TEMPORAL AND SPATIAL PATTERN OF EXPRESSION OF THE HDL RECEPTOR SR-BI DURING MURINE EMBRYOGENESIS

During rodent fetal development, maternal lipoproteins can be sources of cholesterol for the membrane synthesis required for tissue growth i n the developing embryo and for steroid hormone production in the extr aembryonic tissues. Although the mechanisms underlying the maternal fe tal lipoprotein cholesterol transport system are not well de fined, th e placenta and yolk sac seem to play major roles in this process, serv ing as functionally active interfaces between maternal circulation and the embryo. In rodents, the principal cholesterol transporter in the plasma is HDL, and the HDL receptor SR-BI is a physiologically importa nt mediator of cholesterol uptake in adult liver and steroidogenic tis sues. To begin to investigate SR-BI's role in maternal cholesterol upt ake by the fetus, we used immunofluorescence microscopy to determine t he pattern of SR-BI expression during murine embryogenesis. At day E7. 5 in gestation, there was significant SR-BI expression in endothelial cells of the decidua, but little in intraembryonic and extraembryonic tissues. By day E8.5, there was a dramatic increase in SR-BI expressio n in the trophoblast cells which surround the developing embryo. Begin ning at day E10, SR-BI was expressed in both the placenta and yolk sac . The expression in these extraembryonic tissues was correlated with s ignificant uptake of fluorescent dye by the yolk sac visceral endoderm al cells from DiI-labeled HDL injected into pregnant mice. Within the embryo proper. SR-BI expression appeared by day E14.5 at high levels i n the adrenal gland. SR-BI expression was not detected in the embryoni c liver through day E17.5 of gestation; however, it could be observed ill neonatal livers. These findings suggest that SR-BI may play a role in the rodent maternal-fetal lipoprotein cholesterol transport system , supplying HDL cholesterol for either membrane or steroid hormone syn thesis, or both.