Apolipoprotein E gene expression correlates with endogenous lipid nutrition and yolk syncytial layer lipoprotein synthesis during fish development

During embryogenesis of teleost fish, the formation of a yolk syncytial lay er (YSL) enables the resorption of the yolk reserves and development up to the larval stage. We have examined the changes of the yolk cell structure i n relation to yolk and oil-globule lipid utilization during development of the turbot (Scophthalmus maximus). After encapsulation by the YSL, resorpti on of the single, large oil globule occurred predominantly after yolk resor ption and was slower in fasting larvae. The YSL was in contact with an enla rged perisyncytial space, but no vascular network or red blood cells were p resent within the walls of the yolk sac. Intrasyncytial channels infiltrate d by pigmented lining cells were observed in the YSL surrounding the oil gl obule. Apolipoprotein E (apoE) has a prominent role in lipid metabolism bec ause of its ability to interact with lipoprotein receptors. We performed mo lecular cloning of the putative low-density lipoprotein-receptor binding do main of turbot apoE. In situ hybridization analysis revealed a very high le vel of apoE transcripts in the YSL, while no expression could be detected i n the intestine. YSL apoE expression was correlated with the synthesis of v ery low density Lipoprotein (VLDL) particles. An extraordinarily high numbe r of VLDL particles were poured into the perisyncytial space, and intrasync ytial channels enabled the transfer of yolk- and oil globule-derived lipids to the developing embryo or larva. The pattern of apoE mRNA distribution i n relation to YSL lipoprotein synthesis indicates that apoE expression is a suitable molecular marker for monitoring endogenous lipid nutrition during the endo-exotrophic period of teleost fish development.