.:Research
Viral Vectors for Gene Therapy
Herpes Simplex Virus (HSV-1)
The HSV-1 virion is about 20 nm in diameter and consists of four components: envelope, tegument, capsid and viral genome. The envelope is derived from the cellular membrane and contains approximately 12 viral glycoproteins essential for viral entry. The tegument is the protein layer between the capsid and the envelope, and contains at least 10 viral proteins which are involved in shut-off of host protein synthesis, activation of immediate early viral gene expression and assembly functions. Among these are proteins called VP16 (essential for transactivation and virion envelopment), VP22 (membrane translocation domain) and virion host shut off (vhs) protein. The icosahedral capsid consists of 7 viral proteins and contains the linear dsDNA genome, which is 152 Kb in size and is divided into unique long (UL) and unique short (US) regions that are flanked by terminal repeats (TR).
The virus encodes at least 80 viral proteins with very little splicing of genes. The natural infection can result in either lytic replication in mucosal or epithelial cells or a latent state in neurons with persistence of the virus genome (94). Four of the HSV-1 glycoproteins in the virus envelope mediate interaction with the host cell and virus entry. The gB and gC proteins bind to heparan sulfate on the cell surface, and this is followed by gD binding to cellular receptors known as herpes virus entry mediators HVEM/HveA and HveC/nectin-1. The gB, gD, and gH/gL complex are all thought to be required for fusion of the virus envelope with the cell membrane to allow release of the nucleocapsid into the cellular cytoplasm. During the lytic replication cycle, the host cell protein synthesis is shut-off immediately after infection by the tegument vhs protein. Subsequently, the viral capsid releases the viral DNA into the nucleus where it will circularize. Within several hours after infection, protein expression from the circular genome is initiated. Protein expression occurs in a highly regulated fashion and can be divided into three groups of sequentially expressed proteins: alpha- or immediate early genes, beta- or early genes and gamma- or late genes. Once beta-gene products are present, DNA replication and gamma-gene production is initiated, and progeny virus will be produced.
In the nucleus, capsid proteins assemble together with the viral DNA genomes and the newly formed viral capsids bud through the nuclear membrane. On their way to the Golgi apparatus, the virions obtain their tegument and viral envelope, and subsequently the virus is released from the cell through secretory vesicles. Virus replication in the permissive epithelial cells produces virions that can enter the nervous system through axon terminals of local sensory neurons to enable establishment of a latent infection. Upon entry, the virus nucleocapsid is transported to the nucleus in the cell body by retrograde transport along the axon. It is currently unknown, which viral genes are involved in the establishment of a latent infection, but de novo viral protein synthesis is not required. However, latency is related to the expression of latency associated transcripts (LAT), which are expressed from a promoter that is highly active in neurons. The virus persists in the latent state for the lifetime of the infected individual. Reactivation of the latent virus can be induced by different stimuli such as stress and UV irradiation and is results in production of progeny virions that are transported by along axons to establish an active infection at the primary site of infection.
