Département de Biologie, Université de Moncton, Moncton, NB, Canada, E1A 3E9
Département de Biologie, Université de Moncton, Moncton, NB, Canada, E1A 3E9.
J Exp Biol. 2019 Nov 21;222(Pt 22):jeb203901. doi: 10.1242/jeb.203901.
Fish exposed to fluctuating oxygen concentrations often alter their metabolism and/or behaviour to survive. Hypoxia tolerance is typically associated with the ability to reduce energy demand by supressing metabolic processes such as protein synthesis. Arctic char is amongst the most sensitive salmonid to hypoxia, and typically engage in avoidance behaviour when faced with lack of oxygen. We hypothesized that a sensitive species will still have the ability (albeit reduced) to regulate molecular mechanisms during hypoxia. We investigated the tissue-specific response of protein metabolism during hypoxia. Little is known about protein degradation pathways during hypoxia in fish and we predict that protein degradation pathways are differentially regulated and play a role in the hypoxia response. We also studied the regulation of oxygen-responsive cellular signalling pathways [hypoxia inducible factor (HIF), unfolded protein response (UPR) and mTOR pathways] since most of what we know comes from studies on cancerous mammalian cell lines. Arctic char were exposed to cumulative graded hypoxia trials for 3 h at four air saturation levels (100%, 50%, 30% and 15%). The rate of protein synthesis was measured using a flooding dose technique, whereas protein degradation and signalling pathways were assessed by measuring transcripts and phosphorylation of target proteins. Protein synthesis decreased in all tissues measured (liver, muscle, gill, digestive system) except for the heart. Salmonid hearts have preferential access to oxygen through a well-developed coronary artery, therefore the heart is likely to be the last tissue to become hypoxic. Autophagy markers were upregulated in the liver, whereas protein degradation markers were downregulated in the heart during hypoxia. Further work is needed to determine the effects of a decrease in protein degradation on a hypoxic salmonid heart. Our study showed that protein metabolism in Arctic char is altered in a tissue-specific fashion during graded hypoxia, which is in accordance with the responses of the three major hypoxia-sensitive pathways (HIF, UPR and mTOR). The activation pattern of these pathways and the cellular processes that are under their control varies greatly among tissues, sometimes even going in the opposite direction. This study provides new insights on the effects of hypoxia on protein metabolism. Adjustment of these cellular processes is likely to contribute to shifting the fish phenotype into a more hypoxia-tolerant one, if more than one hypoxia event were to occur. Our results warrant studying these adjustments in fish exposed to long-term and diel cycling hypoxia.
鱼类暴露在波动的氧气浓度中通常会改变它们的新陈代谢和/或行为以生存。耐缺氧能力通常与通过抑制蛋白质合成等代谢过程来降低能量需求的能力有关。北极红点鲑是对缺氧最敏感的鲑鱼之一，通常在缺氧时会采取回避行为。我们假设，一个敏感的物种仍然有能力（尽管降低了）在缺氧期间调节分子机制。我们研究了蛋白质代谢在缺氧期间的组织特异性反应。在鱼类缺氧期间，关于蛋白质降解途径知之甚少，我们预测蛋白质降解途径会受到不同的调节，并在缺氧反应中发挥作用。我们还研究了氧反应性细胞信号通路的调节[缺氧诱导因子 (HIF)、未折叠蛋白反应 (UPR) 和 mTOR 通路]，因为我们所知道的大部分内容都来自对癌症哺乳动物细胞系的研究。北极红点鲑被暴露在累积分级缺氧试验中，在四个空气饱和度水平（100%、50%、30%和 15%）下持续 3 小时。使用淹没剂量技术测量蛋白质合成率，而通过测量靶蛋白的转录物和磷酸化来评估蛋白质降解和信号通路。除了心脏外，所有测量的组织（肝脏、肌肉、鳃、消化系统）的蛋白质合成都减少了。鲑鱼的心脏通过发育良好的冠状动脉优先获得氧气，因此心脏可能是最后一个缺氧的组织。自噬标记物在肝脏中上调，而心脏中的蛋白质降解标记物在缺氧期间下调。需要进一步的工作来确定蛋白质降解减少对缺氧鲑鱼心脏的影响。我们的研究表明，在分级缺氧期间，北极红点鲑的蛋白质代谢以组织特异性的方式发生变化，这与三种主要的缺氧敏感途径（HIF、UPR 和 mTOR）的反应一致。这些途径的激活模式及其受其控制的细胞过程在组织之间差异很大，有时甚至朝着相反的方向发展。这项研究提供了关于缺氧对蛋白质代谢影响的新见解。如果发生不止一次缺氧事件，这些细胞过程的调整可能有助于将鱼类表型转变为更耐缺氧的表型。我们的研究结果表明，需要研究长期和昼夜循环缺氧下暴露于这些调整的鱼类。
