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

PTPN11/Corkscrew Activates Local Presynaptic Mapk Signaling to Regulate Synapsin, Synaptic Vesicle Pools, and Neurotransmission Strength, with a Dual Requirement in Neurons and Glia

Cytoplasmic protein tyrosine phosphatase nonreceptor type 11 (PTPN11) and Drosophila homolog Corkscrew (Csw) regulate the mitogen-activated protein kinase (MAPK) pathway via a conserved autoinhibitory mechanism. Disease-causing loss-of-function (LoF) and gain-of-function (GoF) mutations both disrupt this autoinhibition to potentiate MAPK signaling. At the Drosophila neuromuscular junction glutamatergic synapse, LoF/GoF mutations elevate transmission strength and reduce activity-dependent synaptic depression. In both sexes of LoF/GoF mutations, the synaptic vesicles (SV)-colocalized synapsin phosphoprotein tether is highly elevated at rest, but quickly reduced with stimulation, suggesting a larger SV reserve pool with greatly heightened activity-dependent recruitment. Transmission electron microscopy of mutants reveals an elevated number of SVs clustered at the presynaptic active zones, suggesting that the increased vesicle availability is causative for the elevated neurotransmission. Direct neuron-targeted extracellular signal-regulated kinase (ERK) GoF phenocopies both increased local presynaptic MAPK/ERK signaling and synaptic transmission strength in mutants, confirming the presynaptic regulatory mechanism. Synapsin loss blocks this elevation in both presynaptic PTPN11 and ERK mutants. However, csw null mutants cannot be rescued by wild-type Csw in neurons: neurotransmission is only rescued by expressing Csw in both neurons and glia simultaneously. Nevertheless, targeted LoF/GoF mutations in either neurons or glia alone recapitulate the elevated neurotransmission. Thus, PTPN11/Csw mutations in either cell type are sufficient to upregulate presynaptic function, but a dual requirement in neurons and glia is necessary for neurotransmission. Taken together, we conclude that PTPN11/Csw acts in both neurons and glia, with LoF and GoF similarly upregulating MAPK/ERK signaling to enhance presynaptic Synapsin-mediated SV trafficking.

Therapeutic Potential of Natural Compounds from Herbs and Nutraceuticals in Alleviating Neurological Disorders: Targeting the Wnt Signaling Pathway

As an important signaling pathway in multicellular eukaryotes, the Wnt signaling pathway participates in a variety of physiological processes. Recent studies have confirmed that the Wnt signaling pathway plays an important role in neurological disorders such as stroke, Alzheimer's disease, and Parkinson's disease. The regulation of Wnt signaling by natural compounds in herbal medicines and nutraceuticals has emerged as a potential strategy for the development of new drugs for neurological disorders. Purpose: The aim of this review is to evaluate the latest research results on the efficacy of natural compounds derived from herbs and nutraceuticals in the prevention and treatment of neurological disorders by regulating the Wnt pathway in vivo and in vitro. A manual and electronic search was performed for English articles available from PubMed, Web of Science, and ScienceDirect from the January 2010 to February 2023. Keywords used for the search engines were "natural products,″ "plant derived products,″ "Wnt+ clinical trials,″ and "Wnt+,″ and/or paired with "natural products″/″plant derived products", and "neurological disorders." A total of 22 articles were enrolled in this review, and a variety of natural compounds from herbal medicine and nutritional foods have been shown to exert therapeutic effects on neurological disorders through the Wnt pathway, including curcumin, resveratrol, and querctrin, etc. These natural products possess antioxidant, anti-inflammatory, and angiogenic properties, confer neurovascular unit and blood-brain barrier integrity protection, and affect neural stem cell differentiation, synaptic formation, and neurogenesis, to play a therapeutic role in neurological disorders. In various in vivo and in vitro studies and clinical trials, these natural compounds have been shown to be safe and tolerable with few adverse effects. Natural compounds may serve a therapeutic role in neurological disorders by regulating the Wnt pathway. This summary of the research progress of natural compounds targeting the Wnt pathway may provide new insights for the treatment of neurological disorders and potential targets for the development of new drugs.

Monomeric α-synuclein activates the plasma membrane calcium pump

Alpha-synuclein (aSN) is a membrane-associated and intrinsically disordered protein, well known for pathological aggregation in neurodegeneration. However, the physiological function of aSN is disputed. Pull-down experiments have pointed to plasma membrane Ca2+ -ATPase (PMCA) as a potential interaction partner. From proximity ligation assays, we find that aSN and PMCA colocalize at neuronal synapses, and we show that calcium expulsion is activated by aSN and PMCA. We further show that soluble, monomeric aSN activates PMCA at par with calmodulin, but independent of the autoinhibitory domain of PMCA, and highly dependent on acidic phospholipids and membrane-anchoring properties of aSN. On PMCA, the key site is mapped to the acidic lipid-binding site, located within a disordered PMCA-specific loop connecting the cytosolic A domain and transmembrane segment 3. Our studies point toward a novel physiological role of monomeric aSN as a stimulator of calcium clearance in neurons through activation of PMCA.

FM Dye Cycling at the Synapse: Comparing High Potassium Depolarization, Electrical and Channelrhodopsin Stimulation

FM dyes are used to study the synaptic vesicle (SV) cycle. These amphipathic probes have a hydrophilic head and hydrophobic tail, making them water-soluble with the ability to reversibly enter and exit membrane lipid bilayers. These styryl dyes are relatively non-fluorescent in aqueous medium, but insertion into the outer leaflet of the plasma membrane causes a >40X increase in fluorescence. In neuronal synapses, FM dyes are internalized during SV endocytosis, trafficked both within and between SV pools, and released with SV exocytosis, providing a powerful tool to visualize presynaptic stages of neurotransmission. A primary genetic model of glutamatergic synapse development and function is the Drosophila neuromuscular junction (NMJ), where FM dye imaging has been used extensively to quantify SV dynamics in a wide range of mutant conditions. The NMJ synaptic terminal is easily accessible, with a beautiful array of large synaptic boutons ideal for imaging applications. Here, we compare and contrast the three ways to stimulate the Drosophila NMJ to drive activity-dependent FM1-43 dye uptake/release: 1) bath application of high [K+] to depolarize neuromuscular tissues, 2) suction electrode motor nerve stimulation to depolarize the presynaptic nerve terminal, and 3) targeted transgenic expression of channelrhodopsin variants for light-stimulated, spatial control of depolarization. Each of these methods has benefits and disadvantages for the study of genetic mutation effects on the SV cycle at the Drosophila NMJ. We will discuss these advantages and disadvantages to assist the selection of the stimulation approach, together with the methodologies specific to each strategy. In addition to fluorescent imaging, FM dyes can be photoconverted to electron-dense signals visualized using transmission electron microscopy (TEM) to study SV cycle mechanisms at an ultrastructural level. We provide the comparisons of confocal and electron microscopy imaging from the different methods of Drosophila NMJ stimulation, to help guide the selection of future experimental paradigms.

From damage response to action potentials: early evolution of neural and contractile modules in stem eukaryotes

Eukaryotic cells convert external stimuli into membrane depolarization, which in turn triggers effector responses such as secretion and contraction. Here, we put forward an evolutionary hypothesis for the origin of the depolarization-contraction-secretion (DCS) coupling, the functional core of animal neuromuscular circuits. We propose that DCS coupling evolved in unicellular stem eukaryotes as part of an 'emergency response' to calcium influx upon membrane rupture. We detail how this initial response was subsequently modified into an ancient mechanosensory-effector arc, present in the last eukaryotic common ancestor, which enabled contractile amoeboid movement that is widespread in extant eukaryotes. Elaborating on calcium-triggered membrane depolarization, we reason that the first action potentials evolved alongside the membrane of sensory-motile cilia, with the first voltage-sensitive sodium/calcium channels (Nav/Cav) enabling a fast and coordinated response of the entire cilium to mechanosensory stimuli. From the cilium, action potentials then spread across the entire cell, enabling global cellular responses such as concerted contraction in several independent eukaryote lineages. In animals, this process led to the invention of mechanosensory contractile cells. These gave rise to mechanosensory receptor cells, neurons and muscle cells by division of labour and can be regarded as the founder cell type of the nervous system.

Molecular underpinnings of synaptic vesicle pool heterogeneity

Neuronal communication relies on chemical synaptic transmission for information transfer and processing. Chemical neurotransmission is initiated by synaptic vesicle fusion with the presynaptic active zone resulting in release of neurotransmitters. Classical models have assumed that all synaptic vesicles within a synapse have the same potential to fuse under different functional contexts. In this model, functional differences among synaptic vesicle populations are ascribed to their spatial distribution in the synapse with respect to the active zone. Emerging evidence suggests, however, that synaptic vesicles are not a homogenous population of organelles, and they possess intrinsic molecular differences and differential interaction partners. Recent studies have reported a diverse array of synaptic molecules that selectively regulate synaptic vesicles' ability to fuse synchronously and asynchronously in response to action potentials or spontaneously irrespective of action potentials. Here we discuss these molecular mediators of vesicle pool heterogeneity that are found on the synaptic vesicle membrane, on the presynaptic plasma membrane, or within the cytosol and consider some of the functional consequences of this diversity. This emerging molecular framework presents novel avenues to probe synaptic function and uncover how synaptic vesicle pools impact neuronal signaling.

Dynamic regulation of mRNA decay during neural development

Background

Gene expression patterns are determined by rates of mRNA transcription and decay. While transcription is known to regulate many developmental processes, the role of mRNA decay is less extensively defined. A critical step toward defining the role of mRNA decay in neural development is to measure genome-wide mRNA decay rates in neural tissue. Such information should reveal the degree to which mRNA decay contributes to differential gene expression and provide a foundation for identifying regulatory mechanisms that affect neural mRNA decay.

Results

We developed a technique that allows genome-wide mRNA decay measurements in intact Drosophila embryos, across all tissues and specifically in the nervous system. Our approach revealed neural-specific decay kinetics, including stabilization of transcripts encoding regulators of axonogenesis and destabilization of transcripts encoding ribosomal proteins and histones. We also identified correlations between mRNA stability and physiologic properties of mRNAs; mRNAs that are predicted to be translated within axon growth cones or dendrites have long half-lives while mRNAs encoding transcription factors that regulate neurogenesis have short half-lives. A search for candidate cis-regulatory elements identified enrichment of the Pumilio recognition element (PRE) in mRNAs encoding regulators of neurogenesis. We found that decreased expression of the RNA-binding protein Pumilio stabilized predicted neural mRNA targets and that a PRE is necessary to trigger reporter-transcript decay in the nervous system.

Conclusions

We found that differential mRNA decay contributes to the relative abundance of transcripts involved in cell-fate decisions, axonogenesis, and other critical events during Drosophila neural development. Neural-specific decay kinetics and the functional specificity of mRNA decay suggest the existence of a dynamic neurodevelopmental mRNA decay network. We found that Pumilio is one component of this network, revealing a novel function for this RNA-binding protein.

Electronic supplementary material

The online version of this article (doi:10.1186/s13064-015-0038-6) contains supplementary material, which is available to authorized users.

Two protein N-acetylgalactosaminyl transferases regulate synaptic plasticity by activity-dependent regulation of integrin signaling

Using a Drosophila whole-genome transgenic RNAi screen for glycogenes regulating synapse function, we have identified two protein α-N-acetylgalactosaminyltransferases (pgant3 and pgant35A) that regulate synaptic O-linked glycosylation (GalNAcα1-O-S/T). Loss of either pgant alone elevates presynaptic/postsynaptic molecular assembly and evoked neurotransmission strength, but synapses appear restored to normal in double mutants. Likewise, activity-dependent facilitation, augmentation, and posttetanic potentiation are all suppressively impaired in pgant mutants. In non-neuronal contexts, pgant function regulates integrin signaling, and we show here that the synaptic Position Specific 2 (αPS2) integrin receptor and transmembrane tenascin ligand are both suppressively downregulated in pgant mutants. Channelrhodopsin-driven activity rapidly (<1 min) drives integrin signaling in wild-type synapses but is suppressively abolished in pgant mutants. Optogenetic stimulation in pgant mutants alters presynaptic vesicle trafficking and postsynaptic pocket size during the perturbed integrin signaling underlying synaptic plasticity defects. Critically, acute blockade of integrin signaling acts synergistically with pgant mutants to eliminate all activity-dependent synaptic plasticity.

N-glycosylation requirements in neuromuscular synaptogenesis

Neural development requires N-glycosylation regulation of intercellular signaling, but the requirements in synaptogenesis have not been well tested. All complex and hybrid N-glycosylation requires MGAT1 (UDP-GlcNAc:α-3-D-mannoside-β1,2-N-acetylglucosaminyl-transferase I) function, and Mgat1 nulls are the most compromised N-glycosylation condition that survive long enough to permit synaptogenesis studies. At the Drosophila neuromuscular junction (NMJ), Mgat1 mutants display selective loss of lectin-defined carbohydrates in the extracellular synaptomatrix, and an accompanying accumulation of the secreted endogenous Mind the gap (MTG) lectin, a key synaptogenesis regulator. Null Mgat1 mutants exhibit strongly overelaborated synaptic structural development, consistent with inhibitory roles for complex/hybrid N-glycans in morphological synaptogenesis, and strengthened functional synapse differentiation, consistent with synaptogenic MTG functions. Synapse molecular composition is surprisingly selectively altered, with decreases in presynaptic active zone Bruchpilot (BRP) and postsynaptic Glutamate receptor subtype B (GLURIIB), but no detectable change in a wide range of other synaptic components. Synaptogenesis is driven by bidirectional trans-synaptic signals that traverse the glycan-rich synaptomatrix, and Mgat1 mutation disrupts both anterograde and retrograde signals, consistent with MTG regulation of trans-synaptic signaling. Downstream of intercellular signaling, pre- and postsynaptic scaffolds are recruited to drive synaptogenesis, and Mgat1 mutants exhibit loss of both classic Discs large 1 (DLG1) and newly defined Lethal (2) giant larvae [L(2)GL] scaffolds. We conclude that MGAT1-dependent N-glycosylation shapes the synaptomatrix carbohydrate environment and endogenous lectin localization within this domain, to modulate retention of trans-synaptic signaling ligands driving synaptic scaffold recruitment during synaptogenesis.

The role of Drosophila cytidine monophosphate-sialic acid synthetase in the nervous system

While sialylation plays important functions in the nervous system, the complexity of glycosylation pathways and limitations of genetic approaches preclude the efficient analysis of these functions in mammalian organisms. Drosophila has recently emerged as a promising model for studying neural sialylation. Drosophila sialyltransferase, DSiaT, was shown to be involved in the regulation of neural transmission. However, the sialylation pathway was not investigated in Drosophila beyond the DSiaT-mediated step. Here we focused on the function of Drosophila cytidine monophosphate-sialic acid synthetase (CSAS), the enzyme providing a sugar donor for DSiaT. Our results revealed that the expression of CSAS is tightly regulated and restricted to the CNS throughout development and in adult flies. We generated CSAS mutants and analyzed their phenotypes using behavioral and physiological approaches. Our experiments demonstrated that mutant phenotypes of CSAS are similar to those of DSiaT, including decreased longevity, temperature-induced paralysis, locomotor abnormalities, and defects of neural transmission at neuromuscular junctions. Genetic interactions between CSAS, DSiaT, and voltage-gated channel genes paralytic and seizure were consistent with the hypothesis that CSAS and DSiaT function within the same pathway regulating neural excitability. Intriguingly, these interactions also suggested that CSAS and DSiaT have some additional, independent functions. Moreover, unlike its mammalian counterparts that work in the nucleus, Drosophila CSAS was found to be a glycoprotein-bearing N-glycans and predominantly localized in vivo to the Golgi compartment. Our work provides the first systematic analysis of in vivo functions of a eukaryotic CSAS gene and sheds light on evolutionary relationships among metazoan CSAS proteins.

The cell polarity scaffold Lethal Giant Larvae regulates synapse morphology and function

Lethal Giant Larvae (LGL) is a cytosolic cell polarity scaffold whose loss dominantly enhances neuromuscular junction (NMJ) synaptic overgrowth caused by loss of the Fragile X Mental Retardation Protein (FMRP). However, direct roles for LGL in NMJ morphological and functional development have not before been tested. Here, we use confocal imaging and two-electrode voltage-clamp electrophysiology at the Drosophila larval NMJ to define the synaptic requirements of LGL. We find that LGL is expressed both pre- and postsynaptically, where the scaffold localizes at the membrane on both sides of the synaptic interface. We show that LGL has a cell autonomous presynaptic role facilitating NMJ terminal branching and synaptic bouton formation. Moreover, loss of both pre- and postsynaptic LGL strongly decreases evoked neurotransmission strength, whereas the frequency and amplitude of spontaneous synaptic vesicle fusion events is increased. Cell-targeted RNAi and rescue reveals separable pre- and postsynaptic LGL roles mediating neurotransmission. We show that presynaptic LGL facilitates the assembly of active zone vesicle fusion sites, and that neuronally targeted rescue of LGL is sufficient to ameliorate increased synaptic vesicle cycling imaged with FM1-43 dye labeling. Postsynaptically, we show that loss of LGL results in a net increase in total glutamate receptor (GluR) expression, associated with the selective elevation of GluRIIB subunit-containing receptors. Taken together, these data indicate that the presynaptic LGL scaffold facilitates the assembly of active zone fusion sites to regulate synaptic vesicle cycling, and that the postsynaptic LGL scaffold modulates glutamate receptor composition and function.

Glycosylated synaptomatrix regulation of trans-synaptic signaling

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

In search of proteins that are important for synaptic functions in Drosophila visual system

This is the second of two reviews that include some of the studies we, members of the Pak laboratory and collaborators, did from 2000 to 2010 on the mutants that affect synaptic transmission in the Drosophila visual system. Of the five mutants we discuss, two turned out to also play roles in the larval neuromuscular junction. This review complements the one on phototransduction to give a fairly complete account of what we focused on during the 10-year period, although we also did some studies on photoreceptor degeneration in the early part of the decade. Besides showing the power of using a genetic approach to the study of synaptic transmission, the review contains some unexpected results that illustrate the serendipitous nature of research.

Extracellular matrix and its receptors in Drosophila neural development

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

The Cdc42-selective GAP rich regulates postsynaptic development and retrograde BMP transsynaptic signaling

Inhibition of Cdc42 by dRich induces postsynaptic release of the BMP ligand Glass bottom boat.

Retrograde bone morphogenetic protein signaling mediated by the Glass bottom boat (Gbb) ligand modulates structural and functional synaptogenesis at the Drosophila melanogaster neuromuscular junction. However, the molecular mechanisms regulating postsynaptic Gbb release are poorly understood. In this study, we show that Drosophila Rich (dRich), a conserved Cdc42-selective guanosine triphosphatase–activating protein (GAP), inhibits the Cdc42–Wsp pathway to stimulate postsynaptic Gbb release. Loss of dRich causes synaptic undergrowth and strongly impairs neurotransmitter release. These presynaptic defects are rescued by targeted postsynaptic expression of wild-type dRich but not a GAP-deficient mutant. dRich inhibits the postsynaptic localization of the Cdc42 effector Wsp (Drosophila orthologue of mammalian Wiskott-Aldrich syndrome protein, WASp), and manifestation of synaptogenesis defects in drich mutants requires Wsp signaling. In addition, dRich regulates postsynaptic organization independently of Cdc42. Importantly, dRich increases Gbb release and elevates presynaptic phosphorylated Mad levels. We propose that dRich coordinates the Gbb-dependent modulation of synaptic growth and function with postsynaptic development.

The nonsense-mediated decay pathway maintains synapse architecture and synaptic vesicle cycle efficacy

A systematic Drosophila forward genetic screen for photoreceptor synaptic transmission mutants identified no-on-and-no-off transient C (nonC) based on loss of retinal synaptic responses to light stimulation. The cloned gene encodes phosphatidylinositol-3-kinase-like kinase (PIKK) Smg1, a regulatory kinase of the nonsense-mediated decay (NMD) pathway. The Smg proteins act in an mRNA quality control surveillance mechanism to selectively degrade transcripts containing premature stop codons, thereby preventing the translation of truncated proteins with dominant-negative or deleterious gain-of-function activities. At the neuromuscular junction (NMJ) synapse, an extended allelic series of Smg1 mutants show impaired structural architecture, with decreased terminal arbor size, branching and synaptic bouton number. Functionally, loss of Smg1 results in a ~50% reduction in basal neurotransmission strength, as well as progressive transmission fatigue and greatly impaired synaptic vesicle recycling during high-frequency stimulation. Mutation of other NMD pathways genes (Upf2 and Smg6) similarly impairs neurotransmission and synaptic vesicle cycling. These findings suggest that the NMD pathway acts to regulate proper mRNA translation to safeguard synapse morphology and maintain the efficacy of synaptic function.

Sialyltransferase regulates nervous system function in Drosophila

In vertebrates, sialylated glycans participate in a wide range of biological processes and affect the development and function of the nervous system. While the complexity of glycosylation and the functional redundancy among sialyltransferases provide obstacles for revealing biological roles of sialylation in mammals, Drosophila possesses a sole vertebrate-type sialyltransferase, Drosophila sialyltransferase (DSiaT), with significant homology to its mammalian counterparts, suggesting that Drosophila could be a suitable model to investigate the function of sialylation. To explore this possibility and investigate the role of sialylation in Drosophila, we inactivated DSiaT in vivo by gene targeting and analyzed phenotypes of DSiaT mutants using a combination of behavioral, immunolabeling, electrophysiological, and pharmacological approaches. Our experiments demonstrated that DSiaT expression is restricted to a subset of CNS neurons throughout development. We found that DSiaT mutations result in significantly decreased life span, locomotor abnormalities, temperature-sensitive paralysis, and defects of neuromuscular junctions. Our results indicate that DSiaT regulates neuronal excitability and affects the function of a voltage-gated sodium channel. Finally, we showed that sialyltransferase activity is required for DSiaT function in vivo, which suggests that DSiaT mutant phenotypes result from a defect in sialylation of N-glycans. This work provided the first evidence that sialylation has an important biological function in protostomes, while also revealing a novel, nervous system-specific function of alpha2,6-sialylation. Thus, our data shed light on one of the most ancient functions of sialic acids in metazoan organisms and suggest a possibility that this function is evolutionarily conserved between flies and mammals.

The role of cyclophilin D in learning and memory

Cyclophilin D (Cyp D) is implicated in cell death pathway and blockade of Cyp D could be a potent therapeutic strategy for degenerative disorders such as Alzheimer's disease, ischemia, and multiple sclerosis, but physiological role of Cyp D remains elusive. Here, we investigated the ability of learning and memory in several behavioral tasks in mice that lacked Cyp D (Cyp D(-/-)) and the relationship between ability of learning and memory and hippocampal architecture or neuronal transmission in Cyp D(-/-) mice. Cyp D(-/-) mice showed impairments of short-term memory in the Y-maze, object recognition memory in the novel-object recognition test, reference memory in the water maze test, and associative learning in the conditioned fear learning test. Hippocampal infusion of Cyclosporine A, which binds to Cyp D, replicated the defect in hippocampus-dependent cognition observed in Cyp D(-/-) mice. The Cyp D(-/-) mice did not show histopathological abnormalities upon Nissl staining and GFAP immunostaining or irregular expression of neuronal and glial marker proteins on Western blotting. However, release of glutamate and acetylcholine was decreased from the hippocampus in response to high-potassium treatment in the Cyp D(-/-) mice than in the wild-type mice. These results suggest a physiological role for Cyp D in learning and memory via the regulation of neurotransmission.

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

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

Loose coupling between calcium channels and sites of exocytosis in chromaffin cells

Calcium microdomains generated by tight clusters of calcium channels regulate fusion of small vesicles at the synaptic terminal and have also been suggested to trigger exocytosis of large dense-core vesicles from neuroendocrine cells. To test this idea, we have compared sites of exocytosis and the spatial distribution of calcium channels in chromaffin cells. Fusion of individual vesicles was visualized using interference reflection microscopy and the submembranous calcium signal was assessed using total internal reflection fluorescence microscopy. Depolarization triggered a burst of exocytosis from up to seven sites in a membrane area of 11 microm(2), but these sites did not colocalize with calcium microdomains. Instead, calcium influx occurred in large patches (averaging 34 microm(2)) containing a mixture of P/Q- and N-type channels. About 20% of fusion events occurred outside calcium channel patches. Further, the delay between the onset of stimulation and a burst of exocytosis was prolonged for several seconds by increasing the concentration of the slow calcium chelator EGTA from 1.5 to 5 mM. These results demonstrate that while calcium channels and release sites tend to congregate in specialized regions of the surface membrane, these have dimensions of several micrometres. The dominant calcium signal regulating release in chromaffin cells is generated by the cooperative action of many channels operating over distances of many micrometres rather than discrete clusters of calcium channels generating localized microdomains.

Presynaptic CaV2 calcium channel traffic requires CALF-1 and the alpha(2)delta subunit UNC-36

Presynaptic voltage-gated calcium channels provide calcium for synaptic vesicle exocytosis. We show here that a green fluorescent protein-tagged alpha(1) subunit of the Caenorhabditis elegans CaV2 channel, UNC-2, is localized to presynaptic active zones of sensory and motor neurons. Synaptic localization of CaV2 requires the alpha(2)delta subunit UNC-36 and CALF-1 (Calcium Channel Localization Factor-1), a neuronal transmembrane protein that localizes to the endoplasmic reticulum. In calf-1 mutants, UNC-2 is retained in the endoplasmic reticulum, but other active-zone components and synaptic vesicles are delivered to synapses. Acute induction of calf-1 mobilizes preexisting UNC-2 for delivery to synapses, consistent with a direct trafficking role. The alpha(2)delta subunit UNC-36 is likewise required for exit of UNC-2 from endoplasmic reticulum but has additional functions. Genetic and cell biological interactions suggest that CALF-1 couples intracellular traffic to functional maturation of CaV2 presynaptic calcium channels.

Maturation of active zone assembly by Drosophila Bruchpilot

Synaptic vesicles fuse at active zone (AZ) membranes where Ca²⁺ channels are clustered and that are typically decorated by electron-dense projections. Recently, mutants of the Drosophilamelanogaster ERC/CAST family protein Bruchpilot (BRP) were shown to lack dense projections (T-bars) and to suffer from Ca²⁺ channel–clustering defects. In this study, we used high resolution light microscopy, electron microscopy, and intravital imaging to analyze the function of BRP in AZ assembly. Consistent with truncated BRP variants forming shortened T-bars, we identify BRP as a direct T-bar component at the AZ center with its N terminus closer to the AZ membrane than its C terminus. In contrast, Drosophila Liprin-α, another AZ-organizing protein, precedes BRP during the assembly of newly forming AZs by several hours and surrounds the AZ center in few discrete punctae. BRP seems responsible for effectively clustering Ca²⁺ channels beneath the T-bar density late in a protracted AZ formation process, potentially through a direct molecular interaction with intracellular Ca²⁺ channel domains.

Metabotropic glutamate receptor-mediated use-dependent down-regulation of synaptic excitability involves the fragile X mental retardation protein

Loss of the mRNA-binding protein FMRP results in the most common inherited form of both mental retardation and autism spectrum disorders: fragile X syndrome (FXS). The leading FXS hypothesis proposes that metabotropic glutamate receptor (mGluR) signaling at the synapse controls FMRP function in the regulation of local protein translation to modulate synaptic transmission strength. In this study, we use the Drosophila FXS disease model to test the relationship between Drosophila FMRP (dFMRP) and the sole Drosophila mGluR (dmGluRA) in regulation of synaptic function, using two-electrode voltage-clamp recording at the glutamatergic neuromuscular junction (NMJ). Null dmGluRA mutants show minimal changes in basal synapse properties but pronounced defects during sustained high-frequency stimulation (HFS). The double null dfmr1;dmGluRA mutant shows repression of enhanced augmentation and delayed onset of premature long-term facilitation (LTF) and strongly reduces grossly elevated post-tetanic potentiation (PTP) phenotypes present in dmGluRA-null animals. Null dfmr1 mutants show features of synaptic hyperexcitability, including multiple transmission events in response to a single stimulus and cyclic modulation of transmission amplitude during prolonged HFS. The double null dfmr1;dmGluRA mutant shows amelioration of these defects but does not fully restore wildtype properties in dfmr1-null animals. These data suggest that dmGluRA functions in a negative feedback loop in which excess glutamate released during high-frequency transmission binds the glutamate receptor to dampen synaptic excitability, and dFMRP functions to suppress the translation of proteins regulating this synaptic excitability. Removal of the translational regulator partially compensates for loss of the receptor and, similarly, loss of the receptor weakly compensates for loss of the translational regulator.

Cav2-type calcium channels encoded by cac regulate AP-independent neurotransmitter release at cholinergic synapses in adult Drosophila brain

Voltage-gated calcium channels containing alpha1 subunits encoded by Ca(v)2 family genes are critical in regulating release of neurotransmitter at chemical synapses. In Drosophila, cac is the only Ca(v)2-type gene. Cacophony (CAC) channels are localized in motor neuron terminals where they have been shown to mediate evoked, but not AP-independent, release of glutamate at the larval neuromuscular junction (NMJ). Cultured embryonic neurons also express CAC channels, but there is no information about the properties of CAC-mediated currents in adult brain nor how these channels regulate transmission in central neural circuits where fast excitatory synaptic transmission is predominantly cholinergic. Here we report that wild-type neurons cultured from late stage pupal brains and antennal lobe projection neurons (PNs) examined in adult brains, express calcium currents with two components: a slow-inactivating current sensitive to the spider toxin Plectreurys toxin II (PLTXII) and a fast-inactivating PLTXII-resistant component. CAC channels are the major contributors to the slow-inactivating PLTXII-sensitive current based on selective reduction of this component in hypomorphic cac mutants (NT27 and TS3). Another characteristic of cac mutant neurons both in culture and in whole brain recordings is a reduced cholinergic miniature excitatory postsynaptic current frequency that is mimicked in wild-type neurons by acute application of PLTXII. These data demonstrate that cac encoded Ca(v)2-type calcium channels regulate action potential (AP)-independent release of neurotransmitter at excitatory cholinergic synapses in the adult brain, a function not predicted from studies at the larval NMJ.