Enhanced sensitivity of midline glial cells to apoptosis is achieved by HOW(L)-dependent repression of Diap1

RNAi-mediated held-out wing (HOW) gene knockdown inhibits wing expansion of white-backed planthopper, Sogatella furcifera (Hemiptera: Delphacidae)

The white-backed planthopper (Sogatella furcifera), a major migratory pest of rice, has resistance to various chemical treatments. The developmental regulation gene "held-out wing (HOW)" gene can serve as a potential target for RNA interference (RNAi) pesticides. Here, we analyzed spatiotemporal expression pattern and biological function of SfHOW by RNAi-mediated gene silencing. The SfHOW gene contains 628 bp and encodes 196 amino acids. SfHOW was highly expressed in 3rd instar nymphs, followed by 5th instar nymphs. In different tissues, SfHOW is most abundantly expressed in epidermis, with lower levels in wings and legs, and the least expression observed in fat body and gut. RNAi-mediated silencing of SfHOW resulted in a marked decrease in its transcription levels, leading to mortality of 66.8 %. Additionally, 62.0 % emerged adults exhibited folded and curled wings, indicating that SfHOW is crucial for wing expansion of S. furcifera. Following SfHOW silencing, significant reductions in the expression of S. furcifera Apterous (SfAp), suggesting that SfHOW may regulate wing expansion by modulating SfAp, so as to regulate the expression of Dpp gene to participate in the regulation of expansion. This study identified a new target for the development of RNAi-based pesticides for rice pest control and enhances molecular understanding of wing development in Hemipteran insects.

The E2F transcription factor 2: What do we know?

E2F transcription factor 2 (E2F2) is a member of the E2F family of transcription factors. The classical view is that some E2Fs act as "activators" and others "inhibitors" of cell cycle gene expression. However, the so-called "activator" E2F2 is particularly enigmatic, with seemingly contradictory roles in the cell cycle, proliferation, apoptosis, inflammation, and cell migration and invasion. How can we rationalize the apparently opposing functions of E2F2 in different situations? This is difficult because different methods of studying E2F2 have yielded conflicting results, so extrapolating mechanisms from an observed endpoint is challenging. This review will attempt to summarize and clarify these issues. This review focuses on genetic studies that have helped elucidate the biological functions of E2F2 and that have enhanced our understanding of how E2F2 is integrated into pathways controlling the cell cycle, proliferation, apoptosis, inflammation, and cell migration and invasion. This review will also discuss the function of E2F2 in cancer and other diseases. This review provides a strong basis for further research on the biological function and clinical potential of E2F2.

New Insights Into the Role of RNA-Binding Proteins in the Regulation of Heart Development

The regulation of gene expression during development takes place both at the transcriptional and posttranscriptional levels. RNA-binding proteins (RBPs) regulate pre-mRNA processing, mRNA localization, stability, and translation. Many RBPs are expressed in the heart and have been implicated in heart development, function, or disease. This chapter will review the current knowledge about RBPs in the developing heart, focusing on those that regulate posttranscriptional gene expression. The involvement of RBPs at each stage of heart development will be considered in turn, including the establishment of specific cardiac cell types and formation of the primitive heart tube, cardiac morphogenesis, and postnatal maturation and aging. The contributions of RBPs to cardiac birth defects and heart disease will also be considered in these contexts. Finally, the interplay between RBPs and other regulatory factors in the developing heart, such as transcription factors and miRNAs, will be discussed.

The pro-apoptotic activity of Drosophila Rbf1 involves dE2F2-dependent downregulation of diap1 and buffy mRNA

The retinoblastoma gene, rb, ensures at least its tumor suppressor function by inhibiting cell proliferation. Its role in apoptosis is more complex and less described than its role in cell cycle regulation. Rbf1, the Drosophila homolog of Rb, has been found to be pro-apoptotic in proliferative tissue. However, the way it induces apoptosis at the molecular level is still unknown. To decipher this mechanism, we induced rbf1 expression in wing proliferative tissue. We found that Rbf1-induced apoptosis depends on dE2F2/dDP heterodimer, whereas dE2F1 transcriptional activity is not required. Furthermore, we highlight that Rbf1 and dE2F2 downregulate two major anti-apoptotic genes in Drosophila: buffy, an anti-apoptotic member of Bcl-2 family and diap1, a gene encoding a caspase inhibitor. On the one hand, Rbf1/dE2F2 repress buffy at the transcriptional level, which contributes to cell death. On the other hand, Rbf1 and dE2F2 upregulate how expression. How is a RNA binding protein involved in diap1 mRNA degradation. By this way, Rbf1 downregulates diap1 at a post-transcriptional level. Moreover, we show that the dREAM complex has a part in these transcriptional regulations. Taken together, these data show that Rbf1, in cooperation with dE2F2 and some members of the dREAM complex, can downregulate the anti-apoptotic genes buffy and diap1, and thus promote cell death in a proliferative tissue.

The Drosophila RNA-binding protein HOW controls the stability of dgrasp mRNA in the follicular epithelium

Post-transcriptional regulation of RNA stability and localization underlies a wide array of developmental processes, such as axon guidance and epithelial morphogenesis. In Drosophila, ectopic expression of the classically Golgi peripheral protein dGRASP at the plasma membrane is achieved through its mRNA targeting at key developmental time-points, in a process critical to follicular epithelium integrity. However, the trans-acting factors that tightly regulate the spatio-temporal dynamics of dgrasp are unknown. Using an in silico approach, we identified two putative HOW Response Elements (HRE1 and HRE2) within the dgrasp open reading frame for binding to Held Out Wings (HOW), a member of the Signal Transduction and Activation of RNA family of RNA-binding proteins. Using RNA immunoprecipitations, we confirmed this by showing that the short cytoplasmic isoform of HOW binds directly to dgrasp HRE1. Furthermore, HOW loss of function in vivo leads to a significant decrease in dgrasp mRNA levels. We demonstrate that HRE1 protects dgrasp mRNA from cytoplasmic degradation, but does not mediate its targeting. We propose that this binding event promotes the formation of ribonucleoprotein particles that ensure dgrasp stability during transport to the basal plasma membrane, thus enabling the local translation of dgrasp for its roles at non-Golgi locations.

Sex-lethal promotes nuclear retention of msl2 mRNA via interactions with the STAR protein HOW

Female-specific repression of male-specific-lethal-2 (msl2) mRNA in Drosophila melanogaster provides a paradigm for coordinated control of gene expression by RNA-binding complexes. Repression is orchestrated by Sex-lethal (SXL), which binds to the 5' and 3' untranslated regions (UTRs) of the mRNA and inhibits splicing in the nucleus and subsequent translation in the cytoplasm. Here we show that SXL ensures msl2 silencing by yet a third mechanism that involves inhibition of nucleocytoplasmic transport of msl2 mRNA. To identify SXL cofactors in msl2 regulation, we devised a two-step purification method termed GRAB (GST pull-down and RNA affinity binding) and identified Held-Out-Wings (HOW) as a component of the msl2 5' UTR-associated complex. HOW directly interacts with SXL and binds to two sequence elements in the msl2 5' UTR. Depletion of HOW reduces the capacity of SXL to repress the expression of msl2 reporters without affecting SXL-mediated regulation of splicing or translation. Instead, HOW is required for SXL to retain msl2 transcripts in the nucleus. Cooperation with SXL confers a sex-specific role to HOW. Our results uncover a novel function of SXL in nuclear mRNA retention and identify HOW as a mediator of this function.

Multiplexin promotes heart but not aorta morphogenesis by polarized enhancement of slit/robo activity at the heart lumen

The Drosophila heart tube represents a structure that similarly to vertebrates' primary heart tube exhibits a large lumen; the mechanisms promoting heart tube morphology in both Drosophila and vertebrates are poorly understood. We identified Multiplexin (Mp), the Drosophila orthologue of mammalian Collagen-XV/XVIII, and the only structural heart-specific protein described so far in Drosophila, as necessary and sufficient for shaping the heart tube lumen, but not that of the aorta. Mp is expressed specifically at the stage of heart tube closure, in a polarized fashion, uniquely along the cardioblasts luminal membrane, and its absence results in an extremely small heart tube lumen. Importantly, Mp forms a protein complex with Slit, and interacts genetically with both slit and robo in the formation of the heart tube. Overexpression of Mp in cardioblasts promotes a large heart lumen in a Slit-dependent manner. Moreover, Mp alters Slit distribution, and promotes the formation of multiple Slit endocytic vesicles, similarly to the effect of overexpression of Robo in these cells. Our data are consistent with Mp-dependent enhancement of Slit/Robo activity and signaling, presumably by affecting Slit protein stabilization, specifically at the lumen side of the heart tube. This activity results with a Slit-dependent, local reduction of F-actin levels at the heart luminal membrane, necessary for forming the large heart tube lumen. Consequently, lack of Mp results in decreased diastolic capacity, leading to reduced heart contractility, as measured in live fly hearts. In summary, these findings show that the polarized localization of Mp controls the direction, timing, and presumably the extent of Slit/Robo activity and signaling at the luminal membrane of the heart cardioblasts. This regulation is essential for the morphogenetic changes that sculpt the heart tube in Drosophila, and possibly in forming the vertebrates primary heart tube.

Phosphorylation of the Drosophila melanogaster RNA-binding protein HOW by MAPK/ERK enhances its dimerization and activity

Drosophila melanogaster Held Out Wings (HOW) is a conserved RNA-binding protein (RBP) belonging to the STAR family, whose closest mammalian ortholog Quaking (QKI) has been implicated in embryonic development and nervous system myelination. The HOW RBP modulates a variety of developmental processes by controlling mRNA levels and the splicing profile of multiple key regulatory genes; however, mechanisms regulating its activity in tissues have yet to be elucidated. Here, we link receptor tyrosine kinase (RTK) signaling to the regulation of QKI subfamily of STAR proteins, by showing that HOW undergoes phosphorylation by MAPK/ERK. Importantly, we show that this modification facilitates HOW dimerization and potentiates its ability to bind RNA and regulate its levels. Employing an antibody that specifically recognizes phosphorylated HOW, we show that HOW is phosphorylated in embryonic muscles and heart cardioblasts in vivo, thus documenting for the first time Serine/Threonine (Ser/Thr) phosphorylation of a STAR protein in the context of an intact organism. We also identify the sallimus/D-titin (sls) gene as a novel muscle target of HOW-mediated negative regulation and further show that this regulation is phosphorylation-dependent, underscoring the physiological relevance of this modification. Importantly, we demonstrate that HOW Thr phosphorylation is reduced following muscle-specific knock down of Drosophila MAPK rolled and that, correspondingly, Sls is elevated in these muscles, similarly to the HOW RNAi effect. Taken together, our results provide a coherent mechanism of differential HOW activation; MAPK/ERK-dependent phosphorylation of HOW promotes the formation of HOW dimers and thus enhances its activity in controlling mRNA levels of key muscle-specific genes. Hence, our findings bridge between MAPK/ERK signaling and RNA regulation in developing muscles.

By design or by chance: cell death during Drosophila embryogenesis

Cell death plays an essential role during Drosophila embryogenesis. However, it remains an enigma as to what mechanisms determine (or select) the specific cells to be eliminated at a particular developmental stage. Is it mostly dependent on the lineage of the cell, signifying genetic predetermination, or is it due to the failure of a cell to compete for growth factors, which is more or less by chance? Recent developments in studying the molecular mechanism of cell death during Drosophila embryogenesis has provided much insight into our understanding of the relative importance of, and the interaction between, these two mechanisms in shaping the embryo.

Inhibitor of apoptosis proteins in Drosophila: gatekeepers of death

Regulation of apoptosis is crucial to ensure cellular viability, and failure to do so is linked to several human pathologies. The apoptotic cell death programme culminates in the activation of caspases, a family of highly specific cysteine proteases essential for the destruction of the cell. Although best known for their role in executing apoptosis, caspases also play important signalling roles in non-apoptotic processes, such as regulation of actin dynamics, innate immunity, cell proliferation, differentiation and survival. Under such conditions, caspases are activated without killing the cell. Caspase activation and activity is subject to complex regulation, and various cellular and viral inhibitors have been identified that control the activity of caspases in their apoptotic and non-apoptotic roles. Members of the Inhibitor of APoptosis (IAP) protein family ensure cell viability in Drosophila by directly binding to caspases and regulating their activities in a ubiquitin-dependent manner. The observation that IAPs are essential for cell survival in Drosophila, and are frequently deregulated in human cancer, contributing to tumourigenesis, chemoresistance, disease progression and poor patient survival, highlights the importance of this family of caspase regulators in health and disease. Here we summarise recent advances from Drosophila that start to elucidate how the cellular response to caspase activation is modulated by IAPs and their regulators.