METABOLIC IMPLICATIONS OF DIETARY CHOLESTEROL IN SHRIMP  

Among crustaceans, shrimps are the most popular food item and are consumed in larger quantities worldwide than other edible crustaceans such as crabs or lobsters. In shrimp and other crustaceans, lipids are the organic reserves and are usually the second largest biochemical fractions following protein. Among lipid, cholesterol is a major sterol in shrimp which occurs in all cells and in haemolymph either in free form or in combination with fatty acids. Cholesterol serves as an important component of cellular membrane and lipoprotein. It acts as a key precursor of bile acids, hormones (steroid and molting) and vitamin D3. Despite of these distinctive biological repercussions, shrimps and other crustaceans are incapable (unlike vertebrate) of de novo synthesize of cholesterol, therefore, dietary supply of cholesterol is essential for their growth and survival. Optimum levels of cholesterol for shrimp may range between 0.2-2%, depending on species, stage, and other dietary ingredients. For enhancing better growth and for improving current management strategies toward sustainable shrimp farming, the knowledge of cholesterol metabolism is of great interest. Despite of existing information on the dietary fortification and optimization of cholesterol for different species of shrimp; the regulatory mechanisms and allied putative cholesterol metabolic pathways involved in the digestibility, absorption, transport, energy production and ecdysis is largely unknown for shrimp.

Among crustaceans, lipase plays an important physiological role, involved in dietary lipid and triacylglyceride catabolism. A wide class of candidate genes has been identified for lipase in different shrimp species. Cholesterol transportation is facilitated via esters formations packaged into lipoprotein particles which produce a hydrophobic core surrounded by a surface monolayer of polar phospholipids. Although largely undescribed for shrimps, however, in mammalian plasma chylomicrons and VLDL carries triglycerides and a VLDL degenerated product (LDL) is a major cholesterol-carrying lipoprotein. Correspondingly, HDL (high-density lipoproteins) is active in reverse cholesterol transport i.e., the transport of cholesterol from peripheral tissues to the liver, the main organ responsible for cholesterol utilization. The cholesterol level in shrimp (which is authenticated in crab) might be regulated by LDL receptor (LDLR), VLDL receptor (VLDLR) or HDL binding protein by removing cholesterol-rich intermediate IDL (intermediate-density lipoprotein) and LDL (low-density lipoprotein) from haemolymph.  Furthermore, in vertebrates, excess cholesterol in the tissue can be transformed for bile acids production via oxidation into oxysterols. Ecdysteroids or molting hormones are synthesized from cholesterol by steroidogenic enzymes of the cytochrome P450 (CYP) family. In insects these enzymes are encoded by a cluster of genes and the molting hormone 20-hydroxyecdysone (20E) is highly conserved. However, except the insecta, cholesterol based biosynthetic pathway of ecdysteroids remains largely unknown in crustaceans with shrimp in particular. Overall, it can be anticipated that these regulatory pathways involved in cholesterol metabolism might be (partially) conserved among crustacean, however, a substantial evidence is needed for shrimp.  Eventually, there is an urgent need to reduce the knowledge gap so that cholesterol based dietary manipulation can be promoted efficiently in shrimp farming.