Transport and utilization of yolk lipids in the chicken

In egg-laying species, for normal development the embryo depends entirely on components deposited into the egg by the mother. Energy requirements are fulfilled to the major part through mobilization of lipid molecules from the yolk of the oocyte. The important yolk components, such as lipoproteins, are almost exclusively synthesized in and secreted by the liver, and only to a very small extent by other organs, of the sexually mature hen. We have shown that the hepato-oocyte-embryo axis consists of 4 components: i) massive hormonal induction of yolk precursor synthesis in the maternal liver; ii) oocyte-directed receptor-mediated transport of yolk precursors and their limited proteolysis to form yolk; iii) mobilization of yolk components, mainly lipoproteins, via the extraembryonic yolk sac into the embryonic circulation; and iv) lipid utilization by the action of lipases and lipid transfer proteins expressed by the developing embryo. While we have made contributions to the understanding of the processes underlying these four steps, with emphasis on i) interaction of lipoproteins with receptors belonging to the LDL receptor family, ii) the enzyme lipoprotein lipase and its inhibitor, apolipoprotein-VLDL-II, and iii) acellular structures involved in yolk precursor transfer within the follicle, very little is known about the apolipoproteins, lipolytic enzymes, and lipid transfer activities required for the utilization of lipid molecules, both by the embryo and the adult animal. Novel and powerful tools are now available to tackle these questions with both functional and genomic approaches. The delineation of chicken genome sequence, albeit yet incomplete, has raised this long-studied animal model for developmental studies to new levels. Thus, based on our previous work and the exploitation of genome information, we will now take an integrative approach towards the characterization in the chicken of newly identified lipolytic enzymes, relevant apolipoproteins modulating their activity, as well as of phospholipid transfer protein (PLTP), none of which have been studied to date. The results of this research will bring us closer to a complete understanding of the paths leading from the conversion of nutrients to the development of the embryo in a prime investigative model. The newly-gained knowledge related to yet unknown molecular mechanisms will also be of general value to understanding complex lipid-related diseases, a major cause of premature death.

In mature hens of the chicken, the yolk of fully-grown oocytes contains ca. 5,3 g fat including an average of 0,22 g cholesterol, and the oocytes ovulate every 25 hours. Accumulation of yolk, the major source of nutrients and regulatory factors for the developing embryo in egg-laying species, is predominantly the result of massive transport from the bloodstream into the oocyte. The main constituents of yolk are lipid-rich complexes produced in the liver and secreted in the form of yolk precursors, which are directed towards oocytes embedded within ovarian follicles. This is a highly regulated process, as the delivery of yolk precursors must largely bypass other potential uptake sites. Thus, in parallel to the oocyte-targeted flow, lipid homeostasis of other tissues must be maintained simultaneously. This is, at least in part, achieved through regulation of systemic lipoprotein metabolism involving somatic-cell-specific genes including genes for lipases, lipid transfer proteins, apolipoproteins, and low density lipoprotein (LDL) receptor relatives (LRs). These features make the laying hen a prime model for delineating details of molecular mechanisms underlying the more or less unidirectional mass lipid transfer (from the liver to oocytes, and from yolk to embryo) as well as of fine-tuned regulatory events in systemic lipid metabolism. In the research funded by the FWF we concentrated on the mechanism by which the yolk is delivered via the yolk sac (YS) to the embryo. We show that (i) the YS incorporates lipoproteins from the underlying yolk by uptake via specific receptors; (ii) the YSs innermost cells, epithelial cells, degrade the components taken up and resynthesize new lipoproteins; (iii) these lipoproteins are different in many aspects, from those in yolk; and (iv) the lipoproteins are transported through the joint YS-embryo circulation into metabolic organs of the developing embryo. We have also shown, for the first time, that during growth of the YS surrounding the embryo, the formation of blood vessels in the YS and the acquisition of the ability to transport lipoproteins, as described in (i) to (iv), are highly co-ordinated processes. Finally, several of the genes involved in the regulation, such as those encoding lipid metabolic enzymes, receptors, and proteins of lipoprotein particles, have now been extensively characterized. Taken together, the results demonstrate that the YS of egg-laying species is, in every respect, the functional analogue of the mammalian placenta.