The action of the notch locus in Drosophila melanogaster. I. Effects of the notch8 deficiency on mitochondrial enzymes

The mitochondrial ribosomal protein mRpL4 regulates Notch signaling

Mitochondrial ribosomal proteins (MRPs) assemble as specialized ribosome to synthesize mtDNA-encoded proteins, which are essential for mitochondrial bioenergetic and metabolic processes. MRPs are required for fundamental cellular activities during animal development, but their roles beyond mitochondrial protein translation are poorly understood. Here, we report a conserved role of the mitochondrial ribosomal protein L4 (mRpL4) in Notch signaling. Genetic analyses demonstrate that mRpL4 is required in the Notch signal-receiving cells to permit target gene transcription during Drosophila wing development. We find that mRpL4 physically and genetically interacts with the WD40 repeat protein wap and activates the transcription of Notch signaling targets. We show that human mRpL4 is capable of replacing fly mRpL4 during wing development. Furthermore, knockout of mRpL4 in zebrafish leads to downregulated expression of Notch signaling components. Thus, we have discovered a previously unknown function of mRpL4 during animal development.

Iron Sulfur and Molybdenum Cofactor Enzymes Regulate the Drosophila Life Cycle by Controlling Cell Metabolism

Iron sulfur (Fe-S) clusters and the molybdenum cofactor (Moco) are present at enzyme sites, where the active metal facilitates electron transfer. Such enzyme systems are soluble in the mitochondrial matrix, cytosol and nucleus, or embedded in the inner mitochondrial membrane, but virtually absent from the cell secretory pathway. They are of ancient evolutionary origin supporting respiration, DNA replication, transcription, translation, the biosynthesis of steroids, heme, catabolism of purines, hydroxylation of xenobiotics, and cellular sulfur metabolism. Here, Fe-S cluster and Moco biosynthesis in Drosophila melanogaster is reviewed and the multiple biochemical and physiological functions of known Fe-S and Moco enzymes are described. We show that RNA interference of Mocs3 disrupts Moco biosynthesis and the circadian clock. Fe-S-dependent mitochondrial respiration is discussed in the context of germ line and somatic development, stem cell differentiation and aging. The subcellular compartmentalization of the Fe-S and Moco assembly machinery components and their connections to iron sensing mechanisms and intermediary metabolism are emphasized. A biochemically active Fe-S core complex of heterologously expressed fly Nfs1, Isd11, IscU, and human frataxin is presented. Based on the recent demonstration that copper displaces the Fe-S cluster of yeast and human ferredoxin, an explanation for why high dietary copper leads to cytoplasmic iron deficiency in flies is proposed. Another proposal that exosomes contribute to the transport of xanthine dehydrogenase from peripheral tissues to the eye pigment cells is put forward, where the Vps16a subunit of the HOPS complex may have a specialized role in concentrating this enzyme within pigment granules. Finally, we formulate a hypothesis that (i) mitochondrial superoxide mobilizes iron from the Fe-S clusters in aconitase and succinate dehydrogenase; (ii) increased iron transiently displaces manganese on superoxide dismutase, which may function as a mitochondrial iron sensor since it is inactivated by iron; (iii) with the Krebs cycle thus disrupted, citrate is exported to the cytosol for fatty acid synthesis, while succinyl-CoA and the iron are used for heme biosynthesis; (iv) as iron is used for heme biosynthesis its concentration in the matrix drops allowing for manganese to reactivate superoxide dismutase and Fe-S cluster biosynthesis to reestablish the Krebs cycle.

Notch signaling and M1 macrophage activation in obesity-alcohol synergism

Macrophages are a key cell type in the innate immune system, and its proinflammatory (M1) activation in the liver plays a critical role in pathogenesis of alcoholic steatohepatitis. Emerging evidence indicates the involvement of Notch signaling in regulation of innate immune response and cellular metabolism. Metabolic switch to glycolysis characterizes macrophages undergoing M1 activation. It has been proposed that metabolic reprograming in response to extrinsic stimulation, such as bacterial endotoxin, triggers intrinsic signal to dictate cell differentiation. Using an obesity-alcohol synergistic ASH mouse model, we have recently shown that Notch1 pathway promotes M1 activation of hepatic macrophages, through direct upregulation of M1 gene transcription and through reprograming of mitochondrial metabolism to glucose oxidation and subsequent mtROS generation to augment M1 gene expression. Our studies demonstrate a novel mechanism of Notch1 signaling in metabolic reprograming of macrophage for M1 activation in ASH.

Mitochondria: from cell death executioners to regulators of cell differentiation

Most, if not all mitochondrial functions, including adenosine-5'-triphosphate (ATP) production and regulation of apoptosis and Ca(2+) homeostasis, are inextricably linked to mitochondrial morphology and dynamics, a process controlled by a family of GTP-dependent dynamin related 'mitochondria-shaping' proteins. Mitochondrial fusion and fission directly influence mitochondrial metabolism, apoptotic and necrotic cell death, autophagy, muscular atrophy and cell migration. In this review, we discuss the recent evidence indicating that mitochondrial dynamics influence complex signaling pathways, affect gene expression and define cell differentiation. These findings extend the importance of mitochondria to developmental biology, far beyond their mere bioenergetic role.

The Notch signalling system: recent insights into the complexity of a conserved pathway

Notch signalling links the fate of one cell to that of an immediate neighbour and consequently controls differentiation, proliferation and apoptotic events in multiple metazoan tissues. Perturbations in this pathway activity have been linked to several human genetic disorders and cancers. Recent genome-scale studies in Drosophila melanogaster have revealed an extraordinarily complex network of genes that can affect Notch activity. This highly interconnected network contrasts our traditional view of the Notch pathway as a simple linear sequence of events. Although we now have an unprecedented insight into the way in which such a fundamental signalling mechanism is controlled by the genome, we are faced with serious challenges in analysing the underlying molecular mechanisms of Notch signal control.

Gamma-secretase-regulated proteolysis of the Notch receptor by mitochondrial intermediate peptidase

Notch is a transmembrane receptor that controls a diverse array of cellular processes including cell proliferation, differentiation, survival, and migration. The cellular outcome of Notch signaling is dependent on extracellular and intracellular signals, but the complexities of its regulation are not well understood. Canonical Notch signaling involves ligand association that triggers sequential and regulated proteolysis of Notch at several sites. Ligand-dependent proteolysis at the S2 site removes the bulk of the extracellular domain of Notch. Subsequent γ-secretase-mediated intramembrane proteolysis of the remaining membrane-tethered Notch fragment at the S3 site produces a nuclear-destined Notch intracellular domain (NICD). Here we show that following γ-secretase cleavage, Notch is proteolyzed at a novel S5 site. We have identified this S5 site to be eight amino acids downstream of the S3 site. Biochemical fractionation and purification resulted in the identification of the S5 site protease as the mitochondrial intermediate peptidase (MIPEP). Expression of the MIPEP-cleaved NICD (ΔNICD) results in a decrease in cell viability and mitochondria membrane potential. The sequential and regulated proteolysis by γ-secretase and MIPEP suggests a new means by which Notch function can be modulated.

Evidence for a role for notch signaling in the cytokine-dependent survival of activated T cells

Peripheral T cell homeostasis results from a balance between factors promoting survival and those that trigger deletion of Ag-reactive cells. The cytokine IL-2 promotes T cell survival whereas reactive oxygen species (ROS) sensitize T cells to apoptosis. Two pathways of activated T cell apoptosis-one triggered by Fas ligand and the other by cytokine deprivation-depend on ROS, with the latter also regulated by members of the Bcl-2 family. Notch family proteins regulate several cell-fate decisions in metazoans. Ectopic expression of the Notch1 intracellular domain (NICD) in T cells inhibits Fas-induced apoptosis. The underlying mechanism is not known and the role, if any, of Notch in regulating apoptosis triggered by cytokine deprivation or neglect has not been examined. In this study, we use a Notch1/Fc chimera; a blocking Ab to Notch1 and chemical inhibitors of gamma-secretase to investigate the role of Notch signaling in activated T cells of murine origin. We show that perturbing Notch signaling in activated CD4+/CD8+ T cells maintained in IL-2 results in the accumulation of ROS, reduced Akt/protein kinase B activity, and expression of the antiapoptotic protein Bcl-xL, culminating in apoptosis. A broad-spectrum redox scavenger inhibits apoptosis but T cells expressing mutant Fas ligand are sensitive to apoptosis. Activated T cells isolated on the basis of Notch expression (Notch+) are enriched for Bcl-xL expression and demonstrate reduced susceptibility to apoptosis triggered by neglect or oxidative stress. Furthermore, enforced expression of NICD protects activated T cells from apoptosis triggered by cytokine deprivation. Taken together, these data implicate Notch1 signaling in the cytokine-dependent survival of activated T cells.

Differences in teratogenic and toxic properties of alcohol dehydrogenase inhibitors pyrazole and 4-methylpyrazole in Drosophila melanogaster: I. ADH allozymes in variable genetic backgrounds

Pyrazole and 4-methylpyrazole (4-MP) are effective inhibitors of alcohol dehydrogenase (ADH) activity in mammals both in vivo and in vitro. 4-MP has a tenfold higher inhibition specificity compared with pyrazole. Pyrazole proved a teratogenic compound in Drosophila melanogaster. Treatment of third instar larvae of D. melanogaster with pyrazole, in contrast with 4-MP, produced an increase in the number of dorsocentral and scutellar macrochaetae and wing-notches in the adult fly. A large difference in the penetrance of terata in males and females was observed. Similar effects were observed in flies lacking ADH molecules. The teratogenicity of pyrazole must be due to disturbance of processes other than ADH inhibition. Synergistic effects were observed between pyrazole and methoxyacetic acid (MAA), an in vitro inhibitor of sarcosine dehydrogenase activity. Each of these compounds, when fed to early third instar larvae, produced terata resembling the Notch mutant of D. melanogaster.

Temperature-dependency of electron-transport activity in mitochondria with exogenous mitochondrial DNA in Drosophila

In mitochondrial DNA (mtDNA) heteroplasmy induced artificially in Drosophila melanogaster (Matsuura et al., 1989), foreign mtDNA derived from D. mauritiana was selectively transmitted at 25 degrees C but was lost at 19 degrees C (Niki et al., 1989; Matsuura et al., 1990, 1991). To investigate temperature-dependent factors in the selective transmission of mtDNA, the temperature-dependency of electron-transport activity of mitochondria from D. melanogaster in which endogenous mtDNA was completely replaced by the foreign mtDNA was compared with that of D. melanogaster and D. mauritiana. For NADH-oxidase activity, the optimum temperature of D. mauritiana mitochondria was 35 degrees C while for two types of mitochondria from D. melanogaster each possessing either endogenous or exogenous mtDNA, maximum activity was noted at 32 degrees C. This observation suggests that the temperature-dependency of mitochondrial electron-transport activity is mainly determined by a nuclear genome. NADH-cytochrome c reductase and cytochrome c oxidase activities were not significantly different among the three types of mitochondria. The temperature-dependency of mitochondrial function apparently is not involved in the temperature-dependent selective transmission of mtDNA in the heteroplasmic state.

The effects of recessive lethal Notch mutations of Drosophila melanogaster on flavoprotein enzyme activities whose inhibitions cause Notch-like phenocopies

The biochemical action of the Notch locus whose mutants cause morphological aberrations in flies, viz., notches of wings and bristle multiplication, has been analyzed by the addition to the food medium of enzyme inhibitors causing phenocopies of Notch and by comparison of enzyme activity patterns of Notch mutants with different degrees of phenotypic expression. Notch phenocopies were induced by inhibitors of enzyme activities in two biochemical pathways: the de novo pyrimidine synthesis by 5-methylorotate (inhibitor of dihydroorotate dehydrogenase) and the choline shunt by amobarbital (inhibits choline dehydrogenase) and methoxyacetate (inhibits sarcosine dehydrogenase). The inhibition of de novo pyrimidine synthesis prevents the production of deoxyuridine-5-phosphate, the substrate for the synthesis of thymidine-5-phosphate via thymidylate synthase, whereas the inhibition of the choline shunt prevents the production of HCHO groups and glycine, both of which are involved in the synthesis of 5,10-methylenetetrahydrofolate, which is a cofactor of thymidylate synthase. It was already known that the inhibition of the latter enzyme in vivo induces Notch phenocopies. Notch mutants with a strong morphological expression show low enzyme activities for dihydroorotate dehydrogenase and choline dehydrogenase. Both are flavoprotein enzymes linked to the respiratory chain. The correspondence between the low enzyme activities in Notch mutants with a strong morphological expression and the phenocopying effect of antimetabolites on these enzymes in the two biochemical pathways involved strongly suggests that the morphological effects of Notch on flies are a consequence of lowered activities of choline dehydrogenase and dihydroorotate dehydrogenase.

Evidence for a multiple function of the alcohol dehydrogenase allozyme ADH71k of Drosophila melanogaster

Alcohol dehydrogenase of Drosophila melanogaster catalyzes the oxidation of many primary and secondary alcohols. We show that sarcosine, choline and dihydroorotate are substrates of ADH in vitro. The first two substrates are regular substrates of the choline shunt, and the latter of the de novo pyrimidine synthesis. Differences in oxidative ability towards sarcosine and dihydroorotate between two ADH allozymes, ADH71k and ADHF, are observed. The catalytic activity of ADH71k towards sarcosine and dihydroorotate might be responsible for its allelic fixation in Notch8 mutant stocks, in which Notch females have a decreased level of the regular enzymes for these substrates. Their oxidation by ADH71k might act as a bypass, which restores at least part of the decreased activity of enzymes encoded by the Notch locus.

Genetics of the nervous system in Drosophila

The genetic analysis ofDrosophilaneurobiology has expanded into a considerable field. A host of genetic variants is now available for analyzing the development, structure, physiology, and chemistry of the fly's excitable cells and tissues and how they control the animal's behavior.