The G2-to-M transition from a phosphatase perspective: a new vision of the meiotic division

Abstract

Cell division is orchestrated by the phosphorylation and dephosphorylation of thousands of proteins. These post-translational modifications underlie the molecular cascades converging to the activation of the universal mitotic kinase, Cdk1, and entry into cell division. They also govern the structural events that sustain the mechanics of cell division. While the role of protein kinases in mitosis has been well documented by decades of investigations, little was known regarding the control of protein phosphatases until the recent years. However, the regulation of phosphatase activities is as essential as kinases in controlling the activation of Cdk1 to enter M-phase. The regulation and the function of phosphatases result from post-translational modifications but also from the combinatorial association between conserved catalytic subunits and regulatory subunits that drive their substrate specificity, their cellular localization and their activity. It now appears that sequential dephosphorylations orchestrated by a network of phosphatase activities trigger Cdk1 activation and then order the structural events necessary for the timely execution of cell division. This review discusses a series of recent works describing the important roles played by protein phosphatases for the proper regulation of meiotic division. Many breakthroughs in the field of cell cycle research came from studies on oocyte meiotic divisions. Indeed, the meiotic division shares most of the molecular regulators with mitosis. The natural arrests of oocytes in G2 and in M-phase, the giant size of these cells, the variety of model species allowing either biochemical or imaging as well as genetics approaches explain why the process of meiosis has served as an historical model to decipher signalling pathways involved in the G2-to-M transition. The review especially highlights how the phosphatase PP2A-B55δ critically orchestrates the timing of meiosis resumption in amphibian oocytes. By opposing the kinase PKA, PP2A-B55δ controls the release of the G2 arrest through the dephosphorylation of their substrate, Arpp19. Few hours later, the inhibition of PP2A-B55δ by Arpp19 releases its opposing kinase, Cdk1, and triggers M-phase. In coordination with a variety of phosphatases and kinases, the PP2A-B55δ/Arpp19 duo therefore emerges as the key effector of the G2-to-M transition.

Keywords: Cdk1; Cell division; Meiotic division; Oocyte; PKA; PP2A; Protein kinases; Protein phosphatases; Protein phosphorylation.

Fig. 1

The three transitions of the mitotic cycle involving Cdk1-Cyclin B. During the G2-phase, Cyclin B accumulates and binds to Cdk1 that is phosphorylated at T161 by CAK and at Y15 and T14 by Wee1/Myt1. The complex is thus inactive. At the G2-to-M transition, Wee1/Myt1 are turned off while Cdc25 is turned on, promoting Y15 and T14 dephosphorylations and Cdk1 activation. During M-phase, Cdk1-Cyclin B phosphorylates many mitotic substrates, including Wee1/Myt1 and Cdc25. At the end of M-phase, MPF activates the Cyclin proteolytic degradation system, thus inducing its own inactivation

Fig. 2

Resumption of Xenopus oocyte meiosis. G2-arrested oocytes contain pre-MPF, i.e. inactive Cdk1-Cyclin B complexes in which Cdk1 is inhibited by T14 and Y15 phosphorylation. Progesterone initiates a signalling pathway that leads to T14-Y15 dephosphorylation of Cdk1. MPF promotes the breakdown of the nuclear envelope (GVBD for germinal vesicle breakdown, identified by a white spot at the pole of the brown hemisphere) and formation of the metaphase I spindle. After extrusion of the first polar body, the metaphase II spindle is organized and the oocyte arrests at this stage, until fertilization. Top: two pictures of Xenopus oocytes, G2 arrest (left) and GVBD (right)

Fig. 3

From progesterone to Cdk1 activation: the scattered elements of a not fully understood signalling pathway

Fig. 4

Molecular circuitry of the Cdk1 positive feedback loop. In G2-arrested oocytes, Cdk1-Cyclin B is stockpiled in an inactive state, with Cdk1 phosphorylated at T14 and Y15 by Myt1, while Cdc25 is inactive. PP2A-B55δ maintains Cdc25 and Myt1 under a dephosphorylated state. The first active molecules of Cdk1 reverse the balance by phosphorylating Myt1 and Cdc25. They also recruit other kinases that facilitate further phosphorylation of Myt1 and Cdc25. Finally, Cdk1 indirectly inactivates PP2A-B55δ through Gwl and S67-phosphorylated Arpp19

Fig. 5

Generating a Cdk1 starter to launch the Cdk1 feedback loop. In response to progesterone, Cyclin B1 accumulates, binds monomeric Cdk1 and forms Cdk1-Cyclin B1 complexes that escape Myt1 inhibition and cause inactivating phosphorylation of Myt1. Hence, these few active complexes serve as a switch to initiate the MPF activation loop from the stockpiled Cdk1-Cyclin B2 complexes

Fig. 6

At the top of the signalling pathway, the critical Arpp19 dephosphorylation at S109 by PP2A-B55δ. The G2 lock is achieved by high cAMP, high PKA activity and Arpp19 phosphorylation at S109. Arpp19 phosphorylation turns over due to PKA and PP2A-B55δ, with PP2A-B55δ being swamped by PKA. Upon PKA inhibition promoted by progesterone, Arpp19 is dephosphorylated at S109 by PP2A-B55δ and launches the signalling pathway that ends with Cdk1 activation. The targets of Arpp19 are still unknown. It could control the synthesis of new proteins required to activate Cdk1, such as Cyclin B1 or Mos