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O Krebs

Publications and source records attributed to O Krebs.

17 recordsLinked to original sources
PubMed2005

Electrical control of hole spin relaxation in charge tunable InAs/GaAs quantum dots.

We report on optical orientation of singly charged excitons (trions) in charge-tunable self-assembled InAs/GaAs quantum dots. When the charge varies from 0 to -2, the trion photoluminescence of a single quantum dot shows up and under quasiresonant excitation gets progressively polarized from zero to approximately 100%. This behavior is interpreted as the electric control of the trion thermalization process, which subsequently acts on the hole-spin relaxation driven in nanosecond time scale by the anisotropic electron-hole exchange. This is supported by the excitation spectroscopy and time-resolved measurements of a quantum dot ensemble.

Journal Article↗
PubMed2005

Direct observation of the electron spin relaxation induced by nuclei in quantum dots.

We have studied the electron spin relaxation in semiconductor InAs/GaAs quantum dots by time-resolved optical spectroscopy. The average spin polarization of the electrons in an ensemble of p-doped quantum dots decays down to 1/3 of its initial value with a characteristic time T(Delta) approximately 500 ps, which is attributed to the hyperfine interaction with randomly oriented nuclear spins. We show that this efficient electron spin relaxation mechanism can be suppressed by an external magnetic field as small as 100 mT.

Journal Article↗
PubMed2002

Optically driven spin memory in n-doped InAs-GaAs quantum dots.

We show that the spin state of the resident electron in an n-doped self-assembled InAs-GaAs quantum dot can be written and read using nonresonant, circularly polarized optical pumping. A simple theoretical model is presented and accounts for the remarkable dynamics producing counterpolarized photoluminescence.

Journal Article↗
PubMed2001

Insulin-like growth factor-binding protein-5 inhibits growth and induces differentiation of mouse osteosarcoma cells.

The precise role of insulin-like growth factor-binding protein-5 (IGFBP-5) in regulating the growth of tumor cells, especially of bone-derived malignant cells, is not well understood. We have investigated the biological activity of IGFBP-5 by transfecting OS/50-K8 mouse osteosarcoma cells with an expression vector containing the osteocalcin promoter and the complete mouse IGFBP-5 cDNA (OC-IGFBP-5). Overexpression of IGFBP-5 mRNA and secretion of increased amounts of bioactive protein in conditioned media were demonstrated in different clones. For the analysis of cell proliferation, three clones exhibiting high levels of IGFBP-5 expression were selected and compared to a mock clone and to nontransfected parental cells. IGFBP-5-secreting clones displayed reduced proliferation under both anchorage-dependent and -independent conditions (P < 0.05). The increase in proliferation observed in IGFBP-5-secreting clones after addition of exogenous IGF was significantly lower than that observed in mock-transfected or parental cells. A similar result was obtained with long[R3]IGF-I which has a low affinity for all IGFBPs, suggesting that the inhibitory effect of IGFBP-5 is only partially IGF-dependent. OC-IGFBP-5-transfected clones expressed significantly higher amounts of osteocalcin mRNA (P < 0.05) and secreted more osteocalcin protein than a mock clone or parental OS-50/K8 cells. Thus, part of the growth-inhibiting effect of IGFBP-5 may be due to an induction of differentiation in these cells.

Animals↗
PubMed2001

An experimental and computational study on the reactivity and regioselectivity for the nitrosoarene ene reaction: comparison with triazolinedione and singlet oxygen.

The regioselectivities and the reactivities (relative rates) for the ene reaction of the enophile 4-nitronitrosobenzene (ArNO) with an extensive set of regiochemically defined acyclic and cyclic olefins have been determined. These experimental data establish that the ArNO enophile attacks the olefinic substrate along the novel skew trajectory, with preferred hydrogen abstraction at the corner (twix regioselectivity). This is in contrast to the isoelectronic species singlet oxygen ((1)O(2)), which abstracts at the higher substituted side of the double-bond (cis effect), and triazolindione (TAD), which undergoes the ene reaction at the more crowded end (gem effect). Ab initio computations (B3LYP/6-31+g) for the ene reaction of the ArNO with 2-methyl-2-butene reveal that the steric effects between the aryl group of the enophile and the substituents of the olefin dictate the skew trajectory. These computations identify the aziridine N-oxide (AI) as a bona fide intermediate in this ene reaction, whose formation is usually rate-determining and, thus, irreversible along the skew trajectory (twix selectivity). The reversible generation of the AI becomes feasible when conformational constraints outweigh steric effects, as manifested by enhanced twin regioselectivity.

Journal Article↗
PubMed2000

Overexpression of insulin-like growth factor-II in mouse embryonic stem cells promotes myogenic differentiation.

Embryonic stem (ES) cells derived from androgenetic or parthenogenetic mouse embryos are important tools for studying the roles of imprinted genes in early development. Androgenetic ES cells have been shown to preferentially differentiate into the myogenic lineage both in vitro and after formation of teratocarcinomas in vivo. To clarify if the maternally imprinted Igf2 gene which is expected to be overexpressed in androgenetic ES cells is sufficient to induce myogenic differentiation, R1 ES cells were transfected with human IGF-II expression vectors. Stable ES cell clones exhibiting human IGF-II mRNA and protein expression were studied vs ES cell clones without IGF-II overexpression in a standard in vitro differentiation system involving culture in "hanging drops" and observation of differentiation of the recovered embryoid bodies (EBs). EBs derived from IGF-II overexpressing ES cells showed stimulated myogenic differentiation evident by the appearance of myoblasts already 3 days after plating and by higher levels of skeletal muscle-specific transcripts (myf5, myoD, myogenin) at earlier stages. Our study demonstrates for the first time that overexpression of IGF-II enhances and accelerates myogenic differentiation of ES cells, which has implications for ES cell-derived tissue engineering.

Animals↗
PubMed2000

Steric and conformational control of the regioselectivities in the ene reaction with trisubstituted cycloalkenes: comparison of the enophiles singlet oxygen, triazolinedione, and nitrosoarene

The nitrosoarene ene reaction with the cycloalkenes 1-3 and E-4 proceeds in high twix regioselectivity to afford the hydroxylamine ene products 1a-4a (twix) and 1b-4b (twin, except far E-4 twix). Steric interactions in the enophile attack are responsible for the skew trajectory of the nitrosoarene enophile. For Z-1-methylcyclooctene (Z-4), twin abstraction dominates, caused by conformational constraints (transannular interactions) in the hydrgogen-atom abstraction. The balance between these steric and conformational factors dictates the regioselectivity in the ene reaction

Journal Article↗
PubMed2000

Large-scale N-ethyl-N-nitrosourea mutagenesis of mice--from phenotypes to genes.

The most important tool for obtaining insight into the function of genes is the use of mutant model organisms. Homologous recombination in embryonic stem cells allows the systematic production of mouse mutants for any gene that has been cloned. Gene trap strategies have been designed to interrupt even unknown genes which are tagged by the inserted vector and can be characterised structurally and functionally. Complementary to such 'gene-driven' approaches, 'phenotype-driven' approaches are necessary to identify new genes or gene products through a search for mutants with specific defects, uncovering the function of genetic pathways in physiological and pathological processes. Mutagenesis using the alkylating agent N-ethyl-N-nitrosourea (ENU) is a powerful approach for the production of such mouse mutants. Since ENU induces mainly point mutations in premeiotic spermatogonia, this strategy allows the production of multiple alleles of a particular gene, which is pivotal for a fine tuned analysis of its function.

Alkylating Agents↗
PubMed1998

Enzymatic activity of two caspases related to interleukin-1beta-converting enzyme.

Interleukin-1beta-converting enzyme is a member of a family of human cysteine proteases with specificity for aspartic acid, which have been named caspases. Within this family of enzymes, transcript X (TX) and transcript Y (TY) (caspases 4 and 5, respectively) are very similar to ICE (caspase 1) and form the ICE subfamily. Given the high degree of conservation in the sequences of these proteases (more than 50% amino acid identity in the mature enzymes), it was of interest to examine whether they shared similar substrate specificities. The three enzymes, ICE, TX and TY, were therefore expressed in baculovirus-infected insect cells, as 30-kDa proteins lacking the propeptide. Automaturation into p20 and p10 subunits occurred within the cells. Active ICE, TX and TY were collected in the cell culture supernatants. In addition, their production induced the activation of an endogenous 32-kDa putative cysteine protease (CPP32) like caspase. T7-tagged ICE, TX and TY were purified by immunoaffinity and tested for their catalytic efficiency on YVAD-containing synthetic substrates and on the ICE natural substrate, pro-interleukin-1beta. TX cleaved the same synthetic substrates as ICE (Km of 90 microM and k(cat) of 0.4 s(-1) for Suc-YVAD-NH-Mec, where Suc represents succinyl and NH-Mec represents amino-4-methylcoumarin) and could cleave pro-interleukin-1beta into the same peptides as ICE but less efficiently. On the other hand, TY showed very little efficacy on the different ICE substrates (Km of 860 microM for Suc-YVAD-NH-Mec). These results show that the ICE/TX/TY subfamily has functional heterogeneity and that ICE remains the preferred enzyme for pro-interleukin-1beta cleavage.

Amino Acid Sequence↗
PubMed1998

The DNA damaging drug cyproterone acetate causes gene mutations and induces glutathione-S-transferase P in the liver of female Big Blue transgenic F344 rats.

The gestagenic and antiandrogenic drug cyproterone acetate (CPA) is mitogenic, tumorigenic and induces DNA-adducts and DNA-repair synthesis in rat liver. Thus CPA is expected to be mutagenic. However in vitro mutagenicity test systems were negative. To examine whether CPA induces mutations in rat liver, the in vivo mutation assay based on Big Blue transgenic F344 rats was employed. Single oral doses of 25, 50, 75, 100 and 200 mg CPA/kg b.w. respectively were administered to female Big Blue rats. Six weeks after treatment, liver DNA was assayed for mutations. At the highest dose, 200 mg CPA/kg b.w., the frequency of (17 +/- 4) x 10(-6) spontaneous mutations was increased to a maximum of (80 +/- 8) x 10(-6) mutations. One-hundred and 75 mg CPA/kg b.w. resulted in mutation frequencies of (35 +/- 5) and (27 +/- 5) x 10(-6), respectively. The mutation frequency at doses of 50 and 25 mg CPA/kg b.w. was similar to that of vehicle treated controls. Statistical analysis of the dose-effect relationship revealed that it was not possible to decide whether a threshold dose exists or not. DNA adducts were analyzed by the 32P-postlabelling technique. The total level of the major and the two minor adducts observed in the autoradiograms increased between doses of 25 to 75 mg CPA/kg b.w. to a maximum of approximately 12,000 +/- 3000 adducts per 10(9) nucleotides. The level did not further increase significantly with 100 and 200 mg CPA/kg b.w. After CPA treatment no preneoplastic liver foci were observed. However, single glutathione-S-transferase placental form (GST-P) positive hepatocytes were observed and the frequency was dependent on the dose. These cells are not supposed to represent initiated cells, since they occurred only transiently after 6 weeks and disappeared thereafter completely. In conclusion, our results demonstrate that CPA is mutagenic in vivo. The mutation frequency increased at high CPA doses, when the increase of the DNA adduct formation had already ceased. This suggests that the mitogenic activity of CPA is required to express the mutations.

Animals↗
PubMed1997

Somatic and germ cell mutagenesis in lambda lacZ transgenic mice treated with acrylamide or ethylnitrosourea.

The transgenic Muta Mouse in vivo mutagenesis assay was employed to determine the activity of acrylamide and ethylnitrosourea in liver and germ cells after 3, 10 and 100 days following treatment. Each cell of the Muta Mouse carries 80 copies of the lambda gt10 phage including the bacterial lacZ gene, which act as the target gene for the mutagenesis assay. Groups of Muta Mice were given a single intraperitoneal injection of 80 or 160 mg/kg ethylnitrosourea or 50 or 100 mg/kg acrylamide. The tissues were prepared 3, 10 or 100 days post treatment. The liver genomic DNA was extracted with the manufacturer's standard protocol, while the genomic germ cell DNA was extracted with 4 different methods due to problems encountered in DNA yields and packaging efficiency. The mutation analysis of the lacZ gene was carried out by the positive selective assay method [Gossen et al. (1989) Proc. Natl. Acad. Sci. USA, 86, 7971-7975; Dean and Myhr (1994) Mutagenesis, 9, 183-185]. There was a slight increase due to treatment of the observed mutation frequencies in the acrylamide liver group for all three assay times. From the day 3 group to the day 100 group a time dependent decrease in all the absolute mutant frequencies was detectable. The ethylnitrosourea liver group showed a time- and dose-dependent increase in the mutant frequencies from day 3 to day 100. No meaningful results were obtained for the germ cell tissue assays due to the low amount of genomic DNA extracted which was not packageable in the lambda lacZ assay. At present for the mutagenesis assay of isolated spermatozoa in our laboratory we would be forced to pool tissues from animals to obtain enough DNA for an assay. Since 'jackpot'-animals may exist [Heddle et al. (1992) Mutation Res., 272, 195-203] the individual animals of such a pooled analysis group must be tested before pooling.

Acrylamide↗
PubMed1995

Comparison of lacI and lacZ transgenic mouse mutation assays: an EU-sponsored interlaboratory study.

As part of an EU Environment Programme sponsored study, the performance of two transgenic mouse mutation assays has been evaluated in three laboratories using common liver samples. The systems studied were the lacI-(Big Blue) mutation assay, and the GalE- positive selection modified lacZ- (Muta Mouse) assay. The liver samples compared were derived from mice treated with either saline or dimethylnitrosamine (DMN; 14 day recovery). Each assay gave an increased mutation frequency (MF) for the DMN-treated livers when compared to the saline control MFs. Sources of variability in the assays are discussed, and it is concluded that whole liver should be homogenised before DNA extraction, and that concurrent controls should be processed in parallel with test samples.

Animals↗
PubMed1993

Amino acid changes outside the G-H loop of capsid protein VP1 of type O foot-and-mouth disease virus confer resistance to neutralization by antipeptide G-H serum.

Antiserum to a peptide corresponding to the 135-154 sequence of capsid protein VP1 of the foot-and-mouth disease virus O1 Kaufbeuren was raised in a pig. Although this serum contained neutralizing antibodies, the pig showed clinical symptoms after challenge. Virus isolated from this pig was identified as a mutant, with changes at positions 50, 198 and 211 of VP1 and at position 209 of VP2. This mutant, as well as a plaque isolate of it, differing from the challenge virus at positions 198 on VP1 (alanine being substituted for glutamic acid) and 209 on VP2 (histidine being substituted for tyrosine) resisted neutralization by the anti-peptide serum also in vitro. The same was observed with the O1 Kaufbeuren-related strain O1 Burgwedel, isolated from cattle in the field. It had substitutions only at positions 43 and 101 on VP1. The results show that neutralization epitopes flanking positions 145-147 on VP1 are modulated by other capsid protein parts. These parts seem to be important for neutralization escape in natural FMDV host species.

Amino Acid Sequence↗
PubMed1992

Identification and characterization of foot-and-mouth disease virus O1 Burgwedel/1987 as an intertypic recombinant.

The foot-and-mouth disease virus field isolate Burgwedel/1987 subtype O1 was found to differ genetically from the antigenically related strain O1 Kaufbeuren within the region encoding the non-structural proteins. This genetic difference was indicated by the RNase mismatch cleavage method and confirmed by nucleotide sequencing. An alignment of sequences encoding proteinase 3C of the Burgwedel isolate and several other virus strains identified this isolate as an intertypic recombinant; the parent strains were O1 Kaufbeuren and a subtype C1 strain. Recombination occurred between nucleotide positions 5493 and 5521, within the region encoding peptide 3B1. Thus, the 5' three-quarters of the O1 genome were fused to the 3'-terminal quarter of the C1 genome. Other contemporary isolates from the same district are not recombinants. Sequence alignment distinguished four patterns of proteinase 3C-coding sequences among the virus strains analysed: subtypes A12, C1 and O1 exhibit one pattern each, and another pattern is common to subtypes A5, A10 and O2.

Aphthovirus↗
PubMed1991

Foot-and-mouth disease virus O1Lombardy is biochemically related to O2 isolates.

The capsid protein VP1-encoding RNA regions of the foot-and-mouth disease virus isolates O1Lombardy/1946 and O2Brescia/1947 were sequenced and found to be closely related to each other and to O2Normandy/1949, despite some sequence differences. The O1Lombardy sequence was expected to be more closely related to those of the subtype O1 isolates of 1965 and later (e.g., O1Kaufbeuren/1966), but this was not the case. The serological subtyping of both the Lombardy and the Kaufbeuren isolate as O1 strains was possibly due to identical VP1 C-terminal sequences, since all the subtype O2 isolates differ here from the O1 isolates at residue 209. Considerable dissimilarity of other O1Lombardy and O2Brescia genome parts to those of O1Kaufbeuren was qualitatively shown by analyzing the sizes of RNase-treated hybrids formed with virus RNA and defined subgenomic fragments of O1Kaufbeuren-specific antisense cRNA. These hybrids were fragmented into oligonucleotides, but others containing O1Kaufbeuren virus RNA were protected.

Amino Acid Sequence↗
PubMed1991

The capsid protein-encoding sequence of foot-and-mouth disease virus O2Brescia.

Nucleotide sequences encoding the four capsid proteins of foot-and-mouth disease virus subtype O2Brescia/1947 have been determined. These and the deduced amino acid sequences were compared with those of a subtype O1 virus strain. The nucleotide sequences differed at 259 positions, causing only 35 amino acid changes. VP4 and VP2 differed by 2.4 and 1.8%, whereas VP1, known as major viral antigen, and VP3 differed by 8% and 5.5%, respectively. The differences occur mainly in protein domains not involved in the formation of alpha-helices and beta-sheets, suggesting that the surfaces of both viruses are more variable than their scaffolds. The O2Brescia sequence has been submitted to the GenBank data base and has the accession number M 55287.

Amino Acid Sequence↗