Hypoxia impairs monocyte/macrophage function and host defense via ALAS2-mediated heme biosynthesis in Japanese sea bass (Lateolabrax japonicus).

Hypoxia compromises host defense in fish by impairing macrophage function, but the underlying mechanisms remain unclear. This study elucidates the molecular mechanisms by which hypoxia disrupts monocytes/macrophages (MO/MФ) function in Japanese sea bass (Lateolabrax japonicus), with a particular focus on the regulatory role of heme biosynthesis and its key gene, 5'-aminolevulinate synthase 2 (ALAS2). Phylogenetic analysis confirmed high evolutionary conservation of ALAS2 across vertebrates, supporting its fundamental role in heme metabolism. Transcriptomic analysis revealed that hypoxia induced the upregulation of genes involved in heme biosynthesis of MO/MФ, particularly ALAS2. Functional assays demonstrated that heme accumulation under hypoxic conditions impaired MO/MФ phagocytic and bactericidal activities, while also promoted the production of pro-inflammatory cytokines. These dysfunctions were mediated by the ALAS2-heme signaling axis, which orchestrated inflammatory responses and metabolic reprogramming throughout the study. Knockdown of ALAS2 enhanced phagocytic activity and suppressed pro-inflammatory cytokine expression, suggesting its critical role in modulating MO/MФ function under hypoxia. Furthermore, inhibition of heme synthesis improved fish survival rates and reduced bacterial burdens during Vibrio harveyi infection. These findings elucidate the pivotal role of heme in regulating MO/MФ function and host defense under hypoxic conditions, providing mechanistic insights into hypoxia-induced immune suppression in aquaculture.