The innate immune regulator MyD88 dampens fibrosis during zebrafish heart regeneration
The innate immune response is triggered rapidly after injury and its spatiotemporal dynamics are critical for regeneration, but many questions remain about its exact role.  Here we show that MyD88, a key component of the innate immune response, controls not only the inflammatory but also the fibrotic response during zebrafish cardiac regeneration.  We find in cryoinjured myd88-/- ventricles a significant reduction in neutrophil and macrophage numbers as well as the expansion of a collagen-rich endocardial population.  Further analyses reveal compromised PI3K/AKT pathway activation in the myd88-/- endocardium and increased myofibroblasts and scarring.  Notably, endothelial-specific overexpression of myd88 reverses these neutrophil, fibrotic, and scarring phenotypes.  Mechanistically, we identify the endocardial-derived chemokine gene cxcl18b as a target of the MyD88-signaling pathway, and using loss- and gain-of-function tools show that it controls neutrophil recruitment.  Altogether, these findings shed light on the pivotal role of MyD88 in modulating inflammation and fibrosis during tissue regeneration. Overall design: Aim: Investigation of the MyD88 signaling in endocardial cells during zebrafish cardiac regeneration. Methods: Endocardial cells sorted from the ET(krt4:EGFP)sqet33-1A line at 96 hours post cryoinjury (hpci). Conditions: 3 myd88+/+ and 3 myd88-/- samples