Asymmetric Stem Cell Division
Regenerative medicine holds great promise for improving human health by using stem cells to repair damaged tissues or to replace diseased tissues with healthy tissues. Stem cells have the ability to make more stem cells and also to give rise to more differentiated cells that maintain the body's tissues throughout our lives. Both process are key to normal organ maintenance and repair. One mechanism to achieve both tasks is asymmetric cell division (ACD). During ACD, stem cells partition cell fate determinants and orient the mitotic spindle to maintain stem cell numbers while generating differentiated daughter cells. Misregulation of stem cell ACD has been implicated in normal development (i.e. aging) and pathology (i.e. cancer).
Very little is known about how ACD is initiated in mammalian stem cells. We postulate that an external signal can induce ACD. Wnt signaling is known to regulate stem cell self-renewal and thus Wnt proteins are good candidates as external signals that may initiate ACD. To test this hypothesis, we used a bioengineering approach to develop a novel system in which Wnt proteins can be immobilized onto beads and applied locally to embryonic stem (ES) cells.
From single cell analysis to tissue regeneration
1. The molecular mechanism of Wnt-mediated ACD of ES cells. We use real-time 3D super-resolution microscopy to follow the dynamics of components involved in ACD. We focus on Wnt pathway components that have been proposed to be involved in organizing the centrosomes and the mitotic spindle.
2. Exploring Wnt-mediated ACD of adult stem cells and cancer stem cells. We aim to explore the effect of localized Wnt sginals on adult stem cells and compare it to cancer stem cells. The emerging results might pave the path for promoting differentiation of cancer cells.
3. Immobilized Wnt proteins for tissue regeneration. Genetic evidence has implicated the role of Wnt/beta-catenin in the regeneration of a number of tissues. To date, it has been difficult to target Wnt proteins to specific tissue sites. To overcome this obstacle, we will use bioengineering approaches to deliver localized Wnt particles. We aim to target the Wnt particles to different tissues and study their in vivo effects under homeostasis and injury.
Our lab is interested in studying the role of Wnt signaling in asymmetric cell division of mammalian stem cells. To that end, we apply principles from organic chemistry, biochemistry, and stem cell biology in conjunction with advanced imaging techniques to further probe this biological phenomenon
The Role of Localized Wnt signals in Asymmetric Stem Cell Division and Tissue Regeneration
Wnt-mediated asymmetric stem cell division
Using Wnt beads as a localized source of Wnt signaling, We found that during mouse ES cell division at the single cell level, a local source of Wnt could induce polarization of cellular components such as centrosomes and Wnt signaling proteins. The polarization of these molecular components prepared the ES cell to produce two daughter cells with different cell fates. Using advanced three-dimensional live imaging microscopy, we demonstrated that Wnt beads could orient the plane of mitotic division, such that one daughter cell remains in contact with the bead (Wnt-proximal daughter cell) and the other daughter cell is positioned much further from the bead (Wnt-distal daughter cell).
In short, the Wnt bead initiated ACD produces a Wnt-proximal daughter cell that expresses high levels of nuclear ß-catenin (a transducer of Wnt signaling) and pluripotency genes, suggesting that it remained as a stem cell. On the other hand the Wnt-distal daughter cell acquired hallmarks of differentiation towards epiblast stem cell (EpiSC) fate. The results of these studies are published in Science (Habib et al Science 2013). To our knowledge this is the first demonstration of how Wnts can affect single, vertebrate stem cell.
alpha- Tubulin (ES cell)