Pore-forming toxins induce multiple cellular responses promoting survival

Pore-forming toxins (PFTs) are secreted proteins that contribute to the virulence of a great variety of bacterial pathogens. They inflict one of the more disastrous damages a target cell can be exposed to: disruption of plasma membrane integrity. Since this is an ancient form of attack, which bears similarities to mechanical membrane damage, cells have evolved response pathways to these perturbations. Here, it is reported that PFTs trigger very diverse yet specific response pathways. Many are triggered by the decrease in cytoplasmic potassium, which thus emerges as a central regulator. Upon plasma membrane damage, cells activate signalling pathways aimed at restoring plasma membrane integrity and ion homeostasis. Interestingly these pathways do not require protein synthesis. Cells also trigger signalling cascades that allow them to enter a quiescent-like state, where minimal energy is consumed while waiting for plasma membrane damage to be repaired. More specifically, protein synthesis is arrested, cytosolic constituents are recycled by autophagy and energy is stored in lipid droplets.     

Role of AP1 and Gadkin in the traffic of secretory endo-lysosomes

Whereas lysosome-related organelles (LRO) of specialized cells display both exocytic and endocytic features, lysosomes in nonspecialized cells can also acquire the property to fuse with the plasma membrane upon an acute rise in cytosolic calcium. Here, we characterize this unconventional secretory pathway in fibroblast-like cells, by monitoring the appearance of Lamp1 on the plasma membrane and the release of lysosomal enzymes into the medium. After sequential ablation of endocytic compartments in living cells, we find that donor membranes primarily derive from a late compartment, but that an early compartment is also involved. Strikingly, this endo-secretory process is not affected by treatments that inhibit endosome dynamics (microtubule depolymerization, cholesterol accumulation, overexpression of Rab7 or its effector Rab-interacting lysosomal protein [RILP], overexpression of Rab5 mutants), but depends on Rab27a, a GTPase involved in LRO secretion, and is controlled by F-actin. Moreover, we find that this unconventional endo-secretory pathway requires the adaptor protein complexes AP1, Gadkin (which recruits AP1 by binding to the γ1 subunit), and AP2, but not AP3. We conclude that a specific fraction of the AP2-derived endocytic pathway is dedicated to secretory purposes under the control of AP1 and Gadkin.     

Caveolin, cholesterol, and lipid bodies

In mammalian cells a complex interplay regulates the distribution of cholesterol between intracellular membrane compartments. One important aspect of cholesterol regulation is intracellular cholesterol storage in neutral lipid storage organelles called lipid droplets or lipid bodies (LBs). Recent work has thrust the LB into the limelight as a complex and dynamic cellular organelle. LBs play a crucial role in maintaining the cellular levels of cholesterol by regulating the interplay between lipid storage, hydrolysis and trafficking. Studies of caveolins, caveolar membrane proteins linked to lipid regulation, are providing new insights into the role of LBs in regulating cholesterol balance.     

Endosome-to-cytosol transport of viral nucleocapsids

During viral infection, fusion of the viral envelope with endosomal membranes and nucleocapsid release were thought to be concomitant events. We show here that for the vesicular stomatitis virus they occur sequentially, at two successive steps of the endocytic pathway. Fusion already occurs in transport intermediates between early and late endosomes, presumably releasing the nucleocapsid within the lumen of intra-endosomal vesicles, where it remains hidden. Transport to late endosomes is then required for the nucleocapsid to be delivered to the cytoplasm. This last step, which initiates infection, depends on the late endosomal lipid lysobisphosphatidic acid (LBPA) and its putative effector Alix/AIP1, and is regulated by phosphatidylinositol-3-phosphate (PtdIns3P) signalling via the PtdIns3P-binding protein Snx16. We conclude that the nucleocapsid is exported into the cytoplasm after the back-fusion of internal vesicles with the limiting membrane of late endosomes, and that this process is controlled by the phospholipids LBPA and PtdIns3P and their effectors.     

Ultrastructural identification of uncoated caveolin-independent early endocytic vehicles

Using quantitative light microscopy and a modified immunoelectron microscopic technique, we have characterized the entry pathway of the cholera toxin binding subunit (CTB) in primary embryonic fibroblasts. CTB trafficking to the Golgi complex was identical in caveolin-1null (Cav1-/-) mouse embryonic fibroblasts (MEFs) and wild-type (WT) MEFs. CTB entry in the Cav1-/- MEFs was predominantly clathrin and dynamin independent but relatively cholesterol dependent. Immunoelectron microscopy was used to quantify budded and surface-connected caveolae and to identify noncaveolar endocytic vehicles. In WT MEFs, a small fraction of the total Cav1-positive structures were shown to bud from the plasma membrane (2% per minute), and budding increased upon okadaic acid or lactosyl ceramide treatment. However, the major carriers involved in initial entry of CTB were identified as uncoated tubular or ring-shaped structures. These carriers contained GPI-anchored proteins and fluid phase markers and represented the major vehicles mediating CTB uptake in both WT and caveolae-null cells.     

Expression of caveolin-1 enhances cholesterol efflux in hepatic cells

HepG2 cells were stably transfected with human caveolin-1 (HepG2/cav cells). Transfection resulted in expression of caveolin-1 mRNA, a high abundance of caveolin-1 protein, and the formation of caveolae on the plasma membrane. Cholesterol efflux from HepG2/cav cells was 280 and 45% higher than that from parent HepG2 cells when human plasma and human apoA-I, respectively, were used as acceptors. The difference in efflux was eliminated by treatment of cells with progesterone. There was no difference in cholesterol efflux to cyclodextrin. Cholesterol efflux from plasma membrane vesicles was similar for the two cell types. Transfection led to a 40% increase in the amount of plasma membrane cholesterol in cholesterol-rich domains (caveolae and/or rafts) and a 67% increase in the rate of cholesterol trafficking from intracellular compartments to these domains. Cholesterol biosynthesis in HepG2/cav cells was increased by 2-fold, and cholesterol esterification was reduced by 50% compared with parent HepG2 cells. The proliferation rate of transfected cells was significantly lower than that of non-transfected cells. Transfection did not affect expression of ABCA1 or the abundance of ABCA1 protein, but decreased secretion of apoA-I. We conclude that overexpression of caveolin-1 in hepatic cells stimulates cholesterol efflux by enhancing transfer of cholesterol to cholesterol-rich domains in the plasma membrane.     

Arachidonic acid release from mammalian cells transfected with human groups IIA and X secreted phospholipase A(2) occurs predominantly during the secretory process and with the involvement of cytosolic phospholipase A(2)-alpha

Stable expression of human groups IIA and X secreted phospholipases A(2) (hGIIA and hGX) in CHO-K1 and HEK293 cells leads to serum- and interleukin-1beta-promoted arachidonate release. Using mutant CHO-K1 cell lines, it is shown that this arachidonate release does not require heparan sulfate proteoglycan- or glycosylphosphatidylinositol-anchored proteins. It is shown that the potent secreted phospholipase A(2) inhibitor Me-Indoxam is cell-impermeable. By use of Me-Indoxam and the cell-impermeable, secreted phospholipase A(2) trapping agent heparin, it is shown that hGIIA liberates free arachidonate prior to secretion from the cell. With hGX-transfected CHO-K1 cells, arachidonate release occurs before and after enzyme secretion, whereas all of the arachidonate release from HEK293 cells occurs prior to enzyme secretion. Immunocytochemical studies by confocal laser and electron microscopies show localization of hGIIA to the cell surface and Golgi compartment. Additional results show that the interleukin-1beta-dependent release of arachidonate is promoted by secreted phospholipase A(2) expression and is completely dependent on cytosolic (group IVA) phospholipase A(2). These results along with additional data resolve the paradox that efficient arachidonic acid release occurs with hGIIA-transfected cells, and yet exogenously added hGIIA is poorly able to liberate arachidonic acid from mammalian cells.     

Development of novel microsatellite markers for the grassland species Lolium multiflorum,Lolium perenne and Festuca pratensis

Twelve microsatellite markers were isolated from Lolium multiflorum. Allelic variability and cross‐species amplification were assessed on 16 individuals of each of the three grassland species L. multiflorum, Lolium perenne and Festuca pratensis. Cross‐species amplification success was 100% for L. perenne and 83% for F. pratensis. The number of alleles detected ranged from one to 14 with an average of 3.4. While three microsatellite loci were polymorphic in all three species, one marker produced species‐specific alleles in all three species. These microsatellite markers provide a valuable tool for population genetic studies within and among species of the Festuca–Lolium complex.     

Grassland yield declined by a quarter in 5 years of free-air ozone fumigation

The sensitivity of seminatural grasslands to ozone (O₃) pollution is not well known, in spite of the important function of these common ecosystems for agriculture and nature conservation. A 5‐year field experiment was carried out at a rural, mid‐elevation site at Le Mouret (Switzerland) to investigate the effect of elevated O₃ on yield and species composition of a permanent, extensively managed, species‐rich old pasture. Using a free‐air fumigation system, circular plots of 7 m diameter were exposed to either ambient air (control plots) or to air containing O₃ of approximately 1.5 × ambient concentrations. The resulting accumulated O₃ exposure over the threshold of 40 ppb for one season ranged from 13.3 to 59.5 ppm h in the elevated O₃ plots, and from 1.0 to 20.7 ppm h in the control plots. Subplots in each ring were harvested three times each year, and harvested biomass was separated into functional groups (FGs) (grasses, legumes, forbs). Productivity in both treatments decreased over time, but the yield of O₃‐exposed plots decreased faster than that of the control plots, with the reduction being twice as large by the end of the fifth season. Compared with the ambient air control, loss in annual dry matter yield was 23% after 5 years. The change in annual biomass production because of O₃ stress was accompanied by a change in the fractions of FGs, with the legume fraction showing a strong negative response. Such long‐term effects of O₃ stress could have negative implications for the maintenance of biological diversity in rural landscapes across large areas of Europe. The results from this first long‐term experiment show that a moderately elevated O₃ level reduces the productivity of intact grasslands during a 5‐year exposure under real field conditions.