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Retinol-binding protein 4 (RBP4) is the major transport protein for retinol in blood. Recent evidence from genetic mouse models shows that circulating RBP4 derives exclusively from hepatocytes. Because RBP4 is elevated in obesity and associates with the development of glucose intolerance and insulin resistance, we tested whether a liver-specific overexpression of RBP4 in mice impairs glucose homeostasis. We used adeno-associated viruses (AAV) that contain a highly liver-specific promoter to drive expression of murine RBP4 in livers of adult mice. The resulting increase in serum RBP4 levels in these mice was comparable with elevated levels that were reported in obesity. Surprisingly, we found that increasing circulating RBP4 had no effect on glucose homeostasis. Also during a high-fat diet challenge, elevated levels of RBP4 in the circulation failed to aggravate the worsening of systemic parameters of glucose and energy homeostasis. These findings show that liver-secreted RBP4 does not impair glucose homeostasis. We conclude that a modest increase of its circulating levels in mice, as observed in the obese, insulin-resistant state, is unlikely to be a causative factor for impaired glucose homeostasis.
Leveraging large-deviation statistics to decipher the stochastic properties of measured trajectories
(2021)
Extensive time-series encoding the position of particles such as viruses, vesicles, or individualproteins are routinely garnered insingle-particle tracking experiments or supercomputing studies.They contain vital clues on how viruses spread or drugs may be delivered in biological cells.Similar time-series are being recorded of stock values in financial markets and of climate data.Such time-series are most typically evaluated in terms of time-averaged mean-squareddisplacements (TAMSDs), which remain random variables for finite measurement times. Theirstatistical properties are different for differentphysical stochastic processes, thus allowing us toextract valuable information on the stochastic process itself. To exploit the full potential of thestatistical information encoded in measured time-series we here propose an easy-to-implementand computationally inexpensive new methodology, based on deviations of the TAMSD from itsensemble average counterpart. Specifically, we use the upper bound of these deviations forBrownian motion (BM) to check the applicability of this approach to simulated and real data sets.By comparing the probability of deviations fordifferent data sets, we demonstrate how thetheoretical bound for BM reveals additional information about observed stochastic processes. Weapply the large-deviation method to data sets of tracer beads tracked in aqueous solution, tracerbeads measured in mucin hydrogels, and of geographic surface temperature anomalies. Ouranalysis shows how the large-deviation properties can be efficiently used as a simple yet effectiveroutine test to reject the BM hypothesis and unveil relevant information on statistical propertiessuch as ergodicity breaking and short-time correlations.
George Santayana
(2021)
Die Geschichte der Mikrobiologie als Laborwissenschaft des späten 19. Jahrhunderts hat für Ehrenberg keinen Platz. Unmodern, ja fehlerhaft scheinen die Befunde dieses „Humboldt en miniature“, der die Belebtheit von Wasser oder Luft mikroskopisch untersuchte, der Proben aus aller Welt sammelte und so zahlreiche „Infusorien“ genannte Mikroben sowie deren Effekte etwa bei Blutwundern beschrieb, deren Beteiligung an Infektionskrankheiten aber verneinte. Zugleich scheint sein ökologischer Blick auf den Mikrokosmos die Gegenwart auf überraschende Weise anzusprechen – einerseits weil Mikroben omnipräsentes Faszinosum wie Bedrohung bleiben, andererseits weil viele der Prämissen von Pasteur und Koch im Zeitalter der Genomik überholt erscheinen. Ausgehend von der zwiespältigen Position Ehrenbergs fragt dieser Artikel, warum er möglicherweise gerade deswegen spannender ist, als ihn die Historiographie bislang hat erscheinen lassen.
Im Jahr 1820 traten zwei Universitätsabsolventen, Christian Gottfried Ehrenberg und Wilhelm Hemprich, eine Forschungsreise nach Nordafrika an, die eine der ersten vom preußischen Staat maßgeblich getragenen Reiseunternehmungen darstellt. Wesentliches Ziel der Reise war das Sammeln von naturkundlichen Exemplaren für die junge Berliner Museumslandschaft. Der Beitrag ordnet die afrikanische Forschungsreise in die wissenschaftliche Laufbahn Ehrenbergs ein, der als einziger Reisender überlebte, sowie in die Geschichte der Berliner Wissenschaftslandschaft im frühen 19. Jahrhundert. Dabei konzentriert sich der Beitrag auf die Medien – insbesondere Sammlungsverzeichnisse und Sammlungsobjekte – die die Reisenden im Feld einsetzen mussten, um ihre Identität als gelehrte Naturforscher gegenüber ihren Förderern zu inszenieren und damit ihre Position in der preußischen Wissenschaft strategisch zu sichern.
The Russian-Siberian journey, carried out with Alexander von Humboldt and Gustav Rose in 1829 coincided with a decisive phase in Christian Gottfried Ehrenberg’s scientific career. His field diary and the drawings he made on the trip document his intensified focus on the study of microorganisms. The numerous observations on plant and animal geography show how Ehrenberg tested a combination of micro- and macro-perspectives that would shape his research on infusoria in the following decades: In addition to the task of classification at the microscope, Ehrenberg carried out worldwide comparative research on the distribution of microorganisms.
Die Erforschung und Beschreibung mikroskopischer Organismen durch Christian Gottfried Ehrenberg ist durch seine publizierten Texte und Kupfertafeln in den „Infusionsthierchen“, der „Mikrogeologie“ sowie aus Abhandlungen der Akademie der Wissenschaften überliefert. Zentrale Bedeutung haben seine Zeichenblätter, anfangs alleinige Dokumentation, später komplementiert durch tausende mikroskopischer Dauerpräparate rezenter und fossiler Kieselschalen sowie getrockneter Einzeller. Alle Originalmaterialien Ehrenbergs stehen für die aktuelle Forschung in der Ehrenbergsammlung zur Verfügung. Seine detailgenauen Zeichnungen dienten der Erstbeschreibung einer Vielzahl neuer Arten aus allen Weltteilen. Biogeografische und ökologische Fragestellungen, Beschreibungen von Lebenszyklen sowie außerordentliche Phänomene wie die Färbung von Gewässern und Erden wurden mithilfe zeichnerischer Analysen bearbeitet.
Geleitwort
(2021)
Essays on Macroeconomics
(2021)
This dissertation consists of four self-contained papers. Each paper deals with a specific macroeconomic question. The first paper assesses the distributional implications of environmental policies from a general equilibrium macroeconomic perspective. I develop a New-Keynesian model with several types of uncertainties and frictions that incorporates liquidity constrained households. The model is calibrated to match the German economy and the numerical results show that climate policy instruments can be associated with regressive welfare effects. Furthermore, the analysis shows that these effects can be mitigated through an appropriate revenue recycling scheme. The second paper deals with short-run inequality dynamics within a real business cycle model. An empirical evaluation shows that the cyclical components of income inequality, the capital share and real GDP are correlated. We develop tractable representation of common inequality indicators in the general equilibrium model and show that the observed pattern is driven by innovations in the capital share. A Bayesian estimation of the model for the United States with data for the period 1948 to 2017 indicates that the model provides a reasonable fit for the data and successfully replicates the observed pattern of cyclical correlations. The third paper empirically examines the effects of banking regulation on the risk-relationship between sovereigns and banks. Based on a comprehensive data set of the European banking sector, we find that the implementation of the novel European banking regulation framework significantly contributed to a weakening of the risk-link between sovereigns and banks.The fourth paper empirically examines the role of institutional experience for institutional development in transition economies. To capture institutional experience, we develop a novel index, based on historical country records. The results of cross-sectional and panel estimations suggest that institutional experience helps to explain the divergent economic and institutional development in transition economies after the dissolution of the Soviet Union.