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 cells accumulate in a C-mitosis. For many cell types, however, including rat kangaroo kidney PtK(1) 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 delayed from entering division. To differentiate between these possibilities, we fixed PtK(1) cultures 0, 90 and 270 minutes after treatment with nocodazole, colcemid, lumi-colcemid, taxol or cytochalasin D. After 90 minutes, we found that the numbers of prophase cells in cultures treated with nocodazole or colcemid were reduced by approximately 80% relative to cultures treated with lumi-colcemid, cytochalasin D or taxol. Thus, destroying microtubules delays 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 mid-prophase cells were treated with nocodazole, the majority (70%) decondensed their chromosomes and returned to G(2) before re-entering and completing prophase 3-10 hours later. Thus, a pathway exists in vertebrates that delays the G(2)-M transition when microtubules are disassembled during the terminal stages of G(2). As this pathway induces mid-prophase cells to transiently decondense their chromosomes, it is likely that it downregulates the cyclin A-cyclin-dependent kinase 2 (CDK2) complex, which is required in vertebrates for the early stages of prophase [4].