Central afferent projections of proprioceptive sensory neurons in Drosophila revealed with the enhancer-trap technique

Mechanosensory bristles mediate avoidance behavior by triggering sustained local motor activity in Drosophila melanogaster

During locomotion, most vertebrates-and invertebrates such as Drosophila melanogaster-are able to quickly adapt to terrain irregularities or avoid physical threats by integrating sensory information along with motor commands. Key to this adaptability are leg mechanosensory structures, which assist in motor coordination by transmitting external cues and proprioceptive information to motor centers in the central nervous system. Nevertheless, how different mechanosensory structures engage these locomotor centers remains poorly understood. Here, we tested the role of mechanosensory structures in movement initiation by optogenetically stimulating specific classes of leg sensory structures. We found that stimulation of leg mechanosensory bristles (MsBs) and the femoral chordotonal organ (ChO) is sufficient to initiate forward movement in immobile animals. While the stimulation of the ChO required brain centers to induce forward movement, unexpectedly, brief stimulation of leg MsBs triggered a fast response and sustained motor activity dependent only on the ventral nerve cord (VNC). Moreover, this leg-MsB-mediated movement lacked inter- and intra-leg coordination but preserved antagonistic muscle activity within joints. Finally, we show that leg-MsB activation mediates strong avoidance behavior away from the stimulus source, which is preserved even in the absence of a central brain. Overall, our data show that mechanosensory stimulation can elicit a fast motor response, independently of central brain commands, to evade potentially harmful stimuli. In addition, it sheds light on how specific sensory circuits modulate motor control, including initiation of movement, allowing a better understanding of how different levels of coordination are controlled by the VNC and central brain locomotor circuits.

Divergent neural circuits for proprioceptive and exteroceptive sensing of the Drosophila leg

Somatosensory neurons provide the nervous system with information about mechanical forces originating inside and outside the body. Here, we use connectomics to reconstruct and analyze neural circuits downstream of the largest somatosensory organ in the Drosophila leg, the femoral chordotonal organ (FeCO). The FeCO has been proposed to support both proprioceptive sensing of the fly's femur-tibia joint and exteroceptive sensing of substrate vibrations, but it remains unknown which sensory neurons and central circuits contribute to each of these functions. We found that different subtypes of FeCO sensory neurons feed into distinct proprioceptive and exteroceptive pathways. Position- and movement-encoding FeCO neurons connect to local leg motor control circuits in the ventral nerve cord (VNC), indicating a proprioceptive function. In contrast, signals from the vibration-encoding FeCO neurons are integrated across legs and transmitted to auditory regions in the brain, indicating an exteroceptive function. Overall, our analyses reveal the structure of specialized circuits for processing proprioceptive and exteroceptive signals from the fly leg. They also demonstrate how analyzing patterns of synaptic connectivity can distill organizing principles from complex sensorimotor circuits.

Mechanosensory pathways of scorpion pecten hair sensillae-Adjustment of body height and pecten position

Scorpions' sensory abilities are intriguing, especially the rather enigmatic ventral comb-like chemo- and mechanosensory organs, the so-called pectines. Attached ventrally to the second mesosomal segment just posterior to the coxae of the fourth walking leg pair, the pectines consist of the lamellae, the fulcra, and a variable number of pecten teeth. The latter contain the bimodal peg sensillae, used for probing the substrate with regard to chemo- and mechanosensory cues simultaneously. In addition, the lamellae, the fulcra and the pecten teeth are equipped with pecten hair sensillae (PHS) to gather mechanosensory information. Previously, we have analyzed the neuronal pathway associated with the peg sensillae unraveling their somatotopic projection pattern in dedicated pecten neuropils. Little is known, however, regarding the projections of PHS within the scorpion nervous system. Behavioral and electrophysiological assays showed involvement of PHS in reflexive responses but how the information is integrated remains unresolved. Here, we unravel the innervation pattern of the mechanosensory pecten hair afferents in Mesobuthus eupeus and Euscorpius italicus. By using immunofluorescent labeling and injection of Neurobiotin tracer, we identify extensive arborizations of afferents, including (i) ventral neuropils, (ii) somatotopically organized multisegmental sensory tracts, (iii) contralateral branches via commissures, and (iv) direct ipsilateral innervation of walking leg neuromeres 3 and 4. Our results suggest that PHS function as sensors to elicit reflexive adjustment of body height and obstacle avoidance, mediating accurate pecten teeth alignment to guarantee functionality of pectines, which are involved in fundamental capacities like mating or navigation.

Multisensory interactions regulate feeding behavior in Drosophila

The integration of two or more distinct sensory cues can help animals make more informed decisions about potential food sources, but little is known about how feeding-related multimodal sensory integration happens at the cellular and molecular levels. Here, we show that multimodal sensory integration contributes to a stereotyped feeding behavior in the model organism Drosophila melanogaster Simultaneous olfactory and mechanosensory inputs significantly influence a taste-evoked feeding behavior called the proboscis extension reflex (PER). Olfactory and mechanical information are mediated by antennal Or35a neurons and leg hair plate mechanosensory neurons, respectively. We show that the controlled delivery of three different sensory cues can produce a supra-additive PER via the concurrent stimulation of olfactory, taste, and mechanosensory inputs. We suggest that the fruit fly is a versatile model system to study multisensory integration related to feeding, which also likely exists in vertebrates.

Neural Coding of Leg Proprioception in Drosophila

Animals rely on an internal sense of body position and movement to effectively control motor behavior. This sense of proprioception is mediated by diverse populations of mechanosensory neurons distributed throughout the body. Here, we investigate neural coding of leg proprioception in Drosophila, using in vivo two-photon calcium imaging of proprioceptive sensory neurons during controlled movements of the fly tibia. We found that the axons of leg proprioceptors are organized into distinct functional projections that contain topographic representations of specific kinematic features. Using subclass-specific genetic driver lines, we show that one group of axons encodes tibia position (flexion/extension), another encodes movement direction, and a third encodes bidirectional movement and vibration frequency. Overall, our findings reveal how proprioceptive stimuli from a single leg joint are encoded by a diverse population of sensory neurons, and provide a framework for understanding how proprioceptive feedback signals are used by motor circuits to coordinate the body.

Topological and modality-specific representation of somatosensory information in the fly brain

Insects and mammals share similarities of neural organization underlying the perception of odors, taste, vision, sound, and gravity. We observed that insect somatosensation also corresponds to that of mammals. In Drosophila, the projections of all the somatosensory neuron types to the insect's equivalent of the spinal cord segregated into modality-specific layers comparable to those in mammals. Some sensory neurons innervate the ventral brain directly to form modality-specific and topological somatosensory maps. Ascending interneurons with dendrites in matching layers of the nerve cord send axons that converge to respective brain regions. Pathways arising from leg somatosensory neurons encode distinct qualities of leg movement information and play different roles in ground detection. Establishment of the ground pattern and genetic tools for neuronal manipulation should provide the basis for elucidating the mechanisms underlying somatosensation.

Simultaneous activation of parallel sensory pathways promotes a grooming sequence in Drosophila

A central model that describes how behavioral sequences are produced features a neural architecture that readies different movements simultaneously, and a mechanism where prioritized suppression between the movements determines their sequential performance. We previously described a model whereby suppression drives a Drosophila grooming sequence that is induced by simultaneous activation of different sensory pathways that each elicit a distinct movement (Seeds et al., 2014). Here, we confirm this model using transgenic expression to identify and optogenetically activate sensory neurons that elicit specific grooming movements. Simultaneous activation of different sensory pathways elicits a grooming sequence that resembles the naturally induced sequence. Moreover, the sequence proceeds after the sensory excitation is terminated, indicating that a persistent trace of this excitation induces the next grooming movement once the previous one is performed. This reveals a mechanism whereby parallel sensory inputs can be integrated and stored to elicit a delayed and sequential grooming response.

Mechanosensation and Adaptive Motor Control in Insects

The ability of animals to flexibly navigate through complex environments depends on the integration of sensory information with motor commands. The sensory modality most tightly linked to motor control is mechanosensation. Adaptive motor control depends critically on an animal's ability to respond to mechanical forces generated both within and outside the body. The compact neural circuits of insects provide appealing systems to investigate how mechanical cues guide locomotion in rugged environments. Here, we review our current understanding of mechanosensation in insects and its role in adaptive motor control. We first examine the detection and encoding of mechanical forces by primary mechanoreceptor neurons. We then discuss how central circuits integrate and transform mechanosensory information to guide locomotion. Because most studies in this field have been performed in locusts, cockroaches, crickets, and stick insects, the examples we cite here are drawn mainly from these 'big insects'. However, we also pay particular attention to the tiny fruit fly, Drosophila, where new tools are creating new opportunities, particularly for understanding central circuits. Our aim is to show how studies of big insects have yielded fundamental insights relevant to mechanosensation in all animals, and also to point out how the Drosophila toolkit can contribute to future progress in understanding mechanosensory processing.

Parallel Transformation of Tactile Signals in Central Circuits of Drosophila

To distinguish between complex somatosensory stimuli, central circuits must combine signals from multiple peripheral mechanoreceptor types, as well as mechanoreceptors at different sites in the body. Here, we investigate the first stages of somatosensory integration in Drosophila using in vivo recordings from genetically labeled central neurons in combination with mechanical and optogenetic stimulation of specific mechanoreceptor types. We identify three classes of central neurons that process touch: one compares touch signals on different parts of the same limb, one compares touch signals on right and left limbs, and the third compares touch and proprioceptive signals. Each class encodes distinct features of somatosensory stimuli. The axon of an individual touch receptor neuron can diverge to synapse onto all three classes, meaning that these computations occur in parallel, not hierarchically. Representing a stimulus as a set of parallel comparisons is a fast and efficient way to deliver somatosensory signals to motor circuits.

Associative learning between odorants and mechanosensory punishment in larval Drosophila

We tested whether Drosophila larvae can associate odours with a mechanosensory disturbance as a punishment, using substrate vibration conveyed by a loudspeaker (buzz:). One odour (A) was presented with the buzz, while another odour (B) was presented without the buzz (A/B training). Then, animals were offered the choice between A and B. After reciprocal training (A/B), a second experimental group was tested in the same way. We found that larvae show conditioned escape from the previously punished odour. We further report an increase of associative performance scores with the number of punishments, and an increase according to the number of training cycles. Within the range tested (between 50 and 200 Hz), however, the pitch of the buzz does not apparently impact associative success. Last, but not least, we characterized odour-buzz memories with regard to the conditions under which they are behaviourally expressed--or not. In accordance with what has previously been found for associative learning between odours and bad taste (such as high concentration salt or quinine), we report that conditioned escape after odour-buzz learning is disabled if escape is not warranted, i.e. if no punishment to escape from is present during testing. Together with the already established paradigms for the association of odour and bad taste, the present assay offers the prospect of analysing how a relatively simple brain orchestrates memory and behaviour with regard to different kinds of 'bad' events.

Engrailed expression in subsets of adult Drosophila sensory neurons: an enhancer-trap study

Engrailed (En) has an important role in neuronal development in vertebrates and invertebrates. In adult Drosophila, although En expression persists throughout adulthood, a detailed description of its expression in sensory neurons has not been made. In this study, en-GAL4 was used to drive UAS-CD8::GFP expression and the projections of sensory neurons were examined with confocal microscopy. En protein expression was confirmed using immunocytochemistry. In the antenna, En is present in subsets of Johnston's organ neurons and of olfactory neurons. En-driven GFP is expressed in axons projecting to 18 identified olfactory glomeruli, originating from basiconic, trichoid and coeloconic sensilla. In most cases both neurons of a sensillum express En. En expression overlaps with that of Acj6, another transcription factor. En-driven GFP is also expressed in a subset of maxillary palp olfactory neurons and in all mechanosensory and gustatory sensilla in the posterior compartment of the labial palps. In the legs and halteres, en-driven GFP is expressed in only a subset of the sensory neurons of different modalities that arise in the posterior compartment. Finally, en-driven GFP is expressed in a single multidendritic sensory neuron in each abdominal segment.

Atypical expression of Drosophila gustatory receptor genes in sensory and central neurons

Members of the Drosophila gustatory receptor (Gr) gene family are generally expressed in chemosensory neurons and are known to mediate the perception of sugars, bitter substrates, CO(2), and pheromones. The Gr gene family consists of 68 members, many of which are organized in gene clusters of up to six genes, yet only expression of about 15 Gr genes has been characterized in detail prior to this study. Here we describe the first comprehensive expression analysis of six highly conserved Gr genes, Gr28a and Gr28b.a to Gr28b.e. Four of these Gr genes are not only expressed in the characteristic pattern associated with previously analyzed Gr genes-chemosensory neurons of the gustatory and olfactory system-but several other types of sensory neurons and neurons in the brain. Specifically, we show that several of the Gr28 genes are expressed in abdominal multidendritic neurons, putative hygroreceptive neurons of the arista, neurons associated with the Johnston's organ, peripheral proprioceptive neurons in the legs, neurons in the larval and adult brain, and oenocytes. Thus, our findings suggest that some Gr genes are utilized in nongustatory roles in the nervous system and tissues involved in proprioception, hygroreception, and other sensory modalities. It is also possible that the Gr28 genes have chemosensory roles in the detection of internal ligands.

Nociceptive neurons protect Drosophila larvae from parasitoid wasps

BACKGROUND

Natural selection has resulted in a complex and fascinating repertoire of innate behaviors that are produced by insects. One puzzling example occurs in fruit fly larvae that have been subjected to a noxious mechanical or thermal sensory input. In response, the larvae "roll" with a motor pattern that is completely distinct from the style of locomotion that is used for foraging.

RESULTS

We have precisely mapped the sensory neurons that are used by the Drosophila larvae to detect nociceptive stimuli. By using complementary optogenetic activation and targeted silencing of sensory neurons, we have demonstrated that a single class of neuron (class IV multidendritic neuron) is sufficient and necessary for triggering the unusual rolling behavior. In addition, we find that larvae have an innately encoded preference in the directionality of rolling. Surprisingly, the initial direction of rolling locomotion is toward the side of the body that has been stimulated. We propose that directional rolling might provide a selective advantage in escape from parasitoid wasps that are ubiquitously present in the natural environment of Drosophila. Consistent with this hypothesis, we have documented that larvae can escape the attack of Leptopilina boulardi parasitoid wasps by rolling, occasionally flipping the attacker onto its back.

CONCLUSIONS

The class IV multidendritic neurons of Drosophila larvae are nociceptive. The nociception behavior of Drosophila melanagaster larvae includes an innately encoded directional preference. Nociception behavior is elicited by the ecologically relevant sensory stimulus of parasitoid wasp attack.

Development of Johnston's organ in Drosophila

Hearing is a specialized mechanosensory modality that is refined during evolution to meet the particular requirements of different organisms. In the fruitfly, Drosophila, hearing is mediated by Johnston's organ, a large chordotonal organ in the antenna that is exquisitely sensitive to the near-field acoustic signal of courtship songs generated by male wing vibration. We summarize recent progress in understanding the molecular genetic determinants of Johnston's organ development and discuss surprising differences from other chordotonal organs that likely facilitate hearing. We outline novel discoveries of active processes that generate motion of the antenna for acute sensitivity to the stimulus. Finally, we discuss further research directions that would probe remaining questions in understanding Johnston's organ development, function and evolution.

Cellular mechanisms of dendrite pruning in Drosophila: insights from in vivo time-lapse of remodeling dendritic arborizing sensory neurons

Regressive events that refine exuberant or inaccurate connections are critical in neuronal development. We used multi-photon, time-lapse imaging to examine how dendrites of Drosophila dendritic arborizing (da) sensory neurons are eliminated during early metamorphosis, and how intrinsic and extrinsic cellular mechanisms control this deconstruction. Removal of the larval dendritic arbor involves two mechanisms: local degeneration and branch retraction. In local degeneration, major branch severing events entail focal disruption of the microtubule cytoskeleton, followed by thinning of the disrupted region, severing and fragmentation. Retraction was observed at distal tips of branches and in proximal stumps after severing events. The pruning program of da neuron dendrites is steroid induced; cell-autonomous dominant-negative inhibition of steroid action blocks local degeneration,although retraction events still occur. Our data suggest that steroid-induced changes in the epidermis may contribute to dendritic retraction. Finally, we find that phagocytic blood cells not only engulf neuronal debris but also attack and sever intact branches that show signs of destabilization.

Male-specific fruitless specifies the neural substrates of Drosophila courtship behaviour

Robust innate behaviours are attractive systems for genetically dissecting how environmental cues are perceived and integrated to generate complex behaviours. During courtship, Drosophila males engage in a series of innate, stereotyped behaviours that are coordinated by specific sensory cues. However, little is known about the specific neural substrates mediating this complex behavioural programme. Genetic, developmental and behavioural studies have shown that the fruitless (fru) gene encodes a set of male-specific transcription factors (FruM) that act to establish the potential for courtship in Drosophila. FruM proteins are expressed in approximately 2% of central nervous system neurons, at least one subset of which coordinates the component behaviours of courtship. Here we have inserted the yeast GAL4 gene into the fru locus by homologous recombination and show that (1) FruM is expressed in subsets of all peripheral sensory systems previously implicated in courtship, (2) inhibition of FruM function in olfactory system components reduces olfactory-dependent changes in courtship behaviour, (3) transient inactivation of all FruM-expressing neurons abolishes courtship behaviour, with no other gross changes in general behaviour, and (4) 'masculinization' of FruM-expressing neurons in females is largely sufficient to confer male courtship behaviour. Together, these data demonstrate that FruM proteins specify the neural substrates of male courtship.

Mechanisms of dendritic elaboration of sensory neurons in Drosophila: insights from in vivo time lapse

In vivo time-lapse multiphoton microscopy was used to analyze the remodeling of the dendritic arborizing (da) sensory neuron known as dorsal dendritic arborizing neuron E (ddaE) during metamorphosis. After its larval processes have been removed, the cell body of ddaE repositions itself on the body wall between 25 and 40 hr after puparium formation (APF) and begins its adult outgrowth at 40 hr APF. The scaffold of the arbor is laid down between 40 and 54 hr APF, when growth is characterized by high filopodial activity at both terminal and interstitial positions and by branch retraction along with branch establishment. Later in development, filopodial activity remains high but is confined to terminal branches, and branch retraction is no longer seen. Treatment with the insect hormone juvenile hormone (JH), a key regulator of metamorphosis, alters the shape and complexity of the adult dendritic tree in a time-dependent manner. Early treatments with juvenile hormone mimic (JHm) appear to repress extension programs and maintain retraction programs. With later JHm treatments, extension programs appear normal, but retraction programs are maintained beyond their normal time. The JH treatments show the importance of retraction programs in establishing the overall arbor shape.

Persistent larval sensory neurones are required for the normal development of the adult sensory afferent projections inDrosophila

We have tested the hypothesis that larval neurones guide growth of adult sensory axons in Drosophila. We show that ablation of larval sensory neurones causes defects in the central projections of adult sensory neurones. Spiralling axons and ectopic projections indicate failure in axon growth guidance. We show that larval sensory neurones are required for peripheral pathfinding, entry into the CNS and growth guidance within the CNS. Ablation of subsets of neurones shows that larval sensory neurones serve specific guidance roles. Dorsal neurones are required for axon guidance across the midline, whereas lateral neurones are required for posterior growth. We conclude that larval sensory neurones pioneer the assembly of sensory arrays in adults.

Confocal quality imaging of afferent neurons from semi-thin sections of Drosophila ganglia

The aim of this study was to develop protocols for computer imaging of the thoraco-abdominal ganglion of Drosophila from serial semi-thin sections, in which specific neurons were stained and related to neuropilar structures. The central projections of a subset of transgenically labelled sensory neurons were revealed by immunohistochemistry, while Nomarski optics were used to show motor neuron targets in the neuropil. Digital photomicrographs of each section were aligned and the resultant image stacks rendered into three-dimensional (3D) images that can be rotated in real time. The result is a detailed, in-depth visualization of labelled neurons at a resolution comparable with that in confocal reconstructions, which also allows investigation of their relationships with other components of the neuropil.

Behavioral transformations during metamorphosis: remodeling of neural and motor systems

During insect metamorphosis, neural and motor systems are remodeled to accommodate behavioral transformations. Nerve and muscle cells that are required for larval behavior, such as crawling, feeding and ecdysis, must either be replaced or respecified to allow adult emergence, walking, flight, mating and egg-laying. This review describes the types of cellular changes that occur during metamorphosis, as well as recent attempts to understand how they are related to behavioral changes and how they are regulated. Within the periphery, many larval muscles degenerate at the onset of metamorphosis and are replaced by adult muscles, which are derived from myoblasts and, in some cases, remnants of the larval muscle fibers. The terminal processes of many larval motoneurons persist within the periphery and are essential for the formation of adult muscle fibers. Although most adult sensory neurons are born postembryonically, a subset of larval proprioceptive neurons persist to participate in adult behavior. Within the central nervous system, larval neurons that will no longer be necessary die and some adult interneurons are born postembryonically. By contrast, all of the adult motoneurons, as well as some interneurons and modulatory neurons, are persistent larval cells. In accordance with their new behavioral roles, these neurons undergo striking changes in dendritic morphology, intrinsic biophysical properties, and synaptic interactions.

Methods for imaging labeled neurons together with neuropil features in Drosophila

We describe staining protocols for serial semithin sections of Drosophila central ganglia that allow visualization of gene expression in particular neurons with counterstaining to display the ganglion architecture. Green fluorescent protein (GFP), expressed in a subset of sensory neurons from a selected enhancer trap line, is visualized by conventional immunohistochemistry with a peroxidase-linked antibody, and neural architecture is revealed by reduced silver staining. This makes visible in histological sections the same GFP-labeled cells seen with confocal microscopy, but with the especial advantage that neuropil structures are also revealed at the level of individual cells and neuron processes. Not only does this allow the physical relationships among intracellularly labeled neurons to be determined by reference to specific features in the neuropil but it also enables a function to be ascribed provisionally to particular regions of neuropil. These methods have particular utility for mapping morphological information on specific neurons in the context of central nervous system architecture, both in adult Drosophila and during development.

Neuroanatomy of cells expressing clock genes in Drosophila: transgenic manipulation of the period and timeless genes to mark the perikarya of circadian pacemaker neurons and their projections

Subsets of brain neurons expressing the clock genes period (per) and timeless (tim) are involved in the generation of circadian behavioral rhythms. However, current knowledge of projection patterns of these neurons is limited to those immunoreactive to an antibody against a crustacean neuropeptide. The GAL4-expression system was utilized to visualize neuronal processes from all per and tim-expressing neurons in the central nervous system. Each of two types of GAL4-driver fusion genes, per-gal4 or tim-gal4, was combined in transgenic flies with marker genes-lacZ, and sequences encoding green fluorescent protein or TAU protein-under the control of the GAL4-responsive element UAS. This allowed visualization of the cytoplasm of GAL4-expressing cells. Thus, neurites of clock neurons in the adult brain as well as those of larvae and pupae were revealed. Among the anatomical patterns revealed by per-gal4- or tim-gal4-driven marker expression were a previously unknown, dorsally located neuronal cluster, along with the projections of these cells and of other dorsal neurons characterized in earlier studies only by the location of their perikarya. The similarity of projections from PER- or TIM-containing neurons during development to those in the adult implies that these features of mature clock neurons are established by the larval stages. Neurons that have never been identified as PER- or TIM-immunoreactive were also visualized in this assay system, indicating promoter activity of the clock genes in these cells and suggesting that their products cannot accumulate to detectable levels in certain neurons.

Remodeling of the leg sensory system during metamorphosis of the hawkmoth, Manduca sexta

The adult legs of the hawkmoth Manduca sexta are supplied by a diverse array of sensory organs and associated neurons (Kent and Griffin [1990] Cell Tissue Res. 259:209-223) that differ from those in the larval legs. In the present study, a combination of nerve-tracing techniques [biocytin, 1,1;-dioctadecyl-3,3,3;, 3;-tetramethyl-indocarbocyanine perchlorate (DiI)], birth date labeling (5-bromodeoxyuridine), confocal microscopy, and electrophysiology were used to describe the remodeling of the prothoracic leg sensory system. Four primary sensory branches carry the axons of all of the sensory neurons in the larval leg. At the onset of metamorphosis, the imaginal leg epidermis develops underneath the larval cuticle and encircles the sensory neurons, thus separating them from their target-organs. Most of the larval neurons degenerate during the larval-to-pupal transition and are replaced by new-adult sensory neurons that are born and differentiate in the pupa. Six sensory neurons that supply hair sensilla in the larval leg, together with 13 femoral and tibial chordotonal organ neurons, persist into the developing adult leg to serve similar functions. Early in the pupal stage, electrical activity can be recorded from these neurons despite the absence of target sensory structures. During the differentiation of the adult sensory system, the axons of the new-adult sensory neurons contact and fasciculate with the axons of the persistent neurons. Thus, five of the primary sensory branches of the adult leg are built on the preexisting larval sensory trajectories. Two sensory branches, however, are established de novo by the axons of specific adult sensory neurons.

Dynein-dynactin function and sensory axon growth during Drosophila metamorphosis: A role for retrograde motors

Mutations in the genes for components of the dynein-dynactin complex disrupt axon path finding and synaptogenesis during metamorphosis in the Drosophila central nervous system. In order to better understand the functions of this retrograde motor in nervous system assembly, we analyzed the path finding and arborization of sensory axons during metamorphosis in wild-type and mutant backgrounds. In wild-type specimens the sensory axons first reach the CNS 6-12 h after puparium formation and elaborate their terminal arborizations over the next 48 h. In Glued1 and Cytoplasmic dynein light chain mutants, proprioceptive and tactile axons arrive at the CNS on time but exhibit defects in terminal arborizations that increase in severity up to 48 h after puparium formation. The results show that axon growth occurs on schedule in these mutants but the final process of terminal branching, synaptogenesis, and stabilization of these sensory axons requires the dynein-dynactin complex. Since this complex functions as a retrograde motor, we suggest that a retrograde signal needs to be transported to the nucleus for the proper termination of some sensory neurons.

Localization of choline acetyltransferase-expressing neurons in Drosophila nervous system

A variety of approaches have been developed to localize neurons and neural elements in nervous system tissues that make and use acetylcholine (ACh) as a neurotransmitter. Choline acetyltransferase (ChAT) is the enzyme catalyzing the biosynthesis of ACh and is considered to be an excellent phenotypic marker for cholinergic neurons. We have surveyed the distribution of choline acetyltransferase (ChAT)-expressing neurons in the Drosophila nervous system detected by three different but complementary techniques. Immunocytochemistry, using anti-ChAT monoclonal antibodies results in identification of neuronal processes and a few types of cell somata that contain ChAT protein. In situ hybridization using cRNA probes to ChAT messenger RNA results in identification of cell bodies transcribing the ChAT gene. X-gal staining and/or beta-galactosidase immunocytochemistry of transformed animals carrying a fusion gene composed of the regulatory DNA from the ChAT gene controlling expression of a lacZ reporter has also been useful in identifying cholinergic neurons and neural elements. The combination of these three techniques has revealed that cholinergic neurons are widespread in both the peripheral and central nervous system of this model genetic organism at all but the earliest developmental stages. Expression of ChAT is detected in a variety of peripheral sensory neurons, and in the brain neurons associated with the visual and olfactory system, as well as in neurons with unknown functions in the cortices of brain and ganglia.

The 69 bp circadian regulatory sequence (CRS) mediates per-like developmental, spatial, and circadian expression and behavioral rescue in Drosophila

The period (per) gene is an essential component of the circadian timekeeping mechanism in Drosophila. This gene is expressed in a circadian manner, giving rise to a protein that feeds-back to regulate its own transcription. A 69 bp clock regulatory sequence (CRS) has been identified previously upstream of the period gene. The CRS confers wild-type mRNA cycling when used to drive a lacZ reporter gene in transgenic flies. To determine whether the CRS also mediates proper developmental and spatial expression and behavioral rescue, we used the CRS to drive either lacZ or per in transgenic flies. The results show that the CRS is able to activate expression in pacemaker neuron precursors in larvae and essentially all tissues that normally express per in pupae and adults. The CRS is sufficient to rescue circadian feedback loop function and behavioral rhythms in per01 flies. However, the period of locomotor activity rhythms shortens if a stronger basal promoter is used. This study shows that regulatory elements sufficient for clock-dependent and tissue-specific per expression in larvae, pupae, and adults are present in the CRS and that the period of adult locomotor activity rhythms is dependent, in part, on the overall level of per transcripts.

Development of the giant fiber neuron of Drosophila melanogaster

The giant fiber system (GFS) of Drosophila melanogaster provides a convenient system in which to study neural development. It mediates escape behaviour through a small number of neurons, including the giant fibers (GFs), to innervate the tergotrochantral jump muscle (TTM) and the dorsal longitudinal flight muscles. The GFS has been intensively studied physiologically in both wild-type and mutant flies, and is often used as a system to study the effects of neural mutations on the physiology of the adult nervous system. Recently, much information has been gleaned as to how and when synaptogenesis, with its major target neurons, is achieved. However, little is known of the earlier development of this neuron. Here we have used an enhancer-trap, marking parts of the GFS, in conjunction with BrdU labelling, to attempt to follow the birth, axonogenesis, and the early morphological meeting of the GFs with their target neurons. From these anatomical observations we propose that the GF cell is not born during the larval or pupal stages and, therefore. appears to be a persistent embryonic cell. The axons of the GFs develop during the third instar. During the early pupal stages the GFs contact other identified neurons of the GFS. In addition, we see some aberrant development of the network, with some flies carrying only one GF, and yet others with extended axons. We present a model for the initial joining of the GFs and tergotrochanteral motorneurons (TTMns).

Dynamic developmental expression of smallminded, a Drosophila gene required for cell division

Here we describe the expression pattern of the smallminded (smid) gene during Drosophila development and investigate the phenotype of a null mutant. In situ hybridisation reveals the ubiquitous expression of smid transcript throughout early embryonic stages until the extended germ band stage, after which expression becomes localised to the neurogenic ectoderm and gonad. Post-embryonic expression is restricted to tissues engaged in the developmental programme of the adult fly: the re-enlarged neuroblasts; imaginal disks; histoblast nests; and precursors of adult muscles. The correlation of smid expression with mitotic activity suggests a cell cycle function which is confirmed by the observed phenotype of a smid null mutant characterised by an abnormally small CNS, due to defective mitosis of post-embryonic neuroblasts and their subsequent death by apoptosis.

Isolation and characterisation of smallminded, a Drosophila gene encoding a new member of the Cdc48p/VCP subfamily of AAA proteins

Smallminded (smid) encodes a new member of the cdc48p/VCP subfamily of AAA proteins in Drosophila. The gene was isolated by plasmid rescue from a GAL4 enhancer trap line which shows reporter gene expression in neuroblasts, imaginal disks and a subset of sensory neurons. Larvae homozygous for the insert arrest development as second instar larvae and die without pupating. The most obvious defect in these larvae is a significantly reduced CNS, hence the naming of the gene as smallminded. The deduced amino acid sequence of smid contains a tandem duplication of the AAA nucleotide binding domain characteristic of the cdc48p/VCP subfamily. Overall, smid shares 33% identical residues with its closest relative, yeast L0919-chrXII and 26-29% with other members of the cdc48p/VCP subfamily. The most highly conserved regions of the predicted protein structure are found in and around the nucleotide binding domains. The gene is expressed at all developmental stages.

Mutant molecular motors disrupt neural circuits in Drosophila

A dominant negative mutation, Glued1, that codes for a component of the dynactin complex, disrupted the axonal anatomy of leg sensory neurons in Drosophila. To examine neuron structure in mutant animals, a P[Gal4] enhancer trap targeted expression of lacZ to the sensory neurons and thereby labeled neurons in the femoral chordotonal organ and their axons within the central nervous system. When these sensory axons were examined in the Glued1 mutant specimens, they were observed to arborize abnormally. This anatomical disruption of the sensory axons was associated with a corresponding disruption in a reflex. Normally, the tibial extensor motor neurons were excited when the femoral-tibial joint was flexed, but this resistance reflex was nearly absent in mutant animals. We used the P[Gal4] insertion strains to target expression of tetanus toxin light chain to these sensory neurons in wild-type animals and showed that this blocked the resistance reflex and produced a phenocopy of the Glued result. We conclude that disruption of the dynein-dynactin complex disrupts sensory axon path finding during metamorphosis, and this in turn disrupts synaptic connectivity.

Mutations in the 8 kDa dynein light chain gene disrupt sensory axon projections in the Drosophila imaginal CNS

Mutations in an 8 kDa (8x10(3) Mr) cytoplasmic dynein light chain disrupt sensory axon trajectories in the imaginal nervous system of Drosophila. Weak alleles are behaviorally mutant, female-sterile and exhibit bristle thinning and bristle loss. Null alleles are lethal in late pupal stages and alter neuronal anatomy within the imaginal CNS. We utilized P[Gal4] inserts to examine the axon projections of stretch receptor neurons and an engrailed-lacZ construct to characterize the anatomy of tactile neurons. In mutant animals both types of sensory neurons exhibited altered axon trajectories within the CNS, suggesting a defect in axon pathfinding. However, the alterations in axon trajectory did not prevent these axons from reaching their normal termination regions. In the alleles producing these neuronal phenotypes, expression of the cytoplasmic dynein 8 kDa light chain gene is completely absent. These results demonstrate a new function for the cytoplasmic dynein light chain in the regulation of axonogenesis and may provide a point of entry for studies of the role of cellular motors in growth cone guidance.

Central projections of persistent larval sensory neurons prefigure adult sensory pathways in the CNS of Drosophila

We have used a GAL4 enhancer-trap line driving the expression of a lacZ construct to examine the reorganisation of an identified group of proprioceptive sensory neurons during metamorphosis in Drosophila. The results show that whilst most larval sensory neurons degenerate during the first 24 hours of metamorphosis a segmentally repeated array of 6 neurons per segment persists into the adult stages to become functional adult neurons. These sensory neurons retain their axonal projections in the central nervous system intact and unchanged throughout. The adult sensory neuron axons enter the central nervous system at around 44 hours after puparium formation. Most of these axons grow along the pathways defined by the persistent larval sensory axons. The ordering of the adult sensory projections is, therefore, established upon the larval pattern of projections. The possibility that the larval neurons act as guidance cues for organising the ordered arrays of sensory neurons is discussed.