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A Role for Protein Kinase Dbf2-Mob1 in Mitotic Exit
Get PDFExit from mitosis is characterized by precise control of the cyclin-dependent kinase complex (Cdk) activity, breaking down mitotic structures, and completing cytokinesis. In Saccaromyces cerevisiae, protein phosphotase Cdc14 is involved in counteracting mitotic-Cdk activity by promoting the degradation of mitotic cyclin Clb2, and stabilizing the Cdc28 inhibitor Sic1. The activity and the cellular localization of Cdc14 are tightly regulated by the cell cycle. Cdc14 is sequestered and inhibited in the nucleolus by forming the RENT (Regulator of Nucleolar Silencing and Teleophase) complex for most of the cell cycle. It is released and distributed into the cytoplasm in anaphase and telophase, and then returns to the nucleolus in G1 phase. Activation of Cdc14 is achieved via multi-site phosphorylation of Net1 leading to the release of Cdc14. Net1 is first phosphorylated by the Fourteen Early Anaphase Release (FEAR) network, and then by the Mitotic Exit Network (MEN).
In this thesis, we show that a MEN component, protein kinase Dbf2-Mob1, plays a role in phosphorylating Net1 in late anaphase. We identified the effective Dbf2-Mob1 phosphorylation sites in the N-terminal of Net1 by in vitro kinase reaction assay. We found that cells that express mutant Net1 show growth defects and chain-like terminal morphology under restrictive temperatures. Genetic interactions suggested that the MEN kinases and Cdc14 are related to the cell cycle defects caused by the phosphosite mutated Net1. Analyzing the phosphosite mutants with Fluorecence-activated cell sorting (FACS) and immunofluorescence assay, we found that the growth defects and the abnormal cell morphology are due to a defect in releasing Cdc14 in late anaphase, leading to disruption of mitotic exit. This result is further confirmed by western blot assay and the beads releasing assay.
In summary, the regulation of Cdc14 release in late anaphase via phosphorylation of its inhibitor Net1 by Dbf2-Mob1 is demonstrated in this work. This thesis provides a crucial piece of information that furthers our understanding of the mechanism of mitotic exit. It also points to a fascinating mechanism of controlling cytokinesis and meiosis by regulating Net1 phosphorylation by the MEN.</p
Zn₃P₂ and Cu₂O Substrates for Solar Energy Conversion
Get PDFZinc phosphide (Zn3P2) and cuprous oxide (Cu2O) are promising and earth-abundant alternatives to traditional thin film photovoltaics materials such as CIGS, CdTe, and a-Si. We have prepared high purity substrates of Zn3P2 from elemental zinc and phosphorus, and Cu2O by the thermal oxidation of copper foils, to investigate their fundamental material properties and potential for solar energy conversion. Photoluminescence-based measurements of Zn3P2 substrates have revealed a fundamental indirect band gap at 1.38 eV and a direct band gap at 1.50 eV, with time-resolved data indicating minority carrier diusion lengths of ≥7 μm. Solar cells based on Mg/Zn3P2 junctions with solar energy conversion efficiency reaching 4.5% were examined by composition profiling to elucidate the passivation reaction between Mg metal and Zn3P2 surfaces. Semiconductor/liquid junctions incorporating Cu2O substrates exhibited open-circuit voltage, Voc, values in excess of 800 mV and internal quantum yields approaching 100% in the 400–500 nm spectral range
Interaction of Planning Regions in Cortex
Get PDFTo what extent do parietal and frontal areas involved in action planning interact as a monkey plans a movement? This report seeks an answer using the timing relationships between action potentials, local field potentials (LFPs) and behavioral events as a monkey plans reaches and eye movements to remembered targets. Both parietal reach region (PRR) and dorsal premotor cortex (PMd) show similar profiles of activity characteristic of action planning. In some cases, both premotor and intraparietal areas show decision-making activity far earlier than previously anticipated, even before the onset of the trial. However, despite their similarities in action planning, PMd responds tens of milliseconds sooner to targets and movement instructions. These results suggest that PMd precedes PRR, apparently contrary to a common heuristic about the chain of processing from sensation to action. On the other hand, during periods of steady state, as the monkey anticipates information or plans a movement, the apparent directionality of fronto-parietal interaction may reverse. Coherent phase-locking between action potentials and local field potentials (LFPs), which has been implicated in directional influence between brain regions, is highly significant from PRR to PMd, but not vice-versa. Spikes in PRR cohere with LFPs in PMd between 15–25 Hz, whereas spikes in PMd do not cohere with LFPs in PRR at any frequency. This uni-directional spike-LFP coherence varies over the course of the trial, achieving a peak in magnitude and frequency, on average, during the planning period. The phase-locking component of the coherence shows weak but significant variation according to the particular action being planned. The cross-cortical coherence also varies significantly with cortical anatomy. Coherence is stronger between spikes in PRR and LFPs in its anatomical target PMd than between PRR and other recording areas within and beyond the arcuate sulcus (associated with saccades, and not known to be connected with PRR). The asymmetry of spike - LFP coherence, its task–dependence, and variation over cortical territory add to a growing body of knowledge implicating the intraparietal sulcus as the center of a network of beta-band activity characteristic of action planning. This highly specific beta-band oscillation links frontal and parietal planning regions at the single cell level. Overall, these results suggest an interplay between premotor and parietal regions, with influence shifting back and forth according to the phase of behavior
Neural and Behavioral Investigations of Social Reward Processing
Get PDFDespite an extensive literature on the neural substrates of reward, relatively little is known about how social interactions modify decision-making. Here I present three experiments that examine the neural basis of social reward processing both in neurotypicals and individuals with autism spectrum disorder (ASD), a neuropsychiatric syndrome associated with social cognition impairments. Using functional magnetic resonance imaging (fMRI), I recorded brain activity during a probabilistic reward learning task with either social (smiling/frowning faces) or monetary (gaining/losing money) rewards. I found substantial overlap in the neural circuitry associated with social and non-social reward processing, suggesting that social rewards are processed similarly to other types of rewards. In contrast, individuals with ASD showed behavioral impairments in social reward processing, both in probabilistic reward learning and in an ecologically valid charitable donation task. Exploratory neuroimaging in ASD showed hypoactivation of key reward areas during decision-making. Taken together, these findings support the idea of a “common neural currency” in decision-making but also suggest the construction of accurate social reward value signals relies on recruitment of additional regions known to process social information
The SPIDER CMB Polarimeter
Get PDFSPIDER is a balloon-borne millimeter-wave telescope designed to study the polarization of the Cosmic Microwave Background (CMB). SPIDER will map 10% of the full sky with degree-scale beams to search for the distinctive inflationary gravitational wave signal on angular scales between 1 degree and 10 degrees, thereby probing the energy scale of inflation. In its first flight, SPIDER will field 2,400 antenna-coupled bolometers split between two bands centered at 93 GHz and 148 GHz. Slot antenna arrays, band defining microstrip filters and superconducting bolometers are all fabricated photolithographically on a shared silicon substrate. SPIDER's detectors are split amongst six monochromatic on-axis refractors in a shared helium-cooled cryostat. This thesis reviews the design of SPIDER and its antenna-coupled bolometers, and details the currently achieved performance of SPIDER's receivers
I. Chemical-Scale Studies of Ligand-Gated Ion Channels, and II. Novel Methods for Phosphonate Synthesis
Get PDFSection 1: Chemical-Scale Studies of Ligand-Gated Ion Channels : Ligand-gated ion channels are amazing molecular machines that respond to specific small-molecule agonists by opening a central pore to enable ions to flow through them. In the aggregate, they transduce chemical signals into electrical currents, and they have numerous critical physiological functions. The tools of pharmacology and unnatural amino acid (and hydroxy acid) mutagenesis enable us to study these receptors on an atomic level. Two such projects are presented here. First, the synthesis of a new 5-HT3 receptor agonist helps to map the receptor’s binding site. Second, mutant cycle analysis in the nicotinic acetylcholine receptor with the novel unnatural residue α-hydroxyserine (Sah) enables the identification of a crucial hydrogen bond whose formation is part of the pathway leading from acetylcholine binding to pore opening.
Section 2: Novel Methods for Phosphonate Synthesis : Phosphonates are a key functional group in both organic synthesis and biological chemistry. The Arbuzov reaction stands as dominant method available for synthesizing this important class of compounds. Two new methods for phosphonate synthesis are presented here. The first method enables room-temperature phosphonate synthesis from carboxylic acids, taking advantage of a novel Wolff-Kishner-type reductive deoxygenation of an intermediate acyl phosphonate. The second method enables phosphonate synthesis through the reductive coupling of ketones/aldehydes with dialkyl phosphites, mediated by a tosylhydrazone derivative. The latter method requires only mild heating (60 °C) and enables access to phosphonates containing azides, benzyl halides, and other functional groups poorly tolerated by the Arbuzov onditions.</p
Aryne Annulation Reactions Toward the Synthesis of Heterocyclic Molecules
Get PDFThe last decade has seen an outgrowth in the development of synthetic methodologies exploiting benzyne. The unique ability of this reactive intermediate to directly furnish ortho-difuntionalized aromatic systems first stoked interest in this research group as a possible partner in asymmetric arylation reactions. Since our initial forays, we have expanded our synthetic strategies to include bond insertions, cycloadditions, condensations, and multicomponent reactions. The first project discussed in this volume is the development of an aryne annulation strategy for constructing common, synthetically useful heterocyclic structures in a convergent manner. We have developed a convergent approach to indoles and indolines. Likewise, through an orthogonal functional group intallation upon an enamine substrate, isoquinolines, quinolines, and isoquinolones can all be accessed as well. In this manner, we have been able to generate an array of functionalized heterocycles, including some that are prohibited by traditional means of synthesis. We have also begun to understand some of the reactivity trends in this context for the elusive aryne reaction partner. The development of the aryne annulation strategy for the synthesis of isoquinolines directly led to the shortest reported total synthesis of the opiate alkaloid papaverine, and the tetrahydroisoquinoline anticancer antibiotic quinocarcin. Our more recent, ongoing efforts toward the synthesis of the bis-tetrahydroisoquinoline antitumor molecule jorumycin and its many structural relatives are detailed herein. Jorumycin has been targeted through a combination of aryne annulation and acyl-alkylation/condensation methodologies aimed at the synthesis of a functionalized bis-isoquinoline intermediate. Reduction of this key bis-isoquinoline to a bis-tetrahydroisoquinoline and subsequent lactamization will provide the pentacyclic core of jorumycin and related natural products in only three steps from simple isoquinoline building blocks. The final project described is the development of several different aryne multicomponent reactions to form novel carbo- and heterocyclic scaffolds, including iminoisobenzfurans, iminoindenones, dibenzoketocaprolactams, and 2-quinolones
Atomistic Simulations of Material Properties under Extreme Conditions
Get PDFExtreme conditions involve low or high temperatures (> 1500 K), high pressures (> 30 MPa), high strains or strain rates, high radiation fluxes (> 100 dpa), and high electromagnetic fields (> 15T). Material properties under extreme conditions can be extremely different from those under normal conditions. Understanding material properties and performance under extreme conditions, including their dynamic evolution over time, plays an essential role in improving material properties and developing novel materials with desired properties.
To understand material properties under extreme conditions, we use molecular dynamics (MD) simulations with recently developed reactive force fields (ReaxFF) and traditional embedded atom methods (EAM) potentials to examine various materials (e.g., energetic materials and binary liquids) and processes. The key results from the simulations are summarized below.
Anisotropic sensitivity of RDX crystals: Based on the compress-and-shear reactive dynamics (CS-RD) simulations of cyclotrimethylene trinitramine (RDX) crystals, we predict that for mechanical shocks between 3 and 7 GPa, RDX is the most sensitive to shocks perpendicular to the (100) and (210) planes, while it is insensitive to those perpendicular to the (120), (111), and (110) planes. The simulations demonstrate that the molecular origin of anisotropic shock sensitivity is the steric hindrance to shearing of adjacent slip planes.
Mechanisms of hotspot formation in polymer bonded explosives (PBXs): The simulations of a realistic model of PBXs reveal that hotspots may form at the nonplanar interfaces where shear relaxation leads to a dramatic temperature increase that persists long after the shock front has passed the interface. For energetic materials this temperature increase is coupled to chemical reactions that eventually lead to detonation. We show that decreasing the density of the binder eliminates the hotspots or reduces the sensitivity.
Cavitation in binary metallic liquids: We demonstrate the stochastic nature of the cavitation process in binary metallic liquids, and that classical nucleation theory can predict the cavitation rate if we incorporate the Tolman length derived from the MD simulations.
Synthesis the single metallic glass on amorphous substrate: We show that single component metallic glasses (SCMGs) can be synthesized by thermal spray coating of nanodroplets onto an amorphous substrate (ND-AS). The key requirements to form the SCMGs are the rapid cooling rates and the amorphous substrates.
Carbon and hydrogen phases under extreme conditions: we report on the use of electron force fields (eFF) in characterizing the Hugoniot relationships of carbon, which includes consecutive phase transitions also captured by experiments, as well as the Hugonoit states of hydrogen centered at various initial densities compared to experiments and the predictions of other theories.</p
Distributed Control and Computing: Optimal Estimation, Error Correcting Codes, and Interactive Protocols
Get PDFEmerging applications of networked control and distributed computing are characterized by decentralization of information and the need to exchange it over potentially unreliable communication networks. This results in novel interactive communication scenarios that are incompatible with conventional information and coding theoretic approaches. To address this gap, through the early and late 1990's, a new information theoretic notion called anytime reliability and a new coding paradigm called tree codes were proposed. Although the central role of tree codes in several interactive communication problems such as distributed control and computing has been well understood, there have been no practical constructions till date. For the first time, we have an explicit ensemble of linear tree codes with efficient encoding and decoding for the class of erasure channels. In the process, we have developed novel non-asymptotic sufficient conditions on the kind of communication reliability required to stabilize control systems over noisy channels. We also study the application of tree codes to interactive protocols over erasure networks and illustrate their benefits through the example of average consensus
Iron in the Ocean: Laboratory Experiments of Iron Geochemistry in the Presence of Marine Particles
Get PDFIron (Fe) is an important micronutrient for primary productivity in the ocean. The Fe cycle in the ocean is relatively unconstrained, especially when it comes to quantifying sources and sinks related to exchange with particulate matter. This thesis attempts to constrain some of the kinetic and equilibrium particle interactions with Fe bound to the siderophore desferrioxamine B (DFB). Out of five inorganic particle types investigated, ferrihydrite, goethite, opal, foraminifera, and montmorillonite, ferrihydrite has the largest, extended impact on dissolved FeDFB. From experimental and modeling results, ferrihydrite has two primary exchange pathways, absorption, with a rate of 4 ± 2 x 10-4 /(mg/L) per day, and dissolution, with a rate of 0.015 ± 0.01 per day. Uptake appears irreversible and follows a colloidal pumping model. Isotopic fractionation is also the greatest in the presence of ferrihydrite with signals up to +1‰ or higher with excess ligand. Dry montmorillonite has the biggest initial impact on FeDFB, resulting in a nearly instantaneous equilibrium and little isotopic fractionation. Goethite, opal, and foraminifera all have a minimal impact on FeDFB and show slight enriched isotopic fractionation, +0.15‰, in the presence of large particle concentrations. DFB seems to induce heavy Fe desorption or dissolution, while particle uptake seems to favor transfer of lighter Fe. These isotopic and kinetic parameters are important constraints on the ability of particles to control dissolved Fe, since they fall through the water column faster than equilibrium will be obtained