The role of eukaryotic initiation factor 2α during the metabolic depression associated with estivation

Abstract

During adverse environmental conditions, many organisms are able to enter a state of quiescence and survive for extended periods on stored fuels. Associated with this quiescence there is typically a depression of basal metabolic rate by 60-100% (termed metabolic depression; Guppy and Withers, 1999), which necessitates a coordinated change in ATP production and utilization. Estivation is a form of metabolic depression that occurs in some animals in response to a lack of food and/or water or in response to otherwise potentially desiccating situations. It occurs without any change in ambient oxygen partial pressure (PO∑) or temperature. The cellular signaling mechanisms that coordinate the change in energy metabolism during estivation, and indeed the initial cue that initiates estivation, remain unknown.Given that ATP utilization decreases during metabolic depression, energy-consuming pathways must be downregulated. If their mode of regulation during estivation can be elucidated it may be possible to delineate the upstream signaling mechanisms that would then point to the initial cue for metabolic depression. Protein synthesis is a major energyconsuming pathway and, if maintained at a similar rate during metabolic depression, would become an impossibly costly process in terms of its contribution to total energy consumption. Accordingly, the downregulation of protein synthesis is a consistent phenomenon seen in metabolically depressed organisms. The extent of this downregulation has been well characterised in a number of both in vivo and in vitro systems and in a variety of different types of metabolic depression, including that associated with anoxia (Bailey and Driedzic, 1996;Hofmann and Hand, 1994), developmental arrest (Podrabsky and Hand, 2000), hibernation (Frerichs et al., 1998) and estivation (Fuery et al., 1998;Pakay et al., 2002).Despite the often substantial decrease in the rate of protein synthesis during metabolic depression, mRNA pools appear to be maintained during the depressed state. For example, translatable mRNA can be detected in dormant Artemia embryos, and its in vitro translation demonstrates that there is no significant qualitative and/or quantitative differences in mRNA between anoxic and developing embryos (Hofmann and Hand, 1994). Also in Artemia, the addition of exogenous mRNA does not increase the translational capacity of lysates prepared from dormant embryos (Hofmann and Hand, 1994). We have investigated the role of eukaryotic initiation factor 2α (eIF2α) in two estivating organisms previously shown to downregulate protein synthesis during metabolic depression, the land snail Helix aspersa Müller and the desert frog Neobatrachus sutor Main 1957. We have developed a method using a single antibody (which binds specifically to the phosphorylated, conserved phosphorylation region) by which the total levels of eIF2α and the ratio of phosphorylated eIF2α [eIF2α(P)] to total (phosphorylated and unphosphorylated) eIF2α can be determined. In H. aspersa, we have shown that the level of eIF2α mRN...

Metabolic Regulation and Gene Expression During Aestivation

“…2.2) and foot muscle showed a 20-fold higher P-eIF2a (Ser51) content after 14 days (Ramnanan et al 2009). Pakay et al (2003) also found a approximately twofold increase in the relative amount of P-eIF2a in liver extracts from aestivating desert frogs (Neobatrachus sutor), but in H. aspersa the amount of P-eIF2a was below the detection limit in hepatopancreas of both aestivating and awake snails (Pakay et al 2003).…”

Aestivation: signaling and hypometabolism

“…Phosphorylation of eIF2 blocks the protein from being recharged with GTP and thereby inhibits its GTP-dependent function in the delivery and attachment of the initiating methionine residue to the assembling ribosome (Proud, 2007). Phosphorylated eIF2 content also increased in liver of aestivating desert frogs (Pakay et al, 2003). The eIF4 complex is regulated at several points.…”

Biomarker-based assessment of the muscle maintenance and energy status of anurans from an extremely seasonal semi-arid environment, the Brazillian Caatinga

“…Thus, low levels of eIF2α and/or elevated expression of p-eIF2α indicate suppression of the protein synthesis in the muscles of P. diplolister during aestivation. The stable level of eIF2α and p-eIF2α in the other studied P. diplolister muscles might indicate that the regulation of the protein synthesis in these tissues might be dependent on alternative mechanisms such as control of the translation elongation [38] or ribosome (dis)assembly [39]. Suppression of the protein synthesis is a common energy-saving mechanism in estivating species [38] and has been observed in desert amphibians Neobatrachus centralis [40] and in estivating snails Otala lactea [41,42].…”

“…The stable level of eIF2α and p-eIF2α in the other studied P. diplolister muscles might indicate that the regulation of the protein synthesis in these tissues might be dependent on alternative mechanisms such as control of the translation elongation [38] or ribosome (dis)assembly [39]. Suppression of the protein synthesis is a common energy-saving mechanism in estivating species [38] and has been observed in desert amphibians Neobatrachus centralis [40] and in estivating snails Otala lactea [41,42]. Earlier studies on estivating frogs ( Cyclorana alboguttata) showed that muscles are protected against atrophy during prolonged (9 months) estivation with no decline in muscle mass, cross-sectional area or fiber number [7].…”