Year 2

In this research project, we address a major question in the stem cell biology, the ability of stem cells to divide asymmetrically. In particular, we are interested in testing whether asymmetric division can be controlled by external signals, such as those provided by stem cell niches. In earlier work, we found that a single external signal, Wnt, 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. We develop a novel immobilization method to couple the Wnt signal to a local source. This method could be applied to other signals as well, to explore how those would affect stem cell divisions. This is an example of bio-engineering, developing technology based on bio-active molecules that influence stem cell behavior.

We also initiated a genome-wide screen to identify genes implicated in Wnt signaling. This screen is based on the novel CRISPR method to mutate genes at will in cells, including in human cells. These reagents and genes should facilitate stem cell research in the wider community.