Living in underground borrows, Spalax, the blind subterranean mole rat, is often exposed to extreme fluctuations of oxygen level due to changes in the soil's gas permeability caused by seasonal floods that saturate the soil. As a result of their subterranean life-style, Spalax belong to the group of mammals with the greatest degree of hypoxia tolerance. Intrigued by the animal's remarkable hypoxia tolerance, Imad Shams and colleagues from University of Haifa in Israel decided to focus on the expression of two key indicators of hypoxic stress, erythropoietin (Epo) and hypoxia-inducible factor 1α (HIF-1α) to assess the animals' hypoxia tolerance.
HIF1 is a transcription factor that responds rapidly to periods of hypoxia. Under normoxic conditions, one of the HIF1 components, HIF-1α, is constantly broken down, and so HIF1 is unable to activate hypoxia-induced genes. However, under hypoxic conditions, the HIF-1α component is stabilised, and HIF-1 can in turn bind to an enhancer element in hypoxia-induced genes, such as the Epo gene, and activate its transcription; HIF-1α levels are said to be post-translationally regulated. Epo's main, but not its only, function is the regulation of levels of red blood cells. Knowing that the kidney is the main site of Epo production, Shams and coworkers compared Epo mRNA expression levels in the kidneys of white rats and several species of mole rats, Spalax, and found that Epo expression increased more in the hypoxia tolerant subterranean mole rats than in white rats at low oxygen levels. But when they looked at a time course of Epo expression in subterranean mole rat, they were in for a surprise! Although Epolevels increased dramatically over the first 24 h of hypoxia, its expression returned close to normoxic levels after 44 h. Differences between Epo expression were not only found between white and mole rats, but also between different species of Spalax in extreme hypoxic conditions, with animals from damp hypoxic burrows producing higher levels of Epo than animals from well oxygenated burrows. Interestingly, the time course and the fact that the development of erythrocytes takes up to 2 weeks suggests that Epo may have other functions than erythropoiesis in setting hypoxia tolerance limits in Spalax.
Investigating the levels of HIF-1α, the team found that under normal conditions, hypoxia-tolerant blind subterranean mole rats naturally produced twice as much HIF-1α mRNA as hypoxia-sensitive white rats. And when they dropped the oxygen levels to only 3%, blind mole rat's HIF-1α mRNA levels rose dramatically,peaking after 4 h, while the HIF-1α levels of rats exposed to low levels of oxygen didn't change from their normoxic levels. Given that HIF-1α levels are normally regulated at the protein level, the team was surprised to see that the blind subterranean mole rats seemed able to regulate the levels of HIF-1α mRNA, making them suspect that the HIF-1α gene itself might also be regulated by hypoxia.
Next the team compared the HIF-1α levels of two different species of blind mole rats, from flooded and well-aerated areas, and found that the animals from the wetter and more hypoxic environment had higher levels of HIF-1α than the animals from well-aerated burrows. Shams suggests that `this pattern of Epo and HIF-1αexpression is a substantial contribution to the adaptive strategy of hypoxia tolerance in Spalax'.
Having identified that HIF-1α and Epo are probably major players in the rodents' remarkable hypoxia tolerance, Shams and colleagues are keen to know whether the mRNA and protein levels of HIF-1α respond to fluctuating oxygen levels, given that blind mole rats survive such large fluctuations in oxygen level; hopefully making us less blind towards the promising lessons we can learn from an odd mammal living underground.
