Context-specific functions of Notch in Drosophila blood cell progenitors.

Drosophila Larval hematopoiesis takes place at the lymph gland, where myeloid-like progenitors differentiate into Plasmatocytes and Crystal Cells, under regulation of conserved signaling pathways. It has been established that the Notch pathway plays a specific role in Crystal Cell differentiation and maintenance. In mammalian hematopoiesis, the Notch pathway has been proposed to fulfill broader functions, including Hematopoietic Stem Cell maintenance and cell fate decision in progenitors. In this work we describe different roles that Notch plays in the lymph gland. We show that Notch, activated by its ligand Serrate, expressed at the Posterior Signaling Center, is required to restrain Core Progenitor differentiation. We define a novel population of blood cell progenitors that we name Distal Progenitors, where Notch, activated by Serrate expressed in Lineage Specifying Cells at the Medullary Zone/Cortical Zone boundary, regulates a binary decision between Plasmatocyte and Crystal Cell fates. Thus, Notch plays context-specific functions in different blood cell progenitor populations of the Drosophila lymph gland.

A genetic toolkit for the analysis of metabolic changes in Drosophila provides new insights into metabolic responses to stress and malignant transformation.

Regulation of the energetic metabolism occurs fundamentally at the cellular level, so analytical strategies must aim to attain single cell resolution to fully embrace its inherent complexity. We have developed methods to utilize a toolset of metabolic FRET sensors for assessing lactate, pyruvate and 2-oxoglutarate levels of Drosophila tissues in vivo by imaging techniques. We show here how the energetic metabolism is altered by hypoxia: While some larval tissues respond to low oxygen levels by executing a metabolic switch towards lactic fermentation, the fat body and salivary glands do not alter their energetic metabolism. Analysis of tumor metabolism revealed that depending on the genetic background, some tumors undergo a lactogenic switch typical of the Warburg effect, while other tumors do not. This toolset allows for developmental and physiologic studies in genetically manipulated Drosophila individuals in vivo.

Hydroxylation and translational adaptation to stress: some answers lie beyond the STOP codon.

Regulation of protein synthesis contributes to maintenance of homeostasis and adaptation to environmental changes. mRNA translation is controlled at various levels including initiation, elongation and termination, through post-transcriptional/translational modifications of components of the protein synthesis machinery. Recently, protein and RNA hydroxylation have emerged as important enzymatic modifications of tRNAs, elongation and termination factors, as well as ribosomal proteins. These modifications enable a correct STOP codon recognition, ensuring translational fidelity. Recent studies are starting to show that STOP codon read-through is related to the ability of the cell to cope with different types of stress, such as oxidative and chemical insults, while correlations between defects in hydroxylation of protein synthesis components and STOP codon read-through are beginning to emerge. In this review we will discuss our current knowledge of protein synthesis regulation through hydroxylation of components of the translation machinery, with special focus on STOP codon recognition. We speculate on the possibility that programmed STOP codon read-through, modulated by hydroxylation of components of the protein synthesis machinery, is part of a concerted cellular response to stress.

Catecholamine-beta-alanyl ligase in the medfly Ceratitis capitata.

Dopamine (DA) and norepinephrine (NE) derivatives play an important role in the sclerotization and pigmentation of insect cuticles by serving as precursors for cuticular cross-linking. Protein preparations from prepupae of the medfly, Ceratitis capitata, were able to conjugate beta-alanine with DA producing N-beta-alanyldopamine (NBAD) or with NE, synthesizing N-beta-alanylnorepinephrine (NBANE). The latter reaction has been demonstrated for the first time. Apparent kinetic parameters were obtained for both substrates, DA (V(max)=30.7+/-6.0 pmol min(-1) mg(-1); K(m)=29.5+/-3.5 microM) and NE (V(max)=16.1+/-6.6 pmol min(-1)mg(-1); K(m)=89.0+/-8.3 microM). The same protein seems to be responsible for both enzymatic activities, judging from several criteria like identical behavior under heat inactivation as well as identical Mg2+ and Mn2+ dependent stimulation and Co2+ inhibition. Furthermore, the melanic mutants niger of C. capitata and ebony(4) of D. melanogaster, known to be defective for NBAD synthase, were also unable to synthesize NBANE. The protein preparation acylated tyrosine with much less efficiency, to produce sarcophagine (beta-alanyltyrosine). Strikingly, extracts from the melanic mutants were unable to synthesize sarcophagine. Our results strongly suggest that the enzymatic activity previously known as NBAD synthase is in fact a novel catalytic protein showing broad substrate specificity. We propose to identify it as catecholamine-beta-alanyl ligase.

Occurrence of a putative SCF ubiquitin ligase complex in Drosophila.

Many proteins are targeted to proteasome degradation by a family of E3 ubiquitin ligases, termed SCF complexes, that link substrate proteins to an E2 ubiquitin-conjugating enzyme. SCFs are composed of three core proteins-Skp1, Cdc53/Cull, Rbx1/Hrt1-and a substrate specific F-box protein. We have identified in Drosophila melanogaster the closest homologues to the human components of the SCF(betaTrCP) complex and the E2 ubiquitin-conjugating enzyme UbcH5. We show that putative Drosophila SCF core subunits dSkpA and dRbx1 both interact directly with dCu11 and the F-box protein Slmb. We also describe the direct interaction of the UbcH5 related protein UbcD1 with dCul1 and Slmb. In addition, a functional complementation test performed on a Saccharomyces cerevisiae Hrt1p-deficient mutant showed that Drosophila Rbx1 is able to restore the yeast cells viability. Our results suggest that dRbx1, dSkpA, dCullin1, and Slimb proteins are components of a Drosophila SCF complex that functions in combination with the ubiquitin conjugating enzyme UbcD1.

Regulation of the Drosophila bHLH-PAS protein Sima by hypoxia: functional evidence for homology with mammalian HIF-1 alpha.

Hypoxia inducible factor-1 (HIF-1) is a heterodimeric complex of two basic-helix-loop-helix proteins of the PAS family which is critical for oxygen-dependent expression of many mammalian genes. Regulation is mediated by the alpha subunit (HIF-1 alpha) and sequences from HIF-1 alpha can confer hypoxia-inducible activity on a Ga14 fusion protein. To analyse conservation of this system of gene regulation between Drosophila and mammalian cells we constructed Ga14 fusions with a series of Drosophila basic-helix-loop-helix PAS (bHLH-PAS) proteins and tested for hypoxia inducibility in transfected Hep3B cells. We found that Ga14 functions with Similar (Sima) but not other Drosophila bHLH-PAS proteins showed inducible activity following exposure to stimuli which classically activate mammalian HIF-1:hypoxia, cobaltous ions, and desferrioxamine. We also found that Sima protein accumulated in Drosophila SL2 cells following hypoxia. Together these findings indicate the existence of functional homologies between Sima and HIF-1 alpha, and that conservation is such as to enable Sima to interact with the hypoxia signal transduction system in mammalian cells.

Interactions between the EGF receptor and DPP pathways establish distinct cell fates in the tracheal placodes.

The formation of the tracheal network in Drosophila is driven by stereotyped migration of cells from the tracheal pits. No cell divisions take place during tracheal migration and the number of cells in each branch is fixed. This work examines the basis for the determination of tracheal branch fates, prior to the onset of migration. We show that the EGF receptor pathway is activated by localized processing of the ligand SPITZ in the tracheal placodes and is responsible for the capacity to form the dorsal trunk and visceral branch. The DPP pathway, on the contrary, is induced in the tracheal pit by local presentation of DPP from the adjacent dorsal and ventral ectodermal cells. This pathway patterns the dorsal and lateral branches. Elimination of both pathways blocks migration of all tracheal branches. Antagonistic interactions between the two pathways are demonstrated. The opposing activities of two pathways may refine the final determination of tracheal branch fates.

The PAS domain confers target gene specificity of Drosophila bHLH/PAS proteins.

Trachealess (Trh) and Single-minded (Sim) are highly similar Drosophila bHLH/PAS transcription factors. They activate nonoverlapping target genes and induce diverse cell fates. A single Drosophila gene encoding a bHLH/PAS protein homologous to the vertebrate ARNT protein was isolated and may serve as a partner for both Trh and Sim. We show that Trh and Sim complexes recognize similar DNA-binding sites in the embryo. To examine the basis for their distinct target gene specificity, the activity of Trh-Sim chimeric proteins was monitored in embryos. Replacement of the Trh PAS domain by the analogous region of Sim was sufficient to convert it into a functional Sim protein. The PAS domain thus mediates all the features conferring specificity and the distinct recognition of target genes. The normal expression pattern of additional proteins essential for the activity of the Trh or Sim complexes can be inferred from the induction pattern of target genes and binding-site reporters, triggered by ubiquitous expression of Trh or Sim. We postulate that the capacity of bHLH/PAS heterodimers to associate, through the PAS domain, with additional distinct proteins that bind target-gene DNA, is essential to confer specificity.