Drosophila Hephaestus/polypyrimidine tract binding protein is required for dorso-ventral patterning and regulation of signalling between the germline and soma

RNA trafficking and subcellular localization-a review of mechanisms, experimental and predictive methodologies

RNA localization is essential for regulating spatial translation, where RNAs are trafficked to their target locations via various biological mechanisms. In this review, we discuss RNA localization in the context of molecular mechanisms, experimental techniques and machine learning-based prediction tools. Three main types of molecular mechanisms that control the localization of RNA to distinct cellular compartments are reviewed, including directed transport, protection from mRNA degradation, as well as diffusion and local entrapment. Advances in experimental methods, both image and sequence based, provide substantial data resources, which allow for the design of powerful machine learning models to predict RNA localizations. We review the publicly available predictive tools to serve as a guide for users and inspire developers to build more effective prediction models. Finally, we provide an overview of multimodal learning, which may provide a new avenue for the prediction of RNA localization.

Dynamic regulation of ribosome levels and translation during development

The ability of ribosomes to translate mRNAs into proteins is the basis of all life. While ribosomes are essential for cell viability, reduction in levels of ribosomes can affect cell fate and developmental transitions in a tissue specific manner and can cause a plethora of related diseases called ribosomopathies. How dysregulated ribosomes homeostasis influences cell fate and developmental transitions is not fully understood. Model systems such as Drosophila and C. elegans oogenesis have been used to address these questions since defects in conserved steps in ribosome biogenesis result in stem cell differentiation and developmental defects. In this review, we first explore how ribosome levels affect stem cell differentiation. Second, we describe how ribosomal modifications and incorporation of ribosomal protein paralogs contribute to development. Third, we summarize how cells with perturbed ribosome biogenesis are sensed and eliminated during organismal growth.

RNA degradation is required for the germ-cell to maternal transition in Drosophila

In sexually reproducing animals, the oocyte contributes a large supply of RNAs that are essential to launch development upon fertilization. The mechanisms that regulate the composition of the maternal RNA contribution during oogenesis are unclear. Here, we show that a subset of RNAs expressed during the early stages of oogenesis is subjected to regulated degradation during oocyte specification. Failure to remove these RNAs results in oocyte dysfunction and death. We identify the RNA-degrading Super Killer complex and No-Go Decay factor Pelota as key regulators of oogenesis via targeted degradation of specific RNAs expressed in undifferentiated germ cells. These regulators target RNAs enriched for cytidine sequences that are bound by the polypyrimidine tract binding protein Half pint. Thus, RNA degradation helps orchestrate a germ cell-to-maternal transition that gives rise to the maternal contribution to the zygote.

Cell Cycle Kinase Polo Is Controlled by a Widespread 3' Untranslated Region Regulatory Sequence in Drosophila melanogaster

Alternative polyadenylation generates transcriptomic diversity, although the physiological impact and regulatory mechanisms involved are still poorly understood. The cell cycle kinase Polo is controlled by alternative polyadenylation in the 3' untranslated region (3'UTR), with critical physiological consequences. Here, we characterized the molecular mechanisms required for polo alternative polyadenylation. We identified a conserved upstream sequence element (USE) close to the polo proximal poly(A) signal. Transgenic flies without this sequence show incorrect selection of polo poly(A) signals with consequent downregulation of Polo expression levels and insufficient/defective activation of Polo kinetochore targets Mps1 and Aurora B. Deletion of the USE results in abnormal mitoses in neuroblasts, revealing a role for this sequence in vivo We found that Hephaestus binds to the USE RNA and that hephaestus mutants display defects in polo alternative polyadenylation concomitant with a striking reduction in Polo protein levels, leading to mitotic errors and aneuploidy. Bioinformatic analyses show that the USE is preferentially localized upstream of noncanonical polyadenylation signals in Drosophila melanogaster genes. Taken together, our results revealed the molecular mechanisms involved in polo alternative polyadenylation, with remarkable physiological functions in Polo expression and activity at the kinetochores, and disclosed a new in vivo function for USEs in Drosophila melanogaster.

Subcellular Specialization and Organelle Behavior in Germ Cells

Gametes, eggs and sperm, are the highly specialized cell types on which the development of new life solely depends. Although all cells share essential organelles, such as the ER (endoplasmic reticulum), Golgi, mitochondria, and centrosomes, germ cells display unique regulation and behavior of organelles during gametogenesis. These germ cell-specific functions of organelles serve critical roles in successful gamete production. In this chapter, I will review the behaviors and roles of organelles during germ cell differentiation.

Community effects in regulation of translation

Certain forms of translational regulation, and translation itself, rely on long-range interactions between proteins bound to the different ends of mRNAs. A widespread assumption is that such interactions occur only in cis, between the two ends of a single transcript. However, certain translational regulatory defects of the Drosophila oskar (osk) mRNA can be rescued in trans. We proposed that inter-transcript interactions, promoted by assembly of the mRNAs in particles, allow regulatory elements to act in trans. Here we confirm predictions of that model and show that disruption of PTB-dependent particle assembly inhibits rescue in trans. Communication between transcripts is not limited to different osk mRNAs, as regulation imposed by cis-acting elements embedded in the osk mRNA spreads to gurken mRNA. We conclude that community effects exist in translational regulation.

DOI:http://dx.doi.org/10.7554/eLife.10965.001

Genes encode the instructions needed to make proteins and other molecules. To make a protein, the DNA within a gene is copied to produce molecules of messenger ribonucleic acid (mRNA) that are then used as templates to build proteins via a process called translation. This process – which involves protein machines called ribosomes binding to the start of the mRNA – is tightly regulated to control the amounts of particular proteins in cells. For example, in fruit fly ovaries, a protein called Bruno both represses and activates the translation of a gene known as oskar. To achieve this, Bruno binds to regions near the end of the oskar RNA known as Bruno response elements.

It is not clear how Bruno acts to control translation. However, because ribosomes begin translation near the start of the mRNA, while Bruno is bound to regions near the end of the mRNA, there must be long-range interactions between the two ends of the mRNA. It is generally assumed that such long-range interactions only occur between proteins that are bound to the same mRNA molecule. However, in 2010, researchers observed that Bruno response elements within one oskar mRNA could influence the translation of other oskar mRNAs. This is known as “regulation in trans”. Here, Macdonald et al. – including some of the researchers from the earlier work – investigated this observation in more detail in fruit flies.

In cells, multiple mRNA molecules and their associated proteins can assemble into particles. Macdonald et al. proposed that the close proximity of many mRNA molecules in these particles could allow trans regulation to take place. Indeed, the experiments found that blocking the assembly of oskar mRNA into particles inhibited trans regulation as expected. Macdonald et al. also asked if trans regulation can occur between mRNAs that encode different proteins. The experiments show that oskar mRNA could block the translation of an mRNA produced by the gurken gene, even when oskar mRNA was not being translated. More work is needed to find out how widely trans regulation is used to control translation.

DOI:http://dx.doi.org/10.7554/eLife.10965.002

High Throughput Sequencing Identifies Misregulated Genes in the Drosophila Polypyrimidine Tract-Binding Protein (hephaestus) Mutant Defective in Spermatogenesis

The Drosophila polypyrimidine tract-binding protein (dmPTB or hephaestus) plays an important role during spermatogenesis. The heph² mutation in this gene results in a specific defect in spermatogenesis, causing aberrant spermatid individualization and male sterility. However, the array of molecular defects in the mutant remains uncharacterized. Using an unbiased high throughput sequencing approach, we have identified transcripts that are misregulated in this mutant. Aberrant transcripts show altered expression levels, exon skipping, and alternative 5’ ends. We independently verified these findings by reverse-transcription and polymerase chain reaction (RT-PCR) analysis. Our analysis shows misregulation of transcripts that have been connected to spermatogenesis, including components of the actomyosin cytoskeletal apparatus. We show, for example, that the Myosin light chain 1 (Mlc1) transcript is aberrantly spliced. Furthermore, bioinformatics analysis reveals that Mlc1 contains a high affinity binding site(s) for dmPTB and that the site is conserved in many Drosophila species. We discuss that Mlc1 and other components of the actomyosin cytoskeletal apparatus offer important molecular links between the loss of dmPTB function and the observed developmental defect in spermatogenesis. This study provides the first comprehensive list of genes misregulated in vivo in the heph² mutant in Drosophila and offers insight into the role of dmPTB during spermatogenesis.

In the right place at the right time: visualizing and understanding mRNA localization

The spatial regulation of protein translation is an efficient way to create functional and structural asymmetries in cells. Recent research has furthered our understanding of how individual cells spatially organize protein synthesis, by applying innovative technology to characterize the relationship between mRNAs and their regulatory proteins, single-mRNA trafficking dynamics, physiological effects of abrogating mRNA localization in vivo and for endogenous mRNA labelling. The implementation of new imaging technologies has yielded valuable information on mRNA localization, for example, by observing single molecules in tissues. The emerging movements and localization patterns of mRNAs in morphologically distinct unicellular organisms and in neurons have illuminated shared and specialized mechanisms of mRNA localization, and this information is complemented by transgenic and biochemical techniques that reveal the biological consequences of mRNA mislocalization.

mRNA localization in the Drosophila germline

Localization and the associated translational control of mRNA is a well established mechanism for segregating cellular protein expression. Drosophila has been instrumental in deciphering the prevailing mechanisms of mRNA localization and regulation. This review will discuss the diverse roles of mRNA localization in the Drosophila germline, the cis-elements and cellular components regulating localization and the superimposition of translational regulatory mechanisms. Despite a history of discovery, there are still many fundamental questions regarding mRNA localization that remain unanswered. Take home messages, outstanding questions and future approaches that will likely lead to resolving these unknowns in the future are summarized at the end.

Drosophila polypyrimidine tract-binding protein (DmPTB) regulates dorso-ventral patterning genes in embryos

The Drosophila polypyrimidine tract-binding protein (dmPTB or hephaestus) plays an important role during embryogenesis. A loss of function mutation, heph⁰³⁴²⁹, results in varied defects in embryonic developmental processes, leading to embryonic lethality. However, the suite of molecular functions that are disrupted in the mutant remains unknown. We have used an unbiased high throughput sequencing approach to identify transcripts that are misregulated in this mutant. Misregulated transcripts show evidence of significantly altered patterns of splicing (exon skipping, 5′ and 3′ splice site switching), alternative 5′ ends, and mRNA level changes (up and down regulation). These findings are independently supported by reverse-transcription-polymerase chain reaction (RT-PCR) analysis and in situ hybridization. We show that a group of genes, such as Zerknüllt, z600 and screw are among the most upregulated in the mutant and have been functionally linked to dorso-ventral patterning and/or dorsal closure processes. Thus, loss of dmPTB function results in specific misregulated transcripts, including those that provide the missing link between the loss of dmPTB function and observed developmental defects in embryogenesis. This study provides the first comprehensive repertoire of genes affected in vivo in the heph mutant in Drosophila and offers insight into the role of dmPTB during embryonic development.