A presynaptic role of microtubule-associated protein 1/Futsch in Drosophila: regulation of active zone number and neurotransmitter release

Neuronal LRP4 directs the development, maturation and cytoskeletal organization of Drosophila peripheral synapses

Synaptic development requires multiple signaling pathways to ensure successful connections. Transmembrane receptors are optimally positioned to connect the synapse and the rest of the neuron, often acting as synaptic organizers to synchronize downstream events. One such organizer, the LDL receptor-related protein LRP4, is a cell surface receptor that has been most well-studied postsynaptically at mammalian neuromuscular junctions. Recent work, however, identified emerging roles, but how LRP4 acts as a presynaptic organizer and the downstream mechanisms of LRP4 are not well understood. Here, we show that LRP4 functions presynaptically at Drosophila neuromuscular synapses, acting in motoneurons to instruct pre- and postsynaptic development. Loss of presynaptic LRP4 results in multiple defects, impairing active zone organization, synapse growth, physiological function, microtubule organization, synaptic ultrastructure and synapse maturation. We further demonstrate that LRP4 promotes most aspects of presynaptic development via a downstream SR-protein kinase, SRPK79D. These data demonstrate a function for presynaptic LRP4 as a peripheral synaptic organizer, highlight a downstream mechanism conserved with its CNS function in Drosophila, and underscore previously unappreciated but important developmental roles for LRP4 in cytoskeletal organization, synapse maturation and active zone organization.

Neuronal LRP4 directs the development, maturation, and cytoskeletal organization of peripheral synapses

Synapse development requires multiple signaling pathways to accomplish the myriad of steps needed to ensure a successful connection. Transmembrane receptors on the cell surface are optimally positioned to facilitate communication between the synapse and the rest of the neuron and often function as synaptic organizers to synchronize downstream signaling events. One such organizer, the LDL receptor-related protein LRP4, is a cell surface receptor most well-studied postsynaptically at mammalian neuromuscular junctions. Recent work, however, has identified emerging roles for LRP4 as a presynaptic molecule, but how LRP4 acts as a presynaptic organizer, what roles LRP4 plays in organizing presynaptic biology, and the downstream mechanisms of LRP4 are not well understood. Here we show that LRP4 functions presynaptically at Drosophila neuromuscular synapses, acting in motor neurons to instruct multiple aspects of pre- and postsynaptic development. Loss of presynaptic LRP4 results in a range of developmental defects, impairing active zone organization, synapse growth, physiological function, microtubule organization, synaptic ultrastructure, and synapse maturation. We further demonstrate that LRP4 promotes most aspects of presynaptic development via a downstream SR-protein kinase, SRPK79D. SRPK79D overexpression suppresses synaptic defects associated with loss of lrp4. These data demonstrate a function for LRP4 as a peripheral synaptic organizer acting presynaptically, highlight a downstream mechanism conserved with its CNS function, and indicate previously unappreciated roles for LRP4 in cytoskeletal organization, synapse maturation, and active zone organization, underscoring its developmental importance.

The CD63 homologs, Tsp42Ee and Tsp42Eg, restrict endocytosis and promote neurotransmission through differential regulation of synaptic vesicle pools

The Tetraspanin (Tsp), CD63, is a transmembrane component of late endosomes and facilitates vesicular trafficking through endosomal pathways. Despite being widely expressed in the human brain and localized to late endosomes, CD63's role in regulating endo- and exocytic cycling at the synapse has not been investigated. Synaptic vesicle pools are highly dynamic and disruptions in the mobilization and replenishment of these vesicle pools have adverse neuronal effects. We find that the CD63 homologs, Tsp42Ee and Tsp42Eg, are expressed at the Drosophila neuromuscular junction to regulate synaptic vesicle pools through both shared and unique mechanisms. Tsp42Ee and Tsp42Eg negatively regulate endocytosis and positively regulate neurotransmitter release. Both tsp mutants show impaired locomotion, reduced miniature endplate junctional current frequencies, and increased endocytosis. Expression of human CD63 in Drosophila neurons leads to impaired endocytosis suggesting the role of Tsps in endocytosis is conserved. We further show that Tsps influence the synaptic cytoskeleton and membrane composition by regulating Futsch loop formation and synaptic levels of SCAR and PI(4,5)P2. Finally, Tsp42Ee and Tsp42Eg influence the synaptic localization of several vesicle-associated proteins including Synapsin, Synaptotagmin, and Cysteine String Protein. Together, our results present a novel function for Tsps in the regulation of vesicle pools and provide insight into the molecular mechanisms of Tsp-related synaptic dysfunction.

Microtubules as Regulators of Neural Network Shape and Function: Focus on Excitability, Plasticity and Memory

Neuronal microtubules (MTs) are complex cytoskeletal protein arrays that undergo activity-dependent changes in their structure and function as a response to physiological demands throughout the lifespan of neurons. Many factors shape the allostatic dynamics of MTs and tubulin dimers in the cytosolic microenvironment, such as protein–protein interactions and activity-dependent shifts in these interactions that are responsible for their plastic capabilities. Recently, several findings have reinforced the role of MTs in behavioral and cognitive processes in normal and pathological conditions. In this review, we summarize the bidirectional relationships between MTs dynamics, neuronal processes, and brain and behavioral states. The outcomes of manipulating the dynamicity of MTs by genetic or pharmacological approaches on neuronal morphology, intrinsic and synaptic excitability, the state of the network, and behaviors are heterogeneous. We discuss the critical position of MTs as responders and adaptative elements of basic neuronal function whose impact on brain function is not fully understood, and we highlight the dilemma of artificially modulating MT dynamics for therapeutic purposes.

Drosophila Models Rediscovered with Super-Resolution Microscopy

With the advent of super-resolution microscopy, we gained a powerful toolbox to bridge the gap between the cellular- and molecular-level analysis of living organisms. Although nanoscopy is broadly applicable, classical model organisms, such as fruit flies, worms and mice, remained the leading subjects because combining the strength of sophisticated genetics, biochemistry and electrophysiology with the unparalleled resolution provided by super-resolution imaging appears as one of the most efficient approaches to understanding the basic cell biological questions and the molecular complexity of life. Here, we summarize the major nanoscopic techniques and illustrate how these approaches were used in Drosophila model systems to revisit a series of well-known cell biological phenomena. These investigations clearly demonstrate that instead of simply achieving an improvement in image quality, nanoscopy goes far beyond with its immense potential to discover novel structural and mechanistic aspects. With the examples of synaptic active zones, centrosomes and sarcomeres, we will explain the instrumental role of super-resolution imaging pioneered in Drosophila in understanding fundamental subcellular constituents.

Standing genetic variation in laboratory populations of insecticide-susceptible Phlebotomus papatasi and Lutzomyia longipalpis (Diptera: Psychodidae: Phlebotominae) for the evolution of resistance

Insecticides can exert strong selection on insect pest species, including those that vector diseases, and have led to rapid evolution of resistance. Despite such rapid evolution, relatively little is known about standing genetic variation for resistance in insecticide-susceptible populations of many species. To help fill this knowledge gap, we generated genotyping-by-sequencing data from insecticide-susceptible Phlebotomus papatasi and Lutzomyia longipalpis sand flies that survived or died from a sub-diagnostic exposure to either permethrin or malathion using a modified version of the Centers for Disease Control and Prevention bottle bioassay. Multi-locus genome-wide association mapping methods were used to quantify standing genetic variation for insecticide resistance in these populations and to identify specific alleles associated with insecticide survival. For each insecticide treatment, we estimated the proportion of the variation in survival explained by the genetic data (i.e., "chip" heritability) and the number and contribution of individual loci with measurable effects. For all treatments, survival to an insecticide exposure was heritable with a polygenic architecture. Both P. papatasi and L. longipalpis had alleles for survival that resided within many genes throughout their genomes. The implications for resistance conferred by many alleles, as well as inferences made about the utility of laboratory insecticide resistance association studies compared to field observations, are discussed.

The synaptic life of microtubules

In neurons, control of microtubule dynamics is required for multiple homeostatic and regulated activities. Over the past few decades, a great deal has been learned about the role of the microtubule cytoskeleton in axonal and dendritic transport, with a broad impact on neuronal health and disease. However, significantly less attention has been paid to the importance of microtubule dynamics in directly regulating synaptic function. Here, we review emerging literature demonstrating that microtubules enter synapses and control central aspects of synaptic activity, including neurotransmitter release and synaptic plasticity. The pleiotropic effects caused by a dysfunctional synaptic microtubule cytoskeleton may thus represent a key point of vulnerability for neurons and a primary driver of neurological disease.

The microtubule cytoskeleton at the synapse

In neurons, microtubules (MTs) provide routes for transport throughout the cell and structural support for dendrites and axons. Both stable and dynamic MTs are necessary for normal neuronal functions. Research in the last two decades has demonstrated that MTs play additional roles in synaptic structure and function in both pre- and postsynaptic elements. Here, we review current knowledge of the functions that MTs perform in excitatory and inhibitory synapses, as well as in the neuromuscular junction and other specialized synapses, and discuss the implications that this knowledge may have in neurological disease.

Dynamic microtubules at the synapse

Microtubules (MTs) are a fundamental cytoskeletal component that give neurons structure and are the primary polymer system for long distance transport of cargo throughout the cytoplasm. Although neurons are highly polarized and their structure is often maintained throughout the life of an organism, MTs can remain dynamic in axons and dendrites, undergoing bouts of polymerization and depolymerization, referred to as dynamic instability. Furthermore, MTs can be nucleated outside of the centrosome or MT organizing center (MTOC) that is located in the cell body, allowing dynamic formation and branching of MT polymers throughout the neuron. Together, these recent findings point to a much more dynamic landscape of microtubules in developing and mature neurons than was previously appreciated. Here we will focus on recent studies that show MT dynamics are playing a role at the synapse, both post-synaptically in dendrites and pre-synaptically in axons.

TMT-Based Quantitative Proteomic Profiling of Overwintering Lissorhoptrus oryzophilus

Adaptations to low temperature play a critical role in restricting the geographical distribution of insects. Decreasing day lengths and temperatures trigger seasonal cold adaptations in insects. These adaptions include changes in expression at the miRNA, mRNA and protein levels. The rice water weevil (RWW), Lissorhoptrus oryzophilus, introduced from the Mississippi River, is a globally invasive pest of wetland rice that can survive at the northern border of China. To investigate the changes in expression at the protein level in overwintering female RWW adults, 6-plex tandem mass tags (TMTs) were used in overwintering and summer adults. By using a proteome database available for Curculionidae, 1077 proteins were quantified, 183 of which differed significantly between the overwintering and summer samples. To further understand these differentially expressed proteins (DEPs), bioinformatics analyses such as gene ontology (GO) enrichment analyses were performed. DEPs associated with the terms binding, structural molecule activity, catalytic activity, multicellular organismal process, extracellular region, chitin binding, metabolic process, intracellular part and organic cyclic compound binding were altered by selection during winter. The changes in the expression of these proteins suggest that the proteins are important for RWW survival in winter.

dTACC restricts bouton addition and regulates microtubule organization at the Drosophila neuromuscular junction

Regulation of the synaptic cytoskeleton is essential to proper neuronal development and wiring. Perturbations in neuronal microtubules (MTs) are associated with numerous pathologies, yet it remains unclear how changes in MTs may be coupled to synapse morphogenesis. Studies have identified many MT regulators that promote synapse growth. However, less is known about the factors that restrict growth, despite the potential links of synaptic overgrowth to severe neurological conditions. Here, we report that dTACC, which is implicated in MT assembly and stability, prevents synapse overgrowth at the Drosophila neuromuscular junction by restricting addition of new boutons throughout larval development. dTACC localizes to the axonal MT lattice and is required to maintain tubulin levels and the integrity of higher-order MT structures in motor axon terminals. While previous reports have demonstrated the roles of MT-stabilizing proteins in promoting synapse growth, our findings suggest that in certain contexts, MT stabilization may correlate with restricted growth.

Microtubule and Actin Differentially Regulate Synaptic Vesicle Cycling to Maintain High-Frequency Neurotransmission

Cytoskeletal filaments such as microtubules (MTs) and filamentous actin (F-actin) dynamically support cell structure and functions. In central presynaptic terminals, F-actin is expressed along the release edge and reportedly plays diverse functional roles, but whether axonal MTs extend deep into terminals and play any physiological role remains controversial.

Cytoskeletal filaments such as microtubules (MTs) and filamentous actin (F-actin) dynamically support cell structure and functions. In central presynaptic terminals, F-actin is expressed along the release edge and reportedly plays diverse functional roles, but whether axonal MTs extend deep into terminals and play any physiological role remains controversial. At the calyx of Held in rats of either sex, confocal and high-resolution microscopy revealed that MTs enter deep into presynaptic terminal swellings and partially colocalize with a subset of synaptic vesicles (SVs). Electrophysiological analysis demonstrated that depolymerization of MTs specifically prolonged the slow-recovery time component of EPSCs from short-term depression induced by a train of high-frequency stimulation, whereas depolymerization of F-actin specifically prolonged the fast-recovery component. In simultaneous presynaptic and postsynaptic action potential recordings, depolymerization of MTs or F-actin significantly impaired the fidelity of high-frequency neurotransmission. We conclude that MTs and F-actin differentially contribute to slow and fast SV replenishment, thereby maintaining high-frequency neurotransmission.

SIGNIFICANCE STATEMENT The presence and functional role of MTs in the presynaptic terminal are controversial. Here, we demonstrate that MTs are present near SVs in calyceal presynaptic terminals and that MT depolymerization specifically prolongs the slow-recovery component of EPSCs from short-term depression. In contrast, F-actin depolymerization specifically prolongs fast-recovery component. Depolymerization of MT or F-actin has no direct effect on SV exocytosis/endocytosis or basal transmission, but significantly impairs the fidelity of high-frequency transmission, suggesting that presynaptic cytoskeletal filaments play essential roles in SV replenishment for the maintenance of high-frequency neurotransmission.

Tubulin Polymerization Promoting Protein, Ringmaker, and MAP1B Homolog Futsch Coordinate Microtubule Organization and Synaptic Growth

Drosophila Ringmaker (Ringer) is homologous to the human Tubulin Polymerization Promoting Proteins (TPPPs) that are implicated in the stabilization and bundling of microtubules (MTs) that are particularly important for neurons and are also implicated in synaptic organization and plasticity. No in vivo functional data exist that have addressed the role of TPPP in synapse organization in any system. Here, we present the phenotypic and functional characterization of ringer mutants during Drosophila larval neuromuscular junction (NMJ) synaptic development. ringer mutants show reduced synaptic growth and transmission and display phenotypic similarities and genetic interactions with the Drosophila homolog of vertebrate Microtubule Associated Protein (MAP)1B, futsch. Immunohistochemical and biochemical analyses show that individual and combined loss of Ringer and Futsch cause a significant reduction in MT loops at the NMJs and reduced acetylated-tubulin levels. Presynaptic over-expression of Ringer and Futsch causes elevated levels of acetylated-tubulin and significant increase in NMJ MT loops. These results indicate that Ringer and Futsch regulate synaptic MT organization in addition to synaptic growth. Together our findings may inform studies on the close mammalian homolog, TPPP, and provide insights into the role of MTs and associated proteins in synapse growth and organization.

Molecular cross-talk in a unique parasitoid manipulation strategy

Envenomation of cockroach cerebral ganglia by the parasitoid Jewel wasp, Ampulex compressa, induces specific, long-lasting behavioural changes. We hypothesized that this prolonged action results from venom-induced changes in brain neurochemistry. Here, we address this issue by first identifying molecular targets of the venom, i.e., proteins to which venom components bind and interact with to mediate altered behaviour. Our results show that venom components bind to synaptic proteins and likely interfere with both pre- and postsynaptic processes. Since behavioural changes induced by the sting are long-lasting and reversible, we hypothesized further that long-term effects of the venom must be mediated by up or down regulation of cerebral ganglia proteins. We therefore characterize changes in cerebral ganglia protein abundance of stung cockroaches at different time points after the sting by quantitative mass spectrometry. Our findings indicate that numerous proteins are differentially expressed in cerebral ganglia of stung cockroaches, many of which are involved in signal transduction, such as the Rho GTPase pathway, which is implicated in synaptic plasticity. Altogether, our data suggest that the Jewel wasp commandeers cockroach behaviour through molecular cross-talk between venom components and molecular targets in the cockroach central nervous system, leading to broad-based alteration of synaptic efficacy and behavioural changes that promote successful development of wasp progeny.

Coordinated Regulation of Axonal Microtubule Organization and Transport by Drosophila Neurexin and BMP Pathway

Neurexins are well known trans-synaptic cell adhesion molecules that are required for proper synaptic development and function across species. Beyond synapse organization and function, little is known about other roles Neurexins might have in the nervous system. Here we report novel phenotypic consequences of mutations in Drosophila neurexin (dnrx), which alters axonal microtubule organization and transport. We show that dnrx mutants display phenotypic similarities with the BMP receptor wishful thinking (wit) and one of the downstream effectors, futsch, which is a known regulator of microtubule organization and stability. dnrx has genetic interactions with wit and futsch. Loss of Dnrx also results in reduced levels of other downstream effectors of BMP signaling, phosphorylated-Mad and Trio. Interestingly, postsynaptic overexpression of the BMP ligand, Glass bottom boat, in dnrx mutants partially rescues the axonal transport defects but not the synapse undergrowth at the neuromuscular junctions. These data suggest that Dnrx and BMP signaling are involved in many diverse functions and that regulation of axonal MT organization and transport might be distinct from regulation of synaptic growth in dnrx mutants. Together, our work uncovers a novel function of Drosophila Neurexin and may provide insights into functions of Neurexins in vertebrates.

Levels of Par-1 kinase determine the localization of Bruchpilot at the Drosophila neuromuscular junction synapses

Functional synaptic networks are compromised in many neurodevelopmental and neurodegenerative diseases. While the mechanisms of axonal transport and localization of synaptic vesicles and mitochondria are relatively well studied, little is known about the mechanisms that regulate the localization of proteins that localize to active zones. Recent finding suggests that mechanisms involved in transporting proteins destined to active zones are distinct from those that transport synaptic vesicles or mitochondria. Here we report that localization of BRP-an essential active zone scaffolding protein in Drosophila, depends on the precise balance of neuronal Par-1 kinase. Disruption of Par-1 levels leads to excess accumulation of BRP in axons at the expense of BRP at active zones. Temporal analyses demonstrate that accumulation of BRP within axons precedes the loss of synaptic function and its depletion from the active zones. Mechanistically, we find that Par-1 co-localizes with BRP and is present in the same molecular complex, raising the possibility of a novel mechanism for selective localization of BRP-like active zone scaffolding proteins. Taken together, these data suggest an intriguing possibility that mislocalization of active zone proteins like BRP might be one of the earliest signs of synapse perturbation and perhaps, synaptic networks that precede many neurological disorders.

Superresolution imaging of Drosophila tissues using expansion microscopy

The limited resolving power of conventional diffraction-limited microscopy hinders analysis of small, densely packed structural elements in cells. Expansion microscopy (ExM) provides an elegant solution to this problem, allowing for increased resolution with standard microscopes via physical expansion of the specimen in a swellable polymer hydrogel. Here, we apply, validate, and optimize ExM protocols that enable the study of Drosophila embryos, larval brains, and larval and adult body walls. We achieve a lateral resolution of ∼70 nm in Drosophila tissues using a standard confocal microscope, and we use ExM to analyze fine intracellular structures and intercellular interactions. First, we find that ExM reveals features of presynaptic active zone (AZ) structure that are observable with other superresolution imaging techniques but not with standard confocal microscopy. We further show that synapses known to exhibit age-dependent changes in activity also exhibit age-dependent changes in AZ structure. Finally, we use the significantly improved axial resolution of ExM to show that dendrites of somatosensory neurons are inserted into epithelial cells at a higher frequency than previously reported in confocal microscopy studies. Altogether, our study provides a foundation for the application of ExM to Drosophila tissues and underscores the importance of tissue-specific optimization of ExM procedures.

Microtubule organization in presynaptic boutons relies on the formin DAAM

Regulation of the cytoskeleton is fundamental to the development and function of synaptic terminals, such as neuromuscular junctions. Despite the identification of numerous proteins that regulate synaptic actin and microtubule dynamics, the mechanisms of cytoskeletal control during terminal arbor formation have remained largely elusive. Here, we show that DAAM, a member of the formin family of cytoskeleton organizing factors, is an important presynaptic regulator of neuromuscular junction development in Drosophila We demonstrate that the actin filament assembly activity of DAAM plays a negligible role in terminal formation; rather, DAAM is necessary for synaptic microtubule organization. Genetic interaction studies consistently link DAAM with the Wg/Ank2/Futsch module of microtubule regulation and bouton formation. Finally, we provide evidence that DAAM is tightly associated with the synaptic active zone scaffold, and electrophysiological data point to a role in the modulation of synaptic vesicle release. Based on these results, we propose that DAAM is an important cytoskeletal effector element of the Wg/Ank2 pathway involved in the determination of basic synaptic structures, and, additionally, that DAAM may couple the active zone scaffold to the presynaptic cytoskeleton.

Stabilization of dynamic microtubules by mDia1 drives Tau-dependent Aβ1-42 synaptotoxicity

Oligomeric Amyloid β_1-42 (Aβ) plays a crucial synaptotoxic role in Alzheimer's disease, and hyperphosphorylated tau facilitates Aβ toxicity. The link between Aβ and tau, however, remains controversial. In this study, we find that in hippocampal neurons, Aβ acutely induces tubulin posttranslational modifications (PTMs) and stabilizes dynamic microtubules (MTs) by reducing their catastrophe frequency. Silencing or acute inhibition of the formin mDia1 suppresses these activities and corrects the synaptotoxicity and deficits of axonal transport induced by Aβ. We explored the mechanism of rescue and found that stabilization of dynamic MTs promotes tau-dependent loss of dendritic spines and tau hyperphosphorylation. Collectively, these results uncover a novel role for mDia1 in Aβ-mediated synaptotoxicity and demonstrate that inhibition of MT dynamics and accumulation of PTMs are driving factors for the induction of tau-mediated neuronal damage.

Active zone proteins are transported via distinct mechanisms regulated by Par-1 kinase

Disruption of synapses underlies a plethora of neurodevelopmental and neurodegenerative disease. Presynaptic specialization called the active zone plays a critical role in the communication with postsynaptic neuron. While the role of many proteins at the active zones in synaptic communication is relatively well studied, very little is known about how these proteins are transported to the synapses. For example, are there distinct mechanisms for the transport of active zone components or are they all transported in the same transport vesicle? Is active zone protein transport regulated? In this report we show that overexpression of Par-1/MARK kinase, a protein whose misregulation has been implicated in Autism spectrum disorders (ASDs) and neurodegenerative disorders, lead to a specific block in the transport of an active zone protein component- Bruchpilot at Drosophila neuromuscular junctions. Consistent with a block in axonal transport, we find a decrease in number of active zones and reduced neurotransmission in flies overexpressing Par-1 kinase. Interestingly, we find that Par-1 acts independently of Tau-one of the most well studied substrates of Par-1, revealing a presynaptic function for Par-1 that is independent of Tau. Thus, our study strongly suggests that there are distinct mechanisms that transport components of active zones and that they are tightly regulated.

Synapses consist of pre- and postsynaptic partners. Proper function of active zones, a presynaptic component of synapse, is essential for efficacious neuronal communication. Disruption of neuronal communication is an early sign of both neurodevelopmental as well as neurodegenerative diseases. Since proteins that reside in active zones are used so frequently during the neuronal communication, they must be constantly replenished to maintain active zones. Axonal transport of these proteins plays an important role in replenishing these vital components necessary for the health of active zones. However, the mechanisms that transport components of active zones are not well understood. Our data suggest that there are distinct mechanisms that transport various active zone cargoes and this process is likely regulated by kinases. Further, our data show that disruption in the transport of one such active zone components causes reduced neuronal communication emphasizing the importance of the process of axonal transport of active zone protein(s) for neuronal communication. Understanding the processes that govern the axonal transport of active zone components will help dissect the initial stages of pathogenesis in both neurodevelopmental and neurodegenerative diseases.

Human R1441C LRRK2 regulates the synaptic vesicle proteome and phosphoproteome in a Drosophila model of Parkinson's disease

Mutations in leucine-rich repeat kinase 2 (LRRK2) cause late-onset, autosomal dominant familial Parkinson`s disease (PD) and variation at the LRRK2 locus contributes to the risk for idiopathic PD. LRRK2 can function as a protein kinase and mutations lead to increased kinase activity. To elucidate the pathophysiological mechanism of the R1441C mutation in the GTPase domain of LRRK2, we expressed human wild-type or R1441C LRRK2 in dopaminergic neurons of Drosophila and observe reduced locomotor activity, impaired survival and an age-dependent degeneration of dopaminergic neurons thereby creating a new PD-like model. To explore the function of LRRK2 variants in vivo, we performed mass spectrometry and quantified 3,616 proteins in the fly brain. We identify several differentially-expressed cytoskeletal, mitochondrial and synaptic vesicle proteins (SV), including synaptotagmin-1, syntaxin-1A and Rab3, in the brain of this LRRK2 fly model. In addition, a global phosphoproteome analysis reveals the enhanced phosphorylation of several SV proteins, including synaptojanin-1 (pThr1131) and the microtubule-associated protein futsch (pSer4106) in the brain of R1441C hLRRK2 flies. The direct phosphorylation of human synaptojanin-1 by R1441C hLRRK2 could further be confirmed by in vitro kinase assays. A protein-protein interaction screen in the fly brain confirms that LRRK2 robustly interacts with numerous SV proteins, including synaptojanin-1 and EndophilinA. Our proteomic, phosphoproteomic and interactome study in the Drosophila brain provides a systematic analyses of R1441C hLRRK2-induced pathobiological mechanisms in this model. We demonstrate for the first time that the R1441C mutation located within the LRRK2 GTPase domain induces the enhanced phosphorylation of SV proteins in the brain.

Comparative proteomic analysis in Aphis glycines Mutsumura under lambda-cyhalothrin insecticide stress

Lambda-cyhalothrin is now widely used in China to control the soybean aphid Aphis glycines. To dissect the resistance mechanism, a laboratory-selected resistant soybean aphid strain (CRR) was established with a 43.42-fold resistance ratio to λ-cyhalothrin than the susceptible strain (CSS) in adult aphids. In this study, a comparative proteomic analysis between the CRR and CSS strains revealed important differences between the susceptible and resistant strains of soybean aphids for λ-cyhalothrin. Approximately 493 protein spots were detected in two-dimensional polyacrylamide gel electrophoresis (2-DE). Thirty-six protein spots displayed differential expression of >2-fold in the CRR strain compared to the CSS strain. Out of these 36 protein spots, 21 had elevated and 15 had decreased expression. Twenty-four differentially expressed proteins were identified by MALDI TOF MS/MS and categorized into the functional groups cytoskeleton-related protein, carbohydrate and energy metabolism, protein folding, antioxidant system, and nucleotide and amino acid metabolism. Function analysis showed that cytoskeleton-related proteins and energy metabolism proteins have been associated with the λ-cyhalothrin resistance of A. glycines. The differential expression of λ-cyhalothrin responsive proteins reflected the overall change in cellular structure and metabolism after insecticide treatment in aphids. In summary, our studies improve understanding of the molecular mechanism resistance of soybean aphid to lambda-cyhalothrin, which will facilitate the development of rational approaches to improve the management of this pest and to improve the yield of soybean.

The Presynaptic Microtubule Cytoskeleton in Physiological and Pathological Conditions: Lessons from Drosophila Fragile X Syndrome and Hereditary Spastic Paraplegias

The capacity of the nervous system to generate neuronal networks relies on the establishment and maintenance of synaptic contacts. Synapses are composed of functionally different presynaptic and postsynaptic compartments. An appropriate synaptic architecture is required to provide the structural basis that supports synaptic transmission, a process involving changes in cytoskeletal dynamics. Actin microfilaments are the main cytoskeletal components present at both presynaptic and postsynaptic terminals in glutamatergic synapses. However, in the last few years it has been demonstrated that microtubules (MTs) transiently invade dendritic spines, promoting their maturation. Nevertheless, the presence and functions of MTs at the presynaptic site are still a matter of debate. Early electron microscopy (EM) studies revealed that MTs are present in the presynaptic terminals of the central nervous system (CNS) where they interact with synaptic vesicles (SVs) and reach the active zone. These observations have been reproduced by several EM protocols; however, there is empirical heterogeneity in detecting presynaptic MTs, since they appear to be both labile and unstable. Moreover, increasing evidence derived from studies in the fruit fly neuromuscular junction proposes different roles for MTs in regulating presynaptic function in physiological and pathological conditions. In this review, we summarize the main findings that support the presence and roles of MTs at presynaptic terminals, integrating descriptive and biochemical analyses, and studies performed in invertebrate genetic models.

Microtubule-associated protein 1B (MAP1B)-deficient neurons show structural presynaptic deficiencies in vitro and altered presynaptic physiology

Microtubule-associated protein 1B (MAP1B) is expressed predominantly during the early stages of development of the nervous system, where it regulates processes such as axonal guidance and elongation. Nevertheless, MAP1B expression in the brain persists in adult stages, where it participates in the regulation of the structure and physiology of dendritic spines in glutamatergic synapses. Moreover, MAP1B expression is also found in presynaptic synaptosomal preparations. In this work, we describe a presynaptic phenotype in mature neurons derived from MAP1B knockout (MAP1B KO) mice. Mature neurons express MAP1B, and its deficiency does not alter the expression levels of a subgroup of other synaptic proteins. MAP1B KO neurons display a decrease in the density of presynaptic and postsynaptic terminals, which involves a reduction in the density of synaptic contacts, and an increased proportion of orphan presynaptic terminals. Accordingly, MAP1B KO neurons present altered synaptic vesicle fusion events, as shown by FM4-64 release assay, and a decrease in the density of both synaptic vesicles and dense core vesicles at presynaptic terminals. Finally, an increased proportion of excitatory immature symmetrical synaptic contacts in MAP1B KO neurons was detected. Altogether these results suggest a novel role for MAP1B in presynaptic structure and physiology regulation in vitro.

The axonal cytoskeleton: from organization to function

The axon is the single long fiber that extends from the neuron and transmits electrical signals away from the cell body. The neuronal cytoskeleton, composed of microtubules (MTs), actin filaments and neurofilaments, is not only required for axon formation and axonal transport but also provides the structural basis for several specialized axonal structures, such as the axon initial segment (AIS) and presynaptic boutons. Emerging evidence suggest that the unique cytoskeleton organization in the axon is essential for its structure and integrity. In addition, the increasing number of neurodevelopmental and neurodegenerative diseases linked to defect in actin- and microtubule-dependent processes emphasizes the importance of a properly regulated cytoskeleton for normal axonal functioning. Here, we provide an overview of the current understanding of actin and microtubule organization within the axon and discuss models for the functional role of the cytoskeleton at specialized axonal structures.

Active zone stability: insights from fly neuromuscular junction

The presynaptic active zone is a dynamic structure that orchestrates regulated release of neurotransmitters. Developmental and aging processes, and changes in neuronal network activity can all modulate the number, size and composition of active zone and thereby synaptic efficacy. However, very little is known about the mechanism that controls the structural stability of active zone. By studying a model synapse, the Drosophila neuromuscular junction, our recent work shed light on how two scaffolding proteins at the active zone regulate active zone stability by promoting a localized dephosphorylation event at the nerve terminal. Here we discuss the major insights from our findings and their implications for future research.

Advances in imaging ultrastructure yield new insights into presynaptic biology

Synapses are the fundamental functional units of neural circuits, and their dysregulation has been implicated in diverse neurological disorders. At presynaptic terminals, neurotransmitter-filled synaptic vesicles are released in response to calcium influx through voltage-gated calcium channels activated by the arrival of an action potential. Decades of electrophysiological, biochemical, and genetic studies have contributed to a growing understanding of presynaptic biology. Imaging studies are yielding new insights into how synapses are organized to carry out their critical functions. The development of techniques for rapid immobilization and preservation of neuronal tissues for electron microscopy (EM) has led to a new renaissance in ultrastructural imaging that is rapidly advancing our understanding of synapse structure and function.

Molecular Remodeling of the Presynaptic Active Zone of Drosophila Photoreceptors via Activity-Dependent Feedback

Neural activity contributes to the regulation of the properties of synapses in sensory systems, allowing for adjustment to a changing environment. Little is known about how synaptic molecular components are regulated to achieve activity-dependent plasticity at central synapses. Here, we found that after prolonged exposure to natural ambient light the presynaptic active zone in Drosophila photoreceptors undergoes reversible remodeling, including loss of Bruchpilot, DLiprin-α, and DRBP, but not of DSyd-1 or Cacophony. The level of depolarization of the postsynaptic neurons is critical for the light-induced changes in active zone composition in the photoreceptors, indicating the existence of a feedback signal. In search of this signal, we have identified a crucial role of microtubule meshwork organization downstream of the divergent canonical Wnt pathway, potentially via Kinesin-3 Imac. These data reveal that active zone composition can be regulated in vivo and identify the underlying molecular machinery.

Hierarchical microtubule organization controls axon caliber and transport and determines synaptic structure and stability

The dimensions of axons and synaptic terminals determine cell-intrinsic properties of neurons; however, the cellular mechanisms selectively controlling establishment and maintenance of neuronal compartments remain poorly understood. Here, we show that two giant Drosophila Ankyrin2 isoforms, Ank2-L and Ank2-XL, and the MAP1B homolog Futsch form a membrane-associated microtubule-organizing complex that determines axonal diameter, supports axonal transport, and provides independent control of synaptic dimensions and stability. Ank2-L controls microtubule and synaptic stability upstream of Ank2-XL that selectively controls microtubule organization. Synergistically with Futsch, Ank2-XL provides three-dimensional microtubule organization and is required to establish appropriate synaptic dimensions and release properties. In axons, the Ank2-XL/Futsch complex establishes evenly spaced, grid-like microtubule organization and determines axonal diameter in the absence of neurofilaments. Reduced microtubule spacing limits anterograde transport velocities of mitochondria and synaptic vesicles. Our data identify control of microtubule architecture as a central mechanism to selectively control neuronal dimensions, functional properties, and connectivity.

Regulation of synaptic development and function by the Drosophila PDZ protein Dyschronic

Synaptic scaffold proteins control the localization of ion channels and receptors, and facilitate molecular associations between signaling components that modulate synaptic transmission and plasticity. Here, we define novel roles for a recently described scaffold protein, Dsychronic (DYSC), at the Drosophila larval neuromuscular junction. DYSC is the Drosophila homolog of whirlin/DFNB31, a PDZ domain protein linked to Usher syndrome, the most common form of human deaf-blindness. We show that DYSC is expressed presynaptically and is often localized adjacent to the active zone, the site of neurotransmitter release. Loss of DYSC results in marked alterations in synaptic morphology and cytoskeletal organization. Moreover, active zones are frequently enlarged and misshapen in dysc mutants. Electrophysiological analyses further demonstrate that dysc mutants exhibit substantial increases in both evoked and spontaneous synaptic transmission. We have previously shown that DYSC binds to and regulates the expression of the Slowpoke (SLO) BK potassium channel. Consistent with this, slo mutant larvae exhibit similar alterations in synapse morphology, active zone size and neurotransmission, and simultaneous loss of dysc and slo does not enhance these phenotypes, suggesting that dysc and slo act in a common genetic pathway to modulate synaptic development and output. Our data expand our understanding of the neuronal functions of DYSC and uncover non-canonical roles for the SLO potassium channel at Drosophila synapses.