Cardiac Hypoxia Tolerance in Fish: From Functional Responses to Cell Signals

Environmental epigenetics: Exploring phenotypic plasticity and transgenerational adaptation in fish

Epigenetics plays a vital role in the interaction between living organisms and their environment by regulating biological functions and phenotypic plasticity. Considering that most aquaculture activities take place in open or natural habitats that are vulnerable to environmental changes. Promising findings from recent research conducted on various aquaculture species have provided preliminary evidence suggesting a link between epigenetic mechanisms and economically valuable characteristics. Environmental stressors, including climate changes (thermal stress, hypoxia, and water salinity), anthropogenic impacts such as (pesticides, crude oil pollution, nutritional impacts, and heavy metal) and abiotic factors (infectious diseases), can directly trigger epigenetic modifications in fish. While experiments have confirmed that many epigenetic alterations caused by environmental factors have plastic responses, some can be permanently integrated into the genome through genetic integration and promoting rapid transgenerational adaptation in fish. These environmental factors might cause irregular DNA methylation patterns in genes related to many biological events leading to organs dysfunction by inducing alterations in genes related to oxidative stress or apoptosis. Moreover, these environmental issues alter DNA/histone methylation leading to decreased reproductive competence. This review emphasizes the importance of understanding the effects of environmentally relevant issues on the epigenetic regulation of phenotypic variations in fish. The goal is to expand our knowledge of how epigenetics can either facilitate or hinder species' adaptation to these adverse conditions. Furthermore, this review outlines the areas that warrant further investigation in understanding epigenetic reactions to various environmental issues.

Comparative transcriptomics combined with physiological and functional analysis reveals the regulatory mechanism of rainbow trout (Oncorhynchus mykiss) under acute hypoxia stress

Hypoxia, largely triggered by global warming and water contamination, has become an environmental issue of great concern, posing a great threat to aquatic ecosystem. As one of the world's most economically important fish, rainbow trout (Oncorhynchus mykiss) is extremely intolerant of hypoxic environments, however, little is known about the roles of non-coding RNAs (ncRNAs) in the response of rainbow trout to hypoxia stress. Herein, effects of moderate (Tm12L) and severe hypoxia for 12 h (Ts12L) and 12 h reoxygenation on histology, biochemical parameters (antioxidant, metabolism and immunity) and transcriptome (lncRNA, miRNA and mRNA) in rainbow trout liver were investigated. We further validated the regulatory relationships between LOC110519952, novel-m0023-5p and glut1a via dual‑luciferase reporter, overexpression and silencing assays. Compared with Tm12L, the liver in Ts12L showed more severe oxidative damage. Anaerobic, lipid and protein metabolism was enhanced under hypoxia stress, especially in Ts12L. We also found that Tm12L could strengthen innate immune response, which was inhibited in Ts12L. Besides, several hypoxia-related genes (glut1a, vegfaa, hmox, epoa, foxo1a and igfbp1) and ceRNA networks were identified from 1824, 427 and 545 differentially expressed mRNAs, miRNAs and lncRNAs, including LOC118965299-novel-m0179-3p-epoa, LOC110519952-novel-m0023-5p-glut1a, MSTRG.7382.2-miR-184-y-hmox and LOC110520012-miR-206-y-vegfaa. Through in vitro and in vivo functional analysis, we demonstrated that glut1a is a target of novel-m0023-5p, and LOC110519952 can positively regulate glut1a by targeting novel-m0023-5p. Introduction of LOC110519952 could attenuate the promoting effects of novel-m0023-5p on rainbow trout liver cell viability and proliferation. This study highlights the differences in the regulatory mechanism of rainbow trout under different concentrations of hypoxia stress and provides valuable data for further research on the molecular mechanisms of fish adaptation to hypoxic environments.

Functional, structural, and molecular remodelling of the goldfish (Carassius auratus) heart under moderate hypoxia

The goldfish (Carassius auratus) is known for its physiologic ability to survive even long periods of oxygen limitation (hypoxia), adapting the cardiac performance to the requirements of peripheral tissue perfusion. We here investigated the effects of short-term moderate hypoxia on the heart, focusing on ventricular adaptation, in terms of hemodynamics and structural traits. Functional evaluations revealed that animals exposed to 4 days of environmental hypoxia increased the hemodynamic performance evaluated on ex vivo cardiac preparations. This was associated with a thicker and more vascularized ventricular compact layer and a reduced luminal lacunary space. Compared to normoxic animals, ventricular cardiomyocytes of goldfish exposed to hypoxia showed an extended mitochondrial compartment and a modulation of proteins involved in mitochondria dynamics. The enhanced expression of the pro-fission markers DRP1 and OMA1, and the modulation of the short and long forms of OPA1, suggested a hypoxia-related mitochondria fission. Our data propose that under hypoxia, the goldfish heart undergoes a structural remodelling associated with a potentiated cardiac activity. The energy demand for the highly performant myocardium is supported by an increased number of mitochondria, likely occurring through fission events.

Efficacy of quercetin in ameliorating hypoxia-induced hematological and histopathological alterations in rohu Labeo rohita

Hypoxia, a major issue in aquatic ecosystems, in special reference to climate change, and exacerbated by anthropogenic activities. It is causing slow growth, disease outbreaks, and mortality in finfish and shellfish. Therefore, adaptation to lowering oxygen levels through supplementation of herbs or their extracts in diets is imperative. In this study, hypoxia was simulated in controlled conditions with quercetin-enriched diets. Quercetin is a plant pigment (flavonol) possessing anti-oxidant property and is present in vegetables, leaves, seeds, pulses, and fruits. The experiment was conducted on rohu Labeo rohita, which is most widely cultured in India. There were four treatments including T1 (Normoxia: > 5 ppm dissolved oxygen; DO2), T2 (hypoxia: 3-4 ppm DO2), T3 (hypoxia + 50 mg quercetin/kg diet), and T4 (hypoxia + 100 mg quercetin/kg diet). The study was conducted for 30 days, and water quality was measured regularly. The results revealed that the hematological parameters were negatively affected. The tissue micro-architecture illustrated the impairment through degeneration of neurons in the brain, increased pigmentation as melanosis in the kidney, increased thickness of primary lamellae in the gills, and dilatations of sinusoids in the liver in hypoxia groups, while quercetin-enriched diets improved the hematological and histomorphological parameters. The results confirm the utility of hematological and histopathological tools as biomarkers and reflect the possible threats of hypoxia on fish. In conclusion, quercetin in diets appeared to show resistance towards chronic hypoxia by restoring the structure and functions of the vital organs towards normalcy and could be recommended as a potential ameliorative agent.

Effects of environmental hypoxia on the goldfish skeletal muscle: Focus on oxidative status and mitochondrial dynamics

The skeletal muscle is a highly plastic tissue. Its ability to respond to external stimuli and challenges allows it to face the functional needs of the organism. In the goldfish Carassius auratus, a model of hypoxia resistance, exposure to reduced oxygen is accompanied by an improvement of the swimming performance, relying on a sustained contractile behavior of the skeletal muscle. At the moment, limited information is available on the mechanisms underlying these responses. We here evaluated the effects of short- (4 days) and long- (20 days) term exposure to moderate water hypoxia on the goldfish white skeletal muscle, focusing on oxidative status and mitochondrial dynamics. No differences in lipid peroxidation, measured as 2-thiobarbituric acid-reacting substances (TBARS), and oxidatively modified proteins (OMP) were detected in animals exposed to hypoxia with respect to their normoxic counterparts. Exposure to short-term hypoxia was characterized by an enhanced SOD activity and expression, paralleled by increased levels of Nrf2, a regulator of the antioxidant cell response, and HSP70, a chaperone also acting as a redox sensor. The expression of markers of mitochondrial biogenesis (TFAM) and abundance (VDAC) and of the mtDNA/nDNA ratio was similar under normoxia and under both short- and long-term hypoxia, thus excluding a rearrangement of the mitochondrial apparatus. Only an increase of PGC1α (a transcription factor involved in mitochondrial dynamics) was detected after 20 days of hypoxia. Our results revealed novel aspects of the molecular mechanisms that in the goldfish skeletal muscle may sustain the response to hypoxia, thus contributing to adequate tissue function to organism requirements.