Down-regulation of inhibitor of apoptosis levels provides competence for steroid-triggered cell death

A pulse of the steroid hormone ecdysone triggers the destruction of larval salivary glands during Drosophila metamorphosis through a transcriptional cascade that converges on reaper (rpr) and head involution defective (hid) induction, resulting in caspase activation and cell death. We identify the CREB binding protein (CBP) transcriptional cofactor as essential for salivary gland cell death. We show that CBP acts 1 d before the onset of metamorphosis in apparent response to a mid-third instar ecdysone pulse, when CBP is necessary and sufficient for down-regulation of the Drosophila inhibitor of apoptosis 1 (DIAP1). It is only after DIAP1 levels are reduced that salivary glands become competent to die through rpr/hid-mediated cell death. Before this time, high levels of DIAP1 block salivary gland cell death, even in the presence of ectopic rpr expression. This study shows that naturally occurring changes in inhibitor of apoptosis levels can be critical for regulating cell death during development. It also provides a molecular mechanism for the acquisition of competence in steroid signaling pathways

RNA localization in development

Cytoplasmic RNA localization is an evolutionarily ancient mechanism for producing cellular asymmetries. This review considers RNA localization in the context of animal development. Both mRNAs and non-protein-coding RNAs are localized in Drosophila, Xenopus, ascidian, zebrafish, and echinoderm oocytes and embryos, as well as in a variety of developing and differentiated polarized cells from yeast to mammals. Mechanisms used to transport and anchor RNAs in the cytoplasm include vectorial transport out of the nucleus, directed cytoplasmic transport in association with the cytoskeleton, and local entrapment at particular cytoplasmic sites. The majority of localized RNAs are targeted to particular cytoplasmic regions by cis-acting RNA elements; in mRNAs these are almost always in the 3'-untranslated region (UTR). A variety of trans-acting factors—many of them RNA-binding proteins—function in localization. Developmental functions of RNA localization have been defined in Xenopus, Drosophila, and Saccharomyces cerevisiae. In Drosophila, localized RNAs program the antero-posterior and dorso-ventral axes of the oocyte and embryo. In Xenopus, localized RNAs may function in mesoderm induction as well as in dorso-ventral axis specification. Localized RNAs also program asymmetric cell fates during Drosophila neurogenesis and yeast budding

Temporal and spacial control of RNA stability in the early embryo of Drosophila melanogaster

Early embryogenesis in metazoa is controlled by maternally synthesized products. Among these products, the mature egg is loaded with transcripts representing approximately two thirds of the genome. A subset of this maternal RNA pool is degraded prior to the transition to zygotic control of development. This transfer of control of development from maternal to zygotic products is referred to as the midblastula transition (or MBT). It is believed that the degradation of maternal transcripts is required to terminate maternal control of development and to allow zygotic control of development to begin. Until now this process of maternal transcript degradation and the subsequent timing of the MBT has been poorly understood. I have demonstrated that in the early embryo there are two independent RNA degradation pathways, either of which is sufficient for transcript elimination. However, only the concerted action of both pathways leads to elimination of transcripts with the correct timing, at the MBT. The first pathway is maternally encoded, is triggered by egg activation, and is targeted to specific classes of mRNAs through cis-acting elements in the 3' untranslated region (UTR}. The second pathway is activated 2 hr after fertilization and functions together with the maternal pathway to ensure that transcripts are degraded by the MBT. In addition, some transcripts fail to degrade at select subcellular locations adding an element of spatial control to RNA degradation. The spatial control of RNA degradation is achieved by protecting, or masking, transcripts from the degradation machinery. The RNA degradation and protection events are regulated by distinct cis-elements in the 3' untranslated region (UTR). These results provide the first systematic dissection of this highly conserved process in development and demonstrate that RNA degradation is a novel mechanism used for both temporal and spatial control of development

A steroid-controlled global switch in sensitivity to apoptosis during Drosophila development

Precise control over activation of the apoptotic machinery is critical for development, tissue homeostasis and disease. In Drosophila, the decision to trigger apoptosis--whether in response to developmental cues or to DNA damage--converges on transcription of inhibitor of apoptosis protein (IAP) antagonists reaper, hid and grim. Here we describe a parallel process that regulates the sensitivity to, rather than the execution of, apoptosis. This process establishes developmental windows that are permissive or restrictive for triggering apoptosis, where the status of cells determines their capacity to die. We characterize one switch in the sensitivity to apoptotic triggers, from restrictive to permissive, that occurs during third-instar larval (L3) development. Early L3 animals are highly resistant to induction of apoptosis by expression of IAP-antagonists, DNA-damaging agents and even knockdown of the IAP diap1. This resistance to apoptosis, however, is lost in wandering L3 animals after acquiring a heightened sensitivity to apoptotic triggers. This switch in sensitivity to death activators is mediated by a change in mechanisms available for activating endogenous caspases, from an apoptosome-independent to an apoptosome-dependent pathway. This switch in apoptotic pathways is regulated in a cell-autonomous manner by the steroid hormone ecdysone, through changes in expression of critical pro-, but not anti-, apoptotic genes. This steroid-controlled switch defines a novel, physiologically-regulated, mechanism for controlling sensitivity to apoptosis and provides new insights into the control of apoptosis during development

The Hob Proteins: Putative, Novel Lipid Transfer Proteins at ER-PM Contact Sites

Nonvesicular transfer of lipids at membrane contact sites (MCS) has recently emerged as a critical process for cellular function. Lipid transfer proteins (LTPs) mediate this unique transport mechanism, and although several LTPs are known, the cellular complement of these proteins continues to expand. Our recent work has revealed the highly conserved but poorly characterized Hobbit/Hob proteins as novel, putative LTPs at endoplasmic reticulum-plasma membrane (ER-PM) contact sites. Using both S. cerevisiae and D. melanogaster model systems, we demonstrated that the Hob proteins localize to ER-PM contact sites via an N-terminal ER membrane anchor and conserved C-terminal sequences. These conserved C-terminal sequences bind to phosphoinositides (PIPs), and the distribution of PIPs is disrupted in hobbit mutant cells. Recently released structural models of the Hob proteins exhibit remarkable similarity to other bona fide LTPs, like VPS13A and ATG2, that function at MCS. Hobbit is required for viability in Drosophila, suggesting that the Hob proteins are essential genes that may mediate lipid transfer at MCS. </jats:p

A novel superfamily of bridge-like lipid transfer proteins

Lipid transfer proteins mediate nonvesicular transport of lipids at membrane contact sites to regulate the lipid composition of organelle membranes. Recently, a new type of bridge-like lipid transfer protein has emerged; these proteins contain a long hydrophobic groove and can mediate bulk transport of lipids between organelles. Here, we review recent insights into the structure of these proteins and identify a repeating modular unit that we propose to name the repeating β-groove (RBG) domain. This new structural understanding conceptually unifies all the RBG domain-containing lipid transfer proteins as members of an RBG protein superfamily. We also examine the biological functions of these lipid transporters in normal physiology and disease and speculate on the evolutionary origins of RBG proteins in bacteria

RNA LOCALIZATION IN DEVELOPMENT

Cytoplasmic RNA localization is an evolutionarily ancient mechanism for producing cellular asymmetries. This review considers RNA localization in the context of animal development. Both mRNAs and non-protein-coding RNAs are localized in Drosophila, Xenopus, ascidian, zebrafish, and echinoderm oocytes and embryos, as well as in a variety of developing and differentiated polarized cells from yeast to mammals. Mechanisms used to transport and anchor RNAs in the cytoplasm include vectorial transport out of the nucleus, directed cytoplasmic transport in association with the cytoskeleton, and local entrapment at particular cytoplasmic sites. The majority of localized RNAs are targeted to particular cytoplasmic regions by cis-acting RNA elements; in mRNAs these are almost always in the 3′-untranslated region (UTR). A variety of trans-acting factors—many of them RNA-binding proteins—function in localization. Developmental functions of RNA localization have been defined in Xenopus, Drosophila, and Saccharomyces cerevisiae. In Drosophila, localized RNAs program the antero-posterior and dorso-ventral axes of the oocyte and embryo. In Xenopus, localized RNAs may function in mesoderm induction as well as in dorso-ventral axis specification. Localized RNAs also program asymmetric cell fates during Drosophila neurogenesis and yeast budding. </jats:p