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  • Here we examine the impact of hypoxia

    2022-01-10

    Here we examine the impact of hypoxia on lake whitefish (Coregonus clupeaformis) during embryonic and larval development. Lake whitefish are a cold-water salmonid endemic to North America and are found from western Alaska, USA to eastern Labrador in Canada. Spawning occurs from late autumn to early winter in near shore areas with embryos developing over the winter for 3 to 4 months, often under the cover of ice at near freezing temperatures. Lake whitefish are considered a hypoxia-sensitive species and episodes of hypoxia have been linked with increases in mortality (Arend et al., 2011); yet they are still found in lakes that routinely experience hypoxic events throughout the summer (Barica, 1993). Winter hypoxia events also occur in these lakes in part due to ice coverage and can cause increases in over winter mortality (Barica and Mathias, 1979). Lake whitefish embryos are likely developing in these environments, suggesting that embryos may have mechanisms to aid in survival and development under low oxygen environments. The present study characterises the ABT263 patterns of hif-1α mRNA throughout development and identifies when lake whitefish develop the competence to mount a cellular stress response to hypoxia. To further our understanding of the physiological and cellular response to hypoxia we also quantified changes in mRNA levels of key HIF-1-responsive genes known to be associated with various physiological and developmental processes as well as the cellular stress marker, heat shock protein 70 (hsp70). The HIF-1-responsive genes examined (vegfa, epo, igfbp1, ldha and gapdh) are associated with angiogenesis, growth and development, and glucose metabolism. We predict that prior to organogenesis lake whitefish embryos have low oxygen demands and will be hypoxia-tolerant. However, as oxygen demands increase with development, exposure to hypoxic episodes will trigger compensatory mechanisms including increases in the mRNA levels of both hif-1α and HIF-1 target genes to maintain function under reduced oxygen conditions.
    Methods
    Results
    Discussion
    Conclusions Lake whitefish spend much of their embryonic period under the cover of ice before hatching in the early spring with larvae continuing to develop in the near-shore nursery grounds. Both environments are susceptible to periods of low oxygen. Our study demonstrates that lake whitefish at all ages studied can respond to these low oxygen environments by initiating some form of cellular response. We show that hif-1a mRNA levels decreased in response to hypoxia from 38 to 83 dpf. However, this decrease coincided with the increase of several putative HIF-1-responsive genes, suggesting that the HIF-1 pathway is functional and that hif-1a mRNA levels are transiently regulated and that we missed the upregulation of hif-1α mRNA with our 6 h hypoxia exposure and sampling time point. Increases in putative HIF-1-responsive gene transcripts varied with developmental age but were consistent with observed changes in oxygen demand and the importance of these genes for a variety of developmental processes. We concluded that lake whitefish cannot initiate a HIF-1-mediated response to hypoxia during gastrulation (21 dpf) because it is not yet functional. However, hypoxia likely has little direct effects at this early stage of development owing to the low oxygen requirements. Moreover, hsp70 mRNA was strongly elevated at this age and we propose that it is the dominant cellular stress response prior to heart formation. Here we present clear evidence that lake whitefish embryos can upregulate HIF-1-responsive genes that have been shown to drive functional changes important to surviving bouts of low oxygen. Our future work aims to more closely examine both the short-and long-term effects of hypoxia on lake whitefish embryo developmental rate, morphogenesis, survivability and metabolism, and the potential role of HIF-1 response genes in these processes.