A Cell Cycle Timer for Asymmetric Spindle Positioning
Creators: name: Werts, Adam D
name: McCarthy Campbell, Erin K.
name: Goldstein, Bob
Publisher: name: The University of North Carolina at Chapel Hill University Libraries
Year: 2009

description: The displacement of the mitotic spindle to one side of a cell is important for many cells to divide unequally. While recent progress has begun to unveil some of the molecular mechanisms of mitotic spindle displacement, far less is known about how spindle displacement is precisely timed. A conserved mitotic progression mechanism is known to time events in dividing cells, although this has never been linked to spindle displacement. This mechanism involves the anaphase-promoting complex (APC), its activator Cdc20/Fizzy, its degradation target cyclin, and cyclin-dependent kinase (CDK). Here we show that these components comprise a previously unrecognized timer for spindle displacement. In the Caenorhabditis elegans zygote, mitotic spindle displacement begins at a precise time, soon after chromosomes congress to the metaphase plate. We found that reducing the function of the proteasome, the APC, or Cdc20/Fizzy delayed spindle displacement. Conversely, inactivating CDK in prometaphase caused the spindle to displace early. The consequence of experimentally unlinking spindle displacement from this timing mechanism was the premature displacement of incompletely assembled components of the mitotic spindle. We conclude that in this system, asymmetric positioning of the mitotic spindle is normally delayed for a short time until the APC inactivates CDK, and that this delay ensures that the spindle does not begin to move until it is fully assembled. To our knowledge, this is the first demonstration that mitotic progression times spindle displacement in the asymmetric division of an animal cell. We speculate that this link between the cell cycle and asymmetric cell division might be evolutionarily conserved, because the mitotic spindle is displaced at a similar stage of mitosis during asymmetric cell divisions in diverse systems.Author SummaryThroughout animal development, and in stem cells, many cell divisions are asymmetric. The one-cell-stage C. elegans embryo divides asymmetrically, as a result of a displacement of the mitotic spindle to one side of the cell. As in other cell divisions, a mitotic progression machinery ensures that all chromosomes are associated with the metaphase plate before anaphase begins. This machinery involves the anaphase-promoting complex and its activator Cdc20/Fizzy, which target proteins for destruction by the proteasome; the cyclin that is targeted for degradation by the proteasome; and a cyclin-dependent kinase. We have asked whether the same machinery has a second function, delaying movement of the spindle to an asymmetric position until spindle assembly is complete. To address this question, we used genetic, reverse genetic, and pharmacological techniques to disrupt the function of elements of the mitotic progression machinery. We find that the mitotic progression machinery does indeed time spindle positioning, acting to delay spindle displacement until spindle assembly completes. This demonstrates a previously unrecognized link between the mitotic progression machinery and asymmetric spindle positioning in an animal cell.The machinery that times entry into mitotic anaphase has the extra function during an asymmetric cell division of regulating when the mitotic spindle can shift to one side of a cell.