Depolarization of the membrane potential by beta-lactams as a signal to induce autolysis.

The effect of beta-lactam antibiotics that are known to inhibit cell wall biosynthesis and induce cell wall autolysis on the electrophysiological state of the plasma membrane in Streptomyces griseus was studied. Addition of various beta-lactam antibiotics induced a dose- and growth-stage-dependent depolarization of the membrane potential of Streptomyces griseus. The hydrolyzed biologically inactive derivative penicilloic acid had no depolarizing effect on the membrane potential. The ionophore gramicidin D, while depolarizing the membrane potential, also induced a dose-dependent increase in cell wall lysis. These observations suggest that alteration of the transmembrane potential could be an important signal in triggering cell wall autolysis of S. griseus.     

Membrane Potential Depolarization as a Triggering Mechanism for Vpu-Mediated HIV-1 Release

Vpu, a component unique to HIV-1, greatly enhances the efficiency of viral particle release by unclear mechanisms. This Vpu function is intrinsically linked to its channel-like structure, which enables it to interfere with homologous transmembrane structures in infected cells. Because Vpu interacts destructively with host background K⁺ channels that set the cell resting potential, we hypothesized that Vpu might trigger viral release by destabilizing the electric field across a budding membrane. Here, we found that the efficiency of Vpu-mediated viral release is inversely correlated with membrane potential polarization. By inhibiting the background K⁺ currents, Vpu dissipates the voltage constraint on viral particle discharge. As a proof of concept, we show that HIV-1 release can be accelerated by externally imposed depolarization alone. Our findings identify the trigger of Vpu-mediated release as a manifestation of the general principle of depolarization-stimulated exocytosis.     

Ultrafast Depolarization of Transient Absorption as a Probe of Plasmonicity of Optical Transitions in Ag Nanoclusters

Time-resolved polarization-dependent transient absorption has been used to study the plasmonicity of the optical transitions of Ag nanoparticles and nanoclusters. The lack of a measureable polarization anisotropy in the nanoparticles is indicative of the ultrafast electron-electron scattering while the anisotropy with a depolarization timescale of 500 fs observed in the nanoclusters indicates the excitation of a non-plasmonic state. The short lifetime of the anisotropy is a measure of electronic coupling between nearly degenerate states and is thus proposed as a sensitive measurement of the plasmonic content of the optical transitions of nanoclusters.

     

Membrane Depolarization and the Metabolism of Muscle

The respiration of frog twitch muscles rises markedly when [K]₀israised; respiration is stimulated by levels of [K]₀ below thethreshold for contracture(Fenn, 1931).Respiration is also stimulatedby elevated [Rb]₀ and [Cs]₀ in direct relation to their abilityto depolarize the membrane. Respiration is stimulated even whenthe anions in the high [K]_o solution cannot permeate the membrane.If[K]₀is raised to 25 mM there is an increase in respiration whichis sustained for hours. If [K]₀is 30 mM or above, there is atransitory burst of stimulated respiration followed by a declineback to the basal level. The response to elevated[K]₀ can beblocked by divalent cations or by local anesthetics; the blockingagents act rapidly, probably on the cell membrane. Either extracellularcalcium or strontium is needed for a prolonged stimulation ofrespiration. Depolarization seems to increase respiration bycausing the release of calcium into the sarcoplasm. Since respirationis increased by a depolarization below the threshold for producinga contracture, respiration is a sensitive indicator of the sarcoplasmicconcentration of calcium. A model for the relation between sarcoplasmic[Ca] and membrane potential is proposed. Calcium can be releasedfrom a store in the cell, the released Ca⁺⁺ entering the sarcoplasm.The store is replenished by Ca⁺⁺ entering the fiber from theextracellular solution. When the membrane is depolarized, therate of release of calcium in the store is increased; at thesame time the rate at which extracellular calcium can replenishthe store is decreased.This model accounts well for the dataon respiration and also for the contractures of single musclefibers. Calcium probably acts within the cell to activate anATP-ase which causes an increase in ADP and hence an increasein respiration. Other investigators have found changes in theactivity of certain enzymes and in the permeability of the membrane;possibly these changes are also a direct response to an increasein sarcoplasmic calcium.     

Depolarization of the membrane potential by hyaluronan

The membrane potential is mainly maintained by the K⁺ concentration gradient across the cell membrane between the cytosol and the extracellular matrix. Here, we show that extracellular addition of high‐molecular weight hyaluronan depolarized the membrane potential of human fibroblasts, human embryonic kidney cells (HEK), and central nervous system neurons in a concentration‐dependent manner, whereas digestion of cell surface hyaluronan by hyaluronidase caused hyperpolarization. This effect could not be achieved by other glycosaminoglycans or hyaluronan oligosaccharides, chondroitin sulfate, and heparin which did not affect the membrane potential. Mixtures of high‐molecular weight hyaluronan and bovine serum albumin had a larger depolarization effect than expected as the sum of both individual components. The different behavior of high‐molecular weight hyaluronan versus hyaluronan oligosaccharides and other glycosaminoglycans can be explained by a Donnan effect combined with a steric exclusion of other molecules from the water solvated chains of high‐molecular weight hyaluronan. Depolarization of the plasma membrane by hyaluronan represents an additional pathway of signal transduction to the classical CD44 signal transduction pathway, which links the extracellular matrix to intracellular metabolism. J. Cell. Biochem. 111: 858–864, 2010. © 2010 Wiley‐Liss, Inc.     

Depolarization of the liver cell membrane by metformin

Metformin (1,1-dimethylbiguanide; MET) is used in the treatment of type 2 diabetes mellitus. MET's antihyperglycemic action depends at least in part on its inhibitory effect on hepatic gluconeogenesis. As to gluconeogenesis from amino acids (e.g. L-alanine), this is associated with an inhibition of L-alanine uptake into hepatocytes. Since this uptake is mediated by an electrogenic transport mechanism, the aim of the present study was to investigate whether MET has an influence on the liver cell membrane potential which might explain its inhibitory effect on L-alanine uptake. The experiments were performed in vivo in anesthetized rats and in vitro using superfused mouse liver slices with the conventional microelectrode technique. In vivo, MET (160 mg/kg intraperitoneally (i.p.)) significantly depolarized (dV) the liver cell membrane by 6 mV. MET (1 mmol/l) also depolarized the liver cell membrane in vitro (e.g. 15 min after start of superfusion: dV=8 mV). MET's effect was at least partly reversible. Glucagon (10(-7) mol/l), which hyperpolarized the liver cell membrane, abolished MET's effect. Further, the MET-induced depolarization was completely absent during superfusion with low Cl(-) ([Cl(-)]=27 mmol/l) medium, and significantly attenuated by the Cl(-) channel blocker NPPB (25 micromol/l). While MET's effect was only somewhat attenuated by blockade of the Na(+)/K(+)/2Cl(-) cotransporter or by superfusion with (HCO(-)(3)-free) HEPES buffer, the carboanhydrase blocker acetazolamide (1 mmol/l) or blockade of the HCO(-)(3)/Cl(-) exchanger by DIDS (100 micromol/l), which, however, also blocks Cl(-) channels, abolished its effect. The depolarization of the liver cell membrane by MET was unaffected by a blockade of K(+) channels with Ba(2+), a blockade of the Na(+)/K(+) pump or superfusion with low Na(+) medium ([Na(+)]=26 mmol/l). According to these results, the MET-induced depolarization of the liver cell membrane could be due to an activation of the Cl(-)/HCO(-)(3) exchanger and thus depend on intracellular HCO(-)(3) formation. This activation could then lead to a disturbance of the equilibrium between intra- and extracellular Cl(-) and therefore to an enhanced Cl(-) efflux via Cl(-) channels. It is plausible that the depolarizing effect induced by MET is associated with its inhibitory effect on gluconeogenesis by inhibiting uptake of L-alanine and other amino acids into hepatocytes.     

Using Genes as Characters and a Parsimony Analysis to Explore the Phylogenetic Position of Turtles

The phylogenetic position of turtles within the vertebrate tree of life remains controversial. Conflicting conclusions from different studies are likely a consequence of systematic error in the tree construction process, rather than random error from small amounts of data. Using genomic data, we evaluate the phylogenetic position of turtles with both conventional concatenated data analysis and a “genes as characters” approach. Two datasets were constructed, one with seven species (human, opossum, zebra finch, chicken, green anole, Chinese pond turtle, and western clawed frog) and 4584 orthologous genes, and the second with four additional species (soft-shelled turtle, Nile crocodile, royal python, and tuatara) but only 1638 genes. Our concatenated data analysis strongly supported turtle as the sister-group to archosaurs (the archosaur hypothesis), similar to several recent genomic data based studies using similar methods. When using genes as characters and gene trees as character-state trees with equal weighting for each gene, however, our parsimony analysis suggested that turtles are possibly sister-group to diapsids, archosaurs, or lepidosaurs. None of these resolutions were strongly supported by bootstraps. Furthermore, our incongruence analysis clearly demonstrated that there is a large amount of inconsistency among genes and most of the conflict relates to the placement of turtles. We conclude that the uncertain placement of turtles is a reflection of the true state of nature. Concatenated data analysis of large and heterogeneous datasets likely suffers from systematic error and over-estimates of confidence as a consequence of a large number of characters. Using genes as characters offers an alternative for phylogenomic analysis. It has potential to reduce systematic error, such as data heterogeneity and long-branch attraction, and it can also avoid problems associated with computation time and model selection. Finally, treating genes as characters provides a convenient method for examining gene and genome evolution.     

Muscarinic Depolarization of Layer II Neurons of the Parasubiculum

The parasubiculum (PaS) is a component of the hippocampal formation that sends its major output to layer II of the entorhinal cortex. The PaS receives strong cholinergic innervation from the basal forebrain that is likely to modulate neuronal excitability and contribute to theta-frequency network activity. The present study used whole cell current- and voltage-clamp recordings to determine the effects of cholinergic receptor activation on layer II PaS neurons. Bath application of carbachol (CCh; 10–50 µM) resulted in a dose-dependent depolarization of morphologically-identified layer II stellate and pyramidal cells that was not prevented by blockade of excitatory and inhibitory synaptic inputs. Bath application of the M₁ receptor antagonist pirenzepine (1 µM), but not the M₂-preferring antagonist methoctramine (1 µM), blocked the depolarization, suggesting that it is dependent on M₁ receptors. Voltage-clamp experiments using ramped voltage commands showed that CCh resulted in the gradual development of an inward current that was partially blocked by concurrent application of the selective Kv7.2/3 channel antagonist XE-991, which inhibits the muscarine-dependent K⁺ current I_M. The remaining inward current also reversed near E_K and was inhibited by the K⁺ channel blocker Ba²⁺, suggesting that M₁ receptor activation attenuates both I_M as well as an additional K⁺ current. The additional K⁺ current showed rectification at depolarized voltages, similar to K⁺ conductances mediated by Kir 2.3 channels. The cholinergic depolarization of layer II PaS neurons therefore appears to occur through M₁-mediated effects on I_M as well as an additional K⁺ conductance.     

A Contrasting Position Grid as a Diagnostic Measure of the Level of Development of Interpersonal Relations

As a number of theoretical and experimental studies have shown [5,6,8,9,11, and others], the level of group development is an extremely important parameter of interpersonal relations. We refer to a general, integrative, sociopsychological characteristic of the group reflecting the mediation of interpersonal relations by the content and organization of joint activity and by the moral norms and values operating in the group. There is a fundamental difference between the stratometric approach to group processes and the traditional concepts of small-group dynamics in American psychology, in which the size of the group, the time it has existed, its composition, the feeling of belonging, feelings of satisfaction, etc., are taken as the chief parameters of the group. Essentially, the difference lies in the substantive normative definitions given to the group in the stratometric approach. The introduction of the principle of activity as a mediating link in social-psychological experiments has enabled us to construct a typology of groups that takes into account the level of development and distinguishes among collectives, prosocial associations, diffuse groups, asocial associations, and corporations.     

The Dim Light Melatonin Onset as a Marker for Orcadian Phase Position

Masking is known to affect a variety of circadian rhythms, making it difficult to use them as reliable markers of circadian phase position. Melatonin may be unique in that it appears to be masked only by (bright) light. Sleep and activity do not appear to influence the melatonin rhythm. By measuring the onset of melatonin production, a clearly demarcated event, we can reliably assess circadian phase position, provided blood is sampled under dim light (the dim light melatonin onset, or DL.MO). The DLMO has been useful in assessing the phase-shifting properties of bright light and in phase typing patients with chronobiologic disorders, such as winter depression.     

Psychological distress as a risk factor for death from cerebrovascular disease

Background:

Little is known about psychological risk factors in cerebrovascular disease. We examined the association between psychological distress and risk of death due to cerebrovascular disease.

Methods:

We obtained data from 68 652 adult participants of the Health Survey for England (mean age 54.9 [standard deviation 13.9] yr, 45.0% male sex) with no known history of cardiovascular diseases at baseline. We used the 12-item General Health Questionnaire (GHQ-12) to assess the presence of psychological distress. We followed participants for eight years for cause-specific death using linkage to national registers.

Results:

There were 2367 deaths due to cardiovascular disease during follow-up. Relative to participants with no symptoms of psychological distress (GHQ-12 score 0) at baseline, people with psychological distress (GHQ-12 score ≥ 4, 14.7% of participants) had an increased risk of death from cerebrovascular disease (adjusted hazard ratio [HR] 1.66, 95% confidence interval [CI] 1.32–2.08) and ischemic heart disease (adjusted HR 1.59, 95% CI 1.34–1.88). There was also evidence of a dose–response effect with increasing GHQ-12 score (p for trend < 0.001 in all analyses). Associations were only marginally attenuated after we adjusted for possible confounders, including socioeconomic status, smoking and use of antihypertensive medications.

Interpretation:

Psychological distress was associated with increased risk of death due to cerebrovascular disease in a large population-representative cohort. These data suggest that the cardiovascular effects of psychological distress are not limited to coronary artery disease.Psychological constructs are becoming increasingly recognized as risk factors and triggers for cardiovascular events.¹ To date, most cardiovascular research has focused on coronary artery disease rather than cerebrovascular disease. Several studies have examined the association between symptoms of depression and risk of stroke,^2–11 although the results have been inconsistent, and effect sizes are generally modest.¹² For example, some evidence from epidemiological studies has shown a relation between depression and incident stroke.^2–7 However, a recent study showed an association between depression and coronary artery disease but not cerebrovascular disease.¹¹ This finding is consistent with other analyses that have also found null associations for stroke outcomes.^8–10 Other investigators have suggested that psychological distress is a more robust predictor of stroke than are symptoms of depression.¹³In addition to a paucity of evidence linking psychological distress with risk of stroke, extant evidence is hampered by a series of methodological problems. These limitations include small sample sizes (including rarity of outcome), a limited range of confounding data, an unknown influence of existing comorbidity on psychological distress (reverse causality) and few data from women. We addressed these shortcomings by examining the association between psychological distress and risk of death due to cerebrovascular disease in a large, well-characterized cohort of men and women representative of the general population.     

Depolarization induces intersubunit cross-linking in a S4 cysteine mutant of the Shaker potassium channel

Voltage-gated potassium (K(v)) channels are integral membrane proteins, composed of four subunits, each comprising six (S1-S6) transmembrane segments. S1-S4 comprise the voltage-sensing domain, and S5-S6 with the linker P-loop forms the ion conducting pore domain. During activation, S4 undergoes structural rearrangements that lead to the opening of the channel pore and ion conduction. To obtain details of these structural changes we have used the engineered disulfide bridge approach. For this we have introduced the L361C mutation at the extracellular end of S4 of the Shaker K channel and expressed the mutant channel in Xenopus oocytes. When exposed to mild oxidizing conditions (ambient oxygen or copper phenanthroline), Cys-361 formed an intersubunit disulfide bridge as revealed by the appearance of a dimeric band on Western blotting. As a consequence, the mutant channel suffered a significant loss in conductance (measured by two-electrode voltage clamp). Removal of native cysteines failed to prevent the disulfide formation, indicating that Cys-361 forms a disulfide with its counterpart in the neighboring subunit. The effect was voltage-dependent and occurred during channel activation after Cys-361 has been exposed to the extracellular phase. Although the disulfide bridge reduced the maximal conductance, it caused a hyperpolarizing shift in the conductance-voltage relationship and reduced the deactivation kinetics of the channel. The latter two effects suggest stabilization of the open state of the channel. In conclusion, we report that during activation the intersubunit distance between the N-terminal ends of the S4 segments of the L361C mutant Shaker K channel is reduced.     

Successional Position of Dry Andean Dwarf Forest Species as a Basis for Restoration Trials

The successional affinity of nine woody species was inferred from the structure, diversity and disturbance history of the vegetation where these occurred. This was done in order to obtain a basis for a restoration experiment, currently in execution, in the dry Andean dwarf forest zone on the edge of the High Plain of Bogotá (Colombia), at 2600–2950 m.a.s.l. We laid out 101 relevees in grassland and shrubland types in different stages of recovery, and in relatively little disturbed endemic Condalia thomasiana dwarf forest. The disturbance history of sites over the last ~60 years was inferred from aerial photograph series (1941–1991). CCA and logistic regression were applied to relate species composition to diversity, environment and disturbance history. All species showed a preference for certain structural groups. Also, a clear relation between species occurrence and vegetation diversity was found. Baccharis macrantha, and Dalea coerulea appeared relatively tolerant to grazing, while the remaining seven species reacted negatively. Soil clay content, base availability and organic carbon content was also an important factor for occurrence of each species. Invasion of grasslands by woody species is pioneered by Baccharis macrantha and followed by Dodonaea viscosa. Dalea coerulea was predominantly found on truncated clayey soils, which will probably not support Condalia dwarf forest. The hypothesized classification of the nine planted species to either pioneers or late-successional was fine-tuned. This exploratory study will be of use in the set-up of future succession-based restoration experiments, and for converting exotics afforestations to natural vegetation.