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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.
Trueness and precision of milled and 3D printed root-analogue implants: A comparative in vitro study
(2023)
The Chemotype of Chromanones as a Privileged Scaffold for Multineurotarget Anti-Alzheimer Agents
(2022)
Background There is a lack of cardiac magnetic resonance (CMR) data regarding mid- to long-term myocardial damage due to Covid-19 in elite athletes. Objective This study investigated mid-to long-term consequences of myocardial involvement after a Covid-19 infection in elite athletes.
Methods Between January 2020 and October 2021, 27 athletes of the German Olympic centre Rhineland with confirmed Covid-19 infection were analyzed. 9 healthy non-athlete volunteers served as control. CMR was performed in mean 182 days (SD 99) after initial positive test result.
Results CMR did not reveal any signs of acute myocarditis in regard to the current Lake Louise criteria or myocardial damage in any of the 26 elite athletes with previous Covid-19 infection. Nevertheless, 92 % of the athletes experienced a symptomatic course and 54 % reported lasting symptoms for more than 4 weeks. In one male athlete CMR revealed an arrhythmogenic right ventricular cardiomyopathy (ARVC) and this athlete was excluded from the study. Athletes had significantly enlarged left and right ventricle volumes and increased left ventricular myocardial mass in comparison to the healthy control group (LVEDVi 103.4 vs. 91.1 ml/m 2 p=0.031; RVEDVi 104.1 vs. 86.6 ml/m 2 p=0.007; and LVMi 59.0 vs. 46.2 g/m 2 p=0.002).
Conclusion Our findings suggest that the risk for mid-to long-term myocardial damage seems to be very low to negligible in elite athletes. No conclusions can be drawn regarding myocardial injury in the acute phase of infection nor about possible long-term myocardial effects in the general population.
Rosiglitazone and glimeperide: review of clinical results supporting a fixed dose combination
(2007)
This volume of the series Springer Briefs in Space Life Sciences explains the physics and biology of radiation in space, defines various forms of cosmic radiation and their dosimetry, and presents a range of exposure scenarios. It also discusses the effects of radiation on human health and describes the molecular mechanisms of heavy charged particles’ deleterious effects in the body. Lastly, it discusses countermeasures and addresses the vital question: Are we ready for launch?
Written for researchers in the space life sciences and space biomedicine, and for master’s students in biology, physics, and medicine, the book will also benefit all non-experts endeavoring to understand and enter space.