Lipoprotein Receptors in Extraembryonic Tissues of the
Chicken*
Marcela
Hermann
,
Michelle G.
Mahon§,
Ken A.
Lindstedt¶,
Johannes
Nimpf, and
Wolfgang J.
Schneider
From the Institute of Medical Biochemistry, Department of Molecular
Genetics, Biocenter and University of Vienna, Dr. Bohr-Gasse 9/2,
A-1030 Vienna, Austria
Yolk is the major source of nutrients
for the developing chicken embryo, but molecular details of the
delivery mechanisms are largely unknown. During oogenesis in the
chicken, the main yolk components vitellogenin and very low density
lipoprotein (VLDL) are taken up into the oocytes via a member of the
low density lipoprotein receptor gene family termed LR8 (Bujo, H.,
Hermann, M., Kaderli, M. O., Jacobsen, L., Sugawara, S., Nimpf,
J., Yamamoto, T., and Schneider, W. J. (1994) EMBO J. 13, 5165-5175). This endocytosis is accompanied by partial degradation
of the yolk precursor protein moieties; however, fragmentation does not
abolish binding of VLDL to LR8. The receptor exists in two isoforms
that differ by a so-called O-linked sugar domain; the shorter form
(LR8
) is the major form in oocytes, and the longer protein (LR8+)
predominates in somatic cells. Here we show that both LR8 isoforms are
expressed at ratios that vary with embryonic age in the extraembryonic
yolk sac, which mobilizes yolk for utilization by the embryo, and in
the allantois, the embryo's catabolic sink. Stored yolk VLDL interacts
with LR8 localized on the surface of the yolk sac endodermal
endothelial cells (EEC), is internalized, and degraded, as demonstrated
by the catabolism of fluorescently labeled VLDL in cultured EEC. Addition to the incubation medium of the 39-kDa receptor-associated protein, which inhibits all known LR8/ligand interactions, blocks the
uptake of VLDL by EEC. The levels of endogenous receptor-associated protein correspond to those of LR8+ but not LR8, suggesting that it
may play a role in the modulation of surface presentation of LR8+.
Importantly, EEC express significant levels of microsomal triglyceride
transfer protein and protein disulfide isomerase, key components
required for lipoprotein synthesis. Because the apolipoprotein pattern
of VLDL isolated from the yolk sac-efferent omphalomesenteric vein is
very different from that of yolk VLDL, these data strongly suggest that
embryo plasma VLDL is resynthesized in the EEC. LR8 is a key mediator
of a two-step pathway, which affects the uptake of VLDL from the yolk
sac and the subsequent delivery of its components to the growing embryo.
*
This work was supported by Oesterreichische Nationalbank
Grant NB 5700 (to M. H.) and Austrian Science Foundation (Fonds
zur Forderung der wissenschaftlichen Forschung) Grants F-606 (to
J. N.) and F-608 and P-11694 (to W. J. S.).The costs of publication of this
article were defrayed in part by the
payment of page charges. The article
must therefore be hereby marked
"advertisement" in accordance with 18 U.S.C. Section
1734 solely to indicate this fact.
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