Selective dephosphorylation by PP2A-B55 directs the meiosis I - meiosis II transition in oocytes

Samuel J.P. Taylor & Federico Pelisch

School of Life Sciences – University of Dundee

Background

Oocytes in many organisms, including humans, exist in a prolonged state of arrest during meiotic prophase until stimulated to resume cell division (1). Once stimulated, the oocyte undergoes two successive segregation events to generate haploid gametes. These then fuse with the haploid sperm to begin mitotic divisions consisting of diploid genetic material. The progression from oocyte prophase arrest to embryonic mitotic division must be conducted in an organised and timely manner in order to maintain the integrity of cell division and produce viable gametes. Whilst the importance of phosphorylation and dephosphorylation during cell division has been studied for decades (2), a wider understanding of the global phosphoproteome dynamics throughout the transition from oocyte to embryo remained unclear. In this paper, Swartz et al., investigated the global phosphoproteomic changes that take place during the oocyte to embryo transition. Interestingly, this showed that PP2A:B55 plays a key role in dephosphorylation events required to transition from meiosis I to meiosis II. Furthermore, the preference of PP2A:B55 for phospho-threonines plays an important role in the timely dephosphorylation of substrate proteins during this meiotic transition, as has previously been shown to be important in the regulation of mitotic exit (3-5). 

Key Findings

• Protein levels are mostly stable throughout Meiosis: Proteomic analysis showed protein levels changed <2-fold in 98.8% of proteins, with some important exceptions such as cyclin B.  
• Phosphorylation levels altered throughout oocyte to embryo transition: Phosphoproteomic analysis showed the changing phosphorylation abundance at different stages of the oocyte to embryo transition, with the lowest point during prophase arrest and peak during meiosis I before falling again for meiosis II (fig. 1). The authors showed that an activating phosphorylation of p42/ERK was not present during prophase arrest but was during meiosis I and II, whilst inhibitory mutations of Cdk1 are high in prophase arrest and low during meiosis. In this manner, phosphorylation regulates the activity of proteins required at different stages within the oocyte to embryo transition.  
• Phosphatase activity regulates phosphorylation state throughout the oocyte to embryo transition: Inhibitory phosphorylations of PP1 and PP2A:B55 are low during prophase and high during meiosis I and meiosis II, respectively. Inhibiting phosphatase activity released prophase arrest but did not allow full meiotic progression due to widespread errors in cell division machinery. As such, the regulation of phosphatase activity is important for maintaining the low phosphorylation state during prophase arrest and allowing enhanced phosphorylation during meiosis.  
• PP2A:B55 primarily responsible for dephosphorylation events at the meiosis I to meiosis II transition: The authors clustered the phosphoproteomic sites that achieve maximum phosphorylation in meiosis I into sites that are rapidly dephosphorylated by meiosis II, and those that are more slowly dephosphorylated throughout meiosis II and the subsequent stages (fig. 1). Those in the rapidly dephosphorylated cluster were enriched for threonine followed by proline with basic amino acids downstream (TPxK) – a PP2A:B55 consensus sequence (4-6). Disruption of the interaction between substrates and the B55 regulatory subunit resulted in successful meiosis I but disrupted meiosis II, highlighting the importance of PP2A:B55 in meiosis II particularly. This was explored further by substituting T61 of starfish INCENP for a serine residue. Dephosphorylation of T61 is required to translocate INCENP to the central spindle in anaphase, the T61S mutant did not translocate to the central spindle – emphasising the importance of the threonine residue for the timely dephosphorylation by PP2A:B55. 

What we liked about the paper 

Although many individual examples of the importance of phosphorylation events at different stages of cell division are known, the global overview of how phosphorylation states change throughout the oocyte to embryo transition, as well as the confirmation that overall protein levels remain relatively stable throughout the transition, will be highly useful information. The essential nature of these phosphorylation state changes in maintaining prophase arrest and the fidelity of meiotic progression, as well as the importance of phosphatase activity in regulating these changes, is clearly a fundamentally important balance required throughout the transition from oocyte to embryo. Furthermore, it was particularly interesting how large an effect altering T61 to serine had on INCENP localisation changes in anaphase, highlighting the threonine and serine are not necessarily interchangeable residues despite their similar properties. This builds on previous work indicating phospho-threonines are preferentially dephosphorylated during mitotic exit (3-5).  

Questions 

• Was Securin dynamics measured at all? 
• What is the localisation of B55 athroughout these stages?  
• Is the enhanced stability of phospho-serines only observed at points where B55 is particularly important or is this a wider characteristic? Could the reason that most sites identified in phosphoproteomic analyses are serine residues (~80%) rather than threonine residues (~20%) reflect phospho-serines being more stable than phospho-threonines in a wider sense?  

References 

1. Von Stetina, J. R., and Orr-Weaver, T. L. (2011) Developmental control of oocyte maturation and egg activation in metazoan models. Cold Spring Harb. Perspect. Biol. 3, a005553–a005553
2. Moura, M., and Conde, C. (2019) Phosphatases in Mitosis: Roles and Regulation. Biomolecules. 9, 55
3. Hein, J. B., Hertz, E. P. T., Garvanska, D. H., Kruse, T., and Nilsson, J. (2017) Distinct kinetics of serine and threonine dephosphorylation are essential for mitosis. Nat. Cell Biol. 19, 1433–1440
4. Cundell, M. J., Hutter, L. H., Nunes Bastos, R., Poser, E., Holder, J., Mohammed, S., Novak, B., and Barr, F. A. (2016) A PP2A-B55 recognition signal controls substrate dephosphorylation kinetics during mitotic exit. J. Cell Biol. 214, 539–554
5. McCloy, R. A., Parker, B. L., Rogers, S., Chaudhuri, R., Gayevskiy, V., Hoffman, N. J., Ali, N., Watkins, D. N., Daly, R. J., James, D. E., Lorca, T., Castro, A., and Burgess, A. (2015) Global Phosphoproteomic Mapping of Early Mitotic Exit in Human Cells Identifies Novel Substrate Dephosphorylation Motifs. Mol. Cell. Proteomics. 14, 2194–2212
6. Kruse, T., Gnosa, S. P., Nasa, I., Garvanska, D. H., Hein, J. B., Nguyen, H., Samsøe-Petersen, J., Lopez-Mendez, B., Hertz, E. P. T., Schwarz, J., Pena, H. S., Nikodemus, D., Kveiborg, M., Kettenbach, A. N., and Nilsson, J. (2020) Mechanisms of site-specific dephosphorylation and kinase opposition imposed by PP2A regulatory subunits. EMBO J. 39, e103695

Tags: meiosis, phosphorylation, pp2a, sea star

doi: https://doi.org/10.1242/prelights.25250

(No Ratings Yet)