Anomalous power laws of spectral diffusion in quantum dots: A connection to luminescence intermittency

We show that the wandering of transition frequencies in colloidal quantum dots does not follow the statistics expected for ordinary diffusive processes. The trajectory of this anomalous spectral diffusion is characterized by a root t dependence of the squared deviation. The behavior is reproduced when the electronic states of quantum dots are assumed to interact with environments such as, for example, an ensemble of two-level systems, where the correlation times are distributed according to a power law similar to the one generally attributed to the dot's luminescence intermittency

Organogenesis relies on SoxC transcription factors for the survival of neural and mesenchymal progenitors

During organogenesis, neural and mesenchymal progenitor cells give rise to many cell lineages, but their molecular requirements for self-renewal and lineage decisions are incompletely understood. In this study, we show that their survival critically relies on the redundantly acting SoxC transcription factors Sox4, Sox11 and Sox12. The more SoxC alleles that are deleted in mouse embryos, the more severe and widespread organ hypoplasia is. SoxC triple-null embryos die at midgestation unturned and tiny, with normal patterning and lineage specification, but with massively dying neural and mesenchymal progenitor cells. Specific inactivation of SoxC genes in neural and mesenchymal cells leads to selective apoptosis of these cells, suggesting SoxC cell-autonomous roles. Tead2 functionally interacts with SoxC genes in embryonic development, and is a direct target of SoxC proteins. SoxC genes therefore ensure neural and mesenchymal progenitor cell survival, and function in part by activating this transcriptional mediator of the Hippo signalling pathway

Transcriptional and Post-Transcriptional Regulation of SPAST, the Gene Most Frequently Mutated in Hereditary Spastic Paraplegia

Hereditary spastic paraplegias (HSPs) comprise a group of neurodegenerative disorders that are characterized by progressive spasticity of the lower extremities, due to axonal degeneration in the corticospinal motor tracts. HSPs are genetically heterogeneous and show autosomal dominant inheritance in ∼70–80% of cases, with additional cases being recessive or X-linked. The most common type of HSP is SPG4 with mutations in the SPAST gene, encoding spastin, which occurs in 40% of dominantly inherited cases and in ∼10% of sporadic cases. Both loss-of-function and dominant-negative mutation mechanisms have been described for SPG4, suggesting that precise or stoichiometric levels of spastin are necessary for biological function. Therefore, we hypothesized that regulatory mechanisms controlling expression of SPAST are important determinants of spastin biology, and if altered, could contribute to the development and progression of the disease. To examine the transcriptional and post-transcriptional regulation of SPAST, we used molecular phylogenetic methods to identify conserved sequences for putative transcription factor binding sites and miRNA targeting motifs in the SPAST promoter and 3′-UTR, respectively. By a variety of molecular methods, we demonstrate that SPAST transcription is positively regulated by NRF1 and SOX11. Furthermore, we show that miR-96 and miR-182 negatively regulate SPAST by effects on mRNA stability and protein level. These transcriptional and miRNA regulatory mechanisms provide new functional targets for mutation screening and therapeutic targeting in HSP

Identification of a Gene Regulatory Network Necessary for the Initiation of Oligodendrocyte Differentiation

Differentiation of oligodendrocyte progenitor cells (OPCs) into mature oligodendrocytes requires extensive changes in gene expression, which are partly mediated by post-translational modifications of nucleosomal histones. An essential modification for oligodendrocyte differentiation is the removal of acetyl groups from lysine residues which is catalyzed by histone deacetylases (HDACs). The transcriptional targets of HDAC activity within OPCs however, have remained elusive and have been identified in this study by interrogating the oligodendrocyte transcriptome. Using a novel algorithm that allows clustering of gene transcripts according to expression kinetics and expression levels, we defined major waves of co-regulated genes. The initial overall decrease in gene expression was followed by the up-regulation of genes involved in lipid metabolism and myelination. Functional annotation of the down-regulated gene clusters identified transcripts involved in cell cycle regulation, transcription, and RNA processing. To define whether these genes were the targets of HDAC activity, we cultured rat OPCs in the presence of trichostatin A (TSA), an HDAC inhibitor previously shown to inhibit oligodendrocyte differentiation. By overlaying the defined oligodendrocyte transcriptome with the list of ‘TSA sensitive’ genes, we determined that a high percentage of ‘TSA sensitive’ genes are part of a normal program of oligodendrocyte differentiation. TSA treatment increased the expression of genes whose down-regulation occurs very early after induction of OPC differentiation, but did not affect the expression of genes with a slower kinetic. Among the increased ‘TSA sensitive’ genes we detected several transcription factors including Id2, Egr1, and Sox11, whose down-regulation is critical for OPC differentiation. Thus, HDAC target genes include clusters of co-regulated genes involved in transcriptional repression. These results support a de-repression model of oligodendrocyte lineage progression that relies on the concurrent down-regulation of several inhibitors of differentiation

Differential Deployment of REST and CoREST Promotes Glial Subtype Specification and Oligodendrocyte Lineage Maturation

The repressor element-1 (RE1) silencing transcription factor/neuron-restrictive silencer factor (REST/NRSF) is a master transcriptional regulator that binds to numerous genomic RE1 sites where it acts as a molecular scaffold for dynamic recruitment of modulatory and epigenetic cofactors, including corepressor for element-1-silencing transcription factor (CoREST). CoREST also acts as a hub for various cofactors that play important roles in epigenetic remodeling and transcriptional regulation. While REST can recruit CoREST to its macromolecular complex, CoREST complexes also function at genomic sites independently of REST. REST and CoREST perform a broad array of context-specific functions, which include repression of neuronal differentiation genes in neural stem cells (NSCs) and other non-neuronal cells as well as promotion of neurogenesis. Despite their involvement in multiple aspects of neuronal development, REST and CoREST are not believed to have any direct modulatory roles in glial cell maturation.We challenged this view by performing the first study of REST and CoREST in NSC-mediated glial lineage specification and differentiation. Utilizing ChIP on chip (ChIP-chip) assays, we identified distinct but overlapping developmental stage-specific profiles for REST and CoREST target genes during astrocyte (AS) and oligodendrocyte (OL) lineage specification and OL lineage maturation and myelination, including many genes not previously implicated in glial cell biology or linked to REST and CoREST regulation. Amongst these factors are those implicated in macroglial (AS and OL) cell identity, maturation, and maintenance, such as members of key developmental signaling pathways and combinatorial transcription factor codes.Our results imply that REST and CoREST modulate not only neuronal but also glial lineage elaboration. These factors may therefore mediate critical developmental processes including the coupling of neurogenesis and gliogenesis and neuronal-glial interactions that underlie synaptic and neural network plasticity and homeostasis in health and in specific neurological disease states

Chancen und Risiken der Arbeit im E-Business

Bundesministerium für Bildung und Forschung (BMBF

Band structure engineering in II-VI semiconductor core/shell nanocrystals

© 2011 Dr. Christian Alexander PotznerSemiconductor Quantum Dots (QDs) with their size dependent electronic and optical characteristics received immense attention over the last three decades due to the plethora of potential applications, and since they allow the observation and study of quantum confinement under ambient conditions. Coating these particles with another semiconductor material leads to the formation of a core/shell heterostructure which not only can significantly improve the chemical and photo–stability but also enables band gap engineering in these systems. The ability to control the spatial probabilities of charge carriers within such systems opened up a variety of new optical properties previously not accessible with bare semiconductor nanocrystals. In this thesis, core/shell structures with several material combinations are investigated with the aim to relate their optical properties to the confinement system imposed by the shell configuration. Utilising salt precursors and a layer–by–layer deposition technique termed SILAR, the experimental results demonstrate quantitative and highly uniform shell growth. It was found that the effect of lattice strain plays a vital role for the degree of epitaxy attainably in these materials, which significantly affects their uniformity and optical properties. Studying the exciton confinement in classical Type–I CdSe/CdS and CdSe/ZnS heterostructures, the experimentally obtained results of different barrier potentials are in good agreement with theoretically predicted trends. Cryogenic optical measurements revealed an increase in confinement in CdSe/CdS structures as a function of shell growth at low temperature. For the first time, the evolution of the higher order transitions in CdSe/CdS core/shell heterostructures as a function of core size and shell thickness was quantitatively studied via low temperature photoluminescence excitation spectroscopy. The ultimate success of applications based on the fluorescent properties of QDs requires high brightness and strong environmental robustness of these emitters. Since the luminescence efficiency of nanocrystal ensembles is directly related the number of non–radiative exciton recombination channels, present in the individual particles, reduction of these pathways is highly desirable. Following the argument that non–radiative recombination is intimately linked to the number of trap states available to the charge carriers, a graded seal core/shell/shell structure, based on a CdSe/CdS/ZnS system, was proposed with the aim to effectively confine the exciton in the core while minimising trap states. Investigating the effect of the CdS–to–ZnS shell thickness ratio, our results demonstrate the crucial effect of lattice strain in such particles. Strong blinking suppression was observed on the single nanocrystal level for the configuration with a monolayer ratio of 1:4 between CdS and ZnS, demonstrating the viability of our structural considerations. The capabilities of band gap engineering in core/shell heterostructures were further explored by investigation of a so–called Dual Quantum System (DQS) which is thought to host two potential regions for radiative exciton recombination. Thus, the corresponding photoluminescence spectrum should exhibit two emission peaks given that the recombination centres are sufficiently separated. Our results on theoretical calculations based on a core/barrier/shell model (CdSe/ZnS/CdSe) give insight into the principle structural requirements necessary to establish dual emission. Experimentally it was found that the successful preparation of such materials is highly challenging due to the specific structural and electronic requirements of a large barrier potential, a large core size and a thick barrier, which entail the corresponding adverse effects of lattice mismatch and low particle surface reactivity. Establishing dual emission, our optical studies show good agreement with the theoretically predicted trends and indicate de-coupled behaviour of the two recombination centres in these complex nanocrystal heterostructures

Prolonged Sox4 Expression in Oligodendrocytes Interferes with Normal Myelination in the Central Nervous System▿ †

The highly related transcription factors Sox4 and Sox11 are both expressed in oligodendrocyte precursors. Yet whether they have a function in oligodendrocyte development is unknown. By overexpressing Sox4 under the control of 3.1 kb of 5′ flanking sequences of the myelin basic protein gene in transgenic mice, we extended Sox4 expression in the oligodendrocyte lineage from oligodendrocyte precursors to cells undergoing terminal differentiation. As a consequence of transgene expression, mice develop the full spectrum of phenotypic traits associated with a severe hypomyelination during the first postnatal weeks. Myelin gene expression was severely reduced, and myelin dramatically thinned in several central nervous system (CNS) regions. Despite these disturbances in CNS myelination, the number of oligodendrocytic cells remained unaltered. Considering that apoptosis rates were normal and proliferation only slightly increased, oligodendrocytes likely persist in a premyelinating to early myelinating state. This shows that prolonged Sox4 expression in cells of the oligodendrocyte lineage is incompatible with the acquisition of a fully mature phenotype and argues that the presence of Sox4, and possibly Sox11, in oligodendrocyte precursors may normally prevent premature differentiation