Structural Basis of Glucose Transport in the Placental Barrier: Role of GLUT1 and the Gap Junction

Abstract

Glucose transport across cellular membranes is mediated by integral membrane proteins (glucose transporters). GLUT1, an isoform of facilitated-diffusion glucose transporters, is abundant in human and rat placentae. In the human placenta, syncytiotrophoblast serves as the structural basis of the placental barrier. GLUT1 is present along the luminal and basal plasma membranes of this cell. GLUT1 seems to play an important role in the transfer of glucose through a syncytiotrophoblast layer in the human placenta. In the rat placenta, two layers of syncytiotrophoblasts (namely syncytiotrophoblasts I and II from the maternal blood side to the fetal side) serve as a barrier. GLUT1 is abundant in these syncytiotrophoblasts. It is concentrated along the plasma membrane of the maternal side in syncytiotrophoblast I and also along the plasma membrane of the fetal side in the adjacent syncytiotrophoblast II. These two syncytiotrophoblast layers are connected by numerous gap junctions. Connexin 26 is present in these gap junctions. GLUT1 in conjunction with connexin 26 in the gap junction seems to serve as the machinery for trans-placental transfer of glucose in the rat placental barrier.