Endocrine Abstracts

During the metaphase to anaphase transition of mitosis, destruction of the human securin, pituitary tumor-transforming gene (PTTG), and subsequent activation of the cysteine endopeptidase separase, leads to the cleavage of RAD21, a component protein of the cohesin complex. The developing fetal brain has rapidly proliferating neuronal cells, whilst adult neurons no longer proliferate and are maintained in G0 status of the cell cycle. We have previously investigated the expression of PTTG and separase in ontogeny of the human fetal brain. PTTG expression was significantly increased and separase reduced during fetal life. We have now defined expression of mRNA encoding the cohesin subunit RAD21, and compared this with expression of PTTG and separase in the fetal and adult human brain. mRNAs encoding PTTG and RAD21 showed a negative correlation in fetal brain (n=59, r²=0.1, P=0.013) but no correlation in adult cortex (n=11). A strong positive correlation was also apparent between separase and RAD21 mRNAs in fetal brain (r²=0.1, P=0.005) but no association was observed in adult samples. Subsequently, human embryonal neuro-progenitor cells (NT2) were used to examine expression of RAD21 following transfection of PTTG and separase cDNAs in vitro. A reduction of RAD21 mRNA resulted both from overexpression of PTTG (65% reduction, n=8, P<0.05) and from separase (63% reduction, n=7, P<0.001).Transient transfection of a mutant separase, incapable of cleaving RAD21, also significantly reduced RAD21 mRNA (50% reduction, n=8, P<0.05), albeit to a lesser extent. Reduced RAD21 levels were associated with a repression in cell proliferation (r²=0.867, P=0.007). This decrease in RAD21 mRNA expression was accompanied by a parallel induction of apoptosis. We postulate that the influence of the key mitotic regulators, PTTG and separase in fetal embryonal NT2 cells reflects a RAD21-dependent mechanism, and that RAD21 plays an integral role in the development of the fetal brain.