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.
 
 

	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.

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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
 

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)

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   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.

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