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CURRENT RESEARCH: Stem Cells, Wnt signaling and Tissue Repair
(Updated: September 2011) 
 

Roel Nusse Principal Investigator		Our laboratory is interested in the growth, development and integrity of animal tissues. We study multiple different organs, trying to identify common principles, and we extend these investigations to cancer and injury repair. In most organs, different cell types are generated by stem cells - cells that also make copies of themselves and thereby maintain the tissue. An optimal balance between the number of stem and differentiated cells is essential for the proper function of the organs. Locally-acting signals are important to maintain this balance in a spatially-organized manner and these signals are key to understanding the regulation of growth.    A common theme linking our work together are Wnt signals.  Work from many laboratories, including our own, has shown that Wnt proteins are essential for the control over stem cells. How this is achieved is far from clear and is the subject of studies in the lab, both in vivo and in cell culture.  In vivo, a particular question we address is how physiological changes, such as those occurring during hormonal stimuli, injury or programmed tissue degeneration have an impact on the self-renewal signals and on stem cell biology.    Specifically, our research addresses the following main questions: 1. Can we identify Wnt-responding stem cells in tissues? 2. Where are Wnt genes active and how is their expression controlled? 3. Do Wnt proteins act on stem cells directly? 4. Can Wnt proteins control asymmetric cell division?

 

SPECIFIC QUESTIONS

Renée van Amerongen Angela Bowman Xinhong Lim		Can we identify Wnt-responding stem cells in tissues?    Identifying stem cells in tissues requires us to label the cells and to track their developmental fate. We approach this question by lineage tracing and transplantation experiments, using the Wnt target gene Axin2. We have inserted the Cre-ERT2 gene into the Axin2 locus. In combination with reporters such as mTmG, we can mark the Axin2-expressing, Wnt-responsive cells and their descendants. We found that this protocol labels stem cells in multiple organs, including the mammary gland, intestine (images on right), skin, brain and muscle.  The mammary gland is an excellent organ to study stem cell responses to changes in physiology, as its development is regulated by local and systemic hormones as well as by age. A key question in this field is whether long term self-renewing stem cells are the source of the growth of the tissue over multiple rounds of pregnancy. Using the AXCT2 strain, we are able to trace stem cells and assess their developmental potential in the mammary gland. In combination with classical transplantation methods, this approach provides a unique opportunity to ask fundamental questions regarding stem cell biology.  In related research, we use similar labeling protocols to identify stem cells in the skin and the brain. We are also interested in understanding whether stem cells are the precursors to cancer, and we examine the origin of skin cancer cells using lineage tracing.		Intestine Mammary gland
Catriona Logan Makiko Mizutani Si Hui Tan Bruce Wang		Where are Wnt genes active and how is their expression controlled?   External signals are essential for the self-renewal of the stem cells and are often produced by cells in close vicinity, in a structural arrangement commonly called a niche. There is overall very little known, however, about the exact nature of mammalian stem cell niches, including what kind of cells constitute a niche, or even what the self-renewing signals are. During our research on the role of Wnts and stem cells we have identified a transcriptional control element that governs the expression of Wnt signals, in multiple presumed stem cell niches. Reporter genes linked to this enhancer are active in brain, hair follicle, and bone tissues, in each case close to stem cells. Interestingly, these reporters and the adjacent Wnt genes are induced to be expressed after damaging tissues. This suggests that niche-derived Wnt signals are activated by injury, as a first step to recruit stem cells for tissue repair.
Timothy Blauwkamp		Do Wnt proteins act on stem cells directly?   To address this question, we have developed methods to purify active Wnt proteins. During this work, we established that Wnt proteins are unusual, in that they are modified by fatty acids, making the Wnt protein very hydrophobic.  With the purified active Wnt in hand, we tested how the protein can be used to manipulate the behavior of stem cells in culture. These studies have borne fruit in several different stem cell areas, including in embryonic stem (ES) cells and mammary stem cells.  In these cases, we established that we can expand stem cells in an undifferentiated state, with full retention of specific differentiation capacities. The latter is revealed when the Wnt protein is withdrawn and the stem cells are transplanted in vivo. We have also established that Wnt protein is specifically required for preventing the differentiation that ES cells would undergo into epiblast-like stem cells which have different characteristics compared to embryonic stem cell. These findings have significant fundamental and practical implications, as one of the key questions in the stem cell field is how to control the decisions that these cells make to stay undifferentiated or to become committed.
Shukry Habib		Can Wnt proteins control asymmetric cell divisions?   To better understand how Wnt signals affect stem cell division, we study the direct effect of recombinant Wnt proteins on the division mode (symmetric vs. asymmetric) of purified populations of embryonic stem cells. We developed a novel strategy that directs the Wnt proteins to one specific side of the cell and therefore allows the induction of cell polarity. The division mode of the cell is monitored by time lapse imaging. Our data shows that a local source of Wnt proteins is sufficient to induce asymmetric division of mESCs. Furthermore, during the division, the cell in contact with the Wnt source maintains its pluripotency whereas the daughter cell that is farther from the Wnt source is prone to differentiate.