Gastrin/cholecystokinin-like post-prandial variations: quantitative and qualitative changes in the haemolymph of penaeids (Crustacea Decapoda)

Trypsin isozymes in the lobster Panulirus argus (Latreille, 1804): from molecules to physiology

Trypsin enzymes have been studied in a wide variety of animal taxa due to their central role in protein digestion as well as in other important physiological and biotechnological processes. Crustacean trypsins exhibit a high number of isoforms. However, while differences in properties of isoenzymes are known to play important roles in regulating different physiological processes, there is little information on this aspect for decapod trypsins. The aim of this review is to integrate recent findings at the molecular level on trypsin enzymes of the spiny lobster Panulirus argus, into higher levels of organization (biochemical, organism) and to interpret those findings in relation to the feeding ecology of these crustaceans. Trypsin in lobster is a polymorphic enzyme, showing isoforms that differ in their biochemical features and catalytic efficiencies. Molecular studies suggest that polymorphism in lobster trypsins may be non-neutral. Trypsin isoenzymes are differentially regulated by dietary proteins, and it seems that some isoenzymes have undergone adaptive evolution coupled with a divergence in expression rate to increase fitness. This review highlights important but poorly studied issues in crustaceans in general, such as the relation among trypsin polymorphism, phenotypic (digestive) flexibility, digestion efficiency, and feeding ecology.

Dietary protein quality differentially regulates trypsin enzymes at the secretion and transcription level in Panulirus argus by distinct signaling pathways

The effects of pelleted diets with different protein composition (fish, squid or soybean meals as main protein sources) on trypsin secretion and expression were studied in the lobster Panulirus argus. Trypsin secretion was shown to be maximal 4 h after ingestion. At this time, fish- and squid-based diets induced trypsin secretion, as well as up-regulation of the major trypsin isoform at the transcription level. While fish- and squid-based diets elicited a prandial response, soybean-based diet failed to stimulate the digestive gland to secrete trypsin into the gastric fluid or induce trypsin expression above the levels observed in fasting lobsters. In vitro assays showed that intact proteins rather than protein hydrolysates stimulate trypsin secretion in the lobster. However, the signal for trypsin transcription appears to be different to that for secretion and is probably mediated by the appearance of free amino acids in the digestive gland, suggesting a stepwise regulation of trypsin enzymes during digestion. We conclude that trypsin enzymes in P. argus are regulated at the transcription and secretion level by the quality of dietary proteins through two distinct signaling pathways. Our results indicate that protein digestion efficiency in spiny lobsters can be improved by selecting appropriated protein sources. However, other factors like the poor solubility of dietary proteins in dry diets could hamper further enhancement of digestion efficiency.

Release of digestive enzymes from the crustacean hepatopancreas: effect of vertebrate gastrointestinal hormones

Vertebrate gastrointestinal hormones were tested on their ability to liberate digestive enzymes from the crustacean midgut gland. CCK-8 (desulfated form), gastrin, bombesin, secretin, and substance P were detected to release enzymes. Maximal concentrations observed were 5 nM CCK for protease release, 1 nM gastrin for protease and 100 nM for amylase release, 100 nM bombesin for protease release, 10 nM secretin for amylase and protease release, and 100 nM substance P for protease release. Unlike in vertebrates, glucagon was unable to stimulate enzyme release in crustaceans, this also applies to the counterpart insulin. These results may support the assumption that Crustacea possess endogenous factors resembling the above mentioned vertebrate hormones, at least in such a way that the appropriate receptors have the capacity to accept these hormones.

Structure and biological activity of crustacean gastrointestinal peptides identified with antibodies to gastrin/cholecystokinin

Four gastrin/cholecystokinin-like peptides (G/CCK) which cross-react with a specific C-terminal gastrin/CCK antiserum have been isolated from the stomach of the marine crustacean Nephrops norvegicus. The molecular weight of the four peptides was estimated between 1000 and 2000 Da by molecular sieving. By radioimmunoassay, the cross-reactivity of these peptides with human gastrin 17-I was found to be around 0.03%. Pure peptidic fractions were recovered after four successive steps of HPLC. Amino-acid analysis suggested a similarity between the four peptides identified which may belong to a new family. A limited homology between the C-terminus of one Nephrops peptide and vertebrate G/CCK was found after sequencing. Two of the peptides exhibited secretagogue effects on crustacean isolated midgut glands. The Nephrops peptides, although structurally distinct from the vertebrate G/CCKs, appear to serve similar biological functions in crustaceans.

A cholecystokinin-like hormone activates a feeding-related neural circuit in lobster

The peptide hormone cholecystokinin (CCK) contributes to the production of feeding-related behaviour in mammals, but the mechanism by which it exerts its effects remains unclear. The gastric mill neural circuit of lobster is an experimentally accessible model system for studying the hormonal control of feeding-related behaviour. Composed of 11 identified neurons, this circuit produces rhythmic movement of teeth within the stomach. We have previously shown that the gastric mill motor pattern can be modulated by a cholecystokinin-like peptide in vitro. We report here that (1) after feeding, levels of CCK-like peptide in haemolymph increase with the activation of the gastric mill, (2) injections of CCK activate the gastric mill, and (3) a specific CCK antagonist inhibits feeding-induced gastric mill activity. This neatly demonstrates a casual link between in vivo release of a peptide hormone and activation of a neural circuit.