Mechanisms of protein degradation in mantle muscle and proposed gill remodeling in starvedSepia officinalis

Abstract

-Cephalopods have relatively high rates of protein synthesis compared to rates of protein degradation, along with minimal carbohydrate and lipid reserves. During food deprivation on board protein is catabolized as a metabolic fuel. The aim of the current study was to assess whether biochemical indices of protein synthesis and proteolytic mechanisms were altered in cuttlefish, Sepia officinalis, starved for 7 days. In mantle muscle, food deprivation is associated with a decrease in protein synthesis, as indicated by a decrease in the total RNA level and dephosphorylation of key signaling molecules, such as the eukaryote binding protein, 4E-BP1 (regulator of translation) and Akt. The ubiquitinationproteasome system (UPS) is activated as shown by an increase in the levels of proteasome ␤-subunit mRNA, polyubiquitinated protein, and polyubiquitin mRNA. As well, cathepsin activity levels are increased, suggesting increased proteolysis through the lysosomal pathway. Together, these mechanisms could supply amino acids as metabolic fuels. In gill, the situation is quite different. It appears that during the first stages of starvation, both protein synthesis and protein degradation are enhanced in gill. This is based upon increased phosphorylation of 4E-BP1 and enhanced levels of UPS indicators, especially 20S proteasome activity and polyubiquitin mRNA. It is proposed that an increased protein turnover is related to gill remodeling perhaps to retain essential hemolymph-borne compounds.cathepsin; cephalopod; metabolic enzymes; proteasome; polyubiquitin; RNA; starvation; triglyceride CEPHALOPODS TYPICALLY FEED upon protein-rich diets and are generally high in protein content with low reserves of lipid and carbohydrates. Most of the lipid is present in the digestive gland with very low lipid reserves in other tissues (33). Studies tracking the incorporation of radiolabeled phenylalanine into the protein pool show that in feeding cephalopods, protein accretion is favored by relatively low rates of protein degradation compared to rates of protein synthesis (6,18,27). In Southern dumpling squid (Euprymna tasmanica), whole body protein degradation increases in minimally fed animals (18). Furthermore in octopus, Octopus vulgaris, the oxygen consumption-nitrogen excretion ratio increased over the first 5 days of starvation to values that imply a mixed lipid and protein oxidation, and by day 8 the ratio decreased to a predominantly protein-based metabolism (5). In cuttlefish, Sepia officinalis, oxygen consumption is maintained for the first 6 days of starvation, and thus, rates of metabolic fuel delivery must remain constant (15). Collectively, the above implies that during periods of food deprivation, the limited lipid reserves are called upon first, in parallel with, then subsequently, followed by, an obligate dependence on the use of protein as a metabolic fuel. The objective of the current study was to assess whether biochemical indices of protein breakdown and to some extent protein synthesis in various tissues are altered du...

Metabolic rate and rates of protein turnover in food-deprived cuttlefish,Sepia officinalis(Linnaeus 1758)

“…Cathepsin A-, B-, H-, and L-like proteases were elevated, as were indices of the ubiquitin-proteasome pathway. Most notably, there were increases in proteasome (␤-subunit) and polyubiquitin transcript levels, and polyubiquinated protein (21). These biochemical indices imply that protein synthesis is curtailed during the early stages of food deprivation in the mantle of S. officinalis, and as starvation ensues, mantle protein is catabolized.…”

“…We have previously shown that in the mantle of fooddeprived cuttlefish, at a time when digestive gland triglycerides were being mobilized, total RNA levels were decreased and cellular signaling pathways stimulating protein synthesis were disengaged (21). More specifically, the phosphorylation of AKT and 4EBP1 decreased.…”

“…Cephalopods have a "live fast and die young" life strategy. They are mostly carnivorous animals that have a protein-based metabolism (21,27,28,33). Contrary to many other animals, cephalopods do not store substantial amounts of lipids and carbohydrates as energy reserves, although triglycerides in the digestive gland may serve as a short-term fuel to support aerobic metabolism during food limitation.…”

“…Contrary to many other animals, cephalopods do not store substantial amounts of lipids and carbohydrates as energy reserves, although triglycerides in the digestive gland may serve as a short-term fuel to support aerobic metabolism during food limitation. In these animals, surplus energy is instead used for somatic growth, and the protein in the mantle may, thus, be viewed as a form of stored energy (6,21,27). A consequence of this metabolic strategy is that, during starvation, cephalopods need to rely rapidly on this protein reserve to sustain metabolism.…”

“…Once lipids are almost depleted, the animals are forced into phase III and oxidize their proteins as a fuel of last resort, and only then, are they considered to be in a true phase of starvation (23). Cuttlefish (Sepia officinalis) reach phase III following 7 days of food deprivation (19).…”