Steroid and neuronal regulation of ecdysone receptor expression during metamorphosis of muscle in the moth, Manduca sexta

A Novel Perspective on Neuronal Control of Anatomical Patterning, Remodeling, and Maintenance

While the nervous system may be best known as the sensory communication center of an organism, recent research has revealed a myriad of multifaceted roles for both the CNS and PNS from early development to adult regeneration and remodeling. These systems work to orchestrate tissue pattern formation during embryonic development and continue shaping pattering through transitional periods such as metamorphosis and growth. During periods of injury or wounding, the nervous system has also been shown to influence remodeling and wound healing. The neuronal mechanisms responsible for these events are largely conserved across species, suggesting this evidence may be important in understanding and resolving many human defects and diseases. By unraveling these diverse roles, this paper highlights the necessity of broadening our perspective on the nervous system beyond its conventional functions. A comprehensive understanding of the complex interactions and contributions of the nervous system throughout development and adulthood has the potential to revolutionize therapeutic strategies and open new avenues for regenerative medicine and tissue engineering. This review highlights an important role for the nervous system during the patterning and maintenance of complex tissues and provides a potential avenue for advancing biomedical applications.

Integration of proteomic and genetic approaches to assess developmental muscle atrophy

Muscle atrophy, or a decline in muscle protein mass, is a significant problem in the aging population and in numerous disease states. Unraveling molecular signals that trigger and promote atrophy may lead to a better understanding of treatment options; however, there is no single cause of atrophy identified to date. To gain insight into this problem, we chose to investigate changes in protein profiles during muscle atrophy in Manduca sexta and Drosophila melanogaster. The use of insect models provides an interesting parallel to probe atrophic mechanisms as these organisms undergo a normal developmental atrophy process during the pupal transition stage. Leveraging the inherent advantages of each model organism, we first defined protein signature changes during M. sexta intersegmental muscle (ISM) atrophy and then used genetic approaches to confirm their functional importance in the D. melanogaster dorsal internal oblique muscles (DIOMs). Our data reveal an upregulation of proteasome and peptidase components and a general downregulation of proteins that regulate actin filament formation. Surprisingly, thick filament proteins that comprise the A-band are increased in abundance, providing support for the ordered destruction of myofibrillar components during developmental atrophy. We also uncovered the actin filament regulator ciboulot (Cib) as a novel regulator of muscle atrophy. These insights provide a framework towards a better understanding of global changes that occur during atrophy and may eventually lead to therapeutic targets.

A neural mechanism of nuclear receptor expression and regionalization

Spatially restricted expression of genes by global circulating inducers (hormones, secreted proteins, growth factors, neuromodulators, etc.) was a prerequisite for the evolution of animals. Far from a random occurrence, it is a systematically occurring, certain event, implying that specific information is invested for it to happen. In this minireview, we show for the first time that the expression and regionalization takes place at the level of receptors via a neural mechanism and make an attempt to reconstruct the causal chain from neural signaling to expression of nuclear receptors.

Programmed cell death of larval tissues induced by juvenile hormone in the bamboo borer, Omphisa fuscidentalis

Programmed cell death (PCD) plays a critical role during animal development through the destruction of unneeded cells and tissues. In some insects, the prothoracic glands (PGs) and anterior silk glands (ASGs) are larval-specific tissues that are normally eliminated by PCD after pupation. Previous studies report that juvenile hormone analog (JHA) terminates the larval diapause of Omphisa fuscidentalis by increasing the hemolymph ecdysteroids that trigger PCD. Because JHA may indirectly induce the PCD of the PGs and ASGs of Omphisa diapausing larvae, the effects of JHA on the induction of PCD were determined. The application of 1μg JHA induced PCD in the PGs and ASGs of larvae identified as stage G0 (prior to pupation). The injection of 1μg 20E triggered the PCD of the ASGs when the larvae expressed a G0-G1 morphology, whereas PCD occurred in the PGs on day 1 post-injection. Histological studies revealed similar patterns of morphological changes during the PG and ASG PCD in the JHA- and 20E-treated larvae. Furthermore, to confirm that PCD was induced by a high ecdysteroid level that increases after JHA application, the expression profiles of EcR-A and EcR-B1 in the PGs and ASGs from the JHA-treated larvae were examined, and the results showed that the expression levels of EcR-A and EcR-B1 mRNA increased during the G0 stage. These results suggest that JHA may be involved in PCD by increasing the ecdysteroid titer, leading to termination of the larval diapause period in Omphisa fuscidentalis.

Ultrastructure and morphology of midgut visceral muscle in early pupal Aedes aegypti mosquitoes

These studies focus on the pupal Aedes aegypti midgut muscularis for the first 26 h following larval-pupal transition. The midgut muscularis of Ae. aegypti pupae during this first half of the pupal stadium is a grid of both circularly and longitudinally oriented muscle bands, arranged in a manner resembling that of the larvae. While many muscle bands exhibit signs of degeneration during the time period studied, not all bands degrade, nor is this degradation simultaneous. Band deterioration involves destruction of internal elements while the muscle fiber plasma membrane remains intact. Deterioration of contractile elements may involve proteosome-like structures and associated enzymes. Many features of the larval muscularis including cruciform cells, bifurcating circular bands, and bifurcating longitudinal bands of muscle are retained during the time period investigated. Neuromuscular junctions along some muscle bands are retained through at least 16 h into the pupal stadium. The selective nature of muscle fiber degradation, coupled with the retention of larval features and neural input, may allow for limited functionality of the muscularis during metamorphosis. Evidence of sexual dimorphism in the midgut muscularis of male and female Ae. aegypti pupae was not observed during the time period studied.

EcR and Usp, components of the ecdysteroid nuclear receptor complex, exhibit differential distribution of molecular determinants directing subcellular trafficking

Ecdysteroids coordinate development, reproduction and other essential biological processes in insects and other arthropods through the receptor which is a heterodimer of two members of the nuclear receptors superfamily, the ecdysteroid receptor (EcR) and the Ultraspiracle (Usp). Although the transcriptionally active EcR/Usp heterocomplex is believed to be the only functional form of the receptor, there are data indicating that EcR may be involved in the mediation of the non-genomic effects outside of the nucleus. Since the nucleocytoplasmic shuttling could be a key element determining participation of the single nuclear receptor molecule both in the genomic and non-genomic functions we have analyzed nuclear import and export properties of the EcR and Usp from Drosophila melanogaster. We show for the first time that both receptors exhibit differential distribution of the nuclear localization and nuclear export signals (NLSs and NESs). In particular, the Usp which exhibits exclusively nuclear localization in all cell types analyzed, contains apparently only NLS activity within the DNA-binding domain. In contrast, the three known EcR isoforms (A, B1 and B2) are mosaics of elements which can potentially mediate their nucleocytoplasmic shuttling. We have found two active NESs in ligand binding domain and NLS activity within the DNA-binding domain of all isoforms. Simultaneously we demonstrate that B1 and A isoforms possess an additional NLS activity localized in AB regions. We speculate that this characteristic, along with the previously reported structural pliability of the EcR molecule, allows the single receptor to evoke many different genomic as well as non-genomic ecdysteroid-dependent responses.

Remodeling of an identified motoneuron during metamorphosis: hormonal influences on the growth of dendrites and axon terminals

During metamorphosis of the tobacco hawkmoth Manduca sexta, the femoral depressor motoneuron (FeDe MN) undergoes remodeling of its dendrites and motor terminals. Previous studies have established that remodeling of MNs during metamorphosis is mediated by the same hormones that control metamorphosis: the ecdysteroids and juvenile hormone (JH). During the pupal stage, the ecdysteroids promote adult-specific growth of MNs in the absence of JH, but JH or its synthetic mimics can interfere with ecdysteroid-mediated growth if applied during early sensitive periods. Hence, the application of a JH mimic (JHM) either systemically or locally to a target muscle has been used to distinguish those aspects of motor-terminal remodeling that are controlled by ecdysteroid action on the CNS from those that are influenced by ecdysteroid action on the peripheral targets. Here, we have extended the analysis of central and peripheral hormonal influences on MN remodeling by injecting JHM locally into the CNS thus altering the hormonal environment of the FeDe MN soma without altering the hormonal environment of its target muscle. Our results demonstrate that adult dendritic growth and motor-terminal growth can be experimentally uncoupled, suggesting that each is regulated independently. JHM application to the CNS perturbed dendritic growth, but had no measurable impact on motor-terminal growth. Peripheral actions of ecdysteroids, therefore, appear sufficient to promote the development of adult-specific motor terminals but not the development of an adult-specific dendritic arbor.

Segment-specific muscle degeneration is triggered directly by a steroid hormone during insect metamorphosis

During metamorphosis of the hawkmoth, Manduca sexta, some larval muscles degenerate while others are respecified for new functions. In larvae, accessory planta retractor muscles (APRMs) are present in abdominal segments 1 to 6 (A1 to A6). APRMs serve as proleg retractors in A3 to A6 and body wall muscles in A1 and A2. At pupation, all APRMs degenerate except those in A2 and A3, which are respecified to circulate hemolymph in pupae. The motoneurons that innervate APRMs, the APRs, likewise undergo segment-specific programmed cell death (PCD), as a direct, cell-autonomous response to the prepupal peak of ecdysteroids. The segment-specific patterns of APR and APRM death differ. The present study tested the hypothesis that APRM death is a direct, cell-autonomous response to the prepupal peak of ecdysteroids. Prevention of the prepupal peak prevented APRM degeneration, and replacement of the peak by infusion of 20-hydroxyecdysone restored the correct segment-specific pattern of APRM degeneration. Surgical denervation of APRMs did not perturb their segment-specific degeneration at pupation, indicating that signals from APRs are not required for the muscles' segment-specific responses to ecdysteroids. The possibility that instructive signals originate from APRMs' epidermal attachment points was tested by treating the epidermis with a juvenile hormone analog to prevent pupal development. This manipulation likewise did not alter APRM fate. We conclude that both the muscles and motoneurons in this motor system respond directly and cell-autonomously to prepupal ecdysteroids to produce a segment-specific pattern of PCD that is matched to the functional requirements of the pupal body.

Edysteroid receptor (EcR) shows marked differences in temporal patterns between tissues during larval-adult development in Rhodnius prolixus: correlations with haemolymph ecdysteroid titres

The presence of ecdysteroid receptor (EcR) in various tissues was studied throughout larval-adult development of the blood-sucking bug, Rhodnius prolixus, using an antibody to EcR that recognizes all isoforms. On Western blots, the antibody recognizes three peptides of approximate molecular masses of 70, 68 and 64 kDa, from epidermis and fat body of developing larvae, which contain high levels of haemolymph ecdysteroids. These peptides are absent from both unfed larvae and adults, which are devoid of ecdysteroids. In vitro treatment of epidermis and fat body from unfed larvae with 20E induces the appearance of all three EcR immunoreactive peptides. The stage-specific appearance and 20E inducibility of the peptides implies that they represent the native EcR(s) of Rhodnius. Confocal fluorescence analysis using this antibody revealed a great diversity of temporal profiles of EcR in various tissues during development. Developmental profiles of EcR were examined in abdominal epidermis, fat body, spermatocytes, brain (including the medial neurosecretory cells), prothoracic glands (PGs), rectal epithelium and Malpighian tubules. EcR fluorescence was confined to the nuclei in close association with chromatin. EcR was absent from tissues of unfed larvae or adults, supporting the results from Western blots. Different tissues develop EcR at different developmental times and in the presence of radically different concentrations of haemolymph ecdysteroids, retain EcR for different lengths of time and lose EcR at different concentrations of ecdysteroids. These results suggest that each tissue possesses a distinctive response mechanism to ecdysteroids. An exception to this, are the PGs, which exhibited no EcR fluorescence at any time during development.

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.

Characterization of crab EcR and RXR homologs and expression during limb regeneration and oocyte maturation

We report here complete coding sequences for the Uca pugilator homologs of the ecdysteroid (UpEcR) and retinoid-X receptors (UpRXR). Library screenings recovered cDNA clones containing a unique amino terminal open-reading frame (A/B domain) for each gene, most similar to insect B1 EcR and USP1/RXR isoforms. Splicing variants in the UpRXR ligand-binding domain were also identified, in a region critical for folding of Drosophila and lepidopteran USP. UpEcR and UpRXR proteins were able to associate, and both are required for binding to an ecdysteroid HRE; these interactions were not hormone-dependent. Ribonuclease protection assays (RPA) were conducted using A/B domain and 'common' (C or E) domain probes on RNA isolated from various stages of regenerating limb buds and ovaries. For several of the limb bud and ovarian stages examined, the relative level of A/B domain sequence protected was significantly less than common domain suggesting alternative amino terminal isoforms other than those isolated through cloning. This is the first report of UpEcR and UpRXR transcription during ovarian maturation, implicating the ovary as a potential target for hormonal control in Crustacea.

Structure and metamorphic remodeling of the larval nervous system and musculature of Phoronis pallida (Phoronida)

The structure of the larval nervous system and the musculature of Phoronis pallida were studied, as well as the remodeling of these systems at metamorphosis. The serotonergic portion of the apical ganglion is a U-shaped field of cell bodies that send projections into a central neuropil. The majority of the serotonergic cells are (at least) bipolar sensory cells, and a few are nonsensory cells. Catecholaminergic cell bodies border the apical ganglion. The second (hood) sense organ develops at competence and is composed of bipolar sensory cells that send projections into a secondary neuropil. Musculature of the competent larva includes circular and longitudinal muscle fibers of the body wall, as well as elevators and depressors of the tentacles and hood. The juvenile nervous system and musculature are developed prior to metamorphosis and are integrated with those of the larva. Components of the juvenile nervous system include a diffuse neural net of serotonergic cell bodies and fibers and longitudinal catecholaminergic fibers. The juvenile body wall musculature consists of longitudinal fibers that overlie circular muscle fibers, except in the cincture regions, where this pattern is reversed. Metamorphosis is initiated by the larval neuromuscular system but is completed by the juvenile neuromuscular system. During metamorphosis, the larval nervous system and the musculature undergo cell death, and the larval tentacles and gut are remodeled into the juvenile arrangement. Although the phoronid nervous system has often been described as deuterostome-like, these data show that several cytological aspects of the larval and juvenile neuromuscular systems also have protostome (lophotrochozoan) characteristics.

Control of muscle degeneration following autotomy of a hindleg in the grasshopper, Barytettix humphreysii

When the grasshopper, Barrytettix humphreysii, sheds a hindlimb during autotomy, certain thoracic muscles degenerate although they are neither directly damaged nor denervated. Muscle degeneration is induced when a leg nerve (N5) that does not innervate the thoracic muscles is severed. Together these results suggest that transneuronal mechanisms influence muscle survival. To further characterize this autotomy-induced process, we studied the degeneration of a thoracic tergotrochanteral muscle (M#133b,c) following autotomy or experimental manipulation in adult animals. Its degeneration is correlated with reduced activity of its neural input and occurs by programmed cell death (PCD). PCD onset is variable between individual muscle fibers, indicating that the trigger of degeneration is fiber specific. Muscle degeneration appears to be triggered by the loss of proprioceptive input from the autotomized limb, since severing of axons from proprioceptive organs, but not exteroceptive chemo- or mechanoreceptors, leads to muscle degeneration. Muscle disuse, neuronal degeneration, or changes in juvenile hormone titer do not appear to play a role in autotomy-induced degeneration. We propose that the loss of proprioceptive input from proximal campaniform sensilla on the tibia deafferents the thoracic muscle motor neurons and leads to a decrease in their activity. Muscle degeneration is ultimately triggered by the loss of normal neural activity.

Developmental change in the steroid hormone signal for cell-autonomous, segment-specific programmed cell death of a motoneuron

During metamorphosis of the hawkmoth, Manduca sexta, accessory planta retractor (APR) motoneurons undergo a segment-specific pattern of programmed cell death (PCD): e.g., APRs from abdominal segment six [APR(6)s] die at pupation in direct response to the prepupal rise in 20-hydroxyecdysone (20E), whereas APR(4)s survive through the pupal stage and die at eclosion (adult emergence). The hypothesis that the death of APR(4)s is triggered by the decline in 20E at eclosion was supported by findings that injection of 20E into developing pupae to delay the fall in 20E delayed APR(4) death. Furthermore, abdomen isolation to advance the fall in 20E caused precocious APR(4) death. In other experiments, APR(4)s were placed in primary cell culture 4 days before eclosion in medium containing 1 microg/ml 20E. A switch to hormone-free medium induced PCD in a significant proportion of APR(4)s, compared to APR(4)s that remained in 20E. Process fragmentation was a reliable early indicator of PCD. These results show that a decline in 20E triggers cell-autonomous PCD of APR(4)s, in contrast to the rise in 20E that triggers cell-autonomous PCD of APR(6)s. Thus, the PCD of homologous motoneurons in different body segments at different developmental times is triggered by different steroid hormone signals.

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.

Variability in the size, composition, and function of insect flight muscles

In order to fly, insects require flight muscles that constitute at least 12 to 16% of their total mass, and flight performance increases as this percentage increases. However, flight muscles are energetically and materially expensive to build and maintain, and investment in flight muscles constrains other aspects of function, particularly female fecundity. This review examines ways in which insects vary the size of their flight muscles, and how variation in the relative size and composition of flight muscles affects flight performance. Sources of variability in flight muscle size and composition include genetic differences within and between species, individual phenotypic responses to environmental stimuli, and maturational changes that occur before and during the adult stage. Insects have evolved a wide variety of ways to adjust flight muscle size and contractile performance in order to meet demands imposed by variation in life history and ecology.