latheo, a Drosophila gene involved in learning, regulates functional synaptic plasticity

Regulation of excitation-contraction coupling at the Drosophila neuromuscular junction

The Drosophila neuromuscular system is widely used to characterize synaptic development and function. However, little is known about how specific synaptic alterations effect neuromuscular transduction and muscle contractility, which ultimately dictate behavioural output. Here we develop and use a force transducer system to characterize excitation-contraction coupling at Drosophila larval neuromuscular junctions (NMJs), examining how specific neuronal and muscle manipulations disrupt muscle contractility. Muscle contraction force increased with motoneuron stimulation frequency and duration, showing considerable plasticity between 5 and 40 Hz and saturating above 50 Hz. Endogenous recordings of fictive contractions revealed average motoneuron burst frequencies of 20-30 Hz, consistent with the system operating within this plastic range of contractility. Temperature was also a key factor in muscle contractility, as force was enhanced at lower temperatures and dramatically reduced with increasing temperatures. Pharmacological and genetic manipulations of critical components of Ca2+ regulation in both pre- and postsynaptic compartments affected the strength and time course of muscle contractions. A screen for modulators of muscle contractility led to identification and characterization of the molecular and cellular pathway by which the FMRFa peptide, TPAEDFMRFa, increases muscle performance. These findings indicate Drosophila NMJs provide a robust system to correlate synaptic dysfunction, regulation and modulation to alterations in excitation-contraction coupling. KEY POINTS: Larval muscle contraction force increases with stimulation frequency and duration, revealing substantial plasticity between 5 and 40 Hz. Fictive contraction recordings demonstrate endogenous motoneuron burst frequencies consistent with the neuromuscular system operating within the range of greatest plasticity. Genetic and pharmacological manipulations of critical components of pre- and postsynaptic Ca2+ regulation significantly affect the strength and time course of muscle contractions. A screen for modulators of the excitation-contraction machinery identified a FMRFa peptide, TPAEDFMRFa and its associated signalling pathway, that dramatically increases muscle performance. Drosophila serves as an excellent model for dissecting components of the excitation-contraction coupling machinery.

Secreted C-type lectin regulation of neuromuscular junction synaptic vesicle dynamics modulates coordinated movement

The synaptic cleft manifests enriched glycosylation, with structured glycans coordinating signaling between presynaptic and postsynaptic cells. Glycosylated signaling ligands orchestrating communication are tightly regulated by secreted glycan-binding lectins. Using the Drosophila neuromuscular junction (NMJ) as a model glutamatergic synapse, we identify a new Ca2+-binding (C-type) lectin, Lectin-galC1 (LGC1), which modulates presynaptic function and neurotransmission strength. We find that LGC1 is enriched in motoneuron presynaptic boutons and secreted into the NMJ extracellular synaptomatrix. We show that LGC1 limits locomotor peristalsis and coordinated movement speed, with a specific requirement for synaptic function, but not NMJ architecture. LGC1 controls neurotransmission strength by limiting presynaptic active zone (AZ) and postsynaptic glutamate receptor (GluR) aligned synapse number, reducing both spontaneous and stimulation-evoked synaptic vesicle (SV) release, and capping SV cycling rate. During high-frequency stimulation (HFS), mutants have faster synaptic depression and impaired recovery while replenishing depleted SV pools. Although LGC1 removal increases the number of glutamatergic synapses, we find that LGC1-null mutants exhibit decreased SV density within presynaptic boutons, particularly SV pools at presynaptic active zones. Thus, LGC1 regulates NMJ neurotransmission to modulate coordinated movement.

Dystrobrevin is required postsynaptically for homeostatic potentiation at the Drosophila NMJ

Evolutionarily conserved homeostatic systems have been shown to modulate synaptic efficiency at the neuromuscular junctions of organisms. While advances have been made in identifying molecules that function presynaptically during homeostasis, limited information is currently available on how postsynaptic alterations affect presynaptic function. We previously identified a role for postsynaptic Dystrophin in the maintenance of evoked neurotransmitter release. We herein demonstrated that Dystrobrevin, a member of the Dystrophin Glycoprotein Complex, was delocalized from the postsynaptic region in the absence of Dystrophin. A newly-generated Dystrobrevin mutant showed elevated evoked neurotransmitter release, increased bouton numbers, and a readily releasable pool of synaptic vesicles without changes in the function or numbers of postsynaptic glutamate receptors. In addition, we provide evidence to show that the highly conserved Cdc42 Rho GTPase plays a key role in the postsynaptic Dystrophin/Dystrobrevin pathway for synaptic homeostasis. The present results give novel insights into the synaptic deficits underlying Duchenne Muscular Dystrophy affected by a dysfunctional Dystrophin Glycoprotein complex.

Nonreplicative functions of the origin recognition complex

Origin recognition complex (ORC), a heteromeric six-subunit complex, is the central component of the eukaryotic pre-replication complex. Recent data from yeast, frogs, flies and mammals present compelling evidence that ORC and its individual subunits have nonreplicative functions as well. The majority of these functions, such as heterochromatin formation, chromosome condensation, and segregation are dependent on ORC-DNA interactions. Furthermore, ORC is involved in the control of cell division via its participation in centrosome duplication and cytokinesis. Recent findings have also demonstrated a direct interaction between ORC and mRNPs and highlighted an essential role of ORC in mRNA nuclear export. Along with the growth of evolutionary complexity of organisms, ORC complex functions become more elaborate and new functions of the ORC sub-complexes and individual subunits have emerged.

Zebrafish In Situ Spinal Cord Preparation for Electrophysiological Recordings from Spinal Sensory and Motor Neurons

Zebrafish, first introduced as a developmental model, have gained popularity in many other fields. The ease of rearing large numbers of rapidly developing organisms, combined with the embryonic optical clarity, served as initial compelling attributes of this model. Over the past two decades, the success of this model has been further propelled by its amenability to large-scale mutagenesis screens and by the ease of transgenesis. More recently, gene-editing approaches have extended the power of the model. For neurodevelopmental studies, the zebrafish embryo and larva provide a model to which multiple methods can be applied. Here, we focus on methods that allow the study of an essential property of neurons, electrical excitability. Our preparation for the electrophysiological study of zebrafish spinal neurons involves the use of veterinarian suture glue to secure the preparation to a recording chamber. Alternative methods for recording from zebrafish embryos and larvae involve the attachment of the preparation to the chamber using a fine tungsten pin^1,2,3,4,5. A tungsten pin is most often used to mount the preparation in a lateral orientation, although it has been used to mount larvae dorsal-side up⁴. The suture glue has been used to mount embryos and larvae in both orientations. Using the glue, a minimal dissection can be performed, allowing access to spinal neurons without the use of an enzymatic treatment, thereby avoiding any resultant damage. However, for larvae, it is necessary to apply a brief enzyme treatment to remove the muscle tissue surrounding the spinal cord. The methods described here have been used to study the intrinsic electrical properties of motor neurons, interneurons, and sensory neurons at several developmental stages^6,7,8,9.

Drosophila Nesprin-1 controls glutamate receptor density at neuromuscular junctions

Nesprin-1 is a core component of a protein complex connecting nuclei to cytoskeleton termed LINC (linker of nucleoskeleton and cytoskeleton). Nesprin-1 is anchored to the nuclear envelope by its C-terminal KASH domain, the disruption of which has been associated with neuronal and neuromuscular pathologies, including autosomal recessive cerebellar ataxia and Emery-Dreifuss muscular dystrophy. Here, we describe a new and unexpected role of Drosophila Nesprin-1, Msp-300, in neuromuscular junction. We show that larvae carrying a deletion of Msp-300 KASH domain (Msp-300 (∆KASH) ) present a locomotion defect suggestive of a myasthenia, and demonstrate the importance of muscle Msp-300 for this phenotype, using tissue-specific RNAi knock-down. We show that Msp-300 (∆KASH) mutants display abnormal neurotransmission at the larval neuromuscular junction, as well as an imbalance in postsynaptic glutamate receptor composition with a decreased percentage of GluRIIA-containing receptors. We could rescue Msp-300 (∆KASH) locomotion phenotypes by GluRIIA overexpression, suggesting that the locomotion impairment associated with the KASH domain deletion is due to a reduction in junctional GluRIIA. In summary, we found that Msp-300 controls GluRIIA density at the neuromuscular junction. Our results suggest that Drosophila is a valuable model for further deciphering how Nesprin-1 and LINC disruption may lead to neuronal and neuromuscular pathologies.

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

Structural microtubule-associated proteins (MAPs), like MAP1, not only control the stability of microtubules, but also interact with postsynaptic proteins in the nervous system. Their presynaptic role has barely been studied. To tackle this question, we used the Drosophila model in which there is only one MAP1 homolog: Futsch, which is expressed at the larval neuromuscular junction, presynaptically only. We show that Futsch regulates neurotransmitter release and active zone density. Importantly, we provide evidence that this role of Futsch is not just the consequence of its microtubule-stabilizing function. Using high-resolution microscopy, we show that Futsch and microtubules are almost systematically present in close proximity to active zones, with Futsch being localized in-between microtubules and active zones. Using proximity ligation assays, we further demonstrate the proximity of Futsch, but not microtubules, to active zone components. Altogether our data are in favor of a model by which Futsch locally stabilizes active zones, by reinforcing their link with the underlying microtubule cytoskeleton.

Meier-Gorlin syndrome and Wolf-Hirschhorn syndrome: two developmental disorders highlighting the importance of efficient DNA replication for normal development and neurogenesis

Microcephaly represents one of the most obvious clinical manifestations of impaired neurogenesis. Defects in the DNA damage response, in DNA repair, and structural abnormalities in centrosomes, centrioles and the spindle microtubule network have all been demonstrated to cause microcephaly in humans. Work describing novel functional defects in cell lines from individuals with either Meier-Gorlin syndrome or Wolf-Hirschhorn syndrome highlight the significance of optimal DNA replication and S phase progression for normal human development, including neurogenesis. These findings illustrate how different primary defects in processes impacting upon DNA replication potentially influence similar phenotypic outcomes, including growth retardation and microcephaly. Herein, we will describe the nature of the S phase defects uncovered for each of these conditions and highlight some of the overlapping cellular features.

The origin recognition complex in human diseases

ORC (origin recognition complex) serves as the initiator for the assembly of the pre-RC (pre-replication complex) and the subsequent DNA replication. Together with many of its non-replication functions, ORC is a pivotal regulator of various cellular processes. Notably, a number of reports connect ORC to numerous human diseases, including MGS (Meier-Gorlin syndrome), EBV (Epstein-Barr virus)-infected diseases, American trypanosomiasis and African trypanosomiasis. However, much of the underlying molecular mechanism remains unclear. In those genetic diseases, mutations in ORC alter its function and lead to the dysregulated phenotypes; whereas in some pathogen-induced symptoms, host ORC and archaeal-like ORC are exploited by these organisms to maintain their own genomes. In this review, I provide detailed examples of ORC-related human diseases, and summarize the current findings on how ORC is involved and/or dysregulated. I further discuss how these discoveries can be generalized as model systems, which can then be applied to elucidating other related diseases and revealing potential targets for developing effective therapies.

Spartin regulates synaptic growth and neuronal survival by inhibiting BMP-mediated microtubule stabilization

Troyer syndrome is a hereditary spastic paraplegia caused by human spartin (SPG20) gene mutations. We have generated a Drosophila disease model showing that Spartin functions presynaptically with endocytic adaptor Eps15 to regulate synaptic growth and function. Spartin inhibits bone morphogenetic protein (BMP) signaling by promoting endocytic degradation of BMP receptor wishful thinking (Wit). Drosophila fragile X mental retardation protein (dFMRP) and Futsch/MAP1B are downstream effectors of Spartin and BMP signaling in regulating microtubule stability and synaptic growth. Loss of Spartin or elevation of BMP signaling induces age-dependent progressive defects resembling hereditary spastic paraplegias, including motor dysfunction and brain neurodegeneration. Null spartin phenotypes are prevented by administration of the microtubule-destabilizing drug vinblastine. Together, these results demonstrate that Spartin regulates both synaptic development and neuronal survival by controlling microtubule stability via the BMP-dFMRP-Futsch pathway, suggesting that impaired regulation of microtubule stability is a core pathogenic component in Troyer syndrome.

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.

Jelly Belly trans-synaptic signaling to anaplastic lymphoma kinase regulates neurotransmission strength and synapse architecture

In Drosophila, the secreted signaling molecule Jelly Belly (Jeb) activates anaplastic lymphoma kinase (Alk), a receptor tyrosine kinase, in multiple developmental and adult contexts. We have shown previously that Jeb and Alk are highly enriched at Drosophila synapses within the CNS neuropil and neuromuscular junction (NMJ) and postulated a conserved intercellular signaling function. At the embryonic and larval NMJ, Jeb is localized in the motor neuron presynaptic terminal whereas Alk is concentrated in the muscle postsynaptic domain surrounding boutons, consistent with anterograde trans-synaptic signaling. Here, we show that neurotransmission is regulated by Jeb secretion by functional inhibition of Jeb-Alk signaling. Jeb is a novel negative regulator of neuromuscular transmission. Reduction or inhibition of Alk function results in enhanced synaptic transmission. Activation of Alk conversely inhibits synaptic transmission. Restoration of wild-type postsynaptic Alk expression in Alk partial loss-of-function mutants rescues NMJ transmission phenotypes and confirms that postsynaptic Alk regulates NMJ transmission. The effects of impaired Alk signaling on neurotransmission are observed in the absence of associated changes in NMJ structure. Complete removal of Jeb in motor neurons, however, disrupts both presynaptic bouton architecture and postsynaptic differentiation. Nonphysiologic activation of Alk signaling also negatively regulates NMJ growth. Activation of Jeb-Alk signaling triggers the Ras-MAP kinase cascade in both pre- and postsynaptic compartments. These novel roles for Jeb-Alk signaling in the modulation of synaptic function and structure have potential implications for recently reported Alk functions in human addiction, retention of spatial memory, cognitive dysfunction in neurofibromatosis, and pathogenesis of amyotrophic lateral sclerosis.

Molecular and genetic analysis of the Drosophila model of fragile X syndrome

The Drosophila genome contains most genes known to be involved in heritable disease. The extraordinary genetic malleability of Drosophila, coupled to sophisticated imaging, electrophysiology, and behavioral paradigms, has paved the way for insightful mechanistic studies on the causes of developmental and neurological disease as well as many possible interventions. Here, we focus on one of the most advanced examples of Drosophila genetic disease modeling, the Drosophila model of Fragile X Syndrome, which for the past decade has provided key advances into the molecular, cellular, and behavioral defects underlying this devastating disorder. We discuss the multitude of RNAs and proteins that interact with the disease-causing FMR1 gene product, whose function is conserved from Drosophila to human. In turn, we consider FMR1 mechanistic relationships in non-neuronal tissues (germ cells and embryos), peripheral motor and sensory circuits, and central brain circuits involved in circadian clock activity and learning/memory.

The RhoGAP crossveinless-c interacts with Dystrophin and is required for synaptic homeostasis at the Drosophila neuromuscular junction

Duchenne muscular dystrophy is caused by mutations in the Dystrophin gene and is characterized by muscle degeneration and the occurrence of mental deficits in a significant number of patients. Although Dystrophin and its closely related ortholog Utrophin are present at a variety of synapses, little is known about their roles in the nervous system. Previously, we reported that absence of postsynaptic Dystrophin from the Drosophila neuromuscular junction (NMJ) disrupts synaptic homeostasis, resulting in increased stimulus-evoked neurotransmitter release. Here, we show that RhoGAP crossveinless-c (cv-c), a negative regulator of Rho GTPase signaling pathways, genetically interacts with Dystrophin. Electrophysiological characterization of the cv-c-deficient NMJ and the use of presynaptic- and postsynaptic-specific transgenic rescue versus RNA interference reveal that the absence of postsynaptic cv-c results in elevated evoked neurotransmitter release. The cv-c mutant NMJ exhibits an increased number of presynaptic neurotransmitter release sites and higher probability of vesicle release without apparent changes in postsynaptic glutamate receptor numbers or function. Moreover, we find that decreasing expression of the Rho GTPase Cdc42 suppresses the high neurotransmitter release in the cv-c and Dystrophin mutants, suggesting that Cdc42 is a substrate of Cv-c. These results indicate that Dystrophin and the Rho GTPase signaling pathway likely interact at the postsynaptic side of the NMJ to maintain synaptic homeostasis. The absence of this postsynaptic pathway results in presynaptic structural and functional alterations, suggesting that retrograde signaling mechanisms are affected.

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.

DnaA and ORC: more than DNA replication initiators

Mutations in DNA replication initiator genes in both prokaryotes and eukaryotes lead to a pleiotropic array of phenotypes, including defects in chromosome segregation, cytokinesis, cell cycle regulation and gene expression. For years, it was not clear whether these diverse effects were indirect consequences of perturbed DNA replication, or whether they indicated that DNA replication initiator proteins had roles beyond their activity in initiating DNA synthesis. Recent work from a range of organisms has demonstrated that DNA replication initiator proteins play direct roles in many cellular processes, often functioning to coordinate the initiation of DNA replication with essential cell-cycle activities. The aim of this review is to highlight these new findings, focusing on the pathways and mechanisms utilized by DNA replication initiator proteins to carry out a diverse array of cellular functions.

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.

Electrophysiological recording from Drosophila larval body-wall muscles

The Drosophila larval neuromuscular junction (NMJ) shares many structural and functional similarities to synapses in other animals, including humans. These include the basic feature of synaptic transmission, as well as the molecular mechanisms regulating the synaptic vesicle cycle. Because of its large size, easy accessibility, and the well-characterized genetics, the fly NMJ remains an excellent model system for dissecting the cellular and molecular mechanisms of synaptic transmission. This protocol describes the steps for performing intracellular recording from fly larval body-wall muscles and explains how to record and analyze spontaneous and evoked synaptic potentials. Methods used include larval dissection ("filleting"), identification of muscle fibers and their innervating nerves, the use of a micromanipulator and microelectrode in penetrating the muscle membrane, and nerve stimulation to evoke synaptic potentials.

Synaptic degeneration in Alzheimer's disease

Synaptic loss is the major neurobiological substrate of cognitive dysfunction in Alzheimer's disease (AD). Synaptic failure is an early event in the pathogenesis that is clearly detectable already in patients with mild cognitive impairment (MCI), a prodromal state of AD. It progresses during the course of AD and in most early stages involves mechanisms of compensation before reaching a stage of decompensated function. This dynamic process from an initially reversible functionally responsive stage of down-regulation of synaptic function to stages irreversibly associated with degeneration might be related to a disturbance of structural brain self-organization and involves morpho-regulatory molecules such as the amyloid precursor protein. Further, recent evidence suggests a role for diffusible oligomers of amyloid beta in synaptic dysfunction. To form synaptic connections and to continuously re-shape them in a process of ongoing structural adaptation, neurons must permanently withdraw from the cell cycle. Previously, we formulated the hypothesis that differentiated neurons after having withdrawn from the cell cycle are able to use molecular mechanisms primarily developed to control proliferation alternatively to control synaptic plasticity. The existence of these alternative effector pathways within neurons might put them at risk of erroneously converting signals derived from plastic synaptic changes into the program of cell cycle activation, which subsequently leads to cell death. The molecular mechanisms involved in cell cycle activation might, thus, link aberrant synaptic changes to cell death.

Alternative functions of core cell cycle regulators in neuronal migration, neuronal maturation, and synaptic plasticity

Recent studies have demonstrated that boundaries separating a cycling cell from a postmitotic neuron are not as concrete as expected. Novel and unique physiological functions in neurons have been ascribed for proteins fundamentally required for cell cycle progression and control. These "core" cell cycle regulators serve diverse postmitotic functions that span various developmental stages of a neuron, including neuronal migration, axonal elongation, axon pruning, dendrite morphogenesis, and synaptic maturation and plasticity. In this review, we detail the nonproliferative postmitotic roles that these cell cycle proteins have recently been reported to play, the significance of their expression in neurons, mechanistic insight when available, and future prospects.

The origin recognition complex protein family

The proteins of the origin recognition complex are found throughout all eukaryotes and have roles beyond that of DNA replication.

Origin recognition complex (ORC) proteins were first discovered as a six-subunit assemblage in budding yeast that promotes the initiation of DNA replication. Orc1-5 appear to be present in all eukaryotes, and include both AAA+ and winged-helix motifs. A sixth protein, Orc6, shows no structural similarity to the other ORC proteins, and is poorly conserved between budding yeast and most other eukaryotic species. The replication factor Cdc6 has extensive sequence similarity with Orc1 and phylogenetic analysis suggests the genes that encode them may be paralogs. ORC proteins have also been found in the archaea, and the bacterial DnaA replication protein has ORC-like functional domains. In budding yeast, Orc1-6 are bound to origins of DNA replication throughout the cell cycle. Following association with Cdc6 in G1 phase, the sequential hydrolysis of Cdc6 - then ORC-bound ATP loads the Mcm2-7 helicase complex onto DNA. Localization of ORC subunits to the kinetochore and centrosome during mitosis and to the cleavage furrow during cytokinesis has been observed in metazoan cells and, along with phenotypes observed following knockdown with short interfering RNAs, point to additional roles at these cell-cycle stages. In addition, ORC proteins function in epigenetic gene silencing through interactions with heterochromatin factors such as Sir1 in budding yeast and HP1 in higher eukaryotes. Current avenues of research have identified roles for ORC proteins in the development of neuronal and muscle tissue, and are probing their relationship to genome integrity.

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.

Motor deficit in a Drosophila model of mucolipidosis type IV due to defective clearance of apoptotic cells

Disruption of the Transient Receptor Potential (TRP) mucolipin 1 (TRPML1) channel results in the neurodegenerative disorder mucolipidosis type IV (MLIV), a lysosomal storage disease with severe motor impairments. The mechanisms underlying MLIV are poorly understood and there is no treatment. Here, we report a Drosophila MLIV model, which recapitulates the key disease features, including abnormal intracellular accumulation of macromolecules, motor defects, and neurodegeneration. The basis for the buildup of macromolecules was defective autophagy, which resulted in oxidative stress and impaired synaptic transmission. Late-apoptotic cells accumulated in trpml mutant brains, suggesting diminished cell clearance. The accumulation of late-apoptotic cells and motor deficits were suppressed by expression of trpml(+) in neurons, glia, or hematopoietic cells. We conclude that the neurodegeneration and motor defects result primarily from decreased clearance of apoptotic cells. Since hematopoietic cells in humans are involved in clearance of apoptotic cells, our results raise the possibility that bone marrow transplantation may limit the progression of MLIV.

The genetics of symptom-based phenotypes: toward a molecular classification of schizophrenia

OBJECTIVE

Genetic linkage studies in schizophrenia (SZ) have primarily focused on the phenotype of disease susceptibility. A limited number of studies, however, have reported suggestive linkage to specific SZ symptom domains including regions on chromosomes 6, 8, and 20. We examined these chromosomal regions for association to positive, negative, and disorganized symptom clusters, using a dense set of single-nucleotide polymorphisms (SNPs).

METHODS

We ascertained 178 Caucasian patients with SZ for lifetime severity of clinical symptomatology using a structured diagnostic interview. The cohort was genotyped with the Affymetrix 500K microarray, from which we selected, a priori, 4833 intragenic SNPs located within chromosomal regions previously linked to specific SZ symptom clusters. Parametric tests, corrected for multiple testing, were used to compare the effects of allelic variation within these SNPs to the lifetime severity of the specific symptom domain that had been implicated by prior linkage studies.

RESULTS

We were able to extend previous reports of linkage between chromosome 6q and both positive and disorganized symptoms. Lifetime severity of positive symptoms was significantly (P = 2.50 x 10(-5)) associated with a SNP within the origin recognition complex subunit 3-like (ORC3L) gene, a gene implicated in synaptic plasticity. Level of disorganized symptoms was significantly (P < 6.00 x 10(-5)) associated 2 SNPs within the brain-specific angiogenesis inhibitor 3 (BAI3) gene, which is highly expressed in brain during development.

CONCLUSIONS

These data point toward specific candidate genes located within previously implicated linkage peaks for clinical symptomatology. Identification of functional variants within these regions and a characterization of the effect of these risk genotypes on the treatment of specific clinical symptoms are needed.

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

A systematic forward genetic Drosophila screen for electroretinogram mutants lacking synaptic transients identified the fuseless (fusl) gene, which encodes a predicted eight-pass transmembrane protein in the presynaptic membrane. Null fusl mutants display >75% reduction in evoked synaptic transmission but, conversely, an approximately threefold increase in the frequency and amplitude of spontaneous synaptic vesicle fusion events. These neurotransmission defects are rescued by a wild-type fusl transgene targeted only to the presynaptic cell, demonstrating a strictly presynaptic requirement for Fusl function. Defects in FM dye turnover at the synapse show a severely impaired exo-endo synaptic vesicle cycling pool. Consistently, ultrastructural analyses reveal accumulated vesicles arrested in clustered and docked pools at presynaptic active zones. In the absence of Fusl, calcium-dependent neurotransmitter release is dramatically compromised and there is little enhancement of synaptic efficacy with elevated external Ca(2+) concentrations. These defects are causally linked with severe loss of the Cacophony voltage-gated Ca(2+) channels, which fail to localize normally at presynaptic active zone domains in the absence of Fusl. These data indicate that Fusl regulates assembly of the presynaptic active zone Ca(2+) channel domains required for efficient coupling of the Ca(2+) influx and synaptic vesicle exocytosis during neurotransmission.

Stoned B mediates sorting of integral synaptic vesicle proteins

A continuous supply of fusion-competent synaptic vesicles is essential for sustainable neurotransmission. Drosophila mutations of the dicistronic stoned locus disrupt normal vesicle cycling and cause functional deficits in synaptic transmission. Although both Stoned A and B proteins putatively participate in reconstituting synaptic vesicles, their precise function is still unclear. Here we investigate the effects of progressive depletion of Stoned B protein (STNB) on the release properties of neuromuscular synapses using a novel set of synthetic stnB hypomorphic alleles. Decreasing neuronal STNB expression to < or =35% of wild-type level causes a strong reduction in excitatory junctional current amplitude at low stimulation frequencies and a marked slowing in synaptic depression during high-frequency stimulation, suggesting vesicle depletion is attenuated by decreased release probability. Recovery from synaptic depression after prolonged stimulation is also decelerated in mutants, indicating a delayed recovery of fusion-ready vesicles. These phenotypes appear not to be due to a diminished vesicle population, since the docked vesicle pool is ultrastructurally unaffected, and the total number of vesicles is only slightly reduced in these hypomorphs, unlike lethal stoned mutants. Therefore, we conclude that STNB not only functions as an essential component of the endocytic complex for vesicle reconstitution, as previously proposed, but also regulates the competence of recycled vesicles to undergo fusion. In support of such role of STNB, synaptic levels of the vesicular glutamate transporter (vGLUT) and synaptotagmin-1 are strongly reduced with diminishing STNB function, while other synaptic proteins are largely unaffected. We conclude that STNB organizes the endocytic sorting of a subset of integral synaptic vesicle proteins thereby regulating the fusion-competence of the recycled vesicle.

Dendrite development: a surprising origin

Neurons extend elaborate dendrites studded with spines. Unexpectedly, this cellular sculpting is regulated by the origin recognition complex—the core machinery for initiating DNA replication.

The many faces of the origin recognition complex

The hetero-hexameric origin recognition complex (ORC) is well known for its separable roles in DNA replication and heterochromatin assembly. However, ORC and its individual subunits have been implicated in diverse cellular activities in both the nucleus and the cytoplasm. Some of ORC's implied functions, such as cell cycle checkpoint control and mitotic chromosome assembly, may be indirectly related to its roles in replication control and/or heterochromatin assembly. Other suggested roles in ribosomal biogenesis and in centrosome and kinetochore function are based on localization/interaction data and are as yet inconclusive. However, recent findings directly link ORC to sister chromatin cohesion, cytokinesis and neural dendritic branching.

The ubiquitin-proteasome system postsynaptically regulates glutamatergic synaptic function

The ubiquitin-proteasome system (UPS) actively controls protein dynamics and local abundance via regulated protein degradation. This study investigates UPS' roles in the regulation of postsynaptic function and molecular composition in the Drosophila neuromuscular junction (NMJ) genetic system. To specifically impair UPS function postsynaptically, the UAS/GAL4 transgenic method was employed to drive postsynaptic expression of proteasome beta2 and beta6 subunit mutant proteins, which operate through a dominant negative mechanism to block proteasome function. When proteasome mutant subunits were constitutively expressed, excitatory junctional current (EJC) amplitudes were increased, demonstrating that postsynaptic proteasome function limits neurotransmission strength. Interestingly, the alteration in synaptic strength was calcium-dependent and miniature EJCs had significantly smaller mean amplitudes and more rapid mean decay rates. Postsynaptic levels of the Drosophila PSD-95/SAP97 homologue, discs large (DLG), and the GluRIIB-containing glutamate receptor were increased, but GluRIIA-containing receptors were unaltered. With acute postsynaptic proteasome inhibition using an inducible transgenic system, neurotransmission was similarly elevated with the same specific increase in postsynaptic GluRIIB abundance. These findings demonstrate postsynaptic proteasome regulation of glutamatergic synaptic function that is mediated through specific regulation of GluRIIB-containing glutamate receptors.

Multiple functions of the origin recognition complex

The origin recognition complex (ORC), a heteromeric six-subunit protein, is a central component for eukaryotic DNA replication. The ORC binds to DNA at replication origin sites in an ATP-dependent manner and serves as a scaffold for the assembly of other key initiation factors. Sequence rules for ORC-DNA binding appear to vary widely. In budding yeast the ORC recognizes specific ori elements, however, in higher eukaryotes origin site selection does not appear to depend on the specific DNA sequence. In metazoans, during cell cycle progression, one or more of the ORC subunits can be modified in such a way that ORC activity is inhibited until mitosis is complete and a nuclear membrane is assembled. In addition to its well-documented role in the initiation of DNA replication, the ORC is also involved in other cell functions. Some of these activities directly link cell cycle progression with DNA replication, while other functions seem distinct from replication. The function of ORCs in the establishment of transcriptionally repressed regions is described for many species and may be a conserved feature common for both unicellular eukaryotes and metazoans. ORC subunits were found at centrosomes, at the cell membranes, at the cytokinesis furrows of dividing cells, as well as at the kinetochore. The exact mechanism of these localizations remains to be determined, however, latest results support the idea that ORC proteins participate in multiple aspects of the chromosome inheritance cycle. In this review, we discuss the participation of ORC proteins in various cell functions, in addition to the canonical role of ORC in initiating DNA replication.

Linking cell-cycle dysfunction in Alzheimer's disease to a failure of synaptic plasticity

Higher cerebral functions are based upon a dynamic organization of neuronal networks. To form synaptic connections and to continuously re-shape them in a process of ongoing structural adaptation, neurons must permanently withdraw from the cell cycle. In other words, synaptic plasticity can only occur on the expense of the ability to proliferate. Previously, we have put forward a hypothesis, coined "Dr. Jekyll and Mr. Hyde concept" that differentiated neurons after having withdrawn from the cell cycle are able to use those molecular mechanisms primarily developed to control proliferation alternatively to control synaptic plasticity [T. Arendt, Synaptic plasticity and cell cycle activation in neurons are alternative effector pathways The Dr. Jekyll and Mr. Hyde Theory of Alzheimer's disease or The yin and yang of Neuroplasticity. Progr. Neurobiol. 71 (2003) 83-248]. The existence of these alternative effector pathways within a neuron might put it on the risk to erroneously convert signals derived from plastic synaptic changes into cell cycle activation which subsequently leads to cell death. Here we add further evidence to this hypothesis demonstrating a tight association of the origin recognition complex (ORC) with neurofibrillar pathology in AD. The ORC is a critical "guard" of DNA replication and point of convergence of numerous functionally redundant signaling pathways involved in cell cycle progression and transcriptional silencing of apoptotic programmes. ORC subunits in the mammalian brain and their homologes in Drosophila, however, have further been implicated in the regulation of structural neuronal plasticity and cognitive function. We propose that the abnormal subcellular distribution and segregation of ORC proteins in AD might compromise their physiological function in gene silencing and plasticity. This might result in cell cycle activation, DNA-replication and de-repression of apoptotic programmes. ORC subunits might, thus, provide a direct molecular link between synaptic plasticity, DNA replication and cell death.

Requirement of Akt to mediate long-term synaptic depression in Drosophila

Drosophila larval neuromuscular junction (NMJ) is a well established preparation enabling quantitative analyses of synaptic physiology at identifiable synapses. Here, we report the first characterization of synaptic long-term depression (LTD) at the Drosophila NMJ. LTD can be reliably induced by specific patterns of tetanic stimulation, and the level of LTD depends on both stimulus frequency and Ca2+ concentration. We provide evidence that LTD is likely a result of presynaptic changes. Through screening of targeted mutants with defects in memory or signal transduction pathways, we found that LTD is strongly reduced in the akt mutants. This defect can be rescued by acutely induced expression of the normal akt transgene, suggesting that altered LTD is not attributable to developmental abnormalities and that Akt is critical for the induction of LTD. Our study also indicates that the molecular mechanisms of LTD are distinct from that of short-term synaptic plasticity, because akt mutants showed normal short-term facilitation and posttetanic potentiation, whereas LTD was unaffected in mutants that exhibit defective short-term synaptic plasticity, such as dunce and rutabaga. The characterization of LTD allows genetic analysis of the molecular mechanisms of long-term synaptic plasticity in Drosophila and provides an additional assay for studying functions of genes pertaining to synaptic and behavioral plasticity.

Dystrophin is required for appropriate retrograde control of neurotransmitter release at the Drosophila neuromuscular junction

Mutations in the human dystrophin gene cause the Duchenne and Becker muscular dystrophies. The Dystrophin protein provides a structural link between the muscle cytoskeleton and extracellular matrix to maintain muscle integrity. Recently, Dystrophin has also been found to act as a scaffold for several signaling molecules, but the roles of dystrophin-mediated signaling pathways remain unknown. To further our understanding of this aspect of the function of dystrophin, we have generated Drosophila mutants that lack the large dystrophin isoforms and analyzed their role in synapse function at the neuromuscular junction. In expression and rescue studies, we show that lack of the large dystrophin isoforms in the postsynaptic muscle cell leads to elevated evoked neurotransmitter release from the presynaptic apparatus. Overall synapse size, the size of the readily releasable vesicle pool as assessed with hypertonic shock, and the number of presynaptic neurotransmitter release sites (active zones) are not changed in the mutants. Short-term synaptic facilitation of evoked transmitter release is decreased in the mutants, suggesting that the absence of dystrophin results in increased probability of release. Absence of the large dystrophin isoforms does not lead to changes in muscle cell morphology or alterations in the postsynaptic electrical response to spontaneously released neurotransmitter. Therefore, postsynaptic glutamate receptor function does not appear to be affected. Our results indicate that the postsynaptically localized scaffolding protein Dystrophin is required for appropriate control of neuromuscular synaptic homeostasis.

Thirty years of olfactory learning and memory research in Drosophila melanogaster

The last 30 years have witnessed tremendous progress in elucidating the basic mechanisms underlying a simple form of olfactory learning and memory in Drosophila. The application of the mutagenic approach to the study of olfactory learning and memory in Drosophila has yielded insights into the participation of a large number of genes in both the development of critical brain regions as well as in the physiology underlying the acquisition, storage, and retrieval of memory. Newer sophisticated molecular-genetic tools have further allowed for the specification and functional dissection of the neuronal circuitry involved in these processes at a systems level. With these advances in our understanding of the genes, neurons, and circuits involved in learning and memory, the field of Drosophila memory research is nearing a state of integration of the bottom up and top down approaches to understanding this form of behavioral plasticity.

A novel motif governs APC-dependent degradation of Drosophila ORC1 in vivo

Regulated degradation plays a key role in setting the level of many factors that govern cell cycle progression. In Drosophila, the largest subunit of the origin recognition complex protein 1 (ORC1) is degraded at the end of M phase and throughout much of G1 by anaphase-promoting complexes (APC) activated by Fzr/Cdh1. We show here that none of the previously identified APC motifs targets ORC1 for degradation. Instead, a novel sequence, the O-box, is necessary and sufficient to direct Fzr/Cdh1-dependent polyubiquitylation in vitro and degradation in vivo. The O-box is similar to but distinct from the well characterized D-box. Finally, we show that O-box motifs in two other proteins, Drosophila Abnormal Spindle and Schizosaccharomyces pombe Cut2, contribute to Cdh1-dependent polyubiquitylation in vitro, suggesting that the O-box may mediate degradation of a variety of cell cycle factors.

CDK phosphorylation inhibits the DNA-binding and ATP-hydrolysis activities of the Drosophila origin recognition complex

Faithful propagation of eukaryotic chromosomes usually requires that no DNA segment be replicated more than once during one cell cycle. Cyclin-dependent kinases (Cdks) are critical for the re-replication controls that inhibit the activities of components of the pre-replication complexes (pre-RCs) following origin activation. The origin recognition complex (ORC) initiates the assembly of pre-RCs at origins of replication and Cdk phosphorylation of ORC is important for the prevention of re-initiation. Here we show that Drosophila melanogaster ORC (DmORC) is phosphorylated in vivo and is a substrate for Cdks in vitro. Cdk phosphorylation of DmORC subunits DmOrc1p and DmOrc2p inhibits the intrinsic ATPase activity of DmORC without affecting ATP binding to DmOrc1p. Moreover, Cdk phosphorylation inhibits the ATP-dependent DNA-binding activity of DmORC in vitro, thus identifying a novel determinant for DmORC-DNA interaction. DmORC is a substrate for both Cdk2 x cyclin E and Cdk1 x cyclin B in vitro. Such phosphorylation of DmORC by Cdk2 x cyclin E, but not by Cdk1 x cyclin B, requires an "RXL" motif in DmOrc1p. We also identify casein kinase 2 (CK2) as a kinase activity in embryonic extracts targeting DmORC for modification. CK2 phosphorylation does not affect ATP hydrolysis by DmORC but modulates the ATP-dependent DNA-binding activity of DmORC. These results suggest molecular mechanisms by which Cdks may inhibit ORC function as part of re-replication control and show that DmORC activity may be modulated in response to phosphorylation by multiple kinases.

The genes encoding Arabidopsis ORC subunits are E2F targets and the two ORC1 genes are differently expressed in proliferating and endoreplicating cells

Initiation of eukaryotic DNA replication depends on the function of pre-replication complexes (pre-RC), one of its key component being the six subunits origin recognition complex (ORC). In spite of a significant degree of conservation among ORC proteins from different eukaryotic sources, the regulation of their availability varies considerably in different model systems and cell types. Here, we show that the six ORC genes of Arabidopsis thaliana are regulated at the transcriptional level during cell cycle and development. We found that Arabidopsis ORC genes, except AtORC5, contain binding sites for the E2F family of transcription factors. Expression of AtORC genes containing E2F binding sites peaks at the G1/S-phase. Analysis of AtORC gene expression in plants with reduced E2F activity, obtained by expressing a dominant negative version of DP, the E2F heterodimerization partner, and with increased E2F activity, obtained by inactivation of the retinoblastoma protein, led us to conclude that all AtORC genes, except AtORC5 are E2F targets. Interestingly, Arabidopsis contains two AtORC1 (a and b) genes, highly conserved at the amino acid level but with unrelated promoter sequences. AtORC1b expression is restricted to proliferating cells. However, AtORC1a is preferentially expressed in endoreplicating cells based on our analysis in endoreplicating tissues and in a mutant with altered endocycle pattern. This suggests a differential expression of the two ORC1 genes in Arabidopsis.

The origin recognition core complex regulates dendrite and spine development in postmitotic neurons

The origin recognition complex (ORC) ensures exactly one round of genome replication per cell cycle through acting as a molecular switch that precisely controls the assembly, firing, and inactivation of the replication initiation machinery. Recent data indicate that it may also coordinate the processes of mitosis and cytokinesis and ensure the proper distribution of replicated genome to daughter cells. We have found that the ORC core subunits are highly expressed in the nervous system. They are selectively localized to the neuronal somatodendritic compartment and enriched in the membrane fraction. siRNA knockdown of ORC subunits dramatically reduced dendritic branch formation and severely impeded dendritic spine emergence. Expression of ORC ATPase motif mutants enhanced the branching of dendritic arbors. The ORC core complex thus appears to have a novel role in regulating dendrite and dendritic spine development in postmitotic neurons.

Independent regulation of synaptic size and activity by the anaphase-promoting complex

Neuronal plasticity relies on tightly regulated control of protein levels at synapses. One mechanism to control protein abundance is the ubiquitin-proteasome degradation system. Recent studies have implicated ubiquitin-mediated protein degradation in synaptic development, function, and plasticity, but little is known about the regulatory mechanisms controlling ubiquitylation in neurons. In contrast, ubiquitylation has long been studied as a central regulator of the eukaryotic cell cycle. A critical mediator of cell-cycle transitions, the anaphase-promoting complex/cyclosome (APC/C), is an E3 ubiquitin ligase. Although the APC/C has been detected in several differentiated cell types, a functional role for the complex in postmitotic cells has been elusive. We describe a novel postmitotic role for the APC/C at Drosophila neuromuscular synapses: independent regulation of synaptic growth and synaptic transmission. In neurons, the APC/C controls synaptic size via a downstream effector Liprin-alpha; in muscles, the APC/C regulates synaptic transmission, controlling the concentration of a postsynaptic glutamate receptor.

The Drosophila metabotropic glutamate receptor DmGluRA regulates activity-dependent synaptic facilitation and fine synaptic morphology

In vertebrates, several groups of metabotropic glutamate receptors (mGluRs) are known to modulate synaptic properties. In contrast, the Drosophila genome encodes a single functional mGluR (DmGluRA), an ortholog of vertebrate group II mGluRs, greatly expediting the functional characterization of mGluR-mediated signaling in the nervous system. We show here that DmGluRA is expressed at the glutamatergic neuromuscular junction (NMJ), localized in periactive zones of presynaptic boutons but excluded from active sites. Null DmGluRA mutants are completely viable, and all of the basal NMJ synaptic transmission properties are normal. In contrast, DmGluRA mutants display approximately a threefold increase in synaptic facilitation during short stimulus trains. Prolonged stimulus trains result in very strongly increased ( approximately 10-fold) augmentation, including the appearance of asynchronous, bursting excitatory currents never observed in wild type. Both defects are rescued by expression of DmGluRA only in the neurons, indicating a specific presynaptic requirement. These phenotypes are reminiscent of hyperexcitable mutants, suggesting a role of DmGluRA signaling in the regulation of presynaptic excitability properties. The mutant phenotypes could not be replicated by acute application of mGluR antagonists, suggesting that DmGluRA regulates the development of presynaptic properties rather than directly controlling short-term modulation. DmGluRA mutants also display mild defects in NMJ architecture: a decreased number of synaptic boutons accompanied by an increase in mean bouton size. These morphological changes bidirectionally correlate with DmGluRA levels in the presynaptic terminal. These data reveal the following two roles for DmGluRA in presynaptic mechanisms: (1) modulation of presynaptic excitability properties important for the control of activity-dependent neurotransmitter release and (2) modulation of synaptic architecture.

The hereditary spastic paraplegia gene, spastin, regulates microtubule stability to modulate synaptic structure and function

BACKGROUND

Hereditary Spastic Paraplegia (HSP) is a devastating neurological disease causing spastic weakness of the lower extremities and eventual axonal degeneration. Over 20 genes have been linked to HSP in humans; however, mutations in one gene, spastin (SPG4), are the cause of >40% of all cases. Spastin is a member of the ATPases associated with diverse cellular activities (AAA) protein family, and contains a microtubule interacting and organelle transport (MIT) domain. Previous work in cell culture has proposed a role for Spastin in regulating microtubules.

RESULTS

Employing Drosophila transgenic methods for overexpression and RNA interference (RNAi), we have investigated the role of Spastin in vivo. We show that Drosophila Spastin (D-Spastin) is enriched in axons and synaptic connections. At neuromuscular junctions (NMJ), Dspastin RNAi causes morphological undergrowth and reduced synaptic area. Moreover, Dspastin overexpression reduces synaptic strength, whereas Dspastin RNAi elevates synaptic currents. By using antibodies against posttranslationally modified alpha-Tubulin, we find that Dspastin regulates microtubule stability. Functional synaptic defects caused by Dspastin RNAi and overexpression were pharmacologically alleviated by agents that destabilize and stabilize microtubules, respectively.

CONCLUSIONS

Loss of Dspastin in Drosophila causes an aberrantly stabilized microtubule cytoskeleton in neurons and defects in synaptic growth and neurotransmission. These in vivo data strongly support previous reports, providing a probable cause for the neuronal dysfunction in spastin-linked HSP disease. The role of Spastin in regulating neuronal microtubule stability suggests therapeutic targets for HSP treatment and may provide insight into neurological disorders linked to microtubule dysfunction.

Genetic interaction of an origin recognition complex subunit and the Polycomb group gene MEDEA during seed development

The eukaryotic origin recognition complex (ORC) is made up of six subunits and functions in nuclear DNA replication, chromatin structure, and gene silencing in both fungi and metazoans. We demonstrate that disruption of a plant ORC subunit homolog, AtORC2 of Arabidopsis (Arabidopsis thaliana), causes a zygotic lethal mutant phenotype (orc2). Seeds of orc2 abort early, typically producing embryos with up to eight cells. Nuclear division in the endosperm is arrested at an earlier developmental stage: only approximately four nuclei are detected in orc2 endosperm. The endosperm nuclei in orc2 are dramatically enlarged, a phenotype that is most similar to class B titan mutants, which include mutants in structural maintenance of chromosomes (SMC) cohesins. The highest levels of ORC2 gene expression were found in preglobular embryos, coinciding with the stage at which homozygous orc2 mutant seeds arrest. The homologs of the other five Arabidopsis ORC subunits are also expressed at this developmental stage. The orc2 mutant phenotype is partly suppressed by a mutation in the Polycomb group gene MEDEA. In double mutants between orc2 and medea (mea), orc2 homozygotes arrest later with a phenotype intermediate between those of mea and orc2 single mutants. Either alterations in chromatin structure or the release of cell cycle checkpoints by the mea mutation may allow more cell and nuclear divisions to occur in orc2 homozygous seeds.

Nervous Wreck, an SH3 Adaptor Protein that Interacts with Wsp, Regulates Synaptic Growth in Drosophila

We describe the isolation and characterization of nwk (nervous wreck), a temperature-sensitive paralytic mutant that causes excessive growth of larval neuromuscular junctions (NMJs), resulting in increased synaptic bouton number and branch formation. Ultrastructurally, mutant boutons have reduced size and fewer active zones, associated with a reduction in synaptic transmission. nwk encodes an FCH and SH3 domain-containing adaptor protein that localizes to the periactive zone of presynaptic terminals and binds to the Drosophila ortholog of Wasp (Wsp), a key regulator of actin polymerization. wsp null mutants display synaptic overgrowth similar to nwk and enhance the nwk morphological phenotype in a dose-dependent manner. Evolutionarily, Nwk belongs to a previously undescribed family of adaptor proteins that includes the human srGAPs, which regulate Rho activity downstream of Robo receptors. We propose that Nwk controls synapse morphology by regulating actin dynamics downstream of growth signals in presynaptic terminals.

Degradation of origin recognition complex large subunit by the anaphase-promoting complex in Drosophila

The initiation of DNA synthesis is thought to occur at sites bound by a heteromeric origin recognition complex (ORC). Previously, we have shown that in Drosophila, the level of the large subunit, ORC1, is modulated during cell cycle progression and that changes in ORC1 concentration alter origin utilization during development. Here, we investigate the mechanisms underlying cell cycle-dependent degradation of ORC1. We show that signals in the non-conserved N-terminal domain of ORC1 mediate its degradation upon exit from mitosis and in G1 phase by the anaphase-promoting complex (APC) in vivo. Degradation appears to be the result of direct action of the APC, as the N-terminal domain is ubiquitylated by purified APC in vitro. This regulated proteolysis is potent, sufficient to generate a normal temporal distribution of protein even when transcription of ORC1 is driven by strong constitutive promoters. These observations suggest that in Drosophila, ORC1 regulates origin utilization much as does Cdc6 in budding yeast.

Cellular bases of activity-dependent paralysis inDrosophila stress-sensitive mutants

Stress-sensitive mutants in Drosophila have been shown to exhibit activity-dependent defects in neurotransmission. Using the neuromuscular junction (NMJ), this study investigates synaptic function more specifically in two stress-sensitive mutants: stress-sensitive B (sesB), which encodes a mitochondrial ADP/ATP translocase (ANT); and Atpalpha(2206), a conditional mutant of the Na+/K+ ATPase alpha-subunit. Mechanical shock induces a period of brief paralysis in both homozygous and double heterozygous mutants, but further analysis revealed distinct activity-dependent neurotransmission lesions in each mutant. Basal neurotransmission appeared similar to wild-type controls in both mutants under low frequency stimulation. High frequency stimulation, however, caused pronounced synaptic fatigue as well as slow and incomplete synaptic recovery in sesB mutants while Atpalpha(2206) mutants displayed an increase (25-fold) in synaptic failures. Perhaps to compensate for these activity dependent defects, the neuromuscular synapse was found to be overgrown in both mutants. Passive electrotonic stimulation, which initiates synaptic transmission independent of action potentials, ameliorated synaptic failures and resulted in increased neurotransmission amplitude in Atpalpha(2206) mutants. In addition, spontaneous synaptic vesicle fusion rates were increased in Atpalpha(2206) mutants, suggesting that, in the absence of action potential requirements, these synaptic terminals are healthy, if not hyperactive. Dye labeling studies revealed aberrant synaptic vesicle cycling in sesB mutants indicating a reduction of functional synaptic vesicles. We therefore postulate that both stress-sensitive mutants harbor unique neurotransmission defects: Atpalpha(2206) mutants are unable to maintain ionic gradients required during repetitive action potential propagation, and sesB mutants cannot maintain synaptic vesicle cycling during periods of high demand.