Differential environmental-induced heat stresses cause the structural and molecular changes in the spleen of Japanese flounder (Paralichthys olivaceus)

Japanese flounder (Paralichthys olivaceus) is a cold-water fish widely cultured in Northeast Asia. With global warming and increased breeding density, Japanese flounder aquaculture is threatened by heat stress. Therefore, understanding the molecular mechanisms underlying heat stress is important. This study applied two laboratory heat stress conditions, gradual temperature rise (GTR) and abrupt temperature rise (ATR) to imitate the stresses faced by Japanese flounder. The histological damage of spleen and the underlying molecular mechanisms were revealed. Histological observation showed that the size and number of melanoma macrophage centers (MMCs) changed under the applied heat stresses; however, inflammatory infiltration and erythrocytes specifically appeared only under the ATR stress. Transcriptome analysis identified 1073 and 985 differentially expressed genes (DEGs) in the GTR vs. the C and the ATR vs. the C groups, respectively. Functional enrichment analysis illustrated GTR and ATR stresses affected similar categories while simultaneously influencing diverse functions. The “Protein processing in endoplasmic reticulum (ER)” was the most enriched pathway in two kinds of heat stress. The “Apoptosis” pathway was GTR-specific while the“DNA replication” was ATR-specific. In addition, the comparison between GTR and ATR stresses in “Protein processing in ER” pathway were further described. The results illustrated both acute and chronic heat stress could increase misfolded proteins, disrupt the ER function, and cause histological damage. Misfolded proteins in the GTR group were removed by apoptosis, whereas those in the ATR group were cleared via DNA-related pathways. Protein-Protein Interaction (PPI) analysis identified 20 hub genes, eight of which were shared between the two groups (hsp90aa1.1, hspa5, hsp90b1, hsp90ab1, hspa9, hsp70, hspe1 and hspd1); three (hspa4b, fn1a and bmp4) were GTR-specific, whereas nine genes (chek1, pcna, aurkb, rpal, cdc6, rrm1, mcm5, pold1 and mcm3) were ATR-specific. The current study sheds light on the molecular mechanisms underlying the heat stress response in Japanese flounder.