Optical quantal analysis of synaptic transmission in wild-type and rab3-mutant Drosophila motor axons

Ca2+ channel and active zone protein abundance intersects with input-specific synapse organization to shape functional synaptic diversity

Synaptic heterogeneity is a hallmark of nervous systems that enables complex and adaptable communication in neural circuits. To understand circuit function, it is thus critical to determine the factors that contribute to the functional diversity of synapses. We investigated the contributions of voltage-gated calcium channel (VGCC) abundance, spatial organization, and subunit composition to synapse diversity among and between synapses formed by two closely related Drosophila glutamatergic motor neurons with distinct neurotransmitter release probabilities (Pr). Surprisingly, VGCC levels are highly predictive of heterogeneous Pr among individual synapses of either low- or high-Pr inputs, but not between inputs. We find that the same number of VGCCs are more densely organized at high-Pr synapses, consistent with tighter VGCC-synaptic vesicle coupling. We generated endogenously tagged lines to investigate VGCC subunits in vivo and found that the α2δ-3 subunit Straightjacket along with the CAST/ELKS active zone (AZ) protein Bruchpilot, both key regulators of VGCCs, are less abundant at high-Pr inputs, yet positively correlate with Pr among synapses formed by either input. Consistently, both Straightjacket and Bruchpilot levels are dynamically increased across AZs of both inputs when neurotransmitter release is potentiated to maintain stable communication following glutamate receptor inhibition. Together, these findings suggest a model in which VGCC and AZ protein abundance intersects with input-specific spatial and molecular organization to shape the functional diversity of synapses.

The exocyst subunit Sec15 is critical for proper synaptic development and function at the Drosophila NMJ

The exocyst protein complex is important for targeted vesicle fusion in a variety of cell types, however, its function in neurons is still not entirely known. We found that presynaptic knockdown (KD) of the exocyst component sec15 by transgenic RNAi expression caused a number of unexpected morphological and physiological defects in the synapse. These include the development of active zones (AZ) devoid of essential presynaptic proteins, an increase in the branching of the presynaptic arbor, the appearance of satellite boutons, and a decrease in the amplitude of stimulated postsynaptic currents as well as a decrease in the frequency of spontaneous synaptic vesicle release. We also found the release of extracellular vesicles from the presynaptic neuron was greatly diminished in the Sec15 KDs. These effects were mimicked by presynaptic knockdown of Rab11, a protein known to interact with the exocyst. sec15 RNAi expression caused an increase in phosphorylated Mothers against decapentaplegic (pMad) in the presynaptic terminal, an indication of enhanced bone morphogenic protein (BMP) signaling. Some morphological phenotypes caused by Sec15 knockdown were reduced by attenuation of BMP signaling through knockdown of wishful thinking (Wit), while other phenotypes were unaffected. Individual knockdown of multiple proteins of the exocyst complex also displayed a morphological phenotype similar to Sec15 KD. We conclude that Sec15, functioning as part of the exocyst complex, is critically important for proper formation and function of neuronal synapses. We propose a model in which Sec15 is involved in the trafficking of vesicles from the recycling endosome to the cell membrane as well as possibly trafficking extracellular vesicles for presynaptic release and these processes are necessary for the correct structure and function of the synapse.

Spontaneous neurotransmission at evocable synapses predicts their responsiveness to action potentials

Synaptic transmission relies on presynaptic neurotransmitter (NT) release from synaptic vesicles (SVs) and on NT detection by postsynaptic receptors. Transmission exists in two principal modes: action-potential (AP) evoked and AP-independent, "spontaneous" transmission. AP-evoked neurotransmission is considered the primary mode of inter-neuronal communication, whereas spontaneous transmission is required for neuronal development, homeostasis, and plasticity. While some synapses appear dedicated to spontaneous transmission only, all AP-responsive synapses also engage spontaneously, but whether this encodes functional information regarding their excitability is unknown. Here we report on functional interdependence of both transmission modes at individual synaptic contacts of Drosophila larval neuromuscular junctions (NMJs) which were identified by the presynaptic scaffolding protein Bruchpilot (BRP) and whose activities were quantified using the genetically encoded Ca2+ indicator GCaMP. Consistent with the role of BRP in organizing the AP-dependent release machinery (voltage-dependent Ca2+ channels and SV fusion machinery), most active BRP-positive synapses (>85%) responded to APs. At these synapses, the level of spontaneous activity was a predictor for their responsiveness to AP-stimulation. AP-stimulation resulted in cross-depletion of spontaneous activity and both transmission modes were affected by the non-specific Ca2+ channel blocker cadmium and engaged overlapping postsynaptic receptors. Thus, by using overlapping machinery, spontaneous transmission is a continuous, stimulus independent predictor for the AP-responsiveness of individual synapses.

Mechanisms controlling the trafficking, localization, and abundance of presynaptic Ca2+ channels

Voltage-gated Ca2+ channels (VGCCs) mediate Ca2+ influx to trigger neurotransmitter release at specialized presynaptic sites termed active zones (AZs). The abundance of VGCCs at AZs regulates neurotransmitter release probability (Pr ), a key presynaptic determinant of synaptic strength. Given this functional significance, defining the processes that cooperate to establish AZ VGCC abundance is critical for understanding how these mechanisms set synaptic strength and how they might be regulated to control presynaptic plasticity. VGCC abundance at AZs involves multiple steps, including channel biosynthesis (transcription, translation, and trafficking through the endomembrane system), forward axonal trafficking and delivery to synaptic terminals, incorporation and retention at presynaptic sites, and protein recycling. Here we discuss mechanisms that control VGCC abundance at synapses, highlighting findings from invertebrate and vertebrate models.

Probabilities of evoked and spontaneous synaptic transmission at individual active zones: Lessons from Drosophila

Nerve terminals release neuronal transmitters at morphological specializations known as active zones (AZs). Synaptic vesicle fusion at individual AZs is probabilistic, and this property is fundamental for the neuronal information transfer. Until recently, a lack of appropriate tools limited the studies of stochastic properties of neuronal secretion at individual AZs. However, Drosophila transgenic lines that express postsynaptically tethered Ca²⁺ sensor GCaMP enabled the visualization of single exocytic event at individual AZs. The present mini-review discusses how this powerful approach enables the investigation of the evoked and spontaneous transmission at single AZs and promotes the understanding of the properties of both release components.

Botulinum neurotoxin accurately separates tonic vs. phasic transmission and reveals heterosynaptic plasticity rules in Drosophila

In developing and mature nervous systems, diverse neuronal subtypes innervate common targets to establish, maintain, and modify neural circuit function. A major challenge towards understanding the structural and functional architecture of neural circuits is to separate these inputs and determine their intrinsic and heterosynaptic relationships. The Drosophila larval neuromuscular junction is a powerful model system to study these questions, where two glutamatergic motor neurons, the strong phasic-like Is and weak tonic-like Ib, co-innervate individual muscle targets to coordinate locomotor behavior. However, complete neurotransmission from each input has never been electrophysiologically separated. We have employed a botulinum neurotoxin, BoNT-C, that eliminates both spontaneous and evoked neurotransmission without perturbing synaptic growth or structure, enabling the first approach that accurately isolates input-specific neurotransmission. Selective expression of BoNT-C in Is or Ib motor neurons disambiguates the functional properties of each input. Importantly, the blended values of Is+Ib neurotransmission can be fully recapitulated by isolated physiology from each input. Finally, selective silencing by BoNT-C does not induce heterosynaptic structural or functional plasticity at the convergent input. Thus, BoNT-C establishes the first approach to accurately separate neurotransmission between tonic vs. phasic neurons and defines heterosynaptic plasticity rules in a powerful model glutamatergic circuit.

Regulation of presynaptic Ca2+ channel abundance at active zones through a balance of delivery and turnover

Voltage-gated Ca²⁺ channels (VGCCs) mediate Ca²⁺ influx to trigger neurotransmitter release at specialized presynaptic sites termed active zones (AZs). The abundance of VGCCs at AZs regulates neurotransmitter release probability (P_r), a key presynaptic determinant of synaptic strength. Although biosynthesis, delivery, and recycling cooperate to establish AZ VGCC abundance, experimentally isolating these distinct regulatory processes has been difficult. Here, we describe how the AZ levels of cacophony (Cac), the sole VGCC-mediating synaptic transmission in Drosophila, are determined. We also analyzed the relationship between Cac, the conserved VGCC regulatory subunit α2δ, and the core AZ scaffold protein Bruchpilot (BRP) in establishing a functional AZ. We find that Cac and BRP are independently regulated at growing AZs, as Cac is dispensable for AZ formation and structural maturation, and BRP abundance is not limiting for Cac accumulation. Additionally, AZs stop accumulating Cac after an initial growth phase, whereas BRP levels continue to increase given extended developmental time. AZ Cac is also buffered against moderate increases or decreases in biosynthesis, whereas BRP lacks this buffering. To probe mechanisms that determine AZ Cac abundance, intravital FRAP and Cac photoconversion were used to separately measure delivery and turnover at individual AZs over a multi-day period. Cac delivery occurs broadly across the AZ population, correlates with AZ size, and is rate-limited by α2δ. Although Cac does not undergo significant lateral transfer between neighboring AZs over the course of development, Cac removal from AZs does occur and is promoted by new Cac delivery, generating a cap on Cac accumulation at mature AZs. Together, these findings reveal how Cac biosynthesis, synaptic delivery, and recycling set the abundance of VGCCs at individual AZs throughout synapse development and maintenance.

Visualizing synaptic dopamine efflux with a 2D composite nanofilm

Chemical neurotransmission constitutes one of the fundamental modalities of communication between neurons. Monitoring release of these chemicals has traditionally been difficult to carry out at spatial and temporal scales relevant to neuron function. To understand chemical neurotransmission more fully, we need to improve the spatial and temporal resolutions of measurements for neurotransmitter release. To address this, we engineered a chemi-sensitive, two-dimensional composite nanofilm that facilitates visualization of the release and diffusion of the neurochemical dopamine with synaptic resolution, quantal sensitivity, and simultaneously from hundreds of release sites. Using this technology, we were able to monitor the spatiotemporal dynamics of dopamine release in dendritic processes, a poorly understood phenomenon. We found that dopamine release is broadcast from a subset of dendritic processes as hotspots that have a mean spatial spread of ≈ 3.2 µm (full width at half maximum [FWHM]) and are observed with a mean spatial frequency of one hotspot per ≈ 7.5 µm of dendritic length. Major dendrites of dopamine neurons and fine dendritic processes, as well as dendritic arbors and dendrites with no apparent varicose morphology participated in dopamine release. Remarkably, these release hotspots co-localized with Bassoon, suggesting that Bassoon may contribute to organizing active zones in dendrites, similar to its role in axon terminals.

To form the vast and complex network necessary for an organism to sense and react to the world, neurons must connect at highly specialized junctions. Individual cells communicate at these ‘synapses’ by releasing chemical signals (or neurotransmitters) such as dopamine, a molecule involved in learning and motivation.

Despite the central role that synapses play in the brain, it remains challenging to measure exactly where neurotransmitters are released and how far they travel from their release site. Currently, most tools available to scientists only allow bulk measurements of neurotransmitter release.

To tackle this limitation, Bulumulla et al. developed a new way to measure neurotransmitter release from neurons, harnessing a technique which uses fluorescent nanosensors that glow brighter when exposed to dopamine. These sensors form a very thin film upon which neurons can grow; when the cells release dopamine, the sensors ‘light up’ as they encounter the molecule. Dubbed DopaFilm, the technology reveals exactly where the neurotransmitter comes from and how it spreads between cells in real time. In particular, the approach showed that dopamine emerges from 'hot spots' at specific sites in cells; it also helped Bulumulla et al. study how dopamine is released from subcellular compartments that have previously not been well characterized.

Improving the sensors so that the film could detect other neurotransmitters besides dopamine would broaden the use of this approach. In the future, combining this technology with other types of imaging should enable studies of individual synapses with intricate detail.

Influence of T-Bar on Calcium Concentration Impacting Release Probability

The relation of form and function, namely the impact of the synaptic anatomy on calcium dynamics in the presynaptic bouton, is a major challenge of present (computational) neuroscience at a cellular level. The Drosophila larval neuromuscular junction (NMJ) is a simple model system, which allows studying basic effects in a rather simple way. This synapse harbors several special structures. In particular, in opposite to standard vertebrate synapses, the presynaptic boutons are rather large, and they have several presynaptic zones. In these zones, different types of anatomical structures are present. Some of the zones bear a so-called T-bar, a particular anatomical structure. The geometric form of the T-bar resembles the shape of the letter “T” or a table with one leg. When an action potential arises, calcium influx is triggered. The probability of vesicle docking and neurotransmitter release is superlinearly proportional to the concentration of calcium close to the vesicular release site. It is tempting to assume that the T-bar causes some sort of calcium accumulation and hence triggers a higher release probability and thus enhances neurotransmitter exocytosis. In order to study this influence in a quantitative manner, we constructed a typical T-bar geometry and compared the calcium concentration close to the active zones (AZs). We compared the case of synapses with and without T-bars. Indeed, we found a substantial influence of the T-bar structure on the presynaptic calcium concentrations close to the AZs, indicating that this anatomical structure increases vesicle release probability. Therefore, our study reveals how the T-bar zone implies a strong relation between form and function. Our study answers the question of experimental studies (namely “Wichmann and Sigrist, Journal of neurogenetics 2010”) concerning the sense of the anatomical structure of the T-bar.

Similarity and Diversity of Presynaptic Molecules at Neuromuscular Junctions and Central Synapses

Synaptic transmission is essential for controlling motor functions and maintaining brain functions such as walking, breathing, cognition, learning, and memory. Neurotransmitter release is regulated by presynaptic molecules assembled in active zones of presynaptic terminals. The size of presynaptic terminals varies, but the size of a single active zone and the types of presynaptic molecules are highly conserved among neuromuscular junctions (NMJs) and central synapses. Three parameters play an important role in the determination of neurotransmitter release properties at NMJs and central excitatory/inhibitory synapses: the number of presynaptic molecular clusters, the protein families of the presynaptic molecules, and the distance between presynaptic molecules and voltage-gated calcium channels. In addition, dysfunction of presynaptic molecules causes clinical symptoms such as motor and cognitive decline in patients with various neurological disorders and during aging. This review focuses on the molecular mechanisms responsible for the functional similarities and differences between excitatory and inhibitory synapses in the peripheral and central nervous systems, and summarizes recent findings regarding presynaptic molecules assembled in the active zone. Furthermore, we discuss the relationship between functional alterations of presynaptic molecules and dysfunction of NMJs or central synapses in diseases and during aging.

Determinants of synapse diversity revealed by super-resolution quantal transmission and active zone imaging

Neural circuit function depends on the pattern of synaptic connections between neurons and the strength of those connections. Synaptic strength is determined by both postsynaptic sensitivity to neurotransmitter and the presynaptic probability of action potential evoked transmitter release (Pr). Whereas morphology and neurotransmitter receptor number indicate postsynaptic sensitivity, presynaptic indicators and the mechanism that sets Pr remain to be defined. To address this, we developed QuaSOR, a super-resolution method for determining Pr from quantal synaptic transmission imaging at hundreds of glutamatergic synapses at a time. We mapped the Pr onto super-resolution 3D molecular reconstructions of the presynaptic active zones (AZs) of the same synapses at the Drosophila larval neuromuscular junction (NMJ). We find that Pr varies greatly between synapses made by a single axon, quantify the contribution of key AZ proteins to Pr diversity and find that one of these, Complexin, suppresses spontaneous and evoked transmission differentially, thereby generating a spatial and quantitative mismatch between release modes. Transmission is thus regulated by the balance and nanoscale distribution of release-enhancing and suppressing presynaptic proteins to generate high signal-to-noise evoked transmission.

Active zone compaction correlates with presynaptic homeostatic potentiation

Neurotransmitter release is stabilized by homeostatic plasticity. Presynaptic homeostatic potentiation (PHP) operates on timescales ranging from minute- to life-long adaptations and likely involves reorganization of presynaptic active zones (AZs). At Drosophila melanogaster neuromuscular junctions, earlier work ascribed AZ enlargement by incorporating more Bruchpilot (Brp) scaffold protein a role in PHP. We use localization microscopy (direct stochastic optical reconstruction microscopy [dSTORM]) and hierarchical density-based spatial clustering of applications with noise (HDBSCAN) to study AZ plasticity during PHP at the synaptic mesoscale. We find compaction of individual AZs in acute philanthotoxin-induced and chronic genetically induced PHP but unchanged copy numbers of AZ proteins. Compaction even occurs at the level of Brp subclusters, which move toward AZ centers, and in Rab3 interacting molecule (RIM)-binding protein (RBP) subclusters. Furthermore, correlative confocal and dSTORM imaging reveals how AZ compaction in PHP translates into apparent increases in AZ area and Brp protein content, as implied earlier.

SNARE Regulatory Proteins in Synaptic Vesicle Fusion and Recycling

Membrane fusion is a universal feature of eukaryotic protein trafficking and is mediated by the soluble N-ethylmaleimide sensitive factor attachment protein receptor (SNARE) family. SNARE proteins embedded in opposing membranes spontaneously assemble to drive membrane fusion and cargo exchange in vitro. Evolution has generated a diverse complement of SNARE regulatory proteins (SRPs) that ensure membrane fusion occurs at the right time and place in vivo. While a core set of SNAREs and SRPs are common to all eukaryotic cells, a specialized set of SRPs within neurons confer additional regulation to synaptic vesicle (SV) fusion. Neuronal communication is characterized by precise spatial and temporal control of SNARE dynamics within presynaptic subdomains specialized for neurotransmitter release. Action potential-elicited Ca²⁺ influx at these release sites triggers zippering of SNAREs embedded in the SV and plasma membrane to drive bilayer fusion and release of neurotransmitters that activate downstream targets. Here we discuss current models for how SRPs regulate SNARE dynamics and presynaptic output, emphasizing invertebrate genetic findings that advanced our understanding of SRP regulation of SV cycling.

Subsynaptic positioning of AMPARs by LRRTM2 controls synaptic strength

Recent evidence suggests that nano-organization of proteins within synapses may control the strength of communication between neurons in the brain. The unique subsynaptic distribution of glutamate receptors, which cluster in nanoalignment with presynaptic sites of glutamate release, supports this hypothesis. However, testing it has been difficult because mechanisms controlling subsynaptic organization remain unknown. Reasoning that transcellular interactions could position AMPA receptors (AMPARs), we targeted a key transsynaptic adhesion molecule implicated in controlling AMPAR number, LRRTM2, using engineered, rapid proteolysis. Severing the LRRTM2 extracellular domain led quickly to nanoscale declustering of AMPARs away from release sites, not prompting their escape from synapses until much later. This rapid remodeling of AMPAR position produced significant deficits in evoked, but not spontaneous, postsynaptic receptor activation. These results dissociate receptor numbers from their nanopositioning in determination of synaptic function and support the novel concept that adhesion molecules acutely position receptors to dynamically control synaptic strength.

Genetically Encoded Fluorescent Indicators for Imaging Brain Chemistry

Genetically encoded fluorescent indicators, combined with optical imaging, enable the detection of physiologically or behaviorally relevant neural activity with high spatiotemporal resolution. Recent developments in protein engineering and screening strategies have improved the dynamic range, kinetics, and spectral properties of genetically encoded fluorescence indicators of brain chemistry. Such indicators have detected neurotransmitter and calcium dynamics with high signal-to-noise ratio at multiple temporal and spatial scales in vitro and in vivo. This review summarizes the current trends in these genetically encoded fluorescent indicators of neurotransmitters and calcium, focusing on their key metrics and in vivo applications.

Function of Drosophila Synaptotagmins in membrane trafficking at synapses

The Synaptotagmin (SYT) family of proteins play key roles in regulating membrane trafficking at neuronal synapses. Using both Ca²⁺-dependent and Ca²⁺-independent interactions, several SYT isoforms participate in synchronous and asynchronous fusion of synaptic vesicles (SVs) while preventing spontaneous release that occurs in the absence of stimulation. Changes in the function or abundance of the SYT1 and SYT7 isoforms alter the number and route by which SVs fuse at nerve terminals. Several SYT family members also regulate trafficking of other subcellular organelles at synapses, including dense core vesicles (DCV), exosomes, and postsynaptic vesicles. Although SYTs are linked to trafficking of multiple classes of synaptic membrane compartments, how and when they interact with lipids, the SNARE machinery and other release effectors are still being elucidated. Given mutations in the SYT family cause disorders in both the central and peripheral nervous system in humans, ongoing efforts are defining how these proteins regulate vesicle trafficking within distinct neuronal compartments. Here, we review the Drosophila SYT family and examine their role in synaptic communication. Studies in this invertebrate model have revealed key similarities and several differences with the predicted activity of their mammalian counterparts. In addition, we highlight the remaining areas of uncertainty in the field and describe outstanding questions on how the SYT family regulates membrane trafficking at nerve terminals.

Synaptic Properties and Plasticity Mechanisms of Invertebrate Tonic and Phasic Neurons

Defining neuronal cell types and their associated biophysical and synaptic diversity has become an important goal in neuroscience as a mechanism to create comprehensive brain cell atlases in the post-genomic age. Beyond broad classification such as neurotransmitter expression, interneuron vs. pyramidal, sensory or motor, the field is still in the early stages of understanding closely related cell types. In both vertebrate and invertebrate nervous systems, one well-described distinction related to firing characteristics and synaptic release properties are tonic and phasic neuronal subtypes. In vertebrates, these classes were defined based on sustained firing responses during stimulation (tonic) vs. transient responses that rapidly adapt (phasic). In crustaceans, the distinction expanded to include synaptic release properties, with tonic motoneurons displaying sustained firing and weaker synapses that undergo short-term facilitation to maintain muscle contraction and posture. In contrast, phasic motoneurons with stronger synapses showed rapid depression and were recruited for short bursts during fast locomotion. Tonic and phasic motoneurons with similarities to those in crustaceans have been characterized in Drosophila, allowing the genetic toolkit associated with this model to be used for dissecting the unique properties and plasticity mechanisms for these neuronal subtypes. This review outlines general properties of invertebrate tonic and phasic motoneurons and highlights recent advances that characterize distinct synaptic and plasticity pathways associated with two closely related glutamatergic neuronal cell types that drive invertebrate locomotion.

Synaptic Plasticity Induced by Differential Manipulation of Tonic and Phasic Motoneurons in Drosophila

Structural and functional plasticity induced by neuronal competition is a common feature of developing nervous systems. However, the rules governing how postsynaptic cells differentiate between presynaptic inputs are unclear. In this study, we characterized synaptic interactions following manipulations of tonic Ib or phasic Is glutamatergic motoneurons that coinnervate postsynaptic muscles of male or female Drosophila melanogaster larvae. After identifying drivers for each neuronal subtype, we performed ablation or genetic manipulations to alter neuronal activity and examined the effects on synaptic innervation and function at neuromuscular junctions. Ablation of either Ib or Is resulted in decreased muscle response, with some functional compensation occurring in the Ib input when Is was missing. In contrast, the Is terminal failed to show functional or structural changes following loss of the coinnervating Ib input. Decreasing the activity of the Ib or Is neuron with tetanus toxin light chain resulted in structural changes in muscle innervation. Decreased Ib activity resulted in reduced active zone (AZ) number and decreased postsynaptic subsynaptic reticulum volume, with the emergence of filopodial-like protrusions from synaptic boutons of the Ib input. Decreased Is activity did not induce structural changes at its own synapses, but the coinnervating Ib motoneuron increased the number of synaptic boutons and AZs it formed. These findings indicate that tonic Ib and phasic Is motoneurons respond independently to changes in activity, with either functional or structural alterations in the Ib neuron occurring following ablation or reduced activity of the coinnervating Is input, respectively.SIGNIFICANCE STATEMENT Both invertebrate and vertebrate nervous systems display synaptic plasticity in response to behavioral experiences, indicating that underlying mechanisms emerged early in evolution. How specific neuronal classes innervating the same postsynaptic target display distinct types of plasticity is unclear. Here, we examined whether Drosophila tonic Ib and phasic Is motoneurons display competitive or cooperative interactions during innervation of the same muscle, or compensatory changes when the output of one motoneuron is altered. We established a system to differentially manipulate the motoneurons and examined the effects of cell type-specific changes to one of the inputs. Our findings indicate Ib and Is motoneurons respond differently to activity mismatch or loss of the coinnervating input, with the Ib subclass responding robustly compared with Is motoneurons.

Specific Isoforms of the Guanine-Nucleotide Exchange Factor dPix Couple Neuromuscular Synapse Growth to Muscle Growth

Developmental growth requires coordination between the growth rates of individual tissues and organs. Here, we examine how Drosophila neuromuscular synapses grow to match the size of their target muscles. We show that changes in muscle growth driven by autonomous modulation of insulin receptor signaling produce corresponding changes in synapse size, with each muscle affecting only its presynaptic motor neuron branches. This scaling growth is mechanistically distinct from synaptic plasticity driven by neuronal activity and requires increased postsynaptic differentiation induced by insulin receptor signaling in muscle. We identify the guanine-nucleotide exchange factor dPix as an effector of insulin receptor signaling. Alternatively spliced dPix isoforms that contain a specific exon are necessary and sufficient for postsynaptic differentiation and scaling growth, and their mRNA levels are regulated by insulin receptor signaling. These findings define a mechanism by which the same signaling pathway promotes both autonomous muscle growth and non-autonomous synapse growth.

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.

Drosophila Synaptotagmin 7 negatively regulates synaptic vesicle release and replenishment in a dosage-dependent manner

Synchronous neurotransmitter release is triggered by Ca2+ binding to the synaptic vesicle protein Synaptotagmin 1, while asynchronous fusion and short-term facilitation is hypothesized to be mediated by plasma membrane-localized Synaptotagmin 7 (SYT7). We generated mutations in Drosophila Syt7 to determine if it plays a conserved role as the Ca2+ sensor for these processes. Electrophysiology and quantal imaging revealed evoked release was elevated 2-fold. Syt7 mutants also had a larger pool of readily-releasable vesicles, faster recovery following stimulation, and intact facilitation. Syt1/Syt7 double mutants displayed more release than Syt1 mutants alone, indicating SYT7 does not mediate the residual asynchronous release remaining in the absence of SYT1. SYT7 localizes to an internal membrane tubular network within the peri-active zone, but does not enrich at active zones. These findings indicate the two Ca2+ sensor model of SYT1 and SYT7 mediating all phases of neurotransmitter release and facilitation is not applicable at Drosophila synapses.

Rapid regulation of vesicle priming explains synaptic facilitation despite heterogeneous vesicle:Ca2+ channel distances

Chemical synaptic transmission relies on the Ca²⁺-induced fusion of transmitter-laden vesicles whose coupling distance to Ca²⁺ channels determines synaptic release probability and short-term plasticity, the facilitation or depression of repetitive responses. Here, using electron- and super-resolution microscopy at the Drosophila neuromuscular junction we quantitatively map vesicle:Ca²⁺ channel coupling distances. These are very heterogeneous, resulting in a broad spectrum of vesicular release probabilities within synapses. Stochastic simulations of transmitter release from vesicles placed according to this distribution revealed strong constraints on short-term plasticity; particularly facilitation was difficult to achieve. We show that postulated facilitation mechanisms operating via activity-dependent changes of vesicular release probability (e.g. by a facilitation fusion sensor) generate too little facilitation and too much variance. In contrast, Ca²⁺-dependent mechanisms rapidly increasing the number of releasable vesicles reliably reproduce short-term plasticity and variance of synaptic responses. We propose activity-dependent inhibition of vesicle un-priming or release site activation as novel facilitation mechanisms.

Cells in the nervous system of all animals communicate by releasing and sensing chemicals at contact points named synapses. The ‘talking’ (or pre-synaptic) cell stores the chemicals close to the synapse, in small spheres called vesicles. When the cell is activated, calcium ions flow in and interact with the release-ready vesicles, which then spill the chemicals into the synapse. In turn, the ‘listening’ (or post-synaptic) cell can detect the chemicals and react accordingly.

When the pre-synaptic cell is activated many times in a short period, it can release a greater quantity of chemicals, allowing a bigger reaction in the post-synaptic cell. This phenomenon is known as facilitation, but it is still unclear how exactly it can take place. This is especially the case when many of the vesicles are not ready to respond, for example when they are too far from where calcium flows into the cell. Computer simulations have been created to model facilitation but they have assumed that all vesicles are placed at the same distance to the calcium entry point: Kobbersmed et al. now provide evidence that this assumption is incorrect.

Two high-resolution imaging techniques were used to measure the actual distances between the vesicles and the calcium source in the pre-synaptic cells of fruit flies: this showed that these distances are quite variable – some vesicles sit much closer to the source than others.

This information was then used to create a new computer model to simulate facilitation. The results from this computing work led Kobbersmed et al. to suggest that facilitation may take place because a calcium-based mechanism in the cell increases the number of vesicles ready to release their chemicals.

This new model may help researchers to better understand how the cells in the nervous system work. Ultimately, this can guide experiments to investigate what happens when information processing at synapses breaks down, for example in diseases such as epilepsy.

Zebrafish neuromuscular junction: The power of N

In the early 1950s, Katz and his colleagues capitalized on the newly developed intracellular microelectrode recording technique to investigate synaptic transmission. For study they chose frog neuromuscular junction (NMJ), which was ideally suited due to the accessibility and large size of the muscle cells. Paradoxically, the large size precluded the use of next generation patch clamp technology. Consequently, electrophysiological study of synaptic function shifted to small central synapses made amenable by patch clamp. Recently, however, the unique features offered by zebrafish have rekindled interest in the NMJ as a model for electrophysiological study of synaptic transmission. The small muscle size and synaptic simplicity provide the singular opportunity to perform in vivo spinal motoneuron-target muscle patch clamp recordings. Additional incentive is provided by zebrafish lines harboring mutations in key synaptic proteins, many of which are embryonic lethal in mammals, but all of which are able to survive well past synapse maturation in zebrafish. This mini-review will highlight features that set zebrafish NMJs apart from traditional NMJs. We also draw into focus findings that offer the promise of identifying features that define release sites, which serve to set the upper limit of transmitter release. Since its conception several candidates representing release sites have been proposed, most of which are based on distinctions among vesicle pools in their state of readiness for release. However, models based on distinctions among vesicles have become enormously complicated and none adequately account for setting an upper limit for exocytosis in response to an action potential (AP). Specifically, findings from zebrafish NMJ point to an alternative model, positing that elements other than vesicles per se set the upper limits of release.

Homeostatic scaling of active zone scaffolds maintains global synaptic strength

Synaptic terminals grow and retract throughout life, yet synaptic strength is maintained within stable physiological ranges. To study this process, we investigated Drosophila endophilin (endo) mutants. Although active zone (AZ) number is doubled in endo mutants, a compensatory reduction in their size homeostatically adjusts global neurotransmitter output to maintain synaptic strength. We find an inverse adaptation in rab3 mutants. Additional analyses using confocal, STED, and electron microscopy reveal a stoichiometric tuning of AZ scaffolds and nanoarchitecture. Axonal transport of synaptic cargo via the lysosomal kinesin adapter Arl8 regulates AZ abundance to modulate global synaptic output and sustain the homeostatic potentiation of neurotransmission. Finally, we find that this AZ scaling can interface with two independent homeostats, depression and potentiation, to remodel AZ structure and function, demonstrating a robust balancing of separate homeostatic adaptations. Thus, AZs are pliable substrates with elastic and modular nanostructures that can be dynamically sculpted to stabilize and tune both local and global synaptic strength.

A Screen for Synaptic Growth Mutants Reveals Mechanisms That Stabilize Synaptic Strength

Synapses grow, prune, and remodel throughout development, experience, and disease. This structural plasticity can destabilize information transfer in the nervous system. However, neural activity remains stable throughout life, implying that adaptive countermeasures exist that maintain neurotransmission within proper physiological ranges. Aberrant synaptic structure and function have been associated with a variety of neural diseases, including Fragile X syndrome, autism, and intellectual disability. We have screened 300 mutants in Drosophila larvae of both sexes for defects in synaptic growth at the neuromuscular junction, identifying 12 mutants with severe reductions or enhancements in synaptic growth. Remarkably, electrophysiological recordings revealed that synaptic strength was unchanged in all but one of these mutants compared with WT. We used a combination of genetic, anatomical, and electrophysiological analyses to illuminate three mechanisms that stabilize synaptic strength despite major disparities in synaptic growth. These include compensatory changes in (1) postsynaptic neurotransmitter receptor abundance, (2) presynaptic morphology, and (3) active zone structure. Together, this characterization identifies new mutants with defects in synaptic growth and the adaptive strategies used by synapses to homeostatically stabilize neurotransmission in response.SIGNIFICANCE STATEMENT This study reveals compensatory mechanisms used by synapses to ensure stable functionality during severe alterations in synaptic growth using the neuromuscular junction of Drosophila melanogaster as a model system. Through a forward genetic screen, we identify mutants that exhibit dramatic undergrown or overgrown synapses yet express stable levels of synaptic strength, with three specific compensatory mechanisms discovered. Thus, this study reveals novel insights into the adaptive strategies that constrain neurotransmission within narrow physiological ranges while allowing considerable flexibility in overall synapse number. More broadly, these findings provide insights into how stable synaptic function may be maintained in the nervous system during periods of intensive synaptic growth, pruning, and remodeling.

Autophagy within the mushroom body protects from synapse aging in a non-cell autonomous manner

Macroautophagy is an evolutionarily conserved cellular maintenance program, meant to protect the brain from premature aging and neurodegeneration. How neuronal autophagy, usually loosing efficacy with age, intersects with neuronal processes mediating brain maintenance remains to be explored. Here, we show that impairing autophagy in the Drosophila learning center (mushroom body, MB) but not in other brain regions triggered changes normally restricted to aged brains: impaired associative olfactory memory as well as a brain-wide ultrastructural increase of presynaptic active zones (metaplasticity), a state non-compatible with memory formation. Mechanistically, decreasing autophagy within the MBs reduced expression of an NPY-family neuropeptide, and interfering with autocrine NPY signaling of the MBs provoked similar brain-wide metaplastic changes. Our results in an exemplary fashion show that autophagy-regulated signaling emanating from a higher brain integration center can execute high-level control over other brain regions to steer life-strategy decisions such as whether or not to form memories.

Rapid active zone remodeling consolidates presynaptic potentiation

Neuronal communication across synapses relies on neurotransmitter release from presynaptic active zones (AZs) followed by postsynaptic transmitter detection. Synaptic plasticity homeostatically maintains functionality during perturbations and enables memory formation. Postsynaptic plasticity targets neurotransmitter receptors, but presynaptic mechanisms regulating the neurotransmitter release apparatus remain largely enigmatic. By studying Drosophila neuromuscular junctions (NMJs) we show that AZs consist of nano-modular release sites and identify a molecular sequence that adds modules within minutes of inducing homeostatic plasticity. This requires cognate transport machinery and specific AZ-scaffolding proteins. Structural remodeling is not required for immediate potentiation of neurotransmitter release, but necessary to sustain potentiation over longer timescales. Finally, mutations in Unc13 disrupting homeostatic plasticity at the NMJ also impair short-term memory when central neurons are targeted, suggesting that both plasticity mechanisms utilize Unc13. Together, while immediate synaptic potentiation capitalizes on available material, it triggers the coincident incorporation of modular release sites to consolidate synaptic potentiation.

Endogenous Tagging Reveals Differential Regulation of Ca2+ Channels at Single Active Zones during Presynaptic Homeostatic Potentiation and Depression

Neurons communicate through Ca²⁺-dependent neurotransmitter release at presynaptic active zones (AZs). Neurotransmitter release properties play a key role in defining information flow in circuits and are tuned during multiple forms of plasticity. Despite their central role in determining neurotransmitter release properties, little is known about how Ca²⁺ channel levels are modulated to calibrate synaptic function. We used CRISPR to tag the Drosophila Ca_V2 Ca²⁺ channel Cacophony (Cac) and, in males in which all Cac channels are tagged, investigated the regulation of endogenous Ca²⁺ channels during homeostatic plasticity. We found that heterogeneously distributed Cac is highly predictive of neurotransmitter release probability at individual AZs and differentially regulated during opposing forms of presynaptic homeostatic plasticity. Specifically, AZ Cac levels are increased during chronic and acute presynaptic homeostatic potentiation (PHP), and live imaging during acute expression of PHP reveals proportional Ca²⁺ channel accumulation across heterogeneous AZs. In contrast, endogenous Cac levels do not change during presynaptic homeostatic depression (PHD), implying that the reported reduction in Ca²⁺ influx during PHD is achieved through functional adaptions to pre-existing Ca²⁺ channels. Thus, distinct mechanisms bidirectionally modulate presynaptic Ca²⁺ levels to maintain stable synaptic strength in response to diverse challenges, with Ca²⁺ channel abundance providing a rapidly tunable substrate for potentiating neurotransmitter release over both acute and chronic timescales.

SIGNIFICANCE STATEMENT Presynaptic Ca²⁺ dynamics play an important role in establishing neurotransmitter release properties. Presynaptic Ca²⁺ influx is modulated during multiple forms of homeostatic plasticity at Drosophila neuromuscular junctions to stabilize synaptic communication. However, it remains unclear how this dynamic regulation is achieved. We used CRISPR gene editing to endogenously tag the sole Drosophila Ca²⁺ channel responsible for synchronized neurotransmitter release, and found that channel abundance is regulated during homeostatic potentiation, but not homeostatic depression. Through live imaging experiments during the adaptation to acute homeostatic challenge, we visualize the accumulation of endogenous Ca²⁺ channels at individual active zones within 10 min. We propose that differential regulation of Ca²⁺ channels confers broad capacity for tuning neurotransmitter release properties to maintain neural communication.

Inter-relationships among physical dimensions, distal-proximal rank orders, and basal GCaMP fluorescence levels in Ca2+ imaging of functionally distinct synaptic boutons at Drosophila neuromuscular junctions

GCaMP imaging is widely employed for investigating neuronal Ca²⁺ dynamics. The Drosophila larval neuromuscular junction (NMJ) consists of three distinct types of motor terminals (type Ib, Is and II). We investigated whether variability in synaptic bouton sizes and GCaMP expression levels confound interpretations of GCaMP readouts, in inferring the intrinsic Ca²⁺ handling properties among these functionally distinct synapses. Analysis of large data sets accumulated over years established the wide ranges of bouton sizes and GCaMP baseline fluorescence, with large overlaps among synaptic categories. We showed that bouton size and GCaMP baseline fluorescence were not confounding factors in determining the characteristic frequency responses among type Ib, Is and II synapses. More importantly, the drastic phenotypes that hyperexcitability mutations manifest preferentially in particular synaptic categories, were not obscured by bouton heterogeneity in physical size and GCaMP expression level. Our data enabled an extensive analysis of the distal-proximal gradient of GCaMP responses upon genetic and pharmacological manipulations. The results illustrate the conditions that disrupt or enhance the distal-proximal gradients. For example, stimulus frequencies just above the threshold level produced the steepest gradient in low Ca²⁺ (0.1 mM) saline, while supra-threshold stimulation flattened the gradient. Moreover, membrane hyperexcitability mutations (eag¹ Sh¹²⁰ and para^bss1) and mitochondrial inhibition by dinitrophenol (DNP) disrupted the gradient. However, a novel distal-proximal gradient of decay kinetics appeared after long-term DNP incubation. We performed focal recording to assess the failure rates in transmission at low Ca²⁺ levels, which yielded indications of a mild distal-proximal gradient in release probability.

The application of 'kisser' probes for resolving the distribution and microenvironment of membrane proteins in situ

Membrane proteins play a lead role in the formation and function of synapses, but, despite revolutions in immunology and molecular genetics, limitations persist in our ability to investigate membrane proteins in the context of an intact synapse. Here, we introduce a simple but novel approach to resolving the distribution of endogenous membrane proteins in either live or fixed tissues. The technique involves transgenic expression of a protein with an extracellular tag, a generic transmembrane domain, and an intracellular terminus that mimics the intracellular anchoring motifs of the endogenous protein of interest. We provide three examples where these kisser probes can be used to answer questions regarding the synaptic distribution of endogenous proteins and their microenvironment that would be difficult to resolve by other contemporary means: (i) the live distribution of untagged proteins at the neuromuscular junction (Cacophony and Shaker), (ii) the relative distribution of an untagged protein (PMCA) in pre- versus post-synaptic membranes separated by only 20 nm across the cleft of a fixed synapse, and (iii) the live targeting of functional probes (chemical and protein fluorescent pH reporters) to membrane protein-defined subcellular domains.

Shaking-B misexpression increases the formation of gap junctions but not chemical synapses between auditory sensory neurons and the giant fiber of Drosophila melanogaster

The synapse between auditory Johnston's Organ neurons (JONs) and the giant fiber (GF) of Drosophila is structurally mixed, being composed of cholinergic chemical synapses and Neurobiotin- (NB) permeable gap junctions, which consist of the innexin Shaking-B (ShakB). Previous observations showed that misexpression of one ShakB isoform, ShakB(N+16), in a subset of JONs that do not normally form gap junctions results in their de novo dye coupling to the GF. Misexpression of the transcription factor Engrailed (En) in these neurons also has this effect, and in addition causes the formation of new chemical synapses. These results, along with earlier studies suggesting that gap junctions are required for the development of some chemical synapses, led to the hypothesis that ShakB would, like En, have an instructive effect on the distribution of mixed chemical/electrical contacts. To test this, we first confirmed quantitatively that ShakB(N+16) misexpression increased the dye-coupling of JONs with the GF, indicating the formation of ectopic gap junctions. Conversely, expression of the 'incorrect' isoform, ShakB(N), abolished dye coupling. Immunocytochemistry of the ShakB protein showed that ShakB(N+16) increased gap junctional plaques in JON axons but ShakB(N) did not. To test our hypothesis, fluorescently-labeled presynaptic active zone protein (Brp) was expressed in JONs and the changes in its distribution on the GF dendrites was assayed with confocal microscopy in animals with misexpression of ShakB(N+16), ShakB(N) or, as a positive control, En. Using different methods of image analysis, we confirmed our previous result that En misexpression increased the chemical synapses with the GF and the amount of GF medial dendrite branching. However, contrary to our hypothesis, misexpression of ShakB did not increase these parameters. Immunostaining showed no association between presynaptic active zones and the new ShakB plaques, further evidence against the hypothesis. We conclude that both subsets of JON form chemical synapses onto the GF dendrites but only one population forms gap junctions, comprised of ShakB(N+16). Misexpression of this isoform in all JONs does not instruct the formation of new mixed chemical/electrical synapses, but results in the insertion of new gap junctions, presumably at the sites of existing chemical synaptic contacts with the GF.

Characterization of developmental and molecular factors underlying release heterogeneity at Drosophila synapses

Neurons communicate through neurotransmitter release at specialized synaptic regions known as active zones (AZs). Using biosensors to visualize single synaptic vesicle fusion events at Drosophila neuromuscular junctions, we analyzed the developmental and molecular determinants of release probability (P_r) for a defined connection with ~300 AZs. P_r was heterogeneous but represented a stable feature of each AZ. P_r remained stable during high frequency stimulation and retained heterogeneity in mutants lacking the Ca²⁺ sensor Synaptotagmin 1. P_r correlated with both presynaptic Ca²⁺ channel abundance and Ca²⁺ influx at individual release sites. P_r heterogeneity also correlated with glutamate receptor abundance, with high P_r connections developing receptor subtype segregation. Intravital imaging throughout development revealed that AZs acquire high P_r during a multi-day maturation period, with P_r heterogeneity largely reflecting AZ age. The rate of synapse maturation was activity-dependent, as both increases and decreases in neuronal activity modulated glutamate receptor field size and segregation.

To send a message to its neighbor, a neuron releases chemicals called neurotransmitters into the gap – or synapse – between them. The neurotransmitter molecules bind to proteins on the receiver neuron called receptors. But what causes the sender neuron to release neurotransmitter in the first place? The process starts when an electrical impulse called an action potential arrives at the sender cell. Its arrival causes channels in the membrane of the sender neuron to open, so that calcium ions flood into the cell. The calcium ions interact with packages of neurotransmitter molecules, known as synaptic vesicles. This causes some of the vesicles to empty their contents into the synapse.

But this process is not particularly reliable. Only a small fraction of action potentials cause vesicles to fuse with the synaptic membrane. How likely this is to occur varies greatly between neurons, and even between synapses formed by the same neuron. Synapses that are likely to release neurotransmitter are said to be strong. They are good at passing messages from the sender neuron to the receiver. Synapses with a low probability of release are said to be weak. But what exactly differs between strong and weak synapses?

Akbergenova et al. studied synapses between motor neurons and muscle cells in the fruit fly Drosophila. Each motor neuron forms several hundred synapses. Some of these synapses are 50 times more likely to release neurotransmitter than others. Using calcium imaging and genetics, Akbergenova et al. showed that sender cells at strong synapses have more calcium channels than sender cells at weak synapses. The subtypes and arrangement of receptor proteins also differ between the receiver neurons of strong versus weak synapses. Finally, studies in larvae revealed that newly formed synapses all start out weak and then gradually become stronger. How fast this strengthening occurs depends on how active the neuron at the synapse is.

This study has shown, in unprecedented detail, key molecular factors that make some fruit fly synapses more likely to release neurotransmitter than others. Many proteins at synapses of mammals resemble those at fruit fly synapses. This means that similar factors may also explain differences in synaptic strength in the mammalian brain. Changes in the strength of synapses underlie the ability to learn. Furthermore, many neurological and psychiatric disorders result from disruption of synapses. Understanding the molecular basis of synapses will thus provide clues to the origins of certain brain diseases.

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.

Presynaptic active zones of mammalian neuromuscular junctions: Nanoarchitecture and selective impairments in aging

Neurotransmitter release occurs at active zones, which are specialized regions of the presynaptic membrane. A dense collection of proteins at the active zone provides a platform for molecular interactions that promote recruitment, docking, and priming of synaptic vesicles. At mammalian neuromuscular junctions (NMJs), muscle-derived laminin β2 interacts with presynaptic voltage-gated calcium channels to organize active zones. The molecular architecture of presynaptic active zones has been revealed using super-resolution microscopy techniques that combine nanoscale resolution and multiple molecular identification. Interestingly, the active zones of adult NMJs are not stable structures and thus become impaired during aging due to the selective degeneration of specific active zone proteins. This review will discuss recent progress in the understanding of active zone nanoarchitecture and the mechanisms underlying active zone organization in mammalian NMJs. Furthermore, we will summarize the age-related degeneration of active zones at NMJs, and the role of exercise in maintaining active zones.

cAMP Signals in Drosophila Motor Neurons Are Confined to Single Synaptic Boutons

The second messenger cyclic AMP (cAMP) plays an important role in synaptic plasticity. Although there is evidence for local control of synaptic transmission and plasticity, it is less clear whether a similar spatial confinement of cAMP signaling exists. Here, we suggest a possible biophysical basis for the site-specific regulation of synaptic plasticity by cAMP, a highly diffusible small molecule that transforms the physiology of synapses in a local and specific manner. By exploiting the octopaminergic system of Drosophila, which mediates structural synaptic plasticity via a cAMP-dependent pathway, we demonstrate the existence of local cAMP signaling compartments of micrometer dimensions within single motor neurons. In addition, we provide evidence that heterogeneous octopamine receptor localization, coupled with local differences in phosphodiesterase activity, underlies the observed differences in cAMP signaling in the axon, cell body, and boutons.

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Boutons, axon, and cell body are independent cAMP signaling compartments

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Receptors and PDEs are responsible for the compartmentalization of cAMP

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cAMP does not propagate from the bouton to the cell body

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Local cAMP increases provides a basis for site-specific control of synaptic plasticity

The Long 3'UTR mRNA of CaMKII Is Essential for Translation-Dependent Plasticity of Spontaneous Release in Drosophila melanogaster

A null mutation of the Drosophila calcium/calmodulin-dependent protein kinase II gene (CaMKII) was generated using homologous recombination. Null animals survive to larval and pupal stages due to a large maternal contribution of CaMKII mRNA, which consists of a short 3'-untranslated region (UTR) form lacking regulatory elements that guide local translation. The selective loss of the long 3'UTR mRNA in CaMKII-null larvae allows us to test its role in plasticity. Development and evoked function of the larval neuromuscular junction are surprisingly normal, but the resting rate of miniature excitatory junctional potentials (mEJPs) is significantly lower in CaMKII mutants. Mutants also lack the ability to increase mEJP rate in response to spaced depolarization, a type of activity-dependent plasticity shown to require both transcription and translation. Consistent with this, overexpression of miR-289 in wild-type animals blocks plasticity of spontaneous release. In addition to the defects in regulation of mEJP rate, CaMKII protein is largely lost from synapses in the mutant. All phenotypes are non-sex-specific and rescued by a fosmid containing the entire wild-type CaMKII locus, but only viability and CaMKII localization are rescued by genomic fosmids lacking the long 3'UTR. This suggests that synaptic CaMKII accumulates by two distinct mechanisms: local synthesis requiring the long 3'UTR form of CaMKII mRNA and a process that requires zygotic transcription of CaMKII mRNA. The origin of synaptic CaMKII also dictates its functionality. Locally translated CaMKII has a privileged role in regulation of spontaneous release, which cannot be fulfilled by synaptic CaMKII from the other pool.SIGNIFICANCE STATEMENT As a regulator of synaptic development and plasticity, CaMKII has important roles in both normal and pathological function of the nervous system. CaMKII shows high conservation between Drosophila and humans, underscoring the usefulness of Drosophila in modeling its function. Drosophila CaMKII-null mutants remain viable throughout development, enabling morphological and electrophysiological characterization. Although the structure of the synapse is normal, maternally contributed CaMKII does not localize to synapses. Zygotic production of CaMKII mRNA with a long 3'-untranslated region is necessary for modulating spontaneous neurotransmission in an activity-dependent manner, but not for viability. These data argue that regulation of CaMKII localization and levels by local transcriptional processes is conserved. This is the first demonstration of distinct functions for Drosophila CaMKII mRNA variants.

Presynaptic morphogenesis, active zone organization and structural plasticity in Drosophila

Effective adaptation of neural circuit function to a changing environment requires many forms of plasticity. Among these, structural plasticity is one of the most durable, and is also an intrinsic part of the developmental logic for the formation and refinement of synaptic connectivity. Structural plasticity of presynaptic sites can involve the addition, remodeling, or removal of pre- and post-synaptic elements. However, this requires coordination of morphogenesis and assembly of the subcellular machinery for neurotransmitter release within the presynaptic neuron, as well as coordination of these events with the postsynaptic cell. While much progress has been made in revealing the cell biological mechanisms of postsynaptic structural plasticity, our understanding of presynaptic mechanisms is less complete.

Single-Molecule Localization Microscopy in Eukaryotes

Super-resolution fluorescence imaging by photoactivation or photoswitching of single fluorophores and position determination (single-molecule localization microscopy, SMLM) provides microscopic images with subdiffraction spatial resolution. This technology has enabled new insights into how proteins are organized in a cellular context, with a spatial resolution approaching virtually the molecular level. A unique strength of SMLM is that it delivers molecule-resolved information, along with super-resolved images of cellular structures. This allows quantitative access to cellular structures, for example, how proteins are distributed and organized and how they interact with other biomolecules. Ultimately, it is even possible to determine protein numbers in cells and the number of subunits in a protein complex. SMLM thus has the potential to pave the way toward a better understanding of how cells function at the molecular level. In this review, we describe how SMLM has contributed new knowledge in eukaryotic biology, and we specifically focus on quantitative biological data extracted from SMLM images.

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.

Fife organizes synaptic vesicles and calcium channels for high-probability neurotransmitter release

Fife is a Piccolo-RIM–related protein that regulates neurotransmission and motor behavior through an unknown mechanism. Here, Bruckner et al. show that Fife organizes synaptic vesicle docking and coupling to calcium channels to establish and modulate synaptic strength.

The strength of synaptic connections varies significantly and is a key determinant of communication within neural circuits. Mechanistic insight into presynaptic factors that establish and modulate neurotransmitter release properties is crucial to understanding synapse strength, circuit function, and neural plasticity. We previously identified Drosophila Piccolo-RIM-related Fife, which regulates neurotransmission and motor behavior through an unknown mechanism. Here, we demonstrate that Fife localizes and interacts with RIM at the active zone cytomatrix to promote neurotransmitter release. Loss of Fife results in the severe disruption of active zone cytomatrix architecture and molecular organization. Through electron tomographic and electrophysiological studies, we find a decrease in the accumulation of release-ready synaptic vesicles and their release probability caused by impaired coupling to Ca²⁺ channels. Finally, we find that Fife is essential for the homeostatic modulation of neurotransmission. We propose that Fife organizes active zones to create synaptic vesicle release sites within nanometer distance of Ca²⁺ channel clusters for reliable and modifiable neurotransmitter release.

Spermidine Suppresses Age-Associated Memory Impairment by Preventing Adverse Increase of Presynaptic Active Zone Size and Release

Memories are assumed to be formed by sets of synapses changing their structural or functional performance. The efficacy of forming new memories declines with advancing age, but the synaptic changes underlying age-induced memory impairment remain poorly understood. Recently, we found spermidine feeding to specifically suppress age-dependent impairments in forming olfactory memories, providing a mean to search for synaptic changes involved in age-dependent memory impairment. Here, we show that a specific synaptic compartment, the presynaptic active zone (AZ), increases the size of its ultrastructural elaboration and releases significantly more synaptic vesicles with advancing age. These age-induced AZ changes, however, were fully suppressed by spermidine feeding. A genetically enforced enlargement of AZ scaffolds (four gene-copies of BRP) impaired memory formation in young animals. Thus, in the Drosophila nervous system, aging AZs seem to steer towards the upper limit of their operational range, limiting synaptic plasticity and contributing to impairment of memory formation. Spermidine feeding suppresses age-dependent memory impairment by counteracting these age-dependent changes directly at the synapse.

Presynaptic DLG regulates synaptic function through the localization of voltage-activated Ca(2+) Channels

The DLG-MAGUK subfamily of proteins plays a role on the recycling and clustering of glutamate receptors (GLUR) at the postsynaptic density. discs-large1 (dlg) is the only DLG-MAGUK gene in Drosophila and originates two main products, DLGA and DLGS97 which differ by the presence of an L27 domain. Combining electrophysiology, immunostaining and genetic manipulation at the pre and postsynaptic compartments we study the DLG contribution to the basal synaptic-function at the Drosophila larval neuromuscular junction. Our results reveal a specific function of DLGS97 in the regulation of the size of GLUR fields and their subunit composition. Strikingly the absence of any of DLG proteins at the presynaptic terminal disrupts the clustering and localization of the calcium channel DmCa1A subunit (Cacophony), decreases the action potential-evoked release probability and alters short-term plasticity. Our results show for the first time a crucial role of DLG proteins in the presynaptic function in vivo.

Synaptic Vesicle Proteins and Active Zone Plasticity

Neurotransmitter is released from synaptic vesicles at the highly specialized presynaptic active zone (AZ). The complex molecular architecture of AZs mediates the speed, precision and plasticity of synaptic transmission. Importantly, structural and functional properties of AZs vary significantly, even for a given connection. Thus, there appear to be distinct AZ states, which fundamentally influence neuronal communication by controlling the positioning and release of synaptic vesicles. Vice versa, recent evidence has revealed that synaptic vesicle components also modulate organizational states of the AZ. The protein-rich cytomatrix at the active zone (CAZ) provides a structural platform for molecular interactions guiding vesicle exocytosis. Studies in Drosophila have now demonstrated that the vesicle proteins Synaptotagmin-1 (Syt1) and Rab3 also regulate glutamate release by shaping differentiation of the CAZ ultrastructure. We review these unexpected findings and discuss mechanistic interpretations of the reciprocal relationship between synaptic vesicles and AZ states, which has heretofore received little attention.

Rab3-GEF Controls Active Zone Development at the Drosophila Neuromuscular Junction

Synaptic signaling involves the release of neurotransmitter from presynaptic active zones (AZs). Proteins that regulate vesicle exocytosis cluster at AZs, composing the cytomatrix at the active zone (CAZ). At the Drosophila neuromuscular junction (NMJ), the small GTPase Rab3 controls the distribution of CAZ proteins across release sites, thereby regulating the efficacy of individual AZs. Here we identify Rab3-GEF as a second protein that acts in conjunction with Rab3 to control AZ protein composition. At rab3-GEF mutant NMJs, Bruchpilot (Brp) and Ca²⁺ channels are enriched at a subset of AZs, leaving the remaining sites devoid of key CAZ components in a manner that is indistinguishable from rab3 mutant NMJs. As the Drosophila homologue of mammalian DENN/MADD and Caenorhabditis elegans AEX-3, Rab3-GEF is a guanine nucleotide exchange factor (GEF) for Rab3 that stimulates GDP to GTP exchange. Mechanistic studies reveal that although Rab3 and Rab3-GEF act within the same mechanism to control AZ development, Rab3-GEF is involved in multiple roles. We show that Rab3-GEF is required for transport of Rab3. However, the synaptic phenotype in the rab3-GEF mutant cannot be fully explained by defective transport and loss of GEF activity. A transgenically expressed GTP-locked variant of Rab3 accumulates at the NMJ at wild-type levels and fully rescues the rab3 mutant but is unable to rescue the rab3-GEF mutant. Our results suggest that although Rab3-GEF acts upstream of Rab3 to control Rab3 localization and likely GTP-binding, it also acts downstream to regulate CAZ development, potentially as a Rab3 effector at the synapse.

Presynaptic D1 heteroreceptors and mGlu autoreceptors act at individual cortical release sites to modify glutamate release

The aim of this work was to study release of glutamic acid (GLU) from one-axon terminal or bouton at-a-time using cortical neurons grown in vitro to study the effect of presynaptic auto- and heteroreceptor stimulation. Neurons were infected with release reporters SypHx2 or iGluSnFR at 7 or 3 days-in-vitro (DIV) respectively. At 13-15 DIV single synaptic boutons were identified from images obtained from a confocal scanning microscope before and after field electrical stimulation. We further stimulated release by raising intracellular levels of cAMP with forskolin (10µM). Forskolin-mediated effects were dependent on protein kinase A (PKA) and did not result from an increase in endocytosis, but rather from an increase in the size of the vesicle readily releasable pool. Once iGluSnFR was confirmed as more sensitive than SypHx2, it was used to study the participation of presynaptic auto- and heteroreceptors on GLU release. Although most receptor agonizts (carbamylcholine, nicotine, dopamine D2, BDNF) did not affect electrically stimulated GLU release, a significant increase was observed in the presence of metabotropic D1/D5 heteroreceptor agonist (SKF38393 10µM) that was reversed by PKA inhibitors. Interestingly, stimulation of group II metabotropic mGLU2/3 autoreceptors (LY379268 50nM) induced a decrease in GLU release that was reversed by the specific mGLU2/3 receptor antagonist (LY341495 1µM) and also by PKA inhibitors (KT5720 200nM and PKI14-22 400nM). These changes in release probability at individual release sites suggest another level of control of the distribution of transmitter substances in cortical tissue.

A family of photoswitchable NMDA receptors

NMDA receptors, which regulate synaptic strength and are implicated in learning and memory, consist of several subtypes with distinct subunit compositions and functional properties. To enable spatiotemporally defined, rapid and reproducible manipulation of function of specific subtypes, we engineered a set of photoswitchable GluN subunits ('LiGluNs'). Photo-agonism of GluN2A or GluN2B elicits an excitatory drive to hippocampal neurons that can be shaped in time to mimic synaptic activation. Photo-agonism of GluN2A at single dendritic spines evokes spine-specific calcium elevation and expansion, the morphological correlate of LTP. Photo-antagonism of GluN2A alone, or in combination with photo-antagonism of GluN1a, reversibly blocks excitatory synaptic currents, prevents the induction of long-term potentiation and prevents spine expansion. In addition, photo-antagonism in vivo disrupts synaptic pruning of developing retino-tectal projections in larval zebrafish. By providing precise and rapidly reversible optical control of NMDA receptor subtypes, LiGluNs should help unravel the contribution of specific NMDA receptors to synaptic transmission, integration and plasticity.

Phosphorylation of Complexin by PKA Regulates Activity-Dependent Spontaneous Neurotransmitter Release and Structural Synaptic Plasticity

Synaptic plasticity is a fundamental feature of the nervous system that allows adaptation to changing behavioral environments. Most studies of synaptic plasticity have examined the regulated trafficking of postsynaptic glutamate receptors that generates alterations in synaptic transmission. Whether and how changes in the presynaptic release machinery contribute to neuronal plasticity is less clear. The SNARE complex mediates neurotransmitter release in response to presynaptic Ca(2+) entry. Here we show that the SNARE fusion clamp Complexin undergoes activity-dependent phosphorylation that alters the basic properties of neurotransmission in Drosophila. Retrograde signaling following stimulation activates PKA-dependent phosphorylation of the Complexin C terminus that selectively and transiently enhances spontaneous release. Enhanced spontaneous release is required for activity-dependent synaptic growth. These data indicate that SNARE-dependent fusion mechanisms can be regulated in an activity-dependent manner and highlight the key role of spontaneous neurotransmitter release as a mediator of functional and structural plasticity.