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The generation and maintenance of intricate spatiotemporal patterns of gene expression in multicellular organisms requires the establishment of complex mechanisms of transcriptional regulation. Estimations that up to one million enhancers exist in the human genome accentuates the utmost importance of this type of cis-regulatory element for gene regulation. However, surprisingly little is known about the mechanisms used to temporarily or permanently activate or inactivate enhancers during cellular differentiation. The current work addresses the question how enhancer regulation can be achieved.
Using the chemokine (C-C motif) ligand gene Ccl22 as a model, the first example is based on the question how the activation of an enhancer can be prevented in a physiological context. Ccl22 is expressed by myeloid cells, such as dendritic cells, upon exposure to inflammatory stimuli. The expression in other cell types, such as fibroblasts, is prevented by the strong accumulation of H3K9me3 at the enhancer's proximal region. This accumulation is attenuated in myeloid cells through activity of the stimulus-induced demethylase Jmjd2d. To tease out which genomic fragment or fragments in the Ccl22 locus could be responsible for the maintenance of enhancer inactivity, potentially through the recruitment of H3K9 methyltransferases, the enhancer repressing capacity of 1 kb fragments of the gene locus was analysed in retroviral reporter assays. It was found that a fragment adjacent to the Ccl22 enhancer that overlaps with a member of a subfamily of long interspersed nuclear elements (LINEs) showed strong repressive potential on a model enhancer. Subsequent retroviral reporter assays with LINEs from loci of other stimulus-dependent genes identified additional LINE fragments that exhibit strong enhancer repressive capacity. These findings suggest a mechanism for enhancer silencing involving LINEs.
The second example concentrates on the inactivation of an enhancer during colorectal cancer (CRC) progression. The adenoma to carcinoma transition during CRC progression often is accompanied by a downregulation of the tumour suppressor gene EPHB2. The EMT inducing factor SNAIL1 strongly downregulated EPHB2 expression in a CRC cell model. To gain insights into the transcriptional regulation of EPHB2, potential cis-regulatory elements in the EPHB2 upstream region were analysed using reporter assays. A cell-type-specific enhancer was identified and subsequent chromatin analyses revealed a correlation between enhancer chromatin conformation and EPHB2 expression in different CRC cell lines. Additionally, the overexpression of the murine Snail1 induced chromatin changes at the EPHB2 enhancer towards a poised, transcriptionally silent chromatin conformation. Mutational analyses of the minimal enhancer region pinpointed three transcription factor binding motifs to be essential for full enhancer activity. Different binding patterns between CRC cell lines at the TCF/LEF motif were subsequently identified. Furthermore, a switch from TCF7L2 to LEF1 occupancy was found upon overexpression of Snail1 in vitro and in vivo. The generation of LS174T CRC cells overexpressing LEF1 confirmed the involvement of LEF1 in the downregulation of EPHB2 and the competitive displacement of TCF7L2. This part of the work demonstrated that the SNAIL1 induced downregulation of EPHB2 is dependent on the decommissioning of a transcriptional enhancer and led to a hypothetical model involving LEF1 and ZEB1.
In summary, this work highlighted two distinct mechanisms for enhancer regulation. One mechanism is based on enhancer repressive LINE fragments that might prevent stimulus-dependent enhancer activation. In the second, enhancer silencing was shown to be based on a competitive transcription factor binding mechanism.
Fundamentals of Energy Meteorology - Influence of atmospheric parameters on solar energy production
(2015)
Only since the turn of the 21st century have humanitarian organisations developed specific strategies that address climate change impacts as a humanitarian challenge. Taking the International Red Cross / Red Crescent Movement, being the largest humanitarian network, as an empirical case study, the article discusses the Movement’s changes in the areas 1) agenda setting, 2) organisational restructuring, 3) networking, 4) programming, and 5) advocacy. Based on the case study and a theoretical framework of organisational sociology, the article provides conclusions on internal and external factors that can explain why the Movement has been successful in being one of the first actors within the organisational field of humanitarian organisations to focus systematically on the humanitarian implications of climatic changes.
Communicating Climate Risks. A case study of the International Red Cross/Red Crescent Movement
(2015)
In education, finding the appropriate learning pace that fits to the members of a large group is a challenging task. This becomes especially evident when teaching multidisciplinary subjects such as epidemiology in medicine or computer science in most study programs, since lecturers have to face a very heterogeneous state of previous knowledge. Approaching this issue requires an individual supervision of each and every student, which is obviously bounded by the available resources. Moreover, when referring back to the second example, writing computer programs requires a complex installation and configuration of development tools. Many beginning programmers already become stuck at this entry stage. This paper introduces WHELP, a Web-based Holistic E-Learning Platform, which provides an integrated environment enabling the learning and teaching of computer science topics without the need to install any software. Moreover, WHELP includes an interactive feedback system for each programming exercise, where lecturers or tutors can supply comments, improvements, code assistance or tips helping the students to accomplish their tasks. Furthermore, WHELP offers a statistical analysis module as well as a real-time classroom polling system both promoting an overview of the state of knowledge of a course. In addition to that, WHELP enables collaborative working including code-sharing and peer-to-peer learning. This feature enables students to work on exercises simultaneously at distinct places. WHELP has been successfully deployed in the winter term 2013 at the Cologne University of Applied Sciences supporting the 120 students and 3 lecturers to learn and teach basic topics of computer science in an engineering study program.