Characterizations of non-cardiomyocyte populations in ybx1 knockout zebrafish hearts

Cardiomyocyte deficiency results in heart failure, which increasingly kills more people than any other disease in the world. Upon injury, heart regeneration in zebrafish involves the de-differentiation and proliferation of cardiomyocytes, but it is recently understood that tissue repair is also regulated by non-cardiomyocyte (non-CM) cell populations. Moreover, Y-box binding protein (ybx1) is a cytoplasmic binding protein that is vital in the contexts of cancer and zebrafish embryonic development, but is largely unexplored in the context of heart regeneration. To examine the impact of ybx1 knockout in this setting, apex resections and cryoinjuries were employed to induce injuries in both ybx1-/- hearts and wildtype hearts. At the injury site of each heart, the cell activities were visualized by immunostainings and further characterized at various time points by either reverse transcription-quantitative polymerase chain reaction (RT-qPCR) or flow cytometry trials. We found that the ybx1-/- hearts displayed faster regeneration and greater fibroblast abundance at five days and seven days post injuries upon apex resections. We also found a significant reduction of neutrophils at two days post injury in the ybx1-/- hearts upon both injury methods. Therefore, we speculate that the ybx1 protein may have an inhibitory role on the tissue regeneration of a wildtype heart based on greater neutrophil infiltration, which may cause further damage and delay tissue repair in the myocardium11. These results and speculations provide a basis for identifying target genes within the major non-CM populations, especially neutrophils.