Ventilatory support: a dynamical systems approach

Misunderstanding of the dynamical behavior of the ventilatory system, especially under assisted ventilation, may explain the problems encountered in ventilatory support monitoring. Proportional assist ventilation (PAV) that theoretically gives a breath by breath assistance presents instability with high levels of assistance. We have constructed a mathematical model of interactions between three objects: the central respiratory pattern generator modelled by a modified Van der Pol oscillator, the mechanical respiratory system which is the passive part of the system and a controlled ventilator that follows its own law. The dynamical study of our model shows the existence of two crucial behaviors, i.e. oscillations and damping, depending on only two parameters, namely the time constant of the mechanical respiratory system and a cumulative interaction index. The same result is observed in simulations of spontaneous breathing as well as of PAV. In this last case, increasing assistance leads first to an increase of the tidal volume (V_T), a further increase in assistance inducing a decrease in V_T, ending in damping of the whole system to an attractive fixed point. We conclude that instabilities observed in PAV may be explained by the different possible dynamical behaviors of the system rather than changes in mechanical characteristics of the respiratory system.     

Differential regulation of the chick dorsal thoracic dermal progenitors from the medial dermomyotome

The chick dorsal feather-forming dermis originates from the dorsomedial somite and its formation depends primarily on Wnt1 from the dorsal neural tube. We investigate further the origin and specification of dermal progenitors from the medial dermomyotome. This comprises two distinct domains: the dorsomedial lip and a more central region (or intervening zone) that derives from it. We confirm that Wnt1 induces Wnt11 expression in the dorsomedial lip as previously shown, and show using DiI injections that some of these cells, which continue to express Wnt11 migrate under the ectoderm, towards the midline, to form most of the dorsal dermis. Transplantation of left somites to the right side to reverse the mediolateral axis confirms this finding and moreover suggests the presence of an attractive or permissive environment produced by the midline tissues or/and a repellent or inadequate environment by the lateral tissues. By contrast, the dorsolateral dermal cells just delaminate from the surface of the intervening space, which expresses En1. Excision of the axial organs or the ectoderm, and grafting of Wnt1-secreting cells, shows that, although the two populations of dermal progenitors both requires Wnt1 for their survival, the signalling required for their specification differs. Indeed Wnt11 expression relies on dorsal neural tube-derived Wnt1, while En1 expression depends on the presence of the ectoderm. The dorsal feather-forming dermal progenitors thus appear to be differentially regulated by dorsal signals from the neural tube and the ectoderm, and derive directly and indirectly from the dorsomedial lip. As these two dermomyotomal populations are well known to also give rise to epaxial muscles, an isolated domain of the dermomyotome that contains only dermal precursors does not exist and none of the dermomyotomal domains can be considered uniquely as a dermatome.     

TGFbeta2 mediates rapid inhibition of calcium influx in identified cholinergic basal forebrain neurons.

Transforming growth factors betas (TGFbetas) are known to have important roles in neuronal survival and can be upregulated in disease. However, unlike many other trophic factors, nothing is known about the rapid neurotransmitter-like actions of TGFbeta in the CNS. We explored this by examining the effects of TGFbeta on calcium influx of large enzymatically dissociated basal forebrain neurons. We show that brief application of TGFbeta2, but not TGFbeta1, to fura-2AM-loaded neurons reversibly and acutely (within seconds) inhibited K(+)-evoked calcium influx. Moreover, using single-cell RT-PCR, we confirmed that the large TGFbeta2-responsive neurons presented a cholinergic phenotype. Investigation of the signaling mechanism underlying TGFbeta2 actions using whole-cell recordings of calcium currents revealed that TGFbeta2-mediated responses were insensitive to the nonhydrolyzable GTP analogue GTPgammaS. However, TGFbeta2-mediated calcium current reductions were prevented by intracellular perfusion of a Smad2/3 peptide antagonist. Together, these results suggest that TGFbeta2 can acutely regulate the excitability of basal forebrain cholinergic neurons through an atypical signaling mechanism.     

Crucial role of conserved lysine 277 in the fidelity of tRNA aminoacylation by Escherichia coli valyl-tRNA synthetase

Valyl-tRNA synthetase (ValRS) from Escherichia coli undergoes covalent valylation by a donor valyl adenylate synthesized by the enzyme itself. ValRS could also be modified, although to a lesser extent, by the noncognate isosteric substrate L-threonine from a donor threonyl adenylate synthesized by the synthetase itself, or by the nonsubstrate methionine from methionyl adenylate produced by catalytic amounts of methionyl-tRNA synthetase. MALDI mass spectrometry analysis designated lysines 154, 162, 170, 533, 554, 593, 894, 930, and 940 of ValRS as the target residues for the attachment of valine. Following autothreonylation, lysines 162, 170, 178, 277, 291, 554, 580, 593, 861, 894, and 930 were found to be modified. Finally, L-Met-labeled residues were lysines 118, 162, 170, 178, 277, and 938. Alignment of the available ValRS amino acid sequences showed that lysines 277 and 554 are strictly conserved (with the exception concerning replacement of Lys-277 with a methionine or a tyrosine in archaebacteria), suggesting that these residues might be functionally significant. Indeed, lysine 554 of ValRS is the first lysine of the Lys-Met-Ser-Lys-Ser signature of the catalytic site of class I aminoacyl-tRNA synthetases. Lys-277 which is labeled by L-threonine or L-methionine, and not by L-valine, is located at or near the editing site, in the three-dimensional structure of ValRS. The role of lysine 277 was evaluated by site-directed mutagenesis. The Lys277Ala mutant (K277A) exhibited a posttransfer Thr-tRNA(Val) editing rate that was significantly lower than that observed for the wild-type enzyme. In addition, the K277A substitution altered amino acid discrimination in the editing site, resulting in hydrolysis of the correctly charged cognate Val-tRNA(Val). Finally, significant amounts of mischarged Thr-tRNA(Val) were produced by the K277A mutant, and not by wild-type ValRS. Altogether, our results designate Lys-277 as a likely candidate for nucleophilic attack of misacylated tRNA in the editing site of ValRS.     

Overexpression of a soybean cytosolic glutamine synthetase gene linked to organ-specific promoters in pea plants grown in different concentrations of nitrate

A glutamine synthetase gene ( GS15) coding for soybean cytosolic glutamine synthetase (GS1) fused to a constitutive promoter (CaMV 35S), a putative nodule-specific promoter (LBC(3)) and a putative root-specific promoter (rolD) was transformed into Pisum sativum L. cv. Greenfeast. Four lines with single copies of GS15 (one 35S-GS15 line, one LBC (3) -GS15 line, and two rolD-GS15 lines) were tested for the expression of GS15, levels of GS1, GS activity, N accumulation, N(2) fixation, and plant growth at different levels of nitrate. Enhanced levels of GS1 were detected in leaves of three transformed lines (the 35S-GS15 and rolD-GS15 transformants), in nodules of three lines (the LBC (3) -GS15 and rolD-GS15 transformants), and in roots of all four transformants. Despite increased levels of GS1 in leaves and nodules, there were no differences in GS activity in these tissues or in whole-plant N content, N(2) fixation, or biomass accumulation among all the transgenic lines and the wild-type control. However, the rolD-GS15 transformants, which displayed the highest levels of GS1 in the roots of all the transformants, had significantly higher GS activity in roots than the wild type. In one of the rolD-GS15 transformed lines (Line 8), increased root GS activity resulted in a lower N content and biomass accumulation, supporting the findings of earlier studies with Lotus japonicus (Limami et al. 1999 ). However, N content and biomass accumulation was not negatively affected in the other rolD-GS15 transformant (Line 9) and, in fact, these parameters were positively affected in the 0.1 mM treatment. These findings indicate that overexpression of GS15 in various tissues of pea does not consistently result in increases in GS activity. The current study also indicates that the increase in root GS activity is not always consistent with decreases in plant N and biomass accumulation and that further investigation of the relationship between root GS activity and growth responses is warranted.     

Translokin is an intracellular mediator of FGF-2 trafficking

Basic fibroblast growth factor (bFGF or FGF-2) exerts its pleiotropic activities both as an exogenous and an intracellular factor. FGF-1 and FGF-2 are prototypes for this dual signalling, but the mechanisms of their intracellular actions remain unknown. Here we show that Translokin, a cytoplasmic protein of relative molecular mass 55,000 (M(r) 55K), interacts specifically with the 18K form of FGF-2. Translokin is ubiquitously expressed and colocalizes with the microtubular network. As Translokin does not interact with FGF-1, we used a strategy based on FGF-1-FGF-2 chimaeras to map the interacting regions in FGF-2 and to generate Nb1a2, a non-interacting variant of FGF-2. Although most of the FGF-2 properties are preserved in Nb1a2, this variant is defective in intracellular translocation and in stimulating proliferation. The fusion of a nuclear localization signal to Nb1a2 restores its mitogenic activity and its nuclear association. Inhibiting Translokin expression by RNA interference reduces the translocation of FGF-2 without affecting the intracellular trafficking of FGF-1. Our data show that the nuclear association of internalized FGF-2 is essential for its mitogenic activity and that Translokin is important in this translocation pathway.     

Splanchnic free fatty acid kinetics

These studies were conducted to assess the relationship between visceral adipose tissue free fatty acid (FFA) release and splanchnic FFA release. Steady-state splanchnic bed palmitate ([9,10-(3)H]palmitate) kinetics were determined from 14 sampling intervals from eight dogs with chronic indwelling arterial, portal vein, and hepatic vein catheters. We tested a model designed to predict the proportion of FFAs delivered to the liver from visceral fat by use of hepatic vein data. The model predicted that 15 +/- 2% of hepatic palmitate delivery originated from visceral lipolysis, which was greater (P = 0.004) than the 11 +/- 2% actually observed. There was a good relationship (r(2) = 0.63) between the predicted and observed hepatic palmitate delivery values, but the model overestimated visceral FFA release more at lower than at higher palmitate concentrations. The discrepancy could be due to differential uptake of FFAs arriving from the arterial vs. the portal vein or to release of FFAs in the hepatic circulatory bed. Splanchnic FFA release measured using hepatic vein samples was strongly related to visceral adipose tissue FFA release into the portal vein. This finding suggests that splanchnic FFA release is a good indicator of visceral adipose tissue lipolysis.     

Effects of overstory and understory vegetation on the understory light environment in mixed boreal forests

Abstract. The percentage of above-canopy Photosynthetic Photon Flux Density (%PPFD) was measured at 0, 50 and 100 cm above the forest floor and above the main understory vegetation in stands of (1) pure Betula papyrifera (White birch), (2) pure Populus tremuloides (Trembling aspen), (3) mixed broad-leaf-conifer, (4) shade-tolerant conifer and (5) pure Pinus banksiana (Jack pine) occurring on both clay and till soil types. %PPFD was measured instantaneously under overcast sky conditions (nine locations within each of 29 stands) and continuously for a full day under clear sky conditions (five locations within each of eight stands). The percentage cover of the understory layer was estimated at the same locations as light measurements. Mean %PPFD varied from 2% at the forest floor under Populus forests to 15% above the understory vegetation cover under Betula forests. Percent PPFD above the understory vegetation cover was significantly higher under shade intolerant tree species such as Populus, Betula and Pinus than under shade tolerant conifers. No significant differences were found in %PPFD above the understory vegetation cover under similar tree species between clay and till soil types. The coefficient of variation in %PPFD measured in the nine locations within each stand was significantly lower under deciduous dominated forests (mean of 19%) than under coniferous dominated forests (mean of 40%). %PPFD measured at the forest floor was positively correlated with %PPFD measured above the understory vegetation and negatively correlated with cumulative total percent cover of the understory vegetation (R² = 0.852). The proportion of sunflecks above 250 and 500 mmol m^–2 s^–1 was much lower and %PPFD in shade much higher under Populus and Betula forests than under the other forests. Differences in the mean, variability and nature of the light environment found among forest and soil types are discussed in relation to their possible influences on tree succession.