Wiring by fly: the neuromuscular system of the Drosophila embryo

MicroRNA-8 promotes robust motor axon targeting by coordinate regulation of cell adhesion molecules during synapse development

Neuronal connectivity and specificity rely upon precise coordinated deployment of multiple cell-surface and secreted molecules. MicroRNAs have tremendous potential for shaping neural circuitry by fine-tuning the spatio-temporal expression of key synaptic effector molecules. The highly conserved microRNA miR-8 is required during late stages of neuromuscular synapse development in Drosophila. However, its role in initial synapse formation was previously unknown. Detailed analysis of synaptogenesis in this system now reveals that miR-8 is required at the earliest stages of muscle target contact by RP3 motor axons. We find that the localization of multiple synaptic cell adhesion molecules (CAMs) is dependent on the expression of miR-8, suggesting that miR-8 regulates the initial assembly of synaptic sites. Using stable isotope labelling in vivo and comparative mass spectrometry, we find that miR-8 is required for normal expression of multiple proteins, including the CAMs Fasciclin III (FasIII) and Neuroglian (Nrg). Genetic analysis suggests that Nrg and FasIII collaborate downstream of miR-8 to promote accurate target recognition. Unlike the function of miR-8 at mature larval neuromuscular junctions, at the embryonic stage we find that miR-8 controls key effectors on both sides of the synapse. MiR-8 controls multiple stages of synapse formation through the coordinate regulation of both pre- and postsynaptic cell adhesion proteins.

Diversification of muscle types in Drosophila: upstream and downstream of identity genes

Understanding gene regulatory pathways underlying diversification of cell types during development is one of the major challenges in developmental biology. Progressive specification of mesodermal lineages that are at the origin of body wall muscles in Drosophila embryos has been extensively studied during past years, providing an attractive framework for dissecting cell type diversification processes. In particular, it has been found that muscle founder cells that are at the origin of individual muscles display specific expression of transcription factors that control diversification of muscle types. These factors, encoded by genes collectively called muscle identity genes, are activated in discrete subsets of muscle founders. As a result, each founder cell is thought to carry a unique combinatorial code of identity gene expression. Considering this, to define temporally and spatially restricted expression of identity genes, a set of coordinated upstream regulatory inputs is required. But also, to realize the identity program and to form specific muscle types with distinct properties, an efficient battery of downstream identity gene targets needs to be activated. Here we review how the specificity of expression and action of muscle identity genes is acquired.

Regulation and restoration of motoneuronal synaptic transmission during neuromuscular regeneration in the pulmonate snail Helisoma trivolvis

Regeneration of motor systems involves reestablishment of central control networks, reinnervation of muscle targets by motoneurons, and reconnection of neuromodulatory circuits. Still, how these processes are integrated as motor function is restored during regeneration remains ill defined. Here, we examined the mechanisms underlying motoneuronal regeneration of neuromuscular synapses related to feeding movements in the pulmonate snail Helisoma trivolvis. Neurons B19 and B110, although activated during different phases of the feeding pattern, innervate similar sets of muscles. However, the percentage of muscle fibers innervated, the efficacy of excitatory junction potentials, and the strength of muscle contractions were different for each cell's specific connections. After peripheral nerve crush, a sequence of transient electrical and chemical connections formed centrally within the buccal ganglia. Neuromuscular synapse regeneration involved a three-phase process: the emergence of spontaneous synaptic transmission (P1), the acquisition of evoked potentials of weak efficacy (P2), and the establishment of functional reinnervation (P3). Differential synaptic efficacy at muscle contacts was recapitulated in cell culture. Differences in motoneuronal presynaptic properties (i.e., quantal content) were the basis of disparate neuromuscular synapse function, suggesting a role for retrograde target influences. We propose a homeostatic model of molluscan motor system regeneration. This model has three restoration events: (1) transient central synaptogenesis during axonal outgrowth, (2) intermotoneuronal inhibitory synaptogenesis during initial neuromuscular synapse formation, and (3) target-dependent regulation of neuromuscular junction formation.

A combinatorial semaphorin code instructs the initial steps of sensory circuit assembly in the Drosophila CNS

Longitudinal axon fascicles within the Drosophila embryonic CNS provide connections between body segments and are required for coordinated neural signaling along the anterior-posterior axis. We show here that establishment of select CNS longitudinal tracts and formation of precise mechanosensory afferent innervation to the same CNS region are coordinately regulated by the secreted semaphorins Sema-2a and Sema-2b. Both Sema-2a and Sema-2b utilize the same neuronal receptor, plexin B (PlexB), but serve distinct guidance functions. Localized Sema-2b attraction promotes the initial assembly of a subset of CNS longitudinal projections and subsequent targeting of chordotonal sensory afferent axons to these same longitudinal connectives, whereas broader Sema-2a repulsion serves to prevent aberrant innervation. In the absence of Sema-2b or PlexB, chordotonal afferent connectivity within the CNS is severely disrupted, resulting in specific larval behavioral deficits. These results reveal that distinct semaphorin-mediated guidance functions converge at PlexB and are critical for functional neural circuit assembly.

Drosophila as a genetic and cellular model for studies on axonal growth

One of the most fascinating processes during nervous system development is the establishment of stereotypic neuronal networks. An essential step in this process is the outgrowth and precise navigation (pathfinding) of axons and dendrites towards their synaptic partner cells. This phenomenon was first described more than a century ago and, over the past decades, increasing insights have been gained into the cellular and molecular mechanisms regulating neuronal growth and navigation. Progress in this area has been greatly assisted by the use of simple and genetically tractable invertebrate model systems, such as the fruit fly Drosophila melanogaster. This review is dedicated to Drosophila as a genetic and cellular model to study axonal growth and demonstrates how it can and has been used for this research. We describe the various cellular systems of Drosophila used for such studies, insights into axonal growth cones and their cytoskeletal dynamics, and summarise identified molecular signalling pathways required for growth cone navigation, with particular focus on pathfinding decisions in the ventral nerve cord of Drosophila embryos. These Drosophila-specific aspects are viewed in the general context of our current knowledge about neuronal growth.

The divergent TGF-β ligand Dawdle utilizes an activin pathway to influence axon guidance inDrosophila

Axon guidance is regulated by intrinsic factors and extrinsic cues provided by other neurons, glia and target muscles. Dawdle (Daw), a divergent TGF-β superfamily ligand expressed in glia and mesoderm, is required for embryonic motoneuron pathfinding in Drosophila. In dawmutants, ISNb and SNa axons fail to extend completely and are unable to innervate their targets. We find that Daw initiates an activin signaling pathway via the receptors Punt and Baboon (Babo) and the signal-transducer Smad2. Furthermore, mutations in these signaling components display similar axon guidance defects. Cell-autonomous disruption of receptor signaling suggests that Babo is required in motoneurons rather than in muscles or glia. Ectopic ligand expression can rescue the daw phenotype, but has no deleterious effects. Our results indicate that Daw functions in a permissive manner to modulate or enable the growth cone response to other restricted guidance cues,and support a novel role for activin signaling in axon guidance.

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.

Expression of muscarinic acetylcholine receptors and choline acetyltransferase enzyme in cultured antennal sensory neurons and non-neural cells of the developing moth Manduca sexta

Antennal sensory neurons of Manduca sexta emerge from epidermal cells that also give rise to sheath cells surrounding the peripheral parts of the neurons and to glial cells that enwrap the sensory axons in the antennal nerve. Reciprocal interactions between sensory neurons and glial cells are believed to aid in axon growth and guidance, but the exact nature of these interactions is not known. We investigated the possibility of cholinergic interactions in this process by locating muscarinic acetylcholine receptors (mAChRs) and choline acetyltransferase (ChAT) enzyme in cultured antennal sensory neurons and non-neural cells. ChAT and mAChRs were present in the sensory neurons from the first day in culture. Therefore, the sensory neurons are probably cholinergic, as previously suggested, but they may also be controlled by ACh. In 7-day-old cultures a subgroup of small non-neural cells with processes expressed ChAT activity, and in 14-day-old cultures non-neural cells that formed lamellipodia and scaffoldlike structures on the culture substrate were labeled with ChAT antibody. mAChR activity was detected in similar non-neural cells but only in areas surrounding the nuclei. In addition, mAChRs were found in flat lamellipodia and filopodia forming cells that were present in 1-day-old cultures and grew in size during the 2 week investigation period. These findings suggest muscarinic cholinergic interactions between the neural and non-neural cells during the development of Manduca antenna.

Electrophysiological and Morphological Characterization of Identified Motor Neurons in theDrosophilaThird Instar Larva Central Nervous System

We have used dye fills and electrophysiological recordings to identify and characterize a cluster of motor neurons in the third instar larval ventral ganglion. This cluster of neurons is similar in position to the well-studied embryonic RP neurons. Dye fills of larval dorsomedial neurons demonstrate that individual neurons within the cluster can be reproducibly identified by observing their muscle targets and bouton morphology. The terminal targets of these five neurons are body wall muscles 6/7, 1, 14, and 30 and the intersegmental nerve (ISN) terminal muscles (1, 2, 3, 4, 9, 10, 19, 20). All cells except the ISN neuron, which has a type Is ending, display type Ib boutons. Two of these neurons appear to be identical to the embryonic RP3 and aCC cells, which define the most proximal and distal innervations within a hemisegment. The targets of the other neurons in the larval dorsomedial cluster do not correspond to embryonic targets of the neurons in the RP cluster, suggesting rewiring of this circuit during early larval stages. Electrophysiological studies of the five neurons in current clamp revealed that type Is neurons have a longer delay in the appearance of the first spike compared with type Ib neurons. Genetic, biophysical, and pharmacological studies in current and voltage clamp show this delay is controlled by the kinetics and voltage sensitivity of inactivation of a current whose properties suggest that it may be the Shal IAcurrent. The combination of genetic identification and whole cell recording allows us to directly explore the cellular substrates of neural and locomotor behavior in an intact system.

Taste, movement, and death: varying effects of new prospero mutants during Drosophila development

The PGal4 transposon inserted upstream of the pan-neural gene prospero (pros) causes several neural and behavioral defects in the Voila(1) strain. The precise excision of the transposon simultaneously rescued all these defects whereas its unprecise excision created new pros(V) alleles, including the null allele pros(V17). Here, we describe the relationship between the genetic structure of pros locus, larval locomotion, and larval gustatory response. These two behaviors showed varying degrees of variation depending upon the pros allele. We also found a good relation between behavioral alteration, the level of Pros protein in the embryo, and the degree of disorganization in the larval neuromuscular junction. These data suggest that the complete development of the nervous system requires a full complement of Pros, and that a gradual decrease in the levels of this protein can proportionally alter the development and the function of the nervous system.

Synaptic Drosophila UNC-13 is regulated by antagonistic G-protein pathways via a proteasome-dependent degradation mechanism

UNC-13 is a highly conserved plasma membrane-associated synaptic protein implicated in the regulation of neurotransmitter release through the direct modulation of the SNARE exocytosis complex. Previously, we characterized the Drosophila homologue (DUNC-13) and showed it to be essential for neurotransmitter release immediately upstream of vesicular fusion ("priming") at the neuromuscular junction (NMJ). Here, we show that the abundance of DUNC-13 in NMJ synaptic boutons is regulated downstream of GalphaS and Galphaq pathways, which have inhibitory and facilitatory roles, respectively. Both cAMP modulation and PKA function are required for DUNC-13 synaptic up-regulation, suggesting that the cAMP pathway enhances synaptic efficacy via DUNC-13. Similarly, PLC function and DAG modulation also regulate the synaptic levels of DUNC-13, through a mechanism that appears independent of PKC. Our results suggest that proteasome-mediated protein degradation is the primary mechanism regulating DUNC-13 levels at the synapse. Both PLC- and PKA-mediated pathways appear to regulate synaptic levels of DUNC-13 through controlling the rate of proteasome-dependent DUNC-13 degradation. We conclude that the functional abundance of DUNC-13 at the synapse, a key determinant of synaptic vesicle priming and neurotransmitter release probability, is primarily regulated by the rate of protein degradation, rather than translocation or transport, convergently controlled via both cAMP and DAG signal transduction pathways.

Establishing and sculpting the synapse in Drosophila and C. elegans

Genetic approaches in flies and worms continue to dissect the intricate molecular machinery of chemical synapses. Investigations carried out in the last year provide important new insights into the development and modulation of the presynaptic active zones and postsynaptic receptor fields mediating synaptic function. Mutant screens have identified overlapping gene classes mediating synaptogenesis. The leucocyte common antigen-related receptor tyrosine phosphatase interacts with liprin in the formation of the active zone. Spectrins are essential for the spatial restriction of synaptic proteins to define active zones. Glutamate acts as a negative regulator of its cognate postsynaptic receptor to sculpt receptor field size. Finally, protein translation and degradation regulation emerge as possible key regulators of synaptic efficacy.

Drosophila liprin-alpha and the receptor phosphatase Dlar control synapse morphogenesis

Here, we examine the synaptic function of the receptor protein tyrosine phosphatase (RPTP), Dlar, and an associated intracellular protein, Dliprin-alpha, at the Drosophila larval neuromuscular junction. We show that Dliprin-alpha and Dlar are required for normal synaptic morphology. We also find that synapse complexity is proportional to the amount of Dlar gene product, suggesting that Dlar activity determines synapse size. Ultrastructural analysis reveals that Dliprin-alpha and Dlar are required to define the size and shape of the presynaptic active zone. Accordingly, there is a concomitant decrease in synaptic transmission in both mutants. Finally, epistasis analysis indicates that Dliprin-alpha is required for Dlar's action at the synapse. These data suggest a model where Dliprin-alpha and Dlar cooperate to regulate the formation and/or maintenance of a network of presynaptic proteins.

small bristles is required for the morphogenesis of multiple tissues during Drosophila development

We found that mutations in small bristles (sbr) affect several tissues during the development of the fruit fly. In sbr embryos, neurons have defects in pathfinding and the body wall muscles have defective morphology. As adults, sbr flies have smaller and thinner bristles with a reduced diameter, suggesting a defective cytoskeleton within. The phenotypes we observe are consistent with defects in cell morphogenesis. We identified DmNXF1, the Drosophila homolog of a mRNA export protein that has been characterized in human (NXF1/TAP) and yeast (Mex67p) as the protein encoded by the small bristles locus. Given that a global decrease in mRNA export in these mutants is likely, the phenotypes we observe suggest that certain tissues are acutely sensitive to lower levels of cytoplasmic mRNA and the resultant decrease in protein synthesis during key stages of cellular morphogenesis.

Neurotransmitter levels and synaptic strength at the Drosophila larval neuromuscular junction are not altered by mutation in the sluggish-A gene, which encodes proline oxidase and affects adult locomotion

The sluggish-A (slgA) gene of Drosophila melanogaster has been shown to encode for the enzyme proline oxidase, a mitochondrial enzyme which catalyzes the first step in the conversion of L-proline to L-glutamate. The slgA transcript is expressed in both larval and adult Drosophila melanogaster. Mutations in this gene lead to reduced proline oxidase activity and an elevation of free proline levels. Adult mutant flies show a striking reduction of motor activity. Since proline oxidase may contribute to the supply of the neurotransmitter glutamate in the nervous system, a reduction in proline oxidase activity could reduce neural glutamate pools and affect synaptic transmission in neurons utilizing glutamate as a transmitter, including peripheral motor neurons. We tested the hypothesis that glutamate, and synaptic transmission mediated by glutamate, are reduced at synapses of glutamatergic motor neurons in slgA mutants. Levels of glutamate and proline in different cell compartments, and functional properties of synaptic transmission were compared in slgA and control specimens. Proline is elevated in muscle cells of slgA mutants, indicating that the slgA gene regulates tissue proline levels. In nerve terminal varicosities, proline levels were low in both mutants and controls. Glutamate levels in nerve terminal varicosities of slgA mutants and controls were similar. In addition, we found that glutamatergic synaptic transmission at individual nerve endings and at the whole-cell level was similar in slgA mutants and controls. Thus, proline oxidase does not play a major role in generating neuronal glutamate pools at the Drosophila larval neuromuscular junction, and larval neuromuscular performance is not altered significantly in slgA mutants. Metabolic pathways other than that involving proline oxidase are able to sustain glutamatergic synaptic function in Drosophila larvae.

A genetic programme for neuronal connectivity

What is the nature of the genetic programme that allows neurons to extend their axons and connect to other neurons with a high degree of specificity? Work on the sensory neurons of the fly has shown how the control of neuronal identity is embedded in the general developmental programme of the organism. The ongoing analysis of pathfinding mutants suggests plausible mechanisms for the translation of neuronal identity into axonal behaviour.

Development of specific connectivity between premotor neurons and motoneurons in the brain stem and spinal cord

Astounding progress has been made during the past decade in understanding the general principles governing the development of the nervous system. An area of prime physiological interest that is being elucidated is how the neural circuitry that governs movement is established. The concerted application of molecular biological, anatomical, and electrophysiological techniques to this problem is yielding gratifying insight into how motoneuron, interneuron, and sensory neuron identities are determined, how these different neuron types establish specific axonal projections, and how they recognize and synapse upon each other in patterns that enable the nervous system to exercise precise control over skeletal musculature. This review is an attempt to convey to the physiologist some of the exciting discoveries that have been made, within a context that is intended to link molecular mechanism to behavioral realization. The focus is restricted to the development of monosynaptic connections onto skeletal motoneurons. Principal topics include the inductive mechanisms that pattern the placement and differentiation of motoneurons, Ia sensory afferents, and premotor interneurons; the molecular guidance mechanisms that pattern the projection of premotor axons in the brain stem and spinal cord; and the precision with which initial synaptic connections onto motoneurons are established, with emphasis on the relative roles played by cellular recognition versus electrical activity. It is hoped that this review will provide a guide to understanding both the existing literature and the advances that await this rapidly developing topic.

Ectopic expression of NCAM in skeletal muscle of transgenic mice results in terminal sprouting at the neuromuscular junction and altered structure but not function

The neuromuscular system provides an excellent model for the analysis of molecular interactions involved in the development and plasticity of synaptic contacts. The neural cell adhesion molecule (NCAM) is believed to be involved in the development and plasticity of the neuromuscular junction, in particular the axonal sprouting response observed in paralyzed and denervated muscle. In order to explore the role of myofiber NCAM in modulating the differentiation of motor neurons, we generated transgenic mice expressing a GPI-anchored NCAM isoform that is normally found in developing and denervated muscle, under the control of a skeletal muscle-specific promoter. This results in the constitutive expression of NCAM at postnatal ages, a time when the endogenous mouse NCAM is absent from the myofiber. We found that a significant number of neuromuscular junctions in adult transgenic animals displayed terminal sprouting (>20%) reminiscent of that elicited in response to cessation of neuromuscular activity. Additionally, a significant increase in the size and complexity of neuromuscular synapses as a result of extensive intraterminal sprouting was detected. Electrophysiological studies, however, revealed no significant alterations of neuromuscular transmission at this highly efficient synapse. Sprouting in response to paralysis or following nerve crush was also significantly enhanced in transgenic animals. These results suggest that in this ectopic expression model NCAM can directly modulate synaptic structure and motor neuron-muscle interactions. The results contrast with knockout experiments of the NCAM gene, where very limited changes in the neuromuscular system were observed.

Synaptic localization and restricted diffusion of a Drosophila neuronal synaptobrevin--green fluorescent protein chimera in vivo

Fluorescent markers for subcellular compartments in Drosophila neurons should allow one to combine genetic mutant analysis with visualization of subcellular structures in vivo. Here we describe an analysis of two markers which may be used to observe different compartments of live Drosophila synapses. Soluble jellyfish green fluorescent protein (GFP) expressed at high levels in neurons diffuses freely in the neuronal cytosol as evidenced by confocal microscopy and fluorescence recovery from photobleaching experiments. Thus, the distribution pattern of soluble GFP in motor axons and larval motor terminals indicates the expected distribution for diffusible presynaptic molecules. In contrast to GFP, a neurally expressed neuronal synaptobrevin-GFP chimera (n-syb GFP) is transported down axons and specifically localized to nerve terminals. We demonstrate that n-syb GFP labels synaptic-vesicle membrane at larval motor terminals by documenting its restriction to presynaptic varicosities, its colocalization with synaptic vesicle antigens, and its redistribution in Drosophila shits1 mutant nerve terminals transiently depleted of synaptic vesicles. Surprisingly, n-syb GFP expressed in muscle is concentrated at the subsynaptic reticulum (SSR), postsynaptic infoldings of muscle plasma membrane. We suggest, using different membrane markers, that this apparent postsynaptic enrichment simply reflects a concentration of plasma membrane in the SSR, rather than a selective targeting of n-syb GFP to postsynaptic sites. Utilities and implications of these studies are demonstrated or discussed.

CEPU-1, an immunoglobulin superfamily molecule, has cell adhesion activity and shows dynamic expression patterns in chick embryonic spinal cord

In an attempt to isolate novel molecules involved in motoneuron differentiation and target muscle innervation during embryogenesis, we performed mRNA differential display analysis by comparing cDNAs of motoneurons purified by immunopanning from different portions along the rostro-caudal axis of chick embryonic spinal cord, and cloned an immunoglobulin superfamily protein named C30. By sequence comparison, C30 was shown to be an alternatively spliced isoform of CEPU-1, which was formerly reported as a member of the immunoglobulin superfamily specifically expressed in cerebellar Purkinje cells (Spaltmann and Brummendorf, 1996, J. Neurosci. 16, 1770-1779). We analyzed the expression pattern of CEPU-1 both at the mRNA and protein levels in the spinal cord of the chick embryo. Until stage 23, CEPU-1 was expressed faintly in the ventral part of the neural tube but gradually it became localized to a specific group of cells. In the motor column, CEPU-1 was expressed transiently in many columnar layers. A C30-transfected cell line showed Ca(2+)-independent cell-cell binding activity. These results suggest a role for CEPU-1 in specific axon guidance and/or fasciculation of motoneurons during development.

Nitric oxide and cyclic GMP induce vesicle release at Drosophila neuromuscular junction

Nitric oxide (NO) diffuses as short-lived messenger through the plasma membrane and serves, among many other functions, as an activator of the cGMP synthesizing enzyme soluble guanylyl cyclase (sGC). In view of recent genetic investigations that postulated a retrograde signal from the larval muscle fibers to the presynaptic terminals, we looked for the presence of an NO/cGMP signaling system at the neuromuscular junction (NMJ) of Drosophila melanogaster larvae. Application of NO donors induced cGMP immunoreactivity in the presynaptic terminals but not the postsynaptic muscle fibers at an identified NMJ. The NO-induced cGMP immunoreactivity was sensitive to a specific inhibitor (ODQ) of the sGC. Since presynaptic terminals which were surgically isolated from the central nervous system are capable of synthesizing cGMP, we suggest that an NO-sensitive guanylyl cyclase is present in the terminal arborizations. Using a fluorescent dye that is known to stain recycling synaptic vesicles, we demonstrate that NO donors and membrane permeant cGMP analogues cause vesicle release at the NMJ. Moreover, the NO-induced release could be blocked by the specific inhibitor of the sGC. A destaining of synaptic terminals after NO exposure in Ca2+-free solution in the presence of cobalt chloride as a channel blocker suggested that NO stimulates Ca2+-independent vesicle release at the NMJ. The combined immunocytochemical and exocytosis imaging experiments imply the involvement of cGMP and NO in the regulation of vesicle release at the NMJ of Drosophila larvae.

Developmental expression of nitric oxide/cyclic GMP synthesizing cells in the nervous system of Drosophila melanogaster

Nitric oxide (NO) is a membrane-permeant signaling molecule which activates soluble guanylyl cyclase and leads to the formation of cyclic GMP (cGMP). The NO/cGMP signaling system is thought to play essential roles during the development of vertebrate and invertebrate animals. Here, we analyzed the cellular expression of this signaling pathway during the development of the Drosophila melanogaster nervous system. Using NADPH diaphorase histochemistry as a marker for NO synthase, we identified several neuronal and glial cell types as potential NO donor cells. To label NO-responsive target cells, we used the detection of cGMP by an immunocytochemical technique. Incubation of tissue in an NO donor induced cGMP immunoreactivity (cGMP-IR) in individual motoneurons, sensory neurons, and groups of interneurons of the brain and ventral nerve cord. A dynamic pattern of the cellular expression of NADPHd staining and cGMP-IR was observed during embryonic, larval, and prepupal phases. The expression of NADPH diaphorase and cGMP-IR in distinct neuronal populations of the larval central nervous system (CNS) indicates a role of NO in transcellular signaling within the CNS and as potential retrograde messenger across the neuromuscular junction. In addition, the presence of NADPH diaphorase-positive imaginal discs containing NO-responsive sensory neurons suggests that a transcellular NO/cGMP messenger system can operate between cells of epithelial and neuronal phenotype. The discrete cellular resolution of donor and NO-responsive target cells in identifiable cell types will facilitate the genetic, pharmacological, and physiological analysis of NO/cGMP signal transduction in the developing nervous system of Drosophila.

The kakapo mutation affects terminal arborization and central dendritic sprouting of Drosophila motorneurons

The lethal mutation l(2)CA4 causes specific defects in local growth of neuronal processes. We uncovered four alleles of l(2)CA4 and mapped it to bands 50A-C on the polytene chromosomes and found it to be allelic to kakapo (. Genetics. 146:275- 285). In embryos carrying our kakapo mutant alleles, motorneurons form correct nerve branches, showing that long distance growth of neuronal processes is unaffected. However, neuromuscular junctions (NMJs) fail to form normal local arbors on their target muscles and are significantly reduced in size. In agreement with this finding, antibodies against kakapo (Gregory and Brown. 1998. J. Cell Biol. 143:1271-1282) detect a specific epitope at all or most Drosophila NMJs. Within the central nervous system of kakapo mutant embryos, neuronal dendrites of the RP3 motorneuron form at correct positions, but are significantly reduced in size. At the subcellular level we demonstrate two phenotypes potentially responsible for the defects in neuronal branching: first, transmembrane proteins, which can play important roles in neuronal growth regulation, are incorrectly localized along neuronal processes. Second, microtubules play an important role in neuronal growth, and kakapo appears to be required for their organization in certain ectodermal cells: On the one hand, kakapo mutant embryos exhibit impaired microtubule organization within epidermal cells leading to detachment of muscles from the cuticle. On the other, a specific type of sensory neuron (scolopidial neurons) shows defects in microtubule organization and detaches from its support cells.

Regulated spacing of synapses and presynaptic active zones at larval neuromuscular junctions in different genotypes of the flies Drosophila and Sarcophaga

Synapses at larval neuromuscular junctions of the flies Drosophila melanogaster and Sarcophaga bullata are not distributed randomly. They have been studied in serial electron micrographs of two identified axons (axons 1 and 2) that innervate ventral longitudinal muscles 6 and 7 of the larval body wall. The following fly larvae were examined: axon 1--wild-type Sarcophaga and Drosophila and Drosophila mutants dunce(m14) and fasII(e76), a hypomorphic allele of the fasciclin II gene; and axon 2--drosophila wild-type, dunce(m14), and fasII(e76). These lines were selected to provide a wide range of nerve terminal phenotypes in which to study the distribution and spacing of synapses. Each terminal varicosity is applied closely to the underlying subsynaptic reticulum of the muscle fiber and has 15-40 synapses. Each synapse usually bears one or more active zones, characterized by dense bodies that are T-shaped in cross section; they are located at the presumed sites of transmitter release. The distribution of synapses was characterized from the center-to-center distance of each synapse to its nearest neighbor. The mean spacing between nearest-neighbor pairs ranged from 0.84 microm to 1.05 microm for axon 1, showing no significant difference regardless of genotype. The corresponding values for axon 2, 0.58 microm to 0.75 microm, were also statistically indistinguishable from one another in terminals of different genotype but differed significantly from the values for axon 1. Thus, the functional class of the axon provides a clear prediction of the spacing of its synapses, suggesting that spacing may be determined by the functional properties of transmission at the two types of terminals. Individual dense bodies were situated mostly at least 0.4 microm away from one another, suggesting that an interaction between neighboring active zones could prevent their final positions from being located more closely.

The transmembrane Semaphorin Sema I is required in Drosophila for embryonic motor and CNS axon guidance

The semaphorins comprise a large family of conserved glycoproteins, several members of which have been shown to function in repulsive neuronal growth cone guidance. We show here that Drosophila Semaphorin I (Sema I), a transmembrane semaphorin expressed on embryonic motor and CNS axons, is required for correct guidance of motor axons and for the formation of CNS pathways. In mutant embryos lacking Sema I, motor axons stall and fail to defasciculate at specific choice points where normally they would project to their muscle targets. In addition, a specific CNS fascicle fails to form correctly in these embryos. Rescue and ectopic expression experiments show that Sema I is required in neurons to mediate axon guidance decisions. These studies further suggest that like secreted semaphorins, transmembrane semaphorins can function as repulsive guidance cues for specific axon guidance events during neurodevelopment.

Drosophila Rac1 controls motor axon guidance

Previous genetic studies of intersegmental nerve b development have identified several cell-surface proteins required for correct axon guidance to appropriate target muscles. Here we provide evidence that the small GTPase Drac1 also plays a key role in this guidance process. Neuronal expression of the dominant negative mutation Drac1(N17) causes axons to bypass and extend beyond normal synaptic partners. This phenotype is consistently reproduced by pharmacological blockade of actin assembly. Genetic interactions between Drac1(N17) and the receptor-tyrosine phosphatase Dlar suggest that intersegmental nerve b guidance requires the integration of multiple, convergent signals.

Retrograde signaling in the development and modification of synapses

Retrograde signaling from the postsynaptic cell to the presynaptic neuron is essential for the development, maintenance, and activity-dependent modification of synaptic connections. This review covers various forms of retrograde interactions at developing and mature synapses. First, we discuss evidence for early retrograde inductive events during synaptogenesis and how maturation of presynaptic structure and function is affected by signals from the postsynaptic cell. Second, we review the evidence that retrograde interactions are involved in activity-dependent synapse competition and elimination in developing nervous systems and in long-term potentiation and depression at mature synapses. Third, we review evidence for various forms of retrograde signaling via membrane-permeant factors, secreted factors, and membrane-bound factors. Finally, we discuss the evidence and physiological implications of the long-range propagation of retrograde signals to the cell body and other parts of the presynaptic neuron.

Imaging of calcium in Drosophila larval motor nerve terminals

Calcium measurements in the presynaptic terminal are essential in the investigation of mechanisms underlying neurotransmitter release. To enhance the genetic analysis of secretory mechanisms, we have developed Ca2+ imaging techniques for Drosophila larval motor nerve terminals. We studied Ca2+ signals in "big" (type Ib) and "small" (type Is) boutons that innervate ventral longitudinal muscles 6 and 7 in each abdominal segment of Canton-S (CS)-strain 3rd instar larvae. The indicator fluo-3 in conjunction with confocal microscopy was used to detect stimulus-dependent changes in [Ca2+]i. The Ca2+ signals were reliable and reproducible, and the resting fluorescence remained constant throughout the experiments. The Ca2+ signals increased with stimulus frequency from 5 to 20 Hz for both bouton types. No significant differences in the Ca2+ signals were seen between the two bouton types at 5 and 20 Hz, but there was a difference at 10 Hz. The decay of the Ca2+ signal was more prolonged after 20-Hz stimulation than after 5 and 10 Hz. At the single-synapse level, the secretory efficacy of Is synapses is greater than that of Ib synapses, but our data show that factors other than differences in Ca2+ entry may govern the strength of synaptic transmission.

Intercellular communication that mediates formation of the neuromuscular junction

Reciprocal signals between the motor axon and myofiber induce structural and functional differentiation in the developing neuromuscular junction (NMJ). Elevation of presynaptic acetylcholine (ACh) release on nerve-muscle contact and the correlated increase in axonal-free calcium are triggered by unidentified membrane molecules. Restriction of axon growth to the developing NMJ and formation of active zones for ACh release in the presynaptic terminal may be induced by molecules in the synaptic basal lamina, such as S-laminin, heparin binding growth factors, and agrin. Acetylcholine receptor (AChR) synthesis by muscle cells may be increased by calcitonin gene-related peptide (CGRP), ascorbic acid, and AChR-inducing activity (ARIA)/heregulin, which is the best-established regulator. Heparin binding growth factors, proteases, adhesion molecules, and agrin all may be involved in the induction of AChR redistribution to form postsynaptic-like aggregates. However, the strongest case has been made for agrin's involvement. "Knockout" experiments have implicated agrin as a primary anterograde signal for postsynaptic differentiation and muscle-specific kinase (MuSK), as a putative agrin receptor. It is likely that both presynaptic and postsynaptic differentiation are induced by multiple molecular signals. Future research should reveal the physiological roles of different molecules, their interactions, and the identity of other molecular participants.

Neuromuscular target recognition by a homophilic interaction of connectin cell adhesion molecules in Drosophila

Drosophila Connectin (CON) is a cell surface protein of the leucine-rich repeat family. During the formation of neuromuscular connectivity, CON is expressed on the surface of a subset of embryonic muscles and on the growth cones and axons of the motoneurons that innervate these muscles, including primarily SNa motoneurons and their synaptic targets (lateral muscles). In vitro, CON can mediate homophilic cell adhesion. In this study, we generated transgenic lines that ectopically expressed CON on all muscles. In the transformant embryos and larvae, SNa motoneurons often inappropriately innervated a neighboring non-target muscle (muscle 12) that ectopically expressed CON. Furthermore, the ectopic synapse formation was dependent on the endogenous CON expression on the SNa motoneurons. These results show that CON can function as an attractive and homophilic target recognition molecule in vivo.

Genetic analysis of the larval optic nerve projection in Drosophila

The Drosophila larval optic nerve, called Bolwig's nerve (BN), projects into the central brain along a simple invariant path. The growth of the BN proceeds in three phases, during which the nerve changes direction at two intermediate targets, P1 and P2. Here we show that the projection of the BN is amenable to genetic dissection. In a mutagenesis screen, we have isolated mutations in 13 genes that disrupt the BN projection in distinct phases of its development. The mutant phenotypes in combination with the expression patterns of corresponding candidate genes define cellular components necessary for directing the growth of the BN toward P2 and for redirecting its growth at P2, and reveal developmental strategies employed in the establishment of the BN projection.

cdh-3, a gene encoding a member of the cadherin superfamily, functions in epithelial cell morphogenesis in Caenorhabditis elegans

Several genes that encode members of the cadherin superfamily have been identified in Caenorhabditis elegans. Based on the roles of cadherins in vertebrates and Drosophila, it is expected that they function in the control of epithelial morphogenesis, an event which is poorly understood at the molecular level in C. elegans. Reporter genes under the control of upstream sequences from one of these genes, cdh-3, are expressed in developing epithelial cells, but also in a number of neuroectodermal cells that extend processes along some of these epithelial cells. We generated a loss-of-function mutation in cdh-3 by transposon-mediated deletion mutagenesis. This mutation affects the morphogenesis of a single cell, hyp10, which forms the tip of the nematode tail. The lack of detectable defects associated with the other cells expressing cdh-3 reporter constructs hints at the existence of other genes that can compensate for cdh-3 loss of function.

Traffic of dynamin within individual Drosophila synaptic boutons relative to compartment-specific markers

Presynaptic terminals contain several specialized compartments, which have been described by electron microscopy. We show in an identified Drosophila neuromuscular synapse that several of these compartments-synaptic vesicle clusters, presynaptic plasma membrane, presynaptic cytosol, and axonal cytoskeleton-labeled by specific reagents may be resolved from one another by laser scanning confocal microscopy. Using a panel of compartment-specific markers and Drosophila shibire(ts1) mutants to trap an intermediate stage in synaptic vesicle recycling, we have examined the localization and redistribution of dynamin within single synaptic varicosities at the larval neuromuscular junction. Our results suggest that dynamin is not a freely diffusible molecule in resting nerve terminals; rather, it appears localized to synaptic sites by association with yet uncharacterized presynaptic components. In shi(ts1) nerve terminals depleted of synaptic vesicles, dynamin is quantitatively redistributed to the plasma membrane. It is not, however, distributed uniformly over presynaptic plasmalemma; instead, fluorescence images show "hot spots" of dynamin on the plasma membrane of vesicle-depleted nerve terminals. We suggest that these dynamin-rich domains may mark the active zones for synaptic vesicle endocytosis first described at the frog neuromuscular junction.

Presynaptic development at the Drosophila neuromuscular junction: assembly and localization of presynaptic active zones

We describe the extent to which presynaptic structures at the embryonic neuromuscular junction of Drosophila can form in mutants where development of postsynaptic somatic muscles is affected. Although twist mutant embryos lack mesoderm, motor axons still grow out of the CNS and form morphologically normal presynaptic active zones, independent of their target cells. In myoblast city mutant embryos, myoblasts do not fuse but form fully differentiated mononucleate muscles, which make functional neuromuscular synapses with correctly localized presynaptic active zones. Myoblasts also fail to fuse but still attract appropriate innervation in mef2 mutant embryos. However, these myoblasts fail to differentiate into muscles and presynaptic active zones fail to localize at neuromuscular contacts. Thus, the process of synapse formation can be genetically separated from the process of target recognition, revealing that localization of presynaptic active zones requires mef2-dependent muscle differentiation.

New views on synapse-glia interactions

Although glial cells ensheath synapses throughout the nervous system, the functional consequences of this relationship are uncertain. Recent studies suggest that glial cells may promote the formation of synapses and help to maintain their function by providing nerve terminals with energy substrates and glutamate precursors.

A neural tetraspanin, encoded by late bloomer, that facilitates synapse formation

Upon contacting its postsynaptic target, a neuronal growth cone transforms into a presynaptic terminal. A membrane component on the growth cone that facilitates synapse formation was identified by means of a complementary DNA-based screen followed by genetic analysis. The late bloomer (lbl) gene in Drosophila encodes a member of the tetraspanin family of cell surface proteins. LBL protein is transiently expressed on motor axons, growth cones, and terminal arbors. In lbl mutant embryos, the growth cone of the RP3 motoneuron contacts its target muscles, but synapse formation is delayed and neighboring motoneurons display an increase in ectopic sprouting.

Receptor tyrosine phosphatases are required for motor axon guidance in the Drosophila embryo

The receptor tyrosine phosphatases DPTP69D and DPTP99A are expressed on motor axons in Drosophila embryos. In mutant embryos lacking DPTP69D protein, motor neuron growth cones stop growing before reaching their muscle targets, or follow incorrect pathways that bypass these muscles. Mutant embryos lacking DPTP99A are indistinguishable from wild type. Motor axon defects in dptp69D dptp99A double mutant embryos, however, are much more severe than in embryos lacking only DPTP69D. Our results demonstrate that DPTP69D and DPTP99A are required for motor axon guidance and that they have partially redundant functions during development of the neuro-muscular system.