Control of Drosophila perineurial glial growth by interacting neurotransmitter-mediated signaling pathways

NF1-dependent disruption of the blood-nerve-barrier is improved by blockade of P2RY14

The blood-nerve-barrier (BNB) that regulates peripheral nerve homeostasis is formed by endoneurial capillaries and perineurial cells surrounding the Schwann cell (SC)-rich endoneurium. Barrier dysfunction is common in human tumorigenesis, including in some nerve tumors. We identify barrier disruption in human NF1 deficient neurofibromas, which were characterized by reduced perineurial cell glucose transporter 1 (GLUT1) expression and increased endoneurial fibrin(ogen) deposition. Conditional Nf1 loss in murine SCs recapitulated these alterations and revealed decreased tight junctions and decreased caveolin-1 (Cav1) expression in mutant nerves and in tumors, implicating reduced Cav1-mediated transcytosis in barrier disruption and tumorigenesis. Additionally, elevated receptor tyrosine kinase activity and genetic deletion of Cav1 increased endoneurial fibrin(ogen), and promoted SC tumor formation. Finally, when SC lacked Nf1, genetic loss or pharmacological inhibition of P2RY14 rescued Cav1 expression and barrier function. Thus, loss of Nf1 in SC causes dysfunction of the BNB via P2RY14-mediated G-protein coupled receptor (GPCR) signaling.

wde, calpA, if, dap160, and poe genes knock down Drosophila models exhibit neurofunctional deficit

Intellectual disability (ID) is a heterogeneous disorder with high prevalence and remarkable social and cost burdens. Novel genetic variants of ATF7IP, CAPN9, ITGAV, ITSN1, and UBR4 genes are reported to be associated with the ID among Iranian families. However, in vivo validation is required to confirm the functional role of these variants in ID development. Drosophila melanogaster is a convenient model for such functional investigations as its genome bears ortholog of more than 75% of the disease-causing genes in human and represents numerous approaches to study defects in neuronal function. In this connection, RNAi gene silencing was applied to wde, calpA, if, dap160, and poe genes, the Drosophila ortholog of the selected human genes, and then consequent structural and functional changes in neurons were studied by means of immunohistochemistry and confocal microscopy of mushroom bodies (MBs) and validated behavioural assays including larvae and adult conditioning learning and memories, and ethanol sensitivity. Down-regulation of these genes led to neuronal loss which was evident by decline in total fluorescent signal intensity in micrographs of MBs structure. The gene silencing caused neuronal dysfunction and induction of ID-like symptoms manifested by deficits in larval preference learning, and short-term olfactory memory and courtship suppression learning in adults. Moreover, the RNAi flies showed higher sensitivity to ethanol vapour. Interestingly, the poe knock-down flies exhibited the most severe phenotypes among other genes. Altogether, we believe this study is first-of-its-kind and findings are highly applicable to confirm pathogenecity of the selected ID gene variants in Iranian population.

Modeling human brain tumors in flies, worms, and zebrafish: From proof of principle to novel therapeutic targets

For decades, cell biologists and cancer researchers have taken advantage of non-murine species to increase our understanding of the molecular processes that drive normal cell and tissue development, and when perturbed, cause cancer. The advent of whole-genome sequencing has revealed the high genetic homology of these organisms to humans. Seminal studies in non-murine organisms such as Drosophila melanogaster, Caenorhabditis elegans, and Danio rerio identified many of the signaling pathways involved in cancer. Studies in these organisms offer distinct advantages over mammalian cell or murine systems. Compared to murine models, these three species have shorter lifespans, are less resource intense, and are amenable to high-throughput drug and RNA interference screening to test a myriad of promising drugs against novel targets. In this review, we introduce species-specific breeding strategies, highlight the advantages of modeling brain tumors in each non-mammalian species, and underscore the successes attributed to scientific investigation using these models. We conclude with an optimistic proposal that discoveries in the fields of cancer research, and in particular neuro-oncology, may be expedited using these powerful screening tools and strategies.

BIG Regulates Dynamic Adjustment of Circadian Period in Arabidopsis thaliana

Circadian clocks drive rhythms with a period near 24 h, but the molecular basis of the regulation of the period of the circadian clockis poorly understood. We previously demonstrated that metabolites affect the free-running period of the circadian oscillator of Arabidopsis (Arabidopsis thaliana), with endogenous sugars acting as an accelerator and exogenous nicotinamide acting as a brake. Changes in circadian oscillator period are thought to adjust the timing of biological activities through the process of entrainment, in which the circadian oscillator becomes synchronized to rhythmic signals such as light and dark cycles as well as changes in internal metabolism. To identify the molecular components associated with the dynamic adjustment of circadian period, we performed a forward genetic screen. We identified Arabidopsis mutants that were either period insensitive to nicotinamide (sin) or period oversensitive to nicotinamide (son). We mapped son1 to BIG, a gene of unknown molecular function that was shown previously to play a role in light signaling. We found that son1 has an early entrained phase, suggesting that the dynamic alteration of circadian period contributes to the correct timing of biological events. Our data provide insight into how the dynamic period adjustment of circadian oscillators contributes to establishing a correct phase relationship with the environment and show that BIG is involved in this process.

Location and functions of Inebriated in the Drosophila eye

Histamine (HA) is a neurotransmitter in arthropod photoreceptors. It is recycled via conjugation to β-alanine to form β-alanylhistamine (carcinine). Conjugation occurs in epithelial glia that surround photoreceptor terminals in the first optic neuropil, and carcinine (CA) is then transported back to photoreceptors and cleaved to liberate HA and β-alanine. The gene Inebriated (Ine) encodes an Na⁺/Cl^--dependent SLC6 family transporter translated as two protein isoforms, long (P1) and short (P2). Photoreceptors specifically express Ine-P2 whereas Ine-P1 is expressed in non-neuronal cells. Both ine¹ and ine³ have significantly reduced head HA contents compared with wild type, and a smaller increase in head HA after drinking 1% CA. Similarly, uptake of 0.1% CA was reduced in ine¹ and ine³ mutant synaptosomes, but increased by 90% and 84% respectively for fractions incubated in 0.05% β-Ala, compared with wild type. Screening potential substrates in Ine expressing Xenopus oocytes revealed very little response to carcinine and β-Ala but increased conductance with glycine. Both ine¹ and ine³ mutant responses in light-dark phototaxis did not differ from wild-type. Collectively our results suggest that Inebriated functions in an adjunct role as a transporter to the previously reported carcinine transporter CarT.

Bacteria Endosymbiont, Wolbachia, Promotes Parasitism of Parasitoid Wasp Asobara japonica

Wolbachia is the most widespread endosymbiotic bacterium that manipulates reproduction of its arthropod hosts to enhance its own spread throughout host populations. Infection with Wolbachia causes complete parthenogenetic reproduction in many Hymenoptera, producing only female offspring. The mechanism of such reproductive manipulation by Wolbachia has been extensively studied. However, the effects of Wolbachia symbiosis on behavioral traits of the hosts are scarcely investigated. The parasitoid wasp Asobara japonica is an ideal insect to investigate this because symbiotic and aposymbiotic strains are available: Wolbachia-infected Tokyo (TK) and noninfected Iriomote (IR) strains originally collected on the main island and southwest islands of Japan, respectively. We compared the oviposition behaviors of the two strains and found that TK strain females parasitized Drosophila melanogaster larvae more actively than the IR strain, especially during the first two days after eclosion. Removing Wolbachia from the TK strain wasps by treatment with tetracycline or rifampicin decreased their parasitism activity to the level of the IR strain. Morphological and behavioral analyses of both strain wasps showed that Wolbachia endosymbionts do not affect development of the host female reproductive tract and eggs, but do enhance host-searching ability of female wasps. These results suggest the possibility that Wolbachia endosymbionts may promote their diffusion and persistence in the host A. japonica population not only at least partly by parthenogenesis but also by enhancement of oviposition frequency of the host females.

Glial enriched gene expression profiling identifies novel factors regulating the proliferation of specific glial subtypes in the Drosophila brain

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Global gene expression analysis identifies glial specific transcriptomes.

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Different glial subtypes have distinct but overlapping transcriptomes.

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foxO and tramtrack69 are novel regulators of glial subtype specific proliferation.

Glial cells constitute a large proportion of the central nervous system (CNS) and are critical for the correct development and function of the adult CNS. Recent studies have shown that specific subtypes of glia are generated through the proliferation of differentiated glial cells in both the developing invertebrate and vertebrate nervous systems. However, the factors that regulate glial proliferation in specific glial subtypes are poorly understood. To address this we have performed global gene expression analysis of Drosophila post-embryonic CNS tissue enriched in glial cells, through glial specific overexpression of either the FGF or insulin receptor. Analysis of the differentially regulated genes in these tissues shows that the expression of known glial genes is significantly increased in both cases. Conversely, the expression of neuronal genes is significantly decreased. FGF and insulin signalling drive the expression of overlapping sets of genes in glial cells that then activate proliferation. We then used these data to identify novel transcription factors that are expressed in glia in the brain. We show that two of the transcription factors identified in the glial enriched gene expression profiles, foxO and tramtrack69, have novel roles in regulating the proliferation of cortex and perineurial glia. These studies provide new insight into the genes and molecular pathways that regulate the proliferation of specific glial subtypes in the Drosophila post-embryonic brain.

The transcriptomic basis of oviposition behaviour in the parasitoid wasp Nasonia vitripennis

Linking behavioural phenotypes to their underlying genotypes is crucial for uncovering the mechanisms that underpin behaviour and for understanding the origins and maintenance of genetic variation in behaviour. Recently, interest has begun to focus on the transcriptome as a route for identifying genes and gene pathways associated with behaviour. For many behavioural traits studied at the phenotypic level, we have little or no idea of where to start searching for "candidate" genes: the transcriptome provides such a starting point. Here we consider transcriptomic changes associated with oviposition in the parasitoid wasp Nasonia vitripennis. Oviposition is a key behaviour for parasitoids, as females are faced with a variety of decisions that will impact offspring fitness. These include choosing between hosts of differing quality, as well as making decisions regarding clutch size and offspring sex ratio. We compared the whole-body transcriptomes of resting or ovipositing female Nasonia using a "DeepSAGE" gene expression approach on the Illumina sequencing platform. We identified 332 tags that were significantly differentially expressed between the two treatments, with 77% of the changes associated with greater expression in resting females. Oviposition therefore appears to focus gene expression away from a number of physiological processes, with gene ontologies suggesting that aspects of metabolism may be down-regulated during egg-laying. Nine of the most abundant differentially expressed tags showed greater expression in ovipositing females though, including the genes purity-of-essence (associated with behavioural phenotypes in Drosophila) and glucose dehydrogenase (GLD). The GLD protein has been implicated in sperm storage and release in Drosophila and so provides a possible candidate for the control of sex allocation by female Nasonia during oviposition. Oviposition in Nasonia therefore clearly modifies the transcriptome, providing a starting point for the genetic dissection of oviposition.

p600 Plays Essential Roles in Fetal Development

p600 is a multifunctional protein implicated in cytoskeletal organization, integrin-mediated survival signaling, calcium-calmodulin signaling and the N-end rule pathway of ubiquitin-proteasome-mediated proteolysis. While push, the Drosophila counterpart of p600, is dispensable for development up to adult stage, the role of p600 has not been studied during mouse development. Here we generated p600 knockout mice to investigate the in vivo functions of p600. Interestingly, we found that homozygous deletion of p600 results in lethality between embryonic days 11.5 and 13.5 with severe defects in both embryo and placenta. Since p600 is required for placental development, we performed conditional disruption of p600, which deletes selectively p600 in the embryo but not in the placenta. The conditional mutant embryos survive longer than knockout embryos but ultimately die before embryonic day 14.5. The mutant embryos display severe cardiac problems characterized by ventricular septal defects and thin ventricular walls. These anomalies are associated with reduced activation of FAK and decreased expression of MEF2, which is regulated by FAK and plays a crucial role in cardiac development. Moreover, we observed pleiotropic defects in the liver and brain. In sum, our study sheds light on the essential roles of p600 in fetal development.

Glial processes at the Drosophila larval neuromuscular junction match synaptic growth

Glia are integral participants in synaptic physiology, remodeling and maturation from blowflies to humans, yet how glial structure is coordinated with synaptic growth is unknown. To investigate the dynamics of glial development at the Drosophila larval neuromuscular junction (NMJ), we developed a live imaging system to establish the relationship between glia, neuronal boutons, and the muscle subsynaptic reticulum. Using this system we observed processes from two classes of peripheral glia present at the NMJ. Processes from the subperineurial glia formed a blood-nerve barrier around the axon proximal to the first bouton. Processes from the perineurial glial extended beyond the end of the blood-nerve barrier into the NMJ where they contacted synapses and extended across non-synaptic muscle. Growth of the glial processes was coordinated with NMJ growth and synaptic activity. Increasing synaptic size through elevated temperature or the highwire mutation increased the extent of glial processes at the NMJ and conversely blocking synaptic activity and size decreased the presence and size of glial processes. We found that elevated temperature was required during embryogenesis in order to increase glial expansion at the nmj. Therefore, in our live imaging system, glial processes at the NMJ are likely indirectly regulated by synaptic changes to ensure the coordinated growth of all components of the tripartite larval NMJ.

Drosophila glia use a conserved cotransporter mechanism to regulate extracellular volume

The nervous system is protected by blood barriers that use multiple systems to control extracellular solute composition, osmotic pressure, and fluid volume. In the human nervous system, misregulation of the extracellular volume poses serious health threats. Here, we show that the glial cells that form the Drosophila blood-nerve barrier have a conserved molecular mechanism that regulates extracellular volume: the Serine/Threonine kinase Fray, which we previously showed is an ortholog of mammalian PASK/SPAK; and the Na-K-Cl cotransporter Ncc69, which we show is an ortholog of human NKCC1. In mammals, PASK/SPAK binds to NKCC1 and regulates its activity. In Drosophila, larvae mutant for Ncc69 develop a peripheral neuropathy, where fluid accumulates between glia and axons. The accumulation of fluid has no detectable impact on action potential conduction, suggesting that the role of Ncc69 is to maintain volume or osmotic homeostasis. Drosophila Ncc69 has kinetics similar to human NKCC1, and NKCC1 can rescue Ncc69, suggesting that they function in a conserved physiological mechanism. We show that fray and Ncc69 are coexpressed in nerve glia, interact in a yeast-two-hybrid assay, and have an essentially identical bulging nerve phenotype. We propose that normally functioning nerves generate extracellular solutes that are removed by Ncc69 under the control of Fray. This mechanism may perform a similar role in humans, given that NKCC1 is expressed at the blood-brain barrier.

Localization of a GABA transporter to glial cells in the developing and adult olfactory pathway of the moth Manduca sexta

Glial cells have several critical roles in the developing and adult olfactory (antennal) lobe of the moth Manduca sexta. Early in development, glial cells occupy discrete regions of the developing olfactory pathway and processes of gamma-aminobutyric acid (GABA)ergic neurons extend into some of these regions. Because GABA is known to have developmental effects in a variety of systems, we explored the possibility that the glial cells express a GABA transporter that could regulate GABA levels to which olfactory neurons and glial cells are exposed. By using an antibody raised against a characterized high-affinity M. sexta GABA transporter with high sequence homology to known mammalian GABA transporters (Mbungu et al. [1995] Arch. Biochem. Biophys. 318:489-497; Umesh and Gill [2002] J. Comp. Neurol. 448:388-398), we found that the GABA transporter is localized to subsets of centrally derived glial cells during metamorphic adult development. The transporter persists into adulthood in a subset of the neuropil-associated glial cells, but its distribution pattern as determined by light-and electron-microscopic-level immunocytochemistry indicates that it could not serve to regulate GABA concentration in the synaptic cleft. Instead, its role is more likely to regulate extracellular GABA levels within the glomerular neuropil. Expression in the sorting zone glial cells disappears after the period of olfactory receptor axon ingrowth, but may be important during ingrowth if GABA regulates axon growth. Glial cells take up GABA, and that uptake can be blocked by L-2,4-diaminobutyric acid (DABA). This is the first molecular evidence that the central glial cell population in this pathway is heterogeneous.

A PI3-kinase-mediated negative feedback regulates neuronal excitability

Use-dependent downregulation of neuronal activity (negative feedback) can act as a homeostatic mechanism to maintain neuronal activity at a particular specified value. Disruption of this negative feedback might lead to neurological pathologies, such as epilepsy, but the precise mechanisms by which this feedback can occur remain incompletely understood. At one glutamatergic synapse, the Drosophila neuromuscular junction, a mutation in the group II metabotropic glutamate receptor gene (DmGluRA) increased motor neuron excitability by disrupting an autocrine, glutamate-mediated negative feedback. We show that DmGluRA mutations increase neuronal excitability by preventing PI3 kinase (PI3K) activation and consequently hyperactivating the transcription factor Foxo. Furthermore, glutamate application increases levels of phospho-Akt, a product of PI3K signaling, within motor nerve terminals in a DmGluRA-dependent manner. Finally, we show that PI3K increases both axon diameter and synapse number via the Tor/S6 kinase pathway, but not Foxo. In humans, PI3K and group II mGluRs are implicated in epilepsy, neurofibromatosis, autism, schizophrenia, and other neurological disorders; however, neither the link between group II mGluRs and PI3K, nor the role of PI3K-dependent regulation of Foxo in the control of neuronal excitability, had been previously reported. Our work suggests that some of the deficits in these neurological disorders might result from disruption of glutamate-mediated homeostasis of neuronal excitability.

Use-dependent downregulation of neuronal excitability (negative feedback) can act to maintain neuronal activity within specified levels. Disruption of this homeostasis can lead to neurological disorders, such as epilepsy. Here, we report a novel mechanism for negative feedback control of excitability in the Drosophila larval motor neuron. In this mechanism, activation by the excitatory neurotransmitter glutamate of metabotropic glutamate receptors (mGluRs) located at motor nerve terminals decreases excitability by activating PI3 kinase (PI3K), consequently causing the phosphorylation and inhibition of the transcription factor Foxo. Foxo inhibition, in turn, decreases neuronal excitability. These observations are of interest for two reasons. First, our observation that PI3K activity regulates neuronal excitability is of interest because altered PI3K activity is implicated in a number of neurological disorders, such as autism and neurofibromatosis. Our results raise the possibility that altered excitability might contribute to the deficits in these disorders. Second, our observation that group II metabotropic glutamate receptors (mGluRs) activate PI3K is of interest because group II mGluRs are implicated in epilepsy, anxiety disorders, and schizophrenia. Yet the downstream signaling pathways affected by these treatments are incompletely understood. Our results raise the possibility that the PI3K pathway might be an essential mediator of signalling by these mGluRs.

Glial ensheathment of peripheral axons in Drosophila

The ensheathment of neurons and their axons creates an ion-sensitive microenvironment that allows rapid conduction of nerve impulses. One of the fundamental questions about axonal ensheathment is how insulating glial cells wrap around axons. The mechanisms that underlie insulation of axons in invertebrates and vertebrates are not fully understood. In the present article we address cellular aspects of axonal ensheathment in Drosophila by taking advantage of glial mutants that illustrate a range of phenotypic defects including ensheathment of axons. From the findings of these mutant studies, we summarize that loss of glial cells, defects in glial membrane wrapping, failure of glial migration, and loss of specialized ladderlike septate junctions between ensheathing glial membranes result in axon-glial functional defects. These studies provide a broad perspective on glial ensheathment of axons in Drosophila and key insights into the anatomical and cellular aspects of axonal insulation. Given the powerful genetic approaches available in Drosophila, the axonal ensheathment process can be dissected in great detail to reveal the fundamental principles of ensheathment. These observations will be relevant to understanding the very similar processes in vertebrates, where defects in glial cell functions lead to devastating neurological diseases.

The mammalian N-end rule pathway: new insights into its components and physiological roles

The N-end rule pathway is a ubiquitin-dependent proteolytic system, in which destabilizing N-terminal residues of short-lived proteins function as an essential determinant of an N-terminal degradation signal (N-degron). An N-degron can be created from a pre-N-degron through specific N-terminal modifications, providing a means conditionally to destabilize otherwise stable polypeptides. The pathway has been found in all organisms examined, from prokaryotes to eukaryotes. Recent biochemical and proteomic studies identified many components of the mammalian N-end rule pathway, including a family of substrate recognition ubiquitin ligases and their substrates. The genetic dissection in animals and humans revealed its essential role in various vital physiological processes, ranging from cardiovascular development and meiosis to the pathogenesis of human genetic diseases. These discoveries have provided new insights into the components, functions and mechanics of this unique proteolytic system.

Phosphatidylinositol 3-kinase and Akt nonautonomously promote perineurial glial growth in Drosophila peripheral nerves

Drosophila peripheral nerves, structured similarly to their mammalian counterparts, comprise a layer of motor and sensory axons wrapped by an inner peripheral glia (analogous to the mammalian Schwann cell) and an outer perineurial glia (analogous to the mammalian perineurium). Growth and proliferation within mammalian peripheral nerves are increased by Ras pathway activation: loss-of-function mutations in Nf1, which encodes the Ras inhibitor neurofibromin, cause the human genetic disorder neurofibromatosis, which is characterized by formation of neurofibromas (tumors of peripheral nerves). However, the signaling pathways that control nerve growth downstream of Ras remain incompletely characterized. Here we show that expression specifically within the Drosophila peripheral glia of the constitutively active Ras(V12) increases perineurial glial thickness. Using chromosomal loss-of-function mutations and transgenes encoding dominant-negative and constitutively active proteins, we show that this nonautonomous effect of Ras(V12) is mediated by the Ras effector phosphatidylinositol 3-kinase (PI3K) and its downstream kinase Akt. We also show that the nonautonomous, growth-promoting effects of activated PI3K are suppressed by coexpression within the peripheral glia of FOXO+ (forkhead box O) a transcription factor inhibited by Akt-dependent phosphorylation. We suggest that Ras-PI3K-Akt activity in the peripheral glia promotes growth of the perineurial glia by inhibiting FOXO. In mammalian peripheral nerves, the Schwann cell releases several growth factors that affect the proliferative properties of neighbors. Some of these factors are oversecreted in Nf1 mutants. Our results raise the possibility that neurofibroma formation in individuals with neurofibromatosis might result in part from a Ras-PI3K-Akt-dependent inhibition of FOXO within Schwann cells.

Nonvesicular release of glutamate by glial xCT transporters suppresses glutamate receptor clustering in vivo

We hypothesized that cystine/glutamate transporters (xCTs) might be critical regulators of ambient extracellular glutamate levels in the nervous system and that misregulation of this glutamate pool might have important neurophysiological and/or behavioral consequences. To test this idea, we identified and functionally characterized a novel Drosophila xCT gene, which we subsequently named "genderblind" (gb). Genderblind is expressed in a previously overlooked subset of peripheral and central glia. Genetic elimination of gb causes a 50% reduction in extracellular glutamate concentration, demonstrating that xCT transporters are important regulators of extracellular glutamate. Consistent with previous studies showing that extracellular glutamate regulates postsynaptic glutamate receptor clustering, gb mutants show a large (200-300%) increase in the number of postsynaptic glutamate receptors. This increase in postsynaptic receptor abundance is not accompanied by other obvious synaptic changes and is completely rescued when synapses are cultured in wild-type levels of glutamate. Additional in situ pharmacology suggests that glutamate-mediated suppression of glutamate receptor clustering depends on receptor desensitization. Together, our results suggest that (1) xCT transporters are critical for regulation of ambient extracellular glutamate in vivo; (2) ambient extracellular glutamate maintains some receptors constitutively desensitized in vivo; and (3) constitutive desensitization of ionotropic glutamate receptors suppresses their ability to cluster at synapses.

Roles of glia in the Drosophila nervous system

Glial cells have diverse functions that are necessary for the proper development and function of complex nervous systems. Various insects, primarily the fruit fly Drosophila melanogaster and the moth Manduca sexta, have provided useful models of glial function during development. The present review will outline evidence of glial contributions to embryonic, visual, olfactory and wing development. We will also outline evidence for non-developmental functions of insect glia including blood-brain-barrier formation, homeostatic functions and potential contributions to synaptic function. Where relevant, we will also point out similarities between the functions of insect glia and their vertebrate counterparts.

moody encodes two GPCRs that regulate cocaine behaviors and blood-brain barrier permeability in Drosophila

We identified moody in a genetic screen for Drosophila mutants with altered cocaine sensitivity. Hypomorphic mutations in moody cause an increased sensitivity to cocaine and nicotine exposure. In contrast, sensitivity to the acute intoxicating effects of ethanol is reduced. The moody locus encodes two novel GPCRs, Moody-alpha and Moody-beta. While identical in their membrane-spanning domains, the two Moody proteins differ in their long carboxy-terminal domains, which are generated by use of alternative reading frames. Both Moody forms are required for normal cocaine sensitivity, suggesting that they carry out distinct but complementary functions. Moody-alpha and Moody-beta are coexpressed in surface glia that surround the nervous system, where they are actively required to maintain the integrity of the blood-brain barrier in the adult fly. We propose that a Moody-mediated signaling pathway functions in glia to regulate nervous system insulation and drug-related behaviors.

Ras activity in the Drosophila prothoracic gland regulates body size and developmental rate via ecdysone release

BACKGROUND

In Drosophila, each of the three larval instars ends with a molt, triggered by release of steroid molting hormone ecdysone from the prothoracic gland (PG). Because all growth occurs during the larval stages, final body size depends on both the larval growth rate and the duration of each larval stage, which in turn might be regulated by the timing of ecdysone release.

RESULTS

Here, we show that the expression of activated Ras, PI3 kinase (PI3K), or Raf specifically in the PG reduces body size, whereas activated Ras or PI3K, but not Raf, increases PG cell size. In contrast, expression of either dominant-negative (dn) Ras, Raf, or PI3K increases body size and prolongs the larval stages, leading to delayed pupariation, whereas expression of dn-PI3K, but not of dn-Raf or dn-Ras, reduces PG cell size. To test the possibility that altered ecdysone release is responsible for these phenotypes, we measured larval ecdysone levels indirectly, via the transcriptional activation of two ecdysone targets, E74A and E74B. We found that the activation of Ras within the PG induces precocious ecdysone release, whereas expression of either dn-PI3K or dn-Raf in the PG greatly attenuates the [ecdysone] increase that causes growth cessation and pupariation onset.

CONCLUSIONS

We conclude that Ras activity in the PG regulates body size and the duration of each larval stage by regulating ecdysone release. We also suggest that ecdysone release is regulated in two ways: a PI3K-dependent growth-promoting effect on PG cells, and a Raf-dependent step that may involve the transcriptional regulation of ecdysone biosynthetic genes.

A family of mammalian E3 ubiquitin ligases that contain the UBR box motif and recognize N-degrons

A subset of proteins targeted by the N-end rule pathway bear degradation signals called N-degrons, whose determinants include destabilizing N-terminal residues. Our previous work identified mouse UBR1 and UBR2 as E3 ubiquitin ligases that recognize N-degrons. Such E3s are called N-recognins. We report here that while double-mutant UBR1(-/-) UBR2(-/-) mice die as early embryos, the rescued UBR1(-/-) UBR2(-/-) fibroblasts still retain the N-end rule pathway, albeit of lower activity than that of wild-type fibroblasts. An affinity assay for proteins that bind to destabilizing N-terminal residues has identified, in addition to UBR1 and UBR2, a huge (570 kDa) mouse protein, termed UBR4, and also the 300-kDa UBR5, a previously characterized mammalian E3 known as EDD/hHYD. UBR1, UBR2, UBR4, and UBR5 shared a approximately 70-amino-acid zinc finger-like domain termed the UBR box. The mammalian genome encodes at least seven UBR box-containing proteins, which we propose to call UBR1 to UBR7. UBR1(-/-) UBR2(-/-) fibroblasts that have been made deficient in UBR4 as well (through RNA interference) were significantly impaired in the degradation of N-end rule substrates such as the Sindbis virus RNA polymerase nsP4 (bearing N-terminal Tyr) and the human immunodeficiency virus type 1 integrase (bearing N-terminal Phe). Our results establish the UBR box family as a unique class of E3 proteins that recognize N-degrons or structurally related determinants for ubiquitin-dependent proteolysis and perhaps other processes as well.

Association of the human papillomavirus type 16 E7 oncoprotein with the 600-kDa retinoblastoma protein-associated factor, p600

The human papillomavirus type 16 (HPV-16) E7 gene encodes a multifunctional oncoprotein that can subvert multiple cellular regulatory pathways. The best-known cellular targets of the HPV-16 E7 oncoprotein are the retinoblastoma tumor suppressor protein pRB and the related pocket proteins p107 and p130. However, there is ample evidence that E7 has additional cellular targets that contribute to its transforming potential. We isolated HPV-16 E7 associated cellular protein complexes by tandem affinity purification and mass spectrometry and identified the 600-kDa retinoblastoma protein associated factor, p600, as a cellular target of E7. Association of E7 with p600 is independent of the pocket proteins and is mediated through the N terminal E7 domain, which is related to conserved region 1 of the adenovirus E1A protein and importantly contributes to cellular transformation independent of pRB binding. Depletion of p600 protein levels by RNA interference substantially decreased anchorage-independent growth in HPV-positive and -negative human cancer cells. Therefore, p600 is a cellular target of E7 that regulates cellular pathways that contribute to anchorage-independent growth and cellular transformation.

Molecular and comparative genetics of mental retardation

Affecting 1-3% of the population, mental retardation (MR) poses significant challenges for clinicians and scientists. Understanding the biology of MR is complicated by the extraordinary heterogeneity of genetic MR disorders. Detailed analyses of >1000 Online Mendelian Inheritance in Man (OMIM) database entries and literature searches through September 2003 revealed 282 molecularly identified MR genes. We estimate that hundreds more MR genes remain to be identified. A novel test, in which we distributed unmapped MR disorders proportionately across the autosomes, failed to eliminate the well-known X-chromosome overrepresentation of MR genes and candidate genes. This evidence argues against ascertainment bias as the main cause of the skewed distribution. On the basis of a synthesis of clinical and laboratory data, we developed a biological functions classification scheme for MR genes. Metabolic pathways, signaling pathways, and transcription are the most common functions, but numerous other aspects of neuronal and glial biology are controlled by MR genes as well. Using protein sequence and domain-organization comparisons, we found a striking conservation of MR genes and genetic pathways across the approximately 700 million years that separate Homo sapiens and Drosophila melanogaster. Eighty-seven percent have one or more fruit fly homologs and 76% have at least one candidate functional ortholog. We propose that D. melanogaster can be used in a systematic manner to study MR and possibly to develop bioassays for therapeutic drug discovery. We selected 42 Drosophila orthologs as most likely to reveal molecular and cellular mechanisms of nervous system development or plasticity relevant to MR.

Molecular and Comparative Genetics of Mental Retardation

Affecting 1-3% of the population, mental retardation (MR) poses significant challenges for clinicians and scientists. Understanding the biology of MR is complicated by the extraordinary heterogeneity of genetic MR disorders. Detailed analyses of >1000 Online Mendelian Inheritance in Man (OMIM) database entries and literature searches through September 2003 revealed 282 molecularly identified MR genes. We estimate that hundreds more MR genes remain to be identified. A novel test, in which we distributed unmapped MR disorders proportionately across the autosomes, failed to eliminate the well-known X-chromosome overrepresentation of MR genes and candidate genes. This evidence argues against ascertainment bias as the main cause of the skewed distribution. On the basis of a synthesis of clinical and laboratory data, we developed a biological functions classification scheme for MR genes. Metabolic pathways, signaling pathways, and transcription are the most common functions, but numerous other aspects of neuronal and glial biology are controlled by MR genes as well. Using protein sequence and domain-organization comparisons, we found a striking conservation of MR genes and genetic pathways across the ∼700 million years that separate Homo sapiens and Drosophila melanogaster. Eighty-seven percent have one or more fruit fly homologs and 76% have at least one candidate functional ortholog. We propose that D. melanogaster can be used in a systematic manner to study MR and possibly to develop bioassays for therapeutic drug discovery. We selected 42 Drosophila orthologs as most likely to reveal molecular and cellular mechanisms of nervous system development or plasticity relevant to MR.

Pheromone-mediated gene expression in the honey bee brain

We tested the hypothesis that queen mandibular pheromone (QMP) causes changes in gene expression in the brain of the adult worker honey bee, and that these changes can be correlated to the downstream behavioral responses induced by QMP. In support of the first hypothesis, cage experiments revealed that QMP transiently regulated expression of several hundred genes and chronically regulated the expression of 19 genes. Several of these genes were also affected by QMP in experiments with bee colonies in the field, demonstrating robust gene regulation by pheromone. To evaluate the second hypothesis, we focused on one function of QMP: delaying the transition from working in the hive (e.g., brood care, or "nursing") to foraging. We compared the list of QMP-regulated genes with the lists of genes differentially regulated in nurse and forager brains generated in a separate study. QMP consistently activated "nursing genes" and repressed "foraging genes," suggesting that QMP may delay behavioral maturation by regulating genes in the brain that produce these behavioral states. We also report here on an ortholog of the Drosophila transcription factor kruppel homolog 1 that was strongly regulated by QMP, especially in the mushroom bodies of the bee brain. These results demonstrate chronic gene regulation by a primer pheromone and illustrate the potential of genomics to trace the actions of a pheromone from perception to action, and thereby provide insights into how pheromones regulate social life.

Expression of stage-specific genes during zygotic gene activation in preimplantation mouse embryos

The expression of mouse two-cell stage specific genes was studied using the modified DDRT-PCR method, which overcame the paucity of the experimental materials of preimplantation embryos. Embryo tissues equivalent to that of four blastomeres are sufficient for amplification of target genes as visualized using polyacrylamide gel. Sequence analyses and reverse Northern blots indicate that the genes of ATPase 6 and Ywhaz are expressed specifically in two-cell embryos. ATPase 6 is essential for one-cell to two-cell transition and plays an important role in establishment of oxidative phosphorylation, while Ywhaz is related to initiating cellular communication system.

The neurofibromatosis 1 gene product neurofibromin regulates pituitary adenylate cyclase-activating polypeptide-mediated signaling in astrocytes

Individuals with the neurofibromatosis 1 (NF1)-inherited tumor predisposition syndrome develop low-grade astrocytomas. The NF1 tumor suppressor gene product neurofibromin exhibits GTPase-activating activity (GAP) toward RAS, such that loss of neurofibromin expression leads to high levels of activated RAS and increased cell proliferation. Previous work has demonstrated that Nf1 inactivation in astrocytes leads to increased cell proliferation in vitro and in vivo, accompanied by increased RAS pathway activation. Studies on Nf1 mutant Drosophila have suggested that neurofibromin might also regulate cAMP signaling. Because intracellular cAMP levels have profound effects on astrocyte growth control, we sought to determine the contribution of neurofibromin to astrocyte cAMP regulation. In this report, we demonstrate that Nf1 inactivation in astrocytes results in reduced cAMP generation in response to PACAP and attenuated calcium influx and Rap1 activation. Based on the differential effects of forskolin and dibutyryl-cAMP on Nf1-/- astrocytes, neurofibromin likely functions at the level of adenylyl cyclase activation. Last, the reintroduction of a fragment of neurofibromin containing residues sufficient for restoring RAS-GAP function in Nf1-/- cells resulted in only partial restoration of neurofibromin-mediated cAMP regulation. These results demonstrate that neurofibromin positively influences cAMP generation and activation of cAMP growth regulatory targets in astrocytes and expands the role of the NF1 gene in astrocyte growth regulation.

The neurofibromatosis 1 gene product neurofibromin regulates pituitary adenylate cyclase-activating polypeptide-mediated signaling in astrocytes

Individuals with the neurofibromatosis 1 (NF1)-inherited tumor predisposition syndrome develop low-grade astrocytomas. The NF1 tumor suppressor gene product neurofibromin exhibits GTPase-activating activity (GAP) toward RAS, such that loss of neurofibromin expression leads to high levels of activated RAS and increased cell proliferation. Previous work has demonstrated that Nf1 inactivation in astrocytes leads to increased cell proliferation in vitro and in vivo, accompanied by increased RAS pathway activation. Studies on Nf1 mutant Drosophila have suggested that neurofibromin might also regulate cAMP signaling. Because intracellular cAMP levels have profound effects on astrocyte growth control, we sought to determine the contribution of neurofibromin to astrocyte cAMP regulation. In this report, we demonstrate that Nf1 inactivation in astrocytes results in reduced cAMP generation in response to PACAP and attenuated calcium influx and Rap1 activation. Based on the differential effects of forskolin and dibutyryl-cAMP on Nf1-/- astrocytes, neurofibromin likely functions at the level of adenylyl cyclase activation. Last, the reintroduction of a fragment of neurofibromin containing residues sufficient for restoring RAS-GAP function in Nf1-/- cells resulted in only partial restoration of neurofibromin-mediated cAMP regulation. These results demonstrate that neurofibromin positively influences cAMP generation and activation of cAMP growth regulatory targets in astrocytes and expands the role of the NF1 gene in astrocyte growth regulation.

Mutations in the huge Arabidopsis gene BIG affect a range of hormone and light responses

In independent genetic screens, for shade-avoidance response and cytokinin sensitivity, we identified two Arabidopsis mutants, attenuated shade avoidance 1 (asa1) and umbrella1 (umb1), which have very similar pleiotropic phenotypes. asa1 and umb1 are allelic to tir3-1, and are caused by mutations in BIG, which is required for normal auxin efflux. They have a compact rosette, fewer lateral roots, delayed flowering, more secondary inflorescence, smaller seeds and, in the Laer-0 background, much shorter internodes between adjacent flowers, suggesting an interaction between BIG and ERECTA. These mutants have organ-specific defects in response to cytokinins, ethylene, N-1-naphthylphthalamic acid (NPA) and gibberellin (GA). The phenotype of the asa1 ga1-3 double mutant is consistent with defects in GA signalling. There are subtle effects in responses to auxins, abscisic acid and brassinolide. Elongation growth associated with shade avoidance in phyA phyB null mutants is suppressed by asa1 in all organs other than the hypocotyl. Therefore, we here provide evidence that BIG is a key player not just in auxin signalling, but in a multitude of light and hormone pathways.

The architecture of polarized cell growth: the unique status of elongating plant cells

Polarity is an inherent feature of almost all prokaryotic and eukaryotic cells. In most eukaryotic cells, growth polarity is due to the assembly of actin-based growing domains at particular locations on the cell periphery. A contrasting scenario is that growth polarity results from the establishment of non-growing domains, which are actively maintained at opposite end-poles of the cell. This latter mode of growth is common in rod-shaped bacteria and, surprisingly, also in the majority of plant cells, which elongate along the apical-basal axes of plant organs. The available data indicate that the non-growing end-pole domains of plant cells are sites of intense endocytosis and recycling. These actin-enriched end-poles serve also as signaling platforms, allowing bidirectional exchange of diverse signals along the supracellular domains of longitudinal cell files. It is proposed that these actively remodeled end-poles of elongating plant cells remotely resemble neuronal synapses.

Higher brain functions of PACAP and a homologous Drosophila memory gene amnesiac: insights from knockouts and mutants

Neuropeptides usually exert a long-lived modulatory effect on the small-molecule neurotransmitters with which they colocalize via regulation of the response times of second messenger systems. Pituitary adenylate cyclase-activating polypeptide (PACAP) functions as a neuromodulator and neurotransmitter and regulates a variety of physiological processes. PACAP is structurally highly conserved during evolution, implying its vital importance. In Drosophila, loss-of-function mutations in a PACAP-like neuropeptide gene, amnesiac (amn), affect both memory retention and ethanol sensitivity. The amnesiac gene is expressed in neurons innervating the mushroom body lobes, the olfactory associative learning center. Conditional genetic ablation of neurotransmitter release from these neurons mimics the amnesiac memory phenotypes, suggesting an acute role for amnesiac in memory. However, genetic rescue experiments also suggest developmental defects in amnesiac mutants, implying a role in neuronal development. There is a parallel between memory formation in Drosophila and mammals. PACAP-specific (PAC(1)) receptor-deficient mice show a deficit in hippocampus-dependent associative learning and mossy fiber long-term potentiation (LTP). Meanwhile, PACAP-deficient mice display a high early mortality rate and additional CNS phenotypes including behavioral and psychological phenotypes (e.g., hyperlocomotion, intense novelty-seeking behavior, and explosive jumping). A functional comparison between PACAP and amnesiac underlines phylogenetically conserved functions across phyla and may provide insights into the possible mechanisms of action and evolution of this neuropeptidergic system.

In vivo properties of the Drosophila inebriated-encoded neurotransmitter transporter

Altering neurotransmitter levels within the nervous system can cause profound changes in behavior and neuronal function. Neurotransmitter transporters play important roles in regulating neurotransmitter levels by performing neurotransmitter reuptake. It was previously shown that mutations in the Drosophila inebriated (ine)-encoded neurotransmitter transporter cause increased neuronal excitability. Here we report a further functional characterization of Ine. First we show that Ine functions in the short-term (time scale of minutes to a few hours) to regulate neuronal excitability. Second, we show that Ine is able to control excitability from either neurons or glia cells. Third, we show that overexpression of Ine reduces neuronal excitability. Overexpression phenotypes of ine include: delayed onset of long-term facilitation and increased failure rate of transmitter release at the larval neuromuscular junction, reduced amplitude of larval nerve compound action potentials, suppression of the leg-shaking behavior of mutants defective in the Shaker-encoded potassium channel, and temperature-sensitive paralysis. Each of these overexpression phenotypes closely resembles those of loss of function mutants in the para-encoded sodium channel. These data raise the possibility that Ine negatively regulates neuronal sodium channels, and thus that the substrate neurotransmitter of Ine positively regulates sodium channels.

In vivo properties of the Drosophila inebriated-encoded neurotransmitter transporter

Altering neurotransmitter levels within the nervous system can cause profound changes in behavior and neuronal function. Neurotransmitter transporters play important roles in regulating neurotransmitter levels by performing neurotransmitter reuptake. It was previously shown that mutations in the Drosophila inebriated (ine)-encoded neurotransmitter transporter cause increased neuronal excitability. Here we report a further functional characterization of Ine. First we show that Ine functions in the short-term (time scale of minutes to a few hours) to regulate neuronal excitability. Second, we show that Ine is able to control excitability from either neurons or glia cells. Third, we show that overexpression of Ine reduces neuronal excitability. Overexpression phenotypes of ine include: delayed onset of long-term facilitation and increased failure rate of transmitter release at the larval neuromuscular junction, reduced amplitude of larval nerve compound action potentials, suppression of the leg-shaking behavior of mutants defective in the Shaker-encoded potassium channel, and temperature-sensitive paralysis. Each of these overexpression phenotypes closely resembles those of loss of function mutants in the para-encoded sodium channel. These data raise the possibility that Ine negatively regulates neuronal sodium channels, and thus that the substrate neurotransmitter of Ine positively regulates sodium channels.

The Drosophila inebriated-encoded neurotransmitter/osmolyte transporter: dual roles in the control of neuronal excitability and the osmotic stress response

Water reabsorption by organs such as the mammalian kidney and insect Malpighian tubule/hindgut requires a region of hypertonicity within the organ. To balance the high extracellular osmolarity, cells within these regions accumulate small organic molecules called osmolytes. These osmolytes can accumulate to a high level without toxic effects on cellular processes. Here we provide evidence consistent with the possibility that the two protein isoforms encoded by the inebriated (ine) gene, which are members of the Na+/Cl--dependent neurotransmitter/osmolyte transporter family, perform osmolyte transport within the Malpighian tubule and hindgut. We show that ine mutants lacking both isoforms are hypersensitive to osmotic stress, which we assayed by maintaining flies on media containing NaCl, KCl, or sorbitol, and that this hypersensitivity is completely rescued by high-level ectopic expression of the ine-RB isoform. We provide evidence that this hypersensitivity represents a role for ine that is distinct from the increased neuronal excitability phenotype of ine mutants. Finally, we show that each ine genotype exhibits a "threshold" [NaCl]: long-term maintenance on NaCl-containing media above, but not below, the threshold causes lethality. Furthermore, this threshold value increases with the amount of ine activity. These data suggest that ine mutations confer osmotic stress sensitivity by preventing osmolyte accumulation within the Malpighian tubule and hindgut.