Localization of monoamine oxidase mRNA in human placenta

Monoamine oxidase (MAO) oxidatively deaminates vasoactive and biogenic amin es 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 transcriptio n-polymerase chain reaction (RT-PCR) on RNA from human liver, the predicted products for both enzymes were detected. Furthermore, RT-PCR on RNA from h uman placenta, believed to contain predominantly (or only) MAO-A protein, a lso indicated the presence of both A and B gene transcripts. The cellular d istribution of MAO mRNA in placental tissue was analyzed by in situ hybridi zation of MAO-A and MAO-B mRNA-specific cRNA probes on paraffin sections. M AO-A mRNA was mainly evident in the syncytiotrophoblastic layer. None was d etected in the vascular endothelium/smooth muscles. Significantly, MAO-B mR NA signal was also evident in the placental villi, notably in the syncytiot rophoblasts, intermediate trophoblasts, cytotrophoblasts, and the vascular endothelium. To our knowledge, this is the first demonstration of the cellu lar distribution of MAO mRNA in human placenta via in situ hybridization. T he expression of MAO-B in placental tissue rather than in blood elements wi thin placenta is also unequivocally demonstrated. These highly specific cRN A probes can now be used to study the distribution of MAO-A and MAO-B expre ssion in other tissues.