Ovarian germline stem cell dedifferentiation is cytoneme-dependent
Progenitor cell dedifferentiation is important for stem cell maintenance during tissue repair and age-related stem cell decline. Here, we use the Drosophila ovary as a model to study the role of cytonemes in BMP signalling-directed germline stem cell (GSC) maintenance and dedifferentiation of germ cells to GSCs. We provide evidence that differentiating germ cell cysts extend longer cytonemes that are more polarised towards the niche during dedifferentiation to reactivate BMP signalling. The presence of additional somatic cells in the niche is associated with a failure of germ cell dedifferentiation, consistent with the formation of a physical barrier to cytoneme-niche contact and outcompetition of germ cells for BMP. Using BMP beads in vitro, we show that these are sufficient to induce cytoneme-dependent contacts in Drosophila tissue culture cells. We demonstrate that the Enabled (Ena) actin polymerase is localised to the tips of germ cell cytonemes and is necessary for robust cytoneme formation, as its mislocalisation reduces the frequency, length and directionality of cytonemes. During homeostasis, specifically perturbing cytoneme function through Ena mislocalisation impairs GSC fitness by reducing GSC BMP signalling and niche occupancy. Disrupting cytonemes by targeting Ena during dedifferentiation reduces germ cell BMP responsiveness and the ability of differentiating cysts to dedifferentiate. Overall, our results provide evidence that cytonemes play a fundamental role in establishing polarised signalling and niche occupancy during stem cell maintenance and dedifferentiation. Significance StatementFertility depends not only on germline stem cell (GSC) maintenance during homeostasis, but also on the ability of germ cells to reverse their developmental programme and dedifferentiate during ageing- or stress-induced GSC loss. Here, using the Drosophila ovary as a model, we show that cytonemes are required for GSC asymmetric signalling, niche occupancy and fitness during homeostasis. During dedifferentiation, we demonstrate that cytonemes are necessary for differentiating germ cells to re-access the self-renewal signal and establish polarised signalling, recolonise an empty niche and dedifferentiate to germline stem cells. Our data identify cytonemes as a novel target for strategies aimed at improving stem cell fitness or dedifferentiation, which has huge therapeutic potential for the regeneration of damaged tissues.