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HIV:HUMAN IMMUNO DEFIENCY VIRUS

Viruses cannot reproduce themselves without the
aid of a living (host) cell
...
T-cells are an
important part of the immune system because
control the body's response to many types of
infections
...
HIV enters the body
-- through the usual means of unsafe sex, sharing
contaminated needles, blood transfusions or from
mother to child (vertical infection)and waits to
bump into its host cell; the T-cell
...
For this to happen HIV
has to complete its life cycle: The steps of the Viral
life cycle all involve making more viruses
...

On the surface of your T-cells are receptors, Hiv
binds to two in particular, CD4 and a chemokine
receptor, are used by HIV to attach onto the cell in
HIV structure has three components order to gain access
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membrane, or outer surface
...
To do this, HIV uses an enzyme known as Reverse Transcriptase
...
This
enzyme is only found in Retroviruses (Viruses with RNA genomes)
...
These inhibitors are currently part of our best
treatment regime for AIDS
...
Integration is the process by which HIV
inserts its DNA into the cell's DNA
...

Packaging - Once the cell has the viral instructions in its nucleus and becomes activated, it is
ready to make new HIV
...
HIV uses an enzyme called protease to cut the long chains of subunits so
they can come together to form the new virus
...

Once the subunits are cut by the protease enzyme they are ready to be put together to form the
new viruses
...

This process is known as budding
...
Once this process is completed the new HIV
particles leave the cell in search of new cells to infect and start the whole process over
...
Resistance to HIV infection is a result of a defect in
the gene for a chemokine receptor, CKR-5
...
In resistant individuals, the gene for
CKR-5 exhibits a 32 bp deletion which alters the protein product
...


Introduction
Bringing the global HIV epidemic under control will require more effective approaches to prevent the
spread of the retrovirus, as well as broader use of existing and future antiretroviral drugs
...

Understanding the dynamic interplay of HIV with its cellular host provides the biological basis for
controlling the epidemic
...


Binding and Entry
The genetic material of HIV, an RNA molecule 9 kilobases in length, contains 9 different genes
encoding 15 proteins
...
(F To productively infect a target cell, HIV must introduce its genetic material into the
cytoplasm of this cell
...

The two viral envelope proteins, gp120 and gp41, are conformationally associated to form a trimeric
functional unit consisting of three molecules of gp120 exposed on the virion surface and associated
with three molecules of gp41 inserted into the viral lipid membrane
...
These receptors normally play a role in chemoattraction, in which hematopoietic
cells move along chemokine gradients to specific sites
...


Twelve chemokine receptors can function as HIV coreceptors in cultured cells, but only two are
known to play a role in vivo
...
The
other, CXCR4, binds T-cell-tropic, syncytium-inducing (X4) viruses, which are frequently found
during the later stages of disease
...
Individuals homozygous for this mutation (1-2% of the Caucasian
population) are almost completely resistant to HIV infection
...

Both CD4 and chemokine coreceptors for HIV are found disproportionately in lipid rafts in the cell
membrane
...

Removing cholesterol from virions, producer cells, or target cells greatly decreases the infectivity of
HIV
...
The
development of effective microbicides represents an important component of future HIV prevention
strategies
...
Assembled as a trimer on the virion membrane, this coiled-coil protein
springs open, projecting three peptide fusion domains that "harpoon" the lipid bilayer of the target cell
...
The fusion inhibitors T20 and T-1249 act to prevent fusion by blocking the formation of these hairpin structures
...
Usually, productive infection does not result,
presumably reflecting inactivation of these virions within endosomes
...
These cells, which normally process
and present antigens to immune cells, express a specialized attachment structure termed DCSIGN
...
Instead, virions bound to DC-SIGN are internalized into an acidic
compartment and subsequently displayed on the cell surface after the dendritic cell has matured and
migrated to regional lymph nodes, where it engages T cells
...


Cytoplasmic Events
Once inside the cell, the virion undergoes uncoating, likely while still associated with the plasma
membrane
...
Nef associates with a universal proton pump, V-ATPase, which could promote uncoating
by inducing local changes in pH in a manner similar to that of the M2 protein of influenza
...
This
complex includes the diploid viral RNA genome, lysine transfer RNA (tRNALys) which acts as a
primer for reverse transcription, viral reverse transcriptase, integrase, matrix and nucleocapsid
proteins, viral protein R (Vpr), and various host proteins
...
This interaction, mediated by the phosphorylated matrix, is required for
efficient viral DNA synthesis
...

Reverse transcription yields the HIV preintegration complex (PIC), composed of double-stranded viral
cDNA, integrase, matrix, Vpr, reverse transcriptase, and the high mobility group DNA-binding
cellular protein HMGI(Y)
...
Adenovirus and herpes simplex virus 1 also dock with microtubules and use the microtubuleassociated dynein molecular motor for cytoplasmic transport
...
How the switch from actin microfilaments
to microtubules is orchestrated remains unknown
...
Within 30 minutes of infection, select host proteins including the integrase interactor 1 (also
known as INI-1, SNF5, or BAF47), a component of the SWI/SNF chromatin remodeling complex, and
PML, a protein present in promyelocytic oncogenic domains, translocate from the nucleus into the
cytoplasm
...
The binding of integrase to integrase interactor 1 may be a viral adaptation that recruits
additional chromatin remodeling factors
...


Crossing the Nuclear Pore
Unlike most animal retroviruses, HIV can infect nondividing cells, such as terminally differentiated
macrophages
...
With a Stokes radius of
approximately 28 nm or roughly the size of a ribosome, the PIC is roughly twice as large as the
maximal diameter of the central aqueous channel in the nuclear pore
...

One of the most contentious areas of HIV research involves the identification of key viral proteins that
mediate the nuclear import of the PIC
...
Because plusstrand synthesis is discontinuous in reverse transcription, a triple helical DNA domain or "DNA flap"
results that may bind a host protein containing a nuclear targeting signal
...
However, a recent publication calls into question the
contributions both of the nuclear import signal in integrase and of the DNA flap to the nuclear uptake
of the PIC
...
Vpr may bypass the importin system altogether, perhaps mediating the direct docking of the
PIC with one or more components of the nuclear pore complex
...
For example, while Vpr is not needed for infection of nondividing, resting T cells, it
enhances viral infection in nondividing macrophages
...


Integration
Once inside the nucleus, the viral PIC can establish a functional provirus
...
The host proteins HMGI(Y) and barrier to autointegration (BAF) are required for efficient

integration, although their precise functions remain unknown
...
Integrase also catalyzes the subsequent
joining reaction that establishes the HIV provirus within the chromosome
...
The ends of the viral DNA may be
joined to form a 2-LTR circle containing long terminal repeat sequences from both ends of the viral
genome, or the viral genome may undergo homologous recombination yielding a single-LTR circle
...

Although some circular forms may direct the synthesis of the transcriptional transactivator Tat or the
accessory protein Nef, none produces infectious virus
...
This system is responsible for rapid repair of double-strand breaks, thereby preventing an
apoptotic response
...
The
ability of the free ends of the viral DNA to mimic such double-strand chromosomal breaks may
contribute to the direct cytopathic effects observed with HIV
...
HIV's transcriptional latency
explains the inability of potent antiviral therapies to eradicate the virus from the body
...
Understanding latency and
developing approaches to target latent virus are essential goals if eradication of HIV infection is ever
to be achieved
...
For example,
proviral integration into repressed heterochromatin might result in latency
...
However, of the multiple copies of provirus that are usually
integrated in a given infected cell, at least one is likely to be transcriptionally active
...

In the host genome, the 5´ LTR functions like other eukaryotic transcriptional units
...
These regions help position the RNA polymerase II (RNAPII) at the site of initiation of
transcription and to assemble the preinitiation complex
...
NF-[kappa]B and NFAT relocalize to the
nucleus after cellular activation
...
NFAT is dephosphorylated by calcineurin (a reaction inhibited by cyclosporin A) and, after

its nuclear import, assembles with AP1 to form the fully active transcriptional complex
...
Since NF-[kappa]B is activated after several antigen-specific and cytokine-mediated
events, it may play a key role in rousing transcriptionally silent proviruses
When these factors engage the LTR, transcription begins, but in the absence of Tat described below
the polymerase fails to elongate efficiently along the viral genome
...

Tat significantly increases the rate of viral gene expression
...
Within the positive transcription elongation factor b (P-TEFb) complex, Cdk9 phosphorylates the
C-terminal domain of RNAPII, marking the transition from initiation to elongation of eukaryotic
transcription
...
The high efficiency
with which the HIV LTR attracts these negative transcription factors in vivo may explain why the
LTR is a poor promoter in the absence of Tat
...
A shorter ARM in cyclin T1, which is also called the Tat-TAR recognition motif
(TRM), binds the central loop of TAR
...
All of these components are required for Tat transactivation
...
Because murine CycT1
contains a cysteine at position 261, the complex between Tat and murine P-TEFb binds TAR
weakly
...
Cdk9 also must undergo
autophosphorylation of several serine and threonine residues near its C-terminus to allow productive
interactions between Tat, P-TEFb, and TAR
...
All of these
events may contribute to postintegration latency
...

Some are processed cotranscriptionally and, in the absence of inhibitory RNA sequences (IRS),
transported rapidly into the cytoplasm
...

Other singly spliced or unspliced viral transcripts remain in the nucleus and are relatively stable
...

Incomplete splicing likely results from suboptimal splice donor and acceptor sites in viral transcripts
...

Transport of the incompletely spliced viral transcripts to the cytoplasm depends on an adequate supply
of Rev
...
Rev binds first with high affinity to a small region of
the RRE termed the stem-loop IIB
...
In addition to a nuclear localization signal, Rev contains a leucine-rich nuclear export
sequence (NES)
...

The nuclear export of this assembly (viral RNA transcript, Rev, and CRM1/exportin 1) depends
critically on yet another host factor, RanGTP
...
RanGDP is found predominantly in the cytoplasm
because the GTPase activating protein specific for Ran (RanGAP) is expressed in this cellular
compartment
...
The inverse nucleocytoplasmic gradients of RanGTP and
RanGDP produced by the subcellular localization of these enzymes likely plays a major role in
determining the directional transport of proteins into and out of the nucleus
...
However, when the complex
reaches the cytoplasm, GTP is hydrolyzed to GDP, resulting in release of the bound cargo
...

For HIV infection to spread, a balance between splicing and transport of viral mRNA species must be
achieved
...

Although required, the regulatory proteins encoded by multiply spliced transcripts are insufficient to
support full viral replication
...
In many non-primate cells, HIV transcripts may be overly spliced, effectively
preventing viral replication in these hosts
...
The absence of Nef in infected monkeys and humans is
associated with much slower clinical progression to AIDS
...
Nef also promotes the production
and release of virions that are more infectious
...
Indeed, Nef and viral structural proteins colocalize in lipid rafts
...
The envelope protein gp120 binds CD4 in the
endoplasmic reticulum, slowing its export to the plasma membrane,( and Vpu binds the cytoplasmic
tail of CD4, promoting recruitment of TrCP and Skp1p
...

Nef acts by several mechanisms to impair immunological responses to HIV
...
Nef also reduces the expression
of MHC I determinants on the surface of the infected celland so decreases the recognition and killing
of infected cells by CD8 cytotoxic T cells
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Nef also inhibits apoptosis
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Nef also binds the tumor
suppressor protein p53, inhibiting another potiential initator of apoptosis
...

Other viral proteins also participate in the modification of the environment in infected cells
...
Since the viral LTR is more active during G2, this arrest likely enhances viral gene
expression
...
Intermittently, these herniations rupture, causing the mixing of soluble nuclear and
cytoplasmic proteins
...


Viral Assembly
New viral particles are assembled at the plasma membrane
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Several proteins participate in the assembly process, including Gag polyproteins and Gag-Pol, as well
as Nef and Env
...
In primary CD4 T lymphocytes, Vif plays a key but poorly understood role
in the assembly of infectious virions
...
Heterokaryon analyses of cells formed by the fusion of nonpermissive (requiring Vif for

viral growth) and permissive (supporting growth of Vif-deficient viruses) cells have revealed that Vif
overcomes the effects of a natural inhibitor of HIV replication
...
Whether the intrinsic antiviral activity of CEM15 involves such an RNA
editing function remains unknown
...


Virion Budding
The Gag polyproteins are subject to myristylation,and thus associate preferentially with cholesteroland glycolipid-enriched membrane microdomains
...
This lipid composition
likely favors release, stability, and fusion of virions with the subsequent target cell
...

The product of the tumor suppressor gene 101 (TSG101) binds the PTAP motif of p6 Gag and also
recognizes ubiquitin through its ubiquitin enzyme 2 (UEV) domain
...
The MVB is
produced when surface patches on late endosomes bud away from the cytoplasm and fuse with
lysosomes, releasing their contents for degradation within this organelle
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Summary and Conclusion
As the AIDS pandemic continues, advances in antiretroviral therapies have slowed its advance in the
industrialized world, but have had little effect in developing countries
...
The virus ensures that the host cell survives until the viral
replicative cycle is completed
...
Eradication of the virus appears unlikely until effective
methods are developed to purge these latent viral reservoirs
...
A small-animal model that recapitulates the pathogenic mechanisms of HIV is sorely

needed to study the mechanisms underlying viral cytopathogenesis
...
Murine cells support neither
efficient virion assembly nor release of virions from the cell surface
...

Proposed mechanisms for HIV killing of T cells include the formation of giant cell syncytia through
the interactions of gp120 with CD4 and chemokine receptors, the accumulation of unintegrated linear
forms of viral DNA, the proapoptotic effects of the Tat, Nef,and Vpr proteins, and the adverse effects
conferred by the metabolic burden that HIV replication places on the infected cell
...
All of these mechanisms suggest potential points of
therapeutic intervention
...
Clinical trials are already underway to study small
molecules or short peptides that block the binding of HIV to cell-surface chemokine receptors or
interfere with the machinery of viral-host cell fusion
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As a proof of principle, dominant-negative mutants of Tat, Rev, and
Gag proteins have been shown to block viral replication
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Acknowledgements
Warner C
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Pendleton
Charitable Trust, and the J
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B
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