673 research outputs found
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
Alopecia Areata Susceptibility in Rodent Models
With our current view of alopecia areata as an autoimmune disease, it is probable that disease development in an individual is dependent on multiple genetic and environmental factors interacting in a complex system. Rodent models afford the opportunity to investigate alopecia areata development and to define the significance of the different factors involved. Recently, rodent model characterization has been conducted using flow cytometry, microarray analysis, and functional studies. From these a pattern of events in alopecia areata development has emerged. Although the preliminary activation events for the onset of alopecia areata remain unknown, the response of the immune system is characterized by antigen presentation and costimulation of lymphocytes in the lymph nodes and skin, a deficiency of CD4+/CD25+ regulatory cells, and an action of activated lymphocytes on hair follicles via Fas/FasL signaling and cytokines. Thus, onset of disease may require appropriate (or inappropriate) expression of stimulatory antigens within the hair follicle, the breakdown of the putative hair follicle immune privilege, the presentation of antigens to the immune system, a failure of immune system regulation, and the ability of the activated immune system to disrupt anagen-stage hair follicles. Once the sequence of events is initiated, it may become a self-perpetuating cycle, with epitope spreading leading to a wider range of targets in chronic alopecia areata. Rodent model studies have provided significant insight into alopecia areata, but much more remains to be explained about the mechanisms of disease development
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
Self-organization of spatio-temporal earthquake clusters
International audienceCellular automaton versions of the Burridge-Knopoff model have been shown to reproduce the power law distribution of event sizes; that is, the Gutenberg-Richter law. However, they have failed to reproduce the occurrence of foreshock and aftershock sequences correlated with large earthquakes. We show that in the case of partial stress recovery due to transient creep occurring subsequently to earthquakes in the crust, such spring-block systems self-organize into a statistically stationary state characterized by a power law distribution of fracture sizes as well as by foreshocks and aftershocks accompanying large events. In particular, the increase of foreshock and the decrease of aftershock activity can be described by, aside from a prefactor, the same Omori law. The exponent of the Omori law depends on the relaxation time and on the spatial scale of transient creep. Further investigations concerning the number of aftershocks, the temporal variation of aftershock magnitudes, and the waiting time distribution support the conclusion that this model, even "more realistic" physics in missed, captures in some ways the origin of the size distribution as well as spatio-temporal clustering of earthquakes
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
Identification of polymorphisms in Apolipoprotein M gene and their relationship with risk of recurrent venous thromboembolism
Apolipoprotein M (ApoM) plasma levels have been reported to be associated with risk of venous thromboembolism (VTE) recurrence. However, the role of genetic alterations in the ApoM gene in VTE recurrence remains unknown. The aim of this study was to identify genetic aberrations in ApoM gene in VTE recurrence and their role in prediction of VTE recurrence in a prospective follow-up study of 1465 VTE patients. During follow-up, 156 (10.6 %) patients had VTE recurrence. First screening of whole ApoM gene was performed by Sanger's sequencing in selected age and sex matched non-recurrent and recurrent patients (n=95). In total six polymorphisms were identified and two polymorphisms (rs805297 and rs9404941) with minor allele frequency (MAF) ≥5 % were further genotyped in the whole cohort by Taqman PCR. ApoM rs805297 polymorphism was significantly associated with higher risk of VTE recurrence in males but not in females on both univariate (p= 0.038, hazard ratio = 1.72, confidence interval = 1.03-2.88) and on multivariate analysis adjusted with mild and severe thrombophilia, family history, location and acquired risk factors for VTE. However, ApoM rs9404941 polymorphism showed no significant association with risk of VTE recurrence in all patients as well as in different gender groups. Moreover, ApoM rs805297 and rs9404941 polymorphisms were not associated with the ApoM plasma levels. In conclusion, for the first time we have sequenced whole ApoM gene in VTE and identified six polymorphisms. ApoM rs805297 was significantly associated with higher risk of VTE recurrence in male but not in female patients
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