Pyrocumulonimbus affect average stratospheric aerosol composition.

Pyrocumulonimbus (pyroCb) are wildfire-generated convective clouds that can inject smoke directly into the stratosphere. PyroCb have been tracked for years, yet their apparent rarity and episodic nature lead to highly uncertain climate impacts. In situ measurements of pyroCb smoke reveal its distinctive and exceptionally stable aerosol properties and define the long-term influence of pyroCb activity on the stratospheric aerosol budget. Analysis of 13 years of airborne observations shows that pyroCb are responsible for 10 to 25% of the black carbon and organic aerosols in the "present-day" lower stratosphere, with similar impacts in both the North and South Hemispheres. These results suggest that, should pyroCb increase in frequency and/or magnitude in future climates, they could generate dominant trends in stratospheric aerosol.

An aerosol particle containing enriched uranium encountered in the remote upper troposphere.

We describe a submicron aerosol particle sampled at an altitude of 7 km near the Aleutian Islands that contained a small percentage of enriched uranium oxide. 235U was 3.1 ±â€¯0.5% of 238U. During twenty years of aircraft sampling of millions of particles in the global atmosphere, we have rarely encountered a particle with a similarly high content of 238U and never a particle with enriched 235U. The bulk of the particle consisted of material consistent with combustion of heavy fuel oil. Analysis of wind trajectories and particle dispersion model results show that the particle could have originated from a variety of areas across Asia. The source of such a particle is unclear, and the particle is described here in case it indicates a novel source where enriched uranium was dispersed.

The Convective Transport of Active Species in the Tropics (CONTRAST) Experiment.

The Convective Transport of Active Species in the Tropics (CONTRAST) experiment was conducted from Guam (13.5° N, 144.8° E) during January-February 2014. Using the NSF/NCAR Gulfstream V research aircraft, the experiment investigated the photochemical environment over the tropical western Pacific (TWP) warm pool, a region of massive deep convection and the major pathway for air to enter the stratosphere during Northern Hemisphere (NH) winter. The new observations provide a wealth of information for quantifying the influence of convection on the vertical distributions of active species. The airborne in situ measurements up to 15 km altitude fill a significant gap by characterizing the abundance and altitude variation of a wide suite of trace gases. These measurements, together with observations of dynamical and microphysical parameters, provide significant new data for constraining and evaluating global chemistry climate models. Measurements include precursor and product gas species of reactive halogen compounds that impact ozone in the upper troposphere/lower stratosphere. High accuracy, in-situ measurements of ozone obtained during CONTRAST quantify ozone concentration profiles in the UT, where previous observations from balloon-borne ozonesondes were often near or below the limit of detection. CONTRAST was one of the three coordinated experiments to observe the TWP during January-February 2014. Together, CONTRAST, ATTREX and CAST, using complementary capabilities of the three aircraft platforms as well as ground-based instrumentation, provide a comprehensive quantification of the regional distribution and vertical structure of natural and pollutant trace gases in the TWP during NH winter, from the oceanic boundary to the lower stratosphere.

Quantifying global terrestrial methanol emissions using observations from the TES satellite sensor.

We employ new global space-based measurements of atmospheric methanol from the Tropospheric Emission Spectrometer (TES) with the adjoint of the GEOS-Chem chemical transport model to quantify terrestrial emissions of methanol to the atmosphere. Biogenic methanol emissions in the model are based on version 2.1 of the Model of Emissions of Gases and Aerosols from Nature (MEGANv2.1), using leaf area data from NASA's Moderate Resolution Imaging Spectroradiometer (MODIS) and GEOS-5 assimilated meteorological fields. We first carry out a pseudo observation test to validate the overall approach, and find that the TES sampling density is sufficient to accurately quantify regional- to continental-scale methanol emissions using this method. A global inversion of two years of TES data yields an optimized annual global surface flux of 122 Tg yr-1 (including biogenic, pyrogenic, and anthropogenic sources), an increase of 60 % from the a priori global flux of 76 Tg yr-1. Global terrestrial methanol emissions are thus nearly 25 % those of isoprene (~540 Tg yr-1), and are comparable to the combined emissions of all anthropogenic volatile organic compounds (~100-200 Tg yr-1). Our a posteriori terrestrial methanol source leads to a strong improvement of the simulation relative to an ensemble of airborne observations, and corroborates two other recent top-down estimates (114-120 Tg yr-1) derived using in situ and space-based measurements. Inversions testing the sensitivity of optimized fluxes to model errors in OH, dry deposition, and oceanic uptake of methanol, as well as to the assumed a priori constraint, lead to global fluxes ranging from 118 to 126 Tg yr-1. The TES data imply a relatively modest revision of model emissions over most of the tropics, but a significant upward revision in midlatitudes, particularly over Europe and North America. We interpret the inversion results in terms of specific source types using the methanol : CO correlations measured by TES, and find that biogenic emissions are overestimated relative to biomass burning and anthropogenic emissions in central Africa and southeastern China, while they are underestimated in regions such as Brazil and the US. Based on our optimized emissions, methanol accounts for > 25 % of the photochemical source of CO and HCHO over many parts of the northern extratropics during springtime, and contributes ~6 % of the global secondary source of those compounds annually.

Tropospheric methanol observations from space: retrieval evaluation and constraints on the seasonality of biogenic emissions.

Methanol retrievals from nadir-viewing space-based sensors offer powerful new information for quantifying methanol emissions on a global scale. Here we apply an ensemble of aircraft observations over North America to evaluate new methanol measurements from the Tropospheric Emission Spectrometer (TES) on the Aura satellite, and combine the TES data with observations from the Infrared Atmospheric Sounding Interferometer (IASI) on the MetOp-A satellite to investigate the seasonality of methanol emissions from northern midlatitude ecosystems. Using the GEOS-Chem chemical transport model as an intercomparison platform, we find that the TES retrieval performs well when the degrees of freedom for signal (DOFS) are above 0.5, in which case the model:TES regressions are generally consistent with the model:aircraft comparisons. Including retrievals with DOFS below 0.5 degrades the comparisons, as these are excessively influenced by the a priori. The comparisons suggest DOFS >0.5 as a minimum threshold for interpreting retrievals of trace gases with a weak tropospheric signal. We analyze one full year of satellite observations and find that GEOS-Chem, driven with MEGANv2.1 biogenic emissions, underestimates observed methanol concentrations throughout the midlatitudes in springtime, with the timing of the seasonal peak in model emissions 1-2 months too late. We attribute this discrepancy to an underestimate of emissions from new leaves in MEGAN, and apply the satellite data to better quantify the seasonal change in methanol emissions for midlatitude ecosystems. The derived parameters (relative emission factors of 11.0, 0.26, 0.12 and 3.0 for new, growing, mature, and old leaves, respectively, plus a leaf area index activity factor of 0.5 for expanding canopies with leaf area index <1.2) provide a more realistic simulation of seasonal methanol concentrations in midlatitudes on the basis of both the IASI and TES measurements.

Phenomena and mechanisms of crack propagation in glass-ceramics.

Lithium disilicate, leucite and apatite glass-ceramics have become state-of-the-art framework materials in the fabrication of all-ceramic dental restorative materials. The goal of this study was to examine the crack propagation behaviour of these three known glass-ceramic materials after they have been subjected to Vickers indentation and to characterize their crack opening profiles (delta(meas) vs. (a-r)). For this purpose, various methods of optical examination were employed. Optical microscopy investigations were performed to examine the crack phenomena at a macroscopic level, while high-resolution techniques, such as scanning electron microscopy (SEM) and atomic force microscopy (AFM), were employed to investigate the crack phenomena at a microscopic level. The crack patterns of the three glass-ceramics vary from fairly straightforward to more complex, depending on the amount of residual glass matrix present in the material. The high-strength lithium disilicate crystals feature a high degree of crosslinking, thereby preventing crack propagation. In this material, the crack propagates only through the residual glass phase, which constitutes 30%-40% by volume. Having a high glass content of more than 65% by volume, the leucite and apatite glass-ceramics show far more complex crack patterns. Cracks in the leucite glass-ceramic propagate through both the glass and crystal phase. The apatite glass-ceramic shows a similar crack behaviour as an inorganic-organic composite material containing nanoscale fillers, which are pulled out in the surroundings of the crack tip. The observed crack behaviour and the calculated K(tip) values of the three types of glass-ceramics were compared to the K(IC) values determined according to the SEVNB method.

Development of an automated cylindrical ion trap mass spectrometer for the determination of atmospheric volatile organic compounds.

Volatile organic compounds released from the biosphere are known to have a large impact on atmospheric chemistry. Field instruments for the detection of these trace gases are often limited by the lack of instrument portability and the inability to distinguish compounds of interest from background or other interfering compounds. We have developed an automated sampling and preconcentration system, coupled to a lightweight, low-power cylindrical ion trap mass spectrometer. The instrument was evaluated by measuring isoprene concentrations during a field campaign at the University of Michigan Biological Station PROPHET lab. Isoprene was preconcentrated by sampling directly into a short capillary column precooled without the aid of cryogens. The capillary column was then rapidly heated by moving the column to a preheated region to obtain fast separation of isoprene from other components, followed by detection with a cylindrical ion trap. This combination yielded a detection limit of approximately 80 ppt (parts per trillion) for isoprene with a measurement frequency of one sample every 11 min. The data obtained by the automated sampling and preconcentration system during the PROPHET 2005 campaign were compared to those of other field instruments measuring isoprene at this site in an intercomparison exercise. The intercomparisons suggest the new inlet system, when coupled with this ion trap detector, provides a viable field instrument for the fast, precise, and quantitative determination of isoprene and other trace gases over a variety of atmospheric conditions.

Intercomparison of volatile organic carbon measurement techniques and data at La Porte during the TexAQS2000 Air Quality Study.

The Texas Air Quality Study 2000 (TexAQS2000) investigated the photochemical production of ozone and the chemistry of related precursors and reaction products in the vicinity of Houston, TX. The colocation of four instruments for the measurement of volatile organic carbon compounds (VOCs) allowed a unique opportunity for the intercomparison of the different in-situ measuring techniques. The instruments included three gas chromatographs, each with a different type of detector, and a Proton-Transfer-Reaction Mass Spectrometer (PTR-MS) with each system designed to measure a different suite of VOCs. Correlation plots and correlation statistics are presented for species measured by more than one of these instruments. The GC instruments were all in agreement to within 10-20% (slope) with coefficients of variation (r2) of > or = 0.85. The PTR-MS agreement with other instruments was more dependent on species with some very good agreements (r2 values of approximately 0.95 for some aromatics), but isoprene, acetaldehyde and propene were substantially less highly correlated (0.55 < r2 < 0.80). At least part of these differences were undoubtedly due to the timing of sample acquisition in an environment in which VOC levels changed very rapidly on both quantitative and temporal scales.

Syne-1, a dystrophin- and Klarsicht-related protein associated with synaptic nuclei at the neuromuscular junction.

We describe a novel protein, Syne-1, that is associated with nuclear envelopes in skeletal, cardiac, and smooth muscle cells. Syne-1 contains multiple spectrin repeats similar to those found in dystrophin and utrophin, as well as a domain homologous to the carboxyl-terminal of Klarsicht, a protein associated with nuclei and required for a subset of nuclear migrations in Drosophila. In adult skeletal muscle fibers, levels of Syne-1 are highest in the nuclei that lie beneath the postsynaptic membrane at the neuromuscular junction. These nuclei are transcriptionally specialized, expressing genes for synaptic components at higher levels than extrasynaptic nuclei in the same cytoplasm. Syne-1 is the first protein found to be selectively associated with synaptic nuclei. Syne-1 becomes concentrated in synaptic nuclei postnatally. It remains synaptically enriched following denervation or degeneration/regeneration, and is also present at high levels in the central nuclei of dystrophic myotubes. The location and structure of Syne-1 suggest that it may participate in the migration of myonuclei in myotubes and/or their anchoring at the postsynaptic apparatus. Finally, we identify a homologous gene, syne-2, that is expressed in an overlapping but distinct pattern.

Development of the neuromuscular junction: genetic analysis in mice.

Formation of the skeletal neuromuscular junction is a multi-step process that requires communication between the nerve and muscle. Studies in many laboratories have led to identification of factors that seem likely to mediate these interactions. 'Knock-out' mice have now been generated with mutations in several genes that encode candidate transsynaptic messengers and components of their effector mechanisms. Using these mice, it is possible to test hypotheses about the control of synaptogenesis. Here, we review our studies on neuromuscular development in mutant mice lacking agrin alpha CGRP, rapsyn, MuSK, dystrophin, dystrobrevin, utrophin, laminin alpha 5, laminin beta 2, collagen alpha 3 (IV), the acetylcholine receptor epsilon subunit, the collagenous tail of acetylcholinesterase, fibroblast growth factor-5, the neural cell adhesion molecule, and tenascin-C.

Nab1, a corepressor of NGFI-A (Egr-1), contains an active transcriptional repression domain.

Nab proteins constitute an evolutionarily conserved family of corepressors that specifically interact with and repress transcription mediated by three members of the NGFI-A (Egr-1, Krox24, zif/268) family of immediate-early gene transcription factors, which includes NGFI-C, Krox20, and Egr3. We explored the mechanism of Nab1 repression and identified structural domains required for Nab1 function. Nab1 does not act by blocking DNA binding or nuclear localization of NGFI-A. In fact, Nab1 repression is not unique to NGFI-A because multiple types of non-NGFI-A activation domains were repressed, as was a heterologous transcription factor carrying the NGFI-A R1 domain, which is required for Nab1 interaction. Additionally, Nab1 tethered directly to DNA repressed constitutively active promoters. Tethered repression was not dependent on the identity of the basal promoter elements, the presence of a distal enhancer, or the distance separating the binding sites from the promoter. These results suggest that Nab1 repression is not specific to particular activators and that Nab1 is an active repressor that works by a direct mechanism. We identified a bipartite-like nuclear localization sequence and localized the repression function to the Nab conserved domain 2 (NCD2), a region found in the carboxy-terminal half of all Nab proteins. Three small regions of homology between Nab1 and previously characterized corepressors, Dr1 and E1b 55-kDa protein, were identified within NCD2. Replacement mutagenesis of residues conserved between these proteins interfered with Nab1 repression, although Nab1 does not function by the same mechanism as Dr1. The human NAB1 genomic locus was mapped to chromosome 2q32.3-33.

Kinase domain of the muscle-specific receptor tyrosine kinase (MuSK) is sufficient for phosphorylation but not clustering of acetylcholine receptors: required role for the MuSK ectodomain?

Formation of the neuromuscular junction (NMJ) depends upon a nerve-derived protein, agrin, acting by means of a muscle-specific receptor tyrosine kinase, MuSK, as well as a required accessory receptor protein known as MASC. We report that MuSK does not merely play a structural role by demonstrating that MuSK kinase activity is required for inducing acetylcholine receptor (AChR) clustering. We also show that MuSK is necessary, and that MuSK kinase domain activation is sufficient, to mediate a key early event in NMJ formation-phosphorylation of the AChR. However, MuSK kinase domain activation and the resulting AChR phosphorylation are not sufficient for AChR clustering; thus we show that the MuSK ectodomain is also required. These results indicate that AChR phosphorylation is not the sole trigger of the clustering process. Moreover, our results suggest that, unlike the ectodomain of all other receptor tyrosine kinases, the MuSK ectodomain plays a required role in addition to simply mediating ligand binding and receptor dimerization, perhaps by helping to recruit NMJ components to a MuSK-based scaffold.

The Nab2 and Stat6 genes share a common transcription termination region.

The two Nab genes, coding for transcriptional corepressors of NGFI-A (Egr-1, Krox24, zif268) and Krox20, have been localized to two regions of the genome, each of which contains at least two members of the Stat gene family. The association of the two Nab genes with the Stat clusters on mouse chromosomes 1 and 10 (human chromosomes 2 and 12) suggest that a Nab gene was involved in at least one of the duplication events that resulted in dispersion of the primordial Stat gene pair to three different mouse chromosomes. Sequencing of the Nab2 genomic locus revealed that it is situated very close to the Stat6 gene. The transcripts of the two genes converge, such that the 3' ends of the Stat6 and Nab2 mRNAs overlap by 58 bp. Both transcripts terminate within a 78-bp region that is absolutely conserved between mouse and human. Analysis of Nab2 cDNA revealed that there is an alternatively spliced form of the Nab2 transcript (lacking exon 3) that produces a protein that lacks the ability to repress transcription by NGFI-A and Krox20.

Rapsyn is required for MuSK signaling and recruits synaptic components to a MuSK-containing scaffold.

Agrin-induced clustering of acetylcholine receptors (AChRs) in the postsynaptic membrane is a key step in synaptogenesis at the neuromuscular junction. The receptor tyrosine kinase MuSK is a component of the agrin receptor, while the cytoplasmic protein rapsyn is necessary for the clustering of AChRs and all other postsynaptic membrane components studied to date. We show here that MuSK remains concentrated at synaptic sites in rapsyn-deficient mutant mice, suggesting that MuSK forms a primary structural scaffold to which rapsyn attaches other synaptic components. Using nonmuscle cells, we show that rapsyn-MuSK interactions are mediated by the ectodomain of MuSK, suggesting the existence of a transmembrane intermediate. In addition to rapsyn's structural role, we demonstrate that it is required for an early step in MuSK signaling, AChR phosphorylation. This signaling requires the kinase domain of MuSK, but not its ectodomain. Thus, MuSK may interact with rapsyn in multiple ways to play both structural and signaling roles in agrin-induced differentiation.

NAB2, a corepressor of NGFI-A (Egr-1) and Krox20, is induced by proliferative and differentiative stimuli.

Previous work had identified a corepressor, NAB1, which represses transcriptional activation mediated by NGFI-A (also known as Egr-1, zif268, and Krox24) and Krox20. These zinc finger transcription factors are encoded by immediate-early genes and have been implicated in a wide variety of proliferative and differentiative processes. We have isolated and characterized another corepressor, NAB2, which is highly related to NAB1 within two discrete domains. The first conserved domain of NAB2 mediates an interaction with the R1 domain of NGFI-A. NAB2 represses the activity of both NGFI-A and Krox20, and its expression is regulated by some of the same stimuli that induce NGFI-A expression, including serum stimulation of fibroblasts and nerve growth factor stimulation of PC12 cells. The human NAB2 gene has been localized to chromosome 12ql3.3-14.1, a region that is rearranged in several solid tumors, lipomas, uterine leiomyomata, and liposarcomas. Sequencing of the Caenorhabditis elegans genome has identified a gene that bears high homology to both NAB1 and NAB2, suggesting that NAB molecules fulfill an evolutionarily conserved role.

The synapse-associated protein rapsyn regulates tyrosine phosphorylation of proteins colocalized at nicotinic acetylcholine receptor clusters.

Protein tyrosine phosphorylation has been suggested to play an important role in the clustering of the nicotinic acetylcholine receptor (AChR) at the developing neuromuscular junction. Recent studies have shown that the 43-kDa synapse-associated protein rapsyn induces clustering of the AChR in heterologous expression systems. In this study we examined whether tyrosine phosphorylation is involved in this rapsyn-induced AChR clustering. Rapsyn-induced AChR clusters in fibroblasts contain phosphotyrosine, as detected using immunofluorescent labeling with anti-phosphotyrosine antibodies. No anti-phosphotyrosine staining of rapsyn clusters is seen in the absence of AChR expression, indicating that the AChR is required for the appearance of phosphotyrosine at clusters. In addition, coexpression of rapsyn with the AChR induces the tyrosine phosphorylation of the beta amd delta subunits of the AChR. Surprisingly, mutation of the tyrosine phosphorylation sites in the AChR did not inhibit rapsyn-induced clustering of the AChR and clusters of the mutant AChRs still contained high levels of phosphotyrosine. Experiments with single AChR subunits demonstrate that the alpha subunit of the AChR appears to be necessary and sufficient for codistribution of phosphotyrosine with rapsyn-induced clusters of AChR subunits. Finally, transfection of cells with rapsyn activates cellular protein tyrosine kinase activity, resulting in the tyrosine phosphorylation of several membrane-associated proteins. These results suggest that rapsyn may therefore regulate clustering at least in part by regulating the tyrosine phosphorylation of cellular proteins.

Rapsyn may function as a link between the acetylcholine receptor and the agrin-binding dystrophin-associated glycoprotein complex.

The 43 kDa AChR-associated protein rapsyn is required for the clustering of nicotinic acetylcholine receptors (AChRs) at the developing neuromuscular junction, but the functions of other postsynaptic proteins colocalized with the AChR are less clear. Here we use a fibroblast expression system to investigate the role of the dystrophin-glycoprotein complex (DGC) in AChR clustering. The agrin-binding component of the DGC, dystroglycan, is found evenly distributed across the cell surface when expressed in fibroblasts. However, dystroglycan colocalizes with AChR-rapsyn clusters when these proteins are coexpressed. Furthermore, dystroglycan colocalizes with rapsyn clusters even in the absence of AChR, indicating that rapsyn can cluster dystroglycan and AChR independently. Immunofluorescence staining using a polyclonal antibody to utrophin reveals a lack of staining of clusters, suggesting that the immunoreactive species, like the AChR, does not mediate the observed rapsyndystroglycan interaction. Rapsyn may therefore be a molecular link connecting the AChR to the DGC. At the neuromuscular synapse, rapsyn-mediated linkage of the AChR to the cytoskeleton-anchored DGC may underlie AChR cluster stabilization.

Assembly of the postsynaptic apparatus.

Recent research has led to a clearer picture of the molecular organization of the postsynaptic apparatus at the developing neuromuscular junction. In addition, one link between the extracellular signaling molecule agrin and the intracellular events that mediate formation of acetylcholine receptor clusters has been established with the identification of an argin-binding protein.

Functional domains of neuromodulin (GAP-43).

Although neuromodulin (GAP-43, B50, F1, pp46, protein 4) was first identified over a decade ago, the physiological function(s) of the protein and the molecular mechanism(s) for its biological activities are still an area of active investigation. Neuromodulin has been implicated in several biological processes in neurons, including growth and regeneration, synaptic plasticity and neurotransmitter release. The molecular mechanisms underlying these implied physiological roles have not been elucidated, but there are several molecular properties of neuromodulin that may be important for its function in neurons. In this review, we will discuss research which has defined several of the functional domains of neuromodulin, including its phosphorylation sites, calmodulin binding domain, membrane binding domain and growth cone targeting domain. We will also suggest possible molecular functions of neuromodulin based on its biochemical properties.