The ABC of the cell cycle : roles of the mammalian Cdc25 isoforms

Cell cycle progression is mediated by Cyclin dependent kinases (Cdks) in complex with Cyclins. The activity of the Cyclin-Cdk complexes is regulated through a variety of levels: expression and proteolysis of Cyclins, binding of inhibitors, phosphorylations and subcellular localization. One of the key Cyclin-Cdk activating steps, is the removal of inhibitory phosphorylations by the Cdc25 family of dual-specificity phosphatases. This thesis is dedicated to unravel some of the roles of mammalian Cdc25A, Cdc25B and Cdc25C.

The localization of Cdc25A has long been proposed as nuclear. However, both Cdc25B and Cdc25C has been shown to reside in the nucleus and to some extent in the cytoplasm. Therefore, we sought to characterize Cdc25A localization. We find that Cdc25A is in fact a dynamic protein that shuttles between the nucleus and cytoplasm. Furthermore, we characterize an NES, but surprisingly find the export to be Exportin 1 independent. We also find that Cdc25As nuclear localization is dependent on an NLS.

By reducing levels of Cdc25A, Cd25B and Cd25C individually or in combination, using RNA interference, we find that Cdc25A and Cd25B cooperate to induce mitosis. Down regulation of Cdc25C, however, does not influence mitotic entry. We observe that a G2 delay caused by down regulated levels of Cdc25A or Cdc25B is accompanied by reduced activities of both Cyclin B1-Cdk1 and Cyclin A-Cdk2 complexes. In addition, by using 3D time-lapse microscopy, we see that, Cdc25A seems to have a nuclear role and that Cd25B regulates centrosome separation. In addition, siRNA toward Cdc25B leads to higher levels of phosphorylated, centrosome-associated Cyclin B-Cdk1, implying that Cdc25B is involved in the process of activating Cyclin B-Cdk1 on the centrosome.

The surprising finding that Cdc25C does not affect mitotic entry, lead us to investigate the role of Cd25C in mitosis. We used RNA interference to down regulate Cdc25C and monitored mitotic progression, using 3D time-lapse microscopy. We find that the time of mitotic progression of Cdc25C reduced cells is equivalent to that of control cells. Furthermore, we find that Cyclin B-Cdk1 activity is. uninfluenced by siRNA reduced Cdc25C. We conclude that Cdc25C is not necessary for mitotic progression in our system.

List of scientific papers

I. Kallstrom H, Lindqvist A, Pospisil V, Lundgren A, Rosenthal CK (2005). Cdc25A localisation and shuttling: characterisation of sequences mediating nuclear export and import. Exp Cell Res. 303(1): 89-100.
https://pubmed.ncbi.nlm.nih.gov/15572030

II. Lindqvist A, Kallstrom H, Lundgren A, Barsoum E, Rosenthal CK (2005). Cdc25B cooperates with Cdc25A to induce mitosis but has a unique role in activating cyclin B1-Cdk1 at the centrosome. J Cell Biol. 171(1): 35-45.
https://pubmed.ncbi.nlm.nih.gov/16216921

III. Lundgren A, Lindqvist A, Karlsson Rosenthal C (2006). Mitotic progression is undisturbed by reduced levels of Cdc25C. [Manuscript]