Cellular bases of behavioral plasticity: establishing and modifying synaptic circuits in the Drosophila genetic system

Electrophysiological analysis of synaptic transmission in Drosophila

Synaptic transmission is dynamic, plastic, and highly regulated. Drosophila is an advantageous model system for genetic and molecular studies of presynaptic and postsynaptic mechanisms and plasticity. Electrical recordings of synaptic responses represent a wide-spread approach to study neuronal signaling and synaptic transmission. We discuss experimental techniques that allow monitoring synaptic transmission in Drosophila neuromuscular and central systems. Recordings of synaptic potentials or currents at the larval neuromuscular junction (NMJ) are most common and provide numerous technical advantages due to robustness of the preparation, large and identifiable muscles, and synaptic boutons which can be readily visualized. In particular, focal macropatch recordings combined with the analysis of neurosecretory quanta enable rigorous quantification of the magnitude and kinetics of transmitter release. Patch-clamp recordings of synaptic transmission from the embryonic NMJ enable overcoming the problem of lethality in mutant lines. Recordings from the adult NMJ proved instrumental in the studies of temperature-sensitive paralytic mutants. Genetic studies of behavioral learning in Drosophila compel an investigation of synaptic transmission in the central nervous system (CNS), including primary cultured neurons and an intact brain. Cholinergic and GABAergic synaptic transmission has been recorded from the Drosophila CNS both in vitro and in vivo. In vivo patch-clamp recordings of synaptic transmission from the neurons in the olfactory pathway is a very powerful approach, which has a potential to elucidate how synaptic transmission is associated with behavioral learning. WIREs Dev Biol 2017, 6:e277. doi: 10.1002/wdev.277 For further resources related to this article, please visit the WIREs website.

A new Drosophila model to study the interaction between genetic and environmental factors in Parkinson's disease

The fruit fly Drosophila melanogaster has long been used as a model organism for human diseases, including Parkinson׳s disease (PD). Its short lifespan, simple maintenance, and the widespread availability of genetic tools allow researchers to study disease mechanisms as well as potential drug therapies. Many different PD models have already been developed, including ones utilizing mutated α-Syn and chronic exposure to rotenone. However, few animal models have been used to study interaction between the PD causing factors. In this study, we developed a new model of PD for use in the larval stage in order to study interaction between genetic and environmental factors. First, the 3rd instar larvae (90-94 hours after egg laying) expressing a mutated form of human α-Syn (A53T) in dopaminergic (DA) neurons were video-taped and quantified for locomotion (e.g. crawling pattern and speed) using ImageJ software. A53T mutant larvae showed locomotion deficits and also loss of DA neurons in age-dependent manner. Similarly, larvae chronically exposed to rotenone (10 μM in food) showed age-dependent decline in locomotion accompanied by loss of DA neurons. We further show that combining the two models, by exposing A53T mutant larvae to rotenone, causes a much more severe PD phenotype (i.e. locomotor deficit). Our finding shows interaction between genetic and environmental factors underlying development of PD symptoms. This model can be used to further study mechanisms underlying the interaction between genes and different environmental PD factors, as well as to explore potential therapies for PD treatment.

Impaired activity-dependent neural circuit assembly and refinement in autism spectrum disorder genetic models

Early-use activity during circuit-specific critical periods refines brain circuitry by the coupled processes of eliminating inappropriate synapses and strengthening maintained synapses. We theorize these activity-dependent (A-D) developmental processes are specifically impaired in autism spectrum disorders (ASDs). ASD genetic models in both mouse and Drosophila have pioneered our insights into normal A-D neural circuit assembly and consolidation, and how these developmental mechanisms go awry in specific genetic conditions. The monogenic fragile X syndrome (FXS), a common cause of heritable ASD and intellectual disability, has been particularly well linked to defects in A-D critical period processes. The fragile X mental retardation protein (FMRP) is positively activity-regulated in expression and function, in turn regulates excitability and activity in a negative feedback loop, and appears to be required for the A-D remodeling of synaptic connectivity during early-use critical periods. The Drosophila FXS model has been shown to functionally conserve the roles of human FMRP in synaptogenesis, and has been centrally important in generating our current mechanistic understanding of the FXS disease state. Recent advances in Drosophila optogenetics, transgenic calcium reporters, highly-targeted transgenic drivers for individually-identified neurons, and a vastly improved connectome of the brain are now being combined to provide unparalleled opportunities to both manipulate and monitor A-D processes during critical period brain development in defined neural circuits. The field is now poised to exploit this new Drosophila transgenic toolbox for the systematic dissection of A-D mechanisms in normal versus ASD brain development, particularly utilizing the well-established Drosophila FXS disease model.

Drosophila modifier screens to identify novel neuropsychiatric drugs including aminergic agents for the possible treatment of Parkinson's disease and depression

Small molecules that increase the presynaptic function of aminergic cells may provide neuroprotection in Parkinson's disease (PD) as well as treatments for attention deficit hyperactivity disorder (ADHD) and depression. Model genetic organisms such as Drosophila melanogaster may enhance the detection of new drugs via modifier or 'enhancer/suppressor' screens, but this technique has not been applied to processes relevant to psychiatry. To identify new aminergic drugs in vivo, we used a mutation in the Drosophila vesicular monoamine transporter (dVMAT) as a sensitized genetic background and performed a suppressor screen. We fed dVMAT mutant larvae ∼ 1000 known drugs and quantitated rescue (suppression) of an amine-dependent locomotor deficit in the larva. To determine which drugs might specifically potentiate neurotransmitter release, we performed an additional secondary screen for drugs that require presynaptic amine storage to rescue larval locomotion. Using additional larval locomotion and adult fertility assays, we validated that at least one compound previously used clinically as an antineoplastic agent potentiates the presynaptic function of aminergic circuits. We suggest that structurally similar agents might be used to development treatments for PD, depression and ADHD, and that modifier screens in Drosophila provide a new strategy to screen for neuropsychiatric drugs. More generally, our findings demonstrate the power of physiologically based screens for identifying bioactive agents for select neurotransmitter systems.

An injury paradigm to investigate central nervous system repair in Drosophila

An experimental method has been developed to investigate the cellular responses to central nervous system (CNS) injury using the fruit-fly Drosophila. Understanding repair and regeneration in animals is a key question in biology. The damaged human CNS does not regenerate, and understanding how to promote the regeneration is one of main goals of medical neuroscience. The powerful genetic toolkit of Drosophila can be used to tackle the problem of CNS regeneration.

A lesion to the CNS ventral nerve cord (VNC, equivalent to the vertebrate spinal cord) is applied manually with a tungsten needle. The VNC can subsequently be filmed in time-lapse using laser scanning confocal microscopy for up to 24 hr to follow the development of the lesion over time. Alternatively, it can be cultured, then fixed and stained using immunofluorescence to visualize neuron and glial cells with confocal microscopy. Using appropriate markers, changes in cell morphology and cell state as a result of injury can be visualized. With ImageJ and purposely developed plug-ins, quantitative and statistical analyses can be carried out to measure changes in wound size over time and the effects of injury in cell proliferation and cell death. These methods allow the analysis of large sample sizes. They can be combined with the powerful genetics of Drosophila to investigate the molecular mechanisms underlying CNS regeneration and repair.

Developmental changes in expression, subcellular distribution, and function of Drosophila N-cadherin, guided by a cell-intrinsic program during neuronal differentiation

Cell adhesion molecules (CAMs) perform numerous functions during neural development. An individual CAM can play different roles during each stage of neuronal differentiation; however, little is known about how such functional switching is accomplished. Here we show that Drosophila N-cadherin (CadN) is required at multiple developmental stages within the same neuronal population and that its sub-cellular expression pattern changes between the different stages. During development of mushroom body neurons and motoneurons, CadN is expressed at high levels on growing axons, whereas expression becomes downregulated and restricted to synaptic sites in mature neurons. Phenotypic analysis of CadN mutants reveals that developing axons require CadN for axon guidance and fasciculation, whereas mature neurons for terminal growth and receptor clustering. Furthermore, we demonstrate that CadN downregulation can be achieved in cultured neurons without synaptic contact with other cells. Neuronal silencing experiments using Kir(2.1) indicate that neuronal excitability is also dispensable for CadN downregulation in vivo. Interestingly, downregulation of CadN can be prematurely induced by ectopic expression of a nonselective cation channel, dTRPA1, in developing neurons. Together, we suggest that switching of CadN expression during neuronal differentiation involves regulated cation influx within neurons.

Ca(v)2 channels mediate low and high voltage-activated calcium currents in Drosophila motoneurons

Different blends of membrane currents underlie distinct functions of neurons in the brain. A major step towards understanding neuronal function, therefore, is to identify the genes that encode different ionic currents. This study combined in situ patch clamp recordings of somatodendritic calcium currents in an identified adult Drosophila motoneuron with targeted genetic manipulation. Voltage clamp recordings revealed transient low voltage-activated (LVA) currents with activation between –60 mV and –70 mV as well as high voltage-activated (HVA) current with an activation voltage around –30 mV. LVA could be fully inactivated by prepulses to –50 mV and was partially amiloride sensitive. Recordings from newly generated mutant flies demonstrated that DmαG (Ca(v)3 homolog) encoded the amiloride-sensitive portion of the transient LVA calcium current. We further demonstrated that the Ca(v)2 homolog, Dmca1A, mediated the amiloride-insensitive component of LVA current. This novel role of Ca(v)2 channels was substantiated by patch clamp recordings from conditional mutants, RNAi knock-downs, and following Dmca1A overexpression. In addition, we show that Dmca1A underlies the HVA somatodendritic calcium currents in vivo. Therefore, the Drosophila Ca(v)2 homolog, Dmca1A, underlies HVA and LVA somatodendritic calcium currents in the same neuron. Interestingly, DmαG is required for regulating LVA and HVA derived from Dmca1A in vivo. In summary, each vertebrate gene family for voltage-gated calcium channels is represented by a single gene in Drosophila, namely Dmca1D (Ca(v)1), Dmca1A (Ca(v)2) and DmαG (Ca(v)3), but the commonly held view that LVA calcium currents are usually mediated by Ca(v)3 rather than Ca(v)2 channels may require reconsideration.

A new genetic model of activity-induced Ras signaling dependent pre-synaptic plasticity in Drosophila

Techniques to induce activity-dependent neuronal plasticity in vivo allow the underlying signaling pathways to be studied in their biological context. Here, we demonstrate activity-induced plasticity at neuromuscular synapses of Drosophila double mutant for comatose (an NSF mutant) and Kum (a SERCA mutant), and present an analysis of the underlying signaling pathways. comt; Kum (CK) double mutants exhibit increased locomotor activity under normal culture conditions, concomitant with a larger neuromuscular junction synapse and stably elevated evoked transmitter release. The observed enhancements of synaptic size and transmitter release in CK mutants are completely abrogated by: a) reduced activity of motor neurons; b) attenuation of the Ras/ERK signaling cascade; or c) inhibition of the transcription factors Fos and CREB. All of which restrict synaptic properties to near wild type levels. Together, these results document neural activity-dependent plasticity of motor synapses in CK animals that requires Ras/ERK signaling and normal transcriptional activity of Fos and CREB. Further, novel in vivo reporters of neuronal Ras activation and Fos transcription also confirm increased signaling through a Ras/AP-1 pathway in motor neurons of CK animals, consistent with results from our genetic experiments. Thus, this study: a) provides a robust system in which to study activity-induced synaptic plasticity in vivo; b) establishes a causal link between neural activity, Ras signaling, transcriptional regulation and pre-synaptic plasticity in glutamatergic motor neurons of Drosophila larvae; and c) presents novel, genetically encoded reporters for Ras and AP-1 dependent signaling pathways in Drosophila.

Ion channels to inactivate neurons in Drosophila

Ion channels are the determinants of excitability; therefore, manipulation of their levels and properties provides an opportunity for the investigator to modulate neuronal and circuit function. There are a number of ways to suppress electrical activity in Drosophila neurons, for instance, over-expression of potassium channels (i.e. Shaker Kv1, Shaw Kv3, Kir2.1 and DORK) that are open at resting membrane potential. This will result in increased potassium efflux and membrane hyperpolarisation setting resting membrane potential below the threshold required to fire action potentials. Alternatively over-expression of other channels, pumps or co-transporters that result in a hyperpolarised membrane potential will also prevent firing. Lastly, neurons can be inactivated by, disrupting or reducing the level of functional voltage-gated sodium (Nav1 paralytic) or calcium (Cav2 cacophony) channels that mediate the depolarisation phase of action potentials. Similarly, strategies involving the opposite channel manipulation should allow net depolarisation and hyperexcitation in a given neuron. These changes in ion channel expression can be brought about by the versatile transgenic (i.e. Gal4/UAS based) systems available in Drosophila allowing fine temporal and spatial control of (channel) transgene expression. These systems are making it possible to electrically inactivate (or hyperexcite) any neuron or neural circuit in the fly brain, and much like an exquisite lesion experiment, potentially elucidate whatever interesting behaviour or phenotype each network mediates. These techniques are now being used in Drosophila to reprogram electrical activity of well-defined circuits and bring about robust and easily quantifiable changes in behaviour, allowing different models and hypotheses to be rapidly tested.

Neuronal loss of Drosophila NPC1a causes cholesterol aggregation and age-progressive neurodegeneration

The mistrafficking and consequent cytoplasmic accumulation of cholesterol and sphingolipids is linked to multiple neurodegenerative diseases. One class of disease, the sphingolipid storage diseases, includes Niemann-Pick disease type C (NPC), caused predominantly (95%) by mutation of the NPC1 gene. A disease model has been established through mutation of Drosophila NPC1a (dnpc1a). Null mutants display early lethality attributable to loss of cholesterol-dependent ecdysone steroid hormone production. Null mutants rescued to adults by restoring ecdysone production mimic human NPC patients with progressive motor defects and reduced life spans. Analysis of dnpc1a null brains shows elevated overall cholesterol levels and progressive accumulation of filipin-positive cholesterol aggregates within brain and retina, as well as isolated cultured brain neurons. Ultrastructural imaging of dnpc1a mutant brains reveals age-progressive accumulation of striking multilamellar and multivesicular organelles, preceding the onset of neurodegeneration. Consistently, electroretinogram recordings show age-progressive loss of phototransduction and photoreceptor synaptic transmission. Early lethality, movement impairments, neuronal cholesterol deposits, accumulation of multilamellar bodies, and age-dependent neurodegeneration are all rescued by targeted neuronal expression of a wild-type dnpc1a transgene. Interestingly, targeted expression of dnpc1a in glia also provides limited rescue of adult lethality. Generation of dnpc1a null mutant neuron clones in the brain reveals cell-autonomous requirements for dNPC1a in cholesterol and membrane trafficking. These data demonstrate a requirement for dNPC1a in the maintenance of neuronal function and viability and show that loss of dNPC1a in neurons mimics the human neurodegenerative condition.

Ultrastructural analysis of chemical synapses and gap junctions between Drosophila brain neurons in culture

Dissociated cultures from many species have been important tools for exploring factors that regulate structure and function of central neuronal synapses. We have previously shown that cells harvested from brains of late stage Drosophila pupae can regenerate their processes in vitro. Electrophysiological recordings demonstrate the formation of functional synaptic connections as early as 3 days in vitro (DIV), but no information about synapse structure is available. Here, we report that antibodies against pre-synaptic proteins Synapsin and Bruchpilot result in punctate staining of regenerating neurites. Puncta density increases as neuritic plexuses develop over the first 4 DIV. Electron microscopy reveals that closely apposed neurites can form chemical synapses with both pre- and postsynaptic specializations characteristic of many inter-neuronal synapses in the adult brain. Chemical synapses in culture are restricted to neuritic processes and some neurite pairs form reciprocal synapses. GABAergic synapses have a significantly higher percentage of clear core versus granular vesicles than non-GABA synapses. Gap junction profiles, some adjacent to chemical synapses, suggest that neurons in culture can form purely electrical as well as mixed synapses, as they do in the brain. However, unlike adult brain, gap junctions in culture form between neuronal somata as well as neurites, suggesting soma ensheathing glia, largely absent in culture, regulate gap junction location in vivo. Thus pupal brain cultures, which support formation of interneuronal synapses with structural features similar to synapses in adult brain, are a useful model system for identifying intrinsic and extrinsic regulators of central synapse structure as well as function.

Tissue-specific targeting of Hsp26 has no effect on heat resistance of neural function in larval Drosophila

Hsp26 belongs to the small heat-shock protein family and is normally expressed in all cells during heat stress. We aimed to determine if overexpression of this protein protects behavior and neural function in Drosophila melanogaster during heat stress, as has previously been shown for Hsp70. We used the UAS-GAL4 expression system to drive expression of Hsp26 in the whole animal (ubiquitously), in the motoneurons, and in the muscles of wandering third-instar larvae. There were slight increases in time to crawling failure and normalized excitatory junction potential (EJP) area for some of the transgenic lines, but these were not consistent. In addition, Hsp26 had no effect on the temperature at failure of EJPs, normalized EJP peak amplitude, and normalized EJP half-width. Overexpression larvae had a similar number of motoneuronal boutons and length of nerve terminals as controls, indicating that the occasional protective effects on locomotion were not due to changes at the synapse. We conclude that overexpression had a small thermoprotective effect on locomotion and no effect on neural function. As it has been shown that Hsp26 requires action of other Hsps to reactivate the denatured proteins to which it binds, we propose that at least in larvae, the function of Hsp26 was masked in the relative absence of other Hsps.

Preparation of neuronal cultures from midgastrula stage Drosophila embryos

This video illustrates the procedure for making primary neuronal cultures from midgastrula stage Drosophila embryos. The methods for collecting embryos and their dechorionation using bleach are demonstrated. Using a glass pipet attached to a mouth suction tube, we illustrate the removal of all cells from single embryos. The method for dispersing cells from each embryo into a small (5 l) drop of medium on an uncoated glass coverslip is demonstrated. A view through the microscope at 1 hour after plating illustrates the preferred cell density. Most of the cells that survive when grown in defined medium are neuroblasts that divide one or more times in culture before extending neuritic processes by 12-24 hours. A view through the microscope illustrates the level of neurite outgrowth and branching expected in a healthy culture at 2 days in vitro. The cultures are grown in a simple bicarbonate based defined medium, in a 5% CO(2) incubator at 22-24 degrees C. Neuritic processes continue to elaborate over the first week in culture and when they make contact with neurites from neighboring cells they often form functional synaptic connections. Neurons in these cultures express voltage-gated sodium, calcium, and potassium channels and are electrically excitable. This culture system is useful for studying molecular genetic and environmental factors that regulate neuronal differentiation, excitability, and synapse formation/function.

NeuronMetrics: software for semi-automated processing of cultured neuron images

Using primary cell culture to screen for changes in neuronal morphology requires specialized analysis software. We developed NeuronMetrics for semi-automated, quantitative analysis of two-dimensional (2D) images of fluorescently labeled cultured neurons. It skeletonizes the neuron image using two complementary image-processing techniques, capturing fine terminal neurites with high fidelity. An algorithm was devised to span wide gaps in the skeleton. NeuronMetrics uses a novel strategy based on geometric features called faces to extract a branch number estimate from complex arbors with numerous neurite-to-neurite contacts, without creating a precise, contact-free representation of the neurite arbor. It estimates total neurite length, branch number, primary neurite number, territory (the area of the convex polygon bounding the skeleton and cell body), and Polarity Index (a measure of neuronal polarity). These parameters provide fundamental information about the size and shape of neurite arbors, which are critical factors for neuronal function. NeuronMetrics streamlines optional manual tasks such as removing noise, isolating the largest primary neurite, and correcting length for self-fasciculating neurites. Numeric data are output in a single text file, readily imported into other applications for further analysis. Written as modules for ImageJ, NeuronMetrics provides practical analysis tools that are easy to use and support batch processing. Depending on the need for manual intervention, processing time for a batch of approximately 60 2D images is 1.0-2.5 h, from a folder of images to a table of numeric data. NeuronMetrics' output accelerates the quantitative detection of mutations and chemical compounds that alter neurite morphology in vitro, and will contribute to the use of cultured neurons for drug discovery.

Phenotypes of Drosophila brain neurons in primary culture reveal a role for fascin in neurite shape and trajectory

Subtle cellular phenotypes in the CNS may evade detection by routine histopathology. Here, we demonstrate the value of primary culture for revealing genetically determined neuronal phenotypes at high resolution. Gamma neurons of Drosophila melanogaster mushroom bodies (MBs) are remodeled during metamorphosis under the control of the steroid hormone 20-hydroxyecdysone (20E). In vitro, wild-type gamma neurons retain characteristic morphogenetic features, notably a single axon-like dominant primary process and an arbor of short dendrite-like processes, as determined with microtubule-polarity markers. We found three distinct genetically determined phenotypes of cultured neurons from grossly normal brains, suggesting that subtle in vivo attributes are unmasked and amplified in vitro. First, the neurite outgrowth response to 20E is sexually dimorphic, being much greater in female than in male gamma neurons. Second, the gamma neuron-specific "naked runt" phenotype results from transgenic insertion of an MB-specific promoter. Third, the recessive, pan-neuronal "filagree" phenotype maps to singed, which encodes the actin-bundling protein fascin. Fascin deficiency does not impair the 20E response, but neurites fail to maintain their normal, nearly straight trajectory, instead forming curls and hooks. This is accompanied by abnormally distributed filamentous actin. This is the first demonstration of fascin function in neuronal morphogenesis. Our findings, along with the regulation of human Fascin1 (OMIM 602689) by CREB (cAMP response element-binding protein) binding protein, suggest FSCN1 as a candidate gene for developmental brain disorders. We developed an automated method of computing neurite curvature and classifying neurons based on curvature phenotype. This will facilitate detection of genetic and pharmacological modifiers of neuronal defects resulting from fascin deficiency.

Insect neuronal cultures: an experimental vehicle for studies of physiology, pharmacology and cell interactions

The current status of insect neuronal cultures is discussed and their contribution to our understanding of the insect nervous system is explored. Neuronal cultures have been developed from a wide range of insect species and from all developmental stages. These have been used to study the morphological development of insect neurones and some of the extrinsic factors that affect this process. In addition, they have been used to investigate the physiology of sodium, potassium and calcium channels and the pharmacology of acetylcholine and GABA receptors. Insect neurones have also been grown in culture with muscle and glial cells to study cell interactions.

Cholinergic synaptic transmission in adult Drosophila Kenyon cells in situ

Behavioral and genetic studies in Drosophila have contributed to our understanding of molecular mechanisms that underlie the complex processes of learning and memory. Use of this model organism for exploration of the cellular mechanisms of memory formation requires the ability to monitor synaptic activity in the underlying neural networks, a challenging task in the tiny adult fly. Here, we describe an isolated whole-brain preparation in which it is possible to obtain in situ whole-cell recordings from adult Kenyon cells, key members of a neural circuit essential for olfactory associative learning in Drosophila. The presence of sodium action potential (AP)-dependent synaptic potentials and synaptic currents in >50% of the Kenyon cells shows that these neurons are members of a spontaneously active neural circuit in the isolated brain. The majority of sodium AP-dependent synaptic transmission is blocked by curare and by alpha-bungarotoxin (alpha-BTX). This demonstrates that nicotinic acetylcholine receptors (nAChRs) are responsible for most of the spontaneous excitatory drive in this circuit in the absence of normal sensory input. Furthermore, analysis of sodium AP-independent synaptic currents provides the first direct demonstration that alpha-BTX-sensitive nAChRs mediate fast excitatory synaptic transmission in Kenyon cells in the adult Drosophila brain. This new preparation, in which whole-cell recordings and pharmacology can be combined with genetic approaches, will be critical in understanding the contribution of nAChR-mediated fast synaptic transmission to cellular plasticity in the neural circuits underlying olfactory associative learning.

An essential Drosophila glutamate receptor subunit that functions in both central neuropil and neuromuscular junction

A Drosophila forward genetic screen for mutants with defective synaptic development identified bad reception (brec). Homozygous brec mutants are embryonic lethal, paralyzed, and show no detectable synaptic transmission at the glutamatergic neuromuscular junction (NMJ). Genetic mapping, complementation tests, and genomic sequencing show that brec mutations disrupt a previously uncharacterized ionotropic glutamate receptor subunit, named here "GluRIID." GluRIID is expressed in the postsynaptic domain of the NMJ, as well as widely throughout the synaptic neuropil of the CNS. In the NMJ of null brec mutants, all known glutamate receptor subunits are undetectable by immunocytochemistry, and all functional glutamate receptors are eliminated. Thus, we conclude that GluRIID is essential for the assembly and/or stabilization of glutamate receptors in the NMJ. In null brec mutant embryos, the frequency of periodic excitatory currents in motor neurons is significantly reduced, demonstrating that CNS motor pattern activity is regulated by GluRIID. Although synaptic development and molecular differentiation appear otherwise unperturbed in null mutants, viable hypomorphic brec mutants display dramatically undergrown NMJs by the end of larval development, suggesting that GluRIID-dependent central pattern activity regulates peripheral synaptic growth. These studies reveal GluRIID as a newly identified glutamate receptor subunit that is essential for glutamate receptor assembly/stabilization in the peripheral NMJ and required for properly patterned motor output in the CNS.

Determinants of electrical properties in developing neurons

The regulatory mechanisms that orchestrate the developmental acquisition of electrical properties in embryonic neurons are poorly understood. Progress in this important area is dependent on the availability of preparations that allow electrophysiology to be married with genetics. The powerful genetics of the fruitfly Drosophila melanogaster has long been exploited to describe fundamental mechanisms associated with structural neuronal development (i.e. axon guidance). It has not, however, been fully employed to study the final stages of embryonic neural development and in particular the acquisition of electrical activity. This review focuses on the recent development of a Drosophila preparation that allows central neurons to be accessed by patch electrodes at both embryonic and larval stages. This preparation, which allows electrophysiology to be coupled with genetics, offers the prospect of making significant advances in our understanding of functional neuron development.

Development and structure of synaptic contacts in Drosophila

Structural synapses are key regulators of information flow in neuronal networks. To understand the function and formation of neuronal circuits, the development and function of synapses have therefore been intensely studied in both vertebrate and invertebrate species. Precise descriptions of synapses and their amenability to genetic analysis in the model organism Drosophila provide an efficient platform from which to explore mechanisms and principles of synapse formation, which find many counterparts in other animals. Here we summarise our knowledge of the structure of Drosophila synapses. Focussing on neuromuscular junctions and photoreceptor synapses, we provide an overview of mechanisms underlying the development of synaptic structure in Drosophila.

Are dendrites in Drosophila homologous to vertebrate dendrites?

Dendrites represent arborising neurites in both vertebrates and invertebrates. However, in vertebrates, dendrites develop on neuronal cell bodies, whereas in higher invertebrates, they arise from very different neuronal structures, the primary neurites, which also form the axons. Is this anatomical difference paralleled by principal developmental and/or physiological differences? We address this question by focussing on one cellular model, motorneurons of Drosophila and characterise the compartmentalisation of these cells. We find that motorneuronal dendrites of Drosophila share with typical vertebrate dendrites that they lack presynaptic but harbour postsynaptic proteins, display calcium elevation upon excitation, have distinct cytoskeletal features, develop later than axons and are preceded by restricted localisation of Par6-complex proteins. Furthermore, we demonstrate in situ and culture that Drosophila dendrites can be shifted from the primary neurite to their soma, i.e. into vertebrate-like positions. Integrating these different lines of argumentation, we propose that dendrites in vertebrates and higher invertebrates have a common origin, and differences in dendrite location can be explained through translocation of neuronal cell bodies introduced during the evolutionary process by which arthropods and vertebrates diverged from a common urbilaterian ancestor. Implications of these findings for studies of dendrite development, neuronal polarity, transport and evolution are discussed.

Morphogens and synaptogenesis in Drosophila

During development and adult life synapses are remodeled in response to genetic programs and environmental cues. This synaptic plasticity is thought to be the basis of learning and memory. The larval neuromuscular junction of Drosophila is established during embryogenesis and grows during larval development to accommodate muscle growth and maintain synaptic homeostasis. This growth is dependent on bidirectional communication between the motoneuron and the muscle fiber. The best-characterized retrograde signaling pathway is defined by Glass bottom boat (Gbb), a morphogen of the transforming growth factor-beta (TGF-beta) superfamily. Gbb acts as a muscle-derived retrograde signal that activates the TGF-beta pathway presynaptically. This pathway includes the type II receptor Wishful thinking, type I receptors Thick veins and Saxophone, and the second messenger Smads Mothers against dpp (Mad) and Medea. Mutations that block this pathway result in small synapses that are morphologically aberrant and severely impaired functionally. An emerging anterograde signaling pathway is defined by Wingless, a morphogen of the Wnt family that acts as a motoneuron-derived anterograde signal required for both pre- and postsynaptic development. In the absence of Wingless the neuronal microtubule cytoskeleton regulator Futsch is down-regulated and synaptic growth impaired. Some of these morphogens have conserved roles in mammalian synaptogenesis, and genetic analysis suggests that additional signaling molecules are required for synaptic growth at the Drosophila neuromuscular junction.

Regulation of neuronal excitability through pumilio-dependent control of a sodium channel gene

Dynamic changes in synaptic connectivity and strength, which occur during both embryonic development and learning, have the tendency to destabilize neural circuits. To overcome this, neurons have developed a diversity of homeostatic mechanisms to maintain firing within physiologically defined limits. In this study, we show that activity-dependent control of mRNA for a specific voltage-gated Na+ channel [encoded by paralytic (para)] contributes to the regulation of membrane excitability in Drosophila motoneurons. Quantification of para mRNA, by real-time reverse-transcription PCR, shows that levels are significantly decreased in CNSs in which synaptic excitation is elevated, whereas, conversely, they are significantly increased when synaptic vesicle release is blocked. Quantification of mRNA encoding the translational repressor pumilio (pum) reveals a reciprocal regulation to that seen for para. Pumilio is sufficient to influence para mRNA. Thus, para mRNA is significantly elevated in a loss-of-function allele of pum (pum(bemused)), whereas expression of a full-length pum transgene is sufficient to reduce para mRNA. In the absence of pum, increased synaptic excitation fails to reduce para mRNA, showing that Pum is also necessary for activity-dependent regulation of para mRNA. Analysis of voltage-gated Na+ current (I(Na)) mediated by para in two identified motoneurons (termed aCC and RP2) reveals that removal of pum is sufficient to increase one of two separable I(Na) components (persistent I(Na)), whereas overexpression of a pum transgene is sufficient to suppress both components (transient and persistent). We show, through use of anemone toxin (ATX II), that alteration in persistent I(Na) is sufficient to regulate membrane excitability in these two motoneurons.

The Drosophila metabotropic glutamate receptor DmGluRA regulates activity-dependent synaptic facilitation and fine synaptic morphology

In vertebrates, several groups of metabotropic glutamate receptors (mGluRs) are known to modulate synaptic properties. In contrast, the Drosophila genome encodes a single functional mGluR (DmGluRA), an ortholog of vertebrate group II mGluRs, greatly expediting the functional characterization of mGluR-mediated signaling in the nervous system. We show here that DmGluRA is expressed at the glutamatergic neuromuscular junction (NMJ), localized in periactive zones of presynaptic boutons but excluded from active sites. Null DmGluRA mutants are completely viable, and all of the basal NMJ synaptic transmission properties are normal. In contrast, DmGluRA mutants display approximately a threefold increase in synaptic facilitation during short stimulus trains. Prolonged stimulus trains result in very strongly increased ( approximately 10-fold) augmentation, including the appearance of asynchronous, bursting excitatory currents never observed in wild type. Both defects are rescued by expression of DmGluRA only in the neurons, indicating a specific presynaptic requirement. These phenotypes are reminiscent of hyperexcitable mutants, suggesting a role of DmGluRA signaling in the regulation of presynaptic excitability properties. The mutant phenotypes could not be replicated by acute application of mGluR antagonists, suggesting that DmGluRA regulates the development of presynaptic properties rather than directly controlling short-term modulation. DmGluRA mutants also display mild defects in NMJ architecture: a decreased number of synaptic boutons accompanied by an increase in mean bouton size. These morphological changes bidirectionally correlate with DmGluRA levels in the presynaptic terminal. These data reveal the following two roles for DmGluRA in presynaptic mechanisms: (1) modulation of presynaptic excitability properties important for the control of activity-dependent neurotransmitter release and (2) modulation of synaptic architecture.

A new culturing strategy optimises Drosophila primary cell cultures for structural and functional analyses

Neurons in primary cell cultures provide important experimental possibilities complementing or substituting those in the nervous system. However, Drosophila primary cell cultures have unfortunate limitations: they lack either a range of naturally occurring cell types, or of mature physiological properties. Here, we demonstrate a strategy which supports both aspects integrated in one culture: Initial culturing in conventional serum-supplemented Schneider's medium (SM(20K)) guarantees acquisition of all properties known from 30 years of work on cell type-specific differentiation in this medium. Through subsequent shift to newly developed active Schneider's medium (SM(active)), neurons adopt additional mature properties like the ability to carry out plastic morphological changes, neurotransmitter expression and electrical activity. We introduce long-term FM-dye measurements as a tool for Drosophila primary cell cultures demonstrating the presence of increased, action potential-dependent synaptic activity in SM(active). This is confirmed by patch-clamp recordings, which in addition show that SM(active)-cultured neurons display different spiking patterns. Furthermore, we demonstrate that transmission can be evoked in SM(active) cultures, revealing the existence of synaptic plasticity. Thus, these culture conditions support developmental, structural and physiological properties known or expected from the nervous system, enhancing possibilities for future experiments complementing or substituting those in nervous systems of Drosophila.

Fast synaptic currents in Drosophila mushroom body Kenyon cells are mediated by alpha-bungarotoxin-sensitive nicotinic acetylcholine receptors and picrotoxin-sensitive GABA receptors

The mushroom bodies, bilaterally symmetric regions in the insect brain, play a critical role in olfactory associative learning. Genetic studies in Drosophila suggest that plasticity underlying acquisition and storage of memory occurs at synapses on the dendrites of mushroom body Kenyon cells (Dubnau et al., 2001). Additional exploration of the mechanisms governing synaptic plasticity contributing to these aspects of olfactory associative learning requires identification of the receptors that mediate fast synaptic transmission in Kenyon cells. To this end, we developed a culture system that supports the formation of excitatory and inhibitory synaptic connections between neurons harvested from the central brain region of late-stage Drosophila pupae. Mushroom body Kenyon cells are identified as small-diameter, green fluorescent protein-positive (GFP+) neurons in cultures from OK107-GAL4;UAS-GFP pupae. In GFP+ Kenyon cells, fast EPSCs are mediated by alpha-bungarotoxin-sensitive nicotinic acetylcholine receptors (nAChRs). The miniature EPSCs have rapid rise and decay kinetics and a broad, positively skewed amplitude distribution. Fast IPSCs are mediated by picrotoxin-sensitive chloride conducting GABA receptors. The miniature IPSCs also have a rapid rate of rise and decay and a broad amplitude distribution. The vast majority of spontaneous synaptic currents in the cultured Kenyon cells are mediated byalpha-bungarotoxin-sensitive nAChRs or picrotoxin-sensitive GABA receptors. Therefore, these receptors are also likely to mediate synaptic transmission in Kenyon cells in vivo and to contribute to plasticity during olfactory associative learning.

Acetylcholine increases intracellular Ca2+ via nicotinic receptors in cultured PDF-containing clock neurons of Drosophila

Light entrains the biological clock both in adult and larval Drosophila melanogaster. The Bolwig organ photoreceptors most likely constitute one substrate for this light entrainment in larvae. Acetylcholine (ACh) has been suggested as the neurotransmitter in these photoreceptors, but there is no evidence that ACh signaling is involved in photic input onto circadian pacemaker neurons. Here we demonstrate that the putative targets of the Bolwig photoreceptors, the PDF-containing clock neurons (LNs), in the larval brain express functional ACh receptors (AChRs). With the use of GAL4-UAS-driven expression of green fluorescent protein (GFP), we were able to identify LNs in dissociated cell culture. After loading with the Ca(2+)-sensitive dye fura-2, we monitored changes in intracellular Ca(2+) levels ([Ca(2+)](i)) in GFP-marked LNs while applying candidate neurotransmitters. ACh induced transient increases in [Ca(2+)](i) at physiological concentrations. These increases were dependent on extracellular Ca(2+) and Na(+) and were likely caused by activation of voltage-dependent Ca(2+) channels. Application of nicotinic and muscarinic agonists and antagonists showed that the AChRs on cultured LNs have a nicotinic pharmacology. Antibodies to several subunits of nicotinic AChRs (nAChRs) labeled the putative contact site of the Bolwig organ axon terminals with the dendrites of LNs, as well as dissociated LNs in culture. Our findings support a role of ACh as input factor onto the LNs and suggest that Ca(2+) is used as a second messenger mediating cholinergic input within the LNs. Experiments using a more general GAL4-UAS-driven expression of GFP showed that functional expression of nAChRs is a widespread phenomenon in peptidergic neurons.

Neural correlates to flight-related density-dependent phase characteristics in locusts

Locust phase polymorphism is an extreme example of behavioral plasticity; in response to changes in population density, locusts dramatically alter their behavior. These changes in behavior facilitate the appearance of various morphological and physiological phase characteristics. One of the principal behavioral changes is the more intense flight behavior and improved flight performance of gregarious locusts compared to solitary ones. Surprisingly, the neurophysiological basis of the behavioral phase characteristics has received little attention. Here we present density-dependent differences in flight-related sensory and central neural elements in the desert locust. Using techniques already established for gregarious locusts, we compared the response of locusts of both phases to controlled wind stimuli. Gregarious locusts demonstrated a lower threshold for wind-induced flight initiation. Wind-induced spiking activity in the locust tritocerebral commissure giants (TCG, a pair of identified interneurons that relay input from head hair receptors to thoracic motor centers) was found to be weaker in solitary locusts compared to gregarious ones. The solitary locusts' TCG also demonstrated much stronger spike frequency adaptation in response to wind stimuli. Although the number of forehead wind sensitive hairs was found to be larger in solitary locusts, the stimuli conveyed to their flight motor centers were weaker. The tritocerebral commissure dwarf (TCD) is an inhibitory flight-related interneuron in the locust that responds to light stimuli. An increase in TCD spontaneous activity in dark conditions was significantly stronger in gregarious locusts than in solitary ones. Thus, phase-dependent differences in the activity of flight-related interneurons reflect behavioral phase characteristics.

Charting the Drosophila neuropile: a strategy for the standardised characterisation of genetically amenable neurites

Insect neurons are individually identifiable and have been used successfully to study principles of the formation and function of neuronal circuits. In the fruitfly Drosophila, studies on identifiable neurons can be combined with efficient genetic approaches. However, to capitalise on this potential for studies of circuit formation in the CNS of Drosophila embryos or larvae, we need to identify pre- and postsynaptic elements of such circuits and describe the neuropilar territories they occupy. Here, we present a strategy for neurite mapping, using a set of evenly distributed landmarks labelled by commercially available anti-Fasciclin2 antibodies which remain comparatively constant between specimens and over developmental time. By applying this procedure to neurites labelled by three Gal4 lines, we show that neuritic territories are established in the embryo and maintained throughout larval life, although the complexity of neuritic arborisations increases during this period. Using additional immunostainings or dye fills, we can assign Gal4-targeted neurites to individual neurons and characterise them further as a reference for future experiments on circuit formation. Using the Fasciclin2-based mapping procedure as a standard (e.g., in a common database) would facilitate studies on the functional architecture of the neuropile and the identification of candiate circuit elements.