Microtubule disassembly delays the G2-M transition in vertebrates

When cell cultures in growth are treated with drugs that cause microtubules to disassemble, the mitotic index (MI) progressively increases as the cell s accumulate in a C-mitosis. For many cell types, however, including rat ka ngaroo kidney PtK1 cells, the MI does not increase during the first several hours of treatment [1-3] (Figure 1). This 'lag' implies either that cells are entering mitosis but rapidly escaping the block, or that they are delay ed from entering division. To differentiate between these possibilities, we fixed PtK1 cultures 0, 90 and 270 minutes after treatment with nocodazole, colcemid, lumi-colcemid, taxol or cytochalasin D. After 90 minutes, we fou nd that the numbers of prophase cells in cultures treated with nocodazole o r colcemid were reduced by similar to 80% relative to cultures treated with lumi-colcemid, cytochalasin D or taxol. Thus, destroying microtubules dela ys late G(2) cells from entering prophase and, as the MI does not increase during this time, existing prophase cells do not enter prometaphase. When m id-prophase cells were treated with nocodazole, the majority (70%) deconden sed their chromosomes and returned to G(2) before re-entering and completin g prophase 3-10 hours later. Thus, a pathway exists in vertebrates that del ays the G(2)-M transition when microtubules are disassembled during the ter minal stages of G(2). As this pathway induces mid-prophase cells to transie ntly decondense their chromosomes, it is likely that it downregulates the c yclin A-cyclin dependent kinase 2 (CDK2) complex, which is required in vert ebrates for the early stages of prophase[4].