In collaboration with a team from the Institute of Cardiovascular Regeneration and the Department of Cardiology at the Goethe University in Frankfurt, directed by Michael Potente, Franck Dequiedt has revealed another pathway enabling the cell to control the levels of the NICD and therefore the strength and the duration of the Notch signal. The results of this study have been published in the journal Nature (1).

When SIRT1 gets involved

“We have worked together on this study because it concerns the “meeting” of two important proteins: Notch and SIRT1”, explains the FNRS researcher. “In our laboratory, we are working on enzymes  called “histone deacetylases” and SIRT1 belongs to this superfamily of enzymes”, specifies Franck Dequiedt. Thus, SIRT1 removes the acetyl groups (CH3-CO-) which are grafted onto proteins. A protein has different properties depending on whether or not it is acetylated. “For the first time, we have shown that NICD is an acetylated protein that is deacetylated by SIRT1”, reveals Franck Dequiedt. Subsequently, when the SIRT1 levels are modified, this automatically affects the levels of NICD inside the cell. “The more NICD is acetylated, the more stable it is and vice versa”, the scientist explains. “We can therefore say that SIRT1 indirectly controls the stability of NICD”. 

Another observation made by the researchers in Liège: there is competition between ubiquitination and acetylation. “If we promote acetylation of NICD by reducing the levels of SIRT1, we impair its ubiquitination, which leads to an increased stability of the NICD”, explains Franck Dequiedt. And conversely: the more SIRT1, the more unstable the NICD will be. Its action on the expression of the genes involved in the development will therefore be reduced. In short, if there are high levels of SIRT1 in the cell, this causes a reduction in the intensity and duration of the Notch signal, whereas if these levels are low, the signal will last longer..

Notch and the formation of new vessels

In order to provide more concrete proof of the link between SIRT1 and the Notch signalling pathway, the researchers chose to demonstrate that these mechanisms function within the vascular system, one of the major systems controlled by the Notch signalling pathway. “We have been working on what is known as ‘tip cell differentiation'”, specifies Franck Dequiedt. New vascular branches emerge from existing vascular trunks. “A cell will sprout, send out filipodia and will thus be the source of a new vascular branch that will grow in a directional manner", explains the researcher. Within this small new growing vessel, not all the cells are equal. There are tip cells and stalk cells. “At the tip of the growing vessel, a cell directs the vessel’s growth and the others follow. The tip cell sprouts and creates new branches while the stalk cells form the trunk of the new vessel”, continues Franck Dequiedt. But how does that relate to the Notch signalling pathway? The cellular specification of tip cells/stalk cells depends precisely on this signalling pathway! The tip cell makes the DLL4 protein (Delta-like 4 protein) which is a Notch receptor ligand. Once released by the tip cell, this protein activates the Notch receptors of the other cells, thus telling them that they must behave like stalk cells. As a result, a reduction in the Notch pathway increases the number of cells behaving like tip cells, which in turn increases the number of branches resulting in a denser vascular network.