HSV-1 and HSV-2 are very closely related viruses, which have
preferences for oral and genital mucosa, respectively. [11]
Independently these two virus have very similar modes of infection,
varying more in their specific binding motifs but using related
structures. It could be argued that the differences in the nucleotide
sequences of their genomes and the amino acid sequences of their
proteins reflect the difference in the environments in which they
function. [11] For this reason, the following discussion is relative
for each strain.
 
 

Transmission
 

Oral  
Genital
Oral-Genital  
Anogenital  
Oral to anogenital
Autoinoculation [24][77]

Attachment

Once introduced to HSV,  HSV virions undergo a complex multi-step
process initiating fusion to host cells in the parabasal and
intermediate epithelium. [11][77] This fusion process, of HSV, is
intiated by a cascade of dynamic interactions. These interactions are
known to rely on a set of HSVs surface glycoproteins which interact
with cell surface molecules (the complete mechanism is unknown). As of
now, we know HSV DNA encodes for at least 11 glycoproteins . [12] The
initial attachment of the HSV is mediated by glycoprotein C (gC), a
conserved 120-kDa protein, and/or glycoprotein B (gB) which forms a
low affinity attachment to heparan sulfate proteoglycan (HS) on the
cell surface. [12][14][60] These HS have a negative charge and usually
interact with a variety of proteins such as cytokines, extracellular
matrix proteins, enzymes, and enzyme inhibitors. [60] In order to
prove that the binding  protein of HSV was indeed gC, a study
demonstrated that HSV infection can be blocked with antibodies
directed against gC by preventing its primary attachment to host
cells. [15]    However, as indicated previously, the specific features
of the host protein recognized are different for each serotype's gC
protein, although heparan sulfate serves as a receptor for both HSV-1
and HSV-2. [14] Interestingly, it is these types of minor alterations
which dominate the specificity for the realm of infection created by
each serotype. In evolutionary terms, these mutations resemble a
vicariance event, a viral divergence if you will.
                                                                     
                            [82]
 

Future Focus

   In order to understand this interaction further it is important to
take a look at HS. Heparin and related polysaccharide HS are
glycosaminoglycans (GAGs) which are synthesized as proteoglycans with
GAG chains covalently bound to a protein core. The backbone of a GAG
moiety consists of alternating hexuronic acid and hexoamine units.
[60] This has significant applications in developing an
oligosaccharide capable of binding gC which would have an inhibitory
effect on HSV binding. [60] As stated earlier, no known information
regarding the interactive structure of gC/HSV-1 binding domains in HS
are known, but the need for further research is evident. [60]
 



Virion Penetration

After initial binding of HSV by gC/gB, an interaction of glycoprotein
D (gD) with cellular receptors creates an  even higher affinity
binding to the host cell. [12][14][15] Subsequently gD-negative
virions and virions treated with neutralizing antibodies failed to
enter cells, further exemplifying the importance of one of gD's
functions. [14] Then gC, gB, gD, with gH, and gL act alone or in
combination to trigger a pH-independent fusion of the viral envelope
(nuclear) and the host cell plasma membrane. [12][14]  The membrane
receptor that gD binds to has not yet been identified, however one
study found that gD can bind to mannose-6-phosphate receptors.
[14][15]

Summary
 

Surface Protein  Function
gC Initial Attachment to heparan sulfate (low affinity) of free
virion, Immune Evasion (binds complement component C3b)
gB Initial Attachment (low affinity) of free virion, penetration,
required for production of virus in cell cultures
gD Secondary binding (high affinity) of free virion (possibly to
mannose-6-phosphate receptor), fusion for cell-cell spread,
penetration, required for production of virus in cell cultures
gE Immune Evasion (binding IgG Fc region with gI), fusion for
cell-cell spread
gI Immune Evasion (binding of IgG Fc region with gE), fusion for
cell-cell spread
gH HSV fusion to host cell, penetration, required for production of
virus in cell cultures
gL HSV fusion to host cell, penetration, required for production of
virus in cell cultures
gG  ?
gK  Required for production of virus in cell cultures, implicated in
cell fusion
gJ ?
gM ?



Retrograde Axonal Capsid Transport
     Upon virion membrane fusion and penetration of the distal axonal
terminae of a neuron, the HSV capsid undergoes rapid transport by
retrograde axonal flow to the neuronal nucleus. [19][45] The speed of
retrograde transport is 2.2 +/- 0.26 micrometers/sec or 3-5 mm/Hr
(consistent for all viral particles observed). [45] Although this
transport process is unknown, current investigation has revealed that
the transported viruses have lost their envelope and retain some of
their tegument protein, VP16, which has been shown when tegument
proteins are labeled with fluorescent protein. [45] This protein was
shown to be necessary for transport, since no viruses were found by
western blot unless fused with protein fl bound to VP16 [45]
Therefore, it is believed that viral tegument and capsid are
sufficient to recruit cellular machinery for retrograde transport.
Keep in mind that these associations are only starting to be
discovered and the exact mechanism for transport of the viral proteins
is unknown. [45] Discovered thus far are at least 11 tegument proteins
which are most likely the candidates for recruiting the cellular
machinery for transport. [45] Currently it is believed that
microtubules of the cytoskelton play an essential role in retrograde
transport, specifically the protein Kinesin acting in association with
actin filaments. [11] [45]  Currently there is only one known
microtubule based retrograde motor in axons, dynein. Which needs the
presence of a cofactor, dynactin, a large multi-molecular complex
which binds organelles. [45]  In observing active transport, the virus
appears to only travel in a linear manor since it does not typically
alter its plan of focus (Video). [45] At this point the capsid fuses
with the nuclear membrane and the dsDNA from the virion is injected
into the nucleus via nuclear pores. [11] The dsDNA can either initiate
viral production or enter a latency stage in neurons, the signaling
factors for which pathway is intiated is not understood. ( Production
/ Latency)
                                       [82]

  Video: "Live Imaging of HSV during retrograde transport in an
axon."  (need Quick Time to view "try it")
                       Courtesy of Elaine Bearer, MD/PhD, Brown
University  {click "go"} [GO]
 



Viral Production

   Viral replication can occur during: primary infection,
reactivation (neurons), and in secondarily infected cells. Logically
it seems that HSV virions replicate in mainly keratinocytes during
primary and secondary cutaneous infiltration, which leads to cell
lysis and possibly lessen formation. (Pathogensis)In fact, it appears
that HSV replicates in sensory neurons after primary and/or secondary
infection, inducing cell-cell spread in neuronal ganglia without
causing lysis.  As one can see the virus must make "genetical
decisions", if the replication process ensues, a particular gene
expressed by the virus becomes active vs. or inactive (which balances
with perpetuating LAT). This gene is for a protein encoding vhs (for
virion host shut off) whose function causes rapid destabilization of
host RNA's and transitional arrest. (vhs is also released during viral
penetration since it is a component of the tegument.) [11]
Surprisingly, vhs also destabilizes viral messages, resulting in over
accumulation of immediate-early and early genes during lytic
infection. This vhs has been associated with the viral gene UL41 b/c
KO's have shown decreased virulence (gene). [19]
 

Gene Activation

Packaging
     Viral replication now ensues by circular dsDNA replication into
capsid [9][19] The empty capsid binds to an initiation sequence
believed to contain the first Uc domain. {I} [9][11] The dsDNA is then
packaged into the empty capsid. {II}  Until the maximum density is
reached or a reflection of sequence is meet. {III}
An unknown process nicks both strands on opposite sites of the DR1
sequence. {IV} The linear HSV is packaged in the capsid. {V}



Latency

   The latency stage occurs wherever the virus can remain dormate,
typically in the trigeminal ganglia for oral herpes and in the sacral
ganglia for genital herpes. Direct evidence of latency has been
available since early 1970ís [11]  but was hypothesized by E.W.
Pasture in 1929 when he said this statement:

ìIt seems to me probable, from experiment and clinical facts, that
herpetic virus does reside in a latent state within the human body
and, specifically, in the nervous tissues, perhaps primarily within
nerve cells of the ganglia, and that neuron disturbances are
frequently the basis for subsequent outbreaks.î [11]

  We now know that in latently infected neurons, the viral genome
acquires the characteristics of circular dsDNA versus its linear form
in the capsid. [11] While in the latency period it has been reported
that the DNA is not extensively methylated and no free infectious
virus can be detected. [11] [19] However, the HSV's genome is active,
because a gene called "Latency-Associated Transcript (LAT)" has been
found to be abundantly transcribed during latency. [61]  LAT is
located in the long repeat region of the viral genome and is initially
transcribed as a 8.3-kb RNA which is spliced into a family of LAT
RNAs. [42] [61](MAP)The signal received for activation of LAT remains
unknown. [42] [61] It is hypothisised that LAT RNA may regulate the
expression or function of one or more viral and/or cellular genes
directly or by interfering with signaling pathways. Also hypothesised
is that the normal function of LAT is to protect acutely infected
neurons from death. [61] In relation, a protein designated RR1 a
ribonucleotide reductase, is the first protein detected in neurons
during reactivation. [64]



Viron Budding

       Following viral reproduction, during either primary infection
or recurrent infection, encapsulation of the HSV capsid occurs.
Electron microscopic studies have indicated that enveloped capsids
accumulate between the inner and outer nuclear membranes and that
virus particles are not present on the plasma membrane or in the
extracellular space between cells. [13] Specifically, viral capsids
are enveloped at the nuclear membrane, where it has been observed that
the virions enter branched tubular structures similar to the
connections between the outer nuclear membrane and rough endoplasmic
reticulum to ultimately appear in transport vesicles. The transport
vesicles are then transported through the cytoplasm of the infected
cell. [79] Interestingly, the virus may now be involved in cell-cell
spread or anterograde transport in neurons. At the end of the exocytic
pathway, the transport vesicles fuse with the plasma membrane,
releasing virions to the extracellular space causing secondary
infection.  When the virion is involved in anterograde transport it
can be understood why occurrences recur at the site of primary
infection. [19]
 



Recurrences
                                       Reactivation is another
mechanism which is not fully understood,
                                                but these
environmental factors have been associated:
 

 
Common cold
Fever
Severe sunburn
Physical fatigue
Emotional disturbance
Trauma
Gastrointestinal disturbances
Menstruation
Pregnancy
Debilitating illnesses
Food allergy

                                                                 
[3][5]

        Physiologically, latent virus can be triggered by at least
oxidative stress and by drugs that stimulate prostaglandin synthesis.
It is believed that the HSV latency associated transcript (LAT) is
essential for efficient spontaneous reactivation although the
mechanism by which LAT functions remains unknown. [42] Once functional
alteration of LAT is stimulated, synthesis of viral capsids occurs and
the newly synthesized dsDNA  is enveloped, as previously described.
[13] Following capsid production, budding and transport, newly
synthesized HSV virions infect susceptible cells. This secondary
reinfection may give rise to a recurring lesion if the Immune Response
(IR) is not adequate.
[38]

 
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