The stabilizing effect of shear thinning on the onset of purely elastic instabilities in serpentine microflows.

We determine both experimentally and numerically the onset of elastic flow instabilities in viscoelastic polymer solutions with different levels of shear thinning. Previous experiments realized in microfluidic serpentine channels using dilute polymeric solutions showed that the onset of elastic instabilities strongly depends on the channel curvature. The scaling dependence is well captured by the general instability scaling criterion proposed by Pakdel and McKinley [Phys. Rev. Lett., 1996, 76, 2459:1-4]. We determine here the influence of fluid shear thinning on the onset of such purely-elastic flow instabilities. By testing a set of polyethylene oxide solutions of high molecular weight at different polymer concentrations in microfluidic serpentine channels we observe that shear thinning has a stabilizing effect on the microfluidic flow. Three-dimensional numerical simulations performed using the White-Metzner model predict similar trends, which are not captured by a simple scaling analysis using the Pakdel-McKinley criterion.

Flows of living polymer fluids.

We report on the recent progress made on flows of living polymer fluids. Such fluids have been model systems for rheological research for more than twenty years and they continue to be fascinating. Like most if not all soft matter systems, living polymers under flow show a strong feedback between the structure of the fluid and that of the flow, the first influencing the second and vice versa. In our opinion, such interplay between microscopic kinetics and macroscopic kinematics has historically been mostly understood from a "structural perspective", in the tradition of physical chemistry. Nevertheless, in recent years, a more "hydrodynamical perspective" has emerged by making fruitful analogies with elastic and inertio-elastic instabilities known in solutions of regular polymers. We also underline how this new perspective constrains theoretical modelling and calls for the use of new tools of investigation.

Birefringence banding in a micellar solution or the complexity of heterogeneous flows.

We have investigated the shear flow behavior of a classical viscoelastic equimolar wormlike micellar system made of cetyltrimethylammonium bromide and potassium bromide in the semidilute regime using mechanical and optical measurements. The experimental flow curve of this surfactant solution exhibits, above a critical shear rate, a well-defined stress plateau, characteristic of a flow of the shear-banding type. We first focus on the rheological and rheo-optical transient response of the sample after the sudden start-up of flow. The time-dependent stress profiles are strongly similar to those observed on various other systems with the occurrence of an overshoot at short times followed by a stretched exponential relaxation toward the steady state on a long time scale. This behavior is then correlated to the temporal evolution of the birefringence intensity and the extinction angle; the latter exhibits an undershoot just after the inception of the flow. Using direct visualizations of the sheared sample and spatially resolved flow birefringence across the gap of the Couette cell, we have been able to highlight a peculiar banding structure made up of three distinct regions: two layers of homogeneous but strongly differing orientations located against the walls, separated by a mixed layer, the width of which can reach half of the gap as a function of the effective applied shear rate. The induced structures contained in the band adjacent to the inner moving cylinder are found to be almost fully aligned along the flow direction. The relative proportions of each region are derived from the orientation profiles and compared to the predictions of the lever rule. The results suggest that orientation bands and shear bands are not linked in an obvious way, and the observed band structure can finally be interpreted as the coexistence either of three distinct "phases" or of only two homogeneous phases separated by an interface which can be broad, or thin and fluctuating.

Cytoprotective effect of glucosylceramide synthase inhibition against daunorubicin-induced apoptosis in human leukemic cell lines.

Several studies have shown that ceramide (CER) glucosylation contributes to drug resistance in multidrug-resistant cells and that inhibition of glucosylceramide synthase sensitizes cells to various drug treatments. However, the role of glucosylceramide synthase has not been studied in drug-sensitive cancer cells. We have demonstrated previously that the anthracycline daunorubicin (DNR) rapidly induces interphasic apoptosis through neutral sphingomyelinase-mediated CER generation in human leukemic cell lines. We now report that inhibition of glucosylceramide synthase using d,l-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (PDMP) or 1-phenyl-2-palmitoylamino-3-morpholino-1-propanol (PPMP) protected U937 and HL-60 cells from DNR-induced apoptosis. Moreover, blocking CER glucosylation did not lead to increased CER levels but to increased CER galactosylation. We also observed that pretreating cells with galactosylceramide (GalCER) significantly inhibited DNR-induced apoptosis. Finally, we show that GalCER-enriched lymphoblast cells (Krabbe's disease) were significantly more resistant to DNR- and cytosine arabinoside-induced apoptosis as compared with normal lymphoblasts, whereas glucosylceramide-enriched cells (Gaucher's disease) were more sensitive. In conclusion, this study suggests that sphingomyelin-derived CER in itself is not a second messenger but rather a precursor of both an apoptosis second messenger (GD3) and an apoptosis "protector" (GalCER).

Changing the conformation state of cytochrome b558 initiates NADPH oxidase activation: MRP8/MRP14 regulation.

Phagocyte NADPH oxidase generates O2. for defense mechanisms and cellular signaling. Myeloid-related proteins MRP8 and MRP14 of the S100 family are EF-hand calcium-binding proteins. MRP8 and MRP14 were co-isolated from neutrophils on an anti-p47phox matrix with oxidase cytosolic factors and identified by mass spectrometry. MRP8 and MRP14 are absent from Epstein-Barr virus-immortalized B lymphocytes, and, coincidentally, these cells display weak oxidase activity compared with neutrophils. MRP8/MRP14 that was purified from neutrophils enhanced oxidase turnover of B cells in vitro, suggesting that MRP8/MRP14 is involved in the activation process. This was confirmed ex vivo by co-transfection of Epstein-Barr virus-transformed B lymphocytes with genes encoding MRP8 and MRP14. In a semi-recombinant cell-free assay, recombinant MRP8/MRP14 increased the affinity of p67phox for cytochrome b558 synergistically with p47phox. Moreover, MRP8/MRP14 initiated oxidase activation on its own, through a calcium-dependent specific interaction with cytochrome b558 as shown by atomic force microscopy and a structure-function relationship investigation. The data suggest that the change of conformation in cytochrome b558, which initiates the electron transfer, can be mediated by effectors other than oxidase cytosolic factors p67phox and p47phox. Moreover, MRP8/MRP14 dimer behaves as a positive mediator of phagocyte NADPH oxidase regulation.

Daunorubicin- and Ara-C-induced interphasic apoptosis of human type II leukemia cells is caspase-8-independent.

Drug-induced interphasic apoptosis in human leukemia cells is mediated through intracellular signaling pathways, of which the most proximal (initiating) event remains unclear. Indeed, both early ceramide generation and procaspase-8 cleavage have been individually identified as the initial apoptotic signaling events which precede the mitochondrial control of the apoptotic execution phase in Type II cells. In order to evaluate whether or not procaspase-8 cleavage is requisite for initial ceramide generation and rapid interphasic apoptosis, we investigated the chronological ordering of early ceramide generation and caspase-8 cleavage induced by daunorubicin (DNR) and 1-beta-D-arabinofuranosylcytosine (Ara-C) in U937 cells. We further evaluated the impact of these two drugs on initial ceramide generation and apoptosis in wild-type Jurkat cells and Jurkat clones mutated for caspase-8 and Fas-associated death domain. We show that while both DNR and Ara-C similarly induced early ceramide generation (within 5-20 min) and interphasic apoptosis in all cell models, caspase-8 cleavage was only observed farther downstream (4.5 h) and only in DNR-treated cells. Furthermore, neither DNR or Ara-C induced caspase-8 activation. These results demonstrate that caspase-8 cleavage is not requisite for the drug-induced activation of the ceramide-mediated interphasic apoptotic pathway in human Type II leukemic cells.