Two research paths for probing the roles of oxygen in metabolic regulation

Anoxia- and hypoxia-induced expression of LDH-A* in the Amazon Oscar, Astronotus crassipinis

Adaptation or acclimation to hypoxia occurs via the modulation of physiologically relevant genes, such as erythropoietin, transferrin, vascular endothelial growth factor, phosphofructokinase and lactate dehydrogenase A. In the present study, we have cloned, sequenced and examined the modulation of the LDH-A gene after an Amazonian fish species, Astronotus crassipinis (the Oscar), was exposed to hypoxia and anoxia. In earlier studies, we have discovered that adults of this species are extremely tolerant to hypoxia and anoxia, while the juveniles are less tolerant. Exposure of juveniles to acute hypoxia and anoxia resulted in increased LDH-A gene expression in skeletal and cardiac muscles. When exposed to graded hypoxia juveniles show decreased LDH-A expression. In adults, the levels of LDH-A mRNA did not increase in hypoxic or anoxic conditions. Our results demonstrate that, when given time for acclimation, fish at different life-stages are able to respond differently to survive hypoxic episodes.

Developmental response to hypoxia

Molecular mechanisms underlying fetal growth restriction due to placental insufficiency and in utero hypoxia are not well understood. In the current study, time-dependent (3 h-11 days) changes in fetal tissue gene expression in a rat model of in utero hypoxia compared with normoxic controls were investigated as an initial approach to understand molecular events underlying fetal development in response to hypoxia. Under hypoxic conditions, litter size was reduced and IGFBP-1 was up-regulated in maternal serum and in fetal liver and heart. Tissue-specific, distinct regulatory patterns of gene expression were observed under acute vs. chronic hypoxic conditions. Induction of glycolytic enzymes was an early event in response to hypoxia during organ development; consistently, tissue-specific induction of calcium homeostasis-related genes and suppression of growth-related genes were observed, suggesting mechanisms underlying hypoxia-related fetal growth restriction. Furthermore, induction of inflammation-related genes in placentas exposed to long-term hypoxia (11 days) suggests a mechanism for placental dysfunction and impaired pregnancy outcome accompanying in utero hypoxia.

Cross-species studies of glycolytic function

Researchers probing the functional properties of glycogen (glucose) fermentation to lactate typically work within either one of two theoretical frameworks or models. The first assumes that the cell is analogous to a watery bag of enzymes, while the second assumes that three dimensional order and structure constrain the behaviors of glycolytic intermediates, of glycolytic enzymes, and of integrated glycolytic pathway functions per se. The former approach has been quite successful in accounting for many glycolytic functions but has not been able to satisfactorily explain a hallmark property of the pathway: namely, that large scale change in pathway flux is reflected in only modest changes in concentrations of pathway intermediates. Despite being composed of very different kinds of enzymes, the pathway is remarkably homeostatic by criterion of stability of concentrations of its intermediates in different metabolic states. The view of the cell as a system in which enzyme, substrate, and modulator mobilities are constrained by intracellular structures, the second framework above, posits intracellular perfusion or convection as a means for increasing rates of enzyme-substrate encounter and as an explanation for how high glycolytic pathway fluxes and homeostasis of pathway intermediates can be sustained simultaneously.