lso be included in `native’ culture screens along with other agents that were considered inactive in standard culture conditions. Because of their dependency on MPF, the phosphatase inhibitors preferentially induce PCC in G2-phase. Here, we present the unusual morphological changes induced by LSA in human cells and focus on a previously unnoticed nuclear modification: the massive, rapid and reversible chromosome condensation induced by LSA at all phases of the cell cycle. We differentiate LSA from the known PCC inducer okadaic acid and identify key biochemical and epigenetic components of LSAinduced PCC. Cell Cycle Volume 11 Issue 23 RePORT RePORT Results Lasonolide A induces rapid, extensive and reversible premature chromosome condensation. To observe the changes in cellular chromatin architecture after LSA treatment, we stained cellular DNA with propidium iodide in Burkitt lymphoma CA46 cells. Within 1 h treatment at low 25 nM concentrations, LSA modified PI staining and the overall nuclear shape. Nuclei became circular instead of the lobular shape of untreated cells, as chromatin and chromosome condensations were induced by LSA. At 100 nM LSA, 97.5% of the nuclei exhibited PTK/ZK web condensed chromatin or chromosomes together with an overall rounded nuclear shape. Time-course experiments showed that chromosome condensation was induced within 30 min exposure, and that the effects of LSA on chromosome and chromatin condensation were reversible. As shown in Cells were treated with 100 nM Lasonolide A for 1 h. PCC efficiency was determined as the percentage of cells with condensed chromosomes by immunofluorescence microscopy after nuclear staining with propidium iodide. bivalent chromosomes that were well-spread. To further establish that LSA induces chromosome condensation independently of the cell cycle, the effects of LSA were determined on quiescent human peripheral blood lymphocytes. LSA-treated lymphocytes showed 35.5% of the cells with condensed chromatin. These results demonstrate that LSA is a potent inducer of premature chromosome condensation in all phases of the cell cycle and is effective even in quiescent G0 cells. Cellular morphological and structural changes induced by LSA in association with premature chromosome condensation. To further characterize the nuclear changes induced by LSA, we performed electron microscopy. Extensive morphological and structural changes were induced by LSA besides PCC induction. Transmission electron microscopy confirmed that nuclei lost their lobulated contour. They also lost their condensed peripheral chromatin regions, while the overall chromatin PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/19822663 was condensed with enlarged inter-chromosomal space. We also noted the appearance of an outer membrane structure around the nuclear envelope whose significance is unclear. Scanning electron microscopy showed marked smoothening of the plasma membrane, consistent with the transmission electron microscopy images. We also looked at the cellular skeleton networks. Microtubule fibers that cross 
the cell body and the tubulin network node in the center of cells were removed by LSA. LSA also rearranged the actin network by inducing the disappearance of the outer actin stress fibers, while the -tubulin and actin filaments were aggregated around the cell membrane. Together, these morphological studies demonstrate that LSA induces a broad range of cellular changes in association with PCC. In the rest of the study, we will focus on the pathways that are connected with t
