Involvement of DNA topoisomerase II beta in neuronal differentiation

Two isoforms of DNA topoisomerase II (topo II) have been identified in mamm alian cells. While topo II alpha is essential for chromosome segregation in mitotic cells, in vivo function of topo II beta remains to be clarified. H ere we demonstrate that the nucleoplasmic topo II beta, highly expressed in differentiating cerebellar neurons, is the catalytically competent entity operating directly on chromatin DNA in vivo. When the cells reached termina l differentiation, this in vivo activity decreased to a negligible level wi th concomitant loss of the nucleoplasmic enzyme. Effects of topo II-specifi c inhibitors were analyzed in a primary culture of cerebellar granule neuro ns that can mimic the in vivo situation. Only the beta isoform was expresse d in granule cells differentiating in vitro. ICRF-193, a catalytic topo II inhibitor, suppressed the transcriptional induction of amphiphysin I which is essential for mature neuronal activity. The effect decreased significant ly as the cells differentiate. Expression profiling with a cDNA macroarray showed that 18% of detectable transcripts were up-regulated during the diff erentiation and one-third of them were susceptible to ICRF-193. The results suggest that topo II beta is involved in an early stage of granule cell di fferentiation by potentiating inducible neuronal genes to become transcriba ble probably through alterations in higher order chromatin structure.