Asymmetric stem cell division oriented by a local self-renewing signal
When stem cells divide, the two daughter cells have a choice. Commonly, one of the daughter cells becomes a new stem cell while the other one will be more specialized (or differentiated). This property -- the ability to generate more stem cells (self-renewal) while making differentiated cells simultaneously -- defines a stem cell. Stem cells have the unique ability to divide asymmetrically but how this happens is poorly understood. Moreover, there is little knowledge on the mechanisms by which external signals control asymmetric division of stem cells. In tissues, it is also essential that the orientation of stem cell division is properly regulated. At the most fundamental level, asymmetry and the orientation of cell division are at the heart of stem cell biology.
We have found that we can instruct stem cells to divide in an asymmetric way by applying an external signaling molecule (called Wnt) to stem cells in a spatially controlled way. We found that the proximal daughter cell will become another stem cell while the distal cell is differentiated. We propose to examine the organization of human stem cells as they divide asymmetrically. Using live imaging microscopy and other tools, we intend to follow how critical determinants segregate over the two daughter cells. We expect that the new mechanistic insights into asymmetric stem cell division will ultimately lead to a better understanding of the possible use of stem cells for therapy.
This research proposal aims at understanding asymmetric divisions of stem cells, a fundamental biological property. The research will initially increase our insights into the basic biology of stem cells. In the longer term however, this work will also lead to better methods to manipulate stem cells for therapeutic purposes, as it will be essential to understand the ways that stem cells divide and differentiate.
Our work will also lead to technological advances that will be of use to stem cell researchers and stem cell-based applications. In fact, we have already made several advances in designing methods to direct the growth of stem cells, including the use of artificial niches and the use of specific growth factors that influence stem cells.
By dividing asymmetrically, stem cells maintain their numbers and to generate differentiated daughter cells. In this work, we aim to answer fundamental questions on the regulation and mechanisms of asymmetric stem cell division. We address In particular how asymmetric stem cell division is influenced by external signals, such as those provided by stem cell niches. Using asymmetric exposure of single stem cells to a Wnt protein, we have discovered that the Wnt signal governs both stem cell fate and orientation of division simultaneously. We are now extending this work to include human stem cells, those of epidermal origin and embryonic stem cells. We have designed experiments to elucidate the mechanism of asymmetric stem cell division.
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.