Metabolic downregulation during diapause in embryos of Artemia franciscana

Abstract

Encysted embryos of Artemia franciscana undergo a dramatic respiratory depression upon release from the adult female as they enter a state of hypometabolism termed diapause. The mechanisms by which such a respiratory depression is achieved remain unexplained. Evidence presented here shows that strategic enzymes involved in trehalose catabolism are inhibited during diapause, namely trehalase, hexokinase, pyruvate kinase and pyruvate dehydrogenase. Trehalose is the sole source of fuel in the embryos of A. franciscana, and hence downregulation of trehalose catabolism results in severe limitation of metabolic fuel available to the embryo during diapause. Western blot data demonstrates that pyruvate dehydrogenase becomes phosphorylated during entrance into diapause, and as a consequence, one would predict PDH to be strongly inhibited in this state. Restriction of glycolytic flux will lead to metabolic 'starvation' of the mitochondrion, and in turn will reduce mitochondrial oxidative phosphorylation during diapause. Measurements of ATP, ADP and AMP show that substantial decreases occur in ATP:ADP ratio and in adenylate energy charge during diapause. Respiration studies conducted with embryo lysates document a depression of oxidative phosphorylation during diapause in the case where substrates for respiratory complex I (pyruvate+malate) are used as the fuel source. Reduced respiration through complex I is corroborated by the increased phosphorylation of pyruvate dehydrogenase. When substrates for complexes I and II (pyruvate+malate+succinate) are added simultaneously, the increased electron flow through the electron transport system allows the detection of respiratory inhibition by the phosphorylation system (i.e., the F1.Fo-ATP synthase, adenine nucleotide transporter, and phosphate transporter). This inhibition of the phosphorylation system is diminished as diapause lysates are diluted, which suggests the presence of an unidentified inhibitor. Finally, measurements of catalytic activity for respiratory complexes extracted from isolated mitochondria in the presence of phosphatase inhibitors reveal a minor decrease in complex I activity during diapause and a drop in activity of complex IV, the latter effect being minimized by COX excess capacity. Taken together, restriction of glycolytic carbon to the mitochondrion appears to the primary mechanism for the in vivo metabolic arrest in A. franciscana embryos during diapause, which is accentuated by inhibitions within the mitochondrion itself.