Search

Drosophila Orb, the homolog of vertebrate CPEB, is a key translational regulator involved in oocyte polarity and maturation through poly(A) tail elongation of specific mRNAs. orb also has an essential function during early oogenesis that has not been addressed at the molecular level. Here, we show that orb prevents cell death during early oogenesis, thus allowing oogenesis to progress. It does so through the repression of autophagy by directly repressing, together with the CCR4 deadenylase, the translation of Autophagy-specific gene 12 (Atg12) mRNA. Autophagy and cell death observed in orb mutant ovaries are reduced by decreasing Atg12 or other Atg mRNA levels. These results reveal a role of Orb in translational repression and identify autophagy as an essential pathway regulated by Orb during early oogenesis. Importantly, they also establish translational regulation as a major mode of control of autophagy, a key process in cell homeostasis in response to environmental cues., (Copyright © 2015 Elsevier Inc. All rights reserved.)

The Piwi-interacting RNA (piRNA) pathway plays an essential role in the repression of transposons in the germline. Other functions of piRNAs such as post-transcriptional regulation of mRNAs are now emerging. Here, we perform iCLIP with the PIWI protein Aubergine (Aub) and identify hundreds of maternal mRNAs interacting with Aub in the early Drosophila embryo. Gene expression profiling reveals that a proportion of these mRNAs undergo Aub-dependent destabilization during the maternal-to-zygotic transition. Strikingly, Aub-dependent unstable mRNAs encode germ cell determinants. iCLIP with an Aub mutant that is unable to bind piRNAs confirms piRNA-dependent binding of Aub to mRNAs. Base pairing between piRNAs and mRNAs can induce mRNA cleavage and decay that are essential for embryonic development. These results suggest general regulation of maternal mRNAs by Aub and piRNAs, which plays a key developmental role in the embryo through decay and localization of mRNAs encoding germ cell determinants., (Copyright © 2015 The Authors. Published by Elsevier Inc. All rights reserved.)

Translational regulation plays an essential role in Drosophila ovarian germline stem cell (GSC) biology. GSC self-renewal requires two translational repressors, Nanos (Nos) and Pumilio (Pum), which repress the expression of differentiation factors in the stem cells. The molecular mechanisms underlying this translational repression remain unknown. Here, we show that the CCR4 deadenylase is required for GSC self-renewal and that Nos and Pum act through its recruitment onto specific mRNAs. We identify mei-P26 mRNA as a direct and major target of Nos/Pum/CCR4 translational repression in the GSCs. mei-P26 encodes a protein of the Trim-NHL tumor suppressor family that has conserved functions in stem cell lineages. We show that fine-tuning Mei-P26 expression by CCR4 plays a key role in GSC self-renewal. These results identify the molecular mechanism of Nos/Pum function in GSC self-renewal and reveal the role of CCR4-NOT-mediated deadenylation in regulating the balance between GSC self-renewal and differentiation.

Bicaudal-C (Bic-C) encodes an RNA-binding protein required maternally for patterning the Drosophila embryo. We identified a set of mRNAs that associate with Bic-C in ovarian ribonucleoprotein complexes. These mRNAs are enriched for mRNAs that function in oogenesis and in cytoskeletal regulation, and include Bic-C RNA itself. Bic-C binds specific segments of the Bic-C 5' untranslated region and negatively regulates its own expression by binding directly to NOT3/5, a component of the CCR4 core deadenylase complex, thereby promoting deadenylation. Bic-C overexpression induces premature cytoplasmic-streaming, a posterior-group phenotype, defects in Oskar and Kinesin heavy chain:betaGal localization as well as dorsal-appendage defects. These phenotypes are largely reciprocal to those of Bic-C mutants, and they affect cellular processes that Bic-C-associated mRNAs are known, or predicted, to regulate. We conclude that Bic-C regulates expression of specific germline mRNAs by controlling their poly(A)-tail length.

Translational control of maternal mRNA through regulation of poly(A) tail length is crucial during early development. The nuclear poly(A) binding protein, PABP2, was identified biochemically from its role in nuclear polyadenylation. Here, we analyze the in vivo function of PABP2 in Drosophila. PABP2 is required in vivo for polyadenylation, and Pabp2 function, including poly(A) polymerase stimulation, is essential for viability. We also demonstrate an unanticipated cytoplasmic function for PABP2 during early development. In contrast to its role in nuclear polyadenylation, cytoplasmic PABP2 acts to shorten the poly(A) tails of specific mRNAs. PABP2, together with the deadenylase CCR4, regulates the poly(A) tails of oskar and cyclin B mRNAs, both of which are also controlled by cytoplasmic polyadenylation. Both Cyclin B protein levels and embryonic development depend upon this regulation. These results identify a regulator of maternal mRNA poly(A) tail length and highlight the importance of this mode of translational control.