The research in my laboratory is aimed at understanding the regulation of growth, development, and integrity of animal tissues. Stem cells play an important role in these processes; in most organs, stem cells generate the specialized cell types but can also self-renew and thereby maintain the tissue. An optimal balance between the number of stem and differentiated cells is essential for proper organ function. Locally acting signals from stem cell niches are important to maintain this balance in a spatially organized manner, and these signals are key to understanding the regulation of growth.
Wnt proteins are among the most prominent stem cell signals. Work from many laboratories, including our own, has shown that Wnt signals are involved in self-renewal of stem cells. How this is achieved is far from clear, and this question is the subject of studies in our lab, both in vivo and in cell culture. We study multiple different organs and stem cell types, trying to identify common principles, and we extend these investigations to cancer and injury repair.
Our research is centered around the following main themes:
Can we identify Wnt-Responsive Stem Cells in Tissues by Lineage Tracing?
Based on our current understanding of Wnt signaling, we have developed a method to label stem cells genetically and to track their developmental fate. We generated a mouse strain where Wnt signaling activates a recombination enzyme. Subsequently, a green fluorescent protein (GFP)-based reporter becomes activated to mark Wnt-responsive cells and their descendants. By this approach, we are able to label stem cells in multiple organs. In recent work, we have described a novel population of Wnt-responsive cells in the liver that act as hepatocyte stem cells. These cells are present pericentrally in the liver lobule, express the Wnt target gene Axin2 and can be followed over time to contribute to the entire hepatocyte population in the lobule (Figure). Importantly, these cells are diploid, unlike the mostly polyploid mature hepatocytes, and are not dependent on injury. Central vein endothelial cells provide an essential source of Wnt signals and act as a liver stem cell niche.
How Is the Expression of Wnt Signals Regulated, in Normal Tissues and after Injury?
In some tissues, such as the liver and the pancreas, Wnt-responsive progenitor cells can only be detected after injury, and these cells then contribute to the repair of the tissue. This suggests that Wnt signals are induced as the consequence of injury, but there is little known about Wnt gene regulation or the nature of mammalian stem cell niches. During our research on the role of Wnt signals and stem cells we have identified a transcriptional control element, an enhancer that governs the expression of Wnt signals in multiple stem cell niches. Reporter genes linked to this enhancer are active in brain, hair follicle, and bone tissues, in each case in areas close to stem cells.
In several cases, including the lung, we found that these reporters and the Wnt genes become expressed only after damage to tissues, possibly as a first step in linking damage to tissue repair. Using these reporters, we are analyzing the molecular pathway between injury detection and the activation of Wnt signals.
Does Oriented Wnt Signaling Polarize Asymmetrically Dividing Stem Cells?
Asymmetric division of stem cells plays a central role in tissue homeostasis and regeneration. This mode of cell division involves partitioning of cell fate determinants and orientation of the mitotic spindle, to achieve maintenance of stem cell numbers and generation of differentiated daughter cells. As our work has shown that the self-renewal of stem cells is dependent on external Wnt signals, we asked whether Wnt signals could operate in a directional, oriented manner on stem cells to orchestrate their asymmetric division. To test this hypothesis, we have developed a novel technology that includes immobilized Wnt proteins on small beads and the application of these to single stem cells in culture. By this method, we are able to activate one specific side of the cell by the locally acting Wnt. We then follow cell division by time-lapse imaging. Our data show that a local source of Wnt proteins sets up the orientation of cell division and the mitotic spindle (Figure 2, in collaboration with Eric Betzig [HHMI, Janelia Farm Research Campus]). The oriented Wnt signal induces asymmetric division of stem cells: the daughter cell in contact with the Wnt source maintains pluripotency, whereas the distal cell differentiates. This will allow us to study the mechanism of asymmetric division of stem cells in a detailed and time-dependent manner.
Grants from the California Institute for Regenerative Medicine provided partial support for this project.
As of February 19, 2016.