Final Report Summary - NOANEUPLOIDY (Mechanisms that prevent aneuploidy)
The mechanisms that safeguard cells against aneuploidy are of great interest as aneuploidy contribute to tumorigenesis. Using live imaging approaches, we have identified two novel mechanisms that permit the accurate transmission of chromosomes during cell division.
1. The first mechanism involves the faithful segregation of damaged chromosomes.
The presence of DNA double-strand breaks during mitosis is particularly challenging for the cell as it produces broken chromosomes lacking a centromere. This situation can cause genomic instability due to improper segregation of the broken fragments into daughter cells. We have previously uncovered a process by which broken chromosomes are faithfully transmitted, via the BubR1 and Polo-dependent tethering of the two broken chromosome ends. We now demonstrated that BubR1 requires interaction with Bub3 to localize on the broken chromosome fragments and to mediate their proper segregation. We also found that Cdc20, a co-factor of the E3 ubiquitin ligase Anaphase-Promoting-Complex/Cyclosome (APC/C), accumulates on DNA lesions in a BubR1 KEN box-dependent manner. A biosensor for APC/C activity revealed a BubR1-dependent local inhibition of APC/C around the segregating broken chromosomes. We therefore propose that the Bub3/BubR1 complex on broken DNA inhibits the APC/C locally via the sequestration of Cdc20, thus promoting proper transmission of broken chromosomes.
2. The second mechanism involves the coordination of chromosome segregation with cell cleavage.
Chromosome segregation must be coordinated with cell division to ensure proper transmission of the genetic material into daughter cells. Our group identified a mechanism by which Drosophila neuroblats coordinate chromosome segregation with cell cleavage. Cells adapt to trailing chromatids by transiently elongating, thus clearing trailing chromatids from the midzone during anaphase. We now provide new insights into the mechanisms underlying adaptive cell elongation. Cells with trailing chromatids undergo two phases of adaptive elongation. The first phase relies on the establishment of a wide contractile ring. The second phase requires an unsuspected myosin outward flux from the contractile ring toward the polar cortex. We found that myosin efflux is a novel feature of cytokinesis as it occurs in all cell types examined regardless of the presence of trailing chromatids. In control cells, the time of myosin efflux is brief and myosin disappears from the cortex upon completion of nuclear envelope reformation (NER). In contrast, cells with trailing chromatids exhibit an extended duration of myosin efflux concomitant with a delay in NER. During the prolonged efflux, myosin reorganizes transiently into ectopic broad contractile rings, providing forces for the second adaptive cell elongation. A decrease in RhoGEF Pebble/Ect2 activity impedes myosin efflux and adaptive cell elongation. Conversely, the preclusion of Pebble nuclear sequestration at telophase induces prolonged myosin efflux, cell elongation and extensive blebbing, even in the absence of trailing chromatid arms. We propose that the coordination of chromatid segregation, myosin efflux and NER allows adaptive cell elongation to clear chromatid arms from the cleavage plane before the completion of cytokinesis.
1. The first mechanism involves the faithful segregation of damaged chromosomes.
The presence of DNA double-strand breaks during mitosis is particularly challenging for the cell as it produces broken chromosomes lacking a centromere. This situation can cause genomic instability due to improper segregation of the broken fragments into daughter cells. We have previously uncovered a process by which broken chromosomes are faithfully transmitted, via the BubR1 and Polo-dependent tethering of the two broken chromosome ends. We now demonstrated that BubR1 requires interaction with Bub3 to localize on the broken chromosome fragments and to mediate their proper segregation. We also found that Cdc20, a co-factor of the E3 ubiquitin ligase Anaphase-Promoting-Complex/Cyclosome (APC/C), accumulates on DNA lesions in a BubR1 KEN box-dependent manner. A biosensor for APC/C activity revealed a BubR1-dependent local inhibition of APC/C around the segregating broken chromosomes. We therefore propose that the Bub3/BubR1 complex on broken DNA inhibits the APC/C locally via the sequestration of Cdc20, thus promoting proper transmission of broken chromosomes.
2. The second mechanism involves the coordination of chromosome segregation with cell cleavage.
Chromosome segregation must be coordinated with cell division to ensure proper transmission of the genetic material into daughter cells. Our group identified a mechanism by which Drosophila neuroblats coordinate chromosome segregation with cell cleavage. Cells adapt to trailing chromatids by transiently elongating, thus clearing trailing chromatids from the midzone during anaphase. We now provide new insights into the mechanisms underlying adaptive cell elongation. Cells with trailing chromatids undergo two phases of adaptive elongation. The first phase relies on the establishment of a wide contractile ring. The second phase requires an unsuspected myosin outward flux from the contractile ring toward the polar cortex. We found that myosin efflux is a novel feature of cytokinesis as it occurs in all cell types examined regardless of the presence of trailing chromatids. In control cells, the time of myosin efflux is brief and myosin disappears from the cortex upon completion of nuclear envelope reformation (NER). In contrast, cells with trailing chromatids exhibit an extended duration of myosin efflux concomitant with a delay in NER. During the prolonged efflux, myosin reorganizes transiently into ectopic broad contractile rings, providing forces for the second adaptive cell elongation. A decrease in RhoGEF Pebble/Ect2 activity impedes myosin efflux and adaptive cell elongation. Conversely, the preclusion of Pebble nuclear sequestration at telophase induces prolonged myosin efflux, cell elongation and extensive blebbing, even in the absence of trailing chromatid arms. We propose that the coordination of chromatid segregation, myosin efflux and NER allows adaptive cell elongation to clear chromatid arms from the cleavage plane before the completion of cytokinesis.