Developmental consequences of neuromuscular junctions with reduced presynaptic calcium channel function

The Ih Channel Gene Promotes Synaptic Transmission and Coordinated Movement in Drosophila melanogaster

Hyperpolarization-activated cyclic nucleotide-gated “HCN” channels, which underlie the hyperpolarization-activated current (I_h), have been proposed to play diverse roles in neurons. The presynaptic HCN channel is thought to both promote and inhibit neurotransmitter release from synapses, depending upon its interactions with other presynaptic ion channels. In larvae of Drosophila melanogaster, inhibition of the presynaptic HCN channel by the drug ZD7288 reduces the enhancement of neurotransmitter release at motor terminals by serotonin but this drug has no effect on basal neurotransmitter release, implying that the channel does not contribute to firing under basal conditions. Here, we show that genetic disruption of the sole HCN gene (Ih) reduces the amplitude of the evoked response at the neuromuscular junction (NMJ) of third instar larvae by decreasing the number of released vesicles. The anatomy of the (NMJ) is not notably affected by disruption of the Ih gene. We propose that the presynaptic HCN channel is active under basal conditions and promotes neurotransmission at larval motor terminals. Finally, we demonstrate that Ih partial loss-of-function mutant adult flies have impaired locomotion, and, thus, we hypothesize that the presynaptic HCN channel at the (NMJ) may contribute to coordinated movement.

Subclinical Neuromuscular Transmission Abnormality Detected by Single-Fibre EMG is More Pronounced in Cluster Headache Than in Migraine With Aura

The aim was to investigate neuromuscular transmission (NMT) by single-fibre EMG (SFEMG) in a large series of patients having migraine with aura (MA) or cluster headache (CH). Recent studies using SFEMG have shown subclinical dysfunction of NMT in MA and CH. Forty-three patients having MA, 51 with CH and 38 healthy control subjects underwent nerve conduction studies, EMG and SFEMG during voluntary contraction of the extensor digitorum communis muscle. Twenty different potential pairs were recorded and individual, mean and total abnormal individual jitter values were calculated. The results obtained from MA patients were compared with those from CH patients. In MA patients, 32 of 860 jitters were abnormally high, whereas 73 of 1020 of the jitters showed this abnormality in CH patients. None of the control subjects, five MA patients (11.6%) and 11 CH patients (21.6%) were designated as having subclinical NMT abnormality. Thus, patients having junction dysfunction were significantly more common in the CH group. The subclinical NMT abnormality shown by SFEMG is more common in CH than in MA. These two primary headache syndromes may have some shared functional abnormality of NMT constituents which is more evident in CH.

How voltage-gated calcium channels gate forms of homeostatic synaptic plasticity

Throughout life, animals face a variety of challenges such as developmental growth, the presence of toxins, or changes in temperature. Neuronal circuits and synapses respond to challenges by executing an array of neuroplasticity paradigms. Some paradigms allow neurons to up- or downregulate activity outputs, while countervailing ones ensure that outputs remain within appropriate physiological ranges. A growing body of evidence suggests that homeostatic synaptic plasticity (HSP) is critical in the latter case. Voltage-gated calcium channels gate forms of HSP. Presynaptically, the aggregate data show that when synapse activity is weakened, homeostatic signaling systems can act to correct impairments, in part by increasing calcium influx through presynaptic CaV2-type channels. Increased calcium influx is often accompanied by parallel increases in the size of active zones and the size of the readily releasable pool of presynaptic vesicles. These changes coincide with homeostatic enhancements of neurotransmitter release. Postsynaptically, there is a great deal of evidence that reduced network activity and loss of calcium influx through CaV1-type calcium channels also results in adaptive homeostatic signaling. Some adaptations drive presynaptic enhancements of vesicle pool size and turnover rate via retrograde signaling, as well as de novo insertion of postsynaptic neurotransmitter receptors. Enhanced calcium influx through CaV1 after network activation or single cell stimulation can elicit the opposite response-homeostatic depression via removal of excitatory receptors. There exist intriguing links between HSP and calcium channelopathies-such as forms of epilepsy, migraine, ataxia, and myasthenia. The episodic nature of some of these disorders suggests alternating periods of stable and unstable function. Uncovering information about how calcium channels are regulated in the context of HSP could be relevant toward understanding these and other disorders.

Analysis of various physiological salines for heart rate, CNS function, and synaptic transmission at neuromuscular junctions in Drosophila melanogaster larvae

Drosophila serves as a playground for examining the effects of genetic mutations on development, physiological function and behavior. Many physiological measures that address the effects of mutations require semi-intact or cultured preparations. To obtain consistent physiological recordings, cellular function needs to remain viable. Numerous physiological salines have been developed for fly preparations, with emphasis on nervous system viability. The commonly used saline drifts in pH and will cause an alteration in the heart rate. We identify a saline that maintains a stable pH and physiological function in the larval heart, skeletal neuromuscular junction, and ventral nerve cord preparations. Using these common assays, we screened various pH buffers of differing concentrations to identify optimum conditions. Buffers at 25 mM produce a stable heart rate with minimal variation in pH. Excitatory junction potentials evoked directly on larval muscles or through sensory-CNS-motor circuits were unaffected by at buffers at 25 mM. The salines examined did not impede the modulatory effect of serotonin on heart rate or neural activity. Together, our results indicate that the higher buffer concentrations needed to stabilize pH in HL3 hemolymph-like saline do not interfere with the acute function of neurons or cardiac myocytes.

Development and plasticity of the Drosophila larval neuromuscular junction

The Drosophila larval neuromuscular system is relatively simple, containing only 32 motor neurons in each abdominal hemisegment, and its neuromuscular junctions (NMJs) have been studied extensively. NMJ synapses exhibit developmental and functional plasticity while displaying stereotyped connectivity. Drosophila Type I NMJ synapses are glutamatergic, while the vertebrate NMJ uses acetylcholine as its primary neurotransmitter. The larval NMJ synapses use ionotropic glutamate receptors (GluRs) that are homologous to AMPA-type GluRs in the mammalian brain, and they have postsynaptic scaffolds that resemble those found in mammalian postsynaptic densities. These features make the Drosophila neuromuscular system an excellent genetic model for the study of excitatory synapses in the mammalian central nervous system. The first section of the review presents an overview of NMJ development. The second section describes genes that regulate NMJ development, including: (1) genes that positively and negatively regulate growth of the NMJ, (2) genes required for maintenance of NMJ bouton structure, (3) genes that modulate neuronal activity and alter NMJ growth, (4) genes involved in transsynaptic signaling at the NMJ. The third section describes genes that regulate acute plasticity, focusing on translational regulatory mechanisms. As this review is intended for a developmental biology audience, it does not cover NMJ electrophysiology in detail, and does not review genes for which mutations produce only electrophysiological but no structural phenotypes.

Comparison of parallel high-throughput RNA sequencing between knockout of TDP-43 and its overexpression reveals primarily nonreciprocal and nonoverlapping gene expression changes in the central nervous system of Drosophila

The human Tar-DNA binding protein, TDP-43, is associated with amyotrophic lateral sclerosis (ALS) and other neurodegenerative disorders. TDP-43 contains two conserved RNA-binding motifs and has documented roles in RNA metabolism, including pre-mRNA splicing and repression of transcription. Here, using Drosophila melanogaster as a model, we generated loss-of-function and overexpression genotypes of Tar-DNA binding protein homolog (TBPH) to study their effect on the transcriptome of the central nervous system (CNS). By using massively parallel sequencing methods (RNA-seq) to profile the CNS, we find that loss of TBPH results in widespread gene activation and altered splicing, much of which are reversed by rescue of TBPH expression. Conversely, TBPH overexpression results in decreased gene expression. Although previous studies implicated both absence and mis-expression of TDP-43 in ALS, our data exhibit little overlap in the gene expression between them, suggesting that the bulk of genes affected by TBPH loss-of-function and overexpression are different. In combination with computational approaches to identify likely TBPH targets and orthologs of previously identified vertebrate TDP-43 targets, we provide a comprehensive analysis of enriched gene ontologies. Our data suggest that TDP-43 plays a role in synaptic transmission, synaptic release, and endocytosis. We also uncovered a potential novel regulation of the Wnt and BMP pathways, many of whose targets appear to be conserved.

Glycosylated synaptomatrix regulation of trans-synaptic signaling

Synapse formation is driven by precisely orchestrated intercellular communication between the presynaptic and the postsynaptic cell, involving a cascade of anterograde and retrograde signals. At the neuromuscular junction (NMJ), both neuron and muscle secrete signals into the heavily glycosylated synaptic cleft matrix sandwiched between the two synapsing cells. These signals must necessarily traverse and interact with the extracellular environment, for the ligand-receptor interactions mediating communication to occur. This complex synaptomatrix, rich in glycoproteins and proteoglycans, comprises heterogeneous, compartmentalized domains where specialized glycans modulate trans-synaptic signaling during synaptogenesis and subsequent synapse modulation. The general importance of glycans during development, homeostasis and disease is well established, but this important molecular class has received less study in the nervous system. Glycan modifications are now understood to play functional and modulatory roles as ligands and co-receptors in numerous tissues; however, roles at the synapse are relatively unexplored. We highlight here properties of synaptomatrix glycans and glycan-interacting proteins with key roles in synaptogenesis, with a particular focus on recent advances made in the Drosophila NMJ genetic system. We discuss open questions and interesting new findings driving this investigation of complex, diverse, and largely understudied glycan mechanisms at the synapse.

Notch signaling is required for activity-dependent synaptic plasticity at the Drosophila neuromuscular junction

The cell-surface-signaling protein Notch, is required for numerous developmental processes and typically specifies which of two adjacent cells will adopt a non-neuronal developmental fate. It has recently been implicated in long-term memory formation in mammals and Drosophila. Here, we investigated whether activity-dependent synaptic plasticity at the neuromuscular junctions (NMJs) of third instar Drosophila larvae depends on Notch signaling. The length and number of axonal branches and number of presynaptic sites (boutons) in NMJ vary with the level of synaptic activity, so we increased activity at the NMJ by two complementary methods: increasing the chronic growth temperature of third instar larvae from 18 to 28 degrees C and using the double-mutant ether-a-gogo,Shaker (eagSh), both of which increase NMJ size and bouton count. Animals homozygous for the functionally null, temperature-sensitive Notch alleles, N(ts1) and N(ts2), displayed no activity-dependent increase in NMJ complexity when reared at the restrictive temperature. Dominant-negative Notch transgenic expression also blocked activity-dependent plasticity. Ectopic expression of wild-type Notch and constitutively active truncated Notch transgenes also reduced activity-dependent plasticity, suggesting that there is a "happy medium" level of Notch activity in mediating NMJ outgrowth. Last, we show that endogenous Notch is primarily expressed in the presynaptic cell bodies where its expression level is positively correlated with motor neuron activity.

Extracellular matrix and its receptors in Drosophila neural development

Extracellular matrix (ECM) and matrix receptors are intimately involved in most biological processes. The ECM plays fundamental developmental and physiological roles in health and disease, including processes underlying the development, maintenance, and regeneration of the nervous system. To understand the principles of ECM-mediated functions in the nervous system, genetic model organisms like Drosophila provide simple, malleable, and powerful experimental platforms. This article provides an overview of ECM proteins and receptors in Drosophila. It then focuses on their roles during three progressive phases of neural development: (1) neural progenitor proliferation, (2) axonal growth and pathfinding, and (3) synapse formation and function. Each section highlights known ECM and ECM-receptor components and recent studies done in mutant conditions to reveal their in vivo functions, all illustrating the enormous opportunities provided when merging work on the nervous system with systematic research into ECM-related gene functions.

Different mechanisms of Ca2+ regulation that influence synaptic transmission: comparison between crayfish and Drosophila neuromuscular junctions

A brief historical background on synaptic transmission in relation to Ca(2+) dynamics and short-term facilitation is described. This study focuses on the mechanisms responsible for the regulation of intracellular calcium concentration ([Ca(2+)](i)) in high output terminals of larval Drosophila compared to a low-output terminal of the crayfish neuromuscular junction (NMJ). Three processes; plasmalemmal Na(+)/Ca(2+) exchanger [NCX], Ca(2+)-ATPase (PMCA), and sarcoplasmic/endoplasmic Ca(2+)-ATPase (SERCA) are important in regulating the [Ca(2+)](i) are examined. When the NCX is compromised by reduced [Na(+)](o), no consistent effect occurred; but a NCX blocker KB-R7943 decreased the excitatory postsynaptic potential (EPSP) amplitudes. Compromising the PMCA with pH 8.8 resulted in an increase in EPSP amplitude but treatment with a PMCA specific inhibitor carboxyeosin produced opposite results. Thapsigargin exposure to block the SERCA generally decreases EPSP amplitude. Compromising the activity of the above Ca(2+) regulating proteins had no substantial effects on short-term depression. The Kum(170TS) strain (with dysfunctional SERCA), showed a decrease in EPSP amplitudes including the first EPSP within the train. Synaptic transmission is altered by reducing the function of the above three [Ca(2+)](i) regulators; but they are not consistent among different species as expected. Results in crayfish NMJ were more consistent with expected results as compared to the Drosophila NMJ. It is predicated that different mechanisms are used for regulating the [Ca(2+)](i) in high and low output synaptic terminals.

Frequenin/NCS-1 and the Ca2+-channel alpha1-subunit co-regulate synaptic transmission and nerve-terminal growth

Drosophila Frequenin (Frq) and its mammalian and worm homologue, NCS-1, are Ca(2+)-binding proteins involved in neurotransmission. Using site-specific recombination in Drosophila, we created two deletions that removed the entire frq1 gene and part of the frq2 gene, resulting in no detectable Frq protein. Frq-null mutants were viable, but had defects in larval locomotion, deficient synaptic transmission, impaired Ca(2+) entry and enhanced nerve-terminal growth. The impaired Ca(2+) entry was sufficient to account for reduced neurotransmitter release. We hypothesized that Frq either modulates Ca(2+) channels, or that it regulates the PI4Kbeta pathway as described in other organisms. To determine whether Frq interacts with PI4Kbeta with consequent effects on Ca(2+) channels, we first characterized a PI4Kbeta-null mutant and found that PI4Kbeta was dispensable for synaptic transmission and nerve-terminal growth. Frq gain-of-function phenotypes remained present in a PI4Kbeta-null background. We conclude that the effects of Frq are not due to an interaction with PI4Kbeta. Using flies that were trans-heterozygous for a null frq allele and a null cacophony (encoding the alpha(1)-subunit of voltage-gated Ca(2+) channels) allele, we show a synergistic effect between these proteins in neurotransmitter release. Gain-of-function Frq phenotypes were rescued by a hypomorphic cacophony mutation. Overall, Frq modulates Ca(2+) entry through a functional interaction with the alpha(1) voltage-gated Ca(2+)-channel subunit; this interaction regulates neurotransmission and nerve-terminal growth.

Characterization of voltage-dependent Ca2+ currents in identified Drosophila motoneurons in situ

Voltage-dependent Ca2+ channels contribute to neurotransmitter release, integration of synaptic information, and gene regulation within neurons. Thus understanding where diverse Ca2+ channels are expressed is an important step toward understanding neuronal function within a network. Drosophila provides a useful model for exploring the function of voltage-dependent Ca2+ channels in an intact system, but Ca2+ currents within the central processes of Drosophila neurons in situ have not been well described. The aim of this study was to characterize voltage-dependent Ca2+ currents in situ from identified larval motoneurons. Whole cell recordings from the somata of identified motoneurons revealed a significant influence of extracellular Ca2+ on spike shape and firing rate. Using whole cell voltage clamp, along with blockers of Na+ and K+ channels, a Ca2+-dependent inward current was isolated. The Drosophila genome contains three genes with homology to vertebrate voltage-dependent Ca2+ channels: Dmca1A, Dmca1D, and Dmalpha1G. We used mutants of Dmca1A and Dmca1D as well as targeted expression of an RNAi transgene to Dmca1D to determine the genes responsible for the voltage-dependent Ca2+ current recorded from two identified motoneurons. Our results implicate Dmca1D as the major contributor to the voltage-dependent Ca2+ current recorded from the somatodendritic processes of motoneurons, whereas Dmca1A has previously been localized to the presynaptic terminal where it is essential for neurotransmitter release. Altered firing properties in cells from both Dmca1D and Dmca1A mutants indicate a role for both genes in shaping firing properties.

Presynaptic calcium channel localization and calcium-dependent synaptic vesicle exocytosis regulated by the Fuseless protein

A systematic forward genetic Drosophila screen for electroretinogram mutants lacking synaptic transients identified the fuseless (fusl) gene, which encodes a predicted eight-pass transmembrane protein in the presynaptic membrane. Null fusl mutants display >75% reduction in evoked synaptic transmission but, conversely, an approximately threefold increase in the frequency and amplitude of spontaneous synaptic vesicle fusion events. These neurotransmission defects are rescued by a wild-type fusl transgene targeted only to the presynaptic cell, demonstrating a strictly presynaptic requirement for Fusl function. Defects in FM dye turnover at the synapse show a severely impaired exo-endo synaptic vesicle cycling pool. Consistently, ultrastructural analyses reveal accumulated vesicles arrested in clustered and docked pools at presynaptic active zones. In the absence of Fusl, calcium-dependent neurotransmitter release is dramatically compromised and there is little enhancement of synaptic efficacy with elevated external Ca(2+) concentrations. These defects are causally linked with severe loss of the Cacophony voltage-gated Ca(2+) channels, which fail to localize normally at presynaptic active zone domains in the absence of Fusl. These data indicate that Fusl regulates assembly of the presynaptic active zone Ca(2+) channel domains required for efficient coupling of the Ca(2+) influx and synaptic vesicle exocytosis during neurotransmission.

Mutations in a Drosophila alpha2delta voltage-gated calcium channel subunit reveal a crucial synaptic function

Voltage-dependent calcium channels regulate many aspects of neuronal biology, including synaptic transmission. In addition to their alpha1 subunit, which encodes the essential voltage gate and selective pore, calcium channels also contain auxiliary alpha2delta, beta, and gamma subunits. Despite progress in understanding the biophysical properties of calcium channels, the in vivo functions of these auxiliary subunits remain unclear. We have isolated mutations in the gene encoding an alpha2delta calcium channel subunit (d alpha2delta-3) using a forward genetic screen in Drosophila. Null mutations in this gene are embryonic lethal and can be rescued by expression in the nervous system, demonstrating that the essential function of this subunit is neuronal. The photoreceptor phenotype of d alpha2delta-3 mutants resembles that of the calcium channel alpha1 mutant cacophony (cac), suggesting shared functions. We have examined in detail genotypes that survive to the third-instar stage. Electrophysiological recordings demonstrate that synaptic transmission is severely impaired in these mutants. Thus the alpha2delta calcium channel subunit is critical for calcium-dependent synaptic function. As such, this Drosophila isoform is the likely partner to the presynaptic calcium channel alpha1 subunit encoded by the cac locus. Consistent with this hypothesis, cacGFP fluorescence at the neuromuscular junction is reduced in d alpha2delta-3 mutants. This is the first characterization of an alpha2delta-3 mutant in any organism and indicates a necessary role for alpha2delta-3 in presynaptic vesicle release and calcium channel expression at active zones.

Mechanisms underlying the rapid induction and sustained expression of synaptic homeostasis

Homeostatic signaling systems are thought to interface with the mechanisms of neural plasticity to achieve stable yet flexible neural circuitry. However, the time course, molecular design, and implementation of homeostatic signaling remain poorly defined. Here we demonstrate that a homeostatic increase in presynaptic neurotransmitter release can be induced within minutes following postsynaptic glutamate receptor blockade. The rapid induction of synaptic homeostasis is independent of new protein synthesis and does not require evoked neurotransmission, indicating that a change in the efficacy of spontaneous quantal release events is sufficient to trigger the induction of synaptic homeostasis. Finally, both the rapid induction and the sustained expression of synaptic homeostasis are blocked by mutations that disrupt the pore-forming subunit of the presynaptic Ca(V)2.1 calcium channel encoded by cacophony. These data confirm the presynaptic expression of synaptic homeostasis and implicate presynaptic Ca(V)2.1 in a homeostatic retrograde signaling system.

TheDrosophila cacts2mutation reduces presynaptic Ca2+entry and defines an important element in Cav2.1 channel inactivation

Voltage-gated Ca2+ channels in nerve terminals open in response to action potentials and admit Ca2+, the trigger for neurotransmitter release. The cacophony gene encodes the primary presynaptic voltage-gated Ca2+ channel in Drosophila motor-nerve terminals. The cac(ts2) mutant allele of cacophony is associated with paralysis and reduced neurotransmission at non-permissive temperatures but the basis for the neurotransmission deficit has not been established. The cac(ts2) mutation occurs in the cytoplasmic carboxyl tail of the alpha1-subunit, not within the pore-forming trans-membrane domains, making it difficult to predict the mutation's impact. We applied a Ca2+-imaging technique at motor-nerve terminals of mutant larvae to test the hypothesis that the neurotransmission deficit is a result of impaired Ca2+ entry. Presynaptic Ca2+ signals evoked by single and multiple action potentials showed a temperature-dependent reduction. The amplitude of the reduction was sufficient to account for the neurotransmission deficit, indicating that the site of the cac(ts2) mutation plays a role in Ca2+ channel activity. As the mutation occurs in a motif conserved in mammalian high-voltage-activated Ca2+ channels, we used a heterologous expression system to probe the effect of this mutation on channel function. The mutation was introduced into rat Ca(v)2.1 channels expressed in human embryonic kidney cells. Patch-clamp analysis of mutant channels at the physiological temperature of 37 degrees C showed much faster inactivation rates than for wild-type channels, demonstrating that the integrity of this motif is critical for normal Ca(v)2.1 channel inactivation.