Oxygen sensing and signal transduction in metabolic defense against hypoxia: lessons from vertebrate facultative anaerobes.

Earlier studies identified two main defense strategies against hypoxia in hypoxia tolerant animals: (1) reduction in energy turnover, and (2) improved energetic efficiency of those metabolic processes that remain. We used two model systems from the highly anoxia-tolerant aquatic turtle: (1) tissue slices of brain cortex (to probe cell level electrophysiological responses to oxygen limitation), and (2) isolated liver hepatocytes (to probe signalling and defense). In the latter, a cascade of processes underpinning hypoxia defense begins with an oxygen sensor that is probably a heme protein and a signal transduction pathway that leads to the specific activation of some genes (increased expression of several proteins) and to specific down-regulation of other genes (decreased expression of several other proteins). The pathway seems to have characteristics in common with oxygen-regulated control elements in other cells. The probable roles of the oxygen sensing and signal transduction system include coordinate down-regulation of energy demand and energy supply pathways in metabolism. Because of this coordination, hypoxia tolerant cells stay in energy balance even as they down-regulate to extremely low levels of ATP turnover. The main ATP-demanding processes in normoxia (protein synthesis, protein degradation, glucose synthesis, urea synthesis and maintenance of electrochemical gradients) are all turned down to variable degrees during anoxia or extreme hypoxia. Most striking is the observation that ion pumping is the main energy sink in anoxia-despite reductions in cell membrane permeability ("channel arrest"). Neurons also show a much lower permeability than do homologous mammalian cells but, in this case under acute anoxia, there is no further change in cell membrane conductivity. We consider that, through this recent work, it is becoming evident how normoxic maintenance ATP turnover rates can be down-regulated by an order of magnitude or more-to a new hypometabolic steady state that is prerequisite for surviving prolonged hypoxia or anoxia. The implications of these developments extend to many facets of biology and medicine.