses are single -stranded, plus-strand oriented RNA viruses in which the genome is not immediately as the template (mRNA ) can be used in the infection: When the virus RNA has been introduced into the to infesting cell, the RNA to be transcribed into double-stranded DNA. This process is called reverse transcription. For this, the virus brings the enzyme reverse transcriptase in its virus particles. This " write " to the RNA of the virus into the DNA which is then integrated by means of a second brought enzyme integrase into the genome of the host cell. For reverse transcription also the two LTR sequences are generated, which are essential for the further course of infection.

Normally, the transcription of the DNA as a template, extending, with a complementary strand of RNA is synthesized; an exception to the retroviruses and retroelements (also Class I transposons called ) dar. Because the process by the lack of proofreading ability of the reverse transcriptase is relatively inaccurate, carried frequent mutations of the virus. These allow a rapid adaptation of the virus to antiviral drugs and thus a development of resistance.

Overcoming the nuclear envelope

Some genera of retroviruses, such as gamma retroviruses, the nuclear envelope can not actively overcome. Therefore, you only infect cells that are in division and seize the moment of cell division for integration, if the genome is not protected by the nuclear envelope. Other genera such as the Alpha retroviruses and particularly the lentiviruses, the nuclear envelope active overcome and thus also infect resting cells. The entry into the nucleus is performed after formation of the pre-integration complex (PIC) in the cytoplasm. Since the nuclear pores are smaller than the PIC, which is about the size of a ribosome, it must be an active transport process. In this process is not completely understood, both cellular and viral proteins are involved. A well akzeptieres model describes the entry into the nucleus through the nuclear pores using karyophilic signals of the proteins contained in the PIC.

Integration into the host genome

Main article: integrase

The integration of the viral genome into the host genome is an essential step in the replication cycle of the virus. He is catalyzed by an enzyme called integrase, which is found in all retroviruses and retrotransposons. Integrase binds to the viral and host DNA, and with these forms a complex as Präintegrationskomplex (PIC, ENGL. Preintegration complex) is referred to. However, theory allows to integrate in any place in the host genome, depending on the nature of the retrovirus show certain preferred chromosomal regions. What exactly influences the site of integration, is not fully understood, sure, however, that the amino acid sequence of the integrase plays a role, resulting in specific interaction between integrase and factors in chromatin. In the right figure the sequence of the DNA strand breaks and subsequent recomposition in the integration of the viral genome is shown. In gray the integrase monomers, in red, the viral DNA and in black the host DNA are shown. The red dots are the 5 'ends of the viral DNA. (1) The linear viral cDNA is bound to the integrase portion of the pre-integration complex. (2) The integrase from two nucleotides such that the 5 ' overhangs created by the 3' ends of the viral DNA. (3) The integrase intersects the host DNA at a random location so that 5 'overhangs of 4-6 nucleotides occur, and connecting the shortened 3' ends of the viral DNA with the host DNA. ( 4) The pairing of host DNA is dissolved. ( 5) DNA repair enzymes fill in the gaps, ligases include the double strand. (6) During the process, the viral genome will lose the two terminal nucleotides. The 4-6 bases of host DNA flanking be doubled and the resulting provirus.

Expression and particle formation

After integration of cellular transcription factors and RNA polymerase II to be transformed cell to transcribe the DNA provirus. The necessary promoter and enhancer structures are located in the LTR of the provirus. In complex retroviruses some viral proteins act (eg did) additionally as a transcriptional enhancer. It creates different mRNAs by alternative splicing.

The resulting mRNAs are transported into the cytoplasm. There the different viral proteins are translated. The synthesis of the Env proteins occurs at the membrane of the endoplasmic reticulum, so that the Env proteins are directly anchored in the cell membrane, where they assemble into trimers. All other viral proteins are synthesized on free ribosomes. The Gag and Gag / Pol precursor proteins are myristylated amino-terminal and attach themselves to the cell membrane. At the cell membrane, then the particle formation took place: Gag and Gag / Pol precursor proteins accumulate and interact except with glycoproteins of the cell together. Only the unspliced ​​mRNAs from which Gag and Pol were translated, have the packaging signal Psi, and the leader sequence. With the help of Psi signal the mRNAs bind to the zinc finger motifs of Nucleokapsidproteine ​​- so ensures that only unspliced ​​and thus complete genomes are packaged into the virus particle. Upon contact with the mRNAs, the membrane invaginates at the cell surface and a constricted from an immature virus particles. Only within this particle starts the viral protease to aggregate into dimers, is cut out from the precursor proteins in an autocatalytic section and then the Gag and Gag / Pol precursor proteins into the individual components (matrix, capsid, nucleocapsid, reverse transcriptase and integrase ) to split. Within the particle, the capsid proteins associate to give a conical capsid. Only at the end of this maturation process of the particles is infectious.

Endogenous retroviruses and evolutionary development

Main article: retroelement, retrotransposon, Endogenous retrovirus

The exact date of origin of the first retrovirus- like particles is unclear, the oldest sequences define an age of 250 million years suggests they are probably considerably older. Retroviruses have probably evolved from retrotransposons. This means that they represent an infectious, evolved form of the retro elements. Accordingly, the reverse transcription of the oldest mechanisms would be in the retrovirus development, may already incurred in the RNA world. Great similarity exists in any case between retroviruses and transposons from different creatures, such as the Ty elements of baker's yeast and the copia and Ulysses elements from Drosophila melanogaster. These retrotransposons encoding a reverse transcriptase, and having a similar structure as the retroviruses.

The integration into the genome of its host is one of the most unusual and remarkable features of retroviruses. The myriad of similar sequences in vertebrates and retroviruses shows that retroviruses have very often infected in the past and the germ line cells of their hosts. Thus the descendants inherited retroviruses as endogenous retroviruses ( ERV ) designated to distinguish them from the horizontally passed, exogenous retroviruses. The increasing number of sequenced organisms, more and more endogenous retroviruses have been discovered. The amount of retroviral DNA varies in vertebrates 5-10 %, the human genome consists of about 8% of retroviral sequences. These data provide insight into the long host -virus coevolution. With the Endogenous retroviruses were derived from the infectious viruses that originated from transposons, again parts of the genomes.

So far in the human genome 31 different ERV "Families " section, which probably go back to 31 different cases of germ line infections by retroviruses ( known in English as genome invasion event ( genome invasion event ) refers ). This initial event was followed by an increase in ERV copy number, either by re-infection of germline cells or by retrotransposition within the cell. Over the generations, the activity of ERVs steadily decreasing since accumulate mutations and whole sections of the ERVs may be lost, until finally the activity of the virus ceases altogether. The age of the individual ERV ERV families or lines can be estimated from the size and shape of the phylogenetic pedigrees of the lines. Most of the human ERV- lines ( HERV ) arose, therefore, before the evolutionary development of Old World and New World monkeys in front of about 25 to 30 million years.

Diseases caused by retroviruses

From retroviruses are many different creatures are infected. The species affected range from mussels to human beings, most of them are, however, found among the vertebrates. Retroviruses cause in their hosts a large number of different types of diseases, including tumors ( lymphomas, sarcomas), neurological diseases and immune deficiencies. Some of these diseases cause great damage to agriculture, because livestock are affected, or are the cause of human pandemics (AIDS). Other infections remain asymptomatic, which is why these retroviruses are considered to be non-pathogenic.

Some induced by retroviruses diseases of rodents represent model systems by which the infection mechanisms of retroviruses and the formation caused by some retroviruses tumors can be studied in detail. Modern tumor biology based in part on data that could be generated due to these models.