Long‐Term Survival of Anoxia Despite Rapid ATP Decline in Embryos of the Annual KillifishAustrofundulus limnaeus

Abstract

Embryos of the annual killifish Austrofundulus limnaeus can survive for months in the complete absence of oxygen. Survival of anoxia is associated with entry into a state of metabolic dormancy known as diapause. However, extreme tolerance of anoxia is retained for several days of post-diapause development. Rates of heat dissipation in diapause II and 4 days post-diapause II embryos were measured under aerobic conditions and during the transition into anoxia. Phosphorylated adenylate compounds were quantified in embryos during entry into anoxia and after 12 h of aerobic recovery. Rates of heat dissipation were not affected by exposure to anoxia in diapause II embryos, while post-diapause II embryos experienced a profound decrease in heat dissipation. ATP decreased substantially in both developmental stages upon exposure to anoxia, and all indicators of cellular energetic status indicated energetic stress, at least based on the mammalian paradigm. The rate of decline in ATP is the most acute reported for any vertebrate. The mechanisms responsible for cellular survival despite a clear dysregulation between energy production and energy consumption remain to be identified. Necrotic and apoptotic cell death in response to hypoxia contribute to poor survival during many diseases and pathological conditions in mammals. Understanding the mechanisms that are in place to prevent maladaptive cell death in embryos of A. limnaeus may greatly improve treatment strategies in diseases that involve hypoxia and reperfusion injuries.

Role of AMP-activated protein kinase in metabolic depression in animals

“…In the frog Pseudacris crucifer, a drop in ATP, total adenylates and AEC was seen in liver during severe dehydration (Churchill and Storey 1994), and freezing in Rana sylvatica led to a fall in ATP and AEC in liver (Storey and Storey 1986). ATP levels decreased substantially after anoxic exposure of diapausing embryos of the annual killifish Austrofundulus limnaeus (Podrabsky et al 2012). Exposure of diapausing adult firebugs Pyrrhocoris apterus to −5 °C, a non-freezing subzero temperature, resulted in a drop in fat body ATP levels and a fall in AEC (Kostál et al 2004).…”

Avoidance of Apoptosis in Embryonic Cells of the Annual Killifish Austrofundulus limnaeus Exposed to Anoxia

“…Anaerobic metabolism in anoxia tolerant embryos is supported by accumulation of lactate, succinate, and large amounts of γ-aminobutyric acid (GABA) [6]. Post-diapause II embryos respond to anoxia by entering a profound state of metabolic depression with heart rates dropping from over 80 to 0 beats per minute [11] and rates of heat dissipation decreasing to those exhibited by dormant diapause II embryos [12]. In both diapause II embryos and those at 4 days post-diapause II, ATP levels plummet during the initial hours of exposure to anoxia to as low as 20% of normoxic levels [12].…”

Cell cycle regulation during development and dormancy in embryos of the annual killifishAustrofundulus limnaeus

“…8). 36 Anaerobic metabolism is largely supported by lactate production in embryos of A. limnaeus. 20 Embryos can accumulate over 30 mM lactate after extended periods of anoxia.…”

“…9). 36 AMP accumulates as a consequence of the drop in ATP, while ADP values remain relatively constant. While anoxic, ATP/ADP ratios approach 1, and adenylate energy charge drops to near 0.3, indicating severe energy limitation during exposure to anoxia.…”

“…While anoxic, ATP/ADP ratios approach 1, and adenylate energy charge drops to near 0.3, indicating severe energy limitation during exposure to anoxia. 36 The vast majority of vertebrate cells, especially cardiac myocytes and neurons, would be irreversibly damaged within minutes of experiencing this severe of a depletion of ATP. The fact that cells of A. limnaeus can tolerate this level of ATP depletion for months without obvious cell damage is exceptional.…”