Pore formation and uncoupling initiate a Ca2+-independent degradation of mitochondrial phospholipids

Mitochondria contain a type IIA secretory phospholipase A(2) that has been thought to hydrolyze phospholipids following Ca(2+) accumulation and induction of the permeability transition. These enzymes normally require millimolar Ca(2+) for optimal activity; however, no dependence of the mitochondrial activity on Ca(2+) can be demonstrated upon equilibrating the matrix space with extramitochondrial Ca(2+) buffers. Ca(2+)-independent activity is seen following protonophore-mediated uncoupling, when uncoupling arises through alamethicin-mediated pore formation, or upon opening the permeability transition pore. Under the latter conditions, activity continues in the presence of excess EGTA but is somewhat enhanced by exogenous Ca(2+). The Ca(2+)-independent activity is best seen in media of high ionic strength and displays a broad pH optimum located between pH 8 and pH 8.5. It is strongly inhibited by bromoenol lactone but not by arachidonyl trifluoromethyl ketone, dithiothreitol, and other inhibitors of particular phospholipase A(2) classes. Immunoanalysis of mitochondria and mitochondrial subfractions shows that a membrane-bound protein is present that is recognized by antibody against an authentic iPLA(2) that was first found in P388D(1) cells. It is concluded that mitochondria contain a distinct Ca(2+)-independent phospholipase A(2) that is regulated by bioenergetic parameters. It is proposed that this enzyme, rather than the Ca(2+)-dependent type IIA phospholipase A(2), initiates the removal of poorly functioning mitochondria by processes involving autolysis.     

Top2 and Sgs1-Top3 Act Redundantly to Ensure rDNA Replication Termination

Faithful DNA replication with correct termination is essential for genome stability and transmission of genetic information. Here we have investigated the potential roles of Topoisomerase II (Top2) and the RecQ helicase Sgs1 during late stages of replication. We find that cells lacking Top2 and Sgs1 (or Top3) display two different characteristics during late S/G2 phase, checkpoint activation and accumulation of asymmetric X-structures, which are both independent of homologous recombination. Our data demonstrate that checkpoint activation is caused by a DNA structure formed at the strongest rDNA replication fork barrier (RFB) during replication termination, and consistently, checkpoint activation is dependent on the RFB binding protein, Fob1. In contrast, asymmetric X-structures are formed independent of Fob1 at less strong rDNA replication fork barriers. However, both checkpoint activation and formation of asymmetric X-structures are sensitive to conditions, which facilitate fork merging and progression of replication forks through replication fork barriers. Our data are consistent with a redundant role of Top2 and Sgs1 together with Top3 (Sgs1-Top3) in replication fork merging at rDNA barriers. At RFB either Top2 or Sgs1-Top3 is essential to prevent formation of a checkpoint activating DNA structure during termination, but at less strong rDNA barriers absence of the enzymes merely delays replication fork merging, causing an accumulation of asymmetric termination structures, which are solved over time.     

(R)-2-(4-Phenylbutyl)dihydrobenzofuran derivatives as melatoninergic agents

(R)-2-(4-Phenylbutyl)dihydrobenzofuran derivatives (e.g., 3 and 4) were synthesized as novel melatoninergic ligands with significantly lower vasoconstrictive activity in vitro in the rat tail artery. Binding affinity assays were performed on cloned human MT1 and MT2 receptors stably expressed in NIH3T3 cells.     

Measurement of myocardial free radical production during exercise using EPR spectroscopy

Exercise is associated with an increase in oxygen flux through the mitochondrial electron transport chain that has recently been demonstrated to increase the production of reactive oxygen species (ROS) in skeletal muscle. This study examined whether exercise also causes free radical production in the heart. We measured ROS production in seven chronically instrumented dogs during rest and treadmill exercise (6.4 km/h at 10 degrees grade; and heart rate, 204 +/- 3 beats/min) using electron paramagnetic resonance spectroscopy in conjunction with the spin trap alpha-phenyl-tert-butylnitrone (PBN) (0.14 mol/l) in blood collected from the aorta and coronary sinus (CS). To improve signal detection, the free radical adducts were deoxygenated over a nitrogen stream for 15 min and extracted with toluene. The hyperfine splitting constants of the radicals were alpha(N) = 13.7 G and alpha(H) = 1.0 G, consistent with an alkoxyl or carbon-centered radical. Resting aortic and CS PBN adduct concentrations were 6.7 and 6.3 x 10(8) arbitrary units (P = not significant). Both aortic and CS adduct concentrations increased during exercise, but there was no significant difference between the aortic and CS concentrations. Thus, in contrast to skeletal muscle, submaximal treadmill exercise did not result in detectable free radical production by the heart.     

The amyloid-beta rise and gamma-secretase inhibitor potency depend on the level of substrate expression

The amyloid-beta (Abeta) peptide, which likely plays a key role in Alzheimer disease, is derived from the amyloid-beta precursor protein (APP) through consecutive proteolytic cleavages by beta-site APP-cleaving enzyme and gamma-secretase. Unexpectedly gamma-secretase inhibitors can increase the secretion of Abeta peptides under some circumstances. This "Abeta rise" phenomenon, the same inhibitor causing an increase in Abeta at low concentrations but inhibition at higher concentrations, has been widely observed. Here we show that the Abeta rise depends on the beta-secretase-derived C-terminal fragment of APP (betaCTF) or C99 levels with low levels causing rises. In contrast, the N-terminally truncated form of Abeta, known as "p3," formed by alpha-secretase cleavage, did not exhibit a rise. In addition to the Abeta rise, low betaCTF or C99 expression decreased gamma-secretase inhibitor potency. This "potency shift" may be explained by the relatively high enzyme to substrate ratio under conditions of low substrate because increased concentrations of inhibitor would be necessary to affect substrate turnover. Consistent with this hypothesis, gamma-secretase inhibitor radioligand occupancy studies showed that a high level of occupancy was correlated with inhibition of Abeta under conditions of low substrate expression. The Abeta rise was also observed in rat brain after dosing with the gamma-secretase inhibitor BMS-299897. The Abeta rise and potency shift are therefore relevant factors in the development of gamma-secretase inhibitors and can be evaluated using appropriate choices of animal and cell culture models. Hypothetical mechanisms for the Abeta rise, including the "incomplete processing" and endocytic models, are discussed.