The effects of temperature, pressure and peptide incorporation on ternary model raft mixtures--a Laurdan fluorescence spectroscopy study

Initial autophagic protection switches to disruption of autophagic flux by lysosomal instability during cadmium stress accrual in renal NRK-52E cells

The renal proximal tubule (PT) is the major target of cadmium (Cd2+) toxicity where Cd2+ causes stress and apoptosis. Autophagy is induced by cell stress, e.g., endoplasmic reticulum (ER) stress, and may contribute to cell survival or death. The role of autophagy in Cd2+-induced nephrotoxicity remains unsettled due to contradictory results and lack of evidence for autophagic machinery damage by Cd2+. Cd2+-induced autophagy in rat kidney PT cell line NRK-52E and its role in cell death was investigated. Increased LC3-II and decreased p62 as autophagy markers indicate rapid induction of autophagic flux by Cd2+ (5-10 µM) after 1 h, accompanied by ER stress (increased p-PERK, p-eIF2α, CHOP). Cd2+ exposure exceeding 3 h results in p62/LC3-II accumulation, but diminished effect of lysosomal inhibitors (bafilomycin A1, pepstatin A +E-64d) on p62/LC3-II levels, indicating decreased autophagic flux and cargo degradation. At 24 h exposure, Cd2+ (5-25 µM) activates intrinsic apoptotic pathways (Bax/Bcl-2, PARP-1), which is not evident earlier (≤6 h) although cell viability by MTT assay is decreased. Autophagy inducer rapamycin (100 nM) does not overcome autophagy inhibition or Cd2+-induced cell viability loss. The autophagosome-lysosome fusion inhibitor liensinine (5 μM) increases CHOP and Bax/Bcl-2-dependent apoptosis by low Cd2+ stress, but not by high Cd2+. Lysosomal instability by Cd2+ (5 μM; 6 h) is indicated by increases in cellular sphingomyelin and membrane fluidity and decreases in cathepsins and LAMP1. The data suggest dual and temporal impact of Cd2+ on autophagy: Low Cd2+ stress rapidly activates autophagy counteracting damage but Cd2+ stress accrual disrupts autophagic flux and lysosomal stability, possibly resulting in lysosomal cell death.

Lipid nanodomains change ion channel function

Signaling proteins and neurotransmitter receptors often associate with saturated chain and cholesterol-rich domains of cell membranes, also known as lipid rafts. The saturated chains and high cholesterol environment in lipid rafts can modulate protein function, but evidence for such modulation of ion channel function in lipid rafts is lacking. Here, using raft-forming model membrane systems containing cholesterol, we show that lipid lateral phase separation at the nanoscale level directly affects the dissociation kinetics of the gramicidin dimer, a model ion channel.

Behavior of plant plasma membranes under hydrostatic pressure as monitored by fluorescent environment-sensitive probes

We monitored the behavior of plasma membrane (PM) isolated from tobacco cells (BY-2) under hydrostatic pressures up to 3.5kbar at 30 degrees C, by steady-state fluorescence spectroscopy using the newly introduced environment-sensitive probe F2N12S and also Laurdan and di-4-ANEPPDHQ. The consequences of sterol depletion by methyl-beta-cyclodextrin were also studied. We found that application of hydrostatic pressure led to a marked decrease of hydration as probed by F2N12S and to an increase of the generalized polarization excitation (GPex) of Laurdan. We observed that the hydration effect of sterol depletion was maximal between 1 and 1.5 kbar but was much less important at higher pressures (above 2 kbar) where both parameters reached a plateau value. The presence of a highly dehydrated gel state, insensitive to the sterol content, was thus proposed above 2.5 kbar. However, the F2N12S polarity parameter and the di-4-ANEPPDHQ intensity ratio showed strong effect on sterol depletion, even at very high pressures (2.5-3.5 kbar), and supported the ability of sterols to modify the electrostatic properties of membrane, notably its dipole potential, in a highly dehydrated gel phase. We thus suggested that BY-2 PM undergoes a complex phase behavior in response to the hydrostatic pressure and we also emphasized the role of phytosterols to regulate the effects of high hydrostatic pressure on plant PM.

Effects of temperature and pressure on the lateral organization of model membranes with functionally reconstituted multidrug transporter LmrA

To contribute to the understanding of membrane protein function upon application of pressure, we investigated the influence of hydrostatic pressure on the conformational order and phase behavior of the multidrug transporter LmrA in biomembrane systems. To this end, the membrane protein was reconstituted into various lipid bilayer systems of different chain length, conformation, phase state and heterogeneity, including raft model mixtures as well as some natural lipid extracts. In the first step, we determined the temperature stability of the protein itself and verified its reconstitution into the lipid bilayer systems using CD spectroscopic and AFM measurements, respectively. Then, to yield information on the temperature and pressure dependent conformation and phase state of the lipid bilayer systems, generalized polarization values by the Laurdan fluorescence technique were determined, which report on the conformation and phase state of the lipid bilayer system. The temperature-dependent measurements were carried out in the temperature range 5-70 degrees C, and the pressure dependent measurements were performed in the range 1-200 MPa. The data show that the effect of the LmrA reconstitution on the conformation and phase state of the lipid matrix depends on the fluidity and hydrophobic matching conditions of the lipid system. The effect is most pronounced for fluid DMPC and DMPC with low cholesterol levels, but minor for longer-chain fluid phospholipids such as DOPC and model raft mixtures such as DOPC/DPPC/cholesterol. The latter have the additional advantage of using lipid sorting to avoid substantial hydrophobic mismatch. Notably, the most drastic effect was observed for the neutral/glycolipid natural lipid mixture. In this case, the impact of LmrA incorporation on the increase of the conformational order of the lipid membrane was most pronounced. As a consequence, the membrane reaches a mechanical stability which makes it very insensitive to application of pressures as high as 200 MPa. The results are correlated with the functional properties of LmrA in these various lipid environments and upon application of high hydrostatic pressure and are discussed in the context of other work on pressure effects on membrane protein systems.