Furthermore, the largest fraction of circulating T4 and T3 is bound to plasma proteins. Intracellular deiodinases (DIOs) catalyse deiodination to either activate or inactivate THs and the thyromimetically active T3 binds to TH receptor (TR) subtypes TRα and TRβ 13, 14, 15 (Fig. At target tissues, cellular uptake and efflux of THs are facilitated by several transmembrane transporters. 1): TH synthesis is regulated by the hypothalamic-pituitary-thyroid (HPT) axis. development, growth, and energy homeostasis, and are thus tightly regulated on several levels (Fig. The thyroid hormone (TH) 3,3′,5- l-triiodothyronine (T3) and its prohormone thyroxine (T4) are versatile hormones involved in numerous physiological processes in vertebrates, e.g. Previous studies have raised the possibility that the thyroid hormone system might be involved in these adaptations 10, 11, 12.
Low RMR and low body temperature are prerequisites to cope with subterranean environmental conditions, but the regulation at the molecular level is still not understood. Like other subterranean rodents, resting metabolic rate (RMR) of African mole-rats is significantly lower than predicted by allometric equations for rodents, which is considered a mechanism to save energy and to keep body temperature low 8, 9. Moreover, food and water availability are often low.Īfrican mole-rats (family Bathyergidae) are subterranean rodents endemic to sub-Saharan Africa 1. Furthermore, subterranean rodents are likely to face hypoxic and hypercapnic conditions during digging or huddling and have therefore evolved a high tolerance to hypoxia and hypercapnia 5, 6, 7. However, environmental conditions in subterranean burrows are harsh: they are usually characterised by high humidity and sometimes high temperatures, which reduce evaporative water loss and convective cooling, increasing the risk of overheating, especially when animals are digging 3, 4. Thus, our study adds to our knowledge for understanding adaptations to the subterranean habitat.Īpproximately 10% of all rodent species worldwide have evolved a subterranean lifestyle, i.e. These findings indicate a possible convergent adaptation. However, contrary to expectations, we found several species-specific differences in the TH systems of both mole-rat species, although ultimately resulting in similar serum TH concentrations.
Most intriguingly, both mole-rat species had low iodide levels in the thyroid and naked mole-rats showed signs of thyroid gland hyperplasia. Since THs are major regulators of metabolic rate and body temperature, we further characterised the TH system of two African mole-rat species, the naked mole-rat ( Heterocephalus glaber) and the Ansell’s mole-rat ( Fukomys anselli) at the molecular level in a comparative approach involving the house mouse ( Mus musculus) as a well-studied laboratory model in TH research. Measurements of serum thyroid hormone (TH) concentrations in African mole-rats have revealed a unique TH phenotype, which deviates from the typical mammalian pattern. Consequently, many subterranean species have evolved low basal metabolism and low body temperature, but the regulation of these traits at the molecular level were unknown. This habitat entails risks of overheating, hypoxia, and scarce food availability. African mole-rats are subterranean rodents inhabiting underground burrows.