From maternal glucose to fetal glycogen: expression of key regulators in the human placenta

The present study investigated the expression of glycogenin, the protein pr imer for glycogen synthesis, and the high affinity glucose transporter isof orm GLUT3 as a further potential regulator of cellular glycogen metabolism, in first trimester and term human placenta using immunohistochemistry and Western blotting. At term, glycogenin was most abundant in the endothelium of fetal vessels. Trophoblast as well as basal decidual cells were moderate ly stained. The glycogenin distribution pattern in first trimester placenta e resembled that at term, but reactivity was generally less intense. Extrav illous trophoblast and villous cytotrophoblast were the major sites of GLUT 3 expression. Endothelial cells were also strongly labelled with the GLUT3 antiserum. Western blotting identified both free and glucosylated glycogeni n, as well as a 48 kDa band reacting with GLUT3 antiserum in placental vill ous tissue. Glycogenin immunoreactivity remained unaffected by amylolytic g lycogen digestion, although preceding electron microscopical examination de monstrated the presence of glycogen. These data may, indicate that placenta l glycogenin can be recycled from the immature glycogen or that it is locat ed on the surface of the glycogen molecule. In conclusion, the co-expressio n of glycogenin with GLUT3 might enable glycogen-storing cells to exchange glucose quite effectively according to prevailing metabolic demands of glyc ogen synthesis or degradation.