Postembryonic development of the dorsal longitudinal flight muscle and its innervation in Manduca sexta

The promoting effects of pyriproxyfen on autophagy and apoptosis in silk glands of non-target insect silkworm, Bombyx mori

Pyriproxyfen is a juvenile hormone analogue. The physiological effects of its low-concentration drift during the process of controlling agricultural and forestry pests on non-target organisms in the ecological environment are unpredictable, especially the effects on organs that play a key role in biological function are worthy of attention. The silk gland is an important organ for silk-secreting insects. Herein, we studied the effects of trace pyriproxyfen on autophagy and apoptosis of the silk gland in the lepidopteran model insect, Bombyx mori (silkworm). After treating fifth instar silkworm larvae with pyriproxyfen for 24 h, we found significant shrinkage, vacuolization, and fragmentation in the posterior silk gland (PSG). In addition, the results of autophagy-related genes of ATG8 and TUNEL assay also demonstrated that autophagy and apoptosis in the PSG of the silkworm was induced by pyriproxyfen. RNA-Seq results showed that pyriproxyfen treatment resulted in the activation of juvenile hormone signaling pathway genes and inhibition of 20-hydroxyecdysone (20E) signaling pathway genes. Among the 1808 significantly differentially expressed genes, 796 were upregulated and 1012 were downregulated. Among them, 30 genes were identified for autophagy-related signaling pathways, such as NOD-like receptor signaling pathway and mTOR signaling pathway, and 30 genes were identified for apoptosis-related signaling pathways, such as P53 signaling pathway and TNF signaling pathway. Further qRT-PCR and in vitro gland culture studies showed that the autophagy-related genes Atg5, Atg6, Atg12, Atg16 and the apoptosis-related genes Aif, Dronc, Dredd, and Caspase1 were responsive to the treatment of pyriproxyfen, with transcription levels up-regulated from 24 to 72 h. In addition, ATG5, ATG6, and Dronc genes had a more direct response to pyriproxyfen treatment. These results suggested that pyriproxyfen treatment could disrupt the hormone regulation in silkworms, promoting autophagy and apoptosis in the PSG. This study provides more evidence for the research on the damage of juvenile hormone analogues to non-target organisms or organs in the environment, and provides reference information for the scientific and rational use of juvenile hormone pesticides.

Single-cell type analysis of wing premotor circuits in the ventral nerve cord of Drosophila melanogaster

To perform most behaviors, animals must send commands from higher-order processing centers in the brain to premotor circuits that reside in ganglia distinct from the brain, such as the mammalian spinal cord or insect ventral nerve cord. How these circuits are functionally organized to generate the great diversity of animal behavior remains unclear. An important first step in unraveling the organization of premotor circuits is to identify their constituent cell types and create tools to monitor and manipulate these with high specificity to assess their function. This is possible in the tractable ventral nerve cord of the fly. To generate such a toolkit, we used a combinatorial genetic technique (split-GAL4) to create 195 sparse driver lines targeting 198 individual cell types in the ventral nerve cord. These included wing and haltere motoneurons, modulatory neurons, and interneurons. Using a combination of behavioral, developmental, and anatomical analyses, we systematically characterized the cell types targeted in our collection. Taken together, the resources and results presented here form a powerful toolkit for future investigations of neural circuits and connectivity of premotor circuits while linking them to behavioral outputs.

Gap junctions desynchronize a neural circuit to stabilize insect flight

Insect asynchronous flight is one of the most prevalent forms of animal locomotion used by more than 600,000 species. Despite profound insights into the motor patterns¹, biomechanics^2,3 and aerodynamics underlying asynchronous flight^4,5, the architecture and function of the central-pattern-generating (CPG) neural network remain unclear. Here, on the basis of an experiment-theory approach including electrophysiology, optophysiology, Drosophila genetics and mathematical modelling, we identify a miniaturized circuit solution with unexpected properties. The CPG network consists of motoneurons interconnected by electrical synapses that, in contrast to doctrine, produce network activity splayed out in time instead of synchronized across neurons. Experimental and mathematical evidence support a generic mechanism for network desynchronization that relies on weak electrical synapses and specific excitability dynamics of the coupled neurons. In small networks, electrical synapses can synchronize or desynchronize network activity, depending on the neuron-intrinsic dynamics and ion channel composition. In the asynchronous flight CPG, this mechanism translates unpatterned premotor input into stereotyped neuronal firing with fixed sequences of cell activation that ensure stable wingbeat power and, as we show, is conserved across multiple species. Our findings prove a wider functional versatility of electrical synapses in the dynamic control of neural circuits and highlight the relevance of detecting electrical synapses in connectomics.

Myoblast proliferation during flight muscle development in Manduca sexta is unaffected by reduced neural signaling

In holometabolous insects, metamorphosis involves restructuring the musculature to accommodate adult-specific anatomy and behaviors. Evidence from experiments on remodeled muscles, as well as those that develop de novo, suggests that signals from the nervous system support adult muscle development by controlling myoblast proliferation rate. However, the dorsolongitudinal flight muscles (DLMs) of Manduca sexta undergo a mixed developmental program involving larval muscle fibers, and it is not known if neurons play the same role in the formation of these muscles. To address this question, we have blocked the most promising candidate pathways for neural input and examined the DLMs for changes in proliferation. Our results show that DLM development does not depend on neural activity, Hedgehog signaling, or EGF signaling. It remains to be determined how DLM growth is controlled and why neurally mediated proliferation differs between individual muscles.

The larval scaffold controls fascicle number but is not required for formation of the dorsolongitudinal flight muscles in Manduca sexta

During metamorphosis, the dorsolongitudinal flight muscles (DLMs) of both the moth Manduca sexta and the fly Drosophila melanogaster develop from the remnants of larval muscles called larval scaffolds. Although this developmental program has been conserved across highly disparate taxa, the role of the larval scaffold remains unclear. Ablation experiments have demonstrated that the Drosophila DLM does not require the scaffold, but the resulting de novo muscles vary highly in fiber number, and their functional characteristics were not examined. To address this question in Manduca, we have surgically ablated the DLM precursors in Manduca sexta larvae and assayed the resulting DLMs in pharate adults using X-ray micro-CT and phalloidin histology. Following ablation, animals were able to form de novo DLMs with normal myofibril alignment, but these muscles had an altered shape and highly variable number of fascicles. Our results suggest that the larval scaffold is not required for DLM development in Manduca sexta, but appears to define the number of fascicles in the adult muscle, as previously found in Drosophila. Additionally, our ablated animals were able to generate flight, further suggesting that the use of a larval scaffold is a modification on the more ancestral myogenesis program.

Metamorphosis in Insect Muscle: Insights for Engineering Muscle-Based Actuators

One of the major limitations to advancing the development of soft robots is the absence of lightweight, effective soft actuators. While synthetic systems, such as pneumatics and shape memory alloys, have created important breakthroughs in soft actuation, they typically rely on large external power sources and some rigid components. Muscles provide an ideal actuator for soft constructs, as they are lightweight, deformable, biodegradable, silent, and powered by energy-dense hydrocarbons such as glucose. Vertebrate cell lines and embryonic cultures have allowed critical foundational work to this end, but progress there is limited by the difficulty of identifying individual pathways in embryonic development, and the divergence of immortal cell lines from these normal developmental programs. An alternative to culturing muscles from embryonic cells is to exploit the advantages of species with metamorphic stages. In these animals, muscles develop from a predefined pool of myoblasts with well-characterized contacts to other tissues. In addition, the endocrine triggers for development into adult muscles are often known and tractable for experimental manipulation. This is particularly true for metamorphic muscle development in holometabolous insects, which provide exciting new avenues for tissue engineering. Using insect tissues for actuator development confers additional benefits; insect muscles are more robust to varying pH, temperature, and oxygenation than are vertebrate cells. Given that biohybrid robots are likely to be used in ambient conditions and changing environments, this sort of hardiness is likely to be required for practical use. In this study, we summarize key processes and signals in metamorphic muscle development, drawing attention to those pathways that offer entry points for manipulation. By focusing on lessons learned from in vivo insect development, we propose that future culture designs will be able to use more systematic, hypothesis-driven approaches to optimizing engineered muscle. Impact statement This review summarizes our current understanding of metamorphic muscle development in insects. It provides a framework for engineering muscle-based actuators that can be used in robotic applications in a wide range of ambient conditions. The focus is on identifying key processes that might be manipulated to solve current challenges in controlling tissue development such as myoblast proliferation, myotube formation and fusion, cytoskeletal alignment, myotendinous attachment and full differentiation. An important goal is to gather findings that cross disciplinary boundaries and to promote the development of better bioactuators for nonclinical applications.

The Wiring Logic of an Identified Serotonergic Neuron That Spans Sensory Networks

Serotonergic neurons project widely throughout the brain to modulate diverse physiological and behavioral processes. However, a single-cell resolution understanding of the connectivity of serotonergic neurons is currently lacking. Using a whole-brain EM dataset of a female Drosophila, we comprehensively determine the wiring logic of a broadly projecting serotonergic neuron (the CSDn) that spans several olfactory regions. Within the antennal lobe, the CSDn differentially innervates each glomerulus, yet surprisingly, this variability reflects a diverse set of presynaptic partners, rather than glomerulus-specific differences in synaptic output, which is predominately to local interneurons. Moreover, the CSDn has distinct connectivity relationships with specific local interneuron subtypes, suggesting that the CSDn influences distinct aspects of local network processing. Across olfactory regions, the CSDn has different patterns of connectivity, even having different connectivity with individual projection neurons that also span these regions. Whereas the CSDn targets inhibitory local neurons in the antennal lobe, the CSDn has more distributed connectivity in the LH, preferentially synapsing with principal neuron types based on transmitter content. Last, we identify individual novel synaptic partners associated with other sensory domains that provide strong, top-down input to the CSDn. Together, our study reveals the complex connectivity of serotonergic neurons, which combine the integration of local and extrinsic synaptic input in a nuanced, region-specific manner.SIGNIFICANCE STATEMENT All sensory systems receive serotonergic modulatory input. However, a comprehensive understanding of the synaptic connectivity of individual serotonergic neurons is lacking. In this study, we use a whole-brain EM microscopy dataset to comprehensively determine the wiring logic of a broadly projecting serotonergic neuron in the olfactory system of Drosophila Collectively, our study demonstrates, at a single-cell level, the complex connectivity of serotonergic neurons within their target networks, identifies specific cell classes heavily targeted for serotonergic modulation in the olfactory system, and reveals novel extrinsic neurons that provide strong input to this serotonergic system outside of the context of olfaction. Elucidating the connectivity logic of individual modulatory neurons provides a ground plan for the seemingly heterogeneous effects of modulatory systems.

Flight duration and flight muscle ultrastructure of unfed hawk moths

Flight muscle breakdown has been reported for many orders of insects, but the basis of this breakdown in insects with lifelong dependence on flight is less clear. Lepidopterans show such muscle changes across their lifespans, yet how this change affects the ability of these insects to complete their life cycles is not well documented. We investigated the changes in muscle function and ultrastructure of unfed aging adult hawk moths (Manduca sexta). Flight duration was examined in young, middle-aged, and advanced-aged unfed moths. After measurement of flight duration, the main flight muscle (dorsolongitudinal muscle) was collected and histologically prepared for transmission electron microscopy to compare several measurements of muscle ultrastructure among moths of different ages. Muscle function assays revealed significant positive correlations between muscle ultrastructure and flight distance that were greatest in middle-aged moths and least in young moths. In addition, changes in flight muscle ultrastructure were detected across treatment groups. The number of mitochondria in muscle cells peaked in middle-aged moths. Many wild M. sexta do not feed as adults; thus, understanding the changes in flight capacity and muscle ultrastructure in unfed moths provides a more complete understanding of the ecophysiology and resource allocation strategies of this species.

A procession of metabolic alterations accompanying muscle senescence in Manduca sexta

Biological aging profoundly impairs muscle function, performance, and metabolism. Because the progression of metabolic alterations associated with aging muscle has not been chronicled, we tracked the metabolic profiles of flight muscle from middle to advanced age in Manduca sexta to identify key molecules during the progression of muscle aging, as well as to evaluate the utility of the M. sexta system for molecular dissection of muscle aging. We identified a number of differences between Diel Time, Sexes, and Muscle Ages, including changes in metabolites related to energetics, extracellular matrix turnover, and glutathione metabolism. Increased abundances of glycolytic metabolites suggest a shift toward increased glycolysis with advancing age, whereas decreased abundances in lysolipids and acylcarnitines reflect decreasing beta-oxidation. We also observed a shift towards decreased polyamine metabolism with age, which might result in an age-related decline in lipid metabolism possibly due to regulation of energy metabolism by polyamines. Collectively, our findings demonstrate the feasibility of our system and approach and provide a deeper understanding of lepidopteran aging. More importantly, the results identify the key altered metabolic pathways that collectively contribute to the muscle aging phenotype and thereby improve our understanding of muscle senescence.

Apoptotic Activity of MeCP2 Is Enhanced by C-Terminal Truncating Mutations

Methyl-CpG binding protein 2 (MeCP2) is a widely abundant, multifunctional protein most highly expressed in post-mitotic neurons. Mutations causing Rett syndrome and related neurodevelopmental disorders have been identified along the entire MECP2 locus, but symptoms vary depending on mutation type and location. C-terminal mutations are prevalent, but little is known about the function of the MeCP2 C-terminus. We employ the genetic efficiency of Drosophila to provide evidence that expression of p.Arg294* (more commonly identified as R294X), a human MECP2 E2 mutant allele causing truncation of the C-terminal domains, promotes apoptosis of identified neurons in vivo. We confirm this novel finding in HEK293T cells and then use Drosophila to map the region critical for neuronal apoptosis to a small sequence at the end of the C-terminal domain. In vitro studies in mammalian systems previously indicated a role of the MeCP2 E2 isoform in apoptosis, which is facilitated by phosphorylation at serine 80 (S80) and decreased by interactions with the forkhead protein FoxG1. We confirm the roles of S80 phosphorylation and forkhead domain transcription factors in affecting MeCP2-induced apoptosis in Drosophila in vivo, thus indicating mechanistic conservation between flies and mammalian cells. Our findings are consistent with a model in which C- and N-terminal interactions are required for healthy function of MeCP2.

Early metamorphic insertion technology for insect flight behavior monitoring

Early Metamorphosis Insertion Technology (EMIT) is a novel methodology for integrating microfabricated neuromuscular recording and actuation platforms on insects during their metamorphic development. Here, the implants are fused within the structure and function of the neuromuscular system as a result of metamorphic tissue remaking. The implants emerge with the insect where the development of tissue around the electronics during pupal development results in a bioelectrically and biomechanically enhanced tissue interface. This relatively more reliable and stable interface would be beneficial for many researchers exploring the neural basis of the insect locomotion with alleviated traumatic effects caused during adult stage insertions. In this article, we implant our electrodes into the indirect flight muscles of Manduca sexta. Located in the dorsal-thorax, these main flight powering dorsoventral and dorsolongitudinal muscles actuate the wings and supply the mechanical power for up and down strokes. Relative contraction of these two muscle groups has been under investigation to explore how the yaw maneuver is neurophysiologically coordinated. To characterize the flight dynamics, insects are often tethered with wires and their flight is recorded with digital cameras. We also developed a novel way to tether Manduca sexta on a magnetically levitating frame where the insect is connected to a commercially available wireless neural amplifier. This set up can be used to limit the degree of freedom to yawing "only" while transmitting the related electromyography signals from dorsoventral and dorsolongitudinal muscle groups.

Sequential acquisition of cacophony calcium currents, sodium channels and voltage-dependent potassium currents affects spike shape and dendrite growth during postembryonic maturation of an identified Drosophila motoneuron

During metamorphosis the CNS undergoes profound changes to accommodate the switch from larval to adult behaviors. In Drosophila and other holometabolous insects, adult neurons differentiate either from respecified larval neurons, newly born neurons, or are born embryonically but remain developmentally arrested until differentiation during pupal life. This study addresses the latter in the identified Drosophila flight motoneuron 5. In situ patch-clamp recordings, intracellular dye fills and immunocytochemistry address the interplay between dendritic shape, excitability and ionic current development. During pupal life, changes in excitability and spike shape correspond to a stereotyped, progressive appearance of voltage-gated ion channels. High-voltage-activated calcium current is the first current to appear at pupal stage P4, prior to the onset of dendrite growth. This is followed by voltage-gated sodium as well as transient potassium channel expression, when first dendrites grow, and sodium-dependent action potentials can be evoked by somatic current injection. Sustained potassium current appears later than transient potassium current. During the early stages of rapid dendritic growth, sodium-dependent action potentials are broadened by a calcium component. Narrowing of spike shape coincides with sequential increases in transient and sustained potassium currents during stages when dendritic growth ceases. Targeted RNAi knockdown of pupal calcium current significantly reduces dendritic growth. These data indicate that the stereotyped sequential acquisition of different voltage-gated ion channels affects spike shape and excitability such that activity-dependent calcium influx serves as a partner of genetic programs during critical stages of motoneuron dendrite growth.

Putative excitatory and putative inhibitory inputs are localised in different dendritic domains in a Drosophila flight motoneuron

Input-output computations of individual neurons may be affected by the three-dimensional structure of their dendrites and by the location of input synapses on specific parts of their dendrites. However, only a few examples exist of dendritic architecture which can be related to behaviorally relevant computations of a neuron. By combining genetic, immunohistochemical and confocal laser scanning methods this study estimates the location of the spike-initiating zone and the dendritic distribution patterns of putative synaptic inputs on an individually identified Drosophila flight motorneuron, MN5. MN5 is a monopolar neuron with > 4,000 dendritic branches. The site of spike initiation was estimated by mapping sodium channel immunolabel onto geometric reconstructions of MN5. Maps of putative excitatory cholinergic and of putative inhibitory GABAergic inputs on MN5 dendrites were created by charting tagged Dα7 nicotinic acetylcholine receptors and Rdl GABAA receptors onto MN5 dendritic surface reconstructions. Although these methods provide only an estimate of putative input synapse distributions, the data indicate that inhibitory and excitatory synapses were located preferentially on different dendritic domains of MN5 and, thus, computed mostly separately. Most putative inhibitory inputs were close to spike initiation, which was consistent with sharp inhibition, as predicted previously based on recordings of motoneuron firing patterns during flight. By contrast, highest densities of putative excitatory inputs at more distant dendritic regions were consistent with the prediction that, in response to different power demands during flight, tonic excitatory drive to flight motoneuron dendrites must be smoothly translated into different tonic firing frequencies.

Bioelectrical enhancement in tissue-electrode coupling with metamorphic-stage insertions for insect machine interfaces

Implanting microtechnologies into insects with an aim of domesticating its locomotion poses certain challenges, however, performing surgical implantation during the early stages of metamorphic growth was shown to mitigate some of the related detriments. This study reports the bioelectrical enhancement at the tissue-electrode interface allowed with these metamorphic stage insertions, where the electrodes implanted in the insect during the early pupal stages and right after emergence were compared. An average 1 kHz impedance of 8.9 kΩ was obtained with pupal stage inserted electrodes, ten days after the emergence, as compared to 12.1 kΩ observed when electrodes were implanted in the adult state. Charge storage capacity also increased to 52 mC/cm(2) from 38 mC/cm(2) with the early metamorphic insertions. The performed voltage excursion studies also confirmed the enhancement demonstrating an increase from 3.5 mC/cm(2) to 5.1 mC/cm(2) in the injectable amount of charge in the water window.

Developmental changes in dendritic shape and synapse location tune single-neuron computations to changing behavioral functions

During nervous system development, different classes of neurons obtain different dendritic architectures, each of which receives a large number of input synapses. However, it is not clear whether synaptic inputs are targeted to specific regions within a dendritic tree and whether dendritic tree geometry and subdendritic synapse distributions might be optimized to support proper neuronal input-output computations. This study uses an insect model where structure and function of an individually identifiable neuron, motoneuron 5 (MN5), are changed while it develops from a slow larval crawling into a fast adult flight motoneuron during metamorphosis. This allows for relating postembryonic dendritic remodeling of an individual motoneuron to developmental changes in behavioral function. Dendritic architecture of MN5 is analyzed by three-dimensional geometric reconstructions and quantitative co-localization analysis to address the distribution of synaptic terminals. Postembryonic development of MN5 comprises distinct changes in dendritic shape and in the subdendritic distribution of GABAergic input synapses onto MN5. Subdendritic synapse targeting is not a consequence of neuropil structure but must rely on specific subdendritic recognition mechanisms. Passive multicompartment simulations indicate that postembryonic changes in dendritic architecture and in subdendritic input synapse distributions may tune the passive computational properties of MN5 toward stage-specific behavioral requirements.

An ecdysteroid-inducible insulin-like growth factor-like peptide regulates adult development of the silkmoth Bombyx mori

Insulin-like growth factors (IGFs) play essential roles in fetal and postnatal growth and development of mammals. They are secreted by a wide variety of tissues, with the liver being the major source of circulating IGFs, and regulate cell growth, differentiation and survival. IGFs share some biological activities with insulin but are secreted in distinct physiological and developmental contexts, having specific functions. Although recent analyses of invertebrate genomes have revealed the presence of multiple insulin family peptide genes in each genome, little is known about functional diversification of the gene products. Here we show that a novel insulin family peptide of the silkmoth Bombyx mori, which was purified and sequenced from the hemolymph, is more like IGFs than like insulin, in contrast to bombyxins, which are previously identified insulin-like peptides in B. mori. Expression analysis reveals that this IGF-like peptide is predominantly produced by the fat body, a functional equivalent of the vertebrate liver and adipocytes, and is massively released during pupa-adult development. Studies using in vitro tissue culture systems show that secretion of the peptide is stimulated by ecdysteroid and that the secreted peptide promotes the growth of adult-specific tissues. These observations suggest that this peptide is a Bombyx counterpart of vertebrate IGFs and that functionally IGF-like peptides may be more ubiquitous in the animal kingdom than previously thought. Our results also suggest that the known effects of ecdysteroid on insect adult development may be in part mediated by IGF-like peptides.

Expression of two different isoforms of fasciclin II during postembryonic central nervous system remodeling in Manduca sexta

Insect metamorphosis serves as a useful model to investigate postembryonic development in the central nervous system, because the transformation between larval and adult life is accompanied by a remodeling of neural circuitry. Most changes are controlled by ecdysteroids, but activity-dependent mechanisms and cell surface signals also play a role. This immunocytochemical study investigates the expression patterns of two isoforms of the neural cell adhesion molecule, fasciclin II (FasII), during postembryonic ventral nerve cord remodeling in the moth, Manduca sexta. Both the expression of the glycosyl-phosphatidylinositol (GPI)-linked isoform and the transmembrane isoform of Manduca FasII (TM-MFasII) are regulated in a stereotyped spatio-temporal pattern. TM-MFasII is expressed in a stage-specific manner in a subset of neurons. Subsets of central axons express high levels during outgrowth supporting a functional role for TM-FasII during pathfinding. Dendritic localization is not found at any stage of metamorphosis, suggesting no homophilic interactions of TM-MFasII during central synapse development. GPI-MFasII is expressed in a stage-specific manner, most likely only in glial cells. The larval and adult stages show almost no GPI-MFasII expression, whereas during pupal life, positive GPI-MFasII labeling is present around synaptotagmin-negative tracts or commissures, so that either homophilic stabilization of glial boundaries or heterophilic neuron-glial interactions possibly stabilize the axons within their tracts. GPI-MFasII expression is not co-localized with synaptotagmin-positive central terminals, rendering a role for synapse development unlikely. Neither isoform is expressed in all neurons of a specific class at any developmental stage, indicating that MFasII functions are restricted to specific subsets of neurons or to individual neurons.

A constitutive model for muscle properties in a soft-bodied arthropod

In this paper, we examine the mechanical properties of muscles in a soft-bodied arthropod under both passive and stimulated conditions. In particular, we examine the ventral interior lateral muscle of the tobacco hornworm caterpillar, Manduca sexta, and show that its response is qualitatively similar to the behaviour of particle-reinforced rubber. Both materials are capable of large nonlinear elastic deformations, show a hysteretic behaviour and display stress softening during the first few cycles of repeated loading. The Manduca muscle can therefore be considered as different elastic materials during loading and unloading and is best described using the theory of pseudo-elasticity. We summarize the basic equations for transversely isotropic pseudo-elastic materials, first for general deformations and then for the appropriate uniaxial specialization. The constitutive relation proposed is in good agreement with the experimental data for both the passive and the stimulated conditions.

Developmental changes of CaMKII localization, activity and function during postembryonic CNS remodelling in Manduca sexta

Insect metamorphosis is a compelling example of postembryonic remodelling of neuronal structure and synaptic connectivity as larval and adult behaviours place distinct demands on the CNS. Holometabolous insects such as the moth Manduca sexta have long served as suitable models for the study of steroid effects on CNS remodelling, but activity and calcium-dependent mechanisms have been found to act in concert with hormonal signals. This study examines developmental changes in the localization and the activational state of CaMKII during postembryonic Manduca CNS remodelling. Western blotting, CaMKII purification and autophosphorylation with gamma(32)P-ATP indicate that the lepidopteran CNS may contain only one CaMKII isoform. In situ immunohistochemistry reveals developmental changes in the expression patterns of CaMKII in different types of thoracic neurons and in different neuronal compartments. Early pupal life is characterized by an increase in postsynaptic CaMKII localization, which coincides with a developmental increase in CaMKII activation. Both events correlate temporally with motoneuron dendritic filopodia collapse and rapid synaptogenesis, indicating a possible functional role for CaMKII for the postembryonic development of invertebrate motor circuitry. Substrate phosphorylation assays demonstrate that CaMKII activity in the ventral nerve cord reflects changes in calcium influx through voltage-activated channels as occurring in vivo during normal development.

Activity affects dendritic shape and synapse elimination during steroid controlled dendritic retraction in Manduca sexta

Insect metamorphosis is a compelling example for dendritic and synaptic remodeling as larval and adult behaviors place distinct demands on the CNS. During the metamorphosis of the moth, Manduca sexta, many larval motoneurons are remodeled to serve a new function in the adult. During late larval life, steroid hormones trigger axonal and dendritic regression as well as larval synapse elimination. These regressive events are accompanied by stereotypical changes in motor behavior during the so-called wandering stages. Both normally occurring changes in dendritic shape and in motor output have previously been analyzed quantitatively for the individually identified motoneuron MN5. This study tested whether activity affected steroid-induced dendritic regression and synapse disassembly in MN5 by means of chronically implanted extracellular electrodes. Stimulating MN5 in vivo in intact, normally developing animals during a developmental period when it usually shows no activity significantly slowed the regression of high-order dendrites. Both physiological and anatomical analysis demonstrated that reduced dendritic regression was accompanied by a significant reduction in larval synapse disassembly. Therefore, steroid-induced alterations of dendritic shape and synaptic connectivity are modified by activity-dependent mechanisms. This interaction might be a common mechanism for rapid adjustments of rigid, inflexible, hormonal programs.

Motoneurons regulate myoblast proliferation and patterning in Drosophila

Motoneurons directly influence the differentiation of muscle fibers, regulating features such as muscle fiber type and receptor development. Less well understood is whether motoneurons direct earlier events, such as the patterning of the musculature. In Drosophila, the denervation of indirect flight muscles results in a diminished myoblast population and smaller or missing muscle fibers. We have examined whether the neuron-dependent control of myoblast number is due to regulation of cell division, motoneuron-dependent apoptosis, or nerve-dependent localization and migration of myoblasts. We found that denervation resulted in a reduced rate of cell division, as revealed by BrDU incorporation. There was no change in the frequency of apoptotic myoblasts following denervation. Using time lapse imaging of GFP-expressing myoblasts in vivo in pupae, we observed that despite denervation, the migration and localization of myoblasts remained unchanged. In addition to reducing myoblast proliferation, denervation also altered the segregation of myoblasts into the de novo arising dorso-ventral muscles (DVMs). To address this effect on muscle patterning, we examined the expression of the founder-cell marker Dumbfounded/Kirre (Duf) in imaginal pioneer cells. We show that there is a strong correspondence between cells that express Dumbfounded/Kirre and the number of DVM fibers, consistent with a role for these cells in establishing adult muscles. In the absence of innervation the Duf-positive cells are no longer detected, and muscle patterning is severely disrupted. Our results support a model where specialized founder cells prefigure the adult muscle fibers under the control of the nervous system.

Stage-specific activity patterns affect motoneuron axonal retraction and outgrowth during the metamorphosis of Manduca sexta

During the metamorphosis of holometabolous insects, most larval muscles and sensory neurons are replaced by new adult elements, whereas most motoneurons persist and are remodelled to serve new adult functions. In Manduca sexta, the formation of the anlagen of the adult dorsal longitudinal flight muscle (DLM) is characterized by retraction of axonal terminals and dendrites of persisting larval motoneurons, partial target muscle degeneration and myoblast accumulation during late larval life. Most of these structural changes have been attributed to hormonal control, not only because ecdysteroids govern metamorphosis, but also because motoneurons express ecdysteroid receptors and experimental manipulations of ecdysteroid titres perturb normal development. To test whether activity-dependent mechanisms also came into play, chronic extracellular recordings were conducted in vivo from the five future DLM motoneurons throughout the last 3 days of larval life. Motoneuron activity is regulated developmentally. The types of motoneurons recruited, the number of motor spikes and the duration of bursts change in a stereotypical fashion during different stages, indicating an internal control of motor activity. A characteristic cessation in the activity of the five future DLM motoneurons coincides in time with the retraction of their dendrites and their terminal arborizations, whereas their activation during ecdysis coincides with the onset of new outgrowth. Inducing advanced activity by stimulating the motoneurons selectively with ecdysis-like patterns results in significant outgrowth of their terminal arborizations. Therefore, steroids might act in concert with activity-dependent mechanisms during the postembryonic modifications of neuromuscular systems.

Morphometric analysis of dendritic remodeling in an identified motoneuron during postembryonic development

A detailed quantitative description of modifications in neuronal architecture is an important prerequisite to investigate the signals underlying behaviorally relevant changes in neuronal shape. Extensive morphological remodeling of neurons occurs during the metamorphosis of holometabolous insects, such as Manduca sexta, in which new adult behaviors develop postembryonically. In this study, a morphometric analysis of the structural changes of an identified Manduca motoneuron, MN5, was conducted by sampling its metric parameters at different developmental stages. The remodeling of MN5 is divided into three main phases. The regression of most larval dendrites (1) is followed by the formation of dendritic growth-cones (2), and subsequently, adult dendrite formation (3). In contrast, the cell body and link segment surface increase during dendritic regression and regrowth, indicating that different cell compartments receive different signals, or respond differently to the same signal. During dendritic growth-cone formation, the growth of the cell body and the link segment are arrested. Sholl and branch frequency analysis suggest two different modes of dendritic growth. During a first growth-cone-dependent phase, new branch formation occurs at all dendrites. The maximum path length of the major dendritic tree changes little, whereas branch order increases from 20 to 45. Changes in total dendritic length are correlated with strong changes in the number of nodes but with minor changes in the average dendritic segment length, indicating a mode of growth similar to that induced by steroid hormone application to cultured motoneurons. The second phase is growth-cone-independent, and branching is limited to high order dendrites.

Dendritic remodeling and growth of motoneurons during metamorphosis of Drosophila melanogaster

Insect motoneurons display dramatic dendritic plasticity during metamorphosis. Many larval motoneurons survive to adulthood but undergo dendritic regression and outgrowth as they are incorporated into developing circuits. This study explores the dendritic remodeling and development of Drosophila motoneurons MN1-MN5, which innervate indirect flight muscles of the adult. MN1-MN5 are persistent larval neurons exhibiting two distinct metamorphic histories. MN1-MN4 are born in the embryo, innervate larval muscles, and undergo dendritic regression and regrowth during metamorphosis. MN5, which was identified through a combination of intracellular dye injection and retrograde staining at all stages, is also born embryonically but remains developmentally arrested until the onset of metamorphosis. In the larva, MN5 lacks dendrites, and its axon stops in the mesothoracic nerve without innervating a target muscle. It is dye coupled to the peripherally synapsing interneuron, which will become part of the giant fiber escape circuit of the adult fly. During pupal development, MN5 undergoes de novo dendritic growth and extension of its axon to innervate the developing target muscle. Its unique developmental history and identifiability make MN5 well suited for the study of dendritic growth using genetic and neurophysiological approaches.

Changes in calcium signaling during postembryonic dendritic growth in Manduca sexta

Activity-dependent Ca(2+) influx plays crucial roles in adult and developing nervous systems through its influence on signal processing, synaptic plasticity, and neuronal differentiation. The responses to internal Ca(2+) elevations vary depending on the spatial distribution of Ca(2+) accumulation in different cell compartments. In this study, the mechanisms and the distribution of Ca(2+) accumulation are addressed by in situ Ca(2+) imaging of an identified insect motoneuron, MN5, at critical stages of postembryonic life. During metamorphosis of Manduca sexta, MN5 undergoes extensive dendritic regression followed by regrowth. The time course, amplitude, and distribution of Ca(2+) accumulation within MN5 change during development. During the initial stage of rapid dendritic growth and branching, dendritic growth cones are present, and voltage-dependent Ca(2+) currents are small. At this stage, activity-induced elevations of internal Ca(2+) are largest in the distal dendrites, suggesting that the density of voltage-gated Ca(2+) channels is highest in these regions. Later phases of dendritic growth are accompanied by the transient occurrence of prominent Ca(2+) spikes. Single Ca(2+) spikes cause robust Ca(2+) influx of similar amplitudes and time courses in all central compartments of MN5. The resting Ca(2+) levels also increase during development. Ca(2+)-induced Ca(2+) release from intracellular stores did not contribute to the elevations measured at either stage, although Ca(2+) stores are present in the dendrites. These developmental changes of the internal Ca(2+) signaling are consistent with a regulatory role for activity-dependent Ca(2+) influx in postembryonic dendritic growth.

Nerve-muscle interactions regulate motor terminal growth and myoblast distribution during muscle development

Interactions between motoneurons and muscles influence many aspects of neuromuscular development in all animals. These interactions can be readily investigated during adult muscle development in holometabolous insects. In this study, the development of the dorsolongitudinal flight muscle (DLM) and its innervation is investigated in the moth, Manduca sexta, to address the specificity of neuromuscular interactions. The DLM develops from an anlage containing both regressed larval template fibers and imaginal myoblasts. In the adult, each fiber bundle (DLM1-5) is innervated by a single motoneuron (MN1-MN5), with the dorsal-most fiber bundle (DLM5) innervated by a mesothoracic motoneuron (MN5). The DLM failed to develop following complete denervation because myoblasts failed to accumulate in the DLM anlage. After lesioning MN1-4, MN5 retained its specificity for the DLM5 region of the anlage and failed to rescue DLM1-4. Thus specific innervation of the DLM fiber bundles does not depend on interactions among motoneurons. Myoblast accumulation, but not myonuclear proliferation, increased around the MN5 terminals, producing a hypertrophied adult DLM5. Therefore, motoneurons compete for uncommitted myoblasts. MN5 terminals subsequently grew more rapidly over the hypertrophied DLM5 anlage, indicating that motoneuron terminal expansion is regulated by the size of the target muscle anlage.

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.

Remodeling of membrane properties and dendritic architecture accompanies the postembryonic conversion of a slow into a fast motoneuron

The postembryonic acquisition of behavior requires alterations in neuronal circuitry, which ultimately must be understood as specific changes in neuronal structure, membrane properties, and synaptic connectivity. This study addresses this goal by describing the postembryonic remodeling of the excitability and dendritic morphology of an identified motoneuron, MN5, which during the metamorphosis of Manduca sexta (L.) changes from a slow motoneuron that is involved in larval-crawling behavior into a fast adult flight motoneuron. A fivefold lower input resistance, a higher firing threshold, and an increase in voltage-activated K(+) current contribute to a lower excitability of the adult MN5, which is a prerequisite for its newly acquired behavioral role. In addition, the adult MN5 displays larger Ca(2+) currents. The dendrites of MN5 undergo extensive remodeling. Drastic regression of larval dendrites during early pupal stages is followed by rapid growth of new dendrites. Critical changes in excitability take place during the onset of adult dendrite formation. Larval Ca(2+) currents are absent when dendritic remodeling is most dramatic but increase markedly during later development. Changes in Ca(2+) and K(+) currents follow different time courses, allowing the transient occurrence of Ca(2+) spikes during pupal stages when new dendritic branching ceases. The adult MN5 can produce prolonged Ca(2+) spikes after K(+) currents are reduced. We suggest that alterations in Ca(2+) and K(+) currents are necessary for the participation of MN5 in flight behavior and that the transient production of Ca(2+) spikes may influence postembryonic dendritic remodeling.