Pathogen-responsive circSAFB2 promotes coelomocyte proliferation via JAK-STAT in sea cucumber

Circular RNAs (circRNAs) play crucial regulatory roles in various biological processes, yet their functions in invertebrate immunity remain largely unexplored. Here, we identified a novel circRNA, circSAFB2, which is derived from the scaffold attachment factor B2-like protein gene (AjSAFB2) in the sea cucumber Apostichopus japonicus . circSAFB2 is a stable cytoplasmic circRNA and was significantly upregulated upon Vibrio splendidus challenge and lipopolysaccharide (LPS) stimulation. RNA-seq revealed that silencing circSAFB2 led to significant enrichment of differentially expressed genes within the JAK-STAT cascade, a pathway well known to govern proliferation in diverse diseases. Moreover, silencing circSAFB2 markedly inhibited coelomocyte proliferation during pathogen challenge, accompanied by transcriptional and translational repression of JAK-STAT pathway components, including AjJAK2. In silico prediction across ten skin ulcer syndrome (SUS)-associated miRNAs, in addition to luciferase reporter assays, miR-9 was predicted to interact with both circSAFB2 and 3′-UTR of AjJAK2. Loss-of-function experiments further demonstrated that circSAFB2 acts as a competing endogenous RNA (ceRNA) to promote coelomocyte proliferation by sponging miR-9 to derepress AjJAK2. This regulatory axis facilitated AjSTAT5 autophosphorylation and nuclear translocation, promoting the expression of downstream proliferative genes, including AjMyc/Ajβ-Catenin. Furthermore, inhibition of miR-9 rescued the suppressive effects of circSAFB2 knockdown on coelomocyte proliferation, confirming the functional relevance of the circSAFB2-miR-9-AjJAK2-AjSTAT5 signaling cascade. This study provides new insights into circRNA-mediated immune regulation in echinoderms and establishes a mechanistic framework for future investigations into host-pathogen interactions in marine invertebrates. • circSAFB2 is strongly upregulated during bacterial infection, indicating its regulatory role in sea cucumber innate immunity. • circSAFB2 acts as a ceRNA to sponge miR-9, derepressing AjJAK2 and promoting infection-induced coelomocyte proliferation. • The circSAFB2-miR-9-AjJAK2 axis drives AjSTAT5 phosphorylation and nuclear translocation to enhance antibacterial defense.