Unraveling the molecular mechanisms of nitrite-induced physiological disruptions in largemouth bass

Largemouth bass (LMB; Micropterus salmoides) is an economically consequential piscine species spanning North America, Europe, and China. Elevated nitrite concentrations in aquatic environments cause significant physiological changes in fish, including stunted growth, appetite changes, reduced feed intake, and impaired oxygen transport, collectively affecting overall fish health. This study embarked on uncovering the molecular mechanisms underscoring the influence of various nitrite concentrations (0 mg/L, 400 mg/L, 600 mg/L, and 600 mg/L followed by recovery in nitrite-free water) on LMB, leveraging transcriptomics and metabolomics. Our findings revealed that a nitrite exposure of 400/600 mg/L may incite temporary, recoverable damage to the spleen, heart, and gills, but potentially enduring, irreversible hepatic pathologies. The spleen emerged as the most vulnerable organ to nitrite-induced deleterious impacts, causing acute, reversible damage and widespread apoptosis. Nitrite reprogramed splenocyte gene expressions, and prompted apoptosis, autophagy, and changes in metabolic pathways. It also triggered cell-type transitions and promoted adhesion-related gene expressions, as well as inhibited nucleotide metabolism and cell cycle progression. Notably, the immune cell repertoire within the spleen was affected by different nitrite concentrations, possibly linking to nitrite-induced spleen tissue impairment. Following the construction of a LMB protein–protein interaction (PPI) network, several pivotal genes uniquely expressed in distinct nitrite concentrations were identified, such as hdac4, hdac5, and brg1 in the N2 group (400 mg/L), hdac1, hells, and nccrp1 in the N3 group (600 mg/L), and Actr1, brd4, atf7a, and kat2b in the N4 group (600 mg/L recovery). The LMB spleen transcriptome was most profoundly influenced in the N3 group. Applying the MCODE algorithm for network module segmentation, we discerned eight MCODE subnetworks. Detailed analyses of these subnetworks shed light on their core functional pathways and pivotal genes. In particular, the MCODE1 core network implicated mainly Notch-HLH transcription, histone deacetylation, and protein deacetylation pathways, with pivotal genes such as ESRRGB, ATF7A, JDP2A, BRD4, and THRB. Metabolomic analysis results paralleled those of transcriptomic examination, corroborating analogous metabolic pathway alterations. This study holds substantial implications for comprehending the toxicological impact of nitrite on economically important fish species like LMB, and provides insights for refining aquaculture practices to minimize this toxicity.