Localization of monoamine oxidase mRNA in human placenta

Monoamine oxidase (MAO) oxidatively deaminates vasoactive and biogenic amines and exists in two distinct forms (A and B), coded for by separate genes, which exhibit distinct substrate specificities and inhibitor sensitivities. Using specific primers for MAO-A and MAO-B mRNA in a reverse transcription-polymerase chain reaction (RT-PCR) on RNA from human liver, the predicted products for both enzymes were detected. Furthermore, RT-PCR on RNA from human placenta, believed to contain predominantly (or only) MAO-A protein, also indicated the presence of both A and B gene transcripts. The cellular distribution of MAO mRNA in placental tissue was analyzed by in situ hybridization of MAO-A and MAO-B mRNA-specific cRNA probes on paraffin sections. MAO-A mRNA was mainly evident in the syncytiotrophoblastic layer. None was detected in the vascular endothelium/smooth muscles. Significantly, MAO-B mRNA signal was also evident in the placental villi, notably in the syncytiotrophoblasts, intermediate trophoblasts, cytotrophoblasts, and the vascular endothelium. To our knowledge, this is the first demonstration of the cellular distribution of MAO mRNA in human placenta via in situ hybridization. The expression of MAO-B in placental tissue rather than in blood elements within placenta is also unequivocally demonstrated. These highly specific cRNA probes can now be used to study the distribution of MAO-A and MAO-B expression in other tissues.