Superconductivity in Dilute Hydrides of Ammonia under Pressure.

The past decade has witnessed great progress in predicting and synthesizing polyhydrides that exhibit superconductivity under pressure. Dopants allow these compounds to become metals at pressures lower than those required to metallize elemental hydrogen. Here, we show that by combining the fundamental planetary building blocks of molecular hydrogen and ammonia, conventional superconducting compounds can be formed at high pressure. Through extensive theoretical calculations, we predict metallic metastable structures with NHn (n = 10, 11, 24) stoichiometries that are based on NH4+ superalkali cations and complex hydrogenic lattices. The hydrogen atoms in the molecular cation contribute to the superconducting mechanism, and the estimated superconducting critical temperatures, Tc's, are comparable to the highest values computed for the alkali metal polyhydrides. The largest calculated (isotropic Eliashberg) Tc is ∼180 K for Pnma-NH10 at 300 GPa. Our results suggest that other molecular cations can be mixed with hydrogen under pressure, yielding superconducting compounds.

Conventional High-Temperature Superconductivity in Metallic, Covalently Bonded, Binary-Guest C-B Clathrates.

Inspired by the synthesis of XB3C3 (X = Sr, La) compounds in the bipartite sodalite clathrate structure, density functional theory (DFT) calculations are performed on members of this family containing up to two different metal atoms. A DFT-chemical pressure analysis on systems with X = Mg, Ca, Sr, Ba reveals that the size of the metal cation, which can be tuned to stabilize the B-C framework, is key for their ambient-pressure dynamic stability. High-throughput density functional theory calculations on 105 Pm3̅ symmetry XYB6C6 binary-guest compounds (where X, Y are electropositive metal atoms) find 22 that are dynamically stable at 1 atm, expanding the number of potentially synthesizable phases by 19 (18 metals and 1 insulator). The density of states at the Fermi level and superconducting critical temperature, Tc, can be tuned by changing the average oxidation state of the metal atoms, with Tc being highest for an average valence of +1.5. KPbB6C6, with an ambient-pressure Eliashberg Tc of 88 K, is predicted to possess the highest Tc among the studied Pm3̅n XB3C3 or Pm3̅ XYB6C6 phases, and calculations suggest it may be synthesized using high-pressure high-temperature techniques and then quenched to ambient conditions.

A Metastable CaSH3 Phase Composed of HS Honeycomb Sheets that is Superconducting Under Pressure.

Evolutionary searches have predicted a number of ternary Ca-S-H phases that could be synthesized at pressures of 100-300 GPa. P63/mmc CaSH2, Pnma CaSH2, Cmc21 CaSH6, and I4̅ CaSH20 were composed of a Ca-S lattice along with H2 molecules coordinated in a "side-on" fashion to Ca. The H-H bond lengths in these semiconducting phases were elongated because of H2 σ → Ca d donation, and Ca d → H2 σ* back-donation, via a Kubas-like mechanism. P6̅m2 CaSH3, consisting of two-dimensional HS and CaH2 sheets, was metastable and metallic above 128 GPa. The presence of van Hove singularities increased its density of states at the Fermi level and concomitantly the superconducting critical temperature, which was estimated to be as high as ∼100 K at 128 GPa. This work will inspire the search for superconductivity in materials based upon honeycomb HX (X = S, Se, Te) and MH2 (M = Mg, Ca, Sr, Ba) layers under pressure.

The Anthelmintic Drug Niclosamide Inhibits the Proliferative Activity of Human Osteosarcoma Cells by Targeting Multiple Signal Pathways.

Osteosarcoma (OS) is the most common primary malignant tumor of bone with a high propensity for lung metastasis. Despite significant advances in surgical techniques and chemotherapeutic regimens over the past few decades, there has been minimal improvement in OS patient survival. There is an urgent need to identify novel antitumor agents to treat human OS. Repurposing the clinically-used drugs represents a rapid and effective approach to the development of new anticancer agents. The anthelmintic drug niclosamide has recently been identified as a potential anticancer agent in human cancers. Here, we investigate if niclosamide can be developed as an anti-OS drug. We find that niclosamide can effectively inhibit OS cell proliferation and survival at low micromolar concentrations. Cell migration and wounding closure are significantly inhibited by niclosamide. Niclosamide induces cell apoptosis and inhibits cell cycle progression in OS cells. Analysis of niclosamide's effect on 11 cancer-related signal pathway reporters reveals that three of them, the E2F1, AP1, and c-Myc-responsive reporters, are significantly inhibited. To a lesser extent, the HIF1α, TCF/LEF, CREB, NFκB, Smad/TGFß, and Rbpj/Notch pathway reporters are also inhibited, while the NFAT and Wnt/ß-catenin reporters are not significantly affected by niclosamide treatment. We demonstrate that the expression of c-Fos, c-Jun. E2F1, and c-Myc in OS cells is effectively inhibited by niclosamide. Furthermore, niclosamide is shown to effectively inhibit tumor growth in a mouse xenograft tumor model of human osteosarcoma cells. Taken together, these results strongly suggest that niclosamide may exert its anticancer activity in OS cells by targeting multiple signaling pathways. Future investigations should be directed to exploring the antitumor activity in clinically relevant OS models and ultimately in clinical trials.

Sustained high level transgene expression in mammalian cells mediated by the optimized piggyBac transposon system.

Sustained, high level transgene expression in mammalian cells, especially stem cells, may be desired in many cases for studying gene functions. Traditionally, stable transgene expression has been accomplished by using retroviral or lentiviral vectors. However, such viral vector-mediated transgene expression is often at low levels and can be reduced over time due to low copy numbers and/or chromatin remodeling repression. The piggyBac transposon has emerged as a promising non-viral vector system for efficient gene transfer into mammalian cells. Despite its inherent advantages over lentiviral and retroviral systems, piggyBac system has not been widely used, at least in part due to the limited availability of piggyBac vectors with manipulation flexibilities. Here, we seek to optimize piggyBac-mediated transgene expression and generate a more efficient, user-friendly piggyBac system. By engineering a panel of versatile piggyBac vectors and constructing recombinant adenoviruses expressing piggyBac transposase (PBase), we demonstrate that adenovirus-mediated PBase expression significantly enhances the integration efficiency and expression level of transgenes in mesenchymal stem cells and osteosarcoma cells, compared to that obtained from co-transfection of the CMV-PBase plasmid. We further determine the drug selection timeline to achieve optimal stable transgene expression. Moreover, we demonstrate that the transgene copy number of piggyBac-mediated integration is approximately 10 times higher than that mediated by retroviral vectors. Using the engineered tandem expression vector, we show that three transgenes can be simultaneously expressed in a single vector with high efficiency. Thus, these results strongly suggest that the optimized piggyBac system is a valuable tool for making stable cell lines with sustained, high transgene expression.

A simplified and versatile system for the simultaneous expression of multiple siRNAs in mammalian cells using Gibson DNA Assembly.

RNA interference (RNAi) denotes sequence-specific mRNA degradation induced by short interfering double-stranded RNA (siRNA) and has become a revolutionary tool for functional annotation of mammalian genes, as well as for development of novel therapeutics. The practical applications of RNAi are usually achieved by expressing short hairpin RNAs (shRNAs) or siRNAs in cells. However, a major technical challenge is to simultaneously express multiple siRNAs to silence one or more genes. We previously developed pSOS system, in which siRNA duplexes are made from oligo templates driven by opposing U6 and H1 promoters. While effective, it is not equipped to express multiple siRNAs in a single vector. Gibson DNA Assembly (GDA) is an in vitro recombination system that has the capacity to assemble multiple overlapping DNA molecules in a single isothermal step. Here, we developed a GDA-based pSOK assembly system for constructing single vectors that express multiple siRNA sites. The assembly fragments were generated by PCR amplifications from the U6-H1 template vector pB2B. GDA assembly specificity was conferred by the overlapping unique siRNA sequences of insert fragments. To prove the technical feasibility, we constructed pSOK vectors that contain four siRNA sites and three siRNA sites targeting human and mouse ß-catenin, respectively. The assembly reactions were efficient, and candidate clones were readily identified by PCR screening. Multiple ß-catenin siRNAs effectively silenced endogenous ß-catenin expression, inhibited Wnt3A-induced ß-catenin/Tcf4 reporter activity and expression of Wnt/ß-catenin downstream genes. Silencing ß-catenin in mesenchymal stem cells inhibited Wnt3A-induced early osteogenic differentiation and significantly diminished synergistic osteogenic activity between BMP9 and Wnt3A in vitro and in vivo. These findings demonstrate that the GDA-based pSOK system has been proven simplistic, effective and versatile for simultaneous expression of multiple siRNAs. Thus, the reported pSOK system should be a valuable tool for gene function studies and development of novel therapeutics.