Heme oxygenase 1 activity mediates hemoglobin clearance and tail fin regeneration in zebrafish larvae

Tissue injury triggers a tightly regulated cascade of events that transitions 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 hemoglobin accumulated in 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 inhibition of Hmox1 activity impaired hemoglobin clearance and tail fin regeneration. In addition, depletion of macrophages led to impaired hemoglobin clearance, phenocopying Hmox1 inhibition. Altogether, our findings reveal a novel role for Hmox1 in shaping the regenerative microenvironment and identify hemoglobin 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 .