Metabolic and neurobehavioral disturbances induced by purine recycling deficiency in Drosophila

Adenine phosphoribosyltransferase (APRT) and hypoxanthine-guanine phosphoribosyltransferase (HGPRT) are two structurally related enzymes involved in purine recycling in humans. Inherited mutations that suppress HGPRT activity are associated with Lesch-Nyhan disease (LND), a rare X-linked metabolic and neurological disorder in children, characterized by hyperuricemia, dystonia, and compulsive self-injury. To date, no treatment is available for these neurological defects and no animal model recapitulates all symptoms of LND patients. Here, we studied LND-related mechanisms in the fruit fly. By combining enzymatic assays and phylogenetic analysis, we confirm that no HGPRT activity is expressed in Drosophila melanogaster, making the APRT homolog (Aprt) the only purine-recycling enzyme in this organism. Whereas APRT deficiency does not trigger neurological defects in humans, we observed that Drosophila Aprt mutants show both metabolic and neurobehavioral disturbances, including increased uric acid levels, locomotor impairments, sleep alterations, seizure-like behavior, reduced lifespan, and reduction of adenosine signaling and content. Locomotor defects could be rescued by Aprt re-expression in neurons and reproduced by knocking down Aprt selectively in the protocerebral anterior medial (PAM) dopaminergic neurons, the mushroom bodies, or glia subsets. Ingestion of allopurinol rescued uric acid levels in Aprt-deficient mutants but not neurological defects, as is the case in LND patients, while feeding adenosine or N6-methyladenosine (m6A) during development fully rescued the epileptic behavior. Intriguingly, pan-neuronal expression of an LND-associated mutant form of human HGPRT (I42T), but not the wild-type enzyme, resulted in early locomotor defects and seizure in flies, similar to Aprt deficiency. Overall, our results suggest that Drosophila could be used in different ways to better understand LND and seek a cure for this dramatic disease.

Inhibition by pesticides of the DJ-1/Park7 protein related to Parkinson disease

Parkinson's disease is a severe neurodegenerative disease. Several environmental contaminants such as pesticides have been suspected to favor the appearance of this pathology. The protein DJ-1 (or Park7) protects against the development of Parkinson's disease. Thus, the possible inhibitory effects of about a hundred pesticides on human DJ-1 have been studied. We identified fifteen of them as strong inhibitors of DJ-1 with IC50 values between 0.02 and 30 µM. Thiocarbamates are particularly good inhibitors, as shown by thiram that acts as an irreversible inhibitor of an esterase activity of DJ-1 with an IC50 value of 0.02 µM. Thiram was also found as a good inhibitor of the protective activity of DJ-1 against glycation. Such inhibitory effects could be one of the various biological effects of these pesticides that may explain their involvement in the development of Parkinson's disease.

Lamp1 Deficiency Enhances Sensitivity to α-Synuclein and Oxidative Stress in Drosophila Models of Parkinson Disease

Parkinson disease (PD) is a common neurodegenerative condition affecting people predominantly at old age that is characterized by a progressive loss of midbrain dopaminergic neurons and by the accumulation of α-synuclein-containing intraneuronal inclusions known as Lewy bodies. Defects in cellular degradation processes such as the autophagy-lysosomal pathway are suspected to be involved in PD progression. The mammalian Lysosomal-associated membrane proteins LAMP1 and LAMP2 are transmembrane glycoproteins localized in lysosomes and late endosomes that are involved in autophagosome/lysosome maturation and function. Here, we show that the lack of Drosophila Lamp1, the homolog of LAMP1 and LAMP2, severely increased fly susceptibility to paraquat, a pro-oxidant compound known as a potential PD inducer in humans. Moreover, the loss of Lamp1 also exacerbated the progressive locomotor defects induced by the expression of PD-associated mutant α-synuclein A30P (α-synA30P) in dopaminergic neurons. Remarkably, the ubiquitous re-expression of Lamp1 in a mutant context fully suppressed all these defects and conferred significant resistance towards both PD factors above that of wild-type flies. Immunostaining analysis showed that the brain levels of α-synA30P were unexpectedly decreased in young adult Lamp1-deficient flies expressing this protein in comparison to non-mutant controls. This suggests that Lamp1 could neutralize α-synuclein toxicity by promoting the formation of non-pathogenic aggregates in neurons. Overall, our findings reveal a novel role for Drosophila Lamp1 in protecting against oxidative stress and α-synuclein neurotoxicity in PD models, thus furthering our understanding of the function of its mammalian homologs.

Spatially-resolved metabolic profiling of living Drosophila in neurodegenerative conditions using 1H magic angle spinning NMR

Drosophila flies are versatile animal models for the study of gene mutations in neuronal pathologies. Their small size allows performing in vivo Magic Angle Spinning (MAS) experiments to obtain high-resolution 1H nuclear magnetic resonance (NMR) spectra. Here, we use spatially-resolved 1H high-resolution MAS NMR to investigate in vivo metabolite contents in different segments of the fly body. A comparative study of metabolic changes was performed for three neurodegenerative disorders: two cell-specific neuronal and glial models of Huntington disease (HD) and a model of glutamate excitotoxicity. It is shown that these pathologies are characterized by specific and sometimes anatomically localized variations in metabolite concentrations. In two cases, the modifications of 1H MAS NMR spectra localized in fly heads were significant enough to allow the creation of a predictive model.

Neuroprotective effects of PACAP against paraquat-induced oxidative stress in the Drosophila central nervous system

Parkinson's disease (PD) is a progressive neurodegenerative movement disorder that can arise after long-term exposure to environmental oxidative stressors, such as the herbicide paraquat (PQ). Here we investigated the potential neuroprotective action of vertebrate pituitary adenylate cyclase-activating polypeptide (PACAP) against PQ in Drosophila. We found that pre-treatment with this neuropeptide applied to the ventral nerve cord (VNC) at low doses markedly extended the survival of wild-type decapitated flies exposed to neurotoxic levels of PQ or dopamine (DA). In contrast and interestingly, application of a PACAP receptor antagonist, PACAP-6-38, had opposite effects, significantly decreasing the resistance of flies to PQ. PACAP also reduced PQ-induced caspase activation and reactive oxygen species (ROS) accumulation in the VNC. We then searched for the endogenous neuropeptide receptor potentially involved in PACAP-mediated neuroprotection in Drosophila. Knocking down the gene encoding the receptor Han/PDFR of the neuropeptide pigment-dispersing factor (PDF) in all neurons conferred to flies higher resistance to PQ, whereas PDFR downregulation restricted to PDF or DA neurons did not increase PQ resistance, but remarkably suppressed the neuroprotective action of PACAP. Further experiments performed with Pdf and Pdfr-deficient mutant strains confirmed that PDF and its receptor are required for PACAP-mediated neuroprotection in flies. We also provide evidence using split-green fluorescent protein (split-GFP) reconstitution that PDF neurons make synaptic contacts onto DA neurons in the abdominal VNC. Our results therefore suggest that the protective action of PACAP against PQ-induced defects in the Drosophila nervous system involves the modulation of PDFR signaling in a small number of interconnected neurons.

Drosophila ammonium transporter Rh50 is required for integrity of larval muscles and neuromuscular system

Rhesus glycoproteins (Rh50) have been shown to be ammonia transporters in many species from bacteria to human. They are involved in various physiological processes including acid excretion and pH regulation. Rh50 proteins can also provide a structural link between the cytoskeleton and the plasma membranes that maintain cellular integrity. Although ammonia plays essential roles in the nervous system, in particular at glutamatergic synapses, a potential role for Rh50 proteins at synapses has not yet been investigated. To better understand the function of these proteins in vivo, we studied the unique Rh50 gene of Drosophila melanogaster, which encodes two isoforms, Rh50A and Rh50BC. We found that Drosophila Rh50A is expressed in larval muscles and enriched in the postsynaptic regions of the glutamatergic neuromuscular junctions. Rh50 inactivation by RNA interference selectively in muscle cells caused muscular atrophy in larval stages and pupal lethality. Interestingly, Rh50-deficiency in muscles specifically increased glutamate receptor subunit IIA (GluRIIA) level and the frequency of spontaneous excitatory postsynaptic potentials. Our work therefore highlights a new role for Rh50 proteins in the maintenance of Drosophila muscle architecture and synaptic physiology, which could be conserved in other species.

Drosophila p53 integrates the antagonism between autophagy and apoptosis in response to stress

The tumor suppressor TP53/p53 is a known regulator of apoptosis and macroautophagy/autophagy. However, the molecular mechanism by which TP53 regulates 2 apparently incompatible processes remains unknown. We found that Drosophila lacking p53 displayed impaired autophagic flux, higher caspase activation and mortality in response to oxidative stress compared with wild-type flies. Moreover, autophagy and apoptosis were differentially regulated by the p53 (p53B) and ΔNp53 (p53A) isoforms: while the former induced autophagy in differentiated neurons, which protected against cell death, the latter inhibited autophagy by activating the caspases Dronc, Drice, and Dcp-1. Our results demonstrate that the differential use of p53 isoforms combined with the antagonism between apoptosis and autophagy ensures the generation of an appropriate p53 biological response to stress.

The lysosomal membrane protein LAMP2A promotes autophagic flux and prevents SNCA-induced Parkinson disease-like symptoms in the Drosophila brain

The autophagy-lysosome pathway plays a fundamental role in the clearance of aggregated proteins and protection against cellular stress and neurodegenerative conditions. Alterations in autophagy processes, including macroautophagy and chaperone-mediated autophagy (CMA), have been described in Parkinson disease (PD). CMA is a selective autophagic process that depends on LAMP2A (lysosomal-associated membrane protein 2A), a mammal and bird-specific membrane glycoprotein that translocates cytosolic proteins containing a KFERQ-like peptide motif across the lysosomal membrane. Drosophila reportedly lack CMA and use endosomal microautophagy (eMI) as an alternative selective autophagic process. Here we report that neuronal expression of human LAMP2A protected Drosophila against starvation and oxidative stress, and delayed locomotor decline in aging flies without extending their lifespan. LAMP2A also prevented the progressive locomotor and oxidative defects induced by neuronal expression of PD-associated human SNCA (synuclein alpha) with alanine-to-proline mutation at position 30 (SNCAA30P). Using KFERQ-tagged fluorescent biosensors, we observed that LAMP2A expression stimulated selective autophagy in the adult brain and not in the larval fat body, but did not increase this process under starvation conditions. Noteworthy, we found that neurally expressed LAMP2A markedly upregulated levels of Drosophila Atg5, a key macroautophagy initiation protein, and that it increased the density of Atg8a/LC3-positive puncta, which reflects the formation of autophagosomes. Furthermore, LAMP2A efficiently prevented accumulation of the autophagy defect marker Ref(2)P/p62 in the adult brain under acute oxidative stress. These results indicate that LAMP2A can potentiate autophagic flux in the Drosophila brain, leading to enhanced stress resistance and neuroprotection.

ABBREVIATIONS

Act5C: actin 5C; a.E.: after eclosion; Atg5: autophagy-related 5; Atg8a/LC3: autophagy-related 8a; CMA: chaperone-mediated autophagy; DHE: dihydroethidium; elav: embryonic lethal abnormal vision; eMI: endosomal microautophagy; ESCRT: endosomal sorting complexes required for transport; GABARAP: GABA typeA receptor-associated protein; Hsc70-4: heat shock protein cognate 4; HSPA8/Hsc70: heat shock protein family A (Hsp70) member 8; LAMP2: lysosomal associated membrane protein 2; MDA: malondialdehyde; PA-mCherry: photoactivable mCherry; PBS: phosphate-buffered saline; PCR: polymerase chain reaction; PD: Parkinson disease; Ref(2)P/p62: refractory to sigma P; ROS: reactive oxygen species; RpL32/rp49: ribosomal protein L32; RT-PCR: reverse transcription polymerase chain reaction; SING: startle-induced negative geotaxis; SNCA/α-synuclein: synuclein alpha; SQSTM1/p62: sequestosome 1; TBS: Tris-buffered saline; UAS: upstream activating sequence.

[Nobel time for the circadian clock - Nobel Prize in Medicine 2017: Jeffrey C. Hall, Michael Rosbash and Michael W. Young]

L’attribution du prix Nobel 2017 de physiologie ou médecine à trois chercheurs américains - Jeffrey C. Hall (né le 3 mai 1945 à New York – University of Maine), Michael Rosbash (né le 7 mars 1944 à Kansas City - Brandeis University, Waltham et Howard Hughes Medical Institute) et Michael W. Young (né le 28 mars 1949 à Miami - Rockefeller University, New York), est difficilement contestable, tant ces chercheurs incarnent depuis près de 35 ans, l’émergence, puis le foisonnement des études moléculaires et cellulaires des rythmes circadiens. Mais ce prix a fait bien plus que trois heureux. Il apporte, en effet, une reconnaissance éclatante à un domaine, la chronobiologie, qui a longtemps fait figure, au mieux pour certains, d’aimable curiosité… La difficulté à identifier les rouages des horloges biologiques qui rythment nos jours et nos nuits, ou même à seulement les imaginer, y a bien sûr contribué. C’est pourquoi les travaux de Hall, Rosbash et Young – récompensés « pour leurs découvertes des mécanismes moléculaires qui contrôlent les rythmes circadiens » – ont revêtu une telle importance, même si la voie leur avait été ouverte un peu plus d’une décennie auparavant. Paradoxalement, le grand public a peut-être admis l’existence de nos horloges internes avant la communauté scientifique, car chacun peut faire l’expérience intime de rythmes journaliers, à commencer par l’alternance veille-sommeil, qui s’imposent à lui !

Neural Control of Startle-Induced Locomotion by the Mushroom Bodies and Associated Neurons in Drosophila

Startle-induced locomotion is commonly used in Drosophila research to monitor locomotor reactivity and its progressive decline with age or under various neuropathological conditions. A widely used paradigm is startle-induced negative geotaxis (SING), in which flies entrapped in a narrow column react to a gentle mechanical shock by climbing rapidly upwards. Here we combined in vivo manipulation of neuronal activity and splitGFP reconstitution across cells to search for brain neurons and putative circuits that regulate this behavior. We show that the activity of specific clusters of dopaminergic neurons (DANs) afferent to the mushroom bodies (MBs) modulates SING, and that DAN-mediated SING regulation requires expression of the DA receptor Dop1R1/Dumb, but not Dop1R2/Damb, in intrinsic MB Kenyon cells (KCs). We confirmed our previous observation that activating the MB α'β', but not αβ, KCs decreased the SING response, and we identified further MB neurons implicated in SING control, including KCs of the γ lobe and two subtypes of MB output neurons (MBONs). We also observed that co-activating the αβ KCs antagonizes α'β' and γ KC-mediated SING modulation, suggesting the existence of subtle regulation mechanisms between the different MB lobes in locomotion control. Overall, this study contributes to an emerging picture of the brain circuits modulating locomotor reactivity in Drosophila that appear both to overlap and differ from those underlying associative learning and memory, sleep/wake state and stress-induced hyperactivity.

Blue-light induced accumulation of reactive oxygen species is a consequence of the Drosophila cryptochrome photocycle

Cryptochromes are evolutionarily conserved blue-light absorbing flavoproteins which participate in many important cellular processes including in entrainment of the circadian clock in plants, Drosophila and humans. Drosophila melanogaster cryptochrome (DmCry) absorbs light through a flavin (FAD) cofactor that undergoes photoreduction to the anionic radical (FAD•-) redox state both in vitro and in vivo. However, recent efforts to link this photoconversion to the initiation of a biological response have remained controversial. Here, we show by kinetic modeling of the DmCry photocycle that the fluence dependence, quantum yield, and half-life of flavin redox state interconversion are consistent with the anionic radical (FAD•-) as the signaling state in vivo. We show by fluorescence detection techniques that illumination of purified DmCry results in enzymatic conversion of molecular oxygen (O2) to reactive oxygen species (ROS). We extend these observations in living cells to demonstrate transient formation of superoxide (O2•-), and accumulation of hydrogen peroxide (H2O2) in the nucleus of insect cell cultures upon DmCry illumination. These results define the kinetic parameters of the Drosophila cryptochrome photocycle and support light-driven electron transfer to the flavin in DmCry signaling. They furthermore raise the intriguing possibility that light-dependent formation of ROS as a byproduct of the cryptochrome photocycle may contribute to its signaling role.

Drosophila Clock Is Required in Brain Pacemaker Neurons to Prevent Premature Locomotor Aging Independently of Its Circadian Function

Circadian clocks control many self-sustained rhythms in physiology and behavior with approximately 24-hour periodicity. In many organisms, oxidative stress and aging negatively impact the circadian system and sleep. Conversely, loss of the clock decreases resistance to oxidative stress, and may reduce lifespan and speed up brain aging and neurodegeneration. Here we examined the effects of clock disruptions on locomotor aging and longevity in Drosophila. We found that lifespan was similarly reduced in three arrhythmic mutants (ClkAR, cyc0 and tim0) and in wild-type flies under constant light, which stops the clock. In contrast, ClkAR mutants showed significantly faster age-related locomotor deficits (as monitored by startle-induced climbing) than cyc0 and tim0, or than control flies under constant light. Reactive oxygen species accumulated more with age in ClkAR mutant brains, but this did not appear to contribute to the accelerated locomotor decline of the mutant. Clk, but not Cyc, inactivation by RNA interference in the pigment-dispersing factor (PDF)-expressing central pacemaker neurons led to similar loss of climbing performance as ClkAR. Conversely, restoring Clk function in these cells was sufficient to rescue the ClkAR locomotor phenotype, independently of behavioral rhythmicity. Accelerated locomotor decline of the ClkAR mutant required expression of the PDF receptor and correlated to an apparent loss of dopaminergic neurons in the posterior protocerebral lateral 1 (PPL1) clusters. This neuronal loss was rescued when the ClkAR mutation was placed in an apoptosis-deficient background. Impairing dopamine synthesis in a single pair of PPL1 neurons that innervate the mushroom bodies accelerated locomotor decline in otherwise wild-type flies. Our results therefore reveal a novel circadian-independent requirement for Clk in brain circadian neurons to maintain a subset of dopaminergic cells and avoid premature locomotor aging in Drosophila.

A new brain dopamine-deficient Drosophila and its pharmacological and genetic rescue

Dopamine (DA) is a neurotransmitter with conserved behavioral roles between invertebrate and vertebrate animals. In addition to its neural functions, in insects DA is a critical substrate for cuticle pigmentation and hardening. Drosophila tyrosine hydroxylase (DTH) is the rate limiting enzyme for DA biosynthesis. Viable brain DA-deficient flies were previously generated using tissue-selective GAL4-UAS binary expression rescue of a DTH null mutation and these flies show specific behavioral impairments. To circumvent the limitations of rescue via binary expression, here we achieve rescue utilizing genomically integrated mutant DTH. As expected, our DA-deficient flies have no detectable DTH or DA in the brain, and show reduced locomotor activity. This deficit can be rescued by l-DOPA/carbidopa feeding, similar to human Parkinson's disease treatment. Genetic rescue via GAL4/UAS-DTH was also successful, although this required the generation of a new UAS-DTH1 transgene devoid of most untranslated regions, as existing UAS-DTH transgenes express in the brain without a Gal4 driver via endogenous regulatory elements. A surprising finding of our newly constructed UAS-DTH1m is that it expresses DTH at an undetectable level when regulated by dopaminergic GAL4 drivers even when fully rescuing DA, indicating that DTH immunostaining is not necessarily a valid marker for DA expression. This finding necessitated optimizing DA immunohistochemistry, showing details of DA innervation to the mushroom body and the central complex. When DA rescue is limited to specific DA neurons, DA does not diffuse beyond the DTH-expressing terminals, such that DA signaling can be limited to very specific brain regions.

Adaptive preconditioning in neurological diseases - therapeutic insights from proteostatic perturbations

In neurological disorders, both acute and chronic neural stress can disrupt cellular proteostasis, resulting in the generation of pathological protein. However in most cases, neurons adapt to these proteostatic perturbations by activating a range of cellular protective and repair responses, thus maintaining cell function. These interconnected adaptive mechanisms comprise a 'proteostasis network' and include the unfolded protein response, the ubiquitin proteasome system and autophagy. Interestingly, several recent studies have shown that these adaptive responses can be stimulated by preconditioning treatments, which confer resistance to a subsequent toxic challenge - the phenomenon known as hormesis. In this review we discuss the impact of adaptive stress responses stimulated in diverse human neuropathologies including Parkinson׳s disease, Wolfram syndrome, brain ischemia, and brain cancer. Further, we examine how these responses and the molecular pathways they recruit might be exploited for therapeutic gain. This article is part of a Special Issue entitled SI:ER stress.

Dopamine drives Drosophila sechellia adaptation to its toxic host

Many insect species are host-obligate specialists. The evolutionary mechanism driving the adaptation of a species to a toxic host is, however, intriguing. We analyzed the tight association of Drosophila sechellia to its sole host, the fruit of Morinda citrifolia, which is toxic to other members of the melanogaster species group. Molecular polymorphisms in the dopamine regulatory protein Catsup cause infertility in D. sechellia due to maternal arrest of oogenesis. In its natural host, the fruit compensates for the impaired maternal dopamine metabolism with the precursor l-DOPA, resuming oogenesis and stimulating egg production. l-DOPA present in morinda additionally increases the size of D. sechellia eggs, what in turn enhances early fitness. We argue that the need of l-DOPA for successful reproduction has driven D. sechellia to become an M. citrifolia obligate specialist. This study illustrates how an insect's dopaminergic system can sustain ecological adaptations by modulating ontogenesis and development.

Dopamine drives Drosophila sechellia adaptation to its toxic host

Many insect species are host-obligate specialists. The evolutionary mechanism driving the adaptation of a species to a toxic host is, however, intriguing. We analyzed the tight association of Drosophila sechellia to its sole host, the fruit of Morinda citrifolia, which is toxic to other members of the melanogaster species group. Molecular polymorphisms in the dopamine regulatory protein Catsup cause infertility in D. sechellia due to maternal arrest of oogenesis. In its natural host, the fruit compensates for the impaired maternal dopamine metabolism with the precursor l-DOPA, resuming oogenesis and stimulating egg production. l-DOPA present in morinda additionally increases the size of D. sechellia eggs, what in turn enhances early fitness. We argue that the need of l-DOPA for successful reproduction has driven D. sechellia to become an M. citrifolia obligate specialist. This study illustrates how an insect's dopaminergic system can sustain ecological adaptations by modulating ontogenesis and development.

DOI:http://dx.doi.org/10.7554/eLife.03785.001

Many insect species rely on another animal or plant species for their own reproduction. For example, a fruit fly called Drosophila sechellia—which is found in the Seychelles—will only feed and lay its eggs on the fruit of a species of tree called Morinda citrifolia. This pairing is particularly unusual because these fruits, commonly called morinda, are toxic to all other Drosophila species.

Female Drosophila sechellia flies produce fewer eggs than other Drosophila species, which makes it difficult to raise this species in the laboratory. However providing these flies with morinda fruit, or chemicals from this fruit, was known to increase the expression of many genes involved in egg production and stimulate the flies to lay more eggs. Nevertheless, the reasons why this species of fruit fly depends on the toxic morinda fruit were unclear.

Now Lavista-Llanos et al. have confirmed that feeding Drosophila sechellia flies a diet of morinda fruit—instead of a typical laboratory diet—causes these flies to produce six-times as many eggs. Furthermore, this morinda diet had effects that went beyond the previously reported stimulatory effects of acidic chemicals in the fruits triggering the flies to lay more eggs.

Egg production in flies is controlled by dopamine, and a lack of this hormone is known to reduce the size of other fruit flies' ovaries and the number of eggs that they produce. Lavista-Llanos et al. went on to feed female Drosophila sechellia flies the chemical building blocks that make up the dopamine hormone, and one such chemical (called l-DOPA) caused the flies to produce more eggs. This did not occur when the flies were fed dopamine itself.

Lavista-Llanos et al. discovered that Drosophila sechellia flies have very high levels of dopamine but much lower levels of l-DOPA than other Drosophila fly species; and revealed that this was because a gene called Catsup is mutated in Drosophila sechellia. When Lavista-Llanos et al. mutated the same gene in another Drosophila species, the mutant flies produced fewer eggs and abnormally accumulated an enzyme (which makes l-DOPA) inside their developing eggs—just like Drosophila sechellia.

The presence of l-DOPA in morinda fruit partly compensates for the reduced fertility of Drosophila sechellia and the other flies with mutations in the Catsup gene. Lavista-Llanos et al. discovered that removing or replacing l-DOPA in the morinda fruit caused the flies to produce fewer eggs. Furthermore, the l-DOPA present in morinda increases the size of Drosophila sechellia eggs, which in turn helps them to survive their toxic environment.

Lavista-Llanos et al. also discovered that feeding dopamine to vulnerable Drosophila species helps them to cope with the toxic effects of a morinda diet. One of the next challenges will be to uncover how chemicals from the morinda fruit affect the dopamine system of the flies. It is also unknown if the dopamine hormone also influences the strong attraction that Drosophila sechellia feels towards its only host, the morinda fruit.

DOI:http://dx.doi.org/10.7554/eLife.03785.002

A dopamine receptor contributes to paraquat-induced neurotoxicity in Drosophila

Long-term exposure to environmental oxidative stressors, like the herbicide paraquat (PQ), has been linked to the development of Parkinson's disease (PD), the most frequent neurodegenerative movement disorder. Paraquat is thus frequently used in the fruit fly Drosophila melanogaster and other animal models to study PD and the degeneration of dopaminergic neurons (DNs) that characterizes this disease. Here, we show that a D1-like dopamine (DA) receptor, DAMB, actively contributes to the fast central nervous system (CNS) failure induced by PQ in the fly. First, we found that a long-term increase in neuronal DA synthesis reduced DAMB expression and protected against PQ neurotoxicity. Secondly, a striking age-related decrease in PQ resistance in young adult flies correlated with an augmentation of DAMB expression. This aging-associated increase in oxidative stress vulnerability was not observed in a DAMB-deficient mutant. Thirdly, targeted inactivation of this receptor in glutamatergic neurons (GNs) markedly enhanced the survival of Drosophila exposed to either PQ or neurotoxic levels of DA, whereas, conversely, DAMB overexpression in these cells made the flies more vulnerable to both compounds. Fourthly, a mutation in the Drosophila ryanodine receptor (RyR), which inhibits activity-induced increase in cytosolic Ca(2+), also strongly enhanced PQ resistance. Finally, we found that DAMB overexpression in specific neuronal populations arrested development of the fly and that in vivo stimulation of either DNs or GNs increased PQ susceptibility. This suggests a model for DA receptor-mediated potentiation of PQ-induced neurotoxicity. Further studies of DAMB signaling in Drosophila could have implications for better understanding DA-related neurodegenerative disorders in humans.

Lack of miRNA Misregulation at Early Pathological Stages in Drosophila Neurodegenerative Disease Models

Late onset neurodegenerative diseases represent a major public health concern as the population in many countries ages. Both frequent diseases such as Alzheimer disease (AD, 14% incidence for 80-84 year-old Europeans) or Parkinson disease (PD, 1.4% prevalence for >55 years old) share, with other low-incidence neurodegenerative pathologies such as spinocerebellar ataxias (SCAs, 0.01% prevalence) and frontotemporal lobar degeneration (FTLD, 0.02% prevalence), a lack of efficient treatment in spite of important research efforts. Besides significant progress, studies with animal models have revealed unexpected complexities in the degenerative process, emphasizing a need to better understand the underlying pathological mechanisms. Recently, microRNAs (miRNAs), a class of small regulatory non-coding RNAs, have been implicated in some neurodegenerative diseases. The current data supporting a role of miRNAs in PD, tauopathies, dominant ataxias, and FTLD will first be discussed to emphasize the different levels of the pathological processes which may be affected by miRNAs. To investigate a potential involvement of miRNA dysregulation in the early stages of these neurodegenerative diseases we have used Drosophila models for seven diseases (PD, 3 FTLD, 3 dominant ataxias) that recapitulate many features of the human diseases. We performed deep sequencing of head small RNAs after 3 days of pathological protein expression in the fly head neurons. We found no evidence for a statistically significant difference in miRNA expression in this early stage of the pathological process. In addition, we could not identify small non-coding CAG repeat RNAs (sCAG) in polyQ disease models. Thus our data suggest that transcriptional deregulation of miRNAs or sCAG is unlikely to play a significant role in the initial stages of neurodegenerative diseases.

Synaptic activity in serotonergic neurons is required for air-puff stimulated flight in Drosophila melanogaster

Background

Flight is an integral component of many complex behavioral patterns in insects. The giant fiber circuit has been well studied in several insects including Drosophila. However, components of the insect flight circuit that respond to an air-puff stimulus and comprise the flight central pattern generator are poorly defined. Aminergic neurons have been implicated in locust, moth and Drosophila flight. Here we have investigated the requirement of neuronal activity in serotonergic neurons, during development and in adults, on air-puff induced flight in Drosophila.

Methodology/Principal Findings

To target serotonergic neurons specifically, a Drosophila strain that contains regulatory regions from the TRH (Tryptophan Hydroxylase) gene linked to the yeast transcription factor GAL4 was used. By blocking synaptic transmission from serotonergic neurons with a tetanus toxin transgene or by hyperpolarisation with Kir2.1, close to 50% adults became flightless. Temporal expression of a temperature sensitive Dynamin mutant transgene (Shi^ts) suggests that synaptic function in serotonergic neurons is required both during development and in adults. Depletion of IP₃R in serotonergic neurons via RNAi did not affect flight. Interestingly, at all stages a partial requirement for synaptic activity in serotonergic neurons was observed. The status of serotonergic neurons was investigated in the central nervous system of larvae and adults expressing tetanus toxin. A small but significant reduction was observed in serotonergic cell number in adult second thoracic segments from flightless tetanus toxin expressing animals.

Conclusions

These studies show that loss of synaptic activity in serotonergic neurons causes a flight deficit. The temporal focus of the flight deficit is during pupal development and in adults. The cause of the flight deficit is likely to be loss of neurons and reduced synaptic function. Based on the partial phenotypes, serotonergic neurons appear to be modulatory, rather than an intrinsic part of the flight circuit.

Alternative oxidase rescues mitochondria-mediated dopaminergic cell loss in Drosophila

Mitochondrial dysfunction is commonly observed in degenerative disorders, including Alzheimer's and Parkinson's disease that are characterized by the progressive and selective loss of neuronal subpopulations. It is currently unclear, however, whether mitochondrial dysfunction is primary or secondary to other pathogenic processes that eventually lead to age-related neurodegeneration. Here we establish an in vivo Drosophila model of mitochondrial dysfunction by downregulating the catalytic subunit of mitochondrial DNA (mtDNA) polymerase in cholinergic, serotonergic and dopaminergic neurons. The resulting flies are characterized by lowered respiratory chain activity, premature aging, age-related motor deficits as well as adult onset, progressive and cell-type-specific, dopaminergic neurodegeneration. Using this model, we find that associated lethality can be partially rescued by targeting PINK1/parkin signaling or Drp1, both of which have been implicated in mitochondrial dynamics and Parkinson's disease. Bypassing mitochondrial complex III/IV deficiencies with Alternative oxidase (AOX), however, fully restores ATP levels and prevents dopaminergic neurodegeneration. In contrast, ATP levels and neurodegeneration are not rescued when mitochondrial complex I deficiencies are bypassed with NADH-Q oxidoreductase. Our results demonstrate that mtDNA-mediated mitochondrial dysfunction can cause age-related and cell-type-specific neurodegeneration which AOX is able to alleviate and indicate that AOX or its surrogates may prove useful as a therapeutic tool for limiting respiratory chain deficiencies caused by mtDNA decline in healthy aging and neurodegenerative disease.

Dynamics of glutamatergic signaling in the mushroom body of young adult Drosophila

Background

The mushroom bodies (MBs) are paired brain centers located in the insect protocerebrum involved in olfactory learning and memory and other associative functions. Processes from the Kenyon cells (KCs), their intrinsic neurons, form the bulk of the MB's calyx, pedunculus and lobes. In young adult Drosophila, the last-born KCs extend their processes in the α/β lobes as a thin core (α/β cores) that is embedded in the surrounding matrix of other mature KC processes. A high level of L-glutamate (Glu) immunoreactivity is present in the α/β cores (α/βc) of recently eclosed adult flies. In a Drosophila model of fragile X syndrome, the main cause of inherited mental retardation, treatment with metabotropic Glu receptor (mGluR) antagonists can rescue memory deficits and MB structural defects.

Results

To address the role of Glu signaling in the development and maturation of the MB, we have compared the time course of Glu immunoreactivity with the expression of various glutamatergic markers at various times, that is, 1 hour, 1 day and 10 days after adult eclosion. We observed that last-born α/βc KCs in young adult as well as developing KCs in late larva and at various pupal stages transiently express high level of Glu immunoreactivity in Drosophila. One day after eclosion, the Glu level was already markedly reduced in the α/βc neurons. Glial cell processes expressing glutamine synthetase and the Glu transporter dEAAT1 were found to surround the Glu-expressing KCs in very young adults, subsequently enwrapping the α/β lobes to become distributed equally over the entire MB neuropil. The vesicular Glu transporter DVGluT was detected by immunostaining in processes that project within the MB lobes and pedunculus, but this transporter is apparently never expressed by the KCs themselves. The NMDA receptor subunit dNR1 is widely expressed in the MB neuropil just after eclosion, but was not detected in the α/βc neurons. In contrast, we provide evidence that DmGluRA, the only Drosophila mGluR, is specifically expressed in Glu-accumulating cells of the MB α/βc immediately and for a short time after eclosion.

Conclusions

The distribution and dynamics of glutamatergic markers indicate that newborn KCs transiently accumulate Glu at a high level in late pupal and young eclosed Drosophila, and may locally release this amino acid by a mechanism that would not involve DVGluT. At this stage, Glu can bind to intrinsic mGluRs abundant in the α/βc KCs, and to NMDA receptors in the rest of the MB neuropil, before being captured and metabolized in surrounding glial cells. This suggests that Glu acts as an autocrine or paracrine agent that contributes to the structural and functional maturation of the MB during the first hours of Drosophila adult life.

Neurodegeneration: RNA turns number one suspect in polyglutamine diseases

Polyglutamine expansion diseases are triggered by the accumulation of toxic proteins. A new study reports that RNA molecules containing long CAG repeats can also be toxic to neurons and may play a significant role in pathogenesis.

Physiological requirement for the glutamate transporter dEAAT1 at the adult Drosophila neuromuscular junction

L-glutamate is the major excitatory neurotransmitter in the mammalian brain. Specific proteins, the Na+/K+-dependent high affinity excitatory amino acid transporters (EAATs), are involved in the extracellular clearance and recycling of this amino acid. Type I synapses of the Drosophila neuromuscular junction (NMJ) similarly use L-glutamate as an excitatory transmitter. However, the localization and function of the only high-affinity glutamate reuptake transporter in Drosophila, dEAAT1, at the NMJ was unknown. Using a specific antibody and transgenic strains, we observed that dEAAT1 is present at the adult, but surprisingly not at embryonic and larval NMJ, suggesting a physiological maturation of the junction during metamorphosis. We found that dEAAT1 is not localized in motor neurons but in glial extensions that closely follow motor axons to the adult NMJ. Inactivation of the dEAAT1 gene by RNA interference generated viable adult flies that were able to walk but were flight-defective. Electrophysiological recordings of the thoracic dorso-lateral NMJ were performed in adult dEAAT1-deficient flies. The lack of dEAAT1 prolonged the duration of the individual responses to motor nerve stimulation and this effect was progressively increased during physiological trains of stimulations. Therefore, glutamate reuptake by glial cells is required to ensure normal activity of the Drosophila NMJ, but only in adult flies.

Cell-type-specific limitation on in vivo serotonin storage following ectopic expression of the Drosophila serotonin transporter, dSERT

The synaptic machinery for neurotransmitter storage is cell-type specific. Although most elements of biosynthesis and transport have been identified, it remains unclear whether additional factors may be required to maintain this specificity. The Drosophila serotonin transporter (dSERT) is normally expressed exclusively in serotonin (5-HT) neurons in the CNS. Here we examine the effects of ectopic transcriptional expression of dSERT in the Drosophila larval CNS. We find a surprising limitation on 5-HT storage following ectopic expression of dSERT and green fluorescence protein-tagged dSERT (GFP-dSERT). When dSERT transcription is driven ectopically in the CNS, 5-HT is detectable only in 5-HT, dopamine (DA), and a very limited number of additional neurons. Addition of exogenous 5-HT does not dramatically broaden neuronal storage sites, so this limitation is only partly due to restricted intercellular diffusion of 5-HT. Furthermore, this limitation is not due to gross mislocalization of dSERT, because cells lacking or containing 5-HT show similar levels and subcellular distribution of GFP-dSERT protein, nor is it due to lack of the vesicular transporter, dVMAT. These data suggest that a small number of neurons selectively express factor(s) required for 5-HT storage, and potentially for function of dSERT.

Chronic exposure to rotenone models sporadic Parkinson's disease in Drosophila melanogaster

Parkinson's disease (PD) is a movement disorder characterized by the selective degeneration of nigrostriatal dopaminergic neurons. Both familial and sporadic cases present tremor, rigidity, slowness of movement, and postural instability. Although major insights into the genes responsible for some rare hereditary cases have arisen, the etiology of sporadic cases remains unknown. Epidemiological studies have suggested an association with environmental toxins, mainly mitochondrial complex I inhibitors such as the widely used pesticide rotenone. In recent years, Drosophila melanogaster has been used as a model of several neurodegenerative diseases, including a genetic model of PD. Here, we studied the neurodegenerative and behavioral effects of a sublethal chronic exposure to rotenone in Drosophila. After several days, the treated flies presented characteristic locomotor impairments that increased with the dose of rotenone. Immunocytochemistry analysis demonstrated a dramatic and selective loss of dopaminergic neurons in all of the brain clusters. The addition of l-dopa (3,4-dihydroxy-L-phenylalanine) into the feeding medium rescued the behavioral deficits but not neuronal death, as is the case in human PD patients. In contrast, the antioxidant melatonin (N-acetyl-5-methoxytryptamine) alleviated both symptomatic impairment and neuronal loss, supporting the idea that this agent may be beneficial in the treatment of PD. Therefore, chronic exposure to pesticides recapitulates key aspects of PD in Drosophila and provides a new in vivo model for studying the mechanisms of dopaminergic neurodegeneration.

Arousal Mechanisms: Speedy Flies Don’t Sleep at Night

Alertness and behavioral performance depend on an animal's level of arousal. In vertebrates, reinforcement and maintenance of arousal in the cortex are ensured by diffuse inputs from neurons releasing biogenic amine neuromodulators. Fruit flies similarly use dopamine for arousal control, indicating an ancient evolutionary origin of this essential feature of the functioning brain.

High Affinity Transport of Taurine by the Drosophila Aspartate Transporter dEAAT2

Excitatory amino acid transporters (EAATs) are structurally related plasma membrane proteins known to mediate the Na(+)/K(+)-dependent uptake of the amino acids l-glutamate and dl-aspartate. In the nervous system, these proteins contribute to the clearance of glutamate from the synaptic cleft and maintain excitatory amino acid concentrations below excitotoxic levels. Two homologues exist in Drosophila melanogaster, dEAAT1 and dEAAT2, which are specifically expressed in the nervous tissue. We previously reported that dEAAT2 shows unique substrate discrimination as it mediates high affinity transport of aspartate but not glutamate. We now show that dEAAT2 can also transport the amino acid taurine with high affinity, a property that is not shared by two other transporters of the same family, Drosophila dEAAT1 and human hEAAT2. Taurine transport by dEAAT2 was efficiently blocked by an EAAT antagonist but not by inhibitors of the structurally unrelated mammalian taurine transporters. Taurine and aspartate are transported with similar K(m) and relative efficacy and behave as mutually competitive inhibitors. dEAAT2 can mediate either net uptake or the heteroexchange of its two substrates, both being dependent on the presence of Na(+) ions in the external medium. Interestingly, heteroexchange only occurs in one preferred substrate orientation, i.e. with taurine transported inwards and aspartate outwards, suggesting a mechanism of transinhibition of aspartate uptake by intracellular taurine. Therefore, dEAAT2 is actually an aspartate/taurine transporter. Further studies of this protein are expected to shed light on the role of taurine as a candidate neuromodulator and cell survival factor in the Drosophila nervous system.

Expanded polyglutamine peptides disrupt EGF receptor signaling and glutamate transporter expression in Drosophila

Huntington's disease (HD) is a late onset heritable neurodegenerative disorder caused by expansion of a polyglutamine (polyQ) sequence in the protein huntingtin (Htt). Transgenic models in mice have suggested that the motor and cognitive deficits associated to this disease are triggered by extended neuronal and possibly glial dysfunction, whereas neuronal death occurs late and selectively. Here, we provide in vivo evidence that expanded polyQ peptides antagonize epidermal growth factor receptor (EGFR) signaling in Drosophila glia. We targeted the expression of the polyQ-containing domain of Htt or an extended polyQ peptide alone in a subset of Drosophila glial cells, where the only fly glutamate transporter, dEAAT1, is detected. This resulted in formation of nuclear inclusions, progressive decrease in dEAAT1 transcription and shortened adult lifespan, but no significant glial cell death. We observed that brain expression of dEAAT1 is normally sustained by the EGFR-Ras-extracellular signal-regulated kinase (ERK) signaling pathway, suggesting that polyQ could act by antagonizing this pathway. We found that the presence of polyQ peptides indeed abolished dEAAT1 upregulation by constitutively active EGFR and potently inhibited EGFR-mediated ERK activation in fly glial cells. Long polyQ also limited the effect of activated EGFR on Drosophila eye development. Our results further indicate that the polyQ acts at an upstream step in the pathway, situated between EGFR and ERK activation. This suggests that disruption of EGFR signaling and ensuing glial cell dysfunction could play a direct role in the pathogenesis of HD and other polyQ diseases in humans.

Female-specific regulation of cuticular hydrocarbon biosynthesis by dopamine in Drosophila melanogaster

The role of dopamine (DA) is investigated in cuticular hydrocarbon biosynthesis in Drosophila melanogaster with three different approaches: use of DA-deficient mutants (dopa decarboxylase temperature sensitive mutants reared at restrictive temperature, and rescued by dopamine ingestion or by pale mutants partially rescued by a tyrosine hydroxylase construction), pharmacological treatments (tyrosine hydroxylase inhibitors) and topical application on decapitated flies. We report that DA specifically regulates diene hydrocarbon biosynthesis, which is female specific. Our results suggest that DA acts in adult flies within the first hours of imaginal life and that DA production from the brain is crucial for this process. Thus, DA contributes to reproduction in D. melanogaster by acting during a critical period during development of young adults.

Decreasing glutamate buffering capacity triggers oxidative stress and neuropil degeneration in the Drosophila brain

L-glutamate is both the major brain excitatory neurotransmitter and a potent neurotoxin in mammals. Glutamate excitotoxicity is partly responsible for cerebral traumas evoked by ischemia and has been implicated in several neurodegenerative diseases including amyotrophic lateral sclerosis (ALS). In contrast, very little is known about the function or potential toxicity of glutamate in the insect brain. Here, we show that decreasing glutamate buffering capacity is neurotoxic in Drosophila. We found that the only Drosophila high-affinity glutamate transporter, dEAAT1, is selectively addressed to glial extensions that project ubiquitously through the neuropil close to synaptic areas. Inactivation of dEAAT1 by RNA interference led to characteristic behavior deficits that were significantly rescued by expression of the human glutamate transporter hEAAT2 or the administration in food of riluzole, an anti-excitotoxic agent used in the clinic for human ALS patients. Signs of oxidative stress included hypersensitivity to the free radical generator paraquat and rescue by the antioxidant melatonin. Inactivation of dEAAT1 also resulted in shortened lifespan and marked brain neuropil degeneration characterized by widespread microvacuolization and swollen mitochondria. This suggests that the dEAAT1-deficient fly provides a powerful genetic model system for molecular analysis of glutamate-mediated neurodegeneration.

Dopamine and octopamine differentiate between aversive and appetitive olfactory memories in Drosophila

The catecholamines play a major role in the regulation of behavior. Here we investigate, in the fly Drosophila melanogaster, the role of dopamine and octopamine (the presumed arthropod homolog of norepinephrine) during the formation of appetitive and aversive olfactory memories. We find that for the formation of both types of memories, cAMP signaling is necessary and sufficient within the same subpopulation of mushroom-body intrinsic neurons. On the other hand, memory formation can be distinguished by the requirement for different catecholamines, dopamine for aversive and octopamine for appetitive conditioning. Our results suggest that in associative conditioning, different memories are formed of the same odor under different circumstances, and that they are linked to the respective motivational systems by their specific modulatory pathways.

Glutamate transport alteration triggers differentiation-state selective oxidative death of cultured astrocytes: a mechanism different from excitotoxicity depending on intracellular GSH contents

Recent evidence has been provided for astrocyte degeneration in experimental models of neurodegenerative insults associated with glutamate transport alteration. To determine whether astrocyte death can directly result from altered glutamate transport, we here investigated the effects of L-trans-pyrrolidine-2,4-dicarboxylate (PDC) on undifferentiated or differentiated cultured rat striatal astrocytes. PDC induced death of differentiated astrocytes without affecting undifferentiated astrocyte viability. Death of differentiated astrocytes was also triggered by another substrate inhibitor but not by blockers of glutamate transporters. The PDC-induced death was delayed and apoptotic, and death rate was dose and treatment duration-dependent. Although preceded by extracellular glutamate increase, this death was not mediated through glutamate receptor stimulation, as antagonists did not provide protection. It involves oxidative stress, as a decrease in glutathione contents and a dramatic raise in reactive oxygen species preceded cell loss, and as protection was provided by antioxidants. PDC induced a similar percentage of GSH depletion in the undifferentiated astrocytes, but only a slight increase in reactive oxygen species. Interestingly, undifferentiated astrocytes exhibited twofold higher basal GSH content compared with the differentiated ones, and depleting their GSH content was found to render them susceptible to PDC. Altogether, these data demonstrate that basal GSH content is a critical factor of astrocyte vulnerability to glutamate transport alteration with possible insights onto concurrent death of astrocytes and gliosis in neurodegenerative insults.

[Huntington chorea in Drosophila and mice: toward new therapeutic steps]

Huntington's disease is an hereditary dominant neurodegenerative disorder clinically characterised by progressive dyskinesia, cognitive decline and psychiatric disturbances. One decade after the identification of the gene whose mutation is responsible for the disease, this pathology remains incurable. However, major insights into early cellular and molecular basis of Huntington's disease have arisen from transgenic models. Transcriptional dysregulation, abnormal degradation of misfolded proteins as well as excitotoxic processes and mitochondrial dysfunction are involved in Huntington's disease. The present review discusses the recent insights gained from mouse and Drosophila models towards the understanding of pathogenesis and the development of new therapeutic tools.

Tissue-specific developmental requirements of Drosophila tyrosine hydroxylase isoforms

Drosophila tyrosine hydroxylase (DTH) is a key enzyme in dopamine (DA) biosynthesis, which is expressed in neural and hypodermal DA-synthesizing cells. We previously reported that two DTH isoforms are produced in flies through tissue-specific alternative splicing that show distinct regulatory properties. We have now selectively expressed each DTH isoform in vivo in a pale (ple, i.e., DTH-deficient) mutant background. We show that the embryonic lethality of ple can be rescued by expression of the hypodermal, but not the neural, DTH isoform in all DA cells, indicating that the hypoderm- isoform is absolutely required for cuticle biosynthesis and survival in Drosophila. In addition, we report new observations on the consequences of DTH overexpression in the CNS and hypoderm. Our results provide evidence that tissue-specific alternative splicing of the DTH gene is a vital process in Drosophila development.

Targeted gene expression in Drosophila dopaminergic cells using regulatory sequences from tyrosine hydroxylase

Dopamine (DA) is the only catecholaminergic neurotransmitter in the fruit fly Drosophila melanogaster. Dopaminergic neurons have been identified in the larval and adult central nervous system (CNS) in Drosophila and other insects, but no specific genetic tool was available to study their development, function, and degeneration in vivo. In Drosophila as in vertebrates, the rate-limiting step in DA biosynthesis is catalyzed by the enzyme tyrosine hydroxylase (TH). The Drosophila TH gene (DTH) is specifically expressed in all dopaminergic cells and the corresponding mutant, pale (ple), is embryonic lethal. We have performed ple rescue experiments with modified DTH transgenes. Our results indicate that partially redundant regulatory elements located in DTH introns are required for proper expression of this gene in the CNS. Based on this study, we generated a GAL4 driver transgene, TH-GAL4, containing regulatory sequences from the DTH 5' flanking and downstream coding regions. TH-GAL4 specifically expresses in dopaminergic cells in embryos, larval CNS, and adult brain when introduced into the Drosophila genome. As a first application of this driver, we observed that in vivo inhibition of DA release induces a striking hyperexcitability behavior in adult flies. We propose that TH-GAL4 will be useful for studies of the role of DA in behavior and disease models in Drosophila.

Tissue-specific developmental requirements of Drosophila tyrosine hydroxylase isoforms

Drosophila tyrosine hydroxylase (DTH) is a key enzyme in dopamine (DA) biosynthesis, which is expressed in neural and hypodermal DA-synthesizing cells. We previously reported that two DTH isoforms are produced in flies through tissue-specific alternative splicing that show distinct regulatory properties. We have now selectively expressed each DTH isoform in vivo in a pale (ple, i.e., DTH-deficient) mutant background. We show that the embryonic lethality of ple can be rescued by expression of the hypodermal, but not the neural, DTH isoform in all DA cells, indicating that the hypoderm- isoform is absolutely required for cuticle biosynthesis and survival in Drosophila. In addition, we report new observations on the consequences of DTH overexpression in the CNS and hypoderm. Our results provide evidence that tissue-specific alternative splicing of the DTH gene is a vital process in Drosophila development.

Terminal glial differentiation involves regulated expression of the excitatory amino acid transporters in the Drosophila embryonic CNS

The Drosophila excitatory amino acid transporters dEAAT1 and dEAAT2 are nervous-specific transmembrane proteins that mediate the high affinity uptake of L-glutamate or aspartate into cells. Here, we demonstrate by colocalization studies that both genes are expressed in discrete and partially overlapping subsets of differentiated glia and not in neurons in the embryonic central nervous system (CNS). We show that expression of these transporters is disrupted in mutant embryos deficient for the glial fate genes glial cells missing (gcm) and reversed polarity (repo). Conversely, ectopic expression of gcm in neuroblasts, which forces all nerve cells to adopt a glial fate, induces an ubiquitous expression of both EAAT genes in the nervous system. We also detected the dEAAT transcripts in the midline glia in late embryos and dEAAT2 in a few peripheral neurons in head sensory organs. Our results show that glia play a major role in excitatory amino acid transport in the Drosophila CNS and that regulated expression of the dEAAT genes contributes to generate the functional diversity of glial cells during embryonic development.

Blockade of the central generator of locomotor rhythm by noncompetitive NMDA receptor antagonists in Drosophila larvae

The noncompetitive antagonists of the vertebrate N-methyl-D-aspartate (NMDA) receptor dizocilpine (MK 801) and phencyclidine (PCP), delivered in food, were found to induce a marked and reversible inhibition of locomotor activity in Drosophila melanogaster larvae. To determine the site of action of these antagonists, we used an in vitro preparation of the Drosophila third-instar larva, preserving the central nervous system and segmental nerves with their connections to muscle fibers of the body wall. Intracellular recordings were made from ventral muscle fibers 6 and 7 in the abdominal segments. In most larvae, long-lasting (>1 h) spontaneous rhythmic motor activities were recorded in the absence of pharmacological activation. After sectioning of the connections between the brain and abdominal ganglia, the rhythm disappeared, but it could be partially restored by perfusing the muscarinic agonist oxotremorine, indicating that the activity was generated in the ventral nerve cord. MK 801 and PCP rapidly and efficiently inhibited the locomotor rhythm in a dose-dependent manner, the rhythm being totally blocked in 2 min with doses over 0.1 mg/mL. In contrast, more hydrophilic competitive NMDA antagonists had no effect on the motor rhythm in this preparation. MK 801 did not affect neuromuscular glutamatergic transmission at similar doses, as demonstrated by monitoring the responses elicited by electrical stimulation of the motor nerve or pressure applied glutamate. The presence of oxotremorine did not prevent the blocking effect of MK 801. These results show that MK 801 and PCP specifically inhibit centrally generated rhythmic activity in Drosophila, and suggest a possible role for NMDA-like receptors in locomotor rhythm control in the insect CNS.

Selective high-affinity transport of aspartate by a Drosophila homologue of the excitatory amino-acid transporters

Excitatory amino-acid transporters (EAATs) are structurally related plasma membrane proteins that mediate the high-affinity uptake of the acidic amino acids glutamate and aspartate released at excitatory synapses, and maintain the extracellular concentrations of these neurotransmitters below excitotoxic levels [1] [2] [3] [4]. Several members of the EAAT family have been described previously. So far, all known EAATs have been reported to transport glutamate and aspartate with a similar affinity. Here, we report that dEAAT2 - a nervous tissue-specific EAAT homologue that we recently identified in the fruit fly Drosophila [5] - is a selective Na(+)-dependent high-affinity aspartate transporter (K(m) = 30 microM). We found that dEAAT2 can also transport L-glutamate but with a much lower affinity (K(m) = 185 microM) and a 10- to 15-fold lower relative efficacy (V(max)/K(m)). Competition experiments showed that the binding of glutamate to this transporter is much weaker than the binding of D- or L-aspartate. As dEAAT2 is the first known EAAT to show this substrate selectivity, it suggests that aspartate may play a specific role in the Drosophila nervous system.

Differential regulation of Drosophila tyrosine hydroxylase isoforms by dopamine binding and cAMP-dependent phosphorylation

Tyrosine hydroxylase (TH) catalyzes the first step in dopamine biosynthesis in Drosophila as in vertebrates. We have previously reported that tissue-specific alternative splicing of the TH primary transcript generates two distinct TH isoforms in Drosophila, DTH I and DTH II (Birman, S., Morgan, B., Anzivino, M., and Hirsh, J. (1994) J. Biol. Chem. 269, 26559-26567). Expression of DTH I is restricted to the central nervous system, whereas DTH II is expressed in non-nervous tissues like the epidermis. The two enzymes present a single structural difference; DTH II specifically contains a very acidic segment of 71 amino acids inserted in the regulatory domain. We show here that the enzymatic and regulatory properties of vertebrate TH are generally conserved in insect TH and that the isoform DTH II presents unique characteristics. The two DTH isoforms were expressed as apoenzymes in Escherichia coli and purified by fast protein liquid chromatography. The recombinant DTH isoforms are enzymatically active in the presence of ferrous iron and a tetrahydropteridine co-substrate. However, the two enzymes differ in many of their properties. DTH II has a lower Km value for the co-substrate (6R)-tetrahydrobiopterin and requires a lower level of ferrous ion than DTH I to be activated. The two isoforms also have a different pH profile. As for mammalian TH, enzymatic activity of the Drosophila enzymes is decreased by dopamine binding, and this effect is dependent on ferrous iron levels. However, DTH II appears comparatively less sensitive than DTH I to dopamine inhibition. The central nervous system isoform DTH I is activated through phosphorylation by cAMP-dependent protein kinase (PKA) in the absence of dopamine. In contrast, activation of DTH II by PKA is only manifest in the presence of dopamine. Site-directed mutagenesis of Ser32, a serine residue occurring in a PKA site conserved in all known TH proteins, abolishes phosphorylation of both isoforms and activation by PKA. We propose that tissue-specific alternative splicing of TH has a functional role for differential regulation of dopamine biosynthesis in the nervous and non-nervous tissues of insects.