Oxygen, pH_i and arrest of biosynthesis in brine shrimp embryos

Abstract

Embryos of the brine shrimp Artemia franciscana are able to withstand bouts of environmental anoxia for several years by entering a quiescent state, during which time metabolism is greatly depressed. Within minutes of oxygen removal, intracellular pH (pHi) drops at least 1.0 unit. This acidification has been strongly implicated in the arrest of both catabolic and anabolic processes in the cytoplasm. A global arrest of cytoplasmic translation accompanies the transition into anoxia or into aerobic acidosis (artificial quiescence imposed by intracellular acidification with CO2 in the presence of oxygen). Similarly, protein synthesis in isolated mitochondria from these embryos is also reduced markedly in response to acidic pH (80% reduction) or anoxia (79% reduction). The constancy of mRNA levels during quiescence indicates that protein synthesis is likely to be controlled at the translational level. Mitochondrial matrix pH is 8.2 during protein synthesis assays performed at the extramitochondrial pH optimum of 7.5. When this proton gradient is abolished with the K+/H+ ionophore nigericin, the extramitochondrial pH optimum for protein synthesis displays an alkaline shift of approximately 0.7 pH unit. These data suggest the presence of proton-sensitive translational components within the mitochondrion. The oxygen dependency of mitochondrial protein synthesis is not explained simply by blockage of the electron transport chain or by the increased redox state. Whereas oxygen deprivation substantially depresses protein synthesis by 77% after 1 h, normoxic incubations with saturating concentrations of cyanide or antimycin A have only a modest effect (36% reduction, cyanide; 20%, antimycin A). 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.

Rapid effects of hypoxia on H+ homeostasis in articular chondrocytes

“…Intriguingly, oxidants other than O 2 itself have been shown to support chondrocyte function [18], whilst perturbation of intracellular ion homeostasis, including that of pH i , also disrupts matrix synthesis [37]. In this context, it is interesting to note that in some tissues, hypoxia-induced acidification appears to be beneficial, being implicated in suppression of metabolic activity, thereby matching ATP production and consumption [11]; it is possible that scenario also applies in the case of articular chondrocytes.…”

Effects of environmental hypercapnia on animal physiology: A 13C NMR study of protein synthesis rates in the marine invertebrate Sipunculus nudus

“…Up to now, a direct influence of pH on global rates of cellular protein synthesis has, for instance, been found in fish tissues (Reid et al, 1997), rat cardiomyocytes (Schlüter et al, 1998) and in Artemia franciscana dormant embryos (Kwast and Hand, 1996;Hand, 1997). The mechanisms of down-regulation have only been examined in the case of the global arrest of protein synthesis during diapause of embryonic brine shrimps, but not in other metazoans and not in adult specimens altogether.…”

V-ATPase expression during development ofArtemia franciscanaembryos: potential role for proton gradients in anoxia signaling

“…Not surprisingly, these embryos possess a profound mechanism of ensuring long-term survivorship in a hydrated anoxic state. Exposure to anoxia induces one of the most dramatic intracellular acidifications ever recorded for a eukaryotic organism (>1.6 pH units; Busa et al, 1982), and more than two decades of research has convincingly demonstrated that this event is one of the primary effectors of a potentially complete metabolic shutdown (for related reviews, see Clegg, 2001;Guppy and Withers, 1999;Hand, 1997;Hand and Hardewig, 1996;Hochachka and Guppy, 1987;Storey and Storey, 1990). However, while a great deal of progress has been made in describing the role of acidification in metabolic depression (e.g.…”

“…However, while a great deal of progress has been made in describing the role of acidification in metabolic depression (e.g. Hand, 1997Hand, , 1998Hand et al, 2001), very little is yet known about the proximal mechanisms responsible for the decrease in intracellular pH (pH i ) itself. By pushing the limits of assumptions about physiologically relevant variables impacting pH i , a maximum of 72% of the proton equivalents required for the observed acidification could be explained (Kwast et al, 1995).…”