Key Role of Polyphosphoinositides in Dynamics of Fusogenic Nuclear Membrane Vesicles

The role of phosphoinositides has been thoroughly described in many signalling and membrane trafficking events but their function as modulators of membrane structure and dynamics in membrane fusion has not been investigated. We have reconstructed models that mimic the composition of nuclear envelope precursor membranes with naturally elevated amounts of phosphoinositides. These fusogenic membranes (membrane vesicle 1(MV1) and nuclear envelope remnants (NER) are critical for the assembly of the nuclear envelope. Phospholipids, cholesterol, and polyphosphoinositides, with polyunsaturated fatty acid chains that were identified in the natural nuclear membranes by lipid mass spectrometry, have been used to reconstruct complex model membranes mimicking nuclear envelope precursor membranes. Structural and dynamic events occurring in the membrane core and at the membrane surface were monitored by solid-state deuterium and phosphorus NMR. “MV1-like” (PC∶PI∶PIP∶PIP2, 30∶20∶18∶12, mol%) membranes that exhibited high levels of PtdIns, PtdInsP and PtdInsP2 had an unusually fluid membrane core (up to 20% increase, compared to membranes with low amounts of phosphoinositides to mimic the endoplasmic reticulum). “NER-like” (PC∶CH∶PI∶PIP∶PIP2, 28∶42∶16∶7∶7, mol%) membranes containing high amounts of both cholesterol and phosphoinositides exhibited liquid-ordered phase properties, but with markedly lower rigidity (10–15% decrease). Phosphoinositides are the first lipids reported to counterbalance the ordering effect of cholesterol. At the membrane surface, phosphoinositides control the orientation dynamics of other lipids in the model membranes, while remaining unchanged themselves. This is an important finding as it provides unprecedented mechanistic insight into the role of phosphoinositides in membrane dynamics. Biological implications of our findings and a model describing the roles of fusogenic membrane vesicles are proposed

Modified Kepler model of photophoresis

In this study, a phenomena called ’photophoresis’ is observed on a circumstellar disk. The study is motivated by the fact that most of the details on how planetary formations on a type of circumstellar disks known as protoplanetary disks are still unknown and by studying how dust particles move in protoplanetary disks through a Kepler problem, we are able to understand the motion of dust better. Most forces such as gravity, radiation force pressure, and gas drag do not fully explain the missing details of planetary formations. One candidate that might explain the missing details is a phenomena called ’photophoresis’ would be the subject of this study, where a dust particle moves after one of its sides that is heated ends up pushing gas molecules faster that its cooler side. The purpose of the study is to observe how the photophoretic force affects the motion of dust particles that are present in the circumstellar disks. The study is conducted by comparing two mathematical models, one with photophoresis and one without photophoresis. The comparison of models would be done over a sample of 6 different masses, 6 different radii, and 15 different initial positions and velocities, giving us 540 samples of particles to compare the two mathematical models. The study is separated into three parts: A theoretical part, A computational part, and a statistical part. In the theoretical part, the Keplerian differential equations is derived from the equations describing the force experienced by the dust particle. In the computational part, the motion of the particle is simulated through a numerical computation program. In the statistical part, the data gathered from the computational part is then analyzed using an approximation error formula in order to compute the difference between the two mathematical models. The computational and statistical results shows that photophoresis has the greatest effect on the particle with the smallest mass 10^-6 g but with the largest radius 10^-1 m within the samples while having the least effect on a particle that is the complete opposite, one with the largest mass 10^-1 g but with the smallest radius 10^-6 m within the samples. Photophoresis also has the greatest effect on the particle with the highest initial position and highest initial velocity within the samples

Physical and structural properties of barley (1→3),(1→4)-β-D-glucan—III. Formation of aggregates analysed through its viscoelastic and flow behaviour

International audienceThe Theological properties (flow and viscoelastic) of barley β-glucan in aqueous solution have been studied. The results shed light on the association behaviour of these macromolecules and even on the formation of network-like structures under certain circumstances. The concentrations used for this study were both in the dilute and in the semidilute regime, close to the critical value where overlap of macromolecular coils sets in. However, the association behaviour detected in such a regime and together with the present experimental techniques, agrees with previous results obtained with other techniques such as capillary viscometry and light scattering, which operate in the dilute solution regime, where no overlapping of the chains is expected. The influence of the molecular weight of the samples, the temperature and the ionic strength on the flow and oscillatory shear measurements has been studied. The preparation of samples and their thermal history have proved to be important factors in the development of associated structures, both affecting the results

Ecological interactions in a shallow sand-pit lake (Lake Créteil, Parisian Basin, France): a modelling approach

A large data set (n = 154) of phytoplankton production and biomass in relation to physico-chemical environmental factors was collected from 1979 to 1986 in a recently created sand-pit lake (Paris suburbs). These data are well suited to interpret the oligotrophication observed along the 8 years period, characterized by a regular decrease in chlorophyll (from 16 to 4 μg l-1 as annual averages). A model describing the ecological functioning of the lake has been established. Biological processes related to phyto-, bacterio- and zooplankton as well as sediment-water interactions, are described within several submodels. Most of the parameters involved were determined by in situ measurements in this or similar environments The model provides a good simulation of observed data and confirms that the reduction of nutrient loading, resulting from the diversion - in 1981 - of a sewer previously discharging into the lake, was responsible for the oligotrophication of the system. The model allows to explore the response of planktonic compartments accross a gradient of nutrient loading. The role of hydrology is also tested. The systematic run of the model with and without zooplankton leads to a better understanding of top-down control. © 1994 Kluwer Academic Publishers.SCOPUS: ar.jinfo:eu-repo/semantics/publishe