FREE ESSAY ON AIDS - RETROVIRUSES

AIDS - RETROVIRUSES

Abstract
Today, tens of millions of people around the world are going to die young because they
are infected by HIV, the virus that causes AIDS. The primary AIDS virus is HIV-1, which
can be spread via sexual intercourse or drug use (activities, which result in body fluid
exchange like blood and semen). HIV can also be passed from mother to child and can also
be acquired during blood transfusions. AIDS, Acquired Immune Deficiency Syndrome, is a
virus that causes a loss of protection against disease causing microorganisms. People who
are infected by AIDS usually have a decline in the number of T-cells that are responsible
for their immune system. Because the virus reproduces by a reversed transcriptase enzyme,
it can quickly make a copy of its virus and so, it is difficult to treat with its
infinite reproductions. However, scientists have now came up with a combination of drug
called cocktail drugs to slow down the development of the virus in different life cycles.

Introduction
HIV, the Human Immunodeficiency Virus, is a disease that merits international concern,
study, and research. The diseases that infect individuals over the world are causing
theme to lose the functionality of their immune system and finally succumb to one or more
opportunistic diseases. AIDS viruses are unbelievably small particles, invisible under an
ordinary microscope, but observable with an electron microscope. All the viruses are tiny
packets of genetic material, composed of either DNA or RNA and are surrounded by a
protective envelope of protein. HIV belongs to a group of especially tricky viruses
called retrovirus. It reverses the usual flow of genetic information within the host
cell. Retroviral RNA is copied, using its reverse transcriptase enzyme, into a
complementary single strand of DNA. This single-strand retroviral DNA is then copied into
double-stranded retroviral DNA inside the cell's cytoplasm. This retroviral DNA migrates
into the host cell's nucleus and becomes integrated into the host cell DNA. It is then a
provirus. (Hyde & Forsyth, 1996)
1.0 Retroviruses
1.1 History
There are two strains of HIV, HIV-1 and HIV-2. HIV-1 is the more prevalent form and also
the one that causes the greater pathology. HIV-2 is a lesser found strain that is endemic
to countries in West Africa. Presently, the majority of those infected with HIV are
infected with the HIV-1 strain. When the infection progresses, the patient is diagnosed
with acquired immunodeficiency syndrome (AIDS).
The first well-documented case of AIDS was found in San Francisco and New York in 1981. A
young gay man in San Francisco was found to be suffering from a severe fungal infection
to which he had little immune reaction. In fact, his immune system did not appear to
respond to any disease. Then he developed Pneunocystis carinii pneumonia (PCP), a type of
pneumonia that is caused by parasite. This disease is usually found only among severely
malnourished individuals or people whose immune systems have been impaired by drugs such
as those used in the treatment of cancer or in connection with organ transplants. The
young man died by the end of the year. Retroviruses are classified into many different
categories, but Lentivirus is responsible for human immunodeficiency virus (HIV-1).
(Armstrong, 1990)
1.2 T-Cells 
A characteristic feature of AIDS is the progressive decline in the number of circulating
CD4 T lymphocytes . Various strains of HIV are cytopathic for cells from cultured CD4
cell lines suggesting that direct killing, among other mechanisms, may contribute to
depletion of CD4 T-cells in HIV-infected persons. Cell killing may also be involved in
other aspects of HIV pathogenesis, including the induction of neurological dysfunctions.
The molecular events that mediate HIV-induced cytopathology have not yet been elucidated,
although env gene products have been implicated by many lines of evidence. Studies funded
in the previous years of this grant indicate that HIV, like several other cytolytic
viruses, mediates changes in plasma membrane-associated ion transport systems. Scientists
proposed studies focus on defining HIV proteins, which mediate the changes in ion flux.
Their study found out that one of the HIV proteins may be involved is the transmembrane
protein (TM). Among the regions of TM, there are carboxyl terminal sequences involved.
These sequences have the potential to form a strongly amphipathic helix enriched in
arginine, a motif shared with monovalent cation channels. HIV also have affects the
potassium and sodium ion transport systems. (Armstrong,1990)
1.3 Structure of AIDS virus
There is considerable diversity between various types of retrovirus; the following is a
generalized description of the particle. There is a universal nomenclature for retrovirus
proteins:
Protein Function
Matrix matrix protein (gag gene); lines envelope
Capsid capsid protein (gag gene); protects the core; most abundant protein in virus
particle
Nucleocapsid capsid protein (gag gene); protects the genome; forms the core
Protease Essential for gag protein cleavage during maturation
Integrase Encoded by the pol gene; needed for integration of the provirus
Surface glycoprotein The outer envelope glycoprotein; major virus antigen
Transmembrane protein The inner component of the mature envelope glycoprotein
Reverse transcriptase Reverse transcribes the RNA genome; also has RNAseH activity
Retroviruses have enveloped particles and vary in sizes and shapes, but are usually 100nm
in diameter. The envelope carries a virus-encoded glycoprotein, which forms spikes in the
membrane. There are certain structural/functional similarities between the envelope
glycoprotein and the influenza haemagglutinin. The mature protein is cleaved into 2
polypeptides: 
-  The outer envelope glycoprotein is the major antigen of the virus, responsible for
receptor binding, linked by disulphide bonds to: 
-  The trans-membrane glycoprotein holds the SU protein in the envelope, responsible for
membrane fusion.
Inside the membrane is the matrix protein, rather amorphous. Capsid is the most abundant
protein in the particle making 33 per cent of the total weight. Inside the capsid is the
core, which contains RNA, genome+NC and protein+RT+IN.
All retrovirus genomes consist of two molecules of RNA, a 5' cap and a 3' poly-A
(equivalent to mRNA). Retrovirus genomes have 4 unique features: 
 They are viruses that are truly diploid. 
 They are the only RNA viruses whose genome is produced by cellular
transcriptional machinery, that is, without any participation by a virus-encoded
polymerase. 
 They are the only viruses whose genome requires a specific cellular RNA (tRNA)
for replication. 
 They are the only RNA viruses whose genome does not serve directly as mRNA
immediately after infection. 
These two molecules are physically linked by hydrogen bonds. (Retroviruses [Online])
1.4 Replication
Shortly after infection, only low amounts of parental infectious material can be
identified, this is the so-called eclipse phase. At this stage, genome replication has
been initiated but progeny viruses are not yet released. There is then a maturation phase
when viral material accumulates exponentially in the cell or surrounding medium. After a
few hours cells infected with lytic viruses become metabolically disordered and viral
production ceases. Titres then slowly drop. Cells infected with non-lytic viruses can
continue to produce viral particles indefinitely. 
Infection of a cell may be: 
-  productive, the cells are permissive for viral replication and virion progeny are
released.
-  abortive, the cells are non permissive for a viral function and virion particles not
produced. 
-  restrictive, the cell is transiently permissive and a few virus are produced. Viral
production then ceases but the genome persists. This is one of the reasons why some
people do not develop AIDS. 
The replication cycle of HIV is shown: 
Source: Retrovirus (Online)
There are several steps involved in the HIV cycle: 
Source: AIDS fact sheet (Online)
 Attachment
Virus attachment consists of specific binding of a viral attachment protein (VAP) to a
cellular receptor. Receptor molecules can be proteins (glycoproteins), or the sugar
residues present on glycoproteins or glycolipids. Some complex viruses, for example,
Poxviruses and Herpesviruses may have more than one receptor-binding protein, therefore,
there may be alternative routes of uptake into cells. The expression or absence of
receptors on the surface of cells largely determines the tropism of most viruses, that
is, the type of cell in which they are able to replicate.
 Penetration
Unlike attachment, viral penetration is an energy-dependent process; that is, the cell
must be metabolically active for this to occur. Three mechanisms may be involved: 
-  Translocation of the entire virion across the cell membrane 
-  Endocytosis of the virus into intracellular vacuoles. 
-  Fusion of the viral envelope with the cell membrane, which requires the presence of a
viral fusion protein in the virus envelop. 
 Uncoating
A general term for the events which occur after penetration, in which the capsid is
removed and the virus genome exposed, usually in the form of a nucleoprotein complex.
This can happen when retrovirus cores are highly ordered nucleoprotein complexes which
contain, in addition to the diploid RNA genome, the reverse transcriptase enzyme
responsible for converting the viral RNA genome into the DNA provirus. This process
occurs inside the core particle. 
For viruses that replicate in the cytoplasm, the genome is simply released into the cell,
but for viruses that replicate in the nucleus, often with associated nucleoproteins, must
be transported through the nuclear membrane. This is achieved by interactions of the
nucleoproteins or capsid with the cytoskeleton. At the nuclear pores, the capsid is
stripped off, and the genome passes into the nucleus. 
 Gene Expression
Control of viral replication is achieved by tight regulation of gene expression and the
methods used depend on nature of the virus genome/replication strategy. One advantage of
monocistronic mRNAs is that various proteins can be produced in different amounts, rather
than in a constant ratio.
Non-segmented genomes tend to produce polycistronic mRNA, which is translated to form a
polyprotein, processed by proteolytic cleavage to form the mature gene products.
To utilize the cellular machinery, viral mRNAs must contain control signals which are
recognized by the cell. 
 Assembly
This involves the assembly of all the components necessary for the formation of the
mature virion at a particular site in the cell. During this process, the basic structure
of the virus is formed.
 Release 
For lytic viruses, release is a simple process - the cell breaks open and releases the
virus.
Enveloped viruses acquire the lipid membrane as the virus buds out through the cell
membrane. Virion envelope proteins are picked up during this process as the virus is
extruded. Budding may or may not kill the cell, but is controlled by the virus - the
physical interaction of the capsid proteins on the inner surface of the cell membrane
forces the particle out through the membrane:
 Maturation
The stage of the life cycle at which the virus becomes infectious. Usually involves
structural changes in the particle, often resulting from specific cleavage of capsid
proteins to form the mature products, which frequently leads to a conformational change
in the capsid, or the condensation of nucleoproteins with the genome. For some viruses,
assembly and maturation are inseparable, whereas for others, maturation may occur after
the virus particle has left the cell. (Retroviruses [Online])
2.0 Transcription
The transcription of RNA in AIDS is caused by reverse transcriptase enzyme. This enzyme
changes HIV's genetic material (RNA) into the form of DNA. This step has to occur before
HIV's genetic code gets combined with an infected cell's own genetic codes. Within the
host cell nucleus, proviral DNA, when activated, produces new strands of RNA. Some RNA is
used as mRNA to produce proteins for making HIV. Other RNA becomes encased within the
viral core proteins to become the new viruses. The RNA strands that are splices become
the mRNA strands used in protein synthesis. The unspliced RNA strands serve as new viral
strands that are encased in their protein coats (capsids) to become new viruses that bud
out of the cell. 
Source: Retrovirus - Transcription (Online)
There are two distinct phases of transcription follow the HIV infection of an individual
cell. Firstly, the RNA strands produced in the cell's nucleus are snipped into multiple
copies of shorter sequences by cellular splicing enzymes. When they reach the cytoplasm
they are only about 2,000 nucleotides in length. These early-phase short transcripts
encode only the virus' regulatory proteins. The regulatory proteins and the structural
genes that constitute the rest of the genome are among parts that are left behind. In the
second, or late phase, two new size classes of RNA move out of the nucleus and into the
cytoplasm. (Cozic & Swisher, 1991)
3.0 Treatment
Although scientists have not came up with sufficient treatments for AIDS, researchers
have found several ways to suppress HIV reproductions:
3.1 Tat Toxoid
Robert Gallo, a co-discoverer of HIV has published significant findings using an
experimental vaccine for an AIDS-like illness in rhesus macaques (monkeys). The vaccine
did not prevent infection, but did weaken the disease with undetectable viral loads, CD4
counts that remained within normal limits, and other parameters, when compared to control
animals. Tat is a protein produced by HIV that has been linked with several components of
HIV disease. In laboratory tests, immune cells treated with Tat have a lesser response to
new, foreign antigens (infections) and can enhance HIV growth in those cells. Tat also
increases levels of fas ligand on immune mononuclear cells, a precursor to apoptosis
(programmed cell death). This is thought to be one possible component that leads to
decreased CD4 counts. Lastly, Tat increases the number of chemokine (intercellular
messengers) receptors on immune cells that HIV partly uses to enter them. Scientists
found out that among those monkeys, who developed both types of immune responses to Tat,
88% were protected against high level virus replication. It should be emphasized,
however, that the vaccine did not protect against infection, but led to viral load levels
that were attenuated in many animals. (Drugs and AIDS, 1994)
3.2 Chinese Herbal Medicines
The Chinese herbal medicine is known as an energy balancing treatment for people with
unbalanced yin and yang. Chinese herbs do not cure HIV infection. However, many people
believed that the herbs have helped them to improve their overall energy, or deal with
the side effects of antiviral medications. Some people have used herbs to reduce the
upset stomach or diarrhea caused by their medications. The reason for this is because
Chinese herbs generally strengthen the immune system and therefore eliminate the chance
of getting sick of an AIDS infected person. (Drugs and AIDS, 1994)
3.3 Cocktail Drugs
Cocktail drugs are known as a combination of a variety of protease inhibitor Invirase,
Nucleoside Analogues and Hivid ?, which can all help to reduce the time to disease
progression or death by more than 50 percent. These drugs combine to attack and slow down
the replication and transcription of the RNA in the virus. Therefore, the patient can
live longer because the drugs are slowing down the speed of disease development.
(Protease Works [Online])
Conclusion
The true impact of AIDS is now fully known for many years for it produced profound
changes in society. Although there are still questions in the minds of some individuals
about whether AIDS should be treated as a medical or a moral problem, an increasing
number are asking what can be done to best serve the dignity, liberty, and health of
those who are infected, as well as to prevent new infections in the name of public
health. While it is the duty of the infected not to spread the virus, it is the
responsibility of the community to provide education, compassion, and support.