Nucleolar stress in Drosophila neuroblasts, a model for human ribosomopathies

Drosophila Trus, the orthologue of mammalian PDCD2L, is required for proper cell proliferation, larval developmental timing, and oogenesis

Toys are us (Trus) is the Drosophila melanogaster ortholog of mammalian Programmed Cell Death 2-Like (PDCD2L), a protein that has been implicated in ribosome biogenesis, cell cycle regulation, and oncogenesis. In this study, we examined the function of Trus during Drosophila development. CRISPR/Cas9 generated null mutations in trus lead to partial embryonic lethality, significant larval developmental delay, and complete pre-pupal lethality. In mutant larvae, we found decreased cell proliferation and growth defects in the brain and imaginal discs. Mapping relevant tissues for Trus function using trus RNAi and trus mutant rescue experiments revealed that imaginal disc defects are primarily responsible for the developmental delay, while the pre-pupal lethality is likely associated with faulty central nervous system (CNS) development. Examination of the molecular mechanism behind the developmental delay phenotype revealed that trus mutations induce the Xrp1-Dilp8 ribosomal stress-response in growth-impaired imaginal discs, and this signaling pathway attenuates production of the hormone ecdysone in the prothoracic gland. Additional Tap-tagging and mass spectrometry of components in Trus complexes isolated from Drosophila Kc cells identified Ribosomal protein subunit 2 (RpS2), which is coded by string of pearls (sop) in Drosophila, and Eukaryotic translation elongation factor 1 alpha 1 (eEF1α1) as interacting factors. We discuss the implication of these findings with respect to the similarity and differences in trus genetic null mutant phenotypes compared to the haplo-insufficiency phenotypes produced by heterozygosity for mutants in Minute genes and other genes involved in ribosome biogenesis.

Pathological Defects in a Drosophila Model of Alzheimer's Disease and Beneficial Effects of the Natural Product Lisosan G

Alzheimer's disease (AD) brains are histologically marked by the presence of intracellular and extracellular amyloid deposits, which characterize the onset of the disease pathogenesis. Increasing evidence suggests that certain nutrients exert a direct or indirect effect on amyloid β (Aβ)-peptide production and accumulation and, consequently, on AD pathogenesis. We exploited the fruit fly Drosophila melanogaster model of AD to evaluate in vivo the beneficial properties of Lisosan G, a fermented powder obtained from organic whole grains, on the intracellular Aβ-42 peptide accumulation and related pathological phenotypes of AD. Our data showed that the Lisosan G-enriched diet attenuates the production of neurotoxic Aβ peptides in fly brains and reduces neuronal apoptosis. Notably, Lisosan G exerted anti-oxidant effects, lowering brain levels of reactive oxygen species and enhancing mitochondrial activity. These aspects paralleled the increase in autophagy turnover and the inhibition of nucleolar stress. Our results give support to the use of the Drosophila model not only to investigate the molecular genetic bases of neurodegenerative disease but also to rapidly and reliably test the efficiency of potential therapeutic agents and diet regimens.

Reduction of nucleolar NOC1 leads to the accumulation of pre-rRNAs and induces Xrp1, affecting growth and resulting in cell competition

NOC1 is a nucleolar protein necessary in yeast for both transport and maturation of ribosomal subunits. Here, we show that Drosophila NOC1 (annotated CG7839) is necessary for rRNAs maturation and for a correct animal development. Its ubiquitous downregulation results in a dramatic decrease in polysome level and of protein synthesis. NOC1 expression in multiple organs, such as the prothoracic gland and the fat body, is necessary for their proper functioning. Reduction of NOC1 in epithelial cells from the imaginal discs results in clones that die by apoptosis, an event that is partially rescued in a Minute/+ background, suggesting that reduction of NOC1 induces the cells to become less fit and to acquire a 'loser' state. NOC1 downregulation activates the pro-apoptotic Eiger-JNK pathway and leads to an increase of Xrp1, which results in the upregulation of DILP8, a member of the insulin/relaxin-like family known to coordinate organ growth with animal development. Our data underline NOC1 as an essential gene in ribosome biogenesis and highlight its novel functions in the control of growth and cell competition.

A clinically-relevant residue of POLR1D is required for Drosophila development

BACKGROUND

POLR1D is a subunit of RNA Polymerases I and III, which synthesize ribosomal RNAs. Dysregulation of these polymerases cause several types of diseases, including ribosomopathies. The craniofacial disorder Treacher Collins Syndrome (TCS) is a ribosomopathy caused by mutations in several subunits of RNA Polymerase I, including POLR1D. Here, we characterized the effect of a missense mutation in POLR1D and RNAi knockdown of POLR1D on Drosophila development.

RESULTS

We found that a missense mutation in Drosophila POLR1D (G30R) reduced larval rRNA levels, slowed larval growth, and arrested larval development. Remarkably, the G30R substitution is at an orthologous glycine in POLR1D that is mutated in a TCS patient (G52E). We showed that the G52E mutation in human POLR1D, and the comparable substitution (G30E) in Drosophila POLR1D, reduced their ability to heterodimerize with POLR1C in vitro. We also found that POLR1D is required early in the development of Drosophila neural cells. Furthermore, an RNAi screen revealed that POLR1D is also required for development of non-neural Drosophila cells, suggesting the possibility of defects in other cell types.

CONCLUSIONS

These results establish a role for POLR1D in Drosophila development, and present Drosophila as an attractive model to evaluate the molecular defects of TCS mutations in POLR1D.

Comparative transcriptome and proteome reveal synergistic functions of differentially expressed genes and proteins implicated in an over-dominant silkworm heterosis of increased silk yield

We previously observed an over-dominant silkworm heterosis of increased yield in a cross of Bombyx mori nuclear polyhydrosis virus-resistant strain NB with a susceptible strain 306. In the present study, we found that heterosis also exists in crosses of NB with other susceptible strains, indicating it is a more general phenomenon. We performed comparative transcriptome and proteome and identified 1624 differentially expressed genes (DEGs) and 298 differentially expressed proteins (DEPs) in silk glands between parents and F1 hybrids, of which 24 DEGs/DEPs showed consistent expression at mRNA and protein levels revealed by Venn joint analysis. Their expressions are completely non-additive, mainly transgressive and under low-parent, suggesting recombination of parental genomes may be the major genetic mechanism for the heterosis. GO and KEGG analyses revealed that they may function in generally similar but distinctive aspects of metabolisms and processes with signal transduction and translation being most affected. Notably, they may not only up-regulate biosynthesis and transport of silk proteins but also down-regulate other unrelated processes, synergistically and globally remodelling the silk gland to increase yield and cause the heterosis. Our findings contribute insights into the understanding of silkworm heterosis and silk gland development and provide targets for transgenic manipulation to further increase the silk yield.

mRNA Translation Is Dynamically Regulated to Instruct Stem Cell Fate

Stem cells preserve tissue homeostasis by replacing the cells lost through damage or natural turnover. Thus, stem cells and their daughters can adopt two identities, characterized by different programs of gene expression and metabolic activity. The composition and regulation of these programs have been extensively studied, particularly by identifying transcription factor networks that define cellular identity and the epigenetic changes that underlie the progressive restriction in gene expression potential. However, there is increasing evidence that post-transcriptional mechanisms influence gene expression in stem cells and their progeny, in particular through the control of mRNA translation. Here, we review the described roles of translational regulation in controlling all aspects of stem cell biology, from the decision to enter or exit quiescence to maintaining self-renewal and promoting differentiation. We focus on mechanisms controlling global translation rates in cells, mTOR signaling, eIF2ɑ phosphorylation, and ribosome biogenesis and how they allow stem cells to rapidly change their gene expression in response to tissue needs or environmental changes. These studies emphasize that translation acts as an additional layer of control in regulating gene expression in stem cells and that understanding this regulation is critical to gaining a full understanding of the mechanisms that underlie fate decisions in stem cells.

Drosophila to Explore Nucleolar Stress

Nucleolar stress occurs when ribosome production or function declines. Nucleolar stress in stem cells or progenitor cells often leads to disease states called ribosomopathies. Drosophila offers a robust system to explore how nucleolar stress causes cell cycle arrest, apoptosis, or autophagy depending on the cell type. We provide an overview of nucleolar stress in Drosophila by depleting nucleolar phosphoprotein of 140 kDa (Nopp140), a ribosome biogenesis factor (RBF) in nucleoli and Cajal bodies (CBs). The depletion of Nopp140 in eye imaginal disc cells generates eye deformities reminiscent of craniofacial deformities associated with the Treacher Collins syndrome (TCS), a human ribosomopathy. We show the activation of c-Jun N-terminal Kinase (JNK) in Drosophila larvae homozygous for a Nopp140 gene deletion. JNK is known to induce the expression of the pro-apoptotic Hid protein and autophagy factors Atg1, Atg18.1, and Atg8a; thus, JNK is a central regulator in Drosophila nucleolar stress. Ribosome abundance declines upon Nopp140 loss, but unusual cytoplasmic granules accumulate that resemble Processing (P) bodies based on marker proteins, Decapping Protein 1 (DCP1) and Maternal expression at 31B (Me31B). Wild type brain neuroblasts (NBs) express copious amounts of endogenous coilin, but coilin levels decline upon nucleolar stress in most NB types relative to the Mushroom body (MB) NBs. MB NBs exhibit resilience against nucleolar stress as they maintain normal coilin, Deadpan, and EdU labeling levels.

Homeostatic Roles of the Proteostasis Network in Dendrites

Cellular protein homeostasis, or proteostasis, is indispensable to the survival and function of all cells. Distinct from other cell types, neurons are long-lived, exhibiting architecturally complex and diverse multipolar projection morphologies that can span great distances. These properties present unique demands on proteostatic machinery to dynamically regulate the neuronal proteome in both space and time. Proteostasis is regulated by a distributed network of cellular processes, the proteostasis network (PN), which ensures precise control of protein synthesis, native conformational folding and maintenance, and protein turnover and degradation, collectively safeguarding proteome integrity both under homeostatic conditions and in the contexts of cellular stress, aging, and disease. Dendrites are equipped with distributed cellular machinery for protein synthesis and turnover, including dendritically trafficked ribosomes, chaperones, and autophagosomes. The PN can be subdivided into an adaptive network of three major functional pathways that synergistically govern protein quality control through the action of (1) protein synthesis machinery; (2) maintenance mechanisms including molecular chaperones involved in protein folding; and (3) degradative pathways (e.g., Ubiquitin-Proteasome System (UPS), endolysosomal pathway, and autophagy. Perturbations in any of the three arms of proteostasis can have dramatic effects on neurons, especially on their dendrites, which require tightly controlled homeostasis for proper development and maintenance. Moreover, the critical importance of the PN as a cell surveillance system against protein dyshomeostasis has been highlighted by extensive work demonstrating that the aggregation and/or failure to clear aggregated proteins figures centrally in many neurological disorders. While these studies demonstrate the relevance of derangements in proteostasis to human neurological disease, here we mainly review recent literature on homeostatic developmental roles the PN machinery plays in the establishment, maintenance, and plasticity of stable and dynamic dendritic arbors. Beyond basic housekeeping functions, we consider roles of PN machinery in protein quality control mechanisms linked to dendritic plasticity (e.g., dendritic spine remodeling during LTP); cell-type specificity; dendritic morphogenesis; and dendritic pruning.

First person – Sonu S. Baral

First Person is a series of interviews with the first authors of a selection of papers published in Biology Open, helping early-career researchers promote themselves alongside their papers. Sonu S. Baral is first author on ‘Nucleolar stress in Drosophila neuroblasts, a model for human ribosomopathies’, published in BiO. Sonu conducted the research described in this article while a PhD student in Patrick J. DiMario's lab at Louisiana State University, Baton Rouge, LA, USA. She is now a R&D scientist at Thermo Fisher Scientific, Santa Clara, CA, USA, investigating various genetic disorders relevant to reproductive health and newborn screening.