Molecular approaches to studying mammalian regeneration

Mammals have an extremely limited ability to replace damaged heart tissue. On a molecular level this irreversible tissue loss results from failure of cardiac myocytes to proliferate in response to an injury. As myocyte proliferation ceases shortly after birth, identification of genes expressed during this developmental stage may lead to the discovery of key molecules and signaling pathways responsible for this proliferative switch. I have undertaken two different approaches for studying mammalian heart regeneration. First, I screened for genes that are differentially expressed between fetal and adult cardiomyocytes. This approach led to the identification of several differentially-regulated genes, including a seven-transmembrane receptor, ETL. The expression of this novel receptor is developmentally upregulated at birth and may be associated with a variety of cellular responses in the heart including inhibition of cellular division.; I also adapted a candidate gene approach to study mammalian regeneration. In vertebrates that are capable of regeneration, such as urodele amphibians and teleost fish, initiation of regeneration is associated with dedifferentiation of the remaining tissue and coincides with the expression of certain genes. Among them, Msx1 and Msx2 are expressed early in regeneration and potentially represent key regulators of the regeneration cascade. Here, I took an in vivo approach to activate Msx genes in the adult mammal. In particular, I describe a transgenic mouse model with inducible expression of Msx2. Induction of Msx2 in the skeletal muscle results in a myopathy with characteristic muscle damage and appearance of fibers with centrally located nuclei. Using Msx2 as a molecular marker, we show that fibers with centrally located nuclei arise not only from satellite cells as previously thought, but also from the movement of peripheral muscle nuclei to the center. This phenomenon potentially represents a dedifferentiation event during regenerative muscle response because only immature fibers contain centrally located nuclei. In addition, I compare my Msx2-induced model of murine myopathy with the mouse model of Duchenne muscular dystrophy (mdx) using gene expression profiling. Our analysis reveals both unique and shared aspects of muscle regeneration in Msx2-induced and mdx models of myopathy. Thus, Msx2-caused myopathy is the first model of inducible murine dystrophy and regeneration.