Single Shot Characterization of High Transformer Ratio Wakefields in Nonlinear Plasma Acceleration.

Plasma wakefields can enable very high accelerating gradients for frontier high energy particle accelerators, in excess of 10 GeV/m. To overcome limits on single stage acceleration, specially shaped drive beams can be used in both linear and nonlinear plasma wakefield accelerators (PWFA), to increase the transformer ratio, implying that the drive beam deceleration is minimized relative to acceleration obtained in the wake. In this Letter, we report the results of a nonlinear PWFA, high transformer ratio experiment using high-charge, longitudinally asymmetric drive beams in a plasma cell. An emittance exchange process is used to generate variable drive current profiles, in conjunction with a long (multiple plasma wavelength) witness beam. The witness beam is energy modulated by the wakefield, yielding a response that contains detailed spectral information in a single-shot measurement. Using these methods, we generate a variety of beam profiles and characterize the wakefields, directly observing transformer ratios up to R=7.8. Furthermore, a spectrally based reconstruction technique, validated by 3D particle-in-cell simulations, is introduced to obtain the drive beam current profile from the decelerating wake data.

Ataxias and cerebellar dysfunction: involvement of synaptic plasticity deficits?

Adaptive processes within cerebellar circuits, such as long-term depression and long-term potentiation at parallel fiber-Purkinje cell synapses, have long been seen as important to cerebellar motor learning, and yet little attention has been given to any possible significance of these processes for cerebellar dysfunction and disease. Several forms of ataxia are caused by mutations in genes encoding for ion channels located at key junctures in pathways that lead to the induction of synaptic plasticity, suggesting that there might be an association between deficits in plasticity and the ataxic phenotype. Herein we explore this possibility and examine the available evidence linking the two together, highlighting specifically the role of P/Q-type calcium channels and their downstream effector small-conductance calcium-sensitive (SK2) potassium channels in the regulation of synaptic gain and intrinsic excitability, and reviewing their connections to ataxia.

Deregulation of synaptic plasticity in autism.

A puzzling observation in the study of autism spectrum disorder (ASD) in mouse models has been the deregulation of long-term synaptic depression (LTD), a form of experience-dependent synaptic plasticity, across brain areas and across syndromic and non-syndromic forms of autism. This review attempts to approach this phenomenon from a largely, but not exclusively, cerebellar perspective. Three potential consequences of LTD deregulation are discussed that are relevant for ASD phenotypes: resulting impairment of proper developmental synaptic pruning, impairment of motor coordination and motor learning, and impairment of the processing of sensory input.

Long-term depression of the cerebellar climbing fiber--Purkinje neuron synapse.

In classic Marr-Albus-Ito models of cerebellar function, coactivation of the climbing fiber (CF) synapse, which provides massive, invariant excitation of Purkinje neurons (coding the unconditioned stimulus), together with a graded parallel fiber synaptic array (coding the conditioned stimulus) leads to long-term depression (LTD) of parallel fiber-Purkinje neuron synapses, underlying production of a conditioned response. Here, we show that the supposedly invariant CF synapse can also express LTD. Brief 5 Hz stimulation of the CF resulted in a sustained depression of CF EPSCs that did not spread to neighboring parallel fiber synapses. Like parallel fiber LTD, CF LTD required postsynaptic Ca2+ elevation, activation of group 1 mGluRs, and activation of PKC. CF LTD is potentially relevant for models of cerebellar motor control and learning and the developmental conversion from multiple to single CF innervation of Purkinje neurons.

Impaired cerebellar long-term potentiation in type I adenylyl cyclase mutant mice.

Activation of adenylyl cyclase and the consequent production of cAMP is a process that has been shown to be central to invertebrate model systems of information storage. In the vertebrate brain, it has been suggested that a presynaptic cascade involving Ca influx, cAMP production, and subsequent activation of cAMP-dependent protein kinase is necessary for induction of long-term potentiation (LTP) at the cerebellar parallel fiber-Purkinje cell synapse. We have used mutant mice in which the major Ca-sensitive adenylyl cyclase isoform of cerebellar cortex (type I) is deleted to show that this results in an approximately 65% reduction in cerebellar Ca-sensitive cyclase activity and a nearly complete blockade of cerebellar LTP assessed using granule cell-Purkinje cell pairs in culture. This blockade is not accompanied by alterations in a number of basal electrophysiological parameters and may be bypassed by application of an exogenous cAMP analog, suggesting that it results specifically from deletion of the type I adenylyl cyclase.

Expression of a protein kinase C inhibitor in Purkinje cells blocks cerebellar LTD and adaptation of the vestibulo-ocular reflex.

Cerebellar long-term depression (LTD) is a model system for neuronal information storage that has an absolute requirement for activation of protein kinase C (PKC). It has been claimed to underlie several forms of cerebellar motor learning. Previous studies using various knockout mice (mGluR1, GluRdelta2, glial fibrillary acidic protein) have supported this claim; however, this work has suffered from the limitations that the knockout technique lacks anatomical specificity and that functional compensation can occur via similar gene family members. To overcome these limitations, a transgenic mouse (called L7-PKCI) has been produced in which the pseudosubstrate PKC inhibitor, PKC[19-31], was selectively expressed in Purkinje cells under the control of the pcp-2(L7) gene promoter. Cultured Purkinje cells prepared from heterozygous or homozygous L7-PKCI embryos showed a complete blockade of LTD induction. In addition, the compensatory eye movements of L7-PKCI mice were recorded during vestibular and visual stimulation. Whereas the absolute gain, phase, and latency values of the vestibulo-ocular reflex and optokinetic reflex of the L7-PKCI mice were normal, their ability to adapt their vestibulo-ocular reflex gain during visuo-vestibular training was absent. These data strongly support the hypothesis that activation of PKC in the Purkinje cell is necessary for cerebellar LTD induction, and that cerebellar LTD is required for a particular form of motor learning, adaptation of the vestibulo-ocular reflex.

Beyond parallel fiber LTD: the diversity of synaptic and non-synaptic plasticity in the cerebellum.

In recent years, it has become clear that motor learning, as revealed by associative eyelid conditioning and adaptation of the vestibulo-ocular reflex, contributes to the well-established cerebellar functions of sensorimotor integration and control. Long-term depression of the parallel fiber-Purkinje cell synapse (which is often called 'cerebellar LTD') is a cellular phenomenon that has been suggested to underlie these forms of learning. However, it is clear that parallel fiber LTD, by itself, cannot account for all the properties of cerebellar motor learning. Here we review recent electrophysiological experiments that have described a rich variety of use-dependent plasticity in cerebellum, including long-term potentiation (LTP) and LTD of excitatory and inhibitory synapses, and persistent modulation of intrinsic neuronal excitability. Finally, using associative eyelid conditioning as an example, we propose some ideas about how these cellular phenomena might function and interact to endow the cerebellar circuit with particular computational and mnemonic properties.

Oxygen isotope analysis of bacterial and fungal manganese oxidation.

The ability of micro-organisms to oxidize manganese (Mn) from Mn(II) to Mn(III/IV) oxides transcends boundaries of biological clade or domain. Many bacteria and fungi oxidize Mn(II) to Mn(III/IV) oxides directly through enzymatic activity or indirectly through the production of reactive oxygen species. Here, we determine the oxygen isotope fractionation factors associated with Mn(II) oxidation via various biotic (bacteria and fungi) and abiotic Mn(II) reaction pathways. As oxygen in Mn(III/IV) oxides may be derived from precursor water and molecular oxygen, we use a twofold approach to determine the isotope fractionation with respect to each oxygen source. Using both 18 O-labeled water and closed-system Rayleigh distillation approaches, we constrain the kinetic isotope fractionation factors associated with O atom incorporation during Mn(II) oxidation to -17.3‰ to -25.9‰ for O2 and -1.9‰ to +1.8‰ for water. Results demonstrate that stable oxygen isotopes of Mn(III/IV) oxides have potential to distinguish between two main classes of biotic Mn(II) oxidation: direct enzymatic oxidation in which O2 is the oxidant and indirect enzymatic oxidation in which superoxide is the oxidant. The fraction of Mn(III/IV) oxide-associated oxygen derived from water varies significantly (38%-62%) among these bio-oxides with only weak relationship to Mn oxidation state, suggesting Mn(III) disproportionation may account for differences in the fraction of mineral-bound oxygen from water and O2 . Additionally, direct incorporation of molecular O2 suggests that Mn(III/IV) oxides contain a yet untapped proxy of δ18OO2 of environmental O2 , a parameter reflecting the integrated influence of global respiration, photorespiration, and several other biogeochemical reactions of global significance.

Biogenic manganese oxides as reservoirs of organic carbon and proteins in terrestrial and marine environments.

Manganese (Mn) oxides participate in a range of interactions with organic carbon (OC) that can lead to either carbon degradation or preservation. Here, we examine the abundance and composition of OC associated with biogenic and environmental Mn oxides to elucidate the role of Mn oxides as a reservoir for carbon and their potential for selective partitioning of particular carbon species. Mn oxides precipitated in natural brackish waters and by Mn(II)-oxidizing marine bacteria and terrestrial fungi harbor considerable levels of organic carbon (4.1-17.0 mol OC per kg mineral) compared to ferromanganese cave deposits which contain 1-2 orders of magnitude lower OC. Spectroscopic analyses indicate that the chemical composition of Mn oxide-associated OC from microbial cultures is homogeneous with bacterial Mn oxides hosting primarily proteinaceous carbon and fungal Mn oxides containing both protein- and lipopolysaccharide-like carbon. The bacterial Mn oxide-hosted proteins are involved in both Mn(II) oxidation and metal binding by these bacterial species and could be involved in the mineral nucleation process as well. By comparison, the composition of OC associated with Mn oxides formed in natural settings (brackish waters and particularly in cave ferromanganese rock coatings) is more spatially and chemically heterogeneous. Cave Mn oxide-associated organic material is enriched in aliphatic C, which together with the lower carbon concentrations, points to more extensive microbial or mineral processing of carbon in this system relative to the other systems examined in this study, and as would be expected in oligotrophic cave environments. This study highlights Mn oxides as a reservoir for carbon in varied environments. The presence and in some cases dominance of proteinaceous carbon within the biogenic and natural Mn oxides may contribute to preferential preservation of proteins in sediments and dominance of protein-dependent metabolisms in the subsurface biosphere.

Novel real-time PCR detection assay for Brucella suis.

INTRODUCTION: Brucella suis is the causative agent of brucellosis in suidae and is differentiated into five biovars (bv). Biovars 1 and 3 possess zoonotic potential and can infect humans, whereas biovar 2 represents the main source of brucellosis in feral and domestic pigs in Europe. Both aspects, the zoonotic threat and the economic loss, emphasize the necessity to monitor feral and domestic pig populations. Available serological or PCR based methods lack sensitivity and specificity. RESULTS: Here a bioinformatics approach was used to identify a B. suis specific 17 bp repeat on chromosome II (BS1330_II0657 locus). This repeat is common for B. suis bv 1 to 4 and was used to develop a TaqMan probe assay. The average PCR efficiency was determined as 95% and the limit of detection as 12,5 fg/µl of DNA, equally to 3.7 bacterial genomes. This assay has the highest sensitivity of all previously described B. suis specific PCR assays, making it possible to detect 3-4 bacterial genomes per 1 µl of sample. The assay was tested 100% specific for B. suis and negative for other Brucella spp. and closely related non-Brucella species. CONCLUSIONS: This novel qPCR assay could become a rapid, inexpensive and reliable screening method for large sample pools of B. suis 1 to 4. This method will be applicable for field samples after validation.

Different threshold levels of postsynaptic [Ca2+]i have to be reached to induce LTP and LTD in neocortical pyramidal cells.

Changes in [Ca2+]i were measured in layer II-III pyramid cells of the rat visual cortex slices during application of either LTP or LTD inducing stimulation protocols. At dendritic sites activated by the stimulated afferents [Ca2+]i reached higher amplitudes and decayed more slowly with LTP than with LTD inducing stimuli. In the presence of Ca2+ chelators, the stimulation protocol that would normally produce LTP induced either LTD or failed to induce a synaptic modifications altogether. These results support the hypothesis that the polarity of synaptic gain changes depends on the magnitude of postsynaptic [Ca2+]i responses, the induction of LTP requiring a more pronounced surge of [Ca2+]i than the induction of LTD.

Delayed increase of extracellular arginine, the nitric oxide precursor, following electrical white matter stimulation in rat cerebellar slices.

Amino acid levels were measured in perfusates from biplanar slices of rat cerebellum installed in a Krebs-filled three-compartment chamber. The two lateral compartments housed the white matter and a section containing parallel fibres respectively. The central compartment housed cortical structures, including the Purkinje cell and granule cell bodies. This arrangement allows selective electrical stimulation of the parallel fibre or mossy fibre pathways, recording of the evoked responses to such stimulation and collection of the perfusion medium passing through the central chamber for amino acid analysis using high-pressure liquid chromatography (HPLC). Both, 2-Hz and 5-Hz stimulation of white matter caused a delayed increase in arginine levels in the perfusate. Since L-arginine is the physiological precursor of nitric oxide, a neuronal messenger in the brain, the data suggest that physiological stimuli can result in the release of this precursor, possibly to supply the nitric oxide synthase.

Effects of various resin composite (co)monomers and extracts on two caries-associated micro-organisms in vitro.

Previous studies have shown that extracts of various filling materials, e.g., resin composites, may influence the growth of cariogenic micro-organisms. The purpose of this study was to examine the effects of important resin composite (co)monomers (Bis-GMA, UDMA, TEGDMA, EGDMA) and extracts of two commercial dental composites with similar composition (composite A, Arabesk; composite S, Superlux) on the growth of the two cariogenic bacterial pathogens Streptococcus sobrinus and Lactobacillus acidophilus. It was found that neither the monomers Bis-GMA and UDMA, nor the comonomer EGDMA, nor the extract of composite A influenced the growth of S. sobrinus in the log phase. The comonomer TEGDMA and the extract of composite S were found to stimulate growth in the log phase, but this stimulation was not statistically significant. However, EGDMA, TEGDMA, and the extract of composite S did stimulate the total growth of S. sobrinus. In the assays with L. acidophilus, Bis-GMA, UDMA, and the extract of composite A inhibited the growth in the log phase, whereas TEGDMA stimulated it. Furthermore, EGDMA, TEGDMA, and the extract of composite S stimulated the biomass production of L. acidophilus. We conclude from our results that a release of EGDMA and TEGDMA from resin composites should be avoided due to their growth-stimulating effects on the caries-associated micro-organisms S. sobrinus and L. acidophilus.

Comparative proteomics of Mn(II)-oxidizing and non-oxidizing Roseobacter clade bacteria reveal an operative manganese transport system but minimal Mn(II)-induced expression of manganese oxidation and antioxidant enzymes.

Manganese (Mn) is an essential nutrient and precipitates as minerals with technological and environmental relevance. To gain a proteomic understanding of how bacteria respond to Mn(II) and its connection to oxidation, a comparative examination of the proteomic response of Mn(II)-oxidizing (Roseobacter sp. AzwK-3b) and non-oxidizing (Ruegeria sp. TM1040) alphaproteobacteria was conducted. Both bacteria show an operative Mn(II) transport system. In the absence of Mn(II), both bacteria have higher expression of proteins that were homologous to SitA and SitB, known proteins in the Mn(II) transport system of other alphaproteobacteria. Overall, each bacterium demonstrated a varied response to Mn(II). Ru. TM1040 had a greater number of proteins differentially expressed in response to Mn(II) and also had a group of proteins related to chemotaxis at higher concentrations of Mn(II), suggesting a potential stress response. While both bacteria are able to generate extracellular superoxide and Mn(II) is a known antioxidant, the presence of Mn(II) did not significantly alter the expression of proteins related to antioxidant activity. Heme peroxidases, previously connected to Mn(II) oxidation, were found in the soluble protein extract of R. AzwK-3b, but only minor differential expression was observed as a function of Mn(II), indicating that their expression was not induced by Mn(II).

Purkinje cell-specific knockout of the protein phosphatase PP2B impairs potentiation and cerebellar motor learning.

Cerebellar motor learning is required to obtain procedural skills. Studies have provided supportive evidence for a potential role of kinase-mediated long-term depression (LTD) at the parallel fiber to Purkinje cell synapse in cerebellar learning. Recently, phosphatases have been implicated in the induction of potentiation of Purkinje cell activities in vitro, but it remains to be shown whether and how phosphatase-mediated potentiation contributes to motor learning. Here, we investigated its possible role by creating and testing a Purkinje cell-specific knockout of calcium/calmodulin-activated protein-phosphatase-2B (L7-PP2B). The selective deletion of PP2B indeed abolished postsynaptic long-term potentiation in Purkinje cells and their ability to increase their excitability, whereas LTD was unaffected. The mutants showed impaired "gain-decrease" and "gain-increase" adaptation of their vestibulo-ocular reflex (VOR) as well as impaired acquisition of classical delay conditioning of their eyeblink response. Thus, our data indicate that PP2B may indeed mediate potentiation in Purkinje cells and contribute prominently to cerebellar motor learning.

The neuropeptide corticotropin-releasing factor regulates excitatory transmission and plasticity at the climbing fibre-Purkinje cell synapse.

The climbing fibre (CF) input controls cerebellar Purkinje cell (PC) activity as well as synaptic plasticity at parallel fibre (PF)-PC synapses. Under high activity conditions, CFs release not only glutamate, but also the neuropeptide corticotropin-releasing factor (CRF). Brief periods of such high CF activity can lead to the induction of long-term depression (LTD) at CF-PC synapses. Thus, we have examined for the first time the role of CRF in regulating excitatory postsynaptic currents (EPSCs) and long-term plasticity at this synapse. Exogenous application of CRF alone transiently mimicked three aspects of CF-LTD, causing reductions in the CF-evoked excitatory postsynaptic current, complex spike second component and complex spike afterhyperpolarization. The complex spike first component is unaffected by CF-LTD induction and was similarly unaffected by CRF. Application of a CRF receptor antagonist reduced the expression amplitude and induction probability of CF-LTD monitored at the EPSC level. Collectively, these results suggest that under particular sensorimotor conditions, co-release of CRF from climbing fibres could down-regulate excitatory transmission and facilitate LTD induction at CF-PC synapses. Inhibition of either protein kinase C (PKC) or protein kinase A (PKA) attenuated the effects of CRF upon CF-EPSCs. We have previously shown that CF-LTD induction is PKC-dependent, and here demonstrate PKA-dependence as well. These results suggest that both the acute effects of CRF on CF-EPSCs as well as the facilitating effect of CRF on CF-LTD induction can be explained by a CRF-mediated recruitment of PKC and PKA.

Relation between dendritic Ca2+ levels and the polarity of synaptic long-term modifications in rat visual cortex neurons.

Long-term changes of synaptic efficacy, in particular when they are use-dependent, are candidate mechanisms for the storage of information in the nervous system. In a variety of brain structures, including the neocortex and hippocampus, synapses are susceptible to long-term potentiation (LTP) and long-term depression (LTD). It has been hypothesized that the polarity of the synaptic gain change depends on the amplitude of the postsynaptic [Ca2+]i rise, the threshold for the induction of LTD being lower than that for the induction of LTP. To test this assumption, we characterized Ca2+ signals in layer II/III pyramidal cells of rat visual cortex slices, using the fluorescent Ca2+ indicator fura-2, during application of stimulation protocols that had been adjusted to reliably induce either LTP or LTD in cells not loaded with fura-2. At dendritic sites activated by the stimulated afferents the intracellular [Ca2+] concentration ([Ca2+]i) reached higher amplitudes and decayed more slowly with stimuli inducing LTP than with those inducing LTD. To directly analyse the functional significance of the observed difference in the Ca2+ signal amplitude, we examined whether a tetanization protocol suitable for the induction of LTP can be converted into a protocol inducing LTD by injecting the postsynaptic cells with Ca2+ chelators that reduce the concentration of effective free Ca2+. In the presence of fura-2 or BAPTA [bis(2-aminophenoxy) ethane-N,N,N',N'-tetraacetate], the stimulation protocol that would normally produce LTP induced either LTD or failed to induce synaptic modifications altogether. These results support the hypothesis that the amplitude of the postsynaptic rise in [Ca2+]i is a key factor in the determination of the polarity of synaptic gain change.

Characterization of Fe plaque and associated metals on the roots of mine-waste impacted aquatic plants.

Iron plaque on aquatic plant roots are ubiquitous and sequester metals in wetland soils; however, the mechanisms of metal sequestration are unresolved. Thus, characterizing the Fe plaque and associated metals will aid in understanding and predicting metal cycling in wetland ecosystems. Accordingly, microscopic and spectroscopic techniques were utilized to identify the spatial distributions, associations, and chemical environments of Fe, Mn, Pb, and Zn on the roots of a common, indigenous wetland plant (Phalaris arundinacea). Iron forms a continuous precipitate on the root surface, which is composed dominantly of ferrihydrite (ca. 63%) with lesser amounts of goethite (32%) and minor levels of siderite (5%). Although Pb is juxtaposed with Fe on the root surface, it is complexed to organic functional groups, consistent with those of bacterial biofilms. In contrast, Mn and Zn exist as discrete, isolated mixed-metal carbonate (rhodochrosite/hydrozincite) nodules on the root surface. Accordingly, the soil-root interface appears to be a complex biochemical environment, containing both reduced and oxidized mineral species, as well as bacterial-induced organic-metal complexes. As such, hydrated iron oxides, bacterial biofilms, and metal carbonates will influence the availability and mobility of metals within the rhizosphere of aquatic plants.

Seasonal fluctuations in zinc speciation within a contaminated wetland.

The cycling of common sorbents such as metal (hydr)- oxides, carbonates, and sulfides in redox-active environments influences the partitioning of associated trace elements such as zinc. Consequently, fluctuations in redox status may in part determine the availability and mobility of Zn and other trace elements. This research examines changes in Zn speciation in a contaminated wetland soil that undergoes seasonal flooding. X-ray absorption spectroscopy (XAS) was employed to identify and quantify Zn species from soil cores collected over a 1-year cycle as a function of water depth, location, and soil depth. Zinc associated with (hydr)oxide phases in dry, oxidized soils and with sulfides and carbonates in flooded systems. An increase in water level was accompanied by a reversible change in Zn fractionation toward ZnS and ZnC03. However, a small, recalcitrant fraction of Zn associated with (hydr)oxides remained even when the soils were exposed to highly reducing conditions. Water depth and redox potential were the most important factors in determining Zn speciation, although spatial variation was also important. These data indicate that zinc sorption is a dynamic process influenced by environmental changes.