Metabolic basis to Sherpa altitude adaptation

Abstract

The Himalayan Sherpas, a human population of Tibetan descent, are highly adapted to life in the hypobaric hypoxia of high altitude. Mechanisms involving enhanced tissue oxygen delivery in comparison to Lowlander populations have been postulated to play a role in such adaptation. Whether differences in tissue oxygen utilization (i.e., metabolic adaptation) underpin this adaptation is not known, however. We sought to address this issue, applying parallel molecular, biochemical, physiological, and genetic approaches to the study of Sherpas and native Lowlanders, studied before and during exposure to hypobaric hypoxia on a gradual ascent to Mount Everest Base Camp (5,300 m). Compared with Lowlanders, Sherpas demonstrated a lower capacity for fatty acid oxidation in skeletal muscle biopsies, along with enhanced efficiency of oxygen utilization, improved muscle energetics, and protection against oxidative stress. This adaptation appeared to be related, in part, to a putatively advantageous allele for the peroxisome proliferator-activated receptor A (PPARA) gene, which was enriched in the Sherpas compared with the Lowlanders. Our findings suggest that metabolic adaptations underpin human evolution to life at high altitude, and could have an impact upon our understanding of human diseases in which hypoxia is a feature.

Keywords: altitude; hypoxia; metabolism; mitochondria; skeletal muscle.

Fig. 1.

Subject genetics, ascent profile, arterial blood O₂ saturation, muscle hypoxia, and circulating NO metabolites. (A) Genotypes of Lowlanders and Sherpas at three PPARA SNPs. Subjects homozygous for the putatively advantageous allele are shown in black, heterozygous subjects are shown in gray, and subjects homozygous for the nonadvantageous allele are shown in white (digits in segments refer to the number of subjects with a specific genotype). (B) Ascent profile, including timing of biopsies. A1, early-altitude exposure; A2, late-altitude exposure; B, baseline; L, Lowlanders; S, Sherpas. Arterial hemoglobin-O₂ saturations (C), muscle VEGFA expression (D), and plasma nitrogen oxides (E and F) in Lowlanders and Sherpas at baseline and at early and late altitudes are shown. Mean ± SEM (n = 4–15). S_a, arterial blood O₂ saturation. ^†P ≤ 0.05, ^†††P ≤ 0.001 at B vs. A1 within cohort.

Fig. S1.

Circulating nitrogen oxide levels. (A) N-nitrosamine (RNNO) and (B) S-nitrosothiol (RSNO) concentrations in blood plasma of Lowlanders and Sherpas at baseline (B) and at early (A1) and late (A2) altitudes are shown. Mean ± SEM (n = 10–12). ^†P ≤ 0.05 at B vs. A1 within cohort. ^△P < 0.001 at A1 vs. A2 within cohort.

Fig. 2.

FAO and regulation in muscle. PPARA expression (A), CPT1B expression (B), 3-hydroxyacyl-CoA dehydrogenase (HADH) activity (C), oxidative phosphorylation with octanoyl carnitine and malate (FAO_P) (D), total carnitine (E), and long chain/total carnitine ratio (F) in Lowlanders and Sherpas are shown. Gene expression and carnitine levels are expressed relative to Lowlanders at baseline. Mean ± SEM (n = 6–13). *P ≤ 0.05, **P ≤ 0.01 in Lowlanders vs. Sherpas at baseline. ^†P ≤ 0.05 at baseline vs. altitude within cohort.

Fig. S2.

Mitochondrial respiratory function by SUIT protocol 1, in the presence of Oct. FAO-LEAK (OctM_L), FAO-OXPHOS (OctM_P), N-OXPHOS (OctGM_P), NS-OXPHOS (OctGMS_P), NS-ETS capacity (OctGMS_E), and S-ETS capacity (S_E) are illustrated. Lowlanders vs. Sherpas at baseline (B) (A), Lowlanders at B and at early-altitude (A1) and late-altitude (A2) time points (B), and Sherpas at B and A1 time points (C). Mean ± SEM (n = 10–11). **P ≤ 0.01, ***P ≤ 0.001 for Lowlanders vs. Sherpas at B. ^††P ≤ 0.01 at B vs. A1 within cohort. ^△P ≤ 0.05 at A1 vs. A2 within cohort. J_O2, oxygen flux.

Fig. 3.

TCA intermediates and activity in muscle. CS activity (A) and TCA cycle intermediates (B–I) in Lowlanders and Sherpas are shown. Metabolite levels are expressed relative to Lowlanders at baseline. Mean ± SEM (n = 7–14). *P ≤ 0.05, **P ≤ 0.01, ***P ≤ 0.001 in Lowlanders vs. Sherpas at baseline. ^†P ≤ 0.05, ^††P ≤ 0.01 at baseline vs. altitude within cohort.

Fig. 4.

Mitochondrial oxygen consumption, efficiency, and uncoupling protein expression. N-OXPHOS (GM_P) (A), S-ETS capacity (S_E) (B), and NS-OXPHOS capacity (GMS_P) (C) in permeabilized muscle fibers from Lowlanders and Sherpas are shown. Octanoyl carnitine and malate-supported LEAK (FAO_L) (D) and OXPHOS coupling efficiency (E) are shown. (F) Muscle UCP3 expression relative to Lowlanders at baseline. Mean ± SEM (n = 7–11). **P ≤ 0.01, ***P ≤ 0.001 in Lowlander vs. Sherpas at baseline. ^†P ≤ 0.05, ^††P ≤ 0.01 at baseline vs. altitude within cohort. ^△P ≤ 0.05, ^△△P ≤ 0.01 at altitude 1 vs. altitude 2 within cohort. G, glutamate; J_O2, oxygen flux; M, malate, S, succinate.

Fig. S3.

Mitochondrial respiratory function by SUIT protocol 2, in the absence of Oct. N-LEAK (GM_L), N-OXPHOS (GM_P), NS-OXPHOS (GMS_P), NS-ETS capacity (GMS_E), and S_E are illustrated in Lowlanders vs. Sherpas at B (A), Lowlanders at B at A1 and A2 time points (B), and Sherpas at B and A1 time points (C). Mean ± SEM (n = 10–11). **P ≤ 0.01, ***P ≤ 0.001 for Lowlanders vs. Sherpas at B. ^†P ≤ 0.05 at B vs. altitude within cohort. ^△△P ≤ 0.01 at A1 vs. A2 within cohort.

Fig. 5.

Muscle glycolysis and blood glucose homeostasis. Hexokinase (A) and lactate dehydrogenase (LDH) (B) activity are shown. Fasting blood glucose (C) and glucose clearance during OGTT (D) are shown. (E) Total muscle glycolytic intermediates relative to Lowlanders at baseline. Mean ± SEM (n = 5–14). AUC, area under the curve. *P ≤ 0.05 in Lowlanders vs. Sherpas at baseline. ^†P ≤ 0.05, ^††P ≤ 0.01, ^†††P ≤ 0.001 at baseline vs. altitude within cohort.

Fig. 6.

Muscle energetics and oxidative stress. PCr (A), ATP (B), oxidized/reduced glutathione (GSSG/GSH) (C), and sulfoxide/total methionine (MetSO/Met) (D), all expressed relative to Lowlanders at baseline, are shown. Mean ± SEM (n = 8–14). ^††P ≤ 0.01, ^†††P ≤ 0.001 at baseline vs. altitude within cohort. ^△P ≤ 0.05 at altitude 1 vs. altitude 2 within cohort.