PDF(Pubmed)
Abstract：
The extinct Steller\'s sea cow (Hydrodamalis gigas; †1768) was a whale-sized marine mammal that manifested profound morphological specializations to exploit the harsh coastal climate of the North Pacific. Yet despite first-hand accounts of their biology, little is known regarding the physiological adjustments underlying their evolution to this environment. Here, the adult-expressed hemoglobin (Hb; α2β/δ2) of this sirenian is shown to harbor a fixed amino acid replacement at an otherwise invariant position (β/δ82Lys→Asn) that alters multiple aspects of Hb function. First, our functional characterization of recombinant sirenian Hb proteins demonstrates that the Hb-O2 affinity of this sub-Arctic species was less affected by temperature than those of living (sub)tropical sea cows. This phenotype presumably safeguarded O2 delivery to cool peripheral tissues and largely arises from a reduced intrinsic temperature sensitivity of the H. gigas protein. Additional experiments on H. gigas β/δ82Asn→Lys mutant Hb further reveal this exchange renders Steller\'s sea cow Hb unresponsive to the potent intraerythrocytic allosteric effector 2,3-diphosphoglycerate, a radical modification that is the first documented example of this phenotype among mammals. Notably, β/δ82Lys→Asn moreover underlies the secondary evolution of a reduced blood-O2 affinity phenotype that would have promoted heightened tissue and maternal/fetal O2 delivery. This conclusion is bolstered by analyses of two Steller\'s sea cow prenatal Hb proteins (Hb Gower I; ζ2ε2 and HbF; α2γ2) that suggest an exclusive embryonic stage expression pattern, and reveal uncommon replacements in H. gigas HbF (γ38Thr→Ile and γ101Glu→Asp) that increased Hb-O2 affinity relative to dugong HbF. Finally, the β/δ82Lys→Asn replacement of the adult/fetal protein is shown to increase protein solubility, which may have elevated red blood cell Hb content within both the adult and fetal circulations and contributed to meeting the elevated metabolic (thermoregulatory) requirements and fetal growth rates associated with this species cold adaptation.
In 1741, shipwrecked naturalist Georg Wilhelm Steller made detailed observations of large marine mammals grazing on seaweed in the shallow waters surrounding a remote island in the North Pacific Ocean. Within thirty years, these ‘Steller’s sea cows’ had been hunted to extinction. Unlike their remaining tropical relatives – dugongs and manatees – Steller’s sea cows were specialized to cold, sub-Arctic environments. Measuring up to 10 meters long, they were much larger than other sea cow species. This, along with having very thick skin, helped them to reduce heat loss. Previous work showed that the hemoglobin protein – which binds to and carries oxygen around mammalian bodies – of Steller’s sea cows had a decreased affinity for oxygen, resulting in greater delivery of oxygen to organs and tissues. It was thought that this could be an adaptation to fuel heightened metabolic heat production in cold conditions. Studies of ancient DNA also identified the substitution of a single building block in the Steller’s sea cow hemoglobin protein that is not present in other mammals and was suspected to underlie this modification. To determine how this unique substitution affects Steller’s sea cow hemoglobin function – and whether it contributed to their ability to live in cold environments – Signore et al. generated hemoglobin proteins of Steller’s sea cows, dugongs and Florida manatees. Testing their biochemical properties showed that this single exchange profoundly alters multiple aspects of how the Steller’s sea cow hemoglobin works. Alongside reducing hemoglobin’s oxygen affinity, the Steller’s sea cow substitution also makes the protein more soluble, potentially increasing the level of hemoglobin within red blood cells. Additionally, it eliminates hemoglobin sensitivity to a molecule involved in oxygen binding – known as DPG – saving energy by no longer requiring production of this molecule. Furthermore, the same substitution makes hemoglobin less sensitive to changes in temperature, which would have helped to safeguard the delivery of oxygen to cool limbs and other extremities, reducing costly heat loss. Together, these changes in hemoglobin would have helped the Steller’s sea cow to more efficiently transport oxygen around the body. Importantly, generating and testing Steller’s sea cow pre-natal hemoglobins suggested this substitution may have also helped to enhance the fetal growth rate of these immense marine mammals by improving gas exchange between the mother and fetus. Signore et al. have revealed how a mutated form of hemoglobin allowed an extinct mammal to adapt to an extreme environment. Similar methods could be used to understand the physiological attributes of other extinct animals. In the future, this increased understanding of hemoglobin mutations could aid the development of human hemoglobin substitutes for therapeutic uses.
In 1741, shipwrecked naturalist Georg Wilhelm Steller made detailed observations of large marine mammals grazing on seaweed in the shallow waters surrounding a remote island in the North Pacific Ocean. Within thirty years, these ‘Steller’s sea cows’ had been hunted to extinction. Unlike their remaining tropical relatives – dugongs and manatees – Steller’s sea cows were specialized to cold, sub-Arctic environments. Measuring up to 10 meters long, they were much larger than other sea cow species. This, along with having very thick skin, helped them to reduce heat loss. Previous work showed that the hemoglobin protein – which binds to and carries oxygen around mammalian bodies – of Steller’s sea cows had a decreased affinity for oxygen, resulting in greater delivery of oxygen to organs and tissues. It was thought that this could be an adaptation to fuel heightened metabolic heat production in cold conditions. Studies of ancient DNA also identified the substitution of a single building block in the Steller’s sea cow hemoglobin protein that is not present in other mammals and was suspected to underlie this modification. To determine how this unique substitution affects Steller’s sea cow hemoglobin function – and whether it contributed to their ability to live in cold environments – Signore et al. generated hemoglobin proteins of Steller’s sea cows, dugongs and Florida manatees. Testing their biochemical properties showed that this single exchange profoundly alters multiple aspects of how the Steller’s sea cow hemoglobin works. Alongside reducing hemoglobin’s oxygen affinity, the Steller’s sea cow substitution also makes the protein more soluble, potentially increasing the level of hemoglobin within red blood cells. Additionally, it eliminates hemoglobin sensitivity to a molecule involved in oxygen binding – known as DPG – saving energy by no longer requiring production of this molecule. Furthermore, the same substitution makes hemoglobin less sensitive to changes in temperature, which would have helped to safeguard the delivery of oxygen to cool limbs and other extremities, reducing costly heat loss. Together, these changes in hemoglobin would have helped the Steller’s sea cow to more efficiently transport oxygen around the body. Importantly, generating and testing Steller’s sea cow pre-natal hemoglobins suggested this substitution may have also helped to enhance the fetal growth rate of these immense marine mammals by improving gas exchange between the mother and fetus. Signore et al. have revealed how a mutated form of hemoglobin allowed an extinct mammal to adapt to an extreme environment. Similar methods could be used to understand the physiological attributes of other extinct animals. In the future, this increased understanding of hemoglobin mutations could aid the development of human hemoglobin substitutes for therapeutic uses.
摘要：
已灭绝的Steller\的海牛（Hydridamalisgigas；*1768）是一种鲸鱼大小的海洋哺乳动物，表现出深刻的形态专长，以利用北太平洋恶劣的沿海气候。然而，尽管他们的生物学第一手资料，关于他们进化到这种环境的生理调整知之甚少。这里,该sirenian的成人表达的血红蛋白（Hb;a2β/δ2）显示在其他不变的位置（β/δ82Lys→Asn）处具有固定的氨基酸替换，从而改变了Hb功能的多个方面。首先,我们对重组sirenianHb蛋白的功能表征表明，与生活（亚）热带海牛相比，该亚北极物种的Hb-O2亲和力受温度影响较小。这种表型可能保护了O2向冷却的外周组织的递送，并且很大程度上是由于H.gigas蛋白的固有温度敏感性降低。对H.gigasβ/δ82Asn→Lys突变体Hb的其他实验进一步揭示了这种交换使Steller的海牛Hb对有效的红细胞内变构效应物2,3-二磷酸甘油酸无反应，一种自由基修饰，是哺乳动物中这种表型的第一个文献记载的例子。值得注意的是,此外，β/δ82Lys→Asn是血液-O2亲和力降低表型的继发性进化的基础，该表型会促进组织和母体/胎儿O2的分娩增加。通过对两种Steller's海牛产前Hb蛋白（HbGowerI;z2e2和HbF;a2g2）的分析支持了这一结论，这表明了其独特的胚胎期表达模式，并揭示了H.gigasHbF（g38Thr→Ile和g101Glu→Asp）中罕见的替代品，这些替代品相对于儒艮HbF增加了Hb-O2亲和力。最后,成人/胎儿蛋白质的β/δ82Lys→Asn替代显示增加蛋白质溶解度，在成人和胎儿循环中，红细胞Hb含量可能升高，并有助于满足与该物种冷适应相关的代谢（体温调节）要求和胎儿生长速率升高。
