Tiki Casts a Spell on Wnt

Negative feedback is a widespread feature of signaling pathways. In an unexpected twist described in this issue, He and colleagues identify a membrane-tethered metalloprotease named Tiki that inhibits Wnt signaling by removing an essential eight-residue fragment from Wnt itself

On primitive formulation in fluid mechanics and fluid-structure interaction with constant piecewise properties in velocity-potentials of acceleration

Discrete mechanics makes it possible to formulate any problem of fluid mechanics or fluid-structure interaction in velocity and potentials of acceleration; the equation system consists of a single vector equation and potentials updates. The scalar potential of the acceleration represents the pressure stress and the vector potential is related to the rotational-shear stress. The formulation of the equation of motion can be expressed in the form of a splitting which leads to an exact projection method; the application of the divergence operator to the discrete motion equation exhibits, without any approximation, a Poisson equation with constant coefficients on the scalar potential whatever the variations of the physical properties of the media. The a posteriori calculation of the pressure is made explicitly by introducing at this stage the local density. Two first examples show the interest of the formulation presented on classical solutions of Navier-Stokes equations; similarly as other results obtained with this formulation, the convergence is of order two in space and time for all the quantities, velocity and potentials. This formulation is then applied to a two-phase flow driven by surface tension and partial wettability. The last case corresponds to a fluid-structure interaction problem for which an analytical solution exists.Comment: 18 pages, 7 figures, article. Acta Mechanica, 202

The Continuous Skolem-Pisot Problem: On the Complexity of Reachability for Linear Ordinary Differential Equations

We study decidability and complexity questions related to a continuous analogue of the Skolem-Pisot problem concerning the zeros and nonnegativity of a linear recurrent sequence. In particular, we show that the continuous version of the nonnegativity problem is NP-hard in general and we show that the presence of a zero is decidable for several subcases, including instances of depth two or less, although the decidability in general is left open. The problems may also be stated as reachability problems related to real zeros of exponential polynomials or solutions to initial value problems of linear differential equations, which are interesting problems in their own right.Comment: 14 pages, no figur

Price Discrimination and EU Dairy Policy: An Economic Evaluation of Policy Options

In a period of market liberalization and multilateral trade negotiations, price discrimination for commodities with distinct markets provides additional policy options to support farm income. While both the USA and Canada have implemented price discrimination policies in their domestic dairy sector, so far the European Union (EU) has not. This paper evaluates the options of developing a price discrimination policy in the EU dairy sector. The analysis is based on an interregional model of the EU dairy sector, involving milk production, dairy processing, and consumption of ten dairy commodities in nine regions. The paper shows that a price discrimination policy that increases prices for commodities with more inelastic demand (fluid milk, soft dairy products) would generate income that can be redistributed to dairy farmers. The results suggest that, while such a price discrimination policy can be a WTO compatible way to support dairy farm income, the efficiency of the associated income transfers declines in the presence of significant supply response.EU, price discrimination, pooling, dairy policy, Demand and Price Analysis,

Changes in tolerance to herbicide toxicity throughout development stages of phototrophic biofilms

Ecotoxicological experiments have been performed in laboratory-scale microcosms to investigate thesensitivity of phototrophic biofilm communities to the alachlor herbicide, in relation to the stages ofphototrophic biofilm maturation (age of the phototrophic biofilms) and physical structure (intact biofilmversus recolonization). The phototrophic biofilms were initially cultivated on artificial supports in aprototype rotating annular bioreactor (RAB) with Taylor–Couette type flow under constant operatingconditions. Biofilms were collected after 1.6 and 4.4 weeks of culture providing biofilms with differentmaturation levels, and then exposed to nominal initial alachlor concentration of 10 ug L−1in either intactor recolonized biofilms for 15 days in microcosms (mean time-weighted average concentration – TWACof 5.52 ± 0.74 ug L−1).At the end of the exposure period, alachlor effects were monitored by a combination of biomass descrip-tors (ash-free dry mass – AFDM, chlorophyll a), structural molecular fingerprinting (T-RFLP), carbonutilization spectra (Biolog) and diatom species composition. We found significant effects that in terms ofAFDM, alachlor inhibited growth of the intact phototrophic biofilms. No effect of alachlor was observedon diatom composition or functional and structural properties of the bacterial community regardless ofwhether they were intact or recolonized. The intact three-dimensional structure of the biofilm did notappear to confer protection from the effects of alachlor. Bacterial community structure and biomass levelof 4.4 weeks – intact phototrophic biofilms were significantly influenced by the biofilm maturation pro-cesses rather than alachlor exposure. The diatom communities which were largely composed of mobileand colonizer life-form populations were not affected by alachlor.This study showed that the effect of alachlor (at initial concentration of 10 ug L−1or mean TWAC of5.52 ± 0.74 ug L−1) is mainly limited to biomass reduction without apparent changes in the ecologicalsuccession trajectories of bacterial and diatom communities and suggested that carbon utilization spec-tra of the biofilm are not damaged resulting. These results confirmed the importance of consideringthe influence of maturation processes or community age when investigating herbicide effects. This isparticularly important with regard to the use of phototrophic biofilms as bio-indicators

La parada de los monstruos

Abstract not availabl

La venjança d'Abel

Abstract not availabl