64,426 research outputs found
On the theory of Gordan-Noether on homogeneous forms with zero Hessian (Improved version)
We give a detailed proof for Gordan-Noether's results in "Ueber die
algebraischen Formen, deren Hesse'sche Determinante identisch verschwindet"
published in 1876 in Mathematische Annahlen. C. Lossen has written a paper in a
similar direction as the present paper, but did not provide a proof for every
result. In our paper, every result is proved. Furthermore, our paper is
independent of Lossen's paper and includes a considerable number of new
observations.
An earlier version of this paper has been printed in Proceedings of the
School of Science of Tokai University, Vol.49, Mar. 2014. In this version, a
serious error has been corrected and some new results have been added
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Plasma clouds in the magnetosphere
Injection of hot plasma clouds into magnetosphere during magnetospheric substorm
Statistical mechanics for metabolic networks during steady-state growth
Which properties of metabolic networks can be derived solely from
stoichiometric information about the network's constituent reactions?
Predictive results have been obtained by Flux Balance Analysis (FBA), by
postulating that cells set metabolic fluxes within the allowed stoichiometry so
as to maximize their growth. Here, we generalize this framework to single cell
level using maximum entropy models from statistical physics. We define and
compute, for the core metabolism of Escherichia coli, a joint distribution over
all fluxes that yields the experimentally observed growth rate. This solution,
containing FBA as a limiting case, provides a better match to the measured
fluxes in the wild type and several mutants. We find that E. coli metabolism is
close to, but not at, the optimality assumed by FBA. Moreover, our model makes
a wide range of predictions: (i) on flux variability, its regulation, and flux
correlations across individual cells; (ii) on the relative importance of
stoichiometric constraints vs. growth rate optimization; (iii) on quantitative
scaling relations for singe-cell growth rate distributions. We validate these
scaling predictions using data from individual bacterial cells grown in a
microfluidic device at different sub-inhibitory antibiotic concentrations.
Under mild dynamical assumptions, fluctuation-response relations further
predict the autocorrelation timescale in growth data and growth rate adaptation
times following an environmental perturbation.Comment: 12 pages, 4 figure
Reconciling a significant hierarchical assembly of massive early-type galaxies at z<~1 with mass downsizing
Hierarchical models predict that massive early-type galaxies (mETGs) are the
latest systems to be in place into the cosmic scenario (at z<~0.5), conflicting
with the observational phenomenon of galaxy mass downsizing, which poses that
the most massive galaxies have been in place earlier that their lower-mass
counterparts (since z~0.7). We have developed a semi-analytical model to test
the feasibility of the major-merger origin hypothesis for mETGs, just
accounting for the effects on galaxy evolution of the major mergers strictly
reported by observations. The most striking model prediction is that very few
present-day mETGs have been really in place since z~1, because ~90% of the
mETGs existing at z~1 are going to be involved in a major merger between z~1
and the present. Accounting for this, the model derives an assembly redshift
for mETGs in good agreement with hierarchical expectations, reproducing
observational mass downsizing trends at the same time.Comment: 2 pages, 1 figure, Proceedings of Symposium 2 of JENAM 2010,
"Environment and the Formation of Galaxies: 30 years later", ed. I. Ferreras
and A. Pasquali, Astrophysics & Space Science Proceedings, Springe
Removal of AMPA receptors (AMPARs) from synapses is preceded by transient endocytosis of extrasynaptic AMPARs
AMPA receptors (AMPARs) are dynamically regulated at synapses, but the time course and location of their exocytosis and endocytosis are not known. Therefore, we have used ecliptic pHluorin-tagged glutamate receptor 2 to visualize changes in AMPAR surface expression in real time. We show that synaptic and extrasynaptic AMPARs respond very differently to NMDA receptor activation; there is a rapid internalization of extrasynaptic AMPARs that precedes the delayed removal of synaptic AMPARs
Gap junction reduction in cardiomyocytes following transforming growth factor- beta treatment and Trypanosoma cruzi infection
Gap junction connexin-43 (Cx43) molecules are responsible for electrical impulse conduction in the heart and are affected by transforming growth factor-beta (TGF-beta). This cytokine increases during Trypanosoma cruzi infection, modulating fibrosis and the parasite cell cycle. We studied Cx43 expression in cardiomyocytes exposed or not to TGF-beta T. cruzi, or SB-431542, an inhibitor of TGF-beta receptor type I (ALK-5). Cx43 expression was also examined in hearts with dilated cardiopathy from chronic Chagas disease patients, in which TGF-beta signalling had been shown previously to be highly activated. We demonstrated that TGF-beta treatment induced disorganised gap junctions in non-infected cardiomyocytes, leading to a punctate, diffuse and non-uniform Cx43 staining. A similar pattern was detected in T. cruzi-infected cardiomyocytes concomitant with high TGF-beta secretion. Both results were reversed if the cells were incubated with SB-431542. Similar tests were performed using human chronic chagasic patients and we confirmed a down-regulation of Cx43 expression, an altered distribution of plaques in the heart and a significant reduction in the number and length of Cx43 plaques, which correlated negatively with cardiomegaly. We conclude that elevated TGF-beta levels during T. cruzi infection promote heart fibrosis and disorganise gap junctions, possibly contributing to abnormal impulse conduction and arrhythmia that characterise severe cardiopathy in Chagas disease
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