Mustn1b is required for cardiomyocyte dedifferentiation and heart regeneration in zebrafish

Unlike the adult mammalian heart, the adult zebrafish heart can robustly regenerate via processes that include cardiomyocyte (CM) dedifferentiation, proliferation, and repopulation of the damaged tissue. Although CM dedifferentiation is considered indispensable for cardiac regeneration, the factors driving it remain elusive. Here, we examine the role of Mustn1b in CM dedifferentiation during zebrafish cardiac regeneration. Mustn1b is a small protein highly expressed in skeletal, smooth, and cardiac muscle tissues. Using a mustn1b:EGFP reporter line, we first find that its expression is specific to CMs during heart development, and that it peaks at early larval stages. In the regenerating adult heart, mustn1b reporter expression is re-induced in dedifferentiating CMs, establishing it as a novel marker for this subset of CMs. By evaluating two newly generated mustn1b loss-of-function mutant alleles, including a large deletion allele, we find that Mustn1b deficiency leads to decreased CM dedifferentiation and proliferation, and ultimately results in impaired cardiac regeneration. Furthermore, CM-specific overexpression of mustn1b can enhance CM dedifferentiation and proliferation after cardiac cryoinjury. Mechanistically, mass spectrometry data suggest that upon cryoinjury, Mustn1b interacts with sarcomeric and cytoskeletal proteins, pointing towards a role in structural remodeling that could facilitate CM dedifferentiation during heart regeneration. Altogether, our findings identify Mustn1b as a regulator of CM dedifferentiation during zebrafish cardiac regeneration.