The Ras1-mitogen-activated protein kinase signal transduction pathway regulates synaptic plasticity through fasciclin II-mediated cell adhesion

Episodes of Rapid Recovery of the Functional Activity of the ras85D Gene in the Evolutionary History of Phylogenetically Distant Drosophila Species

As assemblies of genomes of new species with varying degrees of relationship appear, it becomes obvious that structural rearrangements of the genome, such as inversions, translocations, and transposon movements, are an essential and often the main source of evolutionary variation. In this regard, the following questions arise. How conserved are the regulatory regions of genes? Do they have a common evolutionary origin? And how and at what rate is the functional activity of genes restored during structural changes in the promoter region? In this article, we analyze the evolutionary history of the formation of the regulatory region of the ras85D gene in different lineages of the genus Drosophila, as well as the participation of mobile elements in structural rearrangements and in the replacement of specific areas of the promoter region with those of independent evolutionary origin. In the process, we substantiate hypotheses about the selection of promoter elements from a number of frequently repeated motifs with different degrees of degeneracy in the ancestral sequence, as well as about the restoration of the minimum required set of regulatory sequences using a conversion mechanism or similar.

RasV12; scrib-/- Tumors: A Cooperative Oncogenesis Model Fueled by Tumor/Host Interactions

The phenomenon of how oncogenes and tumor-suppressor mutations can synergize to promote tumor fitness and cancer progression can be studied in relatively simple animal model systems such as Drosophila melanogaster. Almost two decades after the landmark discovery of cooperative oncogenesis between oncogenic Ras^V12 and the loss of the tumor suppressor scribble in flies, this and other tumor models have provided new concepts and findings in cancer biology that has remarkable parallels and relevance to human cancer. Here we review findings using the Ras^V12; scrib^-/- tumor model and how it has contributed to our understanding of how these initial simple genetic insults cooperate within the tumor cell to set in motion the malignant transformation program leading to tumor growth through cell growth, cell survival and proliferation, dismantling of cell-cell interactions, degradation of basement membrane and spreading to other organs. Recent findings have demonstrated that cooperativity goes beyond cell intrinsic mechanisms as the tumor interacts with the immediate cells of the microenvironment, the immune system and systemic organs to eventually facilitate malignant progression.

FOXO Regulates Neuromuscular Junction Homeostasis During Drosophila Aging

The transcription factor foxo is a known regulator of lifespan extension and tissue homeostasis. It has been linked to the maintenance of neuronal processes across many species and has been shown to promote youthful characteristics by regulating cytoskeletal flexibility and synaptic plasticity at the neuromuscular junction (NMJ). However, the role of foxo in aging neuromuscular junction function has yet to be determined. We profiled adult Drosophila foxo- null mutant abdominal ventral longitudinal muscles and found that young mutants exhibited morphological profiles similar to those of aged wild-type flies, such as larger bouton areas and shorter terminal branches. We also observed changes to the axonal cytoskeleton and an accumulation of late endosomes in foxo null mutants and motor neuron-specific foxo knockdown flies, similar to those of aged wild-types. Motor neuron-specific overexpression of foxo can delay age-dependent changes to NMJ morphology, suggesting foxo is responsible for maintaining NMJ integrity during aging. Through genetic screening, we identify several downstream factors mediated through foxo-regulated NMJ homeostasis, including genes involved in the MAPK pathway. Interestingly, the phosphorylation of p38 was increased in the motor neuron-specific foxo knockdown flies, suggesting foxo acts as a suppressor of p38/MAPK activation. Our work reveals that foxo is a key regulator for NMJ homeostasis, and it may maintain NMJ integrity by repressing MAPK signaling.

Glia-derived exosomal miR-274 targets Sprouty in trachea and synaptic boutons to modulate growth and responses to hypoxia

Secreted exosomal microRNAs (miRNAs) mediate interorgan/tissue communications by modulating target gene expression, thereby regulating developmental and physiological functions. However, the source, route, and function in target cells have not been formally established for specific miRNAs. Here, we show that glial miR-274 non-cell-autonomously modulates the growth of synaptic boutons and tracheal branches. Whereas the precursor form of miR-274 is expressed in glia, the mature form of miR-274 distributes broadly, including in synaptic boutons, muscle cells, and tracheal cells. Mature miR-274 is secreted from glia to the circulating hemolymph as an exosomal cargo, a process requiring ESCRT components in exosome biogenesis and Rab11 and Syx1A in exosome release. We further show that miR-274 can function in the neurons or tracheal cells to modulate the growth of synaptic boutons and tracheal branches, respectively. Also, miR-274 uptake into the target cells by AP-2-dependent mechanisms modulates target cell growth. In the target cells, miR-274 down-regulates Sprouty (Sty) through a targeting sequence at the sty 3' untranslated region, thereby enhancing MAPK signaling and promoting cell growth. miR-274 expressed in glia of an mir-274 null mutant is released as an exosomal cargo in the circulating hemolymph, and such glial-specific expression resets normal levels of Sty and MAPK signaling and modulates target cell growth. mir-274 mutant larvae are hypersensitive to hypoxia, which is suppressed by miR-274 expression in glia or by increasing tracheal branches. Thus, glia-derived miR-274 coordinates growth of synaptic boutons and tracheal branches to modulate larval hypoxia responses.

Maintenance of cell type-specific connectivity and circuit function requires Tao kinase

Sensory circuits are typically established during early development, yet how circuit specificity and function are maintained during organismal growth has not been elucidated. To gain insight we quantitatively investigated synaptic growth and connectivity in the Drosophila nociceptive network during larval development. We show that connectivity between primary nociceptors and their downstream neurons scales with animal size. We further identified the conserved Ste20-like kinase Tao as a negative regulator of synaptic growth required for maintenance of circuit specificity and connectivity. Loss of Tao kinase resulted in exuberant postsynaptic specializations and aberrant connectivity during larval growth. Using functional imaging and behavioral analysis we show that loss of Tao-induced ectopic synapses with inappropriate partner neurons are functional and alter behavioral responses in a connection-specific manner. Our data show that fine-tuning of synaptic growth by Tao kinase is required for maintaining specificity and behavioral output of the neuronal network during animal growth.

The PDZ-GEF Gef26 regulates synapse development and function via FasII and Rap1 at the Drosophila neuromuscular junction

Guanine nucleotide exchange factors (GEFs) are essential for small G proteins to activate their downstream signaling pathways, which are involved in morphogenesis, cell adhesion, and migration. Mutants of Gef26, a PDZ-GEF (PDZ domain-containing guanine nucleotide exchange factor) in Drosophila, exhibit strong defects in wings, eyes, and the reproductive and nervous systems. However, the precise roles of Gef26 in development remain unclear. In the present study, we analyzed the role of Gef26 in synaptic development and function. We identified significant decreases in bouton number and branch length at larval neuromuscular junctions (NMJs) in Gef26 mutants, and these defects were fully rescued by restoring Gef26 expression, indicating that Gef26 plays an important role in NMJ morphogenesis. In addition to the observed defects in NMJ morphology, electrophysiological analyses revealed functional defects at NMJs, and locomotor deficiency appeared in Gef26 mutant larvae. Furthermore, Gef26 regulated NMJ morphogenesis by regulating the level of synaptic Fasciclin II (FasII), a well-studied cell adhesion molecule that functions in NMJ development and remodeling. Finally, our data demonstrate that Gef26-specific small G protein Rap1 worked downstream of Gef26 to regulate the level of FasII at NMJs, possibly through a βPS integrin-mediated signaling pathway. Taken together, our findings define a novel role of Gef26 in regulating NMJ development and function.

Regulation of synaptic architecture and synaptic vesicle pools by Nervous wreck at Drosophila Type 1b glutamatergic synapses

Nervous wreck (Nwk), a protein that is present at Type 1 glutamatergic synapses that contains an SH3 domain and an FCH motif, is a Drosophila homolog of the human srGAP3/MEGAP protein, which is associated with mental retardation. Confocal microscopy revealed that circles in Nwk reticulum enclosed T-shaped active zones (T-AZs) and partially colocalized with synaptic vesicle (SV) markers and both exocytosis and endocytosis components. Results from an electron microscopic (EM) analysis showed that Nwk proteins localized at synaptic edges and in SV pools. Both the synaptic areas and the number of SVs in the readily releasable (RRPs) and reserve (RPs) SV pools in nwk² were significantly reduced. Synergistic, morphological phenotypes observed from eag¹;nwk² neuromuscular junctions suggested that Nwk may regulate synaptic plasticity differently from activity-dependent Hebbian plasticity. Although the synaptic areas in eag¹;nwk² boutons were not significantly different from those of nwk², the number of SVs in the RRPs was similar to those of Canton-S. In addition, three-dimensional, high-voltage EM tomographic analysis demonstrated that significantly fewer enlarged SVs were present in nwk² RRPs. Furthermore, Nwk formed protein complexes with Drosophila Synapsin and Synaptotagmin 1 (DSypt1). Taken together, these findings suggest that Nwk is able to maintain synaptic architecture and both SV size and distribution at T-AZs by interacting with Synapsin and DSypt1.

Tay bridge and extracellular-regulated kinase activity are required for motoneuron function in the Drosophila neural system

Extracellular regulated kinase (Erk) activity is required during neural development for the specification of cell fates in neuroblasts and neuronal lineages, and also regulates several aspects of the activity and survival of mature neurons. The activation of Erk is regulated at multiple levels by kinases and phosphatases that alter its phosphorylation state and by other proteins that regulate its subcellular localization. Here, we find that tay bridge (tay), a negative regulator of Erk in Drosophila imaginal discs, is required in the motoneurons to regulate the number and size of neuromuscular synapses in these cells. The expression of Tay is maximal in motoneurons with low levels of activated ERK, suggesting that Tay modulates the activity of Erk in these cells. We also found that loss of tay expression and increased Erk activity specifically in the motoneurons cause a reversible decrease in walking speed. Impaired motoneurons activity may be caused by alterations in the functionality and number of synaptic boutons developing at the neuromuscular junction in tay mutants.

MAPK3 at the Autism-Linked Human 16p11.2 Locus Influences Precise Synaptic Target Selection at Drosophila Larval Neuromuscular Junctions

Proper synaptic function in neural circuits requires precise pairings between correct pre- and post-synaptic partners. Errors in this process may underlie development of neuropsychiatric disorders, such as autism spectrum disorder (ASD). Development of ASD can be influenced by genetic factors, including copy number variations (CNVs). In this study, we focused on a CNV occurring at the 16p11.2 locus in the human genome and investigated potential defects in synaptic connectivity caused by reduced activities of genes located in this region at Drosophila larval neuromuscular junctions, a well-established model synapse with stereotypic synaptic structures. A mutation of rolled, a Drosophila homolog of human mitogen-activated protein kinase 3 (MAPK3) at the 16p11.2 locus, caused ectopic innervation of axonal branches and their abnormal defasciculation. The specificity of these phenotypes was confirmed by expression of wild-type rolled in the mutant background. Albeit to a lesser extent, we also observed ectopic innervation patterns in mutants defective in Cdk2, Gαq, and Gp93, all of which were expected to interact with Rolled MAPK3. A further genetic analysis in double heterozygous combinations revealed a synergistic interaction between rolled and Gp93. In addition, results from RT-qPCR analyses indicated consistently reduced rolled mRNA levels in Cdk2, Gαq, and Gp93 mutants. Taken together, these data suggest a central role of MAPK3 in regulating the precise targeting of presynaptic axons to proper postsynaptic targets, a critical step that may be altered significantly in ASD.

MICAL-like Regulates Fasciclin II Membrane Cycling and Synaptic Development

Fasciclin II (FasII), the Drosophila ortholog of neural cell adhesion molecule (NCAM), plays a critical role in synaptic stabilization and plasticity. Although this molecule undergoes constitutive cycling at the synaptic membrane, how its membrane trafficking is regulated to ensure proper synaptic development remains poorly understood. In a genetic screen, we recovered a mutation in Drosophila mical-like that displays an increase in bouton numbers and a decrease in FasII levels at the neuromuscular junction (NMJ). Similar phenotypes were induced by presynaptic, but not postsynaptic, knockdown of mical-like expression. FasII trafficking assays revealed that the recycling of internalized FasII molecules to the cell surface was significantly impaired in mical-like-knockdown cells. Importantly, this defect correlated with an enhancement of endosomal sorting of FasII to the lysosomal degradation pathway. Similarly, synaptic vesicle exocytosis was also impaired in mical-like mutants. Together, our results identify Mical-like as a novel regulator of synaptic growth and FasII endocytic recycling.

C-terminal Src Kinase Gates Homeostatic Synaptic Plasticity and Regulates Fasciclin II Expression at the Drosophila Neuromuscular Junction

Forms of homeostatic plasticity stabilize neuronal outputs and promote physiologically favorable synapse function. A well-studied homeostatic system operates at the Drosophila melanogaster larval neuromuscular junction (NMJ). At the NMJ, impairment of postsynaptic glutamate receptor activity is offset by a compensatory increase in presynaptic neurotransmitter release. We aim to elucidate how this process operates on a molecular level and is preserved throughout development. In this study, we identified a tyrosine kinase-driven signaling system that sustains homeostatic control of NMJ function. We identified C-terminal Src Kinase (Csk) as a potential regulator of synaptic homeostasis through an RNAi- and electrophysiology-based genetic screen. We found that Csk loss-of-function mutations impaired the sustained expression of homeostatic plasticity at the NMJ, without drastically altering synapse growth or baseline neurotransmission. Muscle-specific overexpression of Src Family Kinase (SFK) substrates that are negatively regulated by Csk also impaired NMJ homeostasis. Surprisingly, we found that transgenic Csk-YFP can support homeostatic plasticity at the NMJ when expressed either in the muscle or in the nerve. However, only muscle-expressed Csk-YFP was able to localize to NMJ structures. By immunostaining, we found that Csk mutant NMJs had dysregulated expression of the Neural Cell Adhesion Molecule homolog Fasciclin II (FasII). By immunoblotting, we found that levels of a specific isoform of FasII were decreased in homeostatically challenged GluRIIA mutant animals–but markedly increased in Csk mutant animals. Additionally, we found that postsynaptic overexpression of FasII from its endogenous locus was sufficient to impair synaptic homeostasis, and genetically reducing FasII levels in Csk mutants fully restored synaptic homeostasis. Based on these data, we propose that Csk and its SFK substrates impinge upon homeostatic control of NMJ function by regulating downstream expression or localization of FasII.

Homeostasis is a fundamental topic in biology. Individual cells and systems of cells constantly monitor their environments and adjust their outputs in order to maintain physiological properties within ranges that can support life. The nervous system is no exception. Synapses and circuits are endowed with a capacity to respond to environmental challenges in a homeostatic fashion. As a result, synaptic output stays within an appropriate physiological range. We know that homeostasis is a fundamental form of regulation in animal nervous systems, but we have very little information about how it works. In this study, we examine the fruit fly Drosophila melanogaster and its ability to maintain normal levels of synaptic output over long periods of developmental time. We identify new roles in this process for classical signaling molecules called C-terminal Src kinase, Src family kinases, as well as a neuronal cell adhesion molecule called Fasciclin II, which was previously shown to stabilize synaptic contacts between neurons and muscles. Our work contributes to a broader understanding of how neurons work to maintain stable outputs. Ultimately, this type of knowledge could have important implications for neurological disorders in which stability is lost, such as forms of epilepsy or ataxia.

Developmental Axon Pruning Requires Destabilization of Cell Adhesion by JNK Signaling

Developmental axon pruning is essential for normal brain wiring in vertebrates and invertebrates. How axon pruning occurs in vivo is not well understood. In a mosaic loss-of-function screen, we found that Bsk, the Drosophila JNK, is required for axon pruning of mushroom body γ neurons, but not their dendrites. By combining in vivo genetics, biochemistry, and high-resolution microscopy, we demonstrate that the mechanism by which Bsk is required for pruning is through reducing the membrane levels of the adhesion molecule Fasciclin II (FasII), the NCAM ortholog. Conversely, overexpression of FasII is sufficient to inhibit axon pruning. Finally, we show that overexpressing other cell adhesion molecules, together with weak attenuation of JNK signaling, strongly inhibits pruning. Taken together, we have uncovered a novel and unexpected interaction between the JNK pathway and cell adhesion and found that destabilization of cell adhesion is necessary for efficient pruning.

Loss of the Coffin-Lowry syndrome-associated gene RSK2 alters ERK activity, synaptic function and axonal transport in Drosophila motoneurons

Plastic changes in synaptic properties are considered as fundamental for adaptive behaviors. Extracellular-signal-regulated kinase (ERK)-mediated signaling has been implicated in regulation of synaptic plasticity. Ribosomal S6 kinase 2 (RSK2) acts as a regulator and downstream effector of ERK. In the brain, RSK2 is predominantly expressed in regions required for learning and memory. Loss-of-function mutations in human RSK2 cause Coffin-Lowry syndrome, which is characterized by severe mental retardation and low IQ scores in affected males. Knockout of RSK2 in mice or the RSK ortholog in Drosophila results in a variety of learning and memory defects. However, overall brain structure in these animals is not affected, leaving open the question of the pathophysiological consequences. Using the fly neuromuscular system as a model for excitatory glutamatergic synapses, we show that removal of RSK function causes distinct defects in motoneurons and at the neuromuscular junction. Based on histochemical and electrophysiological analyses, we conclude that RSK is required for normal synaptic morphology and function. Furthermore, loss of RSK function interferes with ERK signaling at different levels. Elevated ERK activity was evident in the somata of motoneurons, whereas decreased ERK activity was observed in axons and the presynapse. In addition, we uncovered a novel function of RSK in anterograde axonal transport. Our results emphasize the importance of fine-tuning ERK activity in neuronal processes underlying higher brain functions. In this context, RSK acts as a modulator of ERK signaling.

ERK1/2: Function, signaling and implication in pain and pain-related anxio-depressive disorders

Despite the increasing knowledge regarding pain modulation, the understanding of the mechanisms behind a complex and pathologic chronic pain condition is still insufficient. These knowledge gaps might result in ineffective therapeutic approaches to relieve painful sensations. As a result, severe untreated chronic pain frequently triggers the onset of new disorders such as depression and/or anxiety, and therefore, both the diagnosis and treatment of patients suffering from chronic pain become seriously compromised, prompting a self-perpetuating cycle of symptomatology. The extracellular signal-regulated kinases 1 and 2 (ERK1/2) are molecules strongly implicated in the somatic component of pain at the spinal cord level and have been emerging as mediators of the emotional-affective component as well. Although these molecules might represent good biomarkers, their use as pharmacological targets is still open to discussion as paradoxical information has been obtained. Here we review the current scientific literature regarding ERK1/2 signaling in the modulation of pain, depression and anxiety, including the emotional-affective spheres of the pain experience.

ERK phosphorylation regulates sleep and plasticity in Drosophila

Given the relationship between sleep and plasticity, we examined the role of Extracellular signal-regulated kinase (ERK) in regulating baseline sleep, and modulating the response to waking experience. Both sleep deprivation and social enrichment increase ERK phosphorylation in wild-type flies. The effects of both sleep deprivation and social enrichment on structural plasticity in the LNvs can be recapitulated by expressing an active version of ERK (UAS-ERK^SEM) pan-neuronally in the adult fly using GeneSwitch (Gsw) Gsw-elav-GAL4. Conversely, disrupting ERK reduces sleep and prevents both the behavioral and structural plasticity normally induced by social enrichment. Finally, using transgenic flies carrying a cAMP response Element (CRE)-luciferase reporter we show that activating ERK enhances CRE-Luc activity while disrupting ERK reduces it. These data suggest that ERK phosphorylation is an important mediator in transducing waking experience into sleep.

TRPC6 channel-mediated neurite outgrowth in PC12 cells and hippocampal neurons involves activation of RAS/MEK/ERK, PI3K, and CAMKIV signaling

The non-selective cationic transient receptor canonical 6 (TRPC6) channels are involved in synaptic plasticity changes ranging from dendritic growth, spine morphology changes and increase in excitatory synapses. We previously showed that the TRPC6 activator hyperforin, the active antidepressant component of St. John's wort, induces neuritic outgrowth and spine morphology changes in PC12 cells and hippocampal CA1 neurons. However, the signaling cascade that transmits the hyperforin-induced transient rise in intracellular calcium into neuritic outgrowth is not yet fully understood. Several signaling pathways are involved in calcium transient-mediated changes in synaptic plasticity, ranging from calmodulin-mediated Ras-induced signaling cascades comprising the mitogen-activated protein kinase, PI3K signal transduction pathways as well as Ca(2+) /calmodulin-dependent protein kinase II (CAMKII) and CAMKIV. We show that several mechanisms are involved in TRPC6-mediated synaptic plasticity changes in PC12 cells and primary hippocampal neurons. Influx of calcium via TRPC6 channels activates different pathways including Ras/mitogen-activated protein kinase/extracellular signal-regulated kinases, phosphatidylinositide 3-kinase/protein kinase B, and CAMKIV in both cell types, leading to cAMP-response element binding protein phosphorylation. These findings are interesting not only in terms of the downstream targets of TRPC6 channels but also because of their potential to facilitate further understanding of St. John's wort extract-mediated antidepressant activity. Alterations in synaptic plasticity are considered to play an important role in the pathogenesis of depression. Beside several other proteins, TRPC6 channels regulate synaptic plasticity. This study demonstrates that different pathways including Ras/MEK/ERK, PI3K/Akt, and CAMKIV are involved in the improvement of synaptic plasticity by the TRPC6 activator hyperforin, the antidepressant active constituent of St. John's wort extract.

Human synaptic plasticity gene expression profile and dendritic spine density changes in HIV-infected human CNS cells: role in HIV-associated neurocognitive disorders (HAND)

HIV-associated neurocognitive disorders (HAND) is characterized by development of cognitive, behavioral and motor abnormalities, and occur in approximately 50% of HIV infected individuals. Our current understanding of HAND emanates mainly from HIV-1 subtype B (clade B), which is prevalent in USA and Western countries. However very little information is available on neuropathogenesis of HIV-1 subtype C (clade C) that exists in Sub-Saharan Africa and Asia. Therefore, studies to identify specific neuropathogenic mechanisms associated with HAND are worth pursuing to dissect the mechanisms underlying this modulation and to prevent HAND particularly in clade B infection. In this study, we have investigated 84 key human synaptic plasticity genes differential expression profile in clade B and clade C infected primary human astrocytes by using RT(2) Profile PCR Array human Synaptic Plasticity kit. Among these, 31 and 21 synaptic genes were significantly (≥3 fold) down-regulated and 5 genes were significantly (≥3 fold) up-regulated in clade B and clade C infected cells, respectively compared to the uninfected control astrocytes. In flow-cytometry analysis, down-regulation of postsynaptic density and dendrite spine morphology regulatory proteins (ARC, NMDAR1 and GRM1) was confirmed in both clade B and C infected primary human astrocytes and SK-N-MC neuroblastoma cells. Further, spine density and dendrite morphology changes by confocal microscopic analysis indicates significantly decreased spine density, loss of spines and decreased dendrite diameter, total dendrite and spine area in clade B infected SK-N-MC neuroblastoma cells compared to uninfected and clade C infected cells. We have also observed that, in clade B infected astrocytes, induction of apoptosis was significantly higher than in the clade C infected astrocytes. In conclusion, this study suggests that down-regulation of synaptic plasticity genes, decreased dendritic spine density and induction of apoptosis in astrocytes may contribute to the severe neuropathogenesis in clade B infection.

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.

Extracellular signal-regulated kinase activation in the chronic constriction injury model of neuropathic pain in anaesthetized rats

BACKGROUND

The role of extracellular signal-regulated kinases (ERKs) in nociception has been explored in the last years. While in spinal cord their activation is frequently correlated with pain or acute noxious stimuli, supraspinally, this association is not so evident and remains unclear. This study aims to evaluate ERK1/2 activation in the spinal cord and brainstem nuclei upon neuropathy and/or an additional mechanical stimulus.

METHODS

Acute noxious mechanical stimulation was applied in the left hindpaw of anaesthetized SHAM-operated and chronic constriction injured (CCI, neuropathic pain model) rats. Other SHAM or CCI rats did not receive any stimulus. Immunohistochemistry against the phosphorylated isoforms of ERK1/2 (pERK1/2) was performed in lumbar spinal cord and brainstem sections to assess ERK1/2 activation.

RESULTS

In the spinal cord, stimulation promoted an increase in pERK1/2 expression in the superficial dorsal horn of SHAM rats. No significant effects were caused by CCI alone. At supraspinal level, changes in ERK1/2 activation induced by CCI were observed in A5, locus coeruleus (LC), raphe obscurus (ROb), raphe magnus, dorsal raphe (DRN), lateral reticular and paragigantocellularis nucleus. CCI increased pERK1/2 expression in all these nuclei, with exception of LC, where a significant decrease was verified. Mechanical noxious stimulation of CCI rats decreased pERK1/2 expression in ROb and DRN, but no further changes were detected in either SHAM- or CCI-stimulated animals.

CONCLUSION

ERK1/2 are differentially activated in the spinal cord and in selected brainstem nuclei implicated in nociception, in response to an acute noxious stimulus and/or to a neuropathic pain condition.

Activity-dependent retrograde laminin A signaling regulates synapse growth at Drosophila neuromuscular junctions

Retrograde signals induced by synaptic activities are derived from postsynaptic cells to potentiate presynaptic properties, such as cytoskeletal dynamics, gene expression, and synaptic growth. However, it is not known whether activity-dependent retrograde signals can also depotentiate synaptic properties. Here we report that laminin A (LanA) functions as a retrograde signal to suppress synapse growth at Drosophila neuromuscular junctions (NMJs). The presynaptic integrin pathway consists of the integrin subunit βν and focal adhesion kinase 56 (Fak56), both of which are required to suppress crawling activity-dependent NMJ growth. LanA protein is localized in the synaptic cleft and only muscle-derived LanA is functional in modulating NMJ growth. The LanA level at NMJs is inversely correlated with NMJ size and regulated by larval crawling activity, synapse excitability, postsynaptic response, and anterograde Wnt/Wingless signaling, all of which modulate NMJ growth through LanA and βν. Our data indicate that synaptic activities down-regulate levels of the retrograde signal LanA to promote NMJ growth.

Oxidative stress in synapse development and function

Oxidative stress, caused by increased levels of reactive oxidative species (ROS), is considered a major contributor to the aging process. How oxidative stress may bring about changes to structures and function in the aging brain is poorly understood. Oxidative stress activates a number of cellular responses, including activation of the Jun-N-terminal kinase (JNK) pathway and autophagy. In addition to their pathological role, ROS also act as signaling molecules. ROS such as nitric oxide have a well-known role in learning and memory. In addition, activation of JNK and its transcriptional effector AP-1 are well-known mediators of synaptic function and growth. Both are essential mediators of physiological correlates of learning and memory such as long-term potentiation. JNK and AP-1 are potently activated and regulated by oxidative stress and mediate protective cellular responses such as autophagy. Recent work at the Drosophila neuromuscular junction implicates autophagy as a regulator of synaptic growth via activation of the JNK signaling pathway. We here outline a framework predicating oxidative stress as a major regulator of synaptic function and growth by the activation of JNK/AP-1 and autophagy. Such responses, we suggest, may underpin some forms of synaptic growth responses and synaptic aging.

Notch signaling is required for activity-dependent synaptic plasticity at the Drosophila neuromuscular junction

The cell-surface-signaling protein Notch, is required for numerous developmental processes and typically specifies which of two adjacent cells will adopt a non-neuronal developmental fate. It has recently been implicated in long-term memory formation in mammals and Drosophila. Here, we investigated whether activity-dependent synaptic plasticity at the neuromuscular junctions (NMJs) of third instar Drosophila larvae depends on Notch signaling. The length and number of axonal branches and number of presynaptic sites (boutons) in NMJ vary with the level of synaptic activity, so we increased activity at the NMJ by two complementary methods: increasing the chronic growth temperature of third instar larvae from 18 to 28 degrees C and using the double-mutant ether-a-gogo,Shaker (eagSh), both of which increase NMJ size and bouton count. Animals homozygous for the functionally null, temperature-sensitive Notch alleles, N(ts1) and N(ts2), displayed no activity-dependent increase in NMJ complexity when reared at the restrictive temperature. Dominant-negative Notch transgenic expression also blocked activity-dependent plasticity. Ectopic expression of wild-type Notch and constitutively active truncated Notch transgenes also reduced activity-dependent plasticity, suggesting that there is a "happy medium" level of Notch activity in mediating NMJ outgrowth. Last, we show that endogenous Notch is primarily expressed in the presynaptic cell bodies where its expression level is positively correlated with motor neuron activity.

arouser reveals a role for synapse number in the regulation of ethanol sensitivity

A reduced sensitivity to the sedating effects of alcohol is a characteristic associated with alcohol use disorders (AUDs). A genetic screen for ethanol sedation mutants in Drosophila identified arouser (aru), which functions in developing neurons to reduce ethanol sensitivity. Genetic evidence suggests that aru regulates ethanol sensitivity through its activation by Egfr/Erk signaling and its inhibition by PI3K/Akt signaling. The aru mutant also has an increased number of synaptic terminals in the larva and adult fly. Both the increased ethanol sensitivity and synapse number of the aru mutant are restored upon adult social isolation, suggesting a causal relationship between synapse number and ethanol sensitivity. We thus show that a developmental abnormality affecting synapse number and ethanol sensitivity is not permanent and can be reversed by manipulating the environment of the adult fly.

NFAT regulates pre-synaptic development and activity-dependent plasticity in Drosophila

The calcium-regulated transcription factor NFAT is emerging as a key regulator of neuronal development and plasticity but precise cellular consequences of NFAT function remain poorly understood. Here, we report that the single Drosophila NFAT homolog is widely expressed in the nervous system including motor neurons and unexpectedly controls neural excitability. Likely due to this effect on excitability, NFAT regulates overall larval locomotion and both chronic and acute forms of activity-dependent plasticity at the larval glutamatergic neuro-muscular synapse. Specifically, NFAT-dependent synaptic phenotypes include changes in the number of pre-synaptic boutons, stable modifications in synaptic microtubule architecture and pre-synaptic transmitter release, while no evidence is found for synaptic retraction or alterations in the level of the synaptic cell adhesion molecule FasII. We propose that NFAT regulates pre-synaptic development and constrains long-term plasticity by dampening neuronal excitability.

Small G protein signaling in neuronal plasticity and memory formation: the specific role of ras family proteins

Small G proteins are an extensive family of proteins that bind and hydrolyze GTP. They are ubiquitous inside cells, regulating a wide range of cellular processes. Recently, many studies have examined the role of small G proteins, particularly the Ras family of G proteins, in memory formation. Once thought to be primarily involved in the transduction of a variety of extracellular signals during development, it is now clear that Ras family proteins also play critical roles in molecular processing underlying neuronal and behavioral plasticity. We here review a number of recent studies that explore how the signaling of Ras family proteins contributes to memory formation. Understanding these signaling processes is of fundamental importance both from a basic scientific perspective, with the goal of providing mechanistic insights into a critical aspect of cognitive behavior, and from a clinical perspective, with the goal of providing effective therapies for a range of disorders involving cognitive impairments.

A novel strategy to isolate ubiquitin conjugates reveals wide role for ubiquitination during neural development

Ubiquitination has essential roles in neuronal development and function. Ubiquitin proteomics studies on yeast and HeLa cells have proven very informative, but there still is a gap regarding neuronal tissue-specific ubiquitination. In an organism context, direct evidence for the ubiquitination of neuronal proteins is even scarcer. Here, we report a novel proteomics strategy based on the in vivo biotinylation of ubiquitin to isolate ubiquitin conjugates from the neurons of Drosophila melanogaster embryos. We confidently identified 48 neuronal ubiquitin substrates, none of which was yet known to be ubiquitinated. Earlier proteomics and biochemical studies in non-neuronal cell types had identified orthologs to some of those but not to others. The identification here of novel ubiquitin substrates, those with no known ubiquitinated ortholog, suggests that proteomics studies must be performed on neuronal cells to identify ubiquitination pathways not shared by other cell types. Importantly, several of those newly found neuronal ubiquitin substrates are key players in synaptogenesis. Mass spectrometry results were validated by Western blotting to confirm that those proteins are indeed ubiquitinated in the Drosophila embryonic nervous system and to elucidate whether they are mono- or polyubiquitinated. In addition to the ubiquitin substrates, we also identified the ubiquitin carriers that are active during synaptogenesis. Identifying endogenously ubiquitinated proteins in specific cell types, at specific developmental stages, and within the context of a living organism will allow understanding how the tissue-specific function of those proteins is regulated by the ubiquitin system.

Extracellular signal-regulated kinase and phosphatidylinositol-3-kinase/AKT signalling pathways in the human carotid body and peripheral ganglia

Extracellular signal-regulated kinase (ERK) and phosphatidylinositol-3-kinase (PI3K)/AKT signalling pathways are involved in various cell functions, but their developmental regulation in the carotid body and peripheral ganglia has not yet been fully investigated. ERK and AKT immunolocalisation and activation were studied by anti-ERK, -pERK, -AKT and -pAKT immunohistochemistry in carotid bodies and peripheral (sympathetic and sensory) ganglia, sampled at autopsy from 4 foetuses (mean gestational age 177 days), 8 infants (mean age 10 months), 8 young adults (mean age 38 years) and 6 aged adults (mean age 72.4 years). ERK and AKT immunopositivity and activation were demonstrated in both glomic type I cells and peripheral ganglionic cells and are ascribed to local action by neuromodulators or neurotrophic factors. Mean percentages of ERK- and pERK-immunopositive glomic type I cells were lower in foetuses than in infants and young adults, and those of AKT-immunopositive glomic type I cells were lower in foetuses than in young and old adults, suggesting incomplete maturation of these two signalling pathways in foetal life. Both pERK and pAKT immunoreactions were detected only in post-natal sympathetic and sensory ganglia, demonstrating that, also in peripheral ganglia, these pathways are not yet fully operative during the foetal stage.

A new genetic model of activity-induced Ras signaling dependent pre-synaptic plasticity in Drosophila

Techniques to induce activity-dependent neuronal plasticity in vivo allow the underlying signaling pathways to be studied in their biological context. Here, we demonstrate activity-induced plasticity at neuromuscular synapses of Drosophila double mutant for comatose (an NSF mutant) and Kum (a SERCA mutant), and present an analysis of the underlying signaling pathways. comt; Kum (CK) double mutants exhibit increased locomotor activity under normal culture conditions, concomitant with a larger neuromuscular junction synapse and stably elevated evoked transmitter release. The observed enhancements of synaptic size and transmitter release in CK mutants are completely abrogated by: a) reduced activity of motor neurons; b) attenuation of the Ras/ERK signaling cascade; or c) inhibition of the transcription factors Fos and CREB. All of which restrict synaptic properties to near wild type levels. Together, these results document neural activity-dependent plasticity of motor synapses in CK animals that requires Ras/ERK signaling and normal transcriptional activity of Fos and CREB. Further, novel in vivo reporters of neuronal Ras activation and Fos transcription also confirm increased signaling through a Ras/AP-1 pathway in motor neurons of CK animals, consistent with results from our genetic experiments. Thus, this study: a) provides a robust system in which to study activity-induced synaptic plasticity in vivo; b) establishes a causal link between neural activity, Ras signaling, transcriptional regulation and pre-synaptic plasticity in glutamatergic motor neurons of Drosophila larvae; and c) presents novel, genetically encoded reporters for Ras and AP-1 dependent signaling pathways in Drosophila.

Genomic mapping and expression patterns of C380, OK6 and D42 enhancer trap lines in the larval nervous system of Drosophila

Since its description more than fifteen years ago, the GAL4-UAS system of heterologous transgenic expression has found universal and widespread use in Drosophila research , making it a uniquely powerful analytical tool. Several hundreds of enhancer-trap GAL4 "driver" lines have since been used to express proteins of interest in specific spatio-temporal domains. However, the identities of enhancer elements that regulate GAL4 expression in vivo are often not known. Here, I report the mapping of three GAL4 lines commonly used as motor neuron drivers. Sequencing of genomic DNA flanking these three P-element transposon insertion, C380, (BG380), OK6, and D42, shows that these insertions lie upstream of the futsch, Rapgap 1 and toll-6 gene, respectively . Of the three, OK6-GAL4 (Rapgap 1) expression is most restricted to motor neurons, while C380-GAL4 and D42-GAL4 also show prominent expression in the peripheral nervous system, including body wall sensory nervous system. Albeit with clear differences. Finally, I test if the highly restricted expression pattern of Futsch is maintained in six other species of Drosophilids (D. yakuba, D. ananssae, D.pseudoobscura, D. dimulars, D. willistoni and D. virilis). My results suggest conserved control of Futsch expression across species, most likely through upstream cis-acting elements. A comparative anatomy of the laval central nervous systems and peripheral innervation in these Frosophilids species as revealed by contemporary immunohistochemical markers is also presented.

Generation and characterization of monoclonal antibodies specific to Drosophila presenilin

The development of a monoclonal antibody (MAb) specific to Drosophila presenilin (Psn) proteins in vivo was the major aim of this study, since the absence of specific antibodies recognizing Psn proteins hampered our progress in understanding Psn functions during development, differentiation, and pathogenesis. By dot blot and immunofluorescence screenings, we found that MAb Psn2G6 specifically recognized Psn proteins in wing imaginal discs and brains of wild-type control W1118 larvae. MAb Psn2G6 also transgenically expressed a long form of wild-type Psn (Psn + 14 WT) proteins in wing imaginal discs of two independent transgenic lines. Transgenic expression of Psn + 14 WT proteins in psn(B3) larvae completely rescued the expression patterns of Psn proteins and the development of wing imaginal discs. In addition, neural hyperplasia observed in wing imaginal discs of psn(B3) larvae was also suppressed.

Drosophila RSK negatively regulates bouton number at the neuromuscular junction

Ribosomal S6 kinases (RSKs) are growth factor-regulated serine-threonine kinases participating in the RAS-ERK signaling pathway. RSKs have been implicated in memory formation in mammals and flies. To characterize the function of RSK at the synapse level, we investigated the effect of mutations in the rsk gene on the neuromuscular junction (NMJ) in Drosophila larvae. Immunostaining revealed transgenic expressed RSK in presynaptic regions. In mutants with a full deletion or an N-terminal partial deletion of rsk, an increased bouton number was found. Restoring the wild-type rsk function in the null mutant with a genomic rescue construct reverted the synaptic phenotype, and overexpression of the rsk-cDNA in motoneurons reduced bouton numbers. Based on previous observations that RSK interacts with the Drosophila ERK homologue Rolled, genetic epistasis experiments were performed with loss- and gain-of-function mutations in Rolled. These experiments provided evidence that RSK mediates its negative effect on bouton formation at the Drosophila NMJ by inhibition of ERK signaling.

Fak56 functions downstream of integrin alphaPS3betanu and suppresses MAPK activation in neuromuscular junction growth

Background

Focal adhesion kinase (FAK) functions in cell migration and signaling through activation of the mitogen-activated protein kinase (MAPK) signaling cascade. Neuronal function of FAK has been suggested to control axonal branching; however, the underlying mechanism in this process is not clear.

Results

We have generated mutants for the Drosophila FAK gene, Fak56. Null Fak56 mutants display overgrowth of larval neuromuscular junctions (NMJs). Localization of phospho-FAK and rescue experiments suggest that Fak56 is required in presynapses to restrict NMJ growth. Genetic analyses imply that FAK mediates the signaling pathway of the integrin αPS3βν heterodimer and functions redundantly with Src. At NMJs, Fak56 downregulates ERK activity, as shown by diphospho-ERK accumulation in Fak56 mutants, and suppression of Fak56 mutant NMJ phenotypes by reducing ERK activity.

Conclusion

We conclude that Fak56 is required to restrict NMJ growth during NMJ development. Fak56 mediates an extracellular signal through the integrin receptor. Unlike its conventional role in activating MAPK/ERK, Fak56 suppresses ERK activation in this process. These results suggest that Fak56 mediates a specific neuronal signaling pathway distinct from that in other cellular processes.

Overexpression screen in Drosophila identifies neuronal roles of GSK-3 beta/shaggy as a regulator of AP-1-dependent developmental plasticity

AP-1, an immediate-early transcription factor comprising heterodimers of the Fos and Jun proteins, has been shown in several animal models, including Drosophila, to control neuronal development and plasticity. In spite of this important role, very little is known about additional proteins that regulate, cooperate with, or are downstream targets of AP-1 in neurons. Here, we outline results from an overexpression/misexpression screen in Drosophila to identify potential regulators of AP-1 function at third instar larval neuromuscular junction (NMJ) synapses. First, we utilize >4000 enhancer and promoter (EP) and EPgy2 lines to screen a large subset of Drosophila genes for their ability to modify an AP-1-dependent eye-growth phenotype. Of 303 initially identified genes, we use a set of selection criteria to arrive at 25 prioritized genes from the resulting collection of putative interactors. Of these, perturbations in 13 genes result in synaptic phenotypes. Finally, we show that one candidate, the GSK-3beta-kinase homolog, shaggy, negatively influences AP-1-dependent synaptic growth, by modulating the Jun-N-terminal kinase pathway, and also regulates presynaptic neurotransmitter release at the larval neuromuscular junction. Other candidates identified in this screen provide a useful starting point to investigate genes that interact with AP-1 in vivo to regulate neuronal development and plasticity.

Very-KIND, a KIND domain containing RasGEF, controls dendrite growth by linking Ras small GTPases and MAP2

The regulation of cytoskeletal components in the dendritic shaft core is critical for dendrite elongation and branching. Here, we report that a brain-specific Ras guanine nucleotide exchange factor (RasGEF) carrying two kinase non-catalytic C-lobe domains (KINDs), very-KIND (v-KIND), regulates microtubule-associated protein 2 (MAP2). v-KIND is expressed in developing mouse brain, predominantly in the cerebellar granule cells. v-KIND not only activates Ras small GTPases via the C-terminal RasGEF domain, but also specifically binds to MAP2 via the second KIND domain (KIND2), leading to threonine phosphorylation of MAP2. v-KIND overexpression suppresses dendritic extension and branching of hippocampal neurons and cerebellar granule cells, whereas knockdown of endogenous v-KIND expression promotes dendrite growth. These findings suggest that v-KIND mediates a signaling pathway that links Ras and MAP2 to control dendrite growth.

Genetic modifiers of Drosophila palmitoyl-protein thioesterase 1-induced degeneration

Infantile neuronal ceroid lipofuscinosis (INCL) is a pediatric neurodegenerative disease caused by mutations in the human CLN1 gene. CLN1 encodes palmitoyl-protein thioesterase 1 (PPT1), suggesting an important role for the regulation of palmitoylation in normal neuronal function. To further elucidate Ppt1 function, we performed a gain-of-function modifier screen in Drosophila using a collection of enhancer-promoter transgenic lines to suppress or enhance the degeneration produced by overexpression of Ppt1 in the adult visual system. Modifier genes identified in our screen connect Ppt1 function to synaptic vesicle cycling, endo-lysosomal trafficking, synaptic development, and activity-dependent remodeling of the synapse. Furthermore, several homologs of the modifying genes are known to be regulated by palmitoylation in other systems and may be in vivo substrates for Ppt1. Our results complement recent work on mouse Ppt1(-/-) cells that shows a reduction in synaptic vesicle pools in primary neuronal cultures and defects in endosomal trafficking in human fibroblasts. The pathways and processes implicated by our modifier loci shed light on the normal cellular function of Ppt1. A greater understanding of Ppt1 function in these cellular processes will provide valuable insight into the molecular etiology of the neuronal dysfunction underlying the disease.

Semiparametric approach to characterize unique gene expression trajectories across time

Background:

A semiparametric approach was used to identify groups of cDNAs and genes with distinct expression profiles across time and overcome the limitations of clustering to identify groups. The semiparametric approach allows the generalization of mixtures of distributions while making no specific parametric assumptions about the distribution of the hidden heterogeneity of the cDNAs. The semiparametric approach was applied to study gene expression in the brains of Apis mellifera ligustica honey bees raised in two colonies (A. m. mellifera and ligustica) with consistent patterns across five maturation ages.

Results:

The semiparametric approach provided unambiguous criteria to detect groups of genes, trajectories and probability of gene membership to groups. The semiparametric results were cross-validated in both colony data sets. Gene Ontology analysis enhanced by genome annotation helped to confirm the semiparametric results and revealed that most genes with similar or related neurobiological function were assigned to the same group or groups with similar trajectories. Ten groups of genes were identified and nine groups had highly similar trajectories in both data sets. Differences in the trajectory of the reminder group were consistent with reports of accelerated maturation in ligustica colonies compared to mellifera colonies.

Conclusion:

The combination of microarray technology, genomic information and semiparametric analysis provided insights into the genomic plasticity and gene networks linked to behavioral maturation in the honey bee.

Hypotension and short-term anaesthesia induce ERK1/2 phosphorylation in autonomic nuclei of the brainstem

The aims of this study were: first, to investigate the effects of anaesthesia on phosphorylated extracellular signal-regulated kinase (p-ERK)1/2-immunoreactivity (-ir) in the brainstem; second, to choose the best anaesthetic for p-ERK1/2 studies; and third, to determine the effect of short-term hypotension on p-ERK1/2-ir in the brainstem. Rats were anaesthetized with halothane, sodium pentobarbital or 100% CO2 narcosis, or were cervically dislocated and within 5 min perfused and the brains processed immunohistochemically for pERK1/2-ir. p-ERK1/2-ir was primarily observed in regions associated with cardiovascular and/or respiratory control. Several regions consistently showed dense p-ERK1/2 labelling, including a restricted region of the ventrolateral medulla (VLM). In contrast, other regions showed differential labelling depending on the mode of death. Cervical dislocation showed the least VLM labelling, limited to a discrete area approximately 0.6-1.4 mm caudal to the facial nucleus. Anaesthetics induced labelling throughout the VLM, with halothane inducing the most. Many p-ERK1/2-ir VLM neurons were catecholaminergic following halothane or sodium pentobarbital anaesthesia, but no double labelling was seen following cervical dislocation. Of the anaesthetics, sodium pentobarbital induced the least labelling and was used subsequently. Intravenous hydralazine was used to induce a 20-min period of hypotension, whereas arterial pressure did not change in vehicle-treated animals. Hydralazine evoked more pERK-ir neurons in specific regions, including the VLM, nucleus tractus solitarius (NTS), parabrachial nuclei, Kolliker-Fuse nucleus and locus coeruleus. Approximately twice as many p-ERK1/2-positive neurons were seen in the intermediate NTS and rostral VLM following hydralazine compared with the vehicle. In conclusion, p-ERK1/2-ir identifies neurons in central autonomic regions, and their number and distribution are markedly affected by anaesthetics, and are increased in some regions by short-term hypotension.

Plasticity and second messengers during synapse development

Effective function of the locomotor system in the Drosophila larva requires a continuous adjustment of synaptic architecture and neurotransmission at the neuromuscular junction (NMJ). This feature has made the larval NMJ a favorite model to study the genetic and molecular mechanisms underlying synapse plasticity. This chapter will review experimental strategies used to study plasticity at the NMJ, the cellular parameters affected during plastic changes, and many of the known molecules involved in plastic changes. In addition, signal transduction pathways activated during plasticity will be discussed.

Analysis of 5-HT–Induced Short-Term Facilitation atAplysiaSensorimotor Synapse During Bursts: Increased Synaptic Gain That Does Not Require ERK Activation

The 5-HT–induced synaptic plasticity of Aplysia sensorimotor synapses has typically been probed by firing a single presynaptic spike. In this study, 5-HT–induced synaptic plasticity was probed with brief bursts of spikes (10 Hz, 1 s), which are more behaviorally relevant stimuli. Because such bursts provide a greater challenge to the release machinery than single spikes, their use may reveal additional aspects of synaptic modulation, and, in particular, the role of extracellular signal-regulated protein kinase (ERK), which has recently been implicated in several examples of short- and long-term synaptic plasticity. Excitatory postsynaptic currents (EPSCs) were characterized by their amplitudes. In addition, two kinetic measurements, time to peak and decay time constant, were determined for the initial and last EPSCs of each burst. Application of 5-HT produced a uniform increase in gain by facilitating each EPSC elicited during a burst of spikes without affecting the kinetics of the initial or last EPSC. These data suggest that short-term facilitation during a burst is mediated largely by processes such as those that affect the size of the releasable pool or rate of vesicle mobilization rather than by an increase in the duration of the presynaptic action potential. An ERK cascade inhibitor (U0126) had no effect on the 5-HT–mediated facilitation of either the initial EPSC or EPSCs elicited late in the burst.

Synaptic and genomic responses to JNK and AP-1 signaling in Drosophila neurons

BACKGROUND

The transcription factor AP-1 positively controls synaptic plasticity at the Drosophila neuromuscular junction. Although in motor neurons, JNK has been shown to activate AP-1, a positive regulator of growth and strength at the larval NMJ, the consequences of JNK activation are poorly studied. In addition, the downstream transcriptional targets of JNK and AP-1 signaling in the Drosophila nervous system have yet to be identified. Here, we further investigated the role of JNK signaling at this model synapse employing an activated form of JNK-kinase; and using Serial Analysis of Gene Expression and oligonucleotide microarrays, searched for candidate early targets of JNK or AP-1 dependent transcription in neurons.

RESULTS

Temporally-controlled JNK induction in postembryonic motor neurons triggers synaptic growth at the NMJ indicating a role in developmental plasticity rather than synaptogenesis. An unexpected observation that JNK activation also causes a reduction in transmitter release is inconsistent with JNK functioning solely through AP-1 and suggests an additional, yet-unidentified pathway for JNK signaling in motor neurons. SAGE profiling of mRNA expression helps define the neural transcriptome in Drosophila. Though many putative AP-1 and JNK target genes arose from the genomic screens, few were confirmed in subsequent validation experiments. One potentially important neuronal AP-1 target discovered, CG6044, was previously implicated in olfactory associative memory. In addition, 5 mRNAs regulated by RU486, a steroid used to trigger conditional gene expression were identified.

CONCLUSION

This study demonstrates a novel role for JNK signaling at the larval neuromuscular junction and provides a quantitative profile of gene transcription in Drosophila neurons. While identifying potential JNK/AP-1 targets it reveals the limitations of genome-wide analyses using complex tissues like the whole brain.

Activation of Ras in neurons modifies synaptic vesicle docking and release

We have recently demonstrated that constitutive activation of Ha-Ras in differentiated neurons induces structural remodeling of both axons and dendrites in a transgenic mouse model (referred as synRas mice). Here we show that this activation of neuronal Ras enhances docking of synaptic vesicles to active zones, thereby leading to an increase in the size of the readily releasable pool of vesicles, while the size of the total pool of synaptic vesicles remained unchanged. The morphological phenotype was associated with corresponding increases in the probability of glutamate release as revealed by changes in short-term synaptic plasticity. We, therefore, conclude that neuronal Ras activity contributes to the regulation of synaptic plasticity in adult mammalian brain at the presynaptic level.

Cdk5 inhibits anterograde axonal transport of neurofilaments but not that of tau by inhibition of mitogen-activated protein kinase activity

Cyclin-dependent kinase 5 (cdk5) inhibits neurofilament (NF) anterograde axonal transport while p42/44 mitogen-activated protein kinase (MAPk) promotes it. Since cdk5 is known to inhibit MAP kinase activity, we examined whether or not cdk5 inhibits anterograde NF transport via inhibition of MAPk activity. To accomplish this, we manipulated the activity of these kinases in differentiated NB2a/d1 cells, and monitored anterograde axonal transport of green fluorescent protein-conjugated-NF-M (GFP-M) and cyan fluorescent protein-conjugated (CFP)-tau. The cdk5 inhibitor roscovitine increased anterograde axonal transport of GFP-M and CFP-tau; transfection with cdk5/p25 inhibited transport of both. Inhibition of MAPk activity by PD98059 or expression of dominant-negative MAPk inhibited anterograde GFP-M transport, while expression of constitutively active MAPk enhanced it; these treatments did not affect CFP-tau transport. PD98059 prevented roscovitine-mediated enhancement of GFP-M transport, but did not prevent enhancement of CFP-tau transport. Co-transfection with constitutively activated MAPk prevented the inhibition of GFP-M transport that normally accompanied transfection with cdk5/p25, but did not prevent inhibition of tau transport by cdk5/p25. Finally, the extent of inhibition of GFP-M axonal transport by PD98059 was not additive to that derived from transfection with cdk5/p35, and the increase in NF transport that accompanies roscovitine treatment was not additive to that derived from transfection with constitutively activated MAPk, suggesting that the influence of these kinases on NF transport was within the same, rather than distinct, pathways. These findings suggest that axonal transport of tau and NFs is under the control of distinct kinase cascades, and that cdk5 inhibits NF transport at least in part by inhibiting MAPk.

Patterns of gene dysregulation in the frontal cortex of patients with HIV encephalitis

The neurodegenerative process in HIV encephalitis (HIVE) is associated with extensive damage to the dendritic and synaptic structure that often leads to cognitive impairment. Several mechanisms might be at play, including release of neurotoxins, oxidative stress and decreased activity of neurotrophic factors. Furthermore, HIV-mediated dysregulation of genes involved in neuronal maintenance might play an important role. For this purpose, cRNA was prepared from the brains of 17 AIDS patients for analysis with the Affymetrix Human U95Av2 GeneChip and analyzed with the GeneSpring Expression Analysis Software. Out of 12,625 genes analyzed, 74 were downregulated and 59 were upregulated compared to controls. Initial alternative analysis of RNA was performed by ribonuclease protection assay (RPA). In cases with HIVE, downregulated genes included neuronal molecules involved in synaptic plasticity and transmission (ion channels, synaptogyrin, synapsin II), cell cycle (p35, p39, CDC-L2, CDC42, PAK1) and signaling molecules (PI3K, Ras-Raf-MEK1), transcription factors and cytoskeletal components (MAP-1B, MAP-2, tubulin, adducin-2). Upregulated genes included those involved in neuroimmune (IgG, MHC, beta2microglobulin) and anti-viral responses (interferon-inducible molecules), transcription (STAT1, OLIG2, Pax-6) and signaling modulation (MEK3, EphB1) of the cytoskeleton (myosin, aduccin-3, radixin, dystrobrevin). Taken together, this study suggests that HIV proteins released from infected macrophages might not only induce a neuroinflammatory response, but also may promote neurodegeneration by interfering with neuronal transcription of genes involved in regulating signaling and cytoskeletal molecules important in maintaining synapto-dendritic functioning and integrity.

Dap160/intersectin scaffolds the periactive zone to achieve high-fidelity endocytosis and normal synaptic growth

Dap160/Intersectin is a multidomain adaptor protein that colocalizes with endocytic machinery in the periactive zone at the Drosophila NMJ. We have generated severe loss-of-function mutations that eliminate Dap160 protein from the NMJ. dap160 mutant synapses have decreased levels of essential endocytic proteins, including dynamin, endophilin, synaptojanin, and AP180, while other markers of the active zone and periactive zone are generally unaltered. Functional analyses demonstrate that dap160 mutant synapses are unable to sustain high-frequency transmitter release, show impaired FM4-64 loading, and show a dramatic increase in presynaptic quantal size consistent with defects in synaptic vesicle recycling. The dap160 mutant synapse is grossly malformed with abundant, highly ramified, small synaptic boutons. We present a model in which Dap160 scaffolds both endocytic machinery and essential synaptic signaling systems to the periactive zone to coordinately control structural and functional synapse development.