Cell biological mechanisms underlying stem cell competition

A popular view of a stem cell is that it is a cell that always divides asymmetrically to give rise to another stem cell (self-renewing) and a daughter that goes on to give rise to cells that the tissue needs to function (differentiation). However, recent work has found that although there is asymmetry within a tissue as a whole, any individual stem cell can have symmetric outcomes, meaning a single stem cell can divide to give rise to two stem cells or two differentiating daughters. Balance is achieved by ensuring that whenever a stem cell is lost to differentiation, its neighbour will divide symmetrically to replace it (and conversely, when a stem cell divides in a symmetrical self-renewing division, another is lost to differentiation). This means that every stem cell is constantly jostling with its neighbours to remain in the niche, the environment that supports the self-renewal of stem cells. A second implication is that, over time, some stem cell lineages become dominant and replace the others in the niche. This process can be hijacked by stem cells carrying tumour-inducing mutations, suggesting that it is part of the early steps that lead to cancer.
My work proposes to understand how stem cells replace each other, and what biases their decision to either self-renew or differentiate at the expense of their neighbours.
I use the testis of the fruit fly, Drosophila Melanogaster, as a model system to understand the genetics and cell biology that underlie stem cell competition. During my postdoctoral work, I established that the somatic stem cells in the testis, called cyst stem cells or CySCs, undergo stochastic replacement at the niche. Moreover, increasing or decreasing signals that a single CySC normally receives from the niche can skew that stem cell's ability to compete with its neighbours. These signals fall into two classes : cell proliferation-inducing signals and growth-promoting signals. I found that increasing the rate at which a single CySC proliferates increases its likelihood of replacing its neighbours, while increasing the growth of a CySC increases its likelihood of differentiating compared to its neighbours.
As an independent group leader, I propose to investigate how the basic cellular processes that are division and growth control the behaviour of a stem cell relative to its neighbours. I will use a combination of the advanced genetic tools that only Drosophila can provide, along with live imaging and modern molecular biology approaches to visualise, understand and manipulate the events that make stem cells more or less successful at competing with their neighbours.
In the long term, the ability to manipulate the competitiveness of a stem cell will bring great benefits to human health : by giving a stem cell a competitive advantage in occupying the niche, the efficiency of stem cell therapies could be significantly enhanced, as fewer cells would be needed and more diseased tissue could be replaced by the engineered therapeutic stem cells.

The ability of a stem cell to contribute to long term tissue homeostasis depends on its ability to remain in the niche and compete with its neighbours for limited space. This competition is normally a stochastic process but can be biased in favour of a stem cell carrying an oncogenic mutation, such that the mutant clone displaces all wild type cells. The driving force underlying this is an increased cell cycle rate. Conversely, stem cells that leave the niche and differentiate also undergo a selective process, whereby the cell with the highest activity of the growth-promoting TOR pathway differentiates while its neighbours with lower TOR remain in the niche. Thus, there are two opposing inputs that govern the decision a stem cell makes to either self-renew or differentiate.
I use the genetically tractable Drosophila testis as a model to investigate the cell biological mechanisms that confer competitive abilities to a stem cell relative to its neighbours.
In aim 1, I will use live imaging to observe stem cell competition in vivo and describe the relationship between the cell cycle rate and competition. I will carry out a genetic screen for candidate cell cycle regulators in clonal competition assays. Finally, by performing RNA-seq on FACS purified stem cells, I will identify the transcriptional targets of three signalling pathways that affect competitiveness through increased proliferation. In aim 2, I will use live imaging to determine whether competition during differentiation coordinates the development of two separate lineages (soma and germline) to maintain tissue homeostasis. Moreover, I will determine how qualitative changes in protein translation are necessary to promote differentiation of a stem cell lineage.
My work will provide a better understanding of the cell biology underpinning the ability of stem cells to compete with each other, and point to future applications in which manipulating the competitiveness of stem cells can enhance stem cell therapy.

The aims of this research are to identify the cellular mechanisms that control the competitive behaviour of stem cells. I use the fruit fly Drosophila Melanogaster as a simple and tractable model to understand complex cellular behaviours, and expect that my immediate community will benefit from this work. Moreover, I and the researcher on this project will gain skills from this project, particularly new research skills (imaging techniques, molecular biology techniques), but also transferable skills such as communication skills, project management and experience of managing a team. As a basic science proposal, its immediate beneficiaries are mostly within the academic community but there are several other groups that will be positively impacted, both in the short term, over the duration of this fellowship, and in the long term.
1. Stem cell researchers stand to benefit in the short term through the publication of my results. The work proposed here has the potential to significantly inform the way the stem cell field as a whole perceives basic concepts such as self-renewal.
2. Cell biologists studying the regulation of protein synthesis and of cell cycle progression will also benefit, as this work will link those basic cell cycle processes to cell fate decisions in stem cells.
3. In both the short and medium term, this work will impact the field of oncology research, as stem cell competition can be the first step that spreads an oncogenic lesion throughout a tissue. It may therefore be a productive avenue to identify biomarkers of early cancer progression, particularly in the intestine and lung, where it has been shown that oncogenic mutations bias stem cell competition.
4. A long term aim of this research is to be able to manipulate the behaviour of stem cells to make them more or less competitive. This will ultimately be of benefit to therapeutic design in stem cell therapies. By understanding how stem cells can be made more competitive, a better replacement of resident stem cells by therapeutic ones can be achieved, leading to better outcomes for stem cell transplants. This work will therefore impact scientists working to develop such therapies, and eventually affect human health outcomes and be of benefit to society in general.