Investigating Wnt signaling in cardiac and skeletal muscles [electronic resource]
Keywords: Developmental Biology.

In order for an organ to successfully regenerate in response to various injuries, different cells must communicate with each other and multiple aspects of cellular behavior — such as proliferation, differentiation, and movement — must be tightly regulated. In mammals, while some organs maintain their ability to regenerate robustly throughout the life of an organism, other organs fail to regenerate. The molecular mechanisms that underlie the differences in the regenerative capacities of various organs remain largely unknown. The Wnt signaling pathway is one of the key molecular signaling pathways involved in cell-cell interaction during embryonic development, adult homeostasis, and injury response in multiple organs. In this thesis, I describe studies to investigate the Wnt signaling pathway in different types of muscles — the neonatal heart, the adult heart, and skeletal muscles — that possess different regeneration potential. First, in the neonatal heart, which can regenerate after cardiac injuries, I investigated the activation pattern of Wnt signaling using Axin2, a Wnt signaling target gene, as a marker. I found that epicardial cells in the ventricles, cardiomyocytes in atria, and cardiac valves are Axin2-positive. Using the Axin2-CreERT2 lineage tracing mice, I discovered that after cryoinjury to the neonatal heart, Axin2-CreERT2-labeled epicardial cells proliferate and undergo epithelial-to-mesenchymal transition to give rise to cardiac fibroblasts in the subepicardial space. Furthermore, I describe de novo activation of Axin2-expression in the cardiac fibroblasts within the myocardium after cryoinjury to the neonatal heart. In situ hybridization showed that the Wnt ligands upregulated in the epicardium after neonatal cardiac injury are different from the Wnt ligands upregulated after adult cardiac injury. Next, I investigated the activation pattern of Wnt signaling in the adult heart. Similarly to the neonatal heart, the epicardium, cardiomyocytes of the atria, and cardiac valves of the adult heart were also Axin2-positive. Lineage tracing studies using the Axin2-CreERT2 mice to label these Axin2-positive epicardial cells in the adult heart to follow their fates after cardiac injury revealed that these cells changed their morphology in response to cardiac injury. These studies on the neonatal heart and the adult heart suggest that even in the same organ, activation of Wnt signaling is achieved by different mechanisms depending on the context. Lastly, I examined Wnt signaling activation in the skeletal muscle and found that there were Axin2-positive cells. When regenerative potentials of these Axin2-positive cells were examined using the Axin2-CreERT2 lineage tracing mice, I found no evidence that the Axin2-CreERT2 labeled cells contributed to skeletal muscle regeneration. Taken together, these studies highlight the complexities of the Wnt signaling pathway in regulating various cell-cell interactions. In order to apply insights gained from studying Wnt signaling to future therapies to promote regeneration, it is critical to consider details such as the identities of ligands responsible for activation of the pathway.