Zebrafish are able to efficiently regenerate lost cardiac muscle, and is used as a model to understand why natural heart regeneration is blocked in mammals. Two groups reporting in the issue of Nature used genetic fate-mapping approaches to identify which population of cardiomyocytes contribute prominently to cardiac muscle regeneration after an injury approximating myocardial infarction. They show that cardiac muscle regenerates through activation and expansion of existing cardiomyocytes, and does not involve activation of a stem cell population. Zebrafish are able to replace lost heart muscle efficiently, and are used as a model to understand why natural heart regeneration — after a heart attack, for instance — is blocked in mammals. Here, and in an accompanying paper, genetic fate-mapping approaches reveal which cell population contributes prominently to cardiac muscle regeneration after an injury approximating myocardial infarction. The results show that cardiac muscle regenerates through activation and expansion of existing cardiomyocytes, without involving a stem-cell population. Although mammalian hearts show almost no ability to regenerate, there is a growing initiative to determine whether existing cardiomyocytes or progenitor cells can be coaxed into eliciting a regenerative response. In contrast to mammals, several non-mammalian vertebrate species are able to regenerate their hearts1,2,3, including the zebrafish4,5, which can fully regenerate its heart after amputation of up to 20% of the ventricle. To address directly the source of newly formed cardiomyocytes during zebrafish heart regeneration, we first established a genetic strategy to trace the lineage of cardiomyocytes in the adult fish, on the basis of the Cre/lox system widely used in the mouse6. Here we use this system to show that regenerated heart muscle cells are derived from the proliferation of differentiated cardiomyocytes. Furthermore, we show that proliferating cardiomyocytes undergo limited dedifferentiation characterized by the disassembly of their sarcomeric structure, detachment from one another and the expression of regulators of cell-cycle progression. Specifically, we show that the gene product of polo-like kinase 1 (plk1) is an essential component of cardiomyocyte proliferation during heart regeneration. Our data provide the first direct evidence for the source of proliferating cardiomyocytes during zebrafish heart regeneration and indicate that stem or progenitor cells are not significantly involved in this process.

Background—: Hypoxia plays an important role in many biological/pathological processes. In particular, hypoxia is associated with cardiac ischemia. which, although initially inducing a protective response, will ultimately lead to the death of cardiomyocytes and loss of tissue, severely affecting cardiac functionality. Although myocardial damage/loss remains an insurmountable problem for adult mammals, the same is not true for adult zebrafish, which are able to completely regenerate their heart after extensive injury. Myocardial regeneration in zebrafish involves the dedifferentiation and proliferation of cardiomyocytes to replace the damaged/missing tissue; at present, however, little is known about what factors regulate this process. Methods and Results—: We surmised that ventricular amputation would lead to hypoxia induction in the myocardium of zebrafish and that this may play a role in regulating the regeneration of the missing cardiac tissue. Using a combination of O ₂ perturbation, conditional transgenics, in vitro cell culture, and microarray analysis, we found that hypoxia induces cardiomyocytes to dedifferentiate and proliferate during heart regeneration in zebrafish and have identified a number of genes that could play a role in this process. Conclusion—: These results indicate that hypoxia plays a positive role during heart regeneration, which should be taken into account in future strategies aimed at inducing heart regeneration in humans.

Mechanosensitivity is an inherent property of virtually all cell types, allowing them to sense and respond to physical environmental stimuli. Stretch-activated ion channels represent a class of mechanosensitive proteins which allow cells to respond rapidly to changes in membrane tension; however their identity has remained elusive. The piezo genes have recently been identified as a family of stretch-activated mechanosensitive ion channels. We set out to determine the role of piezo1 during zebrafish development. Here we report that morpholino-mediated knockdown of piezo1 impairs erythrocyte survival without affecting hematopoiesis or differentiation. Our results demonstrate that piezo1 is involved in erythrocyte volume homeostasis, disruption of which results in swelling/lysis of red blood cells and consequent anemia.

The sense of touch allows an organism to detect and respond to physical environmental stimuli. Mechanosensitive proteins play a crucial role in this process by converting the mechanical cue into a biological response. Recently, the Piezo family of stretch-activated ion channels has been identified as genuine mechanosensitive proteins. We set out to determine whether any of these genes are involved in touch response during zebrafish development.In situhybridization indicates thatpiezo2bis specifically expressed in a subset of neurons (Rohon–Beard cells) responsible for detecting light touch. Using morpholino-mediated knockdown, we specifically targetedpiezo2band determined that it is involved in mediating touch-evoked response.