Quiescence in Artemia franciscana embryos: Reversible arrest of metabolism and gene expression at low oxygen levels
Depression of the production and consumption of cellular energy appears to be a prerequisite for the survival of prolonged bouts of anoxia. A correlation exists between the degree of metabolic depression under anoxia and the duration of anoxia tolerance. In the case of brine shrimp (Artemia franciscana) embryos, oxygen deprivation induces a reversible quiescent state that can be tolerated for several years with substantial survivorship. A global arrest of cytoplasmic translation accompanies the transition into anoxia, and rates of protein synthesis in mitochondria from these embryos appears to be markedly reduced in response to anoxia. Previous evidence suggests that the acute acidification of intracellular pH (pH(i)) by over 1.0 unit during the transition into anoxia contributes to the depression of biosynthesis, but message limitation does not appear to play a role in the down-regulation in either cellular compartment. The ontogenetic increase in mRNA levels for a mitochondrial-encoded subunit of cytochrome c oxidase (COX I) and for nuclear-encoded actin is blocked by anoxia and aerobic acidosis (artificial quiescence imposed by intracellular acidification under aerobic conditions). Further, the levels of COX I and actin mRNA do not decline appreciably during 6 h bouts of quiescence, even though protein synthesis is acutely arrested across this same period. Thus, the constancy of mRNA levels during quiescence indicates that reduced protein synthesis is not caused by message limitation but, instead, is probably controlled at the translational level. This apparent stabilization of mRNA under anoxia is mirrored in an extension of protein half-life. The ubiquitin-dependent pathway for protein degradation is depressed under anoxia and aerobic acidosis, as judged by the acute drop in levels of ubiquitin-conjugated proteins. Mitochondrial protein synthesis is responsive to both acidification of pH(i) and removal of oxygen per se. Matrix pH declines in parallel with pH(i), and evidence from experiments with nigericin indicates that mitochondrial protein synthesis is depressed directly by acidification of matrix pH. The oxygen dependency of organellar protein synthesis is not explained by blockage of the electron transport chain or by the increased redox state. Rather, this cyanide- and antimycin-insensitive, but hypoxia-sensitive, inhibitory signature for the arrest of protein synthesis suggests the presence of a molecular oxygen sensor within the mitochondrion.