In this research project, we address a the ability of stem cells to divide asymmetrically, a major question in the stem cell biology. We focus on testing whether asymmetric division can be controlled by external signals, such as those provided by stem cell niches. In earlier work, we found that Wnt signals when applied locally to stem cells, can control stem cell fate and cell division orientation simultaneously. During division, this external cue maintains stem cell fate in the daughter cell that stays in contact with the signal. At the same time, the signal determines the polarity of cell division. In the grant, we aim to understand at the mechanistic level how stem cells divide asymmetrically and how the Wnt protein acts as an external cue.
We approach this question by a combination of new methodologies. We immobilize the self-renewal factor Wnt on small beads, generating a localized and visually traceable source of the signal. In addition, we examine single live stem cells in culture. Using fluorescence-based reporters, we follow individual stem cells by time-lapse microscopy, as the cells are dividing. By the single cell approach, we aim at obtaining a detailed view of the partitioning of Wnt signaling components, centrosomal proteins and transcription factors. By interfering with the expression of regulatory genes and Wnt signaling components, we aim at defining the signaling pathway operating in Wnt-controlled asymmetric stem cell division.
During the past year, we made advances in several ways. found that human embryonic stem cells can indeed divide asymmetrically when activated by Wnt signals. We also established a human embryonic stem cell line that can be grown as single cells, a tool that is essential for studying several cell biological properties of stem cells.