The influence of postsynaptic structure on missing quanta at the Drosophila neuromuscular junction

Postsynaptic BMP signaling regulates myonuclear properties in Drosophila larval muscles

The syncytial mammalian muscle fiber contains a heterogeneous population of (myo)nuclei. At the neuromuscular junction (NMJ), myonuclei have specialized positioning and gene expression. However, it remains unclear how myonuclei are recruited and what regulates myonuclear output at the NMJ. Here, we identify specific properties of myonuclei located near the Drosophila larval NMJ. These synaptic myonuclei have increased size in relation to their surrounding cytoplasmic domain (scaling), increased DNA content (ploidy), and increased levels of transcription factor pMad, a readout for BMP signaling activity. Our genetic manipulations show local BMP signaling affects muscle size, nuclear size, ploidy, and NMJ size and function. In support, RNA sequencing analysis reveals that pMad regulates genes involved in muscle growth, ploidy (i.e., E2f1), and neurotransmission. Our data suggest that muscle BMP signaling instructs synaptic myonuclear output that then positively shapes the NMJ synapse. This study deepens our understanding of how myonuclear heterogeneity supports local signaling demands to fine tune cellular function and NMJ activity.

Structural and Functional Synaptic Plasticity Induced by Convergent Synapse Loss in the Drosophila Neuromuscular Circuit

Throughout the nervous system, the convergence of two or more presynaptic inputs on a target cell is commonly observed. The question we ask here is to what extent converging inputs influence each other's structural and functional synaptic plasticity. In complex circuits, isolating individual inputs is difficult because postsynaptic cells can receive thousands of inputs. An ideal model to address this question is the Drosophila larval neuromuscular junction (NMJ) where each postsynaptic muscle cell receives inputs from two glutamatergic types of motor neurons (MNs), known as 1b and 1s MNs. Notably, each muscle is unique and receives input from a different combination of 1b and 1s MNs; we surveyed multiple muscles for this reason. Here, we identified a cell-specific promoter that allows ablation of 1s MNs postinnervation and measured structural and functional responses of convergent 1b NMJs using microscopy and electrophysiology. For all muscles examined in both sexes, ablation of 1s MNs resulted in NMJ expansion and increased spontaneous neurotransmitter release at corresponding 1b NMJs. This demonstrates that 1b NMJs can compensate for the loss of convergent 1s MNs. However, only a subset of 1b NMJs showed compensatory evoked neurotransmission, suggesting target-specific plasticity. Silencing 1s MNs led to similar plasticity at 1b NMJs, suggesting that evoked neurotransmission from 1s MNs contributes to 1b synaptic plasticity. Finally, we genetically blocked 1s innervation in male larvae and robust 1b synaptic plasticity was eliminated, raising the possibility that 1s NMJ formation is required to set up a reference for subsequent synaptic perturbations.SIGNIFICANCE STATEMENT In complex neural circuits, multiple convergent inputs contribute to the activity of the target cell, but whether synaptic plasticity exists among these inputs has not been thoroughly explored. In this study, we examined synaptic plasticity in the structurally and functionally tractable Drosophila larval neuromuscular system. In this convergent circuit, each muscle is innervated by a unique pair of motor neurons. Removal of one neuron after innervation causes the adjacent neuron to increase neuromuscular junction outgrowth and functional output. However, this is not a general feature as each motor neuron differentially compensates. Further, robust compensation requires initial coinnervation by both neurons. Understanding how neurons respond to perturbations in adjacent neurons will provide insight into nervous system plasticity in both healthy and disease states.

Crayfish and Drosophila NMJs

Many synaptic studies have utilized the experimental advantages of the Arthropod NMJ and the most prominent preparations have been the crayfish and Drosophila larval NMJs. Early cellular studies in the crayfish established the framework for later molecular studies in Drosophila. The two neuromuscular systems are compared including the advantages presented by each preparation for cellular analysis. Beginning with the early work in the crayfish, research developments are followed in the areas of structure/function relationships, activity-dependent synaptic plasticity/development and synaptic homeostasis. A reoccurring theme in these studies is the regulation of active zone structure and function. Early studies in the crayfish focused on the role of active zone number/size and possible functional heterogeneity in regulating transmitter release. Recent studies in Drosophila have begun to characterize this heterogeneity using new approaches that combine imaging of transmitter release, Ca²⁺ influx and molecular composition for individual active zones.

Cul3 and insomniac are required for rapid ubiquitination of postsynaptic targets and retrograde homeostatic signaling

At the Drosophila neuromuscular junction, inhibition of postsynaptic glutamate receptors activates retrograde signaling that precisely increases presynaptic neurotransmitter release to restore baseline synaptic strength. However, the nature of the underlying postsynaptic induction process remains enigmatic. Here, we design a forward genetic screen to discover factors in the postsynaptic compartment necessary to generate retrograde homeostatic signaling. This approach identified insomniac (inc), a putative adaptor for the Cullin-3 (Cul3) ubiquitin ligase complex, which together with Cul3 is essential for normal sleep regulation. Interestingly, we find that Inc and Cul3 rapidly accumulate at postsynaptic compartments following acute receptor inhibition and are required for a local increase in mono-ubiquitination. Finally, we show that Peflin, a Ca²⁺-regulated Cul3 co-adaptor, is necessary for homeostatic communication, suggesting a relationship between Ca²⁺ signaling and control of Cul3/Inc activity in the postsynaptic compartment. Our study suggests that Cul3/Inc-dependent mono-ubiquitination, compartmentalized at postsynaptic densities, gates retrograde signaling and provides an intriguing molecular link between the control of sleep and homeostatic plasticity at synapses.

The authors use a forward genetic screen to discover postsynaptic factors required for homeostatic synaptic plasticity at the Drosophila neuromuscular junction. They identify insomniac and the ubiquitin ligase Cul3, genes involved in sleep regulation, to be necessary for retrograde homeostatic signalling at this synapse.

Climbing the ladder from neuron to brain in Harold Atwood's laboratory

Twenty years spent in one laboratory is sufficient to build a legacy of publications and a body of work to make an impact. However, the impact of our work was highest at the personal level, and time spent in Harold Atwood's laboratory was not a culmination of my career but rather a crucial path toward learning and maturing as a researcher. During that time, I experienced discoveries and lessons that shaped the next steps of my career. This article is written in gratitude for wonderful experiences and describes a few highlights that were especially memorable and influential.

Synapse-specific and compartmentalized expression of presynaptic homeostatic potentiation

Postsynaptic compartments can be specifically modulated during various forms of synaptic plasticity, but it is unclear whether this precision is shared at presynaptic terminals. Presynaptic homeostatic plasticity (PHP) stabilizes neurotransmission at the Drosophila neuromuscular junction, where a retrograde enhancement of presynaptic neurotransmitter release compensates for diminished postsynaptic receptor functionality. To test the specificity of PHP induction and expression, we have developed a genetic manipulation to reduce postsynaptic receptor expression at one of the two muscles innervated by a single motor neuron. We find that PHP can be induced and expressed at a subset of synapses, over both acute and chronic time scales, without influencing transmission at adjacent release sites. Further, homeostatic modulations to CaMKII, vesicle pools, and functional release sites are compartmentalized and do not spread to neighboring pre- or post-synaptic structures. Thus, both PHP induction and expression mechanisms are locally transmitted and restricted to specific synaptic compartments.

Everything we think and do is the result of communication between neurons. This communication takes place at junctions called synapses. When two nerve cells or neurons communicate at a synapse, the output terminal of the first cell releases a chemical called a neurotransmitter. This binds to receiver proteins, or receptors, on the second cell. When this communication is interrupted, synapses can adapt to maintain a stable dialogue between them. This can occur in two ways. Either the first neuron starts to release more neurotransmitter from its output terminal, or the second neuron produces extra receptors with which to detect the neurotransmitter.

But how specific are these changes? The brain contains far more synapses than neurons because each neuron can form synapses with many other cells. Can a neuron adjust how much of the neurotransmitter it releases at some of its synapses while leaving the others unchanged?

Li et al. have now addressed this question by studying a special type of synapse that forms between neurons and muscles, known as a neuromuscular junction. At one particular neuromuscular junction in fruit flies, a single neuron splits into two output terminals, each of which forms a synapse with a different muscle. Li et al. show that when the number of neurotransmitter receptors in one of the muscles is artificially reduced, the associated output terminal compensates by increasing its neurotransmitter release. By contrast, the other output terminal remains unaffected.

This suggests that a neuron can induce remarkably specific changes in a subset of its synapses. This discovery paves the way towards identifying the smallest possible unit of change that can occur in the neurons’ ability to communicate. This unit may in turn be the smallest change that can support learning. Such knowledge will help us understand how the nervous system processes and stabilizes information transfer, both in health and after injury or disease.

Jack bean urease modulates neurotransmitter release at insect neuromuscular junctions

BACKGROUND

Plants have developed a vast range of mechanisms to compete with phytophagous insects, including entomotoxic proteins such as ureases. The legume Canavalia ensiformis produces several urease isoforms, of which the more abundant is called Jack Bean Urease (JBU). Previews work has demonstrated the potential insecticidal effects of JBU, by mechanisms so far not entirely elucidated. In this work, we investigated the mechanisms involved in the JBU-induced activity upon neurotransmitter release on insect neuromuscular junctions.

METHODS

Electrophysiological recordings of nerve and muscle action potentials, and calcium imaging bioassays were employed.

RESULTS AND CONCLUSION

JBU (0.28 mg/animal/day) in Locusta migratoria 2nd instar through feeding and injection did not induce lethality, although it did result in a reduction of 20% in the weight gain at the end of 168 h (n = 9, p ≤ 0.05). JBU (0.014 and 0.14 mg) injected direct into the locust hind leg induced a dose and time-dependent decrease in the amplitude of muscle action potentials, with a maximum decrease of 70% in the amplitude at the highest dose (n = 5, p ≤ 0.05). At the same doses JBU did not alter the amplitude of action potentials evoked from motor neurons. Using Drosophila 3rd instar larvae neuromuscular preparations, JBU (10^-7 M) increased the occurrence of miniature Excitatory Junctional Potentials (mEJPs) in the presence of 1 mM CaCl₂ (n = 5, p ≤ 0.05). In low calcium (0.4 mM) assays, JBU (10^-7 M) was not able to modulate the occurrence of the events. In Ca²⁺-free conditions, with EGTA or CoCl₂, JBU induced a significant decrease in the occurrence of mEPJs (n = 5, p ≤ 0.05). Injected into the 3rd abdominal ganglion of Nauphoeta cinerea cockroaches, JBU (1 μM) induced a significant increase in Ca²⁺ influx (n = 7, p ≤ 0.01), similar to that seen for high KCl (35 mM) condition. Taken together the results confirm a direct action of JBU upon insect neuromuscular junctions and possibly central synapses, probably by disrupting the calcium machinery in the pre-synaptic region of the neurons.

Proteomic profiling of neuronal mitochondria reveals modulators of synaptic architecture

Background

Neurons are highly polarized cells consisting of three distinct functional domains: the cell body (and associated dendrites), the axon and the synapse. Previously, it was believed that the clinical phenotypes of neurodegenerative diseases were caused by the loss of entire neurons, however it has recently become apparent that these neuronal sub-compartments can degenerate independently, with synapses being particularly vulnerable to a broad range of stimuli. Whilst the properties governing the differential degenerative mechanisms remain unknown, mitochondria consistently appear in the literature, suggesting these somewhat promiscuous organelles may play a role in affecting synaptic stability. Synaptic and non-synaptic mitochondrial subpools are known to have different enzymatic properties (first demonstrated by Lai et al., 1977). However, the molecular basis underpinning these alterations, and their effects on morphology, has not been well documented.

Methods

The current study has employed electron microscopy, label-free proteomics and in silico analyses to characterize the morphological and biochemical properties of discrete sub-populations of mitochondria. The physiological relevance of these findings was confirmed in-vivo using a molecular genetic approach at the Drosophila neuromuscular junction.

Results

Here, we demonstrate that mitochondria at the synaptic terminal are indeed morphologically different to non-synaptic mitochondria, in both rodents and human patients. Furthermore, generation of proteomic profiles reveals distinct molecular fingerprints – highlighting that the properties of complex I may represent an important specialisation of synaptic mitochondria. Evidence also suggests that at least 30% of the mitochondrial enzymatic activity differences previously reported can be accounted for by protein abundance. Finally, we demonstrate that the molecular differences between discrete mitochondrial sub-populations are capable of selectively influencing synaptic morphology in-vivo. We offer several novel mitochondrial candidates that have the propensity to significantly alter the synaptic architecture in-vivo.

Conclusions

Our study demonstrates discrete proteomic profiles exist dependent upon mitochondrial subcellular localization and selective alteration of intrinsic mitochondrial proteins alters synaptic morphology in-vivo.

Electronic supplementary material

The online version of this article (10.1186/s13024-017-0221-9) contains supplementary material, which is available to authorized users.

Invaginating Presynaptic Terminals in Neuromuscular Junctions, Photoreceptor Terminals, and Other Synapses of Animals

Typically, presynaptic terminals form a synapse directly on the surface of postsynaptic processes such as dendrite shafts and spines. However, some presynaptic terminals invaginate-entirely or partially-into postsynaptic processes. We survey these invaginating presynaptic terminals in all animals and describe several examples from the central nervous system, including giant fiber systems in invertebrates, and cup-shaped spines, electroreceptor synapses, and some specialized auditory and vestibular nerve terminals in vertebrates. We then examine mechanoreceptors and photoreceptors, concentrating on the complex of pre- and postsynaptic processes found in basal invaginations of the cell. We discuss in detail the role of vertebrate invaginating horizontal cell processes in both chemical and electrical feedback mechanisms. We also discuss the common presence of indenting or invaginating terminals in neuromuscular junctions on muscles of most kinds of animals, and especially discuss those of Drosophila and vertebrates. Finally, we consider broad questions about the advantages of possessing invaginating presynaptic terminals and describe some effects of aging and disease, especially on neuromuscular junctions. We suggest that the invagination is a mechanism that can enhance both chemical and electrical interactions at the synapse.

Input-Specific Plasticity and Homeostasis at the Drosophila Larval Neuromuscular Junction

Synaptic connections undergo activity-dependent plasticity during development and learning, as well as homeostatic re-adjustment to ensure stability. Little is known about the relationship between these processes, particularly in vivo. We addressed this with novel quantal resolution imaging of transmission during locomotive behavior at glutamatergic synapses of the Drosophila larval neuromuscular junction. We find that two motor input types, Ib and Is, provide distinct forms of excitatory drive during crawling and differ in key transmission properties. Although both inputs vary in transmission probability, active Is synapses are more reliable. High-frequency firing "wakes up" silent Ib synapses and depresses Is synapses. Strikingly, homeostatic compensation in presynaptic strength only occurs at Ib synapses. This specialization is associated with distinct regulation of postsynaptic CaMKII. Thus, basal synaptic strength, short-term plasticity, and homeostasis are determined input-specifically, generating a functional diversity that sculpts excitatory transmission and behavioral function.