Helfrid Hochegger
Principal Investigator
Helfrid Hochegger works on cell cycle control using chemical genetic, advanced microscopy and proteomic approaches. He was trained in the Nobel Prize winning lab of Tim Hunt and has spent his career investigating the function of kinases and phosphatases in the regulation of cell cycle transitions. He distinguished himself by establishing a chemical genetics system to target mitotic and interphase Cdk activity in a vertebrate cell line. This work led to a prestigious Wellcome Trust career development award to set up a laboratory at the Genome Damage and Stability Centre at the University of Sussex. His genetic analysis of cell cycle control in DT40 and mammalian cells led to a recent senior CRUK research fellowship. His cellular model systems are specifically designed to analyse Cdk1 activity thresholds at the G2/M transition, which will be extremely useful for the quantitative analysis of mitotic entry models. Furthermore he will contribute valuable expertise in live-cell imaging and in genetic manipulation of the mammalian cell cycle.
Publications contributed to in the context of this project
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Thursday, Apr 30, 2020
EMBO Journal
Cyclin A Triggers Mitosis Either via the Greatwall Kinase Pathway or Cyclin B
Nadia Hegarat, Adrijana Crncec, Maria F Suarez Peredo Rodriguez, Fabio Echegaray Iturra, Yan Gu, Oliver Busby, Paul F Lang, Alexis R Barr, Chris Bakal, Masato T Kanemaki, Angus I Lamond, Bela Novak, Tony Ly, Helfrid Hochegger
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AbstractTwo mitotic cyclin types, cyclin A and B, exist in higher eukaryotes, but their specialised functions in mitosis are incompletely understood. Using degron tags for rapid inducible protein removal, we analyse how acute depletion of these proteins affects mitosis. Loss of cyclin A in G2-phase prevents mitotic entry. Cells lacking cyclin B can enter mitosis and phosphorylate most mitotic proteins, because of parallel PP2A:B55 phosphatase inactivation by Greatwall kinase. The final barrier to mitotic establishment corresponds to nuclear envelope breakdown, which requires a decisive shift in the balance of cyclin-dependent kinase Cdk1 and PP2A:B55 activity. Beyond this point, cyclin B/Cdk1 is essential for phosphorylation of a distinct subset of mitotic Cdk1 substrates that are essential to complete cell division. Our results identify how cyclin A, cyclin B and Greatwall kinase coordinate mitotic progression by increasing levels of Cdk1-dependent substrate phosphorylation.
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Thursday, Nov 15, 2018
Current Biology
Two Interlinked Bistable Switches Govern Mitotic Control in Mammalian Cells
Scott Rata, Maria F. Suarez Peredo Rodriguez, Stephy Joseph, Nisha Peter, Fabio E. Iturra, Fengwei Yang, Anotida Madzvamuse, Jan G. Ruppert, Kumiko Samejima, Melpomeni Platani, Monica Alvarez-Fernandez, Marcos Malumbres, William C. Earnshaw, Bela Novak, Helfrid Hochegger
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AbstractDistinct protein phosphorylation levels in interphase and M phase require tight regulation of Cdk1 activity [1,2]. A bistable switch, based on positive feedback in the Cdk1 activation loop, has been proposed to generate different thresholds for transitions between these cell-cycle states [3–5]. Recently, the activity of the major Cdk1-counteracting phosphatase, PP2A:B55, has also been found to be bistable due to Greatwall kinase-dependent regulation [6]. However, the interplay of the regulation of Cdk1 and PP2A:B55 in vivo remains unexplored. Here, we combine quantitative cell biology assays with mathematical modeling to explore the interplay of mitotic kinase activation a nd phosphatase inactivation in human cells. By measuring mitotic entry and exit thresholds using ATP-analog-sensitive Cdk1 mutants, we find evidence that the mitotic switch displays hysteresis and bistability, responding differentially to Cdk1 inhibition in the mitotic and interphase states. Cdk1 activation by Wee1/Cdc25 feedback loops and PP2A:B55 inactivation by Greatwall independently contributes to this hysteretic switch system. However, elimination of both Cdk1 and PP2A:B55 inactivation fully abrogates bistability, suggesting that hysteresis is an emergent property of mutual inhibition between the Cdk1 and PP2A:B55 feedback loops. Our model of the two interlinked feedback systems predicts an intermediate but hidden steady state between interphase and M phase. This could be verified experimentally by Cdk1 inhibition during mitotic entry, supporting the predictive value of our model. Furthermore, we demonstrate that dual inhibition of Wee1 and Gwl kinases causes loss of cell-cycle memory and synthetic lethality, which could be further exploited therapeutically.
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Wednesday, Aug 1, 2018
Molecular Cell
DNA Replication Determines Timing of Mitosis by Restricting CDK1 and PLK1 Activation
Bennie Lemmens, Nadia Hegarat, Karen Akopyan, Joan Sala-Gaston, Jiri Bartek, Helfrid Hochegger, Arne Lindqvist
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AbstractTo maintain genome stability, cells need to replicate their DNA before dividing. Upon completion of bulk DNA synthesis, the mitotic kinases CDK1 and PLK1 become active and drive entry into mitosis. Here, we have tested the hypothesis that DNA replication determines the timing of mitotic kinase activation. Using an optimized double-degron system, together with kinase inhibitors to enforce tight inhibition of key proteins, we find that human cells unable to initiate DNA replication prematurely enter mitosis. Preventing DNA replication licensing and/or firing causes prompt activation of CDK1 and PLK1 in S phase. In the presence of DNA replication, inhibition of CHK1 and p38 leads to premature activation of mitotic kinases, which induces severe replication stress. Our results demonstrate that, rather than merely a cell cycle output, DNA replication is an integral signaling component that restricts activation of mitotic kinases. DNA replication thus functions as a brake that determines cell cycle duration.
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Tuesday, Sep 19, 2017
Cell Cycle
Interlinked bistable mechanisms generate robust mitotic transitions
Lukas Hutter, Scott Rata, Helfrid Hochegger, Bela Novak
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AbstractThe transitions between phases of the cell cycle have evolved to be robust and switch-like, which ensures temporal separation of DNA replication, sister chromatid separation, and cell division. Mathematical models describing the biochemical interaction networks of cell cycle regulators attribute these properties to underlying bistable switches, which inherently generate robust, switch-like, and irreversible transitions between states. We have recently presented new mathematical models for two control systems that regulate crucial transitions in the cell cycle: mitotic entry and exit and the mitotic checkpoint.Each of the two control systems is characterized by two interlinked bistable switches. In the case of mitotic checkpoint control, these switches are mutually activating, whereas in the case of the mitotic entry/exit network, the switches are mutually inhibiting. In this Perspective we describe the qualitative features of these regulatory motifs and show that having two interlinked bistable mechanisms further enhances robustness and irreversibility. We speculate that these network motifs also underlie other cell cycle transitions and cellular transitions between distinct biochemical states.
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Friday, May 27, 2016
BioEssays
Bistability of mitotic entry and exit switches during open mitosis in mammalian cells
Nadia Hegarat, Scott Rata, Helfrid Hochegger
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AbstractMitotic entry and exit are switch-like transitions that are driven by the activation and inactivation of Cdk1 and mitotic cyclins. This simple on/off reaction turns out to be a complex interplay of various reversible reactions, feedback loops, and thresholds that involve both the direct regulators of Cdk1 and its counteracting phosphatases. In this review, we summarize the interplay of the major components of the system and discuss how they work together to generate robustness, bistability, and irreversibility. We propose that it may be beneficial to regard the entry and exit reactions as two separate reversible switches that are distinguished by differences in the state of phosphatase activity, mitotic proteolysis, and a dramatic rearrangement of cellular components after nuclear envelope breakdown, and discuss how the major Cdk1 activity thresholds could be determined for these transitions.