Metabolic responses of plasma to extreme environments in overwintering Tibetan frogs Nanorana parkeri: a metabolome integrated analysis

Yonggang Niu,Lifeng Zhu,Kenneth B Storey,Xuejing Zhang,Tisen Xu,Qiang Chen,Haiying Zhang

doi:10.1186/s12983-021-00428-7

Yonggang Niu, Lifeng Zhu + Show 5 more

Open Access

https://doi.org/10.1186/s12983-021-00428-7

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Abstract

Many animals lower their metabolic rate in response to low temperatures and scarcity of food in the winter in phenomena called hibernation or overwintering. Living at high altitude on the Tibetan Plateau where winters are very cold, the frog Nanorana parkeri, survives in one of the most hostile environments on Earth but, to date, relatively little is known about the biochemical and physiological adjustments for overwintering by this species. The present study profiled changes in plasma metabolites of N. parkeri between winter and summer using UHPLC-QE-MS non-target metabolomics in order to explore metabolic adaptations that support winter survival. The analysis showed that, in total, 11 metabolites accumulated and 95 were reduced in overwintering frogs compared with summer-active animals. Metabolites that increased included some that may have antioxidant functions (canthaxanthin, galactinol), act as a metabolic inhibitor (mono-ethylhexylphthalate), or accumulate as a product of anaerobic metabolism (lactate). Most other metabolites in plasma showed reduced levels in winter and were generally involved in energy metabolism including 11 amino acids (proline, isoleucine, leucine, valine, phenylalanine, tyrosine, arginine, tryptophan, methionine, threonine and histidine) and 4 carbohydrates (glucose, citrate, succinate, and malate). Pathway analysis indicated that aminoacyl-tRNA biosynthesis, phenylalanine, tyrosine and tryptophan biosynthesis, and nitrogen metabolism were potentially the most prominently altered pathways in overwintering frogs. Changes to these pathways are likely due to fasting and global metabolic depression in overwintering frogs. Concentrations of glucose and urea, commonly used as cryoprotectants by amphibians that winter on land, were significantly reduced during underwater hibernation in N. parkeri. In conclusion, winter survival of the high-altitude frog, N. parkeri was accompanied by substantial changes in metabolomic profiles and this study provides valuable information towards understanding the special adaptive mechanisms of N. parkeri to winter stresses.

Highlights

Amphibians in north temperate climates are generally forced to enter into a dormant state in the winter due to stresses that include low or subzero temperatures, a lack of food resources, and other associated stresses such as hypoxia or dehydration [1,2,3]
To further analyze the differences between summer and winter groups, an orthogonal projections to latent structures discriminant analysis (OPLS-DA) model (Fig. 2c, d) was applied, and it showed an obvious separation between the two groups, indicating that there was a significant seasonal change in the metabolic profiles of plasma
A well-known marker of oxidative damage to DNA is 8-hydroxy-2′-deoxyguanosine [68] and the present study shows that 8-OHdG decreased significantly in plasma of overwintering N. parkeri

Summary

Introduction

Amphibians in north temperate climates are generally forced to enter into a dormant state in the winter due to stresses that include low or subzero temperatures, a lack of food resources, and other associated stresses such as hypoxia or dehydration [1,2,3]. Hibernating amphibians can be exposed to desiccation due to their highly water-permeable skins and, if temperatures fall below about 0 °C, whole body freezing can be triggered. Freeze tolerance is a striking survival strategy for dealing with subzero temperatures and has been extensively studied in multiple terrestrially hibernating amphibian species [8, 9]. Amphibians that overwinter in an aquatic environment are not typically subjected to the above stresses, unless the water in a pond freezes right to the bottom. Amphibians that successfully hibernate underwater face challenges of hypoxia but freezing only rarely [1]. There are two paramount physiological mechanisms that allow amphibians to survive the winter underwater: one is protecting themselves against cold and/or hypoxia injury and the other is maintaining long-term metabolic homeostasis [8, 10,11,12,13]

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