Institut Pasteur, Paris
2007 - 2011
Identification of deubiquitination processes regulating Notch signaling
Notch signaling is a highly conserved pathway from Caenorhabditis elegans to mammals. It is absolutely required from embryonic development, and it controls many different processes such as cell differentiation, cell fate decision, cell proliferation, stem cells maintenance or apoptosis.
The signaling is based on the activity of Notch receptor, which is expressed as an heterodimeric transmembrane protein, and which is activated during intercellular contacts by the binding of its transmembrane ligands expressed by neighboring cells. Consequently to this activation, Notch undergoes two successive proteolytic cleavages, respectively catalyzed by an ADAM metalloprotease and the γ-secretase complex. This last cleavage releases Notch intracellular domain in the cytoplasm, then this activated form is translocated to the nucleus where it finally directly promotes Notch target genes transcription by binding its transcription co-factors CSL and Mastermind.
Notch receptor is regulated at several steps by ubiquitination processes : activated Notch monoubiquitination controls its γ-secretase cleavage, non-activated Notch polyubiquitination controls its lysosomal degradation.
Ubiquitination processes are reversed by deubiquitination events. No deubiquitination step has been highlighted in Notch signaling yet. My thesis project consisted in identifying the deubiquitinating enzymes – the enzymes responsible for deubiquitination – involved in these ubiquitination processes, and in determining their functional role in Notch signaling activity. To this end, we set up two immunofluorescence screens to test an shRNA library targeting each one of the 91 known or putative deubiquitinating enzymes of the human genome.
The first screen allowed me to identify eIF3f, known so far as a subunit of the translation initiation factor 3, as a new protein carrying a deubiquitinating activity. eIF3f is recruited to activated Notch via the E3 ubiquitin ligase Deltex, and eventually enhances Notch signaling by deubiquitinating the activated receptor before the γ-secretase cleavage, thus triggering the production of transcriptionally active Notch.
In addition, the second screen identified USP12 that deubiquitinates non-activated Notch prior to lysosomal degradation of the receptor. As a consequence, USP12 participates in the decrease of Notch molecules expressed at the plasma membrane and negatively regulates Notch signaling strength.
* Publications *
As first author :
The translation initiation factor 3f (eIF3f) exhibits a deubiquitinase activity regulating Notch activation.
Julien Moretti et al., PLoS Biology 2010
The ubiquitin-specific protease 12 (USP12) is a negative regulator of Notch signaling acting on Notch receptor trafficking toward degradation.
Julien Moretti et al., Journal of Biological Chemistry 2012
Ubiquitinations in the Notch signaling pathway (Review).
Julien Moretti et Christel Brou, International Journal of Molecular Sciences 2013
As co-author :
The adaptor-associated kinase 1, AAK1, is a positive regulator of the Notch pathway.
Neetu Gupta-Rossi et al., Journal of Biological Chemistry 2011
TspanC8 tetraspanins regulate ADAM10/Kuzbanian trafficking and promote Notch activation in flies and mammals.
Emmanuel Dornier et al., Journal of Cell Biology 2012
A glycosphingolipid binding domain controls trafficking and activity of the mammalian notch ligand delta-like 1.
Sara F Heuss et al., PLoS One 2013
* Fellowships *
French Ministry of Research - October 2007 to September 2010
Association pour la Recherche sur le Cancer - October 2010 to September 2011