Mechanisms that drive cardiomyocyte proliferation during zebrafish heart regeneration

Heart disease is the leading cause of death in the developed world. Adult mammals cannot replace lost cardiac tissue after injury, leading to reduced quality of life and increased instances of future cardiac issues. Zebrafish possess the ability to regenerate lost cardiac muscle after injury. Upon injury, the zebrafish heart responds in a coordinated fashion resulting in activation of the epicardium and endocardium, cardiomyocyte proliferation, and subsequent vascularization and innervation of the newly formed muscle. Thus zebrafish represent an ideal genetic model to dissect the mechanisms of heart regeneration. Previously, it was discovered that regulatory sequences of the cardiac transcription factor, gata4, become active in the ventricular wall following injury and that these gata4+ cardiomyocytes proliferate and contribute the majority of new muscle to the regenerate. We uncovered that gata4 function is required for cardiomyocyte proliferation and regeneration after injury. Cardiomyocyte proliferation is required to achieve proper regeneration and lack of cardiomyocyte proliferation is a hallmark of failed regeneration in the mammalian system. Therefore, understanding the signals that induce mature cardiomyocyte division is of great scientific and clinical relevance. Utilizing transgenic approaches, we have found that gata4 function and Nrg1 signaling are critical regulators of cardiomyocyte proliferation. We found that Nrg1 was expressed following injury in the zebrafish heart and that inhibition of nrg1-erbb signaling blunted cardiomyocyte proliferation. Using transgenic over-expression of Nrg1, we found that Nrg1 was capable of increasing injury-induced cardiomyocyte proliferation. Furthermore we found that activation of Nrg1 in the uninjured adult heart induces cardiomyocyte proliferation and hallmarks of the regenerative program. Long-term nrg1 expression leads to patterned hyperplastic expansion of the zebrafish ventricle. To our knowledge, this is the first description of a single factor that is sufficient to induce such a dramatic hyperplastic response in an adult heart.