Transcription factor VBP specifically regulates granulocyte apoptosis during the immune response in oysters

Vitellogenin-binding protein (VBP), a PAR bZIP transcription factor with homology to the cell death specification gene 2 (Ces-2) in Caenorhabditis elegans, is recognized for its role in cell apoptosis. In the present study, a VBP homolog CgVBP (also annotated as CgCes-2) containing a typical bZIP domain was identified from the Pacific oyster Crassostrea gigas. The transcript of CgVBP was predominantly expressed in haemocytes, specifically enriched within the granulocyte subpopulation. Temporal expression analysis revealed a positive correlation between CgVBP transcript levels and granulocyte apoptosis during Vibrio splendidus stimulation. At 24 h post-stimulation, granulocytes expressing CgVBP (CgVBP⁺ granulocytes) exhibited apoptotic hallmarks, including membrane blebbing and nuclear condensation. RNA interference-mediated knockdown of CgVBP significantly attenuated V. splendidus-induced granulocyte apoptosis, suggesting that CgVBP was involved in regulating granulocyte apoptosis during the immune response. Furthermore, CgVBP knockdown led to a marked decrease in the expression of the pro-apoptotic gene Bcl2-associated X protein (Bax). A dual-luciferase reporter assay confirmed that CgVBP bound directly and activated the Bax promoter, and significantly increased Bax expression. Collectively, these findings elucidated a critical regulatory mechanism wherein CgVBP orchestrates granulocyte apoptosis by transcriptionally upregulating the pro-apoptotic gene Bax, thereby providing novel insights into the transcriptional regulation of pathogen-induced granulocyte apoptosis in mollusks.