Abstract
Tolerance towards hypoxia is highly pronounced in zebrafish. In this study even beneficial effects of hypoxia, specifically enhanced survival of zebrafish larvae, could be demonstrated. This effect was actually more pronounced in breakdance mutants, which phenotypically show cardiac arrhythmia. Breakdance mutants (bre) are characterized by chronically reduced cardiac output. Despite an about 50% heart rate reduction, they become adults, but survival rate significantly drops to 40%. Normoxic bre animals demonstrate increased hypoxia inducible factor 1 a (Hif-1α) expression, which indicates an activated hypoxic signaling pathway. Consequently, cardiovascular acclimation, like cardiac hypertrophy and increased erythrocyte concentration, occurs. Thus, it was hypothesized, that under hypoxic conditions survival might be even more reduced. When bre mutants were exposed to hypoxic conditions, they surprisingly showed higher survival rates than under normoxic conditions and even reached wildtype values. In hypoxic wildtype zebrafish, survival yet exceeded normoxic control values. To specify physiological acclimation, cardiovascular and metabolic parameters were measured before hypoxia started (3 dpf), when the first differences in survival rate occurred (7 dpf) and when survival rate plateaued (15 dpf). Hypoxic animals expectedly demonstrated Hif-1α accumulation and consequently enhanced convective oxygen carrying capacity. Moreover, bre animals showed a significantly enhanced heart rate under hypoxic conditions, which reached normoxic wildtype values. This improvement in convective oxygen transport ensured a sufficient oxygen and nutrient supply and was also reflected in the significantly higher mitochondrial activity. The highly optimized energy metabolism observed in hypoxic zebrafish larvae might be decisive for periods of higher energy demand due to organ development, growth and increased activity. However, hypoxia increased survival only during a short period of development and starting hypoxia before or after this phase reduced survival, particularly in bre animals. Thus, the physiological plasticity, which enables zebrafish larvae to benefit from a hypoxia, occurs only within a narrow developmental window.
Highlights
Water living animals are frequently exposed to hypoxic conditions [1] and their ability for complex physiological acclimation, to enable their survival, has been reported in recent years [2]
Hypoxia inducible factors (Hif isoforms) transcriptionally regulate a high number of genes involved in energy metabolism, angiogenesis and apoptosis and play a central role in the cellular response to hypoxia
Bre larvae, which have a chronically reduced cardiac output, unexpectedly revealed hypoxic signaling under normoxic conditions, as shown by increased levels of hypoxia inducible factor 1 a protein (Hif-1a) concomitant with cardiovascular acclimation [12]
Summary
Water living animals are frequently exposed to hypoxic conditions [1] and their ability for complex physiological acclimation, to enable their survival, has been reported in recent years [2]. Bre larvae, which have a chronically reduced cardiac output, unexpectedly revealed hypoxic signaling under normoxic conditions, as shown by increased levels of hypoxia inducible factor 1 a protein (Hif-1a) concomitant with cardiovascular acclimation [12]. It was proposed that the Hif-1a accumulation in bre animals might derive from the increased blood cell concentration and enhanced shear stress within the vessels. The latter of which was reported to be closely linked to the endothelial nitric oxide production [13] which in turn is known to inhibit the ubiquitin proteasome
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