393B Poster - 04. Stem cells, regeneration and tissue injury
Friday April 08, 2:00 PM - 4:00 PM

Authors:
Diego Sainz de la Maza ¹; Silvana Hof-Michel ²; Lee Phillimore ¹; Christian Bökel ^2,3; Marc Amoyel ¹

Affiliations:
1) University College London, UK; 2) Philipps University Marburg, Germany; 3) Universität Ulm, Germany

Keywords:
p. other (Testis cyst stem cells); bb. other (Cell cycle exit and differentiation)

Stem cells maintain tissue homeostasis by proliferating to replace cells lost to damage or natural turnover. To do so, they need to balance self-renewal and differentiation. Whereas stem cells proliferate, terminal differentiation is accompanied by exit from the cell cycle, so we asked how cell identity was coordinated with cell cycle exit. We use the Drosophila testis as a model. The testis niche supports two stem cell populations, germ line stem cells and somatic cyst stem cells (CySCs) by providing signals necessary for their self-renewal. CySCs proliferate while their differentiating daughter cells do not, providing an ideal model to ask how stem cell identity is linked to proliferation. We show that Cyclin E is required for CySC self-renewal and blocking the G1/S transition causes premature differentiation. Conversely, knocking down Rbf, the homologue of Retinoblastoma that inhibits the G1/S transition, expanded the CySC population and blocked its differentiation. Rbf functions by inhibiting the complex E2f1/Dp. Surprisingly, E2f1/Dp activity was not required for self-renewal, suggesting that the endogenous role E2f1/Dp activity is not in promoting cell cycle progression but to prevent differentiation in cycling cells. To determine how Rbf inhibited differentiation, we analysed gene expression upon Rbf knockdown. Genes regulating mitochondrial biology and energy production were downregulated in Rbf-deficient testes. Promoting mitochondrial biogenesis and activity in Rbf knockdowns by expressing the PGC1α homologue, Spargel (Srl) and Ets97D/NRF-2 rescued differentiation of CySCs. However, we observed ectopic cycling in rescued, differentiated cells suggesting that oxidative metabolism is essential for CySCs differentiation but is not sufficient to exit the cell cycle. Thus, our results indicate that Rbf coordinates cell cycle exit and differentiation by inhibiting E2f1/Dp and promoting a metabolic state that is permissive for differentiation.