Ontogenetic expression of erythropoietin and hypoxia‐inducible factor‐1 alpha genes in subterranean blind mole rats

Adaptation of the Spalax galili transcriptome to hypoxia may underlie the complex phenotype featuring longevity and cancer resistance

In the subterranean rodent (Nanno)spalax galili, evolutionary adaptation to hypoxia is correlated with longevity and tumor resistance. Adapted gene-regulatory networks of Spalax might pinpoint strategies to maintain health in humans. Comparing liver, kidney and spleen transcriptome data from Spalax and rat at hypoxia and normoxia, we identified differentially expressed gene pathways common to multiple organs in both species. Body-wide interspecies differences affected processes like cell death, antioxidant defense, DNA repair, energy metabolism, immune response and angiogenesis, which may play a crucial role in Spalax's adaptation to environmental hypoxia. In all organs, transcription of genes for genome stability maintenance and DNA repair was elevated in Spalax versus rat, accompanied by lower expression of aerobic energy metabolism and proinflammatory genes. These transcriptomic changes might account for the extraordinary lifespan of Spalax and its cancer resistance. The identified gene networks present candidates for further investigating the molecular basis underlying the complex Spalax phenotype.

Adaptation of mammals to hypoxia

Oxygen plays a pivotal role in the metabolism and activities of mammals. However, oxygen is restricted in some environments-subterranean burrow systems or habitats at high altitude or deep in the ocean-and this could exert hypoxic stresses such as oxidative damage on organisms living in these environments. In order to cope with these stresses, organisms have evolved specific strategies to adapt to hypoxia, including changes in physiology, gene expression regulation, and genetic mutations. Here, we review how mammals have adapted to the three high-altitude plateaus of the world, the limited oxygen dissolved in deep water habitats, and underground tunnels, with the aim of better understanding the adaptation of mammals to hypoxia.

The hypoxia adaptation of small mammals to plateau and underground burrow conditions

Oxygen is one of the important substances for the survival of most life systems on the earth, and plateau and underground burrow systems are two typical hypoxic environments. Small mammals living in hypoxic environments have evolved different adaptation strategies, which include increased oxygen delivery, metabolic regulation of physiological responses and other physiological responses that change tissue oxygen utilization. Multi-omics predictions have also shown that these animals have evolved different adaptations to extreme environments. In particular, vascular endothelial growth factor (VEGF) and erythropoietin (EPO), which have specific functions in the control of O2 delivery, have evolved adaptively in small mammals in hypoxic environments. Naked mole-rats and blind mole-rats are typical hypoxic model animals as they have some resistance to cancer. This review primarily summarizes the main living environment of hypoxia tolerant small mammals, as well as the changes of phenotype, physiochemical characteristics and gene expression mode of their long-term living in hypoxia environment.

Evolution of Hemoglobin Genes in a Subterranean Rodent Species (Lasiopodomys mandarinus)

The Mandarin vole (Lasiopodomys mandarinus), a typical subterranean rodent, has undergone hematological adaptations to tolerate the hypoxic/hypercapnic underground environment. Hemoglobin (Hb) genes encode respiratory proteins functioning principally in oxygen binding and transport to various tissues and organs. To investigate the evolution of α- and β-hemoglobin (Hb) in subterranean rodent species, we sequenced Hb genes of the Mandarin vole and the related aboveground Brandt's vole (L. brandtii). Sequencing showed that in both voles, α-globin was encoded by a cluster of five functional genes in the following linkage order: HBZ, HBA-T1, HBQ-T1, HBA-T2, and HBQ-T2; among these, HBQ-T2 is a pseudogene in both voles. The β-globin gene cluster in both voles also included five functional genes in the following linkage order: HBE, HBE/HBG, HBG, HBB-T1, and HBB-T2. Phylogenetic analysis revealed that the Mandarin vole underwent convergent evolution with its related aboveground species (Brandt's vole) but not with other subterranean rodent species. Selection pressure analyses revealed that α- and β-globin genes are under strong purifying selection (ω < 1), and branch-site analyses identified positive selection sites on HBAT-T1 and HBB-T1 in different subterranean rodent species. This suggests that the adaptive evolution of these genes enhanced the ability of Hb to store and transport oxygen in subterranean rodent species. Our findings highlight the critical roles of Hb genes in the evolution of hypoxia tolerance in subterranean rodent species.

Differential responses of Lasiopodomys mandarinus and Lasiopodomys brandtii to chronic hypoxia: a cross-species brain transcriptome analysis

BACKGROUND

Subterranean rodents have evolved many features to adapt to their hypoxic environment. The brain is an organ that is particularly vulnerable to damage caused by exposure to hypoxic conditions. To investigate the mechanisms of adaption to a hypoxic underground environment, we carried out a cross-species brain transcriptome analysis by RNA sequencing and identified genes that are differentially expressed between the subterranean vole Lasiopodomys mandarinus and the closely related above-ground species Lasiopodomys brandtii under chronic hypoxia [10.0% oxygen (O2)] and normoxia (20.9% O2).

RESULTS

A total of 355 million clean reads were obtained, including 69,611 unigenes in L. mandarinus and 69,360 in L. brandtii. A total of 235 and 92 differentially expressed genes (DEGs) were identified by comparing the hypoxic and control groups of L. mandarinus and L. brandtii, respectively. A Gene Ontology (GO) analysis showed that upregulated DEGs in both species had similar functions in response to hypoxia, whereas downregulated DEGs in L. mandarinus were enriched GO terms related to enzymes involved in aerobic reactions. In the Kyoto Encyclopedia of Genes and Genomes pathway analysis, upregulated DEGs in L. mandarinus were associated with angiogenesis and the increased O2 transport capacity of red blood cells, whereas downregulated DEGs were associated with immune responses. On the other hand, upregulated DEGs in L. brandtii were associated with cell survival, vascular endothelial cell proliferation, and neuroprotection, while downregulated genes were related to the synaptic transmission by neurons.

CONCLUSIONS

L. mandarinus actively adapts its physiological functions to hypoxic conditions, for instance by increasing O2 transport capacity and modulating O2 consumption. In contrast, L. brandtii reacts passively to hypoxia by decreasing overall activity in order to reduce O2 consumption. These results provide insight into hypoxia adaptation mechanisms in subterranean rodents that may be applicable to humans living at high altitudes or operating in other O2-poor environments.

The double benefit of Spalax p53: surviving underground hypoxia while defying lung cancer cells in vitro via autophagy and caspase-dependent cell death

The blind subterranean mole rat, Spalax ehrenbergi, is a model organism for hypoxia tolerance. This superspecies have adapted to severe environment by altering an array of hypoxia-mediated genes, among which an alteration in the p53 DNA binding domain (corresponding to R174K in humans) that hinders its transcriptional activity towards apoptotic genes. It is well accepted that apoptosis is not the only form of programmed cell death and that mechanisms that depend on autophagy are also involved. In the current work we have extended our research and investigated the possibility that Spalax p53 can activate autophagy. Using two complementary assays, we have established that over-expression of the Spalax p53 in p53-null cells (human lung cancer cells, H1299), potently induces autophagy. As Spalax is considered highly resistant to cancer, we further studied the relative contribution of autophagy on the outcome of H1299 cells, following transfection with Spalax p53. Results indicate that Spalax p53 acts as a tumor suppressor in lung cancer cells, inducing cell death that involves autophagy and caspases and inhibiting cell number, which is exclusively caspase-dependent. To conclude, the Spalax p53 protein was evolutionary adapted to survive severe underground hypoxia while retaining the ability to defy lung cancer.

Oxygen stress: impact on innate immune system, antioxidant defence system and expression of HIF-1α and ATPase 6 genes in Catla catla

Catla catla catla (2.28 ± 0.1 g) were exposed to six different levels of dissolved oxygen: 1 (DO-1), 3 (DO-3), 5 (DO-5), 7 (DO-7), 9 (DO-9) and 11 (DO-11) mg/L. DO-5 served as control. In DO-1 and DO-3, the number of red blood cells (RBC), lysozyme, respiratory burst activity and nitric oxide synthase were significantly (p < 0.05) lower compared to the control one. In DO-7 and DO-9, RBC and lysozyme were significantly (p < 0.05) higher compared to the control one. Thiobarbituric acid reactive substances was significantly (p < 0.05) higher in catla exposed at low (1 and 3 mg/L) and high (9 and 11 mg/L) dissolved oxygen compared to others. In muscles and hepatopancreas, reduced glutathione was significantly (p < 0.05) higher in DO-5 and DO-7 and in gills of DO-5 compared to others after 1 h. In muscles, glutathione S-transferase (GST) was significantly (p < 0.05) lower in DO-5 and DO-7 compared to others. In hepatopancreas, GST and glutathione peroxidise (GPx) were significantly (p < 0.05) higher in DO-1 and DO-3 compared to others. In gills, GPx was higher in DO-9 and DO-11 after 48 h. In brain, hypoxia-inducible factor (HIF)-1α mRNA level was induced in DO-1 and DO-3 compared to others after 1 h of exposure. In gills and hepatopancreas, HIF-1α mRNA level was significantly (p < 0.05) higher in DO-1 compared to others after 1 h. The ATPase 6 mRNA level was significantly (p < 0.05) higher in brain and hepatopancreas of DO-1 after 1 h and in gills and hepatopancreas of DO-3 and DO-9, respectively, after 48 h compared to others.

Iron-rich ferritin in the hypoxia-tolerant rodent Spalax ehrenbergi: a naturally-occurring biomarker confirms the internalization and pathways of intracellular macromolecules

The discovery of pits/caveolae in the plasmalemma advanced the study of macromolecule internalization. "Transcytosis" describes the transport of macromolecular cargo from one front of a polarized cell to the other within membrane-bounded carrier(s), via endocytosis, intracellular trafficking and exocytosis. Clathrin-mediated transcytosis is used extensively by epithelial cells, while caveolae-mediated transcytosis mostly occurs in endothelial cells. The internalization pathways were monitored by various markers, including radioisotopes, nanoparticles, enzymes, immunostains, and fluorophores. We describe an internalization pathway identified using a naturally-occurring biomarker, in vivo assembled ferritin, containing electron-dense iron cores. Iron, an essential trace metal for most living species and iron homeostasis, is crucial for cellular life. Ferritin is a ubiquitous and highly conserved archeoprotein whose main function is to store a reserve iron supply inside the cytoplasm in a non-toxic form. Ferritin is present in all organisms which have a metabolic requirement for iron and in even in organisms whose taxonomic rank is very low. The newborns of the blind mole, Spalax ehrenbergi, are born and live in a hypoxic environment and have significant iron overload in their liver and heart, but their iron metabolism has not been previously studied. These newborns, which are evolutionarily adapted to fluctuations in the environmental oxygen, have a unique ability to sequester transplacental iron and store it in ferritin without any signs of iron toxicity. Using the ferrihydrite cores of ferritin, we were able to monitor the ferritin internalization from portals of its entry into the cytosol of hepatocytes and cardiomyocytes and into the lysosomes.

Mitochondria: a multimodal hub of hypoxia tolerance

Decreased oxygen availability impairs cellular energy production and, without a coordinated and matched decrease in energy consumption, cellular and whole organism death rapidly ensues. Of particular interest are mechanisms that protect brain from low oxygen injury, as this organ is not only the most sensitive to hypoxia, but must also remain active and functional during low oxygen stress. As a result of natural selective pressures, some species have evolved molecular and physiological mechanisms to tolerate prolonged hypoxia with no apparent detriment. Among these mechanisms are a handful of responses that are essential for hypoxia tolerance, including (i) sensors that detect changes in oxygen availability and initiate protective responses; (ii) mechanisms of energy conservation; (iii) maintenance of basic brain function; and (iv) avoidance of catastrophic cell death cascades. As the study of hypoxia-tolerant brain progresses, it is becoming increasingly apparent that mitochondria play a central role in regulating all of these critical mechanisms. Furthermore, modulation of mitochondrial function to mimic endogenous neuroprotective mechanisms found in hypoxia-tolerant species confers protection against otherwise lethal hypoxic stresses in hypoxia-intolerant organs and organisms. Therefore, lessons gleaned from the investigation of endogenous mechanisms of hypoxia tolerance in hypoxia-tolerant organisms may provide insight into clinical pathologies related to low oxygen stress.

They live in the land down under: thyroid function and basal metabolic rate in the Blind Mole Rat, Spalax

The Israeli blind subterranean mole rat (Spalax ehrenbergi superspecies) lives in sealed underground burrows under extreme, hypoxic conditions. The four Israeli Spalax allospecies have adapted to different climates, the cool-humid (Spalax galili, 2 n = 52 chromosomes), semihumid (S. golani, 2 n = 54) north regions, warm-humid (S. carmeli, 2 n = 58) central region and the warm-dry S. judaei, 2 n = 60) southern regions. A dramatic interspecies decline in basal metabolic rate (BMR) from north to south, even after years of captivity, indicates a genetic basis for this BMR trait. We examined the possibility that the genetically-conditioned interspecies BMR difference was expressed via circulating thyroid hormone. An unexpected north to south increase in serum free thyroxine (FT4) and total 3, 5, 3'-triiodo-L-thyronine (T3) (p < 0.02) correlated negatively with previously published BMR measurements. The increases in serum FT4 and T3 were symmetrical, so that the T3:FT4 ratio - interpretable as an index of conversion of T4 to T3 in nonthyroidal tissues - did not support relative decrease in production of T3 as a contributor to BMR. Increased north-to-south serum FT4 and T3 levels also correlated negatively with hemoglobin/hematocrit. North-to-south adaptations in spalacids include decreased BMR and hematocrit/hemoglobin in the face of increasing thyroid hormone levels, arguing for independent control of hormone secretion and BMR/hematocrit/hemoglobin. But the significant inverse relationship between thyroid hormone levels and BMR/hematocrit/hemoglobin is also consistent with a degree of cellular resistance to thyroid hormone action that protects against hormone-induced increase in oxygen consumption in a hostile, hypoxic environment.

Pronounced cancer resistance in a subterranean rodent, the blind mole-rat, Spalax: in vivo and in vitro evidence

Background

Subterranean blind mole rats (Spalax) are hypoxia tolerant (down to 3% O₂), long lived (>20 years) rodents showing no clear signs of aging or aging related disorders. In 50 years of Spalax research, spontaneous tumors have never been recorded among thousands of individuals. Here we addressed the questions of (1) whether Spalax is resistant to chemically-induced tumorigenesis, and (2) whether normal fibroblasts isolated from Spalax possess tumor-suppressive activity.

Results

Treating animals with 3-Methylcholantrene (3MCA) and 7,12-Dimethylbenz(a) anthracene/12-O-tetradecanoylphorbol-13-acetate (DMBA/TPA), two potent carcinogens, confirmed Spalax high resistance to chemically induced cancers. While all mice and rats developed the expected tumors following treatment with both carcinogens, among Spalax no tumors were observed after DMBA/TPA treatment, while 3MCA induced benign fibroblastic proliferation in 2 Spalax individuals out of12, and only a single animal from the advanced age group developed malignancy 18 months post-treatment. The remaining animals are still healthy 30 months post-treatment. In vitro experiments showed an extraordinary ability of normal Spalax cultured fibroblasts to restrict malignant behavior in a broad spectrum of human-derived and in newly isolated Spalax 3MCA-induced cancer cell lines. Growth of cancer cells was inhibited by either direct interaction with Spalax fibroblasts or with soluble factors released into culture media and soft agar. This was accompanied by decreased cancer cell viability, reduced colony formation in soft agar, disturbed cell cycle progression, chromatin condensation and mitochondrial fragmentation. Cells from another cancer resistant subterranean mammal, the naked mole rat, were also tested for direct effect on cancer cells and, similar to Spalax, demonstrated anti-cancer activity. No effect on cancer cells was observed using fibroblasts from mouse, rat or Acomys. Spalax fibroblast conditioned media had no effect on proliferation of noncancerous cells.

Conclusions

This report provides pioneering evidence that Spalax is not only resistant to spontaneous cancer but also to experimentally induced cancer, and shows the unique ability of Spalax normal fibroblasts to inhibit growth and kill cancer cells, but not normal cells, either through direct fibroblast-cancer cell interaction or via soluble factors. Obviously, along with adaptation to hypoxia, Spalax has evolved efficient anti-cancer mechanisms yet to be elucidated. Exploring the molecular mechanisms allowing Spalax to survive in extreme environments and to escape cancer as well as to kill homologous and heterologous cancer cells may hold the key for understanding the molecular nature of host resistance to cancer and identify new anti-cancer strategies for treating humans.

Linking oxygen to time: the bidirectional interaction between the hypoxic signaling pathway and the circadian clock

The circadian clock and the hypoxic signaling pathway play critical roles in physiological homeostasis as well as in tumorgenesis. Interactions between both pathways have repeatedly been reported for mammals during the last decade, the molecular basis, though, has not been identified so far. Expression levels of oxygen-regulated and circadian clock genes in zebrafish larvae (Danio rerio) and zebrafish cell lines were significantly altered under hypoxic conditions. Thus, long-term hypoxic incubation of larvae resulted in a dampening of the diurnal oscillation amplitude of the period1 gene expression starting only several hours after start of the hypoxic incubation. A significant decrease in the amplitude of the period1 circadian oscillation in response to hypoxia and in response to the hypoxic mimic CoCl2 was also observed using a zebrafish luciferase reporter cell line in constant darkness. In addition, activity measurements of zebrafish larvae using an infrared-sensitive camera demonstrated the loss of their usual circadian activity pattern under hypoxic conditions. To explore the functional basis of the observed cross-talk between both signaling pathways ChIP assays were performed. Increasing with the duration of hypoxia, a nearly 4-fold occupancy of hypoxia-inducible factor 1 (Hif-1α) at two specific E-box binding sites located in the period1 gene control region was shown, demonstrating therewith the transcriptional co-regulation of the core clock gene by the major transcription factor of the hypoxic pathway. On the other hand, circadian transgenic zebrafish cells, simulating a repressed or an overstimulated circadian clock, modified gene transcription levels of oxygen-regulated genes such as erythropoietin and vascular endothelial growth factor 165 and altered the hypoxia-induced increase in Hif-1α protein concentration. In addition, the amount of Hif-1α protein accumulated during the hypoxic response was shown to depend on the time of the day, with one maximum during the light phase and a second one during the dark phase. The direct binding of Hif-1α to the period1 gene control region provides a mechanistic explanation for the repeatedly observed interaction between hypoxia and the circadian clock. The cross-talk between both major signaling pathways was shown for the first time to be bidirectional and may provide the advantage of orchestrating a broad range of genes and metabolic pathways to cope with altered oxygen availabilities.

Transcription pattern of p53-targeted DNA repair genes in the hypoxia-tolerant subterranean mole rat Spalax

The tumor suppressor gene p53 induces growth arrest and/or apoptosis in response to DNA damage/hypoxia. Inactivation of p53 confers a selective advantage to tumor cells under a hypoxic microenvironment during tumor progression. The subterranean blind mole rat, Spalax, spends its life underground at low-oxygen tensions, hence developing a wide range of respiratory/molecular adaptations to hypoxic stress, including critical changes in p53 structure and signaling pathway. The highly conserved p53 Arg(R)-172 is substituted by lysine (K) in Spalax, identical with a tumor-associated mutation. Functionality assays revealed that Spalax p53 is unable to activate apoptotic target genes but is still capable of activating cell cycle arrest genes. Furthermore, we have shown that the transcription patterns of representative p53-induced genes (Apaf1 and Mdm2) in Spalax are influenced by hypoxia. Cell cycle arrest allows the cells to repair DNA damage via different DNA repair genes. We tested the transcription pattern of three p53-related DNA repair genes (p53R2, Mlh1, and Msh2) under normoxia and short-acute hypoxia in Spalax, C57BL/6 wild-type mice, and two strains of mutant C57BL/6 mice, each carrying a different mutation at the R172 position. Our results show that while wild-type/mutant mice exhibit strong hypoxia-induced reductions of repair gene transcript levels, no such inhibition is found in Spalax under hypoxia. Moreover, unlike mouse p53R2, Spalax p53R2 transcript levels are strongly elevated under hypoxia. These results suggest that critical repair functions, which are known to be inhibited under hypoxia in mice, remain active in Spalax, as part of its unique hypoxia tolerance mechanisms.

Oxygen levels and the regulation of cell adhesion in the nervous system: a control point for morphogenesis in development, disease and evolution?

In this article, I discuss the hallmarks of hypoxia in vitro and in vivo and review work showing that many types of stem cell proliferate more robustly in lowered oxygen. I then discuss recent studies showing that alterations in the levels and the types of cell and substrate adhesion molecules are a notable response to reduced O(2) levels in both cultured primary neural stem cells and brain tissues in response to hypoxia in vivo. The ability of O(2) levels to regulate adhesion molecule expression is linked to the Wnt signaling pathway, which can control and be controlled by adhesion events. The ability of O(2) levels to influence cell adhesion also has far-reaching implications for development, ischemic trauma and neural regeneration, as well as for cancer and other diseases. Finally I discuss the possibility that the fluctuations in O(2) levels known to have occurred over evolutionary time could, by influencing adhesion systems, have contributed to early symbiotic events in unicellular organisms and to the emergence of multicellularity. It is not my intention to be exhaustive in these domains, which are far from my own field of study. Rather this article is meant to provoke and stimulate thinking about molecular evolution involving O(2) sensing and signaling during eras of geologic and atmospheric change that might inform modern studies on development and disease.

Living with stress: regulation of antioxidant defense genes in the subterranean, hypoxia-tolerant mole rat, Spalax

Lack of oxygen is life threatening for most mammals. It is therefore of biomedical interest to investigate the adaptive mechanisms which enable mammalian species to tolerate extremely hypoxic conditions. The subterranean mole rat Spalax survives substantially longer periods of hypoxia than the laboratory rat. We hypothesized that genes of the antioxidant defense, detoxifying harmful reactive oxygen species generated during hypoxia and hyperoxia, are involved in Spalax underground adaptation. Using quantitative RT-PCR, we analyzed the mRNA expression levels of seven antioxidant defense genes (catalase, glutathione peroxidase 1, glutathione-S-transferase Pi1, heme oxygenase 1, superoxide dismutase 1 and 2) and a master regulator of this stress pathway, nuclear factor (erythroid-derived 2)-like 2 (Nrf2) in several tissues of two Israeli Spalax species, S. galili (2n=52) and S. judaei (2n=60), and rat. We also studied the differential expression of these genes after experimental hypoxia and hyperoxia as oxidative stress treatments. We found that mRNA levels and transcriptional responses are species and tissue specific. There are constitutively higher transcript levels of antioxidant genes and their transcription factor Nrf2 in Spalax tissue as compared to rat, suggesting an increased ability in the mole rat to withstand hypoxic/hyperoxic insults. In contrast to Spalax, the rat reacts to experimental oxidative stress by changes in gene regulation. In addition, Spalax Nrf2 reveals unique amino acid changes, which may be functionally important for this transcription factor and indicate positive (Darwinian) selection. Antioxidant defense genes are therefore important targets for adaptive change during evolution of hypoxia tolerance in Spalax.

Methionine sulfoxide reductases and methionine sulfoxide in the subterranean mole rat (Spalax): characterization of expression under various oxygen conditions

The blind subterranean mole rat (Spalax ehrenbergi) exhibits a relatively long life span, which is attributed to an efficient antioxidant defense affording protection against accumulation of oxidative modifications of proteins. Methionine residues can be oxidized to methionine sulfoxide (MetO) and then enzymatically reduced by the methionine sulfoxide reductase (Msr) system. In the current study we have isolated the cDNA sequences of the Spalax Msr genes as well as 23 additional selenoproteins and monitored the activities of Msr enzymes in liver and brain of rat (Rattus norvegicus), Spalax galili, and Spalax judaei under normoxia, hypoxia, and hyperoxia. Under normoxia, the Msr activity was lower in S. galili in comparison to S. judaei and R. norvegicus especially in the brain. The pattern of Msr activity of the three species was similar throughout the tested conditions. However, exposure of the animals to hypoxia caused a significant enhancement of Msr activity, especially in S. galili. Hyperoxic exposure showed a highly significant induction of Msr activity compared with normoxic conditions for R. norvegicus and S. galili brain. It was concluded that among all species examined, S. galili appears to be more responsive to oxygen tension changes and that the Msr system is upregulated mainly by severe hypoxia.

Transcriptome sequencing of the blind subterranean mole rat, Spalax galili: utility and potential for the discovery of novel evolutionary patterns

The blind subterranean mole rat (Spalax ehrenbergi superspecies) is a model animal for survival under extreme environments due to its ability to live in underground habitats under severe hypoxic stress and darkness. Here we report the transcriptome sequencing of Spalax galili, a chromosomal type of S. ehrenbergi. cDNA pools from muscle and brain tissues isolated from animals exposed to hypoxic and normoxic conditions were sequenced using Sanger, GS FLX, and GS FLX Titanium technologies. Assembly of the sequences yielded over 51,000 isotigs with homology to ∼12,000 mouse, rat or human genes. Based on these results, it was possible to detect large numbers of splice variants, SNPs, and novel transcribed regions. In addition, multiple differential expression patterns were detected between tissues and treatments. The results presented here will serve as a valuable resource for future studies aimed at identifying genes and gene regions evolved during the adaptive radiation associated with underground life of the blind mole rat.

Matrix metalloproteinase (MMP)-9 induced by Wnt signaling increases the proliferation and migration of embryonic neural stem cells at low O2 levels

Recent studies have shown that various neural and embryonic stem cells cultured in 1-8% oxygen (O(2)), levels lower than those typically used in cell culture (20.9%), displayed increased rates of proliferation; however, the molecular mechanisms underlying these changes are largely undefined. In this study, using rigorously controlled O(2) levels, we found that neural stem cells (NSCs) from embryonic day 15 rat cortex increased their rate of proliferation and migration in 1% O(2) relative to 20% O(2) without changes in viability. We sought to identify molecular changes in NSCs grown in 1% O(2) that might account for these increases. In 1% O(2), levels of the hypoxia-inducible transcription factor HIF-1α were transiently increased. Reduced adherence of NSCs in 1% O(2) to basement membrane-coated plates was observed, and quantitative RT-PCR analysis confirmed that the levels of mRNA for an assortment of cell adhesion and extracellular matrix molecules were altered. Most notable was a 5-fold increase in matrix metalloproteinase (MMP)-9 mRNA. Specific inhibition of MMP-9 activity, verified using a fluorescent substrate assay, prevented the increase in proliferation and migration in 1% O(2). The canonical Wnt pathway was recently shown to be activated in stem cells in low O(2) via HIF-1α. Inhibition of Wnt signaling by DKK-1 also prevented the increase in proliferation, migration, and MMP-9 expression. Thus, MMP-9 is a key molecular effector, downstream of HIF-1α and Wnt activation, responsible for increased rates of NSC proliferation and migration in 1% O(2).

Antioxidant responses to variations of oxygen by the Harderian gland of different species of the superspecies Spalax ehrenbergi

The subterranean blind mole rats of the superspecies Spalax ehrenbergi (Nehring, 1898) have developed several strategies to cope with changing concentrations of underground oxygen. Such an atmosphere induces the generation of reactive oxygen species that can cause oxidative damage without proper control. To understand how S. ehrenbergi appear to be able to counteract the free radicals and avoid oxidative damage, we studied the oxidative status of the Harderian gland (an organ particularly vulnerable to oxidative stress in many rodents) in two species of the superspecies S. ehrenbergi ( Spalax galili and Spalax judaei ) under different oxygen concentration levels, paying special attention to the antioxidant defences developed by these animals and the resulting macromolecular damage. The results presented herein reinforce the idea that S. ehrenbergi deal better with hypoxic conditions than other rodents by regulating the activity of its antioxidant enzymes. Moreover, S. galili is better adapted to hypoxic conditions, whereas S. judaei appears to be better adapted to hyperoxic conditions.

Granulocyte colony-stimulating factor activating HIF-1alpha acts synergistically with erythropoietin to promote tissue plasticity

Stroke and peripheral limb ischemia are serious clinical problems with poor prognosis and limited treatment. The cytokines erythropoietin (EPO) and granulocyte-colony stimulating factor (G-CSF) have been used to induce endogenous cell repair and angiogenesis. Here, we demonstrated that the combination therapy of EPO and G-CSF exerted synergistic effects on cell survival and functional recovery from cerebral and peripheral limbs ischemia. We observed that even under normoxic conditions, G-CSF activates hypoxia-inducible factor-1alpha (HIF-1alpha), which then binds to the EPO promoter and enhances EPO expression. Serum EPO level was significantly increased by G-CSF injection, with the exception of Tg-HIF-1alpha(+f/+f) mice. The neuroplastic mechanisms exerted by EPO combined with G-CSF included enhanced expression of the antiapoptotic protein of Bcl-2, augmented neurotrophic factors synthesis, and promoted neovascularization. Further, the combination therapy significantly increased homing and differentiation of bone marrow stem cells (BMSCs) and intrinsic neural progenitor cells (INPCs) into the ischemic area. In summary, EPO in combination with G-CSF synergistically enhanced angiogenesis and tissue plasticity in ischemic animal models, leading to greater functional recovery than either agent alone.

Burrow characteristics of the co-existing sibling species Mus booduga and Mus terricolor and the genetic basis of adaptation to hypoxic/hypercapnic stress

BACKGROUND

The co-existing, sibling species Mus booduga and Mus terricolor show a difference in site-preference for burrows. The former build them in flat portion of the fields while the latter make burrows in earthen mounds raised for holding water in cultivated fields. In northern India which experiences great variation in climatic condition between summer and winter, M. booduga burrows have an average depth of 41 cm, as against 30 cm in southern India with less climatic fluctuation.M. terricolor burrows are about 20 cm deep everywhere. The three chromosomal species M. terricolor I, II and III have identical burrows, including location of the nest which is situated at the highest position. In contrast, in M. booduga burrows, the nest is at the lowest position.

RESULTS

The nest chamber of M. booduga is located at greater depth than the nest chamber of M. terricolor. Also, in the burrows of M. booduga the exchange of air takes place only from one side (top surface) in contrast to the burrows of M. terricolor where air exchange is through three sides. Hence, M. booduga lives in relatively more hypoxic and hypercapnic conditions than M. terricolor.We observed the fixation of alternative alleles in M. booduga and M. terricolor at Superoxide dismutase-1 (Sod-1), Transferrin (Trf) and Hemoglobin beta chain (Hbb) loci. All the three are directly or indirectly dependent on oxygen concentration for function. In addition to these, there are differences in burrow patterns and site-preference for burrows suggesting difference in probable adaptive strategy in these co-existing sibling species.

CONCLUSION

The burrow structure and depth of nest of the chromosomal species M. terricolor I, II and III are same everywhere probably due to the recency of their evolutionary divergence. Moreover, there is lack of competition for the well-adapted 'microhabitats' since they are non-overlapping in distribution. However, the co-existing sibling species M. booduga and M. terricolor exhibit mutual "exclusion" of the 'microhabitats' for burrow construction. Thus, location, structure and depth of the burrows might have been the contributory factors for selection of alternative alleles at three loci Sod-1, Trf and Hbb, which reflect difference in probable adaptive strategy in M. booduga and M. terricolor.

Adaptive features of skeletal muscles of mole rats (Spalax ehrenbergi) to intensive activity under subterranean hypoxic conditions

Mole rats of the Spalax ehrenbergi superspecies are blind subterranean rodents that live under fluctuating oxygen supply, reduced to a measured 6% O(2), and mostly probably lower, during the rainy season. Fiber typing of muscles of the neck (trapezius) and leg (gastrocnemius, quadriceps) using standard histochemical techniques (succinic dehydrogenase, myosin ATPase) showed that the muscle fibers of mole rats in natural settings, as well as after extended captivity, were predominantly type IIa. The same muscles in laboratory rats showed the full range of fiber types. In contrast, the hearts of the mole rats and the laboratory rats were very similar. Our results indicate that skeletal muscle in the mole rats appears to have evolved in response to specific environmental demands to permit intensive endurance burrowing activities under conditions of severe or chronic hypoxia.

Cloning and in vivo expression of vascular endothelial growth factor receptor 2 (Flk1) in the naturally hypoxia-tolerant subterranean mole rat

Vascular endothelial growth factor receptor (VEGF) plays a critical role in blood vessel formation and affects nerve growth and survival. VEGF receptor 2 (Flk1) functions as the major signal transducer of angiogenesis, mediating VEGF induction of endothelial tubulogenesis. We have cloned and analyzed expression of Flk1 in the blind subterranean mole rat Spalax ehrenbergi. Spalax experience abrupt and sharp changes in oxygen supply in their sealed underground niche and, hence, are genetically adapted to hypoxia and serve as a unique, natural mammalian model organism for hypoxia tolerance. Spalax Flk1 is relatively conserved at the nucleic acid and amino acid level compared to human, mouse, and rat orthologs. Reverse transcription-quantitative polymerase chain reaction was used to analyze Flk1 expression in muscle and brain of animals exposed to ambient or variant hypoxic oxygen levels at multiple stages of development. Transcript levels were compared with those obtained from Rattus, a primary model for human physiology. Our findings demonstrate that under normoxic conditions Flk1 patterns of expression correlate well with our previous investigations of VEGF expression. Exposure to hypoxic conditions resulted in divergent patterns of Flk1 expression between Spalax and Rattus and between muscle and brain. It appears that the regulatory mechanisms differentiating expression between the species and between tissues are most likely unique, suggesting that Flk1 expression may be regulated by multiple processes, including both angiogenesis and neurogenesis.

An extracellular region of the erythropoietin receptor of the subterranean blind mole rat Spalax enhances receptor maturation

Erythropoietic functions of erythropoietin (EPO) are mediated by its receptor (EPO-R), which is present on the cell surface of erythroid progenitors and induced by hypoxia. We focused on EPO-R from Spalax galili (sEPO-R), one of the four Israeli species of the subterranean blind mole rat, Spalax ehrenbergi superspecies, as a special natural animal model of high tolerance to hypoxia. Led by the intriguing observation that most of the mouse EPO-R (mEPO-R) is retained in the endoplasmic reticulum (ER), we hypothesized that sEPO-R is expressed at higher levels on the cell surface, thus maximizing the response to elevated EPO, which has been reported in this species. Indeed, we found increased cell-surface levels of sEPO-R as compared with mEPO-R by using flow cytometry analysis of BOSC cells transiently expressing HA-tagged EPO-Rs (full length or truncated). We then postulated that unique extracellular sEPO-R sequence features contribute to its processing and cell-surface expression. To map these domains of the sEPO-R that augment receptor maturation, we generated EPO-R derivatives in which parts of the extracellular region of mEPO-R were replaced with the corresponding fragments of sEPO-R. We found that an extracellular portion of sEPO-R, harboring the N-glycosylation site, conferred enhanced maturation and increased transport to the cell surface of the respective chimeric receptor. Taken together, we demonstrate higher surface expression of sEPO-R, attributed at least in part to increased ER exit, mediated by an extracellular region of this receptor. We speculate that these sEPO-R sequence features play a role in the adaptation of Spalax to extreme hypoxia.

Effects of seasonal and latitudinal cold on oxidative stress parameters and activation of hypoxia inducible factor (HIF-1) in zoarcid fish

Acute, short term cooling of North Sea eelpout Zoarces viviparus is associated with a reduction of tissue redox state and activation of hypoxia inducible factor (HIF-1) in the liver. The present study explores the response of HIF-1 to seasonal cold in Zoarces viviparus, and to latitudinal cold by comparing the eurythermal North Sea fish to stenothermal Antarctic eelpout (Pachycara brachycephalum). Hypoxic signalling (HIF-1 DNA binding activity) was studied in liver of summer and winter North Sea eelpout as well as of Antarctic eelpout at habitat temperature of 0 degrees C and after long-term warming to 5 degrees C. Biochemical parameters like tissue iron content, glutathione redox ratio, and oxidative stress indicators were analyzed to see whether the cellular redox state or reactive oxygen species formation and HIF activation in the fish correlate. HIF-1 DNA binding activity was significantly higher at cold temperature, both in the interspecific comparison, polar vs. temperate species, and when comparing winter and summer North Sea eelpout. Compared at the low acclimation temperatures (0 degrees C for the polar and 6 degrees C for the temperate eelpout) the polar fish showed lower levels of lipid peroxidation although the liver microsomal fraction turned out to be more susceptible to lipid radical formation. The level of radical scavenger, glutathione, was twofold higher in polar than in North Sea eelpout and also oxidised to over 50%. Under both conditions of cold exposure, latitudinal cold in the Antarctic and seasonal cold in the North Sea eelpout, the glutathione redox ratio was more oxidised when compared to the warmer condition. However, oxidative damage parameters (protein carbonyls and thiobarbituric acid reactive substances (TBARS) were elevated only during seasonal cold exposure in Z. viviparus. Obviously, Antarctic eelpout are keeping oxidative defence mechanisms high enough to avoid accumulation of oxidative damage products at low habitat temperature. The paper discusses how HIF could be instrumental in cold adaptation in fish.

P53 in blind subterranean mole rats – loss-of-function versus gain-of-function activities on newly cloned Spalax target genes

A tumor suppressor gene, p53, controls cellular responses to a variety of stress conditions, including DNA damage and hypoxia, leading to growth arrest and/or apoptosis. Recently, we demonstrated that in blind subterranean mole rats, Spalax, a model organism for hypoxia tolerance, the p53 DNA-binding domain contains a specific Arg174Lys amino acid substitution. This substitution reduces the p53 effect on the transcription of apoptosis genes (apaf1, puma, pten and noxa) and enhances it on human cell cycle arrest and p53 stabilization/homeostasis genes (mdm2, pten, p21 and cycG). In the current study, we cloned Spalax apaf1 promoter and mdm2 intronic regions containing consensus p53-responsive elements. We compared the Spalax-responsive elements to those of human, mouse and rat and investigated the transcriptional activity of Spalax and human Arg174Lys-mutated p53 on target genes of both species. Spalax and human-mutated p53 lost induction of apaf1 transcription, and increased induction of mdm2 transcription. We conclude that Spalax evolved hypoxia-adaptive mechanisms, analogous to the alterations acquired by cancer cells during tumor development, with a bias against apoptosis while favoring cell arrest and DNA repair.

Oxygen-dependent diseases in the retina: Role of hypoxia-inducible factors

The function of the retina is sensitive to oxygen tension. Any change in the perfusion pressure of the eye affects the retina although the eye is able to autoregulate its hemodynamics. Systemic hypoxemia (lung or heart disease) or a vascular disease in the retina can cause retinal hypoxia. All the hypoxia-dependent events in cells appear to share a common denominator: hypoxia-inducible factor (HIF), which is a heterodimeric transcription factor, a protein. HIF comprises a labile alpha subunit (1-3), which is regulated, and a stable beta subunit, which is constitutively expressed. Both are helix-loop-helix factors and belong to the PAS-domain family of transcription factors. Oxygen plays the key role in stabilizing HIF-1alpha and its function. When the oxygen tension is normal, HIF-1alpha is rapidly oxidized by hydroxylase enzymes, but when cells become hypoxic, HIF-1alpha escapes the degradation and starts to accumulate, triggering the activation of a large number of genes, like vascular endothelial growth factor (VEGF) and erythropoietin. HIF-1alpha has been shown to have, either clinically or experimentally, a mediating or contributing role in several oxygen-dependent retinal diseases such as von Hippel-Lindau, proliferative diabetic retinopathy, retinopathy of prematurity and glaucoma. In retinitis pigmentosa and high-altitude retinopathy, however, the evidence is still indirect. There are three different strategies available for treating retinal diseases, which have all shown promising results: retinal cell transplantation or replacement, gene replacement, and pharmacological intervention. Specifically, recent results show that the HIF pathway can be used as a therapeutic target, although there is still a long way to go from bench to clinic. HIF can be stabilized by inhibiting prolyl hydroxylase or by blocking the VHL:HIF-alpha complex if angiogenesis is the goal, as in retinitis pigmentosa. On the other hand, the downregulation of HIF has a pivotal role if we are to inhibit neovascularization, as in proliferative diabetic retinopathy. To date, several small-molecule inhibitors of HIF have been developed and are entering clinical trials. HIF is a remarkable example of a single transcription factor that can be regarded as a "master switch" regulating all the oxygen-dependent retinal diseases.

Differential expression profiling of the blind subterranean mole rat Spalax ehrenbergi superspecies: bioprospecting for hypoxia tolerance

The blind subterranean mole rat of the Spalax ehrenbergi superspecies, living underground and exposed to fluctuating oxygen and carbon dioxide levels, is an excellent model of hypoxic tolerance. Unique structural and functional adaptations of the cardiovascular and respiratory systems allow these underground mammals to survive at severely reduced oxygen tension. Elucidation of the natural variation and evolutionary changes under hypoxia within this superspecies may have biomedical applications in ischemic syndromes and cancer. In this study, we have compared expression profiles of muscle tissue at normoxic (21%) and hypoxic (3%) levels of oxygen concentration between two allospecies of the S. ehrenbergi superspecies exhibiting differential hypoxia tolerance in accordance with their ecological regimes. Profiling was performed by cross-species hybridization using a mouse cDNA array containing 15,000 gene elements. Results uncover species-specific responses to hypoxic stress among numerous genes involved in angiogenesis, apoptosis, and oxidative stress management. Among the most striking results are differential expressions of cardiac ankyrin repeat protein ( Carp), activating transcription factor 3 ( Atf3), LIM and cysteine-rich domains 1 ( Lmcd1), cysteine and glycine-rich protein 2 ( Csrp2), and ras homolog gene family, member B ( RhoB). These findings support the hypothesis that allospecies of the S. ehrenbergi superspecies are variably adapted to fluctuating oxygen tension. Differences may involve specific metabolic pathways and functional adaptations at the structural and molecular levels.

Blood-gas properties of plateau zokor (Myospalax baileyi)

Plateau zokor (Myospalax baileyi) is one of the blind subterranean mole rats that spend their life solely underground in sealed burrows. It is one of the special species of the Qinghai-Tibet plateau. In their burrows, oxygen is low and carbon dioxide is high and their contents fluctuate with the change of seasons, soil types, rain and depth of burrows. However, plateau zokors show successful adaptation to that extreme environment. In this study, their adapting mechanisms to the hypoxic hypercapnic environment were analyzed through the comparison of their blood-gas properties with that of pikas (Ochotona curzniae) and Sprague-Dawley rats. The results indicated that plateau zokors had higher red blood corpuscle counts (8.11+/-0.59 (10(12)/L)) and hemoglobin concentrations (147+/-9.85 g/L), but hematocrit (45.9+/-3.29%) and mean corpuscular volume (56.67+/-2.57 fL) were lower than the other rodents. Their arterial blood and venous blood pH were 7.46+/-0.07 and 7.27+/-0.07. Oxygen pressure in arterial blood of plateau zokors was about 1.5 times higher than that of pikas and rats, and it was 0.36 and 0.26 times in their venous blood. Partial pressure for carbon dioxide in arterial and venous blood of plateau zokors was 1.5-fold and 2.0-fold higher, respectively, than in rats and pikas. Oxygen saturation of plateau zokors was 5.7 and 9.3 times lower in venous blood than that of pikas and rats, respectively. As result, the difference of oxygen saturation in arterial blood to venous blood was 2- and 4.5-fold higher in plateau zokors as that of pikas and rats, respectively. In conclusion, plateau zokors had a high tolerance to pH changes in tissues, together with strong capabilities to obtain oxygen from their hypoxic-hypercapnic environment.

Antioxidant activity in Spalax ehrenbergi: a possible adaptation to underground stress

The blind subterranean mole rat Spalax ehrenbergi superspecies has evolved adaptive strategies to cope with underground stress. Hypoxia is known to stimulate reactive oxygen species generation; however, mechanisms by which Spalax counteracts oxidative damage have not been investigated before. We studied in Spalax the oxidative status of the Harderian gland (HG), an organ which is particularly vulnerable to oxidative stress in many rodents. With regard to the sexual dimorphism found in this gland, differences between males and females were determined and compared to the surface-dwelling Syrian hamster. Our results show, for the first time, that Spalax exhibits remarkably low biomolecular damage, which implies the existence of physiological strategies to avoid oxidative damage under fluctuating O(2) and CO(2) levels existing in the mole rat's subterranean niche. Correspondingly, main antioxidant enzymes, such as superoxide dismutase (SOD), catalase, and glutathione reductase (GR), exhibited high activities in both genders; in particular, remarkably high levels were measured in SOD. SOD and GR activities showed statistically significant differences between sexes. Melatonin, an important circadian agent is also a very important antioxidant molecule and is synthesized in the Harderian glands (HGs) of Spalax. Therefore, the possible interaction between antioxidant enzymes and melatonin is suggested.

Oxygen and carbon dioxide fluctuations in burrows of subterranean blind mole rats indicate tolerance to hypoxic-hypercapnic stresses

The composition of oxygen (O2), carbon dioxide (CO2), and soil humidity in the underground burrows from three species of the Israeli subterranean mole rat Spalax ehrenbergi superspecies were studied in their natural habitat. Two geographically close populations of each species from contrasting soil types were probed. Maximal CO2 levels (6.1%) and minimal O2 levels (7.2%) were recorded in northern Israel in the breeding mounds of S. carmeli in a flooded, poor drained field of heavy clay soil with very high volumetric water content. The patterns of gas fluctuations during the measurement period among the different Spalax species studied were similar. The more significant differentiation in gas levels was not among species, but between neighboring populations inhabiting heavy soils or light soils: O2 was lower and CO2 was higher in the heavy soils (clay and basaltic) compared to the relatively light soils (terra rossa and rendzina). The extreme values of gas concentration, which occurred during the rainy season, seemed to fluctuate with partial flooding of the tunnels, animal digging activity, and over-crowded breeding mounds inhabited by a nursing female and her offspring. The gas composition and soil water content in neighboring sites with different soil types indicated large differences in the levels of hypoxic-hypercapnic stress in different populations of the same species. A growing number of genes associated with hypoxic stress have been shown to exhibit structural and functional differences between the subterranean Spalax and the above-ground rat (Rattus norvegicus), probably reflecting the molecular adaptations that Spalax went through during 40 million years of evolution to survive efficiently in the severe fluctuations in gas composition in the underground habitat.

Erythropoietin receptor spliced forms differentially expressed in blind subterranean mole rats

Erythropoietin (Epo) is the primary regulator of erythropoiesis, controlling the proliferation, maturation, and survival of erythroid progenitor cells. The functions of Epo are mediated through its specific receptor (EpoR) expressed mainly on the surface of erythroid progenitor cells, and the expression of both responds to hypoxia. The subterranean mole rat (Spalax) is a unique model system to study the molecular mechanisms for adaptation to hypoxia. Here, we cloned two forms of Spalax EpoR: a complete EpoR cDNA as well as a novel truncated bone marrow specific EpoR form. In the full-length Spalax EpoR (sEpoR), two out of the eight conserved tyrosine- phosphorylation sites were substituted (Y481F and Y499G), suggesting that Spalax Epo signaling pathways may be modulated. The level of the sEpoR mRNA in the spleen and in bone marrow was relatively low and similar in Spalax newborns and adults, with no significant response to hypoxia. The truncated sEpoR was not detected in the spleen and comprised only approximately 1% of the sEpoR expressed in the bone marrow. In Rattus, the truncated EpoR form was approximately 15% of the total expressed receptor. The level of Rattus EpoR in newborn spleens was three- to fourfold higher than in Spalax newborns and decreased toward adulthood. Severe hypoxia induces a significant increase in adult Rattus EpoR. Our data provide further insight into the adaptive mechanisms of Spalax to the extreme conditions of hypoxia in its subterranean environment.