Viral Manipulation of Neural Stem/Precursor Cells - Viral Vector Approaches in Neurobiology and Brain Diseases - page 260

fate of hippocampal NPCs in vivo toward the generation of oligo-
dendrocyte at the expenses of newborn neurons [ 49 ]. Finally,
NPCs have been also used as source to generate induced pluripo-
tent stem (iPS) cells by retroviral transduction using a reduced
number of transcription factors, as compared to the four factors
(c-myc, Klf4, Oct4, and Sox2) originally used by Takahashi et al.
[ 50 ]. Indeed, taking advantage of the fact that NPCs normally
express two of these transcription factors (which are Sox2 and
c-myc), iPS cells have been generated from NPCs by direct repro-
gramming with three [ 51 ], two [ 52 , 53 ], or also a single transcrip-
tion factor [ 54 ].
Different types of viral vectors are available to obtain the previ-
ous results, and indeed this represents a fundamental aspect to be
taken into account when considering NPC viral manipulation.
Currently, the most popular method for transducing NPCs is by
means of lentiviruses , thanks to some specific features of these vec-
tors. In particular, they can integrate into the genome of the host
cells, infect also nondividing cells, carry fairly long inserts (up to 12
kb), and they elicit a minimal immune response when injected in
vivo [ 55 ]. The most commonly used envelope protein for lentiviral
vectors is the glycoprotein from the vesicular stomatitis virus (VSV-
G), allowing the transduction with high efficiency of a broad reper-
toire of cells, including NPCs [ 56 ]. It is indeed important to remind
that the injection of lentiviral vectors into neurogenic regions of the
brain (e.g., SVZ and SGZ) might result in the transduction of other
types of cells. In this context the choice of the promoter upstream
the gene of interest is another important point to take into account:
it can drive the transcription ubiquitously or in a lineage-specific
manner. The use of cell-specific promoters allows higher specificity
for the expression of a given gene [ 57 ]: for example, to specifically
target NPCs in the SVZ or in SGZ different promoters have been
used, such as Sox2 [ 58 ], Nestin, and Musashi [ 59 , 60 ].
While sharing most of their features with lentiviruses, retrovi-
ruses only infect dividing cells as they require the disruption of the
nuclear membrane during mitosis to allow the pre-integration
complex to access chromatin and subsequently integrate into the
host cell genome [ 61 ]. Because of this property, one of the most
common applications of retroviral vectors in this field is the identi-
fication and labeling of proliferative cells (and their progeny) in the
neurogenic niches of the brain [ 33 , 62 - 65 ].
Adenoviruses have a double stranded, linear DNA genome of
approximately 36 kb that remains episomal in the nucleus of
infected cells. Recombinant adenoviral vectors have some intere-
sting features making them attractive tools for gene transfers, as
they can carry very long inserts (up to 36 kb), they can be easily
produced at high titer, and they elicit a mild immune response
[ 61 ]. The majority of gene-transfer studies using adenoviral vec-
tors are derived from the human serotype 5 (AdH5), although
other adenoviral vectors have been generated from other serotypes