Abstract
Apostichopus japonicus undergoes evisceration in response to adverse environmental stimuli, and its coelomocytes undergo rapid regeneration within 6–24 h to restore innate immune function. FOS, an immediate early gene, regulates cell proliferation and cycle, but its role in A. japonicus coelomocyte regeneration after evisceration is unclear. In this study, AjFOSL from A. japonicus was cloned, which harbors a 609 bp open reading frame (ORF) encoding 202 amino acids (aa) with a conserved bZIP domain and is localized on chromosome 14. It shares 58% homology with FOS from Holothuria leucospilota and Lytechinus pictus. Phylogenetic analysis revealed that AjFOSL clusters closely with FOS from Magallana gigas and Mytilus edulis. Tissue distribution analysis showed that AjFOSL was widely expressed in various tissues, with the highest expression level detected in the tentacles. Temporal expression profiling demonstrated that AjFOSL was significantly upregulated by 1.75-fold at 6 h after evisceration. After AjFOSL knockdown, the peak expression of Cyclin A, Cyclin B, and E2F was delayed and the coelomocyte number was consistently reduced compared with that in the evisceration-only group. The AjFOSL acted as an immediate early response gene and was associated with the regulation of coelomocyte regeneration by modulating the expression of cell cycle-related genes. This study provides novel insights into the molecular associations underlying coelomocyte regeneration and the evolutionary adaptation of FOS genes in echinoderms.