The spindle assembly checkpoint ensures accurate chromosome segregation by delaying anaphase initiation until all chromosomes are properly attached to the mitotic spindle. Here, we show that the previously reported c-Jun amino-terminal kinase (JNK) inhibitor SP600125 effectively disrupts spindle checkpoint function in a JNK-independent fashion. SP600125 potently inhibits activity of the mitotic checkpoint kinase monopolar spindle 1 (Mps1) in vitro and triggers efficient progression through a mitotic arrest imposed by spindle poisons. Importantly, expression of an Mps1 mutant protein refractory to SP600125-mediated inhibition restores spindle checkpoint function in the presence of SP600125, showing that its mitotic phenotype is induced by Mps1 inhibition in vivo. Remarkably, primary human cells are largely resistant to the checkpoint-inactivating action of SP600125, suggesting the existence of Mps1-independent checkpoint pathways that are compromised in tumour cells.

Acquisition of lineage-specific cell cycle duration is an important feature of metazoan development. In Caenorhabditis elegans, differences in cell cycle duration are already apparent in two-cell stage embryos, when the larger anterior blastomere AB divides before the smaller posterior blastomere P 1 . This time difference is under the control of anterior-posterior (A-P) polarity cues set by the PAR proteins. The mechanisms by which these cues regulate the cell cycle machinery differentially in AB and P 1 are incompletely understood. Previous work established that retardation of P 1 cell division is due in part to preferential activation of an ATL-1/CHK-1 dependent checkpoint in P 1 , but how the remaining time difference is controlled is not known. Here, we establish that differential timing relies also on a mechanism that promotes mitosis onset preferentially in AB. The polo-like kinase PLK-1, a positive regulator of mitotic entry, is distributed in an asymmetric manner in two-cell stage embryos, with more protein present in AB than in P 1 . We find that PLK-1 asymmetry is regulated by A-P polarity cues through preferential protein retention in the embryo anterior. Importantly, mild inactivation of plk-1 by RNAi delays entry into mitosis in P 1 , but not in AB, in a manner that is independent of ATL-1/CHK-1. Together, our findings support a model in which differential timing of mitotic entry in C. elegans embryos relies on two complementary mechanisms: ATL-1/CHK-1-dependent preferential retardation in P 1 and PLK-1-dependent preferential promotion in AB, which together couple polarity cues and cell cycle progression during early development.

The core machinery that drives the eukaryotic cell cycle has been thoroughly investigated over the course of the past three decades. It is only more recently, however, that light has been shed on the mechanisms by which elements of this core machinery are modulated to alter cell cycle progression during development. It has also become increasingly clear that, conversely, core cell cycle regulators can play a crucial role in developmental processes. Here, focusing on findings from Drosophila melanogaster and Caenorhabditis elegans, we review the importance of modulating the cell cycle during development and discuss how core cell cycle regulators participate in determining cell fates. IntroductionExtensive studies have led to a thorough understanding of the core mechanisms that drive the eukaryotic cell cycle (Box 1). It has also become increasingly clear that these core mechanisms are modulated during development. Such modulation is important, for instance, during Drosophila eye organogenesis, in which cell cycle synchronization is crucial for photoreceptor fate determination.Interestingly, recent findings demonstrate that components of the cell cycle machinery can in turn regulate development independently of their roles in cell cycle progression. For example, the cell cycle regulators Polo-like kinase 1 (PLK-1) and PLK-2 contribute to fate determination in early C. elegans embryos by phosphorylating proteins involved in the establishment of cell polarity. Overall, these and other observations underscore the tight coupling between the cell cycle and development.In this review, we first provide an overview of the core features of the eukaryotic cell cycle. We then discuss mechanisms by which the cell cycle is modulated in different developmental settings, from the early embryo through to terminal cell differentiation. Finally, we consider how cell cycle regulators can in turn impart cell fate during development. The focus of this review is on D. melanogaster and C. elegans, two organisms in which the coupling of cell cycle progression and development has been well studied, with other systems included where appropriate. For additional information, we refer readers to reviews on the cell cycle that cover related material, including the link between cell cycle progression and cell growth or cancer, as well as the role of cell cycle modulation in plant development (see Barton et al., 2006; De Veylder et al., 2007; Giacinti and Giordano, 2006;Johnson and Degregori, 2006;Leevers and McNeill, 2005;Potter and Xu, 2001;Stanger, 2008). The core eukaryotic cell cycleThe core eukaryotic cell cycle, which operates in most somatic cells, is composed of a synthesis (S) phase, a mitotic (M) phase and two intervening gap phases (G1 and G2; see Box 1). The core engines that drive the eukaryotic cell cycle consist of protein heterodimer complexes, each containing a cyclin and an associated kinase moiety. This group of kinases is referred to as cyclin-dependent kinases (Cdks), as kinase activity requires the presence of the cyclin ...

The early Caenorhabditis elegans embryo is an attractive model to investigate evolutionarily conserved cellular mechanisms. However, there is a paucity of automated methods to gather quantitative information with subcellular precision in this system. We developed ASSET (Algorithm for the Segmentation and the Standardization of C. elegans Time-lapse recordings) to fill this need. ASSET automatically detects the eggshell and the cell cortex from DIC time-lapse recordings of live one-cell-stage embryos and can also track subcellular structures using fluorescent time-lapse microscopy. Importantly, ASSET standardizes the data into an absolute coordinate system to allow robust quantitative comparisons between embryos. We illustrate how ASSET can efficiently gather quantitative data on the motion of centrosomes and precisely track cortical invaginations, revealing hitherto unnoticed differences between wild-type and saps-1(RNAi) embryos. In summary, we establish ASSET as a novel tool for the efficient quantification and standardization of images from early C. elegans embryos. Developmental Dynamics 239:3285-3296,

Tumor suppressor p53 plays an integral role in DNA-damage induced apoptosis, a biological process that protects against tumor progression. Cell shape dramatically changes when cells undergo apoptosis, which is associated with actomyosin contraction; however, it remains entirely elusive how p53 regulates actomyosin contraction in response to DNA-damaging agents. To identify a novel p53 regulating gene encoding the modulator of myosin, we conducted DNA microarray analysis. We found that, in response to DNA-damaging agent doxorubicin, expression of myotonic dystrophy protein kinase (DMPK), which is known to upregulate actomyosin contraction, was increased in a p53-dependent manner. The promoter region of DMPK gene contained potential p53-binding sequences and its promoter activity was increased by overexpression of the p53 family protein p73, but, unexpectedly, not of p53. Furthermore, we found that doxorubicin treatment induced p73 expression, which was significantly attenuated by downregulation of p53. These data suggest that p53 induces expression of DMPK through upregulating p73 expression. Overexpression of DMPK promotes contraction of the actomyosin cortex, which leads to formation of membrane blebs, loss of cell adhesion, and concomitant caspase activation. Taken together, our results suggest the existence of p53-p73-DMPK axis which mediates DNA-damage induced actomyosin contraction at the cortex and concomitant cell death.