期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2020
卷号：117
期号：31
页码：18574-18581
DOI：10.1073/pnas.2006771117
出版社：The National Academy of Sciences of the United States of America
摘要：Many vertebrates have distinctive blue-green bones and other tissues due to unusually high biliverdin concentrations—a phenomenon called chlorosis. Despite its prevalence, the biochemical basis, biology, and evolution of chlorosis are poorly understood. In this study, we show that the occurrence of high biliverdin in anurans (frogs and toads) has evolved multiple times during their evolutionary history, and relies on the same mechanism—the presence of a class of serpin family proteins that bind biliverdin. Using a diverse combination of techniques, we purified these serpins from several species of nonmodel treefrogs and developed a pipeline that allowed us to assemble their complete amino acid and nucleotide sequences. The described proteins, hereafter named biliverdin-binding serpins (BBS), have absorption spectra that mimic those of phytochromes and bacteriophytochromes. Our models showed that physiological concentration of BBSs fine-tune the color of the animals, providing the physiological basis for crypsis in green foliage even under near-infrared light. Additionally, we found that these BBSs are most similar to human glycoprotein alpha-1-antitrypsin, but with a remarkable functional diversification. Our results present molecular and functional evidence of recurrent evolution of chlorosis, describe a biliverdin-binding protein in vertebrates, and introduce a function for a member of the serpin superfamily, the largest and most ubiquitous group of protease inhibitors. The green coloration of vertebrates is normally attributed to a particular spatial arrangement of chromatophore cells in their skins ( 1 ). Using a complex multilayered system of structural and pigmentary components inside of the cells, animals can attain a diverse array of hues. Interestingly, hundreds of frog species have translucent skins, partially or completely devoid of melanophores and other pigmentary cells ( 2 , 3 ). In these species, it is expected that other extracellular pigments and structures may account for the vivid colors they can display. In fact, recent studies have demonstrated the importance of noncellular, subcutaneous (s.c.), and glandular chromophores and fluorophores in the development of amphibian blue-green coloration ( 2 ). Among the extracellular pigments, it is known that many amphibian species have distinctive blue-green pigments in blood, lymph, other soft tissues, and bones, a phenomenon known as physiological chlorosis ( 4 ), that is prevalent in numerous arboreal species and renders them a characteristic blue-green hue ( 2 , 3 ) ( Figs. 1 and 2 A ). Previous studies have shown that this coloration is caused by high concentrations of the pigment biliverdin (BV) ( 4 , 5 ), which is the first intermediate of heme catabolism from senescent red blood cells. In birds ( 6 , 7 ) and at least some amphibians ( 8 ), fish ( 9 ), and reptiles ( 10 ), BV is the end product of heme catabolism and is excreted directly without further reduction to bilirubin (BR). In most mammals though, BV is rapidly reduced to BR and promptly excreted, thus making BV normally undetectable in their bile or blood even under extreme hemolytic conditions ( 11 ). In fact, BV concentrations reach detectable values in humans only when some conditions are met: severe cirrhotic pathologies, bile duct obstructions, and impairment of metabolic function ( 12 ⇓ – 14 ). Remarkably, chlorotic frog species show plasma BV concentrations at least four times larger than in the described pathologies ( 4 , 12 ⇓ – 14 ), and at least 200 times larger than in nonchlorotic species ( 4 ). Interestingly, attempts to induce chlorosis in nonchlorotic species by hemolysis or direct injection of BV have been unsuccessful, with the animals immediately excreting the excess BV ( 5 , 15 ). These observations suggest an unusual physiology of chlorotic frogs.
