A role for astrocytes in cerebellar deficits in frataxin deficiency: Protection by insulin-like growth factor I.

Inherited neurodegenerative diseases such as Friedreich's ataxia (FRDA), produced by deficiency of the mitochondrial chaperone frataxin (Fxn), shows specific neurological deficits involving different subset of neurons even though deficiency of Fxn is ubiquitous. Because astrocytes are involved in neurodegeneration, we analyzed whether they are also affected by frataxin deficiency and contribute to the disease. We also tested whether insulin-like growth factor I (IGF-I), that has proven effective in increasing frataxin levels both in neurons and in astrocytes, also exerts in vivo protective actions. Using the GFAP promoter expressed by multipotential stem cells during development and mostly by astrocytes in the adult, we ablated Fxn in a time-dependent manner in mice (FGKO mice) and found severe ataxia and early death when Fxn was eliminated during development, but not when deleted in the adult. Analysis of underlying mechanisms revealed that Fxn deficiency elicited growth and survival impairments in developing cerebellar astrocytes, whereas forebrain astrocytes grew normally. A similar time-dependent effect of frataxin deficiency in astrocytes was observed in a fly model. In addition, treatment of FGKO mice with IGF-I improved their motor performance, reduced cerebellar atrophy, and increased survival. These observations indicate that a greater vulnerability of developing cerebellar astrocytes to Fxn deficiency may contribute to cerebellar deficits in this inherited disease. Our data also confirm a therapeutic benefit of IGF-I in early FRDA deficiency.

Causative role of oxidative stress in a Drosophila model of Friedreich ataxia.

Friedreich ataxia (FA), the most common form of hereditary ataxia, is caused by a deficit in the mitochondrial protein frataxin. While several hypotheses have been suggested, frataxin function is not well understood. Oxidative stress has been suggested to play a role in the pathophysiology of FA, but this view has been recently questioned, and its link to frataxin is unclear. Here, we report the use of RNA interference (RNAi) to suppress the Drosophila frataxin gene (fh) expression. This model system parallels the situation in FA patients, namely a moderate systemic reduction of frataxin levels compatible with normal embryonic development. Under these conditions, fh-RNAi flies showed a shortened life span, reduced climbing abilities, and enhanced sensitivity to oxidative stress. Under hyperoxia, fh-RNAi flies also showed a dramatic reduction of aconitase activity that seriously impairs the mitochondrial respiration while the activities of succinate dehydrogenase, respiratory complex I and II, and indirectly complex III and IV are normal. Remarkably, frataxin overexpression also induced the oxidative-mediated inactivation of mitochondrial aconitase. This work demonstrates, for the first time, the essential function of frataxin in protecting aconitase from oxidative stress-dependent inactivation in a multicellular organism. Moreover our data support an important role of oxidative stress in the progression of FA and suggest a tissue-dependent sensitivity to frataxin imbalance. We propose that in FA, the oxidative mediated inactivation of aconitase, which occurs normally during the aging process, is enhanced due to the lack of frataxin.

Mitochondrial single-stranded DNA-binding protein is required for mitochondrial DNA replication and development in Drosophila melanogaster.

The discovery that several inherited human diseases are caused by mtDNA depletion has led to an increased interest in the replication and maintenance of mtDNA. We have isolated a new mutant in the lopo (low power) gene from Drosophila melanogaster affecting the mitochondrial single-stranded DNA-binding protein (mtSSB), which is one of the key components in mtDNA replication and maintenance. lopo(1) mutants die late in the third instar before completion of metamorphosis because of a failure in cell proliferation. Molecular, histochemical, and physiological experiments show a drastic decrease in mtDNA content that is coupled with the loss of respiration in these mutants. However, the number and morphology of mitochondria are not greatly affected. Immunocytochemical analysis shows that mtSSB is expressed in all tissues but is highly enriched in proliferating tissues and in the developing oocyte. lopo(1) is the first mtSSB mutant in higher eukaryotes, and its analysis demonstrates the essential function of this gene in development, providing an excellent model to study mitochondrial biogenesis in animals.

Substitution of the thioredoxin system for glutathione reductase in Drosophila melanogaster.

The disulfide reducing enzymes glutathione reductase and thioredoxin reductase are highly conserved among bacteria, fungi, worms, and mammals. These proteins maintain intracellular redox homeostasis to protect the organism from oxidative damage. Here we demonstrate the absence of glutathione reductase in Drosophila melanogaster, identify a new type of thioredoxin reductase, and provide evidence that a thioredoxin system supports GSSG reduction. Our data suggest that antioxidant defense in Drosophila, and probably in related insects, differs fundamentally from that in other organisms.

Ectopic expression of the neuropeptide pigment-dispersing factor alters behavioral rhythms in Drosophila melanogaster.

To study the function of the neuropeptide pigment-dispersing factor (PDF) in the circadian system of Drosophila, we misexpressed the pdf gene from the grasshopper Romalea in the CNS of Drosophila and investigated the effect of this on behavioral rhythmicity. pdf was either ectopically expressed in different numbers of neurons in the brain or the thoracical nervous system or overexpressed in the pacemaker neurons alone. We found severe alterations in the activity and eclosion rhythm of several but not all lines with ectopic pdf expression. Only ectopic pdf expression in neurons that projected into the dorsal central brain severely influenced activity rhythms. Therefore, we conclude that PDF acts as a neuromodulator in the dorsal central brain that is involved in the rhythmic control of behavior. Overexpression of pdf in the pacemaker neurons alone or in the other neurons that express the clock genes period (per) and timeless (tim) did not disturb the activity rhythm. Such flies still showed a rhythm in PDF accumulation in the central brain terminals. This rhythm was absent in the terminals of neurons that expressed PDF ectopically. Probably, PDF is rhythmically processed, transported, or secreted in neurons expressing per and tim, and additional PDF expression in these cells does not influence this rhythmic process. In neurons lacking per and tim, PDF appears to be continuously processed, leading to a constant PDF secretion at their nerve terminals. This may lead to conflicting signals in the rhythmic output pathway and result in a severely altered rhythmic behavior.

INAF, a protein required for transient receptor potential Ca(2+) channel function.

The trp gene of Drosophila encodes a subunit of a class of Ca(2+)-selective light-activated channels that carry the bulk of the phototransduction current. Transient receptor potential (TRP) homologs have been identified throughout animal phylogeny. In vertebrates, TRP-related channels have been suggested to mediate "store-operated Ca(2+) entry," which is important in Ca(2+) homeostasis in a wide variety of cell types. However, the mechanisms of activation and regulation of the TRP channel are not known. Here, we report on the Drosophila inaF gene, which encodes a highly eye-enriched protein, INAF, that appears to be required for TRP channel function. A null mutation in this gene significantly reduces the amount of the TRP protein and, in addition, specifically affects the TRP channel function so as to nearly shut down its activity. The inaF mutation also dramatically suppresses the severe degeneration caused by a constitutively active mutation in the trp gene. Although the reduction in the amount of the TRP protein may contribute to these phenotypes, several lines of evidence support the view that inaF mutations also more directly affect the TRP channel function, suggesting that the INAF protein may have a regulatory role in the channel function.

Isolation and characterization of the droPIK57 gene encoding a new regulatory subunit of phosphatidylinositol 3-kinase from Drosophila melanogaster.

Mammalian phosphatidylinositol 3-kinase (PI 3-kinase) plays an important role in the regulation of various cellular, receptor tyrosine kinase-mediated processes, such as mitogenesis and transformation. PI 3-kinase is composed of a 110-kDa catalytic subunit and a regulatory subunit of 85 kDa or 55 kDa. We have cloned a gene for a regulatory subunit from Drosophila melanogaster, named droPIK57, from head-specific cDNA libraries. The droPIK57 gene encodes a protein containing two SH2 domains with significant sequence homology to those in p85 and p55. Like the p55 subunits, DroPIK57 is missing the SH3 domain and the bcr homology region of the p85 subunit. The short N-terminus as well as the C-terminus of the DroPIK57 protein show no identity to the known PI 3-kinase subunits, suggesting that it is a new member in the family of regulatory subunits. In-situ hybridization and Northern blot analysis indicate a widespread function of this gene during embryogenesis and in the CNS.

GAL4-responsive UAS-tau as a tool for studying the anatomy and development of the Drosophila central nervous system.

To improve the quality of cytoplasmic labelling of GAL4-expressing cells in Drosophila enhancer-trap and transgenic strains, a new GAL4-responsive reporter UAS-tau, which features a bovine tau cDNA under control of a yeast upstream activation sequence (UAS), was tested. Tau, a microtubule-associated protein, is distributed actively and evenly into all cellular processes. Monoclonal anti-bovine Tau antibody reveals the axonal structure of the labelled cells with detail similar to that of Golgi impregnation. We demonstrate that the UAS-tau system is especially useful for studying processes of differentiation and reorganisation of identified neurones during postembryonic development.

Molecular and genetic analysis of the Drosophila mas-1 (mannosidase-1) gene which encodes a glycoprotein processing alpha 1,2-mannosidase.

Glycosylation is an important mechanism for modulating the physicochemical and biological properties of proteins in a stage- and tissue-specific manner. The enzymology of this process is just beginning to be understood. Here we present the molecular analysis of mas-1 (mannosidase-1), a Drosophila gene with significant homologies to mammalian and Saccharomyces cerevisiae glycoprotein processing alpha 1,2-mannosidases. An enhancer-trap P-element inserted upstream of mas-1 leads to highly specific lacZ expression in the lobula plate giant neurons, cells that mediate the large-field optomotor response. This staining, however, seems to reflect only a small part of the complex expression pattern of the mas-1 gene: Two promoters produce alternative transcripts that show individual spatial distributions during embryonic development, including a maternal contribution. Both transcripts code for type II transmembrane proteins which differ in their N-terminal parts. Null mutants in mas-1 display defects in the embryonic PNS, in the wing, and in the adult eye. These findings illustrate that the processing of N-linked glycans plays a functional role in Drosophila development. There is, however, ample evidence for genetic and biochemical redundancy in the mannose-trimming steps of this pathway.

The Drosophila giant lens gene plays a dual role in eye and optic lobe development: inhibition of differentiation of ommatidial cells and interference in photoreceptor axon guidance.

The Drosophila giant lens (gil) gene encodes a secreted molecule which if absent leads to the recruitment of additional ommatidial cells normally eliminated by apoptosis. Heat induced ectopic gil expression leads to a reduction of ommatidial cells suggesting that gil is secreted by differentiating cells to prevent the development of an excess of cells of a given ommatidial cell type. A second important defect is the misrouting of photoreceptor axons in gil mutants. However, gil function is not required in photoreceptor axons for the establishment of proper connections. We propose that gil acts on the development of lamina cells preventing the correct differentiation of the target region of photoreceptor axons and therefore leading to an axon guidance phenotype.

Giant lens, a gene involved in cell determination and axon guidance in the visual system of Drosophila melanogaster.

Mutations in the Drosophila gene giant lens (gil) affect ommatidial development, photoreceptor axon guidance and optic lobe development. We have cloned the gene using an enhancer trap line. Molecular analysis of gil suggests that it encodes a secreted protein with an epidermal-growth-factor-like motif. We have generated mutations at the gil locus by imprecise excision of the enhancer trap P-element. In the absence of gil, additional photoreceptors develop at the expense of pigment cells, suggesting an involvement of gil in cell determination during eye development. In addition, gil mutants show drastic effects on photoreceptor axon guidance and optic lobe development. In wildtype flies, photoreceptor axons grow from the eye disc through the optic stalk into the larval brain hemisphere, where retinal innervation is required for the normal development of the lamina and distal medulla. The projection pattern of these axons in the developing lamina and medulla is highly regular and reproducible. In gil, photoreceptor axons enter the larval brain but fail to establish proper connections in the lamina or medulla. We propose that gil encodes a new type of signalling molecule involved in the process of axon pathfinding and cell determination in the visual system of Drosophila.

Properties of photoreceptor-specific phospholipase C encoded by the norpA gene of Drosophila melanogaster.

Mutations in the norpA gene drastically affect the phototransduction process in Drosophila. To study the biochemical characteristics of the norpA protein and its cellular and subcellular distributions, we have generated antisera against the major gene product of norpA. The antisera recognize an eye-specific protein of 130-kDa relative molecular mass that is present in wild-type head extracts but not in those of strong norpA mutants. The protein is associated with membranes and can be extracted with high salt. Immunohistochemical analysis at the light and electron microscopic levels indicates that the protein is expressed in all adult photoreceptor cells and specifically localized within the rhabdomeres, preferentially adjacent to, but not within, the rhabdomeric membranes. The results of the present study strongly support the previous suggestion that the norpA gene encodes the major phosphoinositol-specific phospholipase C in the photoreceptors. Moreover, insofar as the rhabdomeres are specialized structures for photoreception and phototransduction, specific localization of the norpA protein within these structures, in close association with the membranes, is consistent with the proposal that it has an important role in phototransduction.

Molecular characterization of Drosophila gene encoding G0 alpha subunit homolog.

A Drosophila melanogaster gene (dgo) encoding a G protein alpha subunit has been isolated by screening genomic and adult head cDNA libraries using bovine transducin alpha subunit cDNA as probe. The gene, which maps to 47A on the second chromosome, encodes two proteins which are both 354 amino acids long but differ in seven amino acids in the amino-terminal region. The deduced amino acid sequences of the two proteins are 81% identical to that of a rat Go alpha subunit. Analysis of genomic clones revealed that there are eight coding exons and that the putative transcripts for the two proteins differ in the 5'-noncoding regions and the first coding exons but share the remaining six coding exons. The arrangement of two different 5'-noncoding regions on the gene suggests that two different promoters regulate the expression of the transcripts encoding the two proteins. RNA blot analysis detected three transcripts: a 3.9-kilobase (kb) transcript found at all stages of development; a 5.4-kb transcript present predominantly in adult heads; and a 3.4-kb transcript present only in adult bodies. In situ hybridizations of a cDNA probe to adult tissue sections showed that the gene is expressed abundantly in neuronal cell bodies in the brain, optic lobe, and thoracic ganglia.

Drosophila ninaA gene encodes an eye-specific cyclophilin (cyclosporine A binding protein).

Mutations in the ninaA gene of Drosophila severely reduce the amount of rhodopsin specifically in R1-6 photoreceptors. Isolation of the ninaA gene by chromosomal walking revealed that it is expressed only in the eye and encodes a 237-amino acid polypeptide that shows strong sequence similarity to cyclophilin, a putative molecular target for cyclosporine A, a potent immunosuppressant used in human organ transplantations. Unlike most cyclophilins characterized to date, the ninaA-encoded protein has a putative signal sequence and a transmembrane domain. Each of the three ehtyl methanesulfonate-induced ninaA mutant alleles analyzed shows a single nucleotide change in the mRNA coding region leading to either a nonsense or a missense mutation. We find no evidence that the ninaA-encoded protein is directly involved in phototransduction. The only detectable mutant phenotype that correlates with the severity of molecular defects in the three mutants is the amount of depletion of R1-6 rhodopsin. The above results and the recent findings that cyclophilin is a peptidylprolyl cis-trans-isomerase suggest that the ninaA-encoded protein may be required for proper folding and stability of R1-6 rhodopsin.

Identification and characterization of a Drosophila homologue to the vertebrate neuropeptide cholecystokinin.

Homologues to the cholecystokinin (CCK)-gastrin peptide family have been cloned from Drosophila. The CCK-like precursor found in Drosophila has been designated drosulfakinin (DSK). Genomic and cDNA clones corresponding to the Drosophila neuropeptide precursor encode for three putative peptides. The three peptides (DSK-0, Asn-Gln-Lys-Thr-Met-Ser-Phe-Gly; DSK-I, Phe-Asp-Asp-Tyr-Gly-His-Met-Arg-Phe-Gly; DSK-II, Gly-Gly-Asp-Asp-Gln-Phe-Asp-Asp-Tyr-Gly-His-Met-Arg-Phe-Gly) are flanked by prohormone processing sites and contain C-terminal glycyl residues, a potential amidation site. Two of the peptides, DSK-I and DSK-II, are homologous to CCK-gastrin peptides. Each of the two homologues include a CCK-gastrin-like C-terminal pentapeptide and a conserved sequence corresponding to the sulfated tyrosine in bioactive CCK. The third peptide encoded by the drosulfakinin precursor represents a novel peptide. In situ tissue hybridization indicates the presence of the transcript in the adult head. Chromosomal localization maps the gene to the third chromosome near 81F.

Isolation of a putative phospholipase C gene of Drosophila, norpA, and its role in phototransduction.

Severe norpA mutations in Drosophila eliminate the photoreceptor potential and render the fly completely blind. Recent biochemical analyses have shown that norpA mutants lack phospholipase C (PLC) activity in the eye. A combination of chromosomal walking and transposon-mediated mutagenesis was used to clone the norpA gene. This gene encodes a 7.5 kb RNA that is expressed in the adult head. In situ hybridizations of norpA cDNA to adult tissue sections show that this gene is expressed abundantly in the retina. The putative norpA protein is composed of 1095 amino acid residues and has extensive sequence similarity to a PLC amino acid sequence from bovine brain. We suggest that the norpA gene encodes a PLC expressed in the eye of Drosophila and that PLC is an essential component of the Drosophila phototransduction pathway.

Redesigning the body plan of Drosophila by ectopic expression of the homoeotic gene Antennapedia.

Genetic and molecular studies on the expression of Antennapedia (Antp) have suggested that this gene specifies mainly the second thoracic segment. On the basis of our molecular analysis of dominant gain-of-function mutants we have postulated that the transformation of antennae into second legs is due to the ectopic overexpression of the Antp+ protein. This hypothesis was tested by inserting the complementary DNA encoding the normal Antp protein into a heat-shock expression vector and subsequent germ-line transformation. As predicted, heat induction at defined larval stages leads to the transformation of antennae into second legs. The dorsal part of the head can also be transformed into second thoracic structures (scutum) indicating that Antp indeed specifies the second thoracic segment. By ectopic overexpression of the Antp protein the body plan of the fruit fly can be altered in a predictable way.

Molecular analysis of the dominant homeotic Antennapedia phenotype.

Most of the dominant alleles of the homeotic gene Antennapedia (Antp) which show a transformation of antennae into legs are associated with large chromosomal inversions. To determine the molecular mechanisms underlying the dominant phenotype, one of the strongest Antp alleles (Antp) was studied in more detail. The mutant chromosome has been cloned and the structure of the inversion has been identified. The inversion breaks two genes apart: the Antp gene and a previously unidentified gene, tentatively called responsible for dominant phenotype (rfd), located at 84D1-2. The two genes are transcribed in opposite directions and the breakpoints lie within introns of both genes. Through the inversion event, a reciprocal exchange of the first exons including promoters occurred leading to the production of new transcripts. The transcripts containing the entire Antp protein coding region which have been fused to the promoter of the rfd gene are lost in revertants of the dominant phenotype indicating a correlation between this fusion gene and the dominant phenotype. The molecular structure of inversion Antp suggests that the dominant phenotype arises via ectopic expression of the normal Antp protein due to a gene fusion event.

Structural organization and sequence of the homeotic gene Antennapedia of Drosophila melanogaster.

The structure of the Drosophila melanogaster Antennapedia (Antp) gene has been investigated by the isolation and sequencing of different cDNAs and genomic clones. Northern analysis, S1 mapping and primer extension experiments reveal a complex and unusual gene structure. The gene is composed of two promoters, eight exons spanning >100 kb, and two termination processing regions. Four major polyadenylated transcripts were found, two of them starting at a second internal promoter in front of exon 3. All four transcripts have extremely long untranslated leader and trailer sequences in the range of 1-2 kb. Despite the complex transcriptional organization, the open reading frame is the same in all transcripts, and starts in exon 5 giving rise to a protein of mol. wt. 42 800. The putative protein is rich in glutamine (18%) and proline (10%). The homeobox, a region which previously has been shown to be highly conserved among homeotic genes, is contained in the open reading frame and located in the last exon. Functional implications of the complex structure with respect to development and its relation to the mutant phenotypes are discussed.