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Borggrefe Group


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Unraveling the Molecular Mechanisms of Notch Signal

Transduction Pathway

Notch signaling is an evolutionary conserved signal transduction pathway that plays pivotal roles in numerous aspects of cell differentiation, tissue homeostasis and tumorigenesis. In normal hematopoiesis Notch1 is essential for T-cell development and has been implicated in the maintenance of hematopoietic stem cells. At the molecular level, ligand binding induces the processing of the Notch transmembrane receptor resulting in the release of the intracellular domain of Notch (NICD), see also Figure 1. Subsequently, the NICD translocates to the nucleus, binds the transcription factor RBP-J and activates transcription of target genes. In the absence of the NICD, RBP-J actively represses Notch target genes through recruitment of corepressor complexes. My laboratory has extensively characterized this molecular switch (corepressor to coactivator) and characterized several Notch-cofactors (Oswald et al., 2016; Giaimo et al., 2018, Ferrante et al., 2020; zusammengefasst in Giaimo et al.  2021).


Chromatin players in the Notch response: 

Over the last few years we and other investigators have postulated that chromatin-based mechanisms play a key role in the control of Notch target gene expression (reviewed in Giaimo et al., 2021). Some of the key questions are how histone methyltransferases and demethylases dynamically regulate histone methylation at specific target genes, how gene responsiveness is set up by activating and repressing methylation marks and how these enzymes work in concert with histone acetyltransferases (HATs), deacetylases (HDACs), kinases and phosphatases. We have for example demonstrated that Notch signaling dynamically regulates H3K4 methylation and showed that the H3K4 histone demethylase KDM5A plays a crucial role in this process (Liefke et al., 2010 and Oswald et al., 2016).


Characterization of the RBP-J repressor complex in leukemogenesis

Notch signaling have been implicated in the development of several forms of leukemia. We could identify cofactor ETO as part of the RBP-J corepressor complex. A chromosomal fusion protein AML1/ETO can disrupt the corepressor complex leading to a derepression of Notch target genes (Agrawal et al., 2020). We want to further characterize the underlying molecular mechanism that leads to leukemogenesis.


Characterization of the Notch-coactivator complex

Posttranslational modifications (ubiquitinylation, phosphorylation, hydroxylation and acetylation) regulate amplitude and duration of the Notch response (reviewed in Giaimo et al., 2016). Mutations in the C-terminal PEST-domain, that stabilize the Notch protein, have been detected in human T-ALL patients. We could show that the NICD is not only ubiquitinylated but also methylated and acetylated (Hein et al., 2015 and Ferrante et al., 2020) and this controls the protein half-life of the NICD and the ubiquitinylation-state. 


In future, we want to focus on chromatin- and RNA-based molecular mechanisms, that control the expression of Notch target genes. We will employ a mix of cell-biological, biochemical and molecular biological techniques to reach this goal. 


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