Stem Cells and Tissue Homeostasis

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Group leader : Allison Bardin
Stem Cells and Tissue Homeostasis

Keywords

Stem cells, Proliferation, Differentiation, Development, Cancer

Plain english

We are using the stem cells in the adult intestine of Drosophila to uncover general mechanisms controlling stem cells and their differentiated daughter progeny. Our work takes advantage of the sophisticated genetic tools available in Drosophila and high-resolution confocal imaging to investigate the cell biology of the intestine. 

 

 

 

 

 

 

 

 

 

Stem cells are essential for development and continued maintenance of tissues and organs. They are characterized by their ability to self-renew as well as to produce differentiated progeny. Understanding the dual capacity of self-renewal and differentiation is an important aim of developmental biology, regenerative medicine, and also has implications for cancer biology.


Regulation of stem cells occurs at many levels: cell intrinsic factors as well as extrinsic signals from the surrounding cells and environment may modulate cell division, stem cell maintenance, and the differentiation process. The aim of work in our group is to identify mechanisms important for these processes and ultimately to understand how they function collectively to promote homeostasis of a tissue. To do so, we are using a simplified model system, the Drosophila midgut (Figure 1). The adult Drosphila midgut intestinal stem cells (ISCs) proliferate to provide differentiated cells over the adult lifetime and also have the capacity to respond rapidly to damage occurring in the gut due to pathogens or abrasion.


Understanding cell fate specification


One of our aims is to understand cell intrinsic signalling required for ISC and daughter cell fate. We are investigating how the Notch signalling pathway controls the fine balance of cell types in the intestine. We found that a modulator of Notch signalling is specifically required for commitment of the ISC daughter cell (EB), thereby acting to limit the number of stem cells. This modulator, however, is completely dispensable for Notch-mediated differentiation of the EB. Our data suggest that a high-threshold Notch signalling barrier must be surmounted in order for proper commitment to occur, whereas differentiation can occur properly with low threshold signalling. We are currently investigating the nature of this barrier, the temporal control of fate decisions, and trying to assess dynamic Notch signalling in this system.  

Identifying novel genes controlling stem and daughter cells

We have also taken genetic and genomic approaches to gain broader insight into control of ISCs. Using an EMS-based forward genetic screen, we identified genes required to limit stem cell number and proliferation, control stem cell maintenance, and regulate differentiated daughter fate. We are currently using molecular and genetic approaches to further investigate the roles of these genes in stem cell biology. In parallel, we have taken a genomic approach to identify RNAs that are differentially expressed in the ISC, ee and EC cells and are investigating their potential as regulators and stem cell specific markers. Using these two complementary approaches, we hope to identify both regulatory genes and cellular mechanisms controlling stem and differentiated cells.

 

Our lab is part of the LABEX DEEP - Development, Epigenesys, Epigenetics, Potential (2012-2020)

Figure 1: The adult Drosophila intestine: Figure 1: The adult Drosophila intestine: The ISC is multipotent: it divides to produce an ISC and a daughter cell, the enteroblast (EB) that will differentiate into one of two types of cells2C an enterocyte (EC) or an enteroendocrine cell (ee) cell. ISCs are located basally next to basement membrane and visceral muscleFigure 1: The adult Drosophila intestine: Figure 1: The adult Drosophila intestine: The ISC is multipotent: it divides to produce an ISC and a daughter cell, the enteroblast (EB) that will differentiate into one of two types of cells2C an enterocyte (EC) or an enteroendocrine cell (ee) cell. ISCs are located basally next to basement membrane and visceral muscle

Fig. 2    High-threshold signaling for commitment: Our data suggest a model in which commitment of stem cell daughters requires a transition through high level Notch signaling.Fig. 2 High-threshold signaling for commitment: Our data suggest a model in which commitment of stem cell daughters requires a transition through high level Notch signaling.

 

 

 

Figure 3   Genetic screen identifying genes affecting stem and differentiated cells: Mitotic clones induced in adult intestines (marked in green) show (A) loss of stem cells,(B) ectopic stem cell-like cells  (ISCs marked by Delta expression in red), or (C) ectopic enteroendocrine-like cells (ee marked by Prospero expression in blue).Figure 3 Genetic screen identifying genes affecting stem and differentiated cells: Mitotic clones induced in adult intestines (marked in green) show (A) loss of stem cells,(B) ectopic stem cell-like cells (ISCs marked by Delta expression in red), or (C) ectopic enteroendocrine-like cells (ee marked by Prospero expression in blue).

Key publications

  • Year of publication : 2012

  • The Drosophila adult posterior midgut has been identified as a powerful system in which to study mechanisms that control intestinal maintenance, in normal conditions as well as during injury or infection. Early work on this system has established a model of tissue turnover based on the asymmetric division of intestinal stem cells. From the quantitative analysis of clonal fate data, we show that tissue turnover involves the neutral competition of symmetrically dividing stem cells. This competition leads to stem-cell loss and replacement, resulting in neutral drift dynamics of the clonal population. As well as providing new insight into the mechanisms regulating tissue self-renewal, these findings establish intriguing parallels with the mammalian system, and confirm Drosophila as a useful model for studying adult intestinal maintenance.
  • Year of publication : 2011

  • Tight regulation of self-renewal and differentiation of adult stem cells ensures that tissues are properly maintained. In the Drosophila intestine, both commitment, i.e. exit from self-renewal, and terminal differentiation are controlled by Notch signaling. Here, we show that distinct requirements for Notch activity exist: commitment requires high Notch activity, whereas terminal differentiation can occur with lower Notch activity. We identified the gene GDP-mannose 4,6-dehydratase (Gmd), a modulator of Notch signaling, as being required for commitment but dispensable for terminal differentiation. Gmd loss resulted in aberrant, self-renewing stem cell divisions that generated extra ISC-like cells defective in Notch reporter activation, as well as wild-type-like cell divisions that produced properly terminally differentiated cells. Lowering Notch signaling using additional genetic means, we provided further evidence that commitment has a higher Notch signaling requirement than terminal differentiation. Our work suggests that a commitment requirement for high-level Notch activity safeguards the stem cells from loss through differentiation, revealing a novel role for the importance of Notch signaling levels in this system.
  • Year of publication : 2010

  • Adult stem cells maintain tissue homeostasis by controlling the proper balance of stem cell self-renewal and differentiation. The adult midgut of Drosophila contains multipotent intestinal stem cells (ISCs) that self-renew and produce differentiated progeny. Control of ISC identity and maintenance is poorly understood. Here we find that transcriptional repression of Notch target genes by a Hairless-Suppressor of Hairless complex is required for ISC maintenance, and identify genes of the Enhancer of split complex [E(spl)-C] as the major targets of this repression. In addition, we find that the bHLH transcription factor Daughterless is essential to maintain ISC identity and that bHLH binding sites promote ISC-specific enhancer activity. We propose that Daughterless-dependent bHLH activity is important for the ISC fate and that E(spl)-C factors inhibit this activity to promote differentiation.