EPAS1/HIF-2α a key regulator for chronic hypoxia across species

Individuals exposed to hypoxia for brief or long periods of time manifest physiological changes that aim to maintain oxygen homeostasis. For example, acute exposure to hypoxia produces hypoxic ventilatory response (HVR) due to activation of the peripheral chemoreceptors of the carotid body, as well as hypoxia-triggered changes in heart rate. Exposure to extended periods of hypoxia (chronic sustained hypoxia) results in different responses, including a secondary increase in ventilation termed ventilatory acclimatization to hypoxia (VAH), enlargement of carotid bodies, and increased hemoglobin concentration.

Experiments involving rodent models and human genetic studies describe the importance of the EPAS1/HIF-2α pathway as one of the main triggers of these hypoxia-induced consequences (Yu et al. 2022; Hodson et al. 2016), supporting the idea that the EPAS1/HIF-2α pathway is crucial for mechanisms of hypoxia adaptation. For instance, pharmacological inhibition of HIF-2α and experiments performed in knockout mice show that disruption of HIF-2α results in a blunted VAH after days of exposure to hypoxia. Studies of human populations living at high altitude demonstrate that genes involved in the EPAS1/HIF-2α pathway show signs of selection and are associated with relatively lower levels of hemoglobin at high altitude, i.e., similar to individuals at sea level but lower when compared to other high-altitude populations such as Andeans. And also, Tibetans at high-altitude show increased ventilation when compared to Andean high-altitude populations. In many cases, the potential adaptive or maladaptive roles of these hypoxia-induced changes remain unknown and many groups are enthusiastically applying novel approaches to answer these questions.

Deer mice (Peromyscus maniculatus) live in a large range of altitudes (sea level to ~ 4,000m) offering the opportunity to study physiological and genomic changes shaped by hypoxia exposure over many generations. In this issue of The Journal of Physiology, Ivy et al. (2022) live-trapped deer mice at high altitude (4350m) and at low altitude (430m). The authors genotyped these deer mice for a specific Epas1 variant previously described (Schweizer et al. 2019), with evidence showing an impaired functionality due to an interrupted interaction of HIF-2α with a transcriptional coactivator (CREB-binding protein). The authors characterized homozygous deer mice with the high-altitude (Epas1^H/H) and low-altitude variants (Epas1^L/L) and cross them to obtain an F₂ progeny of inter-population hybrids with admixed genetic background. Mice from F2 with homozygous genotypes for the high and low-altitude variant (Epas1^H/H and Epas1^L/L) and with heterozygous genotype (Epas1^H/L), were then characterized so the effects of Epas1 genotype could be determined. The authors describe the ventilatory and metabolic responses to hypoxia, the increase in carotid body size due to chronic hypoxia, and measure the levels and affinity of hemoglobin in hypoxic conditions.

Ivy et al. (2022) found that all three genotypes have similar responses to acute hypoxia when acclimated to normoxic conditions (similar HVR). However, after 4 weeks of chronic hypoxia, the authors found that mice with the Epas1^H/H variant had a blunted VAH, with reduced HVR responses compared to Epas1^H/L and Epas1^L/L mice. The authors intentionally maintained CO₂ during acute hypoxia exposure to avoid the inhibition of ventilation caused by decreased levels of CO₂ due to hyperventilation. To study ventilatory effects during poikilocapnic hypoxia, the authors also performed an extended protocol with step-wise hypoxia intervals without controlling CO₂ levels, and they found similar results in ventilatory parameters and increased O₂ consumption rates in Epas1^H/H mice acclimated to chronic hypoxia. This resulted in significantly higher arterial O₂ saturation in chronic hypoxia acclimated Epas1^H/L and Epas1^L/L deer mice in several acute hypoxia steps (16, 12, 10 and 8 kPa of O₂ partial pressure) when compared to non-acclimated responses, whereas chronic hypoxia acclimated Epas1^H/H mice showed increased O₂ saturation only with the most severe hypoxia level. When the authors observed the carotid body morphology in the different groups of deer mice, they found an increase in the number of type I (glomus) cells and the total volume in Epas1^L/L mice exposed to sustained hypoxia, but these morphological changes were not observed in Epas1^H/L and Epas1^H/H mice (Ivy et al. 2022). Interestingly, after chronic hypoxia, the authors observed the expected increase in hemoglobin and hematocrit levels, but this increase was not disrupted depending on Epas1 genotype. The measurement of oxygen-affinity properties (P₅₀) for the different groups did not show effects of chronic hypoxia, with similar values of P₅₀ for all deer mice Epas1 variants in normoxic or chronic hypoxic acclimatization conditions. Thus, the Epas1 variants examined were not associated with changes in hemoglobin concentration nor oxygen-binding properties. This is relevant because other studies performed by the authors with a similar approach describe how evolutionary pressure exerted by hypoxia and high altitude may be favoring variants in other hypoxia-related genes, such as those involved in hemoglobin concentration and O₂ affinity, and those involved in mechanisms of oxygen transport to the tissues (Wearing et al. 2021).

These studies performed in deer mice provide an elegant model for studying the contribution of genetic factors in physiological adaptation to hypoxia. And the emergence of studies about genetic-driven changes in animal models may provide clues to understand adaptations to hypoxia in humans. The comparison of the features of this model to laboratory rodents or human high-altitude populations provide a remarkable example of cross-species evolution at EPAS1/HIF-2α that is associated with more efficient and successful mechanisms for oxygen transport.