CgSMARCC2 modulates chromatin accessibility to regulate haemocyte proliferation in the immune priming of oyster Crassostrea gigas

The switch defective/sucrose non-fermentable (SWI/SNF) chromatin remodeling complex is a key regulator for altering chromatin accessibility and reported to play critical roles in trained immunity of vertebrates by impacting gene expression, but its role in immune priming of invertebrates is rarely reported. The present study identified the SWI/SNF related BAF chromatin remodeling complex subunit C2 (SMARCC2) in the Pacific oyster Crassostrea gigas and explored its role in regulating the accessibility of chromatin and haemocyte proliferation during the immune priming. CgSMARCC2 possesses conserved structural features, including the RSC8 superfamily domain and the SWIRM-assoc_2 domain. Phylogenetic analysis revealed that CgSMARCC2 was clustered with molluscan homologs prior to divergence into the vertebrate lineage. CgSMARCC2 mRNA expression in haemocytes was significantly induced (5.26-fold of that in control group, p < 0.05) at 7 d post Vibrio splendidus stimulation, concomitant with enlarged nuclei and a loose chromatin structure in haemocytes, indicating an open chromatin state. When the expression of CgSMARCC2 was knocked-down by RNAi, the DNase I sensitivity of DNA collected from haemocytes decreased significantly at 7 d post first Vibrio splendidus stimulation, and the chromatin accessibilities of gene promoters of CgTLR3 and CgCEBPβ were also significantly reduced. And after the secondary V. splendidus stimulation, the mRNA expressions of TLR signaling-related genes (CgTLR3, CgMyD88-2, CgRel1 and CgIL17-1) and cell proliferation-related genes (CgCDC-45, CgCDK-2, CgRunx-1 and CgGATA3) all significantly decreased in CgSMARCC2-RNAi oysters, along with a decreased haemocyte proliferation index. These results indicate that CgSMARCC2 functions in regulating chromatin accessibility and gene expression, and participates in promoting haemocyte proliferation in immune priming of oysters, which reveals an evolutionarily conserved role of the SWI/SNF complex in regulating trained immunity from invertebrates to vertebrates.