Generation of single skyrmions by picosecond magnetic field pulses

We numerically demonstrate an ultrafast method to create $\textit{single}$ skyrmions in a $\textit{collinear}$ ferromagnetic sample by applying a picosecond (effective) magnetic field pulse in the presence of Dzyaloshinskii-Moriya interaction. For small samples the applied magnetic field pulse could be either spatially uniform or nonuniform while for large samples a nonuniform and localized field is more effective. We examine the phase diagram of pulse width and amplitude for the nucleation. Our finding could ultimately be used to design future skyrmion-based devices.Comment: 4.5 pages+Supplemental Materia

Switching of chiral magnetic skyrmions by picosecond magnetic field pulses via transient topological states

Magnetic chiral skyrmions are vortex like spin structures that appear as stable or meta-stable states in magnetic materials due to the interplay between the symmetric and antisymmetric exchange interactions, applied magnetic field and/or uniaxial anisotropy. Their small size and internal stability make them prospective objects for data storage but for this, the controlled switching between skyrmion states of opposite polarity and topological charge is essential. Here we present a study of magnetic skyrmion switching by an applied magnetic field pulse based on a discrete model of classical spins and atomistic spin dynamics. We found a finite range of coupling parameters corresponding to the coexistence of two degenerate isolated skyrmions characterized by mutually inverted spin structures with opposite polarity and topological charge. We demonstrate how for a wide range of material parameters a short inclined magnetic field pulse can initiate the reliable switching between these states at GHz rates. Detailed analysis of the switching mechanism revealed the complex path of the system accompanied with the excitation of a chiral-achiral meron pair and the formation of an achiral skyrmion

Novel Function of Lysine Methyltransferase G9a in the Regulation of Sox2 Protein Stability

<div><p>G9a is a lysine methyltransferase (KMTase) for histone H3 lysine 9 that plays critical roles in a number of biological processes. Emerging evidence suggests that aberrant expression of G9a contributes to tumor metastasis and maintenance of a malignant phenotype in cancer by inducing epigenetic silencing of tumor suppressor genes. Here, we show that G9a regulates Sox2 protein stability in breast cancer cells. When G9a lysine methyltransferase activity was chemically inhibited in the ER(+) breast cancer cell line MCF7, Sox2 protein levels were decreased. In addition, ectopic overexpression of G9a induced accumulation of Sox2. Changes in cell migration, invasion, and mammosphere formation by MCF7 cells were correlated with the activity or expression level of G9a. Ectopic expression of G9a also increased Sox2 protein levels in another ER(+) breast cancer cell line, ZR-75-1, whereas it did not affect Sox2 expression in MDA-MB-231 cells, an ER(-) breast cancer cell line, or in glioblastoma cell lines. Furthermore, treatment of mouse embryonic stem cells with a KMT inhibitor, BIX-01294, resulted in a rapid reduction in Sox2 protein expression despite increased Sox2 transcript levels. This finding suggests that G9a has a novel function in the regulation of Sox2 protein stability in a cell type-dependent manner.</p></div

Effect of G9a on the transcription and protein expression of Sox2 in various cell types.

<p>(A) The effect of ectopic expression of G9a on Sox2 and Oct4 protein expression in three breast cancer cell lines (MCF7, ZR-75-1, and MDA-MB-231) and two glioblastoma cell lines (U87-MG and T98-G) was determined by immunoblot analysis. Each cell line was transfected with a G9a-expressing plasmid for 24 h and then harvested for analysis. For quantitative analysis of immunoblotting results, the mean density of the Sox2 band was measured with Multi Gauge V3.0 software, and the band density was divided by the density of β-actin to obtain the normalized band density. (B) Cells were transfected with control or G9a expressing plasmids for 24 h, and the effect of ectopic expression of G9a on the transcription of Sox2 was quantitatively analyzed by real-time RT-PCR. (C) The effect of G9a activity on Sox2 protein stability in mouse ESCs. Mouse ESCs were treated with 0.2 μM BIX-01294 for the indicated times and then harvested for immunoblot analysis. (D) The effect of G9a activity on the transcription of Sox2 in mouse ESCs. Mouse ESCs were treated with 0.2 μM BIX-01294 for the indicated times and then harvested for quantitative analysis of Sox2 transcription by real-time RT-PCR. Abbreviations: G9a, G9a-expressing plasmid; Sox2, Sox2-expressing plasmid; C, control plasmid. *P < 0.05 compared with the control group. Data are expressed as the mean ± SD and are representative of three independent experiments.</p

Chemical inhibition of G9a activity reduces the stability of Sox2 protein in MCF7 cells.

<p>(A) MCF7 cells were treated with the indicated doses of BIX-01294 for 24 h. Whole cell lysates were harvested and analyzed by immunoblotting with G9a-, Sox2-, and Oct4-specific antibodies. (B) MCF7 cells were treated with 0.2 μM of BIX-01294 for 96 h, and whole cell lysates were analyzed for the expression of Sox2 and Oct4. (C) MCF7 cells were treated with 2.0 μM BIX-01294 for 96 h, and the levels of Sox2 transcripts were analyzed by real-time RT-PCR analysis. mRNA levels were normalized to those of GAPDH. (D) Sox2 and Oct4 protein levels in MCF7 cells were analyzed by immunoblotting after cells were treated with 2.0 μM BIX-01294 in the presence of the protein synthesis inhibitor, cycloheximide (5 μM). The graph shows band intensity data normalized according to the controls and β-actin. (E) Cell growth was analyzed by counting cell numbers at the indicated times after treatment with 2.0 μM BIX-01294. For quantitative analysis of the immunoblotting results, the mean density of the Sox2 band was measured with Multi Gauge V3.0 software, and the band density was divided by the density of β-actin to obtain the normalized band density. * P < 0.05 compared with the control group. Data are expressed as the mean ± SD and are representative of three independent experiments.</p

Expression level of G9a is correlated with the migration and invasion of MCF7 cells.

<p>(A) Wound-healing assay examining the effect of chemical inhibition of G9a activity on the migration of MCF7 cells. MCF7 cells were treated with 0.2 μM BIX-01294 and wounded using a micropipette tip when the cells were fully confluent. Cell migration was then monitored. (B) Wound-healing assay to examine the effect of ectopic expression of G9a on the migration of MCF7 cells. MCF7 cells were transfected with control or G9a expression plasmids and then re-seeded for the migration assay. When fully confluent, the cells were wounded using a micropipette tip, and cell migration was photographed at the indicated times. (C) The effect of G9a on the invasiveness of MCF7 cells was examined in a Transwell invasion assay. MCF7 cells were first transfected with control or G9a expression plasmids along with siNS or siSox2. Twenty-four hours later, cells were suspended in serum-free medium and seeded in 24-well Matrigel-coated Transwell chambers. Cells crossing the Matrigel-coated filter were fixed and stained with Crystal violet. The number of stained cells was then counted. Representative pictures are shown (left panel). The average number of stained cells in five random microscopic fields from three independent experiments is presented graphically (right panel). (D) The effect of G9a on the mammosphere-forming ability of MCF7 cells was examined in cells maintained under low-serum non-adherent culture conditions for 7 days in the presence or absence of 2 μM BIX-01294. Abbreviations: NT, no treatment; Control (siNS), cells transfected with a control plasmid and non-specific siRNA; OE (siNS), cells transfected with a G9a-expressing plasmid and non-specific siRNA; OE (siSox2), cells transfected with a G9a-expressing plasmid and siSox2. *P < 0.05 and **P < 0.01 compared with the control group. Data are expressed as the mean ± SD and representative of three independent experiments.</p