The zebrafish heart regenerates after ventricular damage through a process involving inflammation, fibrotic tissue deposition/removal and myocardial regeneration. Using 3D whole-mount imaging, we reveal a highly dynamic endocardium during cardiac regeneration, including changes in cell morphology, behaviour and gene expression. These events lay the foundation for an initial expansion of the endocardium that matures to form a coherent endocardial structure within the injury site. We studied two important endocardial molecules, Serpine1 and Notch, which are implicated in different aspects of endocardial regeneration. Notch signalling regulates developmental gene expression and features of endocardial maturation. Also, Notch manipulation interferes with attenuation of the inflammatory response and cardiomyocyte proliferation and dedifferentiation. serpine1 is strongly expressed very early in the wound endocardium, with decreasing expression at later time points. serpine1 expression persists in Notch-abrogated hearts, via what appears to be a conserved mechanism. Functional inhibition studies show that Serpine1 controls endocardial maturation and proliferation and cardiomyocyte proliferation. Thus, we describe a highly dynamic endocardium in the regenerating zebrafish heart, with two key endocardial players, Serpine1 and Notch signalling, regulating crucial regenerative processes.

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The functional and clinical data showed that this novel Dutch founder mutation leads to severe early onset of FHL3 due to misfolding and degradation of munc13-4(1-899).

Caveolin-1 is the main structural protein of caveolae, small membrane invaginations involved in signal transduction and mechanoprotection. Here, we generated cav1-KO zebrafish lacking Cav1 and caveolae, and investigated the impact of this loss on adult heart function and response to cryoinjury. We found that cardiac function was impaired in adult cav1-KO fish, which showed a significantly decreased ejection fraction and heart rate. Using atomic force microscopy, we detected an increase in the stiffness of epicardial cells and cells of the cortical zone lacking Cav1/caveolae. This loss of cardiac elasticity might explain the decreased cardiac contraction and function. Surprisingly, cav1-KO mutants were able to regenerate their heart after a cryoinjury but showed a transient decrease in cardiomyocyte proliferation. Caveolae are small membrane invaginations present in endothelial cells, fibroblasts and less abundantly, in cardiomyocytes 1-4. Caveolin-1 (Cav1) is the main structural protein of the caveolae 5 , as Cav1 deletion in mice diminishes caveolae formation 6-8. Similarly, the caveolae associated protein Cavin1, is essential for caveolae formation, because its genetic deletion leads to loss of caveolae 9. Caveolae participate in multiple cellular processes, including lipid homoeostasis and signal transduction 1,10,11. In particular, Cav1 interacts directly with Transforming growth factor β receptor-1 (TGFβR1), blocking Smad complex nuclear translocation and, consequently, inhibiting transcriptional activation 12. Furthermore, caveolae are involved in mechanoprotection, as they deliver the extra membrane needed for cells to buffer mechanical forces through rapid disassembly and flattening 13,14. Physiologically, caveolae protect mouse cardiac endothelial cells from rupture caused by increased cardiac output 10. Likewise, caveolae safeguard zebrafish skeletal muscle cells from rupture after vigorous activity 15 and maintain notochord's integrity 16,17. Genetic inactivation of Cav1 in the mouse results in cardiac remodelling. Right ventricle dilatation and left ventricle hypertrophy are among the various cardiac defects associated with loss of caveolae 1,8,18. Additionally, Cav1 mutant mice show defective heart function, including decreased systolic and diastolic function 1,8,18,19 , which is exacerbated after myocardial infarction 20,21. Cardiac insult in Cav1 mutant mice also leads to aberrant fibrosis, mediated by increased Smad2/3 phosphorylation and macrophages infiltration 21,22. In zebrafish, the cav1 gene generates two protein-coding transcripts, cav1a and cav1b, with the Cav1b protein being shorter, as it lacks the first 31 amino acids 23. Resection of the ventricular apex in hearts of cav1a mutant zebrafish leads to regeneration defects 30 days post amputation (dpa), because of decreased cardiomyocyte proliferation and increased fibrosis in the amputation plain 24. In addition, inactivation of both cav1a and cav1b transcripts, results in regeneration defects after ventricular resection only in heterozyg...