Biology Reference
In-Depth Information
7 Augmentation of Nitric Oxide Signaling in
mdx
Mice
The observations that nNOS
m
signaling is disrupted in dystrophin-deficient skeletal,
cardiac, and smooth muscle cells, combined with abnormal nNOS and cGMP
signaling in dystrophin-deficient cardiac muscles, provides a compelling rationale
for the use of approaches that enhance nNOS signaling to treat dystrophinopathies.
To date, such approaches have used exogenous sources of NO to enhance NO
signaling, such as NO synthase transgenes or NO donors. Other approaches have
focused on modulating downstream effector activity, specifically by inhibiting
PDE5A activity to enhance cGMP levels that normally result from NO-mediated
activation of sGC.
Targeting nNOS signaling pathways has proved efficacious for reducing muscle
damage and improving exercise performance in the
mdx
mouse model of DMD.
Reengineering of conventional minidystrophin gene therapy cassettes to restore
sarcolemmal nNOS
m
expression provides significant additional improvements over
conventional microdystrophin cassettes, including enhanced vasomodulation, exercise
performance, and resistance to exercise-induced muscle damage (Lai et al.
2009
).
Thus, the ability to restore sarcolemmal nNOS
m
expression substantially improves
gene therapy-based intervention in
mdx
mice. These data are consistent with
reduced contraction-induced myofiber damage observed after the application of
NO donors (Asai et al.
2007
). Cytosolic expression of an nNOS
a
transgene reduced
muscle damage without affecting membrane permeability in adult
mdx
mice
(Wehling et al.
2001
). Substantial reductions in macrophages densities and cyto-
lytic activity were observed, suggesting that the anti-inflammatory properties of NO
were responsible for the reduction in muscle damage (Wehling et al.
2001
).
In agreement with this proposal, a compensatory increase in utrophin expression
was not observed (Tidball and Wehling-Henricks
2004
). Transduction of
mdx
skeletal muscles by adenovirus carrying a constitutively active eNOS gene also
improved dystrophic pathology and increased myofiber size by upregulating follis-
tatin expression (Colussi et al.
2008
). Histone deacetylase 2 (HDAC 2) inhibition
by NO-dependent S-nitrosylation is responsible for the anti-dystrophic impact of
increased cytosolic NO, highlighting a novel link between NO signaling and
chromatin remodeling in dystrophic skeletal muscle (Colussi et al.
2008
). These
findings provide evidence that NO can also exert beneficial effects on dystrophic
muscle through cGMP-independent pathways. Taken together, these studies
provide a compelling rationale for the potential therapeutic utility of increasing
NO-cGMP signaling in dystrophin-deficient tissues.
As in skeletal muscle, cardioprotective effects of increased NO concentrations
have been reported in
mdx
cardiac muscle. Cardiomyocyte-specific expression of a
nNOS
a
transgene in
mdx
mice significantly reduced interstitial fibrosis (Wehling-
Henricks et al.
2005
). Interestingly, ectopic expression of nNOS
a
in
mdx
hearts
(nNOS
a
is not normally expressed in cardiomyocytes) corrected common impulse
conduction defects including deep Q waves, a decreased S:R wave ratio, polyphasic
R waves and shortened PR interval, as well as preventing cardiac arrhythmias, such
