Application of Viral Vectors to Motor Neuron Disorders - Viral Vector Approaches in Neurobiology and Brain Diseases - page 214

1.2 Viral Gene
Delivery to Motor
Neurons

Technical hurdles that include specifi c targeting and widespread trans-
gene delivery to motor neurons challenge gene therapy for MNDs.
Several viral vectors have proven effi cient at inducing expression of a
transgene in motor neurons. For this purpose, lentiviral and adeno-
associated viral (AAV) vectors have mostly been characterized.

Vectors derived from HIV-1 and equine infectious anemia virus
(EIAV) have been used to overexpress various therapeutic mole-
cules in animal models of motor neuron diseases [ 18 , 19 , 27 ].
Lentiviral vectors display several advantages including the ability to
infect nondividing cells such as neurons and a large cloning capacity
(ca. 9 kb). These vectors usually integrate in the genome and there-
fore allow for stable expression of the transgene in both dividing
and nondividing cells. HIV-1-derived lentiviral vectors have been
shown to readily infect motor neurons following direct intraspinal
injection [ 19 ]. Importantly, functional relevance has been demon-
strated by overexpression of shRNAs directed against human SOD1
in G93A SOD1 mice. Subsequent silencing of the mutant SOD1
could signifi cantly delay disease onset in this ALS mouse model
[ 19 ]. Rabies G-pseudotyped EIAV and HIV-1-derived vectors have
been shown to display retrograde transport abilities and hence to
transduce motor neurons following intramuscular injection [ 18 ,
28 ]. This property has been used to express SOD1 shRNAs or neu-
rotrophic factors in motor neurons of G93A SOD1 mice, which
resulted in prolonged survival of treated animals [ 18 , 27 ].

1.2.1 Lentiviral Vectors

AAV recombinant particles have also been successfully used for
gene delivery to motor neurons. AAVs are nonpathogenic in
humans, and they only rarely integrate in the genome, which con-
siderably reduces the risk of insertional mutagenesis but might lead
to vector dilution in dividing cells. Progressive vector loss is how-
ever not expected to be an issue in post-mitotic cell populations
such as neurons. The tropism of recombinant AAV vectors depends
on the serotype used. AAV9 particles have been demonstrated to
effi ciently cross the blood brain barrier (BBB) in newborn mice to
infect neurons and glial cells following intravenous injections [ 29 -
31 ]. Functionally, AAV9-derived overexpression of the SMN pro-
tein in severe mouse models of SMA has led to a drastic increase in
lifespan from 2 weeks to over 200 days [ 23 , 25 , 26 ]. Some AAV
serotypes such as AAV2 and AAV6 have been shown to display
striking retrograde transport properties as they effi ciently induce
expression of a transgene in motor neurons following intramuscu-
lar injection [ 15 , 32 , 33 ]. As a matter of fact, AAV2-mediated
overexpression of IGF-1 in motor neurons has led to a signifi cant
increase in survival of G93A SOD1 mice [ 15 ]. Considering the
widespread distribution of motor neurons in the central nervous
system (CNS), the route of vector delivery is a crucial variable for
the successful application of gene therapy in MNDs.

1.2.2 Adeno-Associated
Viral Vectors