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In Duchenne muscular dystrophy (DMD), a dysregulated extracellular matrix (ECM) directly exacerbates pathology. Glucocorticoids are beneficial therapeutics in DMD, and have pleiotropic effects on the composition and processing of ECM proteins in other biological contexts. The synthesis and remodelling of a transitional versican-rich matrix is necessary for myogenesis; whether glucocorticoids modulate this transitional matrix is not known. Here, versican expression and processing were examined in hindlimb and diaphragm muscles from mdx dystrophin-deficient mice and C57BL/10 wild type mice. V0/V1 versican (Vcan) mRNA transcripts and protein levels were upregulated in dystrophic compared to wild type muscles, especially in the more severely affected mdx diaphragm. Processed versican (versikine) was detected in wild type and dystrophic muscles, and immunoreactivity was highly associated with newly regenerated myofibres. Glucocorticoids enhanced C2C12 myoblast fusion by modulating the expression of genes regulating transitional matrix synthesis and processing. Specifically, Tgfβ1, Vcan and hyaluronan synthase-2 (Has2) mRNA transcripts were decreased by 50% and Adamts1 mRNA transcripts were increased three-fold by glucocorticoid treatment. The addition of exogenous versican impaired myoblast fusion, whilst glucocorticoids alleviated this inhibition in fusion. In dystrophic mdx muscles, versican upregulation correlated with pathology. We propose that versican is a novel and relevant target gene in DMD, given its suppression by glucocorticoids and that in excess it impairs myoblast fusion, a process key for muscle regeneration.

The transcriptional regulation of muscle development involves several complex processes that must work together in order to form functional, syncytial muscle cells. However, when transcription is mis-regulated, muscle development is often times negatively affected and can lead to muscle diseases such as muscular dystrophy and cardiac myopathies. In order to gain more insight into how transcription is regulated, I use Drosophila melanogaster as a model for understanding muscle development. In chapter one, I use a traditional genetic screen to phenotypically and molecularly identify two Hox co-factors, extradenticle and homothorax, that have the ability to change muscle identity. Additionally, in chapter two, through the identification of a mechanism, I identify a gene critical in adult myoblast fusion and is directly regulated by the transcription factor, Myocyte Enhancer Factor-2 (MEF2). Lastly, in chapter three a computation approach is used to discover new potential co-factor binding sites that may work in conjunction with MEF2 in transcriptional muscle regulation. Together, these results provide new information into how muscle is transcriptionally regulated during different stages of development.

The transcriptional regulation of muscle development involves several complex processes that must work together in order to form functional, syncytial muscle cells. However, when transcription is mis-regulated, muscle development is often times negatively affected and can lead to muscle diseases such as muscular dystrophy and cardiac myopathies. In order to gain more insight into how transcription is regulated, I use Drosophila melanogaster as a model for understanding muscle development. In chapter one, I use a traditional genetic screen to phenotypically and molecularly identify two Hox co-factors, extradenticle and homothorax, that have the ability to change muscle identity. Additionally, in chapter two, through the identification of a mechanism, I identify a gene critical in adult myoblast fusion and is directly regulated by the transcription factor, Myocyte Enhancer Factor-2 (MEF2). Lastly, in chapter three a computation approach is used to discover new potential co-factor binding sites that may work in conjunction with MEF2 in transcriptional muscle regulation. Together, these results provide new information into how muscle is transcriptionally regulated during different stages of development.

The goal of this study was to identify novel factors that mediate skeletal muscle development or function. We began the study by searching the gene expression databases for genes that have no known functions but are preferentially expressed in skeletal muscle. This search led to the identification of the Src homology three (SH3) domain and cysteine rich (C1) domain 3 (Stac3) gene. We experimentally confirmed that Stac3 mRNA was predominantly expressed in skeletal muscle. We determined if Stac3 plays a role in skeletal muscle development or function by generating Stac3 knockout mice. All Stac3 homozygous mutant mice were found dead at birth, were never seen move, and had a curved body and dropping forelimbs. These mice had marked abnormalities in skeletal muscles throughout the body, including central location of myonuclei, decreased number but increased cross-sectional area of myofibers, decreased number and size of myofibrils, disarrayed myofibrils, and streaming Z-lines. These phenotypes demonstrate that the Stac3 gene plays a critical role in skeletal muscle development and function in mice.

The goal of this study was to identify novel factors that mediate skeletal muscle development or function. We began the study by searching the gene expression databases for genes that have no known functions but are preferentially expressed in skeletal muscle. This search led to the identification of the Src homology three (SH3) domain and cysteine rich (C1) domain 3 (Stac3) gene. We experimentally confirmed that Stac3 mRNA was predominantly expressed in skeletal muscle. We determined if Stac3 plays a role in skeletal muscle development or function by generating Stac3 knockout mice. All Stac3 homozygous mutant mice were found dead at birth, were never seen move, and had a curved body and dropping forelimbs. These mice had marked abnormalities in skeletal muscles throughout the body, including central location of myonuclei, decreased number but increased cross-sectional area of myofibers, decreased number and size of myofibrils, disarrayed myofibrils, and streaming Z-lines. These phenotypes demonstrate that the Stac3 gene plays a critical role in skeletal muscle development and function in mice.