Heme oxygenase 1 activity mediates red blood cell clearance and tail fin regeneration in zebrafish larvae.

Tissue injury triggers a tightly regulated cascade of events that transition from inflammation to resolution and ultimately tissue remodeling. Although the cellular dynamics of immune cells during these phases are increasingly well-characterized, the molecular mediators orchestrating the response to injury are yet to be fully elucidated. Based on a zebrafish model of tissue injury and using proteomic, in situ RNA expression analyses, and novel transgenic fluorescent reporters, we aimed to uncover relevant molecular mediators of tissue inflammation, resolution, and regeneration. We found that red blood cells (RBCs) accumulated at the injury site after tail fin amputation in zebrafish larvae, reaching its peak during the inflammatory phase and decreasing together with the resolution of inflammation. Furthermore, we observed that the heme scavenger and cytoprotective enzyme heme oxygenase 1 (Hmox1) is expressed in the injury site of amputated tail fins, and that macrophages were the main source of the functional hmox1a paralog. Pharmacological and morpholino-mediated inhibition of Hmox1 impaired RBC clearance and tail fin regeneration. In addition, depletion of macrophages led to impaired RBC clearance, phenocopying Hmox1 inhibition. Altogether, our findings reveal a novel role for Hmox1 in shaping the regenerative microenvironment and identify RBCs and hmox1a-expressing macrophages as previously overlooked players in the zebrafish injury response. This work underscores a new link between heme metabolism, immune regulation, and tissue regeneration in vivo.