Distribution of selected phospholipid modifying enzymes in rat brain microsomal subfractions prepared by density gradient zonal rotor centrifugation

Bottleneck size and selection level reproducibly impact evolution of antibiotic resistance

During antibiotic treatment, the evolution of bacterial pathogens is fundamentally affected by bottlenecks and varying selection levels imposed by the drugs. Bottlenecks-that is, reductions in bacterial population size-lead to an increased influence of random effects (genetic drift) during bacterial evolution, and varying antibiotic concentrations during treatment may favour distinct resistance variants. Both aspects influence the process of bacterial evolution during antibiotic therapy and thereby treatment outcome. Surprisingly, the joint influence of these interconnected factors on the evolution of antibiotic resistance remains largely unexplored. Here we combine evolution experiments with genomic and genetic analyses to demonstrate that bottleneck size and antibiotic-induced selection reproducibly impact the evolutionary path to resistance in pathogenic Pseudomonas aeruginosa, one of the most problematic opportunistic human pathogens. Resistance is favoured-expectedly-under high antibiotic selection and weak bottlenecks, but-unexpectedly-also under low antibiotic selection and severe bottlenecks. The latter is likely to result from a reduced probability of losing favourable variants through drift under weak selection. Moreover, the absence of high resistance under low selection and weak bottlenecks is caused by the spread of low-resistance variants with high competitive fitness under these conditions. We conclude that bottlenecks, in combination with drug-induced selection, are currently neglected key determinants of pathogen evolution and outcome of antibiotic treatment.

Activation of TRPC6 channels promotes endocannabinoid biosynthesis in neuronal CAD cells

Calcium influx activates biosynthesis of the endogenous cannabinoids 2-arachidonyl glycerol (2-AG) and anandamide (AEA). The calcium channel involved with endocannabinoid synthesis and release in neurons is still unknown. The canonical TRP (TRPC) channels are calcium-permeable channels that are a homology-based subdivision of the broader class of TRP channels. TRPC3, 6, and 7 are G-protein-gated non-selective cation channels that have been localized to lipid rafts and shown to colocalize with caveolin 1. Because endocannabinoid synthesis has been found to occur "on demand" in a calcium-dependent manner and has been linked to lipid rafts, we explored the potential role of transient receptor potential (TRP) channels in this process. Previously, we observed that after metabolism AEA and arachidonic acid (ArA) can be recycled into new endocannabinoid molecules. Consistent with these previous findings, we found that Cath.a differentiated (CAD) cells pretreated with radiolabeled ArA exhibited a robust increase in 2-AG release in response to TRPC stimulation with the diacylglycerol (DAG) analogue, 1-oleoyl-2-acetyl-sn-glycerol (OAG). Furthermore, cells pretreated with [(3)H]AEA produced a significant amount of AEA and 2-AG upon stimulation of TRPC channels. This process was not mediated through protein kinase C activation. Reverse transcriptase-polymerase chain reaction (RT-PCR) analysis revealed that only TRPC6 was present in the CAD cells. siRNA-induced knockdown of TRPC6 in the CAD cells abolished OAG-stimulated production of the endocannabionids. This evidence suggests that TRPC6 may be capable of promoting endocannabinoid synthesis in neuronal cells.

Enrichment of neuronal and glial connexins in the postsynaptic density subcellular fraction of rat brain

The similar dense, protein-rich and detergent-resistant characteristics of postsynaptic densities (PSDs) and gap junctions led us to investigate the distribution of gap junctions and their constituent connexins in CNS subcellular fractions containing PSDs. Western blot analysis showed these fractions to be enriched in both neuronal and glial connexins, namely, connexin26, connexin30, connexin36 and connexin43. Connexins were retained in these fractions after treatment with n-lauroyl sarcosine to remove loosely associated proteins. Confocal double immunofluorescence confirmed the presence of connexins in PSD fractions and showed a near total co-localization of glial connexin30 and connexin43, demonstrating preservation of inter-connexin relationships that have been observed in vivo. In contrast, none of the connexins were co-localized with the PSD structural protein PSD-95, indicating their lack of direct association with PSDs. These results show that PSD preparations contain significant levels of connexin proteins, which appear to remain assembled as gap junctions. Thus, protocols used to isolate PSDs may serve as a basis for development of methods to isolate CNS gap junctions, which would aid biochemical identification of regulatory and structural proteins associated with these structures.

Haloperidol-sensitive (+)[3H]SKF-10,047 binding sites (sigma sites) exhibit a unique distribution in rat brain subcellular fractions

The distribution of haloperidol-sensitive (+)[3H]N-allylnormetazocine ((+)[3H]SKF-10,047) binding sites (sigma sites) in subcellular fractions of rat brain homogenates was extensively characterized. In synaptosomal fractions, enriched in choline acetyltransferase activity, sigma sites were present in lower concentrations than in whole brain homogenates. On the other hand, microsomal and myelin fractions were found to be enriched in sigma sites. A similar pattern of enrichment was seen for 5'-nucleotidase activity, a general plasma membrane marker. However, subsequent experiments in which microsomes were subfractionated on linear sucrose gradients led to the recovery of sigma sites over a significantly lower density range than 5'-nucleotidase activity or ATP-stimulated [3H]ouabain binding, an additional plasma membrane marker. In addition, previously reported distributions of a number of other subcellular markers, including those for endoplasmic reticulum, were found to contrast with the observed distribution of sigma sites. It is concluded that rat brain sigma sites are not concentrated at synaptic regions of plasma membrane. However, the possibility that sigma sites are localized to specialized areas of nonsynaptic plasma membrane cannot be excluded.

Effect of modification of membrane phospholipid composition on phospholipid methylation in aggregating cell culture

The effect of the presence of nitrogenous bases in the growth medium of fetal rat brain aggregating cell cultures was investigated. The presence of either N-methylethanolamine (MME) or N,N-dimethylethanolamine (DME) in the growth medium resulted in significant increase of the corresponding phospholipid, phosphatidyl-N-monomethylethanolamine (PMME) or phosphatidyl-N,N-dimethylethanolamine (PDME). They represented 28% and 32% of the total phospholipids, respectively. The presence of the new phospholipids was accompanied by a significant decrease of phosphatidylethanolamine (PE) and phosphatidylcholine (PC). Cells grown in the presence of ethanolamine or choline had only barely detectable amounts of PMME and PDME. Intact cells previously grown with the bases were incubated with [methyl-3H]methionine. Incubation of cells previously grown in presence of the bases MME and DME resulted in a marked increase of radioactivity in the corresponding phospholipids possessing one additional methyl group, PDME and PC respectively. The incorporation of S-adenosyl[methyl-3H]methionine (AdoMet) was examined in cell homogenates incubated in presence or absence of either PMME or PDME acceptors. The addition of these exogenous phospholipids caused a three-or fourfold stimulation of radioactivity incorporated into the total phospholipids of cells grown in the absence of nitrogen bases. The cells grown in presence of either MME or DME in the culture medium did not show an increased incorporation of methyl groups from AdoMet into the total phospholipids after addition of exogenous acceptors. This work suggests that MME and DME incorporated into the corresponding phospholipids function as effective substrates for phospholipid-N-methylation.

The phospholipid-N-methyltransferase of rat brain microsomes

The phospholipid-N-methyltransferase activity of rat brain microsomes had an optimum pH of 11.0 in the absence or presence of phosphatidylethanolamine (PE) but pH 10.0 in the presence of phosphatidylmonomethylethanolamine (PMME) or phosphatidyldimethylethanolamine (PDME). An apparent Km for S-adenosyl methonine from 0.10 to 0.12 mM was observed with exogenous methylated phospholipids PMME or PDME. Methylated neutral lipid was the major lipid produced in the absence of the exogenous acceptors. Two exogenous phospholipids, PMME and PDME, significantly stimulated microsomal phospholipid-N-methyltransferase activity and the predicted methylated phospholipids were the major products. PE additions did not cause any stimulation of methylated lipid formation. Preincubation of particles at temperatures from 40 to 100 degrees C resulted in a loss in the microsomal phospholipid-N-methyltransferase activity that was stimulated by PMME and PDME.