Gilbert VASSART group


Stem cells of the digestive tract:

role in development, homeostasis and regeneration













Tissues from the digestive tract, such as the small intestine or the stomach, are amongst the epithelia with the highest self-renewing capacity in adult mammals, and thereby represent an excellent model system to study stem cell biology. Under homeostatic conditions, the constant renewal is ensured by an active pool of adult stem cells that are characterized by a high proliferation rate and expression of the G protein coupled receptors Lgr4 and Lgr5 (Leucine-rich repeat G-protein coupled receptors 4/5). In the recent years, increasing evidence is accumulating for the existence of wide cellular plasticity, especially in case of regeneration occurring after epithelial injury. Reserve pools of quiescent stem cells can then be activated to maintain tissue homeostasis and, in some cases, cells committed to a differentiated fate can dedifferentiate to a stem cell-like state [for review, Fernandez-Vallone and Garcia, 2016].
The goal of our research group is to better understand the molecular and cellular mechanisms that contribute to tissue development, homeostasis and regeneration in epithelia of the digestive tract. The system used for our studies is the mouse model. We combine two kind of approaches: i) an in vivo approach, via phenotypical analysis of transgenic knock-in, knockout and Cre recombinase lines and ii) an ex vivo approach, through studies on primary cultures of mouse stem cells which can grow as organoids and keep self-renewing and differentiating for indefinite periods in a Petri dish.










    Stem cells, Lgr receptors and homeostasis



One of our aims is to molecularly dissect the biological function of the Lgrs 4/5/6 receptors under homeostatic conditions. These receptors share structural characteristics with the glycoprotein hormone (TSH, LH/CG and FSH) receptors but their natural agonists and downstream signaling pathways appear different from those of classical GPCRs. Currently, Rspondins are considered redundant ligands of the three Lgrs. The identification by the Clevers group of Lgr5 as a specific marker of epithelial adult stem cells in the digestive tract and other tissues [Barker, 2007] has stimulated intense interest, in particular because it is also expressed by cancer initiating cells, mainly associated with activating mutations in the Wnt signaling pathway
Using mouse transgenic knockin/knockout Lgr4 and Lgr5 models together with the ex vivo culture system of mouse intestinal stem cells [Sato, 2009], we and others have previously demonstrated that LGR4 acts as a permissive regulator of the Wnt pathway in the postnatal period, playing an essential role in epithelial stem cell maintenance and differentiation toward the Paneth cell lineage [Mustata, 2011]. While this observation is compatible with Rspondins as being Lgr4 ligands and leading to enhancement of the Wnt pathway, the role of Lgr5 in this pathway remains controversial. Indeed, its deficiency during the mouse prenatal period leads to Wnt overactivation associated with precocious Paneth cell differentiation [Garcia, 2009;Kinzel 2014]. In line with these in vivo studies, and contrary to the prevalent view, LGR5 has been reported to exert a negative regulatory role on tumorigenicity by antagonizing the Wnt cascade in colorectal cancer cell lines via specific recognition of Rspondin2, which would act as a tumour suppressor gene [Walker, 2011; Wu, 2014]. The molecular mechanisms involved in Lgr receptor activation are under investigation. This would help to develop specific drugs to target cancer initiating cells expressing the Lgr receptors.






    Developmental progenitors and cell markers

We have isolated and characterized the murine fetal intestinal and gastric progenitor cells using the three dimensional ex vivo culture system and by lineage tracing in vivo. Our results have shown that these epithelia in mammals are generated in two separate waves – a fetal and an adult one - which depend on two different types of stem cells. Ex vivo, these two kinds of self-renewing cells generate different types of tridimensional structures, growing as spheroids and organoids, respectively [Mustata, 2013, Fernandez-Vallone et al., 2016]. Some of the progenitor markers have been identified in fetal spheroids, such as Trop2 (tacstd2) or Cnx43 (Gja1).


A general protocol for ex vivo culture of these progenitors is available (Fernadez-Vallone et al, 2017a).




    Stem cells and tissue regeneration

We also study the various kinds of stem/progenitor cells that can be reactivated under regenerative conditions following different types of tissue damages in order to identify new cell markers with potential interest in the pathophysiology of inflammatory bowel diseases or in cancer. The regenerative capacity of an adult tissue has been correlated to the presence of undifferentiated stem cells or to the plasticity of differentiated cells. We have recently shown that epithelial injury in the mouse adult glandular stomach induces a regenerative process associated with partial re-expression of a fetal program (Fernandez-Vallone et al., 2016). The function, if any, of several markers common to the fetal progenitors and adult regenerating cells, is currently under study in vivo and ex vivo.


A general protocol for ex vivo culture of these regenerating cells  is available (Fernadez-Vallone et al, 2017b).






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