Cardiac Remodeling in Anemic Zebrafish Involves Cardiomyocyte Hypertrophy and Hyperplasia: Effects of the Wnt/Beta-Catenin Pathway

Abstract

Background. Accumulating evidence is changing the paradigm that the heart is a post-mitotic organ and spurring efforts to promote cardiac regeneration as a therapeutic strategy for heart disease. As part of that effort, this thesis has established and characterized zebrafish models of cardiac remodeling to assess the therapeutic value of promoting cardiomyocyte hyperplasia and discover the mechanisms involved. Specifically, it analyzed the roles of the Wnt/beta-catenin pathway on cardiomyocyte hypertrophy and hyperplasia during cardiac remodeling and the effects this remodeling had on fish lifespan. In addition, this thesis developed assays to measure cardiac function in adult zebrafish to further increase the usefulness of adult zebrafish for heart disease research.

Methodology/principle findings. Structural, cellular, and genetic analyses revealed that the Band-3, anemia zebrafish mutant tr265 exhibits hallmarks of human cardiac remodeling, such as an enlarged heart, cardiomyocyte hypertrophy, cardiomyocyte death, structural disarray, and cardiac dysfunction. However, unlike in humans, cardiomyocyte hyperplasia contributes significantly to the cardiac remodeling process of tr265.

At week 6 post fertilization, tr265 has cardiomyocyte hypertrophy in the inner trabecular region and hyperplasia in the outer compact region. Modulation of the Wnt/beta-catenin pathway affects the hypertrophic response of the zebrafish trabecular cardiomyocytes, like in mammalian cardiomyocytes. Oppositely, modulation of the pathway affects cardiomyocyte hyperplasia in the compact region, demonstrating the location-dependent effects of the Wnt/beta-catenin pathway in the anemia model. Moreover, upregulation of the pathway accelerates the onset of cardiomyocyte hyperplasia in the trabecular region and improves fish survival by six weeks after modulation of the pathway, suggesting a therapeutic role of cardiomyocyte hyperplasia to the cardiac remodeling process.

Two assays developed to measure cardiac function in the adult zebrafish include red blood cell flow rate and shortening fraction. The red blood cell flow rate assay can be used in most fish to measure how fast the red blood cells travel in the caudal fin as an indirect measure of cardiac function. To measure cardiac function directly via the shortening fraction assay, a semi-transparent fish that contains a red fluorescent heart (casper;Tg(cmlc2:nuDsRed) ) was created.

Conclusions/significance. Cardiomyocyte hyperplasia plays a beneficial role in the cardiac remodeling process of tr265 , as demonstrated by improved fish survival after augmentation of cardiomyocyte hyperplasia via the Wnt/beta-catenin pathway. Exciting research remains to be conducted with tr265 to understand how the pathway affects the onset of cardiomyocyte hyperplasia, as well as why the pathway exhibits location-dependent effects on cardiomyocyte hypertrophy and hyperplasia. Using models such as tr265, mechanisms involved in cardiomyocyte hyperplasia can be discovered and hopefully translated into therapeutic strategies for human disease.