Predicting different adhesive regimens of circulating particles at blood capillary walls.

A fundamental step in the rational design of vascular targeted particles is the firm adhesion at the blood vessel walls. Here, a combined lattice Boltzmann-immersed boundary model is presented for predicting the near-wall dynamics of circulating particles. A moving least squares algorithm is used to reconstruct the forcing term accounting for the immersed particle, whereas ligand-receptor binding at the particle-wall interface is described via forward and reverse probability distributions. First, it is demonstrated that the model predicts with good accuracy the rolling velocity of tumor cells over an endothelial layer in a microfluidic channel. Then, particle-wall interactions are systematically analyzed in terms of particle geometries (circular, elliptical with aspect ratios 2 and 3), surface ligand densities (0.3, 0.5, 0.7 and 0.9), ligand-receptor bond strengths (1 and 2) and Reynolds numbers (Re = 0.01, 0.1 and 1.0). Depending on these conditions, four different particle-wall interaction regimens are identified, namely not adhering, rolling, sliding and firmly adhering particles. The proposed computational strategy can be efficiently used for predicting the near-wall dynamics of particles with arbitrary geometries and surface properties and represents a fundamental tool in the rational design of particles for the specific delivery of therapeutic and imaging agents.

Effect of mitochondrial ascorbic acid synthesis on photosynthesis.

Ascorbic acid (AA) is synthesized in plant mitochondria through the oxidation of l-galactono-1,4-lactone (l-GalL) and then distributed to different cell compartments. AA-deficient Arabidopsis thaliana mutants (vtc2) and exogenous applications of l-GalL were used to generate plants with different AA content in their leaves. This experimental approach allows determining specific AA-dependent effects on carbon metabolism. No differences in O2 uptake, malic and citric acid and NADH content suggest that AA synthesis or accumulation did not affect mitochondrial activity; however, l-GalL treatment increased CO2 assimilation and photosynthetic electron transport rate in vtc2 (but not wt) leaves demonstrating a stimulation of photosynthesis after l-GalL treatment. Increased CO2 assimilation correlated with increased leaf stomatal conductance observed in l-GalL-treated vtc2 plants.

Evaluation of prosthetic-valved devices by means of numerical simulations.

The in vivo evaluation of prosthetic device performance is often difficult, if not impossible. In particular, in order to deal with potential problems such as thrombosis, haemolysis, etc., which could arise when a patient undergoes heart valve replacement, a thorough understanding of the blood flow dynamics inside the devices interacting with natural or composite tissues is required. Numerical simulation, combining both computational fluid and structure dynamics, could provide detailed information on such complex problems. In this work, a numerical investigation of the mechanics of two composite aortic prostheses during a cardiac cycle is presented. The numerical tool presented is able to reproduce accurately the flow and structure dynamics of the prostheses. The analysis shows that the vortical structures forming inside the two different grafts do not influence the kinematics of a bileaflet valve or the main coronary flow, whereas major differences are present for the stress status near the suture line of the coronaries to the prostheses. The results are in agreement with in vitro and in vivo observations found in literature.

Fluid-structure interaction of deformable aortic prostheses with a bileaflet mechanical valve.

Two different aortic prostheses can be used for performing the Bentall procedure: a standard straight graft and the Valsalva graft that better reproduces the aortic root anatomy. The aim of the present work is to study the effect of the graft geometry on the blood flow when a bileaflet mechanical heart valve is used, as well as to evaluate the stress concentration near the suture line where the coronary arteries are connected to graft. An accurate three-dimensional numerical method is proposed, based on the immersed boundary technique. The method accounts for the interactions between the flow and the motion of the rigid leaflets and of the deformable aortic root, under physiological pulsatile conditions. The results show that the graft geometry only slightly influences the leaflets dynamics, while using the Valsalva graft the stress level near the coronary-root anastomoses is about half that obtained using the standard straight graft.

Increased ascorbate content delays flowering in long-day grown Arabidopsis thaliana (L.) Heynh.

Flowering requires the integration of different inductive stimuli, including light, temperature and hormones. In an attempt to assess whether ascorbate (ASC) could contribute to the control of flowering time, we analyzed the effects of increased ASC content on the transition to the reproductive stage in the facultative long-day plant Arabidopsis thaliana. ASC content was increased by spraying leaves with the ASC precursor L-galactono-gamma-lactone. Our data show that increased ASC content did not affect vegetative growth, whereas a significant delay (5 days in average) in flower production occurred in ASC-overproducing plants. Higher ASC availability resulted in delayed expression of LEAFY (LFY), the gene encoding for a key transcription factor integrating different flowering-inductive pathways. On the contrary, spraying with gibberellin under the same condition caused both early LFY expression and early flowering. The possible role of ASC in the transition to the reproductive stage is discussed.

Hopes, disillusions and more hopes from vitamin C.

In the current view of most biochemists and physiologists, the role of L-ascorbic acid (AA) in cell metabolism would be more or less confined to the scavenging of reactive oxygen species. Nevertheless, many data have been collected in our and other laboratories concerning the involvement of AA in many different aspects of cell metabolism. At the present time the molecular sites of action of AA have not been completely elucidated, but recent findings on the specific requirement of AA for the activity of several 2-oxoacid-dependent dioxygenases involved in cell signalling and the activation of transcription factors open new fascinating perspectives for further research.

Short- and long-term effects of dehydroascorbate in Lupinus albus and Allium cepa roots.

Administration of 1 mM dehydroascorbate (DHA) results in a rapid and large increase in cellular ascorbate (AA) content in both Lupinus albus L. and Allium cepa L. root tips. Uptake of DHA from the medium occurs at a high rate within 10-12 h of incubation, and is slowed down thereafter. In the first few h, DHA reduction to AA is apparently correlated to GSH depletion and slightly higher DHA reductase activity. DHA incubation also seems to induce new GSH synthesis. Longer DHA incubation (24 h) affects root growth by inhibiting cell proliferation. At this stage, an apparently generalised oxidation of SH-containing proteins is observed in DHA-treated roots. Treatment with 1 mM L-galactono-gamma-lactone, the last precursor of AA biosynthesis, results in an increase in AA content similar to that obtained with DHA, but stimulates growth and affects the redox state of SH-containing proteins in the opposite way. A possible multi-step mechanism of DHA reduction/removal is suggested and the hypothesis that DHA inhibits cell cycle progression by affecting the redox state of SH-containing proteins is discussed.

Dehydroascorbate and dehydroascorbate reductase are phantom indicators of oxidative stress in plants.

In many physiological studies dehydroascorbate (DHA) reductase is regarded as one of the chloroplast enzymes involved in the protection against oxidative stress. Here, evidence is presented that plant cells do not possess a specific DHA reductase. The DHA reductase activities measured in plant extracts are due to side reactions of proteins containing redox-active dicysteine sites. Native gel electrophoresis combined with specific activity staining revealed three different proteins with DHA reductase activity in leaf and chloroplast extracts. These proteins have been identified as thioredoxins and trypsin inhibitors (Kunitz type) by Western blot analysis. The essential regulatory functions of thioredoxins in chloroplast metabolism are strongly inhibited in the presence of as little as 50 microM DHA. Thus, the intracellular DHA concentration should be kept below 50 microM but not all proteins with DHA reductase activity are effective enough for this purpose. A specific DHA reductase is frequently demanded as part of the enzymatic equipment to avoid oxidative stress. We argue that this is not necessary because in chloroplasts DHA does not accumulate to any significant extent due to the high activities of monodehydroascorbate reductase and of reduced ferredoxin.

The multiplicity of enzymatic DNA reduction: a new purification procedure for a DHA reducing protein from potato tubers.

The multiplicity of DHA reducing proteins has been observed by means of a native-PAGE technique in several plant species, thus demonstrating, in accordance with recent literature, that several differently evolved proteins are likely to perform DHA reduction. Moreover, a research strategy coupling the use of native-PAGE with chromatographic separation procedure, tentatively performed in Solanum tuberosum, proved to be a useful tool for the separation and partial identification of the proteins involved in DHA reduction.

An electrophoretic procedure to detect three key enzymes of the ascorbate system.

A method for detection of ascorbic acid oxidase, ascorbic acid peroxidase and dehydroascorbic acid reductase is reported. This method allows the qualitative determination of the presence of these enzymes, also in conditions where the commonly used spectrophotometric assays are unreliable.

Ascorbate peroxidase activity in resistant and susceptible plants of Lycopersicon esculentum.

Activity of redox-enzymes of AA system and of catalase was measured in two near-isogenic tomato lines, respectively resistant and susceptible to Tobacco Mosaic Virus infection. AFR reductase, DHA reductase and catalase showed quite similar activities in both lines, whereas AA peroxidase activity in resistant plants was 75% higher than in susceptible ones, with Km values about 4-fold lower. These data suggest that hydrogen peroxide scavenging operated by AA peroxidase could play an important role in the development of biological defence mechanisms against pathogens.