Shaping the cardiac response to hypoxia: NO and its partners in teleost fish

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.

Exposure to Intermittent Environmental Hypoxia Promotes Vascular Remodeling through Angiogenesis in the Liver of Largemouth Bass (Micropterus salmoides)

In this study, we explored the effects of 4 weeks of intermittent hypoxic exposure (IHE) on liver angiogenesis and related regulatory mechanisms in largemouth bass (Micropterus salmoides). The results indicated that the O2 tension for loss of equilibrium (LOE) decreased from 1.17 to 0.66 mg/L after 4 weeks of IHE. Meanwhile, the red blood cell (RBC) and hemoglobin concentrations significantly increased during IHE. Our investigation also found that the observed increase in angiogenesis was correlated with a high expression of related regulators, such as Jagged, phosphoinositide-3-kinase (PI3K), and mitogen-activated protein kinase (MAPK). After 4 weeks of IHE, the overexpression of factors related to angiogenesis processes mediated by HIF-independent pathways (such as nuclear factor kappa-B (NF-κB), NADPH oxidase 1 (NOX1), and interleukin 8 (IL8)) was correlated with the accumulation of lactic acid (LA) in the liver. The addition of cabozantinib, a specific inhibitor of VEGFR2, blocked the phosphorylation of VEGFR2 and downregulated the expression of downstream angiogenesis regulators in largemouth bass hepatocytes exposed to hypoxia for 4 h. These results suggested that IHE promoted liver vascular remodeling by the regulation of angiogenesis factors, presenting a potential mechanism for the improvement of hypoxia tolerance in largemouth bass.

Evolution and divergence of teleost adrenergic receptors: why sometimes 'the drugs don't work' in fish

Adrenaline and noradrenaline, released as hormones and/or neurotransmitters, exert diverse physiological functions in vertebrates, and teleost fishes are widely used as model organisms to study adrenergic regulation; however, such investigations often rely on receptor subtype-specific pharmacological agents (agonists and antagonists; see Glossary) developed and validated in mammals. Meanwhile, evolutionary (phylogenetic and comparative genomic) studies have begun to unravel the diversification of adrenergic receptors (ARs) and reveal that whole-genome duplications and pseudogenization events in fishes results in notable distinctions from mammals in their genomic repertoire of ARs, while lineage-specific gene losses within teleosts have generated significant interspecific variability. In this Review, we visit the evolutionary history of ARs (including α1-, α2- and β-ARs) to highlight the prominent interspecific differences in teleosts, as well as between teleosts and other vertebrates. We also show that structural modelling of teleost ARs predicts differences in ligand binding affinity compared with mammalian orthologs. To emphasize the difficulty of studying the roles of different AR subtypes in fish, we collate examples from the literature of fish ARs behaving atypically compared with standard mammalian pharmacology. Thereafter, we focus on specific case studies of the liver, heart and red blood cells, where our understanding of AR expression has benefited from combining pharmacological approaches with molecular genetics. Finally, we briefly discuss the ongoing advances in 'omics' technologies that, alongside classical pharmacology, will provide abundant opportunities to further explore adrenergic signalling in teleosts.

Nitric Oxide Function and Nitric Oxide Synthase Evolution in Aquatic Chordates

Nitric oxide (NO) is a key signaling molecule in almost all organisms and is active in a variety of physiological and pathological processes. Our understanding of the peculiarities and functions of this simple gas has increased considerably by extending studies to non-mammal vertebrates and invertebrates. In this review, we report the nitric oxide synthase (Nos) genes so far characterized in chordates and provide an extensive, detailed, and comparative analysis of the function of NO in the aquatic chordates tunicates, cephalochordates, teleost fishes, and amphibians. This comprehensive set of data adds new elements to our understanding of Nos evolution, from the single gene commonly found in invertebrates to the three genes present in vertebrates.

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

Aquatic animals are increasingly challenged by O₂ fluctuations as a result of global warming, as well as eutrophication processes. Teleost fish show important species-specific adaptability to O₂ deprivation, moving from intolerance to a full tolerance of hypoxia and even anoxia. An example is provided by members of Cyprinidae which includes species that are amongst the most tolerant hypoxia/anoxia teleosts. Living at low water O₂ requires the mandatory preservation of the cardiac function to support the metabolic and hemodynamic requirements of organ and tissues which sustain whole organism performance. A number of orchestrated events, from metabolism to behavior, converge to shape the heart response to the restricted availability of the gas, also limiting the potential damages for cells and tissues. In cyprinids, the heart is extraordinarily able to activate peculiar strategies of functional preservation. Accordingly, by using these teleosts as models of tolerance to low O₂, we will synthesize and discuss literature data to describe the functional changes, and the major molecular events that allow the heart of these fish to sustain adaptability to O₂ deprivation. By crossing the boundaries of basic research and environmental physiology, this information may be of interest also in a translational perspective, and in the context of conservative physiology, in which the output of the research is applicable to environmental management and decision making.

An ACE2-Alamandine Axis Modulates the Cardiac Performance of the Goldfish Carassius auratus via the NOS/NO System

Alamandine is a peptide of the Renin Angiotensin System (RAS), either generated from Angiotensin A via the Angiotensin Converting Enzyme 2 (ACE2), or directly from Ang-(1-7). In mammals, it elicits cardioprotection via Mas-related G-protein-coupled receptor D (MrgD), and the NOS/NO system. In teleost fish, RAS is known to modulate heart performance. However, no information is available on the presence of a cardioactive ACE2/Alamandine axis. To fill this gap, we used the cyprinid teleost Carassius auratus (goldfish) for in silico and in vitro analyses. Via the NCBI Blast P suite we found that in cyprinids ace2 is phylogenetically detectable in a subcluster of proteins including ace2-like isoforms, and is correlated with a hypoxia-dependent pathway. By real-time PCR, Western Blotting, and HPLC, ACE2 and Alamandine were identified in goldfish heart and plasma, respectively. Both increased after chronic exposure to low O₂ (2.6 mg O₂ L^-1). By using an ex-vivo working goldfish-heart preparation, we observed that in vitro administration of exogenous Alamandine dose-dependently stimulates myocardial contractility starting from 10^-11 M. The effect that involved Mas-related receptors and PKA occurred via the NOS/NO system. This was shown by exposing the perfused heart to the NOS inhibitor L-NMMA (10^-5 M) that abolished the cardiac effect of Alamandine and was supported by the increased expression of the phosphorylated NOS enzyme in the extract from goldfish heart exposed to 10^-10 M Alamandine. Our data are the first to show that an ACE2/Alamandine axis is present in the goldfish C. auratus and, to elicit cardiac modulation, requires the obligatory involvement of the NOS/NO system.