Touch and go: guidance cues signal to the growth cone cytoskeleton

Quantitative analysis of microtubule dynamics during adhesion-mediated growth cone guidance

During adhesion-mediated neuronal growth cone guidance microtubules undergo major rearrangements. However, it is unknown whether microtubules extend to adhesion sites because of changes in plus-end polymerization and/or translocation dynamics, because of changes in actin-microtubule interactions, or because they follow the reorganization of the actin cytoskeleton. Here, we used fluorescent speckle microscopy to directly quantify microtubule and actin dynamics in Aplysia growth cones as they turn towards beads coated with the cell adhesion molecule apCAM. During the initial phase of adhesion formation, dynamic microtubules in the peripheral domain preferentially explore apCAM-beads prior to changes in growth cone morphology and retrograde actin flow. Interestingly, these early microtubules have unchanged polymerization rates but spend less time in retrograde translocation due to uncoupling from actin flow. Furthermore, microtubules exploring the adhesion site spend less time in depolymerization. During the later phase of traction force generation, the central domain advances and more microtubules in the peripheral domain extend because of attenuation of actin flow and clearance of F-actin structures. Microtubules in the transition zone and central domain, however, translocate towards the adhesion site in concert with actin arcs and bundles, respectively. We conclude that adhesion molecules guide neuronal growth cones and underlying microtubule rearrangements largely by differentially regulating microtubule-actin coupling and actin movements according to growth cone region and not by controlling plus-end polymerization rates.

Formin-dependent synaptic growth: evidence that Dlar signals via Diaphanous to modulate synaptic actin and dynamic pioneer microtubules

The diaphanous gene is the founding member of a family of Diaphanous-related formin proteins (DRFs). We identified diaphanous in a screen for genes that are necessary for the normal growth and stabilization of the Drosophila neuromuscular junction (NMJ). Here, we demonstrate that diaphanous mutations perturb synaptic growth at the NMJ. Diaphanous protein is present both presynaptically and postsynaptically. However, genetic rescue experiments in combination with additional genetic interaction experiments support the conclusion that dia is necessary presynaptically for normal NMJ growth. We then document defects in both the actin and microtubule cytoskeletons in dia mutant nerve terminals. In so doing, we define and characterize a population of dynamic pioneer microtubules within the NMJ that are distinct from the bundled core of microtubules identified by the MAP1b-like protein Futsch. Defects in both synaptic actin and dynamic pioneer microtubules are correlated with impaired synaptic growth in dia mutants. Finally, we present genetic evidence that Dia functions downstream of the presynaptic receptor tyrosine phosphatase Dlar and the Rho-type GEF (guanine nucleotide exchange factor) trio to control NMJ growth. Based on the established function of DRFs as Rho-GTPase-dependent regulators of the cell cytoskeleton, we propose a model in which Diaphanous links receptor tyrosine phosphatase signaling at the plasma membrane to growth-dependent modulation of the synaptic actin and microtubule cytoskeletons.

Wnt regulates axon behavior through changes in microtubule growth directionality: a new role for adenomatous polyposis coli

Axon guidance and target-derived signals control axonal behavior by regulating the cytoskeleton through poorly defined mechanisms. In particular, how these signaling molecules regulate the growth and directionality of microtubules is not well understood. Here we examine the effect of Wnts on growth cone remodeling, a process that precedes synapse formation. Time-lapse recordings reveal that Wnt3a rapidly inhibits growth cone translocation while inducing growth cone enlargement. These changes in axonal behavior are associated with changes in the organization of microtubules. Time-lapse imaging of EB3-GFP (green fluorescent protein)-labeled microtubule plus-ends demonstrates that Wnt3a regulates microtubule directionality, resulting in microtubule looping, growth cone pausing, and remodeling. Analyses of Dishevelled-1 (Dvl1) mutant neurons demonstrate that Dvl1 is required for Wnt-mediated microtubule reorganization and axon remodeling. Wnt signaling directly affects the microtubule cytoskeleton by unexpectedly inducing adenomatous polyposis coli (APC) loss from microtubule plus-ends. Consistently, short hairpin RNA knockdown of APC mimics Wnt3a function. Together, our findings define APC as a key Wnt signaling target in the regulation of microtubule growth direction.

Emergence of highly neurofilament-immunoreactive zipper-like axon segments at the transection site in scalpel-cordotomized adult rats

Following transection of the spinal cord, severed axonal ends retract from the lesion site and attempt regeneration within 24 h of injury. Molecular mechanisms underlying such rapid axonal reactions after severance are not fully characterized so far. To better understand the early axonal degenerating and regenerating processes, we examined the immunohistological expression of axonal cytoskeletal proteins from 5 min to 48 h after scalpel-transection of adult rat spinal cord white matter. Within 30 min of transection, expression of neurofilament (NF)- and peripherin-like immunoreactivity (-IR) was enhanced in severed axonal ends, which conversely lost beta-III-tubulin-IR expression, indicating differential expression of beta-III-tubulin-IR and NF/peripherin-IR. During the next few hours, the strongly-NF/peripherin-IR-positive severed axonal ends adhered to each other and these cytoskeletal alterations expanded bi-directionally (rostro-caudally) 100-300 microm away from the transection point. Within 6 h of transection, secondary axotomy occurred at about 300 microm-rostral and -caudal to the primary transection point, which finally formed strongly-NF/peripherin-IR-positive zipper-like axon segments at the transection site. Notably, sprouting of secondarily severed axons was observed within 6 h of injury. The regenerative axons, which extended toward the transection site, could not traverse the transection site where the zipper-like axon segments resided. The zipper-like axon segments showed abnormal axolemmal permeability through the leakage of an axonal tracer. Western blot analysis revealed a slight increase in peripherin content in transected spinal cord. Local treatment with cycloheximide suppressed the axotomy-induced peripherin-IR-enhancement in severed ends, suggesting the occurrence of intra-axonal peripherin synthesis in vivo. Treatment with calpain inhibitors frequently formed abnormally swollen microtubule-free ends, which suggests that calpain-activation is critical for functional growth cone formation in adult rat spinal cord. These observations indicate that adult rat cordotomy with a scalpel results in the rapid formation of intensely NF-IR-positive zipper-like axon segments at the transection site, which are similar to "preserved fibers" reported by Ramon y Cajal [Ramon y Cajal S (1928) Degeneration and regeneration in the nervous system. New York: Hafner]. On the other hand, axonal regenerative responses start within 6 h of injury, which may be supported by calpain-activation and intra-axonal protein synthesis.

Signalling by neurotrophins and hepatocyte growth factor regulates axon morphogenesis by differential beta-catenin phosphorylation

Tyrosine phosphorylation of beta-catenin, a component of adhesion complexes and of the Wnt pathway, affects cell adhesion, migration and gene transcription. By reducing beta-catenin availability using shRNA-mediated gene silencing or expression of intracellular N-cadherin, we show that beta-catenin is required for axon growth downstream of brain-derived neurotrophic factor (BDNF) signalling and hepatocyte growth factor (HGF) signalling. We demonstrate that the receptor tyrosine kinases (RTKs) Trk and Met interact with and phosphorylate beta-catenin. Stimulation of Trk receptors by neurotrophins (NTs) results in phosphorylation of beta-catenin at residue Y654, and increased axon growth and branching. Conversely, pharmacological inhibition of Trk or expression of a Y654F mutant blocks these effects. beta-catenin phosphorylated at Y654 colocalizes with the cytoskeleton at growth cones. However, HGF, which also increases axon growth and branching, induces beta-catenin phosphorylation at Y142 and a nuclear localization. Interestingly, dominant-negative DeltaN-TCF4 abolishes the effects of HGF in axon growth and branching, but not that of NTs. We conclude that NT- and HGF-signalling differentially phosphorylate beta-catenin, targeting this protein to distinct compartments to regulate axon morphogenesis by TCF4-transcription-dependent and -independent mechanisms. These results place beta-catenin downstream of growth-factor-RTK signalling in axon differentiation.

Identification of Neurochondrin as a new interaction partner of the FH3 domain of the Diaphanous-related formin Dia1

Mammalian Diaphanous (Dia)-related formins initiate the assembly of filamentous actin downstream of Rho GTPases to regulate cellular processes such as cytokinesis, cell polarity, cell motility and adhesion. In this work, we show that Neurochondrin (NC) is a novel Dia1 interacting protein. NC specifically binds to the formin homology 3 (FH3), but not to the FH1 or FH2 domain of Dia1. Both proteins show a partial co-localization in dissociated primary rat hippocampal neurons. Ectopic expression of both proteins induced neurite outgrowth in Neuro2A cells. Using a series of deletion mutants of NC we could show that the first 100 amino acids were responsible for its effect on neurite outgrowth, whereas the C-terminal part of NC had no neurite outgrowth promoting activity. Moreover, co-expression of the C terminus of NC with Dia1DeltaDAD resulted in a dramatic reduction of Dia1-induced neurite outgrowth. On the basis of actin fractionation assays, SRF-activity assays as well as microtubule stabilization assays, we could demonstrate that the C terminus of NC does not influence the actin polymerizing activity of Dia1, indicating a more specific function of NC in the modulation of Dia1 activity.

Genetic modifier screens reveal new components that interact with the Drosophila dystroglycan-dystrophin complex

The Dystroglycan-Dystrophin (Dg-Dys) complex has a capacity to transmit information from the extracellular matrix to the cytoskeleton inside the cell. It is proposed that this interaction is under tight regulation; however the signaling/regulatory components of Dg-Dys complex remain elusive. Understanding the regulation of the complex is critical since defects in this complex cause muscular dystrophy in humans. To reveal new regulators of the Dg-Dys complex, we used a model organism Drosophila melanogaster and performed genetic interaction screens to identify modifiers of Dg and Dys mutants in Drosophila wing veins. These mutant screens revealed that the Dg-Dys complex interacts with genes involved in muscle function and components of Notch, TGF-β and EGFR signaling pathways. In addition, components of pathways that are required for cellular and/or axonal migration through cytoskeletal regulation, such as Semaphorin-Plexin, Frazzled-Netrin and Slit-Robo pathways show interactions with Dys and/or Dg. These data suggest that the Dg-Dys complex and the other pathways regulating extracellular information transfer to the cytoskeletal dynamics are more intercalated than previously thought.

Angiotensin II type 2 receptor stimulation increases the rate of NG108-15 cell migration via actin depolymerization

Angiotensin II (Ang II) has been reported to induce migration in neuronal cell types. Using time-lapse microscopy, we show here that Ang II induces acceleration in NG108-15 cell migration. This effect was antagonized by PD123319, a selective AT2 receptor antagonist, but not by DUP753, a selective AT1 receptor antagonist, and was mimicked by the specific AT2 receptor agonist CGP42112. This Ang II-induced acceleration was not sensitive to the inhibition of previously described signaling pathways of the AT2 receptor, guanylyl cyclase/cyclic GMP or p42/p44 mapk cascades, but was abolished by pertussis toxin treatment and involved PP2A activation. Immunofluorescence studies indicate that Ang II or CGP42112 decreased the amount of filamentous actin at the leading edge of the cells. This decrease was accompanied by a concomitant increase in globular actin levels. Regulation of actin turnover in actin-based motile systems is known to be mainly under the control of the actin depolymerizing factor and cofilin. Basal migration speed decreased by 77.2% in cofilin-1 small interfering RNA-transfected NG108-15 cells, along with suppression of the effect of Ang II. In addition, the Ang II-induced increase in cell velocity was abrogated in serum-free medium as well as by genistein or okadaic acid treatment in a serum-containing medium. Such results indicate that the AT2 receptor increases the migration speed of NG108-15 cells and involves a tyrosine kinase activity, followed by phosphatase activation, which may be of the PP2A type. Therefore, the present study identifies actin depolymerization and cofilin as new targets of AT2 receptor action, in the context of cellular migration.

Ena/VASP: proteins at the tip of the nervous system

The emergence of neurites from a symmetrical cell body is an essential feature of nervous system development. Neurites are the precursors of axons and dendrites and are tipped by growth cones, motile structures that guide elongating axons in the developing nervous system. Growth cones steer the axon along a defined path to its appropriate target in response to guidance cues. This navigation involves the dynamic extension and withdrawal of actin-filled finger-like protrusions called filopodia that continuously sample their environment. Ena/VASP proteins, a conserved family of actin-regulatory proteins, are crucial for filopodia formation and function downstream of several guidance cues. Here we review recent findings into Ena/VASP function in neurite initiation, axon outgrowth and guidance.

Actin-based propulsive forces and myosin-II-based contractile forces in migratingDictyosteliumcells

It has been suggested that myosin II exerts traction forces at the posterior ends and retracting pseudopodia of migrating cells, but there is no direct evidence. Here, using a combination of total internal reflection fluorescence (TIRF) microscopy and force microscopy with a high spatial resolution of ∼400 nm, we simultaneously recorded GFP-myosin II dynamics and traction forces under migrating Dictyostelium cells. Accumulation of filamentous myosin II and a subsequent increase in traction forces were detected in pseudopodia just before retraction. In the case of motorless myosin II, traction forces did not increase after accumulation, suggesting that the source of the retraction force is the motor activity of accumulated myosin II. Simultaneous recording of F-actin and traction forces revealed that traction forces were exerted under spot-like regions where F-actin accumulated. Cells migrated in a direction counter to the sum of the force vectors exerted at each spot, suggesting that the stress spots act as scaffolds to transmit the propulsive forces at the leading edge generated by actin polymerization.

Rapid activity-dependent modifications in synaptic structure and function require bidirectional Wnt signaling

Activity-dependent modifications in synapse structure play a key role in synaptic development and plasticity, but the signaling mechanisms involved are poorly understood. We demonstrate that glutamatergic Drosophila neuromuscular junctions undergo rapid changes in synaptic structure and function in response to patterned stimulation. These changes, which depend on transcription and translation, include formation of motile presynaptic filopodia, elaboration of undifferentiated varicosities, and potentiation of spontaneous release frequency. Experiments indicate that a bidirectional Wnt/Wg signaling pathway underlies these changes. Evoked activity induces Wnt1/Wg release from synaptic boutons, which stimulates both a postsynaptic DFz2 nuclear import pathway as well as a presynaptic pathway involving GSK-3beta/Shaggy. Our findings suggest that bidirectional Wg signaling operates downstream of synaptic activity to induce modifications in synaptic structure and function. We propose that activation of the postsynaptic Wg pathway is required for the assembly of the postsynaptic apparatus, while activation of the presynaptic Wg pathway regulates cytoskeletal dynamics.

Semi-automated quantification of filopodial dynamics

Cellular motility underlies critical physiological processes including embryogenesis, metastasis and wound healing. Nerve cells undergo cellular migration during development and also extend neuronal processes for long distances through a complex microenvironment to appropriately wire the nervous system. The growth cone is a highly dynamic structure that responds to extracellular cues by extending and retracting filopodia and lamellipodia to explore the microenvironment and to dictate the path and speed of process extension. Neuronal responses to a myriad of guidance cues have been studied biochemically, however, these approaches fail to capture critical spatio-temporal elements of growth cone dynamics. Live imaging of growth cones in culture has emerged as a powerful tool to study growth cone responses to guidance cues but the dynamic nature of the growth cone requires careful quantitative analysis. Space time kymographs have been developed as a tool to quantify lamellipodia dynamics in a semi-automated fashion but no such tools exist to analyze filopodial dynamics. In this work we present an algorithm to quantify filopodial dynamics from cultured neurons imaged by time-lapse fluorescence microscopy. The method is based on locating the end tips of filopodia and tracking their locations as if they were free-moving particles. The algorithm is a useful tool and should be broadly applicable to filopodial tracking from multiple cell types.

L1 interaction with ankyrin regulates mediolateral topography in the retinocollicular projection

Dynamic modulation of adhesion provided by anchorage of axonal receptors with the cytoskeleton contributes to attractant or repellent responses that guide axons to topographic targets in the brain. The neural cell adhesion molecule L1 engages the spectrin-actin cytoskeleton through reversible linkage of its cytoplasmic domain to ankyrin. To investigate a role for L1 association with the cytoskeleton in topographic guidance of retinal axons to the superior colliculus, a novel mouse strain was generated by genetic knock-in that expresses an L1 point mutation (Tyr1229His) abolishing ankyrin binding. Axon tracing revealed a striking mistargeting of mutant ganglion cell axons from the ventral retina, which express high levels of ephrinB receptors, to abnormally lateral sites in the contralateral superior colliculus, where they formed multiple ectopic arborizations. These axons were compromised in extending interstitial branches in the medial direction, a normal response to the high medial to low lateral SC gradient of ephrinB1. Furthermore, ventral but not dorsal L1(Y1229H) retinal cells were impaired for ephrinB1-stimulated adhesion through beta1 integrins in culture. The retinocollicular phenotype of the L1(Tyr1229His) mutant provides the first evidence that L1 regulates topographic mapping of retinal axons through adhesion mediated by linkage to the actin cytoskeleton and functional interaction with the ephrinB/EphB targeting system.

Spastin and microtubules: Functions in health and disease

SPG4, the gene encoding for spastin, a member of the ATPases associated with various cellular activities (AAA) family, is mutated in around 40% of cases of autosomal dominant hereditary spastic paraplegia (AD-HSP). This group of neurodegenerative diseases is characterized by a progressive spasticity and lower limb weakness with degeneration of terminal axons in cortico-spinal tracts and dorsal columns. Spastin has two main domains, a microtubule interacting and endosomal trafficking (MIT) domain at the N-terminus and the C-terminus AAA domain. Early studies suggested that spastin interacts with microtubules similarly to katanin, a member of the same subgroup of AAA. Recent evidence confirmed that spastin possesses microtubule-severing activity but can also bundle microtubules in vitro. Understanding the physiologic and pathologic involvement of these activities and their regulation is critical in the study of HSP.

A role for S1P signalling in axon guidance in the Xenopus visual system

Sphingosine 1-phosphate (S1P), a lysophospholipid, plays an important chemotactic role in the migration of lymphocytes and germ cells, and is known to regulate aspects of central nervous system development such as neurogenesis and neuronal migration. Its role in axon guidance, however, has not been examined. We show that sphingosine kinase 1, an enzyme that generates S1P, is expressed in areas surrounding the Xenopus retinal axon pathway, and that gain or loss of S1P function in vivo causes errors in axon navigation. Chemotropic assays reveal that S1P elicits fast repulsive responses in retinal growth cones. These responses require heparan sulfate, are sensitive to inhibitors of proteasomal degradation, and involve RhoA and LIM kinase activation. Together, the data identify downstream components that mediate S1P-induced growth cone responses and implicate S1P signalling in axon guidance.

Adenomatous polyposis coli is differentially distributed in growth cones and modulates their steering

Axonal steering reactions depend on the transformation of environmental information into internal, directed structures, which is achieved by differential modulation of the growth cone cytoskeleton; key elements are the microtubules, which are regulated in their dynamics by microtubule-associated proteins (MAPs). We investigated a potential role of the MAP adenomatous polyposis coli (APC) for growing axons, employing embryonic visual system as a model system. APC is concentrated in the distalmost (i.e., growing) region of retinal ganglion cell axons in vivo and in vitro. Within the growth cone, APC is enriched in the central domain; it only partially colocalizes with microtubules. When axons are induced to turn toward a cell or away from a substrate border, APC is present in the protruding and absent from the collapsing growth cone regions, thus indicating the future growth direction of the axon. To assess the functional role of the differential distribution of APC in navigating growth cones, the protein was inactivated via micro-scale chromophore-assisted laser inactivation in one half of the growth cone. If the N-terminal APC region (crucial for its oligomerization) is locally inactivated, the treated growth cone side collapses and the axon turns away. In contrast, if the 20 aa repeats in the middle region of APC (which can negatively regulate its microtubule association) are inactivated, protrusions are formed and the growth cone turns toward. Our data thus demonstrate a crucial role of APC for axon steering attributable to its multifunctional domain structure and differential distribution in the growth cone.

Semaphorin regulation of cellular morphology

Semaphorin proteins, although initially characterized as repulsive neuronal guidance cues, are now appreciated as major contributors to morphogenesis and homeostasis for a wide range of tissue types. Semaphorin-mediated long- and short-range repulsive, and attractive, guidance has profound influences on cellular morphology. The diversity of semaphorin receptor complexes utilized by various semaphorin ligands, the ability of semaphorins themselves to serve as receptors, and the myriad of intracellular signaling components that comprise semaphorin signaling cascades all contribute to cell-type-specific responses to semaphorins. Analysis of the molecular and cellular mechanisms underlying semaphorin function in neural and vascular systems provides insight into principles governing how this large protein family contributes to organogenesis, function, and disease.

Expression of unconventional myosin genes during neuronal development in zebrafish

Neuronal migration and growth cone motility are essential aspects of the development and maturation of the nervous system. These cellular events result from dynamic changes in the organization and function of the cytoskeleton, in part due to the activity of cytoskeletal motor proteins such as myosins. Although specific myosins such as Myo2 (conventional or muscle myosin), Myo1, and Myo5 have been well characterized for roles in cell motility, the roles of the majority of unconventional (other than Myo2) myosins in cell motility events have not been investigated. To address this issue, we have undertaken an analysis of unconventional myosins in zebrafish, a premier model for studying cellular and growth cone motility in the vertebrate nervous system. We describe the characterization and expression patterns of several members of the unconventional myosin gene family. Based on available genomic sequence data, we identified 18 unconventional myosin- and 4 Myo2-related genes in the zebrafish genome in addition to previously characterized myosin (1, 2, 3, 5, 6, 7) genes. Phylogenetic analyses indicate that these genes can be grouped into existing classifications for unconventional myosins from mouse and man. In situ hybridization analyses using EST probes for 18 of the 22 identified genes indicate that 11/18 genes are expressed in a restricted fashion in the zebrafish embryo. Specific myosins are expressed in particular neuronal or neuroepithelial cell types in the developing zebrafish nervous system, spanning the periods of neuronal differentiation and migration, and of growth cone guidance and motility.

Microtubule plus-end tracking proteins in differentiated mammalian cells

Differentiated mammalian cells are often characterized by highly specialized and polarized structure. Its formation and maintenance depends on cytoskeletal components, among which microtubules play an important role. The shape and dynamic properties of microtubule networks are controlled by multiple microtubule-associated factors. These include molecular motors and non-motor proteins, some of which accumulate specifically at the growing microtubule plus-ends (the so-called microtubule plus-end tracking proteins). Plus-end tracking proteins can contribute to the regulation of microtubule dynamics, mediate the cross-talk between microtubule ends, the actin cytoskeleton and the cell cortex, and participate in transport and positioning of structural and regulatory factors and membrane organelles. Malfunction of these proteins results in various human diseases including some forms of cancer, neurodevelopmental disorders and mental retardation. In this article we discuss recent data on microtubule dynamics and activities of microtubule plus-end binding proteins important for the physiology and pathology of differentiated mammalian cells such as neurons, polarized epithelia, muscle and sperm cells.

A Role for ARF6 and ARNO in the regulation of endosomal dynamics in neurons

During development, neuronal processes extend, branch and navigate to ultimately synapse with target tissue. We have shown a regulatory role for ARNO and ARF6 in dendritic branching and axonal elongation and branching during neuritogenesis, particularly with respect to cytoskeletal dynamics. Here, we have examined the role of ARF6 and the ARF GEF ARNO in endosomal dynamics during neurite elongation in hippocampal neurons. Axonal and dendritic endosomes were labeled by expression of the endosomal marker, endotubin. Expression of endotubin-green fluorescent protein resulted in targeting to tubular-vesicular structures throughout the somatodendritic and axonal domains. These endosomal structures did not colocalize with conventional early or late endosomal markers or with the synaptic vesicle marker, SV2. However, they did label with internalized lectin, indicating that they are endosomal structures. Expression of catalytically inactive ARNO (ARNO-E156K) or inactive ARF6 (ARF6-T27N) caused a redistribution of endotubin to the cell surface of the axons and dendrites. In contrast, expression of these constructs had no effect upon the distribution of SV2-positive structures. Furthermore, expression of inactive ARF1 (ARF1-T31N) did not change endotubin distribution. These results suggest that endotubin labels a distinct endosomal structure in neurons and that ARNO and ARF6 mediate neurite extension through the regulation of this compartment.

Neurobehavioural and molecular changes induced by methylmercury exposure during development

There is an increasing body of evidence on the possible environmental influence on neurodevelopmental and neurodegenerative disorders. Both experimental and epidemiological studies have demonstrated the distinctive susceptibility of the developing brain to environmental factors such as lead, mercury and polychlorinated biphenyls at levels of exposure that have no detectable effects in adults. Methylmercury (MeHg) has long been known to affect neurodevelopment in both humans and experimental animals. Neurobehavioural effects reported include altered motoric function and memory and learning disabilities. In addition, there is evidence from recent experimental neurodevelopmental studies that MeHg can induce depression-like behaviour. Several mechanisms have been suggested from in vivo- and in vitro-studies, such as effects on neurotransmitter systems, induction of oxidative stress and disruption of microtubules and intracellular calcium homeostasis. Recent in vitro data show that very low levels of MeHg can inhibit neuronal differentiation of neural stem cells. This review summarises what is currently known about the neurodevelopmental effects of MeHg and consider the strength of different experimental approaches to study the effects of environmentally relevant exposure in vivo and in vitro.

RhoA regulates dendrite branching in hippocampal neurons by decreasing cypin protein levels

The way a dendrite is patterned determines how a neuron will receive information. The Rho GTPases have been reported to play increasingly well defined roles in determining dendritic branch and spine development and morphology. Much is known about how these small GTPases regulate the actin cytoskeleton; however, very little is known about how they alter the microtubule cytoskeleton. Our laboratory previously cloned and characterized cypin, a guanine deaminase that increases dendrite number by binding to tubulin heterodimers and promoting microtubule assembly. In the present study, we show that cypin and RhoA are part of a common pathway that regulates dendrite number. Inhibition of Rho kinase activity does not affect cypin-mediated dendrite branching. Furthermore, cypin does not affect the activity of RhoA, as measured by GTP binding to RhoA. In fact, activated RhoA acts to inhibit cypin protein expression and, by doing so, decreases dendrite number. In addition, this decrease in cypin protein occurs via a translation-dependent mechanism. Together, our data suggest that cypin acts downstream of the small GTPase RhoA to regulate dendrite branching in hippocampal neurons, providing a novel mechanism for RhoA action on microtubule dynamics.

Fast regulation of axonal growth cone motility by electrical activity

Axonal growth cones are responsible for the correct guidance of developing axons and the establishment of functional neural networks. They are highly motile because of fast and continuous rearrangements of their actin-rich cytoskeleton. Here we have used live imaging of axonal growth cones of hippocampal neurons in culture and quantified their motility with a temporal resolution of 2 s. Using novel methods of analysis of growth cone dynamics, we show that transient activation of kainate receptors by bath-applied kainate induced a fast and reversible growth cone stalling. This effect depends on electrical activity and can be mimicked by the transient discharge of action potentials elicited in the neuron by intracellular current injections at the somatic level through a patch pipette. Growth cone stalling induced by electrical stimulation is mediated by calcium entry from the extracellular medium as well as by calcium release from intracellular stores that define spatially restricted microdomains directly affecting cytoskeletal dynamics. We propose that growth cone motility is dynamically controlled by transient bursts of spontaneous electrical activity, which constitutes a prominent feature of developing neural networks in vivo.

Altered expression of synaptic protein mRNAs in STOP (MAP6) mutant mice

Stable tubule-only polypeptide (STOP) proteins are a family of microtubule associated proteins (MAPs) important in microtubule stabilization. Data indicating a role for microtubules in synaptic function has come from studies of the STOP null mouse, which exhibits synaptic deficits, in association with behavioural changes that are alleviated by antipsychotic treatment. These findings suggested that STOP mutant mice may be useful in studies of synaptic function, and could be especially relevant to schizophrenia, postulated to be a disorder of the synapse. Moreover, a genetic association between STOP and schizophrenia has been reported. This study aimed to further characterize synaptic alterations in STOP null and heterozygous mice. Using in situ hybridization histochemistry, the mRNA expression of three pre-synaptic (synaptophysin; growth associated protein-43 (GAP-43); vesicular glutamate transporter-1 (VGlut1)) and two post-synaptic (spinophilin; MAP2) proteins, was quantified in female STOP null (n = 7), heterozygous (n = 5) and wild type (n = 6) mice. For STOP null and heterozygous mice, synaptophysin, VGlut1, GAP-43 and spinophilin mRNAs were decreased in the hippocampus, whilst in addition in the null mice, synaptophysin, VGlut1 and spinophilin mRNAs were decreased in the cerebellum. Alterations in synaptic protein mRNA expression were also detected in the frontal and occipital cortex. MAP2 mRNA expression was unchanged in all brain regions. The profile of mRNA changes is broadly similar to that observed in schizophrenia. Together the data provide supporting evidence for a role for microtubules in synaptic function, and suggest that STOP, or other microtubule proteins, may contribute to the synaptic pathology of schizophrenia.

Asymmetric redistribution of GABA receptors during GABA gradient sensing by nerve growth cones analyzed by single quantum dot imaging

During development of the nervous system, the tip of a growing axon, the growth cone (GC), must respond accurately to stimuli that direct its growth. This axonal navigation depends on extracellular concentration gradients of numerous guidance cues, including GABA. GCs can detect even weak directional signals, yet the mechanisms underlying this sensitivity remain unclear. Past studies in other eukaryotic chemotactic systems have pointed to the role of the spatial reorganization of the transduction pathway in their sensitive response. Here we have developed a single-molecule assay to observe individual GABA(A) receptors (GABA(A)Rs) in the plasma membrane of nerve GCs subjected to directional stimuli. We report that in the presence of an external GABA gradient GABA(A)Rs redistribute asymmetrically across the GC toward the gradient source. Single-particle tracking of GABA(A)Rs shows that the redistribution results from transient interactions between the receptors and the microtubules. Moreover, the relocalization is accompanied by an enhancement in the asymmetry of intracellular calcium concentration. Altogether, our results reveal a microtubule-dependent polarized reorganization of chemoreceptors at the cell surface and suggest that this polarization serves as an amplification step in GABA gradient sensing by nerve GCs.

Evaluation of the role of Disabled-2 in nerve growth factor-mediated neurite outgrowth and cellular signalling

Disabled-2 (DAB2) is an adapter protein that plays a key role in cell proliferation and differentiation. We reported here that DAB2 is expressed in various regions of rat central nervous system and is most abundant in the olfactory bulb. The up-regulation of DAB2 upon 5,7-dihydroxytryptamine-induced spinal cord lesion implicates that DAB2 may participate in the regulation of neuronal plasticity. To investigate DAB2 function in the regulation of neurite outgrowth, the rat p59 and p82 form of DAB2 was individually and stably expressed in the PC12 cells. Both p59 and p82 inhibited nerve growth factor (NGF)-induced neurite outgrowth concomitantly with a decrease in the expression of neuron-specific cytoskeleton protein beta-tubulin III. To unveil the molecular mechanism of DAB2 in NGF signaling, we found that RhoA-GTPase activity was up-regulated in DAB2 stable lines whereas the Ras/MAPK and PI3-kinase/Akt signaling was not affected. The inhibitory effect of DAB2 on NGF-mediated neurite outgrowth was reversed by the pretreatment of Rho-kinase (ROCK) inhibitor Y27632, implicating that DAB2 modulates RhoA/ROCK signaling. Together, this study defines a role of DAB2 in the control of neuronal plasticity and demonstrates for the first time that DAB2 is a negative regulator in NGF-mediated neurite outgrowth.

Drosophila as a genetic and cellular model for studies on axonal growth

One of the most fascinating processes during nervous system development is the establishment of stereotypic neuronal networks. An essential step in this process is the outgrowth and precise navigation (pathfinding) of axons and dendrites towards their synaptic partner cells. This phenomenon was first described more than a century ago and, over the past decades, increasing insights have been gained into the cellular and molecular mechanisms regulating neuronal growth and navigation. Progress in this area has been greatly assisted by the use of simple and genetically tractable invertebrate model systems, such as the fruit fly Drosophila melanogaster. This review is dedicated to Drosophila as a genetic and cellular model to study axonal growth and demonstrates how it can and has been used for this research. We describe the various cellular systems of Drosophila used for such studies, insights into axonal growth cones and their cytoskeletal dynamics, and summarise identified molecular signalling pathways required for growth cone navigation, with particular focus on pathfinding decisions in the ventral nerve cord of Drosophila embryos. These Drosophila-specific aspects are viewed in the general context of our current knowledge about neuronal growth.

The F-actin-microtubule crosslinker Shot is a platform for Krasavietz-mediated translational regulation of midline axon repulsion

Axon extension and guidance require a coordinated assembly of F-actin and microtubules as well as regulated translation. The molecular basis of how the translation of mRNAs encoding guidance proteins could be closely tied to the pace of cytoskeletal assembly is poorly understood. Previous studies have shown that the F-actin-microtubule crosslinker Short stop (Shot) is required for motor and sensory axon extension in the Drosophila embryo. Here, we provide biochemical and genetic evidence that Shot functions with a novel translation inhibitor, Krasavietz (Kra, Exba), to steer longitudinally directed CNS axons away from the midline. Kra binds directly to the C-terminus of Shot, and this interaction is required for the activity of Shot to support midline axon repulsion. shot and kra mutations lead to weak robo-like phenotypes, and synergistically affect midline avoidance of CNS axons. We also show that shot and kra dominantly enhance the frequency of midline crossovers in embryos heterozygous for slit or robo, and that in kra mutant embryos, some Robo-positive axons ectopically cross the midline that normally expresses the repellent Slit. Finally, we demonstrate that Kra also interacts with the translation initiation factor eIF2beta and inhibits translation in vitro. Together, these data suggest that Kra-mediated translational regulation plays important roles in midline axon repulsion and that Shot functions as a direct physical link between translational regulation and cytoskeleton reorganization.

Kinetic-structural analysis of neuronal growth cone veil motility

Neuronal growth cone advance was investigated by correlative light and electron microscopy carried out on chick dorsal root ganglion cells. Advance was analyzed in terms of the two principal organelles responsible for protrusive motility in the growth cone – namely, veils and filopodia. Veils alternated between rapid phases of protrusion and retraction. Electron microscopy revealed characteristic structural differences between the phases. Our results provide a significant advance in three respects: first, protruding veils are comprised of a densely branched network of actin filaments that is lamellipodial in appearance and includes the Arp2/3 complex. On the basis of this structural and biomarker evidence, we infer that the dendritic nucleation and/or array-treadmilling mechanism of protrusive motility is conserved in veil protrusion of growth cones as in the motility of fibroblasts; second, retracting veils lack dendritic organization but contain a sparse network of long filaments; and third, growth cone filopodia have the capacity to nucleate dendritic networks along their length, a property consistent with veil formation seen at the light microscopic level but not previously understood in supramolecular terms. These elements of veil and filopodial organization, when taken together, provide a conceptual framework for understanding the structural basis of growth cone advance.

The spinal muscular atrophy gene product regulates neurite outgrowth: importance of the C terminus

Spinal muscular atrophy is a neurodegenerative disease accompanied by a loss of motoneurons. Either mutations or deletions in the survival of motoneuron (SMN) gene are responsible for this defect. SMN is an assembly protein for RNA-protein complexes in the nucleus and is also found in axons of neurons. However, it is unclear which dysfunctions of SMN are important for disease progression. In this study we analyzed the contributions of different SMN regions for localization and neuronal differentiation associated with outgrowth of neurites. Suppression of endogenous SMN protein levels significantly decreased the growth of neurites. Down-regulation of the interacting protein gemin2 had the opposite effect. Surprisingly, selective overexpression of the SMN C-terminal domain promoted neurite outgrowth similar to full-length protein and could rescue the SMN knock-down effects. The knock-down led to a significant change in the G-/F-actin ratio, indicating a role for SMN in actin dynamics. Therefore, our data suggest a functional role for SMN in microfilament metabolism in axons of motoneurons.

A role of local signalling in the establishment and maintenance of the asymmetrical architecture of a neuron

Significant progress has been made in the identification of intrinsic and extrinsic factors involved in the development of nervous system. It is remarkable that the establishment and maintenance of the asymmetrical architecture of a neuron is coordinated by a limited repertoire of signalling machineries. However, the details of signalling mechanisms responsible for creating specificity and diversity required for proper development of the nervous system remain largely to be investigated. An emerging body of evidence suggests that specificity and diversity can be achieved by differential regulation of signalling components at distinct subcellular localizations. Many aspects of neuronal polarization and morphogenesis are attributed to localized signalling. Further diversity and specificity of receptor signalling can be achieved by the regulation of molecules outside the cell. Recent evidence suggests that extracellular matrix molecules are essential extrinsic cues that function to foster the growth of neurons. Therefore, it is important to understand where the signalling machineries are activated and how they are combined with other factors in order to understand the molecular mechanism underlying neuronal development.

Cdc42 and ARP2/3-independent regulation of filopodia by an integral membrane lipid-phosphatase-related protein

Filopodia are dynamic cell surface protrusions that are required for proper cellular development and function. We report that the integral membrane protein lipid-phosphatase-related protein 1 (LPR1) localizes to and promotes the formation of actin-rich, dynamic filopodia, both along the cell periphery and the dorsal cell surface. Regulation of filopodia by LPR1 was not mediated by cdc42 or Rif, and is independent of the Arp2/3 complex. We found that LPR1 can induce filopodia formation in the absence of the Ena/Vasp family of proteins, suggesting that these molecules are not essential for the development of the protrusions. Mutagenesis experiments identified residues and regions of LPR1 that are important for the induction of filopodia. RNA interference experiments in an ovarian epithelial cancer cell line demonstrated a role for LPR1 in the maintenance of filopodia-like membrane protrusions. These observations, and our finding that LPR1 is a not an active lipid phosphatase, suggest that LPR1 may be a novel integral membrane protein link between the actin core and the surrounding lipid layer of a nascent filopodium.

Calcium signaling in neuronal motility

Neuronal motility is a fundamental feature that underlies the development, regeneration, and plasticity of the nervous system. Two major developmental events--directed migration of neuronal precursor cells to the proper positions and guided elongation of axons to their target cells--depend on large-scale neuronal motility. At a finer scale, motility is also manifested in many aspects of neuronal structures and functions, ranging from differentiation and refinement of axonal and dendritic morphology during development to synapse remodeling associated with learning and memory in the adult brain. As a primary second messenger that conveys the cytoplasmic actions of electrical activity and many neuroactive ligands, Ca(2+) plays a central role in the regulation of neuronal motility. Recent studies have revealed common Ca(2+)-dependent signaling pathways that are deployed for regulating cytoskeletal dynamics associated with neuronal migration, axon and dendrite development and regeneration, and synaptic plasticity.

Transient directed motions of GABA(A) receptors in growth cones detected by a speed correlation index

Single-molecule tracking of membrane proteins has become an important tool for investigating dynamic processes in live cells, such as cell signaling, membrane compartmentation or trafficking. The extraction of relevant parameters, such as interaction times between molecular partners or confinement-zone sizes, from the trajectories of single molecules requires appropriate statistical methods. Here we report a new tool, the speed correlation index, designed to detect transient periods of directed motion within trajectories of diffusing molecules. The ability to detect such events in a wide range of biologically relevant parameter values (speed, diffusion coefficient, and durations of the directed period) was first established on simulated data. The method was next applied to analyze the trajectories of quantum-dot-labeled GABA(A) receptors in nerve growth cones. The use of the speed correlation index revealed that the receptors had a "conveyor belt" type of motion due to temporary interactions ( approximately 4.0 s) between the receptors and the microtubules, leading to an average directed motion (velocity approximately 0.3 mum s(-1)) in the growth-cone membrane. Our observations point to the possibility of a cytoskeleton-dependent redistribution of the sensing molecules in the membrane, which could play a role in the modulation of the cell response to external signals.

Contribution of mGluR and Fmr1 functional pathways to neurite morphogenesis, craniofacial development and fragile X syndrome

Fragile X Syndrome is a leading heritable cause of mental retardation that results from the loss of FMR1 gene function. Studies in mouse and Drosophila model organisms have been critical in understanding many aspects of the loss of function of the FMR1 gene in the human syndrome. Here, we establish that the zebrafish is a useful model organism for the study of the human fragile X syndrome and can be used to examine phenotypes that are difficult or inaccessible to observation in other model organisms. Using morpholino knockdown of the fmr1 gene, we observed abnormal axonal branching of Rohon-Beard and trigeminal ganglion neurons and guidance and defasciculation defects in the lateral longitudinal fasciculus. We demonstrate that this axonal branching defect can be rescued by treatment with MPEP [2-methyl-6-(phenylethynyl) pyridine]. This is consistent with an interaction between mGluR signalling and fmr1 function in neurite morphogenesis. We also describe novel findings of abnormalities in the abundance of trigeminal ganglion neurons and of craniofacial abnormalities apparently due to dysmorphic cartilage formation. These abnormalities may be related to a role for fmr1 in neural crest cell specification and possibly in migration.

The Drosophila microtubule associated protein Futsch is phosphorylated by Shaggy/Zeste-white 3 at an homologous GSK3β phosphorylation site in MAP1B

The Drosophila homologue of the microtubule associated protein MAP1B is encoded by the futsch locus. The deduced protein Futsch is about twice the size of MAP1B and shows high homology in the N- and C-terminal domains. The central part of Futsch is characterized by a highly repetitive structure based on a 37 amino acid motif. Futsch, like MAP1B, colocalizes with microtubules and is necessary for the organization of the microtubule cytoskeleton during axonal growth and synaptogenesis. To further analyze the functional relevance of Futsch as a MAP1B-like protein, we performed a molecular analysis of the conserved protein domains. Using a number of antisera, we show that, unlike the MAP1B polyprotein, which is cleaved to generate a heavy and light chain, Futsch is expressed as a single protein. The function of MAP1B is in part regulated by phosphorylation mediated by kinases that include casein kinase 2 and glycogen synthase kinase 3beta (GSK3beta). We show here that at least one GSK3beta phosphorylation site of MAP1B is conserved in Futsch and that this site can be phosphorylated by GSK3beta and its Drosophila homologue, Shaggy/Zeste-white 3. To test the functional relevance of these findings we generated a number of minigenes and assayed their ability to rescue the phenotype of futsch mutants. Our data highlight some differences between MAP1B and Futsch but demonstrate that important structural and functional aspects are conserved between fly and vertebrate members of this protein family.

Phenotypes of Drosophila brain neurons in primary culture reveal a role for fascin in neurite shape and trajectory

Subtle cellular phenotypes in the CNS may evade detection by routine histopathology. Here, we demonstrate the value of primary culture for revealing genetically determined neuronal phenotypes at high resolution. Gamma neurons of Drosophila melanogaster mushroom bodies (MBs) are remodeled during metamorphosis under the control of the steroid hormone 20-hydroxyecdysone (20E). In vitro, wild-type gamma neurons retain characteristic morphogenetic features, notably a single axon-like dominant primary process and an arbor of short dendrite-like processes, as determined with microtubule-polarity markers. We found three distinct genetically determined phenotypes of cultured neurons from grossly normal brains, suggesting that subtle in vivo attributes are unmasked and amplified in vitro. First, the neurite outgrowth response to 20E is sexually dimorphic, being much greater in female than in male gamma neurons. Second, the gamma neuron-specific "naked runt" phenotype results from transgenic insertion of an MB-specific promoter. Third, the recessive, pan-neuronal "filagree" phenotype maps to singed, which encodes the actin-bundling protein fascin. Fascin deficiency does not impair the 20E response, but neurites fail to maintain their normal, nearly straight trajectory, instead forming curls and hooks. This is accompanied by abnormally distributed filamentous actin. This is the first demonstration of fascin function in neuronal morphogenesis. Our findings, along with the regulation of human Fascin1 (OMIM 602689) by CREB (cAMP response element-binding protein) binding protein, suggest FSCN1 as a candidate gene for developmental brain disorders. We developed an automated method of computing neurite curvature and classifying neurons based on curvature phenotype. This will facilitate detection of genetic and pharmacological modifiers of neuronal defects resulting from fascin deficiency.

An essential role for beta-actin mRNA localization and translation in Ca2+-dependent growth cone guidance

Axon pathfinding requires directional responses of growth cones to extracellular cues, which have been shown to involve local synthesis of protein. The identity and functions of the locally produced proteins remain, however, unclear. Here we report that Ca(2+)-dependent bidirectional turning of Xenopus laevis growth cones requires localized distribution and translation of beta-actin messenger RNA. Both beta-actin mRNA and its zipcode-binding protein, ZBP1, are localized at the growth cone and become asymmetrically distributed upon local exposure to brain-derived neurotrophic factor (BDNF). Inhibition of protein synthesis or antisense interference with beta-actin mRNA-ZBP1 binding abolishes both Ca(2+)-mediated attraction and repulsion. In addition, attraction involves a local increase in beta-actin, whereas repulsion is accompanied by a local decrease in beta-actin; thus, both produce a synthesis- and ZBP1 binding-dependent beta-actin asymmetry but with opposite polarities. Together with a similar asymmetry in Src activity during bidirectional responses, our findings indicate that Ca(2+)-dependent spatial regulation of beta-actin synthesis through Src contributes to the directional motility of growth cones during guidance.

Repulsion or adhesion: receptors make the call

Repulsive signaling plays a prominent role in regulating cell-cell interactions and is fundamental to multiple developmental processes. A proper balance between repulsion from and adhesion to other cells or the extracellular matrix is also important. Semaphorin-Plexin and ephrin-Eph ligand-receptor pairs compose two major repulsive signaling systems. Recent advances have elucidated mechanisms by which Semaphorin-Plexin and ephrin-Eph signaling control repulsion versus adhesion. Semaphorins act through a complex signaling pathway to inhibit integrin-mediated adhesion, allowing cell repulsion. Ephrin-Eph interactions can directly mediate cell adhesion and several mechanisms control whether ephrin-Eph binding and signaling induces repulsion or adhesion.

(Make) stick and cut loose--disintegrin metalloproteases in development and disease

"A disintegrin and metalloprotease" (ADAM) proteases form a still growing family of about 40 type 1 transmembrane proteins. They are defined by a common modular ectodomain architecture that combines cell deadhesion/adhesion and fusion motifs (disintegrin and cysteine-rich domains), with a Zn-protease domain capped by a large prodomain. Their ectodomain thus strikingly resembles snake venom disintegrin proteases, which by combined integrin blocking and extracellular proteolysis, can cause extensive tissue damage after snake bites. A surprisingly large proportion (13 ADAMs) is exclusively expressed in the male gonads, and only a minority can be found throughout all tissues. As predicted by their amino acid sequence, a major proportion of this family has not maintained a functional protease domain, most probably rendering them into pure adhesion and/or fusion proteins. For most ADAMs, the respective key function has remained elusive. Despite their overall conserved ectodomain structure, ADAMs appear to be subdivided into those with a predominant role in direct adhesion (e.g., ADAMs 1, 2, and 3) and those mainly acting as proteases (e.g., ADAMs 10 and 17). Only for a few of them are functions of more than one domain documented (e.g., ADAM9 in cell fusion and proteolysis). Several ADAMs exist in both membrane-resident and secreted isoforms; the functional significance of this dichotomy is in most cases still unclear. Knockout phenotypes have been informative only in a few cases (ADAMs 1, 2, 10, 12, 15, 17, and 19) and are mainly related to their protease function. A common denominator of ADAM-mediated proteolysis is the ectodomain shedding of a broad spectrum of substrates, including paracrine growth factors like epidermal growth factor receptor (EGFR) ligands, cell adhesion molecules like CD44 or cadherins, and the initiation of regulated intramembrane proteolysis (RIP), whereby the transmembrane fragment of the respective substrate is further cleaved by an intramembrane cleaving protease to release an intracellular domain acting as a nuclear transcription regulator. Most ADAMs feature a significant overlap of substrate specificities, explaining why an inactivation of individual ADAMs only rarely causes major phenotypes.

Directional guidance of nerve growth cones

The intricate connections of the nervous system are established, in part, by elongating axonal fibers that are directed by complex guidance systems to home in on their specific targets. The growth cone, the major motile apparatus at the tip of axons, explores its surroundings and steers the axon along a defined path to its appropriate target. Significant progress has been made in identifying the guidance molecules and receptors that regulate growth cone pathfinding, the signaling cascades underlying distinct growth cone behaviors, and the cytoskeletal components that give rise to the directional motility of the growth cone. Recent studies have also shed light on the sophisticated mechanisms and new players utilized by the growth cone during pathfinding. It is clear that axon pathfinding requires a growth cone to sample and integrate various signals both in space and in time, and subsequently to coordinate the dynamics of its membrane, cytoskeleton and adhesion to generate specific responses.