/Research

My Research

Without cell division, there would be no sophisticated life on earth. The eukaryotic cell cycle produces two healthy daughter cells with two identical copies of the genetic material. These two biological processes, cell division or mitosis, and DNA replication, are under tight control to ensure an error-free cell cycle. Before replication starts, DNA needs to be licensed to control that no more than a single copy is synthesized. Before cells actually divide, the sister chromatids require bipolar attachments to the mitotic spindle which ultimately assures accurate completion of mitosis.  

 

Cells require turning off specific processes at the right time and in a unidirectional fashion. In this context, the Ubiquitin Proteasome System (UPS) plays a key role by controlling protein degradation in a temporal and spatial fashion. My research can be divided over three research themes (see below). For this, I use human primary and cancer cell lines, stem cell cultures, biochemical methods, flow cytometry, advanced time-lapse microscopy techniques, mass-spectrometry and RNA sequencing. I aim to contribute to a better understanding of the regulation of the mitotic cell cycle, as well as non-traditional endocycles, on multiple levels of life: the molecular, tissue and organism level.

Regulation of transcription by the ubiquitin proteasome system

My goal is to unravel molecular mechanisms that safeguard genomic integrity. To this end, I discovered how transcription factors are degraded during the mitotic cell cycle. Additionally, this event seems crucial not only in mitotic cell cycles, but also in endocycles. The endocycle is a non-conventional cell cycle consisting of DNA synthesis phases intermitted by gap-phases where mitosis is omitted. In endocycling cells, the transcriptional program is tightly coordinated by activation and repression through, on the one hand, activator transcription factors blocking endocycles, and on the other hand, repressor transcription factors enhancing endocycles. The regulated degradation of the transcription factors is crucial for the synthesis phase of the endocycle. I aim to understand this intriguing molecular mechanism and its importance for organ and embryo development. 

Controlling replication licensing by regulated protein degradation 

How are the processes of mitosis and DNA replication linked? Interestingly, during mitosis, cells already prepare for replication, and reversely, during DNA replication, key factors required for cell division are put into action.  I study how degradation mechanisms in mitosis are essential for the next S-phase. My studies led to two main findings: (I) The licensing-inhibitor geminin and the licensing-factor Cdc6, are sequentially degraded in mitosis via APC/C. This event is crucial to create a time-window at the end of mitosis in which DNA replication origins are licensed for DNA synthesis. (II) Destruction of Cdc6 during S-phase requires a previously unidentified PIP-box that is essential for CRL4-Cdt2 dependent destruction.

Controlling separation of chromatid pools over two daughter cells

 

During cell division, the mitotic checkpoint assures correct separation of chromatid pools over two daughter cells. During my PhD work, I studied how the incomplete removal of cyclin B1 at mitotic exit, may cause genomic instability. My studies led to two main findings: (I) The simultaneous separation of sister chromatids and the sudden drop of Cdk1 activity are essential for error-free cell division. (II) The incomplete activation of APC/C-Cdh1 at mitotic exit, prevents Aurora A degradation. Furthermore, I contributed to a review to highlight and discuss the role of the APC/C in the cell cycle and in the understanding of cancer to help the design of treatment. How mitotic processes are timed to assure genomic stability is a continued focus of my research.  

© 2017 by Linda Clijsters.

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