PROJECT SUMMARY/ABSTRACT
Skeletal muscle shows amazing plasticity and has the capacity to continuously regulate its size in response to
external cues such as mechanical load, neural activity, hormones, growth factors, and nutritional status. The
maintenance of muscle mass is controlled by a balance between protein synthesis and protein degradation
pathways, a balance that shifts toward protein degradation during atrophy and protein synthesis during
hypertrophy. To date, protein synthesis and degradation systems have been extensively studied in the context
of muscle growth and wasting, while work to identify and characterize novel modulators of muscle hypertrophy
and atrophy has been less prolific. As an example, the MAP kinase signaling pathway has been found to play
an array of roles in skeletal muscle ranging from the regulation of fiber type development to controlling protein
synthesis and autophagy pathways. Interestingly, gene expression analysis of muscle tissue isolated from mice
following denervation revealed that Dusp4, a known negative regulator of MAP kinase signaling, is significantly
upregulated. Further, previous studies suggest that Dusp4 preferentially regulates the Erk1/2 branch of the MAP
kinase signaling cascade in skeletal muscle. While Dusp4 is rapidly and robustly upregulated in response to
sciatic nerve transection, the functional consequence of upregulation of this dual-specificity phosphatase during
neurogenic skeletal muscle atrophy remains unclear. Therefore, the overall objective of this investigation is to
characterize the functional role of Dusp4 in modulating the molecular mechanisms that regulate muscle size and
strength and determine how Dusp4 contributes to changes in muscle mass. This objective will be accomplished
through the completion of the following specific aims. In aim 1, we will determine if Dusp4 expression is
necessary and sufficient to induce muscle atrophy using in vivo electroporation of skeletal muscle to knockdown
or overexpress Dusp4 for 3- and 14-days followed by measurement of muscle weight, myofiber cross sectional
area, fiber type composition, and expression levels of markers of protein synthesis and atrophy. We hypothesize
that knockdown of Dusp4 in denervated muscles will attenuate muscle atrophy, while overexpression of Dusp4
will promote skeletal muscle wasting. In aim 2, we will explore the hypothesis that Dusp4-mediated
dephosphorylation may serve as a signal to alter the activity, localization, and/or stability of putative substrates
in muscle. This hypothesis will be tested using an in vivo overexpression approach combined with an unbiased
phosphoproteomic analysis to identify and validate the full complement of Dusp4 targets in skeletal muscle. The
successful completion of this project will demonstrate for the first time that Dusp4 acts as a regulator of skeletal
muscle mass through modulation of the Erk1/2 branch of the MAP kinase signaling pathway. Furthermore, if the
findings of this investigation demonstrate that Dusp4 participates in the neurogenic atrophy cascade by acting
as a direct or indirect modulator of muscle wasting, then inhibition of this dual-specificity phosphatase could
prove beneficial in the treatment of skeletal muscle atrophy associated with neuromuscular disorders,
neurodegenerative diseases, and aging.