Current-induced highly dissipative domains in high Tc thin films

We have investigated the resistive response of high Tc thin films submitted to a high density of current. For this purpose, current pulses were applied into bridges made of Nd(1.15)Ba(1.85)Cu3O7 and Bi2Sr2CaCu2O8. By recording the time dependent voltage, we observe that at a certain critical current j*, a highly dissipative domain develops somewhere along the bridge. The successive formation of these domains produces stepped I-V characteristics. We present evidences that these domains are not regions with a temperature above Tc, as for hot spots. In fact this phenomenon appears to be analog to the nucleation of phase-slip centers observed in conventional superconductors near Tc, but here in contrast they appear in a wide temperature range. Under some conditions, these domains will propagate and destroy the superconductivity within the whole sample. We have measured the temperature dependence of j* and found a similar behavior in the two investigated compounds. This temperature dependence is just the one expected for the depairing current, but the amplitude is about 100 times smaller.Comment: 9 pages, 9 figures, Revtex, to appear in Phys. Rev.

Comparative Analysis of Microfluidics Thrombus Formation in Multiple Genetically Modified Mice: Link to Thrombosis and Hemostasis

Genetically modified mice are indispensable for establishing the roles of platelets in arterial thrombosis and hemostasis. Microfluidics assays using anticoagulated whole blood are commonly used as integrative proxy tests for platelet function in mice. In the present study, we quantified the changes in collagen-dependent thrombus formation for 38 different strains of (genetically) modified mice, all measured with the same microfluidics chamber. The mice included were deficient in platelet receptors, protein kinases or phosphatases, small GTPases or other signaling or scaffold proteins. By standardized re-analysis of high-resolution microscopic images, detailed information was obtained on altered platelet adhesion, aggregation and/or activation. For a subset of 11 mouse strains, these platelet functions were further evaluated in rhodocytin- and laminin-dependent thrombus formation, thus allowing a comparison of glycoprotein VI (GPVI), C-type lectin-like receptor 2 (CLEC2) and integrin alpha(6)beta(1) pathways. High homogeneity was found between wild-type mice datasets concerning adhesion and aggregation parameters. Quantitative comparison for the 38 modified mouse strains resulted in a matrix visualizing the impact of the respective (genetic) deficiency on thrombus formation with detailed insight into the type and extent of altered thrombus signatures. Network analysis revealed strong clusters of genes involved in GPVI signaling and Ca2+ homeostasis. The majority of mice demonstrating an antithrombotic phenotype in vivo displayed with a larger or smaller reduction in multi-parameter analysis of collagen-dependent thrombus formation in vitro. Remarkably, in only approximately half of the mouse strains that displayed reduced arterial thrombosis in vivo, this was accompanied by impaired hemostasis. This was also reflected by comparing in vitro thrombus formation (by microfluidics) with alterations in in vivo bleeding time. In conclusion, the presently developed multi-parameter analysis of thrombus formation using microfluidics can be used to: (i) determine the severity of platelet abnormalities;(ii) distinguish between altered platelet adhesion, aggregation and activation;and (iii) elucidate both collagen and non-collagen dependent alterations of thrombus formation. This approach may thereby aid in the better understanding and better assessment of genetic variation that affect in vivo arterial thrombosis and hemostasis

Reversal of stress fibre formation by Nitric Oxide mediated RhoA inhibition leads to reduction in the height of preformed thrombi

Evidence has emerged to suggest that thrombi are dynamic structures with distinct areas of differing platelet activation and inhibition. We hypothesised that Nitric oxide (NO), a platelet inhibitor, can modulate the actin cytoskeleton reversing platelet spreading, and therefore reduce the capability of thrombi to withstand a high shear environment. Our data demonstrates that GSNO, DEANONOate, and a PKG-activating cGMP analogue reversed stress fibre formation and increased actin nodule formation in adherent platelets. This effect is sGC dependent and independent of ADP and thromboxanes. Stress fibre formation is a RhoA dependent process and NO induced RhoA inhibition, however, it did not phosphorylate RhoA at ser188 in spread platelets. Interestingly NO and PGI2 synergise to reverse stress fibre formation at physiologically relevant concentrations. Analysis of high shear conditions indicated that platelets activated on fibrinogen, induced stress fibre formation, which was reversed by GSNO treatment. Furthermore, preformed thrombi on collagen post perfused with GSNO had a 30% reduction in thrombus height in comparison to the control. This study demonstrates that NO can reverse key platelet functions after their initial activation and identifies a novel mechanism for controlling excessive thrombosis

Gene Targeting Implicates Cdc42 GTPase in GPVI and Non-GPVI Mediated Platelet Filopodia Formation, Secretion and Aggregation

Background: Cdc42 and Rac1, members of the Rho family of small GTPases, play critical roles in actin cytoskeleton regulation. We have shown previously that Rac1 is involved in regulation of platelet secretion and aggregation. However, the role of Cdc42 in platelet activation remains controversial. This study was undertaken to better understand the role of Cdc42 in platelet activation. Methodology/Principal Findings: We utilized the Mx-cre;Cdc42 lox/lox inducible mice with transient Cdc42 deletion to investigate the involvement of Cdc42 in platelet function. The Cdc42-deficient mice exhibited a significantly reduced platelet count than the matching Cdc42 +/+ mice. Platelets isolated from Cdc42 2/2, as compared to Cdc42 +/+, mice exhibited (a) diminished phosphorylation of PAK1/2, an effector molecule of Cdc42, (b) inhibition of filopodia formation on immobilized CRP or fibrinogen, (c) inhibition of CRP- or thrombin-induced secretion of ATP and release of P-selectin, (d) inhibition of CRP, collagen or thrombin induced platelet aggregation, and (e) minimal phosphorylation of Akt upon stimulation with CRP or thrombin. The bleeding times were significantly prolonged in Cdc42 2/2 mice compared with Cdc42 +/+ mice. Conclusion/Significance: Our data demonstrate that Cdc42 is required for platelet filopodia formation, secretion an

Thiopental attenuates energetic impairment but fails to normalize cerebrospinal fluid glutamate in brain-injured patients

OBJECTIVES: Brain-injured patients are susceptible to secondary brain damage related to decreased cerebral perfusion pressure associated with edema formation and increased intracranial pressure (ICP). Whenever conventional therapy fails to reduce elevated ICP, barbiturate coma represents an additional intervention that may control ICP. In patients suffering from severe traumatic brain injury, cerebrospinal fluid levels of glutamate, hypoxanthine, and lactate were measured during barbiturate coma and correlated to electroencephalographic recordings and ICP. DESIGN: Prospective, descriptive study. SETTING: Ten-bed surgical intensive care unit in a university hospital. PATIENTS: Twenty-one patients with severe traumatic brain injury (Glasgow Coma Scale score < or = 9); 11 required barbiturate coma because of refractory intracranial hypertension, and 10 were manageable with continuous administration of fentanyl and midazolam. INTERVENTIONS: Thiopental was administered continuously for increased ICP within the first 24 hrs after trauma and adjusted to the burst-suppression pattern (four to six bursts per minute) on continuous electroencephalographic monitoring. MEASUREMENTS AND MAIN RESULTS: Glutamate and hypoxanthine were analyzed using high-performance liquid chromatography, whereas lactate was measured enzymatically. Patients requiring thiopental presented with significantly higher ICP, glutamate, and hypoxanthine levels than patients receiving fentanyl and midazolam (p < .05). Within the first 24 hrs, thiopental significantly reduced cerebrospinal fluid glutamate and hypoxanthine levels in all patients, i.e., the burst-suppression pattern was successfully induced (p < .001). Interestingly, in five patients cerebrospinal fluid glutamate increased to initial values again despite unchanged neuronal activity. In these patients, ICP, hypoxanthine, and lactate remained significantly elevated compared with the six patients with steadily decreasing cerebrospinal fluid glutamate, hypoxanthine, lactate, and ICP values (p < .02). CONCLUSIONS: Barbiturate coma does not unequivocally preserve energetic stability despite successful suppression of neuronal activity. Despite the use of barbiturate coma in patients with refractory intracranial hypertension, persistent release or impaired uptake of glutamate may be associated with continuous anaerobic metabolism, as shown by increases in cerebrospinal fluid hypoxanthine and lactate levels

Thiopental and midazolam do not seem to impede metabolism of glutamate in brain-injured patients

Increased extracellular glutamate levels are related to glial and neuronal damage. Glutamate-mediated toxicity is limited by glial uptake and metabolic transformation of glutamate to glutamine and the energetic compounds alanine and lactate which are utilized by surrounding neurons. Under in vitro conditions, barbiturates have been shown to reduce glutamate uptake and its further metabolism, possibly impeding metabolic coupling between astrocytes and neurons. The aims were to investigate if under clinical conditions, the barbiturate thiopental reduces important detoxification of glutamate, resulting in lower CSF glutamine, alanine and lactate levels as opposed to patients receiving midazolam. During long-term administration of thiopental and midazolam, pathologically elevated ventricular CSF glutamate levels were associated with significantly increased glutamine and alanine levels up to 14 days after trauma. CSF lactate, however, remained normal. These data suggest that long-term administration of thiopental and midazolam under clinical conditions does not impede enzymatic activities responsible for detoxification and metabolism of glutamate

Neurotransmitters in cerebrospinal fluid reflect pathological activity

The excitatory transmitters glutamate and aspartate become toxic whenever their extracellular levels are increased because of neuronal, glial and endothelial impairment. Taurine, a volume-regulating amino acid, is released upon excitotoxin-induced cell swelling. Our aim was to investigate if glutamate and aspartate in cerebrospinal fluid (CSF) reveal neuropathology in neurological patients, and if taurine unmasks glutamate-mediated toxicity. Glutamate and aspartate are doubled in viral meningitis, acute multiple sclerosis (MS) and myelopathy compared with control subjects and patients with peripheral facial nerve palsy. These levels do not coincide with a disturbed blood-brain barrier, as estimated by the albumin ratio, are independent of their precursors (glutamine, asparagine) and are not associated with cell lysis. Taurine is significantly increased in meningitis, acute MS, and myelopathy, suggesting glutamate-mediated toxicity. Analysis of transmitters in lumbar CSF can be used to identify patients with cerebral and spinal pathology who might benefit from specific receptor-modulating agents

Platelet hyperreactivity and a prothrombotic phenotype in mice with a gain-of-function mutation in phospholipase C gamma 2

Agonist-induced platelet activation involves different signaling pathways leading to the activation of phospholipase C (PLC) beta or PLCgamma2. Activated PLC produces inositol 1,4,5-trisphosphate and diacylglycerol, which trigger Ca(2+) mobilization and the activation of protein kinase C, respectively. PLCbeta is activated downstream of Gq-coupled receptors for soluble agonists with only short interaction times in flowing blood. In contrast, PLCgamma2 becomes activated downstream of receptors that interact with immobilized ligands such as the collagen receptor glycoprotein (GP) VI or activated integrins.We speculated that PLCgamma2 activity might be optimized for sustained but submaximal signaling to control relatively slow platelet responses. To test this hypothesis, we analyzed platelets from mice heterozygous for a gain-of-function mutation in the Plcg2 gene (Plcg2(Ali5/+)).Plcg2(Ali5/+) platelets showed enhanced Ca(2+) mobilization, integrin activation, granule secretion and phosphatidylserine exposure upon GPVI or C-type lectin-like receptor-2 stimulation. Furthermore, integrin alpha(IIb)beta(3) outside-in signaling was markedly enhanced in the mutant platelets, as shown by accelerated spreading on different matrices and faster clot retraction. These defects translated into virtually unlimited thrombus formation on collagen under flow in vitro and a prothrombotic phenotype in vivo.These results demonstrate that the enzymatic activity of PLCgamma2 is tightly regulated to ensure efficient but limited platelet activation at sites of vascular injury

Muon spin rotation and relaxation experiments on thin films

The recent development at the Paul Scherrer Institute of a beam of low energy muons allows depth dependent muon spin rotation and relaxation investigations in thin samples, multilayers and near surface regions (low energy mu SR, LE-mu SR). After a brief overview of the LE-mu SR method, some representative experiments performed with this technique will be presented. The first direct determination of the field profile just below the surface of a high-temperature superconductor in the Meissner phase illustrates the power and sensitivity of low energy muons as near-surface probe and is an example of general application to depth profiling of magnetic fields. The evolution of the flux line lattice distribution across the surface of a YBa2Cu3O7 film in the vortex phase has been investigated by implanting muons on both sides of a normal-superconducting boundary. A determination of the relaxation time and energy barrier to thermal activation in iron nanoclusters, embedded in a silver thin film matrix (500 nm), demonstrates the use of slow muons to measure the properties of samples that cannot be made thick enough for the use of conventional mu SR. Other experiments investigated the magnetic properties of thin Cr(001) layers at thicknesses above and below the collapse of the spin density wave