Properties of Foreshocks and Aftershocks of the Non-Conservative SOC Olami-Feder-Christensen Model: Triggered or Critical Earthquakes?

Following Hergarten and Neugebauer [2002] who discovered aftershock and foreshock sequences in the Olami-Feder-Christensen (OFC) discrete block-spring earthquake model, we investigate to what degree the simple toppling mechanism of this model is sufficient to account for the properties of earthquake clustering in time and space. Our main finding is that synthetic catalogs generated by the OFC model share practically all properties of real seismicity at a qualitative level, with however significant quantitative differences. We find that OFC catalogs can be in large part described by the concept of triggered seismicity but the properties of foreshocks depend on the mainshock magnitude, in qualitative agreement with the critical earthquake model and in disagreement with simple models of triggered seismicity such as the Epidemic Type Aftershock Sequence (ETAS) model [Ogata, 1988]. Many other features of OFC catalogs can be reproduced with the ETAS model with a weaker clustering than real seismicity, i.e. for a very small average number of triggered earthquakes of first generation per mother-earthquake.Comment: revtex, 19 pages, 8 eps figure

Abrogation of Experimental Colitis Correlates with Increased Apoptosis in Mice Deficient for Cd44 Variant Exon 7 (Cd44v7)

Experimental colitis in mice is characterized by infiltration of activated T helper (Th) cells and macrophages into the lamina propria. Particularly, these cells expressed CD44 variant exon 7 (CD44v7)-containing isoforms. Upregulation of CD44v7 isoforms was induced by CD40 ligation, an inflammation-driving interaction between activated Th cells and macrophages. To define the role of CD44v7 in colitis, mice bearing a targeted deletion for exon v7 were generated. In trinitrobenzene sulfonic acid–induced colitis, wild-type mice developed severe signs of persistent inflammation. Mice lacking CD44v7 initially showed unspecific inflammation, then recovered completely. The pathogenic origin was shown to reside in bone marrow–derived CD44v7+ cells, because adoptive transfer experiments demonstrated an absolute requirement for CD44v7 on hematopoietic cells for maintenance of colitis. Interleukin (IL)-10–deficient mice, which develop a chronic Th1-driven enterocolitis, were crossbred with CD44v6/v7 null mice. In IL-10 × CD44v6/v7 double deficient mice, intestinal inflammation developed only weakly and at an older age. Analysis of cell death in the inflamed lesions revealed that mononuclear cells in the CD44v7 null infiltrates had higher rates of apoptosis than those from wild-type mice. Thus, the region encoded by CD44v7 appears to be essential for survival of effector lymphocytes, resulting in persistence of inflammation

Ligand binding and conformational dynamics of the E. coli nicotinamide nucleotide transhydrogenase revealed by hydrogen/deuterium exchange mass spectrometry

Nicotinamide nucleotide transhydrogenases are integral membrane proteins that utilizes the proton motive force to reduce NADP+ to NADPH while converting NADH to NAD+. Atomic structures of various transhydrogenases in different ligand-bound states have become available, and it is clear that the molecular mechanism involves major conformational changes. Here we utilized hydrogen/deuterium exchange mass spectrometry (HDX-MS) to map ligand binding sites and analyzed the structural dynamics of E. coli transhydrogenase. We found different allosteric effects on the protein depending on the bound ligand (NAD+, NADH, NADP+, NADPH). The binding of either NADP+ or NADPH to domain III had pronounced effects on the transmembrane helices comprising the proton-conducting channel in domain II. We also made use of cyclic ion mobility separation mass spectrometry (cyclic IMS-MS) to maximize coverage and sensitivity in the transmembrane domain, showing for the first time that this technique can be used for HDX-MS studies. Using cyclic IMS-MS, we increased sequence coverage from 68 % to 73 % in the transmembrane segments. Taken together, our results provide important new insights into the transhydrogenase reaction cycle and demonstrate the benefit of this new technique for HDX-MS to study ligand binding and conformational dynamics in membrane proteins

Solar and Atmospheric Neutrinos: Background Sources for the Direct Dark Matter Searches

In experiments for direct dark matter searches, neutrinos coherently scattering off nuclei can produce similar events as Weakly Interacting Massive Particles (WIMPs). The calculated count rate for solar neutrinos in such experiments is a few events per ton-year. This count rate strongly depends on the nuclear recoil energy threshold achieved in the experiments for the WIMP search. We show that solar neutrinos can be a serious background source for direct dark matter search experiments using Ge, Ar, Xe and CaWO_4 as target materials. To reach sensitivities better than approximatly 10^-10 pb for the elastic WIMP nucleon spin-independent cross section in the zero-background limit, energy thresholds for nuclear recoils should be approximatly >2.05 keV for CaWO_4, >4.91 keV for Ge, >2.89 keV for Xe, and >8.62 keV for Ar as target material. Next-generation experiments should not only strive for a reduction of the present energy thresholds but mainly focus on an increase of the target mass. Atmospheric neutrinos limit the achievable sensitivity for the background-free direct dark matter search to approximatly >10^-12 pb.Comment: accepted by Astroparticle Physic