Original articleExpression of GLUT12 in the fetal membranes of the human placenta
Introduction
The fetal membranes are extra-embryonic tissues that contain the fetus throughout the pregnancy. These membranes provide sufficient strength and elasticity to accommodate fetal growth and movements and eventually undergo programmed rupture during the first stage of labour. Bryant-Greenwood [1] recently reassessed the structural organisation of human fetal membranes, which consist of the fetal-facing amnion and maternal-facing chorion. In summary, the amnion comprises five distinct layers, the innermost layer of which is the amniotic epithelium. This layer is in direct contact with the amniotic fluid on one side and a basement membrane on the other. The other layers consist of the compact layer, the fibroblast layer and the spongy or intermediate layer. The chorion consists of the reticular layer, a basement membrane and the trophoblast cell region, which at term firmly adheres to the maternal decidual tissue. Like the placenta, the fetal membranes play an integral role in fetal development and progression of pregnancy. In addition to autocrine regulatory activities, the membranes secrete substances both into the amniotic fluid, affecting amniotic fluid homeostasis, and towards the uterus, where they may influence maternal cellular physiology. The membranes also play a protective role for the fetus against infection ascending the reproductive tract.
The cells of the fetal membranes, as is the case with all mammalian cells, must obtain the nutrient d-glucose by transporting it across the cell membranes. A family of facilitative glucose carriers (gene symbol SLC2A, protein symbol GLUT) primarily achieves this process [2]. Previously, the cellular expression patterns for GLUT1, GLUT3 and GLUT4 in the human fetal membranes have been described (reviewed in Ref. [3]). In recent years, several new GLUTs have been identified in mammalian cells [4], [5], [6], [7], [8], [9]. One of these has been designated GLUT12 and was originally cloned from malignant breast epithelial (MCF7) cells on the basis of homology to GLUT4 [9]. The sequence of GLUT12 presents all the elements characteristic of the GLUT family, in particular the 12 putative trans-membrane domains required for function as a glucose transporter. GLUT12 is expressed in skeletal muscle, adipose tissue, small intestine, heart, placenta, and in pregnant and lactating mammary gland [9], [10], [11], as well as in cancerous tissue of breast [12] and prostate [13] and widely expressed in the fetus [14].
GLUT12 is a member of the class III hexose transporter family [15]. It exhibits 40% amino acid homology with GLUT10 and 29% with GLUT4. GLUT12 has shared characteristics with GLUT4 and has been postulated to be a second insulin-sensitive glucose transport system [9]. In a recent study, Rogers et al. [12] used the Xenopus laevis oocyte expression system to assay transport of the glucose analogue 2-deoxy-d-glucose and characterize the glucose transport properties and hexose affinities for GLUT12. They found that GLUT12 facilitates transport of glucose with an apparent preferential substrate affinity for glucose over the other hexoses studied. Their result supports the hypothesis that GLUT12 acts to facilitate glucose transport in vivo in the many different cells in which it is expressed. Recently, the cellular expression pattern for GLUT12 was described in the human placenta [10]; however, it is not known whether GLUT12 is expressed in human fetal membranes. The aim of this study was to identify GLUT12 mRNA and protein expression in fetal membranes and to compare its cellular distribution to the known distribution for GLUT1.
Section snippets
Tissue collection
All experimental procedures were approved by the Royal Women's Hospital Research and Ethics Committees and performed in accordance with the National Health and Medical Research Council of Australia guidelines. Informed consent was obtained from all patients participating in the study. Term placentas were obtained immediately after delivery following either elective Caesarean section or spontaneous labour with normal vaginal delivery from healthy women who had medically uncomplicated, singleton
Results
GLUT12 and GLUT1 mRNA were identified in extracts of amnion and choriodecidua for four normal term placentas by RT-PCR (Figure 1a). The GLUT12 mRNA signals were more intense for the choriodecidual extracts compared with those of the amnion. The signals for GLUT1 on the other hand were similar between the two types of tissues. Western blot analysis showed a single band of protein of 60 KDa for GLUT12 in both tissues. The signals for choriodecidua were more intense than those for amnion (Figure 1b
Discussion
This study has characterized the expression of the novel glucose transporter protein GLUT12 in the fetal membranes of the human placenta. GLUT12 protein was predominantly expressed in trophoblast cells of the chorion laeve and to a lesser extent in amniotic epithelial and fibroblast cells and in decidual cells. This observation was supported by apparent higher expression of GLUT12 mRNA in extracts of choriodecidua compared with extracts of amnion and suggests that in fetal membranes at term,
Acknowledgements
The Royal Women's Hospital Research Fund supported this work. We also gratefully acknowledge the help of the clinical research midwife Sue Nisbett, and the midwifery and obstetric staff of the Royal Women's Hospital, and the expert technical advice of Maria Macheda.
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