Early perturbation in mitochondria redox homeostasis in response to environmental stress predicts cell fate in diatoms

Diatoms are ubiquitous marine photosynthetic eukaryotes that are responsible for about 20% of global photosynthesis. Nevertheless, little is known about the redox-based mechanisms that mediate diatom sensing and acclimation to environmental stress. Here we used a redox-sensitive green fluorescent protein sensor targeted to various subcellular organelles in the marine diatom Phaeodactylum tricornutum, to map the spatial and temporal oxidation patterns in response to environmental stresses. Specific organelle oxidation patterns were found in response to various stress conditions such as oxidative stress, nutrient limitation and exposure to diatom-derived infochemicals. We found a strong correlation between the mitochondrial glutathione (GSH) redox potential (E_GSH) and subsequent induction of cell death in response to the diatom-derived unsaturated aldehyde 2E,4E/Z-decadienal (DD), and a volatile halocarbon (BrCN) that mediate trophic-level interactions in marine diatoms. Induction of cell death in response to DD was mediated by oxidation of mitochondrial E_GSH and was reversible by application of GSH only within a narrow time frame. We found that cell fate can be accurately predicted by a distinct life-death threshold of mitochondrial E_GSH (−335 mV). We propose that compartmentalized redox-based signaling can integrate the input of diverse environmental cues and will determine cell fate decisions as part of algal acclimation to stress conditions.     

Structure-based analysis of VDAC1: N-terminus location, translocation, channel gating and association with anti-apoptotic proteins

Structural studies place the VDAC1 (voltage-dependent anion channel 1) N-terminal region within the channel pore. Biochemical and functional studies, however, reveal that the N-terminal domain is cytoplasmically exposed. In the present study, the location and translocation of the VDAC1 N-terminal domain, and its role in voltage-gating and as a target for anti-apoptotic proteins, were addressed. Site-directed mutagenesis and cysteine residue substitution, together with a thiol-specific cross-linker, served to show that the VDAC1 N-terminal region exists in a dynamic equilibrium, located within the pore or exposed outside the β-barrel. Using a single cysteine-residue-bearing VDAC1, we demonstrate that the N-terminal region lies inside the pore. However, the same region can be exposed outside the pore, where it dimerizes with the N-terminal domain of a second VDAC1 molecule. When the N-terminal region α-helix structure was perturbed, intra-molecular cross-linking was abolished and dimerization was enhanced. This mutant also displays reduced voltage-gating and reduced binding to hexokinase, but not to the anti-apoptotic proteins Bcl-2 and Bcl-xL. Replacing glycine residues in the N-terminal domain GRS (glycine-rich sequence) yielded less intra-molecular cross-linked product but more dimerization, suggesting that GRS provides the flexibility needed for N-terminal translocation from the internal pore to the channel face. N-terminal mobility may thus contribute to channel gating and interaction with anti-apoptotic proteins.     

Brain Polyamine Stress Response: Recurrence After Repetitive Stressor and Inhibition by Lithium

Abstract: We recently demonstrated that, unlike in peripheral tissues, the increase in activity of polyamine synthesizing enzymes observed in the brain after acute stress can be prevented by long-term, but not by short-term, treatment with lithium. In the present study we sought to examine the effects of chronic intermittent stress on two key polyamine synthesizing enzymes, ornithine decarboxylase and S-adenosylmethionine decarboxylase, and their modulation by lithium treatment. Adult male rats were subjected to 2 h of restraint stress once daily for 5 days and to an additional delayed stress episode 7 days later. Enzyme activities were assayed 6 h after the beginning of each stress episode. In contrast to the liver, where ornithine decarboxylase activity was increased (300% of the control) only after the first stress episode, the enzyme activity in the brain was increased after each stress episode (to ～170% of the control). Unlike ornithine decarboxylase activity, S-adenosylmethionine decarboxylase activity was slightly reduced after the first episode (86% of the control) but remained unchanged thereafter. After cessation of the intermittent stress period, an additional stress episode 7 days later led again to an increase in ornithine decarboxylase activity in the brain (225% of the control) but not in the liver, whereas S-adenosylmethionine decarboxylase activity remained unchanged. The latter increase in ornithine decarboxylase activity was blocked by lithium treatment during the intervening 7-day interval between stressors. The results warrant the following conclusions: (a) Repetitive application of stressors results in a recurrent increase in ornithine decarboxylase activity in the brain but to habituation of this response in the liver. (b) This brain polyamine stress response can be blocked by long-term (days) lithium treatment. (c) The study implicates an overreactive polyamine response as a component of the adaptive, or maladaptive, brain response to stressful events and as a novel molecular target for lithium action.     

THE PHYSIOLOGICAL ECOLOGY OF A FRESHWATER DINOFLAGELLATE BLOOM POPULATION: VERTICAL MIGRATION,NITROGEN LIMITATION,AND NUTRIENT UPTAKE KINETICS1,2

The motile freshwater dinoflagellate Gymnodinium bogoriense Klebs., which forms dense blooms in Jezre'el Valley water reservoirs (Israel) appears to be physiologically suited to exploit stratified environments, where it outcompetes all other phytoplankton types. The dense summer blooms (“red tides”) were found to be nitrogen-limited. The algae's competitive advantage, however, cannot result from superior uptake capabilities: its K_s (μmol NH₄·L^?1) for NH₄ was higher and its V_maxμmol NH₄·mg chlorophyll a^?1·h^?1) was lower than other phytoplankton types commonly occurring in the region. The competitive advantage of G. bogoriense probably stems from other physiological capabilities: dark ammonia and phosphorus assimilation and the ability to undertake diel vertical migration cycles between the upper photic water layers during the day and nutrient-rich deeper layers at night. These findings confirm the vertical nutrient retrieval hypothesis in migrating phytoplankton.     

Activation of Ca2+ release in isolated sarcoplasmic reticulum

Summary The relationship between Ca²⁺ release from sarcoplasmic reticulum, induced by elevated pH, tetraphenylboron (TPB^–) or chemical modification, and the change in the surface charge of the membranes as measured by the fluorescence intensity of anilinonaphthalene sulfonate (ANS) is examined. The stimulated Ca²⁺ release is inhibited by dicyclohexylcarbodiimide and external Ca²⁺. TPB^–, but not tetraphenylarsonium (TPA⁺), causes a decrease in ANS^– fluorescence, with 50% decrease occurring at about 5 m TPB^–. The decrease in ANS^– fluorescence as well as the inhibition of Ca²⁺ accumulation induced by TPB^– are prevented by TPA⁺. A linear relationship between the decrease in membrane surface potential and the extent of the Ca²⁺ released by TPB^– is obtained. Similar levels of [³H]TPB^– bound to sarcoplasmic reticulum membranes were obtained regardless of whether or not the vesicles have taken up Ca²⁺. The inhibition of Ca²⁺ accumulation and the [³H]TPB^– incorporation into the membranes were correlated. Ca²⁺ release from sarcoplasmic reticulum, by pH elevation, chemical modification or by addition of NaSCN (0.2 to 0.5m) or the Ca²⁺ ionophore ionomycin, is also accompanied by a decrease in ANS^– fluorescence intensity. However, chemical modification and elevated pH affects the surface potential much less than SCN^– or TPB^– do. These results suggest that the enhancement of Ca²⁺ release by these treatments is not due to a general effect on the membrane surface potential, but rather through the modification of a specific protein. They also suggest that membrane surface charges might play an important role in the control mechanism of Ca²⁺ release.     

Atrial natriuretic peptide administration to normal and salt depleted rats--effects on digoxin-like immunoreactive factor, aldosterone, ACTH, and renal function

In view of the known interrelationships between renin, aldosterone, and atrial natriuretic peptide (ANP), we sought to examine whether there also exists an interaction between ANP and digoxin-like immunoreactive factor (DLIF). We therefore studied the effects of ANP administration on normal and salt-depleted rats, and measured the effects on blood pressure, urine output, glomerular filtration rate, sodium excretion, aldosterone, ACTH, and DLIF levels. ANP administration resulted in a significant elevation of sodium excretion and glomerular filtration rate and a fall in blood pressure. DLIF concentrations in plasma rose significantly, as did urinary DLIF excretion. ANP administration resulted in a fall in aldosterone as well as ACTH. These observations suggest that ANP has a direct inhibitory effect on ACTH secretion. Our findings support the concept of an interrelationship between ANP and DLIF.     

Facultative anoxygenic photosynthesis in the cyanobacterium Oscillatoria limnetica.

An isolate from H2S-rich layers of the Solar Lake, the cyanobacterium Oscillatoria limnetica, exhibits both oxygenic and anoxygenic photosynthesis. It can use Na2S as an electron donor for CO2 photoassimilation (photosystem I supplies the energy) in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea or 700-nm light. A stoichiometric ratio of approximately 2 is observed between the Na2S consumed and the photoassimilated CO2. The anoxygenic phototrophic capability of this cyanobacterium explains its growth in nature in high sulfide concentrations and indicates a selective advantage.