Immunohistochemical analyses of DNA topoisomerase II isoforms in developing rat cerebellum

In mammalian cells, two isoforms of DNA topoisomerase II (topo II alpha and topo II beta) have been identified. Topo II alpha is essential in mitotic cells, whereas the function of topo II beta remains unclear. In the present study, we investigated the developmental control of topo II isoforms in tw o different neuronal lineages, cerebellar Purkinje cells and granule cells, by immunohistochemical analysis with isoform-specific monoclonal antibodie s. As expected, proliferating cells in the neuroepithelium and in the exter nal germinal layer (EGL) were topo II alpha immunopositive. The migrating a s well as differentiating Purkinje cells and granule cells showed an enhanc ed topo II beta immunoreactivity. The postmitotic granule cells in the post natal EGL showed an abrupt transition of expressed topo II isoforms from I I alpha to II beta. The transition was clearly coincident with the completi on of final cell division and the initiation of terminal differentiation be cause no increase of the topo II beta immunoreactivity was observed in the spreading EGL cells that are still in the cell division cycle. The topo II beta signal was detected in both nucleoplasm and nucleolus of differentiati ng cells. However, the nucleoplas mic signal decreased significantly as the cells reached terminal differentiation. The residual topo II beta in nucle oli was shown to occupy an unique location with respect to other nucleolar proteins, nucleolin and DNA topoisomerase I. Our findings indicate that bot h Purkinje cells and granule cells express the topo II isoforms in a simila r timing during the cerebellar development and also suggest that topo II be ta localized in nucleoplasm is the functional entity involved in neuronal d ifferentiation. J. Comp. Neurol. 431:228-239, 2001. (C) 2001 Wiley-Liss, In c.