Postsynaptic Mad Signaling at the Drosophila Neuromuscular Junction

BACKGROUND

Cell-to-cell communication at the synapse involves synaptic transmission as well as signaling mediated by growth factors, which provide developmental and plasticity cues. There is evidence that a retrograde, presynaptic transforming growth factor-beta (TGF-beta) signaling event regulates synapse development and function in Drosophila.

RESULTS

Here we show that a postsynaptic TGF-beta signaling event occurs during larval development. The type I receptor Thick veins (Tkv) and the R-Smad transcription factor Mothers-against-dpp (Mad) are localized postsynaptically in the muscle. Furthermore, Mad phosphorylation occurs in regions facing the presynaptic active zones of neurotransmitter release within the postsynaptic subsynaptic reticulum (SSR). In order to monitor in real time the levels of TGF-beta signaling in the synapse during synaptic transmission, we have established a FRAP assay to measure Mad nuclear import/export in the muscle. We show that Mad nuclear trafficking depends on stimulation of the muscle.

CONCLUSIONS

Our data suggest a mechanism linking synaptic transmission and postsynaptic TGF-beta signaling that may coordinate nerve-muscle development and function.

Sexual differentiation of identified motor terminals inDrosophila larvae

In Drosophila, we have found that some of the motor terminals in wandering third-instar larvae are sexually differentiated. In three out of the four body-wall muscle fibers that we examined, we found female terminals that produced a larger synaptic response than their male counterparts. The single motor terminal that innervates muscle fiber 5 produces an EPSP that is 69% larger in females than in males. This is due to greater release of transmitter from female than male synaptic terminals because the amplitude of spontaneous miniature EPSPs was similar in male and female muscle fibers. This sexual difference exists throughout the third-instar: it is seen in both early (foraging) and late (wandering) third-instar larvae. The sexual differentiation appears to be neuron specific and not muscle specific because the same axon produces Is terminals on muscle fibers 2 and 4, and both terminals produce larger EPSCs in females than males. Whereas, the Ib terminals innervating muscle fibers 2 and 4 are not sexually differentiated. The differences in transmitter release are not due to differences in the size of the motor terminals. For the terminal on muscle fiber 5 and the Is terminal on muscle fiber 4, there were no differences in terminal length, the number of branches, or the number of synaptic boutons in males compared to females. These sexual differences in neuromuscular synaptic physiology may be related to male-female differences in locomotion.

Development and structure of synaptic contacts in Drosophila

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

Synapse disassembly and formation of new synapses in postnatal muscle upon conditional inactivation of MuSK

The muscle-specific-kinase MuSK is required for the formation of acetylcholine receptor clusters during embryonic development, but its physiological role in adult muscle is not known. We used the loxP/Cre system in mice to conditionally inactivate MuSK whereby expression of Cre recombinase increases during postnatal development. The MuSK-inactivated mice develop myasthenic symptoms and die prematurely due to severe muscle weakness. The postnatal inactivation of MuSK causes loss of acetylcholine receptors and disassembly of the postsynaptic organization and innervating axons retract but start to grow and branch extensively. Due to the mosaic expression of Cre recombinase, MuSK is not globally inactivated and new synapses are formed aberrantly patterned across the diaphragm. Our findings demonstrate that MuSK kinase activity is required throughout postnatal development to hold up MuSK and AChR levels at endplates. Thus, MuSK and AChR together maintain the functional and structural integrity of the postsynaptic architecture and prevent axon growth.

Localization of neuronal calcium sensor-1 at the adult and developing rat neuromuscular junction

Neuronal calcium sensor (NCS-1; frequenin) is a calcium-binding protein involved in the regulation of neurotransmission in the central and peripheral nervous systems from insects to vertebrates. This study reports the localization of NCS-1 immunoreactivity, by Western blotting and immunohistochemistry, at the adult and developing postnatal rat neuromuscular junction. Our confocal immunofluorescence results on the whole-mount muscle and on semithin cross-sections are indicative of the localization of NCS-1 to motor axon terminals. There is no evidence of immunoreactivity in the postsynaptic side of the neuromuscular junctions or teloglial Schwann cells. These results suggest that NCS-1 is involved in the formation and function of presynaptic nerve terminal part of the neuromuscular junction during synaptogenesis and in adult mammals.

Development of postural muscles and their innervation

Control of posture is a prerequisite for efficient motor performance. Posture depends on muscles capable of enduring contractions, whereas movements often require quick, forceful muscle actions. To serve these different goals, muscles contain fibers that meet these different tasks. Muscles with strong postural functions mainly consist of slow muscle fibers with a great resistance against fatigue. Flexor muscles in the leg and arm muscles are mainly composed of fast muscle fibers producing relatively large forces that are rapidly fatigable. Development of the neuromuscular system continues after birth. We discuss in the human baby and in animal experiments changes in muscle fiber properties, regression from polyneural into mononeural innervation, and developmental changes in the motoneurons of postural muscles during that period. The regression of poly-neural innervation in postural muscles and the development of dendrite bundles of their motoneurons seem to be linked to the transition from the immature into the adult-like patterns of moving and postural control.

Accumulation of Nav1 mRNAs at differentiating postsynaptic sites in rat soleus muscles

Acetylcholine receptors (AChRs) and voltage-gated sodium channels (Na(V)1s) accumulate at different times in the development of the murine neuromuscular junction (NMJ). We used in situ hybridization to study the relationship of Na(V)1 mRNA accumulation to this difference. mRNAs encoding both muscle Na(V)1 isoforms, Na(v)1.4 and Na(v)1.5, were first concentrated at NMJs at birth, when the proteins start to accumulate. Within 4 weeks, Na(v)1.4 mRNA increased 5-fold at the NMJ while Na(v)1.5 mRNA became undetectable. Na(V)1 mRNA accumulation occurred even if the nerve was cut at birth. Like AChR mRNA, Na(V)1 mRNA accumulated at denervated synaptic sites on regenerating muscles and in response to ectopically expressed neural agrin. Clustering of Na(V)1 at the NMJ follows that of its mRNA while AChR clustering precedes its mRNA clustering by several days. This suggests that factors other than local mRNA upregulation determine the timing of clustering of these two important postsynaptic ion channels.

{alpha}-Syntrophin regulates ARMS localization at the neuromuscular junction and enhances EphA4 signaling in an ARMS-dependent manner

EphA4 signaling has recently been implicated in the regulation of synapse formation and plasticity. In this study, we show that ankyrin repeat-rich membrane spanning (ARMS; also known as a kinase D–interacting substrate of 220 kD), a substrate for ephrin and neurotrophin receptors, was expressed in developing muscle and was concentrated at the neuromuscular junction (NMJ). Using yeast two-hybrid screening, we identified a PDZ (PSD-95, Dlg, ZO-1) domain protein, α-syntrophin, as an ARMS-interacting protein in muscle. Overexpression of α-syntrophin induced ARMS clustering in a PDZ domain–dependent manner. Coexpression of ARMS enhanced EphA4 signaling, which was further augmented by the presence of α-syntrophin. Moreover, the ephrin-A1–induced tyrosine phosphorylation of EphA4 was reduced in C2C12 myotubes after the blockade of ARMS and α-syntrophin expression by RNA interference. Finally, α-syntrophin–null mice exhibited a disrupted localization of ARMS and EphA4 at the NMJ and a reduced expression of ARMS in muscle. Altogether, our findings suggest that ARMS may play an important role in regulating postsynaptic signal transduction through the syntrophin-mediated localization of receptor tyrosine kinases such as EphA4.

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.

Neurotrophin-3 null mutant mice display a postnatal motor neuropathy

This paper examines early postnatal development of the neuromuscular system in mice with a null mutation in the gene for neurotrophin-3. We report that alpha-motoneurons at first develop substantially normally, despite a known 15% deficit in their somal size [Woolley et al. (1999)Neurosci. Lett., 272, 107-110.] and the absence of proprioceptive input [Ernfors et al. (1994)Cell, 77, 503-512]. At birth, motor axons have extended into the muscle, forming normal-looking neuromuscular junctions with focal accumulations of acetylcholine receptors. Detailed ultrastructural analysis does however, reveal subtle abnormalities at this time, particularly a decrease in the extent of occupancy of the postsynaptic site by nerve terminals, and a small but significant deficit in myofibre number. After the relative normality of this early neuromuscular development, there then occurs a catastrophic postnatal loss of motor nerve terminals, resulting in complete denervation of hindlimb muscles by P7. In systematic semi-serial samples through the entire muscle endplate zones, no neuromuscular junctions can be found. Intramuscular axons are fragmented, as shown by both electron microscopic observations and neurofilament immunohistochemistry, and alpha-bungarotoxin detection of acetylcholine receptors indicates dispersal of the junctional accumulation. At earlier times (postnatal days three and four) the terminal Schwann cells show ultrastructural abnormalities, and preliminary observations suggest marked disturbance of myelination. Based on comparison with other literature, the peripheral nerve degeneration seems unlikely to have arisen as a secondary effect of de-afferentation. We discuss whether the neural degeneration is secondary to the disturbance of Schwann cell function, or due directly to a loss of neurotrophin-3 based support of the motoneuron.

Shaggy, the homolog of glycogen synthase kinase 3, controls neuromuscular junction growth in Drosophila

A protein-trap screen using the Drosophila neuromuscular junction (NMJ) as a model synapse was performed to identify genes that control synaptic structure or plasticity. We found that Shaggy (Sgg), the Drosophila homolog of the mammalian glycogen synthase kinases 3 alpha and beta, two serine-threonine kinases, was concentrated at this synapse. Using various combinations of mutant alleles of shaggy, we found that Shaggy negatively controlled the NMJ growth. Moreover, tissue-specific expression of a dominant-negative Sgg indicated that this kinase is required in the motoneuron, but not in the muscle, to control NMJ growth. Finally, we show that Sgg controlled the microtubule cytoskeleton dynamics in the motoneuron and that Futsch, a microtubule-associated protein, was required for Shaggy function on synaptic growth.

The physiological and behavioral effects of carbon dioxide on Drosophila melanogaster larvae

Adult and larval insects are rapidly anesthetized by carbon dioxide (CO2); however, the mechanisms have not been addressed. In this study, we use larval Drosophila to investigate the actions of CO2 to explain the behavioral effects of rapid immobilization and cardiac arrest with acute exposure to CO2. To determine if the central nervous system (CNS) is required, studies were performed with and without the CNS. The effects of low pH induced by exposure to CO2 were also examined. An acidic saline increases the heart rate in contrast to saline containing CO2. Synaptic transmission at the skeletal neuromuscular junction (NMJ) is blocked by CO2 but not by low pH. The site of action is postsynaptic by a decreased sensitivity to glutamate, the neurotransmitter at Drosophila NMJs. The CNS remains active in synaptic transmission when exposed to CO2 which is in contrast to the synapses at the NMJ. In summary, the effects of CO2 are directly mediated on the heart to stop it and at skeletal NMJs by a reduced sensitivity to glutamate, the released neurotransmitter, from the motor nerve terminals. The rapid behavioral and physiological effects cannot be accounted for by action on the CNS within the larvae nor by a pH effect indirectly induced by CO2. The glutamate receptors in the D. melanogaster preparation are similar in function to ionotropic glutamate receptors in vertebrates which could account for the observational phenomena of CO2 not yet explained mechanistically in vertebrates.

Population density regulates Drosophila synaptic morphology in a Fasciclin-II-dependent manner

Genetic analysis of the Drosophila larval neuromuscular junction has identified some of the key molecules that regulate synaptic plasticity. Among these molecules, the expression level of Fasciclin II (FasII), a homophilic cell adhesion molecule, is critically important for determining the final form of the neuromuscular junction. Genetic reduction of FasII expression by 50% yields more elaborate nerve terminals, while a greater reduction in expression, to 10% of wild-type, yields a substantial reduction in the nerve terminal morphology. Importantly, regulation of FasII expression seems to be the final output for several genetic manipulations that transform NMJ morphology. In an effort to understand the importance of this regulatory pathway in the normal animal, we have undertaken studies to identify environmental cues that might be important for initiating FasII-dependent changes in synaptic plasticity. Here we report on the relationship between larval population density and synaptic morphology, synaptic strength, and FasII levels. We raised Drosophila larvae under conditions of increasing population density and found an inverse exponential relationship between population density and the number of synaptic boutons, the number of branches, and the length of branches. We also observed population-dependent alteration in FasII levels, with lower densities having less FasII at the synapse. The correlation between density and morphological change was abrogated in larvae constitutively expressing FasII, and in wild-type larvae grown on soft culture medium. Together these data show that environmental cues can induce regulation of FasII. Interestingly, however, the quantal content of synaptic transmission was not different among the different population densities, suggesting that other factors contribute to maintaining synaptic strength at a defined level.

Independent regulation of synaptic size and activity by the anaphase-promoting complex

Neuronal plasticity relies on tightly regulated control of protein levels at synapses. One mechanism to control protein abundance is the ubiquitin-proteasome degradation system. Recent studies have implicated ubiquitin-mediated protein degradation in synaptic development, function, and plasticity, but little is known about the regulatory mechanisms controlling ubiquitylation in neurons. In contrast, ubiquitylation has long been studied as a central regulator of the eukaryotic cell cycle. A critical mediator of cell-cycle transitions, the anaphase-promoting complex/cyclosome (APC/C), is an E3 ubiquitin ligase. Although the APC/C has been detected in several differentiated cell types, a functional role for the complex in postmitotic cells has been elusive. We describe a novel postmitotic role for the APC/C at Drosophila neuromuscular synapses: independent regulation of synaptic growth and synaptic transmission. In neurons, the APC/C controls synaptic size via a downstream effector Liprin-alpha; in muscles, the APC/C regulates synaptic transmission, controlling the concentration of a postsynaptic glutamate receptor.

Effects of neurotrophin-3 gene mutation in the expression of neurocalcin

Neurocalcin (NC) is a neuron-specific "EF-hand" calcium-binding protein present in a non-fully characterized subpopulation of dorsal root ganglion (DRG) neurons, some kinds of mechanoreceptors and proprioceptors, and in motor end-plates. In the present study we have characterized NC expression in spinal sensory and motor neurons, and their endings in newborn mouse. Because the neurotrophic factor neurotrophin-3 (NT-3) appears to plays a major role in the development and maturation of sensory and motor neuronal populations, we have studied NC immunoreactivity in newborn NT-3 null mutant. In NT-3 deficient animals the overall number of NC-immunoreactive DRG neurons was reduced by as much as 70% including all large neurons, but subpopulations of NC expressing small and intermediate-sized neurons survived. As expected no muscle spindles were found in NT-3 mutant mice while they were present and normally innervated by NC-positive nerve fibers in wild-type animals. On the other hand, NC immunoreactivity was dramatically decreased in motoneurons of the spinal cord, ventral root nerves and motor end-plates in the absence of NT-3. The present results demonstrate that NC-containing DRG neurons include all proprioceptive, and a subset of mechanoreceptive and proprioceptive. Furthermore, they strongly suggest that NT-3 is involved in the maturation of motor end-plates.

Pruning of motor neuron branches establishes the DLM innervation pattern in Drosophila

During the Drosophila life-cycle two sets of neuromuscular junctions are generated: the embryonic/larval NMJs develop during the first half, followed by the period of metamorphosis during which the adult counterpart is generated. Development of the adult innervation pattern is preceded by a withdrawal of larval NMJs, which occurs at the onset of metamorphosis, and is followed by adult-specific motor neuron outgrowth to innervate the newly developing adult fibers. Establishment of the adult innervation pattern occurs in the context of a broader restructuring of the nervous system, which results in the development of neural circuits that are necessary to carry out behaviors specific to the adult. In this article, we follow development of the dorsal longitudinal muscle (DLM) innervation pattern through metamorphosis. We find that the initial period of motor neuron elaboration is followed by a phase of extensive pruning resulting in a threefold reduction of neuromuscular contacts. This event establishes the adult pattern of second order branching. Subsequent higher order branching from the second order "contact" points generates the characteristic multiterminal innervation pattern of the DLMs. Boutons begin to appear after the pruning phase, and are much smaller than their larval counterparts. Additionally, we demonstrate that the DLM innervation is altered in the hyperexcitable double mutant, ether a go-go Shaker, and that the phenotype is suppressed by the hypoexcitable mutant, nap(ts1). Our results demonstrate that electrical activity regulates the patterning of DLM innervation during metamorphosis.

Nicotine decreases agrin signaling and acetylcholine receptor clustering in C2C12 myotube culture

The clustering of acetylcholine receptors (AChRs) in skeletal muscle fibers is a critical event in neuromuscular synaptogenesis. AChRs in concert with other molecules form postsynaptic scaffolds in response to agrin released from motor neurons as motor neurons near skeletal muscle fibers in development. Agrin drives an intracellular signaling pathway that precedes AChR clustering and includes the tyrosine phosphorylation of AChRs. In C2C12 myotube culture, agrin application stimulates the agrin signaling pathway and AChR clustering. Previous studies have determined that the frequency of spontaneous AChR clustering is decreased and AChRs are partially inactivated when bound by the acetylcholine agonist nicotine. We hypothesized that nicotine interferes with AChR clustering and consequent postsynaptic scaffold formation. In the present study, C2C12 myoblasts were cultured with growth medium to stimulate proliferation and then differentiation medium to stimulate fusion into myotubes. They were bathed in a physiologically relevant concentration of nicotine and then subject to agrin treatment after myotube formation. Our results demonstrate that nicotine decreases agrin-induced tyrosine phosphorylation of AChRs and decreases the frequency of spontaneous as well as agrin-induced AChR clustering. We conclude that nicotine interferes with postsynaptic scaffold formation by preventing the tyrosine phosphorylation of AChRs, an agrin signaling event that precedes AChR clustering.

Conserved and muscle-group-specific gene expression patterns shape postnatal development of the novel extraocular muscle phenotype

Current models in skeletal muscle biology do not fully account for the breadth, causes, and consequences of phenotypic variation among skeletal muscle groups. The muscle allotype concept arose to explain frank differences between limb, masticatory, and extraocular (EOM) muscles, but there is little understanding of the developmental regulation of the skeletal muscle phenotypic range. Here, we used morphological and DNA microarray analyses to generate a comprehensive temporal profile for rat EOM development. Based upon coordinate regulation of morphologic/gene expression traits with key events in visual, vestibular, and oculomotor system development, we propose a model that the EOM phenotype is a consequence of extrinsic factors that are unique to its local environment and sensory-motor control system, acting upon a novel myoblast lineage. We identified a broad spectrum of differences between the postnatal transcriptional patterns of EOM and limb muscle allotypes, including numerous transcripts not traditionally associated with muscle fiber/group differences. Several transcription factors were differentially regulated and may be responsible for signaling muscle allotype specificity. Significant differences in cellular energetic mechanisms defined the EOM and limb allotypes. The allotypes were divergent in many other functional transcript classes that remain to be further explored. Taken together, we suggest that the EOM allotype is the consequence of tissue-specific mechanisms that direct expression of a limited number of EOM-specific transcripts and broader, incremental differences in transcripts that are conserved by the two allotypes. This represents an important first step in dissecting allotype-specific regulatory mechanisms that may, in turn, explain differential muscle group sensitivity to a variety of metabolic and neuromuscular diseases.

Phosphorylation reactions in activity-dependent synapse modification at the neuromuscular junction during development

We have studied developmental activity-dependent synapse diminution in both an in vitro tissue culture chamber system and at the intact rodent neuromuscular junction (nmj). In both types of preparations, pre- and postsynaptic alterations in synapse structure and function are produced by manipulations of thrombin (Thr) and protein kinase C (PKC) activity. An opposing postsynaptic effect of PKC and protein kinase A (PKA) action on the acetycholine receptor (AChR) can be shown in vitro with PKA stabilizing and PKC destabilizing the nmj synapses. In vivo studies of normal junctional maturation show that changes in axonal inputs and postsynaptic receptor cluster morphology occur, to a substantial degree, independently of one another. Presynaptic actions of PKA are involved in the activity dependent synapse modulation that can be demonstrated in vitro. Late in the elimination process, (>12 days in vivo ) the process becomes independent of PKC, implying that diverse, redundant mechanisms are involved in this important developmental process.

The role of nitric oxide signaling in the formation of the neuromuscular junction

The formation of the vertebrate neuromuscular junction (NMJ) depends on the action of neural agrin on the muscle cell. The requirement for agrin and its receptor, muscle-specific kinase (MuSK), has been well established over the past 20 years. However, the signaling mechanisms through which agrin and MuSK cause synaptic differentiation are not well understood. New evidence from studies of muscle cells in culture and in embryos indicates that nitric oxide (NO) is an effector of agrin-induced postsynaptic differentiation at the NMJ. Cyclic GMP (cGMP) production by guanylate cyclase appears to be an important downstream step in this pathway. Nitric oxide and cGMP regulate the activity of several kinases, some of which may influence interaction of dystrophin and utrophin with the actin cytoskeleton to mediate or modulate postsynaptic differentiation in muscle cells. These signaling molecules could also play a role in retrograde signaling to influence differentiation of presynaptic nerve terminals.

Activity-dependent switch from synapse formation to synapse elimination during development of neuromuscular junctions

The embryonic development of neuromuscular junctions consists of two successive epochs, an early period marked by exuberant synapse formation and a later period marked by synapse elimination. In the frog muscles we have studied, myogenesis is protracted and overlaps the periods of synapse formation and elimination. Thus, the formative and regressive events of synaptic development do not occur in synchrony across different fibers in the muscle. We propose that local activity orchestrates a shift from synaptogenesis to synapse elimination at the level of single muscle fibers. We also present evidence that perisynaptic Schwann cells and the expression of ion channels in the sarcolemma play important roles in the development of neuromuscular junctions. Questions for future study are outlined.

Cytokines, growth factors and sprouting at the neuromuscular junction

The formation of neuronal sprouts, either from synaptic terminals or nearby nodes of Ranvier, is a widely known form of plasticity of motoneurons. Sprouts form in response to several stimuli, but most notably in partially denervated or paralyzed muscle. In search of the cellular or molecular basis of this phenomenon, several largely parallel lines of investigation have been pursued. Strong evidence is presented that at least four cytokines or growth factors may be involved in motoneuron sprouting, each of which uses a distinctive signaling pathway. Three of the different proposed sprouting molecules: neuroleukin, insulin-like growth factor, and neural cell adhesion molecules can be viewed as muscle-derived retrograde signaling molecules of roughly equal potency to induce motoneurons to sprout. A fourth molecule, ciliary neurotrophic factor (CNTF) is likely to form an essential anterograde signal, from Schwann cells to muscle fibers, that ultimately produces sprouting. Other cytokines and growth factors such a neurotrophins or GDNF family members are discussed, but their role in motoneuron sprouting is less clear. These cytokines and growth factors could represent redundant mechanisms for self-repair of the neuromuscular junction or they could interact at different levels of their cellular pathways.

Characterization of development, behavior and neuromuscular physiology in the phorid fly, Megaselia scalaris

The Phoridae is known as 'scuttle flies' because they walk in rapid bursts of movement with short pauses between. In this study, larval locomotive behavior and development was characterized in the phorid, Megaselia scalaris. Comparison was made with the well-characterized fruit fly model, Drosophila melanogaster. Developmentally, the rate of maturation was consistently slower for Megaselia than Drosophila. This disparity was exaggerated at lower temperatures, particularly during larval development. In addition to slower growth, movements in Megaselia were also slower, as evidenced by reduced rates of larval body wall contractions and mouth hook movements. Megaselia larvae also displayed a unique behavior of swallowing air when exposed to a small pool of liquid. This permitted floating upon immersion and, therefore, might prevent drowning in the natural environment. The anatomical and physiological properties of a neuromuscular junction in the phorid larvae were also examined. The innervation of the motor nerve terminals on the ventral abdominal muscle (m6) is innervated by Type Ib and Is axons, similar to Drosophila. As in Drosophila, the Is terminals produce larger excitatory postsynaptic potentials (EPSPs) than the Ib. The amplitudes of the EPSPs in M. scalaris were reduced compared to those of D. melanogaster, but unlike D. melanogaster the EPSPs showed marked facilitation when stimulated with a 20 Hz train. We conclude that there may be differences in synaptic structure of the nerve terminals that could account for the different electrophysiological behaviors.

Development of neuromuscular junctions in the mouse esophagus: Morphology suggests a role for enteric coinnervation during maturation of vagal myoneural contacts

The time course of establishment of motor endplates and the subsequent developmental changes in their enteric and vagal innervation were examined in esophageal striated muscle of perinatal and adult C57/Bl6 mice by using immunocytochemistry and confocal laser scanning microscopy. Nicotinic acetylcholine receptors were visualized with alpha-bungarotoxin; vagal motor nerve terminals with antisera against vesicular acetylcholine transporter; and enteric nerve fibers with antisera against neuronal nitric oxide synthase, vasoactive intestinal peptide, and galanin. Because the various stages of esophageal striated myogenesis advance caudocranially, i.e., more mature stages are found cranial to immature stages, longitudinal cryosections through the esophagus were investigated. Synaptogenesis was divided into several distinct stages. 1) Mononucleated cells express acetylcholine receptors over their entire surface. 2) They start to cluster receptors without nerve fiber contacts. 3) The first nerve contact on a growing receptor cluster is made by a vagal nerve terminal, followed by an enteric terminal. 4) Vagal terminals grow until they match the size of endplate areas, and one to three enteric terminals intertwine with them on every receptor cluster. 5) After vagal terminals have covered the whole endplate area, enteric terminals are withdrawn from the majority of motor endplates. In a minority of endplates, enteric coinnervation persists through adulthood. The enteric innervation of all developing motor endplates, shortly after vagal terminals have contacted them, and the removal of enteric nerve fibers from the majority of mature motor endplates suggest a major role of enteric nerve fibers during maturation of esophageal neuromuscular junctions.