Deterministic walks in random networks: an application to thesaurus graphs

In a landscape composed of N randomly distributed sites in Euclidean space, a walker (``tourist'') goes to the nearest one that has not been visited in the last \tau steps. This procedure leads to trajectories composed of a transient part and a final cyclic attractor of period p. The tourist walk presents universal aspects with respect to \tau and can be done in a wide range of networks that can be viewed as ordinal neighborhood graphs. As an example, we show that graphs defined by thesaurus dictionaries share some of the statistical properties of low dimensional (d=2) Euclidean graphs and are easily distinguished from random graphs. This approach furnishes complementary information to the usual clustering coefficient and mean minimum separation length.Comment: 12 pages, 5 figures, revised version submited to Physica A, corrected references to figure

Humidity observations and column simulations for a warm period at the Mars Phoenix lander site : Constraining the adsorptive properties of regolith

Two recalibrated sets of Phoenix (PHX) near-surface TECP air humidity measurements were compared with results from adsorptive single column model simulations during a warm clear-sky polar midsummer period, PHX sols 50-60. The model's 2 m temperatures were close to the observed values. Relative humidity (RH) is very low during the day but at night RH at 2 m reaches nearly 100% by the Zent et al. (2016) recalibration (Z), and 60-70% by the Fischer et al. (2019) recalibration (F). Model values of RH2m are close to Z and F at night and to F during the day. All three imply low water vapor pressures near the surface at night, 0.03-0.05 Pa, with a rapid increase each morning to 0.3-1 Pa and a decrease in the evening by both F and the model simulation. The model's daily adsorbed and desorbed water is in balance for regolith porosity of 16% (instead of 35% for lower latitudes). The depleted layer of nighttime air moisture extends to only about 200 m above the surface; hence the model's precipitable water content stays around the observed similar to 30 mu m throughout the sol. The model's moisture cycle is not sensitive to tortuosity of the regolith but the in-pore molecular diffusivity should be at least 5 cm(2)/s for fair agreement with the observations. In the adsorption experiments there is no fog and just a hint of ground frost, as observed during this period. Strong night frosts appear if adsorption is made weak or absent in the model.Peer reviewe

Exploratory Behavior, Trap Models and Glass Transitions

A random walk is performed on a disordered landscape composed of $N$ sites randomly and uniformly distributed inside a $d$-dimensional hypercube. The walker hops from one site to another with probability proportional to $\exp [- \beta E(D)]$, where $\beta = 1/T$ is the inverse of a formal temperature and $E(D)$ is an arbitrary cost function which depends on the hop distance $D$. Analytic results indicate that, if $E(D) = D^{d}$ and $N \to \infty$, there exists a glass transition at $\beta_d = \pi^{d/2}/\Gamma(d/2 + 1)$. Below $T_d$, the average trapping time diverges and the system falls into an out-of-equilibrium regime with aging phenomena. A L\'evy flight scenario and applications to exploratory behavior are considered.Comment: 4 pages, 1 figure, new versio

SRSF1-dependent nuclear export of C9ORF72 repeat-transcripts: targeting toxic gain-of-functions induced by protein sequestration as a selective therapeutic strategy for neuroprotection

Microsatellite repeat expansions cause several incurable and lethal neurodegenerative disorders including ataxias, myotonic dystrophy, Huntington's disease and C9ORF72-linked amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Abnormal repeat transcripts generated from the expanded loci are substrates of repeat-associated non-AUG (RAN) translation, an unconventional form of translation leading to the production of polymeric repeat proteins with cytotoxic and aggregating properties. The mechanisms involved in the pathogenesis of microsatellite repeat expansion disorders remain a hotly debated topic. They are shared between toxic loss/gain of functions due to intranuclear RNA foci that sequesters RNA-binding proteins and RAN translation of repeat proteins in the cytoplasm. We recently elucidated the molecular mechanism driving the nuclear export of C9ORF72 repeat transcripts and showed for the first time that this pathway can be manipulated to confer neuroprotection. Strikingly, we discovered that intron-retaining C9ORF72 repeat transcripts hijack the physiological NXF1-dependent export pathway by selective RNA-repeat sequestration of SRSF1. Antagonizing SRSF1 and the nuclear export of C9ORF72 repeat transcripts promoted in turn the survival of patient-derived motor neurons and suppressed neurodegeneration-associated motor deficits in Drosophila (Hautbergue et al. Nature Communications 2017; 8:16063). In this invited Research Highlight review, we aim to place this work in the context of our previous studies on the nuclear export of mRNAs, provide a summary of the published research and highlight the significance of these findings as a novel therapeutic strategy for neuroprotection in C9ORF72-ALS/FTD. In addition, we emphasize that protein sequestration, often thought as of inducing loss-of-function mechanisms, can also trigger unwanted protein interactions and toxic gain-of-functions

Orbital Polarons in the Metal-Insulator Transition of Manganites

The metal-insulator transition in manganites is strongly influenced by the concentration of holes present in the system. Based upon an orbitally degenerate Mott-Hubbard model we analyze two possible localization scenarios to account for this doping dependence: First, we rule out that the transition is initiated by a disorder-order crossover in the orbital sector, showing that its effect on charge mobility is only small. Second, we introduce the idea of orbital polarons originating from a strong polarization of orbitals in the vicinity of holes. Considering this direct coupling between charge and orbital degree of freedom in addition to lattice effects we are able to explain well the phase diagram of manganites for low and intermediate hole concentrations

Radiative decays of light vector mesons in a quark level linear sigma model

We calculate the P0 to gamma gamma, V0 to P0 gamma and V0to V'0 gamma gamma decays in the framework of a U(3)xU(3) linear sigma model which includes constituent quarks. For the first two decays this approach improves results based on the anomalous Wess-Zumino term, with contributions due to SU(3) symmetry breaking and vector mixing. The phi to (omega,rho) gamma gamma decays are dominated by resonant eta' exchange . Our calculation for the later decays improves and update similar calculations in the -closely related- framework of vector meson dominance. We obtain BR(phi to rho gamma gamma)=2.5x10^{-5} and BR(phi to omega gamma gamma)=2.8x10^{-6} within the scope of the high-luminosity phi factories.Comment: 8 pages, submitted to Phys. Rev.

Loss of neuronal 3d chromatin organization causes transcriptional and behavioural deficits related to serotonergic dysfunction

The interior of the neuronal cell nucleus is a highly organized three-dimensional (3D) structure where regions of the genome that are linearly millions of bases apart establish sub-structures with specialized functions. To investigate neuronal chromatin organization and dynamics in vivo, we generated bitransgenic mice expressing GFP-tagged histone H2B in principal neurons of the forebrain. Surprisingly, the expression of this chimeric histone in mature neurons caused chromocenter declustering and disrupted the association of heterochromatin with the nuclear lamina. The loss of these structures did not affect neuronal viability but was associated with specific transcriptional and behavioural deficits related to serotonergic dysfunction. Overall, our results demonstrate that the 3D organization of chromatin within neuronal cells provides an additional level of epigenetic regulation of gene expression that critically impacts neuronal function. This in turn suggests that some loci associated with neuropsychiatric disorders may be particularly sensitive to changes in chromatin architecture

Endothelial loss of Fzd5 stimulates PKC/Ets1-mediated transcription of Angpt2 and Flt1

Aims: Formation of a functional vascular system is essential and its formation is a highly regulated process initiated during embryogenesis, which continues to play important roles throughout life in both health and disease. In previous studies, Fzd5 was shown to be critically involved in this process and here we investigated the molecular mechanism by which endothelial loss of this receptor attenuates angiogenesis. Methods and results: Using short interference RNA-mediated loss-of-function assays, the function and mechanism of signaling via Fzd5 was studied in human endothelial cells (ECs). Our findings indicate that Fzd5 signaling promotes neovessel formation in vitro in a collagen matrix-based 3D co-culture of primary vascular cells. Silencing of Fzd5 reduced EC proliferation, as a result of G0/G1 cell cycle arrest, and decreased cell migration. Furthermore, Fzd5 knockdown resulted in enhanced expression of the factors Angpt2 and Flt1, which are mainly known for their destabilizing effects on the vasculature. In Fzd5-silenced ECs, Angpt2 and Flt1 upregulation was induced by enhanced PKC signaling, without the involvement of canonical Wnt signaling, non-canonical Wnt/Ca2+-mediated activation of NFAT, and non-canonical Wnt/PCP-mediated activation of JNK. We demonstrated that PKC-induced transcription of Angpt2 and Flt1 involved the transcription factor Ets1. Conclusions: The current study demonstrates a pro-angiogenic role of Fzd5, which was shown to be involved in endothelial tubule formation, cell cycle progression and migration, and partly does so by repression of PKC/Ets1-mediated transcription of Flt1 and Angpt2

Escaping from cycles through a glass transition

A random walk is performed over a disordered media composed of $N$ sites random and uniformly distributed inside a $d$-dimensional hypercube. The walker cannot remain in the same site and hops to one of its $n$ neighboring sites with a transition probability that depends on the distance $D$ between sites according to a cost function $E(D)$. The stochasticity level is parametrized by a formal temperature $T$. In the case $T = 0$, the walk is deterministic and ergodicity is broken: the phase space is divided in a ${\cal O}(N)$ number of attractor basins of two-cycles that trap the walker. For $d = 1$, analytic results indicate the existence of a glass transition at $T_1 = 1/2$ as $N \to \infty$. Below $T_1$, the average trapping time in two-cycles diverges and out-of-equilibrium behavior appears. Similar glass transitions occur in higher dimensions choosing a proper cost function. We also present some results for the statistics of distances for Poisson spatial point processes.Comment: 11 pages, 4 figure