Martine Simonelig

Maternal mRNA deadenylation and decay by the piRNA pathway in the early Drosophila embryo

Piwi-associated RNAs (piRNAs), a specific class of 24- to 30-nucleotide-long RNAs produced by the Piwi-type of Argonaute proteins, have a specific germline function in repressing transposable elements. This repression is thought to involve heterochromatin formation and transcriptional and post-transcriptional silencing. The piRNA pathway has other essential functions in germline stem cell maintenance and in maintaining germline DNA integrity. Here we uncover an unexpected function of the piRNA pathway in the decay of maternal messenger RNAs and in translational repression in the early embryo. A subset of maternal mRNAs is degraded in the embryo at the maternal-to-zygotic transition. In Drosophila, maternal mRNA degradation depends on the RNA-binding protein Smaug and the deadenylase CCR4, as well as the zygotic expression of a microRNA cluster. Using mRNA encoding the embryonic posterior morphogen Nanos (Nos) as a paradigm to study maternal mRNA decay, we found that CCR4-mediated deadenylation of nos depends on components of the piRNA pathway including piRNAs complementary to a specific region in the nos 3′ untranslated region. Reduced deadenylation when piRNA-induced regulation is impaired correlates with nos mRNA stabilization and translational derepression in the embryo, resulting in head development defects. Aubergine, one of the Argonaute proteins in the piRNA pathway, is present in a complex with Smaug, CCR4, nos mRNA and piRNAs that target the nos 3′ untranslated region, in the bulk of the embryo. We propose that piRNAs and their associated proteins act together with Smaug to recruit the CCR4 deadenylation complex to specific mRNAs, thus promoting their decay. Because the piRNAs involved in this regulation are produced from transposable elements, this identifies a direct developmental function for transposable elements in the regulation of gene expression.

A complex containing the CCR4 and CAF1 proteins is involved in mRNA deadenylation in Drosophila

The CCR4–NOT complex is the major enzyme catalyzing mRNA deadenylation in Saccharomyces cerevisiae. We have identified homologs for almost all subunits of this complex in the Drosophila genome. Biochemical fractionation showed that the two likely catalytic subunits, CCR4 and CAF1, were associated with each other and with a poly(A)‐specific 3′ exonuclease activity. In Drosophila, the CCR4 and CAF1 proteins were ubiquitously expressed and present in cytoplasmic foci. Individual knock‐down of several potential subunits of the Drosophila CCR4–NOT complex by RNAi in tissue culture cells led to a lengthening of bulk mRNA poly(A) tails. Knock‐down of two individual subunits also interfered with the rapid deadenylation of Hsp70 mRNA during recovery from heat shock. Similarly, ccr4 mutant flies had elongated bulk poly(A) and a defect in Hsp70 mRNA deadenylation. A minor increase in bulk poly(A) tail length was also observed in Rga mutant flies, which are affected in the NOT2 subunit. The data show that the CCR4–NOT complex is conserved in Drosophila melanogaster and plays a role in general and regulated mRNA deadenylation.

Oskar allows nanos mRNA translation in Drosophila embryos by preventing its deadenylation by Smaug/CCR4

Anteroposterior patterning of the Drosophila embryo depends on a gradient of Nanos protein arising from the posterior pole. This gradient results from both nanos mRNA translational repression in the bulk of the embryo and translational activation of nanos mRNA localized at the posterior pole. Two mechanisms of nanos translational repression have been described, at the initiation step and after this step. Here we identify a novel level of nanos translational control. We show that the Smaug protein bound to the nanos 3′ UTR recruits the deadenylation complex CCR4-NOT, leading to rapid deadenylation and subsequent decay of nanos mRNA. Inhibition of deadenylation causes stabilization of nanos mRNA, ectopic synthesis of Nanos protein and head defects. Therefore, deadenylation is essential for both translational repression and decay of nanos mRNA. We further propose a mechanism for translational activation at the posterior pole. Translation of nanos mRNA at the posterior pole depends on oskar function. We show that Oskar prevents the rapid deadenylation of nanos mRNA by precluding its binding to Smaug, thus leading to its stabilization and translation. This study provides insights into molecular mechanisms of regulated deadenylation by specific proteins and demonstrates its importance in development.

Subunits of the Drosophila CCR4-NOT complex and their roles in mRNA deadenylation

The CCR4-NOT complex is the main enzyme catalyzing the deadenylation of mRNA. We have investigated the composition of this complex in Drosophila melanogaster by immunoprecipitation with a monoclonal antibody directed against NOT1. The CCR4, CAF1 (=POP2), NOT1, NOT2, NOT3, and CAF40 subunits were associated in a stable complex, but NOT4 was not. Factors known to be involved in mRNA regulation were prominent among the other proteins coprecipitated with the CCR4-NOT complex, as analyzed by mass spectrometry. The complex was localized mostly in the cytoplasm but did not appear to be a major component of P bodies. Of the known CCR4 paralogs, Nocturnin was found associated with the subunits of the CCR4-NOT complex, whereas Angel and 3635 were not. RNAi experiments in Schneider cells showed that CAF1, NOT1, NOT2, and NOT3 are required for bulk poly(A) shortening and hsp70 mRNA deadenylation, but knock-down of CCR4, CAF40, and NOT4 did not affect these processes. Overexpression of catalytically dead CAF1 had a dominant-negative effect on mRNA decay. In contrast, overexpression of inactive CCR4 had no effect. We conclude that CAF1 is the major catalytically important subunit of the CCR4-NOT complex in Drosophila Schneider cells. Nocturnin may also be involved in mRNA deadenylation, whereas there is no evidence for a similar role of Angel and 3635.

PAP- and GLD-2-type poly(A) polymerases are required sequentially in cytoplasmic polyadenylation and oogenesis in Drosophila

Cytoplasmic polyadenylation has an essential role in activating maternal mRNA translation during early development. In vertebrates, the reaction requires CPEB, an RNA-binding protein and the poly(A) polymerase GLD-2. GLD-2-type poly(A) polymerases form a family clearly distinguishable from canonical poly(A) polymerases (PAPs). In Drosophila, canonical PAP is involved in cytoplasmic polyadenylation with Orb, the Drosophila CPEB, during mid-oogenesis. We show that the female germline GLD-2 is encoded by wispy. Wispy acts as a poly(A) polymerase in a tethering assay and in vivo for cytoplasmic polyadenylation of specific mRNA targets during late oogenesis and early embryogenesis. wispy function is required at the final stage of oogenesis for metaphase of meiosis I arrest and for progression beyond this stage. By contrast, canonical PAP acts with Orb for the earliest steps of oogenesis. Both Wispy and PAP interact with Orb genetically and physically in an ovarian complex. We conclude that two distinct poly(A) polymerases have a role in cytoplasmic polyadenylation in the female germline, each of them being specifically required for different steps of oogenesis.

A Drosophila model of oculopharyngeal muscular dystrophy reveals intrinsic toxicity of PABPN1

Oculopharyngeal muscular dystrophy (OPMD) is an adult‐onset syndrome characterized by progressive degeneration of particular muscles. OPMD is caused by short GCG repeat expansions within the gene encoding the nuclear poly(A)‐binding protein 1 (PABPN1) that extend an N‐terminal polyalanine tract in the protein. Mutant PABPN1 aggregates as nuclear inclusions in OMPD patient muscles. We have created a Drosophila model of OPMD that recapitulates the features of the human disorder: progressive muscle degeneration, with muscle defects proportional to the number of alanines in the tract, and formation of PABPN1 nuclear inclusions. Strikingly, the polyalanine tract is not absolutely required for muscle degeneration, whereas another domain of PABPN1, the RNA‐binding domain and its function in RNA binding are required. This demonstrates that OPMD does not result from polyalanine toxicity, but from an intrinsic property of PABPN1. We also identify several suppressors of the OPMD phenotype. This establishes our OPMD Drosophila model as a powerful in vivo test to understand the disease process and develop novel therapeutic strategies.

Control of poly(A) polymerase level is essential to cytoplasmic polyadenylation and early development in Drosophila

Poly(A) polymerase (PAP) has a role in two processes, polyadenylation of mRNA precursors in the nucleus and translational control of certain mRNAs by cytoplasmic elongation of their poly(A) tails, particularly during early development. It was found recently that at least three different PAP genes exist in mammals, encoding several PAP isoforms. The in vivo specificity of function of each PAP isoform currently is unknown. Here, we analyse PAP function in Drosophila. We show that a single PAP isoform exists in Drosophila that is encoded by the hiiragi gene. This single Drosophila PAP is active in specific polyadenylation in vitro and is involved in both nuclear and cytoplasmic polyadenylation in vivo. Therefore, the same PAP can be responsible for both processes. In addition, in vivo overexpression of PAP does not affect poly(A) tail length during nuclear polyadenylation, but leads to a dramatic elongation of poly(A) tails and a loss of specificity during cytoplasmic polyadenylation, resulting in embryonic lethality. This demonstrates that regulation of the PAP level is essential for controlled cytoplasmic polyadenylation and early development.

Control of maternal mRNA stability in germ cells and early embryos

mRNA regulation is essential in germ cells and early embryos. In particular, late oogenesis and early embryogenesis occur in the absence of transcription and rely on maternal mRNAs stored in oocytes. These maternal mRNAs subsequently undergo a general decay in embryos during the maternal-to-zygotic transition in which the control of development switches from the maternal to the zygotic genome. Regulation of mRNA stability thus plays a key role during these early stages of development and is tightly interconnected with translational regulation and mRNA localization. A common mechanism in these three types of regulation implicates variations in mRNA poly(A) tail length. Recent advances in the control of mRNA stability include the widespread and essential role of regulated deadenylation in early developmental processes, as well as the mechanisms regulating mRNA stability which involve RNA binding proteins, microRNAs and interplay between the two. Also emerging are the roles that other classes of small non-coding RNAs, endo-siRNAs and piRNAs play in the control of mRNA decay, including connections between the regulation of transposable elements and cellular mRNA regulation through the piRNA pathway. This article is part of a Special Issue entitled: RNA Decay mechanisms.

Prevention of oculopharyngeal muscular dystrophy by muscular expression of Llama single-chain intrabodies in vivo

Oculopharyngeal muscular dystrophy (OPMD) is a late onset disorder characterized by progressive weakening of specific muscles. It is caused by short expansions of the N-terminal polyalanine tract in the poly(A) binding protein nuclear 1 (PABPN1), and it belongs to the group of protein aggregation diseases, such as Huntingtons, Parkinsons and Alzheimer diseases. Mutant PABPN1 forms nuclear aggregates in diseased muscles in both patients and animal models. Intrabodies are antibodies that are modified to be expressed intracellularly and target specific antigens in subcellular locations. They are commonly generated by artificially linking the variable domains of antibody heavy and light chains. However, natural single-chain antibodies are produced in Camelids and, when engineered, combined the advantages of being single-chain, small sized and very stable. Here, we determine the in vivo efficiency of Llama intrabodies against PABPN1, using the established Drosophila model of OPMD. Among six anti-PABPN1 intrabodies expressed in muscle nuclei, we identify one as a strong suppressor of OPMD muscle degeneration in Drosophila, leading to nearly complete rescue. Expression of this intrabody affects PABPN1 aggregation and restores muscle gene expression. This approach promotes the identification of intrabodies with high therapeutic value and highlights the potential of natural single-chain intrabodies in treating protein aggregation diseases.

Aubergine iCLIP Reveals piRNA-Dependent Decay of mRNAs Involved in Germ Cell Development in the Early Embryo

Summary 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.