Lentiviral Vectors in Huntington’s Disease Research and Therapy - Viral Vector Approaches in Neurobiology and Brain Diseases - page 185

glial cells [ 12 - 15 ]. In particular, mHtt production by astrocytes
decreases the expression of glutamate transporters, leading to an
accumulation of synaptic glutamate, which is highly toxic to neu-
rons. The contribution of cell-cell interactions to the disease pro-
cess remains unclear but is likely to have a major impact on the
development of treatments. Efforts are currently focused on
understanding pathological mechanisms and developing therapeu-
tic strategies based on small molecules, cell transplantation, and
gene therapy [ 16 - 18 ].

1.2 Lentiviral-
Mediated Gene
Transfer

Lentiviral vectors (LVs) are particularly suitable for central nervous
system (CNS) gene transfer, due to their large cloning capacity and
high transduction effi ciency. Numerous functional and preclinical
studies have demonstrated the potential of such systems to mediate
the overexpression or silencing of specifi c genes. Most LVs are
derived from the type 1 human immunodefi ciency virus (HIV-1),
but some are derived from other lentiviruses. Unlike other vectors
derived from oncoretroviruses [ 19 , 20 ] ( see Chap. 2 on lentiviral
vectors), they effi ciently transduce mitotic or postmitotic cells. In
the original seminal paper by Naldini and coworkers, high trans-
duction frequencies in the CNS were obtained by broadening the
original tropism of HIV-1, through the use of a heterologous
envelope protein, the G glycoprotein of the vesicular stomatitis
virus (VSV-G). In addition to its effect on tropism, VSV-G renders
the viral particles highly stable, making it possible to concentrate
them by ultracentrifugation, which is essential for CNS applica-
tions. Neither the receptor of the VSV-G protein nor specifi c host
factors have been identifi ed. However, in the 1980s, Schlegel and
coworkers suggested that at least some of the VSV-G might bind
to plasma membrane phosphatidylserine [ 21 ]. It was subsequently
shown that phosphatidylserine was not the cellular receptor for
VSV-G, but that it played a potential role in the postbinding step
of virus entry [ 22 ]. It was suggested that an interaction between
the VSV-G protein and an unknown receptor at the cell membrane
induces receptor-mediated endocytosis. The VSV-G protein may
then interact with the phosphatidylserine in the endosome, allow-
ing fusion to occur. The endoplasmic reticulum chaperone gp96
has recently been shown to be essential for the occurrence of func-
tional VSV-G receptors at the cell surface [ 23 ]. These fi ndings pro-
vide evidence for a broad host range of LV, with high transduction
effi ciency in various organs (muscle, retina, liver, and brain).
In the CNS, VSV-G-pseudotyped LVs expressing reporter
genes under the control of ubiquitous promoters mostly transduce
neurons [ 20 , 24 - 31 ]. Following the injection of LV into the
parenchyma, the transgene is expressed throughout the cell bodies
and axons, in the absence of retrograde or anterograde transport of
viral particles. Sustained transgene expression over long periods of
time has been achieved in rodent and primate brains [ 24 , 26 , 32 ].