Mouse embryos express Slc2a2 early during pre-and postimplantation development. Slc2a2 encodes at high KM (~16 mM) glucose transporter, Glut2, which is expressed by pancreatic cells, the liver, kidney, and intestine. Pre- and early postimplantation embryos also express Slc2a1 and Slc2a3, which encode the low KM (~5 mM) glucose transporters, Glut1 and Glut3, respectively. Because embryos would normally be exposed to glucose concentrations from maternal circulation that are near the KM's of Glut 1 and 3, it is expected that they are the predominant glucose transporters under normal circumstances. However, during diabetic pregnancy, blood glucose concentration could approach or surpass the KM of Glut2. We have shown that Slc2a2+/- and Slc2a2-/- embryos are protected from congenital malformations induced by maternal hyperglycemia. However, Glut2 may be important during normal development because functions as a low KM (0.8 mM) glucosamine (GlcN) transporter. We speculated that Glut2 is important for early embryo survival to facilitate GlcN transport from maternal circulation. However, this is difficult to study using i vivo systems. Embryonic stem cells (ESC) that can be differentiated into particular tissue lineages can be very useful to study molecular regulation of embryogenesis. We have used murine ESC that can be induced to form neuronal precursors (NPs) to study pathways that are activated by increased glucose metabolism in embryonic neuroepithelium. A limitation of most existing ESC lines is that they do not express Glut2. We recently showed that ESC isolated in low glucose (100 mg/dl or 5.5 mM) media (called, LG-ESC) express a functional Glut2 transporter and display the same biochemical and gene expression responses to high glucose culture as does the embryo. Using LG-ESC, we have found that Glut2-mediated GlcN transport stimulates ESC self-renewal and anabolic metabolism, and that a pathway that we had found to be regulated by excess glucose metabolism and associated with congenital malformations regulates Slc2a2 expression. The central hypothesis for this proposal is that exogenous GlcN transported by Glut2 promotes proliferation and pluripotency of embryo and stem cells by increasing substrates to drive anabolic metabolism, and that Glut2-mediated GlcN transport is important, not just in nave, undifferentiated cells, but also as they begin to adopt a fate, suchas neuroepithelium; further diabetic embryopathy may in part be due to glucose competition for GlcN uptake. We will test this using LG-ESC, including LG-ESC lines that were established from mouse strains with mutations in gene pathways that are involved in diabetic embryopathy (Aim 1); by testing whether human induced pluripotent stem cells (iPSC) generated in a low glucose environment express SLC2A2, and are respond to exogenous GlcN to promote anabolic metabolism, self-renewal, and pluripotency (Aim 2); by determining whether Glut2-deficient embryos have a survival disadvantage to due insufficient GlcN uptake and metabolism.
Early embryos express Slc2a2, which encodes a solute transporter whose function in embryonic development is not understood. We hypothesize that Slc2a2, also known as Glut2, transports the amino sugar, glucosamine, and that exogenous glucosamine stimulates metabolic pathways for growth and pluripotency; moreover, because Slc2a2/Glut2 is responsible for high rates of glucose transport into the embryo during diabetic pregnancy, part of the mechanism by which diabetes causes congenital malformations may be due to competition by glucose for glucosamine transport. The planned experiments will advance our understanding of metabolic regulation of embryogenesis and may be applied to improved methods for inducing pluripotent stem cells for therapeutic intervention.
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