Cell-Cycle Progression without an Intact Microtubule Cytoskeleton

For mammalian somatic cells, the importance of microtubule cytoskeleton integrity during interphase cell-cycle progression is uncertain. The loss, suppression, or stabilization of the microtubule cytoskeleton has been widely reported to cause a G1 arrest in a variable, and often high, proportion of cell populations, suggesting the existence of a "microtubule damage," "microtubule integrity," or "postmitotic" checkpoint in G1 or G2. We found that when normal human cells (hTERT RPE1 and primary fibroblasts) are continuously exposed to nocodazole, they remain in mitosis for 10-48 hr before they slip out of mitosis and arrest in G1; this finding is consistent with previous reports. To eliminate the persistent effects of prolonged mitosis, we isolated anaphase-telophase cells that were just finishing a mitosis of normal duration, then we rapidly and completely disassembled microtubules by chilling the preparations to 0 degrees C for 10 minutes in the continuous presence of nocodazole or colcemid treatment to ensure that the cells entered G1 without a microtubule cytoskeleton. Without microtubules, cells progressed from anaphase to a subsequent mitosis with essentially normal kinetics. Similar results were obtained for cells in which the microtubule cytoskeleton was partially diminished by lower nocodazole doses or augmented and stabilized with taxol. Thus, after a preceding mitosis of normal duration, the integrity of the microtubule cytoskeleton is not subject to checkpoint surveillance, nor is it required for the normal human cell to progress through G1 and the remainder of interphase.