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    • br Introduction Herpes viruses are members of Herpesviridae

    2022-05-18


    Introduction Herpes viruses are members of Herpesviridae family. This family consists of DNA viruses and it is a very large family. The viruses related to this family cause many diseases in animals and humans [1]. Herpes simplex viruses cause mucocutaneous lesions in the affected humans. These lesions are also known as cold sores. Moreover, viruses may also cause genital lesions which are known as herpes genitalis. These viruses perform their normal functions using many proteins produced by the viruses itself. Moreover, the envelopes of all herpes viruses contain many integral membrane proteins. For example, in alpha herpes virus, more than ten transmembrane glycoproteins are present. Similarly many glycoproteins including gB, gC and gD, are known to be present as multimers [[2], [3], [4], [5]]. These glycoproteins are very significant for virus and absolutely required for different infections cause by herpes viruses [6,7]. Moreover, different glycoproteins and their combinations are necessary and sufficient to induce cell-cell fusion in a transient transfection assay [8]. Furthermore the combination of different glycoproteins specifically gB, gC and gD is required to construct discrete morphological structures of the virus which were observed using immune-electron microscope [9]. The virus may become latent after the primary infection. In latent infection, the virus resides in neurons specifically in sensory and autonomic neurons and cause infection after the incubation period [10]. After infection, attachment and entry of the virus in the host 92 8 are main steps. Hence, these steps are the main targets for vaccines development and treatment of the diseases cause by the herpes viruses. The virus attaches and enters in the host cells using its glycoproteins. Moreover, these biological processes involve 5 of the 12 known surface glycoproteins of the herpes virus [11]. After binding, glycoprotein D (gD) and it's receptors play a key role for virus to enter in the cell. The gD functions with its receptors including herpes virus entry mediator (HVEM), nectin-1 and nectin-2 and specific sites in heparin sulfate generated by certain 3-O-sulfotransferases [12,13]. Other receptors required for entry in host cells include gB and the heterodime rgH/gL [14,15]. Hence gB is also another significant glycoprotein regarding binding and entry of the virus in the host cells. Moreover, this glycoprotein (gB) must carry out some functions other than binding to cell surface heparan sulfate proteoglycan (HSPG). To confirm those functions, various criteria used for gD to show that gD binds receptors, may be applied for gB to demonstrate the existence of a gB receptor. For example, HSV entry can be blocked by using antibodies against gB. While gB can be converted into soluble forms. Moreover, gB receptors and it's antibodies can be converted into soluble forms. Herpes simplex virus (HSV) mutants lacking glycoproteins C (gC) gene exhibit reduced attachment. However, these mutant viruses were still infectious [16]. Moreover, another mutant lacking gB gene (gB null virus) could not penetrate target cells [6]. The possible reason behind that was a polylysine (pK) sequence. This sequence is located in the N terminus of gB which is responsible for interaction of gB with HSPG. Taken together, we may strongly say that virus requires these glycoproteins specifically gB, gC and gD for binding of HSV to HSPG and these are significant steps for viral entry into the host cells. The initial attachment of the virus on cellular surface may also occur due to binding of gB and gC to heparan sulfate [17]. However, herpes virus infection was still observed in the absence of gC or of heparan sulfate but efficiency of the infection was reduced. There are different types of herpes viruses which cause infections in humans and animals. For example, herpes simplex viruses (HSV1 and HSV2) infect human. These viruses belong to alpha herpes virus subfamily. Moreover, pseudorabies virus (PRV1) and bovine herpes virus (BHV1) are other herpes viruses which belong to alpha herpes virus and cause infection in animals. There are 11 glycoproteins on lipid membrane of herpes viruses. Among those 11 glycoproteins, 5 are associated with entry of the virus inside cell. These 5 glycoproteins include gB, gC, gD and the heterodimer gH/gL [18]. Among these glycoproteins, gD is very essential for viral entry inside the cell by binding to the specific cellular receptors. Hence, this binding and entry of the virus can be blocked and infection can be prevented by blocking receptors with soluble gD ectodomains. Moreover, this can also be achieved by blocking gD with soluble receptor ectodomains [12,19,20]. Furthermore, antibodies against gD or gD receptors also blocked virus entry in the cell [[20], [21], [22]]. The gD of HSV1 is a 369 residue glycoprotein with an N-terminal ectodomain of 316 residues and three N-linked oligosaccharide attachment sites [23].