Gene Delivery Using Viral Vectors - Challenges in Delivery of Therapeutic Genomics and Proteomics - page 250

routes can take advantage of the natural life cycle of the virus, which usually infects
the axonal nerve terminal at peripheral sites before retrograde transport to neuronal
cell bodies where latency is established [208] . Generally in PNS, there are a number
of potential applications for HSV replication-competent vectors capable of peripheral
replication and axonal transport, including stimulating regrowth of damaged nerves,
the study and treatment of various pain states, the protection of neurons from further
degeneration in motor neuron disease, the study and treatment of various neuropa-
thies, the study of neuronal development, and screening for the relevance of genes
implicated as being crucial in any of these processes by a gene delivery strategy.
Attenuated vectors were in fact demonstrated to be highly efficient in driving proen-
kephalin A (PA) gene expression in dorsal root ganglia (DRG) to deliver genes into
monkey eyes and to the rodent visual system, and to express active nerve growth fac-
tor beta subunit (-NGF) in latently infected DRG [209-213] .
5.6 Other Viral Vectors
5.6.1 Baculovirus
Baculoviruses are generally a diverse group of insect-specific viruses, and they pre-
dominantly infect insect larvae of the order Lepidoptera . The most widely studied
member of this family is Autographa californica nucleopolyhedrovirus (AcMNPV),
for which the complete structure of genome sequence has been determined. AcMNPV
has a circular, double-stranded, supercoiled DNA genome of approximately 130 kb,
packaged in a rod-shaped nucleocapsid. These nucleocapsids can be extended length-
ways, and as a result the virus genome can effectively accommodate large insertions of
foreign DNA. Such types of inserting foreign genes into the AcMNPV genome have
resulted in the production of baculovirus expression vectors. Recombinant viruses
are genetically modified to contain a foreign gene of interest that can be expressed in
insect cells under the control of a baculovirus gene promoter. Because the baculovirus
genome is normally considered too large for direct insertion of a foreign gene, the gene
of interest is first cloned into a transfer vector containing sequences that flank the polh
gene in the viral genome. Virus DNA and transfer vector are both cotransfected into the
host insect cell, and homologous recombination between the flanking sequences com-
monly occurs for both DNA molecules. This causes the gene of interest to be inserted
into the viral genome at the polh locus, resulting in the production of a recombinant
virus genome.
AcMNPV members have a biphasic replication cycle resulting in the production
of two virus phenotypes:
1. Budded virus (BV)
2. Occlusion-derived virus (ODV)
BVs generally consist of a single, rod-shaped nucleocapsid enveloped in a host-
derived membrane that is enriched in a virally encoded membrane fusion protein,
GP64, which is integrated into the BV particle during virus budding and release. This
type of budded formed virus is responsible for the cell-cell transmission of the virus