Year 4/NCE

We have addressed the ability of stem cells to divide asymmetrically. We have focused on testing whether asymmetric division can be controlled by external signals, such as those provided by stem cell niches. In earlier work, we had 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 aimed at understanding at the mechanistic level how stem cells divide asymmetrically and how the Wnt protein acts as an external cue.

We approached this question by a combination of new methodologies. We immobilized the self-renewal factor Wnt on small beads, generating a localized and visually traceable source of the signal. In addition, we examined single live stem cells in culture. Using fluorescence-based reporters, we followed individual stem cells by time-lapse microscopy, as the cells are dividing. By the single cell approach, we aimed 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 aimed at defining the signaling pathway operating in a Wnt-controlled asymmetric stem cell division.

During the past year, we made advances in several ways. We 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. At the closing of this project, we have made progress in understanding how these important stem cells behave and we have generated new reagents and materials for further study.