Translational control in the Drosophila germline and early embryo – GermlineRNA

Translational regulations are essential at multiple steps during oogenesis and early development. In particular, these regulations are crucial for germline stem cell biology and for early embryogenesis which depends on maternal mRNAs and their regulations. One major regulatory mechanism involves cytoplasmic variations of mRNA poly(A) tail length which regulate mRNA expression by affecting both stability and translation. Poly(A) tail elongation leads to translational activation, whereas poly(A) tail shortening leads to mRNA decay or translational repression. In Drosophila, cytoplasmic polyadenylation and deadenylation play an essential role in embryonic axis formation as they control the production of major determinants for embryonic patterning.
We have been studying cytoplasmic polyadenylation and deadenylation in Drosophila oogenesis and early embryogenesis for several years. Based on our results and the tools and expertise we have developed, we propose an innovative project in which we will address i) the role and mechanisms of translational regulations in germline stem cell biology and ii) the mechanisms of maternal mRNA deadenylation and translational control in the early embryo, in particular, the role of the RNA silencing pathways and of processing (P) bodies and polar granules in this process.
Stem cells are cells that have a high capacity of self-renewal and that can produce at least one type of differentiated progeny. A major question in stem cell biology is to understand how these cells can stay undifferentiated through many rounds of divisions, and how their daughter cells activate a differentiation program. In Drosophila female germline, stem cell maintenance results in part from the repression of the differentiation program by intrinsic translational repressors Nanos and Pumilio. We will analyse the molecular mechanisms of these regulations, including the role of CCR4-mediated deadenylation in translational repression and stem cell maintenance, and the potential role of polyadenylation in translational activation during stem cell differentiation. Stem cell differentiation factors will be identified through RNA co-immunoprecipitation with Pumilio protein.
Recent data have established that CCR4-mediated deadenylation contributes to translational repression by miRNA. Using nanos mRNA, that encodes the posterior morphogen in the Drosophila embryo, as a model, we showed that the three RNA silencing pathways (endosiRNA, miRNA and piRNA pathways) are involved in the deadenylation of nanos mRNA. We will analyse the mechanisms of these regulations. Strikingly, we found that piRNAs target nanos mRNA and are required for its regulation. piRNAs are small RNAs produced from transposable elements and which repress transposable element expression in the germline. Our results, therefore, uncover a novel function of piRNAs and more importantly of transposable elements in gene regulation; they reveal a direct function of transposable elements in development. We will analyse the generality of gene regulation by transposable elements using global bioinformatics and molecular approaches.
mRNA decay and translational regulations take place in cytoplasmic structures, P bodies, whose function has not been clarified. We will address the role of P bodies in vivo, in relation with deadenylation and translational control and their relationships with germline specific granules (polar granules), using imaging techniques.