Velocity Tails for Inelastic Maxwell Models

We study the velocity distribution function for inelastic Maxwell models, characterized by a Boltzmann equation with constant collision rate, independent of the energy of the colliding particles. By means of a nonlinear analysis of the Boltzmann equation, we find that the velocity distribution function decays algebraically for large velocities, with exponents that are analytically calculated.Comment: 4 pages, 2 figure

Chromatin remodelling complex dosage modulates transcription factor function in heart development

Dominant mutations in cardiac transcription factor genes cause human inherited congenital heart defects (CHDs); however, their molecular basis is not understood. Interactions between transcription factors and the Brg1/Brm-associated factor (BAF) chromatin remodelling complex suggest potential mechanisms; however, the role of BAF complexes in cardiogenesis is not known. In this study, we show that dosage of Brg1 is critical for mouse and zebrafish cardiogenesis. Disrupting the balance between Brg1 and disease-causing cardiac transcription factors, including Tbx5, Tbx20 and Nkx2–5, causes severe cardiac anomalies, revealing an essential allelic balance between Brg1 and these cardiac transcription factor genes. This suggests that the relative levels of transcription factors and BAF complexes are important for heart development, which is supported by reduced occupancy of Brg1 at cardiac gene promoters in Tbx5 haploinsufficient hearts. Our results reveal complex dosage-sensitive interdependence between transcription factors and BAF complexes, providing a potential mechanism underlying transcription factor haploinsufficiency, with implications for multigenic inheritance of CHDs

Complex SUMO-1 Regulation of Cardiac Transcription Factor Nkx2-5

Reversible post-translational protein modifications such as SUMOylation add complexity to cardiac transcriptional regulation. The homeodomain transcription factor Nkx2-5/Csx is essential for heart specification and morphogenesis. It has been previously suggested that SUMOylation of lysine 51 (K51) of Nkx2-5 is essential for its DNA binding and transcriptional activation. Here, we confirm that SUMOylation strongly enhances Nkx2-5 transcriptional activity and that residue K51 of Nkx2-5 is a SUMOylation target. However, in a range of cultured cell lines we find that a point mutation of K51 to arginine (K51R) does not affect Nkx2-5 activity or DNA binding, suggesting the existence of additional Nkx2-5 SUMOylated residues. Using biochemical assays, we demonstrate that Nkx2-5 is SUMOylated on at least one additional site, and this is the predominant site in cardiac cells. The second site is either non-canonical or a “shifting” site, as mutation of predicted consensus sites and indeed every individual lysine in the context of the K51R mutation failed to impair Nkx2-5 transcriptional synergism with SUMO, or its nuclear localization and DNA binding. We also observe SUMOylation of Nkx2-5 cofactors, which may be critical to Nkx2-5 regulation. Our data reveal highly complex regulatory mechanisms driven by SUMOylation to modulate Nkx2-5 activity

Density functional approach to helium at finite temperature

We report an application of the general formalism of density functional theory (DFT) for quantum fluids at finite temperature to the case of helium. Using this approach, we compute the liquid-vapour coexistence curve and the surface tension of helium at low temperatures. We observe that the range of the interface is much larger than the usual 10-90 surface thickness, and we find that the DFT reproduces the T7ß temperature dependence of the surface tension. This implies that capillary-wave effects are, at least partially, accounted for by the density functiona

Depletion effects in binary hard-sphere fluids

We report a molecular dynamics computation of the entropic depletion force induced between two large spheres (colloidal particles) immersed in a fluid of small spheres. The effective pair potential obtained by numerical integration of the force is used in a Monte Carlo study of the phase behaviour of the binary mixture. The simulation results are compared with the relevant theoretical predictions that follow from various integral equations for liquid mixtures. The simulations provide evidence for a spinodal instability in a liquid mixture of hard spheres with a size ratio of 0.1

Dispersions and mixtures of particles with complex architectures in shear flow

We review the effect of shear flow on the phase behavior and structure of colloidal dispersions with increasing degree of complexity. We discuss dispersions of colloidal rods, stiff living polymers like wormlike micelles, and colloidal platelets. In addition, a review is presented on sheared binary dispersions. For all cases we discuss the interplay between thermodynamic instabilities and hydrodynamic instabilities

Conversation Among Primate Species

International audienceThe literature in psychology and sociolinguistic suggests that human interlocutors, when conversing, virtually sign a sort of contract that defines the exchange rules in both structural and social domains. These rules make the messages more understandable and the interaction more predictable, but they may also act as a social bond regulator. These rules can be very basic such as speech overlap avoidance, respect of response delays, turn-taking and vocal accommodation to the context and interlocutor’s social status. Interestingly, these rules are universally spread among human cultures questioning their biological basis and motivating the search for possible parallels with our primate cousins. Here, we will review the available literature on monkeys and apes. We will describe the different forms of vocal interactions, the temporal rules underlying these coordinated interactions, the non-random social selection of interlocutors and the context-dependent acoustic plasticity associated to these exchanges. The fact that primate species are socially varied, in terms of both social structure and social organisation, is another interesting aspect, since different social needs may predict different vocal interaction patterns and conversational rules. For example, duets, choruses and dyadic exchanges are not randomly distributed in the primate phylogeny and may even show different functions. Also, age proximity, kin membership, social affinity and hierarchy seem to play species-specific roles. Regarding plasticity, cases of vocal sharing and acoustic matching have been described in some species, notably in contact calls which are the calls the most frequently involved in dyadic exchanges. At last, a few studies also show that these ‘primitive’ conversational rules are often broken by juveniles and that the appropriate way to vocally interact with others may be socially learned, thus another aspect that do not seem strictly human