Greatwall maintains mitosis through regulation of PP2A

Egg Overactivation-An Overlooked Phenomenon of Gamete Physiology

In many vertebrates, mature ovulated eggs are arrested at metaphase II prior to fertilization. The eggs exit meiotic arrest after fertilization-induced or parthenogenetic activation, followed by embryo development or egg degradation, respectively. Calcium-dependent activation of meiotically-arrested eggs has been thoroughly investigated in various species. In addition, several recent studies have detailed the excessive activation of ovulated frog eggs, so-called overactivation. This overview highlights the major events of overactivation observed in mature ovulated eggs of the African clawed frog Xenopus laevis with a focus on similarities and differences between spontaneous, oxidative stress-induced, and mechanical stress-induced overactivation. The dramatically different cell death scenarios that unfold in activated and overactivated eggs are also exposed in the article.

The MAST kinase KIN-4 carries out mitotic entry functions of Greatwall in C. elegans

MAST-like, or Greatwall (Gwl), an atypical protein kinase related to the evolutionarily conserved MAST kinase family, is crucial for cell cycle control during mitotic entry. Mechanistically, Greatwall is activated by Cyclin B-Cdk1 phosphorylation of a 550 amino acids-long insertion in its atypical activation segment. Subsequently, Gwl phosphorylates Endosulfine and Arpp19 to convert them into inhibitors of PP2A-B55 phosphatase, thereby preventing early dephosphorylation of M-phase targets of Cyclin B-Cdk1. Here, searching for an elusive Gwl-like activity in C. elegans, we show that the single worm MAST kinase, KIN-4, fulfills this function in worms and can functionally replace Greatwall in the heterologous Xenopus system. Compared to Greatwall, the short activation segment of KIN-4 lacks a phosphorylation site, and KIN-4 is active even when produced in E. coli. We also show that a balance between Cyclin B-Cdk1 and PP2A-B55 activity, regulated by KIN-4, is essential to ensure asynchronous cell divisions in the early worm embryo. These findings resolve a long-standing puzzle related to the supposed absence of a Greatwall pathway in C. elegans, and highlight a novel aspect of PP2A-B55 regulation by MAST kinases.

ARPP19 phosphorylation site evolution and the switch in cAMP control of oocyte maturation in vertebrates

cAMP-PKA signaling initiates the crucial process of oocyte meiotic maturation in many animals, but inhibits it in vertebrates. To address this 'cAMP paradox', we exchanged the key PKA substrate ARPP19 between representative species, the vertebrate Xenopus and the cnidarian Clytia, comparing its phosphorylation and function. We found that, as in Xenopus, Clytia maturing oocytes undergo ARPP19 phosphorylation on a highly conserved Gwl site, which inhibits PP2A and promotes M-phase entry. In contrast, despite a PKA phosphorylation signature motif recognizable across most animals, Clytia ARPP19 was only poorly phosphorylated by PKA in vitro and in vivo. Furthermore, unlike Xenopus ARPP19, exogenous Clytia ARPP19 did not delay Xenopus oocyte maturation. We conclude that, in Clytia, ARPP19 does not intervene in oocyte maturation initiation because of both poor recognition by PKA and the absence of effectors that mediate vertebrate oocyte prophase arrest. We propose that ancestral ARPP19 phosphorylated by Gwl has retained a key role in M-phase across eukaryotes and has acquired new functions during animal evolution mediated by enhanced PKA phosphorylation, allowing co-option into oocyte maturation regulation in the vertebrate lineage.

Development of luminescent probes for real-time detection of the CDK/PP2A balance during the cell cycle

From a biochemical viewpoint, the cell cycle is controlled by the phosphorylation of cyclin-dependent kinase (CDK) substrates, and the phosphorylation level is determined by the enzymatic balance between CDK and protein phosphatase 2A (PP2A). However, the conventional techniques for analyzing protein phosphorylation using radioisotopes and antibodies involve many operational steps and take days before obtaining results, making them difficult to apply to high-throughput screening and real-time observations. In this study, we developed luminescent probes with a light intensity that changes depending on its phosphorylation state. We modified the Nano-lantern probe (Renilla luciferase-based Ca2+ probe) by introducing a CDK-substrate peptide and a phosphopeptide-binding domain into the luciferase. Our initial trial resulted in new probes that could report the CDK/PP2A balance in a purified system. Further modifications of these probes (replacing the phospho-Ser with phospho-Thr and randomly replacing its surrounding amino acids) improved the dynamic range by up to four-fold, making them practical for use in the Xenopus egg extracts system, where many physiological events can be reproduced. Taken together, our new probes enabled the monitoring of the CDK/PP2A balance in real time, and are applicable to high-throughput systems; the new probes thus appear promising for use in substrate and drug screening.

A genome-wide CRISPR screen in Anopheles mosquito cells identifies essential genes and required components of clodronate liposome function

Anopheles mosquitoes are the sole vector of human malaria, the most burdensome vector-borne disease worldwide. Strategies aimed at reducing mosquito populations and limiting their ability to transmit disease show the most promise for disease control. Therefore, gaining an improved understanding of mosquito biology, and specifically that of the immune response, can aid efforts to develop new approaches that limit malaria transmission. Here, we use a genome-wide CRISPR screening approach for the first time in mosquito cells to identify essential genes in Anopheles and identify genes for which knockout confers resistance to clodronate liposomes, which have been widely used in mammals and arthropods to ablate immune cells. In the essential gene screen, we identified a set of 1280 Anopheles genes that are highly enriched for genes involved in fundamental cell processes. For the clodronate liposome screen, we identified several candidate resistance factors and confirm their roles in the uptake and processing of clodronate liposomes through in vivo validation in Anopheles gambiae, providing new mechanistic detail of phagolysosome formation and clodronate liposome function. In summary, we demonstrate the application of a genome-wide CRISPR knockout platform in a major malaria vector and the identification of genes that are important for fitness and immune-related processes.

PP2A-B55 phosphatase counteracts Ki-67-dependent chromosome individualization during mitosis

Cell cycle progression is regulated by the orderly balance between kinase and phosphatase activities. PP2A phosphatase holoenzymes containing the B55 family of regulatory B subunits function as major CDK1-counteracting phosphatases during mitotic exit in mammals. However, the identification of the specific mitotic roles of these PP2A-B55 complexes has been hindered by the existence of multiple B55 isoforms. Here, through the generation of loss-of-function genetic mouse models for the two ubiquitous B55 isoforms (B55α and B55δ), we report that PP2A-B55α and PP2A-B55δ complexes display overlapping roles in controlling the dynamics of proper chromosome individualization and clustering during mitosis. In the absence of PP2A-B55 activity, mitotic cells display increased chromosome individualization in the presence of enhanced phosphorylation and perichromosomal loading of Ki-67. These data provide experimental evidence for a regulatory mechanism by which the balance between kinase and PP2A-B55 phosphatase activity controls the Ki-67-mediated spatial organization of the mass of chromosomes during mitosis.

Greatwall-Endos-PP2A/B55Twins network regulates translation and stability of maternal transcripts in the Drosophila oocyte-to-embryo transition

The transition from oocyte to embryo requires translation of maternally provided transcripts that in Drosophila is activated by Pan Gu kinase to release a rapid succession of 13 mitotic cycles. Mitotic entry is promoted by several protein kinases that include Greatwall/Mastl, whose Endosulfine substrates antagonize Protein Phosphatase 2A (PP2A), facilitating mitotic Cyclin-dependent kinase 1/Cyclin B kinase activity. Here we show that hyperactive greatwallScant can not only be suppressed by mutants in its Endos substrate but also by mutants in Pan Gu kinase subunits. Conversely, mutants in me31B or trailer hitch, which encode a complex that represses hundreds of maternal mRNAs, enhance greatwallScant . Me31B and Trailer Hitch proteins, known substrates of Pan Gu kinase, copurify with Endos. This echoes findings that budding yeast Dhh1, orthologue of Me31B, associates with Igo1/2, orthologues of Endos and substrates of the Rim15, orthologue of Greatwall. endos-derived mutant embryos show reduced Me31B and elevated transcripts for the mitotic activators Cyclin B, Polo and Twine/Cdc25. Together, our findings demonstrate a previously unappreciated conservation of the Greatwall-Endosulfine pathway in regulating translational repressors and its interactions with the Pan Gu kinase pathway to regulate translation and/or stability of maternal mRNAs upon egg activation.

CDK1-PP2A-B55 interplay ensures cell cycle oscillation via Apc1-loop300

Cell cycle control relies on a delicate balance of phosphorylation with CDK1 and phosphatases like PP1 and PP2A-B55. Yet, identifying the primary substrate responsible for cell cycle oscillations remains a challenge. We uncover the pivotal role of phospho-regulation in the anaphase-promoting complex/cyclosome (APC/C), particularly through the Apc1-loop300 domain (Apc1-300L), orchestrated by CDK1 and PP2A-B55. Premature activation of PP2A-B55 during mitosis, induced by Greatwall kinase depletion, leads to Apc1-300L dephosphorylation, stalling APC/C activity and delaying Cyclin B degradation. This effect can be counteracted using the B55-specific inhibitor pEnsa or by removing Apc1-300L. We also show Cdc20's dynamic APC/C interaction across cell cycle stages, but dephosphorylation of Apc1-300L specifically inhibits further Cdc20 recruitment. Our study underscores APC/C's central role in cell cycle oscillation, identifying it as a primary substrate regulated by the CDK-PP2A partnership.

Deficiency of mastl, a mitotic regulator, results in cell detachment from developing tissues of zebrafish embryos

To form tissues with unique functions and structures, it is important that the cells that comprise them maintain physical contact. On the other hand, with each mitosis, drastic changes in cell shapes, cell adhesion, and cytoskeletal architecture may cause such contacts to be temporarily weakened, risking improper development and maintenance of tissues. Despite such risks, tissues form properly during normal development. However, it is not well understood whether mitotic abnormalities affect tissue formation. Here, analysis of zebrafish embryos with aberrant mitosis shows that proper progression of mitosis is important to maintain cell contact in developing tissues. By screening mutants with abnormal trunk and tail development, we obtained a mutant with perturbed expression of some tissue-specific genes in embryonic caudal regions. The responsible gene is mastl/gwl, which is involved in progression of mitosis. Analysis focusing on the chordo-neural hinge (CNH), the primordium of axial tissues, shows that cell detachment from the CNH is increased in mastl mutant embryos. Time-lapse imaging reveals that this cell detachment occurs during mitosis. These results suggest that cells are unable to maintain contact due to abnormalities in progression of mitosis in mastl mutants.

Increases in cyclin A/Cdk activity and in PP2A-B55 inhibition by FAM122A are key mitosis-inducing events

Entry into mitosis has been classically attributed to the activation of a cyclin B/Cdk1 amplification loop via a partial pool of this kinase becoming active at the end of G2 phase. However, how this initial pool is activated is still unknown. Here we discovered a new role of the recently identified PP2A-B55 inhibitor FAM122A in triggering mitotic entry. Accordingly, depletion of the orthologue of FAM122A in C. elegans prevents entry into mitosis in germline stem cells. Moreover, data from Xenopus egg extracts strongly suggest that FAM122A-dependent inhibition of PP2A-B55 could be the initial event promoting mitotic entry. Inhibition of this phosphatase allows subsequent phosphorylation of early mitotic substrates by cyclin A/Cdk, resulting in full cyclin B/Cdk1 and Greatwall (Gwl) kinase activation. Subsequent to Greatwall activation, Arpp19/ENSA become phosphorylated and now compete with FAM122A, promoting its dissociation from PP2A-B55 and taking over its phosphatase inhibition role until the end of mitosis.

Endosulfine alpha maintains spindle pole integrity by recruiting Aurora A during mitosis

BACKGROUND

The maintenance of spindle pole integrity is essential for spindle assembly and chromosome segregation during mitosis. However, the underlying mechanisms governing spindle pole integrity remain unclear.

METHODS

ENSA was inhibited by siRNA or MKI-2 treatment and its effect on cell cycle progression, chromosome alignment and microtubule alignment was observed by immunohistochemical staining and western blotting. PP2A-B55α knockdown by siRNA was performed to rescue the phenotype caused by ENSA inhibition. The interaction between ENSA and Aurora A was detected by in situ PLA. Furthermore, orthotopic implantation of 4Tl-luc cancer cells was conducted to confirm the consistency between the in vitro and in vivo relationship of the ENSA-Aurora A interaction.

RESULTS

During mitosis, p-ENSA is localized at the spindle poles, and the inhibition of ENSA results in mitotic defects, such as misaligned chromosomes, multipolar spindles, asymmetric bipolar spindles, and centrosome defects, with a delay in mitotic progression. Although the mitotic delay caused by ENSA inhibition was rescued by PP2A-B55α depletion, spindle pole defects persisted. Notably, we observed a interaction between ENSA and Aurora A during mitosis, and inhibition of ENSA reduced Aurora A expression at the mitotic spindle poles. Injecting MKI-2-sensitized tumors led to increased chromosomal instability and downregulation of the MASTL-ENSA-Aurora A pathway in an orthotopic breast cancer mouse model.

CONCLUSIONS

These findings provide novel insights into the regulation of spindle pole integrity by the MASTL-ENSA-Aurora A pathway during mitosis, highlighting the significance of ENSA in recruiting Aurora A to the spindle pole, independent of PP2A-B55α.

The ATM-E6AP-MASTL axis mediates DNA damage checkpoint recovery

Checkpoint activation after DNA damage causes a transient cell cycle arrest by suppressing cyclin-dependent kinases (CDKs). However, it remains largely elusive how cell cycle recovery is initiated after DNA damage. In this study, we discovered the upregulated protein level of MASTL kinase hours after DNA damage. MASTL promotes cell cycle progression by preventing PP2A/B55-catalyzed dephosphorylation of CDK substrates. DNA damage-induced MASTL upregulation was caused by decreased protein degradation, and was unique among mitotic kinases. We identified E6AP as the E3 ubiquitin ligase that mediated MASTL degradation. MASTL degradation was inhibited upon DNA damage as a result of the dissociation of E6AP from MASTL. E6AP depletion reduced DNA damage signaling, and promoted cell cycle recovery from the DNA damage checkpoint, in a MASTL-dependent manner. Furthermore, we found that E6AP was phosphorylated at Ser-218 by ATM after DNA damage and that this phosphorylation was required for its dissociation from MASTL, the stabilization of MASTL, and the timely recovery of cell cycle progression. Together, our data revealed that ATM/ATR-dependent signaling, while activating the DNA damage checkpoint, also initiates cell cycle recovery from the arrest. Consequently, this results in a timer-like mechanism that ensures the transient nature of the DNA damage checkpoint.

Substrate and phosphorylation site selection by phosphoprotein phosphatases

Dynamic protein phosphorylation and dephosphorylation are essential regulatory mechanisms that ensure proper cellular signaling and biological functions. Deregulation of either reaction has been implicated in several human diseases. Here, we focus on the mechanisms that govern the specificity of the dephosphorylation reaction. Most cellular serine/threonine dephosphorylation is catalyzed by 13 highly conserved phosphoprotein phosphatase (PPP) catalytic subunits, which form hundreds of holoenzymes by binding to regulatory and scaffolding subunits. PPP holoenzymes recognize phosphorylation site consensus motifs and interact with short linear motifs (SLiMs) or structural elements distal to the phosphorylation site. We review recent advances in understanding the mechanisms of PPP site-specific dephosphorylation preference and substrate recruitment and highlight examples of their interplay in the regulation of cell division.

CDC6 as a Key Inhibitory Regulator of CDK1 Activation Dynamics and the Timing of Mitotic Entry and Progression

Timely mitosis is critically important for early embryo development. It is regulated by the activity of the conserved protein kinase CDK1. The dynamics of CDK1 activation must be precisely controlled to assure physiologic and timely entry into mitosis. Recently, a known S-phase regulator CDC6 emerged as a key player in mitotic CDK1 activation cascade in early embryonic divisions, operating together with Xic1 as a CDK1 inhibitor upstream of the Aurora A and PLK1, both CDK1 activators. Herein, we review the molecular mechanisms that underlie the control of mitotic timing, with special emphasis on how CDC6/Xic1 function impacts CDK1 regulatory network in the Xenopus system. We focus on the presence of two independent mechanisms inhibiting the dynamics of CDK1 activation, namely Wee1/Myt1- and CDC6/Xic1-dependent, and how they cooperate with CDK1-activating mechanisms. As a result, we propose a comprehensive model integrating CDC6/Xic1-dependent inhibition into the CDK1-activation cascade. The physiological dynamics of CDK1 activation appear to be controlled by the system of multiple inhibitors and activators, and their integrated modulation ensures concomitantly both the robustness and certain flexibility of the control of this process. Identification of multiple activators and inhibitors of CDK1 upon M-phase entry allows for a better understanding of why cells divide at a specific time and how the pathways involved in the timely regulation of cell division are all integrated to precisely tune the control of mitotic events.

The ATM-E6AP-MASTL axis mediates DNA damage checkpoint recovery

Checkpoint activation after DNA damage causes a transient cell cycle arrest by suppressing CDKs. However, it remains largely elusive how cell cycle recovery is initiated after DNA damage. In this study, we discovered the upregulated protein level of MASTL kinase hours after DNA damage. MASTL promotes cell cycle progression by preventing PP2A/B55-catalyzed dephosphorylation of CDK substrates. DNA damage-induced MASTL upregulation was caused by decreased protein degradation, and was unique among mitotic kinases. We identified E6AP as the E3 ubiquitin ligase that mediated MASTL degradation. MASTL degradation was inhibited upon DNA damage as a result of the dissociation of E6AP from MASTL. E6AP depletion reduced DNA damage signaling, and promoted cell cycle recovery from the DNA damage checkpoint, in a MASTL-dependent manner. Furthermore, we found that E6AP was phosphorylated at Ser-218 by ATM after DNA damage and that this phosphorylation was required for its dissociation from MASTL, the stabilization of MASTL, and the timely recovery of cell cycle progression. Together, our data revealed that ATM/ATR-dependent signaling, while activating the DNA damage checkpoint, also initiates cell cycle recovery from the arrest. Consequently, this results in a timer-like mechanism that ensures the transient nature of the DNA damage checkpoint.

Cyclin A and Cks1 promote kinase consensus switching to non-proline-directed CDK1 phosphorylation

Ordered protein phosphorylation by CDKs is a key mechanism for regulating the cell cycle. How temporal order is enforced in mammalian cells remains unclear. Using a fixed cell kinase assay and phosphoproteomics, we show how CDK1 activity and non-catalytic CDK1 subunits contribute to the choice of substrate and site of phosphorylation. Increases in CDK1 activity alter substrate choice, with intermediate- and low-sensitivity CDK1 substrates enriched in DNA replication and mitotic functions, respectively. This activity dependence is shared between Cyclin A- and Cyclin B-CDK1. Cks1 has a proteome-wide role as an enhancer of multisite CDK1 phosphorylation. Contrary to the model of CDK1 as an exclusively proline-directed kinase, we show that Cyclin A and Cks1 enhance non-proline-directed phosphorylation, preferably on sites with a +3 lysine residue. Indeed, 70% of cell-cycle-regulated phosphorylations, where the kinase carrying out this modification has not been identified, are non-proline-directed CDK1 sites.

The MASTL/PP2A cell cycle kinase-phosphatase module restrains PI3K-Akt activity in an mTORC1-dependent manner

The AKT-mTOR pathway is a central regulator of cell growth and metabolism. Upon sustained mTOR activity, AKT activity is attenuated by a feedback loop that restrains upstream signaling. However, how cells control the signals that limit AKT activity is not fully understood. Here, we show that MASTL/Greatwall, a cell cycle kinase that supports mitosis by phosphorylating the PP2A/B55 inhibitors ENSA/ARPP19, inhibits PI3K-AKT activity by sustaining mTORC1- and S6K1-dependent phosphorylation of IRS1 and GRB10. Genetic depletion of MASTL results in an inefficient feedback loop and AKT hyperactivity. These defects are rescued by the expression of phosphomimetic ENSA/ARPP19 or inhibition of PP2A/B55 phosphatases. MASTL is directly phosphorylated by mTORC1, thereby limiting the PP2A/B55-dependent dephosphorylation of IRS1 and GRB10 downstream of mTORC1. Downregulation of MASTL results in increased glucose uptake in vitro and increased glucose tolerance in adult mice, suggesting the relevance of the MASTL-PP2A/B55 kinase-phosphatase module in controlling AKT and maintaining metabolic homeostasis.

ATM phosphorylates PP2A subunit A resulting in nuclear export and spatiotemporal regulation of the DNA damage response

Ataxia telangiectasia mutated (ATM) is a serine-threonine protein kinase and important regulator of the DNA damage response (DDR). One critical ATM target is the structural subunit A (PR65-S401) of protein phosphatase 2A (PP2A), known to regulate diverse cellular processes such as mitosis and cell growth as well as dephosphorylating many proteins during the recovery from the DDR. We generated mouse embryonic fibroblasts expressing PR65-WT, -S401A (cannot be phosphorylated), and -S401D (phospho-mimetic) transgenes. Significantly, S401 mutants exhibited extensive chromosomal aberrations, impaired DNA double-strand break (DSB) repair and underwent increased mitotic catastrophe after radiation. Both S401A and the S401D cells showed impaired DSB repair (nonhomologous end joining and homologous recombination repair) and exhibited delayed DNA damage recovery, which was reflected in reduced radiation survival. Furthermore, S401D cells displayed increased ERK and AKT signaling resulting in enhanced growth rate further underscoring the multiple roles ATM-PP2A signaling plays in regulating prosurvival responses. Time-lapse video and cellular localization experiments showed that PR65 was exported to the cytoplasm after radiation by CRM1, a nuclear export protein, in line with the very rapid pleiotropic effects observed. A putative nuclear export sequence (NES) close to S401 was identified and when mutated resulted in aberrant PR65 shuttling. Our study demonstrates that the phosphorylation of a single, critical PR65 amino acid (S401) by ATM fundamentally controls the DDR, and balances DSB repair quality, cell survival and growth by spatiotemporal PR65 nuclear-cytoplasmic shuttling mediated by the nuclear export receptor CRM1.

Dephosphorylation in nuclear reassembly after mitosis

In most animal cell types, the interphase nucleus is largely disassembled during mitotic entry. The nuclear envelope breaks down and chromosomes are compacted into separated masses. Chromatin organization is also mostly lost and kinetochores assemble on centromeres. Mitotic protein kinases play several roles in inducing these transformations by phosphorylating multiple effector proteins. In many of these events, the mechanistic consequences of phosphorylation have been characterized. In comparison, how the nucleus reassembles at the end of mitosis is less well understood in mechanistic terms. In recent years, much progress has been made in deciphering how dephosphorylation of several effector proteins promotes nuclear envelope reassembly, chromosome decondensation, kinetochore disassembly and interphase chromatin organization. The precise roles of protein phosphatases in this process, in particular of the PP1 and PP2A groups, are emerging. Moreover, how these enzymes are temporally and spatially regulated to ensure that nuclear reassembly progresses in a coordinated manner has been partly uncovered. This review provides a global view of nuclear reassembly with a focus on the roles of dephosphorylation events. It also identifies important open questions and proposes hypotheses.

The MASTL-ENSA-PP2A/B55 axis modulates cisplatin resistance in oral squamous cell carcinoma

Platinum-based chemotherapy is the standard first-line treatment for oral squamous cell carcinoma (OSCC) that is inoperable, recurrent, or metastatic. Platinum sensitivity is a major determinant of patient survival in advanced OSCC. Here, we investigated the involvement of MASTL, a cell cycle kinase that mediates ENSA/ARPP19 phosphorylation and PP2A/B55 inhibition, in OSCC therapy. Interestingly, upregulation of MASTL and ENSA/ARPP19, and downregulation of PP2A/B55, were common in OSCC. MASTL expression was in association with poor patient survival. In established OSCC cell lines, upregulation of MASTL and ENSA, and downregulation of B55 genes, correlated with cisplatin resistance. We further confirmed that stable expression of MASTL in OSCC cells promoted cell survival and proliferation under cisplatin treatment, in an ENSA-dependent manner. Conversely, deletion of MASTL or ENSA, or overexpression of B55α, sensitized cisplatin response, consistent with increased DNA damage accumulation, signaling, and caspase activation. Moreover, GKI-1, the first-in-class small molecule inhibitor of MASTL kinase, phenocopied MASTL depletion in enhancing the outcome of cisplatin treatment in OSCC cells, at a dose substantially lower than that needed to disrupt mitotic entry. Finally, GKI-1 exhibited promising efficacy in a mouse tumor xenograft model, in conjunction with cisplatin therapy.

Structural, enzymatic and spatiotemporal regulation of PP2A-B55 phosphatase in the control of mitosis

Cells require major physical changes to induce a proper repartition of the DNA. Nuclear envelope breakdown, DNA condensation and spindle formation are promoted at mitotic entry by massive protein phosphorylation and reversed at mitotic exit by the timely and ordered dephosphorylation of mitotic substrates. This phosphorylation results from the balance between the activity of kinases and phosphatases. The role of kinases in the control of mitosis has been largely studied, however, the impact of phosphatases has long been underestimated. Recent data have now established that the regulation of phosphatases is crucial to confer timely and ordered cellular events required for cell division. One major phosphatase involved in this process is the phosphatase holoenzyme PP2A-B55. This review will be focused in the latest structural, biochemical and enzymatic insights provided for PP2A-B55 phosphatase as well as its regulators and mechanisms of action.

Uncoupling of Mitosis and Cytokinesis Upon a Prolonged Arrest in Metaphase Is Influenced by Protein Phosphatases and Mitotic Transcription in Fission Yeast

Depletion of the Anaphase-Promoting Complex/Cyclosome (APC/C) activator Cdc20 arrests cells in metaphase with high levels of the mitotic cyclin (Cyclin B) and the Separase inhibitor Securin. In mammalian cells this arrest has been exploited for the treatment of cancer with drugs that engage the spindle assembly checkpoint and, recently, with chemical inhibitors of the APC/C. While most cells arrested in mitosis for prolonged periods undergo apoptosis, others skip cytokinesis and enter G1 with unsegregated chromosomes. This process, known as mitotic slippage, generates aneuploidy and increases genomic instability in the cancer cell. Here, we analyze the behavior of fission yeast cells arrested in mitosis through the transcriptional silencing of the Cdc20 homolog slp1. While depletion of slp1 readily halts cells in metaphase, this arrest is only transient and a majority of cells eventually undergo cytokinesis and show steady mitotic dephosphorylation. Notably, this occurs in the absence of Cyclin B (Cdc13) degradation. We investigate the involvement of phosphatase activity in these events and demonstrate that PP2A-B55^Pab1 is required to prevent septation and, during the arrest, its CDK-mediated inhibition facilitates the induction of cytokinesis. In contrast, deletion of PP2A-B56^Par1 completely abrogates septation. We show that this effect is partly due to this mutant entering mitosis with reduced CDK activity. Interestingly, both PP2A-B55^Pab1 and PP2A-B56^Par1, as well as Clp1 (the homolog of the budding yeast mitotic phosphatase Cdc14) are required for the dephosphorylation of mitotic substrates during the escape. Finally, we show that the mitotic transcriptional wave controlled by the RFX transcription factor Sak1 facilitates the induction of cytokinesis and also requires the activity of PP2A-B56^Par1 in a mechanism independent of CDK.

PP2A-B55: substrates and regulators in the control of cellular functions

PP2A is a major serine/threonine phosphatase class involved in the regulation of cell signaling through the removal of protein phosphorylation. This class of phosphatases is comprised of different heterotrimeric complexes displaying distinct substrate specificities. The present review will focus on one specific heterocomplex, the phosphatase PP2A-B55. Herein, we will report the direct substrates of this phosphatase identified to date, and its impact on different cell signaling cascades. We will additionally describe its negative regulation by its inhibitors Arpp19 and ENSA and their upstream kinase Greatwall. Finally, we will describe the essential molecular features defining PP2A-B55 substrate specificity that confer the correct temporal pattern of substrate dephosphorylation. The main objective of this review is to provide the reader with a unique source compiling all the knowledge of this particular holoenzyme that has evolved as a key enzyme for cell homeostasis and cancer development.

Discovery and Characterization of a Novel MASTL Inhibitor MKI-2 Targeting MASTL-PP2A in Breast Cancer Cells and Oocytes

Although microtubule-associated serine/threonine kinase-like (MASTL) is a promising target for selective anticancer treatment, MASTL inhibitors with nano range potency and antitumor efficacy have not been reported. Here, we report a novel potent and selective MASTL inhibitor MASTL kinase inhibitor-2 (MKI-2) identified in silico through a drug discovery program. Our data showed that MKI-2 inhibited recombinant MASTL activity and cellular MASTL activity with IC₅₀ values of 37.44 nM and 142.7 nM, respectively, in breast cancer cells. In addition, MKI-2 inhibited MASTL kinase rather than other AGC kinases, such as ROCK1, AKT1, PKACα, and p70S6K. Furthermore, MKI-2 exerted various antitumor activities by inducing mitotic catastrophe resulting from the modulation of the MASTL-PP2A axis in breast cancer cells. The MKI-2 treatment showed phenocopies with MASTL-null oocyte in mouse oocytes, which were used as a model to validate MKI-2 activity. Therefore, our study provided a new potent and selective MASTL inhibitor MKI-2 targeting the oncogenic MAST-PP2A axis in breast cancer cells.

The study of the determinants controlling Arpp19 phosphatase-inhibitory activity reveals an Arpp19/PP2A-B55 feedback loop

Arpp19 is a potent PP2A-B55 inhibitor that regulates this phosphatase to ensure the stable phosphorylation of mitotic/meiotic substrates. At G2-M, Arpp19 is phosphorylated by the Greatwall kinase on S67. This phosphorylated Arpp19 form displays a high affinity to PP2A-B55 and a slow dephosphorylation rate, acting as a competitor of PP2A-B55 substrates. The molecular determinants conferring slow dephosphorylation kinetics to S67 are unknown. PKA also phosphorylates Arpp19. This phosphorylation performed on S109 is essential to maintain prophase I-arrest in Xenopus oocytes although the underlying signalling mechanism is elusive. Here, we characterize the molecular determinants conferring high affinity and slow dephosphorylation to S67 and controlling PP2A-B55 inhibitory activity of Arpp19. Moreover, we show that phospho-S109 restricts S67 phosphorylation by increasing its catalysis by PP2A-B55. Finally, we discover a double feed-back loop between these two phospho-sites essential to coordinate the temporal pattern of Arpp19-dependent PP2A-B55 inhibition and Cyclin B/Cdk1 activation during cell division.

Progression of the cell division cycle requires feedback loops including those of phosphorylation and dephosphorylation; however the precise regulation of phosphorylation kinetics of Arpp19, an inhibitor of protein phosphatase 2A, is unclear. Here, the authors report that feedback between phosphorylation states of Ser67 and Ser109 of Arpp19 coordinates Arpp19-dependent inhibition of PP2A-B55 and Cyclin B activation during cell cycle progression.

Spatiotemporal coordination of Greatwall-Endos-PP2A promotes mitotic progression

Mitotic entry involves inhibition of protein phosphatase 2A bound to its B55/Tws regulatory subunit (PP2A-B55/Tws), which dephosphorylates substrates of mitotic kinases. This inhibition is induced when Greatwall phosphorylates Endos, turning it into an inhibitor of PP2A-Tws. How this mechanism operates spatiotemporally in the cell is incompletely understood. We previously reported that the nuclear export of Greatwall in prophase promotes mitotic progression. Here, we examine the importance of the localized activities of PP2A-Tws and Endos for mitotic regulation. We find that Tws shuttles through the nucleus via a conserved nuclear localization signal (NLS), but expression of Tws in the cytoplasm and not in the nucleus rescues the development of tws mutants. Moreover, we show that Endos must be in the cytoplasm before nuclear envelope breakdown (NEBD) to be efficiently phosphorylated by Greatwall and to bind and inhibit PP2A-Tws. Disrupting the cytoplasmic function of Endos before NEBD results in subsequent mitotic defects. Evidence suggests that this spatiotemporal regulation is conserved in humans.

Coupling of Cdc20 inhibition and activation by BubR1

Tight regulation of the APC/C-Cdc20 ubiquitin ligase that targets cyclin B1 for degradation is important for mitotic fidelity. The spindle assembly checkpoint (SAC) inhibits Cdc20 through the mitotic checkpoint complex (MCC). In addition, phosphorylation of Cdc20 by cyclin B1–Cdk1 independently inhibits APC/C–Cdc20 activation. This creates a conundrum for how Cdc20 is activated before cyclin B1 degradation. Here, we show that the MCC component BubR1 harbors both Cdc20 inhibition and activation activities, allowing for cross-talk between the two Cdc20 inhibition pathways. Specifically, BubR1 acts as a substrate specifier for PP2A-B56 to enable efficient Cdc20 dephosphorylation in the MCC. A mutant Cdc20 mimicking the dephosphorylated state escapes a mitotic checkpoint arrest, arguing that restricting Cdc20 dephosphorylation to the MCC is important. Collectively, our work reveals how Cdc20 can be dephosphorylated in the presence of cyclin B1-Cdk1 activity without causing premature anaphase onset.

Back to the new beginning: Mitotic exit in space and time

The ultimate goal of cell division is to generate two identical daughter cells that resemble the mother cell from which they derived. Once all the proper attachments to the spindle have occurred, the chromosomes have aligned at the metaphase plate and the spindle assembly checkpoint (a surveillance mechanism that halts cells form progressing in the cell cycle in case of spindle - microtubule attachment errors) has been satisfied, mitotic exit will occur. Mitotic exit has the purpose of completing the separation of the genomic material but also to rebuild the cellular structures necessary for the new cell cycle. This stage of mitosis received little attention until a decade ago, therefore our knowledge is much patchier than the molecular details we now have for the early stages of mitosis. However, it is emerging that mitotic exit is not just the simple reverse of mitotic entry and it is highly regulated in space and time. In this review I will discuss the main advances in the field that provided us with a better understanding on the key role of protein phosphorylation/de-phosphorylation in this transition together with the concept of their spatial regulation. As this field is much younger, I will highlight general consensus, contrasting views together with the outstanding questions awaiting for answers.

Bora phosphorylation substitutes in trans for T-loop phosphorylation in Aurora A to promote mitotic entry

Polo-like kinase 1 (Plk1) is instrumental for mitotic entry and progression. Plk1 is activated by phosphorylation on a conserved residue Thr210 in its activation segment by the Aurora A kinase (AURKA), a reaction that critically requires the co-factor Bora phosphorylated by a CyclinA/B-Cdk1 kinase. Here we show that phospho-Bora is a direct activator of AURKA kinase activity. We localize the key determinants of phospho-Bora function to a 100 amino acid region encompassing two short Tpx2-like motifs and a phosphoSerine-Proline motif at Serine 112, through which Bora binds AURKA. The latter substitutes in trans for the Thr288 phospho-regulatory site of AURKA, which is essential for an active conformation of the kinase domain. We demonstrate the importance of these determinants for Bora function in mitotic entry both in Xenopus egg extracts and in human cells. Our findings unveil the activation mechanism of AURKA that is critical for mitotic entry.

Bistable, Biphasic Regulation of PP2A-B55 Accounts for the Dynamics of Mitotic Substrate Phosphorylation

The phosphorylation of mitotic proteins is bistable, which contributes to the decisiveness of the transitions into and out of M phase. The bistability in substrate phosphorylation has been attributed to bistability in the activation of the cyclin-dependent kinase Cdk1. However, more recently it has been suggested that bistability also arises from positive feedback in the regulation of the Cdk1-counteracting phosphatase PP2A-B55. Here, we demonstrate biochemically using Xenopus laevis egg extracts that the Cdk1-counter-acting phosphatase PP2A-B55 functions as a bistable switch, even when the bistability of Cdk1 activation is suppressed. In addition, Cdk1 regulates PP2A-B55 in a biphasic manner; low concentrations of Cdk1 activate PP2A-B55 and high concentrations inactivate it. As a consequence of this incoherent feedforward regulation, PP2A-B55 activity rises concurrently with Cdk1 activity during interphase and suppresses substrate phosphorylation. PP2A-B55 activity is then sharply downregulated at the onset of mitosis. During mitotic exit, Cdk1 activity initially falls with no obvious change in substrate phosphorylation; dephosphorylation then commences once PP2A-B55 spikes in activity. These findings suggest that changes in Cdk1 activity are permissive for mitotic entry and exit but that the changes in PP2A-B55 activity are the ultimate trigger.

Mitotic transitions are accompanied by drastic changes in the phosphorylation state of proteins. Kamenz et al. demonstrate biochemically that the major mitotic phosphatase PP2A-B55 is regulated by incoherent feedforward and double-negative feedback loops to promote rapid and switch-like mitotic entry and exit.

Dissection of the Ovulatory Process Using ex vivo Approaches

Ovulation is a unique physiological phenomenon that is essential for sexual reproduction. It refers to the entire process of ovarian follicle responses to hormonal stimulation resulting in the release of mature fertilization-competent oocytes from the follicles and ovaries. Remarkably, ovulation in different species can be reproduced out-of-body with high fidelity. Moreover, most of the molecular mechanisms and signaling pathways engaged in this process have been delineated using in vitro ovulation models. Here, we provide an overview of the major molecular and cytological events of ovulation observed in frogs, primarily in the African clawed frog Xenopus laevis, using mainly ex vivo approaches, with the focus on meiotic oocyte maturation and follicle rupture. For the purpose of comparison and generalization, we also refer extensively to ovulation in other biological species, most notoriously, in mammals.

PP2A-B55 Holoenzyme Regulation and Cancer

Protein phosphorylation is a post-translational modification essential for the control of the activity of most enzymes in the cell. This protein modification results from a fine-tuned balance between kinases and phosphatases. PP2A is one of the major serine/threonine phosphatases that is involved in the control of a myriad of different signaling cascades. This enzyme, often misregulated in cancer, is considered a tumor suppressor. In this review, we will focus on PP2A-B55, a particular holoenzyme of the family of the PP2A phosphatases whose specific role in cancer development and progression has only recently been highlighted. The discovery of the Greatwall (Gwl)/Arpp19-ENSA cascade, a new pathway specifically controlling PP2A-B55 activity, has been shown to be frequently altered in cancer. Herein, we will review the current knowledge about the mechanisms controlling the formation and the regulation of the activity of this phosphatase and its misregulation in cancer.

MKI-1, a Novel Small-Molecule Inhibitor of MASTL, Exerts Antitumor and Radiosensitizer Activities Through PP2A Activation in Breast Cancer

Although MASTL (microtubule-associated serine/threonine kinase-like) is an attractive target for anticancer treatment, MASTL inhibitors with antitumor activity have not yet been reported. In this study, we have presented a novel MASTL inhibitor, MKI-1, identified through in silico screening and in vitro analysis. Our data revealed that MKI-1 exerted antitumor and radiosensitizer activities in in vitro and in vivo models of breast cancer. The mechanism of action of MKI-1 occurred through an increase in PP2A activity, which subsequently decreased the c-Myc protein content in breast cancer cells. Moreover, the activity of MKI-1 in the regulation of MASTL-PP2A was validated in a mouse oocyte model. Our results have demonstrated a new small-molecule inhibitor of MASTL, MKI-1, which exerts antitumor and radiosensitizer activities through PP2A activation in breast cancer in vitro and in vivo.

MASTL: A novel therapeutic target for Cancer Malignancy

Targeting mitotic kinases is an emerging anticancer approach with promising preclinical outcomes. Microtubule‐associated serine/threonine kinase like (MASTL), also known as Greatwall (Gwl), is an important mitotic kinase that regulates mitotic progression of normal or transformed cells by blocking the activity of tumor suppressor protein phosphatase 2A (PP2A). MASTL upregulation has now been detected in multiple cancer types and associated with aggressive clinicopathological features. Apart, an aberrant MASTL activity has been implicated in oncogenic transformation through the development of chromosomal instability and alteration of key oncogenic signaling pathways. In this regard, recent publications have revealed potential role of MASTL in the regulation of AKT/mTOR and Wnt/β‐catenin signaling pathways, which may be independent of its regulation of PP2A‐B55 (PP2A holoenzyme containing a B55‐family regulatory subunit). Taken together, MASTL kinase has emerged as a novel target for cancer therapeutics, and hence development of small molecule inhibitors of MASTL may significantly improve the clinical outcomes of cancer patients. In this article, we review the role of MASTL in cancer progression and the current gaps in this knowledge. We also discuss potential efficacy of MASTL expression for cancer diagnosis and therapy.

AKT Regulates Mitotic Progression of Mammalian Cells by Phosphorylating MASTL, Leading to Protein Phosphatase 2A Inactivation

Microtubule-associated serine/threonine kinase like (MASTL), also known as Greatwall (Gwl) kinase, has an important role in the regulation of mitosis. By inhibiting protein phosphatase 2A (PP2A), it plays a crucial role in activating one of the most important mitotic kinases, known as cyclin-dependent kinase 1 (CDK1). MASTL has been seen to be upregulated in various types of cancers and is also involved in tumor recurrence. It is activated by CDK1 through phosphorylations in the activation/T-loop, but the complete mechanism of its activation is still unclear. Here, we report that AKT phosphorylates MASTL at residue T299, which plays a critical role in its activation. Our results suggest that AKT increases CDK1-mediated phosphorylation and hence the activity of MASTL, which, in turn, promotes mitotic progression through PP2A inhibition. We also show that the oncogenic potential of AKT is augmented by MASTL activation, since AKT-mediated proliferation in colorectal cell lines can be attenuated by inhibiting and/or silencing MASTL. In brief, we report that AKT plays an important role in the progression of mitosis in mammalian cells and that it does so through the phosphorylation and activation of MASTL.

Molecular Basis of the Mechanisms Controlling MASTL

The human MASTL (Microtubule-associated serine/threonine kinase-like) gene encodes an essential protein in the cell cycle. MASTL is a key factor preventing early dephosphorylation of M-phase targets of Cdk1/CycB. Little is known about the mechanism of MASTL activation and regulation. MASTL contains a non-conserved insertion of 550 residues within its activation loop, splitting the kinase domain, and making it unique. Here, we show that this non-conserved middle region (NCMR) of the protein is crucial for target specificity and activity. We performed a phosphoproteomic assay with different MASTL constructs identifying key phosphorylation sites for its activation and determining whether they arise from autophosphorylation or exogenous kinases, thus generating an activation model. Hydrogen/deuterium exchange data complements this analysis revealing that the C-lobe in full-length MASTL forms a stable structure, whereas the N-lobe is dynamic and the NCMR and C-tail contain few localized regions with higher-order structure. Our results indicate that truncated versions of MASTL conserving a cryptic C-Lobe in the NCMR, display catalytic activity and different targets, thus establishing a possible link with truncated mutations observed in cancer-related databases.

Cell Cycle and DNA Repair Regulation in the Damage Response: Protein Phosphatases Take Over the Reins

Cells are constantly suffering genotoxic stresses that affect the integrity of our genetic material. Genotoxic insults must be repaired to avoid the loss or inappropriate transmission of the genetic information, a situation that could lead to the appearance of developmental abnormalities and tumorigenesis. To combat this threat, eukaryotic cells have evolved a set of sophisticated molecular mechanisms that are collectively known as the DNA damage response (DDR). This surveillance system controls several aspects of the cellular response, including the detection of lesions, a temporary cell cycle arrest, and the repair of the broken DNA. While the regulation of the DDR by numerous kinases has been well documented over the last decade, the complex roles of protein dephosphorylation have only recently begun to be investigated. Here, we review recent progress in the characterization of DDR-related protein phosphatases during the response to a DNA lesion, focusing mainly on their ability to modulate the DNA damage checkpoint and the repair of the damaged DNA. We also discuss their protein composition and structure, target specificity, and biochemical regulation along the different stages encompassed in the DDR. The compilation of this information will allow us to better comprehend the physiological significance of protein dephosphorylation in the maintenance of genome integrity and cell viability in response to genotoxic stress.

Protein Phosphatases in G1 Regulation

Eukaryotic cells make the decision to proliferate, to differentiate or to cease dividing during G1, before passage through the restriction point or Start. Keeping cyclin-dependent kinase (CDK) activity low during this period restricts commitment to a new cell cycle and is essential to provide the adequate timeframe for the sensing of environmental signals. Here, we review the role of protein phosphatases in the modulation of CDK activity and as the counteracting force for CDK-dependent substrate phosphorylation, in budding and fission yeast. Moreover, we discuss recent findings that place protein phosphatases in the interface between nutritional signalling pathways and the cell cycle machinery.

Greatwall-Endosulfine: A Molecular Switch that Regulates PP2A/B55 Protein Phosphatase Activity in Dividing and Quiescent Cells

During the cell cycle, hundreds of proteins become phosphorylated and dephosphorylated, indicating that protein kinases and protein phosphatases play a central role in its regulation. It has been widely recognized that oscillation in cyclin-dependent kinase (CDK) activity promotes DNA replication, during S-phase, and chromosome segregation, during mitosis. Each CDK substrate phosphorylation status is defined by the balance between CDKs and CDK-counteracting phosphatases. In fission yeast and animal cells, PP2A/B55 is the main protein phosphatase that counteracts CDK activity. PP2A/B55 plays a key role in mitotic entry and mitotic exit, and it is regulated by the Greatwall-Endosulfine (ENSA) molecular switch that inactivates PP2A/B55 at the onset of mitosis, allowing maximal CDK activity at metaphase. The Greatwall-ENSA-PP2A/B55 pathway is highly conserved from yeast to animal cells. In yeasts, Greatwall is negatively regulated by nutrients through TORC1 and S6 kinase, and couples cell growth, regulated by TORC1, to cell cycle progression, driven by CDK activity. In animal cells, Greatwall is phosphorylated and activated by Cdk1 at G2/M, generating a bistable molecular switch that results in full activation of Cdk1/CyclinB. Here we review the current knowledge of the Greatwall-ENSA-PP2A/B55 pathway and discuss its role in cell cycle progression and as an integrator of nutritional cues.

Triggering mitosis

Entry into mitosis is triggered by the activation of cyclin-dependent kinase 1 (Cdk1). This simple reaction rapidly and irreversibly sets the cell up for division. Even though the core step in triggering mitosis is so simple, the regulation of this cellular switch is highly complex, involving a large number of interconnected signalling cascades. We do have a detailed knowledge of most of the components of this network, but only a poor understanding of how they work together to create a precise and robust system that ensures that mitosis is triggered at the right time and in an orderly fashion. In this review, we will give an overview of the literature that describes the Cdk1 activation network and then address questions relating to the systems biology of this switch. How is the timing of the trigger controlled? How is mitosis insulated from interphase? What determines the sequence of events, following the initial trigger of Cdk1 activation? Which elements ensure robustness in the timing and execution of the switch? How has this system been adapted to the high levels of replication stress in cancer cells?

Getting out of mitosis: spatial and temporal control of mitotic exit and cytokinesis by PP1 and PP2A

Here, we will review the evidence showing that mitotic exit is initiated by regulated proteolysis and then driven by the PPP family of phosphoserine/threonine phosphatases. Rapid APC/CCDC20 and ubiquitin-dependent proteolysis of cyclin B and securin initiates sister chromatid separation, the first step of mitotic exit. Because proteolysis of Aurora and Polo family kinases dependent on APC/CCDH1 is relatively slow, this creates a new regulatory state, anaphase, different to G2 and M-phase. We will discuss how the CDK1-counteracting phosphatases PP1 and PP2A-B55, together with Aurora and Polo kinases, contribute to the temporal regulation and order of events in the different stages of mitotic exit from anaphase to cytokinesis. For PP2A-B55, these timing properties are created by the ENSA-dependent inhibitory pathway and differential recognition of phosphoserine and phosphothreonine. Finally, we will discuss how Aurora B and PP2A-B56 are needed for the spatial regulation of anaphase spindle formation and how APC/C-dependent destruction of PLK1 acts as a timer for abscission, the final event of cytokinesis.

Dephosphorylation of Plk1 occurs through PP2A-B55/ENSA/Greatwall pathway during mitotic DNA damage recovery

Recovery from DNA damage is critical for cell survival. However, serious damage cannot be repaired, leading to cell death for prevention of abnormal cell growth. Previously, we demonstrated that 4N-DNA accumulates via the initiation of an abnormal interphase without cytokinesis and that re-replication occurs during a prolonged recovery period in the presence of severe DNA damage in mitotic cells. Mitotic phosphorylated Plk1 is typically degraded during mitotic exit. However, Plk1 has unusually found to be dephosphorylated in mitotic slippage without cytokinesis during recovery from mitotic DNA damage. Here, we investigated how Plk1 dephosphorylation is established during recovery from mitotic DNA damage. Mitotic DNA damage activated ATM and Chk1/2 and repressed Cdk1 and Greatwall protein kinase, followed by PP2A activation through the dissociation of ENSA and PP2A-B55. Interaction between Plk1 and PP2A-B55α or PP2A-B55δ was strongly induced during recovery from mitotic DNA damage. Moreover, the depletion of PP2A-B55α and/or PP2A-B55δ by siRNA transfection led to the recovery of Plk1 phosphorylation and progression of the cell cycle into the G1 phase. Therefore, to adapt to severe DNA damage, the activated Greatwall/ENSA signaling pathway was repressed by ATM/Chk1/2, even in mitotic cells. Activation of the PP2A-B55 holoenzyme complex induced the dephosphorylation of Plk1 and Cdk1, and finally, mitotic slippage occurred without normal chromosome segregation and cytokinesis.

ENSA and ARPP19 differentially control cell cycle progression and development

The Greatwall kinase substrates ARPP19 and ENSA have been shown to inhibit PP2A-B55 by an identical mechanism. Hached et al. show that, surprisingly, the ARPP19 and ENSA paralogs display specific functions during mouse embryogenesis and differentially control cell cycle progression.

Greatwall (GWL) is an essential kinase that indirectly controls PP2A-B55, the phosphatase counterbalancing cyclin B/CDK1 activity during mitosis. In Xenopus laevis egg extracts, GWL-mediated phosphorylation of overexpressed ARPP19 and ENSA turns them into potent PP2A-B55 inhibitors. It has been shown that the GWL/ENSA/PP2A-B55 axis contributes to the control of DNA replication, but little is known about the role of ARPP19 in cell division. By using conditional knockout mouse models, we investigated the specific roles of ARPP19 and ENSA in cell division. We found that Arpp19, but not Ensa, is essential for mouse embryogenesis. Moreover, Arpp19 ablation dramatically decreased mouse embryonic fibroblast (MEF) viability by perturbing the temporal pattern of protein dephosphorylation during mitotic progression, possibly by a drop of PP2A-B55 activity inhibition. We show that these alterations are not prevented by ENSA, which is still expressed in Arpp19^Δ/Δ MEFs, suggesting that ARPP19 is essential for mitotic division. Strikingly, we demonstrate that unlike ARPP19, ENSA is not required for early embryonic development. Arpp19 knockout did not perturb the S phase, unlike Ensa gene ablation. We conclude that, during mouse embryogenesis, the Arpp19 and Ensa paralog genes display specific functions by differentially controlling cell cycle progression.

Protein phosphatases in the regulation of mitosis

The accurate segregation of genetic material to daughter cells during mitosis depends on the precise coordination and regulation of hundreds of proteins by dynamic phosphorylation. Mitotic kinases are major regulators of protein function, but equally important are protein phosphatases that balance their actions, their coordinated activity being essential for accurate chromosome segregation. Phosphoprotein phosphatases (PPPs) that dephosphorylate phosphoserine and phosphothreonine residues are increasingly understood as essential regulators of mitosis. In contrast to kinases, the lack of a pronounced peptide-binding cleft on the catalytic subunit of PPPs suggests that these enzymes are unlikely to be specific. However, recent exciting insights into how mitotic PPPs recognize specific substrates have revealed that they are as specific as kinases. Furthermore, the activities of PPPs are tightly controlled at many levels to ensure that they are active only at the proper time and place. Here, I will discuss substrate selection and regulation of mitotic PPPs focusing mainly on animal cells and explore how these actions control mitosis, as well as important unanswered questions.

Thrombocytopenia-associated mutations in Ser/Thr kinase MASTL deregulate actin cytoskeletal dynamics in platelets

MASTL, a Ser/Thr kinase that inhibits PP2A-B55 complexes during mitosis, is mutated in autosomal dominant thrombocytopenia. However, the connections between the cell-cycle machinery and this human disease remain unexplored. We report here that, whereas Mastl ablation in megakaryocytes prevented proper maturation of these cells, mice carrying the thrombocytopenia-associated mutation developed thrombocytopenia as a consequence of aberrant activation and survival of platelets. Activation of mutant platelets was characterized by hyperstabilized pseudopods mimicking the effect of PP2A inhibition and actin polymerization defects. These aberrations were accompanied by abnormal hyperphosphorylation of multiple components of the actin cytoskeleton and were rescued both in vitro and in vivo by inhibiting upstream kinases such as PKA, PKC, or AMPK. These data reveal an unexpected role of Mastl in actin cytoskeletal dynamics in postmitotic cells and suggest that the thrombocytopenia-associated mutation in MASTL is a pathogenic dominant mutation that mimics decreased PP2A activity resulting in altered phosphorylation of cytoskeletal regulatory pathways.

Greatwall kinase at a glance

Mitosis is controlled by a subtle balance between kinase and phosphatase activities that involve the master mitotic kinase cyclin-B-Cdk1 and its antagonizing protein phosphatase 2A-B55 (PP2A-B55). Importantly, the Greatwall (Gwl; known as Mastl in mammals, Rim15 in budding yeast and Ppk18 in fission yeast) kinase pathway regulates PP2A-B55 activity by phosphorylating two proteins, cAMP-regulated phosphoprotein 19 (Arpp19) and α-endosulfine (ENSA). This phosphorylation turns these proteins into potent inhibitors of PP2A-B55, thereby promoting a correct timing and progression of mitosis. In this Cell Science at a Glance article and the accompanying poster, we discuss how Gwl is regulated in space and time, and how the Gwl-Arpp19-ENSA-PP2A-B55 pathway plays an essential role in the control of M and S phases from yeast to human. We also summarize how Gwl modulates oncogenic properties of cells and how nutrient deprivation influences Gwl activity.

MASTL inhibition promotes mitotic catastrophe through PP2A activation to inhibit cancer growth and radioresistance in breast cancer cells

BACKGROUND

Although MASTL (microtubule-associated serine/threonine kinase-like) is a key mitotic kinase that regulates mitotic progression through the inactivation of tumor suppressor protein phosphatase 2A (PP2A), the antitumor mechanism of MASTL targeting in cancer cells is still unclear.

METHODS

MASTL expression was evaluated by using breast cancer tissue microarrays and public cancer databases. The effects of MASTL depletion with siRNAs were evaluated in various breast cancer cells or normal cells. Various methods, including cell viability, cell cycle, soft agar, immunoblotting, immunofluorescence, PP2A activity, live image, and sphere forming assay, were used in this study.

RESULTS

This study showed the oncosuppressive mechanism of MASTL targeting that promotes mitotic catastrophe through PP2A activation selectively in breast cancer cells. MASTL expression was closely associated with tumor progression and poor prognosis in breast cancer. The depletion of MASTL reduced the oncogenic properties of breast cancer cells with high MASTL expression, but did not affect the viability of non-transformed normal cells with low MASTL expression. With regard to the underlying mechanism, we found that MASTL inhibition caused mitotic catastrophe through PP2A activation in breast cancer cells. Furthermore, MASTL depletion enhanced the radiosensitivity of breast cancer cells with increased PP2A activity. Notably, MASTL depletion dramatically reduced the formation of radioresistant breast cancer stem cells in response to irradiation.

CONCLUSION

Our data suggested that MASTL inhibition promoted mitotic catastrophe through PP2A activation, which led to the inhibition of cancer cell growth and a reversal of radioresistance in breast cancer cells.

Cyclin A-cdk1-Dependent Phosphorylation of Bora Is the Triggering Factor Promoting Mitotic Entry

Mitosis is induced by the activation of the cyclin B/cdk1 feedback loop that creates a bistable state. The triggering factor promoting active cyclin B/cdk1 switch has been assigned to cyclin B/cdk1 accumulation during G2. However, this complex is rapidly inactivated by Wee1/Myt1-dependent phosphorylation of cdk1 making unlikely a triggering role of this kinase in mitotic commitment. Here we show that cyclin A/cdk1 kinase is the factor triggering mitosis. Cyclin A/cdk1 phosphorylates Bora to promote Aurora A-dependent Plk1 phosphorylation and activation and mitotic entry. We demonstrate that Bora phosphorylation by cyclin A/cdk1 is both necessary and sufficient for mitotic commitment. Finally, we identify a site in Bora whose phosphorylation by cyclin A/cdk1 is required for mitotic entry. We constructed a mathematical model confirming the essential role of this kinase in mitotic commitment. Overall, our results uncover the molecular mechanism by which cyclin A/cdk1 triggers mitotic entry.

MASTL overexpression promotes chromosome instability and metastasis in breast cancer

MASTL kinase is essential for correct progression through mitosis, with loss of MASTL causing chromosome segregation errors, mitotic collapse and failure of cytokinesis. However, in cancer MASTL is most commonly amplified and overexpressed. This correlates with increased chromosome instability in breast cancer and poor patient survival in breast, ovarian and lung cancer. Global phosphoproteomic analysis of immortalised breast MCF10A cells engineered to overexpressed MASTL revealed disruption to desmosomes, actin cytoskeleton, PI3K/AKT/mTOR and p38 stress kinase signalling pathways. Notably, these pathways were also disrupted in patient samples that overexpress MASTL. In MCF10A cells, these alterations corresponded with a loss of contact inhibition and partial epithelial-mesenchymal transition, which disrupted migration and allowed cells to proliferate uncontrollably in 3D culture. Furthermore, MASTL overexpression increased aberrant mitotic divisions resulting in increased micronuclei formation. Mathematical modelling indicated that this delay was due to continued inhibition of PP2A-B55, which delayed timely mitotic exit. This corresponded with an increase in DNA damage and delayed transit through interphase. There were no significant alterations to replication kinetics upon MASTL overexpression, however, inhibition of p38 kinase rescued the interphase delay, suggesting the delay was a G2 DNA damage checkpoint response. Importantly, knockdown of MASTL, reduced cell proliferation, prevented invasion and metastasis of MDA-MB-231 breast cancer cells both in vitro and in vivo, indicating the potential of future therapies that target MASTL. Taken together, these results suggest that MASTL overexpression contributes to chromosome instability and metastasis, thereby decreasing breast cancer patient survival.