Characterisation of cardiomyocyte plasticity and the role of fibroblasts during zebrafish heart regeneration

The zebrafish is an established model organism to study heart regeneration, in which pre-existing cardiomyocytes (CMs) proliferate to replace the lost myocardium. During development, mesodermal progenitors from the first heart field (FHF) form a primitive cardiac tube, to which cells from the second heart field (SHF) are added. Here we investigated whether FHF and SHF derivatives in the zebrafish give rise to distinct CM populations, and examined the degree of cell fate plasticity of SHF derivatives during heart regeneration. Using tbx5a-lineage tracing we found that the adult zebrafish heart is also composed of CM populations from the FHF and SHF. Furthermore, ablation of FHF-derived CMs in the embryo is compensated by expansion of SHF-derived cells. tbx5a lineage-tracing was also employed to investigate the fate of trabecular CMs during adult heart regeneration. While previous clonal analysis suggested that the different myocardial layers are rebuilt by CMs within each layers, we describe that trabecular CMs can switch their fate and differentiate into cortical myocardium. Heart regeneration is preceded by a fibrotic response. Thus, fibrosis and regeneration are not mutually exclusive responses. Upon cardiac cryoinjury, collagen and other extracellular matrix (ECM) components accumulate at the injury site. Unlike the situation in mammals, fibrosis in zebrafish is transient and its regression is concomitant with regrowth of the myocardial wall. We describe that during fibrosis regression, fibroblasts are not fully eliminated and become inactivated. Unexpectedly, limiting the fibrotic response by genetic ablation of col1a2-expressing cells not only failed to enhance regeneration but also impaired CMs proliferation. We conclude that zebrafish regeneration is a process that requires CM plasticity, and involves ECM-producing cells that become inactive and promote CMs proliferation. El pez cebra es un organismo modelo ampliamente usado para estudiar la regeneracion de corazon, en el que los cardiomiocitos preexistentes proliferan y reemplazan el miocardio perdido. Durante el desarrollo, los progenitores mesodermicos del campo cardiaco primario forman un tubo cardiaco, al cual se anaden las celulas del campo cardiaco secundario. Aqui investigamos si los derivados de ambos campos en el pez dan lugar a distintas poblaciones de cardiomiocitos, y el grado de plasticidad durante la regeneracion. El trazado de linaje de las celulas tbx5a-positivas tambien nos permitio investigar el destino de los cardiomiocitos durante la regeneracion en adulto. Mientras que los analisis de trazado de linaje previos sugirieron que cada capa de cardiomiocitos es derivada de la misma capa, aqui describimos que los cardiomiocitos de las trabeculas pueden cambiar su especificacion y diferenciarse en miocardio cortical. La regeneracion del corazon esta precedida de una respuesta fibrotica. Por lo tanto, fibrosis y regeneracion no son respuestas mutuamente excluyentes. Tras una criolesion, colageno y otras proteinas de matriz extracelular se acumulan en el lugar del dano. A diferencia de lo que ocurre en mamiferos, la fibrosis es una respuesta transitoria y simultanea a la regeneracion de la nueva pared miocardica. Aqui describimos que durante la regresion de la fibrosis, los fibroblastos no son completamente eliminados, sino que se inactivan. Sorprendentemente, limitar la respuesta fibrotica por ablacion de las celulas que expresan col1a2 no estimulo la regeneracion, sino que disminuyo la proliferacion de cardiomiocitos. Concluimos que la regeneracion del corazon de pez cebra es un proceso en el que hay una gran plasticidad de cardiomiocitos, y las celulas que producen matriz extracelular y se inactivan, promueven la proliferacion de cardiomiocitos.