The discovery that human lowlanders acclimatize faster on re-ascent to high altitude because erythrocytes retain a hypoxic purinergic memory — plasma adenosine rises, signals through erythrocyte ADORA2B, and triggers PKA-dependent phosphorylation, ubiquitination, and proteasomal degradation of the adenosine transporter ENT1, so that reduced eENT1 persists into a second hypoxic exposure — left four linked questions open: whether the same post-translational mechanism operates in nucleated somatic cells under hypoxia; how long the reduced-eENT1 memory lasts in humans and how variable it is between individuals; what plasma adenosine concentrations and exposure durations are needed to activate erythrocyte ADORA2B and trigger ENT1 degradation in humans; and whether components of the network (ENT1 inhibition, CD73 augmentation, ADORA2B/PKA signaling) can be safely manipulated to enhance acute acclimatization.

Background

When sea-level residents ascend to high altitude, low environmental oxygen pressure triggers a coordinated acclimatization response (hyperventilation, diuresis, pulmonary vasoconstriction, increased hematopoiesis) whose early phase is often symptomatic (acute mountain sickness, AMS). A long-standing physiological observation is that re-ascent after a previous sojourn is accompanied by faster, improved acclimatization, but until recently its molecular basis was unknown. Adenosine is a signaling nucleoside released extracellularly under metabolic stress; it is generated from AMP by CD73 (ecto-5'-nucleotidase, also present as a soluble circulating form, sCD73) and removed by uptake into cells through equilibrative nucleoside transporter 1 (ENT1, gene SLC29A1), a bidirectional plasma-membrane transporter. Extracellular adenosine acts through four G-protein-coupled receptors, of which ADORA2B (the A2B adenosine receptor) has the lowest affinity and is engaged only at micromolar adenosine concentrations, such as those reached during hypoxia. In nucleated cells, hypoxia regulates purinergic genes transcriptionally through hypoxia-inducible factor-1alpha; mature mammalian erythrocytes (red blood cells) have no nucleus and cannot replace proteins by new synthesis. Song and colleagues (Song et al., Nature Communications 8:14108, 2017; human data from the AltitudeOmics cohort of lowlanders ascending to 5260 m) discovered that initial ascent rapidly induces plasma adenosine and sCD73 activity, that adenosine signaling via erythrocyte ADORA2B activates protein kinase A (PKA), which phosphorylates ENT1 and thereby targets it for ubiquitination and proteasomal degradation, and that the resulting reduction in erythrocyte ENT1 (eENT1) lowers adenosine uptake so plasma adenosine accumulates further in a feed-forward loop that limits hypoxic tissue damage (shown in mice with erythrocyte-specific ENT1 deletion). Because degraded eENT1 cannot be replenished until new erythrocytes are produced, reduced eENT1 persists after descent: this is the erythrocyte hypoxic memory, and its presence on re-ascent (higher, faster plasma adenosine rise) accounts for quicker acclimatization. The authors explicitly note that this ADORA2B-mediated post-translational downregulation of a transporter is probably not limited to erythrocytes, suggest that targeting CD73, ENT1 or ADORA2B could enhance acclimatization, exercise performance, and adaptation of hypoxia-exposed patients, and observe that the memory is gradually lost as newly synthesized erythrocytes replace old ones — forward-looking statements that the paper does not itself test.

Problem Statement

The problem is to determine whether and how the discovered erythrocyte hypoxic purinergic memory can be generalized, quantified in humans, and therapeutically exploited to speed acclimatization to high-altitude hypoxia. Four scientifically linked questions articulate this objective, addressed below as (1)-(4); each is independently answerable. (1) Cell-type generality — does hypoxic adenosine signaling via ADORA2B, through PKA-mediated phosphorylation, ubiquitination, and proteasomal degradation, downregulate ENT1 (and thereby raise extracellular adenosine) in nucleated somatic cell types such as endothelial, immune, or epithelial cells under physiological or pathological hypoxia, or is this mechanism specific to anucleate erythrocytes? (2) Human memory kinetics — the primary source already shows membrane eENT1 protein still low at Post7 (after 7 days at 1,525 m) and plasma adenosine higher on re-ascent at Post7 and Post21 than on first exposure; beyond those points, how long does reduced erythrocyte ENT1 persist in humans after a hypoxic exposure, how variable is this duration between individuals, and how does the timescale map onto clinically or operationally relevant re-ascent intervals? (3) Activation thresholds — what plasma adenosine concentrations and hypoxia exposure durations are required to activate erythrocyte ADORA2B and trigger ENT1 degradation in humans? The primary source anchors this only qualitatively, noting that ADORA2B has the lowest affinity of the four adenosine receptors and that its PKA-mediated phosphorylation and ubiquitination of ENT1 do not occur until adenosine reaches the micromolar range (after about 48 h of hypoxia in mice and day 7 at altitude in humans). (4) Therapeutic feasibility — can components of the network (ENT1 inhibition, CD73 or sCD73 augmentation, ADORA2B/PKA signaling modulation) be manipulated to enhance acute acclimatization safely, i.e., can plasma adenosine be augmented transiently for acclimatization benefit without the deleterious consequences associated with sustained adenosine elevation in chronic disease settings? The four questions are scientifically related but distinct: answers to (1)-(3) would inform the design and interpretation of interventions under (4), yet they are not prerequisites for obtaining intervention evidence — the ENT1-inhibition proof-of-concept reported by Ruan et al. (2023) in cardiac ischemia-reperfusion was obtained while question (1) remains unresolved.

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