Differential screening of a PCR-generated mouse embryo cDNA library: glucose transporters are differentially expressed in early postimplantation mouse embryos

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Abstract
Differential screening of a cDNA library constructed using PCR amplification techniques from RNA isolated from the distal portion (embryonic ectoderm, mesoderm and visceral endoderm) of 7.5 days post coitum (dpc) mouse embryos led to the isolation of two cDNA clones expressed at higher levels in 7.5 dpc embryos than 12.5 dpc embryos. Nucleotide sequence analysis revealed that each of these clones was a different member of the family of facilitative glucose transporters (Glut genes). The differentially expressed cDNA clones represent mouse Glut-1 and Glut-3. Levels of the Glut-3 mRNA declined 14-fold between days 7.5 and 12.5 of gestation, and were under our limits of detetction by 14.5 dpc. The levels of the Glut-1 mRNA declined about 3-fold between days 7.5 and 12.5 of gestation. Analysis of the expression of these genes by in situ hybridization revealed striking differences in transcript localization in early postimplantation mouse embryos. At 7.5 dpc, both transporters were expressed more strongly in extraembryonic tissues than in the embryo proper. While both transporters were expressed in the amnion and chorion, only Glut-1 was expressed in the ectoplacental cone. In the yolk sac, Glut-3 appeared to be expressed only in the endoderm while Glut-1, although expressed in both layers, was expressed more strongly in the mesoderm layer. Thus, the two transporters have relatively reciprocal sites of expression in the developing extraembryonic membranes. Expression of Glut-1 was fairly widespread in the embryo at 8.5 dpc, but by 10.5 dpc expression was down-regulated and was observed in the eye and the spinal cord. Expression of Glut-3 was largely confined to non-neural surface ectoderm and was also substantially down-regulated by 10.5 dpc. These results prompted an examination of the RNA expression pattern of two other glucose transporter isoforms, Glut-2 and Glut-4. We did not detect Glut-4 expression, while Glut-2 expression was largely confined to extraembryonic visceral yolk sac endoderm. These data suggest differential roles for these glucose transporter family members during early postimplantation development of mice.