Involvement of the phosphoryl transfer network in gill bioenergetic imbalance of pacamã (Lophiosilurus alexandri) subjected to hypoxia: notable participation of creatine kinase

The Keap1-Nrf2/ARE signaling pathway regulates redox balance and apoptosis in the small yellow croaker (Larimichthys polyactis) under hypoxic stress

Hypoxic stress can result in redox imbalance and apoptosis in teleostean fishes; however, the precise molecular mechanisms underlying this process, including its regulation by the key signaling pathway Kelch-like ECH-associated protein 1 (Keap1)-nuclear factor erythroid 2 related factor (Nrf2)/antioxidant response element (ARE), remain elusive. Therefore, in this study, we chose the Keap1-Nrf2/ARE signaling pathway as the entry point and a combination of in vivo (target organ liver) and in vitro (small yellow croaker fry [SYCF] cell line) experiments to investigate the molecular mechanism by which Larimichthys polyactis (L. polyactis) adapts to hypoxic stress by regulating redox balance and apoptosis. As our previous study found that hypoxic stress could lead to redox imbalance and apoptosis in L. polyactis. First, we observed significant alterations in the expression of key genes Lpkeap1, Lpnrf2, Lpho-1, and Lpnqo1 within the Keap1-Nrf2/ARE signaling pathway in both liver tissue and SYCF cells of L. polyactis under hypoxic stress, indicating activation of this pathway in response to hypoxia. Subsequently, we elucidated the mechanism by which hypoxia activates this pathway, that is, hypoxia weakened the interaction between LpNrf2 and LpKeap1, promoting the nuclear translocation of LpNrf2 and enhancing its binding activity to ARE, thereby activating the transcription of target genes. Furthermore, we found that significant changes occurred in the redox balance and apoptosis-related indicators after LpNrf2 knockdown and exposure to hypoxic stress for 24 h in SYCF cells, indicating that this pathway can regulate redox balance and apoptosis regulation under hypoxic stress in L. polyactis. Additionally, we used DNA affinity purification sequencing (DAP-seq) to identify the ARE sequence (ATGATTTAGC) that bound to LpNrf2 and its target genes. Finally, we conducted a combined analysis of DAP-seq and RNA-seq to identify six key target genes involved in the process: haeme oxygenase-1 (Ho-1), B-cell lymphoma-2 (Bcl2), pituitary homeobox 2 isoform X1 (Pitx2), aquaporin-4 isoform X1 (Aqp4), stress-induced phosphoprotein 1-like isoform X1 (Stip1), and guanine nucleotide-binding protein G (i) subunit alpha-2-like (Gnai2). In summary, hypoxic stress induced by weakening LpNrf2 and LpKeap1 interaction promoted LpNrf2 nuclear entry and enhanced its binding activity to ARE, thereby activating the transcription of multiple target genes to regulate redox balance and apoptosis. The research results not only help deepen our understanding of the adaptive mechanisms of L. polyactis and even marine fish to hypoxic stress and its survival strategies but also provide new ideas and potential targets for breeding new hypoxia-tolerant strains.

Ultrastructural, Antioxidant, and Metabolic Responses of Male Genetically Improved Farmed Tilapia (GIFT, Oreochromis niloticus) to Acute Hypoxia Stress

Tilapia tolerate hypoxia; thus, they are an excellent model for the study of hypoxic adaptation. In this study, we determined the effect of acute hypoxia stress on the antioxidant capacity, metabolism, and gill/liver ultrastructure of male genetically improved farmed tilapia (GIFT, Oreochromis niloticus). Fish were kept under control (dissolved oxygen (DO): 6.5 mg/L) or hypoxic (DO: 1.0 mg/L) conditions for 72 h. After 2 h of hypoxia stress, antioxidant enzyme activities in the heart and gills decreased, while the malondialdehyde (MDA) content increased. In contrast, in the liver, antioxidant enzyme activities increased, and the MDA content decreased. From 4 to 24 h of hypoxia stress, the antioxidant enzyme activity increased in the heart but not in the liver and gills. Cytochrome oxidase activity was increased in the heart after 4 to 8 h of hypoxia stress, while that in the gills decreased during the later stages of hypoxia stress. Hypoxia stress resulted in increased Na+-K+-ATP activity in the heart, as well as hepatic vacuolization and gill lamella elongation. Under hypoxic conditions, male GIFT exhibit dynamic and complementary regulation of antioxidant systems and metabolism in the liver, gills, and heart, with coordinated responses to mitigate hypoxia-induced damage.

Effects of hypoxia on the gill morphological structure, apoptosis and hypoxia-related gene expression in blunt snout bream (Megalobrama amblycephala)

The blunt snout bream (Megalobrama amblycephala) is a typical hypoxia-sensitive fish, and hypoxia stress leads to reduced vitality and yield during aquaculture. To explore the specific adaptation mechanism under hypoxia, the blunt snout bream was treated with hypoxia (DO = 2.0 ± 0.1 mg/L) for 24 h, followed by 3 h of recovery. Our results depicted that the gill filament structure of blunt snout bream changed after hypoxia. During hypoxia for 24 h, the gill filament structure was altered, including a more than 80% expansion of the lamellar respiratory surface area and a proportionate apoptosis decrease in interlamellar cell mass (ILCM) volume. Thus, the water-blood diffusion distance was shortened to less than 46%. During hypoxia for 24 h, the activity of ROS in gill tissue increased significantly (p < 0.05), while the mitochondrial membrane potential decreased significantly (p < 0.05). During hypoxia, mRNA expression level of anti-apoptotic gene Bcl-2 in the gills of blunt snout bream decreased significantly (p < 0.05), while the expression of pro-apoptotic gene Bax mRNA increased significantly (p < 0.05). Thus, the ratio of Bax/Bcl-2 mRNA increased in the gills of blunt snout bream to promote the activity of Caspase-3. Together, our results indicated hypoxia-induced apoptosis in the gills of blunt snout bream through the mitochondrial pathway. In addition, a decreased expression of Phd1 and an increased expression of Hif-1α in gills under hypoxia stress indicates that blunt snout bream may cope with hypoxia-induced apoptosis by enhancing the HIF pathway. These results provide new insights into fish's adaptation strategies and mechanisms of hypoxia.

Essential oil of Ocimum gratissimum (Linnaeus, 1753): efficacy for anesthesia and transport of Oreochromis niloticus

This study aimed to evaluate the essential oil of Ocimum gratissimum L. (EOOG) for anesthesia and in the transport of Oreochromis niloticus. Experiment I determined the time of anesthesia induction and recovery during anesthesia of O. niloticus exposed to different concentrations of EOOG (0, 30, 90, 150, and 300 mg L^-1). Based on data from Experiment I, Experiment II evaluated the effect of 0, 30, and 90 mg L^-1 EOOG on blood parameters and oxidative stress immediately after anesthesia induction and 1 h after recovery. Experiment III evaluated the effect of 0, 5, and 10 mg L^-1 EOOG on blood variables immediately after 4.5 h of transport of juveniles. Concentrations between 90 and 150 mg L^-1 EOOG were efficient for anesthesia and recovery. The use of 90 mg L^-1 of EOOG prevented an increase in plasma glucose. Other changes in blood parameters and oxidative stress are discussed. The use of 10 mg L^-1 EOOG in transport increased plasma glucose and decreased hematocrit values immediately after transport. It is concluded that the use of 90 and 150 mg L^-1 EOOG causes anesthesia and recovery in O. niloticus within the time intervals considered ideal. The use of 90 mg L^-1 EOOG favored stable plasma glucose soon after anesthesia induction and 1 h after recovery, but caused changes in the antioxidant defense system by increasing hepatic and kidney ROS. The transport of 12 g O. niloticus for 4.5 h can be performed with concentration of 5 mg L^-1 of EOOG.