Mechanisms regulating epithelial stem cell behaviour

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Epithelial tissues are central building blocks of the animal and human body. The long-term goal of our research program is to understand how epithelial tissues develop, adopt and maintain specific shapes that determine organ function. During development and homeostasis, many epithelia rely on stem cells for their formation or renewal. It is therefore of fundamental importance to understand how epithelial stem cells are maintained and how their numbers are regulated so that epithelia of normal size and shape can form. This proposal uses the follicle stem cells (FSCs) in the ovary of the fruit fly Drosophila as a model to investigate the molecular and cellular mechanisms that govern the behaviour of epithelial stem cells. Drosophila is a key model in biological research. My laboratory has longstanding experience in using this model to study epithelial biology The FSCs give rise to the follicular epithelium that surrounds the germline cells to form a follicle, which eventually gives rise to an egg. We have identified a so far uncharacterized gene, okapi (oka), as a novel regulator of FSC behaviour. Oka is a conserved FERM domain protein that localizes to adherens junctions (AJs), adhesive structures that link cells together and affect tissue growth and form. Our preliminary observations suggest that Oka has two functions in FSCs. It promotes FSC competitiveness, and it limits the number of FSCs to prevent tissue overgrowth. Our first aim asks whether Oka together with AJs promotes FSC competitiveness. When ovaries are generated that contain oka mutant and wild-type cells, oka mutant FSCs are lost and replaced by more competitive wild-type FSCs. A similar defect was seen for FSCs with compromised AJs. Given that Oka localizes to AJs and considering oka's mutant phenotype, we hypothesize that Oka links AJs to competitiveness. One key feature of AJs is that they are mechanosensitive structures. We will analyze how Oka interacts with AJ proteins and whether mechanosensing has a function in the Oka-AJ interaction that regulates FSC behaviour. Our second aim is to ask whether Oka limits FSC proliferation through the Hippo pathway, a central growth regulator linked to AJs. In contrast to mosaic ovaries that contain oka mutant cells next to wild-type cells, ovaries that lack Oka entirely have too many FSCs and an overabundance of follicle cells, which severely compromises oogenesis and fertility. Our preliminary data suggest that the Hippo pathway shows reduced activity in oka mutant ovaries, and that as a consequence the transcriptional co-activator Yorkie enters the nucleus to stimulate excessive proliferation. This aim will therefore address the hypothesis that Oka acts to limit the proliferation of FSCs as a novel regulator of the Hippo pathway. Overall, this work will clarify the molecular mechanisms of how the maintenance and number of epithelial stem cells are regulated as an essential aspect of normal organ formation and function.