Pax4-Ghrelin mediates the conversion of pancreatic.-cells to beta-cells after extreme beta-cell loss in zebrafish

Pancreatic epsilon-cells producing ghrelin are one type of endocrine cell found in islets, which have been shown to influence other intra-islet cells, especially in regulating the function of beta cells. However, the role of such cells during beta-cell regeneration is currently unknown. Here, using a zebrafish nitroreductase (NTR)-mediated beta-cell ablation model, we reveal that ghrelin-positive epsilon-cells in the pancreas act as contributors to neogenic beta-cells after extreme beta-cell loss. Further studies show that the overexpression of ghrelin or the expansion of epsilon-cells potentiates beta-cell regeneration. Lineage tracing confirms that a proportion of embryonic epsilon-cells can transdifferentiate to beta-cells, and that the deletion of Pax4 enhances this transdifferentiation of epsilon-cells to beta-cells. Mechanistically, Pax4 binds to the ghrelin regulatory region and represses its transcription. Thus, deletion of Pax4 derepresses ghrelin expression and causes producing more ghrelin-positive cells, enhancing the transdifferentiation of epsilon-cells to beta-cells and consequently potentiating beta-cell regeneration. Our findings reveal a previously unreported role for epsilon-cells during zebrafish beta-cell regeneration, indicating that Pax4 regulates ghrelin transcription and mediates the conversion of embryonic epsilon-cells to beta-cells after extreme beta-cell loss.