Biology subjects

Dickman, D.

Publications and source records attributed to Dickman, D..

At least 19 recordsLinked to original sources

bioRxiv2026

An unexpected specialization of the active zone scaffold RIM at high release synapses

The active zone scaffold RIM is canonically viewed as an obligate, universal pillar of the neurotransmitter release machinery. However, whether RIM is strictly required across diverse synapse subtypes with distinct release probabilities has remained an important unresolved question. Here, we report a fundamental revision of this model: RIM is not a constitutive necessity for baseline transmission, but rather a specialized "gain factor" selectively deployed to empower high-release synapses. Utilizing botulinum neurotoxin-based silencing to isolate convergent inputs at the Drosophila neuromuscular junction, we demonstrate that while RIM is essential for high-fidelity transmission at phasic (MN-Is) synapses, it is largely dispensable at low-release tonic (MN-Ib) synapses. This input specificity extends into plasticity: RIM is required for acute presynaptic homeostatic potentiation (PHP) at phasic inputs but is dispensable for the chronic maintenance of PHP at tonic inputs. Mechanistically, super-resolution imaging reveals that RIM is positioned significantly closer to CaV2 Ca2+ channel nanodomains at phasic synapses. During acute plasticity, RIM coordinates the homeostatic compaction of CaV2 channel clusters and drives the rapid expansion of the readily releasable vesicle pool. Our results revise the canonical view of active zone architecture, demonstrating that rather than serving as a static, uniform structural anchor, RIM functions as a tunable, dynamic module deployed to shape the performance and plasticity of high-demand synaptic connections.

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bioRxiv2026

A comprehensive CRISPR screen of the Drosophila glutamate receptome reveals Ekar as a selective regulator of presynaptic homeostatic plasticity

Homeostatic mechanisms protect synapses from destabilizing challenges throughout an organisms lifespan, ensuring stable yet flexible neural network activity. To delineate the molecular basis of presynaptic homeostatic potentiation (PHP), we conducted a comprehensive, in vivo CRISPR/Cas9-based screen of all 16 glutamate receptor (GluR) genes encoded in the Drosophila genome. We first generated a complete expression atlas across larval and adult stages, identifying nine GluRs expressed in presynaptic motor neurons. We then generated null mutants for all 16 GluRs and screened them at the larval neuromuscular junction. While the loss of any single presynaptic GluR did not affect baseline synaptic growth or neurotransmission, our screen revealed a selective and critical requirement for the kainate receptor subunit ekar in the expression of chronic PHP. Further genetic analysis indicates that Ekar functions coordinately with the kainate receptor subunits KaiRID and Ukar within a shared pathway to promote this plasticity. Mechanistically, Ekar acts downstream of active zone remodeling to drive the homeostatic enhancement of presynaptic Ca2+ influx, which is the defining feature of chronic PHP. Together, this genome-wide analysis establishes a definitive functional atlas for the Drosophila glutamate receptome and highlights a specialized, essential role for Ekar in stabilizing long-term synaptic homeostasis.

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bioRxiv2026

Glutamate receptor composition at Drosophila neuromuscular junctions depends on developmental stage and muscle identity

The neuromuscular junction (NMJ) of larval Drosophila is widely used for studying synaptic transmission. Larval body wall muscles express five ionotropic glutamate receptor (iGluR) subunits that assemble into two tetrameric complexes, with subunit composition determining the strength and plasticity of synaptic transmission. Because NMJ function has been extensively characterized in larvae, it has commonly been assumed that adult fly NMJs share this molecular composition, despite substantial differences between life stages. Here, we systematically compare glutamate receptor expression across larval and adult Drosophila muscles. We find that adult leg and flight muscles exhibit different iGluR expression than larvae, lacking expression of several receptors previously considered essential. Adjacent muscles within the adult femur express distinct iGluRs, suggesting specialization of flexor and extensor muscles. Finally, the glutamate-gated chloride channel (GluCl) is expressed extrasynaptically in adult but not larval muscle fibers. Our results reveal unexpected heterogeneity in glutamate receptor expression across muscles and developmental stages, demonstrating the need for muscle-specific analyses in flies and other animals.

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bioRxiv2025

Presynaptic Release Probability Determines the Need for Sleep

Sleep is universal among animals with synapses, yet the synaptic functions determining the need for sleep remain elusive. By directly measuring synaptic transmission at anatomically defined synapses in Drosophila, we found that synaptic strength remained stable or declined after sleep deprivation in a circuit-specific manner. In contrast, presynaptic release probability (Pr) consistently decreased with sleep loss across circuits and species, stemming from reduced Ca2+ influx or weakened vesicle-channel coupling at presynaptic terminals, and recovered after sleep. Bidirectional manipulations of Pr altered sleep pressure, establishing a causal relationship between presynaptic function and sleep need. Non-synaptic sleep-regulatory signaling pathways consistently modulate Pr but not synaptic strength. Thus, our findings identify Pr, rather than synaptic strength, as the conserved synaptic substrate underlying sleep need.

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bioRxiv2024

Distinct input-specific mechanisms enable presynaptic homeostatic plasticity

Synapses are endowed with the flexibility to change through experience, but must be sufficiently stable to last a lifetime. This tension is illustrated at the Drosophila neuromuscular junction (NMJ), where two motor inputs that differ in structural and functional properties co-innervate most muscles to coordinate locomotion. To stabilize NMJ activity, motor neurons augment neurotransmitter release following diminished postsynaptic glutamate receptor functionality, termed presynaptic homeostatic potentiation (PHP). How these distinct inputs contribute to PHP plasticity remains enigmatic. We have used a botulinum neurotoxin to selectively silence each input and resolve their roles in PHP, demonstrating that PHP is input-specific: Chronic (genetic) PHP selectively targets the tonic MN-Ib, where active zone remodeling enhances Ca2+ influx to promote increased glutamate release. In contrast, acute (pharmacological) PHP selectively increases vesicle pools to potentiate phasic MN-Is. Thus, distinct homeostatic modulations in active zone nanoarchitecture, vesicle pools, and Ca2+ influx collaborate to enable input-specific PHP expression.

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bioRxiv2023

Tissue-specific knockout in Drosophila neuromuscular system reveals ESCRT's role in formation of synapse-derived extracellular vesicles

Tissue-specific gene knockout by CRISPR/Cas9 is a powerful approach for characterizing gene functions in animal development. However, this approach has been successfully applied in only a small number of Drosophila tissues. The Drosophila motor nervous system is an excellent model system for studying the biology of neuromuscular junction (NMJ). To expand tissue-specific CRISPR to the Drosophila motor system, here we present a CRISPR-mediated tissue-restricted mutagenesis (CRISPR-TRiM) toolkit for knocking out genes in motoneurons, muscles, and glial cells. We validated the efficacy of this toolkit by knocking out known genes in each tissue, demonstrated its orthogonal use with the Gal4/UAS binary expression system, and showed simultaneous knockout of multiple redundant genes. Using these tools, we discovered an essential role for SNARE pathways in NMJ maintenance. Furthermore, we demonstrate that the canonical ESCRT pathway suppresses NMJ bouton growth by downregulating the retrograde Gbb signaling. Lastly, we found that axon termini of motoneurons rely on ESCRT-mediated intra-axonal membrane trafficking to lease extracellular vesicles at the NMJ. SIGNIFICANCE STATEMENTIn this study, we developed a tissue-specific Cas9 toolkit that enables gene knockout specifically in motor neurons, glial cells, and muscle cells, the three cell types of the Drosophila peripheral motor system. Complementary to existing RNAi methods, this versatile tissue-specific knockout system offers unique advantages for dissecting gene functions at the neuromuscular junction (NMJ). Using these tools, we discovered that SNARE-mediated secretory pathways are required to maintain the integrity of the NMJ and that ESCRT components play critical yet differential roles in the biogenesis of extracellular vesicles, bouton growth, and membrane turnover at the NMJ. This CRISPR toolkit can be applied to study many biological questions in the neuromuscular system.

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bioRxiv2022

Physiologic and nanoscale distinctions define glutamatergic synapses in tonic vs phasic neurons

Neurons exhibit a striking degree of functional diversity, each one tuned to the needs of the circuitry in which it is embedded. A fundamental functional dichotomy occurs in activity patterns, with some neurons firing at a relatively constant "tonic" rate, while others fire in bursts - a "phasic" pattern. Synapses formed by tonic vs phasic neurons are also functionally differentiated, yet the bases of their distinctive properties remain enigmatic. A major challenge towards illuminating the synaptic differences between tonic and phasic neurons is the difficulty in isolating their physiological properties. At the Drosophila neuromuscular junction (NMJ), most muscle fibers are co-innervated by two motor neurons, the tonic "MN-Ib" and phasic "MN-Is". Here, we employed selective expression of a newly developed botulinum neurotoxin (BoNT-C) transgene to silence tonic or phasic motor neurons. This approach revealed major differences in their neurotransmitter release properties, including probability, short-term plasticity, and vesicle pools. Furthermore, Ca2+ imaging demonstrated ~two-fold greater Ca2+ influx at phasic neuron release sites relative to tonic, along with enhanced synaptic vesicle coupling. Finally, confocal and super resolution imaging revealed that phasic neuron release sites are organized in a more compact arrangement, with enhanced stoichiometry of voltage-gated Ca2+ channels relative to other active zone scaffolds. These data suggest that distinctions in active zone nano-architecture and Ca2+ influx collaborate to differentially tune glutamate release at synapses of tonic vs phasic neuronal subtypes.

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bioRxiv2022

Excess glutamate release triggers subunit-specific homeostatic receptor scaling

Ionotropic glutamate receptors (GluRs) are targets for modulation in Hebbian and homeostatic synaptic plasticity and are remodeled by development, experience, and disease. Although much is known about activity-dependent mechanisms that regulate GluR composition and abundance, the role of glutamate itself in these processes is unclear. To determine how glutamate sculpts GluR receptive fields, we have manipulated synaptically released glutamate and generated precise CRISPR mutations in the two postsynaptic GluR subtypes at the Drosophila neuromuscular junction, GluRA and GluRB. We first demonstrate that GluRA and GluRB compete to establish postsynaptic receptive fields, and that proper GluR abundance and localization can be orchestrated in the absence of any synaptic glutamate release. However, excess glutamate release adaptively tunes postsynaptic GluR abundance, echoing GluR receptor scaling observed in mammalian systems. Unexpectedly, when GluRA vs GluRB competition is eliminated, excess glutamate homeostatically regulates GluRA abundance, while GluRB abundance is now insensitive to glutamate modulation. Finally, Ca2+ impermeable GluRA receptors are no longer sensitive to homeostatic regulation by glutamate. Thus, excess glutamate, GluR competition, and Ca2+ signaling collaborate to selectively target GluR subtypes for homeostatic regulation at postsynaptic compartments.

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bioRxiv2022

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 and maintain functional neural circuits. 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 developed 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 inputs disambiguates functional properties of each input. Importantly, the composite values of Is and 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 cleanly separate neurotransmission between tonic vs phasic neurons and defines heterosynaptic plasticity rules in a powerful model glutamatergic circuit.

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bioRxiv2021

Voltage imaging in Drosophila using a hybrid chemical-genetic rhodamine voltage reporter

We combine a chemically-synthesized, voltage-sensitive fluorophore with a genetically encoded, self-labeling enzyme to enable voltage imaging in Drosophila melanogaster. Previously, we showed that a rhodamine voltage reporter (RhoVR) combined with the HaloTag self-labeling enzyme could be used to monitor membrane potential changes from mammalian neurons in culture and brain slice. Here, we apply this hybrid RhoVR-Halo approach in vivo to achieve selective neuron labeling in intact fly brains. We generate a Drosophila UAS-HaloTag reporter line in which the HaloTag enzyme is expressed on the surface of cells. We validate the voltage sensitivity of this new construct in cell culture before driving expression of HaloTag in specific brain neurons in flies. We show that selective labeling of synapses, cells, and brain regions can be achieved with RhoVR-Halo in either larval neuromuscular junction (NMJ) or in whole adult brains. Finally, we validate the voltage sensitivity of RhoVR-Halo in fly tissue via dual-electrode/imaging at the NMJ, show the efficacy of this approach for measuring synaptic excitatory post-synaptic potentials (EPSPs) in muscle cells, and perform voltage imaging of carbachol-evoked depolarization and osmolarity-evoked hyperpolarization in projection neurons and in interoceptive subesophageal zone neurons in fly brain explants following in vivo labeling. We envision the turn-on response to depolarizations, fast response kinetics, and two-photon compatibility of chemical indicators, coupled with the cellular and synaptic specificity of genetically-encoded enzymes, will make RhoVR-Halo a powerful complement to neurobiological imaging in Drosophila. Significance StatementVoltage imaging is a powerful method for interrogating neurobiology. Chemical indicators possess fast response kinetics, turn-on responses to membrane depolarization, and can be compatible with two-photon excitation. However, selective cell labeling in intact tissues and in vivo remains a challenge for completely synthetic fluorophores. Here, we show that a chemical - genetic hybrid approach in Drosophila enables cell-specific staining in vivo and voltage imaging in whole-brain explants and at neuromuscular junction synapses.

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bioRxiv2021

Rapid homeostatic modulation of transsynaptic nanocolumn rings

Robust neural information transfer relies on a delicate molecular nano-architecture of chemical synapses. Neurotransmitter release is controlled by a specific arrangement of proteins within presynaptic active zones. How the specific presynaptic molecular architecture relates to postsynaptic organization, and how synaptic nano-architecture is transsynaptically regulated to achieve stable synaptic transmission remains enigmatic. Using time-gated stimulated emission depletion (gSTED) microscopy at the Drosophila neuromuscular junction, we here find that presynaptic nano-rings formed by the active-zone scaffold Bruchpilot (Brp) precisely align with postsynaptic glutamate receptor (GluR) rings. Individual rings harbor ~5 transsynaptically-aligned Brp-GluR nanocolumns. Intriguingly, acute GluR impairment rapidly triggers the formation of new transsynaptic nanocolumns on the minute time scale during homeostatic plasticity. We reveal distinct phases of structural transsynaptic homeostatic plasticity, with postsynaptic reorganization preceding presynaptic modulation. Finally, the auxiliary GluR subunit Neto-{beta} promotes structural and functional homeostatic plasticity. Thus, transsynaptic nanocolumns arrange in stereotypic rings that are rapidly modulated during homeostatic plasticity to stabilize synaptic efficacy. One Sentence SummaryRapid changes of transsynaptic nanocolumn rings during homeostatic plasticity.

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bioRxiv2020

Spatial representations in macaque hippocampal formation

The hippocampal formation is linked to spatial navigation, but there is little corroboration from freely-moving primates with concurrent monitoring of three-dimensional head and gaze stances. We recorded neurons and local field potentials across hippocampal regions in rhesus macaques during free foraging in an open environment while tracking their head and eye. Theta band activity was intermittently present at movement onset and modulated by saccades. Many cells were phase-locked to theta, with few showing theta phase precession. Most hippocampal neurons encoded a mixture of spatial variables beyond place fields and a negligible number showed prominent grid tuning. Spatial representations were dominated by facing location and allocentric direction, mostly in head, rather than gaze, coordinates. Importantly, eye movements strongly modulated neural activity in all regions. These findings reveal that the macaque hippocampal formation represents three-dimensional space using a multiplexed code, with head orientation and eye movement properties dominating over simple place and grid coding during free exploration.

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bioRxiv2020

Distinct target-specific mechanisms homeostatically stabilize transmission at pre-and post-synaptic compartments

Neurons must establish and stabilize connections made with diverse targets, each with distinct demands and functional characteristics. At Drosophila neuromuscular junctions, synaptic strength remains stable in a manipulation that simultaneously induces hypo-innervation on one target and hyper-innervation on the other. However, the expression mechanisms that achieve this exquisite target-specific homeostatic control remain enigmatic. Here, we identify the distinct target-specific homeostatic expression mechanisms. On the hypo-innervated target, an increase in postsynaptic glutamate receptor (GluR) abundance is sufficient to compensate for reduced innervation, without any apparent presynaptic adaptations. In contrast, a target-specific reduction in presynaptic neurotransmitter release probability is reflected by a decrease in active zone components restricted to terminals of hyper-innervated targets. Finally, loss of postsynaptic GluRs on one target induces a compartmentalized, homeostatic enhancement of presynaptic neurotransmitter release called presynaptic homeostatic potentiation that can be precisely balanced with the adaptations required for both hypo- and hyper-innervation to maintain stable synaptic strength. Thus, distinct anterograde and retrograde signaling systems operate at pre- and post-synaptic compartments to enable target-specific, homeostatic control of neurotransmission.

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bioRxiv2019

Presynaptic depression maintains stable synaptic strength in developmentally arrested Drosophila larvae

Positive and negative modes of regulation typically constrain synaptic growth and function within narrow physiological ranges. However, it is unclear how synaptic strength is maintained when both pre- and post-synaptic compartments continue to grow beyond stages imposed by typical developmental programs. To address whether and how synapses can adjust to a novel life stage for which they were never molded by evolution, we have characterized synaptic growth, structure and function at the Drosophila neuromuscular junction (NMJ) under conditions where larvae are terminally arrested at the third instar stage. While wild type larvae transition to pupae after 5 days, arrested third instar (ATI) larvae persist for up to 35 days, during which NMJs exhibit extensive overgrowth in muscle size, presynaptic release sites, and postsynaptic glutamate receptors. Remarkably, despite this exuberant growth of both pre- and post-synaptic structures, stable neurotransmission is maintained throughout the ATI lifespan through a potent homeostatic reduction in presynaptic neurotransmitter release. Arrest of the larval stage in stathmin mutants reveals a degree of progressive instability and neurodegeneration that was not apparent during the typical larval period. Hence, during a period of unconstrained synaptic growth through an extended developmental period, a robust and adaptive form of presynaptic homeostatic depression can stabilize neurotransmission. More generally, the ATI manipulation provides an attractive system for studying neurodegeneration and plasticity across longer time scales. SIGNIFICANCE STATEMENTIt is unclear whether and how synapses adjust to a novel life stage for which they were never molded by evolution. We have characterized synaptic plasticity at the Drosophila neuromuscular junction in third instar larvae arrested in development for over 35 days. This approach has revealed that homeostatic depression stabilizes synaptic strength throughout the life of arrested third instars to compensate for excessive pre- and post-synaptic growth. This system also now opens the way for the study of synapses and degeneration over long time scales in this powerful model synapse.

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bioRxiv2019

A gravity-based three-dimensional compass in the mouse brain

Head direction cells in the mammalian limbic system are thought to function as an allocentric neuronal compass. Although traditional views hold that the compass of ground-dwelling species is planar, we show that head-direction cells in the rodent thalamus, retrosplenial cortex and cingulum fiber bundle are tuned to conjunctive combinations of azimuth, pitch or roll, similarly to presubicular cells in flying bats. Pitch and roll orientation tuning is ubiquitous, anchored to gravity, and independent of visual landmarks. When head tilts, azimuth tuning is affixed to the head-horizontal plane, but also uses gravity to remain anchored to the terrestrial allocentric world. These findings suggest that gravity defines all three degrees of freedom of the allocentric orientation compass, and only the azimuth component can flexibly remap to local cues in different environments. Collectively, these results demonstrate that a three-dimensional, gravity-based, neural compass is likely a ubiquitous property of mammalian species, including ground-dwelling animals.

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bioRxiv2018

Rapid active zone remodeling consolidates presynaptic potentiation

Synaptic transmission is mediated by neurotransmitter release at presynaptic active zones (AZs) followed by postsynaptic neurotransmitter detection. Plastic changes in transmission maintain functionality during perturbations and enable memory formation. Postsynaptic plasticity targets neurotransmitter receptors, but presynaptic plasticity mechanisms directly regulating the neurotransmitter release apparatus remain largely enigmatic. Here we describe that AZs consist of nano-modular release site units and identify a molecular sequence adding more modules within minutes of plasticity induction. This requires cognate transport machinery and a discrete subset of AZ scaffold proteins. Structural remodeling is not required for the immediate potentiation of neurotransmitter release, but rather necessary to sustain this potentiation over longer timescales. Finally, mutations in Unc13 that disrupt homeostatic plasticity at the neuromuscular junction also impair shot-term memory when central neurons are targeted, suggesting that both forms of plasticity operate via Unc13. Together, while immediate synaptic potentiation capitalizes on available material, it triggers the coincident incorporation of modular release sites to consolidate stable synapse function.

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bioRxiv2018

The sleep gene insomniac ubiquitinates targets at postsynaptic densities and is required for retrograde homeostatic signaling

The nervous system confronts challenges during development and experience that can destabilize information processing. To adapt to these perturbations, synapses homeostatically adjust synaptic strength, a process referred to as homeostatic synaptic plasticity. 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 designed a forward genetic screen to identify factors necessary in the postsynaptic compartment to generate retrograde homeostatic signaling. This approach identified insomniac (inc), a gene that encodes a putative adaptor for the Cullin-3 ubiquitin ligase complex and is essential for normal sleep regulation. Intriguingly, we find that Inc rapidly traffics to postsynaptic densities and is required for increased ubiquitination following acute receptor inhibition. Our study suggests that Inc-dependent ubiquitination, compartmentalized at postsynaptic densities, gates retrograde signaling and provides an intriguing molecular link between the control of sleep behavior and homeostatic plasticity at synapses.

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bioRxiv2018

A screen for genes that regulate synaptic growth 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 remarkably stable throughout life, implying that adaptive countermeasures exist to stabilize neurotransmission. Aberrant synaptic structure and function has been associated with a variety of neural diseases including Fragile X syndrome, autism, and intellectual disability. We have screened disruptions in over 300 genes in Drosophila for defects in synaptic growth at the neuromuscular junction. This effort identified 12 mutants with severe reductions or enhancements in synaptic growth. Remarkably, electrophysiological recordings revealed synaptic strength in all but one of these mutants was unchanged compared to wild type. We utilized a combination of genetic, anatomical, and electrophysiological analyses to illuminate three mechanisms that stabilize synaptic strength in the face of alterations in synaptic growth. These include compensatory changes in 1) postsynaptic receptor abundance; 2) presynaptic morphology; and 3) active zone structure. Together, this analysis identifies new genes that regulate synaptic growth and the adaptive strategies that synapses employ to homeostatically stabilize synaptic strength in response.\n\nAUTHOR SUMMARYThroughout development, maturation, experience, and disease, synapses undergo dramatic changes in growth and remodeling. Although these processes are necessary for learning and memory, they pose major challenges to stable function in the nervous system. However, neurotransmission is typically constrained within narrow physiological ranges, implying the existence of homeostatic mechanisms that maintain stable functionality despite drastic alterations in synapse number. In this study we investigate the relationship between synaptic growth and function across a variety of mutations in neural and synaptic genes in the fruitfly Drosophila melanogaster. Using the neuromuscular junction as a model system, we reveal three adaptive mechanisms that stabilize synaptic strength when synapses are dramatically under- or over-grown. Together, these findings provide insights into the strategies employed at both pre- and post-synaptic compartments to ensure stable functionality while allowing considerable flexibility in overall synapse number.

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