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Small oligomeric forms of amyloid-β (Aβ) are believed to be the culprit for declined brain functions in AD in part through their impairment of neuronal trafficking and synaptic functions. However, the precise cellular actions of Aβ oligomers and underlying mechanisms in neurons remain to be fully defined. Previous studies have identified mitochondria as a major target of Aβ toxicity contributing to early cognitive decline and memory loss in neurodegenerative diseases including Alzheimer's disease (AD). In this study, we report that Aβ oligomers acutely elicit distinct effects on the transport and integrity of mitochondria. We found that acute exposure of hippocampal neurons to Aβ oligomers from either synthetic peptides or AD brain homogenates selectively impaired fast transport of mitochondria without affecting the movement of late endosomes and lysosomes. Extended exposure of hipoocampal neurons to Aβ oligomers was found to result in mitochondrial fragmentation. While both mitochondrial effects induced by Aβ oligomers can be abolished by the inhibition of GSK3β, they appear to be independent from each other. Aβ oligomers impaired mitochondrial transport through HDAC6 activation whereas the fragmentation involved the GTPase Drp-1. These results show that Aβ oligomers can acutely disrupt mitochondrial transport and integrity in a time-dependent and pathway-specific manner. These findings thus provide new insights into Aβ-induced mitochondrial defects that may contribute to neuronal dysfunction and AD pathogenesis. [ABSTRACT FROM AUTHOR]

Background Stromal interaction molecule 1 (STIM1), a Ca2+ sensor in the endoplasmic reticulum, regulates store-operated Ca2+ entry (SOCE) that is essential for Ca2+ homeostasis in many types of cells. However, if and how STIM1 and SOCE function in nerve growth cones during axon guidance remains to be elucidated. Results We report that STIM1 and transient receptor potential channel 1 (TRPC1)-dependent SOCE operates in Xenopus spinal growth cones to regulate Ca2+ signaling and guidance responses. We found that STIM1 works together with TRPC1 to mediate SOCE within growth cones and filopodia. In particular, STIM1/TRPC1-dependent SOCE was found to mediate oscillatory filopodial Ca2+ transients in the growth cone. Disruption of STIM1 function abolished filopodial Ca2+ transients and impaired Ca2+-dependent attractive responses of Xenopus growth cones to netrin-1. Finally, interference with STIM1 function was found to disrupt midline axon guidance of commissural interneurons in the developing Xenopus spinal cord in vivo. Conclusions Our data demonstrate that STIM1/TRPC1-dependent SOCE plays an essential role in generating spatiotemporal Ca2+ signals that mediate guidance responses of nerve growth cones. [ABSTRACT FROM AUTHOR]

Growth cones, found at the tip of axonal projections, are the sensory and motile organelles of developing neurons that enable axon pathfinding and target recognition for precise wiring of the neural circuitry. To date, many families of conserved guidance molecules and their corresponding receptors have been identified that work in space and time to ensure billions of axons to reach their targets. Research in the past two decades has also gained significant insight into the ways in which growth cones translate extracellular signals into directional migration. This review aims to examine new progress toward understanding the cellular mechanisms underlying directional motility of the growth cone and to discuss questions that remain to be addressed. Specifically, we will focus on the cellular ensemble of cytoskeleton, adhesion, and membrane and examine how the intricate interplay between these processes orchestrates the directed movement of growth cones. [ABSTRACT FROM AUTHOR]

Abstract: Axial tension and transverse compression experiments on single fibers were performed to investigate the mechanical behavior of three high-performance fibers (Kevlar®, Kevlar® 129, and Twaron®) with diameters in the order of 9–12 μm. The single fibers were manufactured from 1998 through 2008. A miniaturized tensile Kolsky bar was used to determine the tensile response of PPTA single fibers at a high strain rate. Gage length and strain rate were found to have minimum effects on the tensile strength of PPTA single fibers. Manufacturing time over a decade was found to have negligible effects on the tensile strength of the fibers. Initial transverse compression on the fibers reduces their ultimate tensile strengths. A high resolution scanning electron microscope (SEM) was also used to examine the fracture modes of transversely deformed fibers. Different types of fracture morphology were observed. [Copyright &y& Elsevier]

The mechanical behavior of A265 high-performance fibers was experimentally investigated at both low and high strain rates. Axial, transverse, and torsional experiments were performed to measure the five material constants on a single fiber assumed as a linear, transversely isotropic material. In order to determine the tensile response of the fiber at high rates, a modified Kolsky tensile bar, also known as a split Hopkinson tension bar (SHTB) for single-fiber tests, was used. The diameter of each fiber was measured individually using a high-resolution scanning electron microscope for accurate stress calculation. A pulse shaper technique was adopted to generate a smooth and constant-amplitude incident pulse to produce deformation in the fiber specimen at a nearly constant strain rate. The tensile strength of the fiber exhibits both rate and gage-length effects. [ABSTRACT FROM AUTHOR]

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 filter 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, Ca2+ plays a central role in the regulation of neuronal motility. Recent studies have revealed common Ca2+ -dependent signaling pathways that are deployed for regulating cytoskeletal dynamics associated with neuronal migration, axon and dendrite development and regeneration, and synaptic plasticity. [ABSTRACT FROM AUTHOR]

A key question in brain development is how migration of neuronal precursors is guided to establish the ordered laminar layers. In the April 20, 2007 issue of Cell, Guan et al. show that the leading process of migrating cerebellar granule neurons senses repulsive Slit molecules by generating a Ca2+ wave that propagates to the soma to cause reversal of cell polarity and migration. [Copyright &y& Elsevier]

Ca(2+) signals have profound and varied effects on growth cone motility and guidance. Modulation of Ca(2+) influx and release from stores by guidance cues shapes Ca(2+) signals, which determine the activation of downstream targets. Although the precise molecular mechanisms that underlie distinct Ca(2+)-mediated effects on growth cone behaviours remain unclear, recent studies have identified important players in both the regulation and targets of Ca(2+) signals in growth cones. [ABSTRACT FROM AUTHOR]

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. [Copyright &y& Elsevier]

Presents the results of a study on the use of focal laser-induced photolysis (FLIP) of caged intracellular calcium ions to turn the motile tip, the nerve growth cone. Effect of a direct, spatially restricted elevation of intracellular Ca concentration on side of the growth cone; Impact through the integration of local and global Ca signals on turning responses under different environmental conditions during guidance.

Reports on the positive turning responses of the nerve growth cone in a defined extracellular gradient of the neurotransmitter acetylcholine (ACh). Dependence of the growth response on the activation of neuronal nicotinic ACh receptors; Results of fluorescence imaging of cytosolic Ca2+ concentration.

Examines the long-range signaling in developing neurons after local elevation of cyclic adenylic acid-dependent activity. Local perfusion of the growth cone with forksolin; Effects of uniform exposure to 8-bromoadenosine 3',5'-cyclic monophosphate (8-br-cAMP); Effects on neurite extension of the growth cone by local perfusion of the soma with 8-br-cAMP.

The reduced performance of B4C armor plate for impact against tungsten carbide penetrators beyond a critical velocity has been attributed in the literature to localized amorphization. However, it is unclear if this reduction in strength is a consequence of high pressure or high velocity. Despite numerous fundamental studies of B4C under indentation and impact, the roles of strain rate and pressure on amorphization have not been fully established. Toward this end, rate dependent uniaxial compressive strength and rate dependent indentation hardness, along with Raman spectroscopy, have been employed to show that high strain rate deformation alone (without concurrent high pressure) cannot trigger localized amorphization in B4C. Based on our analysis, it is also suggested that rate dependent indentation hardness can be used to reveal if a given B4C ceramic exhibits amorphization under high pressure and high strain rate loading. It is argued that when amorphization does occur in B4C, its dynamic inelastic properties degrade more severely than its static properties. Finally, it is suggested that dynamic hardness, in conjunction with static hardness, can be used as a measurable mechanical property to reveal the incidence of amorphization in B4C without the need for postmortem TEM or Raman spectroscopy analyses. [ABSTRACT FROM AUTHOR]

We exposed a headform instrumented with 10 pressure sensors mounted flush with the surface to a shock wave with three nominal intensities: 70, 140 and 210 kPa. The headform was mounted on a Hybrid III neck, in a rigid configuration to eliminate motion and associated pressure variations. We evaluated the effect of the test location by placing the headform inside, at the end and outside of the shock tube. The shock wave intensity gradually decreases the further it travels in the shock tube and the end effect degrades shock wave characteristics, which makes comparison of the results obtained at three locations a difficult task. To resolve these issues, we developed a simple strategy of data reduction: the respective pressure parameters recorded by headform sensors were divided by their equivalents associated with the incident shock wave. As a result, we obtained a comprehensive set of non-dimensional parameters. These non-dimensional parameters (or amplification factors) allow for direct comparison of pressure waveform characteristic parameters generated by a range of incident shock waves differing in intensity and for the headform located in different locations. Using this approach, we found a correlation function which allows prediction of the peak pressure on the headform that depends only on the peak pressure of the incident shock wave (for specific sensor location on the headform), and itis independent on the headform location. We also found a similar relationship for the rise time. However, for the duration and impulse, comparable correlation functions do not exist. These findings using a headform with simplified geometry are baseline values and address a need for the development of standardized parameters for the evaluation of personal protective equipment (PPE) under shock wave loading. [ABSTRACT FROM AUTHOR]

Nerve growth cones, the dilated tip of developing axons, are equipped with exquisite abilities to sense environmental cues and to move rapidly through complex terrains of developing brain, leading the axons to their specific targets for precise neuronal wiring. The actin cytoskeleton is the major component of the growth cone that powers its directional motility. Past research has provided significant insights into the mechanisms by which growth cones translate extracellular signals into directional migration. In this review, we summarize the actin-based mechanisms underlying directional growth cone motility, examine novel findings, and discuss the outstanding questions concerning the actin-based growth cone behaviors. [ABSTRACT FROM AUTHOR]

Raman spectroscopy was used to characterize the microstructure of reaction bonded boron carbide ceramics. Compositional and structural gradation in the silicon-doped boron carbide phase (rim), which develops around the parent boron carbide region (core) due to the reaction between silicon and boron carbide, was evaluated using changes in Raman peak position and intensity. Peak shifting and intensity variation from the core to the rim region was attributed to changes in the boron carbide crystal structure based on experimental Raman observations and ab initio calculations reported in literature. The results were consistent with compositional analysis determined by energy dispersive spectroscopy. The Raman analysis revealed the substitution of silicon atoms first into the linear 3-atom chain, and then into icosahedral units of the boron carbide structure. Thus, micro-Raman spectroscopy provided a non-destructive means of identifying the preferential positions of Si atoms in the boron carbide lattice. [ABSTRACT FROM AUTHOR]

Despite recent efforts to understand blast effects on the human brain, there are still no widely accepted injury criteria for humans. Recent animal studies have resulted in important advances in the understanding of brain injury due to intense dynamic loads. However, the applicability of animal brain injury results to humans remains uncertain. Here, we use advanced computational models to derive a scaling law relating blast wave intensity to the mechanical response of brain tissue across species. Detailed simulations of blast effects on the brain are conducted for different mammals using image-based biofidelic models. The intensity of the stress waves computed for different external blast conditions is compared across species. It is found that mass scaling, which successfully estimates blast tolerance of the thorax, fails to capture the brain mechanical response to blast across mammals. Instead, we show that an appropriate scaling variable must account for the mass of protective tissues relative to the brain, as well as their acoustic impedance. Peak stresses transmitted to the brain tissue by the blast are then shown to be a power function of the scaling parameter for a range of blast conditions relevant to TBI. In particular, it is found that human brain vulnerability to blast is higher than for any other mammalian species, which is in distinct contrast to previously proposed scaling laws based on body or brain mass. An application of the scaling law to recent experiments on rabbits furnishes the first physics-based injury estimate for blast-induced TBI in humans. [ABSTRACT FROM AUTHOR]

Mild head trauma, including concussion, can lead to chronic brain dysfunction and degeneration but the underlying mechanisms remain poorly understood. Here, we developed a novel head impact system to investigate the long-term effects of mild head trauma on brain structure and function, as well as the underlying mechanisms in Drosophila melanogaster. We find that Drosophila subjected to repetitive head impacts develop long-term deficits, including impaired startle-induced climbing, progressive brain degeneration, and shortened lifespan, all of which are substantially exacerbated in female flies. Interestingly, head impacts elicit an elevation in neuronal activity and its acute suppression abrogates the detrimental effects in female flies. Together, our findings validate Drosophila as a suitable model system for investigating the long-term effects of mild head trauma, suggest an increased vulnerability to brain injury in female flies, and indicate that early altered neuronal excitability may be a key mechanism linking mild brain trauma to chronic degeneration. [ABSTRACT FROM AUTHOR]

Abstract: We adopted a non-contact laser technique to measure axial small strain (<∼5%) of Kevlar® 129 fibers in a miniaturized tension Kolsky bar. Accurate small strain measurement is necessary to determine the elastic response of the fibers. In this optical arrangement, a laser beam passes through a line generator lens, which is known as a Powell glass, yielding a thin sheet of light with uniform intensity. This laser sheet passes through a gap formed by the fixture holding the fiber specimen. As the specimen is stretched, an increasing amount of light passes through the gap, which is measured by a high-speed, high resolution photo-detector mounted on the other side of the gap. Dynamic tensile experiments performed at a constant strain rate of ∼1500/s on Kevlar® 129 fibers demonstrate the capability of this non-contact laser technique in the determination of dynamic tensile strain histories with low amplitudes. [Copyright &y& Elsevier]

It is generally believed that only the actin cytoskeleton resides in dendritic spines and controls spine morphology and plasticity. Here, we report that microtubules (MTs) are present in spines and that shRNA knockdown of the MT plus-end-binding protein EB3 significantly reduces spine formation. Furthermore, stabilization and inhibition of MTs by low doses of taxol and nocodazole enhance and impair spine formation elicited by BDNF (brain-derived neurotrophic factor), respectively. Therefore, MTs play an important role in the control and regulation of dendritic spine development and plasticity. [ABSTRACT FROM AUTHOR]

Neuronal dendrites have specialized actin-rich structures called dendritic spines that receive and integrate most excitatory synaptic inputs. The stabilization of dendrites and spines during neuronal maturation is essential for proper neural circuit formation. Changes in dendritic morphology and stability are largely mediated by regulation of the actin cytoskeleton; however, the underlying mechanisms remain to be fully elucidated. Here, we present evidence that the nebulin family members LASPI and LASP2 play an important role in the postsynaptic development of rat hippocampal neurons from both sexes. We find that both LASPI and LASP2 are enriched in dendritic spines, and their knockdown impairs spine development and synapse formation. Furthermore, LASP2 exerts a distinct role in dendritic arbor and dendritic spine stabilization. Importantly, the actin-binding N-terminal LIM domain and nebulin repeats of LASP2 are required for spine stability and dendritic arbor complexity. These findings identify LASPI and LASP2 as novel regulators of neuronal circuitry. [ABSTRACT FROM AUTHOR]

Neurons of the CNS elaborate highly branched dendritic arbors that host numerous dendritic spines, which serve as the postsynaptic platform for most excitatory synapses. The actin cytoskeleton plays an important role in dendrite development and spine formation, but the underlying mechanisms remain incompletely understood. Tropomodulins (Tmods) are a family of actin-binding proteins that cap the slow-growing (pointed) end of actin filaments, thereby regulating the stability, length, and architecture of complex actin networks in diverse cell types. Three members of the Tmod family, Tmodl, Tmod2, and Tmod3 are expressed in the vertebrate CNS, but their function in neuronal development is largely unknown. In this study, we present evidence that Tmodl and Tmod2 exhibit distinct roles in regulating spine development and dendritic arborization, respectively. Using rat hippocampal tissues from both sexes, we find that Tmodl and Tmod2 are expressed with distinct developmental profiles: Tmod2 is expressed early during hippocampal development, whereas Tmodl expression coincides with synaptogenesis. We then show that knockdown of Tmod2, but not Tmodl, severely impairs dendritic branching. Both Tmodl and Tmod2 are localized to a distinct subspine region where they regulate local F-actin stability. However, the knockdown of Tmodl, but not Tmod2, disrupts spine morphogenesis and impairs synapse formation. Collectively, these findings demonstrate that regulation of the actin cytoskeleton by different members of the Tmod family plays an important role in distinct aspects of dendrite and spine development. [ABSTRACT FROM AUTHOR]

This paper describes a new witness material for quantifying the back face deformation (BFD) resulting from high rate impact of ballistic protective equipment. Accurate BFD quantification is critical for the assessment and certification of personal protective equipment, such as body armor and helmets, and ballistic evaluation. A common witness material is ballistic clay, specifically, Roma Plastilina No. 1 (RP1). RP1 must be heated to nearly 38°C to pass calibration, and used within a limited time frame to remain in calibration. RP1 also exhibits lot-to-lot variability and is sensitive to time, temperature, and handling procedures, which limits the BFD accuracy and reproducibility. A new silicone composite backing material (SCBM) was developed and tested side-by-side with heated RP1 using quasi-static indentation and compression, low velocity impact, spherical projectile penetration, and both soft and hard armor ballistic BFD measurements to compare their response over a broad range of strain rates and temperatures. The results demonstrate that SCBM mimics the heated RP1 response at room temperature and exhibits minimal temperature sensitivity. With additional optimization of the composition and processing, SCBM could be a drop-in replacement for RP1 that is used at room temperature during BFD quantification with minimal changes to the current RP1 handling protocols and infrastructure. It is anticipated that removing the heating requirement, and temperature-dependence, associated with RP1 will reduce test variability, simplify testing logistics, and enhance test range productivity. [ABSTRACT FROM AUTHOR]

Dendritic spines are small postsynaptic compartments of excitatory synapses in the vertebrate brain that are modified during learning, aging, and neurological disorders. The formation and modification of dendritic spines depend on rapid assembly and dynamic remodeling of the actin cytoskeleton in this highly compartmentalized space, but the precise mechanisms remain to be fully elucidated. In this study, we report that spatiotemporal enrichment of actin monomers (G-actin) in dendritic spines regulates spine development and plasticity. We first show that dendritic spines contain a locally enriched pool of G-actin that can be regulated by synaptic activity. We further find that this G-actin pool functions in spine development and its modification during synaptic plasticity. Mechanistically, the relatively immobile G-actin pool in spines depends on the phosphoinositide PI(3,4,5)P3 and involves the actin monomer-binding protein profilin. Together, our results have revealed a novel mechanism by which dynamic enrichment of G-actin in spines regulates the actin remodeling underlying synapse development and plasticity. [ABSTRACT FROM AUTHOR]

This paper investigates the effect of the inter-fiber friction coefficient on fabric ballistic performance. Firstly, dynamic stress responses within a fabric due to first fiber failure are analyzed. Relations between inter-fiber friction and progressive fiber failure are assessed. Then, ballistic perforation processes of Kevlar KM2 fabrics are simulated using a fiber level micro-approach, the digital element approach (DEA). In this model, each yarn is discretized into many fibers and each fiber is divided into many rod elements. A Monte Carlo process is utilized to assign a unique strength to each element following a bimodal Weibull distribution function. DEA is used to simulate the ballistic perforation processes. A comparative investigation using a broad range of inter-fiber friction coefficients are conducted. Relations of ballistic limits and inter-fiber friction coefficients are presented. Simulation results show that ballistic limit improves as the inter-fiber friction coefficient increases up to a critical value. Beyond that point, ballistic strength decreases slightly as the inter-fiber friction coefficient increases. The inter-fiber friction also changes the ballistic perforation mechanisms. [ABSTRACT FROM AUTHOR]

Two distinct protein cofactors, p35 and p39, independently activate Cyclin-dependent kinase 5 (Cdk5), which plays diverse roles in normal brain function and the pathogenesis of many neurological diseases. The initial discovery that loss of p35 impairs neuronal migration in the embryonic brain prompted intensive research exploring the function of p35-dependent Cdk5 activity. In contrast, p39 expression is restricted to the postnatal brain and its function remains poorly understood. Despite the robustly increased Cdk5 activity during neuronal differentiation, which activator is responsible for enhancing Cdk5 activation and how the two distinct activators direct Cdk5 signaling to govern neuronal network formation and function still remains elusive. Here we report that p39, but not p35, is selectively upregulated by histone acetylation-mediated transcription, which underlies the robust increase of Cdk5 activity during rat and mouse neuronal differentiation. The loss of p39 attenuates overall Cdk5 activity in neurons and preferentially affects phosphorylation of specific Cdk5 targets, leading to aberrant axonal growth and impaired dendritic spine and synapse formation. In adult mouse brains, p39 deficiency results in dysregulation of p35 and Cdk5 targets in synapses. Moreover, in contrast to the proepileptic phenotype caused by the lack of p35, p39 loss leads to deficits in maintaining seizure activity and induction of immediate early genes that control hippocampal excitability. Together, our studies demonstrate essential roles of p39 in neuronal network development and function. Furthermore, our data support a model in which Cdk5 activators play nonoverlapping and even opposing roles to govern balanced Cdk5 signaling in the postnatal brain. [ABSTRACT FROM AUTHOR]

An investigation into mechanical properties and amorphization behavior of ultrafine-grained (0.3 lm) boron carbide (B4C) is conducted and compared to a baseline coarse-grained (10 lm) boron carbide. Static and dynamic uniaxial compressive strength, and static and dynamic Vickers indentation hardness were determined, and Raman spectroscopy was then conducted on indented regions to quantify and compare the intensity of amorphization. In relation to coarse-grained B4C the ultrafinegrained material exhibited, on average, a 33% higher static compressive strength, 20% higher dynamic compressive strength, 10% higher static Vickers hardness, and 23% higher dynamic Vickers hardness. In addition, there was an 18% reduction in indentation-induced radial crack length in ultrafine-grained B4C, which corresponded to an increase in estimated fracture toughness. Although traditional coarse-grained B4C exhibits an 8.6% decrease in hardness from the static to dynamic regimes, ultrafine-grained B4C showed only negligible change under similar conditions, suggesting a reduced propensity for amorphization. Raman spectroscopic analysis confirmed this result by revealing significantly lower amorphization intensity in ultrafine-B4C compared to coarse-grained B4C. These results may have significant positive implications in the implementation of ultrafine-grained boron carbide as a material for improved performance in impact and other highpressure applications. [ABSTRACT FROM AUTHOR]

Synapses are the basic unit of neuronal communication and their disruption is associated with many neurological disorders. Significant progress has been made towards understanding the molecular and genetic regulation of synapse formation, modulation, and dysfunction, but the underlying cellular mechanisms remain incomplete. The actin cytoskeleton not only provides the structural foundation for synapses, but also regulates a diverse array of cellular activities underlying synaptic function. Here we will discuss the regulation of the actin cytoskeleton in dendritic spines, the postsynaptic compartment of excitatory synapses. We will focus on a select number of actin regulatory processes, highlighting recent advances, the complexity of crosstalk between different pathways, and the challenges of understanding their precise impact on the structure and function of synapses. [ABSTRACT FROM AUTHOR]

Axon guidance during neural wiring involves a series of precisely controlled chemotactic events by the motile axonal tip, the growth cone. A fundamental question is how neuronal growth cones make directional decisions in response to extremely shallow gradients of guidance cues with exquisite sensitivity. Here we report that nerve growth cones possess a signal amplification mechanism during gradient sensing process. In neuronal growth cones of Xenopus spinal neurons, phosphatidylinositol-3,4,5-trisphosphate (PIP 3), an important signaling molecule in chemotaxis, was actively recruited to the up-gradient side in response to an external gradient of brain-derived neurotrophic factor (BDNF), resulting in an intracellular gradient with approximate 30-fold amplification of the input. Furthermore, a reverse gradient of phosphatase and tensin homolog (PTEN) was induced by BDNF within the growth cone and the increased PTEN activity at the down-gradient side is required for the amplification of PIP 3 signals. Mechanistically, the establishment of both positive PIP 3 and reverse PTEN gradients depends on the filamentous actin network. Together with computational modeling, our results revealed a double negative feedback loop among PTEN, PIP 3 and actomyosin for signal amplification, which is essential for gradient sensing of neuronal growth cones in response to diffusible cues. • Growth cones can sense very shallow extracellular guidance gradients with highly polarized movement. • Growth cones have a signal amplification mechanism that translates shallow gradients into highly polarized motile activities • An amplified intracellular gradient of PIP 3 mediates the growth cone sensing of extracellular shallow gradient of BDNF • Amplification during growth cone sensing of BDNF involves a double negative feedback loop with PTEN, PIP3 and actomyosin. [ABSTRACT FROM AUTHOR]

Summary: Background: Actin-based cell motility is fundamental for development, function, and malignant events in eukaryotic organisms. During neural development, axonal growth cones depend on rapid assembly and disassembly of actin filaments (F-actin) for their guided extension to specific targets for wiring. Monomeric globular actin (G-actin) is the building block for F-actin but is not considered to play a direct role in spatiotemporal control of actin dynamics in cell motility. Results: Here we report that a pool of G-actin dynamically localizes to the leading edge of growth cones and neuroblastoma cells to spatially elevate the G-/F-actin ratio that drives membrane protrusion and cell movement. Loss of G-actin localization leads to the cessation and retraction of membrane protrusions. Moreover, G-actin localization occurs asymmetrically in growth cones during attractive turning. Finally, we identify the actin monomer-binding proteins profilin and thymosin β4 as key molecules that localize actin monomers to the leading edge of lamellipodia for their motility. Conclusions: Our results suggest that dynamic localization of G-actin provides a novel mechanism to regulate the spatiotemporal actin dynamics underlying membrane protrusion in cell locomotion and growth cone chemotaxis. [ABSTRACT FROM AUTHOR]

Abstract: In boron carbide (B4C), the loss of crystallinity (i.e. amorphization) during high-pressure loading is known to result in reduced hardness and inferior ballistic performance. In this investigation, a systematic procedure is developed to evaluate the size and shape of the amorphized zone as well as the spatial distribution of the amorphization intensity beneath a Vickers indentation. This is accomplished by successive metallographic polishing and material removal from the indented surface at submicron depth increments and then scanning of each surface using micro-Raman spectrometry to construct a three-dimensional map of the amorphized zone. The amorphized zone in B4C extends to a depth of almost seven times the indentation depth. The self-similarity in stress fields at various depths is also confirmed from quantitative description of the Raman peaks for various loads. Damage evolution in the form of a Mescall zone and initiation of radial cracking from the amorphized zone were also observed. It is inferred that the amorphization process initiates as soon as the indenter tip makes contact with the B4C surface and thereafter the indenter penetrates into the weakened amorphized material. The implications of this observation and the utility of the quantitative information on the size and shape of the measured amorphized zone for calibration of multiscale models on structural changes in B4C are discussed. [Copyright &y& Elsevier]

Dendritic spines serve as the postsynaptic platform for most excitatory synapses in the mammalian brain, and their shape and size are tightly correlated with synaptic strength. The actin cytoskeleton plays a crucial role in the spine structure and its modifications during synapse development and plasticity, but the underlying regulatory mechanisms remain to be elucidated. Here, we report that actin capping protein (CP), a regulator of actin filament growth, plays an essential role for spine development and synapse formation. We found that CP expression in rat hippocampus is elevated at and after the stage of substantial synapse formation. CP knockdown in hippocampal cultures resulted in a marked decline in spine density accompanied by increased filopodia-like protrusions. Moreover, the spines of CP knockdown neurons exhibited an altered morphology, highlighted by multiple thin filopodia-like protrusions emerging from the spine head. Finally, the number of functional synapses was reduced by CP knockdown as evidenced by a reduction in the density of paired presynaptic and postsynaptic markers and in the frequency of miniature EPSCs. These findings indicate that capping of actin filaments by CP represents an essential step for the remodeling of the actin architecture underlying spine morphogenesis and synaptic formation during development. [ABSTRACT FROM AUTHOR]

Little is known about how the neuronal cytoskeleton is regulated when a dendrite decides whether to branch or not. Previously, we reported that postsynaptic density protein 95 (PSD-95) acts as a stop signal for dendrite branching. It is yet to be elucidated how PSD-95 affects the cytoskeleton and how this regulation relates to the dendritic arbor. Here, we show that the SH3 (src homology 3) domain of PSD-95 interacts with a proline-rich region within the microtubule end-binding protein EB3. Overexpression of PSD-95 or mutant EB3 results in a decreased lifetime of EB3 comets in dendrites. In line with these data, transfected rat neurons show that overexpression of PSD-95 results in less organized microtubules at dendritic branch points and decreased dendritogensis. The interaction between PSD-95 and EB3 elucidates a function for a novel region of EB3 and provides a new and important mechanism for the regulation of microtubules in determining dendritic morphology. [ABSTRACT FROM AUTHOR]

Dendritic spines undergo actin-based growth and shrinkage during synaptic plasticity, in which the actin depolymerizing factor (ADF)/cofilin family of actin-associated proteins are important. Elevated ADF/cofilin activities often lead to reduced spine size and immature spine morphology but can also enhance synaptic potentiation in some cases. Thus, ADF/cofilin may have distinct effects on postsynaptic structure and function. We found that ADF/cofilin-mediated actin dynamics regulated AMPA receptor (AMPAR) trafficking during synaptic potentiation, which was distinct from actin's structural role in spine morphology. Specifically, elevated ADF/cofilin activity markedly enhanced surface addition of AMPARs after chemically induced long-term potentiation (LTP), whereas inhibition of ADF/cofilin abolished AMPAR addition. We found that chemically induced LTP elicited a temporal sequence of ADF/cofilin dephosphorylation and phosphorylation that underlies AMPAR trafficking and spine enlargement. These findings suggest that temporally regulated ADF/cofilin activities function in postsynaptic modifications of receptor number and spine size during synaptic plasticity. [ABSTRACT FROM AUTHOR]

Expression of the brain-derived neurotrophic factor (BDNF) is under tight regulation to accommodate its intricate roles in controlling brain function. Transcription of BDNF initiates from multiple promoters in response to distinct stimulation cues. However, regard- less which promoter is used, all BDNF transcripts are processed at two alternative polyadenylation sites, generating two pools of mRNAs that carry either a long or a short 3′UTR, both encoding the same BDNF protein. Whether and how the two distinct 3′UTRs may differentially regulate BDNF translation in response to neuronal activity changes is an intriguing and challenging question. We re- port here that the long BDNF 3′UTR is a bona fide cis-acting translation suppressor at rest whereas the short 3′UTR mediates active translation to maintain basal levels of BDNF protein production. Upon neuronal activation, the long BDNF 3′UTR, but not the short 3′UTR, imparts rapid and robust activation of translation from a reporter. Importantly, the endogenous long 3′UTR BDNF mRNA specifically undergoes markedly enhanced polyribosome association in the hippocampus in response to pilocarpine induced-seizure before transcriptional up-regulation of BDNF. Furthermore, BDNF protein level is quickly increased in the hippocampus upon seizure-induced neuronal activation, accompanied by a robust activation of the tropomyosin-related receptor tyrosine kinase B. These observations reveal a mechanism for activity-dependent control of BDNF translation and tropomyosin-related receptor tyrosine kinase B signaling in brain neurons. [ABSTRACT FROM AUTHOR]

The localization of specific mRNAs and their local translation in growth cones of developing axons has been shown to play an important mechanism to regulate growth cone turning responses to attractive or repulsive cues. However, the mechanism whereby local translation and growth cone turning may be controlled by specific mRNA-binding proteins is unknown. Here we demonstrate that brain-derived neurotrophic factor (BDNF) signals the Src-dependent phosphorylation of the β-actin mRNA zipcode binding protein 1 (ZBP1), which is necessary for β-actin synthesis and growth cone turning. We raised a phospho-specific ZBP1 antibody to Tyr396, which is a Src phosphorylation site, and immunofluorescence revealed BDNF-induced phosphorylation of ZBP1 within growth cones. The BDNF-induced increase in fluorescent signal of a green fluorescent protein translation reporter with the 3′ untranslated region of β-actin was attenuated with the Src family kinase-specific inhibitor PP2 [4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine]. Furthermore, a nonphosphorylatable mutant, ZBP1 Y396F, suppressed the BDNF-induced and protein synthesis-dependent increase in β-actin localization in growth cones. Last, the ZBP1 Y396F mutant blocked BDNF-induced attractive growth cone turning. These results indicate that phosphorylation of ZBP1 at Tyr396 within growth cones has a critical role to regulate local protein synthesis and growth cone turning. Our findings provide new insight into how the regulated phosphorylation of mRNA-binding proteins influences local translation underlying growth cone motility and axon guidance. [ABSTRACT FROM AUTHOR]

Background: Synaptic defects represent a major mechanism underlying altered brain functions of patients suffering Alzheimer's disease (AD) [1-3]. An increasing body of work indicates that the oligomeric forms of β-amyloid (Aβ) molecules exert profound inhibition on synaptic functions and can cause a significant loss of neurotransmitter receptors from the postsynaptic surface, but the underlying mechanisms remain poorly understood. In this study, we investigated a potential contribution of mitochondria to Aβ inhibition of AMPA receptor (AMPAR) trafficking. Results: We found that a brief exposure of hippocampal neurons to Aβ oligomers not only led to marked removal of AMPARs from postsynaptic surface but also impaired rapid AMPAR insertion during chemically-induced synaptic potentiation. We also found that Aβ oligomers exerted acute impairment of fast mitochondrial transport, as well as mitochondrial translocation into dendritic spines in response to repetitive membrane depolarization. Quantitative analyses at the single spine level showed a positive correlation between spine-mitochondria association and the surface accumulation of AMPARs. In particular, we found that spines associated with mitochondria tended to be more resistant to Aβ inhibition on AMPAR trafficking. Finally, we showed that inhibition of GSK3b alleviated Aβ impairment of mitochondrial transport, and effectively abolished Aβ-induced AMPAR loss and inhibition of AMPAR insertion at spines during cLTP. Conclusions: Our findings indicate that mitochondrial association with dendritic spines may play an important role in supporting AMPAR presence on or trafficking to the postsynaptic membrane. Aβ disruption of mitochondrial trafficking could contribute to AMPAR removal and trafficking defects leading to synaptic inhibition. [ABSTRACT FROM AUTHOR]

Postsynaptic receptor localization is crucial for synapse development and function, but the underlying cytoskeletal mechanisms remain elusive. Using Xenopus neuromuscular junctions as a model, we found that actin depolymerizing factor (ADF)/cofilin regulated actin-dependent vesicular trafficking of acetylcholine receptors (AChRs) to the postsynaptic membrane. Active ADF/cofilin was concentrated in small puncta adjacent to AChR clusters and was spatiotemporally correlated with the formation and maintenance of surface AChR clusters. Notably, increased actin dynamics, vesicular markers and intracellular AChRs were all enriched at the sites of ADF/cofilin localization. Furthermore, a substantial amount of new AChRs was detected at these ADF/cofilin-enriched sites. Manipulation of either ADF/cofilin activity through its serine-3 phosphorylation or ADF/cofilin localization via 14-3-3 proteins markedly attenuated AChR insertion and clustering. These results suggest that spatiotemporally restricted ADF/cofilin-mediated actin dynamics regulate AChR trafficking during the development of neuromuscular synapses. [ABSTRACT FROM AUTHOR]

Bone morphogenic proteins (BMPs) are involved in axon pathfinding, but how they guide growth cones remains elusive. In this study, we report that a BMP7 gradient elicits bidirectional turning responses from nerve growth cones by acting through LIM kinase (LIMK) and Slingshot (SSH) phosphatase to regulate actin-depolymerizing factor (ADF)/cofilin-mediated actin dynamics. Xenopus laevis growth cones from 4-8-h cultured neurons are attracted to BMP7 gradients but become repelled by BMP7 after overnight culture. The attraction and repulsion are mediated by LIMK and SSH, respectively, which oppositely regulate the phosphorylation-dependent asymmetric activity of ADF/cofilin to control the actin dynamics and growth cone steering. The attraction to repulsion switching requires the expression of a transient receptor potential (TRP) channel TRPC1 and involves Ca[sup 2+] signaling through calcineurin phosphatase for SSH activation and growth cone repulsion. Together, we show that spatial regulation of ADF/cofilin activity controls the directional responses of the growth cone to BMP7, and Ca[sup 2+] influx through TRPC tilts the LIMK-SSH balance toward SSH-mediated repulsion. [ABSTRACT FROM AUTHOR]

The second messenger cyclic adenosine monophosphate (cAMP) plays a pivotal role in axonal growth and guidance, but its downstream mechanisms remain elusive. In this study, we report that type II protein kinase A (PKA) is highly enriched in growth cone filopodia, and this spatial localization enables the coupling of cAMP signaling to its specific effectors to regulate guidance responses. Disrupting the localization of PKA to filopodia impairs cAMP-mediated growth cone attraction and prevents the switching of repulsive responses to attraction by elevated cAMP. Our data further show that PKA targets protein phosphatase-1 (PP1) through the phosphorylation of a regulatory protein inhibitor-1 (I-1) to promote growth cone attraction. Finally, we find that I-1 and PP1 mediate growth cone repulsion induced by myelin-associated glycoprotein. These findings demonstrate that the spatial localization of type II PKA to growth cone filopodia plays an important role in the regulation of growth cone motility and guidance by cAMP. [ABSTRACT FROM AUTHOR]

Defects in axonal transport are often associated with a wide variety of neurological diseases including Alzheimer's disease (AD). β-Amyloid (Aβ) is a major component of neuritic plaques associated with pathological conditions of AD brains. Here, we report that a brief exposure of cultured hippocampal neurons to Aβmolecules resulted in rapid and severe impairment of mitochondrial transport without inducing apparent cell death and significant morphological changes. Such acute inhibition of mitochondrial transport was not associated with a disruption of mitochondria potential nor involved aberrant cytoskeletal changes. Aβ also did not elicit significant Ca2+ signaling to affect mitochondrial trafficking. However, stimulation of protein kinase A (PKA) by forskolin, cAMP analogs, or neuropeptides effectively alleviated the impairment. We also show that Aβ inhibited mitochondrial transport by acting through glycogen synthase kinase 3β(GSK3β). Given that mitochondria are crucial organelles for many cellular functions and survival, our findings thus identify an important acute action of Aβ molecules on nerve cells that could potentially contribute to various abnormalities of neuronal functions under AD conditions. Manipulation of GSK3β and PKA activities may represent a key approach for preventing and alleviating Aβ cytotoxicity and AD pathological conditions. [ABSTRACT FROM AUTHOR]

Dendritic morphology determines many aspects of neuronal function, including action potential propagation and information processing. However, the question remains as to how distinct neuronal dendrite branching patterns are established. Here, we report that postsynaptic density-95 (PSD-95), a protein involved in dendritic spine maturation and clustering of synaptic signaling proteins, plays a novel role in regulating dendrite outgrowth and branching, independent of its synaptic functions. In immature neurons, overexpression of PSD-95 decreases the proportion of primary dendrites that undergo additional branching, resulting in a marked reduction of secondary dendrite number. Conversely, knocking down PSD-95 protein in immature neurons increases secondary dendrite number. The effect of PSD-95 is activity-independent and is antagonized by cypin, a nonsynaptic protein that regulates PSD-95 localization. Binding of cypin to PSD-95 correlates with formation of stable dendrite branches. Finally, overexpression of PSD-95 in COS-7 cells disrupts microtubule organization, indicating that PSD-95 may modulate microtubules to regulate dendritic branching. Whereas many factors have been identified which regulate dendrite number, our findings provide direct evidence that proteins primarily involved in synaptic functions can also play developmental roles in shaping how a neuron patterns its dendrite branches. [ABSTRACT FROM AUTHOR]

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 Ca2+-dependent bidirectional turning of Xenopus laevis growth cones requires localized distribution and translation of β-actin messenger RNA. Both β-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 β-actin mRNA-ZBP1 binding abolishes both Ca2+-mediated attraction and repulsion. In addition, attraction involves a local increase in β-actin, whereas repulsion is accompanied by a local decrease in β-actin; thus, both produce a synthesis- and ZBP1 binding-dependent β-actin asymmetry but with opposite polarities. Together with a similar asymmetry in Src activity during bidirectional responses, our findings indicate that Ca2+-dependent spatial regulation of β-actin synthesis through Src contributes to the directional motility of growth cones during guidance. [ABSTRACT FROM AUTHOR]

Axon pathfinding depends on attractive and repulsive turning of growth cones to extracellular cues. Localized cytosolic Ca2+ signals are known to mediate the bidirectional responses, but downstream mechanisms remain elusive. Here, we report that calcium-calmodulin-dependent protein kinase II (CaMKII) and calcineurin (CaN) phosphatase provide a switch-like mechanism to control the direction of Ca2+-dependent growth cone turning. A relatively large local Ca2+ elevation preferentially activates CaMKII to induce attraction, while a modest local Ca2+ signal predominately acts through CaN and phosphatase-1 (PP1) to produce repulsion. The resting level of intracellular Ca2+ concentrations also affects CaMKII/CaN operation: a normal baseline allows distinct turning responses to different local Ca2+ signals, while a low baseline favors CaN-PP1 activation for repulsion. Moreover, the cAMP pathway negatively regulates CaN-PP1 signaling to inhibit repulsion. Finally, CaMKII/CaN-PP1 also mediates netrin-1 guidance. Together, these findings establish a complex Ca2+ mechanism that targets the balance of CaMKII/CaN-PP1 activation to control distinct growth cone responses. [Copyright &y& Elsevier]

Axon guidance requires signal transduction of extracellular cues through the plasma membrane for directional motility. Here we present evidence that cholesterol- and sphingolipid-enriched membrane microdomains (lipid rafts) mediate specific guidance responses of nerve growth cones. Disruption of lipid rafts by various approaches targeting cholesterol or gangliosides selectively abolished growth cone attraction and repulsion in BDNF and netrin-1 gradients, respectively, without affecting glutamate-induced attraction. Interestingly, local raft disruption on one side of the growth cone in bath BDNF or netrin-1 produced opposite turning responses to that induced by the gradients. Raft manipulation also blocked Semaphorin 3A-induced growth cone repulsion, inhibition, and collapse. Finally, guidance responses appeared to involve raft-dependent activation of p42/p44 MAPK and ligand-induced receptor recruitment to lipid rafts. Together with the observation of asymmetric receptor-raft associations at the growth cone in guidance gradients, our findings indicate that localized signaling through membrane rafts plays a role in mediating guidance actions of extracellular cues on developing axons. [Copyright &y& Elsevier]

Presents a study which reported that synchronized Ca&sup2+; spikes among cultured hippocampal neurons can be modulated by factors that act on G-protein-couples receptors (GPCR). Description of GPCR; Effects of pituitary adenalyte cyclase-activating polypeptide on synchronized Ca&sup2+; spikes; Information on how stromal cell-derived factor-1 was able to affect the network spike activities.

Presents a study which revealed that direct cAMP signaling through G-protein-coupled receptors mediates growth cone attraction induced by pituitary adenylate cyclase-activating polypeptide. Materials and methods; Results of the study; Conclusion.