Skeletal muscle formation is among the most striking examples of cellular plasticity in animal tissue development, where mononucleated, lineage-restricted progenitor cells are epigenetically reprogrammed to produce multinucleated myofibers. While some mediators of epithelial-mesenchymal transition (EMT) have been shown to function in myogenesis, regulation of this process at the interface of multipotency and myogenic differentiation remains poorly understood. The long noncoding RNA (lncRNA) Meg3 is processed from the >200 kb Dlk1-Dio3 polycistron, and while many encoded miRNAs have been shown to regulate skeletal muscle differentiation, regeneration, and aging, the functional relevance of encoded lncRNAs in skeletal muscle remains elusive. Here, I demonstrate that Meg3 is enriched in proliferating post-natal myoblasts, where it epigenetically modulates aspects of cellular plasticity to facilitate myogenic differentiation in vitro, skeletal muscle regeneration in vivo, and safeguard myogenic identity. Chronic inhibition of Meg3 in C2C12 myoblasts compromised cytoarchitectural and transcriptomic cell-state transitions required for myogenic fusion and differentiation. These differentiation defects were primarily driven by TGFβ-dependent Snai2 activation, which correlated with irregular Ezh2 activity and abnormal epigenetic marks in differentiating C2C12 cells. Similarly, adenoviral Meg3 knockdown compromised muscle regeneration in vivo, which manifested as abnormal mesenchymal gene expression, fibrosis, and interstitial cell proliferation in the regenerating milieu. Comparison of Meg3-depleted C2C12 myoblasts and injured skeletal muscle to literature-derived gene sets suggest that Meg3-deficient samples deviate from controls towards abnormal transcriptional states, including immature satellite cell activation, muscle aging, and adoption of an osteoblast-like cell ontology. Thus, Meg3 regulates myoblast identity to maintain proper cell state transitions in postnatal myogenesis.

Efficient targeting of genes to either inhibit or increase their expression in specific tissues in vivo remains a challenge. Adeno-Associated Virus (AAV) has emerged as an efficacious delivery method in both humans and murine model systems. AAV is a non-enveloped, single-stranded DNA virus that is non-integrating with long-term expression. Due to its low immunogenicity and various serotypes with specific tissue tropisms, AAV is a preferred choice for organ specific-gene delivery in many experimental settings. This project focused on protocol optimization for high-volume production of AAV plasmids, improved transfection efficiency, and increased viral yield and purity to specifically target noncoding RNAs (ncRNAs) expressed from the imprinted Dlk1-Dio3 locus. Five AAV9 viruses were produced, each containing one of the following transgenes: 1) human Meg3 cDNA for overexpression of this long noncoding RNA, 2) Meg3-specific short hairpin RNA for knockdown analysis, 3) eGFP cDNA to demonstrate AAV9 tissue tropism, 4) Cas9 cDNA, and 5) gene-specific guide RNAs to target the Meg3 proximal promoter. The AAV9 virus production protocol optimized in this project expands the tools available for in vivo study of the Dlk1-Dio3 ncRNA locus.