Does neocytolysis exist after descent from high altitude?

Neocytolysis is a transient destruction of hypoxia-generated young red cells upon rapid change from hypoxia to normoxia, or upon return from space flight. The term neocytolysis was coined by Alfrey and Rice and colleagues.1, 2 In a recent report, published in Acta Physiol, Klein and coworker questioned the existence of neocytolysis.3 This directly contradicts the data from Alfrey and Rice2 and their study of Andeans descending from Cerro de Pasco at 4380 to Lima at sea level. In those reports, Alfrey et al documented erythron changes by performing red cell mass measurements using two independent methods: 51Cr labelling and carbon monoxide rebreathing methods. They also studied neocytolysis in astronauts, differentiating young red blood cells from old ones by quantitative measurements of erythropoiesis by ferrokinetics and 51Cr labelled red cell survival.1, 2 We also studied neocytolysis, focusing on its molecular basis using a mouse model we generated, wherein we characterized changes in the erythron mediated by hypoxia inducible factors (HIFs).4 We confirmed the existence of neocytolysis and showed that the rapid change from hypoxia (12% O2 corresponding to ~4500 m) to normoxia resulted in overcorrection of the hypoxia-induced increase in red cell mass, by preferential destruction of hypoxia-generated young red cells. We observed that hypoxia increased the expression of miR-21, which inhibits the expression of catalase, thereby decreasing protective mechanisms against reactive oxygen species (ROS). HIFs became transiently undetectable upon return to normoxia, resulting in abolition of HIF-driven mitophagy5 which leads to transient expansion of the mitochondria mass in reticulocytes and an increase in their ROS which diffuse into plasma and destroy preferentially the young red cells generated during hypoxia, which have low catalase. The erythron changes were documented by concomitant separate measurements of red cell mass, plasma volumes and reticulocytes in our mouse model. We showed that marked and rapid decrease in haematocrit upon return to normoxia was caused by decreased red cells mass not by expansion of plasma volume. Only slightly decreased reticulocytes during this time indicated that the considerable decrease in red cell mass occurring within short time after return to normoxia was caused by destruction of red cells rather than by suppressed production. One could argue that studies of mice may not correspond to human physiology; however, in the same paper, we reported the haematocrit levels measured during the first 168 hours of life of 35 000 human newborns.4 We noted a rapid fall during this interval that could only be explained by transient destruction of red cells. Klein and coworkers studied young men after a 19-day stay at an altitude of 3450 m3. Age cohort red cell labelling with glycine, which is incorporated metabolically into heme during synthesis, was performed by feeding the subjects a bolus of 13C2-glycine in a pre-altitude test and with different isotope 15N-glycine labelling newly formed heme at high altitude; 9 days before return to Heidelberg at altitude of 110 m. Searching for haemolysis, they did not detect decreased haptoglobin levels; however, haptoglobin is an indicator of intravascular but not extravascular hemolysis.6 Furthermore, the changes in haemoglobin mass they observed were too great to be explained by transient suppression of erythropoiesis as the average lifespan of red cell is 120 days.6 There are several significant differences that might contribute to recognizing neocytolysis in the Alfery and Rice studies1, 2 and in our work,4 but failure to do so in the Klein report.3 One is the rapidity of change from hypoxia to normoxia; in our animal model return to normoxia happened within minutes. Similarly, in neonates, the transition from foetal to neonatal oxygen content occurs within minutes. Also, in the study of Rice all,2 the descent from higher altitude (and at far greater hypoxia in Cerro de Pasco at 4380 m) to Lima at sea level in Peru happened over 3 hours (Alfrey personal communication).2 In contrast, the descent and return to Heidelberg were by train, although the exact time was not given and could not be ascertained from data presented in their figure 1.3 The other parameter is the degree of hypoxia which was far less in the Klein study than in the manuscripts confirming neocytolysis.1, 2, 4 We conclude that, comparing the previous and recent studies, the article by Klein et al indeed reports some carefully measured data on erythron changes during hypoxia but does not disprove the existence of neocytolysis.